Separation of CH4/CO2/N2 mixtures by layered pressure swing adsorption for upgrade of natural gas

A novel compact adsorption-based process for removal of carbon dioxide and nitrogen from low and medium natural gas flowrates is discussed. The layered pressure swing adsorption (LPSA) process studied is composed of a zeolite 13X to selectively remove carbon dioxide followed by a layer of carbon molecular sieve 3K to make the separation of nitrogen from methane. The advantage of the process is the removal of two different contaminants in the feed step, delivering methane at high pressure without recompression requirements. A four-step cycle was studied comprising countercurrent pressurization, feed, countercurrent blowdown and countercurrent purge with product. The blowdown step was performed in vacuum to remove carbon dioxide from zeolite 13X. Experiments were performed in a single-column LPSA unit at different temperatures and using different ratios of adsorbent layers to study the effects of these parameters in overall performance of the unit. Feeding a mixture of 60% CH4/20% CO2/20% N-2, methane purity of 86.0% with 52.6% recovery was obtained at ambient temperature while 88.8% purity with 66.2% recovery was obtained at 323 K. At both temperatures there was a ratio of adsorbent layers where purity reaches a maximum, while product recovery always decreases for larger zeolite 13X layers. (c) 2006 Elsevier Ltd. All rights reserved.