ChE 211
Quiz # 7
The fresh feed to an ammonia synthesis reactor contains nitrogen, hydrogen and 2.0 mole per cent
inerts. The molar ratio of H2:N2 is 3:1. The product stream consists of pure ammonia. Since
conversion in the reactor is only 15%, a recycle stream is used and in order to avoid build-up of
inerts, a purge stream is withdrawn. The rate of purge stream is adjusted to keep inert
concentration in the recycle stream at 8 mole per cent. For a fresh feed rate of 100 moles/hr,
calculate the ammonia production and purge rates in moles/hr and the overall conversion. Note
that recycle stream contains only nitrogen, hydrogen and inerts. The N 2:H2 ratio of 1:3 is
maintained in every process stream.
Recycle
100.50 N2
35.00 I
Purge
5.75 N2
2.00 I
Fresh Feed 24.50 N2
2.00 I
Pure NH3
37.50
Solution:
The concentration of inerts in the purge stream is 8 mole percent. The amount of inerts must
be 2 moles/hr in order to prevent accumulation of inerts. Therefore the flow rate of purge
stream is 25 moles/hr.
The flow rate of nitrogen and hydrogen in the purge stream is 23 moles/hr (5.75 nitrogen,
17.25 hydrogen). Therefore moles of ammonia produced:
(24.50 – 5.75)x2= 37.50 moles/hr of ammonia.
Moles of nitrogen entering the reactor= (37.50/2)/0.15 = 125 moles/hr
Therefore there are 125 – 24.50= 100.50 moles of nitrogen in the recycle stream.
Total number of moles in the recycle stream = 100.50x4/0.92 = 437 moles/hr
Moles of inerts in the recycle stream= 437- 100.50x4=35 moles/hr
Moles of hydrogen in the recycle stream = 100.50x3 = 301.50 moles/hr