US2650482A

US2650482A - Method of separating gas mixtures

Info

Publication number: US2650482A
Application number: US23933A
Authority: US
Inventors: Walter E Lobo
Original assignee: MW Kellogg Co
Current assignee: MW Kellogg Co
Priority date: 1948-04-29
Filing date: 1948-04-29
Publication date: 1953-09-01
Anticipated expiration: 1970-09-01

Description

Sept. 1, 1953 w. E. LOBO METHOD oF SEPARATING @As MIxTuREs Nm, `N

INVENmR.

ATTONEYs N EULUI I WALTER 'BY 4 r j# @0M Patented Sept. y1, 1953 METHOD F SEPARATING GAS MIXTURES Walter E. Lobo, Westfield, N. J., assigner to The M. W. Kellogg Company, Jersey City, N. J., a corporation of Delaware Application April 29, 1943, Serial No. 23,933

(Cl. (i2-175.5)

Claims.

' This invention relates to improvements in the separation of normally gaseous mixtures, containing higher boiling components as undesirable impurities, by low temperature liquefaction and rectification. More specifically, the invention relates tothe removal of a higher boiling component from a liquefied gas, such as one comprising carbon dioxide, air, oxygen, or hydrocarbons, the concentration being such that as a rule the component does not materially change the boiling point of the liquefied gas, but is otherwise objectionable because it either impairs the purityof the product or by its presence creates an explosive hazard in operating the separation process.

A specic object of this invention is to accomplish the substantially complete removal of an impurity from a liquefied gas in a rectification zone or to reduce the concentration thereof to a concentration Within safe operational limits.

Another object is to remove dissolved impurity from a liquefied gas by the continuous and forced recirculation of the liquefied gas through a body of solid adsorbent material.

' A further object is to remove dissolved normally gaseous impurity from a liquefied gas by the ,continuous and forced recirculation of portions of the liqueed gas through a body of granular particles of solid adsorbent material.

' A still further object is to remove dissolved normally gaseous impurity from a liquefied gas by recirculating continuously through a body of solid adsorbent material a stream of the liqueiied gas maintained in positive flow conditions by a7thermal siphon effect established and maintained by `partial vaporization of the circulating liquefied gas.

It is also an object to utilize the heat derived from condensing vapors of a higher pressure rectification zone to establish and maintain a thermal siphon for the forced recirculation af a stream of a liquefied product of a lower pressure rectiiication zone through a closed circuit including a body of adsorbent material for adsorbinga dissolved impurity from the liquefied product.

' Qther objects will become apparent in the more detailed explanation of the invention with reference to illustrative examples concerning the removal of acetylene from the air in connection with the separation of air by the Well-known procedure of low temperature rectification.

The separation of atmospheric air into oxygen-rich `and nitrogen-rich products by liquefaction and rectification at low subatmospheric temperatures requires the use of relatively large quantities of air. Consequently, even insignicant amounts of impurities in the feed air readily can become greatly magnified if not removed continuously. During the forecooling Step of the process some higher boiling gaseous impurities condense as liquids and solids on the cooling surfaces, and in time accumulate in such quantities as to restrict the gas flows and upset the plant operation. By employing the principle of reversing heat exchange to perform the main cooling step of the process, either by means of regenerators or exchangers, Water and carbon dioxide in particular may be so effectively removed that long time plant operating schedules are possible. Hydrocarbons, however, are seldom present in the feed air in concentrations great enough to cause condensation and thereby their removal in the main forecooling step. Yet these hydrocarbons must be removed from the plant by some means, as they not only interfere with the flows but also introduce a hazard. By the nature of the process, traces of hydrocarbons in the feed air finally accumulate in the reservoir of oxygenrich liquid at the bottom of the main rectification stage. With a sufficient concentration of hydrocarbons in the oxygen-rich liquid, the plant is liable to damaging explosions in the event the reaction is detonated by some priming agent. It is known that acetylene is present in atmospheric air and that it accumulates in the oxygen-rich liquid as a solid explo-sively sensitive to shock, electro static charges, or even sudden temperature changes, particularly if ozone also is present in the liquid. While the acetylene concentrations in air are usually small, the saturation concentrations in the oxygen-rich liquid are also small. For example, solid acetylene may be precipitated when it reaches the low concentration of three parts per million by volume in the oxygen-rich liquid.

Beds of adsorptive materials have been employed at the cold outlet of the forecooling step, on the inlet line to the expander, and on the oxygen-enriched liquid air line of a conventional two stage rectifying column, these beds being designed to take advantage of the high adsorptivity of solid adsorbents Iat low temperatures and under pressure. The beds usually are relatively large `in size, require large amounts of refrigeration for cooling preliminary to being placed in service initially and after each regeneration period, and suffer the additional disadvantage in that they are subject to by-passing either through faulty operation or through defects in construction. These devices situated as they are in the plant processing arrangement, are not capable of being used to remove any impurities that may have by-passed the adsorbent in a period of faulty operation.

Hydrocarbons also have been removed from the oxygen-rich liquid by an evaporative procedure. This procedure, however, concentrates the explosive impurities in a small stream of liquid oxygen that has to be Wasted at the expense of 3 the plants refrigeration and, therefore, lowers the plants power economy. This evaporative method is hazardous because there is no certainty that all of the hydrocarbons are contained in the waste liquid stream since some, for example,

may be entrained in the rising vapors of evapora'-` tion and deposited in a location where they are particularly liable to explode. Also, great care must be exercised to avoid `explosions in the Vdisposal line.

The present invention will be illustrated by its application to the removal of acetylene from the so-called low pressure rectiiica'tion stage of an air separation plant producing an oxygen-rich product. According to this application of the invention, the oxygen-rich liquid at the base of the low pressure stage is recirculated over adsorptive solids which remove the acetylene at such a rate that, over the operating cycle of the adsorber, precipitation of the dangerous solid phase of acetylene at all points of the circuit is avoided'. This procedure eliminates acetylene at its point of final accumulation in the air separation plant. It accomplishes this removal in a safe procedure without allowing the acetylene to reach the dangerous or solid state in the plant or inthe remaining equipment. Because of the recirculation procedure, imperfections in the adsorption step are less serious since traces of acetylene that escape adsorption on their first pass through the adsorptive bed are eventually adsorbed in subsequent passes through the bed. Acetylene escaping adsorption during start-up periods is adsorbed later when the adsorptive bed is funetioning effectively.

