Patent Description:
The combustion of conventional fuels, such as gasoline and diesel, has proven to be essential in a myriad of industrial processes. The combustion of gasoline and diesel, however, may often be accompanied by various drawbacks including increased production costs and increased carbon emissions. In view of the foregoing, recent efforts have focused on alternative fuels with decreased carbon emissions, such as natural gas, to combat the drawbacks of combusting conventional fuels. In addition to providing a "cleaner" alternative fuel with decreased carbon emissions, combusting natural gas may also be relatively safer than combusting conventional fuels. For example, the relatively low density of natural gas allows it to safely and readily dissipate to the atmosphere in the event of a leak. In contrast, conventional fuels (e.g., gasoline and diesel) have a relatively high density and tend to settle or accumulate in the event of a leak, which may present a hazardous and potentially fatal working environment for nearby operators.

While utilizing natural gas may address some of the drawbacks of conventional fuels, the storage and transport of natural gas often prevents it from being viewed as a viable alternative to conventional fuels. Accordingly, natural gas is routinely converted to liquefied natural gas (LNG) via one or more thermodynamic processes. The thermodynamic processes utilized to convert natural gas to LNG may often include circulating one or more refrigerants (e.g., single mixed refrigerants, duel mixed refrigerants, etc.) through a refrigerant cycle. While various thermodynamic processes have been developed for the production of LNG, conventional thermodynamic processes may often fail to produce LNG in quantities sufficient to meet increased demand. Further, the complexity of the conventional thermodynamic processes may often make the production of LNG cost prohibitive and/or impractical. For example, the production of LNG via conventional thermodynamic processes may often require the utilization of additional and/or cost-prohibitive equipment (e.g., compressors, heat exchangers, etc.).

What is needed, then, is an improved, simplified liquefaction system and method for producing liquefied natural gas (LNG).

In <CIT> a natural gas liquefaction process is disclosed using a single refrigeration cycle adopting a mixed refrigerant using two separation units with remixing of the separated liquid portions thereof to provide a single liquid refrigerant part. After the mixed refrigerant is separated in this liquefaction process into a single liquid and a vapour refrigerant parts, the two refrigerant parts are not mixed with each other but go through condensing (cooling), expanding, heat-exchanging, and compressing stages individually. The exit sides of the two compression stages for the two refrigerant parts are connected with each other such that the pressures at the exit sides thereof may be equal to each other, but the pressures at the entrance sides of the two compression stages may be different from each other.

Further, <CIT> discloses a natural gas liquefaction which uses a single closed loop refrigeration cycle employing a mixed refrigerant.

In <CIT> a control system is disclosed for a process of liquefied natural gas production (LNG) from natural gas using a heat exchanger and a closed loop refrigeration cycle employs independent, direct control of both production and temperature by adjusting refrigeration to match a set production.

In <CIT> a system for liquefying and sub-cooling natural gas is disclosed, wherein compression power is shifted off the closed cycle refrigerant by sub-cooling the liquid natural gas to a relatively warm exit temperature and subsequently reducing the pressure and flashing the liquefied natural gas to recover a gaseous phase natural gas in excess of plant fuel requirements, the excess being recompressed and recycled to the feed to the process.

<CIT> discloses a system and a method for liquefying natural gas using single mixed refrigerant as refrigeration medium. The system comprises a two-stage mixed refrigerant compressor driven by a motor, two coolers, a liquid pump, three gas-liquid separators, two throttling devices, a plate-fin heat exchanger group and a LNG storage tank.

Further, in <CIT> a method of gas liquefaction is disclosed wherein the refrigeration to cool and liquefy an essentially water-free feed gas is provided by a single recirculating mixed refrigerant cycle in which refrigeration is provided by the vaporization of two mixed refrigerant streams of different compositions at a lower and higher pressure levels respectively. A lower pressure level vaporizing refrigerant cools the feed gas stream in a first cooling zone and a higher pressure level vaporizing refrigerant further cools and condenses the cooled gas in a second cooling zone to provide the final liquid product. The lower pressure level vaporizing refrigerant is provided by one or more liquids obtained by ambient cooling of compressed mixed refrigerant vapor.

A method for producing liquefied natural gas is provided as set out in the appended set of claims.

Further a liquefaction system is provided as set out in the appended set of claims.

The present invention is best understood from the following detailed description when read with the accompanying Figures.

<FIG> illustrates a process flow diagram of an exemplary liquefaction system for producing liquefied natural gas (LNG) from a natural gas source, according to one or more embodiments disclosed.

<FIG> illustrates a flowchart of a method for producing liquefied natural gas, not according to the present invention.

