1Conceptual Design of Chemical Processes

Jirdsak TscheikunaLecture 03

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Process

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What are we doing in Conceptual Design?

Decide whether the process will be operated as a batch or a continuous flow process.

Identify the input-output structure of the process.

Identify and design general structure of each system in the process.

What are we doing in Conceptual Design?

Identify and design the heat-exchanger network or process energy recovery system.

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2What are we doing in Conceptual Design?

Decide whether the process will be operated as a batch or a continuous flow process.

Identify the input-output structure of the process.

Identify and design general structure of each system in the process.

Batch or Continuous

Production rate Sometimes batch if less than 5,000 ton/yr Usually batch if less than 500 ton/yr

Multiproduct plants High product quality

Batch or Continuous

Scale-up problems Very long reaction times Handling slurries at low flow rates Rapidly fouling materials

Market forces Seasonal production (limitation of feedstock) Short product lifetime

What are we doing in Conceptual Design?

Decide whether the process will be operated as a batch or a continuous flow process.

Identify the input-output structure of the process.

Identify and design general structure of each system in the process.

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3 Focus on raw material (input/feed) and products (output) only.

Utility streams are not considered at this point.

Feed Stream

Feed stream is not a pure component. Feed stream consists of reactant and

impurity. Impurity can be classified as reactive and

inert.

Feed Stream

Reactant is the component which is converted to desired product after the reaction.

Structure Consideration

Should we purify the feed streams before they enter the process or the reactor?

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4Purification of Feed

If a feed impurity is a catalyst poison,remove it.

If a feed impurity is reactive and form difficult-to-separate or hazardous product,remove it.

If an inert impurity is present in large amounts, say >10-20%, remove it.

Purification of Feed

If a feed impurity is an inert, but is easier to separate from the product and by-product than from the feed, it is better to process the impurity.

If an impurity in a liquid feed stream is a product or by-product and is present in large amount, feed the process through the separation system.

Purification of Feed

If separation of impurity is difficult, such as an azeotrope with a reactant, do not separate the impurity.

If an impurity is present in a gas feed stream, as a first guess process the impurity.

Structure Consideration

Should we purify the feed streams before they enter the process or the reactor?

How many product streams will there be?

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5Identify Product Streams

List all the components that are expected to leave the reactor.

Classify each component in the list and assign destination code to each.

Order the components by their normal boiling points

Group neighboring components with the same destination

Classification of Component

Gas/Liquid/Solid Reactant/Product/Byproduct/Impurity Example

Gas Reactant Liquid Byproduct Liquid Reactant

Keep in mind that Gasesare the component which is very difficult to condense or change to liquid phase.

Our Primary Productshould be in liquid phase,if required.

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6Destination Codes for Gases

Vent Recycle and Purge

Destination Codes for Liquid/Solid

Recycle Primary Product Valuable Byproduct Fuel Waste

Evaluation of the Flowsheet

Be certain that all by-products and impurities leave the process!

Process Alternative

If we are not certain that our decision is correct, we list the opposite decision as a process alternative.

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7General Considerationsfor Specifying

Input-Output Structure

General Consideration

Select raw materials and chemical reactions to avoid, or reduce, the handling and storage of hazardous and toxic chemicals.

General Consideration

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

General Consideration

Introduce purge streams to provide exits for species that enter the process as impurities in the feed or are formed in irreversible side reactions, when these species are in trace quantities and/or are difficult to separate from the other chemicals. Lighter species leave in vapor purge streams, and heavier species exit in liquid purge streams.

8General Consideration

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.

General Consideration

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.

General Consideration

For competing reactions, both in series and parallel, 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. Before developing a base-case design, obtain kinetics data and check this assumption.

Profit Margin

PM = Product Value + By-product Value - Raw Material Cost

Profit margin (potential profitability) is a cost per year basis (annual cost)

9Information determined using Input-Output Diagram

All the reactions, both desired and undesired, that take place.

What chemical components must enter with the feed and leave as products

Basic economic analysis on profit margin