The invention is directed to methods for preparing a planar membrane module, a planar membrane module obtainable by said methods, methods for preparing a heat exchanging module, and a heat exchanging module obtainable by said methods. In one aspect the method of the invention comprises:—providing a solid state planar membrane;—injection molding a potting material into one or more potting frames for supporting said membrane;—assembling, preferably by hot pressing, the one or more potting frames with the solid state planar membrane, thereby forming a membrane frame;—optionally providing a spacer at least at one side of the planar membrane; and—joining a stack of said membrane frames in a gastight manner to form a planar membrane module, said joining comprising welding of potting material.