Chapter 2a - Rule of Thumb for Process Synthesis.

8/18/2019 Chapter 2a - Rule of Thumb for Process Synthesis.

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EP426Chemical Process Design and Optimization

Chapter 2 - Synthesis of process flow diagram


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Week Topic Activity Date

1 Introduction 06/01/2016

2 Chapter 1Sustainable process Design.

Aspect of process design, project objective, project classification.11/01/2016

Chapter 1Sustainable process Design.

Hierarchical approach to process design13/01/2016

3 Chapter 2Synthesis of process flow diagram (PFD).

Rule of thumb for process synthesis.18/01/2016

Chapter 2 Synthesis of process flow diagram (PFD). 20/01/2016Chapter 3 Separation train synthesis. (Class Replacement) 23/01/2016

4 Public Holiday : Thaipusam 25/01/2016

Individual Assesement (5%)

Written test based on the group assignment 27/01/2016

5 Public Holiday : Federal Territory 01/02/2016

Chapter 3

Separation train synthesis.

Selection and sequencing (part I). 03/02/2016

6 Public Holiday : Chinese New Year 08/02/2016

Chapter 3Separation train synthesis.

Selection and sequencing (part II).10/02/2016

7 Chapter 3Separation train synthesis.

Selection and sequencing (part III).15/02/2016

MidTerm (20%) 17/02/2016

Teaching plan


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Student attainment

CLO2: Propose a chemical process flow diagram usingthe hierarchy of process design.C5 – Identify component concept and use component skillto solve a problem.

A3 – Proposed a plan for improvement.

PLO3 -Design/Development of Solutions.Design solutions for flowsheet diagram, which the components orprocesses that meet specified needs with appropriate consideration forpublic health and safety, cultural, societal, and environmentalconsiderations

Note:

Teaching method - Lecture & Group Project

Assessment - Test, Final Exam and report presentation.


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Chapter 2

Synthesis of process flow

diagram (PFD)

Rule of thumb forprocess synthesis

Steps in Designing

ChemicalProcesses

Examples &Group Exercise


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EP426Chemical Process Design and Optimization

Chapter 2a – Synthesis of process flow diagram (PFD).

Rule of thumb for process synthesis.

(Process Design Heuristics)


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What You should Know?

• Understand the importance of selecting reaction paths

• Be able to distribute the chemicals in a process flowsheet

• Be able to apply heuristics.

• Be able to remove the exothermic heats of reaction.

• Understand the advantages of pumping a liquid rather

than compressing a vapor.


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Introduction

The heuristic rules is to expedite the selection andpositioning of processing operations as flow-sheetsare assembled.

These rules are based on experience and hold ingeneral.

Note: However, it should be tested to ensure thatthey apply in the specific application.


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Raw Materials and hemical Reactions

Avoid, or reduce, the handling and storage ofhazardous and toxic chemicals.Heuristic 1:

Example: Manufacture of Ethylene Glycol (EG).

C2H4 + O2 CH2 - CH2-2

O

(R.1)

CH2 - CH2 + H2O CH2 - CH2

O OH OH

(R.2)

Since both reactions are highly exothermic, they need to be

controlled carefully. But a water spill into an ethylene-oxide storagetank could lead to an accident similar to the Bhopal incident.

Often such processes are designed with two reaction steps, withstorage of the intermediate, to enable continuous production.


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Distribution of Chemicals

Use an excess of one chemical reactant in a reactionoperation to completely consume chemical reactant.

Example: Consider using excess ethylene in DCE production

Heuristic 2:


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Introduce liquid or vapor purge streams to provide

exits for species.

The species such as

– impurities in the feed

– produced by irreversible side-reactions

Note: when these species are in trace quantities and/or are

difficult to separate from the other chemicals.

Heuristic 4:

Distribution of Chemicals (Cont’d)

Example: NH3 Synthesis Loop.

Purge flow rate selection depends on economics!

What are the

configuration?


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Do not purge valuable species or species thatare toxic and hazardous, even in smallconcentrations.

Heuristic 5:

Example: Catalytic converter in car exhaust system.

• Add separators to recover valuable species.

• Add reactors to eliminate toxic and hazardous species.


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Recycled to extinction.By-products that are produced in reversible reactions, in

small quantities, are usually not recovered in separators or

purged but it being recycled back to the process until its

fully converted.

Heuristic 6:


However, for irreversible reactions,

small quantities of by-products must be

purged, otherwise they will buildup.


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For competing series or parallel reactions,adjust the temperature, pressure, and catalystto obtain high yields of the desired products.In the initial distribution of chemicals, assume thatthese conditions can be satisfied

- obtain kinetics data and check this assumptionbefore developing a base-case design.

