Patent Publication Number: US-10323880-B2

Title: Mixed refrigerant cooling process and system

Description:
BACKGROUND 
     A number of liquefaction systems for cooling, liquefying, and optionally sub-cooling natural gas are well known in the art, such as the single mixed refrigerant (SMR) cycle, the propane-precooled mixed refrigerant (C3MR) cycle, the dual mixed refrigerant (DMR) cycle, C3MR-Nitrogen hybrid (such as AP-X™) cycles, the nitrogen or methane expander cycle, and cascade cycles. Typically, in such systems, natural gas is cooled, liquefied, and optionally sub-cooled by indirect heat exchange with one or more refrigerants. A variety of refrigerants might be employed, such as mixed refrigerants, pure components, two-phase refrigerants, gas phase refrigerants, etc. Mixed refrigerants (MR), which are a mixture of nitrogen, methane, ethane/ethylene, propane, butanes, and pentanes, have been used in many base-load liquefied natural gas (LNG) plants. The composition of the MR stream is typically optimized based on the feed gas composition and operating conditions. 
     The refrigerant is circulated in a refrigerant circuit that includes one or more heat exchangers and a refrigerant compression system. The refrigerant circuit may be closed-loop or open-loop. Natural gas is cooled, liquefied, and/or sub-cooled by indirect heat exchange in one or more refrigerant circuits by indirect heat exchanger with the refrigerants in the heat exchangers. 
     The refrigerant compression system includes a compression sequence for compressing and cooling the circulating refrigerant, and a driver assembly to provide the power needed to drive the compressors. The refrigerant compression system is a critical component of the liquefaction system because the refrigerant needs to be compressed to high pressure and cooled prior to expansion in order to produce a cold low pressure refrigerant stream that provides the heat duty necessary to cool, liquefy, and optionally sub-cool the natural gas. 
     Referring to  FIG. 1 , a typical DMR process of the prior art is shown in liquefaction system  100 . A feed stream, which is preferably natural gas, is cleaned and dried by known methods in a pre-treatment section (not shown) to remove water, acid gases such as CO 2  and H 2 S, and other contaminants such as mercury, resulting in a pre-treated feed stream  101 . The pre-treated feed stream  101 , which is essentially water free, is precooled in a precooling system  134  to produce precooled natural gas stream  102  and further cooled, liquefied, and/or sub-cooled in a main cryogenic heat exchanger (MCHE)  165  to produce LNG stream  104 . The LNG stream  104  is typically let down in pressure by passing it through a valve or a turbine (not shown) and is then sent to LNG storage tank (not shown). Any flash vapor produced during the pressure letdown and/or boil-off in the tank may be used as fuel in the plant, recycled to feed, and/or sent to flare. 
     The pre-treated feed stream  101  is precooled to a temperature below 10 degrees Celsius, preferably below about 0 degrees Celsius, and more preferably below about −30 degrees Celsius. The precooled natural gas stream  102  is liquefied by cooling to a temperature between about −150 degrees Celsius and about −70 degrees Celsius, preferably between about −145 degrees Celsius and about −100 degrees Celsius, and subsequently sub-cooled to a temperature between about −170 degrees Celsius and about −120 degrees Celsius, preferably between about −170 degrees Celsius and about −140 degrees Celsius. MCHE  165  shown in  FIG. 1  is a coil wound heat exchanger with two tube bundles, a warm bundle  166  and a cold bundle  167 . However, any number of bundles and any exchanger type may be utilized. 
     The term “essentially water free” means that any residual water in the pre-treated feed stream  101  is present at a sufficiently low concentration to prevent operational issues associated with water freeze-out in the downstream cooling and liquefaction process. In the embodiments described herein, water concentration is preferably not more than 1.0 ppm and, more preferably between 0.1 ppm and 0.5 ppm. 
     The precooling refrigerant used in the DMR process is a mixed refrigerant (MR) referred to herein as warm mixed refrigerant (WMR), comprising components such as nitrogen, methane, ethane/ethylene, propane, butanes, and other hydrocarbon components. As illustrated in  FIG. 1 , a warm low pressure WMR stream  110  is withdrawn from the bottom of the shell side of precooling heat exchanger  160  and is compressed and cooled in WMR compression system  111  to produce compressed WMR stream  132 . The WMR compression system  111  is described in  FIG. 2 . The compressed WMR stream  132  is cooled in a tube circuit of precooling heat exchanger  160  to produce a cold stream, which is then let down in pressure across first WMR expansion device  137  to produce expanded WMR stream  135 . The expanded WMR stream  135  is injected into the shell-side of precooling heat exchanger  160  and warmed against the pre-treated feed stream  101  to produce the warm low pressure WMR stream  110 .  FIG. 1  shows a coil wound heat exchanger with a single tube bundle for the precooling heat exchanger  160 , however any number of tube bundles and any type of heat exchanger may be employed. 
     In the DMR process, liquefaction and sub-cooling is performed by heat exchanging precooled natural gas against a second mixed refrigerant stream, referred to herein as cold mixed refrigerant (CMR). 
     A warm low pressure CMR stream  140  is withdrawn from the bottom of the shell side of the MCHE  165 , sent through a suction drum (not shown) to separate out any liquids and the vapor stream is compressed in CMR compressor  141  to produce compressed CMR stream  142 . The warm low pressure CMR stream  140  is typically withdrawn at a temperature at or near WMR precooling temperature and preferably less than about −30 degree Celsius and at a pressure of less than 10 bara (145 psia). The compressed CMR stream  142  is cooled in a CMR aftercooler  143  to produce a compressed cooled CMR stream  144 . Additional phase separators, compressors, and aftercoolers may be present. The process of compressing and cooling the CMR after it is withdrawn from the bottom of the MCHE  165  is generally referred to herein as the CMR compression sequence. 