<FIG> illustrates a flowchart of a method for producing liquefied natural gas from a natural gas source, according to one or more embodiments disclosed.

It is to be understood that the following disclosure describes several exemplary embodiments for implementing different features, structures, or functions of the invention. Exemplary embodiments of components, arrangements, and configurations are described below to simplify the present disclosure; however, these exemplary embodiments are provided merely as examples and are not intended to limit the scope of the invention. Additionally, the present disclosure may repeat reference numerals and/or letters in the various exemplary embodiments and across the Figures provided herein. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various exemplary embodiments and/or configurations discussed in the various Figures. Moreover, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the first and second features, such that the first and second features may not be in direct contact. Finally, the exemplary embodiments presented below may be combined in any combination of ways, i.e., any element from one exemplary embodiment may be used in any other exemplary embodiment, without departing from the scope of the disclosure.

Additionally, certain terms are used throughout the following description and claims to refer to particular components. As one skilled in the art will appreciate, various entities may refer to the same component by different names, and as such, the naming convention for the elements described herein is not intended to limit the scope of the invention, unless otherwise specifically defined herein. Further, the naming convention used herein is not intended to distinguish between components that differ in name but not function. Further, in the following discussion and in the claims, the terms "including" and "comprising" are used in an open-ended fashion, and thus should be interpreted to mean "including, but not limited to. " All numerical values in this disclosure may be exact or approximate values unless otherwise specifically stated. Accordingly, various embodiments of the disclosure may deviate from the numbers, values, and ranges disclosed herein without departing from the intended scope. Furthermore, as it is used in the claims or specification, the term "or" is intended to encompass both exclusive and inclusive cases, i.e., "A or B" is intended to be synonymous with "at least one of A and B," unless otherwise expressly specified herein.

<FIG> illustrates a process flow diagram of an exemplary liquefaction system <NUM> for producing liquefied natural gas (LNG) from a natural gas source <NUM>, according to one or more embodiments. As further discussed herein, the liquefaction system <NUM> may be configured to receive natural gas or feed gas from the natural gas source <NUM>, direct or flow the feed gas through a product or feed gas stream to cool at least a portion of the feed gas to the LNG, and discharge or output the LNG. The liquefaction system <NUM> is configured to direct or flow a process fluid containing one or more refrigerants (i.e. a single mixed refrigerant) through one or more refrigerant cycles (e.g., pre-cooling cycle, liquefaction cycle, etc.) to cool at least a portion of the feed gas flowing through the feed gas stream.

The liquefaction system <NUM> may include one or more refrigerant assemblies (one is shown <NUM>) and a single heat exchanger (<NUM>). The refrigerant assembly <NUM> includes a compression assembly <NUM>, one or more pumps (one is shown <NUM>), two or more liquid separators (two are shown <NUM>, <NUM>), fluidly, communicably, thermally, and/or operatively coupled with one another. The refrigerant assembly <NUM> is fluidly coupled with the heat exchanger <NUM>. As illustrated in <FIG>, the refrigerant assembly <NUM> is fluidly coupled with and dispose upstream of the heat exchanger <NUM> via lines <NUM> and <NUM>, and is further fluidly coupled with and disposed downstream from the heat exchanger <NUM> via lines <NUM> and <NUM>. While <FIG> illustrates a single refrigerant assembly <NUM> fluidly coupled with the heat exchanger <NUM>, it should be appreciated that the liquefaction system <NUM> may include a plurality of refrigerant assemblies. For example, two or more refrigerant assemblies may be fluidly coupled with a single heat exchanger <NUM> in series or in parallel.

The natural gas source <NUM> may be or include a natural gas pipeline, a stranded natural gas wellhead, or the like, or any combination thereof. The natural gas source <NUM> may contain natural gas at ambient temperature. The natural gas source <NUM> may contain natural gas having a temperature relatively greater than or relatively less than ambient temperature. The natural gas source <NUM> may also contain natural gas at a relatively high pressure (e.g., about <NUM>,<NUM> kPa to about <NUM>,<NUM> kPa or greater) or a relatively low pressure (e.g., about <NUM> kPa to about <NUM>,<NUM> kPa). For example, the natural gas source <NUM> may be a high pressure natural gas pipeline containing natural gas at a pressure from about <NUM>,<NUM> kPa to about <NUM>,<NUM> kPa or greater. In another example, the natural gas source <NUM> may be a low pressure natural gas pipeline containing natural gas at a pressure from about <NUM> kPa to about <NUM>,<NUM> kPa.

