Volatile Organic Compound Emission Reduction and Abatement ... · colleagues, especially Ricardo Leandro and Paula Martins, for all their good help and welcoming. ... (MLT), ﬁltration

Volatile Organic Compound Emission Reduction andAbatement within the Organic Fine Chemical Industry

Miguel Nuno Guerra Coito Marques

Thesis to obtain the Master of Science Degree in

Biological Engineering

Supervisors: Eng. Francisco Flor da Cruz FerreiraProf. Helena Maria Rodrigues Vasconcelos Pinheiro

Examination CommitteeChairperson: Prof. Arsénio do Carmo Sales Mendes Fialho

Supervisor: Prof. Helena Maria Rodrigues Vasconcelos PinheiroMember of the Committee: Prof. Maria Joana Castelo-Branco de Assis Teixeira Neiva Correia

November 2018

ii

To my parents. To my grandparents. To Fox.

iii

iv

Acknowledgments

My thanks go to my supervisors, Professor Helena Pinheiro and Francisco Ferreira for all their sup-

port throughout this project, to Professor Laura Ilharco for all her good advices, and to my internship

colleagues, especially Ricardo Leandro and Paula Martins, for all their good help and welcoming.

I would like to acknowledge Instituto Superior Tecnico’s efforts behind this work, particularly Profes-

sor Miguel Prazeres, for providing me with the opportunity of learning in an industrial environment with

all the necessary support.

Last, but not least, I want to appreciate my closest family for all their patience and caring throughout

these studying years - my mother and father, my grandmother and grandfathers - as well as Ana and her

dog, Fox.

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Abstract

This document aims at the description of the work performed during a six-month participation of an

engineering master’s student on a Volatile Organic Compound (VOC) reduction and abatement project,

on an industrial scale, within a manufacturing site for active pharmaceutical ingredients.

On a first level, this work oversaw the conceptualization and implementation of a methylene-chloride

emission reduction strategy, through the installation of flow restriction orifices on equipment vent lines,

with monitoring of VOC emissions and Lower Explosion Limit (LEL) percentage. Such work was sup-

ported by a modelling of the affected vent header system, broadly predicting orifice installation impact.

Furthermore, the installation of vent condensers on the affected vent lines was evaluated using Aspen

Plus software, through methylene-chloride condensation simulation.

Secondly, through a gap-analysis of the site’s vent collecting system, all structural and instrumenta-

tion requirements were identified under a safety-by-design perspective. Moreover, a re-assessment of

the current pipeline materials and diameters was performed to ensure the safe collection and treatment

of all present and future process vents.

Finally, a turnkey end-of-line VOC abatement solution was procured, composed by a Regenerative

Thermal Oxidiser (RTO) with integrated LEL control, through the automated inlet of dilution air. A de-

cision was made to install additional quench and scrubber equipment, to prevent dioxin regeneration

and to guarantee the removal of hydrogen chloride. An additional decision was made to install a selec-

tive catalytic reduction system, to ensure the abatement of nitrogen oxides formed during the oxidation

process. The selected system was thoroughly evaluated from an operational safety perspective.

Keywords: Volatile Organic Compounds, Lower Explosion Level control, Flow restriction orifice,

Dichloromethane condensation, Safety-by-design, Regenerative thermal oxidation, Selective catalytic

reduction.

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viii

Resumo

O presente documento descreve o trabalho realizado por um aluno de mestrado de engenharia, no

decorrer da participacao num projecto de mitigacao de Compostos Organicos Volateis (COV) realizado

a escala industrial, numa instalacao de producao de ingredientes activos farmaceuticos.

Foi abordada a conceptualizacao e implementacao de uma estrategia de reducao de emissoes

de diclorometano, instalando-se orifıcios de restricao nas tubagens de respiros de reactores, com

monotorizacao das emissoes de COVs, bem como da percentagem do Limite Inferior de Explosivi-

dade (LIE). A implementacao foi guiada pela modelacao da rede de respiros destes reactores, tendo

o seu impacto global sido previsto adequadamente. Adicionalmente, a instalacao de condensadores

na rede de respiros foi avaliada utilizando o software Aspen Plus para simulacao da condensacao de

diclorometano.

Atraves de uma analise do sistema de recolha de respiros da fabrica, foram identificados requisitos

estruturais e de instrumentacao do mesmo, sob uma perspectiva de seguranca-por-projecto. Foram

reavaliados os respectivos materiais e diametros de tubagem, de modo a garantir a recolha e tratamento

em seguranca dos respiros de processos actuais e futuros.

Foi finalmente seleccionado um sistema terminal de tratamento de COVs, constituıdo por um Re-

actor Termico Regenerativo (RTR) com controlo integrado de LIE, por diluicao automatica com ar.

Definiu-se a instalacao de equipamentos de arrefecimento rapido e lavagem de gases, prevenindo a

regeneracao de dioxinas e garantindo a remocao de acido clorıdrico. Optou-se ainda pela instalacao de

um sistema catalıtico de reducao selectiva, garantindo a remocao dos oxidos de azoto formados no pro-

cesso oxidativo. Este sistema foi ainda avaliado escrupulosamente sob uma perspectiva de seguranca

operacional.

Palavras-chave: Compostos Organicos Volateis, Controlo do Limite Inferior de Explosivi-

dade, Orifıcios de restricao de caudal, Condensacao de diclorometano, Seguranca-por-projecto, Oxidacao

termica regenerativa, Reducao catalıtica selectiva.

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x

Contents

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Resumo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvii

Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxi

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxv

1 Introduction 1

1.1 Industrial background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Atmospheric emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.2 Explosive limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.2.1 The plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.2.2 Waste gas emission data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.2.3 Challenges and objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2 Action Plan 15

2.1 VOC source reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2 VCS upgrade and expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.3 End-of-line VOC abatement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3 Implementation 25

3.1 Engineering concept design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.2 Data collection and analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.2.1 Process historical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.2.2 Process field data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.2.3 Thermodynamic and transport data . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.3 Mass, energy and momentum balances . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.3.1 Vent header system modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.3.2 Restriction orifices impact study . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.3.3 Condenser basic design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

xi

3.3.4 Vent collecting system diameter evaluation . . . . . . . . . . . . . . . . . . . . . . 47

3.4 Procurement and selection of a regenerative thermal oxidizer . . . . . . . . . . . . . . . . 51

3.4.1 Supplier selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.4.2 Technical information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

3.4.3 Environmental compliance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

3.5 Safety analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

3.5.1 Hazard and operability analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

3.5.2 Safety integrity level assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

4 Preliminary results 69

4.1 Operations lead time increase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

4.2 Waste gas monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

5 Conclusions 73

5.1 Achievements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

5.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

Bibliography 77

A Process and instrumentation diagrams 81

A.1 Process and instrumentation diagrams relevant components . . . . . . . . . . . . . . . . 81

xii

List of Tables

1.1 Relevant VOCs in the OFC sector and associated hazards. Listed in bold are the haz-

ards responsible for the definition of the emission limit value (ELV), reporting to half-hour

averages, for a given minimum emission flowrate, in accordance to Portuguese legislation

for API manufacturing facilities dealing with organic solvents [2]. It should be noted that

for TCDD and TCDF (tetrachlorodibenzodioxin and tetrachlorodibenzofuran, respectively)

there is not a ELV defined [2]. DCM, DMF and DMA refer to dichloromethane, dimethyl-

formamide and dimethylacetamide, respectively. . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 VCS inlet gas composition monitoring since 2012. Contains values both from external

and internal monitoring, for main pollutants. Pollutant concentration is shown normalized

against the highest measured pollutant concentration (VOC, maximum - July 2012, in

mg/Nm3), signalled in bold. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.1 Emission limit values (ELV, for facilities using organic solvents for API manufacturing),

associated emission values (AEV) for an RTO/DeNOx system (assuming BAT) and moni-

toring frequency. Values report to half-hour averages. . . . . . . . . . . . . . . . . . . . . 24

3.1 Orifice plate generic design, based on DIN standards for a nominal pressure of 6 bar [35].

Internal diameters refer to 50% and 75% diameter reduction, respectively. . . . . . . . . . 27

3.2 Desired versus expected emission limits, expressed as half-hour measurements, based

on information provided by the chosen supplier for the RTO system. . . . . . . . . . . . . 29

3.3 Project budget distribution (approximate figures, in §), according to the three subprojects

and type of engineering works. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.4 Corrective actions to reduce dichloromethane source emissions, as well as non-ducted

vent emissions, based on gathered data and installation inspection. Associated equip-

ment comprises reactors (R), vent condensers (VC), centrifuges (CG), mother-liquor tanks

(MLT), filtration units (FU) and generic tanks (T). . . . . . . . . . . . . . . . . . . . . . . . 31

xiii

3.5 Average flowrates and piping diameters of the plant’s current vent collecting system, con-

sidering short-term production increase (up to 2021), future production buildings and

short-term production increase. As shown, each scrubber (S) is connected to a subcollec-

tor (collectors 2 to 6). The main collector (collector 1) receives vents from all subcollectors.

Each VP, SF, CT, ET or SDY refers to a vacuum pump, a solvent farm, a charge tank, an

effluent tank or a spray-dryer, respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.6 Current status of all scrubbers connected to the site’s vent collecting system. . . . . . . . 34

3.7 Current instrumentation and automation on the site’s scrubbers. Note that any existing

pressure relief valves (PRVs) should be replaced by pressure safety valves (PSVs). . . . 35

3.8 Prediction results of thermodynamic properties, using the Aspen Plus software and the

NRTL method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.9 Modelled flowrates and pressures on the vent header system for building E, based on

maximum operating pressures for each equipment piece (i.e. connecting points) and

considering the layout displayed by Figure 3.2. The error column translates the sum of all

vectorial flowrates for a given node. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.10 Expected velocity at the restriction orifice for each vent line, as well as restricted flowrates,

expected time increase and pressure drop imposed. Values shown for a 50% diameter

reduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.11 Expected velocity at the restriction orifice for each vent line, as well as restricted flowrates,

expected time increase and pressure drop imposed. Values shown for a 75% diameter

reduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.12 Mass and energy balance results for the inlet of the condensers for MLT03 and MLT01. . 46

3.13 Mass and energy balance results for the outlet of the condensers for MLT03 and MLT01. . 46

3.14 Condenser basic design parameters, including transfer area and thermal fluid flowrate. . . 48

3.15 Flowrates, diameters and predicted maximum flow speeds of the plant’s current vent col-

lecting system, considering short-term production increase (up to 2021). As shown, each

scrubber (S) is connected to a subcollector (collectors 2 to 6). The main collector (collec-

tor 1) receives vents from all subcollectors. Each VP, SF, CT, ET or SDY refers to a vacuum

pump, a solvent farm, a loading vessel, an effluent tank or a spray-dryer, respectively. . . 49

3.16 Flowrates, suggested diamters and predicted maximum flow speeds considering a short-

term vent collecting system revamping. As shown, each scrubber (S) is connected to a

subcollector (collectors 2 to 6). The main collector (collector 1) receives vents from all

subcollectors. Each VP, SF, CT, ET or SDY refers to a vacuum pump, a solvent farm, a

loading vessel, an effluent tank or a spray-dryer, respectively. . . . . . . . . . . . . . . . . 50

3.17 Summarized technical comparison between the three considered suppliers for an RTO

system. Items not included in a given technical solution were signalled as N.I., whereas

items for which no data was given, but are part of the proposed system, are labelled as

N.D.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.18 Approximate cost comparison between the three considered suppliers for an RTO system. 54

xiv

3.19 Estimated power consumption and energy costs associated with operating the RTO at

different flowrates and VOC loads, assuming a cost of 0.0259 § per kWh of natural gas

and of 0.10 § per kWh or electrical power. . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

3.20 Relevant consequences, safeguards, risk ranking and recommendations for node 1. . . . 65



3.23 Safety integrity levels required for the analysed safety instrumented functions. . . . . . . . 68

4.1 Inertization time comparison before and after the installation of flow restriction orifices for

50% diameter reduction, for the affected equipment pieces. . . . . . . . . . . . . . . . . . 70

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xvi

List of Figures

1.1 Emissions of non-methane VOCs to air by industry sector/activity in Europe in 2010 [1]. . 3

1.2 Emissions of nitrogen oxides to air by industry sector/activity in Europe in 2010 [1]. . . . . 4

1.3 Methane-Oxygen-Nitrogen flammability (explosive) range at 25 oC and atmospheric pres-

sure [14]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.4 Standard sign indicating an area where explosive atmospheres may form [19]. . . . . . . 7

1.5 Schematic of the plant’s vent collecting system. . . . . . . . . . . . . . . . . . . . . . . . . 11

1.6 Distribution of the forty LEL events registered in 2016 between the main chemical produc-

tion areas (X, Y and Z). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.7 Distribution of the forty LEL events registered in 2016 between the five VCS sub-collectors

which converge to Collector 1 (main collector). . . . . . . . . . . . . . . . . . . . . . . . . 12

1.8 Scheduled production volume expansion for chemical operations in P until 2023 (blue

trend) and associated process vents expected increase (orange trend). . . . . . . . . . . 13

2.1 Schematic drawing of a shell and tube condenser, where vapour is cooled with cold water,

for condensates removal. [25]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.2 Comparison between a), an orifice plate of fixed geometry [27], and b), a pressure control

globe valve, where the plug’s position can vary from 0% to 100% [28]. . . . . . . . . . . . 18

2.3 Polypropylene, self-extinguishing, electro-conductive. Especially suited for the transporta-

tion of flammable media. Due to carbon black, can be used outdoor. Can effectively

replace stainless steel piping [29]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.4 Decision making diagram with criteria established by BREF OFC [31] for the choice of a

waste gas VOC abatement technique. Table 5.4 (as referred in the picture) is shown of

below the diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.5 Schematic representation of a regenerative thermal oxidizer [32]. Orange lines represent

process vents, blue lines represent clean air used for purging and green lines represent

the oxidized air stream. Methane, natural gas or even liquid waste can be used as auxiliary

fuel (yellow line). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.1 Expected timeline for the current VOC reduction and abatement project, where CapEx

denotes the project’s capital expenditure, or investment. . . . . . . . . . . . . . . . . . . . 30

xvii

3.2 Schematic of building E’s vent header system. In this representation, each equipment

piece is shown as venting independently to the building’s dedicated scrubber (S11) -

however, they actually join in a main header before reaching the scrubber. (VC) Vent

condenser; (LLE) Liquid-liquid extractor; (CG) Centrifuge; (J) Pipeline junction (only rep-

resentative); (R) Reactor; (CT) Charge tank; (DT) Distillate tank. . . . . . . . . . . . . . . 36

3.3 Representation of a multiple-pipe system, where junction j connects pipes from points at

different pressures, forming a node [26]. The sum of the vectorial flowrates from each

pipe towards the node must be zero. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.4 Non-recoverable head loss (resistance coefficient) in Bernoulli obstruction meters in func-

tion of the ratio between orifice diameter and pipe nominal diameter (�) [26]. For thin-

orifice plates with � = 0.5 and � = 0.25 resistance coefficients of 1.8 and 2.5 were

considered, respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.5 Expected temperature profile for the designed vent condensers, where the orange and

blue lines represent the hot and cold fluid temperatures across the heat exchangers, re-

spectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.6 (T, x, y) diagram for a mixture of dichloromethane and nitrogen at atmospheric pressure.

Representation in function of temperature and dichloromethane mass fraction. Thermody-

namic properties predicted by Aspen Plus software, using the NRTL method. The tieline

for a mixture with 70% dichloromethane at -20 oC is represented in red. . . . . . . . . . . 45

3.7 Film coefficient for condensation processes in vertical tubes as a function of the Reynolds

and Prandtl numbers, based on the Nusselt model [36]. . . . . . . . . . . . . . . . . . . . 47

3.8 Mechanical drawing of the designed RTO system. . . . . . . . . . . . . . . . . . . . . . . 56

3.9 Layout drawing of the designed RTO system. . . . . . . . . . . . . . . . . . . . . . . . . . 57

3.10 Process flow diagram of the designed RTO system and its components. On the RTO

(from the right to the left) the first chamber is receiving VOC rich process gas, the second

chamber is exhausting VOC free process gas towards the quench and the third chamber

is being purged of retained VOC containing process gas. HSBP designates the hot side

bypass and LEL the process gas’ lower explosion limit. . . . . . . . . . . . . . . . . . . . . 57

3.11 Estimated energy costs associated with operating the RTO at different flowrates and VOC

loads (in mgC), assuming a cost of 0.0259 § per kWh of natural gas and of 0.10 § per

kWh or electrical power. The blue, orange, grey and yellow series represent, respectively,

operation at 15,000 Nm3/h, at 12,000 Nm3/h, at 8,000 Nm3/h and standby operation with

3,000 Nm3/h of fresh atmospheric air. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

xviii

3.12 Risk matrix example for process safety analysis [41]. Risk rating is given by the prod-

uct of likelihood by severity. (Green) Risk accepted: no measures are recommended;

(Yellow) Risk not desirable, however accepted; (Orange) For existing processes already

running, those can carry on until the due date for control measures implementation, being

reassessed by then or interrupted. For processes under design/development such risk

level is not accepted and additional controls must be implemented to bring the risk down

to an acceptable rating; (Red) Unacceptable risk. . . . . . . . . . . . . . . . . . . . . . . . 64

3.13 Tolerable risk matrix for LOPA method. (Red) Non-tolerable risk; (Green) Tolerable risk. . 67

A.1 Process and instrumentation diagram elaborated by supplier B showing process gas (on

the bottom) and dilution air (on the left) inlet to the regenerative thermal oxidizer system. . 83

A.2 Process and instrumentation diagram elaborated by supplier B showing dilution air heater

and damper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

A.3 Process and instrumentation diagram elaborated by supplier B showing gas bottles and

compressed air to each of the two LEL analyzers. . . . . . . . . . . . . . . . . . . . . . . . 84

A.4 Process and instrumentation diagram elaborated by supplier B showing the two LEL ana-

lyzers, main process fan and the bypass dampers responsible for deviating the flow from

the oxidizer (to the right) to the carbon bed drums (down). . . . . . . . . . . . . . . . . . . 84

A.5 Process and instrumentation diagram elaborated by supplier B showing the oxidizer’s

lower section (including the three alternating chambers) as well as purging channels and

fan, fresh air damper and heater. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

A.6 Process and instrumentation diagram elaborated by supplier B showing the oxidizer’s top

section (including the combustion chamber and gas burner) as well as the hot-side bypass

and the oxidizer’s outlet to the quench. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

A.7 Process and instrumentation diagram elaborated by supplier B showing the gas burner

train as well as the inlet of combustion air including air fan and damper. . . . . . . . . . . 87

A.8 Process and instrumentation diagram elaborated by supplier B showing the quench and

HCl scrubber. The NaOH reservoir is not included in this supplier’s scope of supply and

is to be provided by the plant, downstream of pump P813. The stream exiting the top of

the scrubber is directed to the DeNOx system. . . . . . . . . . . . . . . . . . . . . . . . . 88

A.9 Process and instrumentation diagram elaborated by supplier B showing the DeNOx sys-

tem. The stream entering the heat exchanger’s tubes is exhausted by the top of the HCl

scrubber. Stack not included in scope of supply. . . . . . . . . . . . . . . . . . . . . . . . . 89

A.10 Process and instrumentation diagram elaborated by supplier B showing the inlet of natural

gas into the inline burner for the DeNOx system, as well as the inlet of compressed air

and urea solution into the urea injection nozzles. Urea tank not included in scope of supply. 90

xix

xx

Nomenclature

Greek symbols

� Orifice diameter reduction ratio.

Thermal conductivity coefficient.

⇢ Density.

Roman symbols

�H Enthalpy variation.

�pNR Non-recoverable pressure loss.

�T Temperature variation.

�Tml Logarithmic averaged temperature variation.

M Mass flow.

Q Heat flux.

Kac Average accessory resistance coefficient.

~Q Flowrate.

A Transfer area.

C Mass concentration.

cp Specific heat at constant pressure.

D Diameter.

d0 Orifice diameter.

E Enthalpy.

f Darcy friction factor.

g Gravitational acceleration.

h Film coefficient.

xxi

hf System head loss.

ICL Initial Cause Likelihood.

IEL Intermediate Event Likelihood.

Kac Accessory resistance coefficient.

Kop Orifice plate resistance coefficient.

L Length.

M Mass.

MTRF Maximum Tolerable Risk Frequency.

MW Molecular Weight.

n Number of moles.

Nu Nusselt number.

p Pressure.

Pi Ignition likelihood.

Pp Occupancy factor.

pv Vapour pressure.

Ptr Time-at-risk factor.

PFD Probability of Failure on Demand.

Pr Prandtl number.

R Ideal gas constant.

Rw Conduction resistance.

Re Reynolds number.

s Solubility.

T Temperature.

t Time.

U Global heat transfer coefficient.

V Volume.

v Flow speed.

v0 Flow speed through an orifice.

xxii

VM Molar volume.

z Height.

Subscripts

Cl,DCM,N2, vents Compound designation.

hot, cold Fluid designation.

i, j, k,m Indexes.

rem Removed.

ref Reference condition.

Superscripts

max Maximum.

xxiii

xxiv

Glossary

AEV Associated Emission Value.

AIC Analyser Indicator and Controller.

AI Analog Input.

AO Analog Output.

