Lecture 3(b). 308572 ChE 312 PSD1-Part I, Heuristics for process synthesis.pdf

8/2/2019 Lecture 3(b). 308572 ChE 312 PSD1-Part I, Heuristics for process synthesis.pdf

1/14

1

1

ChE 312 Process Synthesis and Design I

LECTURE 3: HEURISTICS FOR PROCESSSYNTHESIS

Ref.: Daniel R. Lewin, 2004, Lecturenote, Technion, Haifa, Israel

Heuristics2

Introduction

Chemical reaction (to eliminate differences in molecular type)

Mixing and recycle (to distribute the chemicals)

Separation (to eliminate differences in composition)

Temperature, pressure and phase change

Task integration (to combine tasks into unit operations)

Recalling the process operations in process synthesis:

This lecture deals with the heuristic rules that expeditethe selection and positioning of processing operations asflowsheets are assembled.

These rules are based on experience and hold in general,but should be tested (e.g., by simulation) to ensure thatthey apply in the specific application.

Later, in Section B, we will see how algorithmic methodsare used to improve on design decisions.


2/14

2

Heuristics3

Instructional Objectives

Understand the importance of selecting reaction paths that donot involve toxic or hazardous chemicals, and when unavoidable, toreduce their presence by shortening residence times in theprocess units and avoiding their storage in large quantities.

Be able to distribute the chemicals in a process flowsheet, toaccount for the presence of inert species, to purge species thatwould otherwise build up to unacceptable concentrations, toachieve a high selectivity 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,inert diluents, and cold shots, to remove the exothermic heats ofreaction.

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

When you have finished studying this unit, you should:

Heuristics4

Select raw materials and chemical reactions toavoid, or reduce, the handling and storage ofhazardous and toxic chemicals.

Heuristic 1:

Raw Materials and Chemical Reactions

Example: Manufacture of Ethylene Glycol (EG).

C2H4 + O2 CH2 - CH21-2

O

(R.1)

CH2 - CH2 + H2O CH2 - CH2

O OH OH

(R.2)

Since both reactions are highly exothermic, they need to becontrolled carefully. But a water spill into an ethylene-oxide storagetank could lead to an accident similar to the Bhopal incident. Oftensuch processes are designed with two reaction steps, with storage ofthe intermediate, to enable continuous production, even whenmaintenance problems shut down the first reaction operation.


3/14

3

Heuristics5

Alternatives to the two-step EG process

(R.3)

OH OH

CH2=CH2 + Cl2 + 2NaOH(aq) CH2CH2 + 2NaCl

Use chlorine and caustic in a single reaction step, to avoidthe intermediate:

As ethylene-oxide is formed, react it with carbon dioxideto form ethylene-carbonate, a much less activeintermediate that can be stored safely and hydrolyzed,

to form the ethylene-glycol product, as needed:

(R.4)CH2 - CH2 + CO2

O O

O C

CH2 CH2

O

Heuristics6

Distribution of Chemicals

Example: Consider using excess ethylene in DCE production

Use an excess of one chemical reactant in areaction operation to completely consume asecond valuable, toxic, or hazardous chemical

reactant.

Heuristic 2:


4/14

4

Heuristics7

Example:

Distribution of Chemicals (Contd)

When nearly pure products are required,eliminate inert species before the reactionoperations, when the separations are easilyaccomplished, or when the catalyst isadversely affected by the inert

Do not do this when a large exothermic

heat of reaction must be removed.

Heuristic 3:

Heuristics8


Need to decide whetherto remove inerts beforereaction...

Clearly, the ease and cost of the separations must be assessed.This can be accomplished by examining the physical properties uponwhich the separations are based, and implies the use of simulation

or after reaction...


5/14

5

Heuristics9

Introduce liquid or vapor purge streams toprovide exits for species that enter the process as impurities in the feed

produced by irreversible side-reactions

when these species are in trace quantitiesand/or are difficult to separate from theother chemicals.

Heuristic 4:


Example: NH3 Synthesis Loop.

Note: Purge flow rate selection depends on economics!

Heuristics10


Do not purge valuable species or species thatare toxic and hazardous, even in smallconcentrations. Add separators to recover valuable species.

Add reactors to eliminate toxic and hazardous

species.

