Effect of non-condensable gases on the adsorption properties of adsorbent porous media

Bayman, Orhan Ata
Non-condensable gases such as air can penetrate into adsorption machines that utilize adsorbates which work at negative gage pressures. As many adsorption cooling/heat pump applications involve such negative gage pressures, it becomes important to research the effects of such non-condensable gases on the adsorption properties. In this thesis, prescribed amounts of air have been injected into an experimental apparatus containing an adsorbent bed and an evaporator/condenser to observe the effects of air on the adsorption and desorption properties of natural zeolite-water, Zeolite 13X-water and silica gel-water adsorbent-adsorbate pairs, respectively. These different amounts of air have been tested at various evaporater/condenser and adsorbent temperatures. Adsorption and desorption processes were started by lowering and raising the adsorbent temperature to certain temperatures while at equilibrium with evaporator/condenser, respectively. Temperature of evaporator/condenser was held constant during that process. Comparisons are made between experiments with different air amounts, evaporator/condenser temperatures and desorption-adsorption working temperatures. Adsorption and desorption capacities and their reductions from no-air experiments, changes in gas (vapor and air) pressure, gas temperature and average adsorbent temperature over time are tabulated. Significant reductions in both final adsorption and desorption capacities have been observed with increasing amounts of air for all adsorbents, which will affect the cooling and heating capacity of an adsorption refrigerator/heat pump. Both adsorption and desorption capacities reduced less at higher evaporator/condenser temperatures for the same amount of air. Decay in adsorption capacity has been found to be logarithmic for zeolites and exponential for silica gel. Experiment with lower difference between desorption-adsorption temperatures has been found to be more affected than a higher one for the same amount of air. Desorption capacities have been found to be more affected by the presence of air. Hysteresis also increased with increasing amounts of air. It is concluded that precautions should be taken very seriously for avoiding penetration of air into an adsorption machine as only a small amount of residual air (0.3-0.9% of atmospheric pressure) can drastically reduce the adsorption and desorption performance.
Citation Formats
O. A. Bayman, “Effect of non-condensable gases on the adsorption properties of adsorbent porous media,” M.S. - Master of Science, Middle East Technical University, 2014.