Patent Abstract:
A system and method for cooling and liquefying a gas in a heat exchanger that includes compressing and cooling a mixed refrigerant using first and last compression and cooling cycles so that high pressure liquid and vapor streams are formed. The high pressure liquid and vapor streams are cooled in the heat exchanger and then expanded so that a primary refrigeration stream is provided in the heat exchanger. The mixed refrigerant is cooled and equilibrated between the first and last compression and cooling cycles so that a pre-cool liquid stream is formed and subcooled in the heat exchanger. The stream is then expanded and passed through the heat exchanger as a pre-cool refrigeration stream. A stream of gas is passed through the heat exchanger in countercurrent heat exchange with the primary refrigeration stream and the pre-cool refrigeration stream so that the gas is cooled. A resulting vapor stream from the primary refrigeration stream passage and a two-phase stream from the pre-cool refrigeration stream passage exit the warm end of the exchanger and are combined and undergo a simultaneous heat and mass transfer operation prior to the first compression and cooling cycle so that a reduced temperature vapor stream is provided to the first stage compressor so as to lower power consumption by the system. Additionally, the warm end of the cooling curve is nearly closed further reducing power consumption. Heavy components of the refrigerant are also kept out of the cold end of the process, reducing the possibility of refrigerant freezing, as well as facilitating a refrigerant management scheme.

