Carbon molecular sieves for hydrocarbon separations by adsorption

The use of vacuum pressure swing adsorption (VSA-PSA) with carbon molecular sieves (CMS) as selective sorbents is evaluated as an alternative technology for methane-nitrogen and propane-propylene separations. The larger molecules, methane and propane, are very low-diffusing species, resulting in processes where nitrogen and propylene are retained in the bed. For the methane-nitrogen separation, a Skarstrom cycle (pressurization, feed, blowdown, and purge) was used with the advantage of recovering methane in the feed step with a low-pressure drop. At ambient temperature, from a mixture with 20% of nitrogen balanced by methane, a purity > 93% was obtained. For the propane-propylene mixture, a five-step cycle was used-pressurization, feed, rinse, intermediate depressurization, and countercurrent blowdown-where purified propylene is obtained as a low-pressure product. Starting with an equimolar mixture at 373 K, the purity of propylene was 83% with a product recovery of 84%.