Chapter 4: Modelling and evaluation of reaction–separation–recycle flowsheets

Question 1

Why recycle?

Answer 1

Incomplete conversion in reactor
Unreacted raw material
Need to remove impurities from product
Recycling by-products can improve yield
Reaction may benefit from dilutent

Question 2

Benefits of recycling “another” species

Answer 2

Diluent: reduce partial pressure of vapour-phase reactant
Solvent: facilitate liquid-phase reaction
Heat carrier: reduce temperature change in reactor

Question 3

Recycle costs:

Answer 3

Recycle is needed to reduce raw material and waste disposal costs
May required another seperator, higher capital/operating costs. Compression is expensive

Question 4

Recycle with purge:

Answer 4

Purge should be high in concentration of material to be removed.
Purge concentration depends on purge fraction.
Trade off between recycle flow rate and loss of reactant in purge

Question 5

Sources of yield losses:

Answer 5

In unconverted reactants within reactor

Losses from seperation and purges

Question 6

Flowsheet structure will affect:

Answer 6

Operability: recycle may not be available at start-up
Safety: inventory and concentration of hazardous substances
Sustainability: yield, waste generation and energy demands
Flexibility: may not be as adaptable to change in process requirements

Question 7

Important process variables of Reactor:

Answer 7

Flow rates, conversion, operating conditions

Question 8

Important process variables of Recycle:

Answer 8

Purge fraction and recycle flowrate