US2934906A - Processing gas mixtures - Google Patents

Description

May 3, 1960 F. G/PEARCE 2,934,906

I PROCESSING GAS MIXTURES Filed April 22, 1957 2 Sheets-Sheet 1 mi lo 32 22 I so 24 T I FIG] 1 f f INVENTOR.

FRANK G. PEARCE A T TOR/V5 r Un d ws F rm" 6 PROCESSING GAS MIXTURES Frank G. Pearce, Tulsa, Okla., assignor to Pan American Petroleum Corporation, Tulsa, Okla., a corporation of Delaware Application April 22, 1957, Serial No. 654,413

11 Claims. (Cl. 62-13) This invention relates to an improvement in low temperature separation and recovery of gases. More particularly, it is concerned with a novel method for improving the efiiciency of presently known procedures for securing separation of the components in air or other gases.

Gases such as air may be treated to effect liquefaction and subsequent fractionation thereof by first compressing the gas, thereafter cooling it, usually by heat exchange with a cold product gas stream, liquefying a portion of the cooled gas through further heat exchange, and expanding another portion of the cooled compressed gas with the performance of work thereby furnishing the necessary refrigeration to the fractionation system.

Cooling of the air feed stream is generally effected by flowing the latter through one of two sets of regenerators filled with metal packing and which operate in pcriodically reversing cycles between the incoming air feed and the outgoing cold product gas streams. In cooling the air feed stream in this manner, however, water which is carried into the system as vapor is condensed and deposited as liquid at the warm end of the regenerators, i.e., about the last one-eighth to one-fifth of the length of the regenerator. The air flow cycle is continued generally for about two or three minutes at elevated pressures, usually 60 to 100- p.s.i.g. Thereafter, the set of regenerators through which air was flowing is depressurized against. the other group of regenerators through which a cold product gas stream has been flowing. The latter group of regenerators ordinarily operates at about 8 to 10 p.s.i.g. Since this decrease in pressure occurs very rapidly, an appreciable quantity of the liquid water present inthe warm end of the regenerators in which air flow has just been discontinued, is discharged therefrom in the form of a mist or fog. The quantity of water eliminated from the regenerators in this manner, is a substantial amount, i.e., in the range of about 20 to 50 percent of the water introduced into the system as vapor. The liquid water present in the product gas stream is considered to be the direct result of fog formation caused by vaporization of water from the packing in the warm end of the regenerators into the colder gas stream.

As an alternate to the above mentioned regenerative type cold exchangers, countercurrent reversing cold exchangers may be employed. These permit a simultaneous and eflicient heat interchange between passageways containing countercurrently flowing streams of the gas mixture.to be separated and returning cold products. Ex-

changers of this type comprise essentiallya plurality of tact. Reversing cold exchangers of this type, therefore,

are characterized by possessing a high rate of heat transfer and a thermal efliciency unaffected by'cycle time, since little dependence is placed on storage of heat in metal. 1

2,934,90 Patented Ma s, 1960 ice I These reversing cold exchangers are also utilized to remove all of the higherboiling impurities in gaseous mixtures such as air, particularly for separations conducted at relatively low pressures, such removal being accomplished by periodically alternating the flow of warm incoming feed gas and a backward returning cold product stream between at least' two passageways of the exchanger. Thus, during one-half of the reversing cycle when air, for example, is being cooled, water and other condensible impurities are deposited therefrom and accumulate on the metal surfaces of the passageway through which the air at that time is flowing. Then, be fore the accumulation has become great enough to interfere with the operation of that particular passageway the countercurrently flowing-streams are interchanged to enable the backward returning'anhydrous product to flow over the accumulated deposits and re-evaporate them. Theloss in exchanger refrigeration, mentioned above, could be avoided if the total amount of water introduced as vapor were eliminated from the system as vapor. However, if a part of the water entering as a vapor is expelled as a liquid-as would be the case where a water fog is physically entrained with the product gas stream the heat of condensation constitutes a direct heat input to the system. In tonnage oxygen plants, for example, where this problem becomes mostacute, the refrigeration losses of a plant having a capacity of about 1,000 tons of oxygen per day would be as much as 570,000 B. t.u."s/hr. When it is realized that the design 'heat leak for a plant of this size is slightly less than percent of this figure, the significance of such an enthalpy input becomes apparent.

