JP2010514871A - Liquefied natural gas production system and method - Google Patents

Abstract

  Reference is made to a process for the production of liquefied natural gas (LNG) involving the use of air as a refrigerant in an open or closed cycle. The present invention also refers to a system for performing the above process.

Description

  The present invention relates to a process for obtaining liquefied natural gas (LNG) using air as a refrigerant. This process can be performed by using an open air refrigerant cycle or a closed air refrigerant cycle.

  Natural gas is often obtained in an area remote from where it will eventually be used. When transporting natural gas, the natural gas is cooled to a temperature of about −260 ° F. (−160 ° C.) at atmospheric pressure to condense into a liquid called liquefied natural gas (LNG). This LNG is usually transported abroad on a suitable carrier ship.

  A number of process cycles for LNG production have been developed to provide a large amount of cooling requirements. Such a cycle typically uses a mixed refrigerant comprising light hydrocarbons and optionally nitrogen. For example, US Pat. No. 4,274,849 discloses a mixture of at least two hydrocarbons as refrigerant fluids in the process to liquefy natural gas.

US Pat. No. 4,274,849

  The present invention refers to a novel process, system or plant capable of liquefying natural gas from any kind of natural gas field, in particular from an “offshore” gas field, in particular from a strandid natural gas field. This process includes air used as a refrigerant.

  The system of the present invention includes a simple and easily reproducible process in all possible locations, preferably in “offshore” natural gas fields. This system is particularly advantageous when located in a barge to liquefy gas from a small natural gas field located in a remote area far from the coast.

  Accordingly, the first aspect of the present invention refers to a process for obtaining liquefied natural gas containing air used as a refrigerant. This process can be deployed as an air refrigeration cycle independent of the natural gas stream.

According to the process of the present invention, the following steps are performed:
a. Compressing air; and
b. Cooling the compressed air in step (a);
c. Once expanding the compressed air cooled in step (b);
d. Using the expanded air described above to cool natural gas;
An air refrigeration cycle is provided.

In an embodiment of the invention, the process also includes an additional step, namely
e. Adding air replenishment to compensate for possible air loss;
Can be included.

  The air replenishment may be processed to remove CO2 and water that can be carried using processing equipment well known in the art.

  In a further preferred embodiment, process step (a) is carried out in at least one stage, preferably two or more stages.

  The air refrigerant cycle of the present invention may be either an open type or a closed type. If the air refrigeration cycle is open, air is continuously taken from the environment at atmospheric conditions, and according to the above process, to remove CO2 and water used to cool natural gas Processed and returned to the atmosphere.

  If the air refrigeration cycle is closed, the air used to cool the natural gas is returned to the beginning of the process (step (a)).

According to the process of the present invention, the natural gas stream is subjected to the following steps:
a. Cooling natural gas; and
b. Once expanding the natural gas cooled in step (a) to obtain LNG or LNG and gas phase (end flash gas);
Pass through.

  Thus, natural gas can be completely liquefied (LNG) without going through the gas phase or without obtaining two phases, ie, liquid phase and gas phase.

In another embodiment of the invention, if two phases are obtained in step (b), the natural gas stream is further processed, i.e.
c. Separating the above phases, namely the liquid phase (LNG) and the gas phase (end flash gas).

  Optionally, the natural gas stream may further comprise a stage for separating natural gas liquid (NGL) prior to liquefaction.

  The term “natural gas liquid (NGL)” as used herein refers to the low volatility component of natural gas, including small amounts of methane, ethane to higher hydrocarbons (ethane, propane, butane, isobutane and natural gasoline). The latter is sometimes called condensate).

  In another embodiment of the present invention, the natural gas stream can be pretreated before cooling, if desired. Various pretreatment procedures are well known in the art. Appropriate pretreatment depends on the location, type, exact composition, depth and nature of the unwanted contaminants or impurities present in the natural gas feed. Usually, this may include, but is not limited to, removal of Hg, H2O, CO2 or H2S.

  This natural gas stream is often fed to the process at a pressure of at least 1 bar, preferably above 10 bar. The natural gas stream also depends on the location and the type of natural gas in the gas field.

