THE NATURE OF PROCESS DESIGNA Creative Activity

The most effective way of communicating information about a process is through the use of flow diagrams.Block Flow Diagram (BFD)Process Flow Diagram (PFD)Piping and Instrumentation Diagram (P&ID)

Mixed Gas(2,610 kg/h)Benzene(8,210 kg/h)GasSeparatorMixed LiquidsReactorToluene(10,000 kg/h)Hydrogen(820 kg/h)Conversion75% TolueneTolueneReaction : C7H8 + H2 = C6H6 + CH4Figure 1.1 Block flow process diagram for the production of benzeneToluene and hydrogen are converted in a reactor to produce benzene and methane.Thereaction does not go to completion, and excess toluene is required. The noncondensablegases are separated and discharged. The benzene product and the unreacted toluene arethen separated by distillation. The toluene is then recycled back to the reactor and thebenzene removed in the product stream.

Table 1.1 Conventions and Format Recommended for Laying out a Block Flow Process Diagram1. Operations shown by blocks.2. Major flow lines shown with arrows giving direction of flow.3. Flow goes from left to right whenever possible.4. Light stream (gases) toward top with heavy stream (liquids and solids) toward bottom.5. Critical information unique to process supplied.6. If lines cross, then the horizontal line is continuous and the vertical line is broken.7. Simplified material balance provided.

Process Flow Diagram (PFD)

A PFD includes the following items:

1. major equipments;2. principal flow route and control involved from raw material feed to final product;3. key temperature and pressure corresponding to anticipated normal operation;4. material flow rates and compositions;5. design duties and sizes of major equipments.

Table 1.2 Conventions Used for Identifying Process Equipment Process Equipment General Format XX-YZZ A/B

XX are the identification letters for the equipment classification C - Compressor or Turbine E - Heat Exchanger H - Fired Heater P - Pump R - Reactor T - Tower TK - Storage Tank V - Vessel Y designates an area within the plant ZZ are the number designation for each item in an equipment class A/B identifies parallel units or backup units not shown on a PFD Supplemental Information Additional description of equipment given on top of PFD

Table 1.3 Conventions for Identifying Process and Utility Streams Process StreamsAll conventions shown in Table 1.1 apply.Diamond (square) symbol located in flow lines.Numerical identification (unique for that stream) inserted in diamond (square).Flow direction shown by arrows on flow lines. Utility Streamslps Low Pressure Steam: 3-5 barg (sat)mps Medium Pressure Steam: 10-15 barg (sat)hps High Pressure Steam: 40-50 barg (sat)htm Heat Transfer Media (Organic): to 400Ccw Cooling Water: From cooling tower 30C returned at less than 45C+wr River Water: From river 25C returned at less than 35Crw Refrigerated Water: In at 5C returned at less than 15Crb Refrigerated Brine: In at -45C returned at less than 0Ccs Chemical Waste Water with high CODss Sanitary Waste Water with high BOD, etc.el Electric Heat (specify 220, 440, 660V service)ng Natural Gasfg Fuel Gasfo Fuel Oilfw Fuel WaterThese pressure are set during the preliminary design stages and typical values vary within the ranges shown. +Above 45C, significant scaling occurs.

Table 1.4 Information Provided in a Flow Summary Essential InformationStream NumberTemperature (C)Pressure (bar)Vapor FractionTotal Mass Flow Rate (kg/h)Total Mole Flow Rate (kmol/h)Individual Component Flow Rates (kmol/s) Optional InformationComponent Mole FractionsComponent Mass FractionsIndividual Component Flow Rates (kg/h)Volumetric Flow Rates (m3/h)Significant Physical Properties Density Viscosity OtherThermodynamic Data Heat Capacity Stream Enthalpy K-valuesStream Name

Table 1.6 Equipment Descriptions for PFD and P&IDsEquipment TypeDescription of EquipmentTowersSize (height and diameter), Pressure, TemperatureNumber and Type of TraysHeight and Type of PackingMaterials of ConstructionsHeat ExchangersType: Gas-Gas, Gas-Liquid, Liquid-Liquid, Condenser, VaporizerProcess: Duty, Area, Temperature, and Pressure for both streams.No. of shell and Tube PassesMaterials of Construction: Tubes and ShellTanksSee vesselsVesselsHight, Diameter, Orientation, Pressure, Temperature, Materials of ConstructionPumpsFlow, Discharge Pressure, Temperature, P, Driver Type, Shaft Power, Materials of Construction CompressorsActual Inlet Flow Rate, Temperature, Pressure, DrverType, Shaft Power,Materials of ConstructionHeaters (fired)Type, Tube Pressure, Tube Temperature, Duty, Fuel, Material of ConstructionOthersProvide Critical Information

Piping and Instrumentation Diagram (P&ID)

1. All process equipments and pipings required for start-up, shut-down, emergency and normal operation of the plant, including valves, blinds, etc.2. An id number, an identifier of the material of construction, diameter and insulation requirements for each line.3. Direction of flow. 4. Identification of main process and start-up lines.5. All instrumentation, control and interlock facilities with indication of action on instrument air failure.6. Key dimensions or duties of all equipments.7. Operating and design pressures and temperatures for vessels and reactors.8. Equipment elevations.9. Set pressure for relief valves.10.Drainage requirements.11.Special notes on piping configuration as necessary, e.g. gravity drainage.

