US2556850A - Oxygen separation - Google Patents

Description

June 12, 1951 H. J. OGORZALY 2,556,850

OXYGEN SEPARATION Filed June 18, 1946 J, N i9 AHL I 0 O7- All 5 METHAME 1G 6 Q 5O ETHANE ii? 5Q: lOPAN E 9 J, T l

Henry J. Ogor'zalg {Snvenbor abborne Patented June 12, 1951 UNITED PATEI OFFICE OXYGEN SEPARATION Henry J; ogorzaly, Summit, N. J., assignor to Standard Oil Development" Company, acorporation. of Delaware Application June. 18 1946, Serial No. 677,504

3 claims.

1' p The present invention is concerned with an improved Iowtemperature fractionation process for the manufacture of oxygen and nitrogen from air. It is more particularly directed toimprovements in the cooling of the inc'omingfee'd air with the cold oxygen and nitrogen products; In accordance with the present invention, theinle't air stream is cooled in several stages of reversing exchangers or accumulators, utilizing the cold oxygen and nitrogen products; Furthermore, in

accordance with the present process, cascader'e frigeration is employed for auxiliary cooling or the air between the respective stages;

It is well known in the art to manufactureoxygen and nitrogen from air by row tempera= ture fractionation processes. In these" processes the air is cooled" to liquefaction temperatures and fractionated in order to separate an oxygen and a, nitrogen stream free of impurities. For-"coo nomic reasonsit is a matter of major importance that two o erating features be attained: (1) that the minimum possible degree or compressiorr'of the inlet an be employed; and ("2") that the-- air separation unit operate in a contmuousmanner;

'I'norder' to achieve the first of these desid'erata it is customary to go to great lengths t'dafVoid the leakage of" heat into the. cold parts or the system, to deliver as much or" this compression Work as possible in the form of low'temperature refrigeration through the use of expansionerrgine's, and to utilize to the fullest extent the cooling eiiect of the separated products to" chill" the. incoming air. A most ingenious system of fractionation has'also been developed which em= 'pl'oys two towers operating at different pres sures". The" articular advantage or this'system is that by proper adjustment of the-pressures in the two towers, the temperature necessary to vaorize oxygen in thebottom or th'elow pressure tower can be reduced below that required to condense' nitrogen in" tlie upper part of the high pressure tower; and heat flow may therefore be directed from condensing'nitrogento vaporizing oxygen; In this manner the supply" of'liqui'd iiitrogen required as reflux in effect a" complete separation of the oxygen and nitrogen in the air may be obtained; although nitrogen is normally the lower boiling constituent-in air: It will be appreciated that the necessity of maintaining this temperature differential establishes a mini mumpressure differential between the two towers; and this in turn establishes that-when producing separated oxygen and nitrogen at atmospherio pressure; theminimum practicable pressure-of the inlet air is about atmospheres gauge.-

It iseconomic tooperat'ewith inlet air'pres'suie at this minimum level, as fixed by the? fraction ating tower requirements;,' but becauseioftliie heat leak which cannot. beentirely eliminated,

and because the complete utilization of the c ol ing' effect of the separated gases is not practicable, this degree of compression-is found lid-befitadequate' to satisfy the refrigeration require"- merits. It is, accordingly, common practice to employ auxiliary refrigeration means. iiivention applies to optimum methods of distribut-- ing this auxiliary refrigeration.

An equally important desideratum is to achievecontinuous operation of the air separation unit; Difficulties in achieving this end arise front the water and carbon dioxide content of the" inlet air. For example, in a large scale plant pro ducing 40 million cubic feet per day of pure exp gen, the entering air, if saturated with water at: por at F. and 5 atmospheres gaugepres'sure; carries with it about 4000 pounds per hour off water; It also normally carries with it about 300 pounds of carbon dioxide per hour. As the in c'omin'g air is cooled, the water progressively c'o-ndenses out from the saturated air, first as a'li'q'ui'd and then as ice which, in a very short time", will' plug the cooling equipment unless remedial Incasores-are taken. About 10% of the inlet moisture will deposit directly in solid form. Similarly when the saturation temperature of the airwitli res ect to its carbon dioxide. content is reached, the carbon dioxide will begin tocondense out in solid form and will plug the cooling equipment unless prevented by corrective measures.

With the inlet air at 5 atmospheres gauge pressure, the freezing out of water vaporwilllr'e' duce the residual water content of the air" to a very small value at a temperature level of- F1 to 60 F. the carbon dioxide beginsto' deposit at a temperature level of about 207 F1 and is rather completely deposited at a tem erature level of 260 F. It is common ractice to cool the incoming air to a temperature level of about 2T5 before conducting it into the previously described iractionating columns. It is thus anparentthat essentially all of the watervaporand carbon dioxide content of the incoming air Will deposit on the cooling surfaces in the normal course of chilling the incoming air and wiucause frequent'stoppages of flow.

