A membrane system is positioned within an insulated enclosure heated to maintain superheat conditions for the feed gas to the system, wherein individual membrane modules are not insulated. The feed gas compression heat is desirably used to supply the superheat to the feed gas within the insulated enclosure.
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1. An improved air separation process comprising:
(a) compressing a feed air stream containing condensible water vapor to a desired feed air pressure; (b) cooling the said compressed feed air stream to below the design operating temperature level of the air separation process, thereby supersaturating the feed air so that the feed air stream comprises said feed air saturated with condensible water vapor together with free water droplets; (c) removing said free liquid water droplets from the feed air stream; (d) passing the compressed, cooled feed air stream, free of water droplets, into an insulated enclosure adpated to control and/or minimize the lose of heat therefrom, without superheat of said feed air stream prior to its passage into said insulated enclosure and without predrying said feed air stream to a temperature dew point lower than said design operating temperature prior to said passage of the feed air stream into the insulated enclosure; (e) supplying sufficient heat within said insulated enclosure so as to superheat the feed air stream therein to a temperature where the saturation temperature thereof at the feed air pressure, said insulated enclosure serving to control and/or minimize the loss of heat therefrom such that any loss of heat is not greater than that being supplied within said insulated enclosure, so that the feed air within said insulated enclosure is mintained under superheat conditions at a temperature below its dew point to avoid undesired condensation of water present in the feed air within a permeable membrane system positioned within said insulated enclosure to effect the desired air separation; (f) passing the thus- superheated feed air stream to said permeable membrane system positioned within said insulated enclosure, said permeable membrane system containing at least one membrane module capable of selectively permeating oxygen together with condensible water vapor, as a more permeable component of the feed air stream from nitrogen as a less readily permeable component thereof, said membrane module or modules not being individually insulated for the retention of heat therein; (g) withdrawing nitrogen from the membrane system and from said insulated enclosure as non-permeate gas at essentially said feed air pressure; and (h) separately withdrawing oxygen and condensible water vapor from the membrane system and from said insulated enclosure as permeate gas at a lower pressure,
whereby condensation of water from the feed air stream within the membrane system is effectively precluded. 19. An improved air separation process comprising:
(a) compressing a feed air stream containing condensible water vapor to a desired feed air pressure in an oil-flooded compressor; (b) cooling the compressed feed air stream to below the design operating temperature level of the air separation process, thereby supersaturating the feed air so that the feed air stream comprises feed air saturated with condensible water vapor together with free water droplets; (c) removing said free water droplets from the feed air stream; (d) supplying sufficient heat to said feed air stream to superheat the feed air stream to a temperature above the saturation temperature thereof at the feed gas pressure, the heat of compression of the feed air stream heating the oil in the oil-flooded compressor, the heated oil being passed to heat exchange means adapted to supply heat to the feed air stream for the superheating thereof; (e) passing the superheated feed air stream to an insulated enclosure that serves to control and/or minimize the loss of heat therefrom so that the feed air stream within the insulated enclosure is maintained at superheat conditions at a temperature below its dew point to avoid undesired condensation of water present in the feed air within a permeable membrane system positioned within said insulated enclosure to effect the desired air separation, said heat exchange means being positioned outside and upstream of said insulated enclosure, the feed air stream not being predried to a temperature dew point lower than the design operating temperature prior to passage of the feed air stream into the insulated enclosure; (f) passing the superheated feed air stream to said permeable membrane system positioned within said insulated enclosure, said permeable membrane system membrane system containing at least one membrane module capable of selectively permeating oxygen together with condensible water vapor, as a more readily permeable component of the feed air stream, from nitrogen as a less readily permeable component thereof, said membrane module or modules not being individually insulated for the retention of heat therein; (g) withdrawing nitrogen from the membrane system and from said insulated enclosure as non-permeate gas at essentially the feed gas pressure; and (h) separately withdrawing oxygen and condensible water vapor from the membrane system and from said insulated enclosure as permeate gas at a lower pressure,
whereby condensation of water from the feed air steam within the membrane system is effectively precluded. 10. An improved air separation system comprising:
