Abstract:
The present invention is directed to methods and apparatus for improving the recovery of the relatively less volatile components from a methane-rich gas feed under pressure to produce an NGL product while, at the same time, separately recovering the relatively more volatile components which are liquified to produce an LNG product. The methods of the present invention improve separation and efficiency within the NGL recovery column while maintaining column pressure to achieve efficient and economical utilization of the available mechanical refrigeration. The methods of the present invention are particularly useful for removing cyclohexane, benzene and other hazardous, heavy hydrocarbons from a gas feed. The benefits of the present invention are achieved by the introduction to the NGL recovery column of an enhanced liquid reflux lean on the NGL components. Further advantages can be achieved by thermally linking a side reboiler for the NGL recovery column with the overhead condenser for the NGL purifying column. Using the methods of the present invention, recoveries of propane and heavier components in excess of 95% are readily achievable.

Description:
This Appln claims benefit of Prov. No. 60/205,332 filed May 18, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates generally to methods and apparatus for high recovery of hydrocarbon liquids from methane-rich natural gases and other gases, e.g., refinery gases. More particularly, the present invention provides methods and apparatus for more efficiently and economically achieving high recovery of ethane, propane, propylene and heavier hydrocarbon liquids (C 2+  hydrocarbons) in association with liquified natural gas production. 
     II. Description of the Background 
     Due to its clean burning characteristics and the implementation of more stringent environmental regulations, the projected demand for natural gas has been increasing during recent years. In addition to methane, natural gas includes some heavier hydrocarbons and impurities, e.g., carbon dioxide, nitrogen, helium, water and non-hydrocarbon acid gases. After compression and separation of these impurities, natural gas may be further processed to separate and recover heavier hydrocarbons as natural gas liquids (NGL) and produce pipeline quality methane. The pipeline quality methane is then delivered to gas pipelines as the sales gas ultimately transmitted to end-users. 
     In the case of remote gas production or distant gas markets, transportation of produced natural gas via gas pipeline might not be economical or even feasible. Accordingly, liquifaction of natural gas has become a viable and widely adopted scheme. The economics of liquifying natural gas is feasible due mainly to the great reduction in volume as the gas is converted to a liquified state, making it easy to store and transport. Another advantage of converting the produced natural gas to a liquified form is that the produced LNG can be economically stored to supplement energy suppliers during seasonal peak demand periods. Liquified natural gas, typically stored at atmospheric pressures and at temperatures of approximately −260° F., is transported to distant markets via refrigerated tankers. 
     Processes for the liquifaction of natural gas are well known in the art. Natural gas comprising predominantly methane enters an LNG plant at elevated pressures and is pretreated to produce a purified feed stock suitable for liquifaction at cryogenic temperatures. The pretreatment typically includes removal of acid gases, e.g., hydrogen sulfide and carbon dioxide, together with other contaminants, including moisture and mercury. The purified gas is thereafter processed through a plurality of cooling stages using indirect heat exchange with one or more refrigerants to progressively reduce its temperature until total liquifaction is achieved. The pressurized liquid natural gas is sub-cooled to reduce flashed vapor through one or more expansion stages to final atmospheric pressure suitable for storage and transportation. The flashed vapor from each expansion stage, together with the boil off gas produced as a result of heat gain, are collected and used as a source of plant fuel gas with any excess recycled to the liquifaction process. 
     Because a significant amount of refrigeration energy is required for liquifying natural gas, the refrigeration system becomes one of the major units in an LNG facility. Mechanical refrigeration cycles mostly in closed circuit are often employed in LNG projects. A number of liquifaction processes have been developed with the differences mainly found in the refrigeration cycles used. The most commonly used LNG processes can be classified into three categories as follows: 
     1) The cascade process presenting the benefits of easy start-up or shutdown. The cascade process consists of successive refrigeration cycles using propane, ethane or ethylene, and methane. The thermal efficiency can be readily enhanced by the use of multi-compressor stages. U.S. Pat. No. 5,669,234, incorporated herein by reference, represents an exemplary cascade process. 
     2) The propane pre-cooled mixed refrigerant process involves the use of a multi-component mixture of hydrocarbons, typically comprising propane, ethane, methane, and optionally other light components in one cycle, and a separate propane refrigeration cycle to provide pre-cooling of natural gas and the mixed refrigerant to approximately −35° F. The propane mixed refrigerant process advantageously provides improved thermal efficiency. However, a significant disadvantage results from the use of extremely large spiral wound exchangers. Such exchangers are a long lead item requiring special facilities in the field to manufacture. Examples of the propane mixed refrigerant process include those disclosed in U.S. Pat. Nos. 4,404,008 and 4,445,916, incorporated herein by reference. 
     3) The single, mixed refrigerant process includes heavier hydrocarbons, e.g., butanes and pentanes, in the multi-component mixture and eliminates the pre-cooled propane refrigeration cycle. It presents the simplicity of single compression in the heat exchanger line and is particularly advantageous for small LNG plants. U.S. Pat. No. 4,033,735, incorporated herein by reference, represents an exemplary single, mixed refrigerant process. 
     The use of a turbo expander in combination with mechanical refrigeration cycles has also been adopted in many LNG processes. Examples of the use of a turbo expander are disclosed in U.S. Pat. Nos. 3,724,226; 4,065,278; 5,755,114; 4,970,867, 5,537,827; and Int&#39;l Patent No. WO 95/27179. 
     In addition to methane, natural gas typically contains various amounts of ethane, propane and heavier hydrocarbons. The composition varies significantly depending on the source of the gas and gas reserve characteristics. Hydrocarbons heavier than methane need to be removed from LNG for various reasons. Hydrocarbons heavier than pentane, including aromatics, having high freezing points must be reduced to an extremely low level to prevent the freezing and plugging of process equipment in the course of cooling and liquifaction steps. After separation of these heavy components from LNG, they provide excellent gasoline blending stock. Many patents have been directed to methods for removal of these heavy hydrocarbons. For instance, U.S. Pat. No. 5,325,673 discloses the use of a single scrub column in the pretreatment step operated substantially as an absorption column to remove freezable C 5+  components from a natural gas stream feeding to an LNG facility. The heavy liquid recovered subsequently can be fractionated into various fractions for use as make-up refrigerants. U.S. Pat. No. 5,737,940 describes an exemplary system incorporated in a cascade process. 
     Besides being liquified as part of LNG and used as fuel, lighter natural gas liquid (NGL) components, e.g., hydrocarbons having 2-4 carbon atoms, can also be a source of feedstock to refineries or petrochemical plants. Therefore, it is often desirable to maximize the recovery of NGL to enhance revenue. To achieve high recovery of these components, it is common practice to design an NGL recovery plant so that the tail gas produced by the NGL recovery plant and comprising primarily methane is delivered to the LNG facility for liquifaction. U.S. Pat. Nos. 5,291,736 and 5,950,453 are typical examples of such combined facilities. 
