Abstract:
A process for carrying out cryogenic air separation wherein liquid oxygen is pressurized and vaporized against condensing feed air to produce oxygen gas product wherein excess plant refrigeration is generated such that the aggregate warm end temperature difference of the process exceeds the minimum internal temperature difference of the primary heat exchanger by at least 2K.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/265,123, filed Nov. 3, 2005 now abandoned. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to cryogenic air separation and, more particularly, to cryogenic air separation to produce oxygen product. 
     BACKGROUND ART 
     The separation of air into its constituent components by distillation occurs at cryogenic temperatures, and requires some amount of refrigeration. This refrigeration is typically generated by the expansion of a process gas across a turbine. When designing air separation processes, the amount of refrigeration generated by expansion is typically kept at a minimum, as all forms of refrigeration generation are penal to the process, either by degrading the efficiency of the separation or by requiring more compression energy than is minimally required by the needs of the plant&#39;s distillation columns. The efficiency of refrigeration usage for a plant is reflected by the temperature difference between the streams entering and leaving the plant. This temperature difference is referred to as the aggregate warm end temperature difference (WEDT). At the extreme minimum, a WEDT of 0K indicates that only the refrigeration required to drive the air separation was generated. 
     In liquid oxygen pumped cryogenic air separation plants, product oxygen is removed as a liquid from the bottom of a low pressure distillation column, whereupon it is pumped to an elevated pressure, boiled in the primary heat exchanger or a product boiler against a condensing air stream, and the resulting vapor is superheated in the primary heat exchanger to form the gaseous oxygen product. If the liquid oxygen is pumped to its final delivery pressure, the gaseous oxygen product is sent directly to the end user, otherwise it requires further compression. The boiling of this oxygen against the condensing air gives rise to an internal pinch temperature difference. In other words, it gives rise to the minimum aggregate temperature difference between the cooling and warming streams in the primary heat exchanger (PHX). The magnitude of the PHX internal pinch is dictated by the available heat exchanger surface area. The larger the PHX, the tighter the pinch. Typically, in liquid oxygen pumped air separation plants, the PHX pinch DT is approximately 1-2K. 
     The condensing air stream has to be compressed to a higher pressure than that of the main air feed to the plant prior to entering the PHX. This compression is typically accomplished with a separate booster air compressor. The pressure of the condensing air stream is typically higher than that of the boiling oxygen stream. As such, when higher pressure oxygen is required as a product, the booster air compressor consumes a large amount of energy. Because of the rising energy costs, the need exists for improved cryogenic air separation processes that use less total energy. It is a goal of this invention to reduce total power consumption by reducing the compression requirements of the condensing air stream. 
     SUMMARY OF THE INVENTION 
     In a process for the cryogenic separation of feed air wherein feed air is cooled in a primary heat exchanger, is separated by cryogenic rectification in at least one column to produce oxygen-rich liquid and nitrogen-rich vapor, oxygen-rich liquid is increased in pressure, and the pressurized oxygen-rich liquid is vaporized by indirect heat exchange with at least some of the feed air to produce product oxygen, the improvement comprising generating sufficient excess refrigeration beyond that required to carry out the cryogenic rectification such that the aggregate warm end temperature difference of the process exceeds the minimum internal temperature difference of the primary heat exchanger by at least 2K. 
     As used herein, the term “aggregate warm end temperature difference” means the difference between the aggregate temperatures of those streams entering the primary heat exchanger and of those streams leaving the primary heat exchanger. 
     As used herein, the term “minimum internal temperature difference of the primary heat exchanger” means the smallest difference between the aggregate temperatures of the warming and cooling streams inside the primary heat exchanger. 
     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 . A double column comprises a higher pressure column having its upper end in heat exchange relation with the lower end of a lower pressure column. 
     Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The higher vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the lower 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 phases is generally adiabatic 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 “feed air” means a mixture comprising primarily oxygen and nitrogen, such as ambient air. 
     As used herein, the terms “upper portion” and “lower portion” of a column mean those sections of the column respectively above and below the mid point of the column. 
     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 “cryogenic air separation plant” means the column or columns wherein feed air is separated by cryogenic rectification to produce nitrogen, oxygen and/or argon, as well as interconnecting piping, valves, heat exchangers and the like. 
     As used herein, the term “compressor” means a machine that increases the pressure of a gas by the application of work. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of one cryogenic air separation process which may be used with, and which can benefit by the application of, the process of this invention. 
         FIG. 2  is a graphical representation of the temperature difference between the composite warm and cold streams in the primary heat exchanger of the process illustrated in  FIG. 1  as a function of heat exchanger duty when the process is carried out with conventional practice. 
         FIG. 3  is a graphical representation of the temperature difference between the composite warm and cold streams in the primary heat exchanger of the plant and process illustrated in  FIG. 1  as a function of heat exchanger duty when the process is carried out with the practice of this invention. 
     
