Test Bank for Fundamentals

Test Bank for Fundamentals of Chemical Engineering Thermodynamics 1 st Edition By Kevin Dahm, Donald Visco (All Chapters 1-15, 100% Original Verified, A+ Grade)

All Chapters Arranged Reverse: 15-1

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Dahm/Visco Fundamentals of Chemical Engineering Thermodynamics Chapter 15 1.An ammonia synthesis process 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 a fraction of the entering nitrogen and hydrogen are converted into ammonia. 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 almost pure ammonia, is the product. A fraction of the vapor stream leaving the condenser is purged, while the rest is recycled. Which of the following statements is accurate?A.Raoult’s Law would be a reasonable way to model the VLE for ammonia in the condenser.B.Henry’s Law would be a reasonable way to model the VLE for oxygen in the condenser.C.If we increase the size of the reactor, allowing a larger fraction of the nitrogen and hydrogen to be converted to ammonia, it won’t lead to an increase in the liquid product flow rate, because the product flow rate is limited by the equilibrium constraint in the condenser.D.If we maintain the same flow rate of entering feeds, but we purge a smaller fraction of the vapor leaving the condenser, then the mole fraction of oxygen in the purge stream will go down.E.None of the above are true.

Analysis:

A.Incorrect. Raoult’s Law incorporates the ideal gas law for vapor phase modeling. A pressure of 10 MPa is far too high to be considered ideal.B.Correct. The liquid stream is described as almost pure ammonia. Henry’s Law is used to model a system in which a trace of gas dissolved in a liquid. It might well be acceptable to assume the liquid is pure ammonia, but if one was going to account for the presence of gases dissolved in the ammonia, Henry’s Law would indeed be a reasonable way to model the VLE.C.Incorrect. The “equilibrium constraint” in VLE involves mole fractions, not absolute numbers of moles. If the flow rate of ammonia entering the condenser increases, then the liquid product flow rate can increase without a change in the mole fraction of ammonia in either the liquid or the vapor product.D.Incorrect. The amount of oxygen purged (mol/time) essentially equals the amount of oxygen entering at steady state, since none is consumed in the reaction and very little leaves through the liquid product. If we lower the total flow rate of the purge stream, then the mole fraction of oxygen in that stream, which is constant, will increase.E.Incorrect. One of these statements is reasonable.© 2015 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 2 / 4

Dahm/Visco Fundamentals of Chemical Engineering Thermodynamics Chapter 15 2.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. Which of the following would you NOT expect to be true at the new steady state (compared to normal operation)?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.

Analysis:

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 ξ 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 © 2015 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 3 / 4

Dahm/Visco Fundamentals of Chemical Engineering Thermodynamics Chapter 15 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.© 2015 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.

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