Table 1.8 Exclusions from Piping and Instrumentation Diagram1. Operating conditions T,P2. Stream flows3. Equipment locations4. Pipe routing a. Pipe lengths b. Pipe fittings5. Supports, structures, and foundations

Table 1.9 Conventions in Constructing Piping and Instrumentation DiagramsFor Equipment - Shown Every Piece IncludingSpare unitsParallel unitsSummary details of each unitFor Piping - Include All Lines Including Drains, Sample Connections and SpecifySize (use standard sizes)Schedule (thickness)Materials of constructionInsulation (thickness and type)For Instruments - IdentifyIndicatorsRecordersControllersShow instrument linesFor Utility - IndentifyEntrance utilitiesExit utilitiesExit to waste treatment facilities

Activities of Process Design(1)Synthesis The step where one conjectures the building blocks and their interconnections to create a structure which can meet the stated design requirements.(2)Analysis (Simulation) The activity of modeling and then solving the resulting equations to predict how a selected structure should behave if it were constructed.(3)Evaluation The activity of placing a worth on the structure where the worth might be its cost, its safety, or its net energy consumption.(4)Optimization The systematic searching over the allowed operating conditions to improve the evaluation as much as possible.Parameterstructure

Process Synthesis A design task where one conjectures the building blocks and their interconnections to create a structure which can meet the stated design requirements.

IMPORTANCE OF PROCESS STRUCTURE

(1) Recycle AP

o r

(2)separation Sequence ?

A (propane) B (1-Butene) C(n-Butane)

ABC

A

B

C

BC

ABC

o r

C

AB

BC

A

(3)Heat Recovery ?

H

o r

Figure 1.1 Synthesis is the creation of a process to transform feed streams into product streams. Simulation predicts how it would behave if it was constructed.

PROCESS ?

(a) Process design starts with the synthesis of a process to convert raw materials into desired products.

Feed Streams

Product Streams

PROCESS

(b) Simulation predicts how a process would behave if it was constructed.

Feed Streams

Product Streams ?

Reactor Separation and Recycle SystemHeat Exchanger NetworkUtilities Figure 1.6 The onion model of process design. A reactor design in needed before the separation and recycle system can be designed, and so on. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66:195, 1988; reproduced by permission of the Institution of Chemical Engineers.)

Example Hydrodealkylation of Toluene

A HIERARCHICAL APPROACHToluene + H2 Benzene + CH42 Benzene Diphenyl + H21150 F 1300 F500 psia

ENERGY INTEGRATIONToluene feed

compressor

Flashhh

Reactor

furnace

Purge

Liquid recycle

Gas recycle

H2, feed

CW

Benzene product

H2, CH4

Diphenyl

FIGURE 1.2-2

Hydrodealkylation of toluene; maximum energy recovery.

Reactor

compressor

Vapor Recovery System

Purge

Flash Drum

Benzene

Toluene Col.

Benzene Col.

Stablizer

H2 Feed

Toluene

Feed

Toluene

Recycle

Diphenyl

Distillation Train

ALTERNATIVES OF DISTILLATION TRAIN(1) Recycle Diphenyl(2)(3)

H2, CH4

Feed

Benzene

Toluene (recycle)

Diphenyl

H2

CH4 Benzene Toluene

(recycle)

Diphenyl

ALTERNATIVES OF VAPOR RECOVERY SYSTEM(1) Condensation;(2) Absorption; (3) Adsorption;(4) Membrane.

Vapor recoverysystemPhasesplitReactorsystemLiquid separationsystemPurgeH2 , CH4BenzeneDipheny1H2 , CH4TolueneSimplified Flowsheet for the Separation System

ReactorsystemSeparationsystemGas recyclePurgeH2 , CH4BenzeneDipheny1H2 , CH4TolueneToluene recycleRecycle Structure of the Flowsheet

PurgeH2 , CH4BenzeneDipheny1H2 , CH4TolueneInput-Output Structure of the Flowsheet

Hierarchy of decisions

1. Batch versus continuous

2. Input-output structure of the flowsheet

3. Recycle structure of the flowsheet

4. General structure of the separation system Ch.5

a. Vapor recovery system

b. Liquid recovery system

5. Heat-exchanger network Ch.6, Ch.7, Ch.16

Ch. 4