     Among several different NGL recovery processes, the cryogenic expansion process has become the preferred process for deep hydrocarbon liquid recovery. In a conventional turbo-expander process, the feed gas at elevated pressure is pretreated for the removal of acid gases, moisture and other contaminants to produce a purified feed stock suitable for further processing at cryogenic temperatures. The purified feed gas is then cooled to partial condensation by heat exchange with other process streams and/or external propane refrigeration, depending upon the richness of the gas. The condensed liquid after removal of the less volatile components is then separated and fed to a fractionation column, operated at medium or low pressure, to recover the heavy hydrocarbon constituents desired. The remaining non-condensed vapor portion is turbo-expanded to a lower pressure, resulting in further cooling and additional liquid condensation. With the expander discharge pressure typically the same as the column pressure, the resultant two-phase stream is fed to the top section of the fractionation column where the cold liquids act as the top reflux to enhance recovery of heavier hydrocarbon components. The remaining vapor combines with the column overhead as a residue gas, which is then recompressed to a higher pressure suitable for pipeline delivery or for liquifaction in an LNG facility after being heated to recover available refrigeration. 
     Because a column operated as described above acts mainly as a stripping column, the expander discharge vapor leaving the column overhead that is not subject to rectification still contains many heavy components. These components could be further recovered through an additional rectification step. Ongoing efforts attempting to achieve a higher liquid recovery have mostly concentrated on the addition of a rectification section and the generation of an enhanced reflux stream to the expanded vapor. Many patents exist purporting to disclose an improved design for recovering ethane and heavier components in an NGL plant. For example, see U.S. Pat. Nos. 4,140,504; 4,251,249; 4,278,457; 4,657,571; 4,690,702; 4,687,499; 4,851,020; and 5,568,737. At best, these processes are capable of recovering 95%+ of ethane and heavier hydrocarbons. However, they typically involve a significant capital expenditure during construction of the NGL plant as well as increased operational costs during its lifetime. 
     It will be recognized that all NGL components have higher condensing temperatures than methane so that all will be liquified in the course of operating an LNG process. A substantial cost savings may be realized, if the NGL recovery could be effectively integrated within the liquifaction process instead of building a separate facility. 
     Recovery of NGL in the LNG facility has also been suggested in the literature. For example, it has been suggested that lighter NGL components could be recovered in conjunction with the removal of C 5+  hydrocarbons by using a scrub column in a propane pre-cooled, mixed refrigerant process. See U.S. Pat. Nos. 4,445,917 and 5,325,673. A cryogenic stripping column in a cascaded process was suggested in U.S. Pat. No. 5,737,940 for recovery of heavy hydrocarbons from a natural gas feed stream. In a further modification, U.S. Pat. Nos. 5,950,453 and 5,016,665 disclose a method wherein a demethanizer is incorporated in the process for liquifying natural gas for recovering heavier hydrocarbon liquids. 
     The NGL recovery column in these systems is often required to operate at a relatively high pressure, typically above 550 psig, in order to maintain an efficient and economical utilization of mechanical refrigeration employed in the LNG process. While benefitting from lower refrigeration energy by maintaining a high liquifaction pressure, the separation efficiency within the recovery column may be significantly reduced due to less favorable separating conditions, i.e., lower relative volatility inside the column. In addition, prior art processes fail to effectively provide reflux to the recovery column. As a result, none of these processes are capable of efficiently maintaining a high NGL recovery, i.e., the NGL recovery does not typically exceed 80% with these processes. 
     As can be seen from the foregoing description, those skilled in the art have long sought methods and apparatus for improving the efficiency and economy of processes for separating and recovering ethane and heavier natural gas liquids in an NGL plant. While prior art methods have been capable of recovering more than 95% of the ethane and heavier hydrocarbons in a stand-alone NGL recovery plant, those processes fail to maintain the same recovery when integrated with an LNG facility. Accordingly, there has been a long felt but unfulfilled need for more efficient, more economical methods of integrating these processes while improving, or at least maintaining, their economics. 
     SUMMARY OF THE INVENTION 
     The present invention provides an integrated process for recovery of the components of a feed gas containing methane and heavier hydrocarbons while maximizing NGL recovery and minimizing capital expenditures and operating costs incurred with the LNG facility. The present invention is also intended to improve separation efficiency within an NGL recovery column while maintaining column pressure as high as practically possible to achieve an efficient and economical utilization of mechanical refrigeration in the liquifaction process. This is achieved by the introduction of an enhanced liquid reflux specifically suitable for the purpose of the recovery column. 
     Historically, the price of liquid ethane has been cyclical, rising and falling in response to the demand for use as petrochemical feed stock. When the price of liquid ethane is high, gas processors can generate additional revenues by increasing the recovery of ethane. On the other hand, when the ethane market is depressed, it may be desirable to effectively reject ethane, allowing it to remain in the LNG, but still maintain high recovery of propane and heavier components. Due to the cyclical nature of the liquid ethane market, designing a facility which can selectively and efficiently recover or reject ethane will allow producers to quickly respond to changing market conditions, a phenomenon that seems to occur ever more frequently in today&#39;s market. Accordingly, the present invention is designed to permit flexible transition between operation for ethane recovery or ethane rejection. 
     A number of liquifaction processes developed in the prior art have been described above. These processes may differ significantly depending on the mechanical refrigeration cycle used. The methods of the present invention may be integrated with any of those processes. The methods of the present invention are applicable independent of the type of mechanical refrigeration used in the LNG process. 
     The present invention, in the broadest sense, provides an integrated process and apparatus for cryogenically recovering ethane, propane and heavier components during natural gas liquifaction processes via a distillation column, in which the reflux derived from various sources in the liquifaction process is essentially free of the components to be recovered. The provision of an enhanced liquid reflux, which is lean on the NGL components, to the distillation column permits a high recovery of NGL components even when the column is operated at a relatively high pressure. The process involves introducing a cooled gas/condensate feed into a first distillation column, e.g., an NGL recovery column, at one or more feed trays. The gas/condensate feed is separated into a first liquid stream primarily comprising NGL components to be recovered and a methane rich overhead stream essentially free of NGL components. The methane-rich overhead stream is further cooled to total liquifaction. Preferably the liquified methane-rich stream is further sub-cooled. This liquified, and preferably sub-cooled, methane-rich stream under pressure is subsequently flashed to near atmospheric pressure in one or more steps with the liquid collected in the final flash step being delivered to the LNG tank for storage. The flashed vapor is heated and compressed to a higher pressure for delivery as fuel gas. Excess flashed vapor, if any, is recycled to the liquifaction process in which it is ultimately liquified as pressurized LNG or as liquid reflux to the NGL recovery column. The first liquid stream is introduced into a second distillation column, e.g., an NGL purifying column, at one or more feed trays. In the second column, the first liquid stream is separated into an NGL product stream from the bottom and a first vapor portion primarily comprising all of the remaining lighter components from the overhead. 