In accordance with my invention I am able to avoid such refrigeration losses by adding liquid water to the air feed stream in an amount such that the quantity of water entering the regenerators as vapor is substantially the same as that leaving the system with the product gas. The amount of liquid water thus added to the system to obtain a vaporization duty substantiallyequal to the condensation duty during the air flow cycle can be determined rather accurately since the percentage of vaporized water reverting to fog is not influenced to any subs'tantial degree by the quantity of liquid water on the packing at the end of the air flow cycle and just prior to product flow. As previously pointed out, the amount of water discharged as liquid at the warm end of the regenerators with the product gas amounts to from about 20 to 50 percent of the amount introduced as vapor with the air feed stream. The quantity of liquid water introduced into the air feed stream to overcome this refrigeration loss resulting from fog formation, .as described above, may vary from about 20 to about 200 percent of the water vapor originally present in said stream. The actual amount of liquid water injected, however, will depend upon the performance characteristics of the regenerators used. In any event the required amount of liquid water added to the air feed stream will correspond to the quantity necessary to give a quantity of water vapor in the product gas stream substantially equal to the quantity of water vapor in the air fed to the system.

-In the drawings, Figure 1 shows a regenerative type exchanger with associated piping required for use of the present invention. Figure 2 is a' flow diagram of an over-all plant with exchangers and low temperature fractionation equipment illustrating the adaptation of the present invention totreatment of the air intake feed streams for such plants. Figure 3 is a diagram of a reversing exchanger operating on an intake air stream containing liquidwater, ascontemplated by my invention.

Referring now to the drawings, Figures 1 and 3 show a 'single regenerator with appropriate flow lines and their function. with respect to operation of a regenerator of the type used in oxygen plants. The flow rates as well as other quantities appearing in the description of Figure 1 of the drawings are given on the basis of an operation capable of producing 1,000 tons of oxygen per day. Figure 1 shows a single regenerator 2 filled with aluminum packing 4. The upper end of the regeneratoris connected to a flow line 6 which in turncommunicates directly with common header 8 and indirectly on alternate cycles with air feedheader 10 and product gas header 12. At the lower end of regenerator 2 a flow line 14 in similar fashion communicates with common header 16, cold product header 18 and cold air header 20. It will be understood, of course, that in actual practice other genera-, tors such as 2 are similarly connected to headers 8, 10, 12, 16, 18 and 20. I

. Air containing 0.2 volume percent water as vapor, is introduced at aboutl00 p-.s.i.g. and 60 F. into line 22 atthe rate of 5,700 M s.c.f;.h. and mixed with liquid Water added in the form of a mist at the rate of 1004000 pounds per hour through line 24-. I This mixture then flows through air feed header 10, valved line 26, common header 8, line 6, and into regenerator 2. The air is rapidly cooled by coming into contact with cold aluminum packing 4. At the Warm end of regenerator 2, generally designated at 30, water vapor condenses and is deposited on the pack-v ing along with the liquid water which has been added through line 24. Exclusive of the water added through line 24 the water condensed from the air feed stream and deposited on the packing amounts to about 500 pounds per hour. After the air has passed the warm end of the regenerator it is substantially anhydrous and becomes progressively colder until it is withdrawn from the base of regenerator 2 at a temperatureof about -265 F. through line 14; From this point the cold air stream is taken through common header 16, valved line 32, cold air header 20 and out through line 34. The stream in this line is then subsequently split with a major portion ageaeooe ing the system with the product gas through line 28 is less than the water vapor brought in via the air feed stream. Thus, where no additional liquid water is added to the system in accordance with my invention, a portion of the water entering the regenerator as vapor will leave it as a liquid. The amount, of refrigeration lost in this manner therefore correspoiids to the evaporative cooling that would have been furnished the system if such liquid water had been converted to vapor in the regenerator prior to removal therefrom. In the instant case, however, a suflicient amount of water is present in the warm end of regenerator 2 to provide a quantity of water vapor in the product gas streamsubstantially equal. to that introduced into the system by the aforesaid air stream, notwithstanding the fact that 20 to 50 percent or more of the water in the product gas stream leaves the regenerator in the form of a fog or mist.

. In Figure 2, air compressed to about 95 p.s.i., and at a temperature of 60 F., is lead into the system through line 60, mixed with a controlled amount of water injected through line 62, and the resulting mixture sent to regenerative exchanger 64 via line 66 and reversing valve 68. As the moisture laden air flows into exchanger 64, liquid water is laid down on thecold packing therein. When the gas reaches midway of the exchanger, practically all of the water at that point is in the form of ice. As the air travels on, it is reduced in temperature until at the bottom of the exchanger it is withdrawn through line 70 and reversing valve,72 at a temperature of 265 F. Thereafter a stream that is mainly liquid air at this temperature flows through line 73 into adsorber 74 where the last trace of impurities, such as carbon dioxide, are re moved ,by means of a suitable adsorbent, for example,

going to a suitable high pressure fractionating tower and the remainder passing through an expander after which the resulting cold vapors are fed to an appropriate low pressure fractionating tower to furnish the required refrigeration to the system. Since the processing of the air stream after it reaches line 34 forms no part of my invention and inasmuch as. methods of processing this stream are well known to the art, no further discussion of the procedure and equipment used for such purpose is considered necessary.

