US2550679A

US2550679A - Air fractionating column and process of operating same

Info

Publication number: US2550679A
Application number: US794260A
Authority: US
Inventors: Edward G Engel; John A Hufnagel
Original assignee: Hydrocarbon Research Inc
Current assignee: Hydrocarbon Research Inc
Priority date: 1947-12-29
Filing date: 1947-12-29
Publication date: 1951-05-01
Anticipated expiration: 1968-05-01

Description

E. G. ENGEL ET AL AIR FRACTIONATING COLUMN AND PROCESS OF OPERATING SAME Filed Dec. 29, 1947 T INVENTORS Edward (11 1 John 19. Ha/nqgid ATTORNEY Patented May 1, 1951 AIR FRACTIONATING COLUMN AND PROCESS OF OPERATING SAME Edward G. EngeLRoselle, N. J and John A. Hufnagel, Mine-ola, N. Y., assignors to Hydrocarbon Research, Inc., New York, N. Y., a corporation of New Jersey Application December 29, 1947, Serial No. 794,260

Claims. 1

This invention relates to the production of oxygen by the liquefaction and rectification of air, and moreuparticularly to the construction and operation of the rectification column or system.

In the liquefaction and rectificationof air it ise-common to employ a rectification column in which a body of liquid oxygen is boiled, the

- vapors thus produced rising up through the column countercurrent to a descending stream .oiireflux liquid, the rising vapors removing from the reflux liquid nitrogen and otherrelatively low boiling constituents contained therein and the oxygen vapors being condensed resulting in the production of a product oxygen of desired purity whichis withdrawn as product from the column. Air thus subjected to liquefaction and rectification invariably contains acetylene, usually in minute amounts of the order of a few parts per million. It has been found, however, that while the acetylene content of the air is so small as to have heretofore been considered insignificant, in the continued operation of the rectification sys- [tem the acetylene tends to accumulateand build iupxin the body of liquid oxygen reboiled to pro- .duce-- the oxygenvapors which arerectified by flow in contact with the liquid reflux. To avoid the acetylene concentration reaching a point wherean explosive mixture would result, it has been suggested to withdraw continuously or periodically from the base of the body of liquid oxygen subjected to reboiling a purge stream of this oxygen in order to prevent the accumulation of acetylene in the body of liquid oxygen to a dangerous concentration: One proposal calls for Epa'sf'sing this purge oxygen stream downwardly through a heat exchanger wherein the oxygen along with the acetylene contained therein is vaporized, allegedly without danger of an explosion. Observation of this precaution of withdrawing a purge stream of oxygen, however, does not completely eliminate the danger of violent explosions taking place.

.rfictllfioation srstem surges oi the level cities body of liquid oxygen maintained in the rectifica tion column inevitably take place. Thus, for example, small surges take place whenever the reversing valves are operated, usually every three minutes, to cause reversal of flow of the rectification product or products and air through their respective flow paths in the heat exchangers, as is customary practice. In those systems in which the refrigeration necessary to compensate for enthalpy losses and heat leaks into the system is provided by expanding a portion of the relatively high pressure air and introducing the expanded air into the rectification column, such surges, which at times are quite extensive, occur due to fluctuations in the operation of the expander. Such surges of liquid level may occur for other reasons, such as fluctuations in the volume of ,air

supplied to the system, etc. Such surges, we have found, cause precipitation of acetylene on the walls of the column, which precipitate tends to build up until it reaches an amount such that an explosion may occur due to the reaction between the oxygen and the acetylene deposited on the walls.

In accordance with .this invention the side .walls-of the rectification column just above'the portion in which the body of liquid oxygen is maintained are continuously wetted or washed with reflux oxygen to dissolve or prevent the accumulation of acetylene on these walls.

In accordance with a preferred embodiment of this invention, in addition to the above step, a stream of oxygen is continuously withdrawn from the base of the body of liquid oxygen subjected to reboiling. Preferably the major portion, say about of this stream is vaporized and the vapors returned to the column where they flow countercurrent to the descending reflux liquid. The remaining minor portion of the stream is passed through a second reboiler where all but a small proportion, say about 1% of the original stream, is vaporized, the vapors thus produced being returned to the column or admixed with the product oxygen stream withdrawn from the column. The residual unvaporized-oxygen contains the acetylene and is suitably disposed of, for example, it may be wasted as by introduction into the effluent rectification product stream such as the nitrogen stream withdrawn from the system, or may be introduced into the product oxygen stream passed in heat exchange relation with the incoming air, so that the acetylene is vaporized. By purging from the system a small amount of liquid oxygen continuously or periodically w thd awn from the base of the body of liquid oxygen subjected to reboiling, accumulation of acetylene within this body of oxygen such as to form an explosive mixture is prevented. Also, as noted above, by continuously wetting or washing with liquid oxygen the walls above the body or" liquid oxygen subjected to reboiling, accumulation of solidified acetylene on these walls such'as to constitute a serious hazard of explosion is prevented.

