Heavy hydrocarbon recovery from pressure swing adsorption unit tail gas

Abstract

A pressure swing adsorption process for the separation of impurities such as nitrogen and carbon dioxide and recovery of hydrocarbons from a natural gas stream utilizes two separate adsorption systems, the first containing an adsorbent selective for nitrogen, carbon dioxide or both and the second containing a hydrocarbon-selective adsorbent. In the process, the natural gas stream is passed through a first adsorbent to form a product stream enriched with methane and to adsorb nitrogen and/or carbon dioxide and which further co-adsorbs at least a portion of the hydrocarbons contained in the feed stream. The hydrocarbons are recovered by passing a low pressure waste stream from the first pressure swing adsorption stage which contains co-adsorbed nitrogen and/or carbon dioxide and hydrocarbons and directing the waste stream to the second pressure swing adsorption stage to adsorb the hydrocarbons and produce a product stream enriched in nitrogen and/or carbon dioxide. The hydrocarbons are recovered from the hydrocarbon-selective adsorbent by an intermediate pressure methane-containing stream from the first pressure swing adsorption stage which purges the adsorbent in the second stage and forms a combined stream comprising methane and C hydrocarbons. The C hydrocarbons can be separated from the methane such as by compression with flash separation or refrigeration.