Food Processing Unit Operations Questions and Answers for Experienced people on “Evaporation of Heat Sensitive Materials”.
1. What are conditions to achieve maximum retention of volatile constituents?
a) Short residence time and low pressure
b) Short residence time and high pressure
c) Long residence time and low pressure
d) Long residence time and high pressure
Answer: a
Clarification: Under short residence time, the contact between product and evaporator is less and under the low pressure, the boiling point is also reduced therefore volatile components are retained in the product.
2. What type of evaporators used for heat sensitive materials?
a) Vacuum evaporators
b) Vertical type evaporators
c) Climbing film type evaporators
d) Forced circulation evaporators
Answer: a
Clarification: Pressure is directly proportional to boiling point and vacuum evaporators work under low pressure or at high vacuum which results in reduced boiling point.
3. What are the applications of agitated thin-film evaporators?
a) Purification of sensitive organic chemicals
b) Vapor recompression for energy recovery
c) Devolatilization of thermoplastics
d) Devolatilization of thermoplastics and purification of sensitive organic chemicals
Answer: d
Clarification: Purification of sensitive organic chemicals like natural oils, fatty acids, herbicides and insecticides and isocyanates, and devolatilization of thermoplastics, phenolicresins, acrylic resins, silicone polymers and nylon.
4. Are these statements about the gasketed plate-and-frame evaporators true?
Statement 1: Gasketed plate-and-frame evaporators are not used for heat sensitive materials.
Statement 2: These systems are operated as rising-film, falling-film evaporators.
a) True, False
b) True, True
c) False, True
d) False, False
Answer: c
Clarification: The gasketed plate-and-frame evaporator is constructed by mounting a number of embossed plates with corner openings. These systems are operated as rising-film, falling-film evaporators. It is used for heat-sensitive, viscous and foamy materials.
5. Which of the following can be evaporated in plate-and-frame or agitated thin-film evaporator?
a) High viscosity, heavy solids, high tendency to foul
b) Heavy solids, high tendency to foul
c) High viscosity, heavy solids,
d) High viscosity, high tendency to foul
Answer: a
Clarification: Car radiator and air conditioner evaporator coil uses crossflow heat exchangers. Crossflow heat exchangers are widely used for transfer of heat between a gas and a liquid. In air conditioner evaporator coil, the air flows past the evaporator coils are cooled by the cold refrigerant that is flowing inside the tubes of the coil.
6. Are these statements about the distillateto concentrate ratio in evaporators true?
Statement 1: Lack of wall wetting causes fouling.
Statement 2: Fouling depends on the fluid velocity.
a) True, False
b) True, True
c) False, True
d) False, False
Answer: b
Clarification: Enough liquid should be passing through the evaporator to keep the heated walls wet. Lack of wall wetting and fluid velocity may cause serious fouling and salting of solids on the heat transfer surfaces, leading to degradation of product quality. In processes where high distillate-to-concentrate ratios are required, recycling of concentrate is done.
7. Which of the following evaporator is not used for heat sensitive materials?
a) Vacuum evaporators
b) Vertical type evaporators
c) Plate-and-frame type evaporators
d) Agitated thin-film evaporators