(a) a permeable membrane system containing at least one membrane module capable of selectively permeating oxygen, together with condensible water vapor, as a more readily permeable component of a feed air stream from nitrogen as a less readily permeable component thereof, said system including means for passing said feed air stream to the feed side of each said membrane module at a desired feed gas prssure, pressure, and for separately withdrawing nitrogen at essentially said the feed air pressure level and oxygen and condensible water vapor as permeate gas at a lower pressure from each said module, each membrane module not being individually insulated for the retention of heat therein; (b) heat supply means suitable for supplying sufficient heat to the feed air stream so as to superheat said feed air stream to a temperature above the saturation temperature of the feed air at the feed air pressure prior to the passage of said feed air to the permeable membrane system; (c) an insulated enclosure surround said membrane system and said heat supplied by said heat supply means, said enclosure being insulated to control and/or minimize the loss of heat therefrom such that any loss of heat therefrom is not greater than the heat being supplied by said heat supply means so that the feed air passed to the uninsulated membrane module(s) is maintained under superheat conditions at a temperature above its dew point to avoid undesired condensation of water present in the feed air within the membrane system, said insulated enclosure being sufficiently large to permit operating personnel access thereto for servicing of the membrane system of uninsulated individual module(s); (d) compression means for compressing the feed air stream containing condensible water vapor to a desired feed air pressure; (e) an air cooling zone suitable for cooling the compressed feed air stream to below the design operating temperature level of the membrane system, thereby supersaturating the feed air so that said feed air stream comprises said feed air saturated with condensible water vapor together with free water droplets; (f) liquid separation means for removing said free water droplets from the feed air stream; and (g) conduit means for passing the compressed, cooled feed air stream, free of water droplets, into said insulated enclosure without superheat of said feed air stream prior to its passage into said insulated enclosure containing membrane module(s) not individually insulated and without predrying of said feed air stream to a temperature dew point lower than said the design operating temperature prior to said passage of the feed air stream into the insulated enclosure, whereby condensation of water from the feed air stream within the membrane system is effectively precluded.
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7. The process of
8. The process of
9. The process of
11. The system of
12. The system of
13. The system of
14. The system of
15. The system of claim 15 14 in which said heat exchange means is adapted to supply heat directly to said feed air stream.
16. The system of
17. The system of
18. The system of
20. The process of
(a) a permeable membrane system containing at least one membrane module capable of selectively permeating oxygen, together with condensible water vapor, as a more readily permeable component of a feed air stream from nitrogen as a less readily permeable component thereof, said system including means for passing said feed air stream to the feed side of each membrane module at a desired feed gas pressure, and for separately withdrawing nitrogen at essentially said feed air pressure level, and oxygen and condensible water vapor as permeate gas at a lower pressure from each said module, each module not being individually insulated for the retention of heat therein; (b) oil-flooded compressor means for compressing the feed air stream containing condensible water vapor to a desired feed air pressure; (c) an air cooling zone suitable for cooling the compressed feed air stream to below the design operating temperature level of the membrane system, thereby supersaturating the feed air so that the feed air stream comprises feed air saturated with condensible water vapor together with free water droplets; (d) liquid separation means for removing said free water droplets from the feed air stream; (e) heat exchange means adapted to supply heat to the cooled feed air stream, free of water droplets, for the superheating of the feed air stream to superheat conditions at a temperature above its dew point to avoid undesired condensation of water present in the feed air within the membrane system; (f) an insulated enclosure surrounding said membrane system, said enclosure being insulated to control and/or minimize the loss of heat therefrom such that any loss of heat therefrom is not greater than the heat supplied for the superheating of the feed air stream so that the feed air passed to the uninsulated membrane module(s) is maintained under superheat conditions at a temperature above its dew point to avoid undesired condensation of water present in the feed air within the membrane system, the insulated enclosure being sufficiently large to permit operating personnel access thereto for servicing of the membrane system of uninsulated individual module(s), said heat exchange means being positioned outside and upstream of said insulated enclosure; (g) conduit means for passing the compressed, cooled, superheated feed air stream, free of water droplets, into said insulated enclosure without predrying of said feed air stream to a temperature dew point lower than the design operating temperature prior to said passage of the feed air stream into the insulated enclosure; and (h) conduit means for passing oil heating in said oil-flooded compressor
means to said heat exchange means. 22. The system of claim 21 and including control means for adjusting the amount of heated oil passed to said heat exchange means. 23. The system of claim 21 and including means for passing additional heat generated external to the air separation operation to said insulated enclosure. |
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1. Field of the Invention
The invention relates to permeable membrane gas separation systems. More particularly, it relates to the prevention of condensation in such systems.