APA Portuguese environmental agency.

API Active Pharmaceutical Ingredient.

ATEX Atmospheres Explosibles [European directive].

AT Analyser Transmitter.

AZH Analyser safety function: High reading.

BAT Best Available Techniques, as defined by the

European Commission.

BREF Best Available Techniques Reference Docu-

ment.

Brine Thermal fluid consisting of a mixture of ethy-

lene glycol and water, at or below -20 oC.

CDMO Contract Development and Manufacturing Or-

ganization.

CE Conformite Europeenne [marking].

CG Centrifuge.

CMR Carcinogenic, Mutagenic or Reprotoxic.

CT Charge Tank.

CapEx Capital Expenditure.

DCM Dichloromethane (Methylene-chloride).

DIN Deutsches Institut fur Normung [standards].

DI Digital Input.

DMA N,N-Dimethylacetamide.

DMF N,N-Dimethylformamide.

DN Nominal diameter, in millimetres.

DO Digital Output.

DT Distillate Tank.

xxv

DeNOx Selective reduction of nitrogen oxides.

ELV Emission Limit Value.

ET Effluent Tank.

FIC Flow Indicator and Controller.

GMP Good Manufacturing Practices.

HAZOP Hazard and Operability.

HSBP Hot Side Bypass.

LEL Lower Explosive Limit.

LLE Liquid-Liquid Extractor.

LOPA Layer Of Protection Analysis.

LT Level Transmitter.

L Pipe length, in meters.

MEK Methyl Ethyl Ketone.

MLT Mother Liquor Tank.

M Motor.

N.D. Not defined/No data.

NRTL Non-Random Two-Liquid.

OFC Organic Fine Chemistry.

PCV Pressure Control Valve.

PDT Pressure Differential Transmitter.

PPs-el Polypropylene, self-extinguishing, electro-

conductive.

PRV Pressure Relief Valve.

PSV Pressure Safety Valve.

PT Precipitator.

PT Pressure Transmitter.

P&ID Piping and Instrumentation Diagram.

P Production plant where this project was imple-

mented.

RTO Regenerative Thermal Oxidizer.

R Reactor.

SCR Selective Catalytic Reduction.

SC Variable speed drive.

SDI Safety Device Interlock.

SDY Spray-Dryer.

SF Solvent Farm.

SIL Safety Integrity Level.

SNCR Selective non-Catalytic Reduction.

xxvi

S Scrubber.

TCDD 2,3,7,8-Tetrachlorodibenzo-P-dioxin.

TCDF 2,3,7,8-Tetrachlorodibenzofuran.

TE Thermocouple.

TIC Temperature Indicator and Controller.

TT Temperature Transmitter.

TV Temperature control Valve.

UEL Upper Explosive Limit.

VCS Vent Collecting System.

VC Vent Condenser.

VOC Volatile Organic Compound.

VP Vacuum Pump.

iBMK Isobutyl Methyl Ketone.

xxvii

xxviii

Chapter 1

Introduction

This chapter includes a brief description of the main challenges regarding waste gas emission manage-

ment within the organic fine chemistry (OFC) industry (specifically, active pharmaceutical ingredient -

API - synthesis), as well as the justification for the implementation of a volatile organic compound (VOC)

reduction and abatement strategy on an industrial facility in Portugal, herein referenced as P.

1.1 Industrial background

1.1.1 Atmospheric emissions

The main environmental challenge regarding waste gas emission within the OFC sector are volatile or-

ganic compounds (or VOCs), which mainly arise from diffusive (non-ducted) sources, thus being difficult

to measure and qualify [1]. These emissions arise mostly form point, linear, surface or volume sources

throughout the plant (such as equipment openings, the filling of storage drums or even from equip-

ment/piping leaks) but can however be efficiently minimised or even captured. Once captured, diffusive

emissions can be treated alongside exhaust gas from process operations [1].

Some of these VOCs are associated with serious health and/or environmental hazards and can be

often found on the exhaust gases of chemical processes - as such they must comply with specific limit

concentrations, prior to atmospheric discharge - this is demonstrated by Figure 1.1.

As exemplified by Figure 1.1, although oil and gas industries make up the largest share of VOC

emission sources, basic chemical and pharmaceutical synthesis contributes significantly to overall at-

mospheric emissions, especially when considering that the mass ratio (kg per kg) between generated

waste and final product in the OFC sector is 50 to 500 times higher than that of the oil and gas industry

[9].

An additional challenge concerning waste gas is the emission of nitrogen oxides (hereby designated

as NOx, comprising both NO and NO2). These oxides are generated through the combustion of hy-

drocarbons in the presence of nitrogen (which is the case when using atmospheric air), even when not

burning nitrogen-containing compounds. Regarding the formation of NOx, four different mechanisms

must be taken into consideration [10, 11]:

1

Table 1.1: Relevant VOCs in the OFC sector and associated hazards. Listed in bold are the hazardsresponsible for the definition of the emission limit value (ELV), reporting to half-hour averages, for a givenminimum emission flowrate, in accordance to Portuguese legislation for API manufacturing facilitiesdealing with organic solvents [2]. It should be noted that for TCDD and TCDF (tetrachlorodibenzodioxinand tetrachlorodibenzofuran, respectively) there is not a ELV defined [2]. DCM, DMF and DMA refer todichloromethane, dimethylformamide and dimethylacetamide, respectively.

Compound Expected emission Hazard Associated hazards [3–6] ELVID source statements (mg/Nm3)

Methylene-chlorideCommonly used assolvent in the OFC

sector [7]

H302 Harmful if swallowed

20(> 100 g/h)

H315 Causes skin irritationH319 Causes serious eye irritation

H335 May cause respiratory irrita-tion

H336 May cause drowsiness ordizziness

H341 Suspected of causing ge-netic defects

H351 Suspected of causing cancer

H373Causes damage to organsthrough prolonged or re-peated exposure

DimethylformamideCommonly used assolvent in the OFC

sector [7]

H226 Flammable liquid and vapour

2(> 1 g/h)

H312 Harmful in contact with skinH319 Causes serious eye irritationH332 Harmful if inhaled

H360 May damage fertility or theunborn child

DimethylacetamideCommonly used assolvent in the OFC

sector [7]

H220 Extremely flammable gas

2(> 1 g/h)

H312 Harmful in contact with skinH319 Causes serious eye irritationH332 Harmful if inhaled


(Tetrachlorodibenzo)Dioxin

Generated throughthe combustion of

organics in thepresence of chlorine,

at temperaturesbetween 450 - 850 oC [8]

H300 Fatal if swallowed

Notdefined

H310 Fatal in contact with skinH315 Causes skin irritationH319 Causes serious eye irritation

H341 Suspected of causing ge-netic defects

H350 May cause cancer


H370 Causes damage to organs

H372Causes damage to organsthrough prolonged or re-peated exposure

(Tetrachlorodibenzo)Furan

H300 Fatal if swallowedH310 Fatal in contact with skinH330 Fatal if inhaledH400 Very toxic to aquatic life

H410 Very toxic to aquatic life withlong lasting effects

2

Figure 1.1: Emissions of non-methane VOCs to air by industry sector/activity in Europe in 2010 [1].

1. Thermal NOx: described by the Zeldovich mechanism [10], where NOx is formed by the heating

of oxygen and nitrogen in the presence of a flame - this is the predominant source of NOx at

temperatures greater than 1200 oC [11];

2. Prompt NOx: describes the interaction of oxygen and nitrogen with active hydrocarbon species

(i.e. free radicals generated from the fuel in the flame), thus generating HCN and CN, which are

then converted to NOx - this is most predominant at fuel-rich low temperature flames (under 1000oC) and only occurs in flames of carbon-containing fuels [10, 11];

3. N2O mediated NOx: N2O is generated by the oxidation of nitrogen at low temperatures, subse-

quently reacting with hydrogen to form NOx - particularly relevant in premixed lean flames [10];

4. Fuel NOx: formed through the oxidation of nitrogen-containing compounds, being the predominant

mechanism for temperatures between 1000-1200 oC [11].

When present in the atmosphere, NOx is associated with phenomena such as acid rain (by reacting

with water, thus forming nitric acid) or smog generation (by dissociation into active oxygen species [12]).

Additionally, NOx can form small particulate matter, associated with respiratory problems in sensitive

individuals. Visually, NOx has a distinct brownish haze-like effect in the atmosphere [13].

As Figure 1.2 shows, despite not having the same weight as with VOC emission, the synthesis of

basic organic chemicals is nonetheless responsible for a significant amount of waste NOx emission, with

132 ktons in 2010.

Other relevant air pollutants associated with organic fine chemical processes include:

3

Figure 1.2: Emissions of nitrogen oxides to air by industry sector/activity in Europe in 2010 [1].

• CO2, CO (which is toxic and asphyxiant) or other incomplete combustion compounds depending

on combustion parameters (time, temperature and turbulence) [13];

• Sulphur oxides (SOx), formed when burning sulphur-containing fuels [11] - associated with acid

rain phenomena and particulate matter formation, as well as serious respiratory problems [13];

• Cl2, HCl, which may cause material corrosion when condensed, or other halogen-containing coum-

pounds depending on chemical processes and solvents;

• Particulate matter, formed by the atmospheric reaction of sulphur or nitrogen oxides, being poten-

tially hazardous for the respiratory system if smaller than 10 micrometer (diameter) [13].

1.1.2 Explosive limits

One critical safety factor which must be considered on all the stages of design and operation of vent

collection systems is the explosion risk associated with flammable gases and vapours.

Explosions are exothermic reactions triggered by the interaction between a fuel and an oxidizer

in the presence of an ignition source (like a spark or a flame), where the energy release imposes a

sudden increase in temperature, usually accompanied by a flame or glow. The extent and energy

released in combustion will vary according to the explosive mixture’s properties, whilst also depending

on environmental factors such as residence time, temperature, turbulence and pressure [14].

Regarding a potentially explosive gas in atmospheric air, its lower explosion limit (LEL) can be defined

as the fuel concentration above which flames will begin to propagate if ignited [14]. Similarly, its upper

explosion limit (UEL) refers to the concentration of fuel above which combustion ceases and flames no

4

longer propagate [14]. Whenever the ratio of oxygen to nitrogen in a given environment may differ from

that of air, it is useful to represent the explosive (or flammable) range graphically, as a function of fuel,

oxygen and nitrogen concentrations, as shown for methane on Figure 1.3.

Figure 1.3: Methane-Oxygen-Nitrogen flammability (explosive) range at 25 oC and atmospheric pressure[14].

Because of this, when handling flammable gases, which is usually the case of waste gas in the OFC

sector, there is a constant risk of a propagating combustion. A combustion of this kind often begins

as a deflagration but may transition to a detonation - in this case, if the mixture is ignited in a duct at

atmospheric pressure, flame acceleration and pressure piling can result in peak pressures up to 100

times the initial absolute pressure [15]. This kind of occurrence poses naturally as a serious risk to the

operation of waste gas collection/transport systems and must be thoroughly addressed in its design, as

well as in the operation of vent-generating equipment, given that the risk of uncontrolled ignition sources

cannot be realistically fully eliminated (due to human error or static electricity [15], per example).

One way to achieve this is by maintaining the off-gas composition far from the explosive limits, either

by maintaining oxygen below limiting concentrations (e.g. by flushing the system with nitrogen), by

5

operating the system in fuel-rich conditions (i.e. above the UEL) or in fuel-lean conditions (i.e. below

the LEL) [15]. Working below the LEL can be achieved either by diluting the waste gas stream with

atmospheric air, by condensing a fraction of the gaseous pollutants or by adding a resistance on the

vent line from a given equipment, thus decreasing the vent flowrate. This latter option can be particularly

useful in a contract development and manufacturing organization (CDMO) environment where highly

variable batch content often leads to fuel concentration peaks on the exhaust gas; by introducing gas

flow restrictions, fuel concentration measurements in the off-gas will show flatter and smaller peaks, with

larger duration, without changing global peak area (i.e. the same amount of pollutant is released, but

over a larger time span).

Additional measures can be taken to protect vent header systems from potential explosion [15]:

• Chemical suppressant systems with fire-extinguishing properties;

• High speed isolation valves to prevent fire propagation;

• Flame and detonation arresters for flame cooling (quenching) and stopping its propagation;

• Explosion relief vents, which discharge quantities of off-gas to the atmosphere whenever punctual

pollution is preferable to explosion;

• Explosion containment structures, which may direct shock waves to a target structure, such as a

blast-off wall.

Portuguese legislation, in alignment with European ATEX (Atmospheres Explosibles) directives [16,

17], defines specific regulations for the protection of workers and installations against the risks of ex-

posure to explosive atmospheres, created by dusts or gases [18]. Concerning explosive atmospheres

created by the mixture of air with flammable gases, vapours or mists, three types of areas are defined in

this legislation:

• Zone 0: A place in which an explosive atmosphere consisting of a mixture with air of dangerous

substances in the form of gas, vapour or mist is present continuously, for long periods or frequently;


substances in the form of gas, vapour or mist is likely to occur in normal operation occasionally;


substances in the form of gas, vapour or mist is not likely to occur in normal operation but, if it does

occur, will persist for a short period only.

These areas must be properly identified, classified, signalled (as on Figure 1.4) and subject to tech-

nical/organizational measures either to avoid the formation of explosive atmospheres, mitigate explosion

consequences or minimizing contact with potential ignition sources. One crucial aspect regarding the

latter is the selection of the equipment and materials of construction, which must ensure the desired

level of protection without compromising operational parameters [18].

6

Figure 1.4: Standard sign indicating an area where explosive atmospheres may form [19].

1.2 Motivation

As previously mentioned, the aim of this work was to implement a strategy for the revamping of the

vent management, collection and treatment system of an industrial facility based in Portugal. As such, in

order to comprehend the basis for the decisions taken, it is crucial to understand the waste management

strategy and the systems under implementation before the beginning of this project, as well as its results.

1.2.1 The plant

The plant in scope for this project is used for API manufacturing and works primly as a batch produc-

tion CDMO. As such, it is characterized by a waste gas flow of highly variable composition, depending

on current campaigns and production scheduling. Production comprises two main types of processes:

chemical and pharmaceutical operations (the latter being mostly wet-polishing and spray-drying pro-

cesses). Chemical operations comprise three main areas devoted to multipurpose fine chemistry: X, Y

and Z.

The measured off-gas itself comprises over 3,000 Nm3/h and originates from around 100 different

equipment pieces, spread all over the plant, whose vents are continuously collected and distributed

across 15 different scrubbers.

These scrubbers act as structured bed adsorption towers where the exhaust gas contacts with a

liquid phase, in counter-current, in order to promote the transfer of specific pollutants present in the gas

into the liquid phase [13]. The existing scrubbers in P use water as the absorbent liquid (being the most

cost-effective option [13]), which is sprayed from the top of the tower onto the ascending gas stream.

The pollutant-rich liquid phase is then collected at the bottom of the scrubbers and directed to industrial

liquid effluent line for further treatment. This type of system allows for the removal of pollutants such

as particulate matter, ammonia, hydrochloric acid or hydrofluoric acid [13] - such kind of intermediate

treatment additionally allows for the removal of corrosive compounds from the waste gas streams [20].

The choice of liquid water as the absorbent exerts however a constraint upon operational flexibility,

as clean water alone is usually not enough to remove some persistent pollutants, namely non-polar

molecules. For this reason, scrubbing operations operate with the addition of chemicals to the sprayed

water (e.g. lime/limestone, sulphuric acid or sodium hydroxide), leading to the precipitation of salts on

the scrubbers’ liquid waste [21]. The added chemicals, often present in a storage tank next to each

scrubber, are chosen according to the target pollutants and desired treatment efficiency - therefore,

different waste gas streams may require different treatment. This is particularly relevant in a CDMO,

where waste gas content is highly variable, and thus P uses two different strategies for scrubber design

7

and operation:

1. Product devoted scrubbers: this strategy focuses on avoiding cross-contamination between

batches, connecting equipment devoted to the same product to the same scrubber;

2. Acid - alkali scrubbers: this strategy defines the manual connection of an equipment to one of

two scrubbers - one for acidic and another for alkaline gas streams - relying on pressure monitoring

and valve control to ensure that in case of back-pressure the equipment pieces do not contact with

the vent header system content, thus avoiding cross-contamination.

The scrubbers are connected to the plant’s vent collecting system (VCS), which is composed of

five different sub-collectors converging to one main collector, fully controlled and automated, with two

redundant fans for extraction at the end of the main collector. The latter feeds the waste gas stream to

the a carbon filter unit, which acts as the present end-of-line VOC treatment system. The treated gas is

afterwards discharged to the atmosphere through an end-of-line outlet stack (Figure 1.5).

The carbon filter unit is a solid activated carbon bed whose surface contacts continuously with the

collected waste gas stream, as an end-of-line treatment unit. This contact promotes the adsorption of

gas molecules (namely VOCs) by the pore surface, providing a VOC depletion up to 98% at maximum

efficiency, prior to discharging to the atmosphere. Regarding the saturation capacity of the bed, it should

be noted that it reaches significant saturation after some days in operation (associated with a loss of

efficiency). Upon reaching saturation there is the growing risk of breakthrough, that is, the breaking of

adsorbed contaminants out of the bed [20]. This treatment unit is non-regenerative, which means that

once saturated it must be replaced by a new unit - besides generating waste, this is associated with high

operating costs. Because of this, carbon bed adsorbers in the industry are usually used for vents with

low VOC content and poor in aldehydes and ketones, as they are poorly adsorbed [20]. However, this is

not the case in P, where the carbon filters deal with most of the plant’s waste gas.

Regarding safety, it should be noted that activated carbon beds have a potential for flammability [20].

Therefore, this unit is equipped with adequate control - including oxygen and carbon monoxide levels,

as well as pressure, temperature, flowrate, VOC content, and LEL percentage. The latter parameter

reports to how close the waste stream concentration is from reaching its LEL. For safety purposes,

mixtures inside pipelines and manifolds should be kept under 25% LEL. Upon 25% LEL is reached, an

alarm is triggered. When reaching 50% LEL it is considered a high LEL event and thus the waste gas is

bypassed directly to the atmosphere, for safety purposes. Additionally, the gas flow to the carbon filters

is diverted to the atmosphere upon reaching 80 oC, for the same reasons as the LEL.

1.2.2 Waste gas emission data

Based on internal and external emission monitoring on the outlet stack, the main pollutants on the vent

collecting system are discriminated on Figure 1.2, with the exception of DMA, which is not represented

on this data. As the discrepancy between average and maximum emissions shows, the plant is subject

to emission peaks, characterized by pollutant concentration values considerably higher than the average

recordings. It should be noted that this data is prior to the carbon filters installation (dated October 2017).

8

Table 1.2: VCS inlet gas composition monitoring since 2012. Contains values both from external andinternal monitoring, for main pollutants. Pollutant concentration is shown normalized against the highestmeasured pollutant concentration (VOC, maximum - July 2012, in mg/Nm3), signalled in bold.

Date March 2012 July 2012 January 2017 Since October 2017Status External External External Internal

Temperature, average (oC) - 30.0 20.6 -Molecular weight, average (g/mol) - 28.6 28.7 -

Humidity, average (mmHg) - 2.1 2.2 -Velocity, average (m/s) - 13.6 18.5 -

Dry volumetric flow, average (m3/h) - 2,662 3,764 2,528CO2, average (%) - < 2 < 0.5 -O2, average (%) - 20.9 20.5 -

VOC, average (%) 3.7 12.1 1.6 7.7VOC, maximum (%) 10.7 100 9.6 17.9DCM, average (%) 7.3 30.4 1.0 -

DCM, maximum (%) 71.7 65.2 90.2 -DMF, average (%) 0.9 1.7 - -

DMF, maximum (%) 5.2 - - -Methanol, average (%) 1.2 1.9 - -

Methanol, maximum (%) 4.5 1.9 - -Methanol+Ethanol, average (%) - - 0.8 -

Methanol+Ethanol, maximum (%) - - 8.7 -Hexane, average (%) - 9.4 - -

Hexane, maximum (%) - 85.4 - -Heptane, average (%) 0.2 1.0 - -

Heptane, maximum (%) 3.6 27.9 - -Hexane+Heptane, average (%) - - 0.3 -

Hexane+Heptane, maximum (%) - - 3.1 -Acetone, average (%) - - 0.5 -

Acetone, maximum (%) - - 3.1 -Methyl butyl ether, average (%) - - 0.3 -

Methyl butyl ether, maximum (%) - - 1.9 -Various (MEK, iBMK), average (%) 7.1 - - -Various (MEK, iBMK), average (%) 20.2 - - -

9

Some emission peaks have been associated to specific operations and equipment pieces, throughout

external monitoring, which has lead to an internal revision of vent lines associated to equipment pieces

in production areas present in buildings such as building E and F. Such study was performed by the

environmental department, prior to the beginning of the present work.

It is worth noting that in 2016 alone 40 LEL events were registered. As exemplified by figures 1.6

and 1.7, Y and Z production areas (associated with collectors 4 and 5, respectively) were identified as

the main responsibles for LEL emission peaks. Concerning production area Z, distillation was found to

be the main unit operation contributing to these peaks.

