Design process Bo Hu John Nieber. Outline Steps in Design Process Creation Heuristics in Process Design.

Design process

Bo HuJohn Nieber

Outline

• Steps in Design• Process Creation• Heuristics in Process Design

Heuristics in Process Design

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)

Recall 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.

Design Heuristics : IntroductionWhat is a heuristic? ANS : A method to help solve problems (a rule of thumb)

Design heuristics can be generated for :Chemical reactions, mixing and recycle, separation, temperature/pressure/phase change, task integration.

Heuristics can be collected together an put into an expert system (computer program like a help menu on a computer).

The Seider et al. (SSL) Text lists 53 heuristic rules related to several design categories. TABLE 5.2 at back of chapter lists them in order.

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 that would otherwise build up to unacceptable concentrations, to achieve a high selectivity to the desired products.

• Be able to apply heuristics in selecting separation processes to separate 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 - CH2

1-2

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. 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 inert

Do 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 inert 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:

Distribution of Chemicals (Cont’d)

Example: NH3 Synthesis Loop.

Note: Purge flow rate selection depends on economics!

Distribution of Chemicals (Cont’d)

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.

By-products that are produced in reversible reactions, in small quantities, are usually not recovered in separators or purged. Instead, they

are usually recycled to extinction.

Heuristic 6:

Distribution of Chemicals (Cont’d)

Often small quantities of chemicals are produced in side-reactions. When the reaction proceeds irreversibly, small quantities of by-products must be purged, otherwise they will buildup in the process continuously until the process must be shut down. When, however, the reaction proceeds reversibly, it becomes possible to achieve an equilibrium conversion at steady state by recycling product species without removing them from the process. In so doing, it is often said that

undesired byproducts are recycled to extinction.

Distribution of Chemicals (Cont’d)

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/ lbmole

ko

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

9.6

0E-0

4

9.7

0E-0

4

9.8

0E-0

4

9.9

0E-0

4

1.00E-0

3

1.01E

-03

1.02E-0

3

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 ? (E1/E2>1, E1/E3<1)

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:

Distribution of Chemicals (Cont’d)

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 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 22:

Although heat transfer in reactors is better discussed in the context of heat and power integration, 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 convertersExample:

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 early in 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 24:

Endothermic reactors are treated similarly:

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.

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 that would otherwise build up to unacceptable concentrations, to achieve a high selectivity to the desired products.

• Be able to apply heuristics in selecting separation processes to separate 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 covered 16 design heuristics, enabling you to:

Homework III• Use Matlab to do the following calculation related to vinyl chloride

production. – Use the case study feature to calculate and plot the recycle flow rate of

dichloroethane to the second reactor as a function of the single pass conversion of the second reactor.

– Develop a theoretical equation for this curve and show that the Matlab calculation matches your equation.

– Explain in a couple of sentences why, if we consider decreasing the size of the second reactor, there might be increasing costs associated with the increased recycle.

• It may be desirable to feed excess ethylene to the plant to help ensure full conversion of chlorine. Assuming any ethylene that leaves the plant will just be burned as fuel (i.e., we have no intension of repurifying it for sale), use the price structure from the PowerPoint, how much ethylene could we feed before the maximum possible gross profit would fall to zero?

• Develop a base case design for your project and think about heuristics that can be applied to your project.