     The compressed cooled CMR stream  144  is then cooled against evaporating WMR in precooling system  134  to produce a precooled CMR stream  145 , which may be fully condensed or two-phase depending on the precooling temperature and composition of the CMR stream.  FIG. 1  shows an arrangement where the precooled CMR stream  145  is two-phase and is sent to a CMR phase separator  164  from which a CMR liquid (CMRL) stream  147  and a CMR vapor (CMRV) stream  146  are obtained, which are sent back to MCHE  165  to be further cooled. Liquid streams leaving phase separators are referred to in the industry as MRL and vapor streams leaving phase separators are referred to in the industry as MRV, even after they are subsequently liquefied. 
     Both the CMRL stream  147  and CMRV stream  146  are cooled, in two separate circuits of the MCHE  165 . The CMRL stream  147  is cooled and partially liquefied in the warm bundle of the MCHE  165 , resulting in a cold stream that is let down in pressure across CMRL expansion device  149  to produce an expanded CMRL stream  148 , that is sent back to the shell-side of MCHE  165  to provide refrigeration required in the warm bundle  166 . The CMRV stream  146  is cooled in the first and second tube bundles of MCHE  165 , and reduced in pressure across the CMRV expansion device  151  to produce expanded CMRV stream  150  that is introduced to the MCHE  165  to provide refrigeration required in the cold bundle  167  and warm bundle  166 . 
     MCHE  165  and precooling heat exchanger  160  can be any exchanger suitable for natural gas cooling and liquefaction such as a coil wound heat exchanger, plate and fin heat exchanger or a shell and tube heat exchanger. Coil wound heat exchangers are the state of art exchangers for natural gas liquefaction and include at least one tube bundle comprising a plurality of spiral wound tubes for flowing process and warm refrigerant streams and a shell space for flowing a cold refrigerant stream. 
       FIG. 2  shows the details of the WMR compression system  211 . Any liquid present in warm low pressure WMR stream  210  is removed by passing through a phase separator (not shown) and the vapor stream from the phase separator is compressed in low pressure WMR compressor  212  to produce medium pressure WMR stream  213  that is cooled in low pressure WMR aftercooler  214  to produce cooled medium pressure WMR stream  215 . The low pressure WMR aftercooler  214  may further comprise multiple heat exchangers such as a desuperheater and a condenser. The cooled medium pressure WMR stream  215  may be two-phase and sent to WMR phase separator  216  to produce a WMR vapor (WMRV) stream  217  and WMR liquid (WMRL) stream  218 . The WMRV stream  217  is compressed in high pressure WMR compressor  221  to produce high pressure WMR stream  222  and cooled in high pressure WMR desuperheater  223  to produce desuperheated high pressure WMR stream  224 . The WMRL stream  218  is pumped to produce pumped WMRL stream  220  at a pressure comparable to that of the desuperheated high pressure WMR stream  224 . The pumped WMRL stream  220  and the desuperheated high pressure WMR stream  224  are mixed to produce mixed high pressure WMR stream  225  that is cooled in high pressure WMR condenser  226  to produce compressed WMR stream  232 . The mixed high pressure WMR stream  225  is two-phase with a vapor fraction of about 0.5. 
     The high pressure WMR condenser  226  may be a plate and fin heat exchanger or brazed aluminum heat exchanger and must be designed to handle two-phase inlet flow. One of the challenges in doing so is that the liquid and vapor phases will distribute unevenly in the high pressure WMR condenser  226 . As a result, the compressed WMR stream  232  will likely not be fully condensed, which will in turn imply reduced process efficiency for the precooling and liquefaction processes. Additionally, the two entry heat exchanger may involve operational challenges. 
     One approach to address these problems is to compensate for the mal-distribution of liquid and vapor in the design of high pressure WMR condenser  226  and design it to be significantly larger than in the case without mal-distribution, such that the compressed WMR stream  232  is fully condensed. However, there are two drawbacks associated with this method. First, since the degree of mal-distribution in the condenser is unpredictable, this method is somewhat arbitrary and may result in non-zero vapor fraction in compressed WMR stream  232 . Second, this method results in increased capital cost and plot space, which is undesirable. 
     Another solution to address the problem is to cool the WMRL stream  218  and the compressed WMR stream  232  in separate tube circuits of the precooling heat exchanger  260  to about the same precooling temperature. Each cooled stream would be letdown in pressure across separate expansion devices (similar to the first WMR expansion device  237 ) and sent as shellside refrigerant into the precooling heat exchanger  260 . Alternatively, both cooled streams could be combined and letdown in pressure in a common expansion device. This approach eliminates the issue of two-phase entry in the high pressure WMR condenser  226 , however it reduces the overall efficiency of the liquefaction process, in some cases up to 4% lower efficiency as compared to  FIG. 2 . Further, this solution would imply an additional tube circuit in the coil wound heat exchanger or additional passages in a plate and fin heat exchanger which imply increased capital cost. 
     Another solution involves fully condensing the desuperheated high pressure WMR stream  224  prior to mixing with the pumped WMRL stream  220 . This method further involves cooling the mixed streams in a tube circuit of the precooling heat exchanger  260 . However, this method has the same drawbacks as described for the previous solution with separate tube circuits. 
     A further solution involves dividing the precooling heat exchanger  260  into two sections, a warm section and a cold section. In case of a coil wound heat exchanger, the warm and cold sections may be separate tube bundles within the precooling heat exchanger  260 . The WMRL stream  218  is cooled in a separate tube circuit in the warm section of precooling heat exchanger  260 , reduced in pressure across an expansion device, and returned as shell side refrigerant to provide refrigeration to the warm section. The compressed WMR stream  232  is cooled in a separate tube circuit in the warm and cold sections of the precooling heat exchanger  260 , reduced in pressure across an expansion device, and returned as shell side refrigerant to provide refrigeration to the cold and warm sections. This arrangement eliminates the issues of two phase entry and also improve the overall efficiency of the liquefaction process as compared to  FIG. 2 . However, they result in significant increase in capital cost due to breaking up the precooling heat exchanger into multiple sections, and is often not desirable. 
