On the Effect of Grain Size on the Kinetics of Water Vapor Adsorption and Desorption into/from Loose Pellets of FAM-Z02 under a Typical Operating Condition of Adsorption Heat Pumps

Abstract

The kinetics of water vapor adsorption/desorption under quasi isobaric conditions have been studied on one layer of loose pellets of FAM-Z02 placed on a metal plate. Temperature of the sample holding plate has been changed as it takes place in real adsorption heat pumps (AHPs), while the vapor pressure over the adsorbent was maintained almost constant (saturation pressures corresponding to an evaporator temperature of 5°C and a condenser temperature of 35°C). Adsorption- and desorption-end temperatures have been adjusted to 35 and 90°C, respectively. Measurements have been carried out on grains having the following size distributions: 0.7 to 1.0 mm, 1.4 to 1.6 mm and 2.0 to 2.6 mm. An average equilibrium differential water loading of 19.2 g/100 g has been measured during desorption compared to 16.4 g/100 g during adsorption. The adsorption/desorption rates have been characterized by the times required to reach 50, 80 and 90% of the equilibrium loading. It has been found that the adsorption kinetics are mainly dominated by inter-crystalline diffusion, while the desorption kinetics are mostly influenced by surface resistance to the heat and mass transfer. Average specific evaporator cooling/condenser heating capacities in the order of 1 kW/kg have been estimated for the grain size 0.7 to 1.0 mm, putting in evidence the possibility of developing highly compacted AHPs with FAM-Z02 for heating and cooling applications. The influencing parameters on the combined heat and mass transfer process have been discussed.