Separation - Final Exam

How can heat transfer in a conventional single-effect evaporator be improved?

A. By increasing the external steam pressure

B. By lowering the external stem temperature

C. By increasing the evaporator internal pressure

D. By increasing the feed rate

(A) Increasing the heating‑steam pressure raises its saturation temperature, which increases the temperature difference ΔT between the heating side and the boiling liquid. A larger ΔT increases the driving force for heat transfer, so the overall heat transfer rate goes up (as long as product quality or fouling limits are not exceeded).

Which of the following statements about mechanical evaporator recompression (MVR) is false?

A. MVR is less thermodynamically efficient than thermal vapor recompression.

B. MVR compresses steam by radial fan or centrifugal compressor

C. MVR requires a small quantity of condensate to be injected to cool down the superheated compressed vapor

D. MVR is only applied when a small driving force for evaporation is needed

(A) False. MVR (using a mechanical compressor or fan to recompress vapor) is generally more energy‑ and thermodynamically efficient than TVR because it recovers more vapor energy and needs far less fresh high‑pressure steam.

Which of the following phenomena does not cause a boiling point elevation during evaporation?

A. Fouling. (Note: Fouling affects mainly the efficiency of the heat transfer but not at which temperature the product inside will boil)

B. Product residence time

C. Tube length

D. Product recirculation

(B)

For boiling point elevation (BPE) in an evaporator, what matters is mainly solution composition (non‑volatile solute concentration) and pressure; none of the listed “operational” phenomena directly change the colligative boiling point.

• Fouling
  Fouling adds thermal resistance and decreases heat transfer efficiency, so more temperature difference is needed to get the same evaporation rate. It does not change the thermodynamic boiling point of the liquid; it only slows heat transfer.

• Product residence time
  Residence time is how long the liquid stays in the evaporator. Longer or shorter residence time can increase concentration (and thus indirectly BPE) only because more evaporation occurs, but as a factor by itself it does not define or “cause” boiling point elevation; BPE at a given composition and pressure is independent of how long the liquid has been there. This is the one listed that does not inherently cause BPE.

• Tube length
  Longer tubes can allow more evaporation and hence higher solute concentration at the outlet; higher concentration increases boiling point elevation. So tube length can contribute indirectly to higher BPE through greater concentration.

• Product recirculation
  Recirculation passes the concentrate repeatedly through the heater, increasing solids content over time. As solute concentration rises, boiling point elevation increases (classic colligative effect), so recirculation can lead to higher BPE.

Evaporation of liquid foods is applied for the following reasons:

1. To improve the microbiological stability of tomato products

2. To improve the physical-chemical stability of milk

3. to improve the enzymatic stability of orange juice

4. To preconcentrate milk before drying

5. to reduce storage and transportation cost of orange juice

6. to obtain convenience ingredients and foods

Answer:

1, 4, 5, 6

Indicate the correct statement about steam economy.

A. The steam economy is used to express the operating performance of an evaporator system and equals the sum of vapor mass flow rates of all effects of a forward feed multiple evaporator divided by the mass flow rate of steam

B. The seam economy is used to express the extent to which the phase change has progressed and is expressed as a percentage indicating the heat content of the vapor-liquid mixture

C. The steam economy is used to express the operating performance of an evaporator system and equals the mass flow rate of vapor in the first effect of a forward feed multiple evaporator divided the mass flow rate of steam

Answer: (A)

How can you increase the permeate flux by changing the operational parameters of a crossflow membrane filtration system?

A. Lower the feed solids concentration

B. Lower the hydraulic pressure difference across the membrane

C. Lower the feed temperature

D. Lower the crossflow velocity

(A)

Short reasoning for each option:

• Lower the feed solids concentration
  Less solids means less osmotic pressure and a thinner/concentration‑polarization layer at the membrane surface, so hydraulic resistance decreases and permeate flux increases.

• Lower the hydraulic pressure difference across the membrane
  The transmembrane pressure is the driving force for permeation; decreasing it directly lowers permeate flux.

• Lower the feed temperature
  Lower temperature increases viscosity and decreases diffusion, both of which reduce permeate flux.

• Lower the crossflow velocity
  Lower crossflow makes concentration polarization and fouling worse at the membrane surface, which decreases permeate flux; higher crossflow is usually used to maintain or increase flux.

Membrane separation is an innovate unit operation that can be applied. Indicate the false application.

A. Production of alcohol-free beer by nanofiltration

B. Cold pasteurization of skimmed milk by microfiltration

C. Preconcentration of apple juice by reversed osmosis

D. Standardization of protein content of milk by ultrafiltration

(A)

• Beer is usually dealcoholized by reverse osmosis or pervaporation.

• True, microfiltration can remove vegetative MO at low temperature and is often described as cold pasteurization of milk.

• RO is widely used to preconcentrate fruit juices without heating.

• UF is routinely used to adjust and standardize milk protein content.