Abstract:
A method for generating refrigeration for application to a heat load, especially at very cold temperatures, using an environmentally benign working gas such as air and using an upstream precooling circuit to reduce or eliminate inefficiencies stemming from warm end pinch.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates generally to the provision of refrigeration and is particularly useful for the provision of refrigeration at a very cold temperature.  
         BACKGROUND ART  
         [0002]    Historically, refrigeration systems have used various fluorocarbons and hydrofluorocarbons as refrigerant fluids to generate refrigeration and to provide the refrigeration to a heat load. Recently the use of some such refrigerants has been questioned on environmental and other grounds. Systems which use an environmentally friendly working fluid, such as air, are known. However, such systems typically are less efficient than systems using the more conventional refrigerants. For example, air systems commonly have a pinch at the warm end of the heat exchanger used in the system which limits the refrigeration capacity of the system. This is particularly a problem when the provision of the refrigeration is desired at a very cold temperature.  
           [0003]    Accordingly it is an object of this invention to provide an improved method for generating a cold gas for the provision of refrigeration.  
           [0004]    It is another object of this invention to provide an improved method for generating a cold gas for the provision of refrigeration which employs an environmentally benign working fluid.  
           [0005]    It is a further object of this invention to provide an improved method for generating a cold gas for the provision of refrigeration which employs an environmentally benign working fluid and can efficiently provide the refrigeration at a very cold temperature.  
         SUMMARY OF THE INVENTION  
         [0006]    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:  
           [0007]    A method for generating a cold gas for supplying refrigeration comprising:  
           [0008]    (A) compressing a nitrogen-containing working gas and cooling the compressed nitrogen-containing working gas;  
           [0009]    (B) expanding the cooled nitrogen-containing working gas to produce a cold nitrogen-containing working gas, and warming the cold nitrogen-containing working gas to supply refrigeration to a heat load;  
           [0010]    (C) further warming the nitrogen-containing working gas by indirect heat exchange with the compressed nitrogen-containing working gas to effect a portion of said cooling of the compressed nitrogen-containing working gas; and  
           [0011]    (D) compressing a refrigerant fluid, expanding the compressed refrigerant fluid to cool the refrigerant fluid, and warming the cooled refrigerant fluid by indirect heat exchange with the compressed nitrogen-containing working gas to effect another portion of said cooling of the compressed nitrogen-containing working gas.  
           [0012]    Another aspect of the invention is:  
           [0013]    A method for generating a cold gas for supplying refrigeration comprising:  
           [0014]    (A) compressing a nitrogen-containing working gas and cooling the compressed nitrogen-containing working gas;  
           [0015]    (B) expanding the cooled nitrogen-containing working gas to produce a cold nitrogen-containing working gas;  
           [0016]    (C) warming a first portion of the cold nitrogen-containing working gas to supply refrigeration to a heat load; and  
           [0017]    (D) warming a second portion of the cold nitrogen-containing working gas by indirect heat exchange with the compressed nitrogen-containing working gas to effect said cooling of the compressed nitrogen-containing working gas.  
           [0018]    As used herein the term “very cold temperature” means a temperature within the range of from −30° F. to −300° F.  
           [0019]    As used herein the term “nitrogen-containing working gas” means a gas having a nitrogen concentration of at least 78 mole percent.  
           [0020]    As used herein the term “expansion” means to effect a reduction in pressure.  
           [0021]    As used herein the term “refrigeration” means the capability to reject heat from a subambient temperature system.  
           [0022]    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.  
           [0023]    As used herein the term “refrigerant fluid” means a pure component or mixture used as a working fluid in a refrigeration process which undergoes changes in temperature, pressure and possibly phase to absorb heat at a lower temperature and reject it at a higher temperature. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a schematic representation of one preferred embodiment of the invention employing a precooler system.  
         [0025]    [0025]FIG. 2 is a schematic representation of another preferred embodiment of the invention wherein the cooled, compressed nitrogen-containing gas is further cooled prior to expansion.  