For a more complete understanding of the process of the invention, reference is had to the following detailed description taken in connection with the accompanying drawings, in which:

Figure l diagrammatically represents a portion of a two-stage rectification tower and a closed recirculation system for passing liquefied gas through a body of adsorbent in accordance with a preferred modification.

Figure 2 diagrammatically shows an alternative arrangement of apparatus for recirculating liquefied gas through a body of adsorbent material.

Figure 3 and 4 diagrammatically illustrate a further alternative arrangement for recirculating continuously through a body of adsorbent material a stream of liquefied gas maintained in positive flow conditions.

Figures 5 and 6 show diagrammatically the application of the process of the invention according to the preferred modification in connection with a single stage rectification tower.

In the preferred modification, as applied to a two-stage rectification operation, the method of the invention utilizes a closed recirculation system wherein the liqueiied oxygen-rich liquid accumulating in the bottom of the low pressure, or main, rectication stage is passed'continuously through a body of adsorbent material. A portion of this liquid is reboiled to provide vaporized product and the vapor upiiow for rectification. Circulation of liquid through the adsorbent preferably is induced by a thermal flow effect established by the Siphon action set up by a reboiler which may be within or external to the rectifying column and may provide either part or all of the reboiling required. Heat for reboiling is obtained by heat exchange between vaporizing oxygen-rich liquid and. condensing vapors of nitrogen from the top of the preliminary, or high pressure, rectification stage.

According to one processing arrangement incorporating the preferred modification wherein recirculation is effected. byA an` external reboiler, the i'eboiling step is performed downstream from the adsorption step and in this event the vaporized and unvaporized portions of acetylene-denuded liquid preferably are returned through a common conduit to the vapor space below the bottom tray of the main rectification stage. After vaporliquidseparation the unvaporized portion from the external reboiler is commingled with the main body of oxygen-rich liquid in the rectifying tower. The vaporized portions are partially withdrawn as product material and the remainder is permitted to pass upwardly/through the rectification stage. According to an alternative processing arrangement, the reboiling step to establish liquid circulation is performed prior to the adsorption step in which case an internal reboiler preferably promotes positive fluid circulation andl the partial vaporizat'ion of the oxygen-rich liquid in the external reboiler is carried out primarily to produce only the residual vaporization required. The primary requisite of the method is toradjust the rate of flow of liquid through the adsorption step to keep the acetylene concentration in the liquid returning to the rectification stage sufciently low so that when this liquid again is commingled with the main body ofoXygen-rich liduid the overall acetylene concentration therein is maintained below the concentration at which crystals acetylene precipitate for the calculated amount of vaporization in the rectification column. This rate of flow therefore isa Variable condition dependent upon the eiiiciency of' acetylene adsorption per pass in conjunction with the acetylene content inthe liquid downfiow of the main rectification stage.

Referring now to Figure 1, rectification tower l which is constructed in the conventional manner for rectifying air, is separated into two compartments or stages denoted by the numerals 2 and 3'. For the purpose of this description only the top portion of stage 2 and the bottom portion of stage 3 are shown, as these portions only are necessary to the description. Stage 2 represents the first, or high pressure, rectification stage wherein the precooled compressed air is rectified primarily into liquefied oxygen-enriched air by removing overhead a part of its nitrogen content. The nitrogen thus separated is condensed and both products of the primary rectification, preferably also subcooled, are then expanded into the second, or main, rectification stage 3, together with another portion of cold expanded air, for separation into nitrogen-rich or oxygen-rich iinal products. This latter rectification is performed under a lower pressure than the pressure of the rst rectiiication stage.

The liquid downiiow of the second rectification accumulates in a pool at the bottom of stage 3 around the outside of the tubes of the reboiler apparatus 4. Although the apparatus for reboiler 4 is shown in the figure as being a capped bundle of tubes located in the bottom of stage 3 but having the inside of each tube opening into the vapor space at the top of stage 2,` the design of this reboile'r is notto be limited to this specific arrangement. For example, a reboler designed somewhat similar to reboiler 50 of Figure 2 having suitable liquid and vapor connecting lines to

stages

2 and 3 would be just as satisfactory for carrying out the function of reboiler 4. In any event theliquid at the bottom of rectiiication stage 3 is reboiled to provide the vapors for rectification and vaporous product to be withdrawn from the system. To prevent build-up of liquid around the reboiler the liquid revaporized necessarily must be equivalent to the liquid passing down the rectication column into the pool. Since acetylene has a negligible vapor pressure under the conditions at which this vaporization takes place, only a negligible quantity of this material which enters the pool is removed in the evolved vapors. The highest acetylene concentration in the air separation system exists at this point since vaporization, particularly when producing a vaporous product, is the primary means for removing material from the pool.

Figure l illustrates the removal of acetylene according to the invention in an air separation process in which approximately 30,750 pounds per hour of oxygen-rich liquid enters the liquid pool surrounding reboiler 4 from the bottom tray of rectification stage 3 of which 11,165 pounds per hour is subsequently removed through line 46 as vaporous product. Stage 3 of the rectification operates under a pressure of pounds per square inch absolute and this necessitates a reboiling temperature in the liquid pool of -290 F. The heat required to boil vapors from the pool, equivalent to the quantity of liquid entering it, is derived from the condensation of 21,449 pounds per hour of nitrogen vapors reaching the top of the high pressure rectification stage 2. This later stage operates under a pressure of 105 pounds per square inch absolute so that the nitrogen vapors entering into heat exchange relation with the oxygen-rich liquid are approximately at -280 F. Inasmuch as the atmospheric air charged to the separation process may contain on the average about one part of acetylene per million parts of air on a gaseous volume basis, hereinafter referred to simply as a certain number of parts per million of acetylene, the downflowing oxygen-rich liquid continuously is bringing into the liquid pool around reboiler 4, about two parts per million of acetylene. This acetylene must be removed with enough efficiency to keep the concentration of acetylene in the liquid pool around reboiler 4 below three parts per million so that the rectification can be carried Vout within .safe operational limits.

