Abstract:
A system for liquefying industrial gas wherein industrial gas is compressed to two levels using a first and a second compression system and then is processed in a heat exchanger having horizontally oriented sensible heat exchange passages and vertically oriented condensing heat exchange passages.

Description:
TECHNICAL FIELD 
     This invention relates generally to cryogenic heat exchange for the liquefaction of industrial gases. 
     BACKGROUND ART 
     Liquefaction of low boiling point gases, such as oxygen and nitrogen, is both capital and energy intensive. Typically practitioners have addressed the issue of improving liquefier performance by using multiple turbines and by using liquid expanders. Generally heat exchangers used with these systems are oriented in the vertical plane due to process hydraulic effects. This conventional practice leads to long piping runs of large bore warm end piping and also requires the utilization of significant footprint space for aftercooler heat exchangers and associated piping. 
     Accordingly it is an object of this invention to provide an industrial gas liquefaction system having an improved design and lower costs than conventional industrial gas liquefaction systems. 
     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 liquefying an industrial gas comprising: 
     (A) compressing industrial gas to produce compressed industrial gas and further compressing a portion of the compressed industrial gas to produce a compressed first industrial gas portion and a further compressed second industrial gas portion; 
     (B) cooling the first industrial gas portion, turboexpanding the cooled first industrial gas portion, and warming the turboexpanded first industrial gas portion by horizontal countercurrent flow indirect heat exchange with the further compressed second industrial gas portion to cool the further compressed second industrial gas portion; 
     (C) dividing the cooled second industrial gas portion into a first part and a second part, turboexpanding said first part and warming the turboexpanded first part by indirect heat exchange with the second part of the cooled second industrial gas portion in vertical flow to liquefy said second part; and 
     (D) recovering the liquefied second industrial gas part as product liquefied industrial gas. 
     Another aspect of the invention is: 
     Apparatus for liquefying an industrial gas comprising: 
     (A) a heat exchanger having horizontally oriented heat exchange passages and having vertically oriented heat exchange passages in flow communication with the horizontally oriented heat exchange passages; 
     (B) a first compression system, a second compression system, means for providing industrial gas to the first compression system and from the first compression system to a horizontally oriented passage of the heat exchanger, and means for providing industrial gas from the first compression system to the second compression system and from the second compression system to a horizontally oriented passage of the heat exchanger; 
     (C) a first turboexpander, a second turboexpander, means for passing industrial gas from a horizontally oriented passage of the heat exchanger to the first turboexpander and from the first turboexpander to another horizontally oriented passage of the heat exchanger and means for passing industrial gas from the heat exchanger to the second turboexpander and from the second turboexpander either to a vertically oriented passage or to a horizontally oriented passage of the heat exchanger; and 
     (D) means for recovering liquefied industrial gas from a vertically oriented passage of the heat exchanger. 
     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 “compressor” means a device which accepts gaseous fluid at one pressure and discharges it at a higher pressure. 
     As used herein, the terms “turboexpansion” and “turboexpander” 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 terms “subcooling” and “subcooler” mean respectively method and apparatus for cooling a liquid to be at a temperature lower than the saturation temperature of that liquid for the existing pressure. 
     As used herein the term “industrial gas” means a fluid comprised primarily of one or more of nitrogen, oxygen, natural gas or one or more other hydrocarbons. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The sole FIGURE is a simplified schematic representation of one particularly preferred embodiment  10  of the cryogenic industrial gas liquefaction system of this invention. 
    
    
     DETAILED DESCRIPTION 
     The invention will be described in detail with reference to the Drawing. Referring now to the FIGURE, industrial gas  1 , e.g. nitrogen, generally having a pressure up to about 20 pounds per square inch absolute (psia), such as from an air separation plant, is passed to a first compression system comprising feed compressor  2  and recycle compressor  3 . In the embodiment illustrated in the FIGURE, industrial gas feed stream  1  is combined with recycle stream  4  to form combined stream  5  for passage to feed compressor  2 . 
     Within feed compressor  2  the industrial gas feed is compressed to a pressure generally within the range of from 50 to 85 psia and resulting industrial gas stream  6  is cooled of the heat of compression in cooler  7 . Resulting industrial gas stream  8  is passed to recycle compressor  3  of the first compression system. In the embodiment of the invention illustrated in the FIGURE a medium pressure return stream  9 , an additional stream  10  from the air separation plant, and a recycle stream  11  from compressor  3  are all passed into industrial gas stream  8  to form industrial gas stream  12  for passage into recycle compressor  3 . 
     Within recycle compressor  3  the industrial gas in stream  12  is compressed to a pressure generally within the range of from 190 to 380 psia to form compressed industrial gas stream  13 . The heat of compression is removed from stream  13  by passage through cooler  14  and resulting compressed industrial gas stream  15  is divided into a first portion  16  and into a second portion  17 . 
