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in the reactor is only 15%, a recycle stream is used and in order to avoid build-up of inerts, a purge stream is withdrawn.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, and calculate the number of moles, moles of inerts and moles of hydrogen in the recycle stream?

(a) 437 moles/hr, 35 moles/hr, 301.5 moles/hr

(b) 237 moles/hr, 30 moles/hr, 200 moles/hr

(c) 567 moles/hr, 35 moles/hr, 205 moles/hr

(d) 347 moles/hr, 30 moles/hr, 500 moles/hr

The question was posed to me in an interview.

My doubt is from Material Balances with Recycle, By topic in chapter Material Balance of Bioprocess Engineering

Answer 1

Right option is (a) 437 moles/hr, 35 moles/hr, 301.5 moles/hr

To explain I would say: Total number of moles in the recycle stream = 100.50×4/0.92 = 437 moles/hr

Moles of inerts in the recycle stream = 437 – 100.50×4=35 moles/hr

Moles of hydrogen in the recycle stream = 100.50×3 = 301.50 moles/hr.