Heuristic 5:

Example: Catalytic converter in car exhaust system.


6/14

6

Heuristics11

By-products that are produced in reversiblereactions, in small quantities, are usually notrecovered in separators or purged. Instead,

they are usually recycled to extinction.

Heuristic 6:


Often small quantities of chemicals are produced in side-reactions. When the reaction proceeds irreversibly, smallquantities of by-products must be purged, otherwise theywill buildup in the process continuously until the processmust be shut down. When, however, the reaction proceedsreversibly, it becomes possible to achieve an equilibriumconversion at steady state by recycling product specieswithout removing them from the process. In so doing, it is

often said that undesired byproducts are recycled toextinction.

Heuristics12


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, assumethat these conditions can be satisfied - obtainkinetics data and check this assumption beforedeveloping a base-case design.

Heuristic 7:

Example: Manufacture of allyl-chloride.


7/14

7

Heuristics13

Allyl Chloride Manufacture (Contd)Example: Manufacture of allyl-chloride.

Reaction HR

Btu/lbmole

ko

lbmole/(hr ft3atm2)E/R (oR)

1 -4,800 206,000 13,600

2 -79,200 11.7 3,4303 -91,800 4.6 x 108 21,300

Kinetic data

Heuristics14

Allyl Chloride Manufacture (Contd)

-1.6

-1.2

-0.8

-0.49.6

0E-04

9.7

0E-04

9.8

0E-04

9.9

0E-04

1.0

0E-03

1.0

1E-03

1.0

2E-03

1/T (980


8/14


9/14

9

Heuristics17

Ref: Douglas (1988)

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

among similar operations.

Heuristic 9:

Separations

Select fromdistillation, enhanceddistillation, strippingtowers, liquid-liquidextraction, etc.

Heuristics18

Ref: Douglas (1988)

Separations (Contd)

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

Heuristic 10:

Select from partialcondensation,cryogenic distillation,absorption, adsorption,membrane separation,

etc.

Select fromdistillation, enhanceddistillation, strippingtowers, liquid-liquidextraction, etc.

Attempt to coolreactor productsusing cooling water


10/14

10

Heuristics19

Ref: Douglas (1988)

Separations (Contd)

Separate vapor mixtures using partialcondensers, cryogenic distillation, absorption

towers, adsorbers, and/or membrane devices.

Heuristic 11:

Combination of theprevious two flowsheets

Heuristics20

To remove a highly-exothermic heat ofreaction, consider the use of excess reactant,an inert diluent, and cold shots. These affectthe distribution of chemicals and should be

inserted early in process synthesis.

Heuristic 12:

Heat Transfer in Reactors

For less exothermic heats of reaction,circulate reactor fluid to an external cooler,or use a jacketed vessel or cooling coils. Also,consider the use of intercoolers.

Heuristic 13:

Although heat transfer in reactors is better discussed in thecontext of heat and power integration (see Lectures 6- 8), itis treated here because many methods dealing with heattransfer in reactors also affect the distribution of chemicals.Treated first are exothermic reactors.


11/14

11

Heuristics21

Heat Transfer in Reactors (Contd)

To remove a highly-exothermic heat ofreaction, consider the use of

Heuristic 12:

excess reactant

cold shots.

an inert diluent

Heuristics22

Heat Transfer in Reactors (Contd)For less exothermic heats of reaction,circulate reactor fluid to an external cooler,or use a jacketed vessel or cooling coils. Also,consider the use of intercoolers.

Heuristic 13:


12/14

12

Heuristics23

Heat Transfer in Reactors (Contd)TVA design for NH3 synthesis convertersExample:

Heuristics24

To control temperature for a highly-endothermic heat of reaction, consider the useof excess reactant an inert diluent, and hotshots. These affect the distribution ofchemicals and should be inserted early inprocess synthesis.

Heuristic 14:

Heat Transfer in Reactors (Contd)

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:


13/14


14/14

Reference

Seider, Seader and Lewin (1999), Chapter4

Heuristics27

Lecture 3(b). 308572 ChE 312 PSD1-Part I, Heuristics for process synthesis.pdf

Documents

Transcript of Lecture 3(b). 308572 ChE 312 PSD1-Part I, Heuristics for process synthesis.pdf