Membrane gas separation

Abstract: Highly efficient membrane drying and other gas separations are carriet out by the use of shallow vacuum levels on the permeate side of the membrane without the requirement of incorporating a vacuum pump.

Description: Background of the Invention Field of the Invention - The invention relates to a membrane gas separation process. More particularly, it relates to a membrane gas separation process using a purge gas to enhance the drying operation. Description of the Prior Art - There are many commercial circumstances in which it is necessary or desirable to remove moisture from a gas stream. Water vapor is a common impurity in many raw or process gases, and it often acts as a contaminant or corrosive agent that must be removed, or reduced in concentration, before the gas can be used. For example, dry air is often required in pneumatic systems, such as the instrument air used in chemical processing plants. Gases that are used for inert atmospheres must also be highly dried, since residual water vapor can be reactive rather than inert. In other instances, contained moisture can condense or freeze, thereby inhibiting the flow of process streams. Effective means for drying gas streams are, therefore, needed in the art. Many different means have been customarily employed for drying gas streams. In some cases, mere compression of the gas is sufficient to condense the water vapor to liquid, which can be drained away from the gas stream. Likewise, chillers and cryogenic traps can condense and remove the water as a liquid or as solid ice. Condensation methods are very useful for some applications, but they are often inadequate when a very dry gas stream is required. Adsorption processes are also often employed for gas drying purposes, since many adsorbents have a strong adsorptive affinity for water. Such adsorbents soon become saturated, however, and must be regenerated periodically if the drying process is to operate continuously. In pressure swing adsorption (PSA) processing, the adsorption is carried out at an upper adsorption pressure. Some of the dry product gas is depressurized and used as a countercurrent purge stream to facilitate desorption of water from the adsorbent bed at a lower desorption pressure. This PSA process can produce very dry gas streams, but some of the product gas must necessarily be recycled for such purge gas purposes and discharged from the system as a waste gas. Membrane permeation is a particularly attractive drying approach, offering certain advantages over other drying means. It is well known that water vapor is very highly permeable in many synthetic polymer membranes. When a moisture-laden gas is passed over such a membrane, the water vapor will tend to penetrate the membrane and pass through it from the feed to the permeate side provided that a sufficient drying force is present to facilitate the permeation of the water vapor through the membrane. For a commercially suitable drying process, the gas to be dried must be exposed to a large surface area of membrane that is very thin so that the diffusion path in the membrane material is very short. A pressure differential must also be maintained across the membrance to provide the drawing force for a suitable permeation action. In addition, a flow pattern must be established that enables the gas stream being processed to be progressively exposed to additional membrane surface so that the remaining moisture in the gas stream can continue to permeate and be removed from the membrane system. Such processes can conveniently be carried out in a permeation module comprising a large number of so-called composite or asymmetric hollow fibers. Such permeation modules are well known in the art and are becoming widely used for an increasingly broad range of commercial gas separation operations. It has been determined that so-called 3-port permeators have genuine limitations when used for the drying of low-permeability gases. Such 3-port permeations have a feed gas inlet port and separate outlet ports for the permeate and non-permeate portions of the feed gas. Although the water is highly permeable, it can be effectively removed from the low-pressure passages of the membrane only when there is sufficient permeation of other gases. To act as effective dryers, such permeators must operate with a high stage-cut, which means that a considerable amount of the gas being dried must also be permeated, and thus lost as dry gas product. It has been determined that improved drying can be achieved when a 4-port permeator is employed, provided that there is a high degree of radial mixing within the hollow fibers. With the 4-port permeator, a separate dry purge stream is introduced through the fourth port for passage on the permeate side of the hollow fibers, thereby flushing moisture from the low-pressure passages of the fibers. As a result, purge drying is found to be more effective than permeation drying. Even when dry product gas is used for purge purposes as it is known from EP-A-0324675 or from GB-A-1440963, the purge drying process is superior because forcing the permeation of product gas requires a high pressure difference or a large membrane surface area, both of which are unnecessary when a