In the accompanying drawing forming a part of this specification and showing, for purposes of exemplification, preferred forms of this invention without limiting the claimed invention to such illustrative instances:

Figure 1 is a composite fragmentary view, the upper portion being a side elevation and the lower portion a vertical section of a fractionating column embodying this invention;

Figure 2 is a diagrammatic view of a rectification system comprising two. columns, one a high pressure and the other a low pressure column, embodying our invention; and

Figure 3 is a diagrammatic elevational view of a rectification system comprising a two-stage rectification column embodying our invention in association with a reversing heat exchanger.

In Figure 1, ill indicates a rectification column provided as usual with rectification plates H of the bubble cap or other desired type. Plates H shown in the drawing are provided with bubble caps l2 and downcomers l3 for maintaining a body [5 of liquid reflux at the desired level on each of the plates H. A downcomer [5 leading from the lowermost plate I! in the column iii, in

accordance with this invention, leads into an annular trough or other container to suitably disposed in the base portion of the column above the level i"! of the body of liquid oxygen is subjected to reboiling. Trough I6 is suitably mounted in column i l in the relative position shown in Figure 1 above the body of liquid oxygen [8 and just below lowermost plate H. It comprises an inner wall E9 of a height greater than that of outer wall insuring that liquid reflux overflows the depending lip 2i spaced from inner walls '22 of the column H] to provide annular clearance 23 through which clearance liquid from trough iii flows along the side walls 22 on to the body of liquid i8. If desired, side wall 20 is provided with a series of closely spaced openings or ports 24 spaced at suitable intervals 'circumferentially of this side wall through which liquid within the annular trough I6 is projected on to the side walls 22. The liquid flowing over 'the lip :2! through the clearance 23 and through ports 24 continually 'wets and washes the side walls 22 throughout the full circumferential area thereof, this liquid flowing on to the body of liquid'oxygen l8.

Column Iii is provided with a base outlet port "25 through which liquid oxygen from the body 18 flows continuously to a heat exchanger as hereinafter more fully described. This column is pro- "vided, as customary, with a top nitrogen exit port 25, an inlet 2? for addition of liquid nitrogen refiux at the top of the column and an inlet 28 disposed at an intermediate point in the column for the introduction of liquid crude oxy- (containing approximately 40% oxygen) which is fractionated as it flows down through the column from plate to plate. A port 29 hav- "ing its inlet terminal 33 extending well beyond the inner wall 22 is positioned below trough Hi, the end 39 desirably being disposed so that it occurs beneath cylindrical opening 3! in the anill nular trough l6. Thus, oxygen vapors introduced through port 29 rise through opening 3| and are introduced into the bottom of the column it! at a point spaced remotely from the inner walls 22 having the film of liquid oxygen flowing thereover so that the rising vapor stream entering through port 23 does not entrain liquid flowing from trough i6 along the side wall 22 into the body of liquid oxygen l8.

An exit port 32 extends in an upward direction and leads from side wall 22 just below annular trough [5. Product gaseous oxygen may be withdrawn through this port, By having this port inclined in an upward direction, any liquid oxygen entrained in the gas stream flowing through port 32 separates out in this port, flows downwardly along the walls of the port and is thus returned to the column Iii. It will be understood that the construction of ports 29 and 32 may be varied; thus, instead of an inclined exit port 32, an exit port similar to port 29 may be used in which the inlet to this port may be disposed spaced remotely from the side walls 22, or instead of a horizontally extending inlet port 29, an upwardly inclined inlet port corresponding to the exit port 32 may be used. This construction is shown in Figure 2.

