E3.17 Recycle Stream

Example 3.17: 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 Liquid ammonia is withdrawn from the bottom of the separator.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.

Ammonia synthesis reaction proceed as follows:

Conversion = 22%

N₂ consumed in the reactor = 100(0.22) = 22 lbmoles

Amount of N₂ at the reactor outlet = 100 - 22 = 78 lbmoles

Liquid stream that leaves the separator just contains NH₃.

Amount of N₂ in the separator off -gases = 78 lbmol

Amount of N₂ purged = 0.05(78) = 3.9 lbmol

Amount of N₂ recycled = 78 - 3.9 = 74.1 lbmol

Fresh N₂ from the feed = 100 - 74.1 = 25.9 lbmol

Argon from the feed = 25.9(0.994)/78.006 = 0.33 lbmol

Argon in the purge stream = 0.33 lbmol

It should be noted that the composition of purge gas is the same as recycle stream.

Argon in the recycle stream = 0.33/3.9(74.1) = 6.27 lbmol

Argon in the reactor feed stream = 0.33 + 6.27 = 6.6 lbmol

H₂ feed rate in the combined feed to the reactor = 3(100) = 300 lbmol

NH₃ produced = 2(22) = 44 lbmol

H₂ consumed = 3(22) = 66 lbmol

H₂ at the reactor outlet = 300 - 66 = 234 lbmol

H₂ in the purge stream = 0.05(234) = 11.7 lbmol

Amount of H₂ in the recycle stream = 0.95H 234 = 222.3 lbmol

Fresh H₂ feed = 300 - 222.3 = 77.7 lbmol