Low cost oxygen production by pressure swing adsorption with oxygen selective adsorption

© 2015 Dr. Willie Ngee Hon TangOxygen and nitrogen represent two of the four most produced chemicals globally. Adsorption-based processes offer a more energy friendly alternative to cryogenic air separation process in smaller-scale applications. The use of oxygen selective adsorbents in these processes over traditional nitrogen selective adsorbents can provide better separation performance as atmospheric air contains four times more nitrogen than oxygen. Unfortunately, oxygen selective adsorbents are currently limited by low selectivity, capacity and stability. Therefore there is a motivation to develop oxygen selective adsorbents and evaluate their use for air separation. The main purpose of this project is to develop oxygen selective adsorbents and the advantages of utilising these adsorbents over conventional nitrogen selective adsorbents. This study covers materials development, followed by process design and simulations. Post synthesis modification of zeolites by impregnation of AgBr and CoCl2 onto ZSM-5 proved successful in reversing the O2/N2 selectivity of the zeolite from 0.95 to 1.18. On the other hand, deposition of tetraethyl orthosilicate (TEOS) on Chabazite improved the O2/N2 selectivity from 0.54 to 0.62, but did not reverse the selectivity. Zeolites can be altered to be oxygen selective, but may not have sufficient oxygen selectivity and capacity for air separation applications. A series of porphyrin-based porous organic polymers (POPs) were synthesized and characterised, including Fe-POPs and Co-POPs. The POPs were shown to be oxygen selective, with an O2/N2 selectivity up to 1.3 at 273.15 K. A molecular sieving effect was observed at 91 K as nitrogen was excluded and the O2/N2 selectivity increased to 14.7. The high oxygen loading (13 wt% at 1 atm) and high selectivity suggests the potential of these materials for use in hybrid adsorption-cryogenic air separation systems. The POPs also showed good reversibility, in contrast to most oxygen selective adsorbents. 6 VSA cycles were designed and simulated to evaluate the separation performance of the oxygen selective adsorbents in a VSA process for oxygen and nitrogen production. Simulation results of the as-made POPs at cryogenic temperatures suggest that 90% oxygen and 99.99% nitrogen can be achieved. However, the cooling load required was very significant. Adsorption performance at ambient temperatures is more important. The oxygen selectivity of the POPs studied was found to be linearly proportional to its cobalt loading. Based on this, we introduced an ideal, hypothetical POP with increased cobalt loading. This material can achieve an oxygen capacity of 1.36 mmol/g with an O2/N2 selectivity of 13.6 at 273.15 K. Simulations using this ideal POP indicated that 90-95% purity oxygen and 99.99% purity nitrogen can be easily achieved with significant improvements on the performance of current practices. The challenge is then to further develop these oxygen selective adsorbents for large scale applications