1.2.3 Challenges and objectives

In addition to current waste gas emissions, ongoing plans for site operations growth must be taken into

account - in fact, based on expansion plans until 2021, process vents are expected to increase by 145%

over the next five years, as shown by Figure 1.8. Besides the growth of chemical operations in P, a

new area with need for vent collection and dedicated to waste water treatment is scheduled for the near

future. This not only motivated the intensification of VOC reduction and abatement in the plant, but also

brought forth the need for revamping the current VCS, as some of the scheduled production increase is

planned to take place in buildings C and G, which are to be connected to the VCS.

Regarding the current waste gas abatement strategy it should be noted that non-regenerative carbon

beds are not considered a best available technique (BAT) by the European Commission [21]. Moreover,

despite the clear environmental enhancement provided, the carbon filters are not sustainable, with heavy

operational costs for 2018, expected to nearly double in 2019. It should be noted that the strong depen-

dence of the carbon bed’s saturation on inlet VOC concentration is far from ideal, especially when taking

into consideration the highly variable production campaigns associated with a CDMO - the gross of the

LEL events registered originate indeed from production area Z, which is devoted to contract manufac-

turing (Figure 1.6).

In conclusion, based on the information discussed within this chapter, it is the opinion of the envi-

ronmental department and of the P’s management that the main goals for this project are achieving full

environmental compliance and economical sustainability (in accordance the BAT, preconized by BREF

OFC [22] and BREF CWW [23]), by providing a robust and safe strategy for the collection, transport and

treatment of all VOCs originating from process and storage equipment.

10

Outlet stack

Carbonfilters

Collector 1 (main)

Collector 2

Scrubber 1 (Building A)



Scrubber 4 (Building B)

Collector 3

Scrubber 5 (Building B)

Scrubber 6(Building B)

Scrubber 7 (Building C)

Scrubber 8 (Building C)

Scrubber 9 (Building D)

Collector 4

Scrubber 10 (Building E)

Collector 5

Scrubber 12 (Building F)




Collector 6

Scrubber 11 (Building E)

Figure 1.5: Schematic of the plant’s vent collecting system.

11

50%

25%

10%

15%

Z Y X Other

Figure 1.6: Distribution of the forty LEL events registered in 2016 between the main chemical productionareas (X, Y and Z).

14%

8%

19%56%

3%

Collector 2 Collector 3 Collector 4Collector 5 Collector 6

Figure 1.7: Distribution of the forty LEL events registered in 2016 between the five VCS sub-collectorswhich converge to Collector 1 (main collector).

12

0

2 000

4 000

6 000

8 000

10 000

12 000

14 000

0

50

100

150

200

250

300

350

400

450

2016 2017 2018 2019 2020 2021 2022 2023 2024Pr

oces

s ve

nt fl

ow (N

m3 /h

)

Che

mic

al re

actio

n ca

paci

ty (m

3 )

Year

Figure 1.8: Scheduled production volume expansion for chemical operations in P until 2023 (blue trend)and associated process vents expected increase (orange trend).

13

14

Chapter 2

Action Plan

The aim of this chapter is to describe the requirements for this project, presented by the environmental

department and approved by the plant’s management, in January 2018. This requirement specification

laid the grounds for all the engineering concept design and provided rationale for the major project

decisions carried out throughout this work. The requirement specifications for this project are divided

into three different sub-projects, as presented in this chapter: volatile organic compound (VOC) source

reduction, vent collecting system (VCS) upgrade and expansion and end of line VOC abatement.

2.1 VOC source reduction

This sub-project comprises the connection of non-ducted emission points to the VCS, as well as the

reduction of VOC release from vent emission sources. This reduction will allow for the mitigation of LEL

events as well as the optimization of the VOC load to treat at the future end of line treatment unit - with

the aim of reducing the operational costs associated with VOC treatment.

With respect to building localized ventilations (i.e., vent emission sources non-connected with the

VCS), a dedicated scrubber or carbon adsorption system must be ensured for each building, as well

as an independent exhaust stack. This type of ventilation should be equipped with automatic control in

order to ensure the effective closure of the ventilation duct, when not in use. Additionally, all hydrogen

process vents must be collected separately from the VCS and conducted to a knockout tank associated

to a dedicated stack. These vents should be equipped with a forced air dilution system.

Regarding vacuum vents, effective condensation systems using cold water as thermal fluid must be

provided at the compression side of the vacuum pump. All vacuum vents should be pretreated on a gas

scrubber prior to conduction to the VCS.

Condensation is regarded as a primary measure for VOC load reduction, as it allows for the removal

of pollutants based on the respective vapour pressure and on the condenser’s heat removal capacity.

The generated liquid waste should then be directed to the industrial effluent line as exemplified on

Figure 2.1. Heat transfer on a condenser or heat exchanger (Q, Watt) takes place across a well defined

transfer area (A), as function of the temperature difference between the hot and cold streams - since the

15

Figure 2.1: Schematic drawing of a shell and tube condenser, where vapour is cooled with cold water,for condensates removal. [25].

temperature of a stream may vary throughout the length of a condenser’s tubes or plates, the logarithmic

average of temperature difference between both streams is taken into consideration (�Tml) [24]. These

variables correlate to the global heat transfer coefficient (U), which translates the resistances to heat

conduction and convection across the heat exchanger, as shown in equation 2.1 [24].

Q = U ⇥A⇥�Tml (2.1)

Vent condensers are mandatory to be available and operational on the vent line for each process

operation (inertization, solvent charge, reaction, discharge, among others) and interlocked so that the

equipment cannot operate in the absence of a vent condenser connection nor without a thermal fluid

available for effective vent condensation (bypass to a vent condenser is not allowed). The condensers

should be designed so that adequate heat transfer area is available for removal of worst case solvent

from the waste gas stream. Regarding thermal fluid choice, a mixture of ehylene-glycol and water at

-20 oC (henceforth referred to as brine) should be considered for the condensation of organic solvents,

or else chilled water (at or below 7 oC) for water distillation processes. Nevertheless, the thermal fluid

must run at a lower temperature than that of the condensing vapour’s dew point and have sufficient flow

to allow for complete condensation; the thermal fluid flowrate should also be automatically controlled by

the temperature difference between the condenser’s inlet and outlet in order to optimize condensation.

Being the case of a GMP installation (i.e. compliant of Good Manufacturing Practices), process

vents must be ensured to be collected separately from vacuum vents. The latter comprise only vents

from vacuum pumps, whereas the former comprise exhaust gas emissions from reactors, centrifuges,

charge tanks, distillate tanks, filters and filter dryers, among others - these vents must be equipped

with effective condensation (e.g., by using brine) and with slow relief devices such as pressure control

valves or restriction orifices. Similar to vacuum vents, all equipment process vents must be collected,

pretreated on a gas scrubber for neutralization an particle removal and ultimately conduced to the VCS.

The use of slow relief devices aims at imposing a resistance on the vent header system, in order to

16

minimize waste gas flow (without reducing total pollutant quantity). As such, a reduction in overall VOC

concentration (and consequently LEL peaks) is expected. It should be noted however that not all VOCs

contribute to the LEL. Two significant options were considered for this: the use of pressure control valves

on the vent line for each equipment (associated with pressure transmitters and automatic valves down-

stream in order to mitigate cross-contamination between equipment pieces in case of backpressure)

allows for a more robust control on equipment vent flow, whereas a more conventional strategy such

as the use of restriction orifices can prove to be a more rigid - although reliable - approach. As shown

on Figure 2.2, whilst control valves can provide a proportional control strategy, the static geometry of

restriction orifices (which are basically thin plates with calibrated orifices, installed as piping obstruc-

tions) ensures that, if appropriately designed, the pressure drop imposed will be always adequate, as

with valves there can be uncertainty regarding the position of the actuator (hence the importance of

equipping valves with limit switches for obtaining position signals).

The impact of these devices can be roughly predicted as long as stationary flow is considered, as

the Bernoulli principle [26] can be applied to correlate flow conditions between two points (fluid density,

⇢, pressure, pi, flow speed, vi, height zi, and head loss in the system, hf), as shown in equation 2.2.

p1

⇢⇥ g+

v12

2g+ z1 =

p2

⇢⇥ g+

v22

2g+ z2 + hf (2.2)

Furthermore, hf comprises piping head loss (a function of each pipe section’s diameter, Dk, length,

Lk, flow speed, vk, and associated Darcy friction factor, fk) as well as accessory head loss (considering

the resistance coefficient for each accessory, such as a valve or a junction, Kac, m, and flow speed

through the accessory, vm) - in the specific case of an orifice plate, the resistance coefficient, Kop, is

a function of the restriction imposed by the orifice (i.e., the ratio between the orifice diameter and the

pipe’s diameter, �) [26] and relates to the flow speed across the orifice (v0). All this is shown in equation

2.3 [26].

hf =X⇣

vk2

2gf k

Lk

Dk

⌘+X⇣

vm2

2gKm

⌘+

v02

2gKop (2.3)

It should be noted that given the risk for flammability and being P a multipurpose installation, all

instruments and flexible piping included at the design and assembly of this project must be ATEX rated

and with anti-static characteristics at the inner and outer walls. Moreover, the materials of construction

for vent pipelines should be anti-static, self-extinguishable and compatible with acidic and basic gases

(one good example of this is conductive polypropylene, or PPs-el [29]). This sub-project also includes

the replacement and repair of damaged or non-suitable equipment pieces, piping or instruments present

in the production areas under the scope of this work, as well as the elimination of potential piping leaks.

2.2 VCS upgrade and expansion

This sub-project encompasses the expansion of the current vent collecting system (VCS) in order to not

only connect non-ducted emission sources to the VCS main collector for further treatment, but also to

17

Figure 2.2: Comparison between a), an orifice plate of fixed geometry [27], and b), a pressure controlglobe valve, where the plug’s position can vary from 0% to 100% [28].

accommodate the increase of production capacity in P in terms of vent emission. Additionally, this sub-

project includes a revision of the current VCS sub-collectors in order to resolve potential issues, using a

safety-by-design approach.

Full environmental compliance is the main goal of this project and, as such, all process and vacuum

vents in P must be collected for treatment prior to atmospheric discharge through a stack - as such,

the VCS’s sub-collectors must allow for the connection of all vent emission sources (which do not have

already dedicated emission treatment systems) to its main collector.

All VCS sub-collectors must be guaranteed to have adequate capacity to conduce the expected gas

flow from current, future and currently non-ducted emission sources - this implies an adequate sizing

in sub-collector diameter in order to ensure adequate pressure drop and gas velocity (between 4 - 8

m/s). Additionally, low point drains are to be included in the design of the VCS, by installing mechanical

automatic valves - alternatively, siphoning on the VCS can be effectively improved by installing level

switches with alarm to the VCS control panel (all siphons must naturally be ensured to connect to the

industrial effluent line).

Another critical aspect to consider is the need to operate the VCS under lean fuel, for safety purposes

(i.e. a maximum of 25% LEL) - a LEL control strategy is mandatory for the VCS, meaning the existence

of one LEL detector per sub-collector, connected to a dilution air proportional control system, activated

at 12.5% LEL, to ensure LEL < 25%, by means of an air inlet control valve. A bypass pipeline must

ensure that during LEL analyser maintenance the exhaust gas is still directed to the main collector.

Besides LEL, pressure, temperature, flow, oxygen and VOC trends must be registered on the VCS

control panel, for each sub-collector. For this purpose, VOC sampling ports with air tight clamp closing

should be installed in every sub-collector. Moreover, pressure measurement (which has so far taken

place in the connection between each sub-collector and the main collector) is to take place in the end

section of each sub-collector (i.e., opposite to its connection to the main collector), in order to ensure

better control for pressure control valves (PCVs).

18

During future HAZOP analysis (hazard and operability study), the requirement for the existence of

flame arresters on each collector should be re-evaluated, given the new basis of safety of lean fuel

transport, LEL continuous monitoring and control. Any flame arrester that is installed under the scope of

this project should however be chosen in such a way as to minimize pressure drop on the VCS, as well

as having the appropriate materials of construction in order to prevent corrosion and plugging on the

system (as well as being anti-static and dissipative). All flame arresters must be additionally equipped

with temperature analysers as well as a differential pressure monitoring system (for plugging detection)

with alarm to the VCS control panel.

The VCS’ main fans (two redundant ventilators, at the end of the main collector’s pipeline) should

be reviewed in order to ensure their capacity allows for adequate extraction based on the total flowrate

expected (considering future expansion and currently non-ducted emission sources); these ventilators

are to be equipped with variable speed drives, controlled by the pressure on the main collector, upstream

from the fans. A noise reduction canopy should be provided for these fans, alongside with the possibility

to work with one unit in standby.

Regarding all existing scrubbers, various factors are to be taken into consideration regarding their

revamping. Firstly, adequate capacity must be ensured for the scrubber and associated fan(s), whilst

also assuring that the latter are equipped with variable speed drives, controlled by the pressure of the

gas feed to the scrubber. Secondly, the outlet of the scrubbers’ fans should be equipped with a pressure

safety valve (PSV) designed adequately for the expected flowrate, acting as an exclusive bypass to a

safe location (which by company standard should be a knockout tank), with signal of state to the VCS

control panel to register its occurrence and duration. Regarding monitoring and control strategies, the

scrubbers are to be equipped with pH sensors and automatic control and with automatic level control

by water/solution reposition (both systems with alarms to the control room and VCS control panel).

Additionally, a level switch with alarm should be provided in order to improve siphoning on the scrubber

(or, alternatively, the top up scrubber water can be added through the siphon) - in both cases it is

essential to ensure the aerial connection of the siphon to the industrial effluent line.

Other factors to be taken into consideration for the scrubbers include the need for an adequate

demister for water removal in each scrubber, as well as the installation of sampling ports with air tight

clamp closing, for VOC monitoring, at the gas inlet and outlet for each scrubber. Moreover, the possi-

bility of optimizing scrubbing capacity by installing cooling systems for the scrubbing media should be

assessed,in order to minimize the gases’ potential for flammability and to reduce vent flowrates.

It should be noted that each scrubbing system (including the scrubber and reagent tanks, when

applied) should be present inside a retention basin with 25% of the largest tank’s volume or 110% of

the total capacity of all tanks, whichever the greatest - as per P standard. These retention basins act

as an immediate risk mitigation system in case of any liquid spillage or leak - as such, the basins must

be provided with adequate pumping facilities to remove any liquids from within and no valves may be

present at the basins’ walls.

It should be noted that given the risk for flammability and being P a multipurpose installation, all

instruments, fans and flexible piping included at the design and assembly of this project must be ATEX

19

Figure 2.3: Polypropylene, self-extinguishing, electro-conductive. Especially suited for the transportationof flammable media. Due to carbon black, can be used outdoor. Can effectively replace stainless steelpiping [29].

rated and with anti-static characteristics at the inner and outer walls. Moreover, the materials of con-

struction for vent pipelines should be anti-static, self-extinguishable and compatible with acidic and basic

gases (such as PPs-el [29], as shown on Figure 2.3). This sub-project also includes the replacement and

repair non-suitable equipment pieces, piping or instruments associated to the vent collecting system.

2.3 End-of-line VOC abatement

The last item on this project is the installation of an end-of-line gaseous treatment unit, with the objective

of complying both with Portuguese legislation and the best available techniques defined by the European

Commission (BAT), whilst allowing for a robust operation given the needs of P for short-term expansion.

Two best available techniques reference documents (BREFs) are included in the scope of VOC

abatement units for the organic fine chemistry sector: BAT OFC covers integrated pollution control

measures for the batch manufacture of organic fine chemicals in multipurpose installations [22], whereas

BAT CWW covers waste water or gas treatment and management systems within the chemical sector

[23].The main goal of BAT is to simultaneously implement procedures and technical measures to limit

risks associated to hazardous substances and to design/operate new plants in such a way that emissions

are minimized, ultimately providing an auditable trail for the integration of environmental, health and

safety considerations into process development [30].

According to BAT OFC selection criteria [31], the best treatment technique for VOC-rich exhaust gas

containing very toxic or CMR (Carcinogenic, Mutagenic or Reprotoxic) category 1 or 2 substances is

thermal or catalytic oxidation, as shown on Figure 2.4. Initially, the possibility of conducing the VCS

to the plant’s liquid waste oxidizer was considered. For this, the oxidizer’s incineration chamber would

only have to be redesigned for waste gas admission and a maximum capacity of around 5,200 Nm3/h,

covering both current waste gas emissions and the future production building C, with little investment cost

needed, despite high yearly operational costs associated - additionally not only would the liquid oxidizer

lose over 20% of its current liquid waste capacity, but also would not suffice to cover the expected gas

waste produced by the future production building G (as seen on Figure 1.8). The outcome of HAZOP

studies regarding this project conduced in 2017 showed high risk of explosion and caused this option

to be discarded. The carbon bed unit that was installed was a provisional measure until a new VOC

20

abatement unit was operational - it should be noted however that non-regenerative carbon adsorption

systems are not a BAT [21].

As such, and given the motivation for reducing operational costs by recovering heat for further ener-

getic efficiency, the chosen technology was a regenerative thermal oxidizer (RTO). As shown on Figure

2.5, a RTO consists of a series of alternating chambers (usually in pairs or trios), with ceramic filling and

connecting to a combustion chamber. Waste gas is fed at the bottom of the first chamber and travels

through the hot ceramic material (which transfers heat to the gas) towards the combustion chamber (usu-

ally at 800 - 1000 oC) where the oxidation takes place. Oxidized air then travels through the last chamber

and leaves the RTO, being cooled down and simultaneously heating the cold ceramic media within. After

each cycle, the chambers are interchanged and thus the last chamber will receive the waste gas with

its ceramic media already hot. This energy integration allows for very high thermal efficiencies (around

95%) and so the system is usually in an autothermal state (that is, temperature, time, turbulence, oxygen

and VOC load are enough for the desired oxidation of the waste gas) - whenever necessary, or during

startup, a pre-mixture of natural gas and combustion air can be fed into the combustion chamber in order

to create a flame to increase or maintain temperature. Whenever the chambers are arranged in trios,

the second chamber is used for purging any waste gas that may have accumulated during the admission

and could lead to emissions peaks at the outlet of the RTO; as such, in these circumstances, the typical

cycle for each chamber is admission-purging-exhaustion. Besides temperature (which should be 200

- 400 oC above auto-ignition temperature), other critical factors in the design of an RTO are residence

time, turbulence and the availability of oxygen - all of these affecting directly mass and heat transfer

inside the RTO, as well as combustion efficiency and kinetics [21].

As referred on Table 1.1, dioxins and furans (TCDD and TCDF, respectively) can become a major

issue when burning waste gas streams containing halogenated VOCs as these compounds tend to

regenerate immediately after combustion (de novo synthesis), unless specific requirements are met

[33]:

• Minimum combustion chamber temperature of 1100 oC (or 850 oC, if incinerating waste gas con-

taining less than 1% in weight of halogenated organic compounds);

• Residence time inside the combustion chamber of at least two seconds;

• At least 3% oxygen content (by weight).

One additional and cost-effective solution is to promote a fast cooling (or quench) of the treated air,

immediately after leaving the combustion chamber [21]. This allows for the treated air’s temperature to

drop rapidly below the temperature window of PCDD/PCDF formation, which begins at 450 oC and is

guaranteed to cease at 840 oC (worst-case scenario approach) [8]. A scrubber for HCl removal should

follow immediately downstream of the quench.

In this current project, given the high emission values for dichloromethane (DCM, see Table 1.2),

the installation of a quench and HCl scrubber downstream of the RTO was a specific request by the

environmental department.

21

Figure 2.4: Decision making diagram with criteria established by BREF OFC [31] for the choice of awaste gas VOC abatement technique. Table 5.4 (as referred in the picture) is shown of below thediagram.

22

Figure 2.5: Schematic representation of a regenerative thermal oxidizer [32]. Orange lines representprocess vents, blue lines represent clean air used for purging and green lines represent the oxidized airstream. Methane, natural gas or even liquid waste can be used as auxiliary fuel (yellow line).

Additional attention was devoted to the emission of nitrogen oxides (or NOx), which arise during

combustion through the fuel NOx mechanism (but also possibly thermal NOx) and can however be

controlled by selective reduction of nitrogen oxides (also known as DeNOx) - this kind of system was as

well requested. These systems involve the addition of a reducing agent onto the gas stream, namely

NH2-X compounds (where X = H, CN or CONH2) - usually ammonia or urea solutions, as well as nitrolime

or cyanamide are used [21]. The reduction process itself can be catalytic or non-catalytic, both options

being considered a BAT.

When using selective non-catalytic reduction (SCNR) systems, for nitrogen oxides, the injection of

reagent takes place after the combustion and before further treatment, at temperatures between 930oC and 1050 oC (depending on the reagent in use). The critical process parameters here are tempera-

ture, stoichiometry (reagent versus NOx) and residence time. As shown by equations 2.4, 2.5 and 2.6,

ammonia is always present in such reactions, even when using urea, which decomposes into ammonia

(2.4). Inadequate process parameters will lead to the emission of unconverted ammonia - known as

ammonia slip [21].

NH2CONH2 +H2O ��! 2NH3 +CO2 (2.4)

4NO + 4NH3 +O2 ��! 4N2 + 6H2O (2.5)

2NO2 + 4NH3 +O2 ��! 3N2 + 6H2O (2.6)

With selective catalytic reduction of nitrogen oxides (SCR) the reactions are identical, with the (sig-

23

Table 2.1: Emission limit values (ELV, for facilities using organic solvents for API manufacturing), as-sociated emission values (AEV) for an RTO/DeNOx system (assuming BAT) and monitoring frequency.Values report to half-hour averages.

