Membrane air drying and separation operations

Abstract: Clean, dry air and nitrogen product gas are produced in an integrated system including an air separation membrane system and a membrane dryer operated with a significant degree of countercurrency of performance. Waste or recycle streams from the air separation membrane are used as purge in the membrane dryer.

Description: Background of the Invention Field of the Invention - The invention relates to air drying and separation operations. More particularly, it relates to the production of both dry air and nitrogen products. Description of the Prior Art - Permeable membrane processes and systems have been proposed and used increasingly for the production of nitrogen by air separation. In such operations, feed air is brought into contact with the surface of the membrane, and oxygen, as the more readily permeable component of air, passes through the membrane while nitrogen, the less readily permeable component of air, is withdrawn from the membrane system as a non-permeate product stream. Conventional membrane systems typically produce nitrogen having purity levels of up to about 99.5%. Higher nitrogen purities can be achieved if a post-purification feature is employed. Such processing may also include the use of a countercurrent membrane dryer with permeate gas from an air separation membrane being employed as purge gas in the dryer membrane. In some industrial operations, a source of clean, dry compressed air is also desired to prevent corrosion and condensation in instrumentation, piping, pneumatic tools, ventilators and other plant operations. For such latter purposes, separate adsorptive or refrigerative dryers have typically been utilized. Such units, however, tend to have significant product losses, due to purge and blowdown steps, or generally require a significant expenditure of added energy, e.g. for regeneration or refrigeration. The ability of various membrane materials to permeate moisture is well known in the art. Thus, a membrane system utilizing such a material would be used to replace the function of the adsorptive (or regenerative) dryer for producing clean, dry air for such variety of purposes. However, many practical commercial asymmetric and composite-type membranes are characterized by the crossflow permeation model performance, wherein the composition of the bulk gas on the permeate side of the membrane has little effect on the permeation rates achieved. Accordingly, the removal of moisture from compressed air streams by means of such membranes requires the co-permeation of significant amounts of valuable product gas as well. Operation at stage cuts, i.e., the permeate to feed flow ratio, on the order of 10% to 30% might be required to achieve the dew points typically provided by adsorptive or refrigerative dryers. As a result, the substitution of crossflow-type membrane dryers to produce clean, dry compressed air is not economically attractive. Advances in membrane product development and theory have resulted in the recognition in the art that some composite and asymmetric membranes are characterized by a significant degree of countercurrency in operation, and thus follow, at least in part, the countercurrent permeation mathematical model of the art rather than the crossflow model. Membranes designed so that permeation there-through to a significant extent can be described by the countercurrent model can benefit from the use of purge gas on the low pressure permeate side of the membrane. It is possible, in such circumstances, to achieve lower stage cuts to achieve the same moisture removal level as in crossflow permeation designs, provided that dry purge gas is available at the job site. In a method of separating a gas mixture comprising at least two main components and water vapor, known from EP-A-0 430 304, water vapor is separated by a preliminary membrane unit from a compressed stream of the gas mixture and the resulting water-depleted stream is then separated by a further membrane unit located downstream of the preliminary membrane unit to produce a dry permeate gas enriched in one main component and a pressurized product stream enriched in the other main component, wherein a stream of the dry permeate gas is passed over the membrane of the preliminary membrane unit on the permeate side thereof whereby to lower the partial pressure of water vapor on the permeate side of the membrane of the preliminary membrane unit and thereby facilitate separation of the water vapor from the pressurized gas mixture. The apparatus used to carry out this method comprises a preliminary membrane separation vessel having an inlet for a pressurized gas stream, comprising at least two main components and water vapor, and containing membranes adapted to separate water vapor from the gas mixture, wherein the preliminary membrane vessel has an outlet for pressurized gas mixture communicating with an inlet to a main membrane gas separation vessel located downstream of the preliminary membrane separation vessel and containing membranes adapted to effect separation as between two main components of the gas mixture, wherein the main vessel has on its permeate side an outlet which communicates with an inlet on the permeate side