     In one embodiment of the present invention, the first vapor portion is combined with at least a portion of the excess flashed vapor. The combined stream is compressed and cooled to substantial condensation and thereafter introduced to the top of the NGL recovery column as a liquid reflux. This reflux stream will contain an extremely low concentration of the heavy components to be recovered in the NGL product. This stream enhances the recovery efficiency within the column and reduces the loss of NGL components in the methane-rich overhead stream to a minimum. A high NGL recovery is therefore achieved even with a relatively high operating pressure, i.e., a pressure of about 600 psig, for the NGL recovery column. 
     The economic advantages of the present invention can be further improved by thermally linking a side reboiler for the first distillation column with the overhead condenser for the second distillation column. More specifically, the first vapor portion is cooled in countercurrent heat exchange with a liquid withdrawn from a tray located below the feed trays of the first distillation column. The cooled first vapor portion is separated into a liquid fraction for introduction into the second distillation column as a top liquid reflux and a lighter, vapor fraction with further reduced NGL components for introduction into the first distillation column as a top reflux. Thus the NGL recovery efficiency in the second column is enhanced. The heat carried by the liquid withdrawn returns to the first distillation column where it provides a stripping action in the bottom portion of the column, thereby reducing volatile components, e.g., methane, in the first liquid stream from the bottom. 
     The recovery efficiency can be improved in another embodiment of the present invention by introduction of a second liquid reflux to the upper, rectification section of the first distillation column. The second reflux enters the distillation column preferably in the middle of the rectification section, as a middle reflux which provides a bulk rectification effect and reduces the NGL components to be recovered in the up-flow vapor stream. Any residual NGL components in the upward vapor stream can be recovered by the top and leaner liquid reflux. A slipstream from the feed gas can be taken and cooled to substantial condensation or even sub-cooling to form the second liquid reflux. In some cases, the feed gas contains much heavier components, e.g., hexane and aromatics, which tend to freeze at cryogenic temperatures. The feed gas can be first cooled to partial condensation where most of these components will be condensed in the liquid and separated out in a separator. A slipstream can then be taken from the non-condensed vapor portion and further cooled to substantial condensation to form the second liquid reflux. Optionally, this liquid reflux can be fed to the top of the NGL recovery column. 
     In another embodiment of the present invention, the top reflux to the first distillation column is generated by recycling a small portion of LNG under pressure prior to undergoing flashing. This reflux also has an extremely low content of the NGL components to be recovered and, accordingly, enhances separation efficiency. This reflux scheme can be advantageous for the liquifaction process where the LNG can be deeply sub-cooled using very cold mechanical refrigeration to reduce the vapor produced in the flashing steps to a minimum. Typical examples of this embodiment include liquifaction processes using mixed refrigerant with or without propane pre-cooling, cascaded refrigeration in a closed circuit. 
     Another feature providing a significant economic advantage in the present invention is the cooling of the feed gas by countercurrent heat exchange with a refrigerant stream comprising a portion of the first liquid stream or liquid withdrawn from the lower portion of the first distillation column. As a result, the refrigerant stream is partially vaporized and may be separated into a second liquid stream for introduction into the second distillation column and a second vapor stream for introduction into the first distillation column as a stripping gas after compression and cooling. The introduction of stripping gas supplements the heat requirements in the first distillation column for stripping volatile components off the first liquid stream. It also enhances the relative volatility of the key components and, accordingly, the separation efficiency in the column, particularly when the column is operated at a relatively high pressure as in the NGL recovery column of the present invention. 
     The methods and apparatus of the present invention efficiently integrate NGL recovery into the natural gas liquifaction process and permit high recoveries of propane and heavier components, e.g., recoveries exceeding 95% of those components originally present in the feed gas. In fact, the methods of the present invention, properly optimized, permit the recovery of at least 99% of the propane and heavier hydrocarbons originally found in the feed gas. The high recovery of heavier hydrocarbons achieved with the methods of the present invention may be advantageously used to clean gas feeds contaminated by cyclohexane, benzene and other heavy hydrocarbons which have been determined to create potential freezing problems and, accordingly, must be thoroughly removed. This high NGL recovery is achieved while eliminating the NGL plant, as typically employed in the prior art, in the front-end of the LNG facility. Thus, significant savings of capital, as well as operating costs, are achieved. In addition, the flexible design of the present invention permits an easy transition between operations designed to either recover or reject ethane in order to accommodate rapidly changing values of liquid ethane. The integration methods proposed in the present invention can also be easily adapted for use with any liquifaction process regardless the refrigeration system used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The application and advantages of the present invention will become more apparent by referring to the following detailed description in connection with the accompanying drawings, wherein: 
     FIG. 1 illustrates a schematic representation of a system employing a natural gas liquifaction process incorporating the improvements of the present invention; 
     FIG. 2 illustrates a schematic representation of a system employing a typical propane pre-cooled, mixed refrigeration process incorporating the improvements of the present invention for liquifying natural gas; 
     FIG. 3 illustrates a schematic representation of a system employing a typical single, mixed refrigeration process incorporating the improvements of the present invention for liquifying natural gas; 
     FIG. 4 illustrates an alternative embodiment of a system employing a natural gas liquifaction process incorporating the improvements of the present invention and introducing a second liquid reflux; 
     FIG. 5 illustrates another alternative embodiment of a system employing a natural gas liquifaction process incorporating the improvements of the present invention and introducing as a liquid reflux a portion of liquified natural gas recycled under pressure; 
     FIG. 6 illustrates another alternative embodiment of a system employing a natural gas liquifaction process incorporating the improvements of the present invention and introducing a stripping gas refrigeration system; and 
     FIG. 7 illustrates another alternative embodiment of a system employing a natural gas liquifaction process incorporating the improvements of the present invention and employing a simplified NGL purifying system. 
    
