Abstract:
A cryogenic air separation system having a main heat exchanger for processing feed air, said heat exchanger having a reversing heat exchanger function in an upward feed airflow countercurrent section and having a desuperheating function in a downward feed air flow cocurrent section.

Description:
TECHNICAL FIELD 
     This invention relates generally to cryogenic air separation and, more particularly, to cryogenic air separation employing a reversing heat exchanger to clean and cool feed air prior to its passing into the cryogenic air separation plant. 
     BACKGROUND ART 
     In the practice of cryogenic air separation to produce one or more products such as nitrogen and oxygen, the feed air to the cryogenic air separation plant must be cleaned of high boiling impurities, such as carbon dioxide and water vapor, before it enters the cryogenic air separation plant because such high boiling impurities will freeze within the plant at the very low temperatures at which the plant operates, thus reducing the operating efficiency of the cryogenic air separation plant. 
     One very important system for cleaning the feed air is by the use of a reversing heat exchanger wherein the high boiling impurities freeze out onto the heat exchanger passages as the feed air is cooled against return streams such as product and waste streams, and periodically the flow of the feed air stream and a waste stream are alternated in the heat exchanger passages so that the deposited high boiling impurities are passed out of the heat exchanger with the waste stream. 
     Another important function in the operation of a cryogenic air separation system is the desuperheating of the compressed feed air before it enters the column(s) of the cryogenic air separation plant. 
     The heat exchangers employed to cool, clean and desuperheat the feed air for a cryogenic air separation plant involve considerable capital costs. Any improvement to such heat exchanger arrangement would be highly desirable. 
     Accordingly, it is an object of this to provide an improved heat exchanger system for treating feed air to a cryogenic air separation plant. 
     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 cryogenic air separation method comprising: 
     (A) cooling feed air by upward flow in a main heat exchanger to produce cooled feed air, and dividing the cooled feed air into a first portion and a second portion; 
     (B) partially condensing the first portion of the cooled feed air by downward flow in the main heat exchanger, and passing the partially condensed first portion of the cooled feed air and the second portion of the cooled feed air into a cryogenic air separation plant; 
     (C) separating the feed air by cryogenic rectification in the cryogenic air separation plant to produce at least one product; and 
     (D) warming said at least one product by downward flow in the main heat exchanger, and recovering said at least one product. 
     Another aspect of the invention is: 
     Cryogenic air separation apparatus comprising: 
     (A) a main heat exchanger and a cryogenic air separation plant comprising at least one column; 
     (B) means for passing feed air upwardly through a first section of the main heat exchanger, and means for passing feed air downwardly through a second section of the main heat exchanger; 
     (C) means for passing feed air from the main heat exchanger to the cryogenic air separation plant; and 
     (D) means for passing product from the cryogenic air separation plant to the main heat exchanger, and means for recovering product from the main heat exchanger. 
     As used herein, the term “feed air” means a mixture comprising primarily nitrogen and oxygen, such as ambient air. 
     As used herein, the terms “turboexpansion” and “turboexpander” mean respectively method and apparatus for the flow of high pressure fluid through a turbine to reduce the pressure and the temperature of the fluid thereby generating refrigeration. 
     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 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 Engineers&#39; 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 more 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 she vapor and liquid phases 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 (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 “cryogenic air separation plant” means the column or columns wherein feed air is separated by cryogenic rectification, as well as interconnecting piping, valves, heat exchangers and the like. 
     As used herein, the terms “upper portion” and “lower portion” of a column means those portions respectively above and below the midpoint of the column. 
     As used herein, the term “top condenser” means a heat exchange device that generates column downflow liquid from column vapor. 
     As used herein, the term “product nitrogen” means a fluid having a nitrogen concentration equal to or greater than 99 mole percent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The sole FIGURE is a schematic representation of one preferred embodiment of the invention wherein the cryogenic air separation plant comprises a single column with a top condenser for producing product nitrogen. 
    
    
     DETAILED DESCRIPTION 
     In general the invention combines the reversing heat exchanger function in a countercurrent section of a vertically oriented main heat exchanger for treating feed air for a cryogenic air separation plant with the desuperheating function in a cocurrent section of the vertically oriented main heat exchanger. 
     The invention will be described in detail with reference to the Drawing. Referring now to the FIGURE, compressed feed air  1 , generally at a pressure within the range of from 100 to 200 pounds per square inch absolute (psia), is passed through valve  20 A and as stream  60  into pass A of vertically oriented main heat exchanger  61  which comprises a countercurrent or first section  10  and a cocurrent or second section  15 . Countercurrent section  10  is at the lower end of vertically oriented main heat exchanger  61  and cocurrent section  15  is at the upper end of vertically oriented main heat exchanger  61 . 
     The feed air is cooled by upward flow in countercurrent section  10  by countercurrent indirect heat exchange with return streams as will be more fully described below. High boiling impurities such as water vapor and carbon dioxide freeze out onto the inner surface of pass A as the cooling feed air passes upwardly in countercurrent section  10 . The resulting cooled feed air is withdrawn from main heat exchanger  61  in cooled feed air stream  2  which is passed through valve  25 A and as stream  62  to gel trap  45  wherein it is further cleaned of any residual contaminants down to trace levels. 