Full Description:
FIELD OF THE INVENTION 
     The present invention generally relates to processes and systems for cooling or liquefying gases and, more particularly, to an improved mixed refrigerant system and method for cooling or liquefying gases. 
     BACKGROUND 
     Natural gas, which is primarily methane, and other gases, are liquefied under pressure for storage and transport. The reduction in volume that results from liquefaction permits containers of more practical and economical design to be used. Liquefaction is typically accomplished by chilling the gas through indirect heat exchange by one or more refrigeration cycles. Such refrigeration cycles are costly both in terms equipment cost and operation due to the complexity of the required equipment and the required efficiency of performance of the refrigerant. There is a need, therefore, for gas cooling and liquefaction systems having improved refrigeration efficiency and reduced operating costs with reduced complexity. 
     Liquefaction of natural gas requires cooling of the natural gas stream to approximately −160° C. to −170° C. and then letting down the pressure to approximately ambient.  FIG. 1  shows typical temperature—enthalpy curves for methane at 60 bar pressure, methane at 35 bar pressure and a mixture of methane and ethane at 35 bar pressure. There are three regions to the S-shaped curves. Above about −75° C. the gas is de-superheating and below about −90° C. the liquid is subcooling. The relatively flat region in-between is where the gas is condensing into liquid. Since the 60 bar curve is above the critical pressure, there is only one phase present; but its specific heat is large near the critical temperature, and the cooling curve is similar to the lower pressure curves. The curve containing 5% ethane shows the effect of impurities which round off the dew and bubble points. 
     A refrigeration process is necessary to supply the cooling for liquefying natural gas, and the most efficient processes will have heating curves which closely approach the cooling curves in  FIG. 1  to within a few degrees throughout their entire range. However, because of the S-shaped form of the cooling curves and the large temperature range, such a refrigeration process is difficult to design. Because of their flat vaporization curves, pure component refrigerant processes work best in the two-phase region but, because of their sloping vaporization curves, multi-component refrigerant processes are more appropriate for the de-superheating and subcooling regions. Both types of processes, and hybrids of the two, have been developed for liquefying natural gas. 
     Cascaded, multilevel, pure component cycles were initially used with refrigerants such as propylene, ethylene, methane, and nitrogen. With enough levels, such cycles can generate a net heating curve which approximates the cooling curves shown in  FIG. 1 . However, the mechanical complexity becomes overwhelming as additional compressor trains are required as the number of levels increases. Such processes are also thermodynamically inefficient because the pure component refrigerants vaporize at constant temperature instead of following the natural gas cooling curve and the refrigeration valve irreversibly flashes liquid into vapor. For these reasons, improved processes have been sought in order to reduce capital cost, reduce energy consumption and improve operability. 
     U.S. Pat. No. 5,746,066 to Manley describes a cascaded, multilevel, mixed refrigerant process as applied to the similar refrigeration demands for ethylene recovery which eliminates the thermodynamic inefficiencies of the cascaded multilevel pure component process. This is because the refrigerants vaporize at rising temperatures following the gas cooling curve and the liquid refrigerant is subcooled before flashing thus reducing thermodynamic irreversibility. In addition, the mechanical complexity is somewhat less because only two different refrigerant cycles are required instead of the three or four required for the pure refrigerant processes. U.S. Pat. No. 4,525,185 to Newton; U.S. Pat. No. 4,545,795 to Liu et al.; U.S. Pat. No. 4,689,063 to Paradowski et al. and U.S. Pat. No. 6,041,619 to Fischer et al. all show variations on this theme applied to natural gas liquefaction as do U.S. Patent Application Publication Nos. 2007/0227185 to Stone et al. and 2007/0283718 to Hulsey et al. 
     The cascaded, multilevel, mixed refrigerant process is the most efficient known, but a simpler, efficient process which can be more easily operated is desirable for most plants. 
     U.S. Pat. No. 4,033,735 to Swenson describes a single mixed refrigerant process which requires only one compressor for the refrigeration process and which further reduces the mechanical complexity. However, for primarily two reasons, the process consumes somewhat more power than the cascaded, multilevel, mixed refrigerant process discussed above. 
     First, it is difficult, if not impossible, to find a single mixed refrigerant composition which will generate a net heating curve closely following the typical natural gas cooling curves shown in  FIG. 1 . Such a refrigerant must be constituted from a range of relatively high and low boiling components, and their boiling temperatures are thermodynamically constrained by the phase equilibrium. In addition, higher boiling components are limited because they must not freeze out at the lowest temperatures. For these reasons, relatively large temperature differences necessarily occur at several points in the cooling process.  FIG. 2  shows typical composite heating and cooling curves for the process of the Swenson &#39;735 patent. 
     Second, for the single mixed refrigerant process, all of the components in the refrigerant are carried to the lowest temperature level even though the higher boiling components only provide refrigeration at the warmer end of the refrigerated portion of the process. This requires energy to cool and reheat these components which are “inert” at the lower temperatures. This is not the case with either the cascaded, multilevel, pure component refrigeration process or the cascaded, multilevel, mixed refrigerant process. 
     To mitigate this second inefficiency and also address the first, numerous solutions have been developed which separate a heavier fraction from a single mixed refrigerant, use the heavier fraction at the higher temperature levels of refrigeration, and then recombine it with the lighter fraction for subsequent compression. U.S. Pat. No. 2,041,725 to Podbielniak describes one way of doing this which incorporates several phase separation stages at below ambient temperatures. U.S. Pat. No. 3,364,685 to Perret; U.S. Pat. No. 4,057,972 to Sarsten, U.S. Pat. No. 4,274,849 to Garrier et al.; U.S. Pat. No. 4,901,533 to Fan et al.; U.S. Pat. No. 5,644,931 to Ueno et al.; U.S. Pat. No. 5,813,250 to Ueno et al; U.S. Pat. No. 6,065,305 to Arman et al.; U.S. Pat. No. 6,347,531 to Roberts et al. and U.S. Patent Application Publication 2009/0205366 to Schmidt also show variations on this theme. When carefully designed they can improve energy efficiency even though the recombining of streams not at equilibrium is thermodynamically inefficient. This is because the light and heavy fractions are separated at high pressure and then recombined at low pressure so they may be compressed together in the single compressor. Whenever streams are separated at equilibrium, separately processed and then recombined at non-equilibrium conditions, a thermodynamic loss occurs which ultimately increases power consumption. Therefore the number of such separations should be minimized. All of these processes use simple vapor/liquid equilibrium at various places in the refrigeration process to separate a heavier fraction from a lighter one. 
