Abstract:
A refrigeration method reduces compressor energy usage in a hydrocarbon refrigeration system by incorporating a distillation/adsorption cycle. The method and related systems can use waste or other heat sources to replace a portion of the mechanical energy of the compression cycle in a novel process scheme easily adapted to either new or existing refrigeration systems. The present hybrid vapor compression-adsorption cycle combines both the refrigerant and adsorption medium in the compression cycle and separates these components using conventional multi-stage distillation to then separate them for the refrigeration and adsorption cycles.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     None. 
     BACKGROUND 
     1. Field of the Disclosure 
     The present disclosure relates to a hybrid vapor compression-absorption cooling system utilizing a refrigerant pair comprising at least one refrigerant and at least one absorbent. 
     2. Background of the Disclosure 
     The use of compression cycles and adsorption cycles are known in the art. Both of these systems require raising the pressure of the used low pressure refrigerant to a higher pressure so that said refrigerant may be cooled and condensed using a warmer medium. Mechanical compression systems use compressors, either mechanical or jet eductors, for this purpose while adsorption systems pump a liquid adsorption fluid to a pressure great enough to allow the heating, separation and condensing of the refrigerant. 
     The combining of these two types of systems is also known and has been practiced with advantage since there are advantages which accrue to the overall reduction of mechanical energy, which is relatively expensive, by the substitution of heat energy which may be relatively cheaper. These systems have generally sought to utilize refrigerant/adsorption pairs which can be easily separated such as LiBr and Water, or Ammonia and Water. Despite these theoretical improvements the hybrid compression adsorption cycle has not been extensively used due to limitations of the art using these refrigerant/adsorption pairs. 
     The present disclosure provides novel refrigerant/adsorption pairs that can enhance the efficiency of refrigerant/adsorption cycles by reducing the need for mechanical energy. 
     SUMMARY 
     In aspects, the present disclosure uses the perfect or near-perfect miscibility of various light hydrocarbons to reduce the energy requirements associated with compressing the refrigerants. The described refrigeration cycles use various light hydrocarbon mixtures which might include alkanes such as ethane, propane, butane, etc, and alkenes such as ethylene, propylene, butylenes, and so forth. In non-limiting embodiments, the described refrigeration cycles use only paraffinic hydrocarbon pairs such as propane and butane/pentane/hexane. These hydrocarbons are combined in a compression/condensing portion of the cycle and then separated into the lighter refrigerant component and heavier adsorption component in a distillation operation. 
     In one non-limiting implementation, a hydrocarbon vapor (the refrigerant) of molecular weight equal to or greater than ethane is compressed to a pressure less than that normally needed for condensation in a conventional system. Next, this hydrocarbon vapor is mixed with a hydrocarbon liquid (sorbent) of at least one carbon greater molecular weight, which condenses this mixture. The condensed mixture is pumped to a higher pressure. Thereafter, this mixture is distilled into an overhead product and a bottom product, the refrigerant and the sorbent, respectively. By mixing the heavier bottom product of this distillation with the compressed hydrocarbon vapor as previously described, the discharge pressure of the compression step may be reduced, which reduces the energy requirements of the system for the same quantity of refrigeration. 
     In another non-limiting implementation, a process begins with a condensed liquid refrigerant, which is either a pure hydrocarbon or a mixture of several hydrocarbons. The hydrocarbon refrigerant may be in the molecular weight range of ethane, propane and butane. The liquid refrigerant is delivered to the chiller, where the refrigerant is vaporized, thereby cooling another process stream and performing the main objective of any refrigerant system. 
     The cold vaporized refrigerant from the chiller is then compressed using the gas compression device to a higher pressure but a pressure less than that normally required for the condensation of such a refrigerant. At this point, the sorption stream, which is a hydrocarbon stream, of at least one additional carbon atom higher molecular weight, is mixed with the compressed refrigerant stream, causing the partial condensation of this refrigerant stream. 
