Membrane devices are used to selectively separate at least one fluid component from a mixture of fluids. Membrane devices are used in a wide variety of separation applications including reverse osmosis and gas separation. Particular gas separations of interest include the recovery of an enriched oxygen stream from air for use in enhanced combustion processes. Alternately, an enriched nitrogen stream may be obtained from air for use as an inert atmosphere over flammable fluids or for food storage. In other embodiments, nitrogen, hydrogen, helium, carbon dioxide or other organic or inorganic gases may be separated from hydrocarbons.
Different membrane device configurations suitable for fluid separations are described in the art, including plate and frame, tubular, spiral wound, and hollow fiber configurations. The hollow fiber configuration is generally preferred because a higher surface area per unit volume of device can be obtained, resulting in increased device productivity compared to other configurations.
It is known to utilize a bundle of hollow fiber membranes in which the fibers are substantially parallel and the bundles are positioned in a vessel, shell or housing for separating one fluid from a mixture of fluids by allowing the one fluid to permeate from the mixture through the walls of the hollow fiber membranes to the bores thereof, or vice versa. The hollow fiber membranes are normally made from a polymeric material. The housing is typically cylindrical.
Hollow fiber membrane separation devices are typically fabricated by parallel or bias wrapping a plurality of hollow fibers about a core. The hollow fibers are embedded in at least one tubesheet and the assembly is inserted into the housing. The tubesheet(s) sealingly engages along the inside surface of the housing so that two fluid regions are defined, commonly referred to as the shellside and the tubeside regions. The shellside region lies on the outside of the hollow fibers and is defined by the inside wall of the housing and the tubesheets which sealingly engage along the inside wall of the housing. The tubeside region lies on the inside of the hollow fibers and the housing cap. Communication between the two regions is accomplished by selective permeation of a component(s) through the membrane.
To separate a fluid mixture, such as a gas mixture, into two portions, one richer and one leaner in at least one component, the mixture is brought into contact with one side of the semipermeable hollow fiber membrane through which at least one of the gaseous components selectively permeates. A gaseous component which selectively permeates through the membrane passes through the membrane more rapidly than the other component(s) of the mixture. The gas mixture is thereby separated into a stream which is enriched in the selectively permeating component(s) and a stream which is depleted in the selectively permeating component(s). The stream which is depleted in the selectively permeating component(s) is enriched in the relatively nonpermeating component(s). A relatively nonpermeating component permeates more slowly through the membrane than the other component(s). An appropriate membrane material is chosen for the mixture at hand so that some degree of separation of the fluid mixture can be achieved.
These separation devices use the rigid housing to hold and support the bundle of hollow fiber membranes, provide a rigid assembly, provide a rigid surface to sealingly engage the tubesheet(s) and/or end caps, contain the pressurized fluid passing along the membrane outer surfaces, define the shellside region and protect the bundle of hollow fiber membranes from handling damage. Such a rigid housing may be made of a metal, plastic or composite tube, which for the particular size required may have to be machined to fit for sealing with the tubesheets. It is also possible to have a so-called "shell-less" hollow fiber permeator such as described in U.S. Pat. No. 4,871,379.
For bore-fed hollow fiber permeators, however, the fluid pressure in the shellside region is very low preferably atmospheric, so the housing need not provide the function of containing pressurized fluids.
The rigid housing also presents a problem of inserting the fibers without damage and at the same time providing a tight uniform packing within the housing to ensure efficient fluid operation. Hollow fiber membranes which are tightly packed in the permeation apparatus exhibit higher selectivity to the components in a fluid stream. This tight fit is especially difficult as different bundles do not have predictable outer dimensions. Several solutions to this packing problem have been suggested.
U.S. Pat. No. 4,361,481 describes an assembly of a sheet wrapped tightly around the bundle before inserting the assembly into the housing, and leaving the sheet in place or removing it after assembly.
U.S. Pat. No. 4,315,819 describes means for axially compacting the fiber bundles within the housing to thereby expand the bundle radially to contact the housing walls.
The devices described in the preceding paragraphs take advantage of the inherently high strength of small, thin-walled, hollow polymeric fibers. They also take advantage of the large surface area per unit volume available which results from the fact that theoretically the entire circumferential outer surface of each fiber is available for exposure to the feed fluid to be separated. However, several problems develop from such devices.
Prior art devices use a costly rigid housing extending the entire length of the permeator. The housing also adds significant weight to the device. Other low-cost permeators (U.S. Pat. No. 4,871,379) do not have individual housings at all, but instead are grouped together in a sheet-metal enclosure for collection of the permeate. They are lightweight and low cost but are not practical as stand-alone permeators. Prior art devices may also use a separate means to compact the bundle of hollow fiber membranes and to achieve countercurrent flow. Moreover, certain separation devices which have a housing that encases a high-pressure section of the permeator inlet must undergo pressure vessel testing. Results of the pressure test must be submitted to regulatory agencies. If the high-pressure inlet section of the separation device is separate from the housing, but exceeds a certain volume, testing and submission of results may also be required. Such testing and reporting adds to the cost of the unit. Therefore, there is a need for a hollow fiber separation device which provides a large capacity, but which is also low cost, lightweight, and able to be used as a stand-alone separation device, and which may use the best practice of countercurrent flow to improve efficiency.
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