    
     While the invention will be described in connection with the presently preferred embodiment, it will be understood that this is not intended to limit the invention to that embodiment. To the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included in the spirit of the invention as defined in the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention permits the separation and recovery of substantially all of the NGL components, i.e., ethane, propane and heavier hydrocarbons, from a compressed natural gas in an LNG process. The present invention achieves these high recoveries while eliminating the need for a separate NGL plant in the front-end of the LNG facilities by introducing to the distillation column an enhanced liquid reflux having an extremely low content of the NGL components to be recovered. The introduction of lean reflux permits the column to be operated at higher pressures while still maintaining high recovery of NGL and, accordingly, the refrigeration system can be utilized more efficiently in the liquifaction process. As a result of this more efficient integration, the capital requirements, as well as operating costs, for recovering substantially all of the NGL components present in the feed gas in an LNG process may be greatly reduced. 
     The foregoing merely provides an exemplary description of the use of the present invention in a conventional system for liquifying inlet gas and should not be considered as limiting the methods of the present invention. While various values of temperature, pressure and composition are recited in association with the specific examples described below, those conditions are approximate and merely illustrative, and are not meant to limit the invention. For purposes of this invention, when the term lean reflux is used with respect to a distillation column, it refers to the components to be recovered in the bottom liquid stream. For example, a lean reflux for recovery of propane and heavy hydrocarbons means that the reflux stream has a low content of the cited components. Furthermore, with respect to the terms upper and lower as used with respect to a distillation column, these terms are to be understood as relative to one another, i.e., that withdrawal of a stream from an upper region of a column is at a higher position than a stream withdrawn from a lower region thereof. In an exemplary, but non-limiting embodiment, upper may refer to the upper half of a column, whereas lower may refer to the lower half of a column. In another embodiment, where the term middle is used, it is to be understood that a middle region is intermediate to an upper region and a lower region. However, where upper, middle, and lower are used to refer to a cryogenic distillation column, it should not be understood that the column is strictly divided into thirds by these terms. 
     FIG. 1 illustrates a schematic configuration of one exemplary embodiment of the invention where at least about 95%, preferably above 98%, of the propane, propylene and heavier hydrocarbons, i.e., the C 3+  hydrocarbons, from a feed gas which will be ultimately liquified as LNG product may be recovered. Referring to FIG. 1, a dry feed gas at a flow rate of about 400 MMSCFD is introduced into the illustrated process through inlet stream  10  at a pressure of about 1000 psia and a temperature close to ambient, i.e., about 70° F. This dry feed gas stream has been pre-treated as necessary to remove undesirable components, including acid gases, mercaptans, mercury and moisture, from the natural gas delivered to the facility. Stream  10  is split into two streams  12  and  14 . The smaller portion, stream  14 , is directed through gas/liquid exchanger  18  in NGL recovery block  100  where it is in countercurrent heat exchange with liquid withdrawn from the bottom of NGL recovery column  50  and liquid from separator  22 . This inlet gas provides heat for NGL recovery column  50 , while chilling the inlet gas to a temperature of about −60° F. The larger portion of inlet gas, stream  12 , flows to exchanger block  300  where it is cooled to about −42° F. by utilizing refrigeration in the liquifaction process. The cooling steps in the refrigeration system used in the liquifaction process may differ significantly, depending on the process used, and are collectively illustrated as simplified exchanger block  300 , which will be described in more detail later. 
     Cooled feed gas stream  16  from exchanger block  300  is combined with the cooled feed gas  14   a  from gas/liquid exchanger  18 . The combined stream  20  is directed and separated into liquid stream  24  comprising any condensed heavier hydrocarbons and into cooled vapor stream  30  comprising lighter and more volatile components in separator  22 . Liquid stream  24  is expanded through expansion device  26  and preheated in gas/liquid exchanger  18  prior to introduction into a distillation column, e.g., NGL recovery column  50 , as stream  28  for further fractionation. Depending upon feed gas composition and overall refrigeration, the preheating of expanded liquid stream  24  in exchanger  18  can be bypassed in some cases. Cooled vapor stream  30  flows to expander  34  where it is expanded to a pressure slightly above the operating pressure of NGL recovery column  50 . Alternatively, the vapor in stream  30  may by-pass expander  34  through control valve  34   a . Stream  32  from expander discharge at about −84° F. is fed to NGL recovery column  50  right below the upper rectifying section. It should be noted that, in cases where the feed gas pressure is close to the operating pressure of NGL recovery column  50 , cooled stream  16  leaving exchanger block  300  can be directly fed to NGL recovery column as indicated in dashed line  38 . Similarly, cooled feed gas  14   a  can be delivered directly to NGL recovery column  50  either alone or after being combined with the cooled gas in line  38 . 
     The NGL recovery column operated at approximately  600  psia is a conventional distillation column containing a plurality of mass contacting devices, trays or packings, or some combinations of the above. It is typically equipped with one or more liquid draw trays in the lower section of the column to permit heat inputs to the column for stripping volatile components off from the bottom liquid product. Liquid collected in draw tray  50   a  is withdrawn via stream  46   a  and heated by countercurrent heat exchanger in side reboiler  48  prior to re-introduction to the NGL recovery column. Similarly, liquid condensed in the lower draw tray  50   b  is withdrawn via stream  46   b , partially vaporized in gas/liquid exchanger  18 , and re-introduced to the NGL recovery column. 
     The bottom liquid stream  44  containing substantially all of the heavier hydrocarbons is withdrawn from NGL recovery column  50  and directly introduced into the middle portion of a second distillation column, i.e., NGL purifying column  70 . The liquid feed stream is separated in NGL purifying column  70  operated at a pressure of about 440 psia into an NGL product stream  64  comprising mainly propane, propylene and heavier hydrocarbons, i.e., the C 3+  hydrocarbons, and a vapor comprising mainly ethane and lighter hydrocarbons. The purity of the NGL product stream is controlled by external heat input via bottom reboiler  62 . The NGL product stream exits column  70  at about 230° F. and is cooled to about 120° F. via exchanger  66  for delivery to product stream  68 . 
     The vapor phase is withdrawn from the top of NGL purifying column  70  through overhead line  52 . This vapor phase is cooled to partial condensation in side reboiler  48  prior to return to reflux drum  54  at a temperature of about −16° F. The heat carried by vapor stream  52  is effectively transferred to the NGL recovery column as external heat input. This is accomplished by a unique thermal integration between the overhead condenser and the side reboiler for NGL purifying column  70 . The partially condensed stream is separated in reflux drum  54  into vapor and liquid phases. The liquid accumulated in reflux drum  54  is withdrawn via line  58  where it is pumped via reflux pump  60  for re-introduction to the NGL purifying column as top reflux. 
     The vapor phase withdrawn from reflux drum  54  via line  72  comprises mainly methane and ethane which were present in liquid feed stream  44 . The concentration of propane and higher components in the vapor phase of line  72  is very low. This vapor phase is directed into exchanger block  300  for recovering available refrigeration. In cases where the available refrigeration is limited, stream  72  can bypass exchanger block  300  and simplify the exchanger block design. A combined stream formed by warmed stream  72   a  and excess flashed vapor  102 , if any, is compressed to a higher pressure at about 625 psia in compressor  96  prior to being cooled in after-cooler  98 . The cooled, combined vapor stream  104  returns to exchanger block  300  where it is further cooled to substantial condensation in stream  42  using refrigeration employed in the liquifaction process. The substantially condensed stream  42  is introduced to NGL recovery column  50  as top reflux. Reflux stream  42 , characterized by a very low content of C 3+  hydrocarbons, reduces the equilibrium loss of C 3+  hydrocarbons in the overhead vapor to a minimum. The introduction of a lean reflux stream in the present invention permits the column to be operated at a relatively high pressure, e.g., about 600 psia in this example, while maintaining high recovery of C 3+  hydrocarbon liquids. It should be noted that the lean reflux stream  42  may also be the overhead vapor stream  72  from NGL purifying column  70  or a portion of flashed vapor  80  alone, or any combination of these two streams. 