The rectification system of FigureZ comprises two columns 34, 35. Column 35 desirably is operated at a pressure of from about 60 to about pounds per square inch gauge, and column 34 at a pressure of from about '5 to about 12 pounds per square inch gauge; column 35 is therefore the high presure column and 34 is the low pressure column. Both columns, as customary, are provided with rectification plates of the bubble cap or other desired type and, in accordance with this invention, column 34 has leading from lowermost plate 36 a downcomer 31 extending into an annular tubular ring 38 vprovided with ports 3811 or perforations. Hence the reflux liquid oxygen flows from the lowermost plate 36 through the downcomer 31 into the annular ring 38, thence through the ports or perforations in this ring onto the inner side walls .of column 34 so as to continuously wetthe entire area of the inner side walls below ring 38, the liquid oxygen from the side walls flowing into the body of liquid oxygen maintained at the base of column 34. i

As customary, column 34 has a top nitrogen outlet 39, liquid nitrogen reflux inlet 40 and a crude oxygen reflux inlet 4!. The base of the column is provided with an outlet 42 through which a portion of the liquid oxygen from the body thereof maintained in the column flows continuously through a line 43 into a heat exchanger 44. Outlet 42 communicates with a valve controlled line 42; by opening the valve in line 42 the body of liquid oxygen normally maintained at the base of column 34 may be removed therefrom. Vapors generated in heat exchanger 44 flow through line 45 into inlet port 46 leading into the base portion of column 34 just above the level of the body of liquid oxygen maintained therein. From heat exchanger 44 a line 4'! leads to a second heat exchanger 48 which is provided with a line 49 leading to a centrifugal or other separator 50. A vapor line 5| leads from this separator 53 to product gaseous oxygen line 52 leading from exit port 53 disposed at the base of column 34 just above the'level of the :body of liquid oxygen therein. A valve controlled liquid discharge line 54 leads .from'therseparator 50.

A nitrogen line 35 leads from the top of column into the nitrogen return line 58.

35. Y Thisline is provided with two

branches

56 and 51. Branch 56 passes through heat exchanger 44 and leads into nitrogen return line 58 leading'into top tray 59 of ,the column 35. Branch 51 passes through exchanger 48 and leads Thus the ex;

changers

44 and 48 function as a pair of reboilers forfthe liquid'oxygen continuously withdrawn through the base port 42. .Column 35, as customary, is provided with a nitrogen line 68 leading from the top tray 59; this line passes through a suitable exchanger (not shown), has an expansion valve therein and communicates with inlet 48 of column 34. The base of column 35 is provided with a crude oxygen exit line 6| which passes through a suitable exchanger (not shown), is provided with an expansion valve and communicates with the crude oxygen'reflux inlet 4|. An air inlet line 62 leads into the base portion of column 35.

.In. operation the major portion, say about 75%, of the oxygen flowing through port 42 is vaporized by the nitrogen flowing in indirect heat exchangell'elation through exchanger 44, the vaporsro'f oxygen thus generated flowing through line 45, port 46 into and up through column 34. The residual liquid oxygen, say about -25%,'flows throughline 41 into exchanger 48 where substantially' all, say 24% of the oxygen originally withdrawn through port 42, is vaporized by the nitrogen flowing in indirect heat exchange relation through exchanger 48., The mixture of oxygen vapor and liquid thus produced flows into separator 58 through line 49, the vapors leaving separator '58 through line 5| and entering the product'oxygen stream 52 leading from column 34. The liquid oxygen entering the separator which contains the acetylene in solution and in suspension is withdrawn through line 54. By "continuously removing a stream of liquid oxygen, as hereinabove described, and purging a small proportion of this stream, say 1%, in unvapcrized form, the concentration of acetylene within the. body'of liquid oxygen in column 34 is maintained below that at which there isanydanger of a violent explosion taking place." By continuously washing the side walls of column '34 below ring 38 and above the body ofliquid oxygen, accumulation of acetylene on these side walls is prevented.

The rectification system of Figure '3 comprises a two-stagecolumn 63, lower section 64 of' which is oper'ated at a pressure of about pounds to about 100 "pounds gauge, and the upper se'ction 65 of" which is operated at a pressure" of from about 5 pounds to about 12 pounds gauge. This column, as customary, is provided with rectificationplates of the bubble cap or other desired type; In accordance with this invention,low-

ermofst plate 66 of section 65 is provided with a downcomer 61 leading intoan annular trough 68 of the same type as trough l6 hereinabove described. a A body of liquid oxygen is maintained in section 65 at a height indicated by line 69 above condenser-reboiler 10 which has a liquid collecting shelf II disposed immediately therebelow for col lecting' liquid nitrogen. As customary, section 64 provided with an air inlet line 12, crude oxygen withdrawal line 13 which has an expansion valve therein and communicates through a heat exchanger (not shown) with oxygen reflux line 14 and a nitrogen withdrawal line 15 leading from collecting shelf H through a heat exchanger (not w re grav t s w h e r sig ng va ve and lead.-

