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OGILVIE, Jeremy James E. 5ChEB Introduction Crystallization is a heat and mass transfer process that includes the solidification of materials in slurry where the particles arrange themselves to form regular geometric patterns called crystals. In most food and pharmaceutical companies, crystallizers have been widely used to produce their products to be sold in the market that can be used in daily households and in hospitals. This process has been helpful in the chemical industry because of its method of purification and in providing crystallized materials in a desired size range. Compared to distillation, crystallization can save more power when trying to retrieve solid materials. The crystallization process comes in three steps. Some solids have this property to dissolve in a solvent at different temperatures called solubility. Once a solution is saturated, the solute can no longer be dissolved in the solvent therefore the solute remains visible on the solution making the solution supersaturated. This is one step in crystallization, to induce supersaturation. This process can be carried out by either heating the solution (to vaporize the solvent), cooling the solution (solubility decreases) or by salting out (adding another substance that may induce crystallization). After achieving supersaturation, nucleation will take place. During the random motion of the solute’s atoms or molecules they form aggregates called clusters. These clusters would further form lattices called embryo and when the embryos combine the nuclei crystals are formed. And one the nuclei crystals are formed crystal growth begins. Different types of crystallizers have developed through the years with different shapes and sizes and different ways of inducing supersaturation like an Agitated Batch Crystallizer, Krystal Crystallizer, and Vacuum Crystallizer. But in this paper a scraped surface crystallizer will be discussed specifically a Swenson-Walker Crystallizer and on how this certain equipment will be designed. In 1920, Swenson Co. conceived a continuous, semi cylindrical, horizontal, hollow trough crystallizer that cools the slurry to induce supersaturation. The refrigerating fluid passes counter currently with the feed through a jacket or a double wall. The heat transfer wall is scraped or agitated such that the deposits cannot build up. At the end of the crystallizer, the products leave a drain where the mother liquor is then centrifuged in order to obtain the crystals. Typically this equipment is used in processing inorganic salts like sodium phosphate which has a high solubility with water. Problem Statement A solution containing 23 mass % of sodium phosphate is cooled from 313 to 298 K in a SwensonWalker crystallizer to form crystals of Na3PO4.12H2O. The company needs to produce 300 kg of crystals in an hour. The mean heat capacity of the solution is 3.2 kJ/kg deg K and the heat of crystallization is 146.5 kJ/kg. If cooling water enters and leaves at 288 and 293 K, respectively. Given the following data design a Swenson Walker Crystallizer. In this process an overall material balance, solute balance and energy balance, were used in order to acquire the other data needed in order to get the dimensions of the equipment. Overall Material Balance F= L + C Solute Balance Fxf = Lxl + Cxc Heat Balance q= wCp(t2-t1) = FCpf(tf-tl) +CLc = UAΔTlm Given and solving the following data we obtained: Feed Properties F= 870.872 kg/hr xf=0.237 Cpf=3200 J/kgK tf= 313K Crystals and Liquor Properties C= 300 kg/hr xc=0.432 L=570.872 xl=0.134 Lc=14600 J/kg tc=tl=298K Cooling Water Properties t1=288K t2=293K Cp=4184 J/kgK U=350 W/m2K w=4100 kg/hr Dimensions of Swenson Walker Crystallizer A= 4.717 m2 L= 3 m r=0.25 m Data like xf, xc, and xl were obtained using Table 2-122 from Perry’s Chemical Engineering Handbook. The overall heat transfer coefficient for a jacketed carbon and stainless steel vessel was obtained from an internet source. There were no rules of thumb available neither for the type of agitator needed nor its diameter. So the a helical ribbon agitator was used since this was the common agitator used in a Swenson Walker Crystallizer and it was stated that the agitator’s diameter should not touch the walls of the equipment so a diameter of 0.2 meters was used. In this equipment swaged and welded carbon steel would be used. Carbon steel can withstand temperature until 900 °F. The design temperature for this process is only 154°F. The thickness of the vessel would be 1.4mm according to the American Society of Mechanical Engineers Boilers and Pressure Vessels Code Sec. VIII D.1 and a corrosion allowance of 4mm since the fluid being processed is an alkali solution which can cause corrosion and a 2mm allowance for the double wall side since only water was being used. Rendered 3D Diagram Equipment Specification Sheet Swenson Walker Crystallizer Specification Sheet Service of Unit: Trisodium Phosphate Crystals Size Type Horizontal Performance of Unit Fluid Allocation Fluid Name Fluid Quantity(kg/hr) Temperature (K) in out Specific Heat (J/kgK) Latent Heat (J/kg) Double Wall Shell Side Side trisodium phoshpate slurry water ~870 ~4100 313 298 3200 146500 Heat Exchanged (J/hr) Construction of Shell Design Temperature (Max/Min F) Wall thickness (mm) Corrosion Allowance (mm) Length (m) Inside Diameter Supports Saddles 288 293 4184 85750 J/hr 154/79 1.5 4 3 0.5 1.5 2 3 0.54 Conclusion and Recommendation A Swenson walker crystallizer producing trisodium phosphate crystals was designed using all the knowledge gained from heat and mass transfer and equipment design. I recommend anyone to try designing Swenson Walker Crystallizers in series or overlapping equipment and compare which of the two arrangements would give a better yield. Also to show the effects of using an insulating material like 85% magnesia or asbestos and diatomaceous earth comparing the results or yield with an equipment without insulation. And lastly to have a further research on designing a helical ribbon agitator like maximum diameter, number of ribbons, and position of the agitator in order to achieve a better yield. References: McCabe & Smith (2000). Unit Operations of Chemical Engineering' McGraw-Hill, New York Perry, R. H., & Green, D. W. (2008). Perry's Chemical Engineers' Handbook. New York: McGraw-Hill Anantharam, N., & Begum, K.M. (2011). Mass Transfer: Theory and Practice. New Delhi: PHI Learning Private Limited http://www.pharmainfo.net/sudha/blog/crystallization http://faculty.ksu.edu.sa/13042/Documents/crystalization%208-9.pdf http://www.uobabylon.edu.iq/uobColeges/ad_downloads/4_13137_558.pdf http://www.thermopedia.com/content/547/ Appendix Feed Properties tf 313 K xf 31 0.237 131 J Cpf 3200 kg K Crystals and Liquor Properties xl 15.5 115.5 0.134 164 xc tl 298 K 164 12( 18) J Lc 146500 kg 0.432 kg C 300 hr Cooling Water Properties t1 288 t2 293 K Cp 4184 K J kg K W U 350 2 mK Given FL 1 1 xf xl C M F xf L xl C xc F 870.872 870.872 570.872 lsolve( M N ) L 570.872 w F Cpf ( tf tl) C Lc Cp ( t2 t1) Kg 3 4.099 10 hr J 7 q F Cpf ( tf tl) C Lc 8.575 10 Tlm A ( tf t2) ( tl t1) tf t2 ln tl t1 q U Tlm 3600 hr 14.427 4.717 2 m For common Swenson Walker Crystallizers: L 3 m r A 2 L 0.25 m C C xc N Tables Used Table 2-122