Question 1. (a) Draw simple graphs to show how solubility varies with temperature for:
(i) a solute whose solubility increases rapidly with increase in temperature
(ii) a solute whose solubility increases very little with increase in temperature.

Label the axes and indicate appropriate units.

(b) Use one of your curves to indicate the following:

(i) the saturation line
(ii) unsaturated region
(iii) supersaturated region
(iv) metastable region
(v) region of spontaneous nucleation.

(c) Using brief notes, explain the meaning and significance of the terms listed in Question 1 (b) above.
(d) Use your sketches to explain how the shape of a solubility curve can indicate the most appropriate process for crystallisation operations.
(e) What is meant by the term 'seeding' and when might it be used in crystallisation operations.

Question 2. (a) State three factors that affect the rate of crystal formation from a supersaturated solution.
(b) State three factors that affect the growth of crystals within a crystal magma.
(c) The speed of circulation of a solution during crystallisation influences the rate of crystal growth, suggest:
(i) one advantage and one disadvantage of using a very slow circulation rate
(ii) two limiting factors to the upper speed of circulation.

Question 3. (a) Sketch, label and explain the principles behind the operation of one example of each of the following:
(i) a cooling crystalliser
(ii) an evaporative crystalliser.

(b) Explain the principles behind the operation of an evaporative- cooling type crystalliser.

Question 4. Oslo crystallisers are available for operation using the three mechanisms for production of crystals, i.e.
• cooling type
• evaporative type
• evaporative cooling type.

Discuss the relative advantages and disadvantages of each type.

Question 5. (a) Discuss the reasons for and the principles behind the operation of crystallisers at reduced pressure.
(b) Explain what is meant by multiple effect crystallisation and discuss why it is used.

Question 6. (a) Select a suitable crystalliser (or crystallisation procedure) for the following feed solutions. Give reasons for your choices. (You may use the solubility curves included in the lessons of Section 2.)
(i) A solution of sodium chloride (NaCl) at 40°C containing 32 kg of solute per 100 kg of solvent to produce sodium chloride crystals.
(ii) A solution of copper sulphate at 70°C containing 40 kg of solute per 100 kg of solvent to produce copper sulpate 5H₂O crystals.
(iii) A solution of calcium acetate at 45°C containing 30 kg of solute per 100 kg solvent to produce calcium acetate 2H₂O crystals.
(iv) A solution of zinc sulphate at 80°C containing 74 kg of solute per 100 kg solvent to produce:
(1) zinc sulphate 6H₂O crystals
(2) zinc sulphate 7H₂O crystals
(3) zinc sulphate 1H₂O crystals.

(b) Crystal size is important for the copper sulphate 5H₂O. Suggest a crystalliser that would produce large crystals of uniform size.

Question 7. The solubility data for anhydrous sodium acetate (CH₃COONa) and sodium acetate trihydrate (CH₃COONa3H₂O), measured in kg anhydrous sodium acetate per 100 kg of solvent, is shown in the table below.

(a) Plot the solubility curves for both substances on the same axes.

(b) Calculate the yield (in kg s-1) of sodium acetate trihydrate and evaporation rate (in kg s-1) under the conditions stated below for an evaporative cooling crystalliser.
Feed rate of solution to the crystalliser = 0.8 kg s-1 Temperature of feed = 80°C, concentration of feed = 50% by weight sodium acetate
Crystalliser operated under vacuum giving a final solution temperature of 40°C
Specific latent heat of vaporisation at 40°C = 2433 kJ kg-1 Atomic weights: Na = 23, C = 12, O = 16, H = 1.
The mean specific heat capacity of the solution = 3.5 kJ kg-1 K-1 and specific heat of crystallisation = 144 kJ kg-1.

(c) The same solution could have simply been cooled from 80°C to 40°C.
In this case:

(i) Determine the yield assuming a 2% evaporation loss (based on solvent feed)
(ii) Determine the cooling load if the mean latent heat of vaporisation is 2358 kJ kg-1. Assume the vessel to be at 'steady state' conditions.
(d) Which method of crystallisation, (b) or (c), would you choose? Give reasons for your choice.
(e) What conditions would be required to obtain anhydrous sodium acetate crystals?
(f) Name one type of crystalliser that would be suitable for producing anhydrous sodium acetate crystals.
(g) Explain the possible effects on yield of the addition of a small amount of sodium chloride to the sodium acetate solution.