The natural gas from the natural gas source <NUM> may include one or more hydrocarbons. For example, the natural gas may include methane, ethane, propane, butanes, pentanes, or the like, or any combination thereof. Methane may be a major component of the natural gas. For example, the concentration of methane in the natural gas may be greater than about <NUM>%, greater than about <NUM>%, greater than about <NUM>%, or greater than about <NUM>%. The natural gas may also include one or more non-hydrocarbons. For example, the natural gas may be or include a mixture of one or more hydrocarbons and one or more non-hydrocarbons. Illustrative non-hydrocarbons may include, but are not limited to, water, carbon dioxide, helium, nitrogen, or the like, or any combination thereof. The natural gas may be treated to separate or remove at least a portion of the non-hydrocarbons from the natural gas. For example, the natural gas may be flowed through a separator (not shown) containing one or more adsorbents (e.g., molecular sieves, zeolites, metal-organic frameworks, etc.) configured to at least partially separate one or more of the non-hydrocarbons from the natural gas. In an exemplary embodiment, the natural gas may be treated to separate the non-hydrocarbons (e.g., water and/or carbon dioxide) from the natural gas to increase a concentration of the hydrocarbon and/or prevent the natural gas from subsequently crystallizing (e.g., freezing) in one or more portions of the liquefaction system <NUM>. For example, in one or more portions of the liquefaction system <NUM>, the feed gas containing the natural gas may be cooled to or below a freezing point of one or more of the non-hydrocarbons (e.g., water and/or carbon dioxide). Accordingly, removing water and/or carbon dioxide from the natural gas may prevent the subsequent crystallization of the feed gas in the liquefaction system <NUM>.

The compression assembly <NUM> of the refrigerant assembly <NUM> is configured to compress the process fluid (i.e. mixed refrigerant process fluid) directed thereto. The compression assembly <NUM> includes two or more compressors (two are shown <NUM>, <NUM>) configured to compress the process fluid. In an exemplary embodiment, the compression assembly <NUM> may include only two compressors <NUM>, <NUM>. As illustrated in <FIG>, a first compressor <NUM> of the compression assembly <NUM> is fluidly coupled with and disposed downstream from the heat exchanger <NUM> via line <NUM> and line <NUM>, and a second compressor <NUM> is fluidly coupled with and disposed downstream from a first liquid separator <NUM> via line <NUM>. It should be appreciated that utilizing only two compressors <NUM>, <NUM> in the compression assembly <NUM> may reduce the cost, energy consumption, and/or complexity of the liquefaction system <NUM>. For example, utilizing only two compressors <NUM>, <NUM> may reduce the number of drivers <NUM>, coolers <NUM>, <NUM>, liquid separators <NUM>, <NUM>, and/or pumps <NUM> utilized in the liquefaction system <NUM>. In another embodiment, the compression assembly <NUM> may include any number of compressors. For example, the compression assembly <NUM> may include three, four, five, or more compressors. Illustrative compressors may include, but are not limited to, supersonic compressors, centrifugal compressors, axial flow compressors, reciprocating compressors, rotating screw compressors, rotary vane compressors, scroll compressors, diaphragm compressors, or the like, or any combination thereof.

Each of the compressors <NUM>, <NUM> may include one or more stages (not shown). For example, each of the compressors <NUM>, <NUM> may include a first stage, a final stage, and/or one or more intermediate stages disposed between the first stage and the final stage. The first stage (not shown) of the first compressor <NUM> is fluidly coupled with and disposed downstream from the heat exchanger <NUM> via line <NUM>, and an intermediate stage (not shown) of the first compressor <NUM> is fluidly coupled with and disposed downstream from the heat exchanger <NUM> via line <NUM>. As further described herein, the first compressor <NUM> is configured to receive a heated or "spent" first portion of a refrigerant (i.e. a single mixed refrigerant) from the heat exchanger <NUM> at the first stage thereof, and a sidestream of a "spent" second portion of the refrigerant (i.e. the single mixed refrigerant) from the heat exchanger <NUM> at the intermediate stage thereof. For example, the first compressor <NUM> may have a first inlet (not shown) fluidly and/or operably coupled with the first stage and configured to receive the spent first portion of the single mixed refrigerant, and a second inlet (not shown) fluidly and/or operably coupled with the intermediate stage and configured to receive the sidestream of the "spent" second portion of the single mixed refrigerant.

