lecture 05 heuristics for process synthesisdlewin/focapd_2004/lecture_05_h… · lecture 05 product...

PRODUCT and PROCESS DESIGNLECTURE 05

Daniel R. Lewin, Technion1

Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin1

WORKSHOP on PRODUCT AND PROCESS DESIGN

LECTURE 05: HEURISTICS FOR PROCESS SYNTHESIS

Daniel R. LewinDepartment of Chemical Engineering

Technion, Haifa, Israel Ref: Seider, Seader and Lewin (2004), Chapter 5


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 changeTask integration (to combine tasks into unit operations)

Recalling the process operations in process synthesis:

This lecture deals with the heuristic rules that expedite the selection and positioning of processing operations as flowsheets are assembled.These rules are based on experience and hold in general, but should be tested (e.g., by simulation) to ensure that they apply in the specific application.Later, in Part 2, we will see how algorithmic methods are used to improve on design decisions.




Instructional Objectives

Understand the importance of selecting reaction paths that do not involve toxic or hazardous chemicals, and when unavoidable, to reduce their presence by shortening residence times in the process units 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 thatwould otherwise build up to unacceptable concentrations, to achieve 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 of reaction.Understand the advantages of pumping a liquid rather than compressing a vapor.

When you have finished studying this unit, you should:


Select raw materials and chemical reactions to avoid, or reduce, the handling and storage of hazardous 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 be controlled carefully. But a water spill into an ethylene-oxide storage tank could lead to an accident similar to the Bhopal incident. Often such processes are designed with two reaction steps, with storage of the intermediate, to enable continuous production, even when maintenance problems shut down the first reaction operation.




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 avoid the intermediate:

As ethylene-oxide is formed, react it with carbon dioxide to form ethylene-carbonate, a much less active intermediate 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


Distribution of Chemicals

Example: Consider using excess ethylene in DCE production

Use an excess of one chemical reactant in a reaction operation to completely consume a second valuable, toxic, or hazardous chemical reactant.

Heuristic 2:




Example:

Distribution of Chemicals (Cont’d)

When nearly pure products are required, eliminate inert species before the reaction operations, when the separations are easily accomplished, or when the catalyst is adversely affected by the inertDo not do this when a large exothermic heat of reaction must be removed.

Heuristic 3:


Distribution of Chemicals (Cont’d)Need to decide whether to remove inerts before reaction...

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

… or after reaction...




Introduce liquid or vapor purge streams to provide exits for species that– enter the process as impurities in the feed– produced by irreversible side-reactions

when these species are in trace quantities and/or are difficult to separate from the other chemicals.

Heuristic 4:


Example: NH3 Synthesis Loop.

Note: Purge flow rate selection depends on economics!



Do not purge valuable species or species that are toxic and hazardous, even in small concentrations. – Add separators to recover valuable species.– Add reactors to eliminate toxic and hazardous

species.

Heuristic 5:

Example: Catalytic converter in car exhaust system.





For competing series or parallel reactions, adjust the temperature, pressure, and catalyst to obtain high yields of the desired products. In the initial distribution of chemicals, assume that these conditions can be satisfied - obtain kinetics data and check this assumption before developing a base-case design.

Heuristic 7:

Example: Manufacture of allyl-chloride.


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

Reaction ∆HR

Btu/lbmoleko

lbmole/(hr ft3atm2) 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




Allyl Chloride Manufacture (Cont’d)

-1.6

-1.2

-0.8

-0.4

9.60

E-04

9.70

E-04

9.80

E-04

9.90

E-04

1.00E

-03

1.01E

-03

1.02E

-03

1/T (980<T<1042 deg R)

ln(k

)

ln(k1)ln(k2)ln(k3)

What range of operating temperatures favor production of Allyl Chloride ?


For reversible reactions, especially, consider conducting them in a separation device capable of removing the products, and hence, driving the reactions to the right. Such reaction-separation operations lead to very different distributions of chemicals.

