Chapter 6

Pollution Prevention for Unit Operations – Part 2

Pollution Prevention for Chemical Reactors

• From an environmental perspective, reactors are the most important unit operation in a chemical process.

• The degree of conversion of feed to desired products influences all subsequent separation processes, recycle structure for reactors, waste treatment options, energy consumption, and ultimately pollution releases to the environment.

• Once a chemical reaction pathway has been chosen, the inherent product and byproduct (waste) distribution for the process are to a large extent established.

REACTOR PERFORMANCE

Conversion (x)

= (reactant consumed in the reactor)/(reactant fed to the reactor)

Selectivity (S)

=[(desired product produced)/(reactant consumed in the reactor)]*SF

Reactor Yield (Y)

=[(desired product produced)/(reactant fed to the reactor)]*SF

STOICHIOMETRIC FACTOR (SF)

The stoichiometric moles of reactant required per mole of product

Reactorsystem

Separationsystem

Gas recycle PurgeH2 , CH4

Benzene

Dipheny1

H2 , CH4

Toluene

Toluene recycle

Material Balance of the Limiting Reactant (Toluene)

x

x1

TolueneBenzene

Diphenyl

x1Sx

xS)1(2

1

Sx

xS)1(2

1

Assumption: completely recover and recycle the limiting reactant.

Design Considerations

• The raw materials, products, and byproducts should have a relatively low environmental and health impact potential.

• The yield and selectivity should both be high.

• Energy consumption should be low.• The life-cycle impacts reactants, products

and byproducts should be relatively low.

Waste Reduction Methods in Reactor Design

• Changing process chemistry (precursors and/or catalysts);

• Avoiding storage of hazardous materials (in situ, on-demand generation) ;

• Maximizing selectivity;

• Prolonging catalyst life;

• Combining reactor and separator.

Material Use and Selection• Raw materials and feedstocks

– New process chemistry– Purer raw material

• Solvents– Substitute solvent

• Catalysts– can allow the use of more environmentally benign che

micals as raw materials,– can increase selectivity toward the desired product and

away from the unwanted by product (waste),– can convert waste chemicals to raw materials,– can create environmentally acceptable products directly

from the reactions.

Parallel Reaction Networks

• Parallel reactions

• Rate expressionsWR

PRW

P

k

k

0 0exp exp WP nn WPP P R W RW

EEr k C r k C

RT RT

1p wn n

The reaction selectivity is constant and independent of residence time for 1st-order, irreversible, isothermal parallel reactions.

Series Reaction Networks

• Series reactions

• Rate expressions

WPR WP kk

0 0exp exp WP nn WPP P R W PW

EEr k C r k C

RT RT

To minimize waste generation in series reactions, it is important to operate the reactor so that the ratio is as large as possible and to control the reaction residencetime.

Reversible Reactions

Impact of Temperature on Selectivity

111

00 0

/0

/0

( ) ///

( ) // /

if 1p

w

p wWP

p wP W

E RT

p pp wE RT

w w

E E RTE RTE RTp

E E RTE RT E RTw

r k en n

r k e

r e e e

r ee e

Impact of Concentration on Selectivity

• The selectivity ratio for parallel reactions

If then selectivity is improved by increasing the conc. of R; If otherwise, then the conc. of R should be decreased.

• The analysis of series reactions is more complex.

P

P W

W

np p R p n n

Rnw w R w

r k C kC

r k C k

WP nn

Impact of Mixing on Selectivity

• Improve physical mixing in the reactor, which will improve selectivity if the reaction order is greater than 1.

• Distribute feeds better to avoid short-circuiting.• Premixing of reactants may result in better

selectivity.• Provide a separate reactor for recycle streams.• Examine heating and cooling techniques to avoid

cool spots and hot spots. Eliminate direct steam injection.

Wastes Generated by Separation Devices

Separation unit operations generate waste because

• the separation steps themselves are not 100% efficient, and

• require– additional energy input or– waste treatmentto deal with off-spec products.

Choice of Mass Separating Agent

• A poor choice may result in exposure to toxic substances fro not only facility workers but also consumers who use the end product.

• A poor choice may lead to excessive energy consumption and the associated health impacts of the emitted criteria air pollutants.

Pollution Prevention Approaches for Separation

Equipments

1. Minimize the wastes and emissions that are routinely generated;

2. Control excursions in operating conditions;

3. Improve the design efficiency.

Process Wastes Generated from Distillation

• By allowing impurities to remain in a product,

• By forming waste within the column itself (in reboiler),

• By inadequate condensing of overhead product (through the condenser vent), and

• By excessive energy use.

Pollution Prevention Methods for Distillation Columns

1. Increase the reflux ratio, add a section to the column, retray/repack the column, or improve feed distribution to increase column efficiency.

2. Changing the feed location may increase product purity.

3. Insulate or preheat feed to reduce the load on the reboiler.

4. Reduce the pressure drop in column, which reduce the load on reboiler.

5. Vacuum distillation may reduce reboiler requirements.

Separative Reactors

The key feature allowing for the prevention of waste generation and maximizing product yield is the ability to control the addition of reactant and the removal of product more precisely than in traditional designs.

Separation units that have been integrated with reaction include distillation, membrane separation, and adsorption.

Combined Reactor/Separator – Catalytic Distillation

• The conventional MTBE (methyl tert-butyl ether) producing process (from methanol and isobutylene) is given in Figure 6-9a.

• The alternative process is to feed the raw materials to a distillation column in which some of the packing material has been replaced by catalyst.

• Fugitive and process emissions are reduced.

• Fewer heat exchangers are required.

• Water is not needed to separate the components.

• Reaction equilibrium can be shifted since MTBE is less volatile than the reactants. In other words, it moves down the distillation column and away from the reaction zone as it is formed.

Combined Reactor/Separator – Membrane Technology

• Applicable when the product molecules are smaller than the reactant molecules.

• Both types of membrane in Figure 6-10 hold particular promise for reversible reactions because the product is removed as it is formed.

Applications of Membrane Separative Reactors

• Thermodynamically-limited reactions, e.g., C6H12↔C6H6+3H2

• Parallel reactions in which product formation has a lower reaction order than byproduct generation

• Series reactions such as selective dehydrogenations and partial oxidations

• Series-parallel reactions

Sources of Waste from Heat Exchangers

• Heat exchangers can be a direct source of waste when high temperatures cause the fluids they contain to form sludges.

• Because it reduces efficiency and increase energy requirements, sludge buildup in heat exchangers is an indirect source of combustion-related emissions.

Sludge Reduction Methods

1. Reduce the temperature used in the heat exchanger: (a) thermocompressor (Figure 6-12); (b) staged heating (Figure 6-13).

2. Plate-and-frame exchangers3. Scraped-wall exchangers4. Noncorroding tubes5. Antifoulants 6. On-line cleaning techniques

Fugitive Air Emissions

These releases include equipment leaks from valves, pump seals, piping connectors, pressure relief valves, flanges, compressor seals, sampling connections, open-ended lines, and air releases from building ventilation system, etc.

They are not easily identifiable and relatively large in number.

Methods to Reduce Fugitive Emissions

• Leak detection and repair (LDAR) of leaking equipment

• Equipment modification or replacement with emission-less technologies.

Leak Detection and Repair

In a LDAR program, equipment such as pumps and valves are monitored periodically using an organic vapor analyzer (OVA).

If the source registers an OVA reading over a threshold value (>10000ppm), the equipment is said to be leaking and repair is required.