The compression assembly <NUM> may also include one or more drivers (one is shown <NUM>) operatively coupled with and configured to drive each of the compressors <NUM>, <NUM> and/or the respective compressor stages thereof. For example, as illustrated in <FIG>, the driver <NUM> may be coupled with and configured to drive both of the compressors <NUM>, <NUM> via a rotary shaft <NUM>. In another example, separate drivers (not shown) may be coupled with and configured to drive each of the compressors <NUM>, <NUM> via separate rotary shafts (not shown). Illustrative drivers may include, but are not limited to, motors (e.g., electric motors), turbines (e.g., gas turbines, steam turbines, etc.), internal combustion engines, and/or any other devices capable of driving each of the compressors <NUM>, <NUM> or the respective compressor stages thereof. The rotary shaft <NUM> may be a single segment or multiple segments coupled with one another via one or more gears (not shown) and/or one or more couplers. It should be appreciated that the gears coupling the multiple segments of the rotary shaft <NUM> may allow each of the multiple segments of the rotary shaft <NUM> to rotate or spin at the same or different rates or speeds.

The compression assembly <NUM> also includes two or more heat exchangers or coolers (two are shown <NUM>, <NUM>) configured to absorb or remove heat from the process fluid (i.e. the refrigerant) flowing therethrough. The coolers <NUM>, <NUM> are fluidly coupled with and disposed downstream from the respective compressors <NUM>, <NUM>. As illustrated in <FIG>, a first cooler <NUM> is fluidly coupled with and disposed downstream from the first compressor <NUM> via line <NUM>, and a second cooler <NUM> is fluidly coupled with and disposed downstream from the second compressor <NUM> via line <NUM>. As further illustrated in <FIG>, the first cooler <NUM> and the second cooler <NUM> are fluidly coupled with and disposed upstream of the first liquid separator <NUM> and a second liquid separator <NUM> via line <NUM> and line <NUM>, respectively. The first and second coolers <NUM>, <NUM> may be configured to remove at least a portion of the thermal energy or heat generated in the first and second compressors <NUM>, <NUM>, respectively. For example, compressing the process fluid (e.g., the refrigerant) in the compressors <NUM>, <NUM> may generate heat (e.g., heat of compression) in the process fluid, and the coolers <NUM>, <NUM> may be configured to remove at least a portion of the heat of compression from the process fluid and/or the refrigerants contained therein.

In at least one embodiment, a heat transfer medium may flow through each of the coolers <NUM>, <NUM> to absorb the heat in the process fluid flowing therethrough. Accordingly, the heat transfer medium may have a higher temperature when discharged from the coolers <NUM>, <NUM> and the process fluid may have a lower temperature when discharged from the coolers <NUM>, <NUM>. The heat transfer medium may be or include water, steam, a refrigerant, a process gas, such as carbon dioxide, propane, or natural gas, or the like, or any combination thereof. In an exemplary embodiment, the heat transfer medium discharged from the coolers <NUM>, <NUM> may provide supplemental heating to one or more portions and/or assemblies of the liquefaction system <NUM>. For example, the heat transfer medium containing the heat absorbed from the coolers <NUM>, <NUM> may provide supplemental heating to a heat recovery unit (HRU) (not shown).

The liquid separators <NUM>, <NUM> are fluidly coupled with and disposed downstream from the respective coolers <NUM>, <NUM> of the compression assembly <NUM>. As illustrated in <FIG>, a first liquid separator <NUM> and a second liquid separator <NUM> are fluidly coupled with and disposed downstream from the first cooler <NUM> and the second cooler <NUM> via line <NUM> and line <NUM>, respectively. As further illustrated in <FIG>, the first liquid separator <NUM> is fluidly coupled with and disposed upstream of the second compressor <NUM> and the pump <NUM> via line <NUM> and line <NUM>, respectively, and the second liquid separator <NUM> is fluidly coupled with and disposed upstream of the heat exchanger <NUM> via lines <NUM> and <NUM>. The first and second liquid separators <NUM>, <NUM> are each configured to receive a process fluid containing a liquid phase (e.g., a liquid refrigerant) and a gaseous phase (e.g., a vapor or gaseous refrigerant), and separate the liquid phase and the gaseous phase from one another. For example, as further described herein, the first and second liquid separators <NUM>, <NUM> are configured to separate a liquid phase containing relatively high boiling point refrigerants (e.g., liquid refrigerant) and a gaseous phase containing relatively lower boiling point refrigerants (e.g., a vapor or gaseous refrigerant) from one another. Illustrative liquid separators may include, but are not limited to, scrubbers, liquid-gas separators, rotating separators, stationary separators, or the like.

