Abstract:
A method and apparatus for providing direct contact refrigeration to a heat source wherein refrigeration is generated using a recirculating defined multicomponent refrigerant fluid, and transferred to a direct contact refrigerant fluid which directly contacts the heat source.

Description:
TECHNICAL FIELD 
     This invention relates generally to the generation of refrigeration and the provision of the refrigeration by direct contact with a heat source. 
     BACKGROUND ART 
     Refrigeration to provide cooling and/or freezing duty to a heat source is widely required in industrial processes such as in the cooling of exothermic reactors and the cooling of crystallizers. This refrigeration may be provided by indirect heat exchange of the refrigerant with the heat source. Direct contact heat exchange of the refrigerant with the heat source is advantageous because the heat exchange is more efficient than indirect heat exchange but such direct contact heat exchange adds complexity to the system. Moreover conventional direct contact refrigeration provision systems are characterized by high costs to generate the requisite refrigeration. 
     Accordingly, it is an object of this invention to provide an improved method for providing direct contact refrigeration wherein the requisite refrigeration may be generated with lower power costs than conventional 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 providing direct contact refrigeration comprising: 
     (A) compressing a multicomponent refrigerant fluid comprising at least two components from the group consisting of hydrocarbons having from 1 to 6 carbon atoms, fluorocarbons having from 1 to 6 carbon atoms, and inert gases; 
     (B) cooling the compressed multicomponent refrigerant fluid, expanding the cooled compressed multicomponent refrigerant fluid to generate refrigeration, and warming the refrigeration bearing multicomponent refrigerant fluid by indirect heat exchange with said cooling compressed multicomponent refrigerant fluid and also by indirect heat exchange with clean direct contact refrigerant to produce cold direct contact refrigerant; 
     (C) contacting the cold direct contact refrigerant with a heat source to cool the heat source producing warmed direct contact refrigerant which contains contaminants from the heat source; and 
     (D) treating the direct contact refrigerant to remove contaminants and to produce clean direct contact refrigerant for indirect heat exchange with the refrigeration bearing multicomponent refrigerant fluid. 
     Another aspect of the invention is: 
     Apparatus for providing direct contact refrigeration comprising: 
     (A) a multicomponent refrigerant circuit comprising a compressor, a heat exchanger, an expansion device, means for passing multicomponent refrigerant fluid from the compressor to the heat exchanger, from the heat exchanger to the expansion device, from the expansion device to the heat exchanger, and from the heat exchanger to the compressor; 
     (B) a heat source, means for passing direct contact refrigerant to the heat exchanger, and means for passing direct contact refrigerant from the heat exchanger to the heat source; 
     (C) a cleaning device, means for passing direct contact refrigerant from the heat source to the heat exchanger and means for passing direct contact refrigerant from the heat exchanger to the cleaning device; and 
     (D) means for passing direct contact refrigerant from the cleaning device to 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 “contaminants” means one or more substances which will adulterate the direct contact refrigerant used in the method of this invention. 
     As used herein, the term “inert gases” means nitrogen, carbon dioxide and noble gases such as helium and argon. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic representation of one preferred embodiment of the direct contact refrigeration method of this invention. 
     FIG. 2 is a simplified schematic representation of another preferred embodiment of the invention wherein the cooling compressed multicomponent refrigerant fluid is partially condensed. 
     FIG. 3 is a simplified schematic representation of another preferred embodiment of the invention wherein the direct contact refrigeration is provided at two temperature levels. 
    
