Mass Transfer and Separation practice questions

Answer: C) Momentum diffusivity to mass diffusivity

• AIncorrect. That's Reynolds.
• BIncorrect. That's the Lewis number.
• CCorrect. Schmidt number Sc = ν/D.
• DIncorrect. That's Nusselt.

Answer: D) External heat and mass transfer to the wet surface

• AIncorrect. It's external transfer.
• BIncorrect. That governs the falling-rate period.
• CIncorrect. It's external transfer.
• DCorrect. The surface stays saturated.

Answer: A) 4 m

• ACorrect. Z = HTU × NTU.
• BIncorrect. Multiply, don't divide.
• CIncorrect. Multiply, don't add.
• DIncorrect. Multiply HTU by NTU.

Answer: A) Contacting it with a gas or steam

• ACorrect. The reverse of absorption.
• BIncorrect. That's extraction.
• CIncorrect. It uses a gas phase.
• DIncorrect. It uses a gas phase.

Answer: D) Leaching (solid–liquid extraction)

• AIncorrect. That forms solids from solution.
• BIncorrect. That uses volatility.
• CIncorrect. That uses a gas phase.
• DCorrect. A solvent dissolves the solute from a solid.

Answer: D) An infinite number of stages

• AIncorrect. A pinch needs infinitely many stages.
• BIncorrect. That's total reflux.
• CIncorrect. Distillation still needs heat.
• DCorrect. A pinch forms where the operating and equilibrium lines meet.

Answer: A) Lower pressure drop per theoretical stage, which favors vacuum service

Packed columns have a lower pressure drop and lower liquid holdup than tray columns, which makes them attractive for vacuum distillation. Trays are generally better for fouling service, side draws and long residence times.

• ACorrect. Random and structured packings have open, high-voidage structures, so they give a lower pressure drop than trays. This matters in vacuum distillation, where a lower pressure drop keeps bottoms temperatures low.
• BIncorrect. Packing beds tend to trap solids and foul. Trays, especially those with large openings, are usually preferred for dirty or fouling service.
• CIncorrect. Side draws and intermediate heat exchange are usually simpler with trays. Packed beds need extra collectors and liquid redistributors at each draw point.
• DIncorrect. Packed columns have lower liquid holdup than tray columns. Trays are chosen when a long liquid residence time is needed.

Answer: A) 2.0

The K-value is the vapor mole fraction divided by the liquid mole fraction at equilibrium: K = y/x = 0.60/0.30 = 2.0.

• ACorrect. K = y/x = 0.60/0.30 = 2.0.
• BIncorrect. This is x/y, the reciprocal of the K-value.
• CIncorrect. This is the product y·x. The K-value is a ratio, so divide y by x.
• DIncorrect. This is the difference y − x, which is not the distribution ratio.

Answer: D) Rising vapor prevents liquid from flowing down

• AIncorrect. Total reflux doesn't imply flooding.
• BIncorrect. That isn't flooding.
• CIncorrect. That stops vapor flow.
• DCorrect. Efficiency collapses and pressure drop climbs.

Answer: A) Adding stages can push solute recovery close to 100%

With A = L/(mG) > 1 the operating line is steeper than the equilibrium line, so adding stages can drive recovery toward 100%. When A < 1, recovery is limited to about A no matter how many stages are added.

• ACorrect. When A > 1 the operating line is steeper than the equilibrium line, so they do not pinch at the lean end. Adding stages can then approach complete solute recovery.
• BIncorrect. That limit applies when A < 1. The lines then pinch, and fractional recovery cannot exceed about A even with infinite stages.
• CIncorrect. A is an equilibrium/flow-ratio parameter. Flooding depends on hydraulic loading and column diameter, not on A.
• DIncorrect. Parallel operating and equilibrium lines correspond to A = 1, not A well above 1.