Parameter ELV (mg/Nm3) AEV (mg/Nm3) MonitoringVOC (in mgC) 20 [2] 5 [31] Twice a year [2] a

DCM 20 [2] - Twice a year [2] a

DMA, DMF 2 [2] - Twice a year [2] a

NOx None [34] b 50 [31] None [34] b

HCl None [34] c 7.5 [31] None [34] c

Cl2 None [34] c 1 [31] None [34] c

CO None [34] d 50 [31] None [34] d

NH3 (SCR or SCNR) - 2 [31] -TCDD, TCDF - - -

a Under Portuguese legislation [2], the emission levels for these compounds must be monitoredtwice a year, for carbon emission levels between 2 and 10 kg/h, which is the case for P.

b Under Portuguese legislation [34], the emission levels for these compounds only need to bemonitored for NO2 emission flowrates above, or equal, to 2 kg/h, which is not the case for P.

c Under Portuguese legislation [34], the emission levels for these compounds only need to bemonitored for emission flowrates above, or equal, to 0.05 kg/h, which is not the case for P.

d Under Portuguese legislation [34], the emission levels for CO only needs to be monitored foremission flowrates above, or equal, to 5 kg/h, which is not the case for P.

nificant) addition a catalyst bed. As such, depending on the catalyst, operation temperatures will range

from 200 oC to 500 oC, putting a lower strain on materials of construction and allowing for a more

significant NOx abatement efficiency. This however requires the process to occur downstream of the

quench/scrubber system, given high the potential for TCDD/TCDF generation in these temperatures,

when in the presence of halogens [21] - as such, a SCR requires the preheating of the scrubber’s outlet,

as well as the cooling of the catalytic system’s outlet, before discharging to the atmosphere.

One additional request by the environmental department was that no visible plume would be allowed

for the RTO. Technically speaking, this simply means that the purified air stream’s temperature would

have to be controlled, so that it could be high enough not to produce (visible) vapours at the exhaustion

stack.

This project’s implementation has as its primary objective full environmental compliance, under the

scope of current legislation and of the BAT. With this in mind, objectives were set for the emission value

of specific compounds, as shown on Table 2.1.

The RTO system should be designed to operate below 25% of the mixture’s explosive limit. As such,

LEL should be continuously monitored at the connection between the VCS and the RTO, with dilution

air control. The system should be able to bypass the waste gas stream to the fixed bed carbon bed

filters, should the LEL reach 25%. Additionally, the RTO should be equipped with a detonation safety

lock device, as well as a flame arrester and/or water seals, for risk mitigation.

As a final note, all pipelines belonging to the RTO system should be earthed and bonded and any oil

lubricated vacuum pumps should have an associated mist separator. Moreover, all automated solutions

must be controlled by a distributed automation system for vents, condensers, scrubbers, VCS and RTO

system.

24

Chapter 3

Implementation

This chapter describes the implementation course followed by the engineering team with the direct

involvement of the master’s student, guided by the request specifications explained in chapter 2.

The implementation of this project ranged from an initial conceptual stage to the later steps of design.

The description herein enclosed focused in greater detail on those tasks where the student had consid-

erable direct involvement. It should be noted that the time-frame of this dissertation did not encompass

the final handover of ”good for construction” drawings and diagrams to an assembly team and, as such,

this project is without physical implementation at the publishing date of this dissertation - with the excep-

tion of the installation of restriction orifices in building E at P, with its assembly and environmental/safety

impacts being further discussed in chapter 4.

3.1 Engineering concept design

In accordance to P’s site operating procedures, this engineering project began with a formal request

by the user area to the engineering department, as shown in chapter 2 - only when this request was

deemed viable, could engineering project design begin.

Conceptual design marked the first milestone in this engineering project. This was the stage that

allowed for the drafting of project budgeting and timelines, including main equipment, materials and

instruments, as well as basic layouts and legal requirements (from ATEX classification to environmental

licensing). Only after evaluation of the conceptual design could budget approval by P’s management

take place. Once project investment was approved, basic design was then allowed to begin. As soon as

basic project design is concluded and approved, detailed design may begin. As such, each of the three

stages of design (concept, basic and detail) require an evaluation meeting before proceeding to the next.

However, at the the conclusion of this dissertation, basic design was still under implementation.

With all engineering design concluded, mechanical, electrical and automation teams will ensure the

installation of all necessary equipment, wiring, software and hardware - this is followed by commissioning

and qualification, prior to user area and maintenance training. Finally, surrogate testing will take place

and, as soon as qualification approval takes place, the installation’s ownership will be handed over

25

by engineering to the user areas (which in this case are both the environmental department and all

the production units with affected equipment). From that moment onwards, any modifications to the

installation must be formally requested by the user area to engineering.

Regarding VOC source reduction, a decision was made by the project manager to focus the initial im-

plementation stages on methylene-chloride emission reduction. As such, this dissertation describes the

implementation of vent condensation and flow restriction techniques applied only to dichloromethane-

rich waste gas.

With respect to the increase in vent condensation capacity - through the installation of condensers

on all vent lines from reactors, centrifuges and tanks dealing with dichloromethane - the basis for de-

sign was that the condensers’ transfer area should amount to 2 m2 (corresponding to a company stan-

dard for vent condensers), whilst allowing for the condensation of the most critical solvent present (i.e.

dichloromethane) at -15 oC. As such, brine (a short-name for a mixture of ethylene-glycol and water

at around -20 oC) was considered as cooling media. Due to the imperative of chemical compatibil-

ity of the condensers with the vent composition, Hastelloy was chosen as the material of construction

for condenser associated with glass-lined or Hastelloy equipment, whilst stainless steel condensers

were chosen for stainless steel equipment. Regarding installation, it should be noted the importance

of installing these equipment pieces with a slope, in order to ensure proper separation between the

condensed solvents and the outlet vent.

Concerning dichloromethane emission flow control, two technical solutions were considered: for

areas presenting a distributed control system, the possibility of installing a control valve, regulated by

a pressure transmitter on the vent lines was considered, whilst for older, non-automated buildings the

installation of restriction orifices was contemplated. In the case of P, the areas housing processes

associated with high dichloromethane emissions were indeed the oldest facilities on the site, lacking the

possibility of an automated solution to the problem. Therefore, flow restriction orifices were the only

approach taken throughout this work. It was additionally defined to initially install restriction orifices for

50% vent diameter reduction and, if not effective, proceed to a 75% vent diameter reduction. In order

to avoid any welding works, the orifice plates were designed for installation between piping flanges -

this implied efforts to design the external diameter of the plate to fit the bolt circle location on a DIN

(Deutsches Institut fur Normung) flange, as shown on Table 3.1.

This strategy for dichloromethane vent flow reduction was assigned to all reactors’ and tanks’ vent

and vacuum lines. In cases where a vent condenser was present at the equipment’s vent/vacuum outlet

it was defined to install the orifice plate downstream of the condenser in order to avoid the restriction of

a flow with potentially high LEL (before condensation) and to avoid losing condensation capacity after

restricting the vent flow.

Two major identified drawbacks associated with restriction orifices were the increase in operation

time and pressure change on vent lines. To illustrate this, consider a reaction/distillation system com-

posed by a reactor venting into a condenser refluxing back into the reactor with an additional vent outlet

- if a restriction orifice were placed in the condenser vent outlet not only would the distillation time in-

crease due to flow reduction but also there would be a pressure build-up on the system with the risk

26

Table 3.1: Orifice plate generic design, based on DIN standards for a nominal pressure of 6 bar [35].Internal diameters refer to 50% and 75% diameter reduction, respectively.

Vent linediameter

(mm)

Orifice plateinternal

diameter (mm)

Flange bolt circlediameter (DIN)

(mm), [35]

Orifice plateexternal

diameter (mm)

Orifice platethickness

(mm)

Orifice platematerial

25 12.50 / 6.25 75 70 10 Teflon40 20 / 10 100 92 10 Teflon50 25 / 12.5 110 107 10 Teflon

of changing reflux conditions or stressing the materials of construction of the system. Therefore, this

strategy demanded a detailed study on possible product quality and process safety impacts, which saw

no objections to the proposed changes. It was additionally decided not to install restriction orifices on

vent/vacuum lines with glass piping or equipment (like condensers), due to the risk of damage to the

equipment through pressure build-up - this was only considered for equipment pieces where the pres-

sure safety valves were set to actuate at pressures higher than 1 barg, which corresponds to glass

equipment pieces’ material resistance.

As previously mentioned, since the initial scope of this work only contemplated dichloromethane

emissions, vacuum pump condensers were not addressed at this stage - in spite of many equipment

pieces dealing with dichloromethane working under vacuum, vacuum pumps cover entire production

areas and not specific reactors. As such, the installation of vacuum pump condensers was assigned to

a later project stage.

The connection of non-ducted vents to the site’s vent collecting system (VCS) was considered an

imperative action for all process vents. As such, the installation of conductive polypropylene (PPs-EL)

piping with adequate flow control devices for VCS connection was assigned for all identified non-ducted

vent sources.

Concerning all fifteen scrubbers connected to the VCS, a decision was made to install inlet and

outlet of sampling ports with 50 mm diameter, with airtight clamp closing, and no more than 100 mm in

length. A butterfly valve was assigned to each sampling port, for line isolation. Moreover, a perforated

spare clamp closing (for probe insertion during monitoring) should be provided for each sampling port.

Additional requirements for sampling points included the availability of a monophasic power feed for the

VOC analyser and a weather protection cover.

With respect to instrumentation on all scrubbers, one pressure safety valve, one variable speed drive

for fan speed control (associated to one pressure transmitter on the vent header), one level sensor (as-

sociated to one automatic on/off valve for level reposition), one pH sensor (associated to one automatic

on/off valve for acid/alkaline addition) and one level switch (for siphon level control), as well as automa-

tion connection to the VCS control panel (for both pneumatic and electrical instruments) was defined for

each of the scrubbers.

The vent collecting system itself was conceptualized to be expanded, in order to receive waste gas

from two different solvent farms; its material of construction was determined to be upgraded to PPs-EL

and the diameter adjusted to ensure a flow speed between 4 - 8 m/s, due to ATEX constraints. Induced

27

draft was defined for the VCS, through the installation of two redundant fans at its terminal section.

Concerning instrumentation on the VCS, one LEL analyser was assigned to each of the five sub-

collectors (with a bypass line to the LEL analyser available), each associated with one pressure relief

valve and to the existing dilution air system; additionally each of the five subcollectors required one VOC

sampling port (as described above), one flowmeter, one temperature sensor and one oxygen analyser.

The installation of a dilution air system was only a requirement for collector five, as all other already

fulfilled this requirement. All eleven siphons present on the VCS required the installation of one level

switch. Should the requirement of having two flame arresters inline be confirmed on a later stage (i.e.

during the safety analysis of the VCS, not yet addressed at the time of this publication), these must be

equipped with one bypass line for maintenance purposes, one temperature sensor and one differential

pressure transmitter each.

With regards to the installation of an end-of-line VOC abatement system, efforts were directed to a

turnkey purchase. For the regenerative thermal oxidizer (RTO) itself, the need for an oxidation chamber

operating at 1100 oC with 2 seconds residence time for the process gas was identified, as maximum

Chlorine content (%Clmax, in g / 100 g) was estimated to be near 1% of the exhaust gas with risk of dioxin

regeneration, especially when considering the multipurpose character of P and the high dichloromethane

concentrations achieved (C maxDCM , in mg/Nm3) - performed calculations are shown on equation 3.1, where

MWi and VM designate, respectively, molecular weights and molar volume (at normal temperature and

pressure, 2.24⇥10-2 m3/mol).

%Clmax = C

maxDCM

2MWCl

MWDCM

VM

MWvents⇥ 100% (3.1)

The choice of a selective reduction system for nitrogen oxides fell on a catalytic one (SCR), given

the higher efficiencies achieved by this type of process [10]. Urea was the chosen reagent, given the

environmental department’s previous experience with urea addition at the site’s liquid oxidizer, finding

urea more effective and economical than ammonia.

The purchase of a continuous emission monitoring station, to be installed alongside a new outlet

stack (to replace current outlet stack, vide Figure 1.5) was additionally defined, in order to ensure full and

continuous environmental compliance, regardless of the legally defined (and scarce) required monitoring

frequencies.

Quotations were demanded from diverse RTO suppliers (as discriminated on chapter 3.4) who were

responsible for providing information on maximum emission limits expected. Table 3.2 displays these

maximum expected values (associated with the chosen supplier) against the target emission limit values,

as defined by P.

Based on this project’s needs and expected costs, its budget was drafted by the project manager

and reviewed by the student, being divided into the three sub-projects. As shown on Table 3.3, this

intervention represents a substantial investment by P, with the largest share of capital being directed to

the RTO system purchase, alongside a major investment in piping and instrumentation for the site’s vent

collecting system.

28

Table 3.2: Desired versus expected emission limits, expressed as half-hour measurements, based oninformation provided by the chosen supplier for the RTO system.

ParameterSupplier guaranteed emission limit

(mg/Nm3)Target emission limit

(mg/Nm3)VOC (in mgC) 5 5

CO 100 100NOx 50 50HCl 3 7.5Cl2 1 1

NH3 (SCR or SCNR) 2 2TCDD, TCDF not expected none

Table 3.3: Project budget distribution (approximate figures, in §), according to the three subprojects andtype of engineering works.

CategoryVOC

sourcereduction

VCSupgrade andexpansion

VOCend-of-lineabatement

Fullproject

Civil 0 50,000 250,000 300,000Piping 100,000 620,000 100,000 820,000

Electricity 60,000 100,000 20,000 180,000Equipment 600,000 300,000 1,600,000 2,500,000

Instrumentation 280,000 450,000 180,000 910,000Automation 100,000 130,000 60,000 290,000

Projectbudget (§)

1,140,000 1,650,000 2,210,000 5,000,000

Figure 3.1 shows the expected project’s timeline, as elaborated by the project manager. Engineering

works began as soon as investment (or CapEx - Capital Expenditure) was approved and were estimated

to last three months as they included full revision of the current vent collecting system. Long lead time

items referred mainly to the RTO delivery time of 6 months (after supplier consultation), as it was ex-

pected to be the item with longest delivery time (as opposed to condensers, scrubbers or fans). Civil

works were expected to last two months, covering ground preparation and weatherproofing for the RTO

installation as well as the creation of pipeline support structures for the VCS. Equipment installation (8

months) was expected to be the limiting step in this project, as VOC source reduction works take place

across different buildings and the VCS expansion will affect all collectors and scrubbers at P. Commis-

sioning was assigned to last the final two months of this project - it should be noted that qualification

works were not considered as this project deals with only vents and utilities, which are non-GMP and

therefore do not require qualification.

The investment for this project was ultimately approved on September 2018, thus beginning en-

gineering works (namely restriction orifice installation) and establishing August 2019 as the foreseen

handover of the entire installation’s ownership to the user areas.

29

Figure 3.1: Expected timeline for the current VOC reduction and abatement project, where CapEx de-notes the project’s capital expenditure, or investment.

3.2 Data collection and analysis

With the conclusion of conceptual design, the next step was to gather and analyse a panoply of process

and thermodynamic data considered necessary for project design. As such, this section comprises

three main types of gathered data: historical process data, which was used for defining the scope of this

project (from identifying equipment needing dichloromethane emission reduction, to gathering emission

monitoring data, as shown on Table 1.2), field process data, crucial for identifying operational conditions

of existing equipment and for providing a basis for equipment/utilities design, and finally thermodynamic

data, which were mainly predicted using Aspen Plus software, in order to provide the most robust design

as possible.

3.2.1 Process historical data

This step relied on data gathered over the past years by the environmental department and included

both internal and external monitoring values at the outlet of the vent collecting system’s (VCS) main

fans (already summarized on Table 1.2), as well as the statistical distribution of LEL events (i.e. LEL

above 25%, vide figures 1.6 and 1.7) and an internal revision of the status of the vent lines belonging

to all equipment in production areas defined as critical: production area Y in building E, pharmaceutical

operations in building F and production area Z in building F - these areas were defined as Y and Z

production areas contributed with 75% of the 2016 high LEL events (analysing by production area),

whilst buildings E and F contributed another 75% (when analysing by VCS subcollector).

This assessment became the groundwork for the definition of the scope of the VOC reduction and

VCS expansion sub-projects, as it allowed for the identification of equipment lacking in vent condensation

capacity, with high LEL probability or posing as non-ducted vent sources, requiring VCS connection. It

should be noted that this assessment did not cover emergency safety vents, which were not included in

the scope of this project. After visit of production areas, contact with its staff and after analysis of existing

piping and equipment, performed by the student alongside members of environmental department, a list

of corrective actions was elaborated and approved, as listed on Table 3.4. It should be noted that at this

stage only non-ducted vent sources or dichloromethane emission sources were considered.

30

Table 3.4: Corrective actions to reduce dichloromethane source emissions, as well as non-ducted ventemissions, based on gathered data and installation inspection. Associated equipment comprises re-actors (R), vent condensers (VC), centrifuges (CG), mother-liquor tanks (MLT), filtration units (FU) andgeneric tanks (T).

Associated equipment Area (Building) Corrective action to be implemented

R01

VC01Y (E)

Restriction orifice in vent and vaccum

lines exiting the condenser

R02

VC02Y (E)


lines exiting the condenser

R03

VC03Y (E)


lines exiting the reactor;

Restriction orifice in vaccum

line exiting the condenser

R04

VC04Y (E)

Restriction orifice in vent

line exiting the reactor;

Restriction orifice in vacuum


R05

VC05Y (E)

Restriction orifice in vent

line exiting the reactor;

Restriction orifice in vacuum


CG01

MLT01

MLT02

Y (E)Vent condenser in joint vent line to S11;

Condensates to be sent to MLT02

CG02

CG03

MLT03

Y (E)Vent condenser in joint vent line to S11;

Condensates to be sent to MLT03

CG04

MLT04Z (F) Connect joint vent line to VCS

CG05

MLT05Z (F) Connect joint vent line to VCS

T01 Z (F) Connect vent line to VCS

FU01

T02Z (F) Connect joint vent line to VCS

31

3.2.2 Process field data

Field data is one crucial aspect to most engineering works on a manufacturing site (at least those where

older installations are implicated) as visiting facilities and contacting the people responsible for each

area is sometimes the only way to access non-centralized information.

Piping and instrumentation diagrams (or simply P&IDs) provide essential support to field tasks, since

this type of documentation - as long as it is appropriately updated as installations are subject to change -

provides a high level of detail regarding piping material and diameter, as well as equipment type, vessel

volume, instrumentation and automation, thus allowing an easy understanding of an installation and

its components interconnection. Additionally, layout drawings can provide information on the physical

distribution of a process, providing information on distances, equipment size, geographic orientation or

even piping accidents (such as elbows).

Considering the scope of this project and for organization purposes, gathered field data is presented

on this chapter from a broader perspective (that is, data on the site’s vent collecting system) to a more

specific approach (data on individual processes or specific plant areas). As the installation of an end-

of-line VOC treatment unit constitutes a whole new installation and not a revamping/expansion, no field

data gathering was needed.

The vent collecting system

As described above, the vent collecting system receives waste gas from all scrubbers present in the site

and directs it to an outlet stack. Additionally, this system includes three vacuum pumps, two spray-dyers,

one charge tank and one industrial effluent tank that are not connected to any scrubber (VP, SDY, CT

and ET, respectively).

The current system will expand to receive waste gas from the plant’s solvent farms (SF) as well as

new production buildings (buildings C and G), short-term plans for production increase and the afore-

mentioned (currently) non-ducted vent sources (vide Table 3.4). As such, average flowrates and piping

diameters associated with every scrubber and VCS collector were gathered (whenever possible), as

displayed on Table 3.5.

Current instrumentation on the vent collecting system was assessed to confirm if some of it already

complied with the requests of the environmental department - collector 5 was found to be the only

subcollector lacking a dilution air system - this information was considered on the budget shown on

section 3.1.

The scrubbers

Each scrubber present in the plant was visually inspected to determine its current condition and com-

pared with the environmental department’s requests.

Scrubber capacity was compared with average vent flowrate for each scrubber, taking additionally

into consideration each scrubber’s ventilator capacity - it should be noted that all scrubbers operate

under negative pressure, imposed by a ventilator connecting the scrubber outlet to the VCS. In terms of

32

Table 3.5: Average flowrates and piping diameters of the plant’s current vent collecting system, consid-ering short-term production increase (up to 2021), future production buildings and short-term productionincrease. As shown, each scrubber (S) is connected to a subcollector (collectors 2 to 6). The main col-lector (collector 1) receives vents from all subcollectors. Each VP, SF, CT, ET or SDY refers to a vacuumpump, a solvent farm, a charge tank, an effluent tank or a spray-dryer, respectively.

Source Average flowrate (Nm3/h) Piping diameter (mm)S01 200 120S02 200 150S03 200 120S04 200 150

VP01 N.D. N.D.SF01 N.D. a 150

Collector 2 800 150S05 350 200S06 1100 200S07 350 b 100S08 350 b 100S09 800 150

VP02/CT01 N.D. N.D.ET01 N.D. 150

Collector 3 2950 200S10 500 150

Collector 4 500 150S12 400 200S13 400 200S14 400 200S15 400 200

VP03 N.D. 80SDY01 N.D. 100SDY02 N.D. 100

Collector 5 1600 200S11 500 150

SF02 N.D. a 150Collector 6 500 150Building G 3620 a 150

Non-ducted vents 2600 a N.D.Short-term expansion 750 a N.D.

