Abstract:
A cryogenic rectification system comprising an upstream krypton/xenon knockout column and a downstream oxygen upgrader column wherein the knockout column processes a crude feed for removal of heavy components including hydrocarbons and the upgrader column produces ultra high purity oxygen.

Description:
TECHNICAL FIELD 
     This invention relates generally to cryogenic rectification and, more particularly, to the use of cryogenic rectification to produce ultra high purity oxygen. 
     BACKGROUND ART 
     Ultra high purity oxygen is required in manufacturing processes that are very sensitive to contaminants, such as in the production of semiconductors and other electronic components. As the demand for ultra high purity oxygen increases, there is a need for a system which can efficiently produce ultra high purity oxygen. 
     Accordingly, it is an object of this invention to provide an improved system for producing ultra high purity oxygen. 
     SUMMARY OF THE INVENTION 
     The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is: 
     A method for producing ultra high purity oxygen by cryogenic rectification comprising: 
     (A) providing a feed comprising oxygen, argon, krypton and xenon, and passing the feed into the upper portion of a first column; 
     (B) separating the feed by cryogenic rectification within the first column into a top fluid comprising oxygen and argon, and into a bottom fluid comprising krypton and xenon; 
     (C) passing top fluid from the upper portion of the first column into the upper portion of a second column, and separating the top fluid by cryogenic rectification within the second column into argon-enriched fluid and ultra high purity oxygen; and 
     (D) withdrawing ultra high purity oxygen from the lower portion of the second column and recovering the withdrawn ultra high purity oxygen as product. 
     Another aspect of the invention is: 
     Apparatus for producing ultra high purity oxygen comprising: 
     (A) a first column having a top condenser and a bottom reboiler, and means for passing a feed comprising oxygen, argon, krypton and xenon into the upper portion of the first column; 
     (B) a second column having a top condenser and a bottom reboiler, and means for passing fluid from the upper portion of the first column into the upper portion of the second column; 
     (C) means for passing fluid from the bottom reboiler of the first column to the top condenser of the first column, and means for passing fluid from the bottom reboiler of the second column to the top condenser of the second column; and 
     (D) means for recovering ultra high purity oxygen from the lower portion of the second column. 
     As used herein the term “ultra high purity oxygen” means a fluid having an oxygen concentration of at least 99.99 mole percent. 
     As used herein the term “column” means a distillation or 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 of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. 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. Distillation is the separation process whereby heating of a liquid mixture can be used to concentrate the more volatile component(s) in the vapor phase and thereby the less volatile component(s) 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 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 phases can be adiabatic or nonadiabatic and can include integral (stagewise) or differential (continuous) 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 (K). 
     As used herein, the term “indirect heat exchange” means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other. 
     As used herein, the term “upper portion” and “lower portion” mean those sections of a column respectively above and below the mid point of the column. 
     As used herein, the term “tray” means a contacting stage, which is not necessarily an equilibrium stage, and may mean other contacting apparatus such as packing having a separation capability equivalent to one tray. 
     As used herein, the term “equilibrium stage” means a vapor-liquid contacting stage whereby the vapor and liquid leaving the stage are in mass transfer equilibrium, e.g. a tray having 100 percent efficiency or a packing element height equivalent to one theoretical plate (HETP). 
     As used herein, the term “top” when referring to a column means that section of the column above the column mass transfer internals, i.e. trays or packing. 
     As used herein, the term “bottom” when referring to a column means that section of the column below the column mass transfer internals, i.e. trays or packing. 
     As used herein, the term “bottom reboiler” means a heat exchanger for generating column upflow vapor from column liquid. 
     As used herein, the term “top condenser” means a heat exchanger for generating column downflow liquid from column vapor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of one preferred embodiment of the ultra high purity oxygen production system of this invention. 
     FIG. 2 is a schematic representation of another preferred embodiment of the ultra high purity oxygen production system of this invention. 
    
    
     The numerals in the Drawings are the same for the common elements. 
     DETAILED DESCRIPTION 
     The invention will be described in detail with reference to the Drawings. 
     Referring now to FIG. 1, feed stream  101  is passed into the upper portion of first column  100 , which is operating at a pressure generally within the range of from 15 to 50 pounds per square inch absolute (psia). Stream  101  may be liquid, gaseous or mixed phase. Typically stream  101  is taken from a cryogenic air separation plant such as a single column nitrogen plant, a double column plant producing oxygen or both nitrogen and oxygen, or a triple column plant producing argon in addition to oxygen and nitrogen. 
     Feed  101  comprises oxygen, argon, krypton and xenon. Generally feed  101  has an oxygen concentration within the range of from 98 to 99.9 mole percent. Because the product of this invention is ultra high purity oxygen, it is imperative that the presence of flammables such as hydrocarbons be minimized in the process. It is preferred in the practice of this invention that feed  101  undergo hydrocarbon removal prior to passage into first column  100 . In one preferred hydrocarbon removal procedure, the hydrocarbons are removed by heating the feed to about 1000° F. and passing the heated feed over a rare earth catalyst such as platinum. The hydrocarbons will combine with oxygen in the feed and be converted to carbon dioxide and water. The resulting feed which is now substantially free of hydrocarbons is cooled and passed through a molecular sieve dryer to remove the carbon dioxide and water and then further cooled, preferably to just above its saturation point, prior to passage into first column  100 . 
