Measurement of water vapour diffusion coefficient in the packed bed of zeolite particles for the adsorption heat pump.

Waste heat utilization is one of key technologies to reduce the energy consumption and environmental impacts from the fossil fuel use. We studied adsorption heat pump systems designed to recover the exhaust heat from industrial plants in the intermediate temperature range (80-120 degree C), because the adsorption heat pump system can utilize this waste heat. The adsorption heat pump systems have not been widely used as air conditioning systems, because of its low volumetric power density of the adsorber. The volumetric power density of the adsorber is proportional to sorption rate, packing density and latent heat. In general, the sorption rate is determined by mass transfer resistance in single particle of an adsorbent and heat and mass transfer resistance in packed bed. In the present study, we focused on the mass transfer in packed bed, because the mass transfer resistance of the water vapour in packed bed is high due to low water vapour pressure. We developed an apparatus to measure the diffusion coefficient of the water vapour which comprises two vessels with pressure gauges in a thermostatic chamber with different pressures. The packed bed of the adsorbent was placed between the vessels, and the diffusion coefficient was identified from the change rates of the pressure of the vessels. In this paper, we investigated the effect of the pressure on the diffusion coefficient, and kozeny constant and hydraulic mean depth were discussed.