     The cooled feed air in stream  3  emerging from gel trap  45  is divided into first portion  4  and second portion  6 . First portion  4  generally comprises from 3 to 20 percent, preferably from 5 to 15 percent, of the cooled feed air, and second portion  6  generally comprises the remainder of the cooled feed air, i.e. from 80 to 97 percent, preferably from 85 to 95 percent, of stream  3 . First portion  4  is passed to main heat exchanger  61  and is cooled and partially condensed by downward flow through cocurrent section  15  by cocurrent indirect heat exchange with return streams as will be more fully described below, thereby serving to desuperheat the feed air. The partially condensed feed air first portion is withdrawn from main heat exchanger  61  in stream  5 . Generally stream  5  comprises from about 20 to 90 percent liquid and from about 80 to 10 percent vapor, preferably from 30 to 70 percent liquid and from 70 to 30 percent vapor, most preferably from 40 to 60 percent liquid and from 60 to 40 percent vapor. In a particularly preferred embodiment, the partially condensed feed air first portion comprises about 55 percent liquid and about 45 percent vapor. Partially condensed feed air first portion  5  is combined with cooled feed air second portion  6 , which has been passed through valve  63 , to form combined feed air stream  7  for passage into the cryogenic air separation plant. Alternatively the first and second portions of the feed air may be passed separately into the cryogenic air separation plant. 
     In the embodiment of the invention illustrated in the FIGURE, the cryogenic air separation plant comprises a single column  64  and a top condenser  50 . The feed air is passed into the lower portion of column  64  of the cryogenic air separation plant. Column  64  is operating at a pressure generally within the range of from 90 to 190 psia. Within column  64  the feed air is separated by cryogenic rectification into nitrogen-enriched vapor and oxygen-enriched liquid. 
     Oxygen-enriched liquid is withdrawn from the lower portion of column  64  in stream  8 , throttled through valve  65  and passed as stream  9  into top condenser  50 . Nitrogen-enriched vapor is withdrawn from the upper portion of column  64  as stream  12 . A portion  12  of stream  66  is warmed by downward flow through main heat exchanger  61  by cocurrent indirect heat exchange with the partially condensing first feed air portion and by countercurrent indirect heat exchange with the cooling feed air stream. The warmed nitrogen-enriched vapor is withdrawn from main heat exchanger  61  as stream  16 , passed through valve  67  and recovered as product nitrogen in stream  68 . 
     Another portion  13  of the nitrogen-enriched vapor  66  is passed into top condenser  50  wherein it is condensed by heat exchange with boiling oxygen-enriched liquid introduced into top condenser  50  in stream  9 . Resulting nitrogen-enriched liquid is withdrawn from top condenser  50  in stream  14 . Some or all of the nitrogen-enriched liquid is passed into the upper portion of column  64  as reflux. In the embodiment of the invention illustrated in the FIGURE a portion  27  of the nitrogen-enriched liquid is passed into column  64  as reflux, and another portion is passed through valve  69  and recovered as product nitrogen liquid in stream  11 . A small portion of-the oxygen-enriched liquid provided into top condenser  50  is removed through valve  70  in stream  29  so as to remove contaminants that may accumulate in the condenser pool. 
     Oxygen-enriched vapor, generally comprising from about 25 to 40 mole percent oxygen, is withdrawn from top condenser  50  in stream  17  and is divided into stream  18  and stream  71 . Stream  18  is warmed by downward flow cocurrent heat exchange against the partially condensing first portion of the feed air and then by countercurrent heat exchange with the cooling feed air in the countercurrent section of main heat exchanger  61 . The warmed oxygen-enriched vapor stream is withdrawn from main heat exchanger  61  in stream  19 . Stream  71  is passed through valve  72  and as stream  26  is combined with stream  19  to form oxygen-enriched vapor stream  31 . Stream  31  is passed to turboexpander  55  wherein it is turboexpanded to generate refrigeration. Thereafter resulting refrigeration bearing turboexpanded stream  21  is passed from turboexpander  55  to main heat exchanger  61 . 
     The turboexpanded oxygen-enriched vapor is warmed by downward flow through cocurrent section  15  by indirect cocurrent heat exchange with the partially condensing first feed air portion. Resulting warmed oxygen-enriched vapor  22  is passed through valve  35 B and is then further warmed by downward flow in pass B through countercurrent section  10  by indirect countercurrent heat exchange with the cooling upwardly flowing feed air in pass A. Resulting further warmed oxygen-enriched vapor, which contains contaminants picked up by passage through pass B which were deposited there in a previous cycle, is then removed from main heat exchanger  61  in stream  73 , passed through valve  30 B and removed from the system as waste stream  28 . 
     Periodically the flow of the feed air in pass A and the flow of the waste stream in pass B is reversed. In such periods, feed air  1  flosses through valve  20 B and as stream  74  is passed into pass B of main heat exchanger  61  and then is passed through valve  25 B for passage to gel trap  45 . Simultaneously, warmed oxygen-enriched vapor  22  is passed through valve  35 A and then through pass A wherein it is further warmed and picks up the contaminants deposited therein by the feed air which passed through pass A in the previous period. The resulting waste stream is removed from main heat exchanger  61  in stream  74 , passed through valve  30 A and removed from the system in stream  28 . 
     Although the invention has been described in detail with reference to a certain preferred embodiment those skilled in the art will recognize that there are other embodiments of the invention within the spirit and the scope of the claims. For example, another expansion device such as a Joule-Thomson valve may be used in place of turboexpander  55 . In addition liquid nitrogen may be added to the top condenser arid/or the column to help sustain the cryogenic rectification. Moreover, the internal passage configuration of stream  4  in cocurrent section  15  may be of the cocurrent cross-flow type instead of the simple cocurrent flow arrangement shown in the Drawing.