     Lighter and more volatile gases primarily rich in methane are withdrawn from the top of NGL recovery column  50  via overhead stream  40 . This stream is compressed in expander/compressor  36  utilizing work extracted from expander  34  before delivery to exchanger block  300 . It should be noted that overhead stream  40  can be directly sent to exchanger block  300  without further compression as shown with dashed line  40   a  in cases where expander compressor  36  is not available or is used for other services. 
     The methane-rich overhead stream from NGL recovery column  50  at about −101° F. and about 600 psia is totally liquified and in most cases deeply sub-cooled in exchanger block  300  utilizing appropriate refrigeration from refrigeration block  200 . Sub-cooled LNG at an elevated pressure is delivered via stream  74  from exchanger block  300  to expansion block  400  where it is expanded to near atmospheric pressure through one or more expansion steps. Expansion block  400  illustrates a typical arrangement with one expansion step. Sub-cooled LNG is expanded through expansion means  76  to about 20 psia causing partial vaporization in discharge line  78 . An hydraulic turbine optionally can be employed as an expansion means to reduce flashing as a result of pressure reduction. Any flashed vapor in expanded LNG stream  78  is separated from the liquid portion in separator  82 . The liquid portion withdrawn from separator  82  comprises LNG product  84  for delivery to storage. Although illustrated with a single expansion step, the expansion provided in expansion block  400  can also be carried out in multiple stages. 
     Flashed vapor  80  from separator  82 , primarily comprising methane, nitrogen and other lighter components, enters exchanger block  300  for recovery of available cold refrigeration. The warmed, flashed vapor  86  leaves exchanger block  300  at about 65° F. and is compressed to a fuel gas at a pressure of about 420 psia via fuel gas compressor  88 . The compressed vapor is then cooled to about 100° F. through after-cooler  90  prior to being used as fuel gas  92 . It should be noted that, depending upon the pressures of the expansion steps and the final fuel gas supply pressure, more than one compression and cooling step may be required. Any portion of excess flashed vapor  102  may be combined with the warm vapor stream  72   a  for recycle to the top of NGL recovery column  50  as liquid reflux after being further compressed and cooled to substantial condensation. 
     As mentioned previously, mechanical refrigeration cycles mostly in closed circuit are often employed and dictate the detailed cooling and liquifaction steps in the LNG process. FIG. 2 illustrates in more detail a typical arrangement of exchanger block  300  and refrigeration block  200  utilizing the propane pre-cooled mixed refrigeration cycle in conjunction with the embodiment of the present invention illustrated in FIG.  1 . An exemplary three-stage propane refrigeration circuit is illustrated. Referring to FIG. 2, propane refrigerant  202   a  withdrawn from propane surge drum  220  is directed to a pressure reduction device, e.g., expansion valve  204   a , and expanded to a lower pressure, thereby flashing a portion of the propane refrigerant and lowering its temperature. The resulting two-phase stream is directed into high-stage propane chiller  310   a  as a coolant in indirect heat exchange with the main feed gas portion  12  and mixed refrigerant vapor  502  via conduits  302   a  and  206   a , respectively. 
     The flashed propane vapor from chiller  310   a  is fed to the high-stage inlet port of propane compressor  212  through high-stage suction line  210   a . The remaining liquid propane  202   b  is directed to pressure reduction valve  204   b  to further reduce its pressure, thereby flashing an additional portion of propane refrigerant and further lowering its temperature. 
     The resulting two-phase stream is directed into inter-stage propane chiller  310   b  as a coolant in indirect heat exchange with the cooled feed gas split from conduit  302   a  and mixed refrigerant vapor from conduit  206   a  via conduits  302   b  and  206   b , respectively. 
     The flashed propane vapor from chiller  310   b  is fed to the inter-stage inlet port of propane compressor  212  through inter-stage suction line  210   b . The remaining liquid propane  202   c  is further directed to pressure reduction valve  204   c  to reduce its pressure, thereby flashing another portion of propane refrigerant and lowering its temperature still further. The resultant two-phase stream is directed into low-stage propane chiller  310   c  as a coolant in indirect heat exchange with the cooled feed gas split from conduit  302   b  and mixed refrigerant vapor from conduit  206   b  via conduits  302   c  and  206   c , respectively. 
     The flashed propane vapor from chiller  310   c  is fed to the low-stage inlet port of propane compressor  212  through low-stage suction line  210   c . Propane vapor is compressed in three-stage propane compressor  212  typically driven by a gas turbine. Although they may be separate units tandem driven by a single driver, the three stages preferably form a single unit. Compressed propane vapor  214  flows through condenser  216  where it is liquified at about 100° F. and about 192psia in the illustrated system, prior to being returned via line  218  to propane surge drum  220 . Exemplary temperatures for the three propane refrigeration levels, respectively, in the illustrated example are 60° F., 10° F., and −30° F. 
     Partially condensed, mixed refrigerant leaving conduit  206   c  via stream  502   a  from low-stage propane refrigeration is introduced into separator  504 . The condensed portion is removed from the bottom of separator  504  as stream  506  at about −26° F. and about 640psia. Condensed refrigerant  506  is further cooled in exchanger  320  via conduit  506   a  to about −188° F. Sub-cooled refrigerant  514  is directed to a pressure reduction means, e.g., expansion valve  516 , to lower the pressure. Expanded refrigerant  518  returns to exchanger  320  as a coolant. 
     Non-condensed vapor refrigerant  508  from separator  504  is divided into two portions  510  and  512 . Main portion  510  flows through exchanger  320  where it is liquified and, optionally, sub-cooled to about −235° F. via conduit  510   a . Remaining vapor portion  512  passes through exchanger  340  where it is liquified and sub-cooled in indirect heat exchange with flashed vapor stream  80  from expansion block  400  in FIG.  1 . Other streams entering exchanger  340  include combined vapor stream  104  from after-cooler  98  and overhead vapor stream  72  from reflux drum  54  as depicted in FIG.  1 . Inside exchanger  340 , streams  72  and  80  are warmed before exiting exchanger  340  at about 65° F. as streams  72   a  and  86 , respectively. On the other hand, stream  104  is cooled and exits exchanger  340  as stream  104   a  at about −26° F. 
     Sub-cooled refrigerant  524  exiting from exchanger  340  at about −245° F. is combined with the other sub-cooled refrigerant from conduit  510   a  and thereafter directed to a pressure reduction means, e.g., expansion valve  526 , to a lower pressure before being returned to exchanger  320  as a coolant. After providing the coldest portion of refrigeration, expanded refrigerant  528  is combined with the other expanded refrigerant  518  in exchanger  320 . The combined refrigerant provides the refrigeration necessary for cooling the following: 
     feed gas  12   a  from low-stage propane chiller  310   c;    
     methane-rich vapor stream  40   a  from NGL recovery column  50  in FIG. 1; and 
     cooled vapor stream  104   a  from exchanger  340 , 
     via conduits  322   a ,  322   b , and  322   c , respectively. 
     Although not illustrated in FIG. 2, an hydraulic turbine may be used as a pressure reduction means for the sub-cooled refrigerant in place of expansion valves  516  or  526  illustrated therein. During the expansion process, work can also be extracted by a hydraulic turbine, thereby lowering the refrigerant temperature further and enhancing liquifaction efficiency and overall plant throughput. 
     After providing refrigeration, the combined mixed refrigerant exits exchanger  320  as warmed and vaporized stream  520  at about −30° F. and about 50psia. Warmed refrigerant  520  is then compressed and cooled. An exemplary compression and cooling configuration is illustrated in FIG. 2 with two stages. Stream  520  is first compressed to about 250psia via low stage refrigerant compressor  522   a  and then cooled to about 100° F. via low stage refrigerant after-cooler  524   a . The cooled and compressed stream is further compressed and cooled to form stream  502  at about 655 psia and about 100° F. via high stage refrigerant compressor  522   b  and after-cooler  524   b , thus completing the closed circuit. 
     Table 1 summarizes the inlet and overall performance of the embodiment of the invention illustrated above for a target recovery of C 3+  hydrocarbons exceeding 98%. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Components Flow (lbmol/hr) 
               