     Simple one stage vapor/liquid equilibrium separation, however, doesn&#39;t concentrate the fractions as much as may be accomplished using multiple equilibrium stages with reflux. Greater concentration allows greater precision in isolating a composition which will provide refrigeration over a specific range of temperatures. This enhances the process ability to follow the S-shaped cooling curves in  FIG. 1 . U.S. Pat. No. 4,586,942 to Gauthier and U.S. Pat. No. 6,334,334 to Stockmann et al. describe how fractionation may be employed in the above ambient compressor train to further concentrate the separated fractions used for refrigeration in different temperature zones and thus improve the overall process thermodynamic efficiency. A second reason for concentrating the fractions and reducing their temperature range of vaporization is to ensure that they are completely vaporized when they leave the refrigerated part of the process. This fully utilizes the latent heat of the refrigerant and precludes the entrainment of liquids into downstream compressors. For this same reason heavy fraction liquids are normally re-injected into the lighter fraction of the refrigerant as part of the process. Fractionation of the heavy fractions reduces flashing upon re-injection and improves the mechanical distribution of the two phase fluids. 
     As illustrated by U.S. Patent Application Publication No. 2007/0227185 to Stone et al., it is known to remove partially vaporized refrigeration streams from the refrigerated portion of the process. Stone et al. does this for mechanical reasons (not thermodynamic) and in the context of a cascaded, multilevel, mixed refrigerant process requiring two, separate, mixed refrigerants. In addition, the partially vaporized refrigeration streams are completely vaporized upon recombination with their previously separated vapor fractions immediately prior to compression. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graphical representation of temperature—enthalpy curves for methane at pressures of 35 bar and 60 bar and a mixture of methane and ethane at a pressure of 35 bar; 
         FIG. 2  is a graphical representation of the composite heating and cooling curves for a prior art process and system; 
         FIG. 3  is a process flow diagram and schematic illustrating an embodiment of the process and system of the invention; 
         FIG. 4  is a graphical representation of composite heating and cooling curves for the process and system of  FIG. 3   
         FIG. 5  is a process flow diagram and schematic illustrating a second embodiment of the process and system of the invention; 
         FIG. 6  is a process flow diagram and schematic illustrating a third embodiment of the process and system of the invention; 
         FIG. 7  is a process flow diagram and schematic illustrating a fourth embodiment of the process and system of the invention; 
         FIG. 8  is a graphical representation providing enlarged views of the warm end portions of the composite heating and cooling curves of  FIGS. 2 and 4 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In accordance with the invention, and as explained in greater detail below, simple equilibrium separation of a heavy fraction is sufficient to significantly improve the mixed refrigerant process efficiency if that heavy fraction isn&#39;t entirely vaporized as it leaves the primary heat exchanger of the process. This means that some liquid refrigerant will be present at the compressor suction and must beforehand be separated and pumped to a higher pressure. When the liquid refrigerant is mixed with the vaporized lighter fraction of the refrigerant, the compressor suction gas is greatly cooled and the required compressor power is further reduced. Equilibrium separation of the heavy fraction during an intermediate stage also reduces the load on the second or higher stage compressor(s), resulting in improved process efficiency. Heavy components of the refrigerant are also kept out of the cold end of the process, reducing the possibility of refrigerant freezing. 
     Furthermore, use of the heavy fraction in an independent pre-cool refrigeration loop results in near closure of heating/cooling curves at the warm end of the heat exchanger, giving a more efficient use of the refrigeration. This is best illustrated in  FIG. 8  where the curves from  FIGS. 2  (open curves) and  4  (closed curves) are plotted on the same axes with the temperature range limited to +40° C. to −40° C. 
     A process flow diagram and schematic illustrating an embodiment of the system and method of the invention is provided in  FIG. 3 . Operation of the embodiment will now be described with reference to  FIG. 3 . 
     As illustrated in  FIG. 3 , the system includes a multi-stream heat exchanger, indicated in general at  6 , having a warm end  7  and a cold end  8 . The heat exchanger receives a high pressure natural gas feed stream  9  that is liquefied in cooling passage  5  via removal of heat via heat exchange with refrigeration streams in the heat exchanger. As a result, a stream  10  of liquid natural gas product is produced. The multi-stream design of the heat exchanger allows for convenient and energy-efficient integration of several streams into a single exchanger. Suitable heat exchangers may be purchased from Chart Energy &amp; Chemicals, Inc. of The Woodlands, Tex. The plate and fin multi-stream heat exchanger available from Chart Energy &amp; Chemicals, Inc. offers the further advantage of being physically compact. 
     The system of  FIG. 3 , including heat exchanger  6 , may be configured to perform other gas processing options, indicated in phantom at  13 , known in the prior art. These processing options may require the gas stream to exit and reenter the heat exchanger one or more times and may include, for example, natural gas liquids recovery or nitrogen rejection. Furthermore, while the system and method of the present invention are described below in terms of liquefaction of natural gas, they may be used for the cooling, liquefaction and/or processing of gases other than natural gas including, but not limited to, air or nitrogen. 
     The removal of heat is accomplished in the heat exchanger using a single mixed refrigerant and the remaining portion of the system illustrated in  FIG. 3 . The refrigerant compositions, conditions and flows of the streams of the refrigeration portion of the system, as described below, are presented in Table 1 below. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Stream Table 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Stream Number 
                 9 
                 10 
                 12 
                 14 
                 18 
                 28 
                 46 
                 52 
                 58 
               