Heuristic 8:


Example: Manufacture of Ethyl-acetate using reactive distillation. Conventionally, this would call for reaction:

followed by separation of products using a sequence of separation towers.

MeOH + HOAc MeOAc + H2O,←→




MeOAc Manufacture using Reactive Distillation

Reactionzone

MeOAc

HOAc

MeOH

H2O

MeOH + HOAc MeOAc + H2O←→


Ref: Douglas (1988)

Separate liquid mixtures using distillation and stripping towers, and liquid-liquid extractors, among similar operations.

Heuristic 9:

Separations

Select from distillation, enhanced distillation, stripping towers, liquid-liquid extraction, etc.




Ref: Douglas (1988)

Separations (Cont’d)

Attempt to condense vapor mixtures with cooling 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


Ref: Douglas (1988)

Separations (Cont’d)

Separate vapor mixtures using partial condensers, cryogenic distillation, absorption towers, adsorbers, and/or membrane devices.

Heuristic 11:

Combination of the previous two flowsheets




To remove a highly-exothermic heat of reaction, consider the use of excess reactant, an inert diluent, and cold shots. These affect the distribution of chemicals and should be inserted early in process synthesis.

Heuristic 21:

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 22:

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


Heat Transfer in Reactors (Cont’d) To remove a highly-exothermic heat of

reaction, consider the use of… Heuristic 21:

excess reactant

cold shots.

an inert diluent




Heat Transfer in Reactors (Cont’d) 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 22:


Heat Transfer in Reactors (Cont’d) TVA design for NH3 synthesis converters Example:




Heat Transfer in Reactors (Cont’d) NH3 synthesis converter cold-shot optimization Example:


Heat Transfer in Reactors (Cont’d) NH3 synthesis converter cold-shot optimization Example:

Before optimization – φφφφ = [0.100, 0.100]T After optimization – φφφφ = [0.277, 0.240]T




To increase the pressure of a stream, pump a liquid rather than compress a gas; that is, condense a vapor, as long as refrigeration (and compression) is not needed, before pumping.

Heuristic 43:

Pumping and Compression

∫= 2

1

P P dPVW

Since work done by pumping or compressions is given by:

It follows that it is more efficient to pump a liquid than to compress a gas. Thus, it is almost always preferable to condense a vapor, pump it, and vaporize it, rather than compress it. Exception: if condensation requires refrigeration.


Example : Feed Preparation of EthylbenzeneEthylbenzene is to be taken from storage at25 °C and 1 atm and fed to a styrene

reactor at 400°C and 5 atm at 100,000 lb/h.

Show two alternatives for positioning the temperature and pressure-increase operations.

Pumping and Compression





Process Design Heuristics - Summary

Understand the importance of selecting reaction paths that do not involve toxic or hazardous chemicals, or to reduce their presence by shortening residence times in the process units 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 thatwould otherwise build up to unacceptable concentrations, to achieve 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 to remove exothermic heats of reaction.Understand the advantages of pumping a liquid rather than compressing a vapor.

We have focused on 16 design heuristics (out of 53 in the book), enabling you to:




Parting Thought – When working on design projects,students need to be made aware of important sources of heuristics;e.g.,Walas, S. M., Chemical Process Equipment – Selection and Design, Butterworths, Stoneham, MA, 1988.

Turton, R., R. C. Bailie, W. B. Whiting, and J. A. Shaeiwitz, Analysis, Synthesis, and Design of Chemical Processes, Second Edition, Prentice- Hall, 2003. Chapter 9 –Utilizing Experience-based Principles to Confirm the Suitability of a Process Design.

Happel, J., and D. G. Jordan, Chemical Process Economics, Second Edition, Marcel Dekker, New York, 1975 – Appendix C.

Ulrich, G. D., A Guide to Chemical Engineering Process Design and Economics, Wiley, 1984 – Appendix B.

Process Design Heuristics - Sources

lecture 05 heuristics for process synthesisdlewin/focapd_2004/lecture_05_h… · lecture 05 product...

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