US2671324A - Method of gas separation, including impurity removing steps - Google Patents

Description

P. R. TRUMPLER METHOD OF GAS SEPARATION, INCLUDING Marli 5. 1954 IMPURITY REMOVING STEPS 2 Sheets-Sheet l Fld Jan. 26, 1949 PAUL R. RUMPLER BYCQ INVENTR.

ATTORNEYS March 9, 1954 R. TRUMPLER METHOD OF GAS SEPARATION, INCLUDING IMPURITY REMOVING STEPS 2 Sheets-Sheet 2 Filed Jan. 26, 1949 R R. E E N m n R m M o V U W m Eln y A R// /T f L f u Aff@ Dl v. B o m umm umm om com 02% www, T l UmNl u v 7 /w f A@ fmnnnlvmm umm uuu.. L nur umm ovm umm EN atentecl Mar. 9, 1954 METHD GF GAS SEPARATION, INCLUDING IMPURITY REMOVING STEPS Paul E. Trumpler, Olean, N. Y., assigner to The M. W. Kellogg Company, Jersey City, N. J., a

corporation of Delaware Application January 26, 1949, Serial No. 72,783

9 Claims. 1

This application is a continuation-in-part of my co-pending application Serial No. 533,608, filed May 1, 1944, now Patent No. 2,460,859.

The invention relates to improvements in the treatment of a gas or gaseous mixture to effect liquefaction and/or fractionation thereof. More particularly, the invention relates to improvements in such treatments as applied to air.

Gases such as air may be treated to effect liquefaction and/or fractionation thereof by compressing the gas, cooling compressed gas by heat interchange with cold product, liquefying a portion of the precooled gas by further heat interchange, expanding another portion `with the performance of external work, and passing expanded gas through one cr more heat interchange steps, particularly the above-mentioned precooling step, with or without intermediate fractionation of the expanded gas.

The precooling of compressed air in such methods may be carried out in reversing regenerators which operate in pairs in periodically alternating, or reversing, time cycles. When the air is separated into nitrogen-rich and oxygen-rich gaseous fractions one portion of the feed air is cooled by heat exchange with the nitrogen-rich fraction in one set of regenerators while another portion is simultaneously cooled in a separate set of regenerators by the oxygen-rich product.

At times, however, it is desired to obtain one of n and the oxygen fraction is utilized to cool a portion of the feed air in a heat exchanger. A different method for recovering the cold from oxygen and yet maintain its high purity involves a heat exchange procedure wherein all the feed air is passed through one of the nitrogen regenerators while the oxygen simultaneously is caused to flow through conduits positioned therein. In this way the air is cooled regeneratively by the nitrogen and by direct heat exchange with the oxygen. However, in the case where a fraction of the air is recovered as a liquid product fraction, the feed air is cooled only by that fraction which remains in the gaseous phase.

An important feature of regenerators is their ability to bring about the removal of water, carbon dioxide or other relatively higher boiling impurities. The impurities are deposited in a regenerator as the air is cooled and are sufficiently evaporated and removed therefrom during the opposite phase of the reversing time cycle by (Cl. (i2-175.5)

counterflowing cold product to permit continuous operation for a suitable period of time. Thus, when reversing regenerators are utilized as means for heat interchange to cool inlowing compressed air, during one-half of a reversing time cycle as the air is cooled, Water and carbon dioxide precipitate will accumulate as liquid or solid deposits on the metal surfaces of a passageway. Then, before accumulation of solid deposits become great enough to obstruct the passageway, the iiow of air therein is replaced by one of the counterlowing streams of cold product to permit the gaseous product to be utilized as a scavenging medium by passing in reverse direction over the accumulated deposits, thus evaporating and carrying the deposits out of the heat interchange zone. Meanwhile, the inowing air is caused to be cooled and to deposit further quantities of impurities on the metal surfaces of the passageway through which the product fraction previously had been flowing. This reversing procedure for continuously scavenging impurities usually is carried out with the product fraction whose recovery in the pure state is not desired, or from which the impurities subsequently can be more readily removed. The direction of gaseous flow in either reversing passageway is reversed as a result of the interchange of passageways between the reversing streams, but each of the gaseous streams undergoing reversal always ilows through the heat interchange zone in the same direction, iirst in one passageway and then in the other. Another stream, or a plurality of streams, of the product fractions, of course, may be caused to pass through other passageways in the same heat interchange zone, which may or may not be reversing.

Under certain conditions the removal of the precipitate by the evaporating or scavenging gas is limited in that the capacity of the latter for removing the precipitate under such conditions is insufcient to remove such deposits at the same rate per cycle at which they are precipitated. Under these conditions the amount of precipitated material in the heat exchange zone ordinarily requires periodic shutdowns to eiect removal of the accumulation. In general this condition occurs in operations in which the cold stream or streams pass through the heat exchange zone at a lower Weight rate of flow than the Warm stream being treated or in operations in which the specic heat of the cold stream or streams is less than the specific heat of the warm stream. When either or both of these conditions exist, the diiference in temperature between the warm stream and the cold stream or streams at corresponding points in the heat exchange zone increases toward the cold end of the zone. rhis divergence of the temperatures of the cold and warm streams toward the cold end of the heat exchange zone may be so great as to limit the capacity of the available scavenging gas for removing all of the deposits at the same rate at which they are formed. .A relatively great ternperature difference between the warm and cold streams at the cold end o the heat exchange zone may be tolerated when there is available a relatively large volume of scavenging gas whereas a relatively small volume of scavenging gas will remove the deposited material at the necessary rate only if the said temperature dierence is substantialiy restricted.

In connection with air it may be said that the removal of a deposited impurity in the heat exchange zone may be maintained at a rate equal to the rate of precipitation if the ratio of the vapor pressure or" the impurity at the temperature of the evaporating or scavenging gas to the vapor pressure of the impurity at the temperature of the gas being treated, is approximately equal to the ratio o1' the volume of the gas being treated to the volume of the scavenging gas.

It is an object of my invention to provide an improved method for treating a gas or a mixture of gases in a reversing regenerative heat exchange system of the type described above, wherein the cold streams pass through the heat exchange zone at a lower weight rate of ow than the warm stream or streams being treated or wherein the specinc heat of the cold stream or streams is less than the specific heat of the warm stream, which provides for restricting the difference in temperature between the cold stream or streams and the warm stream near the coid end or the heat exchange zone to a maximum which permits complete removal of precipitated deposits by the cold stream or streams employed as a scavenging media.