    
    
     DETAILED DESCRIPTION 
     In general the liquid oxygen pumped cryogenic air separation method of this invention is characterized by an aggregate warm end temperature difference (WEDT) that is at least 2K more than the primary heat exchanger&#39;s minimum internal temperature difference (PHX pinch DT). More preferably, the difference between the WEDT and the PHX pinch DT will be greater than 3K, and most preferably it is greater than 4K. The extra refrigeration required for this invention is generated by the expansion of a process gas across a turbine. In many cases the savings that will be realized by reducing the compression energy of the condensing air stream will more than offset the penalties associated with extra refrigeration production. This is particularly the case at higher oxygen boiling pressures. 
     The invention will be described in greater detail with reference to the Drawings. Referring now to  FIG. 1 , compressed, chilled, pre-purified feed air  1 , which has been compressed in a main air compressor, is split into two streams; stream  2  enters the warm end of primary heat exchanger  101  and stream  3  enters booster compressor  109 . In booster compressor  109 , this portion of the feed air is elevated to a pressure sufficiently high for it to condense against boiling oxygen product. High pressure air stream  4  passes through cooler  110  and cooled high pressure air stream  5  enters the warm end of the primary heat exchanger. Medium pressure air  6  exits heat exchanger  101  cooled to near the dew point. The cold air  6  then enters the bottom of higher pressure rectification column  102  which forms a double column along with lower pressure column  104 . The high pressure air stream  5  is liquefied in the primary heat exchanger against boiling high pressure oxygen and exits the primary heat exchanger as a subcooled liquid. Subcooled liquid air stream  7  is expanded across liquid turbine  111  to provide a portion of the refrigeration needs of cryogenic air separation plant. The liquid air stream is expanded to approximately the operating pressure of column  102 . Liquid air stream  8  is split into three streams; stream  9  enters column  102  a few stages above that point at which stream  6  enters the column, stream  10  is fed to intermediate pressure column  103  a number of stages from the bottom, and stream  11  is fed to heat exchanger  108 . In heat exchanger  108 , stream  11  is further cooled against warming nitrogen vapor, whereupon subcooled liquid air stream  27  is fed to low pressure column  104  a number of stages from the top. 
     In column  102 , the air is separated into oxygen-enriched and nitrogen-enriched portions. Oxygen-enriched liquid  12  is removed from the bottom of the column, introduced into heat exchanger  108 , cooled against warming nitrogen vapor, exits as a subcooled liquid  21 , and is fed to an intermediate point of column  103 , below the feed point for stream  10  but above the bottom of the column. Nitrogen vapor  13  exits the top of the medium pressure column  102 . A portion of that vapor stream  14  is removed as medium pressure nitrogen product, and is fed to the cold end of primary heat exchanger  101 . Stream  14  is warmed in primary heat exchanger  101  against cooling air streams and leaves at the warm end as warmed medium pressure nitrogen stream  39 . The remaining portion  15  of stream  13  enters the condensing side of condenser/reboiler  105 . Stream  15  is liquefied against vaporizing bottoms liquid in column  104 . Liquid nitrogen  16  leaving condenser/reboiler  105  is split into two streams; stream  17  is sent to heat exchanger  108  and stream  18  is returned to column  102  as reflux. Stream  17  is subcooled against warming nitrogen vapor and resulting subcooled liquid nitrogen stream  28  enters low pressure column  104  at or near the top. A nitrogen enriched vapor stream  19  is removed at least one stage below the top of column  102  and enters the condensing side of condenser/reboiler  106 . Stream  19  is liquefied against vaporizing bottoms liquid in column  103  and is returned to column  102  as liquid stream  20 . Stream  20  enters column  102  at or above the withdrawal point for stream  19 . 
     The intermediate pressure column  103  is used to further supplement the nitrogen reflux sent to low pressure column  104 . Nitrogen vapor  23  exits the top of the intermediate pressure column  103  and enters the condensing side of condenser/reboiler  107 . Stream  23  is liquefied against vaporizing liquid in the middle of column  104 . Liquid nitrogen  24  leaving condenser/reboiler  107  is split into two streams; stream  25  is returned to the top of column  103  and stream  26  is fed to heat exchanger  108 . Stream  26  is subcooled against warming nitrogen vapor and resulting subcooled liquid nitrogen stream  29  is fed at or near the top of low pressure column  104 . Oxygen-enriched liquid  22  is removed from the bottom of column  103  and is fed to an intermediate point of low pressure distillation column  104 , a number of stages above condenser/reboiler  107 . 