The oxygen-rich liquid is circulated continuously through a body of adsorbent material to remove its acetylene constituent. In the present illustrative process, a maximum of 55% of the liquid surrounding the tubes of reboiler 4 is vaporized thereat so that the acetylene concentration in the remaining liquid cannot do more than double its value. The process must be operated, however, in a manner to preclude precipitation of solid particles of acetylene even in the event that the acetylene concentration in the liquid downow is doubled. 'I'his is done by continuous recirculation of the oxygen-rich liquid through an adsorbent which eliminates any disadvantage arising from lowered adsorption eiciency, a deficiency attendant with once through procedure, and thus assures greater certaintyof removing enough acetylene to maintain the required low concentration thereof in the reboiler liquid.

The adsorptive capability of the flow arrangement of Figure 1, therefore, is adaptable to variations in the concentration of acetylene in the atmospheric air being charged to the separation system. The present process for describing Figure 1 is based upon an exemplary feed air mixture initially containing an average of acetylene 6 concentration of one part per million. With a feed of this character and with a design in which the adsorption capacity of a bed of adsorbent is of such proportions as to be capable of effectively removing substantially all of the acetylene from the oxygen-rich liquid contacting it in a single pass, the adsorption system can readily maintain a maximum tolerable acetylene concentration of two parts per million in the liquid surrounding the tubes of reboiler 4. On the basis of completely removing the acetylene from the oxygen-rich liquid in one pass through the adsorber, a circulation rate equal to the total rate of vaporizing the oxygen-rich liquid is sufficient to prevent the accumulation of acetylene. However, as it is always advisable in commercial installations to provide for certain inefficiencies of adsorption such as may be encountered when a bed of adsorptive material has been on stream for some time and is approaching need for regeneration, this description is presented from the point of view that the adsorber effectively operates at efficiency. Thus oxygen-rich liquid which contacts the adsorber with two parts per million of acetylene and leaves this contact with one part per million necessarily must have a circulation rate approximately double the minimum circulation rate of complete adsorption, or equal to the feed rate in mols per hour of the atmospheric air to the process. A circulatory flow through the adsorption system approximating at least a circulation rate equal to the feed air rate to the separation process eliminates the phenomenon of total vaporization heretofore employed in the bottom of rectification stage 3 and thus eliminates the certainty of acetylene precipitation at this point. Furthermore, even if the feed air were to have an acetylene concentration of two parts per million increasing the circulation rate of liquid through the adsorber permits the adsorbent to reduce the acetylene concentration in the pool of liquid around the tubes of reboiler 4 to below one and a half parts per million and thus definitely prevents formation of solid particles of acetylene.

Figure 1 illustrates a preferable method vfor circulating the oxygen-rich liquid by thermal flow. By this method, reboiler 4 is operated to vaporize 17,000 pounds of liquid per hour. Meanwhile, 57,200 pounds per hour of liquid is withdrawn through line 5 and taken through one or the other of connecting lines 0 or l, having valves 8 and 9 respectively, for introduction into either of the adsorbers I0 or II. These vessels contain granular adsorbent material Which preferably is silica gel. The beds may be of any suitable arrangement but conveniently the silica gel is placed in an annular space between two perforated cylindrical walls to form the bodies of adsorbent denoted by the numerals I2 and I3 respectively. The beds of adsorptive material I2 and I3 are necessarily designed to accomplish the desired degree of adsorption and in the present illustration, have a volumetric capacity of 18.7 cubic feet. This capacity is Sullicient to retain approximately 748 pounds of silica gel in a mass of such density as to cause only about 0.2 pound per square inch pressure drop through the adsorber which represents a design based on twice the required circulation rate and 41% of the maximum allowable space velocity through the beds. The incoming liquid enters the inner space of beds I2 and I3 and then passes more or less horizontally therethrough into an outer space confined by the walls of adsorbers egebogesa .lfand I.I. FInspassingmthrough the adsorbent ,at .the .'flowfconditions of this operation, the ,oxygen-rich liquid is substantially completely denudedof its acetylene content by the vtime it is .withdrawn from ,the adsorbers through either .offtheconnecting lines I4 or i5, havingvalves 1:6 ,and A|'| respectively. It is possible that the actionbyfbeds I2 and -|'3 is not effected by adsorption ybut `by absorption orby both actions. Whenever the term adsorption is used hereinafter, `itconsequently also includes absorption .-.At pcriodicintervals, which may conveniently be,.once=everye24 hours, it becomes necessary to .regenerate the adsorbers. This is done conveniently by-.closing valves l8 and it or 3 and VEl?, ,depending .upon -which ofthe adsorbers iii or is'undergoing regeneration, opening the corresponding pair of .valves for the other adsorber vandopening-valve |-8rin line i9. When valve |53 isopened, nitrogen vapor is taken from line 20 and. passed throughline i9 to heater 2 Atthis pointthe temperatureof the nitrogen is raised y ltoa.temperature .suitable for purging acetylene .from the adsorbent material, lThe lheated nitro- .gen yvaponis then taken from` the heater through .line-22 and passed by Way of either or" the connecting lines 23 or .24, valve 2.5 or 25 respectively .being.open,.into.1ine.6 or l for passage through. .the adsorber undergoing regeneration. The regenerating vapor commingled with vaporized acetyleneleaveseither-adsorber it or li by way .of line yHl or |.5 .andis .purged from the system byr.either,purge line..2.| or=28, valve 2.9 or 35E being .open for .this .purpose.