     Heat exchanger  18  is comprised of four zones identified as zones  1 ,  2 ,  3  and  4  in the FIGURE. The heat exchange passages in zone  1  are oriented vertically and the heat exchange passages in zone  2 ,  3  and  4  are oriented horizontally. Preferably in zone  1  all of the heat exchange passages are oriented vertically. However, the invention may also be practiced with horizontal parting sheets and cross flow orientation such that the return streams in zone  1  are oriented horizontally while the product stream is oriented vertically. It will be understood by those skilled in the art that small deviations from absolute vertical or absolute horizontal are allowable in the practice of this invention without unduly compromising the effectiveness of the invention. 
     First compressed industrial gas portion  16  is passed to an input of a horizontal heat exchange passage in zone  4  and is cooled by flow through that passage to form cooled first compressed industrial gas portion which is withdrawn from zone  4  of heat exchanger  18  in stream  19 . The cooled first industrial gas portion in stream  19  is turboexpanded by passage through warm or first turboexpander  20  and resulting turboexpanded first industrial gas portion  21  is warmed by passage through zones  3  and  4  of heat exchanger  18 , emerging therefrom as the aforementioned return stream  9 . 
     Compressed second industrial gas portion  17  is further compressed by passage through a second compression system which in the embodiment illustrated in the FIGURE comprises warm booster compressor  22  and cold booster compressor  23 . Stream  17  is compressed by passage through compressor  22  to a pressure generally within the range of 300 to 540 psia and resulting industrial gas stream  24  is cooled of the heat of compression by passage through cooler  25 . Resulting stream  26  is compressed by passage through compressor  23  to a pressure generally within the range of from 450 to 760 psia emerging therefrom as further compressed second industrial gas portion in stream  27 . Further compressed second industrial gas portion  27  is cooled of the heat of compression by passage through cooler  28  and resulting further compressed second industrial gas portion is passed in stream  29  into a horizontal heat exchange passage in zone  4  of heat exchanger  18 . 
     The further compressed second industrial gas portion is cooled by passage through zones  4 ,  3  and  2  of heat exchanger  18  by indirect heat exchange with countercurrently flowing warming streams such as stream  21 , as was previously described, to form cooled second industrial gas portion, a first part of which is withdrawn from heat exchanger  18  in stream  30  and passed to cold or second turboexpander  31 . Stream  30  is turboexpanded by passage through turboexpander  31  and resulting turboexpanded stream  32  is passed into a preferably vertically oriented heat exchange passage in zone  1  of heat exchanger  18 . 
     The remaining or second part of the cooled second industrial gas portion is passed downwardly through zone  1  of heat exchanger  18  preferably countercurrently to upwardly flowing streams such as aforementioned stream  32  and is liquefied by indirect heat exchange therewith to form liquefied industrial gas second part in stream  33 . As illustrated in the FIGURE, stream  32 , after the heat exchange with the cooled industrial gas second part, passes horizontally through zone  2  of heat exchanger  18  and then combines with stream  21  for further passage through zones  3  and  4  of heat exchanger  18  before emerging as previously described stream  9 . 
     Stream  33  may be recovered as product liquefied industrial gas. The FIGURE illustrates a particularly preferred embodiment of the invention wherein stream  33  is subcooled prior to recovery. In accordance with this particularly preferred embodiment, stream  33  which may be a liquid or pseudo-liquid depending on its composition and pressure, is throttled through valve  34  to a pressure generally within the range of from 80 to 120 psia and resulting stream  35  is subcooled by passage through subcooler  36 , from which it is withdrawn as subcooled stream  37 , some or all of which is recovered as product liquefied industrial gas in stream  38 . In the embodiment illustrated in the FIGURE, not all of stream  37  is recovered directly but rather a stream  39  from stream  37  is throttled through valve  40  to a pressure typically within the range of from 16 to 19 psia and passed as stream  41  through subcooler  36  wherein it is warmed by indirect heat exchange to effect the subcooling of stream  35 . Resulting stream  42  is passed from subcooler  36  to heat exchanger  18  and is warmed by passage through heat exchanger  18  preferably countercurrently by indirect heat exchange with the aforesaid cooling or condensing streams. Stream  42  flows upwardly in zone  1  of heat exchanger  18  and horizontally through zones  2 ,  3  and  4  of heat exchanger  18 , emerging therefrom as warm stream  43 , which is passed through valve  44  to form recycle stream  4  as was previously described. 
     With the use of horizontal countercurrent indirect heat exchange in the sensible heat exchange zones and vertical countercurrent heat exchange in the condensing zone of the liquefier heat exchanger, a more efficient industrial gas liquefaction is achieved. Shorter piping runs to unit operations outside the cold box package can be used, and equipment skid design is facilitated. Sensible heat exchange is maximized while fluid distribution especially in the condensing zone is facilitated. 
     Although the invention has been described in detail with reference to a particularly 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, a parallel turbine arrangement could also be employed to carry out the invention.