The pump <NUM> is fluidly coupled with and disposed downstream from the first liquid separator <NUM> via line <NUM>, and is further fluidly coupled with and disposed upstream of the heat exchanger <NUM> via lines <NUM> and <NUM>. The pump <NUM> is configured to direct a process fluid containing a liquid phase (e.g., a liquid refrigerant) from the first liquid separator <NUM> to the heat exchanger <NUM>. The pump <NUM> is configured to pressurize the liquid phase from the first liquid separator <NUM> to direct the liquid phase to the heat exchanger <NUM>. The pump <NUM> may be configured to pressurize the process fluid from the first liquid separator <NUM> to a pressure equal or substantially equal to the process fluid discharged from the second compressor <NUM> and/or the process fluid flowing through line <NUM>. The pump <NUM> may be an electrically driven pump, a mechanically driven pump, a variable frequency driven pump, or the like.

The heat exchanger <NUM> is fluidly coupled with and disposed downstream from the pump <NUM> and two or more of the liquid separators <NUM>, <NUM>, and configured to receive two or more process fluids therefrom. As illustrated in <FIG>, the heat exchanger <NUM> is fluidly coupled with and disposed downstream from the second liquid separator <NUM> via line <NUM> and line <NUM> and configured to receive process fluids therefrom. The heat exchanger <NUM> is also fluidly coupled with and disposed downstream from the pump <NUM> via lines <NUM> and <NUM> and configured to receive a process fluid therefrom. The heat exchanger <NUM> is also fluidly coupled with and disposed upstream of the compression assembly <NUM> and configured to direct two or more process fluids thereto. As illustrated in <FIG>, the heat exchanger <NUM> is fluidly coupled with and disposed upstream from the first compressor <NUM> of the compression assembly <NUM> via line <NUM> and line <NUM>. As further illustrated in <FIG>, the heat exchanger <NUM> may be fluidly coupled with and disposed downstream from the natural gas source <NUM> via line <NUM> and configured to receive the feed gas therefrom.

The heat exchanger <NUM> may be any device capable of directly or indirectly cooling and/or sub-cooling at least a portion of the feed gas flowing therethrough. For example, the heat exchanger <NUM> may be a wound coil heat exchanger, a plate-fin heat exchanger, a shell and tube heat exchanger, a kettle type heat exchanger, or the like. In at least one embodiment, the heat exchanger <NUM> may include one or more regions or zones (two are shown <NUM>, <NUM>). For example, as illustrated in <FIG>, a first zone <NUM> of the heat exchanger <NUM> may be a pre-cooling zone, and a second zone <NUM> of the heat exchanger <NUM> may be a liquefaction zone. As further described herein, the heat exchanger <NUM> may be configured to pre-cool the refrigerants and/or the feed gas flowing through the pre-cooling zone <NUM>. The heat exchanger <NUM> may also be configured to liquefy at least a portion of the feed gas from the natural gas source <NUM> to the LNG in the liquefaction zone <NUM>.

The liquefaction system <NUM> may include one or more expansion elements (two are shown <NUM>, <NUM>) configured to receive and expand a process fluid to thereby decrease a temperature and pressure thereof. Illustrative expansion elements <NUM>, <NUM> may include, but are not limited to, a turbine or turbo-expander, a geroler, a gerotor, an expansion valve, such as a Joule-Thomson (JT) valve, or the like, or any combination thereof. In at least one embodiment, any one or more of the expansion elements <NUM>, <NUM> may be a turbo-expander (not shown) configured to receive and expand a portion of the process fluid to thereby decrease a temperature and pressure thereof. The turbo-expander (not shown) may be configured to convert the pressure drop of the process fluid flowing therethrough to mechanical energy, which may be utilized to drive one or more devices (e.g., generators, compressors, pumps, etc.). In another embodiment, illustrated in <FIG>, any one or more of the expansion elements <NUM>, <NUM> may be expansion valves, such as JT valves. As illustrated in <FIG>, each of the expansion valves <NUM>, <NUM> may be fluidly coupled with the heat exchanger <NUM> and configured to receive and expand a process fluid (e.g., the refrigerant) from the heat exchanger <NUM> to thereby decrease a temperature and pressure thereof. For example, a first expansion valve <NUM> may be disposed downstream from the heat exchanger <NUM> via line <NUM>, and may further be disposed upstream of the heat exchanger <NUM> via line <NUM>. In another example, a second expansion valve <NUM> may be disposed downstream from the heat exchanger <NUM> via line <NUM>, and may further be disposed upstream of the heat exchanger <NUM> via line <NUM>. In at least one embodiment, the expansion of the process fluid through any one or more of the expansion valves <NUM>, <NUM> may flash the process fluid into a two-phase fluid including a gaseous or vapor phase and a liquid phase.

