Chemical Engineering Plant Design

Lek WanthaLecture 05

Input-Output Structure of the Flowsheet

A Hierarchical Approach to Conceptual Process Design

1. Decide whether the process will be batch or continuous

2. Identify the input-output structure of the process

3. Identify and define the recycle structure of the process

4. Identify and design the general structure of the separation system

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

Flowsheet Alternatives

Process

Process

Product

By-product

Product

By-product

Feedstreams

Feedstreams

Purge

Structure Consideration• Should we purify the feed streams before they enter the process?• Should we remove or recycle a reversible by-product?• Should we use a gas recycle and purge stream?• Should we not bother to recover and recycle some reactants?

Structure Consideration• How many product streams will there be?• What are the design variables for the input-output structure, and what economic trade-offs are associated with these variables?

Purification of Feed• If a feed impurity is not inert, remove it.• If an impurity is present in a gas feed stream, as a first guess process the impurity.• If an impurity in a liquid feed stream is a product or by-product, usually feed the process through the separation system.• If an impurity is present in large amounts, remove it.

Purification of Feed• If an impurity is present as an azeotrope with a reactant, process the impurity.• 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 a feed impurity is a catalyst poison, remove it.

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

Recover or Recycle Reversible By-products

• Should we recycle the reversible by-products or should we remove it?• Example

Toluene + H2 Benzene + CH4

2Benzene Diphenyl + H2

Gas Recycle and Purge• If we have a “light” reactant and either a “light” feed impurity or a “light” by-product produced by a reaction, it used to be common practice to use a gas recycle and a purge stream for the first design.

• We define a light component as one which boils lower than propylene (-55 F, -48 C).

Do Not Recover and Recycle Some Reactants

• For example, water and air are much less valuable than organic ones, we normally do not bother to recover and recycle unconverted amounts of these components.

• Often we feed them as an excess to try to force some more valuable reactant to complete conversion.

Destination Codes and Component Classifications

Vent Gaseous by-products and feed impurities

Recycle and purge Gaseous reactants plus inert gases and/or gaseous by-product

Recycle Reactants, reaction intermediates Azeotropes with reactants (sometimes) Reversible by-product (sometimes)

None Reactants-if complete conversion or unstable reaction intermediates

Excess-vent Gaseous reactant not recovered and recycled

Destination Codes and Component Classifications

Excess-waste Liquid reactant not recovered and recycled

Primary product Primary productValuable by-product Separate destination for different

by- productsFuel By-products to fuelWaste Byproduct to waste treatment

Number of 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.

Number of Product Streams• Example 1

Number of Product Streams• Example 2: Hydroalkylation (HDA) of toluene to produce benzene

ProcessBenzene

Diphenyl

H2, CH4

Purge: H2, CH4

Toluene

Evaluation of 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.

Overall Material Balances• Start with specified production rate.

•From the stoichiometry find the by-product flows and the reaction requirements.

• Calculate the impurity inlet and outlet flows for the feed streams where the reactants are completely recovered and recycled.

Overall Material Balances• Calculate the outlet flows of reactants in terms of a specified amount of excess for streams where the reactants are not recovered and recycled.

• Calculate the inlet and outlet flows for the impurities entering with the reactant stream in step 4.

Overall Material Balances• Example: See the accompanied sheet for HDA process

Stream Table

• Example: Stream table

Economic Potential (EP)• EP = Product Value + By-Product Value

- Raw Material Cost• Economic potential is a cost per year basis (annual cost)• EP for HDA example would be EP = Bezene Value + Fuel Value of Diphenyl + Fuel Value of Purge – Toluene Cost - Makeup Gas Cost

Economic Potential (EP)• Example: HDA process

Economic Potential (EP)• Example: HDA process