     A reliable and efficient solution is desired that eliminates two-phase entry in the condenser, at the same time does not increase the capital cost of the facility significantly. This invention provides novel WMR configurations that eliminate two-phase inlet into the high pressure WMR condenser  226  as well as eliminates the WMR pump  268 , thereby reducing capital cost and improving operability and design of the DMR process. The inventions may also be applied to any cooling, liquefaction or subcooling processes involving multiple component refrigerants. 
     SUMMARY 
     Aspect 1: A method of cooling a hydrocarbon feed stream by indirect heat exchange with a first refrigerant stream in a cooling heat exchanger wherein the method comprises:
         a) compressing a warm low pressure first refrigerant stream in one or more compression stages to produce a compressed first refrigerant stream;   b) cooling the compressed first refrigerant stream in one or more cooling units to produce a compressed cooled first refrigerant stream;   c) introducing the compressed cooled first refrigerant stream into a first vapor-liquid separation device to produce a first vapor refrigerant stream and a first liquid refrigerant stream;   d) introducing the first liquid refrigerant stream into the cooling heat exchanger;   e) cooling the first liquid refrigerant stream in the cooling heat exchanger to produce a cooled liquid refrigerant stream;   f) expanding the cooled liquid refrigerant stream to produce a cold refrigerant stream, introducing the cold refrigerant stream into the cooling heat exchanger to provide refrigeration duty required to cool the hydrocarbon feed stream, the first liquid refrigerant stream, and a second refrigerant stream;   g) compressing the first vapor refrigerant stream in one or more compression stages to produce a compressed vapor refrigerant stream;   h) cooling and condensing the compressed vapor refrigerant stream to produce a condensed refrigerant stream;   i) expanding the condensed refrigerant stream to produce an expanded refrigerant stream;   j) introducing the expanded refrigerant stream into the first vapor-liquid separation device;   k) introducing the second refrigerant stream into the cooling heat exchanger;   l) introducing the hydrocarbon feed stream in the cooling heat exchanger; and   m) cooling the hydrocarbon feed stream in the cooling heat exchanger to produce a cooled hydrocarbon stream; and further cooling and liquefying the cooled hydrocarbon stream in a main heat exchanger to produce a liquefied hydrocarbon stream.       

     Aspect 2: The method of Aspect 1, wherein step (i) comprises introducing the expanded refrigerant stream into the first vapor-liquid separation device by mixing the expanded refrigerant stream with the compressed cooled first refrigerant stream upstream of the first vapor-liquid separation device. 
     Aspect 3: The method of any of Aspects 1-2, wherein the only first refrigerant stream to be cooled in the cooling heat exchanger is the first liquid refrigerant stream. 
     Aspect 4: The method of any of Aspects 1-3, wherein:
         step (e) further comprises cooling the first liquid refrigerant stream in the cooling heat exchanger by passing the first refrigerant stream through a first tube circuit of the cooling heat exchanger, wherein the cooling heat exchanger is a coil wound heat exchanger;   step (m) further comprises cooling the hydrocarbon feed stream in the cooling heat exchanger by passing the hydrocarbon feed stream through a second tube circuit of the cooling heat exchanger; and   step (f) further comprises introducing the cold refrigerant stream into a shell-side of the cooling heat exchanger.       

     Aspect 5: The method of any of Aspects 1-4, further comprising:
         n) cooling the second refrigerant stream in the cooling heat exchanger to produce a cooled second refrigerant stream;   o) further cooling the cooled second refrigerant stream in the main heat exchanger to produce a further cooled second refrigerant stream;   p) expanding the further cooled second refrigerant stream to produce an expanded second refrigerant stream;   q) returning the expanded second refrigerant stream to the main heat exchanger; and   r) further cooling and condensing the cooled hydrocarbon stream by indirect heat exchange with the expanded second refrigerant stream in the main heat exchanger to produce the liquefied hydrocarbon stream.       

     Aspect 6: The method of any of Aspects 1-5, further comprising, prior to performing step (d), cooling at least a portion of the first liquid refrigerant stream by indirect heat exchange with at least a portion of the expanded refrigerant stream in a first heat exchanger. 
     Aspect 7: The method of Aspect 6, further comprising cooling at least a portion of the hydrocarbon feed stream in the first heat exchanger prior to performing step (l). 
     Aspect 8: The method of any of Aspects 6-7, further comprising cooling at least a portion of the second refrigerant stream in the first heat exchanger prior to performing step (k). 
     Aspect 9: The method of any of Aspects 1-8, further comprising:
         k) introducing the expanded refrigerant stream into a second vapor-liquid separation device to produce a second vapor refrigerant stream and a second liquid refrigerant stream;   l) introducing the second vapor refrigerant stream into the first vapor-liquid separation device;   m) cooling the first liquid refrigerant stream by indirect heat exchange with the second liquid refrigerant stream in a first heat exchanger prior to cooling the first liquid refrigerant stream in the cooling heat exchanger in step (d); and   n) after performing step (m), introducing the second liquid refrigerant stream into the first vapor-liquid separation device.       

     Aspect 10: The method of Aspect 9, wherein the second vapor refrigerant stream and the second liquid refrigerant stream are mixed with the compressed cooled first refrigerant stream of step (b) upstream of the first vapor-liquid separation device prior to the introduction of the second vapor refrigerant stream and the second liquid refrigerant stream into the first vapor-liquid separation device. 
     Aspect 11: The method of any of Aspects 1-10, wherein step (c) comprises introducing the compressed cooled first refrigerant stream into a first vapor-liquid separation device comprising a mixing column to produce a first vapor refrigerant stream and a first liquid refrigerant stream. 
     Aspect 12: The method of Aspect 11, wherein the compressed cooled first refrigerant stream is introduced into the mixing column at or above a top stage of the mixing column and the expanded first refrigerant stream is introduced to the mixing column at or below a bottom stage of the mixing column. 