As previously discussed, the liquefaction system <NUM> is configured to direct or flow a process fluid (i.e. the refrigerant) through one or more refrigerant cycles to cool at least a portion of the feed gas flowing through the feed gas stream. The refrigerant cycles is a closed-loop refrigerant cycle. The process fluid directed through the refrigerant cycles is a single mixed refrigerant. The single mixed refrigerant may be a multicomponent fluid mixture containing one or more hydrocarbons. Illustrative hydrocarbons may include, but are not limited to, methane, ethane, propane, butanes, pentanes, or the like, or any combination thereof. In at least one embodiment, the single mixed refrigerant may be a multicomponent fluid mixture containing one or more hydrocarbons and one or more non-hydrocarbons. For example, the single mixed refrigerant may be or include a mixture of one or more hydrocarbons and one or more non-hydrocarbons. Illustrative non-hydrocarbons may include, but are not limited to, carbon dioxide, nitrogen, argon, or the like, or any combination thereof. In another embodiment, the single mixed refrigerant may be or include a mixture containing one or more non-hydrocarbons. In an exemplary embodiment, the process fluid directed through the refrigerant cycles may be a single mixed refrigerant containing methane, ethane, propane, butanes, and/or nitrogen. In at least one embodiment, the single mixed refrigerant may include R42, R410a, or the like.

The process fluid containing the single mixed refrigerant is discharged from the first compressor <NUM> of the compression assembly <NUM> and directed to the first cooler <NUM> via line <NUM>. The process fluid discharged from the first compressor <NUM> may have a pressure of about <NUM>,<NUM> kPa to about <NUM>,<NUM> kPa or greater. The first cooler <NUM> receives the process fluid from the first compressor <NUM> and cools at least a portion of the single mixed refrigerant contained therein. The first cooler <NUM> may cool at least a portion of the single mixed refrigerant to a liquid phase. For example, as previously discussed, the single mixed refrigerant may be a multicomponent fluid mixture containing one or more hydrocarbons, and relatively high molecular weight hydrocarbons (e.g., ethane, propane, etc.) may be compressed, cooled, and/or otherwise condensed to the liquid phase before relatively low molecular weight hydrocarbons (e.g., methane). Accordingly, the relatively high molecular weight hydrocarbons of the single mixed refrigerant contained in line <NUM> may be in the liquid phase, and the relatively low molecular weight hydrocarbons of the single mixed refrigerant in line <NUM> may be in the gaseous phase. It should be appreciated that relatively high molecular weight hydrocarbons may generally have a boiling point relatively higher than relatively low molecular weight hydrocarbons. In an exemplary embodiment, the first cooler <NUM> may cool the process fluid from the first compressor <NUM> to a temperature of about <NUM> to about <NUM> or greater.

The process fluid containing the cooled single mixed refrigerant is directed to the first liquid separator <NUM> via line <NUM>, and the first liquid separator <NUM> may separate at least a portion of the liquid phase and the gaseous phase from one another. For example, the first liquid separator <NUM> may separate at least a portion of the liquid phase containing the relatively high molecular weight hydrocarbons from the gaseous phase containing the relatively low molecular weight hydrocarbons. The liquid phase from the first liquid separator <NUM> is directed to the pump <NUM> via line <NUM>, and the gaseous phase from the first liquid separator <NUM> is directed to the second compressor <NUM> via line <NUM>.

The second compressor <NUM> receives and compresses the process fluid containing the gaseous phase from the first liquid separator <NUM>, and directs the compressed process fluid to the second cooler <NUM> via line <NUM>. In an exemplary embodiment, the second compressor <NUM> may compress the process fluid containing the gaseous phase to a pressure of about <NUM>,<NUM> kPa to about <NUM>,<NUM> kPa or greater. Compressing the process fluid in the second compressor <NUM> may generate heat (e.g., the heat of compression) to thereby increase the temperature of the process fluid. Accordingly, the second cooler <NUM> cools or removes at least a portion of the heat (e.g., the heat of compression) contained therein. The second cooler <NUM> cools at least a portion of the process fluid (e.g., the relatively high molecular eight hydrocarbons) to a liquid phase. The cooled process fluid from the second cooler <NUM> is directed to the second liquid separator <NUM> via line <NUM>.