separate purge gas is used. US-A-4,931,070 describes the use of a 4-port membrane module, operated in a countercurrent flow pattern, as a gas dryer. In particular, this reference relates to the production of nitrogen, wherein feed air is passed through two membrane permeator stages, wherein the bulk of the oxygen in the air is removed from the nitrogen. The residual oxygen impurity is removed by catalytic reaction with hydrogen in a "deoxo" unit. The water generated by this reaction is largely removed by passing the wet nitrogen gas through a cooler and liquid water separator. Nevertheless, a substantial amount of water impurity remains in the thus-processed nitrogen stream. This residual moisture is removed by a membrane dryer, the low pressure passages on the permeate side thereof being purged by air, the dry permeate from the second stage membrane or by dry nitrogen product. DE-A-3726431 discloses for a commercially acceptable operation the use of a vacuum pump providing for a high vacuum with a pressure of about 2 to 30 mbar on the product side. On the other hand a pressure of about 1 to 1.3 bar is provided on the feed side. The resulting pressure relationship is typical for conventional processes as it is indicated to be preferred in the process of US-A-4931070, too. Despite such advantageous drying processes, further improvements are desired in the art to enhance the membrane drying of gases in practical commercial operations. When product gas is used for purging, a certain amount of product gas is, of course, lost in the purge waste stream. When an external source of dry purge is used some undesired contamination of the product stream can occur by back diffusion of some of the non-product components present in the purge gas. These factors create a practical limitation on the ultimate usefullness of the membrane drying methods referred to above. It is an object of the invention to provide an improved membrane process for the separation of gases. It is another object of the invention to provide a membrane gas separation drying process wherein the back diffusion of impurities from an external source of purge gas is minimized. It is another object of the invention to provide an improved membrane drying process wherein the amount of product gas or external purge gas used as purge is minimized. It is a further object of the invention to provide an enhanced process for removing water vapor from high purity nitrogen without recontamination of product nitrogen during drying and with a high degree of product nitrogen recovery. With these and other objects in mind, the invention is hereafter described in detail, the novel features thereof being particularly pointed out in the appended claims. Summary of the Invention The membrane gas separation process of the present invention comprises the steps defined in claim 1. Brief Description of the Drawings The invention is hereinafter further described with reference to the accompanying drawings in which: Fig. 1 is a schematic flow diagram of an embodiment of the invention utilizing a small portion of dry product gas as purge gas; and Fig. 2 is a schematic flow diagram for the drying of high purity nitrogen produced in a membrane-deoxo system, using a small portion of the dry nitrogen product as purge gas. Detailed Description of the Invention The objects of the invention are accomplished by effectively utilizing the driving force for permeation through a membrane based on optimal use of purge gas on the permeate side of the membrane. Such driving force for permeation is the difference in the partial pressures of the feed gas on the high pressure feed, or non-permeate side, and the low pressure permeate side of the membrane. Permeation through the membrane ceases when these partial pressures become equal. If the mole fraction concentration of water is Yhi in a wet feed stream at high pressure, Phi, the maximum concentration, Ylo, of water in the permeate stream at low pressure, Plo, is: Ylo = (Phi/Plo)Yhi If the feed flow rate of the gas being dried is Ff, then, for complete removal of water therefrom, a mass balance for the membrane system requires that the waste stream flow, Fw, be: Fw = (Yhi/Ylo)Ff ≥ (Plo/Phi)Ff This flow must come from either dry gases permeated by the membrane or from a dry purge stream. For membranes that advantageously have a very high separation factor for water relative to nitrogen, for example, the amount of nitrogen that will be permeated is insufficient to provide the waste stream flow required by equation (2). In the limiting case of no product permeation, all of the waste flow would come from the purge stream, Fp. Thus, the purge-to-feed ratio for complete impurity removal is as follows: Fp/Ff ≥ Plo/Phi The effects of pressure and flow can be combined so as to define a "cleaning ratio", as follows: CR = FpPhi/FfPlo = (Fp/Ff)(Phi/Plo) In theory, complete drying can be achieved only when CR is unity or larger. It will be appreciated, however, that circumstances may exist where only partial drying is needed, and a smaller cleaning ratio can be employed. Conversely, for thorough drying, a cleaning ratio considerably larger than one may be employed. Thus, a cleaning ratio of unity represents only the limiting