             
          
           
               
                   
                 Temp 
                 Pressure 
                   
                   
                   
                   
                   
                 Non- 
                   
               
               
                 Stream 
                 ° F. 
                 psia 
                 methane 
                 ethane 
                 propane 
                 butanes 
                 C5+ 
                 hydrocarbons 
                 total 
               
               
                   
               
             
          
           
               
                 10 
                 70 
                 1000 
                 39527 
                 2152 
                 1010 
                 878 
                 307 
                 44 
                 43918 
               
               
                 32 
                 −84 
                 608 
                 36523 
                 1629 
                 544 
                 299 
                 51 
                 42 
                 39088 
               
               
                 40 
                 −101 
                 600 
                 41877 
                 2132 
                 16 
                 0 
                 0 
                 61 
                 44086 
               
               
                 68 
                 120 
                 440 
                 0 
                 20 
                 994 
                 878 
                 307 
                 0 
                 2199 
               
               
                 72 
                 −16 
                 440 
                 1495 
                 1118 
                 10 
                 0 
                 0 
                 1 
                 2624 
               
               
                 84 
                 −251 
                 20 
                 35805 
                 2131 
                 16 
                 0 
                 0 
                 16 
                 37968 
               
               
                 92 
                 100 
                 415 
                 3722 
                 1 
                 0 
                 0 
                 0 
                 27 
                 3750 
               
             
          
           
               
                   
                 Liquid product recovery: 
                   
               
               
                   
                 % Propane recovery 
                  98.4 
               
               
                   
                 % Butanes recovery 
                 100.0 
               
               
                   
                 % C5+ recovery 
                 100.0 
               
               
                   
                 Total compression brake horsepower 
               
               
                   
                 Gas compression 88 and 96 
                  13,235 
               
               
                   
                 Refrigerant compression 
                 107,150 
               
               
                   
                 Total 
                 120,385 
               
               
                   
                   
               
             
          
         
       
     
     As indicated in Table 1, recovery of 98.4% of propane and 100% of all C 4+  hydrocarbons can be achieved in the LNG process with the present invention. Total compression horsepower required for the integrated liquifaction process includes 13,235 BHP for fuel gas compressors  88  and  96 , and 107,150 BHP for refrigerant compressors  212 ,  522   a  and  522   b . This compares favorably with a total compression horsepower exceeding 125,000 HP required for a separate, up-front NGL plant to recover NGL components, followed by a liquifaction facility to produce LNG. 
     In addition to the propane pre-cooled, mixed, refrigeration cycle represented in FIG. 2, other mechanical refrigeration cycles for liquifying natural gas known to the art can also be integrated with the present invention. Alternative arrangements of exchanger block  300  and refrigeration block  200  utilizing other refrigeration cycles commonly employed in the LNG process are discussed below. The systems described herein merely provide exemplary illustrations of the use of the present invention with other refrigeration processes for liquifying inlet gas and should not be considered as limiting the methods of the present invention to the specific refrigeration processes described. 
     The single, mixed refrigerant process includes heavier hydrocarbons, e.g., butanes and pentanes, in the multi-component, mixed, refrigeration stream and eliminates the need for a propane pre-cooled refrigeration cycle. FIG. 3 illustrates the embodiment of the present invention as depicted in FIG. 1 further including the single, mixed, refrigeration process via exchanger block  300  and refrigeration block  200 . 
     Referring to FIG. 3, mixed refrigerant  502  exits the final compression and cooling stage from high stage after-cooler  524   b  partially condensed as it contains some heavier components in the mixture. The partially condensed refrigerant  502  is introduced into separator  504  from which the condensed portion is removed from the bottom of the separator as stream  506 . The non-condensed vapor refrigerant  508  from separator  504  is divided into two portions  510  and  512 . The condensed refrigerant  506  is pumped via high stage refrigerant pump  538  as stream  536  for combination with the main vapor portion  510 . The combined stream flows through exchanger  320  where it is liquified and in most cases sub-cooled in conduit  510   a . The remaining vapor portion  512  passes through exchanger  340  where it is also liquified and sub-cooled in indirect heat exchange with the flashed vapor stream  80  from expansion block  400  and the overhead vapor stream  72  from reflux drum  54  as illustrated in FIG.  1 . Streams  72  and  80  are warmed inside exchanger  340  before exiting as streams  72   a  and  86 , respectively. Sub-cooled refrigerant  524  from exchanger  340  is combined with the other sub-cooled refrigerant exiting from conduit  510   a  in exchanger  320 . The combined stream is then directed to a pressure reduction means, e.g., expansion valve  526 , and expanded to a lower pressure for return to exchanger  320  as coolant stream  528 . The combined refrigerant provides via conduit  528   a  the refrigeration necessary for cooling the following: 
     feed gas  12 ; 
     methane-rich vapor stream  40   a  from NGL recovery column  50  in FIG. 1; and 
     combined vapor stream  104  from after-cooler  98  as depicted in FIG.  1 . 
     via conduits  322   a ,  322   b , and  322   c , respectively. 
     Although not illustrated in FIG. 3, an hydraulic turbine may be used as a pressure reduction means for the sub-cooled refrigerant in place of expansion valve  526 . During the expansion process, work may also be extracted by an hydraulic turbine, thereby lowering the refrigerant temperature further. Consequently, liquifaction efficiency and overall plant throughput are further enhanced. Alternatively, instead of being combined, liquid refrigerant  536  and vapor refrigerant  510  can enter exchanger  320  in separate paths and be expanded at different pressure levels. 
     After providing refrigeration, the mixed refrigerant exiting exchanger  320  has been warmed and vaporized to form stream  520 . Warmed refrigerant  520  is then compressed and cooled again. FIG. 3 illustrates an exemplary two stage system for performing this compression and cooling. Stream  520  is first compressed via low stage refrigerant compressor  522   a  and then cooled via low stage refrigerant after-cooler  524   a . Cooled refrigerant  526  is directed to high stage suction scrubber  528  for removal of any condensed refrigerant. The non-condensed refrigerant withdrawn from scrubber  528  is subsequently compressed to final pressure via high stage refrigerant compressor  522   b . The condensed refrigerant separated in scrubber  528  is pumped via refrigerant pump  530  and conduit  534  for combination with compressed refrigerant  532 . After passing through after-cooler  524   b , it is cooled to form stream  502 , thus completing the closed circuit. 
     Recovery efficiency is further improved in another embodiment of the present invention wherein a second liquid reflux is introduced to the NGL recovery column. FIG. 4 represents a schematic embodiment illustrating this improvement to further enhance recovery efficiency. The system illustrated in FIG. 4 is essentially identical to that in FIG.  1  and operates in a similar manner with the exception of the differences detailed below. The same reference numerals have been used to represent the same system components in each figure. 
     With reference to FIG. 4, a small slipstream  106 , about 12.5% in the illustrative example, from the pre-cooled feed gas stream  12   a  in exchanger block  300  is taken for further cooling to substantial condensation by utilizing appropriate refrigeration. In some cases, slipstream  106  may be sub-cooled depending upon the refrigeration level available for the liquifaction process. Sub-cooled stream  108  exits exchanger block  300  at about −170° F. and about 975 psia. Stream  108  is thereafter introduced into the middle of the rectification section of NGL recovery column  50  as a middle reflux after pressure reduction to the column pressure via expansion valve  110 . 
     The introduction of a middle reflux provides a bulk rectification effect while substantially retaining the NGL components for recovery in the downward liquid flow, thereby minimizing the recoverable NGL components in the up-flow vapor stream. Any residual NGL components in the upward vapor can all be substantially recovered by the top and leaner liquid reflux. As a result, the same NGL recovery can be achieved with a significantly reduced top reflux flow. LNG stream  74  from exchanger block  300  can be further sub-cooled to reduce flashed vapor  80  from expansion block  400  to the minimum required for the fuel gas requirements. Consequently, the excess flash vapor flow  102  can be eliminated, leading to a substantial reduction in the compression HP required for fuel gas compressor  88 . Thus, overall recovery efficiency can be significantly enhanced. 
     In some cases, the feed gas contains much heavier components, e.g., hexane, C 6+  alkanes and aromatics, which tend to freeze when cooled to cryogenic temperatures, in particular temperatures below −120° F. For those cases, slipstream  30   a  taken from the vapor portion withdrawn from the top of separator  22  as illustrated with a dashed line in FIG. 4 can be used as stream  106 . The feed gas is pre-cooled to a temperature where most of the components having high freezing points are condensed and separated in the liquid phase in separator  22 . The vapor stream withdrawn from separator  22  comprises very few of these high freezing point components, thus eliminating the concerns of freezing. 
     An example employing the embodiment illustrated in FIG. 4 is demonstrated using the same inlet gas and conditions for the example using FIG.  1  and reported in Table 1. Table 2 below summarizes the overall performance of an LNG process incorporating a second reflux stream as described with reference to FIG.  4 . 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Components Flow (lbmol/hr) 
               
