Semipermeable membranes in the form of hollow fibers have been used to separate components in fluid mixtures for many years. Hollow fibers, which can be spun from a wide variety of materials which have suitable membrane separation properties, provide high surface area for contact with the fluid feed mixture. Each component of the feed mixture coming in contact with the membrane material has a characteristic permeation rate that is a function of its ability to dissolve in and diffuse through the membrane material. This enables the separation of the faster permeating components from the slower permeating components. The driving force for the separation is the difference between each component's partial pressure across the inner and outer walls of the hollow fiber.
Large numbers of hollow fibers of similar length are generally grouped together in a bundle in a pressurizable shell or housing in which opposite ends of the fibers are potted and sealed in a material which serves to form a tubesheet at each end, similar in fashion to a shell and tube heat exchanger. One of the tubesheet's potting material is then cut to open the bores of the fibers and thus allow the faster permeating components to pass through this tubesheet into a permeate compartment. The volume within the shell which has access to the exteriors of the fibers (shell side) is effectively sealed by the tubesheets and other peripheral sealing devices from the volumes within the shell which permeate into the fiber bores and subsequently collect in the permeate compartment.
Such devices can be used to separate liquid mixtures or to separate vapors or gases from liquids, but have found particular utility in the separation of gases, such as in air separation to purify nitrogen.
A number of methods for making bundles of hollow fibers suitable for fabrication of membrane modules are taught in the patent literature. For example, U.S. Pat. No. 3,228,877, Mahon (1966), describes the concept of using hollow fibers in a gas separation apparatus. In a subsequent development, U.S. Pat. No. 3,422,008, McLain (1969) discloses a method to wind hollow fibers spirally around a cylinder core. This permits the winding of hollow fibers to form a bundle shape such that the bundle is in an annular form with narrow flow channels which improve fluid flow distribution on the shell side of the fibers. A method to make a coreless annular array of helically wound fibers is described in U.S. Pat. No. 4,045,851 to Ashare, et al. (1977), and another method for making coreless hollow fiber membrane bundles is described in U.S. Pat. No. 4,351,092, Sebring, et al. (1982) wherein the fibers are interlaced with one another in left-hand and right-hand helices at angles to the common axis of rotation of the rotary members which form the fiber bundle. The foregoing disclosures describe modules which are typical and in which the fluid mixture to be separated is exposed to substantially all of the fibers at one time. Modules have been developed, however, in which the fiber bundles are partitioned into sections in order to manipulate the flow distribution of the feed material or of the permeate.
The design of conventional countercurrent flow membrane separation modules wherein a shellside feed is introduced to a bundle of hollow fiber membranes has a deficiency with respect to the flow patterns that develop in the interior of the fiber bundle at both ends of the module. It is the object of the present invention to increase the efficiency of such modules by reducing this deficiency.