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```Crystallisation
(3 periods)
References
1. Geankoplis, C. J., Transport Processes and Separation
Process Principles, 4th edition, Prentice Hall, New
Jersey, 2003.
2. Badger, W. L. and Banchero, J. T., Introduction to
Chemical Engineering, McGraw-Hill, Singapore, 1957.
3. McCabe, W. L., Smith, J. C. a;nd Harriott, P., Unit
Operations of Chemical Engineering, 4th edition,
McGraw-Hills, Singapore, 1985.
4. Foust, A. S., Wenzel, L. A., Clump, C. W., Maus, L. and
Andersen, L. B., Principles of Unit Operations, 2nd
edition, John Wiley & Sons, New York, 1980.
Overview
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Equipment for crystallisation
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Systems with heat transfer (external cooling systems)
Equilibrium Solubility and Solubility Curves
Enthalpy-Composition Diagrams/Enthalpy-Concentration Diagrams
Material Balances in Crystallisation
– Calculation using equilibrium solubility or solubility curves
– Yield of Crystallisation Process
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Heat Balances in Crystallisation
– Heat evolved and removed from the system
– Calculation using enthalpy-composition diagrams
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Nucleation of Crystals
Crystal Growth
Particle-Size Distribution of Crystals
Model for Mixed Suspension – Mixed Product Removal (MSMPR) Crystalliser
Equilibrium Solubility and
Solubility Curves
Material Balances in Crystallisation
• Calculation using equilibrium solubility or
solubility curves
• Yield of Crystallisation Process
Enthalpy-Composition Diagrams
Example 1 (Geankoplis, p. 819)
• A salt solution weighing 10,000 kg with 30
wt% Na2CO3 is cooled to 293 K (20 oC). The
salt crystallises as the decahydrate. What will
be the yield of Na2CO3.10H2O crystals if the
solubility is 21.5 kg anhydrous Na2CO3/100 kg
of total water? Do this for the following cases:
(a) Assume that no water is evaporated.
(b) Assume that 3% of the total weight of the
solution is lost by evaporation of water in
cooling.
Heat Balances in Crystallisation
• Heat evolved and removed from
the system
Example 2 (Geankoplis, p. 821)
A feed solution of 2,268 kg at 327.6 K (54.4 oC) containing
48.2 kg MgSO4/100 kg total water is cooled to 293.2 K (20
oC), where MgSO .7H O crystals are removed. Calculate
4
2
the yield of crystals and make a heat balance to
determine the heat evolved and removed from the
crystalliser, q, assuming that no water is vaporised.
(Information : The solubility of the salt is 35.5 kg Mg
MgSO4/100 kg total water. The average heat capacity of
the feed solution can be assumed as 2.93 kJ/kg.K. The
heat of solution (endothermic) at 291.2 K (18 oC) is
13.31x103 kJ/kg mol MgSO4.7H2O.)
Example 3 (Badger & Banchero, p. 534)
• A crystalliser is to be used to produce 1 ton/hr of copperas
(FeSO4.7H2O) crystals by the cooling of a saturated solution
entering at 120 oF. The slurry leaving the crystalliser will be
at 80 oF. Cooling water enters the crystalliser jacket at 60 oF
and leaves at 70 oF. It may be assumed that the over-all
coefficient of heat transfer for the crystalliser is 35
Btu/(hr)(ft2)(oF). Each 10-ft section of the crystalliser has 35
ft2. Average specific heat of initial solution = 0.70
Btu/(lb)(oF). Heat of solution of copperas at 18 oC = 4400
cal/gmol (endothermic).
– (a) Calculate the yield
– (b) Estimate the cooling water required in gpm
– (c) Determine the number of crystalliser sections to be used
Heat Balances in Crystallisation
• Calculation using enthalpy-composition
diagrams
Example 4
• Solve the problem in the Example 2 using
enthalpy-composition diagram.
Problems
• 1. A solution consisting of 30% MgSO4 and 70% H2O is cooled to 60 oF.
During cooling, 5% of the total water in the system evaporates. How many
kg of crystals are obtained per kg original mixture?
• 2. A 32.5% solution of MgSO4 at 120 oF is cooled, without appreciable
evaporation, to 70 oC in a batch water-cooled crystalliser. How much heat
must be removed from the solution per ton of crystals?
• 3. One pound each of Na2SO4 and H2O at 50 oF are mixed and allowed to
reach equilibrium at atmospheric pressure. If the system is perfectly
insulated so that equilibrium is reached with no gain or loss of enthalpy,
what will be the temperature and phase condition of the product?
• 4. A single-stage, continuous, Krystal crystalliser is to be used to obtain
CaCl2.4H2O product from a feed containing 40 wt% CaCl2 in water at 180
oF. The vacuum system on the crystalliser will give an equilibrium magma
at 90 oF. (a) What range of heat input per pound of feed solution can be
used to obtain a product containing only CaCl2.4H2O crystals?, (b) What
would be the maximum yield of crystal in the product (pounds of CaCl2 as
crystal per pound of CaCl2 in feed).
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