Chemical Engineering Plant Design

Lek WanthaLecture 06

Recycle Structure of the Flowsheet and Reactor Design

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

Decisions that Determine the Recycle Structure

• How many reactors are required?• Should some components be separated between the reactors? • How many recycle streams are required?• Do we want to use an excess of one reactant at the reactor inlet• Is a gas-recycle compressor required?• How does it affect the cost?

Decisions that Determine the Recycle Structure

• Should the reactor be operated adiabatically, with direct heating or cooling, or is a diluent or heat carrier needed?• Do we want to shift the equilibrium conversion? How?

Number of Reactor Systems

• If reactions take place at different temperatures and pressures and/or they require different catalysts, then a separate reactor system is required for each operating condition.

Number of Reactor Systems

• Example 1:

•Example 2:

Number of Recycle Streams• Depend on process requirement.• Do not separate two components and then remix them at a reactor inlet.• Components recycled to the same reactor that have neighboring boiling points should be recycled in the same stream.• Separate gas and liquid recycle streams.• A gas-recycle compressor is required if the recycled components boil at a temperature lower than that of propylene.

Number of Recycle Streams• Example 1: Acetic anhydride

Number of Recycle Streams• Example 2: HDA process

Number of Recycle Streams• Example 2: HAD process

Number of Recycle Streams• Example 3: Acetic anhydride

Number of Recycle Streams• Example 3: Acetic anhydride

Excess Reactants• Shift the product distribution.• Force another component to be close to complete conversion.• Shift the equilibrium conversion.• If an excess reactant is desirable, these is an optimum amount of the excess.

Recycle Material Balance Example• See accompanied sheet

Reactor Heat Effect• Adiabatic• Direct heating or cooling (Isothermal)• Addition of diluent or heat carrier

Reactor Heat Effect• Reactor Heat Load - Reactor Heat Load = Heat of Reaction x Fresh Feed Rate• Adiabatic Temperature Change - QR = FCp(TR,in – TR,out)

Reactor Heat Effect

Reactor Heat Effect• Heat Transfer Area - Area = QR /UdT

• Heat Carrier - recycle more reactant or a product or by product - extraneous component

Reactor Heat Effect• For endothermic processes with a heat load of less than 6 to 8 E6 Btu/hr, we use an isothermal reactor with direct heating. For larger heat load we may add a diluent or heat carrier.

Reactor Heat Effect• For exothermic processes we use an adiabatic reactor if the adiabatic temperature rise is less than 10 t0 15% of the inlet temperature. If the adiabatic temperature rise exceeds this value, we use direct cooling if the reactor heat load is less than 6 to 8 E6 Btu/hr. Otherwise, we introduce a diluent or a heat carrier.

Equilibrium Limitations

Equilibrium Limitations

Reactor Heat Effect• For single reactions we choose a conversion of 0.96 or 0.98 of the equilibrium conversion.• The most expensive reactant (or the heaviest reactant) is usually the limiting reactant.• If the equilibrium constant of a reversible by-product is small, recycle the reversible by-product.

Reactor Design

Reactor Design