requirement for complete impurity removal. Many other factors are pertinent in the design of practical processes and systems, and a cleaning ratio of 2 or larger is typically employed for essentially complete drying. It should be understood that there is a "trade-off" to be considered between the amount of purge gas employed, above the limiting requirement, and the surface area of the membrane employed. Thus, a large surface area is needed when the driving forces become too low for efficient permeation. The surface area can be decreased, on the other hand, if the amount of purge gas employed is increased and the cleaning ratio is substantially greater than one. When a portion of the desired product gas is used for purge purposes, the product gas actually delivered from the system is the amount of retentate, Fr, which is given by: Fr = Ff-Fp In order for a high product recovery to be achieved, the purge stream, Fp, must be small. From equations (3) and (4), it will be seen that this requires a high pressure ratio, i.e. Phi/Plo. In a typical drying application, the waste stream on the permeate side of the membrane is withdrawn from the membrane at slightly above atmospheric pressure, e.g. 103 kPa (15 psia), and the feed stream may be passed to the membrane system at about 1034 kPa (150 psia), for a pressure ratio of 10:1. For complete drying, at least about 10%, and typically 15% or more, of the desired product gas must be recycled as purge gas and thus lost as part of the permeate side waste stream from the system, thereby reducing the product recovery of the system. Product recovery and process energy efficiency can be economically increased by carrying out the drying operation using purge under relatively deep vacuum conditions on the permeate side of the membrane. By employing subatmospheric pressure conditions for Plo, it is possible to attain very high pressure ratios and, thereby to reduce the purge flow to a very small portion of the product flow on the non-permeate side of the membrane. While the purge flow may be reduced and the cleaning ratio increased by increasing the pressure ratio Phi/Plo, it is common practice in the art to limit the minimum value of Plo to slightly above atmospheric pressure, so that the waste gas may conveniently be discharged from the membrane drying system to the ambient air and to avoid the cost and power expenditure of a vacuum pump. Under such circumstances, the pressure ratio can be increased only by raising the high pressure, Phi, level. However, this requires additional feed compression, and produces dry product at a pressure that may be higher than that required for the intended use. The use of the vacuum conditions of the invention on the permeate side of the membrane is contrary to the conventional practice of the art. For most drying applications, a pressurized product stream is required, and, therefore, a compressor is needed to pressurize the feed stream to a desired superatmospheric pressure level. If the permeate side of the membrane is operated below atmospheric pressure, an additional vacuum pump must also be employed. Thus, two costly machines are required for such a transatmospheric process, while a single compressor suffices for a superatmospheric process. Therefore, it was believed in the art that the operating power requirements would necessarily be higher for such a transatmospheric process operation. Furthermore, for comparable pressure ratios, vacuum pumps are commonly more costly than compressors and operate at lower efficiencies. For these reasons, it has not been considered worthwhile to operate a transatmospheric drying process with a shallow vacuum, such as about 69 kPa (10 psia) or greater, to increase the membrane pressure ratio, but instead to employ the conventional superatmospheric pressure conditions. While vacuum levels of about 69 kPa (10 psia) or greater were indicated above as being undesirable vis-a-vis conventional superatmospheric conditions because of the cost and efficiency considerations relating to the use of vacuum pumps at such vacuum levels, it has been found that subatmospheric pressure from levels of about 69 kPa (10 psia) to about 90 kPa (13 psia) may be used in the practice of the invention if such subatmospheric pressure levels can be generated without the requirement of incorporating a vacuum pump into the system. Thus, the purge exit line of the membrane system can be connected to the suction of a suitable existing compressor, for example, the feed gas compressor of the membrane system, or a third stage membrane permeate recycle compressor, or venturi means or the like can be used to provide such suction. With reference to Fig. 1 of the drawings, wet feed gas to be dried is introduced through line 1 to compressor 2, wherein the feed gas pressure is elevated to the desired upper membrane pressure Phi. In most cases, this compression will result in condensation of some of the water present in compressor discharge 3. Further reduction in the water vapor content of the feed gas can be achieved by reducing the temperature of this stream in a chiller aftercooler 4. The condensed moisture is then removed from the feed gas stream in knock-out phase separator 5. The resulting saturated feed gas stream is