ing into the low pressuresection 65 at 16. 'A valve controlledoxy'gen withdrawal line 11 leads from the base of section 65. A vent line 18 leads from the condenser 10 through which line a minor portion of the nitrogen may be vented to remove incondensible gases. from the top of low pressure section 65 to reversing heat exchanger 85. Line serves for the withdrawal of product gaseous oxygen from above the oxygen liquid level 69. Reversing heat exchanger isprovided with flow paths 86 and 81 through which alternately streams of air and nitrogen flow. Reversing valve 88 has an air supplyyline 90 and a nitrogen vent line 9| and reversing valve 92 at the opposite end of exchanger 85 is connected to line 12 which feeds the chilled air to section 64 of column 63 and iscOnnected to line 19 to receive nitrogen from section 65 of column 63. For simplicity, other conventional heat exchangers usually interposed in lines 12 and 19 between the reversing exchanger 85 and column 63 have been omitted.

In the operation of Figure 3, condenser 10 operates as a reboiler to efiect vaporization of a portion of the liquid oxygen in section 65, which vapors rise upwardly through ection 65 where they are fractionated by flowing countercurrent to the descending liquid. reflux and throu Qhe bodies of liquid reflux on each plate. Liquid oxygen reflux flows fromplate to plate to'the lowermostplate 66, then through downcomer 6'! to the annular trough 68. From this trough liquid oxygen overflows continuously along the sidewalls of section 65 wetting these walls and preventing accumulation of acetylene thereon due tojsurges of level 69 of the body of oxygen or for other reasons. Oxygen is continuously or periodically withdrawn from the body of liquid oxygen in which condenser 10 is immersed through line 11 and may bepassed through the usual heat exchangers in indirect heat exchange relation with the incoming air, whereby the oxygen and the acetylene contained therein vaporized. Thus, an accumulation of acetylene on the side walls. above the body of liquid oxygen subjected to reboiling and within this body of oxygen to a concentration'which would create an explo- 'sion hazard is prevented. It will .be noted this invention provides a process .of operating a rectification column which .the rectification system, and therefore provides an improved and safer design of rectification column.

:. Since certain changes in carrying out the above process and certain modifications in the rectification column which embody the invention may be made without departing from the scope of this invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative'and not in a limiting sense. Thus, for ex ample, the invention comprehends a rectification v column corresponding to the high pressure unit 35 of Figure 2 in which a trough, closed ring provided with ports or othercontainer is provided for producing a continuously flowing stream'of liquid reflux rectification product over the inner walls of this column above the body of crude liquid oxygen therein to prevent accumulation of acetylene on these inner walls.

The term oxygen is used in the claims in a comprehensive sense and is intended to include relati y t me we e! a well as we 9% A nitrogen line 19 leads taining impurities usually associated therewith, suchas the crudeoxygen (containing .up..to about 40% oxygen) produced in the high pressuresece tion or column of the modifications of Figures .2 and 3 or produced by processes-involving asingle rectification stepor column. a

-What is claimed is:

1. In a process of producing oxy en :by the liquefaction and rectification of air involving ithe maintenance in a vesselofa'body of .liquidoxygen containing acetylene and the boiling of saidibody to produce vapors passed in countercurrentflow to a descending stream of reflux liquid oxygen, the improvement which comprises continuously wetting with said reflux liquid all of the wallsof the vessel in the region wherethesurfaceof'said body of liquid oxygen containing acetylene contacts said walls to prevent accumulation of acetylene on said walls.

2. In a process of producing oxygen by the liquefaction and rectification of air involving-the maintenance in a vessel of abody of liquid-oxygen containing acetylene and theboiling of said body to produce vapors passed in countercurrent flow to a descending stream of reflux liquid oxygen,

the improvement which comprises continuously wetting with said reflux liquid all of the walls o'f the vessel in the'region where the surface of said body of liquid oxygen containing acetylene-contacts said walls to prevent accumulation of ace tylene on said walls and removing liquid oxygen containing acetylene from said body of oxygen to prevent the concentration of acetylene insaid body of oxygen reaching the point such as to'form an explosive mixture.