After the required air flow cycle through regenerator 2, which may be aperiod of two or three minutes, valves 36 and 38 are closed and valve 40 is opened. Operation of these valves is controlled by an automatic timing device 42 actuating line 44 which in turn is operatively connected to valves 36, 38 and 40. A returning cold product nitrogen or oxygen stream at a temperature of about 275 F. then surges rapidly at a pressure of about 8 to 10 p.s.i.g. through line 46, cold product header 18, line 46 and check valve 48, common header 16 and into regenerator 2 via line 14. As the cold product gas rises through the regenerator it cools off packing 4 so that the latter will be suitable for use in the next air flow cycle. This gas as it flows upwardly through the regenerator is not only cold but extremely dry. As it approaches the Warm end of regenerator '2 it is at a temperature of about 40 F. Water that has been deposited on packing 4 during the air flow cycle is already partially vaporized and the gas phase in the warm end of regenerator 2 is saturated with water vapor. However, as the colder (40 F.) product gas stream contacts the water vapor in the warm end of the regenerator, conditions are produced which favor fog formation and accordingly, an appreciable portion, e.g-., 20 to 50 percent of said vapor is converted to a fog or mist of liquid water droplets. In instances where ad ditional liquid water has not been added to the air feed stream, it is apparent that a net loss in refrigeration. from the regenerators will be incurred if the water vapor leav:

silica gel; Fro-m adsorber 74, the air flows through line 76 and is split; with a portion being diverted through line '78-, heat exchanger 80 and throttled into low-pressure fractionating tower 82 through valve 79. The balance of the air in line 76 flows through heat exchanger 84 and intorhigh-pressure fractionating column 86. An impure liquid oxygen bottoms from high-pressure tower 86 is taken off via line 88 and sent, after expansion through valve $0, to the upper part of tower 82. Liquid nitrogen producedin the high-pressure section is removed through line 92, heat exchanged in exchanger '94 and returned to the upper part of column 82 as reflux.

Product nitrogen removed from lowepressure tower 82 through line 96 is heat exchanged with oxygen in line 2 and thereafter :gives up additional cold. by means of heat exchanger 80 to that portion of the cold air in line 78 "goingtothetop of low-pressure tower 82. The. warmed product nitrogen then proceeds through switching valve 72, line 98 and exchanger 100, which has just completed a production cycle resulting in the deposition of substantial quantities of liquid water in the upper part of the exchanger. When the latter goes over to the regeneration cycle, i.e., when product nitrogen flows upwardly through it, cooling olf thepacking, the pressure in exchanger 100 is reduced to approximately .15 p.s.i.g. there by causing liquid water, aswell as water vapor, to be blown out the top .along with product nitrogen through switch valve 68 and line 102.

Simultaneously with the operation of exchangers 64 and 100,-theair feed stream in-lin'e 60 is split and the remainder-of the air in this stream proceeds along line #1104, mi-x'ed with 'a controlled amount of liquid water injected'ginto the system through line 106, and the rein operation of the fractionation system, liquid oxygen is removed from the base of low-pressure fractionating column 82 through line 12%), heat exchanged with the partially liquified air stream in line 76 and then taken through reversing valve 116 and line 122 to regenerative exchanger 124. As described above, in connection with the flow of product nitrogen through exchanger 100, the shift from the high-pressure production cycle in line 122 to the lower pressure regenerative cycle'causes water to be removed from exchanger-124 and the rest of the system in the form of both vapor and liquid via line 126, reversing valve 110 and line 128. Since the water vapor content in line 128 is substantially equal to that in the air feed stream entering the system through line 104, relatively little refrigeration is lost from the system due to heat of the condensation.