     The low pressure distillation column  104  further separates its feed streams into oxygen-rich liquid and nitrogen-rich vapor. An oxygen-rich liquid stream  30  is removed from the lower portion of column  104 , passed to cryogenic oxygen pump  112  and raised to slightly above the final oxygen delivery pressure. High pressure liquid stream  32  is fed to the cold end of primary heat exchanger  101  where it is warmed and boiled against the condensing high pressure feed air stream. Warmed, high pressure oxygen vapor product  42  exits the warm end of primary heat exchanger  101 . Nitrogen-rich vapor  31  exits the upper portion of the low pressure column  104 , is fed to heat exchanger  108 , is warmed against cooling liquids, and leaves as superheated nitrogen vapor stream  33 . 
     Stream  33  enters the cold end of primary heat exchanger  101  where it is partially warmed against cooling air streams and is split into two streams. The portion of this stream not needed to complete the nitrogen product requirement is removed from an intermediate point of primary heat exchanger  101 , and this stream  34  is fed to waste turbine  113  and expanded to a lower pressure. Along with liquid turbine  111 , waste turbine  113  is used to generate the cryogenic air separation plant&#39;s refrigeration. Low pressure nitrogen stream  35  exits waste turbine  113 , is fed to primary heat exchanger  101 , and leaves the warm end as warmed, low pressure waste nitrogen  36 . Stream  37  leaves the warm end of heat exchanger  101  as warmed, low pressure product nitrogen and is fed to the first stages of the nitrogen compressor  114  and cooled in those stages&#39; intercoolers  115 . Cooled compressed nitrogen stream  38  is mixed with nitrogen stream  39 , which is at the same pressure to form stream  40 . Nitrogen stream  40  is fed to the remaining stages of the nitrogen compressor  116  and cooled in those stages&#39; intercoolers  117 . The resulting high pressure nitrogen stream is cooled (aftercooler not shown) to form product nitrogen stream  41  delivered to the end user. 
     For this given example, the required oxygen delivery pressure is 1115 pounds per square inch absolute (psia) and the required nitrogen delivery pressure is 335 psia. Ideally, the high pressure air stream  5  would be elevated to at least 2300 psia to accommodate the oxygen boiling above 1115 psia. There are limitations, however, to the pressures that can be tolerated by a brazed aluminum heat exchanger (BAHX). In this case we have limited stream  5  to a pressure of 1215 psia based on the economics and pressure limitations of the BAHX. Somewhat higher pressures are possible for a BAHX, but may not be economical. An alternative technology, such as spiral wound heat exchangers, would be required to handle stream pressures of 2300 psia. However, this is very expensive. 
     By the conventional paradigm, power is minimized when the upper column pressure is raised just enough that expansion of all the waste nitrogen provides the desired primary heat exchanger warm end temperature difference. If the pressure is raised higher than this, the waste expander would provide more than the needed refrigeration. When waste expansion is employed according to the conventional paradigm, the pressure of column  102  is only about 95 psia and the pressure of column  104  is about 25 psia. 
     Because of the high boiling pressure of the oxygen in the primary heat exchanger and the ceiling placed upon the allowable pressure of the condensing high pressure air stream, a significant portion of the feed to the plant must enter booster compressor  109 . In this example, the flowrate of stream  5  is approximately 35% that of stream  1 . This high flowrate coupled with the high discharge pressure means that booster compressor  109  is responsible for a large portion of the plant&#39;s total energy consumption. In this case, over 25% of the plant&#39;s energy consumption comes from booster compressor  109 .  FIG. 2  shows the primary heat exchanger&#39;s cooling curve for the system with the pressure minimized such that the waste nitrogen expander refrigeration gives a primary heat exchanger temperature difference (WEDT) of 3.0K. The internal pinch (PHX pinch DT) of 2.0K is due to the warming of the supercritical (1115 psia) oxygen against cooling supercritical air (1215 psia). The substantial high pressure air flow provides an excess of refrigeration at the cold end of the primary heat exchanger, as evidenced by the large temperature difference at the cold end. The difference between the WEDT and the PHX pinch DT is 1.0K. 
