US2977771A

US2977771A - Processing gas mixtures

Info

Publication number: US2977771A
Application number: US665916A
Authority: US
Inventors: Frank G Pearce; Scott W Walker
Original assignee: Pan American Petroleum Corp
Current assignee: Pan American Petroleum Corp
Priority date: 1957-06-17
Filing date: 1957-06-17
Publication date: 1961-04-04
Anticipated expiration: 1978-04-04

Description

April 1961 F. G. PEARCE ETAL 2,977,771

PROCESS GAS MIXTURES Filed June 1'7, 195'? 2 Sheets-Sheet 1 BY SCOTT W. WALKER FIG. I M 7 A r roan/5r April 4, 1961 F. G. PEARCE EIAL PROCESS GAS MIXTURES Filed June 17, 1957 2 Sheets-Sheet 2 I I l |ZONE 0F FOG FORMATION I +60 I PACKING TEMPERATURE 0 1.; NO RECIRCULATION g AT WARM END 2 PRODUCT GAS 35 TEMPERATURE RECIRCULATION u.| AT WARM END l- C l I o 20 40 so so DISTANCE (PERCENT) FROM WARM END OF REGENERATOR FIG. 2

INVENTORS FRANK G. PEARCE By SCOTT W. WALKER ATTOF/V Y "United States Patent() PROCESSING GAS MIXTURES Frank G. Pearce and Scott W. Walker, Tulsa, Okla., as-

signors to Pan American Petroleum Corporation,.Tulsa, Okla, a corporation of Delaware V Filed June 17, 1957, Ser. No. 665,916 9 Claims. (Cl. 62-13 and subsequent fractionation thereof by first compressing I 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 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 periodically the performance of work thereby furnishing the necessary reversing cycles between the incoming air feed and the I 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 thelast 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 d'epressurized against the other group of regenerators through which a cold product gas stream has beenflowing. 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 in the 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 presentin 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 efficient 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 essentially a plurality of parallel paths for the stream in each passageway. These paths are so metal bonded together as to establish a metal-to-metal thermal contact throughout the entire contact length of the unit. Likewise the individual passageways of an exchanger are joined with metal-to-metal contact. Reversing cold exchangers of this type, therefore, are characterized by possessing a high rate of heat transfer and a thermal efficiency unaffected by cycle time, since little dependence is placed on storage of heat in metal.

'per day would be as much as 570,000 B.t.u.s/hr.

2,977,771 Patented Apr. 4, 1961 These'reversing cold exchangers are also utilized to remove all of the higher boiling 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, before 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.

The loss 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 liquidas would be the case where a water fog is physically entrained with the produce gas streamthe heat of condensation constitutes a direct heat input to the system. In tonnage oxygen plants, for example, where this problem becomes most acute, the refrigeration losses of a plant having a capacity of about 1,000 tons of oxygen 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.

Accordingly, it isan object of our invention to prevent substantial refrigeration losses from the system, such losses being caused by entrainment with the cold product gas of appreciable quantities of liquid water and/or similar substances deposited in the warm end of the exchanger during the air flow cycle. Briefly, this object is accomplished by providing means for decreasing the difference in temperature between said warm end and the cold product gas present in said warm end during the refrigeration cycle.

In accordance with our invention, we are able to avoid refrigeration losses of the above type by taking a portion of the product stream issuing from the warm end of the regenerator or reversing exchanger, and returning such portion to the regenerator or exchanger at about the level therein farthest removed from said warm end where liquid water exists and where substantial quantities of ice begin to form.

The level at which the warm product (50 to 60 F.) stream is recirculated to the unit ordinarily is located at a distance from the warm end thereof corresponding to from about 10 to about 20 percent of the length of the unit. Under such conditions the warm recirculated stream raises the temperature of the metal packing in the regenerator, or of the metal walls of the reversing exchanger, from the point of injection to the opposite end of the warm portion of the unit. This, in turn, decreases the difference in temperature between the cold product stream flowing through the unit and that portion of the packing or wall surface on which liquid water is ordinarily deposited.

As previously pointed out, the amount of water discharged as liquid from the warm end of the unit, together with the product gas, amount to from about 20 to about 50 percent of the amount introduced into the system via the air feed stream. Accordingly, in order to prevent the aforesaid quantity of water from being rejected from the system as liquid along with the product gas, from about 10 to about 30 percent of the warm product stream ordinarily should be recirculated as contemplated herein. The actual amount of warm product recirculated in accordance with our invention, however, will depend upon the performance characteristics of the regenerators or reversing exchangers used. In any event, the quantity of warm product gas recirculated will-correspond approximately to the amount required to bring the temperature of the packing, for example, in the case of a regenerative type unit, up to a level such that the quantity of liquid water remaining in the warm end is held to a minimum during the refrigeration cycle.