    
     DETAILED DESCRIPTION 
     The invention will be described in detail with reference to the Drawings. Referring now to FIG. 1, multicomponent refrigerant fluid  114  is compressed to a pressure generally within the range of from 30 to 500 pounds per square inch absolute (psia) by passage through compressor  16 . Resulting compressed multicomponent refrigerant fluid  130  is cooled of the heat of compression in aftercooler  17  and then passed in stream  111  to heat exchanger  11 . 
     The multicomponent refrigerant fluid useful in the practice of this invention comprises two or more components which can be hydrocarbons having from 1 to 6 carbon atoms, fluorocarbons having from 1 to 6 carbon atoms, and inert gases. Examples of hydrocarbons having from 1 to 6 carbon atoms include methane, ethane, ethylene, propane, propylene, n-butane, n-pentane and n-hexane. Examples of fluorocarbons having from 1 to 6 carbon atoms include tetrafluoromethane, perfluoroethane, fluoroform, pentafluoroethane, difluoromethane, chlorodifluoromethane, and trifluoromethoxy-perfluoromethane. The multicomponent refrigerant fluid useful in the practice of this invention may comprise a mixture of solely hydrocarbons or a mixture of solely fluorocarbons, or may comprise a mixture of one or more hydrocarbons and one or more fluorocarbons, a mixture of one or more hydrocarbons and one or more inert gases, a mixture of one or more fluorocarbons and one or more inert gases, or a mixture having at least one hydrocarbon, at least one fluorocarbon, and at least one inert gas. 
     The compressed multicomponent refrigerant fluid  111  is cooled in heat exchanger  11  by indirect heat exchange with warming refrigeration bearing multicomponent refrigerant fluid, as will be more fully described below, to produce cooled compressed multicomponent refrigerant fluid  112  which may be entirely in the vapor phase or may be partially or totally condensed. Cooled compressed multicomponent refrigerant fluid  112  is expanded to generate refrigeration. The embodiment of the invention illustrated in FIG. 1 is a preferred embodiment wherein the expansion is an isenthalpic expansion through Joule-Thomson valve  18 . The resulting refrigeration bearing multicomponent refrigerant fluid  113  is warmed by passage through heat exchanger  11  to provide the aforesaid cooling of the compressed multicomponent refrigerant fluid and is then passed in stream  114  to compressor  16  and the multicomponent refrigerant fluid refrigeration cycle begins anew. 
     Clean direct contact refrigerant  108  is cooled by indirect heat exchange with warming multicomponent refrigerant fluid preferably, as shown in FIG. 1, by passage through heat exchanger  11  which is a unitary piece. Alternatively, heat exchanger  11  could comprise more than one piece with the multicomponent refrigerant fluid autorefrigeration occurring in one piece and other heat exchange steps occurring in one or more other pieces. Most or all of multicomponent refrigerant fluid  113  which is in the liquid phase is vaporized by the indirect heat exchange with the compressed multicomponent refrigerant fluid and the clean direct contact refrigerant. The indirect heat exchange with the warming refrigeration bearing multicomponent refrigerant fluid results in the production of cold direct contact refrigerant  103 . Preferably the direct contact refrigerant comprises nitrogen. The direct contact refrigerant may be comprised of one or more components. Other components which may comprise the direct contact refrigerant useful in the practice of this invention include argon and helium. The direct contact refrigerant is such that it does not contaminate the process fluid or other heat source that it cools by direct contact. 
     Cold direct contact refrigerant  103  is provided in gaseous and/or liquid form to a process or system which requires refrigeration, shown in representation form in FIG. 1 as item  10 . Examples of such systems or processes include exothermic reactors and direct contact crystallizers. 
     Refrigeration requiring system or process  10  has a heat source, shown in FIG. 1 as input  101 , which receives refrigeration by direct contact with cold direct contact refrigerant  103 , resulting in refrigerated fluid or other substance  102 . The heat source is a source of contaminants for the direct contact refrigerant. Direct contact refrigerant  104  leaves process or system  10  as a vapor containing one or more contaminants such as chemical species which it picks up as a result of directly contacting heat source  101 . For example in a paraxylene crystallization process, the contaminants in stream  104  may include input  101  constituents such as paraxylene, metaxylene, orthoxylene and ethylbenzene. 
     Contaminant containing direct contact refrigerant  104  is passed to heat exchanger  11  wherein it is warmed by indirect heat exchange with the cooling clean direct contact refrigerant and the resulting warmed contaminant containing direct contact refrigerant  105  is cleaned of contaminants in a cleaning device. The embodiment of the invention illustrated in FIG. 1 is a preferred embodiment wherein the cleaning device is an adsorption unit and the contaminant containing direct contact refrigerant is cleaned of contaminants by passage through one of two beds of adsorption system  12 . The beds contain suitable adsorbent material such as zeolite molecular sieve to remove contaminants by adsorption onto the adsorbent as the direct contact refrigerant passes through the bed, emerging therefrom as clean direct contact refrigerant  106 . When the adsorbent bed becomes loaded with contaminants the flow of contaminant containing direct contact refrigerant is directed into the other bed while the loaded bed is cleaned by the passage therethrough of purge gas, shown in FIG. 1 as streams  109  and  115 . This continues until the adsorbing bed becomes loaded with contaminants whereupon the flows are changed again. The adsorption system continues cycling in this manner. 
     If desired, make-up direct contact refrigerant  110  may be added to clean direct contact refrigerant  106  to make up for the loss of refrigerant in the direct contacting of the heat source. The clean direct contact refrigerant is cooled in cooler  13  and passed in stream  107  of compressor  14  wherein it is compressed to a pressure generally within the range of from 50 to 400 psia. Resulting compressed clean direct contact refrigerant  131  is cooled of the heat of compression in aftercooler  15  and then passed in stream  108  to heat exchanger  11  for indirect heat exchange with the refrigeration bearing multicomponent refrigerant fluid and then is recycled to provide further direct contact refrigeration to the heat source. 
     The following example is provided for illustrative purposes and is not intended to be limited. In this example the process or system which requires refrigeration is the direct contact cryogenic crystallizer system disclosed in U.S. Pat. Nos. 5,362,455—Cheng and 5,394,827—Cheng, the direct contact refrigerant is nitrogen, and the multicomponent refrigerant fluid is a mixture of 14 mole percent methane, 40 mole percent ethylene, 28 mole percent propane, 4 mole percent n-butane, 6 mole percent n-pentane and 8 mole percent n-hexane. The refrigeration load is one million BTU/hr. The numerals refer to those of FIG.  1 . 
     Mixed xylenes  101  (mixture of paraxylene (p-xylene), metaxylene (m-xylene) and orthoxylene (o-xylene) with minor quantities of other hydrocarbons) and cold nitrogen gas  103  are fed to direct contact crystallization system  10 . The cold nitrogen gas  103  is supplied at a temperature 5° F. to 100° F. below the crystallizer operating temperature. The cold nitrogen gas is supplied at a pressure which is 5 to 50 psi, and preferably 5 to 15 psi above the crystallizer operating pressure to ensure adequate contact with the liquids, heat removal and gas-liquid-solid fluid dynamics that facilitate formation of desired paraxylene crystals. The liquid product  102  rich in paraxylene crystals is withdrawn and subjected to other unit operations to obtain high purity paraxylene product. The direct contact crystallizer is designed to capture liquid and/or crystalline hydrocarbons entrained in the effluent nitrogen gas above the liquid/gas interface. The effluent nitrogen gas  104  in phase equilibrium with the crystallizer contents is warmed up to near ambient temperature in multi-stream heat exchanger  11 . The resulting nitrogen gas  105  is treated in regenerative dual bed adsorption system  12  to remove the organic contaminants. A small quantity of nitrogen  109  is used to regenerate the off-line adsorption bed, resulting in vent stream  115 . The purified nitrogen  106  is mixed with fresh nitrogen  110  (to compensate for losses) and the resulting nitrogen stream  107  is compressed for recycle. The compressor  14  is sized to deliver the recycle nitrogen  108  to the crystallizer at the required operating pressure, which could be in the range of 100 to 400 psia, preferably 150 to 300 psia, and more preferably 200 to 250 psia. Since the direct contact crystallizer design results in efficient gas-liquid-solid contact, the gas and slurry effluents leave the crystallizer at or near crystallizer operating temperature. Thus, the recycle nitrogen flow and its temperature at the crystallizer inlet are related by the crystallizer refrigeration duty. Colder nitrogen means relatively less nitrogen flow. The multicomponent refrigerant fluid closed loop comprising of streams  111 ,  112 ,  113  and  114 , and associated process equipment is designed and operated to enable the cold nitrogen gas serve as the source of refrigeration in the crystallizer. In this particular example, cold nitrogen gas flow is calculated to supply half of the refrigeration by warming from −130° F. to −87° F., and the balance by warming to −58° F. Stream  111  is compressed to 205 psia in compressor  16 , cooled against cooling water or air in the cooler  17 . It is further cooled to −130° F. against warming stream  113 , which results from isenthalpic expansion of stream  112  upon flowing through valve  18 . Stream  113  serves as the primary source of refrigeration for delivering cold nitrogen gas to the crystallization application. Warmed stream  114  is compressed and thus completes the closed loop. The electricity requirement was calculated as 537 kW. The electricity requirement for a comparable system using a conventional ethylene/propane cascade cycle to generate the refrigeration was calculated to be 634 kW. These results are summarized in Table 1. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 PRIOR ART 
                 INVENTION 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Cold Nitrogen T, F 
                 −130   
                 −130   
               
