Vacuum pressure swing adsorption (VPSA) process with vacuum exhaust step for concentrating ventilation air methane (VAM) was studied experimentally. The effect of vacuum exhaust ratio on methane concentration and recovery are investigated. The results shows that vacuum exhaust step can increase the methane concentration of product gas without increasing pressure. As the ratio increases, the concentrations of methane in both product and effluent firstly increase, and then reach steady state after a certain vacuum exhaust ratio. The process of vacuum exhaust gas recovered can increase methane recovery and concentration, but it will decrease the methane concentration of product when the vacuum exhaust ratio is lower than 1.0. The results could provide reference for the industrial application in VAM enrichment.
Based on pressure swing adsorption, Proportion Pressure Swing Adsorption (PPSA), a new enrichment method with high safety, was suggested to enrich the low concentration coal bed gas. A mixture of active carbon (CA) and carbon molecular sieve (CMS) was employed as adsorbent in the process of PPSA. The methane and oxygen in coal bed gas were adsorbed simultaneously. The experimental study was carried out in order to verify the feasibility of this method. The equilibrium adsorption characteristic of CA and CMS were tested. The results showed that both concentrations of CH4 and O2 in desorption gas could be controlled well and would not exceed the explosion limit in enrichment process by adjusting the mass ratio of CMS5/AC1. Therefore, the new method, PPSA, can guarantee the safety in enrichment of coal bed methane.
Enrichment of ventilation air methane(VAM) is crucial to gather acceptable concentration to utilize. In this work, the adsorption characteristics of activated carbon have been tested. A novel three-bed VPSA cycle for recovering methane from methane/nitrogen mixtures using activated carbon have been done experimentally, and the VPSA process studied contains an effluent pressurization step to improve product concentration and recovery. The VPSA process was carried out under adsorption pressure at 155 kPa abs., and desorption pressure at 20 kPa abs., feeding VAM with the methane purity of 0.2%, methane purity of 0.4% with 70% recovery was obtained at ambient temperature while 0.36% purity with 87% recovery. This process has the potential of significantly increasing the product concentration and recovery with respect to the two-bed cycle process by VPSA.
Conventional PSA methods with a single adsorbent can result in an explosive CH4 concentration of 5 to 15%, making low concentration coal mine methane (LCCMM) or risky in terms of safety at a concentration of 30%. Proportion Pressure Swing Adsorption (PPSA) is a new and safer enrichment method suggested for LCCMM enrichment that uses a mixture of active carbon (AC) and carbon molecular sieves (CMS) as adsorbents. The effect of adsorption Pressure, adsorption time and bed length on the PPSA process were investigated to verify the feasibility of this method. The experiment results show that the methane concentration in desorption gas and the oxygen concentration in exhaust gas increased and decreased, respectively, with an increase in adsorption pressure or bed length. And the methane concentration in desorption gas is increased with the adsorption time expanding, but the recovery of methane is reduced. So the methane concentration in desorption gas can be increased to more than 30% and the oxygen concentration in exhaust gas be decrease to less than 12% by increasing the adsorption pressure or lengthening the bed.
Nine types of active carbon’s adsorption isotherm of nitrogen and methane at 298K, 308K and 318K were measured, and their selectivity factor and PSA sorption selection parameters were calculated. The results show the nine active carbons have desirable selective adsorption characters on methane to nitrogen. AC3 has the highest selectivity factor. It reaches to 5.11 at 298K. And AC3 is the optimal adsorbent in 9 different adsorbents. Its PSA sorption selection parameter is higher than others with the methane concentration of 0.2% at the operating pressure between 50kPa to 150kPa abs. In above conditions the higher the selectivity factor is, the higher the PSA sorption selection parameter will be.
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