Abstract:
An air boiling cryogenic rectification system wherein additional feed air streams are used for vaporizing pressurized liquid oxygen and, by turboexpansion, for the generation of refrigeration prior to being passed into the column system.

Description:
TECHNICAL FIELD 
     This invention relates generally to cryogenic rectification using air boiling and is particularly advantageous for the production of elevated pressure oxygen having an oxygen concentration within the range of from 70 to 85 mole percent. 
     BACKGROUND ART 
     The cryogenic rectification of air to produce oxygen and nitrogen is a well established industrial process. Typically the feed air is separated in a double column system wherein nitrogen shelf or top vapor from a higher pressure column is used to reboil oxygen bottom liquid in a lower pressure column. 
     The demand for lower purity oxygen is increasing in applications such as glassmaking, steelmaking and energy production. Less vapor boilup in the stripping sections of the lower pressure column, and less liquid reflux in the enriching sections of the lower pressure column are necessary for the production of lower purity oxygen which has an oxygen purity of less than 98.5 mole percent, than are typically generated by the operation of a double column. 
     Accordingly, lower purity oxygen is generally produced in large quantities by a cryogenic rectification system wherein feed air at the pressure of the higher pressure column is used to reboil the liquid bottoms of the lower pressure column and is then passed into the higher pressure column. The use of air instead of nitrogen to vaporize the lower pressure column bottoms reduces the air feed pressure requirements, and enables the generation of only the necessary boil-up in the stripping sections of the lower pressure column either by feeding the appropriate portion of the air to the lower pressure column reboiler or by partially condensing a larger portion of the total feed air. 
     While the conventional air boiling cryogenic rectification system would be effective for the production of lower purity oxygen, its ability to generate liquid nitrogen reflux for supply to the top of the lower pressure column is limited. This results from the lower component relative volatilities at the operating pressure of the higher pressure column which is similar to that of the main air feed and because of the large fraction of liquid air produced. More power is consumed because oxygen recovery is reduced as a result of the reduced capability to generate liquid nitrogen reflux. 
     Accordingly, it is an object of this invention to provide a cryogenic rectification system for producing lower purity oxygen wherein the liquid bottoms of a lower pressure column are reboiled by indirect heat exchange with feed air and which operates with reduced power requirements over that of conventional air boiling systems especially while producing oxygen at a concentration less than 90 mole percent. 
     Often it is desired to recover the product oxygen gas at an elevated pressure. Generally this is carried out by compressing the product gas to a higher pressure by passage through a compressor. Such a system is effective but is quite costly. Moreover, air boiling cryogenic rectification systems have heretofore been most useful for the production of lower pressure oxygen. 
     Accordingly, it is another object of this invention to provide an air boiling cryogenic rectification system which can effectively produce elevated pressure oxygen gas without the need for oxygen gas compression. 
     SUMMARY OF THE INVENTION 
     The above and other objects which will become apparent to one skilled in the art upon a reading of this disclosure are attained by the present invention, one aspect of which is: 
     In a cryogenic air separation process employing a higher pressure column and a lower pressure column wherein feed air is employed to boil the bottom liquid of the lower pressure column and is thereafter passed into the higher pressure column and wherein liquid oxygen is produced in the lower pressure column, the improvement comprising: 
     (A) turboexpanding a second portion of feed air to generate refrigeration and passing turboexpanded second feed air into the higher pressure column; 
     (B) withdrawing liquid oxygen from the lower pressure column and increasing the pressure of the withdrawn liquid oxygen; 
     (C) vaporizing the pressurized liquid oxygen by indirect heat exchange with a third feed air portion which is at a pressure higher than that of the feed air employed to boil the bottom liquid of the lower pressure column, resulting in the production of oxygen gas and liquid feed air; 
     (D) passing resulting liquid feed air into at least one of the higher pressure column and the lower pressure column; and 
     (E) recovering resulting oxygen gas as elevated pressure oxygen gas product. 