     This combined hydrocarbon stream is then cooled further, typically using air or cooling water, until it is largely condensed. This condensed hydrocarbon stream is then pumped using the liquid pumping device to a pressure sufficiently high that the adsorbed refrigerant phase, when separated, will be condensable, again typically using air or cooling water. 
     The mixed high pressure hydrocarbon stream is then delivered to the reboiled distillation column at a point between a rectifying and stripping sections. Vapor leaving the overhead portion of the distillation column is condensed. A portion of the condensed vapor serves as a reflux to the column. The remainder constitutes the liquid refrigerant used for cooling the separate process stream as discussed above. The bottom of the distillation column produces the sorbent stream consisting of one or more hydrocarbons of at least one greater carbon number than the refrigerant. Heat for the reboiling of the distillation column may be derived from any waste heat or other heat source of greater temperature than the column bottom product. 
     The liquid sorbent stream is then cooled to roughly ambient temperature and combined as previously described with the refrigerant vapor stream leaving the compression device as previously described. 
     In aspects, the present disclosure provides a mixed refrigerant process. The process may include the steps of: supplying a hydrocarbon refrigerant with a molecular weight of between 30 and 70 to a pressure reducer, the hydrocarbon refrigerant being a liquid when supplied; reducing the pressure of the liquid hydrocarbon refrigerant to produce a two phase refrigerant; delivering the two phase hydrocarbon refrigerant to at least one heat exchanger to cool to at least one process stream, wherein the two phase hydrocarbon refrigerant is vaporized in the at least one heat exchanger; compressing the vaporized two phase hydrocarbon refrigerant to a pressure less than a baseline refrigerant pressure, the baseline refrigerant pressure being a pressure required to condense a vapor fraction of the two phase hydrocarbon refrigerant at an ambient condition surrounding the hydrocarbon refrigerant; mixing a sorbent hydrocarbon of a molecular weight at least 14 greater than the hydrocarbon refrigerant with the two phase hydrocarbon refrigerant to form a hydrocarbon mixture; cooling the hydrocarbon mixture until the hydrocarbon mixture is substantially condensed; increasing a pressure of the hydrocarbon mixture to a pressure less than the baseline refrigerant pressure; fractionally distilling the hydrocarbon mixture into a lighter overhead hydrocarbon stream and a heavier hydrocarbon liquid in a distillation tower, the lighter overhead hydrocarbon stream being the liquid hydrocarbon refrigerant and the heavier hydrocarbon liquid being the sorbent hydrocarbon, and cooling the sorbent hydrocarbon. 
     In aspects, the present disclosure also provides a system using mixed refrigerants. The system may include a hydrocarbon refrigerant with a molecular weight of between 30 and 70; a sorbent hydrocarbon of a molecular weight at least 14 greater than the hydrocarbon refrigerant; a pressure reducer reducing a pressure of the hydrocarbon refrigerant to produce a two phase hydrocarbon refrigerant; at least one heat exchanger using the two phase hydrocarbon refrigerant to cool at least one process stream, wherein the two phase hydrocarbon refrigerant is vaporized in the at least one heat exchanger; a first pumping device compressing the vaporized two phase hydrocarbon refrigerant to a pressure less than a baseline refrigerant pressure, the baseline refrigerant pressure being a pressure required to condense a vapor fraction of the two phase hydrocarbon refrigerant at an ambient condition; a mixer mixing the sorbent hydrocarbon with the hydrocarbon refrigerant from the pumping device to form a hydrocarbon mixture; a cooling device cooling the hydrocarbon mixture until the hydrocarbon mixture is substantially condensed; a second pumping device increasing a pressure of the hydrocarbon mixture to a pressure less than the baseline refrigerant pressure; and a distillation tower fractionally distilling the hydrocarbon mixture into a lighter overhead hydrocarbon stream and a heavier hydrocarbon liquid, the lighter overhead hydrocarbon stream being the liquid hydrocarbon refrigerant and the heavier hydrocarbon liquid being the sorbent hydrocarbon stream. 
