Page 65 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

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CHEMICAL ENGINEERING
In this example the small loss of DCE to carbon and other products has been neglected.
All the DCE reacted has been assumed to be converted to VC.
Yield
Yield is a measure of the performance of a reactor or plant. Several different definitions
of yield are used, and it is important to state clearly the basis of any yield figures. This
is often not done when yield figures are quoted in the literature, and the judgement has
to be used to decide what was intended.
For a reactor the yield (i.e. relative yield, Volume 3, Chapter 1) is defined by:
mols of product produced ð stoichiometric factor
Yield D 2.9
mols of reagent converted
Stoichiometric factor D Stoichiometric mols of reagent required per mol
of product produced
With industrial reactors it is necessary to distinguish between “Reaction yield” (chemical
yield), which includes only chemical losses to side products; and the overall “Reactor
yield” which will include physical losses.
If the conversion is near 100 per cent it may not be worth separating and recycling
the unreacted material; the overall reactor yield would then include the loss of unreacted
material. If the unreacted material is separated and recycled, the overall yield taken over
the reactor and separation step would include any physical losses from the separation
step.
Plant yield is a measure of the overall performance of the plant and includes all chemical
and physical losses.
Plant yield (applied to the complete plant or any stage)
mols product produced ð stoichiometric factor
D 2.10
mols reagent fed to the process
Where more than one reagent is used, or product produced, it is essential that product
and reagent to which the yield figure refers is clearly stated.

Example 2.11

In the production of ethanol by the hydrolysis of ethylene, diethyl ether is produced as a
by-product. A typical feed stream composition is: 55 per cent ethylene, 5 per cent inerts,
40 per cent water; and product stream: 52.26 per cent ethylene, 5.49 per cent ethanol, 0.16
per cent ether, 36.81 per cent water, 5.28 per cent inerts. Calculate the yield of ethanol
and ether based on ethylene.

Solution
Reactions: C 2 H 4 C H 2 O ! C 2 H 5 OH a
2C 2 H 5 OH ! C 2 H 5 2 O C H 2 O b
Basis: 100 mols feed (easier calculation than using the product stream)