of the preliminary membrane vessel, there also being an outlet for moist gas from the membrane side of the preliminary vessel. It is indicated that examples of suitable membranes for drying the incoming gas are described in US-A-4 783 201 from which it is known that countercurrent flow of the permeate versus nonpermeate streams is preferred to maximize the sweep gas effect. While the membrane drying art has thus progressed and membrane air separation systems have been employed in commercial operations, it is desirable to achieve further advances in the art, particularly in light of the ever increasing performance requirements applicable to membrane systems in satisfying the needs of industrial applications. In particular, it is desirable in the art that membrane systems be provided for the enhanced integration of membrane air separation and air drying applications. It is an object of the invention to provide improved air treatment membrane systems and processes. It is another object of the invention to provide membrane systems and processes suitable to provide both air separation and air drying operations. It is a further object of the invention to provide improved membrane systems and processes for the production of both nitrogen product and dry air. With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims. Summary of the Invention The present invention, in conformity with one aspect thereof provides for a system for the production of clean, dry air and nitrogen from feed air, as defined in claim 1. A further aspect of the present invention is a process for the production of clean, dry air and nitrogen from feed air, as defined in claim 5. Brief Description of the Drawing The invention is hereinafter described in detail with respect to the accompanying single figure which is a schematic flow diagram of an embodiment of the invention for producing dry clean air and nitrogen product using moisture resistant membrane materials in the membrane air separation system. Detailed Description of the Invention The objects of the invention are accomplished by the use of a hybrid membrane separation system that produces both clean dry air and moderate purity nitrogen product. A membrane dryer characterized by a high permeability for water relative to air, i.e. a high separation factor or selectivity, is used to produce clean dry air. The membrane material of the membrane air separation system for nitrogen production is relatively insensitive to the pressure of water vapor. The saturated compressed feed air to the overall system is split into two streams. One such stream is passed to a membrane air separation system comprising two membrane stages, while the other stream is passed to the membrane dryer. The permeate gas from the second stage of the air separation unit is be used as purge gas for the membrane dryer, and said second stage permeate can then, if desired, be recycled to the inlet of the feed air compressor for the system. The membrane selected for the dryer should be characterized by a high water separation factor, i.e. ratio of the permeability of water to air, of at least 50, preferably greater than 1,000, for advantageous moisture removal from air. In addition, the membrane should have a relatively low permeability rate for air, i.e. for nitrogen and oxygen. Cellulose acetate is a representative example of a suitable membrane material for such purpose. However, various other materials, such as ethyl cellulose, silicone rubber, polyurethane, polyamide, polystyrene and the like can also be employed. The membrane dryer is operated with a countercurrent flow pattern. As indicated above, in preferred hollow fiber membrane configurations or in other such configurations, e.g. spiral wound, bundle designs providing for cross-flow type flow patterns have been commonly employed. In such cross-flow operation the flow of permeate gas on the permeate side of the membrane is at right angles to the flow of feed air on the feed side of the membrane. Using hollow fiber bundles with the passage of feed gas on the outside of the hollow fiber membranes, the flow direction of permeate in the bores of the fibers is generally at right angles to the flow of feed over the external surface of the hollow fibers. Using the inside-out approach in which the feed gas is passed through the bores of the hollow fibers, the permeate gas generally passes from the surface of the hollow fibers in a direction generally at right angles to the direction of the flow of feed within the bores of the hollow fibers and then, within the outer shell of the membrane module, in the direction of the outlet means for the permeate gas. As shown in EP-A-0 226 431, countercurrent flow patterns can be created by encasing the hollow fiber bundle within an impervious barrier over the entirety of its longitudinal outer surface except for a non-encased circumferential region to enable feed gas or permeate gas, depending on whether outside-in or inside-out operation is employed, to pass in countercurrent flow outside the hollow fibers parallel to the direction of permeate gas or feed gas in the bores of the hollow fibers. 