     In still another aspect, the present disclosure provides a mixed refrigerant process that includes the steps of cooling a process stream using a hydrocarbon refrigerant having a molecular weight less than 70; mixing the hydrocarbon refrigerant with a sorbent hydrocarbon to form a hydrocarbon mixture, wherein the sorbent hydrocarbon has a molecular weight that is greater than the molecular weight of the hydrocarbon refrigerant; and fractionally distilling the hydrocarbon mixture into a lighter overhead hydrocarbon stream and a heavier hydrocarbon liquid in a distillation tower, the lighter overhead hydrocarbon stream being the hydrocarbon refrigerant and the heavier hydrocarbon liquid being the sorbent hydrocarbon. 
     It should be understood that examples of certain features of the disclosure have been summarized rather broadly in order that detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the disclosure that will be described hereinafter and which will form the subject of the claims appended hereto. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For detailed understanding of the present disclosure, references should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals and wherein: 
         FIG. 1  is a flow chart depicting an illustrative method for providing low temperature refrigeration in one embodiment of the present disclosure; 
         FIG. 2  schematically illustrates a refrigeration system according to one embodiment of the present disclosure; and 
         FIG. 3  schematically illustrates a refrigeration system according to one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to using mixed hydrocarbons in refrigeration systems to significantly reduce the mechanical energy requirements of this service. The described refrigeration cycles use various light hydrocarbon mixtures may might include alkanes such as ethane, propane, butane, etc, and alkenes such as ethylene, propylene, butylenes, and so forth. In embodiments, the hydrocarbon refrigerants may consist of light hydrocarbons in a molecular weight range of ethane to octane. The present teachings may be advantageous where there is excess or waste heat available in a processing plant as this heat is used to reduce the overall energy requirements of the refrigeration compression service. 
     Referring initially to  FIG. 1 , there is shown a general flow schematic of an illustrative process  50  of the present disclosure. The process  50  consists of an evaporation step  52  in which a refrigerant stream is vaporized as with any refrigeration application, which provides the desired cooling. This is followed by a vapor compression step  54  in which the vaporized refrigerant is compressed. Next, a mixing step  56  occurs in which another less volatile hydrocarbon liquid sorbent stream is mixed with the compressed refrigerant vapors. Thereafter, a cooling step  58  is used to cool and condense the mixture. In the following pumping step  60 , the liquid mixture is pumped to a higher pressure for fractionation. Finally, in a fractionating step  62 , the liquid mixture of refrigerant and sorbent is fractionated in a distillation column with the liquid overheads forming the refrigerant stream and the bottom liquids forming the sorbent stream. 
       FIG. 2  illustrates a mixed refrigerant refrigeration system  70  in accordance with one embodiment of the present disclosure. The system  70  uses a refrigerant stream and a sorbent stream that each use a hydrocarbon in a molecular weight range from that of ethane to that of octane. 
     In the mixed refrigeration system  70 , a liquid refrigerant at a temperature near ambient is delivered via line  100  from a condensing unit  146  to a letdown valve  102 . As used herein, an ambient temperature is the temperature of the environment in which the system  70  operates. The letdown valve  102  reduces the pressure of the liquid refrigerant to produce a colder two phase refrigerant stream in line  104 . The two phase stream is delivered via line  104  to a phase separation device  106 , typically called an economizer. There, the liquid and vapor phases are separated and conveyed out of the economizer  106  via lines  110  and  108 , respectively. The line  110  conveys the liquid refrigerant stream to another letdown valve  112 , which further reduces the pressure in the liquid refrigerant stream and produces a colder two phase refrigerant stream in line  114 . This two phase refrigerant stream is then completely vaporized in heat exchanger  116 , which is commonly referred to as a chiller. The chiller provides the refrigeration service. A cold vapor refrigerant stream exits the chiller  116  via line  118 . 