The second liquid separator <NUM> receives the process fluid and separates the process fluid into a liquid phase and a gaseous phase. The second liquid separator <NUM> separates at least a portion of the liquid phase containing the condensed portions of the single mixed refrigerant (e.g., the relatively high molecular weight hydrocarbons) from the gaseous phases containing the non-condensed portions of the single mixed refrigerant (e.g., the relatively low molecular weight hydrocarbons). The separated liquid and gaseous phases are then directed from the second liquid separator <NUM> to the heat exchanger <NUM>. The gaseous phase from the second liquid separator <NUM> is directed to the heat exchanger <NUM> as a second portion of the single mixed refrigerant via line <NUM>. According to the invention the liquid phase from the first liquid separator <NUM> is combined with the liquid phase from the second liquid separator <NUM>, and the combined liquid phases are directed to the heat exchanger <NUM> as the first portion of the single mixed refrigerant. The pump <NUM> pressurizes or transfers the liquid phase from the first liquid separator <NUM> to line <NUM> via line <NUM>. Accordingly, the process fluid in line <NUM> includes the liquid phase from the second liquid separator <NUM> and the pressurized liquid phase from the pump <NUM>.

The first portion of the single mixed refrigerant (e.g., the liquid phase) may be directed through the pre-cooling zone <NUM> of the heat exchanger <NUM> from line <NUM> to line <NUM> to pre-cool the second portion of the single mixed refrigerant (e.g., the gaseous phase) flowing through the heat exchanger <NUM> from line <NUM> to line <NUM>. The first portion of the single mixed refrigerant may also be directed through the pre-cooling zone <NUM> from line <NUM> to line <NUM> to pre-cool the feed gas flowing through the feed gas stream from line <NUM> to line <NUM>. The first portion of the single mixed refrigerant may then be directed to the second expansion valve <NUM> via line <NUM>, and the second expansion valve <NUM> may expand the first portion of the single mixed refrigerant to thereby decrease the temperature and pressure thereof. The first portion of the single mixed refrigerant from the second expansion valve <NUM> may be directed to and through the heat exchanger <NUM> from line <NUM> to line <NUM> to provide further cooling or pre-cooling to the second portion of the single mixed refrigerant and/or the feed gas flowing through the heat exchanger <NUM>.

The second portion of the single mixed refrigerant (i.e. the gaseous phase) from the second liquid separator <NUM> may be directed through the pre-cooling zone <NUM> of the heat exchanger <NUM> from line <NUM> to line <NUM>. As discussed above, the second portion of the single mixed refrigerant flowing through the heat exchanger <NUM> from line <NUM> to line <NUM> may be pre-cooled by the first portion of the single mixed refrigerant in the pre-cooling zone <NUM>. The pre-cooled second portion of the single mixed refrigerant may then be directed to the first expansion valve <NUM> via line <NUM>, and the first expansion valve <NUM> may expand the second portion of the single mixed refrigerant to thereby decrease the temperature and pressure thereof. The second portion of the single mixed refrigerant from the first expansion valve <NUM> may then be directed to and through the heat exchanger <NUM> from line <NUM> to line <NUM> to cool at least a portion of the feed gas flowing through the feed gas stream from line <NUM> to line <NUM>. According to the invention, the first and second portions of the single mixed refrigerant flowing through the heat exchanger <NUM> may sufficiently cool at least a portion of the feed gas flowing through the feed gas stream to the LNG. The LNG produced may be discharged from the heat exchanger <NUM> via line <NUM>. The discharged LNG in line <NUM> may be directed to a storage tank <NUM> via flow control valve <NUM> and line <NUM>.

The heated or "spent" first portion of the single mixed refrigerant and the "spent" second portion of the single mixed refrigerant from the heat exchanger <NUM> are directed to the first compressor <NUM> of the compression assembly <NUM> via line <NUM> and line <NUM>, respectively. The "spent" first and second portions of the single mixed refrigerant may have a pressure relatively greater than ambient pressure. The "spent" first and second portions of the single mixed refrigerant have different pressures. For example, the "spent" first portion of the single mixed refrigerant in line <NUM> may have a pressure from about <NUM> kPa to about <NUM> kPa, and the "spent" second portion of the single mixed refrigerant in line <NUM> may have a pressure from about <NUM>,<NUM> kPa to about <NUM>,<NUM> kPa. The "spent" first portion of the single mixed refrigerant is directed to the first stage of the first compressor <NUM>, and the "spent" second portion of the single mixed refrigerant is directed to one of the intermediate stages of the first compressor <NUM>. Accordingly, the "spent" second portion of the single mixed refrigerant from the heat exchanger <NUM> may be directed to the first compressor <NUM> as a sidestream. The first compressor <NUM> receives the "spent" first portion of the single mixed refrigerant and a sidestream of the "spent" second portion of the single mixed refrigerant, and compresses the "spent" first and second portions of the single mixed refrigerant through the stages thereof.