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, can be wound in helical fashion around the central axis. In any event, the impermeable barrier material 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 feed air to be dried and the permeate gas comprising purge gas and the moisture together with other gases that selectively permeates through the membrane material in the membrane dryer. Relatively dry purge gas, preferably a portion of the relatively dry permeate from the air separation membrane system, either the first stage permeate or the second stage permeate in various aspects of the invention in which a two-stage membrane air separation system is employed for the desired production of nitrogen product, is used to purify the permeate side of the membrane dryer. When the membrane material employed in the air separation membrane is essentially moisture resistant, the use of saturated feed air therein has only a modest impact on the performance of the air separation membrane. If a two stage air separation membrane system is employed, the presence of water vapor may cause a slight, e.g. less than 15%, decrease in gas permeability for the first stage. In such cases, it is preferable to split the compressed saturated feed air stream into two streams as shown in the single figure. Thus, feed air in line 31 is passed to air compressor 32, from which compressed air is passed in line 33 to moisture separator 34, from which water is removed through line 35. The feed air is passed in line 36 to first stage air separation membrane 37 for the selective permeation of oxygen therefrom. The partially purified non-permeate nitrogen stream removed therefrom through line 38 is passed to second stage air separation membrane 39 from which product nitrogen is recovered as non-permeate gas through line 40. A portion of the feed air in line 36 is diverted through line 46 for passage to membrane dryer 43, from which dry air product is recovered in line 48. Selectively permeable oxygen and moisture are removed from first stage air separation membrane 37 through line 41 for discard from the system. The permeate from second stage air separation membrane 39 is removed through line 42 and passed to the permeate side of membrane dryer 43 as purge gas to facilitate the desired drying of a portion of the feed air. The purge and permeate gas are removed from said dryer 43 through line 44 and can, optionally, be recycled through line 45 for passage, with additional quantities of feed air from line 31, to air compressor 32 and to the membrane dryer and membrane air separation systems. In general, it is desirable to recycle the nitrogen rich waste purge gas to the inlet of the feed compressor to boost the nitrogen recovery of the overall system. If the oxygen concentration of the dry air product recovered from membrane dryer 43 is of concern, however, the waste reflux purge and permeate from said dryer 43 can simply be vented from line 44. The amount of reflux to the membrane dryer depends on a number of variables, including, but not necessarily limited to, the pressure ratio employed across the membrane, its selectivity or separation factor, the degree of countercurrency of performance achieved, the quantity of membrane area available per unit flowrate, the humidity of the reflux purge and the desired product and dew point. For a pressure dew point of -40°C (-40°F), at 7.9 bar (100 psig) feed, a purge ratio, i.e. reflux/total product air, of at least 15%, preferably 20% or above, is desired at a purge pressure of 1.03 bar (15 psia) to keep the membrane dryer surface area requirements and product loss to a minimum. It is generally desirable to keep the air loss due to co-permeation to less than 1%, and preferably to less than 0.5%, of the total product air flow. The amount of reflux available also depends on a number of variables, such as the properties and staging of the membrane air separation system and the desired nitrogen product purity. The driest reflux will be the permeate from the second stage of the air separation membrane system, and the amount of this flow relative to that permeated in the first stage of the air separation membrane system will depend on the membrane surface area distribution between the two stages. In general, a purge of 30% to 70% of the feed air to the air separation membrane system will be available from all sources. It should be noted that the ratio of available purge to the total feed air in the membrane dryer will be somewhat lower depending on the fraction recovered as clean dry air product. As those skilled in the art will appreciate, the membrane geometry should desirably be optimized to keep the pressure drop on both the high and low pressure sides of the membrane dryer within acceptable levels. The use of a membrane dryer to produce clean dry air for both plant use and as feed for an air separation membrane is feasible, from a practical commercial viewpoint, if the membrane dryer is operated, as indicated above, with a significant degree of countercurrency of permeation in contrast to the common approach of employing a cross-flow permeator. A relatively dry gas stream in the form of permeate from the air separation membrane system is available as reflux purge for the low pressure permeate side of the membrane dryer. Those skilled in the art will appreciate that the