• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.2
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• pp.129-135
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• 2011

An ice-making model has been developed and analyzed in this study. The effects of the following on the ice formation on the outer surface of a tube in which a refrigerant flows and boils are numerically investigated: thermal resistance of the refrigerant and thermal resistance of the ice formed on the outer surface of the tube. The ice thickness and related variables are analyzed in the case of the refrigerants R22 and R134a by using the expressions for phase-change heat transfer and boiling heat transfer coefficient. Vapor qualities of the refrigerants range from 0 to 0.8. As a result, up to the first 30 min, the internal convection resistance is higher than the thermal resistance of the ice on the external surface of the tube. However, after about 30 min, the thermal resistance of the ice increases remarkably due to the increase in the ice thickness. Thus, the heat flux to the refrigerant decreases, and further, the refrigerant quality and the boiling heat transfer coefficient also decrease. As the heat transfer coefficient of R22 is higher than that of R134a, the mass of the ice formed when R22 is used is higher than that formed when R134a is used.

• Journal of the Korean Chemical Society
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• v.33 no.1
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• pp.135-142
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• 1989

Methyl tert-butyl ether(MTBE) was synthesized from vapor phase reaction of methanol with iso-butylene over HZSM-5 catalysts, and effects of SiO$_2/Al_2O_3$ ratio in the HZSM-5 catalysts and reaction conditions on products distribution have been examined. Acid strength and acid type of each catalyst with different SiO$_2/Al_2O_3$ ratio were measured using pyridine adsorption followed by temperature programmed desorption(TPD) and IR analysis. Reactants and products adsorption characteristics on different acid sites have also been examined. As the SiO$_2/Al_2O_3$ ratio of HZSM-5 catalyst was increased, selectivity to MTBE was improved as a result of decrease in dimethylether(DME) formation at the strong acid sites. Conversion and selectivity to MTBE were also greatly enhanced as $i-C_4H_8/CH_3OH$ reactant ratio was increased, and overall about 80$^{\circ}$C was adequate for the MTBE synthesis. The properties of deposited coke on spent catalysts were examined by TG, DTA and IR spectrum analysis, indicating the amount of the coke deposit in the order of HY > H-Mordenite > HZSM-5. Even if the coke deposited on H-Mordenite was little more in amount than to that on HZSM-5, the former deactivated quickly due to its non-interconnected channel structure. For HY, owing to its lange pore size, significant $i-C_4H_8$ polymerization was occured, and rapid deactivation and severe coke formation has resulted within few hours.