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 • Equipment for crystallisation • • • • • Systems with heat transfer (external cooling systems) Adiabatic systems (Vacuum 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 • Heat Balances in Crystallisation – Heat evolved and removed from the system – Calculation using enthalpy-composition diagrams • • • • 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).