quantity of product air lost as permeate in the membrane dryer should desirably be minimized. However, it should be noted that a portion of the actual operating loss is due to the rejection of water vapor and is thus unavoidable if a dry product is to be obtained. Therefore, it is the dry product loss resulting from co-permeation of air that should be minimized. For a membrane with, a typical water/air separation factor of 6,000 and a typical water permeability of 10 -7 cm 3 (STP) cm/cm 2 S cmHg (1,000 Barrers) the dry product loss can be kept very low, e.g., to less than 0.5%, for purge ratios above about 15%. By contrast, a completely crossflow membrane would lose more than 20% of the dry product to achieve the same dew point level. Because of its high permeability to moisture, the membrane dryer requires a small surface area relative to the membrane air separation system. In addition, the stage cut of the dryer will be much lower than that of the air separation membrane. As a result, the membrane system and the membrane bundle geometry should be arranged, in accordance with conventional practice in the art, so that the pressure drop of both the purge and product flows are kept to reasonable levels. In the illustrated embodiment suitable in cases where the use of saturated feed air has only a modest impact on the performance of the air separation membrane, the compressed saturated feed air is simply split into two streams as indicated above. The first stream is passed to the two stage air separation membrane, while the second stream is passed to the membrane dryer. The dryer is then refluxed by the relatively dry permeate from the second stage of the air separation membrane. In general, it is desirable to recycle the nitrogen rich waste purge from the membrane dryer to the inlet of the feed air compressor to boost the recovery of the air separation membrane system. However, if the oxygen purity of the dry air product is critical, the waste reflux purge of the membrane dryer may simply be vented rather than being recycled. It will be appreciated that various changes and modifications can be made in the details of the process and system as herein described without departing from the scope of the invention as set forth in the appended claims. In this regard, it should be noted that membrane drying operations are commonly carried out using a dense fiber membrane. The thickness thereof is also the wall thickness, and is very large in comparison to the skin portion of an asymmetric membrane or to the separation layer of a composite membrane. Dense fibers have very low permeability rates and require the use of a very large surface area for adequate drying of the feed air. Asymmetric or composite membranes have very thin membrane separation layers, with the relatively more porous substrate portion of the membranes providing mechanical strength and support for the very thin portion that determines the separation characteristics of the membrane. Because of the inherently improved permeation fluxes obtainable by use of asymmetric or composite membranes, their use is preferred for the drying and air separation purposes of the invention, although dense fiber membranes can also be employed. The membranes employed for the air separation purposes may comprise composites using a suitable membrane material, such as cellulose acetate or ethyl cellulose on a substrate, such as polysulfone; or asymmetric Or dense membranes of one material, e.g. polysulfone, with the asymmetric membranes having two separate morphological regions, one comprising a thin, dense skin region and a less dense, porous support region, or variations thereof such as multicomponent membranes. While dense membranes are commonly used for product drying applications, the use of such dense membranes is not preferred because of the inherent limitations thereof. It will be understood that air separation membranes, capable of selectively permeating a more permeable component of feed air, generally comprise membrane materials capable of selectively permeating oxygen as the more readily permeable component, with nitrogen, as the less permeable component, being recovered as non-permeate. It is within the scope of the invention, however, to employ an air separation membrane system in which nitrogen is the more readily permeable component, and an oxygen rich stream is removed as the non-permeate stream. As indicated above, the membrane dryer should be characterized by a significant degree of countercurrency. In this regard, countercurrency is the degree to which membrane performance approaches the countercurrent mathematical permeation model for the particular membrane. At 100% countercurrency, the membrane performance would be identical to that predicted by the countercurrent permeation model. At 0% countercurrency, membrane performance would be identical to the cross-flow permeation model. A significant degree of countercurrency, for purposes of the invention, is at least 10%, typically 20% or more. For such purposes, the membrane dryer should be operated in a countercurrent flow pattern as by the use of an impervious barrier over the longitudinal outer surface thereof except for a non-encased region for the desired flow of feed gas