For a better understanding of our invention, reference is made to the accompanying drawings in which:

Figure 1 represents a diagrammatic form of apparatus included within the scope of our invention, and

Figure 2 is a plot showing the temperature profile in said apparatus with and without the application of our invention.

The flow rates, as well as other quantities appearing in the description of the drawings, are given on the basis of an operation capable of producing 1,000 tonsof oxygen per day. Referring now to Figure 1, there isshown a single regenerator 2 filled with aluminum packing 4. The upper end of the regenerator is connected to a flow line 6, which in turn communicates directly with common header 8 and indirectly on alternate cycles with air feed header 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 generators such as 2, are similarly connected to headers S, 10, 12, 16, 18 and 20.

Clean air is introduced at about 100 p.s.i.g. and 60 F. into line 22 at the rate of 5,700 M s.c.f.h. This compressed air stream then flows through air feed header 10, valved line 24, 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 26, water vapor condenses and is deposited on the packing. The water condensed from the air feed stream and thus deposited on the packing amounts to about 500 lbs./ hour. After the air has passed the warm end of the regenerator, it is essentially anhydrous and becomes progressively colder until it is withdrawn from the base of regenerator 2 at a temperature of about 265 F. through line 14. From this point the cold air stream is taken through common header 16, valved line 28, cold air header and out through line 30. The stream in the line is then subsequently split with a major port-ion 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 30 forms no part of our 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 this purpose is considered necessary.

After the required air flow cycle through regenerator 2, which may be a period of two or three minutes,

valves

32 and 34 are closed and valve 36 is opened. Operation of these valves is controlled by automatic timing device 38 actuating line 40 which in turn is operatively connected to

valves

32, 34 and 36. 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 42, cold product header 18, line 44 and check valve 46, 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 re- 7 generator is not only cold but is extremely dry. As the 4 product gas 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 vater'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 this vapor is converted to a fog or mist of liquid water droplets. In conventional procedures it is apparent that a net loss in refrigeration from the regenerators will be incurred if the water vapor leaving the system together with the product gas in line 6 is less than thewater vapor brought in via the air feed system. The amount of refrigeration lost in this manner, therefore, corresponds to the evaporative cooling that would have been furnished this system if such liquid water has been converted to vapor in the regenerator prior to removal therefrom. In accordance with our invention, however, conditions favoring fog formation are minimized by bleeding off between about 10 to about 30 percent of this product gas from line 6 through line 50 and forcing this warm stream (at about 60 F.) back into regenerator 2 at the level indicated by means of blower 52.

By inj-cting the warmer product gas back into the regenerator in this manner, it will be seen that the temperature of the cold product gas entering the warm end of regenerator 2 is increased, thereby decreasing the difference in temperature between said product gas and aluminum packing 4, which in turn renders conditions for fog formation less favorable. For any given flow conditions, regenerator design, etc., the quantity and rate of warm product gas recirculated through line 50 to regenerator 2 can readily be determined and regulated by means of valve 54, once the process is properly lined out. Thus, in applying the principles of our invention to a conventional oxygen plant, the amount of warm product gas injected into regenerator 2 via line 50 during a given cycle, should gradually be increased until the Water vapor content of the product stream issuing through line 48, is substantially equal to the amount of water vapor in air feed line 22.

The effect of recirculating warm product gas to the warm end of the regenerator, as described above, on the difference in temperature of the metal packing and of the product gas travelling toward said warm end, is clearly shown in Figure 2. The improvement afforded, i.e., decrease in temperature difference between the packing and product gas at the warm end of the regenerator, by our invention is evident from a comparison of the packing temperature profile (curve A) with broken lined curve'B (obtained by warm product gas recirculation). The substantial difference in temperature existing at the warm end between the packing (curve A) and the product gas temperature when no warm product gas is recirculated (curve C) is likewise quite apparent. From these curves it is obvious that the opportunity for fog formation has been materially decreased by recirculation of warm product gas in accordance with our invention. The area of the regenerator where this result is particularly noticed is that portion of Figure 2 designated zone of fog formation.

At the flow rates given above, the increase in refrigeration efficiency obtained in regenerator performance, when employing the process of our invention, over regenerator efficiency secured without such improvement, but under otherwise identical conditions, amounts to from approximately 5 to about 15 percent, measured in terms of oxygen production.

From the foregoing description it will be apparent that our invention has many possible applications in gasseparation and related'fields. It is particularly applicable to processes for separation of gases containing vapors condensible under the conditions of operation and which ordinarily remain liquid in the warm end of the exchanger during the refrigeration cycle.

