Method for preparing composite membranes for enhanced gas separation

In the process of preparing a composite membrane capable of selectively permeating a more readily permeable component of a gas mixture in gas separation operations and having a separation layer that determines the separation characteristics of the membrane and a porous support layer, the improvement comprising:

• (a) coating the porous support layer with a wet separation layer of membrane material, said support layer containing a controlled amount of liquid in the range of from about 10% to about 90% by weight of the liquid present in said support layer in fully wet form, said liquid being a solvent or non-solvent for the material of the separation layer;
• and
• (b) drying said separation layer on the porous support layer, the presence of said liquid in the porous support layer precluding any appreciable penetration of the membrane material into the pores of said porous support layer, the separation layer thereby being of a non-occlusive nature with enhanced permeation characteristics, and having thickness of about 0.4 microns or less,
• whereby the composite membrane exhibits essentially the separation characteristics of the very thin, non-occlusive membrane layer while achieving enhanced permeation and an advantageous combination of selectivity and permeability for the desired gas separation operations.