Evaporation Chapter (Week 2)

Which of the following is an example of an industrial evaporation process?

• Fruit juice concentration

• Oil refining

• Fermentation

• Paper manufacturing

Answer explanation

Fruit juice concentration is an industrial evaporation process where water is removed from juice to increase its concentration. This method is widely used in the food industry, making it the correct choice among the options.

Which factor affects the boiling point elevation during evaporation?

• Solubility

• Heat transfer surface area

• Solution concentration

• Type of evaporator used

Answer explanation

The boiling point elevation during evaporation is primarily affected by solution concentration. Higher concentrations of solute increase the boiling point due to the colligative properties, which depend on the number of solute particles.

Which type of evaporator is best suited for heat-sensitive materials?

• Horizontal-tube evaporator

• Falling-film evaporator

• Open-pan solar evaporator

• Long-tube vertical evaporator

Answer explanation

The falling-film evaporator is ideal for heat-sensitive materials as it operates at lower temperatures and has a short residence time, minimizing thermal degradation compared to other types.

What is the main drawback of using open kettle or pan evaporators?

• High complexity

• Poor heat economy

• High operational cost

• Risk of contamination

Answer explanation

The main drawback of open kettle or pan evaporators is their poor heat economy, as they tend to lose a significant amount of heat during the evaporation process, making them less efficient compared to other types of evaporators.

In a multiple-effect evaporator, what happens to the steam from the first effect?

• It is condensed and discarded

• It is used as a heating medium in the next effect

• It is released into the atmosphere

• It is reused in the same effect

Answer explanation

In a multiple-effect evaporator, the steam generated in the first effect is not wasted; instead, it is utilized as a heating medium for the subsequent effect, enhancing energy efficiency.

Which type of evaporator involves mechanically agitating the liquid film to improve heat transfer?

• Open-pan evaporator

• Agitated-film evaporator

• Vertical-tube natural circulation evaporator

• Forced-circulation evaporator

Answer explanation

The agitated-film evaporator uses mechanical agitation to enhance the liquid film's movement, improving heat transfer efficiency. This distinguishes it from other types of evaporators listed.

The overall heat transfer coefficient (U) in an evaporator is influenced by all of the following EXCEPT:

• Steam-side condensing coefficient

• Thermal conductivity of the metal wall

• Resistance of the scale on the liquid side

• Vapor pressure of the solution

Answer explanation

The vapor pressure of the solution does not directly affect the overall heat transfer coefficient (U) in an evaporator, unlike the other factors which influence heat transfer resistance.

What is the typical heat transfer mechanism in horizontal-tube natural circulation evaporators?

• Convection

• Conduction

• Radiation

• Forced convection

Answer explanation

In horizontal-tube natural circulation evaporators, heat transfer occurs primarily through convection. The fluid circulates due to density differences caused by heating, facilitating efficient heat transfer.

Duhring's Rule is used to estimate:

• Steam economy

• Overall heat transfer coefficient

• Boiling point rise of solutions

• Heat of solution

Answer explanation

Duhring's Rule is specifically used to estimate the boiling point rise of solutions, making it the correct choice. It relates the boiling point elevation to the concentration of solute in a solvent.

Which of the following is true for a backward-feed multiple-effect evaporator?

• The fresh feed enters the first and hottest evaporator

• It is used when the product is not heat-sensitive

• Fresh feed enters the last and coldest evaporator

• It is more energy-efficient than forward-feed systems

Answer explanation

In a backward-feed multiple-effect evaporator, the fresh feed enters the last and coldest evaporator, allowing it to utilize the heat from the previous effects, making it efficient for non-heat-sensitive products.