Product recovery in pressure swing adsorption process and system

Claim 1

In a pressure swing adsorption process for the selective adsorption of at least one component of a feed gas mixture in an adsorption system having at least four adsorbent beds, each of which undergoes, on a cyclic basis, higher pressure adsorption, cocurrent depressurization from said higher adsorption pressure to upper intermediate pressure with release of void space gas from the product end of the bed for passage directly to another bed initially at lower pressure for pressure equalization therebetween at said upper intermediate pressure, further cocurrent depressurization from said upper intermediate pressure to lower intermediate pressure with release of void space gas from the product end of the bed for passage directly to another bed to provide purge gas to said other bed for the purge thereof at its lower desorption pressure, countercurrent depressurization from said lower intermediate pressure to said lower desorption pressure, purge at said lower desorption pressure with void space gas being passed directly from another bed being cocurrently depressurized from upper to lower intermediate pressure to said bed as purge gas for said purge at lower desorption pressure, partial repressurization from said lower desorption pressure to said upper intermediate pressure with the passage of void space gas from the product end of another bed initially at higher pressure for pressure equalization therebetween at said upper intermediate pressure, and final repressurization from upper intermediate pressure to said higher adsorption pressure, in which a portion of the void space gas released from one bed is passed to a bed(s) initially at a lower pressure to equalize the pressure therebetween and another portion thereof is used to provide purge gas for a bed undergoing said purge the improvement consisting essentially of: (a) simultaneously with the passage of released void space gas from the product end of the bed directly to another bed to provide purge gas thereto upon further cocurrent depressurization of said bed from upper intermediate pressure to lower intermediate pressure introducing a portion of said released void space gas into an external purge drum; and (b) passing said void space gas introduced into said external drum during step (a) above from said external drum and introducing said void space gas into a bed to be purged at lower desorption pressure for the purging thereof at said lower desorption pressure, in addition to purge gas introduced to said bed directly from another bed, prior to partial repressurization thereof by pressure equalization and final repressurization to higher adsorption pressure, the total purge time in each bed being significantly greater than said step (a) time, whereby the relatively short provide-purge time/long purge time enhances product recovery in each bed and in the overall adsorption system.