It is a further object of my invention to provide an improved method for treating air in a reversing regenerative heat exchange system of the type described above, wherein the air being treated is cooled in said heat exchange system by cold backward-flowing products which flow through the heat exchange Zone at a weight rate of iow lower than that or" the warm air stream, which provides for restricting the difference in temperature between the warm and cold streams near the cold end of the heat exchange zone to permit complete removal of deposits precipitated from the air stream by the scavening media employed.

It is a further object of the invention to provide an improved method for fractionating air in which the air stream is cooled by backwardflowing product stream or streams in a reversing regenerative heat exchange system of the type described above and wherein the specinc heat of such backward-flowing product streams is less than the specic heat of the air stream, and in i.

4 occur, to a temperature lower than that produced in the zone of precipitation by the cold stream normally passed through the regenerative heat exchange Zone. Such further cooling is produced by passing a cooling fluid, separate from the cold stream normally passed through the heat exchange zone, through a separate path in indirect heat exchange relation, preferably countercurrent to the Warm stream flow. The separate cooling .I stream may be passed in heat exchange relation with the zone of precipitation of the regenerator being cooled or with the corresponding zone o the regenerator which is acting on the gas stream or, preferably, the separate cooling stream is y passed simultaneously in indirect heat exchange with both the regenerator being cooled and the regenerator in contact with the warm stream.

This arrangement reduces the diiierence in temperature between the warm stream and the c cold stream in those portions of the regenerative heat exchange zones in which precipitation occurs, whereby the complete evaporation and removal of such deposits by the scavenging stream is accomplished in cach cycle of the operation.

The separate cooling stream may be obtained in any suitable part of the system of which the regenerative heat exchange zones form a part. The separate stream may be derived from the cooled warm stream emerging from the heat exi) change zone, the diverted portion being recombined with the cooled Warm stream after passage of the diverted portion through the separate heat exchange path or paths. Alternatively, the separate cooling stream may be obtained by4 divert- ,i ing a portion of the cold stream abouttc be passed through the regenerative heat exchange system, this diverted portion being recombined with the cold stream prior to passage thereof through the regenerator. If the system in use involves merely liquefaction of a portion of the warm stream, the cold stream passing through the regenerator constitutes that part of the warm stream which has been expanded to produce refrigeration in the liquefaction step. In such a system a portion of the cooled Warm stream about to be expanded may be diverted temporarily for passage through the separate heat exchange path or a portion of the expanded uid, prior to passage to the liquefaction zone, may be passed through the separate heat exchange path. Alternatively, a portion of the expanded fluid, after use in the liquefaction zone and prior to passage through the regenerative heat exchange zone, may be diverted for passage through the said separate heat exchange path.

An important application of the invention relates to the fractionation oi air to produce oxygen-rich and nitrogen-rich streams. In such a system the cooled compressed air is passed to a fractionating system comprising a fractionating zone, conduit lines for conducting the cooled compressed air to the fractionating zone, and conduit lines for conducting one or more product fractions from the fractionating zone to the regenerative heat exchange zone. In such a fractionation treatment of air the separate cooling uid may be obtained from the cooled compressed air or from the cold product stream to be passed through the 'regenerative heat exchange zone. Or, the separate cooling luid may be obtained from any part of the fractionating system, such separate cooling uid stream being returned to the system after passage through the separate cooling path or paths.

In the fractionation treatment of air, the system may be arranged toproduce a relatively pure liquid oxygen product, in which case the other fraction consisting of relatively impure nitrogen, is passed through the regenerative heat exchange system to supply substantially all of the cooling and scavenging treatment. Alternatively, the fractionation system may be arranged to produce a relatively pure gaseous oxygen product stream and a nitrogen fraction. In this system the gaseous oxygen product stream may be passed through the regenerative heat exchange zone in indirect heat exchange stream with the incoming air stream, in which arrangement the nitrogen stream supplies a part of the cooling of the air stream and all the scavenging of the regenerators. Alternatively, the oxygen gas product stream may be employed for both cooling and scavenging. In this arrangement, two separate sets of reversing regenerative heat exchangers are employed, one for the nitrogen stream and one for the oxygen stream. In this arrangement, the air to be treated is split into portions proportion-ate to the volumes of nitrogen and oxygen product streams. The improved method of this invention may be applied to all these various arrangements in accordance with the general principles described above.

Further explanation of the present invention will be made with reference to the `accompanying drawings. It is to be understood that reference to the drawings is by way of example only and is not restricted to the physical limitations of the apparatus indicated in the gures, wherein:

Figure 1 is a diagrammatic representation of a process flow arrangement for the liquefaction and fractionation of air under relatively moderate superatmospheric pressure to show a modification for unbalancing a normal countercurrent heat exchange relationship by illustrating heat exchange between compressed air and colder, less compressed, nitrogen-rich product fraction in the colder portions of a reversing heat exchange zone exemplied by reversing regenerators,

Figure 2 illustrates another modification for unbalancing the normal countercurrent heat exchange relation between the gaseous streams exemplified by the air and nitrogen-rich streams passing through the regenerators of Figure 1, and,

Figures 3 and 4 illustrate further modications for unbalancing the heat exchange relation between counterowing gaseous streams again exemplified by the air and nitrogen-rich streams passing through the regenerators of Figure 1.

Referring to the drawing of Figure 1, illustrating the operation of a process to make liquid oxygen, air is brought into the processing system through line I to air lter 2 for removal of dust, or other suspended matter, before being taken through line 3 to the low pressure inlet of a twostage compressor d. Compressor 4 raises the pressure of the air, with interstage cooling in cooler 5, to a relatively moderate pressure of about 100 pounds per square inch absolute. From the last stage of compression, the air iiows at about 215 F. through line 5 into aftercooler 'i for removal of heat of compression by a cooling step with water. A major part of the water vapor, drawn into the system with the air, is condensed as the result of the temperature reduction in the aftercooler. This water condensate is separated in separator vessel 8 and nally disposed of by means of the valved drain line 9. The partially dried air is subjected to another ltering procedure in lter i 0 as a nal precautionary measure to eliminate suspended 0r entrained matter from the air. The inflowing compressed air is then conveyed by line II at about 76 F. and 99 pounds per square inch absolute to either of reversing regenerators I2 or I3 for cooling therein to a relatively low temperature which may be relatively near its liquefaction temperature.