Collector 1 13320 c 200 to 300N.D.: No data available or not defined.

a Future.b Currently only 50 m3/h.c Currently only around 5750 m3/h.

33

Table 3.6: Current status of all scrubbers connected to the site’s vent collecting system.

ScrubberID

Flowrate(m3/h)

Scrubbercapacity

(m3/h)

Fancapacity

(m3/h)

Material ofconstruction

Retentionbasin status

Samplingports

S01 200 250 N.D.*Polypropylene;Polyethylene

No basin None

S02 200 250 1500 Polypropylene No basin None

S03 200 50 100 PolyethyleneBasin present;Valves on wall

None

S04 200 500 N.D.* Polypropylene Damaged basin NoneS05 350 2450 N.D.* Polypropylene Functional NoneS06 1100 1100 1100 Polypropylene Functional NoneS07 50 500 500 PPs-el Functional NoneS08 50 500 500 PPs-el Functional None

S09 800 600 1000

Polyvinylidenefluoride

Fiber reinforcedplastic

Functional None

S10 500 200 N.D.* Polypropylene Functional NoneS11 500 600 5000 Polypropylene Functional Outlet onlyS12 400 550 5000 Polypropylene Functional None

S13 400 700 5000Glassfiberpolyester

No basin None

S14 400 400 5000Glassfiberpolyester

No basin None

S15 400 750 5000 Polypropylene No basin None* No data register available.

construction, visual inspection (supported by P&ID revision) allowed for an assessment of the scrubbers’

materials of construction, retention basin current status and on whether sampling ports were present. A

summary of this assessment is displayed on Table 3.6.

With regards to instrumentation and automation, and based on intensive P&ID revision of the current

scrubbers, a revision was made on whether pH monitoring/control, ventilator speed control based on

pressure measurements (through a variable speed drive) and automatic liquid refill was a possibility for

any scrubbers. Additionally, the presence of bypass, pressure safety or pressure relief valves, as well

as siphoning conditions was assessed. A summary of the revision of all the scrubbers on site is shown

on Table 3.7.

Based on gathered data it was possible to verify that nearly all scrubbers needed extensive interven-

tion during the revamping of the site’s vent collecting system.

Building E local vent header system

Building E, associated to the scrubber S11, belongs to production area Y and houses equipment subject

to corrective action regarding dichloromethane source reduction, as seen on Table 3.4.

34

Table 3.7: Current instrumentation and automation on the site’s scrubbers. Note that any existing pres-sure relief valves (PRVs) should be replaced by pressure safety valves (PSVs).

ScrubberID

pH monitoringand automated

control

Automaticliquid refill

Variablespeed drive(controlled

by pressure)

Level switchwith alarmon siphon

Bypassvalve

PRV orPSV?

S01 x PRVS02 x PRVS03 x PRVS04 x PRVS05 x PRVS06 x x PRVS07 x x x PSVS08 x x x PSVS09 x PRVS10 x PRVS11 x PRVS12 x PRVS13 x PRVS14 * x PRVS15 x PRV

* No siphon on this scrubber.

All the target equipment pieces present in this building are connected to the same local vent header

system, which directs its vent lines to S11 - this comprises reactors (denoted as R), vent condensers

(VC), centrifuges (CG), charge tanks (CT), mother-liquor vessels (MLT), distillate tanks (DT), precipitate

tanks (PT) and one liquid-liquid extractor (LLE). Maximum working pressures for all the equipment were

provided by the area’s responsible staff and are displayed on Figure 3.2. Working pressure on S11 was

estimated to be around -50 mbarg, as the final ventilators on the vent collecting system operate at an

estimated -70 mbarg.

From equipment P&IDs and building E’s layout drawings it was possible to obtain all vent piping di-

ameter and length - Figure 3.2 provides a simplified schematic of how the vents from each equipment

piece connect to the vent header system and to its dedicated scrubber S11. Note that all equipment

pieces are shown in this picture as venting independently towards the scrubber, as was considered for

calculations and balances on section 3.3 - however, they actually join in a main header before reach-

ing the scrubber. Represented nodes are only representative components of the multiple-pipe system

considered for vent header calculations.

Besides estimated pipe lengths and piping nominal diameter, an approximate height of 5 meters

was estimated between each floor. Concerning pipe roughness, a value of 1.5 ⇥ 10 - 3 millimetres, cor-

responding to plastic (polypropylene) piping was considered [26]. As such, relative roughness values

(corresponding to the ratio between roughness and nominal diameter) were also estimated and used for

the determination of the Darcy factor, assuming low turbulence [26].

It should finally be noted that no flowrate or temperature values were known or available for the vent

35

R06500 mbarg

R03500 mbarg

R07600 mbarg

R05500 mbarg

R04300 mbarg

R02600 mbarg

R01500 mbarg

R08500 mbarg

CT02500 mbarg

CT03300 mbarg

CT040 mbarg

CT05500 mbarg

CT06500 mbarg

DT010 mbarg

CG01

CG02

CG03

CG04

MLT0320 mbarg

MLT0420 mbarg

MLT02

MLT0120 mbarg

VC06

VC03

VC02

VC01

LLE010 mbarg

S11- 50 mbarg

J1

J2

J3

J4

J5

J6

J7

J13

J22

J9

J11

J17

J16

J15

J12

J10

J18

J19

PT010 barg

J14

Figure 3.2: Schematic of building E’s vent header system. In this representation, each equipment pieceis shown as venting independently to the building’s dedicated scrubber (S11) - however, they actually joinin a main header before reaching the scrubber. (VC) Vent condenser; (LLE) Liquid-liquid extractor; (CG)Centrifuge; (J) Pipeline junction (only representative); (R) Reactor; (CT) Charge tank; (DT) Distillatetank.

36

header system. However, an ambient temperature of 20 oC was assumed as the temperature of the

vents.

3.2.3 Thermodynamic and transport data

This section is devoted to the explanation of the methods that were used to obtain all thermodynamic

parameters, as well as those related to the transport of mass, heat or momentum.

The global heat transfer coefficient (U) for a condensing organic vapor (like dichloromethane) on

a shell and tube vent condenser using brine was estimated to be a minimum of 450 W/(m2 oC) [36],

although this may vary depending on available utilities/process flowrates; e.g., a lower flow would mean

lower turbulence and, as a consequence, decreased heat transfer by convection. This is explained as

a lower fluid speed decreases the Reynolds value (Re), translating into a lower Nusselt number (Nu),

which in turn implies a higher resistance to heat transfer through convection - i.e., lower film coefficient

(h) value, as per equation 3.2, where D is the pipe’s diameter, the pipe’s thermal conductivity, Pr is

Prandtl’s number and C, m and n denote positive constants. This way, decreasing the film coefficient

would decrease the value of the global heat transfer coefficient (vide equation 3.3, for heat transfer

between two fluids, separated by a tubular wall with a given heat transfer area A and a conduction

resistance Rw, assuming no fouling of the heat exchanger’s walls occurs) [24].

Nu ⌘ h⇥D

= C ⇥Re

m ⇥ Prn (3.2)

1

U ⇥A=

1

(h⇥A)cold+

1

(h⇥A)hot+Rw (3.3)

Considering the saturation curve of dichloromethane (DCM) in nitrogen, at 20 oC there can be up

to 2.5 kg of DCM vapor per kilogram of nitrogen on a vent line, during the inertization of an equipment

dealing with dichloromethane [37].

Aspen Plus software was chosen for predicting thermodynamic data, using a generic application

of the NRTL method (non-random two-liquid) - this method is ideal for the prediction of liquid-liquid

or liquid-vapor equilibrium data [38] and, as such, it was selected as calculations envolving saturated

or condensing fluids comprised a one crucial aspect of this work. As such, predicted thermodynamic

proprieties are displayed on Table 3.8, alongside with used prediction parameters.

Throughout calculations, gravity acceleration (g) was defined as 9.81 m/s2 and the ideal gas constant

(R) as 8.3145 J/(K mol).

3.3 Mass, energy and momentum balances

This section envisions the mathematical description of the engineering problems formulated on the sec-

tions above, which ultimately aim at confirming the necessary transfer area for the vent condensers for

building E, as well as predicting the impact of the installation of restriction orifices on building E.

37

Table 3.8: Prediction results of thermodynamic properties, using the Aspen Plus software and the NRTLmethod.

Property Compound State ValueSpecific heat

(J/(kg.oC))Nitrogen Vapor at 20 oC 1042

Specific heat(J/(kg.oC))

Dichloromethane Vapor at 20 oC 609.3

Specific heat(J/(kg.oC))

Ethylene-glycol (20% volume) and water Liquid mixture at -20 oC 3648

Enthalpy ofvaporization

(J/kg)Nitrogen 0 oC 0

Enthalpy ofvaporization

(J/kg)Dichloromethane 0 oC 3.577⇥105

Density(kg/m3)

Dichloromethane (70%) and nitrogen Vapor mixture at 20 oC 2.252

3.3.1 Vent header system modeling

As described in equation 2.1, the determination of a heat exchanger’s transfer area requires knowledge

of process flowrates; similarly, understanding the impact of restriction orifices on a vent header system

is only possible when flow speeds (or flowrates) are known, as shown in equation 2.2.

Given the fact that building E, an older production area, lacked inline flowrate measurements for the

vent header system, as referred in section 3.2, a decision was made to attempt a rough model of this

very system, based on the Bernoulli principle, with the ultimate goal of providing indications for the con-

densers and restriction orifices design. However, it was observed that this vent header system provided

no pressure measurements, which naturally complicated the application of the Bernoulli equation to this

system, with the only known pressure points being thus the equipment pieces and scrubber S11.

Considering that building E was scheduled for the installation of two vent condensers (one on the

joint vent line from CG01 and its mother liquor tanks, MLT01 and MLT02, and another on the joint

vent line from CG02, CG03 and the shared mother liquor tank, MLT03) as well as eleven restriction

orifices between vent and vacuum lines arising from different reactors, the vent header system for this

building was roughly modelled by the student, in order to determine maximum vent flowrates associated

with each equipment piece. Figure 3.2 provides a simplified schematic of how the vents from each

equipment piece connect to the vent header system, which is directed to a dedicated scrubber (S11).

Vacuum lines, through which equipment pieces vent whenever vacuum is applied, were not considered

for this modelling since they are collected by vacuum pumps located on another building, consequently

being directed to a different scrubber (S10) - therefore only the vent header system associated with S11

was evaluated in this work.

As Figure 3.2 shows, this vent header constitutes a multiple-pipe system. As multiple pipes are

involved, all arising from equipment pieces at different pressures and through piping with distinct diam-

eter, the vent flowrate will vary from source to source and from pipe to pipe. As such, in order to apply

38

Figure 3.3: Representation of a multiple-pipe system, where junction j connects pipes from points atdifferent pressures, forming a node [26]. The sum of the vectorial flowrates from each pipe towards thenode must be zero.

laws based on the conservation of mass and momentum, the system was divided into its constituent

nodes, defined herein as the junction of two or more distinct pipelines. Balances were thus performed

individually for each node, using the Bernoulli equation under three essential considerations [26]:

1. The Bernoulli equation is only valid for steady-state - this approximation requires the consideration

that, for each node, the sum of vectorial flowrates reaching the junction must be zero (assuming

non-compressible flow), as shown on equation 3.4 (for a node j, where N pipes connect, consider-

ing the vectorial flowrate through each pipe and towards node j as ~Qi, j) and Figure 3.3;

2. For each pipe on each node, the pressure drop must satisfy the Bernoulli principle (equation 2.2);

3. The pressure at each junction constitutes one degree of freedom - as such, it should be arbitrated

in a way that the application of the Bernoulli principle for each pipe of the same node complies

with the steady-state requirement - through an iterative process, the pressure for each node can

be determined consequently determining the flowrate on each pipe.

NX

i=1

~Qi, j = 0 (3.4)

Assuming steady-state, flowrate will not vary across each pipe’s length. Given the relatively small

diameter variation along the pipelines, flow velocity was considered identical between two points. Sim-

ilarly, and given the small height variation between pipes in a node, as well as the vents’ low vapor

density (assuming vents are mainly constituted by nitrogen saturated in DCM; vide Table 3.8), varia-

tions in potential energy were neglected. Concerning head losses for each piping accident (Kac), an

average contribution was considered for all pipes, excluding for that of restriction orifices, which was

not accounted for at this stage, since the objective was to model the vent header prior and post orifice

installation. Considering all these points, the application of the Bernoulli principle between two points on

a node, such as a reactor and the junction (equation 2.2), can be written as shown in function of the flow

velocity. If the average velocity of the fluid on the pipe is considered (i.e. the ratio between flowrate and

sectional area), the former can be re-written as equation 3.5, which allows for the determination of the

flowrate between two points i and j (where the latter is the junction), ~Qi, j, once pressure at the junction, pj

has been arbitrated. It should be noted that if the pressure difference pi-pj is positive (that is, if the vent

39

is flowing towards the junction) the positive root value should be taken and vice-versa. In accordance

with equation 2.2, the indexes k and m refer respectively to all different pipelines and accidents present

between points i and j, whilst d0 refers to the orifice plate’s internal diameter.

~Qi, j = ± ⇡p8⇢

vuut|pi � pj|X⇣

fk⇥LkDk

5

⌘+X⇣

KacDm4

⌘+

Kopd04

(3.5)

However, in this case there were twenty-two nodes and the only known pressures on this piping

network were those operating on each equipment piece and that on the scrubber, i.e. all the pressures

on the vent header system itself were unknown - this meant that the iteration of the pressure at each

junction was intertwined with the iteration at the adjacent junctions, which significantly challenged the

iteration process and imposed several restrictions - e.g., the condition that pressure on the vend header

ought to decrease as moving downstream. Consequently, a different approach was implemented where

each equipment piece was considered to be venting independently to the scrubber and the total flowrate

(500 m3/h, as shown on Table 3.5) considered as the sum of each individual vent flowrate; in some

cases where the same equipment presented two vent lines close to each other (e.g. a reactor and

its vent condenser), the vent flowrate was determined considering a multiple-pipe system between the

two vent lines and the pipeline leading them to the scrubber. In the remaining cases, although a direct

application of the Bernoulli principle was possible, a node connecting two different points (the equipment

and the scrubber) was considered nonetheless, to maintain model consistency. As this model took under

consideration maximum working pressures at each equipment piece, the estimated flowrates correspond

to the maximum flowrates possible, which were considered to correspond to steady-state on a ”worst-

case scenario” approach.

It should be finally noted that this approach took the assumption of considering all flow as incom-

pressible, meaning all fluids’ density would not vary throughout the pipelines. This approach is only valid

should the Mach’s number associated with the flow (ratio between average flow velocity and the speed

of sound) be lower than 0.3 [39], an hypothesis which was validated later on.

Table 3.9 displays the modelling results that were obtained through the following steps:

1. For each node, a pressure value was arbitrated to an initial value of 999.99 mbar (between the

pressure applied on the equipment pieces and that of S11) and flowrates were estimated;

2. The average accident contribution (Kac) was estimated so that the sum of all vent flowrates leaving

the equipment pieces matched the 500 m3/h associated with scrubber S11 - this was done by using

Microsoft Excel’s Solver tool;

3. All arbitrated node pressures were rectified so that the cumulative flowrate at the junction was the

closest possible to zero - using Microsoft Excel’s Solver tool under the conditions that the pressure

at the junction should be between that of the connecting points;

4. Steps 2. and 3. were repeated until the cumulative flowrate at each node (or error) was no larger

than 10 -6 m3/s (absolute value).

40

Table 3.9: Modelled flowrates and pressures on the vent header system for building E, based on maxi-mum operating pressures for each equipment piece (i.e. connecting points) and considering the layoutdisplayed by Figure 3.2. The error column translates the sum of all vectorial flowrates for a given node.

Node (j)Connecting

points (i)pi (mbar) pj (mbar) ~Qi,j (m3/s) Error (m3/s)

J1R06 1500

958.801.780⇥10�2

-3.613⇥10�7VC06 1500 4.449⇥10�3

S11 950 -2.225⇥10�2

J2R03 1500

951.434.479⇥10�3

-1.099⇥10�7VC03 1500 4.479⇥10�3

S11 950 -8.958⇥10�3

J3R07 1600

950.424.874⇥10�3

7.601⇥10�7

S11 950 -4.873⇥10�3

J4R05 1500

950.364.483⇥10�3

5.428⇥10�7

S11 950 -4.483⇥10�3

J5R04 1300

950.233.576⇥10�3

1.845⇥10�7

S11 950 -3.576⇥10�3

J6VC02 1600

952.751.246⇥10�2

6.595⇥10�7

S11 950 -1.246⇥10�2

J7VC01 1500

952.101.791⇥10�2

-1.329⇥10�7

S11 950 -1.791⇥10�2

J9MLT03 1050

950.104.894⇥10�3

1.408⇥10�8

S11 950 -4.894⇥10�3

J10CT02 1500

950.084.484⇥10�3

3.283⇥10�7

S11 950 -4.484⇥10�3

J11MLT01 1050

950.104.894⇥10�3

8.485⇥10�7

S11 950 -4.893⇥10�3

J12CT03 1300

950.053.577⇥10�3

9.374⇥10�7

S11 950 -3.576⇥10�3

J13R08 1500

950.531.148⇥10�2

-1.261⇥10�7

S11 950 -1.148⇥10�2

J14PT01 1000

950.053.460⇥10�3

8.013⇥10�7

S11 950 -3.460⇥10�3

J15CT04 1000

950.011.352⇥10�3

2.366⇥10�7

S11 950 -1.352⇥10�3

J16CT05 1500

950.531.148⇥10�2

-6.451⇥10�8

S11 950 -1.148⇥10�2

J17CT06 1500

950.084.484⇥10�3

5.238⇥10�7

S11 950 -4.484⇥10�3

J18LLE01 1000

950.185.396⇥10�3

-4.246⇥10�7LLE02 1000 1.343⇥10�3

S11 950 -6.739⇥10�3

J19DT01 1000

950.053.460⇥10�3

3.489⇥10�7

S11 950 -3.460⇥10�3

J22MLT04 1000

950.054.095⇥10�3

-5.853⇥10�7

S11 950 -4.096⇥10�3

41

Table 3.10: Expected velocity at the restriction orifice for each vent line, as well as restricted flowrates,expected time increase and pressure drop imposed. Values shown for a 50% diameter reduction.

Line� = 0.5

v0 (m/s) Q (m3/h) Time increase (%) �pNR (mbar)Direct vent from R03 35.63 4.372⇥10�3 2.43 25.73Direct vent from R05 35.66 4.377⇥10�3 2.43 25.78Direct vent from R04 28.45 3.491⇥10�3 2.43 16.40

R02 vent through VC02 38.71 1.216⇥10�2 2.43 30.36R01 vent through VC01 35.62 1.748⇥10�2 2.43 25.70

The implemented model was conceived for a Kac of 195.0, which translates the average resistance

coefficient for all piping accidents on the modelled vent header system. By aggregating the error val-

ues, a total deviation of 0.016 m3/h was observed, which amongst the estimated 500 m3/h vent outlet

constituted a deviation of 0.003%, with this deviation was considered acceptable.

3.3.2 Restriction orifices impact study

After a previous assessment and approval of the installation of restriction orifices by the Product Quality,

Process Development and Process Safety departments, the impact of this installation was assessed

by the student in terms of lead time increase and non-recoverable pressure loss on the system - such

evaluation was entirely based on the elaborated model for building E’s vent header system.

Lead time increase was one of the greatest concerns regarding the impact of restriction orifices from

an operational point of view, as the same amount of gas would be forced to vent over a longer period of

time - operations such as the inertization of an equipment piece using nitrogen would thus take longer,

increasing overall process duration.

In order to estimate increase in lead time, new vent flowrates were estimated by introducing new

resistances on the previously used model. These resistances are simply additional accident contribu-

tions, accounted for separately from Kac in pipes where the installation of vent restriction orifices was

scheduled - in fact, this parameter (Kop) was estimated for thin-orifice plates with 50% and 75% diam-

eter restriction, as shown on Figure 3.4. With knowledge of the restricted vent flowrate, the expected

operation time increase was estimated by dividing the non-restricted by the restricted vent flowrate value.

Furthermore, the non-recoverable pressure drop on the system (�pNR, imposed by each orifice plate)

was quantified using the mathematical definition of the resistance coefficient for thin-orifice plates, Kop

(see equation 3.6), where v0 denotes flow velocity through the orifice.

Kop =2�pNR

⇢⇥ v02 (3.6)

The expected lead time increase, non-recoverable pressure drop, restricted flowrate and orifice ve-

locity for this model, considering 50% and 75% diameter reduction, are displayed on tables 3.10 and

3.11, respectively.

As shown by the obtained results, all average flow velocities through the restriction orifices (where

42

Figure 3.4: Non-recoverable head loss (resistance coefficient) in Bernoulli obstruction meters in functionof the ratio between orifice diameter and pipe nominal diameter (�) [26]. For thin-orifice plates with� = 0.5 and � = 0.25 resistance coefficients of 1.8 and 2.5 were considered, respectively.