     Aspect 13: The method of any of Aspects 1-12, wherein the hydrocarbon feed stream is natural gas. 
     Aspect 14: The method of any of Aspects 1-12, wherein the condensed refrigerant stream is fully condensed. 
     Aspect 15: The method of any of Aspects 1-14, wherein steps a) and c) further comprise:
         a) compressing a warm low pressure first refrigerant stream in one or more compression stages to produce a compressed first refrigerant stream, wherein the warm low pressure first refrigerant stream has a first composition;   c) introducing the compressed cooled first refrigerant stream into a first vapor-liquid separation device to produce a first vapor refrigerant stream and a first liquid refrigerant stream, wherein the first vapor refrigerant stream has a second composition, the second composition having a higher percentage (on a molar basis) of components lighter than ethane than the first composition.       

     Aspect 16: The method of any of Aspects 1-15, wherein step a) further comprises:
         a) compressing a warm low pressure first refrigerant stream in one or more compression stages to produce a compressed first refrigerant stream, wherein the warm low pressure first refrigerant stream has a first composition consisting of less than 10% components lighter than ethane.       

     Aspect 17: The method of any of Aspects 1-16, wherein step c) further comprises:
         c) introducing the compressed cooled first refrigerant stream into a first vapor-liquid separation device to produce a first vapor refrigerant stream and a first liquid refrigerant stream, wherein the first vapor refrigerant stream has a second composition consisting of less than 20% components lighter than ethane.       

     Aspect 18: An apparatus for cooling a hydrocarbon feed stream comprising:
         a cooling heat exchanger including a first hydrocarbon feed circuit, a first refrigerant circuit, a second refrigerant circuit, a first refrigerant circuit inlet located at an upstream end of the first refrigerant circuit, a first pressure letdown device located at a downstream end of the first refrigerant circuit, and an expanded first refrigerant conduit downstream from and in fluid flow communication with the pressure letdown device, the cooling heat exchanger being operationally configured to cool, by indirect heat exchange against a cold refrigerant stream, the hydrocarbon feed stream as it flows through the first hydrocarbon feed circuit, thereby producing a pre-cooled hydrocarbon feed stream, a first refrigerant flowing through the first refrigerant circuit, and a second refrigerant flowing through the second refrigerant circuit; and   a compression system comprising:
           a warm low pressure first refrigerant conduit in fluid flow communication with a lower end of the cooling heat exchanger and a first compressor;   a first aftercooler in fluid flow communication with and downstream from the first compressor;   a first vapor-liquid separation device having a first inlet in fluid flow communication with and downstream from the first aftercooler, a first vapor outlet located in an upper half of the first vapor-liquid separation device, a first liquid outlet located in a lower half of the first vapor-liquid separation device, the first liquid outlet being upstream from and in fluid flow communication with the first refrigerant circuit inlet;   a second compressor downstream from and in fluid flow communication with the first vapor outlet;   a condenser downstream from and in fluid flow communication with the second compressor; and   a second pressure letdown device downstream from and in fluid flow communication with the condenser, the second pressure letdown device being upstream from and in fluid flow communication with the first vapor-liquid separation device, so that all fluid that flows through the second pressure letdown device flows through the first vapor-liquid separation device before flowing to the cooling heat exchanger.   
               

     Aspect 19: The apparatus of Aspect 18, further comprising:
         a main heat exchanger having a second hydrocarbon circuit that is downstream from and in fluid flow communication with the first hydrocarbon circuit of the cooling heat exchanger, the main heat exchanger being operationally configured to at least partially liquefy the pre-cooled hydrocarbon feed stream by indirect heat exchange against the second refrigerant.       

     Aspect 20: The apparatus of any of Aspects 18-19, further comprising:
         a first heat exchanger having a first heat exchange circuit that is operationally configured to provide indirect heat exchange against a second heat exchange circuit, the first heat exchange circuit being downstream from and in fluid flow communication with the second pressure letdown device and the second heat exchange circuit being downstream from and in fluid flow communication with the first liquid outlet of the first liquid-vapor separation device.       

     Aspect 21: The apparatus of any of Aspects 18-20, further comprising:
         a second vapor-liquid separation device having a third inlet in fluid flow communication with and downstream from the second pressure letdown device, a second vapor outlet located in an upper half of the second vapor-liquid separation device, a second liquid outlet located in a lower half of the second vapor-liquid separation device, the first liquid outlet being upstream from and in fluid flow communication with the first heat exchange circuit of the first heat exchanger.       

     Aspect 22: The apparatus of any of Aspects 18-21, wherein the first heat exchanger further comprises a third heat exchange circuit and a fourth heat exchange circuit, the third heat exchange circuit being upstream from and in fluid flow communication with the first refrigerant circuit, the fourth heat exchange circuit being upstream from and in fluid flow communication with the first hydrocarbon feed circuit, the first heat exchanger being operationally configured to cool fluids flowing through the second heat exchange circuit, third heat exchange circuit, and fourth heat exchange circuit against the first heat exchange circuit. 
     Aspect 23: The apparatus of any of Aspects 18-22, wherein the first vapor-liquid separation device is a mixing column. 
     Aspect 24: The apparatus of Aspect 23, wherein the first inlet of the first liquid-vapor separation device is located at a top stage of the mixing column and the second inlet of the first liquid-vapor separation device is located at a bottom stage of the mixing column. 
     Aspect 25: The apparatus of any of Aspects 18-24, wherein the cooling heat exchanger is a coil-wound heat exchanger. 
     Aspect 26: The apparatus of any of Aspects 18-25, further comprising a desuperheater downstream from and in fluid flow communication with the second compressor and upstream from and in fluid flow communication with the condenser. 
     Aspect 27: The apparatus of any of Aspects 18-26, wherein the first refrigerant consists of a first mixed refrigerant. 