               
                   
               
               
                 Temperature, ° C.  
                 35.0  
                 −165.7 
                 4.8 
                 90.5 
                 35.0 
                 35.0 
                 122.8 
                 35.0 
                 35.0 
               
               
                 Pressure, BAR 
                 59.5 
                 59.1 
                 2.5 
                 14.0 
                 13.5 
                 13.5 
                 50.0 
                 49.5 
                 49.5 
               
               
                 Molar Rate, KGMOL/HR  
                 5,748 
                 5,748 
                 13,068  
                 13,068  
                 13,068 
                 10,699 
                 10,699 
                 10,699 
                 3,157 
               
               
                 Mass Rate, KG/HR 
                 92,903 
                 92,903 
                 478,405 
                 478,405 
                 478,405 
                 341,702 
                 341,702 
                 341,702 
                 137,246 
               
               
                 Liquid Mole Fraction 
                 0.0000 
                 1.0000 
                 0.0000 
                 0.0000  
                 0.1808 
                 0.0000 
                 0.0000 
                 0.2951 
                 1.0000 
               
               
                 Mole Percents 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 NITROGEN 
                 1.00 
                 1.00 
                 9.19 
                 9.19 
                 9.19 
                 11.15 
                 11.15 
                 11.15 
                 2.12 
               
               
                 METHANE 
                 99.00 
                 99.00 
                 24.20 
                 24.20 
                 24.20 
                 29.03 
                 29.03 
                 29.03 
                 11.37 
               
               
                 ETHANE 
                 0.00 
                 0.00 
                 35.41 
                 35.41 
                 35.41 
                 40.08  
                 40.08 
                 40.08 
                 39.05 
               
               
                 PROPANE 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 N-BUTANE 
                 0.00 
                 0.00 
                 21.45 
                 21.45 
                 21.45 
                 15.20  
                 15.20 
                 15.20 
                 35.14 
               
               
                 ISOBUTANE 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 ISOPENTANE 
                 0.00 
                 0.00 
                 9.75 
                 9.75 
                 9.75 
                 4.53 
                 4.53 
                 4.53 
                 12.31 
               
               
                   
               
               
                 Stream Number 
                 68 
                 74 
                 84 
                 24 
                 32 
                 34 
                 38 
                 42 
                 56 
               
               
                   
               
               
                 Temperature, ° C. 
                 −134.1  
                 −132.8 
                 4.8 
                 5.6 
                 35.0 
                 −79.2  
                 −78.7 
                 30.0 
                 35.0 
               
               
                 Pressure, BAR 
                 49.3 
                 2.8 
                 2.5 
                 13.5 
                 13.5 
                 13.3 
                 2.8 
                 2.6 
                 49.5 
               
               
                 Molar Rate, KGMOL/HR  
                 3,156 
                 3,156 
                 21 
                 21 
                 2,390 
                 2,391 
                 2,391 
                 2,391 
                 7,541 
               
               
                 Mass Rate, KG/HR 
                 137,183 
                 137,183 
                 1,317 
                 1,317 
                 138,020 
                 138,067 
                 138,067 
                 138,067 
                 204,455 
               