Table 3.11: Expected velocity at the restriction orifice for each vent line, as well as restricted flowrates,expected time increase and pressure drop imposed. Values shown for a 75% diameter reduction.

Line� = 0.25

v0 (m/s) Q (m3/h) Time increase (%) �pNR (mbar)Direct vent from R03 100.9 3.095⇥10�3 45.04 571.7Direct vent from R05 101.0 3.098⇥10�3 45.04 572.8Direct vent from R04 80.57 2.472⇥10�3 45.04 364.5

R021 vent through VC02 109.6 8.608⇥10�3 45.05 674.7R01 vent through VC01 100.8 1.238⇥10�2 45.06 571.2

43

flow velocity is maximum) are far below Mach 0.3 (around 103 m/s) for a beta value of 0.5, which

validates the hypothesis of incompressible flow for the 50% restriction orifice plates [39]. However, for

the 75% restriction orifices the velocity through the orifices is very close to Mach 0.3, even exceeding it

slightly for R02 - albeit a small discrepancy, it was considered acceptable for the applied purposes.

3.3.3 Condenser basic design

Based on estimated vent flowrates for the centrifuges CG01, CG02 and CG03 (venting through tanks

MLT01 and MLT03, vide Table 3.9), minimum transfer areas were established for both associated vent

condensers, as exemplified on Table 3.14. Moreover, as shown by the simulation results on Table 3.9,

the vent flowrates exiting the two tanks are expected to be identical and consequently both condensers

present the same transfer area.

The total mass flowrate (M ) was determined by multiplying the volumetric flowrate by the mixture’s

density estimated on Table 3.8 - as referred above, the exhaust gas stream was considered as nitro-

gen saturated in dichloromethane. As such, using the saturation value for dichloromethane in nitrogen

(sDCM, N2) from section 3.2, the dichloromethane mass flow (MDCM) was determined for all of the tanks’

vents, as shown on equation 3.7. The nitrogen mass flow was then obtained by subtracting the DCM

mass flow to the total vent flowrate.

MDCM = M

⇣sDCM, N2

sDCM, N2 + 1

⌘(3.7)

The utility choice for these condensers was a mixture of ethylene-glycol and water at -20 oC, in

countercurrent with the process stream. Given that the process stream enters the condenser at approx-

imately 20 oC, an outlet temperature of -15 oC was defined, assuming an increase in temperature of 5oC for the cold stream - this is shown on Figure 3.5.

From here, the composition of the condensed and vapour streams exiting the condenser were de-

termined by the student, through analysis of the (T, x, y) diagram for DCM/nitrogen at atmospheric

pressure. This diagram, depicted by Figure 3.6, was simulated using Aspen Plus software, using the

NRTL method. The tieline established at -15 oC for an initial mixture with 70% DCM (weight percent-

age) shows the composition of the liquid and vapor phase exiting the condenser, as well as the initial

mixture’s mass distribution between the two vapour and liquid phases (approximately 6 : 1.5). Further-

more, observation of the (T, x, y) diagram for dichloromethane and nitrogen shows that at -15 oC most

of the dichloromethane remains in the vapor phase - in order to achieve higher DCM removal other con-

densation processes, at lower temperatures should be considered (e.g. cryogenic condensation at -70oC).

The enthalpy for each component (E i) was determined using equation 3.8 (where cpi refers to the

specific heat of the component, M i designates the component’s flowrate, Ti designates the component’s

temperature, for a reference state of 0 oC, 1 atm, with liquid DCM and gaseous nitrogen - these latter

parameters affect the component’s reference temperature (Tref) and the enthalpy variation of a given

component, in relation to the established reference state (�H i); in this current study the only enthalpy

44

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

5.00

10.00

15.00

20.00

25.00

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Flui

d te

mpe

ratu

re (º

C)

Fluid path

Figure 3.5: Expected temperature profile for the designed vent condensers, where the orange and bluelines represent the hot and cold fluid temperatures across the heat exchangers, respectively.

Figure 3.6: (T, x, y) diagram for a mixture of dichloromethane and nitrogen at atmospheric pressure.Representation in function of temperature and dichloromethane mass fraction. Thermodynamic prop-erties predicted by Aspen Plus software, using the NRTL method. The tieline for a mixture with 70%dichloromethane at -20 oC is represented in red.

45

Table 3.12: Mass and energy balance results for the inlet of the condensers for MLT03 and MLT01.IN

Stream Component State Weight fraction Mass flow (kg/s)Temperature

(oC)Enthalpy (W)

VentDCM

Liquid0.7143 7.871⇥10�3

20.002.911⇥103

N2 0.2857 3.148⇥10�3 6.560⇥101

TOTAL 1.102⇥10�2 2.976⇥103

Table 3.13: Mass and energy balance results for the outlet of the condensers for MLT03 and MLT01.OUT

Stream Component State Weight fractionMass flow

(kg/s)Temperature

(oC)Enthalpy (W)

VentDCM

Vapor0.6300 5.553⇥10�3

-15.00

1.935⇥103

N2 0.3700 3.262⇥10�3 -5.097⇥101

CondensatesDCM

Liquid0.9990 2.202⇥10�3 -2.012⇥101

N2 0.0010 2.204⇥10�6 -3.444⇥10�1

Removed heat (W) 1.112⇥103

TOTAL 1.102⇥10�2 -15.00 2.976⇥103

considered was the vaporization enthalpy for nitrogen and dichloromethane, as defined on Table 3.8.

It should be noted that for a component with a liquid reference state, given that enthalpy constitutes a

state function, either the vaporization enthalpy at Tref can be considered whilst considering the specific

heat for the vapor phase, or vice-versa, i.e. accounting for the specific heat for the liquid phase and the

vaporization enthalpy at Ti.

E i = M i[cpi(T i � T ref) +�H i] (3.8)

As an approximation, the enthalpy for each stream was considered as a sum of its components’

enthalpies. By energy balance, and assuming steady-state, removed heat by the condenser (Qrem) was

calculated, as shown in equation 3.9 as well as tables 3.12 and 3.13, for N streams entering the system

and M streams exiting it. It is important to note that on these balances the inlet/outlet DCM and nitrogen

mass flows have a small deviation, which was attributed to the uncertainty associated to the reading of

the (T, x, y) diagram.

NX

i=1

E i =MX

i=1

E i + Qrem (3.9)

By using equation 2.1 the minimum transfer area for the two condensers was determined, as well as

the necessary thermal fluid mass flowrate - this is shown on Table 3.14.

The estimated minimum transfer area was indeed quite small - as such, the original decision to

install condenser with 2 m2 of heat transfer area more than fulfils the requirements from mass and

energy balances. Under these conditions, the installed heat transfer capacity is given by equation 2.1

for all condensers, using a 2 m2 transfer area, as shown on Table 3.14. This means that the condensers’

46

Figure 3.7: Film coefficient for condensation processes in vertical tubes as a function of the Reynoldsand Prandtl numbers, based on the Nusselt model [36].

installed capacity exceeds current process needs - by regulating thermal fluid flow however (M cold),

the desired heat transfer rate can be achieved (as long as keeping in mind its effect on fluid outlet

temperatures), as shown by equation 3.10 [24] (where cpcold designates the themal fluid’s specific heat

and �T cold its temperature variation) and the results on Table 3.14.

Qrem = M cold ⇥ cpcold ⇥�T cold (3.10)

Despite a 2 m2 vent condenser being quite oversized it should be able to fulfil the purpose of con-

densing dichloromethane (although in a less efficient way), however special attention must be paid to

whether fluid flow under these circumstances will result in a low-turbulence/laminar flow, thus compro-

mising heat transfer and consequently condensation. With regards to this matter it should be noted that,

when considering condensation phenomena inside a vertical tube, the Nusselt model, which correlates

the film coefficient and Reynolds number [36], shows that heat transfer is only significantly more effi-

cient in turbulent flow than in laminar flow for a minimum Reynolds (Re) number of 10000 (assuming a

typical value for the Prandtl in number in condensers of at least 5, corresponding to the ratio between

momentum diffusivity and thermal diffusivity [40]); this can be seen on Figure 3.7.

3.3.4 Vent collecting system diameter evaluation

Considering the expected flowrate increase on the VCS until 2021, its impact on average vent speed

was determined by the student (as shown by Table 3.15). Moreover, a safety-by-design decision was

made by the project manager to always guarantee an average gas speed on the VCS of approximately

4 - 8 m/s.

Evidence showed that most of the site’s vent collecting system required an increase in diameter in

order to reduce flow speeds. In fact, some of the average speeds for the exhaust gas, calculated by

the student, exceed 20 m/s - naturally this had no physical significance as before reaching significant

gas speed the associated ventilators would either trip or the pressure build-up inside the collectors

47

Table 3.14: Condenser basic design parameters, including transfer area and thermal fluid flowrate.Parameter Value

Removed heat (W) 1.117⇥103

Processinlet temperature (oC)

20.00

Processoutlet temperature (oC)

-15.00

Thermal fluidinlet temperature (oC)

-20.00

Thermal fluidoutlet temperature (oC)

-15.00

Log meantemperature difference (oC)

15.42

Global heat transfercoefficient (W/(m2.oC))

450.0

Transfer area(minimum) (m2)

0.1603

Transfer area(designed) (m2)

2.000

Thermal fluid flowrate(ethylene-glycol+ water)

(kg/h)219.5

would force the opening of any safety relief devices present therein. It should be noted that all unknown

flowrates are expected to increase slightly the average speed inside each collector, as they are expected

to comprise relatively low amounts of off-gas when comparing with the remaining scrubbers. With all

these aspects considered, current pipe nominal diameters (DN, in millimetres) were re-evaluated, thus

leading to the following decisions:

• Pipeline from S09 to collector 3 increase from DN150 to DN200;

• Pipeline from building G (future) to main collector increase from DN150 to DN400;

• Collector 2 to increase from DN150 to DN250;

• Collector 3 increase from DN200 to DN400;

• Collector 5 increase from DN200 to DN350;

• Collector 6 increase from DN150 to DN200 (despite current average flow speed being below 8

m/s, this collector will receive waste gas from solvent farm 3);

• Main collector increase from DN300 to DN1000.

These changes’ impact on average flow speed on the vent collecting system is listed on Table 3.16.

48

Table 3.15: Flowrates, diameters and predicted maximum flow speeds of the plant’s current vent collect-ing system, considering short-term production increase (up to 2021). As shown, each scrubber (S) isconnected to a subcollector (collectors 2 to 6). The main collector (collector 1) receives vents from allsubcollectors. Each VP, SF, CT, ET or SDY refers to a vacuum pump, a solvent farm, a loading vessel,an effluent tank or a spray-dryer, respectively.

Source Average flowrate (Nm3/h) Pipeline diameter (mm) Maximum speed (m/s)S01 200 120 4.91S02 200 150 3.14S03 200 120 4.91S04 200 150 3.14

VP01 N.D. N.D. N.D.SF01 N.D. a 150 N.D.

Collector 2 800 150 12.58S05 350 200 3.09S06 1100 200 9.73S07 350 b 100 3.09S08 350 b 100 3.09S09 800 150 12.58

VP02/CT01 N.D. N.D. N.D.ET01 N.D. 150 N.D.

Collector 3 2950 200 26.08S10 500 150 7.86

Collector 4 500 150 7.86S12 400 200 3.54S13 400 200 3.54S14 400 200 3.54S15 400 200 3.54

VP03 N.D. 80 N.D.SDY01 N.D. 100 N.D.SDY02 N.D. 100 N.D.

Collector 5 1600 200 14.15S11 500 150 7.86

SF02 N.D. a 150 N.D.Collector 6 500 150 7.86Building G 3620 a 150 56.90

Non-ducted vents 2600 a N.D. N.D.Short-term expansion 750 a N.D. N.D.

Collector 1 13320 200 to 300 117.8N.D.: No data available or not defined.


49

Table 3.16: Flowrates, suggested diamters and predicted maximum flow speeds considering a short-term vent collecting system revamping. As shown, each scrubber (S) is connected to a subcollector(collectors 2 to 6). The main collector (collector 1) receives vents from all subcollectors. Each VP,SF, CT, ET or SDY refers to a vacuum pump, a solvent farm, a loading vessel, an effluent tank or aspray-dryer, respectively.

Source Average flowrate (Nm3/h) Pipeline diameter (mm) Maximum speed (m/s)S01 200 120 4.91S02 200 150 3.14S03 200 120 4.91S04 200 150 3.14

VP01 N.D. N.D. N.D.SF01 N.D. a 150 N.D.

Collector 2 800 250 4.53S05 350 200 3.09S06 1100 200 9.73S07 350 b 100 3.09S08 350 b 100 3.09S09 800 200 7.07

VP02/CT01 N.D. N.D. N.D.ET01 N.D. 150 N.D.

Collector 3 2950 400 6.52S10 500 150 7.86

Collector 4 500 150 7.86S12 400 200 3.54S13 400 200 3.54S14 400 200 3.54S15 400 200 3.54

VP03 N.D. 80 N.D.SDY01 N.D. 100 N.D.SDY02 N.D. 100 N.D.

Collector 5 1600 350 4.62S11 500 150 7.86

SF02 N.D. a 150 N.D.Collector 6 500 200 4.42Building G 3620 a 400 8.00

Non-ducted vents 2600 a N.D. N.D.Short-term expansion 750 a N.D. N.D.

Collector 1 13320 1000 4.71N.D.: No data available or not defined.


50

3.4 Procurement and selection of a regenerative thermal oxidizer

This section aims at describing the process of selecting a supplier for the purchasing of a regenerative

thermal oxidizer (RTO), as a turnkey installation solution. Moreover, critical mechanical, instrumentation,

automation and environmental aspects associated with the chosen design are discussed herein.

3.4.1 Supplier selection

The first step towards the selection of a RTO supplier had already been performed prior to the enrolment

of the student in this project - in fact, five initial candidates had already been identified by the project

manager and reduced to three companies, which are henceforth referred to as Supplier A, Supplier B

and Supplier C, all of which presented CE marking (Conformite Europeenne).

All the three companies (A, B and C) had at such point already presented initial proposals for the

purchase of an RTO, based on emission monitoring records from the plant and considering the request

to allow operation of the RTO for flowrates between 3,000 Nm3/h (current average flowrate) and 15,000

Nm3/h, considering the expansion forecast of the site over the next five years (as shown on Figure 1.8.

Moreover, all suppliers were duly informed about the site’s targets for waste gas limit emission levels,

as shown on Table 3.2. The three proposed technical solutions by each supplier are summarized on

Table 3.17, for the requested end-of-line VOC abatement system, composed by an RTO, a quench, an

HCl scrubber, a SCR DeNOx system with urea injection, allowing no visible plume. It was decided that

the acquisition of a continuous emission monitoring station (to be installed at the outlet stack) was to be

made from a different vendor.

It should be noted that the requested solutions are free of visible plume emissions since the selective

reduction system operates at around 500 oC, thus requiring a preheating of the gas exiting the scrubber

(see DeNOx heat exchanger, on Table 3.17) - this preheating could be performed on an economizer in

countercurrent with the clean gas exiting the catalyst bed, without generation of visible plume.

The main technical distinctions between the three companies consist on the type of draft and on the

integration of fresh air and dilution air. Dilution air can be used as part of an LEL control strategy (i.e. for

keeping it below the 25% limit), and is considered by supplier A and B. However, supplier A considered

that a preheating of the process gas occurs (on the exhaust air heater, vide Table 3.17) prior to its mixture

with the dilution air, whereas supplier B considered only the dilution air is preheated (on the dilution air

heater, vide Table 3.17) and, as a consequence, supplier B relied on a duplex steel inlet duct and lower

part for the RTO, unlike supplier A which used carbon steel for these parts - this can be explained by

the fact that the lower the temperature of the process gas, the higher the risk of HCl condensation and

damage to the material; as a consequence, supplier B proposed building these colder sections of the

RTO in a more resistant (and expensive) type of steel than that of supplier A. Furthermore, supplier A

offered a quotation for the option of upgrading the materials of construction of the system’s colder parts

contacting with untreated gas to Alloy/Hastelloy, allowing for the saving of approximately 645 kg of steam

per hour in the preheating of exhaust process air.

Regarding fresh air, supplier A uses it as a source for combustion air whenever the RTO is on standby

51

Table 3.17: Summarized technical comparison between the three considered suppliers for an RTO sys-tem. Items not included in a given technical solution were signalled as N.I., whereas items for which nodata was given, but are part of the proposed system, are labelled as N.D..

Vendor Supplier A Supplier B Supplier CCountry Germany Germany United Kingdom

Delivery time(months) 9 6 7

Nr. of chambers 3, alternating 3, alternating 3, alternating

Air draftForced:

ventilator upstreamof RTO

Forced:ventilator upstream

of RTO

Induced:ventilator downstram

of scrubberLEL control

strategy Optional Included Not included

Oxygen minimumcontent Not relevant Not relevant Not relevant

Automation system Siemens Siemens Allen BradleyChamber

temperature (oC) 850.0 850.0 900.0

Residence time(s) 1.0 1.0 1.5Noise @ 1m

(dB(A)) 85 80 N.D.

Exhaust air heater Stainless steel 1.4539 N.I. N.D.Exhaust air damper Duplex steel 1.4462 N.I N.D.Dilution air damper N.I. Stainless steel 304 N.I.Dilution air heater N.I. Stainless steel 304 N.I.Bypass damper N.I. Stainless steel 304 N.I.

Fresh air damper Stainless steel 316 Stainless steel 304 Stainless steel 316LFresh air heater N.I. Stainless steel 304 N.D.

Fans Stainless steel 316Stainless steel 304+ wheel in duplex

steel 2205 (ATEX 02)Carbon steel

Inlet channel Carbon steel+ internal coating Duplex steel 2205 Stainless steel 316L

Switching valves Stainless steel Duplex steel 2205 Stainless steel 316L

Lower part Carbon steel sheet+ internal coating Duplex steel 2205

Hastelloy C276+ carbon steel

+ internal coating

Heat exchangemedia

Ceramic structuredpacking

+ support grids instainless steel 316Ti


+ support grids induplex steel 2205


+ support grids inHastelloy C276

Heat exchangetowers

Carbon steel sheet+ internal coating Stainless steel 304L

Hastelloy C276+ carbon steel

+ internal coatingCombustion

chamber Carbon steel sheet Stainless steel 304L Carbon steel+ internal coating

Externalinsulation

Mineral wool cladding+ galvanized steel

Corrugated sheets+ powder coated layer N.I.

Outlet channel Carbon steel Duplex steel 2205 Stainless steel 316L

Quench Hastelloy C22 / Alloy 59 Stainless steel 904L Hastelloy C276+ carbon steel support

ScrubberGlass-reinforced plastic

+ polyethylenepacking/demister

Fibre-reinforced plasticFibre-reinforced plastic

+ polypropylenepacking/demister

DeNOxheat exchanger Stainless steel 904L Stainless steel 304 N.D.

52

(i.e. running exclusively on atmospheric ”fresh” air) or below autothermal oxidation due to low VOC load.

Supplier B proposed a strategy unlike this one where fresh air is preheated and fed to the bottom of the

RTO exclusively for standby operation whilst a fixed flowrate of combustion air is used only when the

process is below autothermal conditions.

Supplier C opted for a induced draft strategy, with the ventilator positioned at the outlet of the scrub-

ber, unlike suppliers A and B that chose a forced draft approach. Induced draft constitutes a more

safety-by-design type of approach, since it applies a negative pressure on the RTO, thus reducing the

probability of leaks of untreated process gas and the loss of energetic efficiency.

It should be noted that none of the suppliers suggested the operation of the combustion chamber at

1100 oC with 2 seconds residence time, unlike the European Commission’s recommendations for waste

gas containing more than 1% chlorine (by weight). Therefore, a request was made for the suppliers to

quote such option on the proposed design solution.

Different plants with operational RTO systems provided by each of the suppliers were visited by

the project manager, with lifetimes ranging between 1 and 10 years, and no operational or corrosion

problems were identified. With the exception of Supplier A, which had had problems related to the

working of damper valves on one of the visited sites, no additional maintenance problems were identified.

Table 3.18 lists the pricing for the different components of the systems proposed by each supplier,

including optional aspects. As observed, suppliers A and B had a much similar base cost, with the grand

total differing only due to the additional options cost. In fact, the high costs of operating at 1100 oC with

2 seconds residence time, as well as those associated with the desired DeNOx system, inflated largely

the price presented by supplier A. Supplier C presented the highest base price and optional costs,

which were mainly due to the choice of Hastelloy as material of construction (being the most chemically

resistant material, although expensive, when comparing to carbon, stainless or duplex steel) and to the

increased cost of the DeNOx system provided. Despite having been the only vendor to propose an

induced draft strategy, Supplier C was dismissed due to the absence of a robust LEL control strategy,

adding to the fact of being the least cost-effective option.

Considering only suppliers A and B it was noted that, apart from differences in materials of construc-

tion, both systems provided similar technical solutions, with large pricing discrepancies, largely due to

the pricing of the DeNOx system and that of the operation at 1100 oC and 2 seconds. Moreover, the

construction of the colder parts of the RTO in duplex steel, as per Supplier B’s proposal, was considered

a very cost-effective compromise between the weaker materials of construction initially suggested by

Supplier A and the Hastelloy/Alloy upgrade, quoted as an option by the latter vendor.