In Figure 3, liquid water in line 150 is mixed in controlled amounts with the air feed stream in line-152. This mixture, which is at a pressure of about lp.s.i. and at a temperature of about 60 F then enters reversing exchanger 154 via reversing valve 156 and line 158. In the particular cycle shown, the wet intake air travels through path 160 where it is cooled by means of indirect heat exchange with a countercurrently flowing stream of product nitrogen in path 162 and entering the exchanger via line 164. The resulting cold air in path 160 is removed through line 166 and taken to the fractionation system where product separation is effected. Product oxygen enters reversing exchanger 154 through line 168 and flows through path 170'and, after giving up its cold to the exchanger, is removed therefrom by means of line 172. It will, of course, be appreciated that'in operating reversing exchanger'154, the individual paths through which air, nitrogen and oxygen flow are periodically changed. When, for example, the flow of nitrogen in path 162 is changed to path 160, the problem of liquid water entrainment with product nitrogen in line 174 is presented." However, as in the case of the conventional regenerative exchangers previously described, the balance between the water vapor content leaving the system through line 174 and that entering the system via line 152 is maintained at about the same level by injection of liquid water into the plant intake air, as provided by the present invention.

For any given flow conditions, regenerator design, etc., the quantity and rate of liquid Water added through line 24 can readily be determined by simple experiment once steady state conditions have been reached. Thus, in applying the principles of my invention to a conventional oxygen plant the amount of water introduced into the air feed as a liquid during a given cycle should be gradually increased until the quantity of water vapor in the air feed -rative cooling, the procedures formerly suggested involve procurement of such cooling by injecting the liquid into the warm end of the regenerators during the production side of the cycle. Such a method, of necessity, is awkward and complex because of the numerous individual injection lines that must lead to the warm end of each regenerator. On the other hand, by the process of my invention loss of valuable refrigeration can be very substantially minimized merely by injecting liquid water into a single line during the air feed cycle after which such Water is equally distributed to all generators receiving the air feed during a given period.

It should be pointed out that in construing the scope of the claims which'follow, the expression pair of re generative cooling paths is intended to mean either two individual regenerators or two individual groups of such regenerators. Also, for the purposes of this description, regenerative and reversing exchangers are to be considered equivalents. r

I claim:

1. In a process for fractionating a gaseous mixture containing water vapor and having other components that can be readily separated by means of known low temperature fractionation techniques, wherein said mixture is first compressed, cooled, expanded, liquefied and at least a part of the resulting liquid mixture is evaporated in a fractionating system, wherein a compressed inflowing stream of said gaseous mixture is cooled by pas sage through a first heat exchange path and the water vapor in said inflowing stream is thereby converted to liquid and deposited in the warm end of said first path while an outflowing cold product stream is simultaneously counterflowed through a second heat exchange path, periodically interchanging the flow of said inflowing stream and said outflowing product stream in said paths so that each undergoes alternate charging and refrigeration cycles, the method for improving the refrigeration capacity of said paths which comprises injecting liquid water into saidinfiowing stream and depositing such liquid water together with the condensed water vapor in said inflowing stream in the farm end of said first path, discontinuing the flow of said inflowing stream through said first path, counterflowing a cold product stream through said first path, whereby the latter is cooled and said cold product stream together with liquid water and water vapor are withdrawn from the warm end of said first path, the quantity of liquid water injected into said inflowing stream being such that the quantity of water vapor in said cold productstream thus withdrawn is at least about equal to the quantity of water vapor originally present in said inflowing stream, and repeating the above cycle with respect to said second path.

2. In a process for fractionating a gaseous mixture containing water vapor and having other components that can be readily separated by means of known low temperature fractionation techniques, wherein said mixture is first compressed, cooled, expanded, liquefied and at least a part of the resulting liquid mixture is evaporated in a fractionating system, wherein a compressed inflowing stream of said gaseous mixture is cooled by passage through a first path in areversing heat exchange zone in heat exchange with a counterflowing cold product stream along a second path in said zone, said first path progressively decreasing in temperature from end to end to effect cooling of said inflowing stream whereby thewater vapor in said stream is converted to liquid and deposited in the warm end of said first path, and wherein the flow in said paths is periodically reversed so that each undergoes alternate charging and refrigeration cycles, the method for improving the refrigeration capacity of said paths which :comprises injecting liquid water into said inflowing stream and depositing such liquid water to gether with the condensed water vapor in said inflowing stream in the warm end of said first path, discontinuing the flow of said inflowing stream through said first path, counterflowing a cold product stream through said first path, whereby the latter is cooled and said cold product stream together with liquid water and water vapor are withdrawn from the warm end of said first path, the quantity of liquid water injected into said inflowing stream being such that the quantity of water vapor in said cold product stream thus withdrawn is about equal to the quantity of water vapor originally present in said inflowing stream, and repeating the above cycle with respect to said second path.