then passed through line 6 to the high pressure side 7 of membrane separation module 8. Due to the high water selectivity of the membrane used for drying, most of the water vapor will selectively permeate through the membrane to the low pressure permeate side 9 of the membrane. The thus dried non-permeate gas is passed from membrane 8 through line 10. In many typical applications, some of this dry gas is expanded through valve 11 to subatmospheric pressure and is passed through line 12 to serve as purge gas for the low pressure side of membrane 8. The remaining dry gas passes through line 13 as the desired dry product gas of the process. The purge gas, as illustrated in Fig. 1, flows, countercurrent to the feed stream, through the low pressure, permeate side 9 of membrane 8 where it serves to sweep the permeated gases, including said water vapor, from membrane 8 through purge exit line 16 connected to the suction of compressor 2. In an alternative embodiment, where an external source of dry gas is available for use as purge gas, valve 11 can be closed, or omitted, and the dry external purge gas can be passed through line 17 and line 12 to serve as the desired purge stream in membrane 8. In this case, the entire non-permeate or retentate stream removed from membrane 8 through line 10 can be recovered through line 13 as dry product gas. In those applications wherein the drying process is an adjunct to another process, such as the production of nitrogen from air, the feed gas stream may be available at high pressure, and feed compressor 2, chiller 4 and phase separator 5 may not be required as elements of the drying process, although functionally present in other parts of an overall separation process of which the subject drying process and system are a part. In the embodiment of the invention illustrated in Fig. 2 of the drawings, feed air is separated in a two-stage membrane system to produce a partially purified nitrogen stream that is subject to deoxo treatment to remove residual oxygen therefrom and produce a wet, high purity nitrogen stream. The latter is dried in a membrane dryer to produce dry, high purity nitrogen product. Thus, feed air in line 21 passes through compressor 22 to first stage membrane 23 from which an oxygen-containing permeate waste stream is separated through line 24. The non-permeate gas therefrom passes in line 21A to second stage membrane 25 from which the partially purified nitrogen stream passes in line 26 to deoxo system 27. Hydrogen is introduced to deoxo system 27 through line 28 for catalytic reaction with residual oxygen so that a wet, high purity nitrogen stream is removed therefrom in line 29 containing chiller 30 and phase separator 31 from which condensed water is removed through line 32. The saturated high purity nitrogen stream in line 29 is then more fully dried by passage to membrane dryer 33 from which dry, high purity nitrogen product is withdrawn through line 34. In this embodiment, a small portion of said dry, high purity nitrogen product is recycled to membrane dryer 33 through line 35 containing valve 36 for passage therethrough as dry purge in countercurrent flow to the nitrogen gas flow on the feed or non-permeate side of the membrane. The purge effluent is discharged from membrane dryer 33 through line 37 for recycle to line 21 for compression and passage to the two-stage membrane system for recovering of additional quantities of nitrogen product. The permeate gas from second membrane stage 25 is also desirably passed in line 39 to line 37 for return to the system. It will be appreciated that the membrane composition used in the membrane dryer of the invention should, as indicated above, be one having a high selectivity for water over the gas being dried, i.e. nitrogen and oxygen in air drying. That is moisture must be permeated much more rapidly than air or other gas being dried. For example, the water/air separation factor should be at least 50, preferably greater than 1,000, for advantageous moisture removal from feed air. In addition, the membrane composition should have a relatively low permeability rate for the gas being dried, e.g. for both nitrogen and oxygen in air drying applications. Cellulose acetate is an example of a membrane material satisfying such criteria. A variety of other membrane materials can also be employed, such as ethyl cellulose, polyurethane, polyamide, polystyrene and the like. While various membrane configurations can be employed in the practice of the invention, e.g. spiral wound membranes, hollow fiber membrane configurations are particularly desirable because of the enhanced surface area and packing density provided thereby. In preferred embodiments of the invention using hollow fiber bundles, the passage of the feed air or other gas being dried may be inside-out, wherein the feed gas is passed through the bores of the hollow fibers, or outside-in, wherein the feed gas is passed to the outer or shell side of the membrane bundles with permeate gas being recovered from within the bores of the hollow fibers. As shown in EP-A-0,226,431 countercurrent flow patterns can be created by encasing the hollow fiber bundle with an impervious barrier over its longitudinal outer surface except for a non-encased