Example 3.17: Recycle stream: Ammonia is produced in a
reactor that gives 22% conversion of hydrogen.
Gases from the reactor are cooled in a series of coolers and chillers. Liquid ammonia is withdrawn from the bottom
of the separator. Unconverted gases
(free of ammonia) are returned to the reactor.
Five percent of the gases are purged and remaining 95% are
recycled. Hydrogen to nitrogen ratio is
maintained at 3:1. Find the composition
of streams at salient locations. Air
composition (by volume) is 21% oxygen, 78.006% nitrogen, and 0.994% argon.
Solution: Basis of calculation: 100 moles of nitrogen in the combined feed
to the reactor.
C
Ammonia synthesis reaction proceed as
follows:
|
|
Conversion = 22%
C
N2 consumed in the reactor =
100(0.22) = 22 lbmoles
C
Amount of N2 at the reactor
outlet = 100 - 22 = 78 lbmoles
Liquid stream that leaves the separator just contains NH3.
C
Amount of N2 in the
separator off -gases = 78 lbmol
C
Amount of N2 purged =
0.05(78) = 3.9 lbmol
C
Amount of N2 recycled = 78 -
3.9 = 74.1 lbmol
C
Fresh N2 from the feed = 100
- 74.1 = 25.9 lbmol
C
Argon from the feed =
25.9(0.994)/78.006 = 0.33 lbmol
C
Argon in the purge stream = 0.33 lbmol
It should be noted that the composition of purge gas is the
same as recycle stream.
C
Argon in the recycle stream =
0.33/3.9(74.1) = 6.27 lbmol
C
Argon in the reactor feed stream = 0.33
+ 6.27 = 6.6 lbmol
C
H2 feed rate in the combined
feed to the reactor = 3(100) = 300 lbmol
C
NH3 produced = 2(22) = 44
lbmol
C
H2 consumed = 3(22) = 66
lbmol
C
H2 at the reactor outlet =
300 - 66 = 234 lbmol
C
H2 in the purge stream =
0.05(234) = 11.7 lbmol