For this `precooling of air, regenerators I2 and I3 are merely illustrative of a plurality of such vessels which may be employed for this purpose, being dependent upon the volume of air undergoing separ-ation. Furthermore, the temperature reduction need not be accomplished entirely in one vessel since in some instances, it may be expeditious to cool step-wise in one or more separate vessels. The regenerators are filled with a thermal regenerative packing material which comprises, for example, a metal in the form of tiers of pancakes of wound corrugated ribbons. The regenerators thus are provided with regenerative packing of large surface area, good heat conductivity and having substantial heat storage capacity. Hence, in its operative relation to the cooling of air, a regenerator is cooled to a desired low temperature by passing a cold product fraction derived from the air in heat interchange relation with the metallic packing whereby the metal gives up heat and becomes exceedingly cold. Subsequently, the warm inlowing compressed air is passed in heat interchange relation with the thus chilled metal packing `and gives up heat thereto whereby the compressed air is substantially cooled. For continuous operative efciency, it is common practice to cool air by passing it in one direction over the regenerative packing of one regenerator while simultaneously passing a cold product fraction over the metal packing of another regenerator in counterow to the direction of air flow. Then, the flows of compressed air and cold product fraction are periodically interchanged between the two regenerators so that the direction of fluid flow in either of the regenerators is reversed by each interchange but the iiow of air and product is always in the same direction. Regenerators I2 and I3, therefore, are illustrative of any pair of such vessels for carrying out their cooling function in the described manner. The precooling of the compressed air to the relatively low temperature required to precipitate carbon dioxide need not necessarily be conducted entirely in a single regenerator or exchanger since it may be expeditious in some events to employ a plurality of vessels for such cooling. Likewise, more than two v regenerators may be used for the reversing procedure. For example, two regenerators may be subjected to cooling while the third is being warmed by the air.

In Figure 1, when regenerator I2 is employed to cool air and the regenerative packing of regenerator I3 is being cooled, the inflowing compressed air is withdrawn from line I I through line I4 and taken through a, reversing valve I5 and line I6 into the bottom portion of regenerator I2. The air passes upwardly over the previously chilled metallic packing I1 of this vessel and this passage results in a lowering of the air temperature to a degree relatively near its liquefaction temperature, for example about -239 F., by the time it reaches the top of the regenerator. The cooled air passes from this cooling step through line I8 and is caused to ow through check valve I9 and line 20 to transfer line 2I for conductance therethrough to surge drum 22. Meanwhile, nitrogen-rich product, derived by subsequent fractionation of the air, is taken at a. relatively low temperature, for example at about -269 F.from line 23 through check valve 24 and line 25 into the top of regenerator I3. The cold product fraction passes downwardly over previously warmed regenerative packing 26 in this vessel and is warmed thereby to an exit temperature of about 68 F. when it leaves the regenerator by way of line 27, reversing valve 28 and lines 29 and 30. Alternatively, to the foregoing ow procedures, during the period when regenerator I3 is employed for cooling air and the regenerative packing I1 is being cooled by cold product, the inflowing compressed air is withdrawn from line II through reversing valve 3| and passed through line 21 into the bottom of regenerator I3. The air then passes upwardly over the previously cooled regenerative packing 26 and also is cooled thereby to the same degree as in regenerator I2. The cooled air then passes out of regenerator I3 by way of line 25 and check valve 32 to transfer line 2l. Meanwhile, the cold` product fraction from line 23, since it is under lower pressure than the compressed air in line 25 is unable to pass check valve 24, is directed through line 33 and check valve 34 into line I8 and then through regenerator I 2 to cool the regenerative packing Il. After absorbing heat from the latter packing and being warmed thereby to about 68 F., the nitrogen-rich product fraction is withdrawn from regenerator I2 by way of line I6, reversing valve 35 and line 3i?. It should be understood, of course, that the transfer of heat through the medium of a body of regenerative material fluctuates and in view of this fact regenerative temperatures are approx- Imate average temperatures.

It is the function of reversing valves I and 3| and 28 and 35 to periodically direct the flow of the compressed air and product fraction alternately to and from regenerators I2 and I3. These valves, therefore, are diagrammatically shown in Figure 1 as being reversing valves. The valves usually are automatically operated by a timing mechanism, not illustrated in the drawing, which is cooperatively connected with the valves to change the valve settings at frequent intervals, generally in periods of two to ve minutes. Such changes thus control iiuid flow through the regenerators in accordance with the foregoing description. Check valves I9, 24, 32, and 34 function as a result of the pressure changes created in the regenerators because of the actuating movements of the reversing valves andY in this way permit the product fraction and compressed air to pass from and to the lines connecting at the top of the regenerators.

As the temperature of the inflowing compressed air is lowered from '76 F. to its relatively low exit temperature by the cooling effected in either ofthe regenerators, water vaporY and carbon dioxide precipitate and become deposited upon the surfaces of the metallic packing in colder portions of each of these vessels alternatively. The automatic timing mechanism is regulated so that during the relatively brief period of about two to ve minutes between the actuation of the reversing valves, the solid phase depositions from this precipitation do not accumulate in quantities suiiicient to obstruct the small iiow passages in the regenerative packing. The valve reversals are regulated to permit satisfactory temperature fluctuations in a regenerative packing and to prevent too great an accumulation of solid on any part of the surface thereof. In the aforementioned description of the operation of regenerators I2 and I3, when the coldy nitrogen-rich product fraction 1s utilized to cool either regenerative packing I1 or 26, this gaseous product likewise evaporates and carries away in vaporous form the deposits of water and carbon dioxide left on the packing by the air as it was cooled below the precipitation temperature of these components.