Heuristic 7:

Example: Manufacture of allyl-chloride.


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Allyl Chloride Manufacture (Cont’d)Example: Manufacture of allyl-chloride.

Reaction HR

Btu/lbmole

ko

lbmole/ hr ft3

atm2

E/R (oR)

1 -4,800 206,000 13,600

2 -79,200 11.7 3,430

3 -91,800 4.6 x 108 21,300

Kinetic data


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Allyl Chloride Manufacture (Cont’d)

-1.6

-1.2

-0.8

-0.4 9 . 6

0 E - 0

4

9 . 7

0 E - 0

4

9 . 8

0 E - 0

4

9 . 9

0 E - 0

4

1 . 0

0 E - 0 3

1 . 0

1 E - 0 3

1 . 0

2 E - 0 3

l n ( k )

1/T (980


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MeOAc Manufacture using Reactive

Distillation

Reactionzone

MeOAc

HOAc

MeOH

H2O

MeOH + HOAc MeOAc + H2O


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Ref: Douglas (1988)

Separate liquid mixtures using distillation andstripping towers, and liquid-liquid extractors, among

similar operations.

Heuristic 9:

Separations

Select from distillation,

enhanced distillation,

stripping towers, liquid-

liquid extraction, etc.


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Ref: Douglas (1988)

Separations (Cont’d)

Attempt to condense vapor mixtures withcooling water. Then, use Heuristic 9.Heuristic 10:

Select from partial

condensation, cryogenic

distillation, absorption,

adsorption, membrane

separation, etc.

Select from distillation,

enhanced distillation,

stripping towers, liquid-

liquid extraction, etc.

Attempt to cool

reactor products

using cooling water


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Ref: Douglas (1988)

Separations (Cont’d)

Separate vapor mixtures using partialcondensers, cryogenic distillation, absorption

towers, adsorbers, and/or membrane devices.

Heuristic 11:


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Heat Transfer in Reactors

Although heat transfer in reactors is betterdiscussed in the context of heat and powerintegration (Chapter 5),

Heat transfer in reactors affect thedistribution of chemicals.


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Heat Transfer in Reactors (Cont’d)

Treat exothermic reactors first.To remove a highly-exothermic heat of reaction, consider

the use of…

Heuristic 12:

excess reactant

cold shots.

an inert diluent


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Heat Transfer in Reactors (Cont’d)TVA design for NH

3

synthesis convertersExample:

• Feed gases pass up the

reactor through tubes.

•The reaction take placeat the catalyst chamber.


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To control temperature for a highly-endothermic heat

of reaction, consider the use of excess reactant an

inert diluent, and hot shots. These affect the

distribution of chemicals and should be inserted earlyin process synthesis.

Heuristic 14:

Heat Transfer in Reactors (Cont’d)

For less endothermic heats of reaction,

circulate reactor fluid to an external heater,or use a jacketed vessel or heating coils. Also,consider the use of interheaters.

Heuristic 15:

Endothermic reactors are treated similarly:


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To increase the pressure of a stream, pump a liquidrather than compress a gas;

that is, condense a vapor, as long as refrigeration (and

compression) is not needed, before pumping.

Heuristic 16:

Pumping and Compression

2

1

P

PdPVW

Since work done by pumping or compressions is given by:

It follows that it is moreefficient to pump a liquid thanto compress a gas. Thus, it is

almost always preferable tocondense a vapor, pump it, andvaporize it, rather thancompress it.

Exception: if condensation requires

refrigeration.


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What You should?

• Understand the importance of selecting reaction paths that do notinvolve toxic or hazardous chemicals, and when unavoidable, toreduce their presence by shortening residence times in the processunits and avoiding their storage in large quantities.

• Be able to distribute the chemicals in a process flowsheet, to account

for the presence of inert species, to purge species that wouldotherwise build up to unacceptable concentrations, to achieve a highselectivity to the desired products.

• Be able to apply heuristics in selecting separation processes toseparate liquids, vapors, and vapor-liquid mixtures.

• Be able to distribute the chemicals, by using excess reactants, inertdiluents, and cold shots, to remove the exothermic heats of reaction.

• Understand the advantages of pumping a liquid rather thancompressing a vapor.


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To be continued…Chapter 2b - Synthesis of process flow diagram (PFD).Steps in Designing Chemical Processes

Chapter 2a - Rule of Thumb for Process Synthesis.

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Transcript of Chapter 2a - Rule of Thumb for Process Synthesis.