Gas separation

A gas separation apparatus comprises a hollow fibre separation membrane arrangement for separating a feed gas into a permeate gas fraction and a retentate gas fraction. The hollow fibre separation membrane arrangement has an inlet 42 for receiving the feed gas, a permeate passage 50 for passing the permeate gas fraction, and a retentate passage for passing a retentate gas fraction. Also provided is a pump arrangement for compressing the feed gas and a pump arrangement for decompressing the permeate gas. The pump arrangements are commonly driven. Preferably, a double acting pump comprising a double acting piston in a double acting cylinder defines two chambers wherein each of the two pump chambers provides one of the said pump arrangements. Preferably the feed gas is air, the permeate is an oxygen (O2) enriched air fraction and the retentate is a nitrogen (N2) enriched air fraction.