In the present description and claims the expression warm end used with reference to that portion of the regenerator or other reversing heat exchange zones from whichproduct gas is recovered, is intended to designate that part of the regenerator or reversing heat exchange zone in which liquid water is present. Also it is to be pointed out that in construing the scope of the claims which follow, the expression pair of regenerative cooling paths is intended to mean either two individual regenerators or two individual groups of such regenerators. For the purpose of this description, regenerative and reversing exchangers are to be considered equivalents.

We claim:

1. In a process for recovering a gaseous component from a gaseous mixture containing water vapors wherein a compressed stream of said gaseous mixture is cooled by passage of said stream through a first heat exchange path progressively decreasing in temperature from end to end, whereby said vapors are condensed and deposited as liquid water in thewarm end of said first path while' an outflowing cold product stream is simultaneously oounterflowed through a second heat exchange path progressively increasing in temperature from end to end, withdrawing said product steam from the warm end of said second path, periodically interchanging the flow of said compressed stream and said product stream in said paths so that each path undergoes alternate charging and refrigeration cycles, the method of improving the refrigeration capacity of said paths which comprises recirculating to a point upstream from which said product stream was withdrawn a portion of said withdrawn product stream to the warm end of said second path and in a portion of .said second path where liquid water exists while the latter is undergoing said refrigeration cycle whereby the temperature difference is decreased between the warm end in said second path and said product stream 7 in the warm end of said second path, the amount ofsaid product stream circulated being sufficient to render the' amount of said vapors in said withdrawn product stream about equal tovthe amount of said vapors in said compressed stream. 2. The. process ofclaim l wherein said recirculated portion is introduced into the warm end of said second path at a distance from the outlet of said warm end corresponding to not more than about 20 percent of the length of said path.

3. The process ofclaim 1 in which the amount of product stream recirculated corresponds from about 10 counter-flowed through the other of said paths thereby refrigerating said other path, withdrawing said product steam from the warm end of said other path, and wherein the flow in said paths is periodically reversed so that each path undergoes alternate charging and refrigeration cycles, the improvement which comprises recirculating to a point upstream from which said product stream was withdrawn a portion of said product stream thus withdrawn to the Warm end of said other of said paths and in a portion of said other of said paths where liquid Water exists while said other of said paths is undergoing said refrigeration cycle whereby the temperature difference is decreased between the warm end of said other path and said product stream in the warm end of said other path, the amount of said product stream recirculated being suflicient to render the amount of said vapors in said withdrawn product stream about equal to the amount end of said other path.

7. The process of claim 5 wherein said recirculated portion is introduced into the warm end of said other of said paths at a distance from the outlet of said warm end corresponding to not more than about 20 percent of I .the length of said other of said paths.

' 8. In a process for recovering a gaseous component from a gaseous mixture containing water vapors wherein a condensible stream of said gaseous mixture is passed in one direction of fiow through a reversing heat exchange zone in indirect heat exchange relation with a counterflowing cold product stream along a path therein progressively decreasing in temperature from end to end to cool said stream thereby converting said vapors into liquid water and depositing the latter in the warm end of said path, wherein a cold product stream is passed subsequently to about 30 percent of that withdrawn from the warm end of the heat exchange path undergoing the refrigeration cycle.

4. The process of claim 3 wherein the recirculated product stream is returned to the heat exchange path undergoing said refrigeration cycle at about the lowermost level in such path where saidli'quid water is present.

5. In a process for recovering'a' gaseous component from a gaseousmixture containing water vapors wherein a compressed stream of said gaseous mixture is cooled by passage through one of a pair of regenerative cooling paths and said vapors are thereby converted to liquid water and'deposited in the warm end of said one of said paths while outflowing cold product gas is simultaneously vthrough the same path in the opposite direction of flow after said compressed stream has ceased fiow therein, and

withdrawing said product stream from the warm end of said path, the method forimproving the refrigeration efficiency of said path which comprises recirculating to a point upstream from which said product stream was withdrawn a portion of said product stream thus withdrawn -to'the warm end of said path and in a portion of said path where liquid water exists 'while additional quantities of said product stream continue to flow through said path whereby the temperature ditierence is decreased between said warm end and said product stream in said warm end, the amount of said product stream circulated being sufficient to render the amount of said vapors in said withdrawn product stream about equal to the amount of said vapors in said compressed stream.

9. The process of claim 8 in which the vapors of the condensible liquid are water vapors and the amount of product stream recirculated corresponds to from about 10 to about 30 percent of that withdrawn from the warm end of said path.

References Cited in the file of this patent UNITED STATES PATENTS Palmer et al. July 10, 1956