             
          
           
               
                   
                 Temp 
                 Pressure 
                   
                   
                   
                   
                   
                 Non- 
                   
               
               
                 Stream 
                 ° F. 
                 psia 
                 methane 
                 ethane 
                 propane 
                 butanes 
                 C5+ 
                 hydrocarbons 
                 total 
               
               
                   
               
             
          
           
               
                 10 
                 70 
                 1000 
                 39527 
                 2152 
                 1010 
                 878 
                 307 
                 44 
                 43918 
               
               
                 32 
                 −83 
                 608 
                 33702 
                 1512 
                 508 
                 281 
                 48 
                 39 
                 36090 
               
               
                 40 
                 −100 
                 600 
                 39532 
                 2132 
                 9 
                 0 
                 0 
                 44 
                 41717 
               
               
                 68 
                 120 
                 440 
                 0 
                 20 
                 1001 
                 878 
                 307 
                 0 
                 2206 
               
               
                 72 
                 −4 
                 440 
                 1382 
                 1475 
                 11 
                 0 
                 0 
                 1 
                 2869 
               
               
                 84 
                 −251 
                 20 
                 35805 
                 2131 
                 9 
                 0 
                 0 
                 16 
                 37961 
               
               
                 92 
                 100 
                 415 
                 3722 
                 1 
                 0 
                 0 
                 0 
                 27 
                 3750 
               
             
          
           
               
                   
                 Liquid product recovery: 
                   
               
               
                   
                 % Propane recovery 
                  99.1 
               
               
                   
                 % Butanes recovery 
                 100.0 
               
               
                   
                 % C5+ recovery 
                 100.0 
               
               
                   
                 Total compression brake horsepower 
               
               
                   
                 Gas compression 88 and 96 
                  7,600 
               
               
                   
                 Refrigerant compression 
                 112,365 
               
               
                   
                 Total 
                 119,965 
               
               
                   
                   
               
             
          
         
       
     
     As indicated in Table 2, propane recovery is improved to 99.1%. Total compression HP required for the integrated liquifaction process reduces to 119,965 BHP with 7600 BHP for fuel gas compressors  88  and  96 , and 112,365 BHP for refrigerant compressors  212 ,  522   a  and  522   b.    
     It should also be noted that the second liquid reflux may be fed to the top of the NGL recovery column alone or in combination with the other top reflux stream  42 . While this will simplify the design of the upper rectification section, the recovery efficiency may be reduced slightly. 
     In yet another embodiment of the present invention, illustrated in FIG. 5, high recovery of NGL components can also be achieved by recycling a portion of the sub-cooled LNG at elevated pressure as the top liquid reflux to NGL recovery column  50 . The LNG stream, again containing a very low content of NGL components, serves as an enhanced lean reflux to achieve high recovery efficiency in this embodiment. The system illustrated in FIG. 5 is essentially the same as that illustrated in FIG.  1  and operates in a similar manner. The difference resides in the source of the top feed (reflux) to NGL recovery column  50 . 
     Referring to FIG. 5, lighter and more volatile methane-rich gases  40  withdrawn from the overhead of NGL recovery column  50  are totally liquified and, in most cases, sub-cooled in exchanger block  300  via conduit  112 . Appropriate refrigeration from refrigeration block  200  is used for this liquifaction and sub-cooling. Prior to introduction into exchanger block  300 , methane-rich overhead stream  40  may be raised in pressure via expander/compressor  36  utilizing work extracted from expander  34  when available as previously described. At least a portion of the sub-cooled LNG is re-introduced to the top of NGL recovery column  50  as reflux via line  42 . In some cases where the expander/compressor  36  is not present, a cryogenic pump  116  may be used to return this liquid reflux to the top of the recovery column as illustrated in dashed line. 
     The main portion of sub-cooled LNG is further cooled before exiting the exchanger block  300  as stream  74  at a much colder temperature of about −242° F. Accordingly, flashed vapor flow  80  from expansion block  400  is greatly reduced before being directed to the fuel gas system after recovering refrigeration and compression. Additional heat input is provided to the lower stripping section of recovery column  50  to further strip lighter components off bottom liquid stream  44 . This also leads to a reduction in overhead vapor  72  from reflux drum  54  associated with NGL purifying column  70 . This overhead vapor stream  72  is also directed to the fuel gas system. Further, a second liquid reflux such as that disclosed in FIG. 4 may be incorporated to further improve recovery efficiency as illustrated previously. 
     While the integration of NGL recovery in an LNG facility in accord with the present invention has been demonstrated effectively for high C 3+  recovery, the aforementioned methods can also be easily modified by adjusting the operating parameters either for enhanced ethane recovery or for the recovery of C 5+  components alone in cases where recovery of lighter NGL components is not desirable. To achieve high ethane recovery, the temperature profile inside NGL recovery block  100  typically needs to be reduced, the reflux stream needs to be leaner and the flow should be increased. Table 3 summarizes the results of the operation of the system illustrated in FIG. 1 under ethane recovery conditions with the same feed gas composition and conditions as used in FIG.  1 . As illustrated, ethane recovery above 84% is achieved using the process of the present invention illustrated in FIG. 1, but optimized for enhanced ethane recovery. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Components Flow (lbmol/hr) 
               