After .undergoing contact with the silicagel .in the adsorption .,step, the acetylene-denuded liquidis passed to the .outside reboiler 3| by way of line .32. For the case in which the product is Withdrawn 'from the rectification in the .liquid phase, it may be. conveniently removed from line 3.2'by line 44, valve 45 being opened for this purpose. In any event, it is thefunction of out- .side reboiler 3| to take acetylene-denuded liquid `passing theretoin line 32 and to begin and maintain a positive circulatory flow by a so-called thermal siphon action created by vaporizing that portion of the liquid which must be vaporized over and above the vapors evolved by reboiler 4 so as to make the total reboiled vapors, or withdrawn liquid product and reboiled vapors, equivalent to the liquid downiiow. In the present operation with a vaporized product, these vapors amount to 13,750 pounds per hour and, forsupplying the necessary heat to produce these vapors. .valve 33 is actuated to bring 17,351 pounds per hour of vaporous nitrogen from the top of the high pressure rectification stage through line 20 Vto reboiler 3| at a temperature of approximately 280 F. These Warm vapors pass through the shell side of reboiler 3| on the outside of tubes 34, in countercurrent heat exchange relation with the acetylene-denuded liquid passing upwardly through the tubes, and are condensed thereby. Nitrogen condensate leaving the reboiler at a temperature of approximately 281 F. through line 35 ,may .be combined with additional amounts Withdrawn from tray 35 through line 3l if such is necessary and then passed to an upper point in rectification stage 3 in a manner not shown on the drawing. In the event it becomes necessary to employ a larger quantity of nitrogen vapors for the above heat transfer heat relationship, the excess condensate is returned to high pressure rectification stage 2 by way of line 38, having check valve 39 positioned therein to vprevent backward-return ofany liquid from recti- .tion .between .the liquid .and vapor phases.

iication:v stage .-2. :The-vaporovus fandrunvaporized portions iof the Hacetylene-denuded liquid :leave reboiler .3| at about 290 F. through .line '.40 and Vare :conducted: through this :line backftothe low .pressure rectification vsta-ge 3. Should .it -become necessary ordesirable .to changethecirculation rate throu'ghithe -adsorption system, ,this is done by employing Vvalve 33 asacontrol valve. For instance, thevalve may be openedesomewhat .to divert-more of the condensing nitrogen ,to outside reboiler Y3|. .Thefresultantsefect 4of-this diversion .would bef-to increase-the vaporization ratein the reboiler 'therebyrestablishing Aan increase of liquid .iiow through the y.circulatory adsorption .system and correspondingly .decreasing the -vaporizing n.eifectof .inside reboiler 24.

.In'the present illustration, to ;achieve.a..suitable thermal .-ilow Vof liquidthrough .thek circulatory adsorptionsystemflines- 5 .and 32 andthe connecting lines vii, l, lr-and .I5 have vbeen-designed so as to create nomore than'about 0.33 pound per .squareinchpressurer drop` tor theinlet of reboiler 3| plus, .ofcourse-the 0.2 pound,per square inch pressure-drop througheitheiuof ,the adsorbers. The pressuredrop through .theputside. reboiler ..3 .has 4beendetermined to vbe .of .the order.of.0.24 pound per square inch whileline te is constructed of suitable.cross-sectional..area to have a pressuredropof about. .22..po.und .per square inch. The summation of .pressure drops throughy each part .of the system. determines the required. driving forceforactuating. the. speciiied circulation rate of the oxygen-rich. liquid .therethrough. 'I'his .driving force is created .by .the

vdensity diierencebetweentheffluids in reboiler ,3i and the bottomof rectification stage "3 as Well as by the positioning of Voutside reboiler 3| ata level below theaverage level .of the .liquid1pool surrounding the tubes of .reboiler '.4 while .also taking into .consideration the ,pressure .'liead necessary .for Vreturning condensed nitrogen ,to thetop tray of rectiiication stage 2.

The materialfromline VAllis returnedtorecti- Acation stage 3 in the vapor space above .the

surface ofthe liquid.aroundreboiler-A andbe- 10W the bottom rectifying tray to effecta separa- `In this illustrative ,procedure, 13,750 ,pounds lper hour of vapors .are derived by -thejheating .ob- .tained in outside Areboiler 13| .and .these vapors, combined with the 17,000 .pounds .per hourpof vapors derivedfrom. the' heatingV effect. of reboiler y1|,.provided both the ,11,165-pounds .per hour .of

oxygen-.rich product, Withdrawnthroughline 46, and .19,585 pounds per hour vaporous upilowior .the rectiiication. Simultaneously, `43,450 v.pounds per hour .of acetylene-.free liquid likewise are .derivedfrom .line 40 and this liquid .once ,again becomes commingled with the. oxygen-rich ,liquid surrounding the tubes of .reboiler.4, and by .this commingling the acetylene Aconcentration in the liquid pool at the base of rectiiication stage 3 is maintained at approximately one part .per million.

Alternatively, tothe process 4iovv arrangement described hereinabove, reboiler 4 maybe -omitted .and its functionincluded in Htheperformance of outside reboiler 3|. Reboiler 'mayberemoved .from operation-by any-meanssuch as iilling'fthe tubes With inert gas or convenientlyby blocking off the tubes thereof to the flow of nitrogen vapors, as by a movableperforatedcover plated This .plate Ais retained in position fby suitable vbracketsin a manner to permit a desired free- .dom of movement. The platecontainsperforations comparable in location and size to the location and inside diameter of the tubes of reboiler 4. A rod 42 is attached to plate 4| and extended through the wall of the rectification column being properly fbushed to prevent leakage around the rod. By means of a hand device 43 outside the column, rod 42 can be manipulated so as to change the position of plate 4| in its retaining brackets enough to entirely block the upward flow of vapors into the tubes of reboiler 4. In this event, the pool of oxygen-rich liquid at the bottom of rectification stage 3 becomes a reservoir to receive and hold both the liquid downflow of rectification and unvaporized portions of the acetylene denuded liquid from line 40. The oxygen-rich liquid for circulation through the adsorbingand vaporizing steps is then withdrawn through line from this reservoir. Circulation of the oxygen-rich liquid from and to the bottom of rectification stage 3, as before, must be at such rate as necessary to maintain the acetylene concentration therein below the main concentration required for safe operation. With omission of reboiler 4 all the vapors of nitrogen passing to the top of rectification stage 2 are taken through line 2|] for condensation through outside reboiler 3|. The total liquid reflux for the high pressure rectification step isthen returned from reboiler 3| by way of line 38.