     Another aspect of the invention is: 
     In a cryogenic rectification apparatus having a first column, a second column with a bottom reboiler and means for passing a feed stream to the bottom reboiler and from the bottom reboiler into the first column, the improvement comprising: 
     (A) a turboexpander, means for passing a second feed stream to the turboexpander and from the turboexpander into the first column; 
     (B) means for withdrawing liquid from the second column and means for increasing the pressure of the liquid withdrawn from the second column to produce elevated pressure liquid; 
     (C) a product boiler, means for passing a third feed stream to the product boiler and means for passing said elevated pressure liquid to the product boiler; 
     (D) means for passing liquid feed from the product boiler into at least one of the first column and the second column; and 
     (E) means for recovering gas product from the product boiler. 
     As used herein the term &#34;liquid oxygen&#34; means a liquid having an oxygen concentration within the range of from 70 to 98 mole percent. 
     As used herein, the term &#34;feed air&#34; means a mixture comprising primarily nitrogen and oxygen, such as air. 
     As used herein, the terms &#34;turboexpansion&#34; and &#34;turboexpander&#34; mean respectively method and apparatus for the flow of high pressure-gas through a turbine to reduce the pressure and the temperature of the gas thereby generating refrigeration. 
     As used herein, the term &#34;column&#34; means a distillation of fractionation column or zone, i.e., a contacting column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting or the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements which may be structured packing and/or random packing elements. For a further discussion of distillation columns, see the Chemical Engineer&#39;s Handbook fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous Distillation Process. 
     Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phase is adiabatic and can include integral or differential contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin. 
     As used herein, the term &#34;indirect heat exchange&#34; means the bringing of two fluid streams into heat exchange relation without any physical contact or intermixing of the fluids with each other. 
     As used herein, the term &#34;top condenser&#34; means a heat exchange device which generates column downflow liquid from column top vapor. 
     As used herein, the term &#34;bottom reboiler&#34; means a heat exchange device which generates column upflow vapor from column bottom liquid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of one preferred embodiment of the invention. 
     FIG. 2 is a schematic representation of another preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     The invention will be described in detail with reference to the Drawings. 
     Referring now to FIG. 1, feed air 1, at a pressure generally within the range of from 40 to 65  pounds per square inch absolute (psia), is cooled by indirect heat exchange with return streams in heat exchanger 300 and then resulting feed air stream 2 is further cooled by passage through heat exchanger 301. Resulting feed air stream 3 is passed into bottom reboiler 306 wherein it is partially condensed while serving to boil the bottom liquid of lower pressure column 200 which is operating at a pressure generally within the range of from 18 to 25 psia. Resulting feed air is passed in stream 4 from bottom reboiler 306 into higher pressure column 100 which is operating at a pressure greater than that of lower pressure column 200 and generally within the range of from 30 to 60 psia. 
     Another feed air stream 10, at a pressure greater than that of stream 1 and generally at a pressure within the range of from 80 to 1400 psia, is cooled by passage through heat exchanger 300. Resulting feed air stream 11 is divided into stream 25 and stream 12. Stream 25 comprises a second portion of the feed air which is turboexpanded by passage through turboexpander 35 to generate refrigeration. Resulting feed air stream 26 is desuperheated by passage through heat exchanger 309 and passed as stream 27 into high pressure column 100. 
     Stream 12 is further cooled by passage through heat exchanger 301 to near its saturation point and resulting feed air stream 14 is divided into stream 5 and stream 15. Stream 5 is liquefied by passage through heat exchanger 305 and the resulting liquefied feed air 6 is passed into the columns as will be described more fully later. 
     Stream 15 comprises a third portion of the feed air and is at a pressure which is higher than the pressure of the feed air used to boil the bottoms of lower pressure column 200. Stream 15 is passed into product boiler 307 wherein it is condensed by indirect heat exchange with vaporizing pressurized liquid oxygen and then passed into at least one of column 100 and column 200. The embodiment illustrated in FIG. 1 is a preferred embodiment wherein the resulting liquid feed air is passed in line 16 to subcooler 308 wherein it is subcooled by indirect heat exchange with pressurized liquid oxygen. Subcooled liquid feed air 17 is then combined with stream 6 to form feed air stream 18 which is further subcooled by passage through heat exchanger 304 to form stream 19. 