3. In a process of producing oxygen by the liquefaction and rectification of air involving the maintenance of a bo'dy of liquid oxygen in 2, rectification column at a pressure of aboutfi to-ahout 12 pounds per square inch gauge, subjecting .a portion of said body of liquid oxygen to vaporization and passing'the resultant oxygen vaporslin 'countercurrent flow to descending reflux liquid oxygen, the improvement which comprisescollecting the reflux liquid oxygen at a point above said body of oxygen, and causing the "collected oxygen-to flow on to saidbody by flowing in con-.- tact with the walls of said column above .the said body of "liquid oxygen and below .said' .pointof collection so as to completely wet the entirearea of said wallsiabove said body and belowssaidpoint of collection.

4. The process defined in claim? in'whicha stream of liquid oxygen'is continuously removed from the said body of liquid oxygen, a:.;major portion of .saidstream is vaporizedandthe vapors thus produced mtroducediintothecolumnito provide .said vapors flowing in couhtercur-rent flow to "the descending refluxliquid, substantially all of the remainder of said oxygen stream is :continuously vaporized except :for a "small portion containing acetylene, and the said. small portion containing acetylene is continuously cremoved."

*5. "A fractionating "column having at jitsibase a chamber adapted to contain. a body of liqui-d, aseries of fractionating plates disposed in spaced relationship above said chambergand an annular collecting container for receiving ,reflux liqugid from said lowermost plate positioned-at the top of said chamber and below said lowermost plate, said container being provided on its outer wall aip'llilifilitXEOf-ClOafilY spaced openings whereby liquid flowing through said openings passes into. contact with the inner walls of said column and flows downwardly there'along into said-chamber.

6. .A'fractionating column as defined in claim 5 in which the container is .a closed annular ring.

7. A fractionating column as defined in claim 5 in which the container is a trough open at its top and having said closely spaced openings in a side wall thereof opposite the inner wall of said column.

3.111 a process of producing oxygen by the liquefaction and rectification of air involvingthe periodic reversal of flow of air and a rectification product through their respective ,fiow paths .in a heat exchange zone 'so' that the air recovers the cold content of the rectification product, the maintenance in a rectification column of a body of liquid containing oxygen, vapors from which body of liquid are passed in countercurrent flow to descending reflux liquid containing oxygen and in which column surges of the level of the body of liquid containing oxygen take place due in part to said reversal of flow, which surges cause precipitation of acetylene on the walls of said rectification column just above 'said body of liquid containing oxygen to take place, the improvement which comprises continuously wetting the said walls of the rectification column with said reflux liquid to prevent accumulation of acetylene on said walls.

'9. Afractionating column comprising a chamber at the base thereof adapted to contain a body of liquid, a series of fractionating plates disposed in spaced relationship above said chamber, a container for receiving liquid from said plates and openings associated with said container for feeding said liquid on to all of the inner walls .of said c hamb,er in the region where the surface of said body of liquid contacts said inner walls to wash down said .inner walls with said liquid throughout the operationgofsaid column.

,10. In a'processof fractionating a gaseous-mixture by the liquefaction andrectification of said gaseous mixture involving the maintenance in a vessel of a body of liquefied gas containing a solidifiable impurity in solution and the'boiling ofsaidbody to produce vapors passed in countercurrent flow -to a descending stream of reflux liquefied gas, -the improvement which comprises continuously wetting with said reflux all of the walls of the vessel in the region where thesurface ofsaidbody of liquefied gas containing said impurity in solution contacts said walls to prevent accumulation of said impurity in solidified form on said walls.

' EDWARD G. ENG-EL.

JOHN A. HUFNAGEL.

Name 7 Date Lafierty July 15,1924

Number

Claims

1. IN A PROCESS OF PRODUCING OXYGEN BY THE LIQUEFACTION AND RECTIFICATION OF AIR INVOLVING THE MAINTENANCE IN A VESSEL OF A BODY OF LIQUID OXYGEN CONTAINING ACETYLENE AND THE BOILING OF SAID BODY TO PRODUCE VAPORS PASSED IN COUNTERCURRENT FLOW TO A DESCENDINGSTREAM OF REFLUX LIQUID OXYGEN THE IMPROVEMENT WHICH COMPRISES CONTINUOUSLY WETTING WITH SAID REFLUX ALL OF THE WALLS OF THE VESSEL IN THE REGION WHERE THE SURFACE OF SAID BODY OF LIQUID OXYGEN CONTAINING ACETYLENE CONTACTS SAID WALLS TO PREVENT ACCUMULATION OF ACETYLENE ON SAID WALLS.