or permeate gas. In hollow fiber embodiments of the membrane dryer, this enables either inside-out or outside-in operation with countercurrent flow outside the hollow fibers parallel to the mean flow direction of permeate gas or feed gas in the bores of the hollow fibers. In the practice of the invention, therefore, the membrane dryer is not designed to provide flow patterns of the cross-flow type as have commonly been employed in commercial practice. In such cross-flow operation, the bulk flow direction of permeate gas on the permeate side of the membrane is at right-angles to the flow of feed gas on the feed side of the membrane. The use of a countercurrent flow pattern, together with the use of purge gas on the permeate side of the preferred hollow fiber membrane, or other configurations, enables the membrane performance to be characterized by a high degree of countercurrency as contrasted to membrane configurations adapted to follow the cross-flow mathematical model of membrane performance. In this regard, it should be understood that if the membrane dryer is configured so as to have a cross-flow type of flow pattern, i.e., by the positioning of openings in the membrane bundle to provide for bulk permeate flow on the permeate side at right angles to the flow of feed air on the feed side of the membrane, the overall membrane performance will be characterized by the cross-flow model of membrane performance regardless of other factors. It will also be understood that, for a membrane dryer having a significant degree of countercurrency of performance, i.e., a membrane that follows to the indicated extent the countercurrency model rather than the cross-flow model, further features can be employed to enhance the countercurrency of performance as may be desired for particular applications. Thus, while the membrane hollow fibers of the invention may be organized either in straight assemblies parallel to the central axis of the membrane bundle, or in helical fashion around the central axis, it is desirable to employ helical windings wound, as shown in the art, to provide uniform fiber lengths so as to provide for a uniform flow of gas through the fibers. In addition, the substrate portions of asymmetric or composite membranes can have a porosity such as to prevent concentration polarization across the substrate, with dense membranes inherently being free of such concentration polarization that precludes desired radial mixing across the substrate portion of the membrane. Furthermore, the purge ratio can be optimized for the subject drying operation. All of these features, outside the scope of the invention, can nevertheless be employed to enhance the countercurrency of membrane dryer performance, so as to achieve a high degree of countercurrency, i.e. at least about 50%, desirably above 80%, in highly advantageous embodiments. The invention will be seen to provide a highly advantageous system and process for producing both clean dry air and nitrogen product gas. By such a hybrid membrane system and process, the highly desirable membrane technology is further advanced in efficiency and in convenient application to the air separation and air drying needs of practical commercial operations.

Claims: A system for the production of clean, dry air and nitrogen from feed air comprising: (a) a membrane dryer unit (43) capable of selectively permeating moisture present in feed air, said membrane dryer unit being characterized by a high separation factor for water/air and being adapted for a countercurrent flow pattern between the feed air and the permeate gas, with permeate gas flowing on the low pressure permeate side thereof generally parallel to the flow of wet feed air in the feed side thereof, said membrane dryer unit thereby being characterized by a significant degree of countercurrency of permeation; (b) a two stage air separation membrane system (37, 39) capable of selectively permeating oxygen or nitrogen, as the more readily permeable component of air, from feed air, the membrane material therein being relatively insensitive to the presence of water: vapor in feed air; (c) conduit means (46) for passing wet feed air to the feed end of the membrane dryer unit (43) and to the feed end of the first stage (37) of the air separation membrane system; (d) conduit means (48) for recovering clean, dry air from the product end of the membrane dryer unit (43) as a clean, dry air product stream; (e) conduit means (38) for passing a partially purified nitrogen stream from the product end of the first stage (37) of the air separation membrane system to the feed end of the second stage (39) thereof; (f) conduit means (41) for discharging oxygen rich gas from the first stage (37) of the air separation membrane system as a waste stream; (g) conduit means (40) for recovering a product nitrogen stream from the second stage (39) of said air separation membrane system; (h) conduit means (42) for passing an oxygen-containing stream from the second stage (39) of the air separation membrane system to the low pressure permeate side of the product end of said membrane dryer unit (43) for use as purge gas therein; and (i) conduit means (44) for removing purge and permeate gas from the feed end of the membrane