             
          
           
               
                   
                 Temp 
                 Pressure 
                   
                   
                   
                   
                   
                 Non- 
                   
               
               
                 Stream 
                 ° F. 
                 psia 
                 methane 
                 ethane 
                 propane 
                 butanes 
                 C5+ 
                 hydrocarbons 
                 total 
               
               
                   
               
             
          
           
               
                 10 
                 70 
                 1000 
                 39527 
                 2152 
                 1010 
                 878 
                 307 
                 44 
                 43918 
               
               
                 32 
                 −111 
                 588 
                 13680 
                 517 
                 177 
                 115 
                 28 
                 18 
                 14535 
               
               
                 40 
                 −122 
                 580 
                 42566 
                 333 
                 7 
                 0 
                 0 
                 65 
                 42971 
               
               
                 68 
                 120 
                 440 
                 36 
                 1819 
                 1003 
                 878 
                 307 
                 1 
                 4044 
               
               
                 72 
                 −102 
                 440 
                 1162 
                 87 
                 0 
                 0 
                 0 
                 1 
                 1250 
               
               
                 84 
                 −252 
                 20 
                 35768 
                 333 
                 7 
                 0 
                 0 
                 15 
                 36123 
               
               
                 92 
                 100 
                 415 
                 3723 
                 0 
                 0 
                 0 
                 0 
                 27 
                 3750 
               
             
          
           
               
                   
                 Liquid product recovery: 
                   
               
               
                   
                 % Ethane recovery 
                  84.5 
               
               
                   
                 % Propane recovery 
                  99.3 
               
               
                   
                 % Butanes recovery 
                 100.0 
               
               
                   
                 % C5+ recovery 
                 100.0 
               
               
                   
                 Total compression brake horsepower 
               
               
                   
                 Gas compression 88 and 96 
                  14,770 
               
               
                   
                 Refrigerant compression 
                 119,670 
               
               
                   
                 Total 
                 134,440 
               
               
                   
                   
               
             
          
         
       
     
     Another aspect of the present invention which offers a significant economic advantage is the cooling of the feed gas by countercurrent heat exchange with a refrigerant stream comprising a portion of bottom liquid stream  44  or liquid withdrawn from the lower portion of NGL recovery column  50 . Illustrated in FIG. 6 is an alternative arrangement of a cryogenic NGL recovery process incorporating this modification. A side liquid is withdrawn from the lower portion of NGL recovery column  50  via line  120 . This liquid is directed to pressure reduction valve  122  to reduce its pressure and thereby flash a portion of the liquid refrigerant. Expanded liquid refrigerant  124  at a lower temperature flows through gas/liquid exchanger  18  to provide additional refrigeration to cool inlet gas portion  14 . Stream  126  carries the partially vaporized liquid exiting exchanger  18  to suction knockout drum  128  where it is separated into vapor and liquid portions. The vapor portion withdrawn from the top of knockout drum  128  through line  130  is directed to recycle compressor  132  where it is compressed to a pressure slightly higher than that of the NGL recovery column. The compressed gas from compressor  132  is cooled in cooler  134  prior to re-introduction to NGL recovery column  50  as a stripping gas. 
     The liquid portion accumulated at the bottom of knockout drum  128  is withdrawn via line  136 . This liquid portion, comprising primarily propane and heavier hydrocarbons, is pumped by recycle pump  138  to NGL purifying column  70  for further fractionation. 
     The introduction of stripping gas (sometimes referred to as enrichment gas) supplements the heat requirements in NGL recovery column  50  for stripping volatile components from the bottom liquid stream  44 . It also enhances the relative volatility of the key components and, accordingly, improves the separation efficiency of the column, particularly when the column is operated at a relatively high pressure as in the NGL recovery column illustrated here. 
     Yet another embodiment of the present invention is illustrated in FIG.  7 . The NGL purifying system can be simplified by eliminating the overhead reflux system, resulting in savings on capital investment. Referring to FIG. 7 where only NGL recovery block  100  is illustrated, bottom liquid stream  44  from NGL recovery column  50  is split into two portions. One portion  44   b  is directly introduced into the middle portion of the NGL purifying column  70 , as illustrated in FIGS. 1,  4 ,  5  and  6 . The other portion  44   a  is directed to reflux exchanger  48  where it is substantially sub-cooled. The sub-cooled liquid  44   c  from reflux exchanger  48  is introduced to the top of NGL purifying column  70  as liquid reflux to reduce the equilibrium loss of heavy hydrocarbons in vapor stream  72 . An exemplary source for the cold stream for reflux exchanger  48  is a liquid side-draw from NGL recovery column  50  as illustrated in FIG.  7 . Consequently, the reflux drum and pumps can be eliminated. 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof, and has been demonstrated as effective in providing methods for maximizing the recovery of NGL components from a natural gas stream within an LNG facility. However, it will be evident to those skilled in the art that various modifications and changes can be made thereto without departing from the true spirit or scope of the invention. Accordingly, the specification is to be regarded in an illustrative rather than a restrictive sense. There may be other ways of configuring and/or operating the integration system of the present invention differently or in association with different liquifaction processes from those explicitly described herein which nevertheless fall within the true spirit and scope of the invention. For example, it is anticipated that by routing certain streams differently or by adjusting operating parameters, different optimizations and efficiencies may be obtained which would nevertheless not cause the system to fall outside of the scope of the present invention. Additionally, it must also be noted that, while the foregoing embodiments have been described in considerable detail for the purpose of disclosure, many variations, e.g., the arrangement and number of heat exchangers and compression stages, may be made therein. Therefore, the invention is not restricted to the preferred embodiments described and illustrated but covers all modifications which may call within the scope of the appended claims.