Page 70 - Chemical engineering design

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FUNDAMENTALS OF MATERIAL BALANCES
separated from the recycle stream in the separation units these inerts would accumulate in
the recycle stream until the stream eventually consisted entirely of inerts. Some portion
of the stream would have to be purged to keep the inert level within acceptable limits. A
continuous purge would normally be used. Under steady-state conditions:
Loss of inert in the purge D Rate of feed of inerts into the system
The concentration of any component in the purge stream will be the same as that in
the recycle stream at the point where the purge is taken off. So the required purge rate
can be determined from the following relationship:
[Feed stream flow-rate] ð [Feed stream inert concentration] D
[Purge stream flow-rate] ð [Specified (desired) recycle inert concentration]

Example 2.14
In the production of ammonia from hydrogen and nitrogen the conversion, based on either
raw material, is limited to 15 per cent. The ammonia produced is condensed from the
reactor (converter) product stream and the unreacted material recycled. If the feed contains
0.2 per cent argon (from the nitrogen separation process), calculate the purge rate required
to hold the argon in the recycle stream below 5.0 per cent. Percentages are by volume.

Solution
Basis: 100 mols feed (purge rate will be expressed as mols per 100 mol feed, as the
production rate is not given).
Process diagram

Purge 5% argon
Recycle
Feed Liquid
0.2% Reactor
NH 3
argon Condenser
Volume percentages are taken as equivalent to mol per cent.
Argon entering system with feed D 100 ð 0.2/100 D 0.2mol.
Let purge rate per 100 mol feed be F.
Argon leaving system in purge D F ð 5/100 D 0.05F.
At the steady state, argon leaving D argon entering

0.05F D 0.2
0.2
F D D 4
0.05
Purge required: 4 mol per 100 mol feed.

2.16. BY-PASS
A flow stream may be divided and some part diverted (by-passed) around some units.
This procedure is often used to control stream composition or temperature.