               
                 Liquid Mole Fraction 
                 1.0000 
                 0.9821 
                 1.0000 
                 1.0000  
                 1.0000  
                 1.0000  
                 1.0000 
                 0.3891 
                 0.0000 
               
               
                 Mole Percents 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 NITROGEN 
                 2.12 
                 2.12 
                 0.04 
                 0.04 
                 0.32 
                 0.32 
                 0.32 
                 0.32 
                 14.94 
               
               
                 METHANE 
                 11.37 
                 11.37 
                 0.43 
                 0.43 
                 2.35 
                 235 
                 2.35 
                 235 
                 36.43 
               
               
                 ETHANE 
                 39.05 
                 39.05 
                 4.14 
                 4.14 
                 14.24 
                 14.24 
                 14.24 
                 14.24 
                 40.51 
               
               
                 PROPANE 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 N-BUTANE 
                 35.14 
                 35.14 
                 42.13 
                 42.13 
                 49.63 
                 49.63  
                 49.63 
                 49.63 
                 6.84 
               
               
                 ISOBUTANE 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 ISOPENTANE 
                 12.31 
                 12.31 
                 53.25 
                 53.25 
                 33.47 
                 33.46  
                 33.46 
                 33.46 
                 1.28 
               
               
                   
               
             
          
           
               
                   
                 Stream Number 
                 62 
                 66 
                 67 
                 76 
                 78 
               
               
                   
               
               
                   
                 Temperature, ° C. 
                 −165.7  
                 −169.7 
                 −128.6 
                 −128.5 
                 30.0 
               
               
                   
                 Pressure, BAR 
                 49.3 
                 3.0 
                 2.8 
                 2.8 
                 2.6 
               
               
                   
                 Molar Rate, KGMOL/HR 
                 7,542 
                 7,542 
                 7,542 
                 10,698  
                 10,698 
               
               
                   
                 Mass Rate, KG/HR 
                 204,471 
                 204,471 
                 204,471 
                 341,655 
                 341,655 
               
               
                   
                 Liquid Mole Fraction 
                 1.0000 
                 0.9132 
                 0.5968  
                 0.7257  
                 0.0000 
               
               
                   
                 Mole Percents 
                   
                   
                   
                   
                   
               
               
                   
                 NITROGEN 
                 14.94 
                 14.94 
                 14.94 
                 11.16  
                 11.16 
               
               
                   
                 METHANE 
                 36.43 
                 36.43 
                 36.43 
                 29.04  
                 29.04 
               
               
                   
                 ETHANE 
                 40.51 
                 40.51 
                 40.51 
                 40.08  
                 40.08 
               
               
                   
                 PROPANE 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                   
                 N-BUTANE 
                 6.84 
                 6.84 
                 6.84 
                 15.19 
                 15.19 
               
               
                   
                 ISOBUTANE 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                   
                 ISOPENTANE 
                 1.28 
                 1.28 
                 1.28 
                 4.53 
                 4.53 
               
               
                   
               
             
          
         
       
     