Consequently,.various proposals have been made for overcoming this problem. Forrexample it'has been proposed that the-water and carbon dioxide be removed chemically, butthis is a costly method. It hasalso been suggested-that the warm air and cold-products be passed alternately and in reverse directionthroughpacked heateace cumulatorssothat aportion 01" the heat or cold in one streamis stored in the accumulator-during the first phase of the cycle and is transfer-red to the: other stream during thesecond-phase-ot the cycle,. while at the same time the carbon dioxide and ice deposited on the. packing fromthe cooled airis Sublimated and carriedoutin the rewarihed ice and carbon dioxide snow which have been de-' posited on the cooling surface by the air being cooled during the preceding stage of the cycle are revaporized into the low pressure product gases because of the greater carrying capacity of the low pressure product gases. The extent of the temperature differential between the gas streams which can be tolerated at any point within the exchanger or system is limited by the pressure differential employed between the inlet and product streams. In other words, for a given delta in pressure the deposited ice or carbon dioxide will be revaporized into the product streams only if the delta temperature is below a critical value. If it exceeds this critical value, the pressure of vaporous carbon dioxide or water which can be supported in the exit gas at saturation at any point, falls below the level required to carry all the solid deposit at that point and consequently the solid cannot be completely Sublimated.

In any air separation process a certain amount of refrigeration is usually supplied in excess of the work of compression of the inlet air stream. The quantity of this refrigeration is dependent on the temperature approach of the air stream entering the system and the separated products leaving the system, and also on the degree of heat leak into the system. It has previously been disclosed that this auxiliary refrigeration can be supplied to the inlet air stream at a point intermediate to the deposition of substantially all the water and the beginning of deposition of carbon dioxide snow.

The essence of my invention lies in the fact that if the required auxiliary refrigeration is added at this point only, temperature differentials in excess of those admissible for the removal of carbon dioxide will generally be encountered at a subsequent stage of the cooling or else the average delta temperature for effecting heat transfer in these latter stages will be excessively low. What I propose in accordance with my invention is to distribute the auxiliary refrigeration supplied to various points of the cooling stage so that relatively high differential temperatures are always available for heat transfer but at no point does the delta temperature become excessive for the carbon dioxide removal.

The process of my invention may be readily understood by referring to the attached drawing which shows diagrammatically a preferred embodiment of my invention involving a pair of multistage reversible heat exhangers coupled with auxiliary air cooling stages. This drawing illustrates a process wherein three main stages for recovering the refrigeration available in the separated gases (oxygen and nitrogen) are utilized in conjunction with two intermediate and auxiliary chilling stages. Chilled oxygen and nitrogen are introduced into the tube sides of tertiary stages I and I, to the pair of heat exchangers shown in the drawing by means of feed lines 2 and 3, respectively. The nitrogen and oxygen streams are withdrawn from said stages I and I and introduced into the tube sides of secondary stages 6 and 6. The warm nitrogen and oxygen streams are withdrawn from secondary stages 6 and 6 and introduced into the tube sides of initial stages 9 and 9. The warm oxygen and nitrogen streams are withdrawn from the initial stages 9 and 9' and handled in any manner desirable. As stated, the initial stage, the secondary stage and the tertiary stage may be any suitable reversible exchangers or accumulators.

The feed air is introduced by means of line l2 into the shell sides of initial stages 9 and 9 wherein heat exchange between the oxygen andnitrogen streams and the air stream is secured. It will be understood that if accumulators are used in place of exchangers, the stages for air cooling will be separate vessels from the stages for product stream warming. In either case, reversals of the pattern of flow through the stages, but not its direction will periodically take place, so that any given point on the heat exchange sur face will alternately be exposed to inlet air under pressure and to separated products at atmospheric pressure.

