Page 64 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

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FUNDAMENTALS OF MATERIAL BALANCES
2.13. CONVERSION AND YIELD
It is important to distinguish between conversion and yield (see Volume 3, Chapter 1).
Conversion is to do with reactants (reagents); yield with products.
Conversion
Conversion is a measure of the fraction of the reagent that reacts.
To optimise reactor design and to minimise by-product formation, the conversion of a
particular reagent is often less than 100 per cent. If more than one reactant is used, the
reagent on which the conversion is based must be specified.
Conversion is defined by the following expression:
amount of reagent consumed
Conversion D
amount supplied
(amount in feed stream) (amount in product stream)
D 2.8
(amount in feed stream)

This definition gives the total conversion of the particular reagent to all products.
Sometimes figures given for conversion refer to one specific product, usually the desired
product. In this instance the product must be specified as well as the reagent. This is
really a way of expressing yield.

Example 2.10

In the manufacture of vinyl chloride (VC) by the pyrolysis of dichloroethane (DCE), the
reactor conversion is limited to 55 per cent to reduce carbon formation, which fouls the
reactor tubes.
Calculate the quantity of DCE needed to produce 5000 kg/h VC.

Solution

Basis: 5000 kg/h VC (the required quantity).

Reaction: C 2 H 4 Cl 2 ! C 2 H 3 Cl + HCl
mol weights DCE 99, VC 62.5

5000
kmol/h VC produced D D 80
62.5
From the stoichiometric equation, 1 kmol DCE produces 1 kmol VC. Let X be DCE feed
kmol/h:
80
Per cent conversion D 55 D ð 100
X
80
X D D 145.5kmol/h
0.55