     With reference to the upper right portion of  FIG. 3 , a first stage compressor  11  receives a low pressure vapor refrigerant stream  12  and compresses it to an intermediate pressure. The stream  14  then travels to a first stage after-cooler  16  where it is cooled. After-cooler  16  may be, as an example, a heat exchanger. The resulting intermediate pressure mixed phase refrigerant stream  18  travels to interstage drum  22 . While an interstage drum  22  is illustrated, alternative separation devices may be used, including, but not limited to, another type of vessel, a cyclonic separator, a distillation unit, a coalescing separator or mesh or vane type mist eliminator. Interstage drum  22  also receives an intermediate pressure liquid refrigerant stream  24  which, as will be explained in greater detail below, is provided by pump  26 . In an alternative embodiment, stream  24  may instead combine with stream  14  upstream of after-cooler  16  or stream  18  downstream of after-cooler  16 . 
     Streams  18  and  24  are combined and equilibrated in interstage drum  22  which results in separated intermediate pressure vapor stream  28  exiting the vapor outlet of the drum  22  and intermediate pressure liquid stream  32  exiting the liquid outlet of the drum. Intermediate pressure liquid stream  32 , which is warm and a heavy fraction, exits the liquid side of drum  22  and enters pre-cool liquid passage  33  of heat exchanger  6  and is subcooled by heat exchange with the various cooling streams, described below, also passing through the heat exchanger. The resulting stream  34  exits the heat exchanger and is flashed through expansion valve  36 . As an alternative to the expansion valve  36 , another type of expansion device could be used, including, but not limited to, a turbine or an orifice. The resulting stream  38  reenters the heat exchanger  6  to provide additional refrigeration via pre-cool refrigeration passage  39 . Stream  42  exits the warm end  7  of the heat exchanger as a two-phase mixture with a significant liquid fraction. 
     Intermediate pressure vapor stream  28  travels from the vapor outlet of drum  22  to second or last stage compressor  44  where it is compressed to a high pressure. Stream  46  exits the compressor  44  and travels through second or last stage after-cooler  48  where it is cooled. The resulting stream  52  contains both vapor and liquid phases which are separated in accumulator drum  54 . While an accumulator drum  54  is illustrated, alternative separation devices may be used, including, but not limited to, another type of vessel, a cyclonic separator, a distillation unit, a coalescing separator or mesh or vane type mist eliminator. High pressure vapor refrigerant stream  56  exits the vapor outlet of drum  54  and travels to the warm side of the heat exchanger  6 . High pressure liquid refrigerant stream  58  exists the liquid outlet of drum  54  and also travels to the warm end of the heat exchanger  6 . It should be noted that first stage compressor  11  and first stage after-cooler  16  make up a first compression and cooling cycle while last stage compressor  44  and last stage after-cooler  48  make up a last compression and cooling cycle. It should also be noted, however, that each cooling cycle stage could alternatively features multiple compressors and/or after-coolers. 
     Warm, high pressure, vapor refrigerant stream  56  is cooled, condensed and subcooled as it travels through high pressure vapor passage  59  of the heat exchanger  6 . As a result, stream  62  exits the cold end of the heat exchanger  6 . Stream  62  is flashed through expansion valve  64  and re-enters the heat exchanger as stream  66  to provide refrigeration as stream  67  traveling through primary refrigeration passage  65 . As an alternative to the expansion valve  64 , another type of expansion device could be used, including, but not limited to, a turbine or an orifice. 
     Warm, high pressure liquid refrigerant stream  58  enters the heat exchanger  6  and is subcooled in high pressure liquid passage  69 . The resulting stream  68  exits the heat exchanger and is flashed through expansion valve  72 . As an alternative to the expansion valve  72 , another type of expansion device could be used, including, but not limited to, a turbine or an orifice. The resulting stream  74  re-enters the heat exchanger  6  where it joins and is combined with stream  67  in primary refrigeration passage  65  to provide additional refrigeration as stream  76  and exit the warm end of the heat exchanger  6  as a superheated vapor stream  78 . 
     Superheated vapor stream  78  and stream  42  which, as noted above, is a two-phase mixture with a significant liquid fraction, enter low pressure suction drum  82  through vapor and mixed phase inlets, respectively, and are combined and equilibrated in the low pressure suction drum. While a suction drum  82  is illustrated, alternative separation devices may be used, including, but not limited to, another type of vessel, a cyclonic separator, a distillation unit, a coalescing separator or mesh or vane type mist eliminator. As a result, a low pressure vapor refrigerant stream  12  exits the vapor outlet of drum  82 . As stated above, the stream  12  travels to the inlet of the first stage compressor  11 . The blending of mixed phase stream  42  with stream  78 , which includes a vapor of greatly different composition, in the suction drum  82  at the suction inlet of the compressor  11  creates a partial flash cooling effect that lowers the temperature of the vapor stream traveling to the compressor, and thus the compressor itself, and thus reduces the power required to operate it. 