3. In a process for fractionating a gaseous mixture containing water vapor and other components that can be readily separated by means of known low temperature fractionation techniques, wherein said mixture is first compressed, cooled, expanded, liquefied and. at least :a part of the resulting liquid mixture is .evaporated'rin a fractionating system wherein a compressed :inflowing stream of said gaseous mixture is passed in :one :direction of flow through a reversing heat exchange zone in indirect heat exchange relation with a counterfiowing product stream along a path vtherein progressively decreasing in temperature from .end to :end to cool said stream thereby converting .the 'water vapor in said infiowing stream {to liquid and depositing the latter in the warmend of said path, and wherein a cold product stream is passed subsequently through the same path in the opposite .direction of flow :after said infiowing stream has :ceased flow therein, .the method for improving the refrigeration efficiency of said path which comprises injecting liquid water into said infiowing stream and depositing such liquid water together with the condensed water vapor in said infiowing stream in the warm .end of said path, discontinuing the flow of said infiowing stream through said path, counterflowing a cold product stream through said path, whereby the latter is .cooled and said cold product stream together with water and water vapor are withdrawn from the warm end of said path, the quantity of liquid water injected into said infiowing stream being such that the quantity of water vapor in said cold product stream thus withdrawn is about equal to the quantity ;of water vapor present in said infiowing stream, and repeating the above cycle.

4. In a process for fractionating a gaseous mixture containing water vapor and other components that can be readily separated by means of known low temperature fractionation techniques, wherein said mixture is first compressed, cooled, expanded, liquefied and at least a part of the resulting liquid mixture is evaporated in a fractionating system, wherein a compressed infiowing stream of said gaseous mixture containing water vapor is cooled by passage through one oi? a pair of regenerative cooling paths and the water vapor in said infiowing stream is thereby converted to liquid and deposited in the warm end of said one of said paths while an outflowing cold product is simultaneously counterfiowed through the other of said paths thereby refrigerating said other path, and wherein the flow in said paths is periodically reversed so that each undergoes alternate charging and refrigeration cycles, the method for improving the refrigeration capacity of said paths which comprises injecting liquid water into said infiowing stream and depositing such liquid water together with the condensed water vapor in said infiowing stream in the warm end of said one of said paths,discontinuing the flow of said infiowing stream through the last mentioned path, whereby the latter is cooled and said cold product stream together with liquid water and water vapor are withdrawn from the warm end of said last mentioned path, the quantity of liquid water water vapor originally presentin said infiowing stream,

and repeating the above cycle with respect to the other of said paths.

5. ha process for fractionating a gaseous mixture containing water vapor and other components that can be readily separated by means of known low temperature fractionation techniques, wherein said mixture is first compressed, cooled, expanded, liquefied and at least a part of the resulting liquid mixture is evaporated in a iractionating system, wherein a compressed infiowing stream 'of said gaseous mixture is cooled by passage through the first of a pair of regenerative cooling paths and the water vapor in said infiowing stream is thereby converted to liquid and deposited in the warm end of said first path while an outflowing gaseous product is simultaneously counterflowed through the second of said paths thereby-refrigerating said second path, and wherein the 'fiow in said paths is periodically reversed so that each undergoes alternate charging and refrigeration cycles the method for improving the refrigeration capacity of said paths which comprises injecting liquid water into said infiowing stream and depositing such liquid water together with the condensed water vapor in said infiowing stream in the warm end of the first of said paths, discontinuing the flow of said infiowing stream through said first path, counterflowing a cold product gas stream through said first path, whereby the latter is cooled and said cold product gas together with liquid water and water vapor are withdrawn from the warm end of said first path, the quantity of liquid water injected into said infiowing stream being such that the quantity of water vapor in said cold product stream thus withdrawn is about equal to the quantity of water vapor originally present in said infiowing stream, and repeating the above cycle with respect to said second path.

6. The process of claim 5 in which the liquid water added to said infiowing stream amounts to from about 20 to about 200 percent of the water vapor originally present in said incoming stream.

7. The process of claim 3 in which the gaseous mixture is air. 7

8. The process of claim 4 in which the gaseous mixture is air. q

9. The process of claim 6 in which the gaseous mixture is air.

10. The process ofclaim 3 in which the infiowing stream is saturated with water vapor prior to injection with liquid water.

11. The process of claim 4 in which the infiowing stream is saturated with water vapor prior to injection with liquid water.

References Cited in the file of this patent UNITED STATES PATENTS

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