The average rate of evaporation by the colder and lower pressure conditions of the nitrogenrich product fraction must be at least the equivalent of the average rate of deposition from the warmer and compressed air to keep any of the deposited water or carbon dioxide from remaining upon the surfaces of the packing when the settings of the reversing valves are next changed. As stated heretofore, it is indicated that a material balance between the amount of an impurity carried out of any region by the evaporative gaseous medium and the amount carried into that region by the inowing gaseous medium may be obtained if the ratio of the vapor pressure of an impurity at the temperature of the evaporating gas to its vapor pressure at the temperature ofthe precipitating gas is approximately equal to the ratio of the volume of the precipitating gas to the volume of the evaporating gas. For carbon dioxide removal, it is convenient to consider material balances for sections of the regenerators which always include the cold end as it is then possible to neglect the relatively insignificant quantity of this impurity which may be in the cold air leaving the regenerator and in the evaporative product fraction entering. The amount of carbon dioxide entering any such section, for

.I example, a section A-A as indicated at the cold end of regenerators I2 and I3, can be determined from the ilow rate of the air, its pressure and temperature, assuming the air to be saturated with this impurity at said temperature. The same amount of carbon dioxide must be contained subsequently in the nitrogen-rich product whose iow rate and pressure are also known. Since the amount of carbon dioxide and flow rate of the nitrogen-rich product determine the actual concentration of carbon dioxide in this product, and the pressure is known, an assumption that the nitrogen-rich product becomes saturated with carbon dioxide makes it possible to calculate the saturation tempei-ature of the product which would be suitable to satisfy the carbon dioxide vapor pressure ratio in the foregoing material balance. This temperature, being always lower than the air temperature because the product fraction is at a lower pressure than the air, when subtracted from the air temperature, determines a temperature difference which is defined as the critical maximum allowable temperature difference. It is evident that as the degree of saturation of the evaporative product fraction decreases, the allowable temperature difference also decreases. Thus, taking any reference air temperature, which determines, or iixes, the actual concentration of carbon dioxide in the nitrogen-rich product fraction required for complete evaporation, it is understood that as the degree of saturation decreases, the saturation concentration, and therefore saturation temperature, of the product fraction must increase. As a result of this, the allowable temperature difference decreases. On the other hand, as the pressure difference between the air and the evaporative product fraction increases, the allowable temperature difference increases. In other words, when the -air pressure increases relative to any reference nitrogen-rich product fraction pressure and air temperature, less carbon dioxide is present in the air at any point, and therefore the actual concentration of carbon dioxide in the evaporative nitrogen-rich product may decrease for com plete evaporation. Hence, a lower actual carbon dioxide concentration requires a lower saturation concentration, lower nitrogen-rich product temperature and therefore higher allowable temperature diiference.

From the foregoing, it can be understood that two variables are to be considered in connection with the evaporative function of the regenerators. One, the pressure difference between the air and the nitrogen-rich product, favors evaporation of carbon dioxide but the second, the ternperature difference, hinders it. Since the pressure dierence in processing arrangements as illustrated by Figure 1 is determined by the refrigeration and distillation requirements of the system, regenerators i12 and I3 must be operated yat temperature differences below the maximum allowable tempera .ire differences deiined hereinbefore.

The specific heat of air increases slightly with pressure in the operating range of air separation methods involving liquefaction and fractionation and, as a result of this fact, the normal temperature diiference between the compressed air and the lower pressure nitrogen-rich product for the countercurrent heat exchange therebetween increases toward the cold end cf regenerators l2 and I3 to values that in time would make the regenerators inoperable because of solid carbon dioxide obstruction. That is, the 30 F. difference between the 239 F. temperature of the cooled air and the 269 F. temperature of the nitrogen-rich product fraction at the cold end of the regenerators constitutes a greater diiference than the maximum allowable temperature diiference for complete evaporation of solid carbon dioxide and operation of regenerators i2 and I3 under such conditions will build up obstructing deposits of this material.

According to the modification of the present invention shown in Figure 1 the foregoing temperature dierences in the colder portions of these regenerators are decreased by separating and diverting a portion of the nitrogen-rich product fraction owing through line 23 to the regenerators into lines 35 and 31. This portion which, in this instance represents about 20% of the nitrogen-rich product, is passed through separate paths as a cooling stream in countercurrent heat exchange relation with the compressed air in the colder portions of both regenerators i2 and i3. illustrated as tubular coils Si! and 39 respectively and these may have to improve heat transfer. However, the physical arrangement of apparatus is by no means limited in this respect as other arrangements `are equally applicable. For example, hollow disc-shaped sections suitably nnned and located, or nnned tubular sections for the air and nitrogen-rich product with the separate portion flowing on the shell side of these tubes may be used. Or again, the heat regenerative elements in the colder ends of the regenerators may be in the form of a packing of granular or globular particles of metal and the separate paths may be embedded therein. In some cases, particularly for small sized regenerators. the separate path may consist of an external jacket surrounding the cold end of the regenerator. Likewise, the separate heat exchange need not necessarily be positioned in a continuous countercurrent relationship with the compressed in. the figure the separate paths are L air in the colder portions since in some events it may become expeditious to by-pass certain longitudinal regions along the length of the regenerator, or to concentrate the countercurrent heat exchange effect of the separate cooling stream in a longitudinal region other than where carbon dioxide precipitates, for instance, the region of frost formation. After passage through coils 38 and 39 the separated portion of the nitrogen-rich product flows back to line 23 by way of lines 40 and M, entering line 23 again downstream from the control valve 42 at a temperature in this case of about 150 F. The recombined nitrogenrich product attains a temperature of Iabout 246 F., which is thus only 7 F. colder than the air, and then passes to one or the other of the regenerators for heat exchange and evaporating duty, in accordance with the operative settings of the reversing valves.

As a result of this procedure, the mass rate of iow of low pressure cold streams is made to eX- ceed the mass rate of flow of the compressed air by an amount necessary to make the eifect of the larger mass of cold streams cause the abovementioned temperature difference to decrease' toward the cold ends of the regenerators. 'Ihe term unbalance is used to emphasize the fact that the cold streams exceed the warm stream and coils 38 and 39, or equivalent separate passages, are designated as unbalance passages. By proper control of the unbalance flow it is possible to overcome the tendency of the higher specific heat of compressed air to increase the temperature difference toward the cold end of the regenerators, and in fact to bring about as great a decrease thereof as may be desired. Thus, when employing the unbalance passage in a normal countercurrent heat exchange relation, it may be seen that the temperature diierences in the illustrative process start at 8 F. at the warm end of the regenerators and increase in a normal manner until the unbalance passages are reached. The actual location of the warm end of these passages is dependent upon various con- 1- siderations in design of the reversing heat exchange vessels, such as heat exchange surface and mass volume of the unbalance stream. In the present arrangement, where about 20% of the nitrogen-rich product flows through the unbalance passages, the unbalance passages start at the location in the regenerators where the compressed air attains a temperature of about 100 F. and extends to the cold end of these vessels.

