Normal butane/iso-butane separation

Claim 1

An improved pressure swing adsorption process for the separation of iso-butane from normal butane in an adsorption system having at least three adsorbent beds, each bed of which undergoes, on a cyclic basis, a processing sequence comprising: (a) introducing a feed gas mixture of iso-butane and normal butane at an upper adsorption pressure to the feed end of the bed capable of selectively adsorbing normal butane as the more selectivity adsorbable component of the gas mixture, while iso-butane as the less readily adsorbable component passes through the bed and is discharged from the discharge end thereof, such feed gas introduction being continued as a normal butane adsorption front is formed in the bed and passes through the bed from the feed end thereof and breaks through at the discharge end of the bed, a portion of the iso-butane effluent stream thus discharged being diverted for passage as purge gas to another bed in the system; (b) countercurrently depressurizing said bed with release of gas from the feed end thereof, thereby depressurizing said bed to an intermediate pressure, said released gas being passed to another bed initially at a lower pressure for pressure equalization at said intermediate pressure; (c) further countercurrently depressurizing said bed with release of additional amounts of gas from the feed end thereof, thereby depressurizing said bed to a lower desorption pressure, the gas released being recovered as normal butane product gas; (d) countercurrently purging said bed by the addition of iso-butane purge gas to the discharge end thereof, thereby purging residual normal butane present in the void spaces of the bed to enhance the recovery of normal butane product gas and the purity thereof; e) partially repressurizing said bed by the passage of countercurrent depressurization effluent gas from a bed in the system initially at a higher pressure for pressure equalization therebetween at said intermediate pressure level; and (f) further repressurizing said bed from the intermediate pressure to said upper adsorption pressure by the addition of feed gas thereto, whereby enhanced bed utilization and productivity are achieved at advantageous adsorbent and captial equipment requirements, and advantageous operating expenses, for the desired gas separation operation.