     A low stage absorption device  120  mixes the cold vapor refrigerant stream received from line  118  with a liquid sorbent stream received from a line  158 . The low stage absorption device  120  may be a simple mixer or a mixing chamber. The liquid sorbent completely condenses and adsorbs the cold vapor refrigerant stream. The resultant liquid hydrocarbon mixture flowing in line  122  is then pumped using pumping device  124  to a pressure approximately that of the separator vessel  106 . At this point in the process, the vapor refrigerant stream in line  108  is combined with the liquid hydrocarbon mixture in line  126  in a high stage absorption device  128 , which may be similar to the low stage absorption device  120 . Here, the liquid hydrocarbon mixture received from line  126 , which acts as a sorbent, mixes with the cold vapor refrigerant stream from line  108 . The mixture completely condenses and adsorbs this cold vapor refrigerant stream. A resultant liquid hydrocarbon stream exits the low stage adsorption device  120  via line  130 . 
     The resultant liquid hydrocarbon stream in line  130  is pumped using pumping device  132  to a pressure sufficiently high to permit the condensation of the refrigerant vapor fraction  100  at ambient temperatures using air, cooling water or other cooling medium. The pumping  132  may be a pump or any other conventional device configured to increase the pressure of a fluid. The pumping device  132  discharges a pressurized liquid hydrocarbon stream in line  134 . The discharge may be preheated in a feed/bottoms cross exchanger,  136 , before being directed via line  138  to a generator tower  140 . 
     The generator tower  140  separates the liquid hydrocarbon stream received via line  138  into the refrigerant stream and the sorbent stream. The generator tower  140  may be a refluxed, reboiled fractionating tower containing a rectifying section  142  and a stripping section  144 . The overhead product of this distillation tower  140  can be condensed in the overhead condenser  146  in order to produce a tower reflux stream as well as the liquid refrigerant stream fed into line  100 . The distillation tower bottom product exits via line  152  and is the sorbent stream. The sorbent stream in line  152  may be cooled sequentially in the aforementioned feed/bottoms exchanger  136  and a sorbent air cooler  156 . The cooled sorbent stream in line  158  is then returned to the adsorption mixer  120 . 
       FIG. 3  illustrates another embodiment of mixed refrigerant system  72  in accordance with the present disclosure. In this embodiment, the system  72  uses a liquid hydrocarbon sorbent stream of higher molecular weight than a refrigerant stream by at least one carbon number. 
     In this embodiment, a liquid refrigerant in line  200  is delivered from a condensing unit  250  at some temperature near ambient to a letdown valve  202  and is reduced in pressure, thereby producing a colder two phase refrigerant stream in line  204 . The two phase refrigerant stream is then delivered via line  204  to a phase separation device  206 , the economizer, wherein the liquid and vapor phases are separated. The liquid refrigerant stream exits via line  210  and the vapor refrigerant stream exits via line  208 . 
     The liquid refrigerant stream  210  is reduced in pressure through another letdown valve  212 , thereby producing a colder two phase refrigerant stream  214 . This refrigerant stream is then completely vaporized in a chiller  216 . A cold vapor refrigerant stream exits the chiller  216  via line  218  and flows to the low stage compressor  220 . The low stage compressor  220  compresses the cold vapor refrigerant stream to a pressure intermediate between the final condensing pressure (e.g., the pressure in line  200 ) and the pressure in the chiller  216 . The refrigerant stream discharged from the low stage compressor via line  222  is at a pressure approximately that of the separator vessel  206 . 
     At this point, a mixer  224  mixes the vapor refrigerant stream that exits the economizer  206  via line  208  with the refrigerant stream in line  222 . The resultant refrigerant vapor stream exiting the mixer  224  via line  226  is further compressed in a high stage compressor  228 . 