D-ue to the cooling effect of the extra portion of the nitrogen-rich product in the unbalance passages, the temperature differences decrease so that the actual temperature diiferences become smaller than the maximum allowable temperature differences for complete evaporation. It is necessary, of course, to provide for more heat exchange surface in the regenerators to continue to cool the air to 239 F. because of the smaller temperature difference at the cold end of these vessels caused by the higher temperature of the reversing nitrogen-rich product stream after it is blended with the unbalance stream. A part of the cold nitrogen-rich product is capable of being used twice to extract heat in these heat exchange relationships in this way because the heat pick-up in the unbalance passage is limited so that the increase in the temperature of the main nitrogen-rich product stream lafter being commingled with the unbalance stream decreases only a portion of the ternperature difference at the cold end of the regenerator and the product remains sufficiently cold to extract a suitable quantity of heat from the air. In the arrangement of Figure iV sub stantially equal volumes flow through each of the unbalance passage coils 38 and 39 continuously. Because of this fact, during the phase of the reversing time cycle presently being described Where air is being cooled in regenerator Ai2 and nitrogen-rich product is being warmed in regenerator I3, the colder unbalance stream in coil 39 abstracts heat from the colder portions of the latter regenerator. The heat thus abstracted is returned to the main stream of nitrogen-rich product and this in eect represents simply a closed circuit of recycling heat around the cold end of the regenerator, having negligible effect, if any, upon the diverted portion of nitrogen-rich product flowing from and to line 23 through lines 36 and 4| respectively.

In the colder portions of regenerator l2, however, the nitrogen-rich product flowing through the unbalance passage 38 abstracts heat from the compressed air since this fluid, being substantially warmer, is the ultimate heat source.

The overall effect of the cold unbalance stream upon the compressed air is that of another countercurrent heat exchanging medium. Since the heat exchange surface of the regenerator is adjusted in accordance with the smaller temperature difference at the cold end of this vessel, created by the higher inlet temperature of the nitrogen-rich product, to maintain the same outlet air temperature of 239 F., the cooling effect of the unbalance stream in coil 31 is reflected on the character of the air cooling curve. That is, the air temperature as compared to the length of the cooling zone is changed within the regenerator from the former normal relationship resulting from a countercurrent relationship to one which enables the quantity of carbon dioxide ovving into colder regions of the regenerator in the air to be equal to the quantity passing from these regions in the evaporative fluid.

Without entering upon detailed theoretical considerations, it may be noted from the foregoing that the thermodynamic irreversibility of the heat exchange has not been changed by the presence of the unbalance stream. Since the temperature differences have been decreased in the heat exchanging paths, heat is transferred more reversibly. Any gain in this respect, however, is cancelled by the irreversibility introduced in mixing the warmed unbalance stream with the remainder of the nitrogen-rich product in line 23. Thus, use of the unbalance passage changes the location of the irreversibility, but cannot change the extent of irreversibility since terminal conditions have not been changed. The important evaporative result is achieved and the regenerators are capable of continuous use under non-obstructing conditions from accumulations of solid carbon dioxide.

The cooled and purified compressed air owing through line 23 is rst taken into surge drum 22 which has the function of separating any carbon dioxide snow from the air, particularly during starting-up periods before the regenerators are chilled to their normal operating temperatures. Upon leaving the surge drum through line 'IEE the air is divided into two portions. The smaller portion, representing about 29% of the air originally drawn into the separating system, is taken through line 43 to liqueer M for heat exchange with cold, backward-returning, nitrogen-rich product. In this heat exchange the temperature of the air is further lowered to about 278 F. to effect partial condensation. The mixture of liquid and gaseous air then flows through line 45 and is introduced through valve 45 into the inside of reboiler 41 of fractionator 48. Since the temperature of the partially liquefied air is of the order of about 278 F., it is Warmer than the liquid oxygen pool 53 in which reboiler 4l is submerged. Consequently, the compressed air gives up heat to reboil the liquid oxygen and in so doing is still further cooled to about -289 F. At this temperature the compressed air is totally condensed. The liquid air is continuously Withdrawn from the bottom of reboiler 41 through line 5l. Valved line 52 represents a drain line attached to the bottom of the reboiler. From line 5l the liquid air undergoes a filtering step in filter 53 as a precautionary measure to remove any solid particles present in the air at this point. It then passes by Way of line 54 into and upwardly through subcooler 55 wherein the temperature is lowered to about 298 F. by heat exchange, again with the cold, nitrogen-rich product. Upon leaving the subcooler through line 56 the subcooled liquefied air is expanded through expansion valve 57 and introduced into the upper portion of fractionator 48.

Meanwhile, the larger portion of the compressed air in line l, representing about 67% of the air originally drawn into the system through line I, is introduced into expansion engine 53 under an inlet pressure at this point of about 93 pounds per square inch absolute. The air pressure is then reduced with the performance of external Work to about 'I pounds per square inch absolute and its temperature resultantly lowered to about *303 F. Under these conditions the expanded air is caused to pass through line 59, having surge drum 60 disposed therein, to fractionator 48 and introduced as vapor feed to the fractionator at an intermediate point somewhat below the point of introduction of the expanded liquefied air from line 55. By-pass line 1|, having valve Si, connects line 59 with line 23 for use during starting-up periods when the airis incompletely cooled and diverts air not needed as vapor feed in fractionator 48.

The vapor-liquid contact obtained by the fractionating elements 62 which may be bubble cap trays or other fractionating means in fractionator d8 separates the air by fractionation and rectilcation into a liquid bottoms product which is essentially pure oxygen and a vaporous overhead product which contains a preponderance of nitrogen. Substantially pure liquid oxygen, representing approximately 4% by weight of the incoming air is continuously Withdrawn from the liquid oxygen pool 5D through valved line 53. In the event that the oxygen product is withdrawn in the vapor phase, the cold loss presently imposed on the system by reason of the liqueed condition of one of the `product fractions becomes recoverable. This cold may be recovered by cooling a part of the inflowing compressed air with the cold oxygen-rich product in either a complemental pair of reversing regenerators, in a heat exchanger or by means of separate heat exchanging passageways in the nitrogen regenerators l2 and I3. The nitrogen-rich vapors rcsulting from the fractionation, representing about 37% of the air initially entering line i, are re moved as a product fraction from the top o fractionator 48 through line 64 at a temperature of about 308 F. These vapors first pass through subcooler 55 and then by way of line 65 at about 291 F. into liquefier 44 for employment in the heat exchange relationships already mentioned. In the event that not all of the nitrogen-rich product is required for heat exchange in the liqueer, the part not so needed may by-pass the liqueer by way of the valvedline -49 which connects line 65 with line 23. In any event the nitrogen-rich product nally is taken through line 23 at about 269 F. for the forementioned cooling and evaporative duty in regenerators I2 and I3.

