Food Processing Unit Operations Questions and Answers – Evaporation of Heat Sensitive Materials

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This set of Food Processing Unit Operations Questions and Answers for Experienced people focuses 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
View Answer

Answer: a
Explanation: 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
View Answer

Answer: a
Explanation: 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
View Answer

Answer: d
Explanation: 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.
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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
View Answer

Answer: c
Explanation: 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
View Answer

Answer: a
Explanation: 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
View Answer

Answer: b
Explanation: 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
View Answer

Answer: b
Explanation: Vacuum evaporators, plate-and-frame type evaporators and agitated thin-film evaporators are used for heat sensitive materials. Vertical type evaporators have long residence time and high pressure which can degrade the quality of the product.
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8. Which of the following evaporator separates volatile and nonvolatile compounds?
a) Falling film evaporators
b) Vertical type evaporators
c) Climbing film type evaporators
d) Forced circulation evaporators
View Answer

Answer: a
Explanation: In falling film evaporators, the liquid product usually enters the evaporator at the head of the evaporator. The liquid enters the heating tube and forms a thin film on the tube wall where it flows downwards at boiling temperature and is partially evaporated. In most cases, steam is used for heating the evaporator.

9. What is the important factor which is employed for heat sensitive materials in an evaporator ?
a) High pressure
b) Vacuum
c) High residence time
d) High temperature
View Answer

Answer: b
Explanation: Under short residence time, the contact between product and evaporator is less and the low pressure creates vacuum where the boiling point is also reduced therefore there is no degradation of heat sensitive materials.
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10. Which of the following evaporator cannot be used for milk?
a) Scraped surface evaporators
b) Fluidized bed evaporators
c) Plate-and-frame type evaporators
d) Batch Pan
View Answer

Answer: b
Explanation: Batch pan is the oldest method which evaporates the milk where the heat transfer per unit volume is small requiring long residence times. Plate-and-frame type evaporators and scraped surface evaporators are commonly used in dairy industry where heat transfer per unit volume is largeand has short residence times.

Sanfoundry Global Education & Learning Series – Food Processing Unit Operations.

To practice all areas of Food Processing Unit Operations for Experienced people, here is complete set of 1000+ Multiple Choice Questions and Answers.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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