     This refrigerant vapor stream is compressed to a pre-determined operating pressure that is lower than a baseline operating pressure. The baseline operating pressure is a pressure which would be required to permit the substantially complete condensation of the refrigerant vapor fraction at an ambient temperature. The vapor fraction may be cooled to ambient using air, cooling water or other cooling medium. The baseline condensation pressure can be experimentally or theoretically determined in a manner well known in the art. By substantially complete condensation, it is meant greater than 95% condensation. In some embodiments, the refrigerant vapor stream is compressed to a pressure significantly lower than the baseline condensation pressure. By significantly lower, it is meant that the pre-determined operating pressure is at least 20% lower than the baseline condensation pressure. For example, in one non-limiting example, a refrigerant vapor fraction that includes propane may have a baseline condensation pressure of 250 PSIA at an ambient temperature of 120 degrees F. Thus, the pre-determined operation pressure may be selected to be 135 PSIA or lower. 
     The compressed refrigerant vapor stream exits the compressor  228  via line  230 . At this point, a mixer  232  mixes the liquid sorbent stream received from line  262  with the vapor refrigerant stream received from line  230 . The resulting two phase hydrocarbon stream is then cooled and condensed with either air, cooling water or other cooling medium, in a condensing heat exchanger  236 . The combined liquid hydrocarbon stream that leaves the condensing exchanger  236  via line  238  is then delivered to the distillation tower feed pump  240 . The pump  240  discharges a pressurized liquid hydrocarbon stream via line  242  and may be preheated in a feed/bottoms cross exchanger (not shown) before being directed to a refrigerant generator tower  244 . 
     The generator tower  244  separates the liquid hydrocarbon stream received via line  242  into the refrigerant stream and the sorbent stream. The refrigerant generator tower may be a refluxed, reboiled fractionating tower that contains a rectifying section  246  and a stripping section  248 . The overhead product of this distillation tower  244  may be condensed in the overhead condenser  250  in order to produce a tower reflux stream as well as the liquid refrigerant fed into line  200 . The refrigerant stream in line  200  is then recycled to the inlet letdown valve  202  as previously described. The distillation tower bottom product that exits a tower reboiler  252  via line  254  is the sorbent stream. This sorbent stream can be cooled sequentially in a feed/bottoms exchanger (not shown) and a sorbent air cooler  256 . The cooled sorbent stream in line  258  is then reduced in pressure across letdown valve  260  returned to the adsorption mixer  232 . 
     It should be understood that the  FIG. 3  system is merely one non-limiting embodiment of a compression/absorption refrigeration system according to the present disclosure. 
     It should be appreciated that an aspect of the present disclosure is the use of one or more hydrocarbons for the refrigerant and the sorbent. The refrigerant may be a hydrocarbon having a molecular weight equal to or greater than ethylene and the sorbent may be a hydrocarbon having at least one carbon greater molecular weight. In embodiments, the selected hydrocarbon refrigerant may have a molecular weight of between 28 and 72. In such embodiments, the sorbent hydrocarbon has a molecular weight at least 14 greater than the selected hydrocarbon refrigerant. In embodiments, the hydrocarbon refrigerant may include methane, ethane, propane, butane, pentane, hydrocarbons having molecular weights between methane and pentane, and mixtures thereof. In embodiments, the sorbent hydrocarbon may include propane, butane, pentane, hexane, heptane, octane, nonane, and decane, hydrocarbons having molecular weights between pentane and decane, and mixtures thereof. These hydrocarbon mixtures may also combine corresponding alkene hydrocarbons such as ethylene, propylene, and butylenes. 
     The teachings of the present disclosure may be especially suitable for the natural gas separation and hydrocarbon processing fields, which have immediate access to the refrigerants called for by this disclosure. However, the present teachings may be advantageously applied in any number of other industrial or consumer applications. 
     As a person skilled in the art will recognize from the previous detailed description and from the figures and claims, modifications and changes can be made to the preferred embodiments of the invention without departing from the scope of this invention defined in the following claims.