Reference no: EM132242493

Question 1. Outline the construction and operational principles of one example of each of the following:

(i) short-tube (natural circulation) evaporator

(ii) short-tube (forced circulation) evaporator

(iii) long-tube (climbing film) evaporator

(iv) plate evaporator.

For each evaporator listed, your answers should include:

(a) descriptions of how the heat in the thermal fluid passes to the process fluid and produces evaporation

(b) comparisons of relative performances

(c) Relative advantages and disadvantages.

NOTE: You may need to refer to standard reference books to answer some parts of this question.

Question 2. (i) Outline the principles behind the design and operation of multiple effect evaporators.
(ii) Explain how the use of vacuum affects the operation and efficiency of multiple effect evaporation.
(iii) Explain the meaning of the following as they apply to multiple effect evaporators:
(a) forward feed
(b) backward feed
(c) parallel feed.

Give one major advantage and one major disadvantage of each type of feed.
(iv) Discuss the factors that must be considered in order to achieve optimum performance in multiple effect evaporation.

Question 3. Briefly outline the principal effects on evaporator operation, efficiency and running costs of the following process features and parameters:
(i) boiling point elevation
(ii) increased feed temperature
(iii) vacuum operation
(iv) vapour recompression
(v) increased calandria tube length.

Question 4. Suggest an appropriate type of evaporator system to carry out each of the processes listed below. Justify your choices by giving one reason for each.
(i) Producing drinking water from sea water.
(ii) Evaporation of a highly scale forming solution
(iii) Evaporation of a solution which becomes highly viscous when concentrated.
(iv) Evaporation of a heat sensitive solution.
(v) The batch evaporation of different materials.

Question 5. A single effect evaporator concentrates 1.1 kg s^-1 of caustic soda (NaOH) solution from 8% (NaOH) to 30% (NaOH). The feed solution is at 60°C, and its mean specific heat capacity at constant pressure is 4.2 kJ kg^-1 K^-1. A 30% solution of caustic soda has a boiling point elevation of 30°C.

The heating steam is supplied at 1.4 bar absolute pressure, and the pressure in the separator is 0.1 bar absolute.

Condensate is collected at a rate of 1 kg s^-1. Assume it is at its condensing temperature.

The overall heat transfer coefficient is 3000 W m^-2 K^-1.

(i) Calculate:

(a) the flowrate of vapour leaving the separator, and the rate of product flow (kg s^-1) from a mass balance.

(b) the heating surface required in the calandria (m²)

(c) the economy of the evaporator

(d) the heat required to raise the feed solution to its boiling point (kW)

(e) the available heat for evaporation and hence a predicted rate of evaporation expected (kg s^-1). Explain why this evaporation rate differs from the evaporation rate obtained in (a).

Data

At 1.4 bar absolute, steam has a condensing temperature of 109.3°C and a latent heat of vaporisation of 2232 kJ kg^-1. At 0.1 bar absolute, steam has a saturation temperature of 45.8°C and a latent heat of vaporisation of 2392 kJ kg^-1.

(ii) If the rate of condensation falls from the initial 1 kg s^-1 to 0.9 kg s^-1, calculate the new overall heat transfer coefficient assuming all other terms remain unaltered.

(iii) What is the likely cause of this reduction in the amount of steam condensation?