circumferential region conveniently located near one end of the bundle. This or other such means serve to enable the feed gas or permeate gas, depending on the desired manner of operation, i.e. inside-out or outside-in to pass in countercurrent flow outside the hollow fibers parallel to the flow direction of permeate gas or feed gas in the bores of the hollow fibers. The feed gas on the outside of the hollow fiber bundle, for example, is caused to flow parallel to, rather than at right angle to, the central axis of the fiber bundle. It will be understood that the membrane fibers may be organized either in straight assemblies parallel to the central axis of the bundle, or alternatively, and preferably, can be wound in helical fashion around the central axis. In any event, the impermeable barrier may be a wrap of impervious film, e.g. polyvinylidene or the like. Alternatively, the impermeable barrier may be an impervious coating material, e.g. polysiloxane, applied from an innocuous solvent, or a shrink sleeve installed over the membrane bundle and shrunk onto said bundle. The impermeable barrier thus encases the hollow fiber or other membrane bundle and, as disclosed in said publication, has an opening therein permitting the flow of gas into or from the bundle so that the fluid flows in a direction substantially parallel to the axis of the fiber bundle. For purposes of the invention, the flow pattern should be one of countercurrent flow of the wet feed air or other gas stream relative to the permeate gas comprising purge gas supplied as indicated above, together with the moisture that permeates through the membrane material. For purposes of the invention, asymmetric or composite membranes are preferred because of their very thin membrane separation regions or layers supported by more porous substrates for mechanical strength and support. Dense fiber membranes can also be used, although they have very low permeability rates because of the inherently greater separation region thickness thereof. While the invention has been described above particularly with respect to the highly desirable gas drying application thereof, it will be understood that the invention can also be practiced with respect to other commercially important gas separations. Thus, the invention can be used for applications in which it is desired to remove fast permeating components, other than water, from a feed gas stream, e.g. CO 2 and ammonia cleanup from process gas streams. In addition, the invention is of significance for air separation applications for the recovery of nitrogen product gas, particularly where the membrane systems employed, e.g. facilitated transport membranes, exhibit an enhanced permeability of the oxygen component of feed air. It is within the scope of the invention, therefore, to separate fast permeating components, such as said CO 2 , ammonia and oxygen, from feed gas streams, apart from the highly desirable gas drying applications of the invention disclosed above. It will be appreciated from the above that the invention represents a significant advance in the membrane separation art. Thus, the invention enables highly efficient drying and other gas separation operations to be carried out using desirably small amounts of purge gas, with low overall energy requirements.

Claims: A membrane gas separation process comprising: a) passing a feed gas stream to the non-permeate side of a membrane system, capable of selectively permeating a fast permeating component from said feed gas, at a feed gas pressure at or above atmospheric pressure; b) passing purge gas to the permeate side of the membrane system in countercurrent flow to the flow of said feed gas stream in order to facilitate carrying away of said fast permeating component from the surface of the membrane and maintaining the driving force for removal of the fast permeating component through the membrane from the feed gas stream, said permeate side of the membrane being maintained at a subatmospheric pressure within the range of from about 70 to about 90 KPa (about 10 to about 13 psia) without the use of vacuum pump means; c) recovering a product gas stream from the non-permeate side of the membrane; and d) discharging purge gas and the fast permeating component that has permeated the membrane from the permeate side of the membrane, whereby the vacuum conditions maintained on the permeate side of the membrane without the use of vacuum pump means enhance the efficiency of the gas separation operation, thereby reducing the overall energy requirements thereof. The process of claim 1 in which said feed gas comprises moisture-laden air and the product gas comprises dry air. The process of any one of the preceding claims in which the purge gas comprises a portion of the product gas stream recovered from the membrane. The process of any one of the preceding claims in which said feed gas pressure is from about atmospheric pressure to about 12 bar (170 psia). The process of any one of claims 1 to 4 in which said feed gas stream comprises air, and said product gas comprises nitrogen.

Inventors: PRASAD RAVI, PRASAD, RAVI
Applicants: PRAXAIR TECHNOLOGY INC, PRAXAIR TECHNOLOGY, INC.
Application year: 1992
Main classification: B01D53
IPC codes: B01D53