     At least a portion 22 of liquid feed air 19 is throttled to the pressure of higher pressure column 100 by passage through valve 40 and the resulting feed air stream 23 is passed into higher pressure column 100. If desired, a portion 20 of liquid feed air 19 is throttled to the pressure of lower pressure column 200 by passage through valve 50 and the resulting feed air portion 21 is passed into lower pressure column 200. 
     Within high pressure column 100 the feeds into that column are separated by cryogenic rectification into nitrogen-enriched vapor and oxygen-enriched liquid. Nitrogen-enriched vapor 70 is passed into top condenser 302 wherein it is condensed. Resulting liquid 71 is divided into reflux streams 46 and 73. Reflux stream 73 is passed as reflux into higher pressure column 100. Reflux stream 46 is subcooled by passage through heat exchanger 303 and resulting stream 47 is throttled to the pressure of lower pressure column 200 by passage through valve 48 and passed as reflux stream 49 into lower pressure column 200. If desired, a portion 42 of the nitrogen-enriched vapor may be warmed by passage through heat exchangers 301 and 300 and recovered as high pressure nitrogen gas product having a purity of up to about 99.9 mole percent. 
     Oxygen-enriched liquid is passed in stream 28 through heat exchanger 304 wherein it is subcooled. Resulting stream 29 is throttled by passage through valve 37 and resulting stream 30 is passed into top condenser 302 wherein it is partially vaporized by indirect heat exchange with condensing nitrogen-enriched vapor. Resulting oxygen-enriched vapor and remaining oxygen-enriched liquid are passed in streams 32 and 31 respectively through valves 38 and 39 respectively wherein they are throttled to the pressure of lower pressure column 200. Respective resulting vapor stream 34 and liquid stream 33 are then passed into lower pressure column 200. 
     The various feeds into lower pressure column 200 are separated by cryogenic rectification within column 200 to produce nitrogen vapor and liquid oxygen. Nitrogen vapor is withdrawn from column 200 as stream 51, warmed by passage through heat exchangers 303, 304, 305, 301 and 300, and, if desired, recovered as lower pressure nitrogen gas product 55 having a nitrogen purity of up to about 99.5 mole percent. 
     Liquid oxygen is withdrawn from lower pressure column 200 in stream 58 and is increased in pressure such as by passage through liquid pump 59. Resulting pressurized liquid oxygen 60 is then warmed against subcooling liquid feed air in heat exchanger 308 and then passed as stream 61 into product boiler 307 wherein it is vaporized by indirect heat exchange with the elevated pressure feed air. Resulting oxygen gas produced in the product boiler is passed as stream 62 through heat exchangers 309, 301 and 300 wherein it is warmed and then recovered as elevated pressure oxygen gas product generally having a pressure within the range of from 40 to 800 psia and an oxygen concentration within the range of from 70 to 98 mole percent. 
     FIG. 2 illustrates another embodiment of the invention. The numerals in FIG. 2 correspond to those of FIG. 1 for the common elements and these common elements will not be described again in detail. The embodiment illustrated in FIG. 2 differs from that illustrated in FIG. 1 primarily in that turboexpanded feed air 26 is not passed directly into higher pressure column 100 after passage through heat exchanger 309. Rather turbo expanded stream 26 is combined with stream 3 to form feed air stream 91 which is then passed through heat exchanger 310 before being passed through bottom reboiler 306 and then as stream 4 into higher pressure column 100. In the practice of the embodiment illustrated in FIG. 2, the higher pressure feed air stream 14, as well as oxygen gas stream 62 and nitrogen gas streams 42 and 51 also pass through heat exchanger 310. 
     The invention is advantageous over conventional air boiling systems in the ability to efficiently produce oxygen at purity levels less than 90 mole percent, and particularly in the range from 70 to 85 mole percent. With conventional processes, at oxygen purities less than 90 mole percent, there may arise the situation wherein the pressure ratio across the turbine is too small to produce enough refrigeration to sustain the process. The invention overcomes this problem because a high pressure feed air stream provides the flow to the turbine. 
     Although the invention has been described in detail with reference to certain preferred embodiments, those skilled in the art will recognize that there are other embodiments of the invention within the spirit and the scope of the claims.