The heretofore described unbalance arrangement is not restricted to the modification illustrated by the process shown by Figure 1, but is capable of other modiiications adaptable to the air separation process in which it is used. The important function of the unbalance passage is to modify the relative flows of warm and cold streams so as to overcome the tendency of the high specic heat of the compressed air to increase the temperature differences at the cold end of the reversing heat exchange zone to inoperable values. An alternative modification to that illustrated by Figure 1 is shown by Figure 2 according to which a portion of the cold puriiied air leaving the regenerators l2 and I3 is utilized as the cooling medium for employment in the unbalance passages of coils. This alternative modification may be designated as high-pressure unbalance while the modication shown by Figure 1 may be designated as low-pressure unbalance to distinguish the employment of the compressed air from the employment of the lower pressure nitrogen-rich product in the unbalance passage.

Referring to Figure 2, which diagrammatically represents regenerators l2 and i3 in the process of Figure 1 and flow lines connecting therewith, inflowing compressed air from line Ila is'caused to ow either to regenerator E', or 13a by way of line Ilia, reversing valve 15a. and line 16a, or through reversing Valve 34a and line 21a. The nitrogen-rich product fraction then will alternatively pass from these regenerators by way of line 21a, reversing valve 28a, line 29a and line 39a, or through line Ilia, reversing valve 35a and line a respectively. At the cold end of the regenerators, the cooled compressed air outilows by way of line 18a, check valve 19a, line 22a and line 2m or by line 25a, check valve 32a and line Zia while the cold nitrogen-rich product at a temperature of about 269 F. alternatively enters the regenerators by Way of line 23a, check valve 24a and line 25a or by line 23a, line 33a, check valve Srila and line itc respectively. According to this modication, approximately 20% of the cooled compressed air as controlled by valve 42a is caused to be diverted from line 21a and to pass continuously through the coils 39a and 39a representing the unbalance passages in regenerators 12a and i3d respectively. Equal portions of the diverted air ow through lines 31a and 36a to the respective unbalance passages. Thereafter, these portions leave coils 36a and 39a by lines a and da respectively and, after being commingled in line Ha are returned therethrough to line 21a downstream to valve 32a at a temperature of about 159 F.

In this modication, the total nitrogen-rich product fraction from line 23a reaches the regenerators at about 269 F. This temperature of the cooling medium by countercurrent heat exchange with the compressed air, as stated before. would normally cool the air to about 239 F. and thus establish an inoperable temperature difference of 30 F. between the heat exchanging fluids. By passing the diverted portions of the cooled compressed air continuously through the unbalance passages of the regenerators, however, it becomes possible to cool the air to a temperature level lower than 239 F. and thus obtain smaller temperature differences between ,the air and nitrogen-rich product in the colder portions of the regenerators than is obtainable in the absence of added cooling eiect of the unbalance stream. The added cooling eieot by this internal recycling of part of the cooled compressed air through the unbalance passages lowers the air temperature in the present instance from 239 F. to 262 F. The resultant temperature difference of 7 F. at the cold end of the regenerators is less than the maximum allowable temperature difference for complete evaporation of carbon dioxide by the nitrogen-rich product and the regenerators become capable of continuous operation under non-obstructing conditions.

Inasmuch as substantially equal volumes of the diverted portion of the cooled air passes continuously through each unbalance passage simultaneously, the heat exchange relationships in these two passages alternatively are aiected rst by the temperature of the compressed air and then the temperature of the nitrogen-rich product, or vice versa. When air, for example, is being cooled in regenerator 12a after equilibrium conditions have become established throughout the air separation system, the unbalance stream enters coil 32a at 262 F. and therefore has no immediate heat exchange effect on the air, also at 262 F. The cooling efrect of the unbalance stream gradually increases, however, as it flows toward the warmer regions of the regenerator. But, since the mass volume of the unbalance stream is much less than the mass volume of the compressed air stream, the temperature of the former changes comparatively more rapidly and approaches that of the compressed air. The relative length of the unbalance passage must be such that the temperature diierences are modied to be less than the allowable in the region where carbon oxide impurity is deposited.

`While one part of the diverted portion of cold air is exchanging heat with the main air stream in regenerator 12a, the other part is exchanging heat with the nitrogen-rich product in regenerator |311. In this vessel the product fraction initially is 7 F. colder than the unbalance stream and hence the latter at ilrst gives up heat to the product fraction and thereafter merely ows concurrently with the product to proportionately add its cooling effect to that of the product. Because this additional cooling of the metallic packing 26a will subsequently be taken up by air during the next reversal, the heat pick-up by the unbalance stream in coil 33a, when added to the heat pick-up in coil 38a makes the combined heat pick-up essentially the same as the total heat which would have been abstracted by the fluid in coil 38a entirely from the air in regencrator i211 had not the latter regenerator likewise been similarly further cooled by the unbalance stream during the next previous period of reversal. In other words, the resultant effect of the unbalance arrangement in the present modication is the same as if the diverted portion was exchanging heat only with the compressed air. The amount of heat pick-up by each of the two portions of the through which the compressed air is owing, i

simultaneously with the alternation or" the air therethrough. The opposite extreme of such control would be that embodiment wherein valve a is entirely closed. rlhen the unbalance eiect is obtained by causing the unbalance stream of cooled air to alternate in the regeneratcrs through which the nitrogen-rich product fraction is flowing, simultaneously with the alternation of the product fraction therethrough. It is understood, of course, that the aforementioned extremes of control is equally applicable to the unbalance effect obtainable according to the arrangement shown in Figure l by diverting all of the nitrogen rich product fraction alternately through coils 38 and 39 by periodically closing valves 66 and 61. Modified processing arrangements for conducting unbalance pursuant to these lastmentioned embodiments are illustrated by Figures 3 and 4 respectively Which diagrammatically represent the regenerators l2 and l5 in the process of Figure 1 and the ow lines connecting7 therewith with suitable connections for operating in raccordance with the described embodiments.