`While circulation of the oxygen-rich liquid through the circulatory adsorption system has been described as being sustained by the thermal flow conditions provided by the outside reboiler 3|, the same iiow conditions may be maintained by interchanging the functions of the inside and outside reboilers. Figure 2 diagrammatically represents a process arrangement wherein the positive flow of liquid through the adsorption system is provided by a reboiler located within rectification column According to this processing method, the design of the inside reboiler is changed somewhat, for example by a heat exchanger 50 for reboiling which is positioned intermediately between

rectification stages

2 and 3 but entirely separated therefrom by a bottom plate 5| and a top plate 52. All of the nitrogen vapors are now withdrawn from the top of recti` flcation stage` 2 through line 20 butpart of these vapors are diverted from line 20 into line 53, introduced therethrough into the shell side of the tubes 54 of reboiler 50, and are condensed by heat exchange relation with oxygen-rich liquid Vpassing upwardly within the tubes. The condensed nitrogen is then withdrawn fromreboiler 50 and passed to the top tray 36 of rectification stage 2 by way of line 55 having check valve 56. The other part of the nitrogen vapors passing through line 20 are introduced into the shell side of the tubes 34 of outside reboiler 3|. The quantity of vapors fiowing through line 53, is controlled by the setting of control valve 33. Within reboiler 3| the nitrogen vapors pass downwardly in countercurrent heat exchange with oxygenrich liquid rising through the inside of tubes 34 and are condensed thereby after which the condensate is removed through line 35 as heretofore described. Any of the nitrogen condensate needed for refluxing purposes in rectification stage 2 is taken from line 35 through line 38 to the upper tray 36. Check valve 39 prevents back-flow of liquid through line 33 in the event of pressure fiuctuations in rectification stage 2.

i Referring now specifically to rectification stage `3.of Figure 2, oxygen-rich liquid downowing from the vapor-liquid contacted in this stage, is collected on tray 51. This liquid is removed from the tray through line 58. Simultaneously, an out-flowing mixture of vapor and liquid from the top of reboiler 50 is taken through line 59, having check valve 60, and passed into line 58. Thecommingled streams in this line are introduced into the bottom of reboiler 3| andV brought into heat exchange relation with condensing nitrogen vapors from line 20 for further vaporization. The vapors are removed from'the top of reboiler 3| and returned through line 40 to the vapor space below the lower most tray of rectification stage 3. Product vapors are withdrawn from this space through the valved line 6| and the vapors not so withdrawn pass upwardly through the several rectifying trays of the stage. The product vapors alternatively may be` withdrawn directly from line 40 through the valved line 62. That part ofthe oxygen-rich liquid not vaporized in reboiler 3| is removed from the base of the reboiler through line 63 to the adsorption stage. There, the liquid undergoes adsorption in the manner as heretofore described in connection with Figure l after which it is subsequently caused to flow thorugh line 64 into the bottom section of reboiler 50 to undergo the necessary heatingfor promoting the positive flow through the adsorption circuit. When a liquefied product is desired, this product may be withdrawn from line 64 by valved line 66 and conditions adjusted in the operation of the liquid recirculation to permit this withdrawal. In case there is an accumulation of liquid in the bottom of rectification stage 3, such liquid may be drained from the stage through valved line 65 and added to the liquid flowing through line 64.

Figure 3 illustrates the application of the method of the invention in a rectification zone wherein all of the reboiling for thelow pressure stage is effected by inside reboiler 4. In this figure only the essential parts of the apparatus that is necessary for the description has been shown as it is understood that rectification tower I separates air according to the conventional two stage operation for such rectification. Likewise, only one absorber vessel is shown, it being understood that this vessel may be taken off stream for regeneration and a second one used for adsorption in the manner heretofore described. The method of purifying the oxygen-rich liquid follows the ,procedure as described in connection with the Aforegoing figures. Line |00, having an expansion valve |ll|, connects the bottom of high pressure rectification stage 2 with an intermediate point of low pressure rectification stage 3 for the purpose of transferring liquefied oxygen-enriched `air from the former to the latter-mentioned rectification stage. In the present instance, however, 57,200 pounds per hour of the oxygen-rich liquid `are recirculated from rectification stage 3 through line 5 under positive flow conditions maintained by pump 1G to adsorber I0. Upon its passage through the bed of adsorbent in absorber I0 the oxygen-rich liquid is substantially denuded of its acetylene constituent when it is passed through line 1| back to rectification stage 3. The continuous commingling of the liquid from line 'Il with the body of liquid surrounding the heat exchange tubes of reboiler 4 maintains the acetylene concentration of the liquid body at approximately one part per` million. The present procedure is particularly adaptable for separations producing substantially pure liquid oxygen as such product may be removed conveniently from line 1l by the valved line 12. A vaporous product may be recovered alternatively directly from the rectication tower through valved line 13.

While it is a convenient practice to Super-im- .pose rectication stage 3 above stage 2 in a single integral vessel, such an arrangement is not a necessary requisite for the rectification. The modication of the circulatory acetylene adsorption system illustrated by Figure 4 is particularly suitable in its application to rectication stages which perform their function in separate vessels positioned side by side. In this case positive circulation of the oxygen-rich liquid between the bottom of low pressure rectication stage 3 and reboiler 14 is maintained by a pump 18 by way of

lines

19 and 80 and the intermediately disposeduadsorber l0. By employing a pump to maintain positive ow conditions, the function of reboiling for stage 3 may be carried out entirely by condensing nitrogen vapors in reboiler 14 at the top of the high pressure rectification stage 2. This permits the convenient return of reflux liquid to stage 2 by gravity flow through line15. Condensation of the nitrogen vapors outside the tubes in reboiler 14V partially vaporizes the oxygen-rich liquid passing upwardly inside the tubes vafter delivery thereto from the low pressure recti'cation stage 3. The mixture of vapors and liquid thus formedv in reboiler i4 are passed through line 16 into the vapor space 11 below the bottom tray of stage 3 which space serves in the dual capacity oi separator and liquid reservoir. From'this reservoir the oxygen-rich liquid is pumped by pump 18 by way of' line 19 through adsorber vIll and then line 86 back into the botltom section of reboiler 1li.v Lines 91 and 98 connect between tower 2 and tower 3 for transferring oxygen-enriched liqueed air and liquefied nitrogen respectively from the higher pressure to the low pressure rectication stage. Expansion valves 102 and |03 are positioned in

lines

91 and 98 respectively to exp-and the material being transferred therethrough from the higher pressure of rectification stage 2 to the lower pressure of rectication stage 3. An oxygen-rich liquid product may be removed from line 89 through the valved line 8l or in the event a vaporous vproduct is desired such product may be removed directly from the low pressure rectification stage 3 through valved line 82.

