Which of the following would you NOT expect to be true at the new steady state (compared to normal operation)?

Question

An ammonia synthesis process normally works as follows: Fresh feed air and hydrogen are combined with a recycle stream and the combined stream is compressed to 10 MPa. The compressed stream enters a reactor, and the entering nitrogen and hydrogen are converted into ammonia until the reaction reaches equilibrium. The reactor effluent, still at 10 MPa, enters a condenser. The liquid and vapor streams leaving the condenser are in VLE. The liquid stream, which is essentially pure ammonia, is the product. A fraction of the vapor stream leaving the condenser is purged, while the rest is recycled.

There is a problem with the compressors and the pressures of the reactor and the condenser both drop to 9 MPa. The fresh feed rate and the fraction of vapor leaving the condenser that is purged each remain the same, as do the reactor and condenser temperatures. The system reaches a new steady state at 9 MPa.
A. The mole fraction of ammonia in the vapor being purged will be higher than in normal operation.
B. The extent of reaction will be lower than in normal operation.
C. The mass flow rate of liquid product will be lower than in normal operation.
D. The composition of the purge stream will change, but its mass flow rate will remain the same as in normal operation.
E. All of the above would be expected to occur.

Answer 1

A. Incorrect. The vapor pressure of ammonia is a function of temperature, and is unchanged. We thus expect the partial pressure of ammonia to be essentially unchanged, but a lower total pressure does indeed mean a higher mole fraction of ammonia.
B. Incorrect. The lower pressure will not drive the reaction as far, hence ? will be lower than normal. The product, ammonia, is favored by higher pressure because there are fewer moles of gas on the product side of the reaction.
C. Incorrect. The lower pressure will cause the reactor to generate less ammonia. There will be less ammonia flowing into the condenser, but the vapor pressure of the ammonia in the vapor phase is a function of temperature and will be unchanged. Consequently the new steady state flow of liquid product will be lower than in normal operation.
D. Correct. Steady state requires that all mass entering the system leave the system. So if the purge stream was the only stream leaving the system, then its mass flow rate would indeed need to be the same at both steady states. But there is also a liquid product leaving. If the flow rate of liquid product decreases, then the flow rate of the purge stream increases.
E. Incorrect. One of these is not a likely expectation.