Vapour Liquid Equilibria Questions and Answers

Answer: C

The feed composition and the K values are given, and the Rachford-Rice equation is solved for the vapour fraction. The second and third options describe bubble and dew point calculations instead. Each answers a different question about the same mixture.

Answer: D

The check comes first because a feed outside that range is a single phase and the flash has no solution. Compositions follow once the vapour fraction is known, and the K values may then be updated and the loop repeated.

Answer: A

If the feed is below its bubble point it is all liquid and if above its dew point it is all vapour, and in either case the vapour fraction falls outside the range from zero to one. Checking the bracket first avoids a meaningless answer.

Answer: B

Correct answer: 45 per cent A

An overall balance on A across the flash, with equal vapour and liquid, gives \[ 0.75 \times 0.5 + x \times 0.5 = 0.60, \qquad 0.375 + 0.5x = 0.60, \qquad x = 0.45. \] The check is that the feed composition must lie between the two products, and 0.45 and 0.75 straddle 0.60 as they should - which is what rules out the 50 per cent offered.

Answer: C

Two components minus two phases plus two gives two, so fixing temperature and pressure fixes both phase compositions. This is why a Txy diagram is drawn at a stated pressure. Composition then follows from the two curves.

Answer: D

Both are true and the reason gives the necessity: the same pair of components produces a different diagram at every pressure, so an unlabelled diagram cannot be used for any quantitative work. The pressure belongs in the caption of every such plot.

Answer: A

At constant temperature high pressure favours the liquid, so the all-liquid region lies above and its boundary is the bubble curve. The arrangement is inverted from the temperature composition diagram, which catches many people out.

Answer: B

The Mollier chart is a property diagram for a single substance rather than a representation of phase equilibrium in a mixture. The other three are the standard ways of plotting binary data. Each suits a different kind of design calculation.

Answer: C

The first two are correct. The third is wrong, since even an ideal solution gives a curved line unless the relative volatility happens to be one, in which case it lies on the diagonal itself. Curvature away from the diagonal measures the ease of separation.

Answer: D

Correct answer: reduces the relative volatility and makes separation harder

Relative volatility falls as the pressure rises, because the components’ vapour pressures converge as the critical region is approached, so more stages or more reflux are needed for the same separation. Pressure is raised nonetheless where the condenser needs it - to allow cooling water rather than refrigeration - so the choice is a trade between separation cost and utility cost.