As such, and considering both suppliers complied with the desired environmental emission limits

(and consequently Portuguese/European legislation as well as the Best Available Techniques), Supplier

B was selected for further negotiations and thus invited to participate in a safety assessment of the

process (vide section 3.5).

53

Table 3.18: Approximate cost comparison between the three considered suppliers for an RTO system.Cost (§) Supplier A Supplier B Supplier C

Base price 1,300,000 1,200,000 1,800,000Shipment, installation,

startup and training80,000 90,000 35,000

LEL control strategy 100,000 included not quotedDeNOx system

(SCR, urea injection)400,000 120,000 800,000

Presence for HAZOP (2 days) not quoted 4,000 3,000Hastelloy/Alloy construction 200,000 not quoted included

Chamber temperature: 1100oCResidence time: 2 seconds

170,000 70,000 not quoted

Redundant process ventilatorand variable frequency drive

not quoted 35,000 not quoted

Grand total 2,250,000 1,519,000 2,638,000

3.4.2 Technical information

The first technical challenge associated with the mechanical installation of the chosen RTO system was

the selection of an adequate location inside the plant.

The initially planned space presumed a rather small installation (approximately 10x5 meters), but the

initial technical solution provided by the vendor assumed dimensions of, approximately 25x25 meters,

which proved to be non-feasible. This was explained by the increased residence time in the chamber,

which forced it to be longer, as well as the unusually low ammonia slip requested (which forced the

installation of a residence tank in order to guarantee full evaporation of the urea droplets, minimizing

deposits and ensuring a complete decomposition of the urea into ammonia), as complying with the BREF

suggestion for the NH3 slip is not a common practice in this industry. Furthermore, these dimensions

include a minimum of 1.5 free meters around the RTO system to ensure maintenance access and good

accessibility to the main fans.

As such, throughout discussions attended with the vendors and the design team, the compromise

was reached for building a compact version of the RTO system, where the DeNOx subsystem stacked

vertically instead of horizontally, as shown by figures 3.8 and 3.9.

The final process integration is represented by Figure 3.10. As evidenced, two redundant LEL sen-

sors (through flame temperature analysis) monitor the percentage of the lower explosion limit of the inlet

process gas: for LEL greater than 15%, the dilution air damper opens and the process air is diluted,

whilst for LEL above 25% the bypass damper opens and the RTO inlet damper closes, thus deviating

the process gas towards the carbon bed drums, followed by the opening of the fresh air damper - this

ensures the RTO is kept in standby, without need of shutting down and restarting the system each time

a LEL event occurred. It should be remembered that the frequency of LEL events is expected to de-

crease over the near future, given the mitigating measures to be added to the current vent collecting

system (such as more dilution air tools) as well as the installation of slow relief devices on problematic

equipment pieces.

54

The presence of two ATEX 02 redundant fans, with an integrated variable frequency drive each

(allowing for flowrate control based on pressure on the vent header) allow that in case of mechanical

failure the system is kept running while the fan is replaced, with no need of stopping production or

bypassing VOC treatment.

The regenerative thermal oxidizer itself is controlled through the action of the switching valves on

the bottom, by temperature measurement on the chamber. Should the chamber temperature rise above

the setpoint of 1100 oC due to abnormally high VOC loading, the hot side bypass (or HSBP) damper is

programmed to open and allow the release of VOC free process gas through the side of the RTO, without

energy recovery, in order to protect the integrity of the ceramic beds. In case of the temperature dropping

below the mixture’s autothermal regime, the combustion air fan starts and natural gas is burnt to ensure

a high enough chamber temperature for VOC oxidation - this process also occurs during startup and

ceases once VOC oxidation returns to autothermal conditions.

The purging process itself is essential to prevent emission peaks during chamber switch-over, as

some of the untreated process gas may be retained in the ceramic matrix during admission and would

thus be expelled after valve switch-over - by purging the chamber between inlet and outlet of process

gas, any persistent VOCs will be returned to the admission of the RTO. It should be noted that although

the purging fan is always operational, the fresh air damper only opens when the RTO is in standby.

The VOC free gas is afterwards directed to a quench where it is sprayed with cooled water and rapidly

cooled below the dioxin regeneration window to a maximum temperature of 90 oC at which it enters

the HCl scrubber, where NaOH addition is automatically controlled by continuous pH measurement;

meanwhile NaCl removal through discharge at the bottom is automatically controlled through continuous

conductivity measurement. At this point it should be noted that the NaOH reservoir is not included in the

scope of this design and is to be supplied by P at a later stage. The scrubber/quench system itself has

make-up water supplied, which is then recirculated until automatic discharge, whilst the HCl/VOC-free

air is exhausted through the top.

As previously mentioned, the off-gas before reaching the inline gas burner is previously heated by

means of an economizer where it circulates in countercurrent with the clean process gas that reaches

the stack. After the burning of natural gas inline with the process gas, liquid urea is injected (at approx-

imately 3 bar) and evaporates onto the process gas inside the residence tank - a static mixer placed

afterwards ensures an adequate dispersion of the urea and consequent decomposition into ammonia

as per equation 2.4.

The ammonia-rich process air reaches the DeNOx reactor, consisting of a double bed amounting to 4

m3 of catalyst, at a temperature around 300 oC, where the reactions from equations 2.5-2.6 take place.

Once the clean gas is cooled down by the process gas entering the DeNOx system, it is exhausted

through a dedicated stack. Given the high gas flow expected (up to 15,000 Nm3/h plus a maximum of

8,000 Nm3/h of dilution air), a decision was made to build a new dedicated stack, where the continuous

emission monitoring station is to be installed. This option had the advantage of placing a stack close to

the RTO system, without need of piping works all the way to the existing outlet stack, thus saving the

heavy costs of building further piping as well as reducing total head loss on the system, consequently

55

Figure 3.8: Mechanical drawing of the designed RTO system.

decreasing the necessary fan capacity.

Piping and instrumentation diagrams (or simply P&IDs) were supplied by the vendor for main process

components: RTO, quench/scrubber and DeNOx. These diagrams provided essential information which

the process flow diagram omits, such as piping dimensions and valves but also (and most importantly)

instrumentation types and locations, as well as associated control and safety functions. Relevant parts

associated with these components are displayed on annex A. It should be noted that throughout this

process all digital inputs (DI) and digital outputs (DO) are associated exclusively to safety functions

whilst control functions are based on analog inputs (AI) and analog outputs (AO).

Figure A.1 displays the junction of process and dilution air, whilst Figure A.2 displays the dilution air

preheater (steam to air) and damper. As observed, the temperature control valve on the steam inlet to

the heat exchanger (TV) is controlled by the readings provided by two temperature transmitters (TT),

displayed by a dotted line: the first temperature transmitter is located on the outlet of warm process air

on the heat exchanger and the second is located on the main process line, after the addition of dilution

air. The objective of this control loop is to provide an inlet temperature setpoint (to the RTO) of 30 oC.

Figure A.3 represents the inlet of compressed air as well as the gas bottles (to be placed on an ATEX

rated area) containing the hydrogen and test gas (methane) necessary for the operation and calibration

of the two LEL sensors located upstream of the redundant process fans and of the bypass damper

responsible for the deviation of the waste gas towards the carbon beds (Figure A.4 - in fact, these LEL

analysers are equipped with one independent safety device interlock each (AZH) that in case of a high

LEL measurement (i.e., above 25%) actuates on the opening of the bypass damper X602, followed

56

Figure 3.9: Layout drawing of the designed RTO system.

Figure 3.10: Process flow diagram of the designed RTO system and its components. On the RTO(from the right to the left) the first chamber is receiving VOC rich process gas, the second chamber isexhausting VOC free process gas towards the quench and the third chamber is being purged of retainedVOC containing process gas. HSBP designates the hot side bypass and LEL the process gas’ lowerexplosion limit.

by the closing of the bypass damper X603 and the opening of the fresh air damper X501. For safety

purposes, X602 and X501 are fail-open while X601 is fail-close - this means that in case of malfunction

57

a spring will force the opening of the former and the opening of latter, in order to avoid at all costs

the feeding of a high-LEL mixture onto the RTO; additionally, none of these dampers take intermediate

positions, being always either open or shut.

At this point, an assessment was performed by the student in order to estimate the minimum pipe

distance required between the LEL sensors and the bypass dampers. This was performed assuming

a response time (tsensor response) of 4 seconds by the sensors (2 seconds plus 0.5 seconds per meter of

sampling line, assuming a line length of 4 meters) and a pipe diameter (D) of 1000 mm, corresponding

to that suggested for the VCS’s main collector, as per Table 3.16. The considered flowrate was the

maximum possible (Qmax), that is 15,000 Nm3/h of process gas, not considering the dilution air, which

is only added close to the dampers. As such, a minimum length of 22 meters was estimated (vide

Lsensors - bypass on equation 3.11), which can be reduced further either by building by compacting this

pipeline into a spiral-like shape, or into a vertical tank (with adequate siphoning or low-point drainage).

Lsensors - bypass =Q

max ⇥ tsensor response

⇡D24

= 22 meters (3.11)

The dilution air itself constitutes a control function where the position of the dilution damper Y600 on

Figure A.2 is regulated by the readings coming from both LEL transmitters (AT), which are compared

before serving as an input to the LEL controller/indicator (AIC) which actuates on the position of the

dilution damper Y600 (fail-open to ensure dilution even in a worst case scenario). Furthermore, the

dilution damper is controlled by the pressure transmitter (PT) upstream of the main process fans P600,

should the process air flowrate drop below the minimum of 3,000 Nm3/h.

The two main process fans are equipped with a variable frequency (or speed) drive (SC) each,

regulating the fan’s motor (M) and consequently the drafted flowrate, based on the reading from the

pressure transmitter located directly upstream.

When considering the RTO’s bottom part, including the purge ducts and the damper/heater for fresh

air on Figure A.5 it is possible to observe a control loop similar to that of the dilution air inlet: fan P500

is thus being controlled by the respective variable frequency drive based on the pressure readings from

the pressure transmitter upstream as well as on the pressure drop on the RTO (which may increase in

case of clogging, requiring an increase purge flowrate), supplied by the pressure differential transmitter

(PDT) located on the section designated by ”E3-BOX”. Additionally, it is possible to observe a control

system actuating on the temperature control valve (TV) on the steam inlet of the fresh air heater, based

on measures from a temperature transmitter downstream on the warm fresh air outlet, thus guaranteeing

that in case of low ambient temperature no condensation occurs inside of the RTO.

Figure A.6 represents the top part of the RTO and its outlet towards the quench, were many ther-

mocouples (TE) fulfil different functions. The thermocouples located on the ceramic beds form a control

loop with the temperature indicators/controllers (TIC) and the hot-side bypass (HSBP) damper X101,

with the latter opening for operating temperatures higher than 1100 oC allowing for the release of VOC-

free air through the side of the RTO. Additionally, the HSBP is also controlled by the readings from a

thermocouple shown on the bottom outlet of the oxidizer. An additional control function regulates the

58

burning of natural gas using combustion air, through the measurements performed by the thermocouples

on the top of the combustion chamber, communicating with the temperature indicator/controller which

actuates on the valves X512/X516 on the combustion air/gas feed to the burner (as per Figure A.7).

The automatic switching of the valves is performed by a safety device interlock (SDI) associated with

additional thermocouples on the top of the combustion chamber and on the bottom outlet of the RTO.

One final SDI is associated with a thermocouple on the line to the quench which activates the bypass

mechanism towards the carbon bed filters should the gas temperature on this line rise above 355 oC,

due to high VOC load, thus dangerously approaching the dioxin window of formation.

Based on expected gas and power consumption values estimated by the vendor for different flowrates

and VOC loads, an estimation of the energy consumption was performed by the student, as shown on

Table 3.19 and Figure 3.11. It is possible to observe that the energy consumed by the RTO is optimal for

a minimum VOC load of 3,000 mg/Nm3 or 2,000 mg/Nm3 if operating at maximum flowrate. Nonetheless,

it was observed that energy consumption due to the fan duty increased non-linearly with gas flowrate.

It is expected that this simulation will provide the basic guidelines for utility management on all vent

condensers, in order that the VOC reaching the RTO allows for maximum energy efficiency and minimum

natural gas burning.

It should be noted that, before production increase and after the VCS is upgraded and all pressure

relief valves on it are replaced by pressure safety valves, the waste gas reaching the RTO will rise from

the current 3,000 - 4,000 Nm3/h to over 5,000 Nm3/h. This means that until the waste gas reaches

the expected 12,000 - 15,000 Nm3/h, a minimum operational cost of 0.80 §/h is expected. However,

once the exhaust gas flowrate reaches 12,000 Nm3/h, such value will increase to 2.40 §/h, hence the

importance of routinely controlling vent condensation processes upstream.

On Figure A.8 it is possible to observe the chosen design for the quench/scrubber subsystem. The

cooling water used on the quench is continuously recirculated from the bottom of the scrubber, through

pumps P811 (redundant) which are controlled by the measurements on the flowmeter downstream of it

(FIT). Additionally, in case of high temperature readings from the quench’s temperature transmitters (and

therefore dioxin generation risk) a safety function actuates on the opening of valve XV835, allowing for

the inlet of 15 m3/h of emergency water, locally supplied by the plant at 18 - 20 oC. This valve is fail-open

in order to never compromise the maximum temperature of 90 oC for the gas reaching the scrubber.

The HCl scrubber itself is not ATEX rated since it will be receiving VOC-free air and, even if an

LEL peak should reach the scrubber, an explosion on the oxidizer would take place beforehand. Fresh

make-up water is supplied by the plant to the scrubber at a flowrate of 3.4 m3/h - however, in case of

high liquid level on the scrubber, its level transmitter (LT) will provide information to shut valve XV830,

which is fail-close in order to prevent any overflow events. Scrubber liquid discharge is controlled by

XV826 (fail-close, to prevent drying the scrubber) which will open in case of high liquid conductivity

(NaCl concentration measured by QE807) and will close in case of low level (information provided by the

same level transmitter aforementioned). Similarly to P811, pumps P812 are redundant and controlled

by a flowmeter on the scrubber’s water recirculation line to the top nozzles.

The supply of 300 kg/h of NaOH to the scrubber is controlled by dosing pump P813, which in case

59

Table 3.19: Estimated power consumption and energy costs associated with operating the RTO at dif-ferent flowrates and VOC loads, assuming a cost of 0.0259 § per kWh of natural gas and of 0.10 § perkWh or electrical power.

Exhaust air flow (Nm3/h) VOC (mgC/Nm3) Gas (kW) Fan (kW) Cost (§/h)15,000 1,000 166 43 8.60

2,000 29 43 5.053,000 0 43 4.305,000 0 43 4.3010,000 0 43 4.30

12,000 1,000 116 24 5.402,000 21 24 2.943,000 0 24 2.405,000 0 24 2.4010,000 0 24 2.40

8,000 0 145 8 4.561,000 72 8 2.661,500 35 8 1.712,000 10 8 1.063,000 0 8 0.805,000 0 8 0.8010,000 0 8 0.80

3,000 (Standby) 0 60 3 1.85

of high/low pH (measured on QE806) will turn off/on the pump’s motor - it should be noted that if the

aim were to control the pH to a setpoint (and not based on a high/low alarm) a variable frequency drive

would be necessary on P813.

After an average residence time of 1 minute in scrubber the waste gas reaches the economizer

(where it is heated by the outlet clean gas) before reaching the inline gas burner. The injection of liquid

urea is performed downstream of the burner, through a dedicated nozzle, at an average temperature

of 300 oC. The urea is supplied by the plant as a 40% liquid solution and is mixed with compressed air

(also supplied by the plant) prior to injection - this system is depicted by the diagram on Figure A.10.

After the evaporation of the urea droplets and its decomposition into ammonia, followed by mixing,

the gas reaches the DeNOx, as shown by Figure A.9. This reactor will be vertically stacked, containing

two layers of catalyst, being additionally equipped with two thermocouples (TE, upon inlet and outlet)

and one differential pressure transmitter (PDT) which will detect any clogging of the equipment or excess

in temperature (i.e., above 460 oC) and/or bypass the system to the carbon bed drums. After leaving

the reactor and being cooled on the economizer, a continuous emission monitoring station (yet to be

defined), will monitor VOC (generic but also DCM, DMF, DMA and dioxins), CO, Cl2, HCl, NOx and

NH3. The two latter measurements will integrate a control loop that regulates urea supply to the system

through a variable speed drive (vide Figure A.10), in order to achieve the desired ammonia slip and NOx

emission setpoints.

60

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

0 2,000 4,000 6,000 8,000 10,000

Ga

s/e

lect

rica

l co

nsu

mp

tion

(€

/h)

Volatile organic compound load reaching the oxidizer (mg/Nm3)

Figure 3.11: Estimated energy costs associated with operating the RTO at different flowrates and VOCloads (in mgC), assuming a cost of 0.0259 § per kWh of natural gas and of 0.10 § per kWh or electricalpower. The blue, orange, grey and yellow series represent, respectively, operation at 15,000 Nm3/h, at12,000 Nm3/h, at 8,000 Nm3/h and standby operation with 3,000 Nm3/h of fresh atmospheric air.

3.4.3 Environmental compliance

Expected maximum limits by the supplier were previously shown on Table 3.2, allowing for confirmation

that without performance deviations the designed end-of-line VOC abatement unit will comply with Por-

tuguese/European environmental legislation, whilst fulfilling scrupulously the best available techniques

and recommendations of the European Commission regarding waste gas management and treatment

for the organic fine chemistry industry. With this information, and considering the basic process de-

sign, approval of this project by the Portuguese Environmental Agency (or APA, in Portuguese) followed

shortly.

Moreover, given the robust design of the RTO system allowing for the oxidation of waste gas ranging

from 3,000 Nm3/h to 15,000 Nm3/h, without compromising safety or environmental aspects, it is safe to

say that the plant is equipped with the possibility to compliantly accommodate future production increase

and expansion. Finally, and from a continuous improvement perspective, VOC source reduction should

be continuously optimized beyond the lifetime of this project - indeed, as soon as the RTO is installed,

the paradigm should shift from VOC source reduction to VOC source management, as the final goal is

to have the RTO capably of processing a maximum of 15,000 Nm3/h with maximum energy efficiency,

61

without any untreated emission being sent to atmosphere.

3.5 Safety analysis

This section describes the safety analysis of the end-of-line VOC abatement subproject, which was

performed throughout several meetings between July and September 2018. In these meetings, to which

the student had the opportunity to attend, representatives were present from the site’s engineering,

safety and environment teams, as well as technical representatives from supplier B and external process

safety consultants.

Safety analysis is an essential milestone for every project, taking place after the closing of basic

design (elaborated in this case by supplier B) and before the beginning of detailed design. Safety

analysis for this project constituted three essential work layers:

1. HAZOP analysis: the HAZard and OPerability analysis aimed at identifying all foreseeable causes

and consequences for the deviation of a process from normal operation - once all consequences

were identified, their severity and likelihood was rated and, should it not be tolerable, recommen-

dations were made for reducing risk severity and/likelihood;

2. SIL assessment: the Safety Integrity Level assessment intended to determine the safety level

required for all safety instrumented systems associated with safety functions identified as critical

on a given process;

3. Safety requirement analysis will follow the SIL assessment in a near future, in order to ensure that

all instrumentation present on a safety instrumented system allows for the desired risk reduction

level - once this work is concluded, detailed design begins.

At the conclusion of this master’s dissertation only HAZOP analysis and SIL assessment were per-

formed over the designed RTO system. As such, only these evaluations will be discussed in this section.

3.5.1 Hazard and operability analysis

The HAZOP method itself constitutes a brainstorming process, conduced by a leader (in this case,

an external consultant) that uses a predefined set of guide-words applied to various process parame-

ters, constituting deviations from normal operations (e.g. ”no/low/more/inverse flow”, ”low/high level”,

”low/high temperature”, ”low/high pressure”, ”change composition” or ”no services”). The aim of this

methodology is to provide insight on the causes and consequences of such deviations, ultimately leading

to the identification of possible unacceptable consequences, followed by recommendations for process

improvement. This approach usually divides the process into independent nodes, for which deviations

are assessed separately. For this current study, the following nodes were suggested by the HAZOP

leader:

1. Regenerative thermal oxidizer towers and combustion chamber, including burner and feed circuits

to the RTO (dilution air, fresh air, combustion air, and natural gas);

62

2. Quench and HCl scrubber, including feed circuits (emergency water, make-up water) and outlets

(scrubber discharge, bleed and siphoning);

3. DeNOx subsystem, including inline gas burner, residence tank, static mixer, catalytic reactor and

feed circuits of natural gas and compressed air;

4. Gas bottles and compressed air feed to the LEL analysers;

5. Urea feed to the DeNOx subsystem;

6. NaOH feed to the HCl scrubber;

7. System startup, normal shutdown and emergency shutdown sequences.

Following the HAZOP method, node by node, foreseeable causes and consequences for each of

the aforementioned deviations were identified. It should be noted that several causes can be associ-

ated with one single deviation or consequence, whilst one deviation can nonetheless produce various

consequences - consequences refer to all operational hazards and operability problems affecting safety,

environmental, financial aspects, ranging from VOC emissions above limit to plant shutdown.