     The invention is applied to this cycle by elevating the pressure of the entire plant. When the pressure of column  102  is raised from 95 psia to 180 psia and the pressure of column  104  is raised from 25 psia to 57 psia, excess refrigeration is generated by the waste expansion turbine since all the nitrogen not needed as product is still passed through the waste expander. As a result, the cooling curve for the PHX opens considerably as is illustrated in  FIG. 3 . The difference between the WEDT and the PHX pinch DT is now greater than 7K. The result is that for the same primary heat exchanger  101 , much less high pressure air  5  from the booster air compressor  109  is needed to properly boil all of the high pressure oxygen. With this excess refrigeration, the flowrate of stream  5  falls from 35% to 25% of feed stream  1  and the fraction of the plant&#39;s energy consumed by booster compressor  109  falls from 25% to 12.5%. Another benefit of the application of the invention to this cycle is a significant increase in the generated turbine power that would be realized from waste expansion turbine  113 . Additionally, because the pressure of the entire plant is elevated in order to generate the excess refrigeration, nitrogen product streams  37  and  39  exit the plant at higher pressures, and thereby the power requirements of the nitrogen compressor fall. 
     In this specific example, there is also a very substantial capital cost benefit realized by the application of the invention. By preferentially operating the plant at elevated pressures, the sizes of the plant&#39;s pieces of equipment are allowed to be much smaller, thereby avoiding the need to construct two separate air separation unit trains, as would likely be required for such a large capacity plant operating at low pressures. Among the pieces of equipment that can be made smaller by this elevated pressure operation are all of the BAHX&#39;s, distillation columns, and pipes, as well as the plant&#39;s prepurifier. Additionally, operating the plant at an elevated pressure affords efficient, direct integration with the gas turbine air compressor (GTAC); operating the plant at elevated pressures allows for the optimal usage of the GTAC&#39;s extraction air. 
     Despite the higher power requirement of the main feed air compressor, the practice of this invention provides advantages over conventional practice. This is demonstrated in Table 1 which shows normalized power consumption for the cycle illustrated in  FIG. 1  for conventional practice (A) and with the practice of this invention (B). The numerals in Table 1 refer to those of  FIG. 1 . In this example, which is presented for illustrative and comparative purposes and is not intended to be limiting, oxygen product stream  42  leaves the plant at 1115 psia and nitrogen product stream  41  is compressed to 335 psia. Additionally the practice of this invention allows for the efficient production of a modest amount of liquid product. Some of the excess turbine refrigeration can be used to make liquid product and the unit power associated with doing so would be very low. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Improvement 
               
               
                   
                 A 
                 B 
                 (Normalized % 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Main Air Compressor 
                 554 
                 694 
                 −14.1% 
               
               
                 Booster Air Compressor 109 
                 253 
                 122 
                 13.2% 
               
               
                 Nitrogen Compressor 39 + 43 
                 195 
                 167 
                 2.8% 
               
               
                 Oxygen Pump 112 
                 6 
                 6 
                 0.0% 
               
               
                 Liquid Turbine 111 
                 −7 
                 −4 
                 −0.3% 
               
               
                 Waste Expansion Turbine 113 
                 −2 
                 −22 
                 2.0% 
               
               
                   
                 1000 
                 964 
                 3.6% 
               
               
                   
               
             
          
         
       
     
     The benefits of the practice of this invention will be particularly beneficial when the pressure of the oxygen product is at least 250 psia. Typically with the practice of this invention, the pressure of the oxygen product will be within the range of from 200 to 1500 psia. 
     Although the invention has been described in detail with reference to a certain embodiment and with reference to a certain cryogenic air separation cycle, those skilled in the art will recognize that there are other embodiments of the invention and other cryogenic air separation cycles within the spirit and the scope of the claims.