     Aspect 28: The apparatus of any of Aspects 18-27, wherein the second refrigerant consists of a second refrigerant having a different composition than the first mixed refrigerant. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic flow diagram of a DMR system in accordance with the prior art; 
         FIG. 2  is a schematic flow diagram of a precooling system of a DMR system in accordance with the prior art; 
         FIG. 3  is a schematic flow diagram of a precooling system of a DMR system in accordance with a first exemplary embodiment of the invention; 
         FIG. 4  is a schematic flow diagram of a precooling system of a DMR system in accordance with a second exemplary embodiment of the invention; 
         FIG. 5  is a schematic flow diagram of a precooling system of a DMR system in accordance with a third exemplary embodiment of the invention; 
         FIG. 6  is a schematic flow diagram of a precooling system of a DMR system in accordance with a fourth exemplary embodiment of the invention; and 
         FIG. 7  is a schematic flow diagram of a precooling system of a DMR system in accordance with a fifth exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The ensuing detailed description provides preferred exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the claimed invention. Rather, the ensuing detailed description of the preferred exemplary embodiments will provide those skilled in the art with an enabling description for implementing the preferred exemplary embodiments of the claimed invention. Various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the claimed invention. 
     Reference numerals that are introduced in the specification in association with a drawing figure may be repeated in one or more subsequent figures without additional description in the specification in order to provide context for other features. 
     The term “fluid flow communication,” as used in the specification and claims, refers to the nature of connectivity between two or more components that enables liquids, vapors, and/or two-phase mixtures to be transported between the components in a controlled fashion (i.e., without leakage) either directly or indirectly. Coupling two or more components such that they are in fluid flow communication with each other can involve any suitable method known in the art, such as with the use of welds, flanged conduits, gaskets, and bolts. Two or more components may also be coupled together via other components of the system that may separate them, for example, valves, gates, or other devices that may selectively restrict or direct fluid flow. 
     The term “conduit,” as used in the specification and claims, refers to one or more structures through which fluids can be transported between two or more components of a system. For example, conduits can include pipes, ducts, passageways, and combinations thereof that transport liquids, vapors, and/or gases. 
     The term “natural gas”, as used in the specification and claims, means a hydrocarbon gas mixture consisting primarily of methane. 
     The terms “hydrocarbon gas” or “hydrocarbon fluid”, as used in the specification and claims, means a gas/fluid comprising at least one hydrocarbon and for which hydrocarbons comprise at least 80%, and more preferably at least 90% of the overall composition of the gas/fluid. 
     The term “mixed refrigerant” (abbreviated as “MR”), as used in the specification and claims, means a fluid comprising at least two hydrocarbons and for which hydrocarbons comprise at least 80% of the overall composition of the refrigerant. 
     The term “heavy mixed refrigerant”, as used in the specification and claims, means an MR in which hydrocarbons at least as heavy as ethane comprise at least 80% of the overall composition of the MR. Preferably, hydrocarbons at least as heavy as butane comprise at least 10% of the overall composition of the mixed refrigerant. 
     The terms “bundle” and “tube bundle” are used interchangeably within this application and are intended to be synonymous. 
     The term “ambient fluid”, as used in the specification and claims, means a fluid that is provided to the system at or near ambient pressure and temperature. 
     In the claims, letters are used to identify claimed steps (e.g. (a), (b), and (c)). These letters are used to aid in referring to the method steps and are not intended to indicate the order in which claimed steps are performed, unless and only to the extent that such order is specifically recited in the claims. 
     Directional terms may be used in the specification and claims to describe portions of the present invention (e.g., upper, lower, left, right, etc.). These directional terms are merely intended to assist in describing exemplary embodiments, and are not intended to limit the scope of the claimed invention. As used herein, the term “upstream” is intended to mean in a direction that is opposite the direction of flow of a fluid in a conduit from a point of reference during normal operation of the system being described. Similarly, the term “downstream” is intended to mean in a direction that is the same as the direction of flow of a fluid in a conduit from a point of reference during normal operation of the system being described. 
     As used in the specification and claims, the terms “high-high”, “high”, “medium”, and “low” are intended to express relative values for a property of the elements with which these terms are used. For example, a high-high pressure stream is intended to indicate a stream having a higher pressure than the corresponding high pressure stream or medium pressure stream or low pressure stream described or claimed in this application. Similarly, a high pressure stream is intended to indicate a stream having a higher pressure than the corresponding medium pressure stream or low pressure stream described in the specification or claims, but lower than the corresponding high-high pressure stream described or claimed in this application. Similarly, a medium pressure stream is intended to indicate a stream having a higher pressure than the corresponding low pressure stream described in the specification or claims, but lower than the corresponding high pressure stream described or claimed in this application. 
     Unless otherwise stated herein, any and all percentages identified in the specification, drawings and claims should be understood to be on a weight percentage basis. Unless otherwise stated herein, any and all pressures identified in the specification, drawings and claims should be understood to mean gauge pressure. 
     As used herein, the term “cryogen” or “cryogenic fluid” is intended to mean a liquid, gas, or mixed phase fluid having a temperature less than −70 degrees Celsius. Examples of cryogens include liquid nitrogen (LIN), liquefied natural gas (LNG), liquid helium, liquid carbon dioxide and pressurized, mixed phase cryogens (e.g., a mixture of LIN and gaseous nitrogen). As used herein, the term “cryogenic temperature” is intended to mean a temperature below −70 degrees Celsius. 
     Unless otherwise stated herein, introducing a stream at a location is intended to mean introducing substantially all of the said stream at the location. All streams discussed in the specification and shown in the drawings (typically represented by a line with an arrow showing the overall direction of fluid flow during normal operation) should be understood to be contained within a corresponding conduit. Each conduit should be understood to have at least one inlet and at least one outlet. Further, each piece of equipment should be understood to have at least one inlet and at least one outlet. 