     A low pressure liquid refrigerant stream  84 , which has also been lowered in temperature by the flash cooling effect of mixing, exits the liquid outlet of drum  82  and is pumped to intermediate pressure by pump  26 . As described above, the outlet stream  24  from the pump travels to the interstage drum  22 . 
     As a result, in accordance with the invention, a pre-cool refrigerant loop, which includes streams  32 ,  34 ,  38  and  42 , enters the warm side of the heat exchanger  6  and exits with a significant liquid fraction. The partially liquid stream  42  is combined with spent refrigerant vapor from stream  78  for equilibration and separation in suction drum  82 , compression of the resultant vapor in compressor  11  and pumping of the resulting liquid by pump  26 . The equilibrium in suction drum  82  reduces the temperature of the stream entering the compressor  11 , by both heat and mass transfer, thus reducing the power usage by the compressor. 
     Composite heating and cooling curves for the process in  FIG. 3  are shown in  FIG. 4 . Comparison with the curves of  FIG. 2  for an optimized, single mixed refrigerant, process, similar to that described in U.S. Pat. No. 4,033,735 to Swenson, shows that the composite heating and cooling curves have been brought closer together thus reducing compressor power by about 5%. This helps reduce the capital cost of a plant and reduces energy consumption with associated environmental emissions. These benefits can result in several million dollars savings a year for a small to middle sized liquid natural gas plant. 
       FIG. 4  also illustrates that the system and method of  FIG. 3  results in near closure of the heat exchanger warm end of the cooling curves (see also  FIG. 8 ). This occurs because the intermediate pressure heavy fraction liquid boils at a higher temperature than the rest of the refrigerant and is thus well suited for the warm end heat exchanger refrigeration. Boiling the intermediate pressure heavy fraction liquid separately from the lighter fraction refrigerant in the heat exchanger allows for an even higher boiling temperature, which results in an even more “closed” (and thus more efficient) warm end of the curve. Furthermore, keeping the heavy fraction out of the cold end of the heat exchanger helps prevent the occurrence of freezing. 
     It should be noted that the embodiment described above is for a representative natural gas feed at supercritical pressure. The optimal refrigerant composition and operating conditions will change when liquefying other, less pure, natural gases at different pressures. The advantage of the process remains, however, because of its thermodynamic efficiency. 
     A process flow diagram and schematic illustrating a second embodiment of the system and method of the invention is provided in  FIG. 5 . In the embodiment of  FIG. 5 , the superheated vapor stream  78  and two-phase mixed stream  42  are combined in a mixing device, indicated at  102 , instead of the suction drum  82  of  FIG. 3 . The mixing device  102  may be, for example, a static mixer, a single pipe segment into which streams  78  and  42  flow, packing or a header of the heat exchanger  6 . After leaving mixing device  102 , the combined and mixed streams  78  and  42  travel as stream  106  to a single inlet of the low pressure suction drum  104 . While a suction drum  104  is illustrated, alternative separation devices may be used, including, but not limited to, another type of vessel, a cyclonic separator, a distillation unit, a coalescing separator or mesh or vane type mist eliminator. When stream  106  enters suction drum  104 , vapor and liquid phases are separated so that a low pressure liquid refrigerant stream  84  exits the liquid outlet of drum  104  while a low pressure vapor stream  12  exits the vapor outlet of drum  104 , as described above for the embodiment of  FIG. 3 . The remaining portion of the embodiment of  FIG. 5  features the same components and operation as described for the embodiment of  FIG. 3 , although the data of Table 1 may differ. 
     A process flow diagram and schematic illustrating a third embodiment of the system and method of the invention is provided in  FIG. 6 . In the embodiment of  FIG. 6 , the two-phase mixed stream  42  from the heat exchanger  6  travels to return drum  120 . The resulting vapor phase travels as return vapor stream  122  to a first vapor inlet of low pressure suction drum  124 . Superheated vapor stream  78  from the heat exchanger  6  travels to a second vapor inlet of low pressure suction drum  124 . The combined stream  126  exits the vapor outlet of suction drum  124 . The drums  120  and  124  may alternatively be combined into a single drum or vessel that performs the return separator drum and suction drum functions. Furthermore, alternative types of separation devices may be substituted for drums  120  and  124 , including, but not limited to, another type of vessel, a cyclonic separator, a distillation unit, a coalescing separator or mesh or vane type mist eliminator. 