The chilled air stream is withdrawn from initial stages 9 and 9' by means of line I3 and I3 and passed to a secondary intermediate and auxiliary stage I4 wherein the air stream is further chilled by means of a medium which, for'the purpose of illustration, is assumed to be an ethane exchanger. The chilled air is withdrawn from secondary intermediate stage I4 by means of lines I5 and I5 and introduced into secondary main stages 6 and 6'. In secondary main stages 6 and 6 the air thermally contacts nitrogen and oxygen streams in a countercurrent manner and is further chilled. The chilled air is withdrawn from secondary main stages 6 and 6 by means of line I6 and I6 and passed through an initial intermediate and auxiliary cooler II. For the purpose of description it is assumed that the initial intermediate cooler comprises a methane cooler. The air is withdrawn from zone I! by means of lines I8 and I8 and introduced into tertiary main stages I and I. In tertiary stages I and I the air makes thermal contact with the cold nitrogen and oxygen streams and is further chilled to the desired temperature. The air is withdrawn from tertiary stages I and I by means of line I9 and is usually passed at this point to low temperature fractionation equipment which is not shown for the purpose of simplicity. Any suitable fractionating equipment may be employed. In general, a two-tower high and low pressure system isemployed for the fractionation and segregation of the nitrogen and oxygen product streams as previously disclosed. As indicated in the drawing, by manipulation of the valves shown therein, the air on the one hand and the separated oxygen and nitrogen product streams on the other hand flow in reverse directions through a pair of multistage heat exchangers, the air flowing through the tube sides of the heat exchangers during certain periods and during alternate periods flowing through the shell sides of the heat exchangers, while the cold product streams similarly flow through the said heat exchangers in paths previously traversed by the air during immediately prior alternate periods. In this manner the cold product streams pick up by vaporization the ice and carbon dioxide snow previously laid down on the cooling surfaces by the air. However, air only is allowed to flow in a continuous manner through intermediate auxiliary stages [4. and I1.

In accordance with a preferred, modification of my invention, I supply b'oilin'g methane to auxiliary cooler I 1, and extract heat from the air passing through this cooler by vaporization of the methane. The vaporized methane is withdrawn from exchanger [1 and compressed by means of compressor 5! to a pressure level such that it can be condensed by boiling ethane in condenser 2|. The liquefied methane is throttled through valve 5! to the low pressure level at which it boils in auxiliary cooler I1. I

Boiling ethane is introduced into the auxiliary intermediate cooler l4, and partially vaporized therein to abstract additional heat from the air stream. The remainder of the liquid ethane is vaporized in exchanger 2|, serving to condense the compressed methane. The vaporized ethane is compressed by means of compressor 52 to a pressure level at which it may be condensed in exchanger 24 by boiling propane. The liquefied ethane is throttled by valve 53 to the pressure at which it boils in cooler I4.

Boiling propane, supplied to exchanger 24, is vaporized therein while condensing the ethane circulated through the next colder refrigeration cycle. The vaporized propane is compressed at 54 to a pressure level such that it can be liquefied by ordinary cooling water in condenser 55. The liquid propane is throttled into cooler 24 by means of valve 56.

It will be seen that I have employed a cascade of successively colder refrigerating cycles in order to effect Very low temperature cooling and also to have available refrigeration at intermediate temperature levels. If desired, I could supply the refrigeration to the lowest temperature level by a vapor compression cycle operating directly between the desired low temperature and coolillg water temperature; and refrigeration at higher temperatures could be obtained by operatll'lg this cycle at several pressure levels, or by operating several cycles independently. I find, however, that it is preferable to operate in the manner described, using several fluids in distinct vapor-compression cycles and discarding the heat abstracted in each cycle and the heat of compression in each cycle, to the next higher cycle. In this manner the heat taken from the air at several temperature levels in the auxiliary cooling means is eventually discarded to cooling water without having excessive pressures at any point in the auxiliary refrigeration system.

My invention generally comprises utilizin at least three main reversible stages, which may be exchangers or accumulators in which the inlet air is cooled by exchange with the separated products, and using in conjunction with these main stages at least two intermediate cooling zones wherein auxiliary refrigerants, such as methane and ethane, are utilized. The refrigerants may be other liquefiable materials condensing within the desired temperature range at suitable pressures, and may, for example, include ethylene and nitrogen.

As pointed out heretofore, in processes for separatin oxygen of high purity from atmospheric air, which employ low temperature fractionation of liquefied air, the successful continuous preliminary cooling of the air to be liquefied and separated is a crucial phase of the process, but is made difficult by the fact that first the water vapor and then the carbon dioxide content associated with the entering air deposit on cooling and tend to plug the heat exchange paths. In reversing accumulators or exchangers, the pressure differential existin betweenthe inlet air and the issuing products permits the transfer of condensables deposited from the air at one temperature by sublimation into the product stream at a lower temperature, corresponding to the difference in partial pressure. Obviously, if the temperature differential between the two streams is too great, the deposited ma terial will not sublime into the product stream, the exchanger or accumulator will become plugged, and the entire system will be shut down. The permissible temperature differential is greater in the range of water deposition than in the range of carbon dioxide deposition, and

in general when operating with reversing exchangers or accumulators, the carbon dioxide condensing zone is the critical zone.