       FIG. 3  shows a first embodiment of the invention. Any liquid present in warm low pressure WMR stream  310  is removed by passing through a phase separator (not shown) and the vapor stream from the phase separator is compressed in low pressure WMR compressor  312  to produce medium pressure WMR stream  313  that is cooled in low pressure WMR aftercooler  314  to produce cooled medium pressure WMR stream  315 . The low pressure WMR aftercooler  314  may further comprise multiple heat exchangers such as a desuperheater and a condenser. The cooled medium pressure WMR stream  315  may be two-phase and sent to WMR phase separator  316  to produce a WMRV stream  317  and WMRL stream  318 . The WMRL stream  318  is further cooled in a tube circuit of precooling heat exchanger  360  to produce a further cooled WMRL stream  319  that is letdown in pressure across first WMR expansion device  337  to produce expanded WMR stream  335  that is then returned to the precooling exchanger  360  as shell-side refrigerant. The pre-treated feed stream  301  is precooled in the precooling heat exchanger  360  to produce a precooled natural gas stream  302 . 
     The WMRV stream  317  is compressed in high pressure WMR compressor  321  to produce high pressure WMRV stream  322  that is cooled in high pressure WMR desuperheater  323  to produce cooled high pressure MRV stream  324  that is further cooled and condensed in high pressure WMR condenser  326  to produce condensed high pressure WMR stream  327 , that is at least partially and preferably totally condensed. Since the warm low pressure WMR stream  310  is used to precool the natural gas stream, it has a low concentration of light components such as nitrogen and methane, and predominantly contains ethane and heavier components. The warm low pressure WMR stream  310  may comprise less than 10% of components lighter than ethane, preferably less than 5% of components lighter than ethane, and more preferably less than 2% of components lighter than ethane. The light components accumulate in the WMRV stream  317 , which may comprise less than 20% of components lighter than ethane, preferably less than 15% of components lighter than ethane, and more preferably less than 10% of components lighter than ethane. Therefore, it is possible to fully condense the WMRV stream  317  to produce a totally condensed high pressure WMR stream  327  without needing to compress to very high pressure. The high pressure WMRV stream  322  may be at a pressure between 450 psia (31 bara) and 700 psia (48 bara), and preferably between 500 psia (34 bara) and 650 psia (45 bara). If precooling heat exchanger  360  was a liquefaction heat exchanger used to fully liquefy the natural gas, the warm low pressure WMR stream  310  would have a higher concentration of nitrogen and methane and therefore the pressure of the high pressure WMRV stream  322  would have to be higher in order for the condensed high pressure WMR stream  327  to be fully condensed. Since this may not be possible to achieve, the condensed high pressure WMR stream  327  would not be fully condensed and would contain significant vapor concentration that may need to be liquefied separately. 
     The condensed high pressure WMR stream  327  is let down in pressure in second WMR expansion device  328  to produce an expanded high pressure WMR stream  329  at about the same pressure as the cooled medium pressure WMR stream  315  which may be at a pressure between 200 psia (14 bara) and 400 psia (28 bara), and preferably between 300 psia (21 bara) and 350 psia (24 bara). The expanded high pressure WMR stream  329  may be at a temperature between −10 degrees Celsius and 20 degrees Celsius and preferably between −5 degrees Celsius and 5 degrees Celsius. The expanded high pressure WMR stream  329  may have a vapor fraction of 0.1 to 0.6 and preferably between 0.2 and 0.4. The conditions of the said streams will vary based on ambient temperature and operating conditions. The expanded high pressure WMR stream  329  is returned to the WMR phase separator  316 . 
     Alternatively, the expanded high pressure WMR stream  329  may be returned to a location upstream of the WMR phase separator  316  (shown by the dashed line  329   a  in  FIG. 3 ), for instance, by mixing with the cooled medium pressure WMR stream  315 . The first WMR expansion device  337  and the second WMR expansion device  328  may be a hydraulic turbine, a Joule-Thomson (J-T) valve, or any other suitable expansion device known in the art. 
     A benefit of the embodiment shown in  FIG. 3  over prior art is that the high pressure WMR condenser  326  needs to be designed only for vapor phase inlet. This helps eliminate any design issues and mitigate potential vapor-liquid distribution issues in the condenser. Additionally, the configuration shown in  FIG. 3  eliminates the WMR pump  268  shown in prior art  FIG. 2  and thereby reduces capital cost, equipment count, and footprint of the LNG facility. 
     An alternative to  FIG. 3  involves the use of an ejector/eductor wherein the cooled medium pressure WMR stream  315  and the condensed high pressure WMR stream  327  are sent to an eductor to produce two-phase stream that is sent to WMR phase separator  316 . 
       FIG. 4  shows a preferred embodiment of the invention. Referring to  FIG. 4 , any liquid present in warm low pressure WMR stream  410  is removed by passing through a phase separator (not shown) and the vapor stream from the phase separator is compressed in low pressure WMR compressor  412  to produce medium pressure WMR stream  413  that is cooled in low pressure WMR aftercooler  414  to produce cooled medium pressure WMR stream  415 . The low pressure WMR aftercooler  414  may further comprise multiple heat exchangers such as a desuperheater and a condenser. The cooled medium pressure WMR stream  415  may be two-phase and sent to WMR phase separator  416  to produce a WMRV stream  417  and WMRL stream  418 . 
     The WMRV stream  417  is compressed in high pressure WMR compressor  421  to produce high pressure WMRV stream  422  that is cooled in high pressure WMR desuperheater  423  to produce cooled high pressure MRV stream  424  that is further cooled and condensed in high pressure WMR condenser  426  to produce condensed high pressure WMR stream  427 . The condensed high pressure WMR stream  427  is letdown in pressure in second WMR expansion device  428  to produce an expanded high pressure WMR stream  429 . The expanded high pressure WMR stream  429  is warmed in WMR heat exchanger  430  to produce warm expanded high pressure WMR stream  431  that is returned to the WMR phase separator  416 . The second WMR expansion device  428  is adjusted such that the pressure of the warm expanded high pressure WMR stream  431  is about the same as the pressure of the cooled medium pressure WMR stream  415 . 