The first compressor <NUM> combines the "spent" first and second portions of the single mixed refrigerant with one another to thereby provide the compressed process fluid containing the single mixed refrigerant in line <NUM>. The compressed process fluid containing the single mixed refrigerant is then re-directed through the refrigerant cycle as described above. It should be appreciated that the ability to receive the first portion of the single mixed refrigerant and the second portion of the single mixed refrigerant (e.g., sidestream) at separate stages of a single compressor (e.g., the first compressor <NUM>) may reduce the cost, energy consumption, and/or complexity of the liquefaction system <NUM>. For example, the ability to receive the first portion of the single mixed refrigerant and the second portion of the single mixed refrigerant in a single compressor (e.g., the first compressor <NUM>) at a first pressure (e.g., about <NUM> kPa to about <NUM> kPa) and a second pressure (e.g., about <NUM>,<NUM> kPa to about <NUM>,<NUM> kPa), respectively, may reduce the number of compressors <NUM>, <NUM> utilized in the liquefaction system <NUM>. In another example, the ability to receive the first portion of the single mixed refrigerant at the first stage of the single compressor (e.g., the first compressor <NUM>) and the second portion of the single mixed refrigerant (e.g., as a sidestream) at an intermediate stage of the single compressor may reduce energy consumption and increase an efficiency of the liquefaction system <NUM>.

<FIG> illustrates a flowchart of a method <NUM> for producing liquefied natural gas, not according to the invention. The method <NUM> may include feeding natural gas through a heat exchanger, as shown at <NUM>. The method <NUM> may also include compressing a first portion of a single mixed refrigerant in a first compressor, as shown at <NUM>. The method <NUM> may further include compressing a second portion of the single mixed refrigerant in the first compressor, as shown at <NUM>. The method <NUM> may also include combining the first portion of the single mixed refrigerant with the second portion of the single mixed refrigerant in the first compressor to produce the single mixed refrigerant, as shown at <NUM>. The method <NUM> may also include cooling the single mixed refrigerant in a first cooler to produce a first liquid phase and a gaseous phase, as shown at <NUM>. The method <NUM> may also include separating the first liquid phase from the gaseous phase in a first liquid separator, as shown at <NUM>. The method <NUM> may also include compressing the gaseous phase in a second compressor, as shown at <NUM>. The method <NUM> may also include cooling the compressed gaseous phase in a second cooler to produce a second liquid phase and the second portion of the single mixed refrigerant, as shown at <NUM>. The method <NUM> may also include separating the second liquid phase from the second portion of the single mixed refrigerant in a second liquid separator, as shown at <NUM>. The method <NUM> may also include pressurizing the first liquid phase in a pump, as shown at <NUM>. The method <NUM> may also include combining the first liquid phase with the second liquid phase to produce the first portion of the single mixed refrigerant, as shown at <NUM>. The method <NUM> may also include feeding the first portion of the single mixed refrigerant and the second portion of the single mixed refrigerant to the heat exchanger to cool at least a portion of the natural gas flowing therethrough to thereby produce the liquefied natural gas, as shown at <NUM>.

Claim 1:
A method for producing liquefied natural gas, comprising:
feeding natural gas through a single heat exchanger (<NUM>);
feeding a first portion of a single mixed refrigerant from the heat exchanger (<NUM>) to a first stage of a first compressor (<NUM>), compressing the first portion of the single mixed refrigerant in the first compressor (<NUM>);
feeding a second portion of the single mixed refrigerant from the heat exchanger (<NUM>) to an intermediate stage of the first compressor (<NUM>),
compressing the second portion of the single mixed refrigerant in the first compressor (<NUM>);
combining the first portion of the single mixed refrigerant with the second portion of the single mixed refrigerant in the first compressor (<NUM>) to produce the single mixed refrigerant;
cooling the single mixed refrigerant in a first cooler (<NUM>) to produce a first liquid phase and a gaseous phase;
separating the first liquid phase from the gaseous phase in a first liquid separator (<NUM>);
compressing the gaseous phase in a second compressor (<NUM>);
cooling the compressed gaseous phase in a second cooler (<NUM>) to produce a second liquid phase and a gaseous phase as the second portion of the single mixed refrigerant;
separating the second liquid phase from the gaseous phase as the second portion of the single mixed refrigerant in a second liquid separator (<NUM>);
pressurizing the first liquid phase in a pump (<NUM>) fluidly coupled with the first liquid separator (<NUM>);
combining the first liquid phase from the pump (<NUM>) with the second liquid phase from the second liquid separator (<NUM>) to produce the first portion of the single mixed refrigerant;
feeding the first portion of the single mixed refrigerant to the heat exchanger (<NUM>) and feeding the second portion of the single mixed refrigerant to the heat exchanger (<NUM>) to cool at least a portion of the natural gas flowing therethrough to thereby produce the liquefied natural gas.