With the higher inlet pressure, the difference in heat capacity between the inlet air and returning products is such that a direct interchange of heat results in a greater spread of temperature at the cold end than exists at the hot end of the exchanger; and it happens that with normal pressure differentials of about 5 atmospheres between the entering air and the products returning at near atmospheric pressure, the differential temperature at the cold end becomes excessive and the exchanger tends to plug with carbon dioxide snow.

As described, it is also found that heat leaks and imperfections in recovery of the cooling available in the product gases are of such magnitude that auxiliary refrigeration must be supplied at some point in the system to maintain the heat balance. It is old to supply this auxiliary refrigeration to the air stream, particularly at a point intermediate between the zones of water deposition and carbon dioxide deposition, where it has the further advantage of bringing the temperature of the issuing products and the incoming air into closer approach, and thus facilitating the sublimation of deposited solids on alternation of the streams. I have found, however, that in many cases after addition of all the required auxiliary refrigeration at one point the temperature approach of the two streams, after the auxiliary cooling, is so close as to increase the area required for recovering the cooling available in the cold separated gases to an undesirable degree.

A small amount of carbon dioxide will deposit in the initial auxiliary cooler H, but this will represent only a small fraction of the carbon dioxide initially present in the air stream because of the small temperature drop employed. A satisfactory method of avoiding interruptions in operation due to possible plugging at this point is to provide the small exchangers required at this point in duplicate.

The present invention is not to be limited by any theory or mode of operation but only in and by the following claims in which it is desired to claim all novelty insofar as the prior art permits.

I claim:

1. In a process for the production of oxygen and nitrogen from air by low temperature fractionation and wherein heat transfer between inlet air and the returning products is accomplished by causing said inlet air to flow in indirect counter-current heat exchange relationship with the cold oxygen and nitrogen products proceeding from the said low temperature fractionation and wherein the travel paths of the air on the one hand and the oxygen and nitrogen on the other are periodically alternated for the purpose of revaporization by the oxygen and nitrogen of ice and solid carbon dioxide deposited in said travel paths by the said inlet air and wherein the indirect countercurrent heat exchange flow is carried out in a plurality of separate stages, the improvement which comprises supplying auxiliary refrigeration to the inlet air stream between said stages at at least two temperature levels in the form of low temperature evaporating liquids.

2. Process according to claim 1 in which auxiliary refrigeration is applied in two stages, the first at a temperature level of the entering air of l10 to l50 F. and the second at a temperature level of -220 to 260 F.

3. The method set forth in claim 1 in which the inlet air is cooled by heat exchange with the cold oxygen and nitrogen in three separate stages and REFERENCES CITED The following references are of record in the file of this patent:

5 UNITED STATES PATENTS Number Name Date 1,808,494 Carney June 2, 1931 1,843;043 Patart Jan. 26, 1932 w 2,02'2i165 Twomey Nov. 26, 1935 2,050,511 Twomey Aug. 11, 1936 2,070,098 Twomey Feb. 9, 1937 2,070,099 Twomey Feb. 9, 1937 2,460,859 Trumpler Feb. 8, 1949 15 OTHER REFERENCES The Separation of Air, Ruhemann (published 1940)", page 163, Figure 104.

Claims (1)

1. IN A PROCESS FOR THE PRODUCTION OF OXYGEN AND NITROGEN FROM AIR BY LOW TEMPERATURE FRACTIONATION AND WHEREIN HEAT TRANSFER BETWEEN INLET AIR AND THE RETURNING PRODUCTS IS ACCOMPLISHED BY CAUSING SAID INLET AIR TO FLOW IN INDIRECT COUNTER-CURRENT HEAT EXCHANGE RELATIONSHIP WITH THE COLD OXYGEN AND NITROGEN PRODUCTS PROCEEDING FROM THE SAID LOW TEMPERATURE FRACTIONATION AND WHEREIN THE TRAVEL PATHS OF THE AIR ON THE ONE HAND AND THE OXYGEN AND NITROGEN ON THE OTHER ARE PERIODICALLY ALTERNATED FOR THE PURPOSE OF REVAPORIZATION BY THE OXYGEN AND NITROGEN OF ICE AND SOLID CARBON DIOXIDE DEPOSITED IN SAID TRAVEL PATHS BY THE SAID INLET AIR AND WHEREIN THE INDIRECT COUNTERCURRENT HEAT EXCHANGE FLOW IS CARRIED OUT IN A PLURALITY OF SEPARATE STAGES, THE IMPROVEMENT WHICH COMPRISES SUPPLYING AUXILIARY REFRIGERATION TO THE INLET AIR STREAM BETWEEN SAID

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