A separation process that involves the use of a semi-permeable membrane that permits the diffusion of one or more selective components to pass. The components able to pass through the membrane are known as the permeate, while the components unable to pass are known as the retentate. The membrane is highly porous and the size of the pores can be controlled during its manufacture using an organic polymer evaporating solvent. For example, the pore size selectivity can be used to control the separation of nitrogen molecules from air. The membranes are manufactured as sheets or as hollow fibres and packed into bundles called modules.