     Feed  101  preferably is passed into first column  100  at a level from 20 to 30 equilibrium stages below the top of column  100 . Within first column  100 , feed  101  is separated by cryogenic rectification into a top fluid comprising oxygen and argon and into a bottom fluid which contains most of the krypton and xenon which was in feed  101 . 
     First column  100  has top condenser  150  and bottom reboiler  160 . Bottom reboiler  160  is driven by vapor  110 , such as oxygen, which is condensed in reboiler  160  by indirect heat exchange with the bottom fluid to provide vapor upflow for the column. Preferably vapor  110  is from the cryogenic air separation system from which feed stream  101  is taken. The resulting condensed reboiler driving fluid  111  is throttled through valve  35  and then passed into top condenser  150  as stream  107 . If necessary, additional liquid  114  may be passed into top condenser  150  to ensure that sufficient refrigeration is supplied to top condenser  150  to adequately reflux first column  100 . Bottom liquid, having a higher concentration of krypton and/or xenon as a result of the reboiling, is withdrawn from the system in stream  109 . Typically the bottom fluid in stream  109  will have a krypton and xenon concentration of at least 90 mole percent. Stream  109  may contain a small fraction of oxygen, generally from 1 to 10 mole percent, as well as trace amounts of other components having boiling points higher than oxygen. 
     Top fluid is withdrawn from the upper portion of the first column and passed into the upper portion of a second column. In the embodiment of the invention illustrated in FIG. 1, top fluid comprising oxygen and argon is withdrawn from the top of first column  100  in vapor stream  102  which is passed into top condenser  150 . Within top condenser  150  top fluid  102  is condensed by indirect heat exchange with driving fluid  107  which is at least partially vaporized and withdrawn from the system in stream  108 . Resulting condensed top fluid  103  from top condenser  150  is passed as reflux stream  104  back into first column  100 , and as stream  105  into the upper portion of second column  200  as was previously described. Preferably feed stream  105  containing oxygen and argon is passed into second column  200  at a level from 3 to 40 equilibrium stages below the top of second column  200 . Top condenser  150  is operated in such a manner to ensure that any hydrocarbons that may be in feed stream  101  are washed down the column by the downflowing reflux and removed from the system in stream  109 , thereby enabling the feed to the second column to be essentially free of any hydrocarbons. 
     Second or upgrader column  200  is operating at a pressure generally within the range of from 15 to 50 psia. Within second column  200  the top fluid passed into this column from first column  100  is separated by cryogenic rectification into argon-enriched fluid and ultra high purity oxygen. 
     Second column  200  has a top condenser  250  and a bottom reboiler  260 . Bottom reboiler  260  is driven by vapor  210  such as nitrogen, which is condensed in reboiler  260  by indirect heat exchange with ultra high purity oxygen liquid at the bottom of second column  200 , serving to boil a portion of this liquid to provide vapor upflow for the column and in the process increasing the oxygen concentration of the ultra high purity oxygen liquid. Preferably vapor  210  is taken from the air separation system from which feed  101  is taken. The resulting condensed reboiler driving fluid  211  is throttled through valve  36  and then passed into top condenser  250  as stream  207 . If necessary, additional liquid  212  may be passed into top condenser  250  to ensure that sufficient refrigeration is supplied to top condenser  250  to reflux second column  200 . Ultra high purity oxygen bottom liquid, having a higher concentration of oxygen as a result of the reboiling, is withdrawn from the lower portion of second column  200  in stream  209  and recovered as product ultra high purity oxygen. If desired, in addition to or in place of liquid stream  209 , ultra high purity oxygen may be recovered from second column  200  from above bottom reboiler  260  as shown by stream  38 . 
     Argon-enriched fluid is withdrawn from the upper portion of second column  200  in vapor stream  201  and a portion  204  is removed from the system as an overhead waste stream. Another portion  202  is passed into top condenser  250  wherein it is condensed by indirect heat exchange with driving fluid  207  which is at least partially vaporized and removed from the system in stream  208 . Resulting condensed argon-enriched fluid  203  from top condenser  25  is passed back into the upper portion of second column  200  as reflux. 
     FIG. 2 illustrates another embodiment of the invention. The elements of the embodiment illustrated in FIG. 2 which are common with those elements of the embodiment illustrated in FIG. 1 will not be described again in detail. Referring now to FIG. 2, top fluid stream  102  is divided into portion  112 , which is removed from the system, and into portion  106  which is passed into top condenser  150  for condensation to produce reflux liquid  103  which is passed back into first column  100  in its entirety. In the embodiment of the invention illustrated in FIG. 2, the top fluid for passage into the upper portion of the second column is taken directly from the upper portion of first column  100  as stream  37  without first going through the top condenser as in the embodiment illustrated in FIG.  1 . An oxygen stream  113 , having an oxygen concentration generally within the range of from 99 to 99.99 mole percent, is withdrawn from the upper portion of first column  100 , but from a level at least one equilibrium stage below the withdrawal level of the top fluid, e.g., stream  37 . Oxygen stream  113  may be recovered as product or may be passed on to another column for further processing. 
     In the embodiment of the invention illustrated in FIG. 2, vaporized first column top condenser driving fluid  108  is not directly removed from the system but instead is passed into second column bottom reboiler  260  wherein it is condensed by indirect heat exchange with ultra high purity oxygen liquid and then passed on to top condenser  250  as was previously described. 
     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.