     A first stage compressor  131  receives the low pressure vapor refrigerant stream  126  and compresses it to an intermediate pressure. The compressed stream  132  then travels to a first stage after-cooler  134  where it is cooled. Meanwhile, liquid from the liquid outlet of return separator drum  120  travels as return liquid stream  136  to pump  138 , and the resulting stream  142  then joins stream  132  upstream from the first stage after-cooler  134 . 
     The intermediate pressure mixed phase refrigerant stream  144  leaving first stage after-cooler  134  travels to interstage drum  146 . While an interstage drum  146  is illustrated, alternative separation devices may be used, including, but not limited to, another type of vessel, a cyclonic separator, a distillation unit, a coalescing separator or mesh or vane type mist eliminator. A separated intermediate pressure vapor stream  28  exits the vapor outlet of the interstage drum  146  and an intermediate pressure liquid stream  32  exits the liquid outlet of the drum. Intermediate pressure vapor stream  28  travels to second stage compressor  44 , while intermediate pressure liquid stream  32 , which is a warm and heavy fraction, travels to the heat exchanger  6 , as described above with respect to the embodiment of  FIG. 3 . The remaining portion of the embodiment of  FIG. 6  features the same components and operation as described for the embodiment of  FIG. 3 , although the data of Table 1 may differ. The embodiment of  FIG. 6  does not provide any cooling at drum  124 , and thus no cooling of the first stage compressor suction stream  126 . In terms of improving efficiency, however, the cool compressor suction stream is traded for a reduced vapor molar flow rate to the compressor suction. The reduced vapor flow to the compressor suction provides a reduction in the compressor power requirement that is roughly equivalent to the reduction provided by the cooled compressor suction stream of the embodiment of  FIG. 3 . While there is an associated increase in the power requirement of pump  138 , as compared to pump  26  in the embodiment of  FIG. 3 , the pump power increase is very small (approximately 1/100) compared to the savings in compressor power. 
     In a fourth embodiment of the system and method of the invention, illustrated in  FIG. 7 , the system of  FIG. 3  is optionally provided with one or more pre-cooling systems, indicated at  202 ,  204  and/or  206 . Of course the embodiments of  FIG. 5 or 6 , or any other embodiment of the system of the invention, could be provided with the pre-cooling systems of  FIG. 7 . Pre-cooling system  202  is for pre-cooling the natural gas stream  9  prior to heat exchanger  6 . Pre-cooling system  204  is for interstage pre-cooling of mixed phase stream  18  as it travels from first stage after-cooler  16  to interstage drum  22 . Pre-cooling system  206  is for discharge pre-cooling of mixed phase stream  52  as it travels to accumulator drum  54  from second stage after-cooler  48 . The remaining portion of the embodiment of  FIG. 7  features the same components and operation as described for the embodiment of  FIG. 3 , although the data of Table 1 may differ. 
     Each one of the pre-cooling systems  202 ,  204  or  206  could be incorporated into or rely on heat exchanger  6  for operation or could include a chiller that may be, for example, a second multi-stream heat exchanger. In addition, two or all three of the pre-cooling systems  202 ,  204  and/or  206  could be incorporated into a single multi-stream heat exchanger. While any pre-cooling system known in the art could be used, the pre-cooling systems of  FIG. 7  each preferably includes a chiller that uses a single component refrigerant, such as propane, or a second mixed refrigerant as the pre-cooling system refrigerant. More specifically, the well-known propane C3-MR pre-cooling process or dual mixed refrigerant processes, with the pre-cooling refrigerant evaporated at either a single pressure or multiple pressures, could be used. Examples of other suitable single component refrigerants include, but are not limited to, N-butane, iso-butane, propylene, ethane, ethylene, ammonia, freon or water. 
     In addition to being provided with a pre-cooling system  202 , the system of  FIG. 7  (or any of the other system embodiments) could serve as a pre-cooling system for a downstream process, such as a liquefaction system or a second mixed refrigerant system. The gas being cooled in the cooling passage of the heat exchanger also could be a second mixed refrigerant or a single component mixed refrigerant. 
     While the preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the appended claims.

Technology Classification (CPC): 5