dryer unit (43), whereby such integrated membrane dryer unit and air separation membrane system (37,39) enables both clean, dry air and nitrogen product streams to be efficiently obtained from feed air. The system of Claim 1 in which said air separation membrane system is adapted for the selective permeation of oxygen as the more readily permeable component of air, with nitrogen product being recovered as the less readily permeable component of air, said conduit means (41) for discharging oxygen rich gas from the first stage (37) of the air separation membrane system comprising conduit means for discharging said oxygen rich gas from the low pressure permeate side thereof, said conduit means (38) for passing an oxygen-containing stream from the second stage (39) of the the air separation membrane system comprises conduit means from the low pressure permeate side of said second stage of the air separation membrane system, and said conduit means (40) for recovering a product nitrogen stream comprises conduit means for recovering said product nitrogen stream as non-permeate gas from the product end of said second stage. The system of Claim 1 or 2 and including conduit means (45) for recycling purge and permeate gas removed from the feed end of the membrane dryer unit (43) for introduction to the feed end of said membrane dryer unit and to the first stage (37) of the air separation membrane system with additional quantities of feed air. The system of any one of the preceding Claims in which the membrane dryer unit (43) comprises hollow fibers having an impervious barrier on the longitudinal outer surface thereof such as to provide for said countercurrent flow pattern between the feed air and the bulk permeate gas. A process for the production of clean, dry air and nitrogen from feed air comprising: (a) passing wet feed air to the feed end of a membrane dryer unit (43) and of the first stage (37) of an air separation membrane system, the membrane dryer unit being capable of selectively permeating moisture present in the feed air, said membrane dryer unit being characterized by a high separation factor for water/air and being adapted for a countercurrent flow pattern between the feed air and the permeate gas, with permeate gas flowing on the low pressure permeate side generally parallel to the flow of wet feed air on the feed side thereof, said membrane dryer unit thereby being characterized by a significant degree of countercurrency of drying performance, said air separation membrane system being capable of selectively permeating oxygen or nitrogen, as the more readily permeable component of air, the membrane material therein being relatively insensitive to the presence of water vapor in the feed air; (b) recovering clean, dry air from the product end of the membrane dryer unit (43) as a clean, dry air product stream; (c) passing a partially purified nitrogen stream from the product end of the first stage (37) of the air separation membrane system to the feed end of the second stage (39) thereof; (d) discharging oxygen rich gas from the first stage (37) of the air separation membrane system as a waste stream; (e) recovering a product nitrogen stream from the second stage (39) of said air separation membrane system; (f) passing an oxygen-containing stream from the second stage (39) of the air separation membrane system to the low pressure permeate side of the product end of said membrane drying unit (43) for use as purge gas therein; and (g) removing purge and permeate gas from the feed end of the membrane dryer unit (43), whereby both clean, dry air and nitrogen product streams are efficiently obtained from feed air in said integrated membrane dryer unit and air separation membrane system (37, 39). The process of Claim 5 in which said air separation membrane system (37, 39) is adapted for the selective permeation of oxygen as the more readily permeable component of air; with nitrogen product being recovered as the less readily permeable component of air, with the oxygen rich gas discharged from the first stage (37) of the air separation membrane system comprising oxygen from the low pressure permeate side thereof, the oxygen-containing stream from the second stage (39) of the air separation membrane system comprising said gas from the low pressure permeate side of the second stage of the air separation membrane system, with the product nitrogen stream being recovered as non-permeate gas from the product end of said second stage. The process of Claim 5 or 6 and including recycling purge and permeate gas removed from the feed end of the membrane dryer unit (43) to the feed end of the membrane dryer unit and to the first stage (37) of the air separation membrane system with additional quantities of feed air. The process of anyone of Claims 5 to 7 in which the membrane dryer unit (43) comprises hollow fibers having an impervious barrier on the longitudinal outer surface thereof such as to provide for said countercurrent flow pattern between the feed air and the permeate gas.

Inventors: NOTARO FRANK, PRASAD RAVI, NOTARO, FRANK, PRASAD, RAVI
Applicants: PRAXAIR TECHNOLOGY INC, PRAXAIR TECHNOLOGY, INC.
Application year: 1992
Main classification: B01D53
IPC codes: C01B21, C01B13, B01D53