         [0026]    [0026]FIG. 3 is a schematic representation of another preferred embodiment of the invention wherein a portion of the expanded cold nitrogen-containing working gas is used to carry out the precooling of the working gas. 
     
    
     DETAILED DESCRIPTION  
       [0027]    The invention will be described in detail with reference to the Drawings. Referring now to FIG. 1, nitrogen-containing working gas  1  is passed to compressor  70  wherein it is compressed to a pressure generally within the range of from 100 to 1500 pounds per square inch absolute (psia). Preferably the nitrogen-containing working gas is air or nitrogen gas having a nitrogen concentration exceeding that of air up to 99 mole percent or more. In the practice of this invention it is important that the nitrogen-containing working gas be substantially free of high boiling impurities such as water vapor and carbon dioxide. The working gas may be passed through a purifier, such as a molecular sieve adsorbent purifier, to ensure that it is cleaned of such high boiling impurities. Resulting compressed nitrogen-containing working gas  2  is cooled of the heat of compression by passage through cooler  71  and then passed as gas stream  3 , generally at about ambient temperature, to heat exchanger  72 . As the nitrogen-containing working gas passes through heat exchanger  72  it is cooled by indirect heat exchange with two different warming fluids, as will be more fully described below, to produce cooled, compressed nitrogen-containing working gas  4 , having a temperature generally within the range of from 300 to 150K.  
         [0028]    Cooled, compressed nitrogen-containing working gas  4  is expanded, such as by passage through turboexpander  73 , to a pressure generally within the range of from 15 to 1000 psia, to generate refrigeration and to produce cold nitrogen-containing working gas  6  having a temperature generally within the range of from 100 to 250K. The cold nitrogen-containing working gas is warmed to supply refrigeration to a heat load. In the embodiment illustrated in FIG. 1, cold gas  6  is passed to heat load heat exchanger  74  wherein it is warmed by indirect heat exchange with fluid  31  to produce cooled fluid  32  and warmed nitrogen-containing working gas  7 . Representative examples of heat loads for use in the practice of this invention include cooling and/or freezing of foods, cooling a vapor stream for the purpose of condensing volatile organic compounds, and absorbing heat from a low temperature heat transfer fluid.  
         [0029]    Nitrogen-containing working gas  7  is then passed to heat exchanger  72  wherein it is further warmed to supply a portion of the cooling necessary to cool the working gas to the temperature of gas  4 . Resulting further warmed nitrogen-containing working gas  8  is then recycled back to compressor  70  in stream  1  and the cycle repeats. If necessary, make up gas  11 , which is substantially free of high boiling impurities, may be added to the feed into compressor  70  to compensate for system losses.  
         [0030]    Refrigerant fluid  21  is compressed to a pressure generally within the range of from 50 to 500 psia by passage through compressor  75 . Any effective refrigerant fluid may be used in the practice of this invention. Examples include ammonia, R-410A, R-507A, R-134A, propane, R-23 and mixtures such as mixtures of fluorocarbons, hydrofluorocarbons, hydrochlorofluorocarbons and/or hydrocarbons.  
         [0031]    Compressed refrigerant fluid  22  is cooled of the heat of compression by passage through cooler  76  and resulting refrigerant fluid  23  is expanded through valve  77  to generate refrigeration and produce cooled refrigerant fluid  24  having a temperature generally within the range of from 150 to 300K. Cooled refrigerant fluid  24  is then warmed by passage through heat exchanger  72  to provide another portion of the cooling necessary to cool the working gas to the temperature of gas  4 . The resulting warmed refrigerant fluid then passes as stream  21  to compressor  75  and the cycle repeats. Although FIG. 1 illustrates the heat exchange of the cooling nitrogen-containing working gas with the warming working gas and the warming refrigerant fluid as occurring in the same heat exchanger, those skilled in the art will recognize that this cooling could take place using different heat exchangers. The use of the precooling circuit employing the recirculating refrigerant fluid serves to reduce or eliminate the warm end pinch enabling efficient downstream generation of very cold temperature refrigeration using an environmentally friendly working gas.  