Referring to Figure 3, inowing compressed air at about 76 F. from line Il b is caused to flow to either of the regenerators 52h or 13b by way of line Mb, reversing valve |5b and line Ifib, or through reversing valve 3lb and line 2lb. The nitrogen-rich product fraction will alternatively pass from these regenerators at about 68 F. by way of line 2lb, reversing valve 28h, line 29h and line 30h, or through line |617, reversing valve 35h and line 36h respectively. At the cold end of the regenerators, the cooled compressed air leaves by way of line lb, check valve |917, line h and line 2lb, or by line 25h, check valve 32h and line 2lb While the cold nitrogen-rich product at a temperature of about 269 F. alternatively enters the regenerators by way of line 23h, check valve 24h and line 25h, or by line 23h, line 33h, check valve 34h and line l8b respectively.

According to this modied flow arrangement, about 20% of the cooled compressed air leaving regenerator 12b when this vessel Yis being employed to cool air, as controlled by valve 421) is caused to be diverted from line I8b into line 31h for use as the unbalance stream. Since the air is at the higher pressure, check valve 68h permits the diverted portion to ow through 31h and then through coil 38h and line 40h at -a temperature of about 150 F. Whereafter it is again combined with the remainder of the air in line 2lb by way of line Mb. The temperature of the combined portions are then about 239 F. Check valve 69h acts to prevent passage of any of the unbalance stream at this time into line h. In this manner the unbalance stream is utilized for heat exchange entirely with the air and to further cool the air sufficiently to maintain its cold end temperature of about 262 F. and

establish the necessary temperature diierence of '7 F. at this point to be below the maximum allowable temperature difference.

During the next succeeding phase of the reversing time cycle, when the compressed air is upflowng through regenerator |3b, the unbalance stream is automatically diverted from line 2512 into line 36h. The higher pressure lof `the air causes check Valve 69h to be open and permit the unbalance stream to ow through coil 39h while at the Ysame time it makes check valve 68h block any flow from this stream into line IBD when the unbalance stream subsequently passes through line Mb to line 2lb. The function of the unbalance stream in coil 39h is carried out in the manner described for it in coil 38h. Hence. by simultaneous change of the unbalance air stream with the main compressed air stream, the main air stream is constantly leaving the regenerators at about 262 F., the unbalance air stream is continuously iiowing from line Mb at about 150 F. and the recombined portions of the air pass through line 2lb to the succeeding stages at about 239 F. Yet there has been no change in the ultimate inlet and outlet temperature conditions of the compressed air or the nitrogen-rich product from what existed prior to employment of the unbalance arrangement, the former still is cooled from about 76 F. to about 239 F. by Warming cold product from about 269 F. to about 68 F.

Referring now to Figure 4, according to this process arrangement inflowing compressed air at about 76 F. from line llc is taken to either of the regenerators I2c or l3c by way of line Mc, reversing valve and line |60, or through reversing valve 3| c and line 21o. The nitrogenrich product fraction will alternatively pass from these regenerators at about 68 F. by way of line zic, reversing valve 28a, line 29e and line 343e,l or through line i60, reversing valve 35e, and line 30e respectively. At the cold end of the regenerators, the cooled air leaves at a temperature of about 262 F. by way of line 8c, check valve |0c, line 20c and line 2Ic, or through line 25e, check valve 32e and line 2 Ic while the cold product fraction enters the regenerators at about 269 F. by way of line 23e, check valve 24e and line 25o, or by line 23e, line 33C, check valve 34e and line l8c respectively.

According to this modied flow arrangement, about 20% of the cooled compressed air leaving regenerator I 2c, when this vessel is being employed to cool the air, as controlled by valve 42e is caused to be diverted from line [Bc into line 36e for use as the unbalance stream. Since the air is at the higher pressure, check valve 69e permits this diverted portion to ow through line 36e and then through coil 39e and line 4| c at a temperature of about F., whereafter it is again combined with the remainder of the air in line 2|c. The temperature of the commingled portions of the air average to about 239 F. Throughout this ow check valve 68e acts to prevent passage of any of the unbalance stream into line 25C. In this manner the unbalance stream is utilized for heat exchange entirely with the metallic packing material 26e and to further cool the packing over and above the cooling by the cold product fraction suiiciently to maintain the final temperature of the compressed air as it leaves regenerator I3c during the next succeeding phase of the reversal at about 262 F. lNhile this cooling of packing 26o is being effected, the compressed air from which the unbalance stream is derived is leaving regenerator I 2c at this latter temperature by reason of the further cooling of packing I 1c accomplished by the unbalance stream in coil 38o during the next preceding yphase of the reversal.

After actuation of the reversing valves and during the next succeeding phase of the reversing time cycle when the compressed air is upfiowing e through regenerator I3c, the unbalance stream is automatically diverted from line 25o into line 31e. The higher pressure of the air causes check valve 68e to be open and permit the unbalance stream to flow through coil 38e while at the same time it makes check valve 69e block any ow from this stream into line 18e when the unbalance stream subsequently passes through line 41o to line 2| c. The function of the unbalance stream in coil 38e is carried out in the manner described for it in coil 38o. Hence, by simultaneous change of the unbalance air stream with the main compressed air stream, the main air stream is constantly leaving the regenerators at about 262 F., the unbalance air stream is continuously flowing from line llc at about 150 F. and the recombined portions of the air pass through line 2|c to the succeeding stages of the separation at about 239 F. yet, again, there has been no change in the desired nal temperatures of the compressed air or the nitrogen-rich product from what existed prior to employment of the unbalance arrangement, the former is still cooled from 76 F. to about 239 F. by warming cold product from about 269 F. to about 68 F.

It is to be understood, of course, that the invention is applicable to the treatment of any gas or mixture of gases. For example, the invention is applicable to the treatment of a mixture of normally gaseous hydrocarbons, such as one containing hydrogen, methane, ethylene, ethane and minor amounts of higher boiling components to obtain, for example, an ethylene product stream. In a treatment of such a gaseous mixture the higher boiling components may be the source of a precipitate in the regenerators and the scavenging stream may comprise the hydrogen and/or methane. The appplication of the various modications of this invention in such a system will be apparent to one skilled in the art.