2. Description of the Prior Art
Permeable membranes capable of selectively permeating one component of a gas mixture are considered in the art as a convenient, potentially highly advantageous means for accomplishing desirable gas separations. To realize this potential in practical commercial operations, membrane systems must be capable of achieving and maintaining a desired degree of process efficiency, without undue maintenance or an unacceptable decrease in membrane life because of environmental factors associated with their use.
One such factor relates to the condensation of the constituents of the feed gas on the surface of the membrane. Such condensation can lead to lower permeation rates, corrosion increased maintenance and decreased membrane life. In addition, condensation in membrane systems can result, in some instances, in a contamination of desired product streams. Because of such condensation, therefore, more membrane surface area is commonly required for a given gas separation operation. As a result, both capital costs and maintenance costs are increased over those that would be incurred for membrane systems free of condensation problems.
It is important, therefore, that efforts be made in the art to minimize or eliminate condensation in membrane systems. One approach that has been employed for this purpose is to superheat the feed to the membrane system and to individually insulate the membrane modules included in a membrane system in order to maintain the superheat conditions therein. The superheat is typically supplied from external sources, such as steam or electrical heaters. Another approach involves predrying the feed stream by means of an adsorbent or a refrigerant dryer to a temperature dew point that is lower than the membrane operating temperature.
While such approaches serve to minimize or eliminate condensation, it will be appreciated that the capital and operating costs associated therewith are relatively high. Preheaters thus typically require an external energy source, and the insulation for individual membrane assemblies is relatively expensive and can make access to the membrane for maintenance purposes troublesome. Dryer systems likewise tend to be expensive, both in terms of operating costs and capital expense.
While solutions to the problem of condensation in membrane systems have thus been developed, a need remains for further improvement in the art, such development to enable condensation to be minimized or eliminated at reduced initial capital cost and lower operating and maintenance costs than are obtainable in the prior art practices. Such improvement in the art would contribute to the technical and economic feasibility of the use of permeable membrane systems in a wide variety of commercially significant gas separation operations.
It is an object of the invention, therefore, to provide an improved membrane separation system and process in which the problem of condensation is obviated.
It is another object of the invention to provide a membrane separation system and process containing improved means for eliminating or minimizing condensation of feed gas constituents on membrane surfaces.
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.
An insulated heated enclosure is used to supply and/or contain superheat to feed gas being passed to a membrane system contained in said enclosure, such superheat serving to prevent condensation on the surfaces of the membrane material. It is not required that individual membrane modules be heated, or that the feed gas be preheated or predried before passage to the membrane system. Heat recovered from feed gas compression operations is desirably employed as said superheat.
The invention is further described herein with reference to the accompanying drawings in which:
FIG. 1 is a schematic diagram of an embodiment of the temperature control system of the invention, and
FIG. 2 is a schematic diagram of a preferred embodiment of said invention providing advantageous heat recovery and temperature control.