     The WMRL stream  418  is cooled in WMR heat exchanger  430  against the expanded high pressure WMR stream  429  to produce a cooled WMRL stream  433 . The warm expanded high pressure WMR stream  431  may be at a temperature of −20 degrees Celsius and 15 degrees Celsius and preferably between −10 degrees Celsius and 0 degrees Celsius. The temperature of the said stream will vary based on ambient temperature and operating conditions. 
     The cooled WMRL stream  433  is further cooled in a tube circuit of the precooling heat exchanger  460  to produce a further cooled WMRL stream  319  that is letdown in pressure across a first WMR expansion device  437  to produce an expanded WMR stream  435  that is then returned to the precooling exchanger  460  as shell-side refrigerant. 
     WMR heat exchanger  430  may be a plate and fin, brazed aluminum, coil wound, or any other suitable type of heat exchanger known in the art. WMR heat exchanger  430  may also comprise multiple heat exchangers in series or parallel. 
     The embodiment shown in  FIG. 4  retains all the benefits of  FIG. 3  over the prior art. Additionally, this embodiment improves the process efficiency of the process shown in  FIG. 3  by about 2% thereby reducing the required power for the same amount of LNG produced. The increase in efficiency observed is primarily due to colder temperature of the liquid stream being sent into the precooling heat exchanger. 
     An alternative embodiment is a variation of  FIG. 4  wherein the heat exchanger  430  provides indirect heat exchange between the expanded high pressure WMR stream  429  and the WMRV stream  417  (instead of the WMRL stream  418 ). This embodiment results in colder conditions at the suction of high pressure WMR compressor  421 . 
     A further embodiment is a variation of  FIG. 4  wherein the heat exchanger  430  provides indirect heat exchange between the expanded high pressure WMR stream  429  and the cooled medium pressure WMR stream  415 . This embodiment results in cooling both the inlet of high pressure WMR compressor  421  and cooled WMRL stream  433 . 
     The expanded high pressure WMR stream  429  may be two-phase. However, it is expected that the performance of the WMR heat exchanger  430  is not significantly affected due to the low amount of vapor typically present in the expanded high pressure WMR stream  429 . In scenarios wherein higher amounts of vapor are present in the expanded high pressure WMR stream  429 ,  FIG. 5  provides an alternative embodiment. 
     Referring to  FIG. 5 , expanded high pressure WMR stream  529  is sent to a second WMR phase separator  538  to produce a second WMRV stream  539  and a second WMRL stream  536 . The second WMRV stream  539  is returned to a WMR phase separator  516 . The second WMR expansion device  528  is adjusted such that the second MRV stream  539  is about the same pressure as the cooled medium pressure WMR stream  515 . 
     The second WMRL stream  536  is warmed in WMR heat exchanger  530  to produce a warm expanded high pressure WMR stream  531  that is returned to the WMR phase separator  516 . Alternatively, the warm expanded high pressure WMR stream  531  could be mixed with the cooled medium pressure WMR stream  515  upstream from the WMR phase separator  516  (shown by dashed line  531   a  in  FIG. 5 ). The WMRL stream  518  from WMR phase separator  516  is cooled in the WMR heat exchanger  530  against the second WMRL stream  536  to produce a cooled WMRL stream  533 . The cooled WMRL stream  533  is further cooled in a tube circuit of the precooling heat exchanger  560  to produce a further cooled WMRL stream  319  that is letdown in pressure across a first WMR expansion device  537  to produce an expanded WMR stream  535  that is then returned to the precooling exchanger  560  as shell-side refrigerant. 
     The embodiment disclosed in  FIG. 5  possesses all the benefits of  FIG. 4 . It includes an additional piece of equipment and is beneficial in scenarios with high vapor flow from the second WMR expansion device  528 . 
     In an alternative embodiment, the second WMRV stream  539  is warmed by passing through a separate passage of the WMR heat exchanger  530  prior to being returned to the WMR phase separator  516 . 
       FIG. 6  shows a further embodiment of the invention and is a variation of  FIG. 3 . Warm low pressure WMR stream  610  is compressed in a low pressure WMR compressor  612  to produce a medium pressure WMR stream  613  that is cooled in a low pressure WMR aftercooler  614  to produce a cooled medium pressure WMR stream  615 . The low pressure WMR aftercooler  614  may further comprise multiple heat exchangers such as a desuperheater and a condenser. The cooled medium pressure WMR stream  615  is sent to a top stage of a mixing column  655  to produce a WMRV stream  617  from a top stage of the mixing column  655  and a WMRL stream  618  from a bottom stage of the mixing column  655 . The WMRL stream  618  is further cooled in a tube circuit of precooling heat exchanger  660  to produce a further cooled WMRL stream  319  that is letdown in pressure across first WMR expansion device  637  to produce expanded WMR stream  635  that is then returned to the precooling exchanger  660  as shell-side refrigerant. 
     The WMRV stream  617  is compressed in a high pressure WMR compressor  621  to produce a high pressure WMRV stream  622  that is cooled in a high pressure WMR desuperheater  623  to produce a cooled high pressure MRV stream  624  that is further cooled and condensed in high pressure WMR condenser  626  to produce condensed high pressure WMR stream  627 . The condensed high pressure WMR stream  627  is letdown in pressure in second WMR expansion device  628  to produce an expanded high pressure WMR stream  629 . The expanded high pressure WMR stream  629  is returned to the bottom stage of the mixing column  655 . This embodiment possesses all the benefits of  FIG. 3  and results in higher process efficiency as compared to  FIG. 3  due to cooling the liquid stream being sent to the precooling heat exchanger. 
     Mixing columns, such as mixing column  655 , operate on the same thermodynamic principles as a distillation column (also referred to in the art as a separation or fractionation column). However, the mixing column  655  performs a task opposite to a distillation column. It reversibly mixes fluids in a plurality of equilibrium stages, instead of separating the components of a fluid. In contrast to a distillation column, the top of the mixing column is warmer than the bottom. The mixing column  655  may contain packing and/or any number of trays. A top stage refers to the top tray or top section of the mixing column  655 . A bottom stage refers to the bottom tray or bottom section of the mixing column  655 . 