The aforedescribed applications of the invention has been illustrated by Figures 1 and 2 with lreference to two stage rectification for a separation. It is to be understood that the invention is equally as applicable to single stage rectification which may be more clearly understood by reference to the diagrammatic process flow arrangements illustrated in Figures and 6. For example, vby now referring to Figure 5, compressed and partially liquefied air is introduced by Way of line 33 into reboiler S4 wherein it relinquishes its heat of condensation and is totally in the liquid phase as it ilows through line 85 and is expanded through expansion valve 85 into `the top of rectiiication tower 81 as reflux liquid. In tower 81 the liqueed air is rectied into a vaporous nitrogen-rich top product which is withdrawn from the tower through line 88 and an oxygen-rich liquid bottom product which accuinulates inthe base of the tower. The heat liberated in reboiler 84 by the nal condensation of the partially liquefied feed air from line 83 creates a thermal Siphon effect and by thermal flow causes the circulation of the oxygenrich liquid from the base of tower 81 through line 89, the'bdy cf adsorbent in. adsorber 9o and line 9| into reboiler 84. In this vessel the` liquid is partially vaporized by the heat exchange with the condensing air so that it is a mixture of acetylene-denuded vapors and liquid which are returned to tower 81 through line 92. Within the tower the vapors are separated from thev liquid and a portion of them taken `as product by way of line Q3, while the remainder pass upwardly through the rectifying steps of the towei` invap-or-liquid contact with the downlowing reflux liquid. The unvaporized material from line 92 commingles with the oxygen-rich liquidi at the bottom of tower 81 and subsequently is recirculated through the adsorption circuit. When a liqueed oxygen-rich product` is desired the bottom product of the rectication may be removed by way of line 9d instead of line 93 and the operating conditions of the air separation plant re,- adjusted to'supply the air to inlet line 83 at the proper temperature to maintain the described flow procedure.

The process arrangement diagrammatically shown by Figure 6 is essentially the same as that liust described for Figure 5. It differs therefrom in that a portion of the partially liqueiied feed air to rectifying tower 81 is totally liquefied ina b ottom section of the tower by heat exchange relation with the liquid bottom product of rectification in reboiler 95. The liquid air thus obtained is thereafter taken through line 96 and combined with the liquid air passing through line Vfrom reboiler t4. Optionally, the liquefied air inline 3bv may be separately introduced into the topof tower 81 through a separate expansion valve. In this arrangement, part of the vapors from the oxygen-rich liquid at the base of the tower are produced by an inside reboiler and the remainder in the. outside reboiler 84 which functions in the manner as described for outside reboiler 3| of Figure l.

The oxygen-rich product again maybe withdrawn either vin the vliquidphase through line `9-I or in the vapor phase through line 93, the conditions of operation being suitably adjusted for producing either type of product.

Having thus described my invention, whatI claim and desire to secure by Letters Patent is:

l, In the fractionation of air, containing acetylene as anv impurity, by liquefaction and'rectiiication at relatively low temperatures, wherein oxygen-rich liquid is continuously added toa pool in the 'bottom of a rectication zone, and V.continuously"kreboile'd from said pool, a method for preventing explosions which comprises: continually circulating a. stream of liquidcontaining dissolved'acetylene from saidfpool through a body of acetylene adsorbent material to remove dissolved. acetylene, and back to said'pool at a rate at least equivalent to the amount of liquid va- `porized by the reboiling of-said pool, and suiiicient to maintain the acetylene in said pool at less than saturation, thereby preventing the existence of acetylene crystals in Contact with said pool.

2. rIn the fractionation of air, containing'acetylene as an impurity,y by liquefaction and recti- -cation in two stages at low temperatures, wherein said air is fractionated in afirst stage under Substantial pressure into a substantially pure nitrogen vapor fraction and a liquid fraction of increased oxygen and acetylene concentration, and wherein said liquid'fraction is fractionated in a second stage under lower pressure into vapor fractions whicliare rectified in a vrectification zone above a reboilingpool ofliquid oxygen containing -dissolved acetylene in still higher concentrations than in said first stage, the method of preventing explosions which comprises: continually circulating a stream of liquid oxygen, containing dissolved acetylene, from said reboiling pool through a body of acetylene adsorbent material and back to said pool at a rate of liquid flow not less than the `rate of vaporization from said reboiling pool and sufficient to maintain the acetylene concentration within said reboiling pool below saturation, thereby preventing the existence of acetylene crystals in contact with said pool.

3. The method in accordance with claim 1 in which the acetylene adsorbent material comprises silica gel.

4. In the fractionation of air containing acetylene as an impurity, wherein compressed air is rectified in a first rectification stage into an oxygen-enriched liquid fraction and a substantially pure nitrogen fraction, wherein the fractions are again rectified together under lower pressure in a second rectification stage and a liquid predominantly oxygen rectification product containing dissolved acetylene is passed into a liquid body thereof in a lower part of the second rectification stage for reboiling by heat supplied by condensation of the substantially pure nitrogen fraction of the first rectification stage; the improvement which comprises continuously withdrawing portions of said liquid body, passing the withdrawn portions by gravity flow through silica gel to a second body of the liquid predominantly oxygen rectification product externally positioned with respect to said rectification stages thereby adsorbing dissolved acetylene from the withdrawn portions, reboiling and partially Vaporizing said second liquid body by condensing at least part of the substantially pure nitrogen fraction to produce an outfiowing overhead mixture of vapors and liquid from said second liquid body sufficient to establish and mantain the gravity flow of the withdrawn portions through the silica gel, and continuously returning unvaporized portions of the outfiowing mixture from said reboiled second liquid body to the first mentioned liquid body of the liquid predominantly oxygen rectification product and thereby preventing precipitation of solid particles of acetylene from the liquid body of predominantly rectification product during the reboiling thereof.