The embodiment referred to above in which said heat exchange means is adapted to supply heat generated upon compression of the feed air stream in oil-flooded compression means, as part of the air separation operation itself, to said heat exchange means positioned outside and upstream of the insulated enclosure is illustrated in FIG. 2A. Thus, the feed gas stream passes from liquid separator 28' through line 30' to heat exchanger zone 36', such as a shell and tube exchanger, where the feed gas is heated by heat oil from feed gas compressor 22'. The outlet temperature of the gas is conveniently controlled by controlling the amount of oil sent through said heat exchanger zone 36' positioned outside and upstream of insulated enclosure 31'. As in the FIG. 2 embodiment, heated oil is passed from the oil-flooded compressor 22' in line 42' to oil cooler 43', from which cooled oil is returned to the compressor through line 44'. For the heat utilization purposes of the invention, a portion of the oil in line 42' can be diverted in line 45', so as to by-pass oil cooler 43', for passage to heat exchanger zone 36' positioned outside of said insulated enclosure 31'. Cooled oil leaving insulated enclosure 31' is passed through line 46' to join the cooled oil in line 44' for recycle to the oil-flooded compressor. The desirable control of the operating temperature in this embodiment may be readily achieved by the use of a suitable control valve 47' positioned in by-pass line 48' to control the amount of heated oil desired to be passed to heat exchanger 36', with the remaining oil being passed through said line 48' for joining with cooled oil being recycled from heat exchanger 36' through line 46'. As in the FIG. 2 embodiment, control valve 47' can be operated in response to suitable temperature measurement means, such as a gauge in the feed gas line leading into the membrane system 38' positioned within insulated enclosure 31' and in communication with said control valve 47' by conventional means. Those skilled in the art will appreciate that coalescing filler zone 33' can be employed in line 30' upstream of heat exchanger 36' as in the FIG. 2 embodiment.
Those skilled in the art will appreciate that various changes and modifications can be made in the details of the invention without departing from the scope of the invention as set forth in the appended claims. In the air separation application referred to above, it is common to employ a permeable membrane material capable of permeating oxygen as the more readily permeable component of the feed air stream. Nitrogen thus comprises the less readily permeable component of the feed air stream, and a nitrogen-rich product stream, if so desired, would be recovered as the non-permeate stream with the permeable gas comprising the residual oxygen-nitrogen stream that is enriched in oxygen as compared to the feed air stream. In other applications of the invention, it would be possible to use a permeable membrane material having the opposite permeation characteristics so that, for example in the air separation application, the permeable membrane would permeate nitrogen, rather than oxygen, as the more readily permeable component of the feed air stream. Those skilled in the art will appreciate that the improved membrane separation system and process of the invention is generally applicable to any desired gas separation operation in which condensation of feed gas constituents is a problem that is desirably to be overcome and/or operations in which it may otherwise be necessary or desirable to achieve a superheated, constant, stable temperature environment, with desirable temperature control capability beyond that obtainable using individually insulated membrane modules. The purification of hydrogen from an off-gas also containing methane, ethane and other hydrocarbons is an example of such gas separation applications, as are the recovery of hydrogen from ammonia purge gas and carbon dioxide and methane separations.
As indicated above, the permeable membranes comprising the membrane system positioned within the insulated enclosure of the invention may be in any desirable form, with hollow fiber membranes being generally preferred. It will be appreciated that the membrane material employed in any particular gas separation application can be any suitable material capable of selectively permeating a more readily permeable component of a gas or fluid mixture containing a less readily permeable component. Cellulose derivatives, such as cellulose acetate, cellulose acetate butyrate, and the like; polyamides and polyimides, including aryl polyamides and aryl polyimides; polysulfones; polystyrenes and the like, are representative examples of such materials. It will be understood in the art that numerous other permeable membrane materials are known in the art and suitable for use in a wide variety of separation operations. As noted, the membranes, as employed in the practice of the invention, may be in composite membrane form, in asymmetric form or in any such form that is useful and effective for the particular gas separation being carried out using the system and process of the invention.
By the effective and convenient overcoming of the condensation problems encountered in practical commercial operations, the invention thus provides a highly desirable advance in the membrane art as it pertains to gas separation operations. The invention also provides a highly desirable means for achieving a constant, stable temperature environment that further enhances the efficiency of the membrane system and process for gas separation, thus enabling permeable membranes to more effectively serve the need for practical and convenient means for achieving gas separations on a practical commercial basis.
Campbell, Michael J., Smolarek, James, Van Lente, Timothy S.
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| Jun 11 1992 | Union Carbide Industrial Gases Technology Corporation | PRAXAIR TECHNOLOGY, INC | CHANGE OF NAME SEE DOCUMENT FOR DETAILS EFFECTIVE ON 06 12 1992 | 006337 | /0037 |
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