Page 57 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

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Now consider recovery system
Column
Monomer
Water CHEMICAL ENGINEERING
54,488 kg
monomer
1122 kg
Effluent
With 98 per cent recovery, recycle to reactor

D 0.98 ð 1122 D 1100 kg

Composition of effluent 23 kg monomer, 54,488 kg water.
Consider reactor monomer feed
Fresh Reactor
feed feed

Recycle
1100 kg
Balance round tee gives fresh monomer required

D 11,223 1100 D 10,123 kg

2.7. CHOICE OF BASIS FOR CALCULATIONS
The correct choice of the basis for a calculation will often determine whether the calcu-
lation proves to be simple or complex. As with the choice of system boundaries, no
all-embracing rules or procedures can be given for the selection of the right basis for any
problem. The selection depends on judgement gained by experience. Some guide rules
that will help in the choice are:

1. Time: choose the time basis in which the results are to be presented; for example
kg/h, tonne/y.
2. For batch processes use one batch.
3. Choose as the mass basis the stream flow for which most information is given.
4. It is often easier to work in mols, rather than weight, even when no reaction is
involved.
5. For gases, if the compositions are given by volume, use a volume basis, remembering
that volume fractions are equivalent to mol fractions up to moderate pressures.

2.8. NUMBER OF INDEPENDENT COMPONENTS

A balance equation can be written for each independent component. Not all the compo-
nents in a material balance will be independent.