I claim:

l. In a process for iractionating a gaseous mixture containing a relatively high-boiling impurity, by compressing said mixture, cooling said compressed mixture, expanding, liquefying, and evaporating at least part of said mixture, in a fractionating system, wherein an inowing stream of compressed mixture is cooled by passage through one of a pair of regenerative cooling paths, depositing said high-boiling impurity in a cold portion of said regenerative cooling path, while an outowing gaseous product is counterowed through the second oi said regenerative cooling paths, reirigerating said second path and scavenging the high-boiling impurity therefrom by revaporization, and wherein the ow in said paths is periodically interchanged so that each experiences alternate charging and refrigerating periods, a method for preventing the excessive accumulation or" said precipitated impurity in said cold portions of said regenerative cooling paths during several cycles of operation, which method includes the steps of z obtaining an auxiliary stream of cold fluid from at least one of the streams owing in said system subsequent to the passage of said cold portion by said inowing mixture but prior to the passage of said cold portion by said outowing product; and flowing said auxiliary cold stream through an auxiliary path in heat exchange with at least one of said regenerative cooling paths and streams flowing therein, said auxiliary stream having a direction of flow from the colder to the warmer end of said auxiliary path.

2. A method as described in claim 1, in which said auxiliary stream of cold fluid flows through said auxiliary path only during charging periods.

3. A method as described in claim 1, in which said auxiliary stream of cold fluid nows through said auxiliary path only during refrigerating periods.

e. In a process for iractionating a gaseous mixture containing a relatively high-boiling impurity, by compressing said mixture, cooling said compressed mixture, expanding, liquefying, and evaporating at least part of said mixture, in a fraotionating system, wherein an inflowing stream of compressed mixture is cooled by passage through one of a pair of regenerative cooling paths, depositing said high-boiling impurity in a cold portion of said regenerative cooling path, while an outowing gaseous product is counterowed through the second of said regenerative cooling paths, refrigerating said second path and scavenging the high-boiling impurity therefrom by revaporization, and wherein the flow in said paths is periodically interchanged so that each experiences alternate charging and refrigerating periods, a method for preventing the excessive accumulation of said precipitated impurity in said cold portions of said regenerative cooling paths during several cycles of operation, which method Aincludes the steps of: obtaining two auxiliary streams of cold fluid from at least one of the streams iiowing in said system subsequent to the passage of said cold portion by said iniiowing mixture but prior to the passage of said cold portion by said outflowing product; flowing one of said auxiliary streams through an auxiliary path in indirect countercurrent heat exchange with said cold portion of said inflowing stream, without simultaneous heat exchange with said oui-,flowing stream; simultaneously flowing the second of said auxiliary streams through a second auxiliary path in indirect concurrent heat exchange with said outiiowing stream, without simultaneous heat exchange with said iniiowing stream and reintroducing at least part of said auxiliary stream into said fractionating system at a point upstream from the outflow of said gaseous product through said cold portion.

5. A method as described in claim 4, in which both of said auxiliary streams are obtained from said inflowing stream of compressed mixture subsequent to its incoming passage through said cold portion during said charging period.

6. A method as described in claim 4, in which both of said auxiliary streams are obtained from said outowing product stream prior to its outward passage through said cold portion during said refrigerating periods.

7. In a process for separating air into at least oxygen-rich and nitrogen-rich product fractions by compressing, expanding, liquefying and evaporating at least part of said air, in a fractionating system, wherein an inlowing stream of compressed air is cooled by direct heat exchange with one of a pair of regenerators, precipitating carbon dioxide within said regenerator in a cold portion thereof, while outflowing nitrogen-rich product is simultaneously counterflowed through the second of said regenerators, reirigerating said second regenerator and scavenging precipitated carbon dioxide therefrom, and wherein the iiow in said regenerators is periodically interchanged so that each experi- 19 ences alternate charging and refrigerating periods, a method for preventing the excessive accumulation of said precipitated carbon dioxide in said cold portions of said regenerators during several cycles of operation which method includes the steps of continuously diverting a ,first minor part of said nitrogen-rich product stream, prior to its outward passage through said cold portion during said refrigerating peri- 0d and flowing said rst minor part in indirect concurrent heat exchange with said outowing nitrogen-rich product stream in said cold portion during said refrigerating period; diverting a second minor part of said out-owing nitrogenrich product stream prior to its outward passage through said cold portion and iiowing said second minor part in indirect countercurrent heat exchange with said inflowing compressed air in said cold portion during said charging periods; and subsequently recombining both said first and said second minor parts of said nitrogen-rich stream with said outflowing nitrogen-rich stream at a point downstream from their withdrawal.

8. In a process for separating air into at least oxygen-rich and nitrogen-rich product fractions by compressing, expanding, liquefying and evaporating at least part of said air, in a fractionating system, wherein an inowing stream of compressed air is cooled by direct heat exchange with one of a pair of regenerators, precipitating carbon dioxide Within said regenerator in a cold portion thereof, while outflowing nitrogen-rich product is simultaneously counterflowed through the second of said regenerators, refrigerating said second regenerator and scavenging precipitated carbon dioxide therefrom, and wherein the flow in said regenerators is periodically interchanged so that each experiences alternate charging and refrigerating periods, a method for preventing the excessive accumulation of said precipitated carbon dioxide in said cold portions of said regenerators during several cycles of operation which method includes the steps of: withdrawing a.v first minor part of said inowing stream of compressed air after its inward passage through said cold portion and flowing said rst minor part in indirect countercurrent heat exchange with said inflowing air stream in said cold portion without simultaneous heat exchange between said rst minor part and said outowng stream; withdrawing a second minor part of said inflowing air stream subsequent to its inward passage through said cold portion and owing said second minor part in indirect concurrent heat exchange with outflowing product stream without simultaneous heat exchange with said inflowing air stream; and subsequently introducing both said first and said second minor parts of said inowing air stream into said fractionating system downstream from their withdrawal.

9. A method as described in claim 8 in which said rst and second minor parts of inflowing air are introduced into said fractionating system subsequent to heat exchange with said inowing and outflowing streams respectively, at a point downstream from their withdrawal and in a part of said fractionating stream upstream from said expansion step.

PAUL R. TRUMPLER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,460,859 Trumpler Feb. 8, 1949 2,584,985 Cicalese Feb. 12 1952 2,586,811 Garbo Feb. 26, 1952 FOREIGN PATENTS Number Country Date 276,381 Great Britain Aug. 18, 1927

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