The use of membranes to effect separation of gas/gas, liquid/liquid, and liquid/solid mixtures and solutions has achieved general industrial applicability by various methods, among them being ultrafiltration, reverse osmosis and dialysis. In general, membrane elements associated with these processes are contained in vessels called permeators, generally comprising a container or housing having various inlet and outlet ports and an assembly of membranes within said container. The internal configurations are so arranged as to permit the introduction of a feed stream with or without pressure on the upstream face of the membranes, means for collecting permeate which passes through the membranes and emerges on their downstream faces, and means for keeping feed and permeate materials from commingling.
The subject invention is concerned with hollow fibers assembled in modular form to provide the desired separation. Hollow fiber permselective separation devices are particularly useful for separating the components of gaseous mixtures. The invention is particularly useful for permeators in which the so-called shell side is at about atmospheric pressure.
Hollow fiber membrane devices are used for selective separation of at least one fluid component from a mixture of fluids or a solution. Such devices generally comprise a plurality of hollow fiber membranes, the membranes being selectively permeable to at least one component of the fluid mixture. The hollow fiber membranes are disposed inside a shell, housing or vessel. The housing has at least one inlet for bringing a fluid feed into contact with one surface of the hollow fiber membranes. A means to promote uniform distribution of the fluid feed is desirable. At least one outlet for discharge of the fluid which does not permeate through the membrane and at least one outlet for fluid which permeates through the membrane is also required. The hollow fibers are embedded in at least one tubesheet. The fiber bores communicate either with the permeate outlet or the fluid inlet. The tubesheet sealingly engages the inner surface of the housing. The vessel serves to support the pressure of one of the fluid streams and to isolate the fluid streams. The vessel and the ancillary hardware also assists in, maintaining the position of the separation device during operation. Multiple separation devices may be stacked or connected in series or parallel alignment.
The membrane material and form employed may be any suitable material capable of effecting the desired separation. For example, cellulose derivatives, polyamides, polyimides, polysulfones, and polystyrenes or blends thereof have found utility. Also, hollow fiber membranes may be composite, asymmetric or dense film.
The variety of membrane materials to effect various separations is abundant; however, the utility of many membranes is limited to narrow applications. For example, one membrane may be suitable for the separation of oxygen/nitrogen gas mixtures, but may be unsuitable for the separation of carbon dioxide/nitrogen gas mixtures. Likewise, one membrane may be suitable for desalination of brackish water but may be unsuitable for the desalination of sea water. The concentrations of the components in the fluid to be separated and the desired flow rates are also factors to be considered.
Because of the limited utility of many membranes, it is desirable to have a separation device which readily allows the exchange or replacement of the bundle of membranes.
It is also known that the operational properties of most membranes deteriorate over time. The deterioration of operational properties may be a result of degradation of the membrane material, damage to the membranes or fouling. Fouling of the membranes is a problem which requires immediate attention and often requires replacement of the separation device.
The problem of membrane fouling due to retention of suspended particles is especially prominent in conventional separation devices. Feed streams inevitably contain varying amounts of suspended particulate matter or biota. Although extreme precautions are generally taken to prefilter the feed stream or otherwise remove the suspended particles and biota prior to admitting the feed to the permeators, the conventional hollow fiber separation devices, ultimately become collectors for much of this material. Bundles, especially tightly packed hollow fiber bundles, develop occluded regions and lose effective membrane areas.
Another problem with the operation of hollow fiber membrane separation device arises from the lack of control of flow of the permeate and/or sweep fluid on the shell side of the separation device in order to enhance the performance of the device.
Permeation membranes are also known to break or burst, particularly at high operating temperatures and pressures. The separation of gases is most efficient at high pressures because the rates of permeation of gases through polymeric membranes are, in general, proportional to the differences in the pressures of the gases on the two sides of the membranes; such separations also are more rapid at higher temperatures. Operation at the highest practical pressures and temperatures is, therefore, important in the commercial use of such separation devices. The breakage of membranes is a particular problem in the areas adjacent to the tubesheet, especially at high operating pressures. As few as 0.1% broken fibers can have a great adverse effect on the fluid separation properties of the separation device.
Operation of permeators at high temperatures (often 80.degree. C.) often necessitates the use of insulation around the permeator. The permeators are conventionally wrapped with an insulating material, for example fiberglass, foam, blankets and the like. Alternatively, the separation device may be placed inside a container containing an insulating material or encased in an insulating material. In any event, an insulator must be added to the separation device in order to reduce heat loss and maintain steady-state operation.
Bundles of membranes must be carefully packed for shipment. Typically, the bundle is wrapped with a "bubble wrap", cardboard, foam or other material to protect the membranes. The wrapped bundle may then be placed in a cardboard container, usually a cylindrical container. One or more cardboard containers may then be placed in a wooden crate for shipment. The packaging material tends to be expensive and bulky. In addition, the packing and shipping process subjects the membranes to excessive handling, often resulting in damage or distortion to the fragile membranes ultimately decreasing the performance of the separation device. Moreover, the bundles must ultimately be unpacked, unwrapped and inserted into the housing of the separation device prior to operation. At that time, the membranes may be damaged from exposure to the environment.
Bundles of membranes may, alternatively, be shipped in steel or fiberglass vessels; i.e., the shells used during operation. The vessels, usually cylindrical shells, may be strapped to a wooden pallet. Packing the bundles directly in the pressure vessels is more convenient and reduces handling, but results in bulky, heavy shipments, ultimately increasing the cost of shipping.
Prior art separation devices are abundant. For example, U.S. Pat. No. 3,339,341 discloses a standard device for the separation of the components of a fluid mixture by the selective permeation of the components through the walls of hollow fiber membranes. The device consists essentially of a bundle of suitable hollow fiber membranes surrounded longitudinally by at least one elongated flexible porous sleeve member, a cast resin tubesheet at each end of the bundle, the hollow fiber membranes being embedded in and extending through the tubesheet, a shell surrounding the bundle and sealed thereto at each end, means for introducing a fluid mixture into the interiors of the hollow fiber membranes at one end of the bundle (that is, fiber side or bore side feed), means for removing from the interiors of the hollow fiber membranes at the other end of the bundle the portion of the fluid mixture which does not permeate through the walls of the hollow fiber (that is, fiber side or bore side recovery), and means for removing from the shell the portion of the fluid mixture which permeates through the hollow fiber walls (that is, shell side recovery). Similar separation devices are disclosed in U.S. Pat. Nos. 3,228,876; 3,228,877; 3,422,008; 3,455,460; 3,475,331; 3,526,001; and 3,536,611.
These prior art separation devices do not, however, provide a permeator which may readily be opened and reclosed to repair, clean or replace the bundle of separation membranes. Moreover, the prior art separation devices do not provide a device for the convenient, cost-effective, lightweight shipment of membrane bundles which may simultaneously be used as a recloseable housing or shell for the membranes. Still further, prior art separation devices do not provide a device in which the housing is fabricated from an insulating material, while still maintaining the advantages of prior art devices. Also, prior art separation devices do not readily permit modifications to the size and configuration of the annular space around the bundles so as to permit control of the flow of the permeate and/or sweep fluid on the shell side of the device.