         [0032]    [0032]FIG. 2 illustrates a preferred embodiment of the system illustrated in FIG. 1 wherein the cooled working gas is further cooled prior to the expansion. The numerals in FIG. 2 are the same as those of FIG. 1 for the common elements, and these common elements will not be described again in detail Referring now to FIG. 2, cooled fluid  32  is passed to freezer  40  wherein it serves to cool and/or freeze articles. Resulting fluid  41 , which still retains significant refrigeration is passed through heat exchanger  42  wherein it is warmed by indirect heat exchange with cooled, compressed nitrogen-containing working gas  4  to produce further cooled, compressed nitrogen-containing working gas  43  for passage to turboexpander  73  and for further processing as previously described with reference to FIG. 1. Generally, in the practice of the invention in accord with the embodiment illustrated in FIG. 2, the cooled, compressed nitrogen-containing working gas has a temperature generally within the range of from 300 to 150K, and the further cooled, compressed nitrogen-containing working gas has a temperature generally within the range of from 100 to 250K.  
         [0033]    [0033]FIG. 3 illustrates another embodiment of the invention wherein a portion of the cold nitrogen-containing working gas is used to carry out the precooling of the working as prior to the turboexpansion. Referring now to FIG. 3, nitrogen-containing working gas  50 , e.g. air, is passed to compressor  51  wherein it is compressed to a pressure generally within the range of from 50 to 250 psia. Resulting compressed nitrogen-containing working gas  52  is passed to membrane unit  53  wherein its nitrogen concentration is increased and wherein high boiling impurities such as carbon dioxide and water vapor are removed. Resulting increased concentration nitrogen-containing working gas  54  is passed to recycle compressor  55  as part of recycle compressor input stream  56 . Within recycle compressor  55  the nitrogen-containing working gas is compressed to a pressure generally within the range of from 50 to 1500 psia to form compressed working gas stream  57  for input to heat exchanger  67 .  
         [0034]    Within heat exchanger  67  the compressed nitrogen-containing working gas is cooled to form cooled, compressed nitrogen-containing working gas  58  which is expanded through turboexpander  59  to generate refrigeration and to produce cold nitrogen-containing working gas  60 . A first portion  61  of cold nitrogen-containing working gas  60  is warmed to supply refrigeration to a heat load. In the embodiment of the invention illustrated in FIG. 3 the heat load is freezer  62 . The resulting warmed nitrogen-containing working as  63  is then cleaned by passage through purifier  64  and resulting purified nitrogen-containing working gas  65  is combined with other streams to form stream  56  for passage to recycle compressor  55 .  
         [0035]    A second portion  66  of cold nitrogen-containing working gas  60  is warmed by passage through heat exchanger  67  by indirect heat exchange with the compressed nitrogen-containing working gas  57  to effect the precooling of the nitrogen-containing working gas prior to the turboexpansion of the nitrogen-containing working gas to generate the cold gas. The resulting warmed second portion  68  is then combined with other nitrogen-containing gas streams to form recycle compressor input stream  56  which is processed as was previously described. Generally second portion  66  comprises from 5 to 50 percent of cold nitrogen-containing working gas  60 . If desired, nitrogen-containing working gas  63  may be passed through the warm end portion of heat exchanger  67  to provide further precooling of stream  57 , with the resulting further warmed nitrogen-containing working gas  63  then passed to purifier  64 .  
         [0036]    With the use of this invention wherein precooling of the working fluid, using either an exogeneous circuit or a recycle circuit, prior to the expansion of the working fluid to generate the cold gas, a nitrogen-containing environmentally friendly working fluid may be used while mitigating to a large extent the process inefficiencies heretofore experienced with the use of such fluids, especially when the refrigeration is supplied to a heat load at a very cold temperature.  
         [0037]    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.