Question

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in the reactor is only 15%, a recycle stream is used and in order to avoid build-up of inerts, a purge stream is withdrawn.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. Note that recycle stream contains only nitrogen, hydrogen and inerts. The N2:H2 ratio of 1:3 is maintained in every process stream. Calculate the moles of ammonia produced.

(a) 38.90 moles/hr

(b) 28.90 moles/hr

(c) 37.50 moles/hr

(d) 27.50 moles/hr

I had been asked this question by my college director while I was bunking the class.

This interesting question is from Material Balances with Recycle, By in division Material Balance of Bioprocess Engineering

Answer 1

Right choice is (c) 37.50 moles/hr

To explain I would say: 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) x 2 = 37.50 moles/hr of ammonia.