               
                 Electricity, kWh/MMBtu Refrigeration 
                 634 
                 537 
               
               
                 Load 
               
               
                   
               
             
          
         
       
     
     FIG. 2 illustrates another embodiment of the invention employing a phase separator to counteract potential maldistribution. The numerals of 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, refrigeration bearing multicomponent refrigerant stream  113  has both vapor and liquid phases and is fed to phase separator  19  wherein it is separated into its vapor and liquid phases. The vapor phase and liquid phase are passed separately from phase separator  19  in streams  116  and  117  respectively to separate passages of heat exchanger  11  wherein they are warmed and the liquid phase vaporized to cool the compressed multicomponent refrigerant fluid  111  and to provide refrigeration to the clean direct contact refrigerant  108 . Streams  116  and  117  exit heat exchanger  11  as streams  118  and  119  respectively. These streams are combined to form stream  114  for passage to compressor  16  for further processing as previously described. 
     FIG. 3 illustrates another embodiment of the invention similar to that illustrated in FIG. 2 but with the added aspect of providing the cold direct contact refrigerant to the heat source at two temperature levels. The numerals of FIG. 3 are the same as those of FIG. 2 for the common elements, and these common elements will not be described again in detail. 
     Referring now to FIG. 3, only a portion of clean direct contact refrigerant  108  completely traverses heat exchanger  11  to emerge therefrom as stream  103 . Another portion  132  of stream  108  is withdrawn from heat exchanger  11  after only partial traverse thereof. Accordingly cold direct contact refrigerant in stream  132  is at a warmer temperature than is cold direct contact refrigerant in stream  103 . These two different temperature cold direct contact refrigerant streams are provided to system or process  10  at different points to more optimally employ the refrigeration by direct contact with the heat source. The contaminant containing direct contact refrigerant from both streams  103  and  132  emerges from system or process  10  as stream  104  and is further processed 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.