5. In the fractionation of air, containing a-cetylene as an impurity, wherein compressed air is rectified to produce an oxygen-enriched air fraction and a substantially pure nitrogen fraction, wherein the fractions are again rectified together under lower pressure with the collection and reboiling of a liquid body of oxygen-rich liquid containing dissolved acetylene and the heat for reboiling is supplied 'by condensing vapors of the substantially pure nitrogen fraction; the improvement which comprises continuously passing a stream of oxygen-rich liquid from the liquid body through acetylene-adsorbent material at a rate not less than the rate of collection of oxygenrich liquid in said body whereby the adsorbent removes dissolved acetylene from the stream at least at the same rate at which acetylene is entering the liquid body, then separately heating the purified oxygen-rich liquid of said stream by a separate heat exchange with a controlled amount of vapors of the substantially pure nitrogen fraction and vaporizing enough of the liquid of said stream to effect by thermal syphon ow passage `of the stream of oxygen-'rich liquid through the adsorbent material at said rate not less than the rate of collection, and thereafter returning to said liquid body of oxygen-rich liquid being reboiled an unvaporized portion of the oxygen-rich liquid from said separate heat exchange in an amount and at an acetylene concentration sufficient to maintain the acetylene concentration in the liquid body below the acetylene saturation point thereby preventing precipitation of solid particles of acetylene from said liquid body.

6. In the fractionation of air, containing acetylene as an impurity. wherein compressed air is rectified to produce an oxygen-enriched air fraction and a substantially pure nitrogen fraction, wherein the fractions are again rectified together in a rectification zone under lower pressure with the collection of an oxygen-rich liquid containing dissolved acetylene; the improvement which -comprisese maintaining a body of the oxygenrich liquid in said lower pressure rectification Zone, continuously passing a stream of oxygenrich liquid, containing dissolved acetylene, from the liquid body at a rate not less than the rate of collection thereof through acetylene-adsorbent material to adsorb dissolved acetylene, passing the purified stream of oxygen-rich liquid to a second liquid body, heating the second liquid body of oxygen-rich liquid by heat exchange with vapors of the substantially pure nitrogen fraction in a heat exchange zone externally positioned with respect to said lower pressure rectification zone, vaporizing at least enough of the liquid of the second liquid body to carry a mixture of vapors and liquid overhead therefrom sufficient to effect movement and passage by thermal syphon flow of said stream of oxygenrich liquid through the adsorbent material at said rate not less than the rate of collection, thereafter separating from said mixture an unvaporized portion of oxygen-rich liquid and passing it to said first-mentioned liquid body of oxygen-rich liquid in an amount and at an acetylene concentration sufcient to maintain the acetylene concentration in the first-mentioned liquid body below the acetylene saturation point thereby preventing precipitation of solid particles gf dacetylene from said first-mentioned liquid '7. In the fractionation of air, containing acetylene as an impurity, wherein compressed air is rectified to produce an oxygen-enriched air fraction and a substantially pure nitrogen fraction, and wherein the fractions are again rectified together under lower pressure with the collection of a liquid 'body of oxygen-rich liquid contain-l mg dissolved acetylene which body is reboiled by heat supplied from condensing vapors of the substantially pure nitrogen fraction to produce a vaporous oxygen-rich rectification product and vapors for the last-mentioned rectification; the improvement which -comprises continuously passing oxygen-rich liquid from said body through an acetylene-selective adsorbent material at a rate not less than the rate of said collection of oxygen-rich liquid from the second-mentioned rectification and sufficient to adsorb at least two parts of acetylene per million parts of oxygenrich liquid passed, then separately heating the passed oxygen-rich liquid by a separate heat exchange with a controlled amount of vapors o1' the substantially pure nitrogen fraction and vaporizing enough of the last-mentioned reboiling assai-Be liquid toproduce== an outowingf mix-ture of Yvai-- porsiV and liquid?i andz effect' by`- athermal syphon iovf'thepassage I ofV oxygen-rich liquid from said b'o dy and lthrough''-thefadsorbentmaterialat said rate; separati-ng liquidi portionY of4 oxygen-rich liquidi:- fionrthe;V outiowing mixture, and:` intro'- dicingAL trie-liquid portion into'saidl bodyfinamounti andy at'- anacetylene concentration'- sui-V 'cient' to maintainIl the acetylene concentration in-saidf body' below-1 about three parts V4of-"acetylene per' miliionl parts off' oxygen-richf liquid" thereby preventing precipitation vof solid particles-of acetylene from-said-liquidbody-during-the-reboiling--thereorf-.

