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Design and modeling of a novel rectifier with ceramic hollow fiber membrane contactor for miniaturized absorption cooling devices

Özkan, Onur
Miniaturized and mobile absorption cooling devices may be used in many applications, but effective and small size heat and mass exchangers are required. Ceramic hollow fiber membranes are thermally stable so that they can be used for the rectification of ammonia in an absorption cooling device. In this study the simultaneous heat and mass transfer between the ammonia-water vapor and the reflux liquid is modeled in a rectifier and stripper with hydrophobic ceramic hollow fiber membranes. A similar model is used to simulate the same process in another rectifier with random packing. The stagnant film model is used to calculate the heat and mass fluxes. The rectifier and the stripper are divided into segments and the equations for all segments are solved simultaneously. The model is verified by comparing the results with the literature. The effects of the packing density, effective porosity, pore radius and heat and mass transfer coefficients on the distillate concentration and the flow rate are discussed. The sensitivity of the concentration profiles to the grid refinement is shown. The hollow fiber membrane module results vi in a higher ammonia concentration and higher mass flow rate at the outlet compared to the random packing because it provides higher vapor-liquid interfacial area. Then, the hollow fiber membrane module model is integrated into the absorption cycle model. The integration of the new design of the stripping and rectifying sections increases the system COP from 0.44 to 0.50 at the design conditions. More importantly, the new design is able to purify the refrigerant at higher desorber heat input rate and absorber coolant temperature. Therefore, the absorption cycle may be operated at different conditions with a smaller performance loss compared to the old design. The effects of absorber coolant temperature and desorber heat input rate on the refrigerant concentration, refrigerant flow rate, system COP and cooling power are investigated. The COP trends for different operating conditions are explained in a relation with the refrigerant ammonia concentration and flow rate. The effects of the sizes and packing densities of stripping and rectifying sections are discussed in detail.