For each identified consequence, a classification procedure was followed encompassing not only the

rating of the respective deviation likelihood (or frequency) but also the rating of the consequence’s sever-

ity. This evaluation was performed over a risk matrix, which allowed for a two-dimensional representation

of likelihood and severity levels, being that the admission or refusal of a given risk was based on its two-

dimensional position over the risk matrix, which had predefined ”acceptable” and ”non-acceptable” re-

gions. The number of existing levels and the definition of risk acceptability ranges is defined internally by

each company. A possible risk matrix is represented by Figure 3.12, where deviation likelihood ranges

from 1 (extremely rare) to 5 (frequent) and impact severity varies from 1 (minor) to 5 (catastrophic) - the

risk rate was obtained multiplying likelihood by severity, leading to a decision on whether the risk level

was acceptable.

For each possible consequence, safeguards were identified and ultimately considered for the risk

rating - whenever present and applicable, safeguards were considered for the reduction of likelihood

or the mitigation of severity, thus decreasing the initially estimated risk rating. It should be noted that

safeguards may encompass not only automatic control systems and safety interlocks, but also routine

detection procedures and alarms, as well as safety-by-design implementations and relief devices, such

as a safety valve or a blast-off wall. Moreover, a total of 42 recommendations were allocated to certain

consequences (whenever applicable), regarding procedural measures and technical modifications to

be further studied by the project team, incorporated into process design and operation, or followed up

during SIL assessment.

The most relevant risks identified for nodes 1, 2 and 4 are shown on tables 3.20, 3.21 and 3.22,

respectively. It should be noted that this section focuses on risks associated with functions requiring

further SIL assessment, as well as recommendations that were critical for the lowering of a given risk to

an acceptable level. With regards to the remaining recommendations for risk reduction a special mention

63

Figure 3.12: Risk matrix example for process safety analysis [41]. Risk rating is given by the productof likelihood by severity. (Green) Risk accepted: no measures are recommended; (Yellow) Risk notdesirable, however accepted; (Orange) For existing processes already running, those can carry onuntil the due date for control measures implementation, being reassessed by then or interrupted. Forprocesses under design/development such risk level is not accepted and additional controls must beimplemented to bring the risk down to an acceptable rating; (Red) Unacceptable risk.

should be made to the decision of installing recirculation lines on both the urea and NaOH tanks’ outlets

to the system, to ensure these reagents are not spent needlessly during startup sequence.

Moreover the need was identified to define the startup, emergency shutdown and normal shutdown

sequences of the system.

3.5.2 Safety integrity level assessment

The objective of the described SIL assessment was to determine the safety integrity level (or SIL) re-

quired for the eight safety instrumented systems associated with the safety instrumented functions iden-

tified as relevant during the HAZOP study.

Safety instrumented systems, alongside with control systems, alarms, operator actions and mitigation

measures (such as emergency relief systems, fire and gas systems or emergency response plans)

constitute the protection layers present in industrial facilities against the associated level of risk and

must be thoroughly addressed during the design of new facilities and equipment. Moreover, safety

instrumented systems aim to direct operations to a safe state whenever certain predetermined conditions

take place, in order to avoid hazards. Such systems are formed by a measuring device (i.e. a sensor),

a logic converter and final control elements or actuators.

LOPA (layer of protection analysis) methodology [42] was used for this assessment which, much alike

the HAZOP analysis, also proceeded in a brainstorming environment with the same participants. This

methodology was applied to the consequences identified during the HAZOP analysis to estimate the

intermediate event frequency (IEL) for a given scenario on a yearly basis, based on its initiating cause

likelihood (ICL), the fraction of time hazard present (or time at risk factor, Ptr), the likelihood of people

being present in the area (or occupancy factor, Pp), the likelihood of ignition (Pi) and the probability of

64

Table 3.20: Relevant consequences, safeguards, risk ranking and recommendations for node 1.Deviation Consequence Safeguards Risk ranking Recommendations

Low/no flow ofprocess gasdue to mainfan failure

Emissionsout of limitsProduction

stops

Replacementfan in stock

HighInstall astandbyduty fan

Low/no flow offresh air due

to recirculationfan failure

No purging of theceramic mediaPotential VOCrelease peaks

Manual bypassto carbon beds

Medium

Evaluate thepossibility ofan automatic

bypass

More flow offresh air due to

fresh air dampersmalfunction

Condensationand corrosion

Preventivemaintenance plan

High

Install safetyinterlock to

bypass RTOin case of low

temperature onthe recirculationline, with alarm;

Evaluate SIL

High temperatureinside the RTO dueto high VOC load

Damage tothe RTO

Dilution air forhigh temperature;

Hot-sidebypass to quench

Medium Evaluate SIL

Low temperatureinside the RTOdue to hot-sidebypass open

Loss of treatmentcapacity

UncontrolledVOC release

Thermocouples incombustion chamber

bypass the RTOMedium Evaluate SIL

Change compositioninside the RTO dueto LEL above 25%

Explosion inthe RTO

Dilution air admissionat 15% LEL;

LEL analyser safetyinterlock to bypassRTO at 25% LEL

High

Install a secondFTA LEL analyzerin the process lineImplementation of

a controlledexplosion system

Evaluate SIL

65


Low flow ofgas reaching

the quench dueto line rupture

Emissions ofHCl and NOx to

atmosphere

Differentialpressure analyserinside scrubber;

Preventivemaintenance plan

Medium

Install safetyinterlock to

bypass RTOin case of

pressure drop;Evaluate SIL

No flow ofmakeup water

due to water linerupture

Emission ofHCl to DeNOx

and atmosphere.Possible corrosion.

Online HClmonitoring;

Safety interlockto bypass RTOin case of low

level in scrubber

Medium

Install pressureswitch on makeup

water line, interlockedto bypass RTO in

case of low pressure;Evaluate SIL

No flow ofemergency waterdue to automaticvalve malfunction

Damage tothe scrubber

Automaticvalve fail-open

MediumTest the valveonce a week;Evaluate SIL

High temperaturein discharge water

from scrubber(above 40 oC)

Possible explosiveatmosphere inwaste water

network

- Very high

Install thermocouplewith alarm in

discharge line;Redesign discharge


High pressurein hydrogenbottle due to

regulator failure

Analyser failure;Bypass of the RTO;

Uncontrolledemissions toatmosphere;

Possible explosion

PSV in analyserfeeding line

High

Install thehydrogen bottle

on an ATEXrated area

Change compositionof the test gas due

to human error(connection of

the wrong bottle)

LEL analysermalfunction;Uncontrolledemissions toatmosphere;

Possible explosion

Standardoperating

proceduresVery high

Install differentbottle of test gas

on the second LELanalyser;

Include doublechecking intoprocedures;

Ensure bottleconnectiononly fits the

correct bottle

66

failure on demand for each of the n independent protection layers on the system (PFD), as shown by

equation 3.12.

IEL = ICL⇥ Ptr ⇥ Pp ⇥ Pi ⇥nY

i=1

PFDi (3.12)

Whilst the time at risk factor is easily estimated by dividing the time that the process is subject to a

hazard by the time corresponding to one calendar year, presence factors and ignition probabilities are

tabled according to human presence duration and type of fluids present in the process, respectively. In

turn, initiating cause likelihoods and probabilities of failure on demand are tabled for initiating events

(e.g. human error on a high-stress non-routine task) and independent protection layers (e.g. pressure

safety valve) - with all these values considered, the intermediate event frequency can be estimated.

Based on the tolerable risk matrix for LOPA assessments shown on Figure 3.13 and based the anal-

ysed consequence’s severity, the maximum tolerable risk frequency (MTRF) was identified: e.g., for a

level 3 consequence there is a maximum tolerable risk frequency of 10�4. Finally, the probability of

failure on demand for the safety instrumented function was obtained dividing the latter value by the esti-

mated intermediate event frequency (vide equation 3.13) - the ultimate identification of the required SIL

(ranging from 1 to 4) corresponds to the logarithmic reduction of risk provided by the safety instrumented

function (e.g. a probability of failure on demand between 10�3 and 10�2 corresponds to SIL 2.

PFDSIF =IEL

MTRF(3.13)

Figure 3.13: Tolerable risk matrix for LOPA method. (Red) Non-tolerable risk; (Green) Tolerable risk.

The SIL classification displayed on Table 3.23 was obtained for the eight safety instrumented func-

tions identified during the HAZOP studies. It is relevant to highlight that during the detailed design of

these safety instrumentation systems, all the installed instruments must present a probability of failure

on demand (provided by the supplier) low enough as to not compromise SIL classification.

67

Table 3.23: Safety integrity levels required for the analysed safety instrumented func-tions.

Safety instrumentedfunction

Probability of failureon demand

Safety integritylevel

RTO bypass to carbonbed filters interlock,

in case of fresh air damper fully open1.8⇥ 10�1 a*


in case of hot-side bypass open,with low temperature in

the combustion chamber

1.1⇥ 10�3 2


in case of high temperaturein the process gas line

1.1⇥ 10�3 2

RTO bypass to carbonbed filters interlock,in case of high LEL

in the process gas line

1.1⇥ 10�2 1


in case of pressure dropin the scrubber

(recommendation from node 2)

1.1⇥ 10�1 a*


in case of low low levelin the HCl scrubber

1.1⇥ 10�3 2

RTO bypass to carbonbed filters interlock, in case

of pressure drop in the makeup waterline to the HCl scrubber

(recommendation from node 2)

1.0 a*

Emergency water supplyto the quench interlock,

in case of high temperaturein the quench

1.1⇥ 10�3 2

* No special integrity requirements.

68

Chapter 4

Preliminary results

This chapter aims at the analysis of the results of this project, up to the publishing date of this document,

with the ultimate aim of validating the defined design strategies, which were the main objective of this

study.

As already referred in chapter 3, with the approval of funding for this project coming only shortly

before the publishing of this document, only a few works had been physically implemented, namely the

installation of restriction orifices on the vent and vacuum lines associated with reactors R01, R02 and

R05.

With this atypical circumstance in mind, this chapter focuses both on the analysis of the inertization

time increase on the affected reactors, after orifice installation, and the impact on measured LEL and

dichloromethane emissions, on the associated scrubber (S11).

4.1 Operations lead time increase

Reactor inertization times were compared for equipment pieces where restriction orifices were installed,

since it was for this kind of type of operation that the vent header system from building E was modelled

(i.e., considering the waste gas made up by nitrogen and dichloromethane, the most relevant pollutant

at P).

As already mentioned, the installation of restriction orifices (50% diameter reduction) was only con-

cluded for three glasslined reactors and associated vent condensers: R01, R02 and R03, all of which

working with dichloromethane. Based on simulated data from Table 3.10, the inertization time increase

was estimated and compared to values of inertization lead times provided by the user area, prior and

post orifice installation, as shown on Table 4.1.

In the first place, it is important to highlight that the definition of inertization herein refers to the

application of vacuum, followed by nitrogen flushing the headspace of the reactor at positive pressure,

over two identical cycles.

Secondly, it should be noted that although the prediction for R05 was accurate, inertization lead

times for R01 and R02 (much larger reactors) exceeded considerably the predicted values, nonetheless

69

Table 4.1: Inertization time comparison before and after the installation of flow restriction orifices for 50%diameter reduction, for the affected equipment pieces.

Affectedequipment

Affected vent andvacuum lines

Initial leadtime (min)

Lead time afterinstallation (min)

Expected leadtime after

installation (min)R01

VC01Vent and vacuum linesexiting the condenser

20 25 20.5

R02VC02

Vent and vacuum linesexiting the condenser

20 32 20.5

R05VC05

Direct vent line;Vacuum line exiting the condenser

4 4* 4.1

* No considerable time increase was verified.

providing the useful qualitative information that the increase in lead time would not compromise overall

operation scheduling (even if failing by 5 or 10 minutes this was already useful information as it did not

compromise operations).

The most relevant approximations contributing to these discrepancies were identified as the following:

• Steady-state assumption: probably the most considerable approximation, since the affected

equipment pieces do not work continuously or at constant pressure (even during an inertization

cycle);

• Vacuum lines not considered: as already mentioned in section 3.3, the vacuum lines, through

which reactors vent whenever under negative (gauge) pressure, on which restriction orifices were

inserted have not been considered for the vent header system modelling, since they were directed

to a different building, associated to a different scrubber than the vent lines. Had these lines been

considered, further resistances would have increased the expected operation time.

Furthermore, it is interesting to note that R05 was the only reactor where a restriction orifice was

installed on its direct vent line to the scrubber, instead of on a condenser’s outlet. It may prove useful to

verify whether (throughout future orifice installation) the predicted time increases are more accurate for

orifices placed on direct vent lines exiting a reactor, when comparing to orifices installed on a vent con-

denser’s outlet, where flowrate may be considerably lower due to partial vent condensation, depending

on available utilities at the time.

On a final note, it is interesting to note that predicted time increase values (percentage) were almost

identical for the same diameter reduction percentages, as shown by the predictions from tables 3.10

and 3.11. This was explained by the fact that the estimation of time increase was done by dividing the

non-restricted vent flowrate by the restricted vent flowrate - according to equation 3.5, which translates

the basis for the modelling, it is shown that the only difference between the restricted and non-restricted

flows was the introduction of the resistance associated to the orifice plate.

Given the fact that the estimated pipeline length and diameter variation were approximately the same

for every reactor in this building when venting to S11 (which is approximately 20 meters away from

building E, connected by a DN250 pipeline), and that the average accident contribution (Kac) was, by

70

definition, the same for every node, operation time increase thus depended almost exclusively on the

diameter reduction percentage (through Kop) and on the restriction orifice’s internal diameter, to the

fourth power (d04). With orifice diameter ranging only from 12.50 to 25.00 millimetres (for 50% diameter

reduction) and from 6.25 to 12.50 millimetres (for 75% reduction) and Kop taking the value of either 1.8

or 2.5, it becomes evident that the diameter percentage reduction is the most determinant parameter

affecting the inertization time increase.

4.2 Waste gas monitoring

Having taken measures to reduce dichloromethane source emissions and to mitigate LEL percentage

on the waste gas from building E reaching the scrubber S11, efforts were directed to the quantification

of these impacts.

As such, the installation of a clamp-tight sampling port on the outlet of S11 was guided by the student,

with the aim of obtaining representative dichloromethane and VOC measurements. The installation was

concluded on June 2018 as there was a need to establish baseline measurements (i.e. dichloromethane

and LEL levels prior to restriction orifice installation).

Data was gathered by the two sensors, one VOC photoionization detector and one LEL flame tem-

perature analyser. Although it was possible to observe a reduction in VOC emissions and LEL peaks

after restriction orifice installation (thus validating its conceptual design and application), after a few

days, high VOC and LEL measurements were obtained - although this was attributed to abnormal pro-

cess operation, at the publishing date of this document no other valid measurements were gathered

by the environmental department, thus preventing full validation of the engineering works. Nonethe-

less dichloromethane load and LEL peaks are still expected to be reduced further once the new vent

condensers and restriction orifices are installed.

71

72

Chapter 5

Conclusions

This final chapter summarizes main deliverables fulfilled by the master’s student throughout his disser-

tation and internship period within the organic fine chemistry (API manufacturing) sector, approaching

implemented strategies towards VOC reduction and abatement, as well as listing future potential im-

provement opportunities.

5.1 Achievements

Throughout the herein described six-month period, the student successfully participated in the imple-

mentation of a dichloromethane emission reduction strategy, through the installation of flow restriction

orifices on the vent and vacuum lines of specific equipment pieces.

Based on obtained measurements for dichloromethane emissions and LEL percentage levels, an

apparently successful impact was observed for the installation of restriction orifices with 50% diameter

reduction - this accomplishment served as proof that a simple and conservative approach to LEL mitiga-

tion such as restriction orifices can nonetheless yield very positive (and cost-effective) results, especially

in installations deprived of a distributed control system, where the installation of pressure control valves

is not facilitated. This task additionally provided the student with the responsibility of coordinating the in-

stallation of sampling port on a gas scrubber, which allowed for the monitoring process and consequent

result validation.

The student additionally implemented a simplified model of the vent header system for the building

where where installation works took place, in order to predict maximum vent flowrates on the header

system - this not only allowed for an adequate estimation of operations lead time increase due to the

restriction orifice installation (with a maximum prediction error of 12 minutes), but also facilitated vent

condenser design (transfer area estimative), as no measures of vent flowrate were available inline or on

record. Aspen Plus software was used by the student for the simulation of thermodynamic data used

in this simulation, through the NRTL method, providing additional liquid-vapour equilibrium data for the

condensation of dichloromethane-rich vent streams.

An intensive gap-analysis of the site’s current vent collecting system and associated gas scrubbers,

73

based both on comprehensive field data and on process and instrumentation diagrams (P&ID), was

performed by the student with the aim of identifying structural and instrumentation requirements of the

current facilities for complying with current company standards and safety/environmental requirements.

An additional evaluation on the vent collecting system’s subcollector pipeline diameter was performed

under a safety-by-design perspective, to estimate new piping requirements for maintaining off-gas speed

below 8 m/s, taking into account both current and future vent gas flow, based on future expansion plans

on site, by the company.

Throughout this dissertation period, the student also participated in the selection process of a sup-

plier company for the purchase of a turnkey RTO system, serving as an end-of-line VOC abatement unit

with up to 95% energy efficiency. The purchased system presented an LEL control strategy with dilution

air injection, as well as the possibility of deviating the waste gas away from the unit and into an activated

carbon bed filter; this system was constituted by a combustion chamber (with the possibility to operate at

1100 oC and two seconds residence time) atop of three ceramic bed towers, for heat recovery between

the oxidized and VOC-rich streams and the ceramic media. During normal operation the system would

run in autothermal regime, but with the possibility of burning natural gas in case of deviation. Down-

stream of the RTO, the installation of a quench and scrubber, followed by a catalytic selective reduction

system (with urea injection) allowed control over dioxin, HCl and NOx emissions, whilst simultaneously

not generating any form of visible plume, due to the high temperature of the exhaust clean gas. It should

be noted that although this solution complies with national and European legislation, as well as the Best

Available Techniques [22, 23], its performance is still to be tested at this publication’s date, given the

expected installation date of this equipment is August 2019.

Finally, the student had the opportunity to participate in an HAZOP analyis to the RTO system,

integrated on a multidisciplinary team, leading to the risk rating of the process and the identification

of needed design alterations (namely the addition of blast-off panels on the RTO, alterations on the

scrubber liquid discharge line to the liquid waste collector and other modifications on the test gas circuit

on the LEL analysers). Attendance to a SIL assessment concluded the safety analysis of the RTO

system, with the identification of four SIL 2 and one SIL 1 safety instrumented functions, whilst providing

deeper understanding of safety and control functions associated to the RTO system.

5.2 Future work

Considering that the final handover of the described engineering project is only August 2019 (nearly one

year away from this publication date), several aspects of this work were left outstanding. Additionally,

and from a continuous improvement perspective, many of the aspects associated with this work may

deserve further investigation in the future.

Restriction orifice installation on all dichloromethane associated equipment still needs to be con-

cluded and its effects quantified through representative LEL and VOC monitoring. Should the overall

environmental impact be below the desired levels, the installation of orifices with 75% diameter reduc-

tion should be assessed, as long as not compromising inertization operations duration.

74

The implemented model for the vent header system should be validated by comparison with the iner-

tization times for the remaining reactors and, given the high dependency of the operation time increase

on the diameter reduction percentage, the possibility of simplifying the model’s calculations should be

assessed in the future (e.g. by exploring the variation of the lead time increase with reduction percent-

age variation, through a sensitivity analysis). Furthermore, any need of modulating vent header systems

would be discarded should flowmeters or pressure gauges be installed on the vent lines, thus highlight-

ing the importance of gathering and centralizing operational knowledge regarding parameters without

direct impact on the product.

Presently, the scheduled condenser installation on the designated centrifuges, as well as the con-

nection of identified non-ducted vent lines to the vent collecting system should be of the utmost priority

for the follow up of these works. With the conclusion of this stage, VOC reduction can be expanded to

cover hazardous solvents other than dichloromethane.

It is relevant to note that, given the low condensation capacity of vapour mixtures of dichloromethane

and nitrogen at -15 oC (vide Figure 3.6), alternatives should be sought, either considering condensation

at lower temperatures (e.g. by means of cryogenic coolant systems), or by foccusing research on the

application of less toxic solvents than dichloromethane, especially for processes still in development and

not yet validated. Moreover, during the near future, efforts should be directed towards the optimization

of company standards for vent condenser sizing procedures.

Concerning the upgrade and expansion of the site’s vent collecting system, it is necessary to start

producing project documentation based on gathered data (namely instrumentation lists and P&IDs)

corresponding to the desired solution, which requires future HAZOP analysis nonetheless.

As a final remark, the importance of managing (not just reducing) VOC emissions should never be

discarded from an operational point of view - in fact, the installation of a regenerative thermal oxidiser

would become pointless should all VOC load be condensed at its source and transformed into liquid

waste; such liquid waste would have to be addressed regardless and the VOC-free exhaust gas would

reach the RTO, leading to an unnecessary burning of natural gas to keep the combustion chamber at

1100 oC, thus creating an environmental and financial singularity.

75

76

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Appendix A

Process and instrumentation

diagrams

A.1 Process and instrumentation diagrams relevant components

This section contains all relevant sections of the P&IDs of the designed RTO system supplied by the

chosen vendor. It should be noted that all digital inputs (DI) and digital outputs (DO) are associated

exclusively to safety functions whilst control functions are based on analog inputs (AI) and analog outputs

(AO).

Due to confidentiality constraints, these diagrams could not be disclosed for this document’s public

version.

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