In `thefractionation. of air-",ccntaining-V acete ylene -a's animpurity; whereinairfisrectiiied into nitrogen-richandli oxygen-richA vaporousI output products-byreboilingivapor'sifroml af body ofoxygeni-ricli liquid, .ini av lowerip'ortion.- ofi thefrectification VLz'on'ei; the method hof; preventing.l the pre' cipitation': of acetylene` crystals from saidi. liquid bod-yf which comprises continuously` removing a stream-:off oxy-e'en-richlliquid'.containing-.dissolved acetylene from saidlbody, continuously passi-ng the; removedi streaml through; an acetylene -selectire.-k adsorbentff materiali at a'. ratei not.- less than the':rate-ofz'vaporization from said oxygen-rich liquidbodyi to continually` adsorbfdissolved acetyleneI fromsthe stream, heating the thus puri-- fied; oxygen-rich? liquider. said streamby heat exc-hangelfwith an' innowing stream ofi partially condensed'i air?under.- higher pressure passing to said rectiiicationn zone-:ina heat exchange zone externally.- positionedwithirespect to said" rootincation-mono;r vaporizing enough` of the purined liquid.y to'produce:` an outowing mixture of` vapers: and liquid r suicienttol supply the vaporous oxygenerich-v output product and: vapors for' said rectification and'to: effectby a thermal syphon flow the: passage of` the stream withdrawn from theA liquidsbody through the. adsorbentmaterial at said: rate;,. separating-1 ai liquid portionl of' oxygen-rich liquid fromthe. outowing mixture and inti-oducimg` the liquid. portion.- into said' bo'dyin anaamount-and'y at an acetylene concentration sufficient to maintain the acetylene concentrationfin-4 theliquidbodyy below theacetylene saturationpointt-herein.` l 9;. Inf the l fractionation ofV air,Y containing acetylene as an impurity, wherein partially condensed compressed air is rectiiiedl in'. a-= iirst-rectication stage intoA a; liquidf oxygen-enriched fraction' and a substantially pure nitrogenf fraction; wherein the frac-tionsareaganrectiied.- together under lowerv pressure: ina secondi rectification stage withthe. collection and,v reboilingof? ani oxygen rich:liquidcontainingldissolved acetylenes by heat supplied-from-condensing vapors of the substantially pure-nitrogen" fraction' inthe rstllrecti cationistage. to: produce a Vaporous oxygen-rich rectificationproduct and vapors for the second stagerectication; theY improvementwhich' com'- prises withdrawingfrom the liquidi;v bodycollected under lower pressure a stream of" oxygenrich liquid-v containing dissolved acetylene;l passing'i'thie withdrawn streainivunder thermal syphon flow conditions ata rate not-less than the'rate ofivap'orization' from said liquid body throughan acetylene-selective" adsorbent material. to. adsorb dissolved-'- acetylene from' said stream, thenpassing the thuspuried oxygen-rich liquid. of. said streamfin-heat-exchange withV a separate stream of partially-f condensed compressed air passing to the secondffrectiiication stage in a heat" exchange zone externally positioned| withlresp'ect to" said second" rectification stage,- thereafter" combiningtheseparate stream off partially condensed com'- pressedV air' withastream of the liquidoxygenenriched? fraction Vof theA ii'rst rectiiicationV stage and.. ezrpanding tl''eV combined'streams into said Secondv rectication stage', vaporizing4 eno'ughof the puriied oxygen-rich liquid in the heat exchange zone to'prodiice an outflowin'gf mixture of! vapors and liquid to establish and maintain thev ther-mali` syphorr flowV ofthe withdrawn4 stream ati saidfr'ate, separating liquid* portion of oxygenrich liquid from theJ outflowingf mixture and' inv troducing the liquid portion into the liquid body of oxygen-rich liquid being reboile'dinthelsecond rectiiic'ation stage in-an amount and at anacetylene con-centration suicient to maintain the acetylene?l concentration in the liquid body below the' acetylene saturation point therein-andftliere-f byl preventing precipitation of solid particles' of acetylene from saidliquid body duringr the re'- boilingthere'of. Y

10. f In thefractionation of air, containing acetylene as an impurity, wherein partially con'- densedcompiessedair is rectied'in a firstrectiiication'V stage into-A a liquid oxygen-enriched fraction and'laV substantially pure nitrogen-frac tion, whereinthe fractions are again rectifiedtqgether under lower pressure in-a secondrectification-stage with the'collection of a body of oxy= gen-rich liquid, containing dissolvedY acetylene in thesecon'd stageA rectification; therimprove'- ment whichk comprises withdrawing from the liquidbodyin the se-cond rectification-stage a'stream orv thev oxygen-rich liquid containing dissolved acetylenazpassing-'the withdrawn stream through an acetyleneeselective adsorbent material at` a ratenot less than the rate-of said'collection' of oxygen-rich liquid in the liquid body toadsorb acetylene-from' thestream at least atl the same rate'y at' which acetylene is entering theV liquid body, removing" the purified oxygen-rich liquid strearn` from contact-'with theadsorbent material. sepa-rating fromv the-purified'stream a liquid oxygen-rich--outputproduct of the air fractionation, passing. the remainder of thepuried stream. in heat exchange.relationwith vapors ofthe substantially' pure'nitrogen-fraction of the first Irectnt-icatonstage' to' produce at least sufficienti vapors .forthe'second stage rectification, separating anunva-poriz'ed portion of the puriedoxygenrich liquid after the last-mentioned heat" exchange: and'` introducing 'said unvaporized portion into-the. body-of oxygen-rich liquidl in anamount and at an acetylene concentration sufiicient to maintain the-acetylene concentration in` the liq.- uld body belowtl'iey acetylene saturation point therein and thereby preventing precipitation of sol-1d particlesofxacetylene-from said liquid body-` WALTER E. LOBO.V

References Cited in the lle ofv this patent' UNITED-f STATES PATENTS Numberr-

Claims

1. IN THE FRACTIONATION OF AIR, CONTAINING ACETYLENE AS AN IMPURITY, BY LIQUEFACTION AND RECTIFICATION AT RELATIVELY LOW TEMPERATURES, WHEREIN OXYGEN-RICH LIQUID IS CONTINUOUSLY ADDED TO A POOL IN THE BOTTOM OF A RECTIFICATION ZONE, AND CONTINUOUSLY REBOILED FROM SAID POOL, A METHOD FOR PREVENTING EXPLOSIONS WHICH COMPRISES: CONTINUALLY CIRCULATING A STREAM OF LIQUID CONTAINING DISSOLVED ACETYLENE FROM SAID POOL THROUGH A BODY OF ACETYLENE ADSORBENT MATERIAL TO REMOVE DISSOLVED ACETYLENE, AND BACK TO SAID POOL AT A RATE AT LEAST EQUIVALENT TO THE AMOUNT OF LIQUID VAPORIZED BY THE REBOILING OF SAID POOL, AND SUFFICIENT TO MAINTAIN THE ACETYLENE IN SAID POOL AT LESS THAN SATURATION, THEREBY PREVENTING THE EXISTENCE OF ACETYLENE CRYSTALS IN CONTACT WITH SAID POOL.