     An alternative embodiment involves replacing the mixing column with a distillation column. In this embodiment, the expanded high pressure WMR stream  629  is inserted at a top stage of the distillation column to provide reflux, while the cooled medium pressure WMR stream  615  is inserted at a lower stage of the column. Additional reboiler duty or condensing duty may be provided. 
     The embodiment shown in  FIG. 7  is a variation of that shown in  FIG. 4 . In this embodiment, the pre-treated feed stream  701  and the compressed cooled CMR stream  745  are also cooled by indirect heat exchange with the expanded high pressure WMR stream  729  in WMR heat exchanger  730  to produce cooled pre-treated feed stream  752  and compressed twice-cooled CMR stream  753  respectively. The cooled pre-treated feed stream  752  and the compressed twice-cooled CMR stream  753  are further cooled in separate tube circuits of the precooling heat exchanger  760 . 
     This embodiment further improves the efficiency of the process by reducing the temperature of the feed streams in the precooling heat exchanger  760  as well as ensuring that the feed streams to the precooling heat exchanger  760  are at similar temperatures. In an alternate embodiment, only one of the pre-treated feed stream  701  and the compressed cooled CMR stream  745  are cooled in the WMR heat exchanger  730 . 
     For all the embodiments described herein, the composition of the WMR stream may be adjusted with varying feed composition, ambient temperature, and other conditions. Typically, the WMR stream contains over 40 mole percent and preferably over 50 mole percent of components lighter than butane. 
     The embodiments of the invention described herein are applicable to any compressor design including any number of compressors, compressor casings, compression stages, presence of inter or after-cooling, etc. Further, the embodiments described herein are applicable to any heat exchanger type such as plate and fin heat exchangers, coil wound heat exchangers, shell and tube heat exchangers, brazed aluminum heat exchangers, kettle, kettle-in-core, and other suitable heat exchanger designs. Although the embodiments described herein refer to mixed refrigerants comprising hydrocarbons and nitrogen, they are also applicable to any other refrigerant mixture such as fluorocarbons. The methods and systems associated with this invention can be implemented as part of new plant design or as a retrofit for existing LNG plants. 
     Example 1 
     The following is an example of the operation of an exemplary embodiment of the invention. The example process and data are based on simulations of a DMR process in an LNG plant that produces about 5.5 million metric tons per annum of LNG and specifically refers to the embodiment shown in  FIG. 4 . In order to simplify the description of this example, elements and reference numerals described with respect to the embodiment shown in  FIG. 4  will be used. 
     Warm low pressure WMR stream  410  at 51 degrees Fahrenheit (11 degrees Celsius), 55 psia (3.8 bara) and 42,803 lbmol/hr (19,415 kmol/hr) is compressed in low pressure WMR compressor  412  to produce medium pressure WMR stream  413  at 207 degrees Fahrenheit (97.5 degrees Celsius) and 331 psia (22.8 bara) that is cooled in low pressure WMR aftercooler  414  to produce cooled medium pressure WMR stream  415  at 77 degrees Fahrenheit (25 degrees Celsius) and 316 psia (21.8 bara). The cooled medium pressure WMR stream  415  is sent to WMR phase separator  416  to produce a WMRV stream  417  and WMRL stream  418 . 
     The WMRV stream  417  of 15,811 lbmol/hr (7172 kmol/hr) is compressed in high pressure WMR compressor  421  to produce high pressure WMRV stream  422  at 146 degrees Fahrenheit (63 degrees Celsius) and 598 psia (41 bara) that is cooled in high pressure WMR desuperheater  423  to produce cooled high pressure MRV stream  424  that is further cooled and condensed in high pressure WMR condenser  426  to produce condensed high pressure WMR stream  427  at 77 degrees Fahrenheit (25 degrees Celsius), 583 psia (40.2 bara), and vapor fraction of 0. The condensed high pressure WMR stream  427  is letdown in pressure in second WMR expansion device  428  to produce an expanded high pressure WMR stream  429  at 34 degrees Fahrenheit (1.4 degrees Celsius) and 324 psia (22.2 bara). The expanded high pressure WMR stream  429  is warmed in WMR heat exchanger  430  to produce warm expanded high pressure WMR stream  431  at 53 degrees Fahrenheit (11.8 degrees Fahrenheit) and 316 psia (21.8 bara) that is returned to the WMR phase separator  316 . In this example, the warm low pressure WMR stream  410  contains 1% of components lighter than ethane and the vapor fraction of the expanded high pressure WMR stream  429  is 0.3. 
     The WMRL stream  418  of 42,800 lbmol/hr (19,415 kmol/hr) is cooled in WMR heat exchanger  430  against the expanded high pressure WMR stream  429  to produce a cooled WMRL stream  433  at 38 degrees Fahrenheit (3.11 degrees Celsius) and 308 psia (21.2 bara). 
     The pre-treated feed stream  401  enters the precooling heat exchanger  460  at 68 degrees Fahrenheit (20 degrees Celsius), 1100 psia (76 bara) to produce precooled natural gas stream  402  at −41 degrees Fahrenheit (−40.5 degrees Celsius) and vapor fraction of 0.74. The compressed cooled CMR stream  444  enters the precooling heat exchanger  460  at 77 degrees Fahrenheit (25 degrees Celsius), 890 psia (61 bara) to produce the precooled CMR stream  445  at −40 degrees Fahrenheit (−40 degrees Celsius) and vapor fraction of 0.3. 
     In this example, the efficiency of the process was found to be 2-3% higher than that corresponding to  FIG. 3 . Therefore, this example demonstrates that the invention provides an efficient and low cost method and system to eliminate two-phase entry in the WMR condenser heat exchanger and also eliminate the WMR liquid pump.