  Natural gas supplies may be scarce with heavy hydrocarbons and do not require separation of natural gas liquids. Alternatively, even though natural gas is partially enriched in heavy hydrocarbons, the natural gas liquid can be trapped in the final LNG and separated by fractional distillation after transport to final purpose. In both cases, the natural gas is liquefied per se, which is achieved using one heat exchanger.

  In a further preferred embodiment, the natural gas stream, which is particularly rich in heavy hydrocarbons, is cooled in two stages.

  First, natural gas is cooled by air to a suitable temperature that allows it to condense as a liquid to the required amount of natural gas liquid (NGL). This stage is performed through the first heat exchanger. This temperature may depend on specific requirements in the composition of the natural gas feed, LNG specifications or heavier component recovery and / or purity. This temperature is −100 ° C. or higher.

  The result of the previous stage is a light hydrocarbon gas stream called lean natural gas. The term “lean natural gas” refers to a stream containing nearly all of the initial feed methane and nitrogen, the desired amount of ethane, and a small amount of low volatile components (Proban and higher hydrocarbons).

  Second, the lean natural gas passes through a second heat exchanger that is cooled with air until the gas is substantially fully or completely liquefied. The temperature at which NG exits this heat exchanger is -163 ° C or higher.

  As an alternative embodiment, the cooling and liquefaction of the natural gas stream, particularly rich in heavy hydrocarbons, can be performed in a single heat exchanger. In this case, NGL extraction is performed after gas pretreatment and before cooling and liquefaction.

  In another embodiment of the invention, an end flash gas stream obtained as a by-product in natural gas liquefaction cools the natural gas and air streams in the process to recover its cryogenic energy as fuel gas. Used for. This fuel can be supplied to the gas turbine and usually needs to be pressurized by the compressor before being introduced into the gas turbine. The amount of end flash gas obtained may correspond to, or be part of, the amount of fuel gas required, may be excessive, and partially used for other purposes. .

  The second aspect of the present invention refers to a system for carrying out the aforementioned process comprising a continuous stream of natural gas, a gas processing facility and an air refrigerant cycle.

  Air is used as a refrigerant in the refrigerant cycle of this system to obtain liquefied natural gas. This cycle may preferably be a closed loop or an open cycle.

In other embodiments, the system includes the following parts of the equipment:
-Heat exchanger. Although any type of heat exchanger may be used in the present invention, a plate fin heat exchanger is preferred. On the natural gas side, the minimum number of heat exchangers is one, but any number of heat exchangers are possible.

  In an embodiment of the invention, the system can have heat exchange between the compressed air stream and the exhaust after cooling and liquefaction in a separate heat exchanger.

  -Expanders. Examples of suitable expanders are JT-valves (Joule Thomson valve) and turbine expanders, but any type of expander may be employed.

  On the air side, at least one expander is required for air expansion. Due to power limitations, two or more air expanders can be in parallel. The air expander can be coupled to one or more air compressors to recover power. Alternatively, their power can be utilized for other process purposes such as power generation.

  On the natural gas side, at least one expander is necessary to expand the liquid obtained from the heat exchanger.

  -Compressor. At least one is required to compress the air. The number of compression stages in the air cycle depends on process optimization. The number is not a fixed number.

  If more than one compressor is used and there is one or more heat exchangers (rear coolers) after the last compressor, the compression section is one or more between the compressors. It is preferable to include a heat exchanger (intercooler). The intercooler and rear cooler preferably use water as the refrigerant, but air can also be used. A shell and tube heat exchanger is preferred.

  Along with the cooling system, another compressor may be required to supply the end flash gas to the gas turbine.

  -Compressor drive. This device drives all of the compression stages except the compression stage connected to the air expander. A gas turbine or electric motor can be used.

  In a further preferred embodiment, the system includes additional equipment:

  -Column for NGL extraction. If NGL fractionation is required, more than one column may be required. If it is not necessary to extract the natural gas liquid, the NGL extraction column can be bypassed, so in this case the natural gas stream is directed to a further heat exchanger for natural gas liquefaction. be able to.

  In a further preferred embodiment, the system includes additional equipment:

  -End flash vessel.

  In a second embodiment of the invention, the system may be located on either a stationary structure such as a platform or a movable structure such as a barge or ship. Both structures may be used in all types of natural gas fields, including inland gas fields and offshore gas fields. This allows development in any kind of deposit, even in the case of the development of stranded gas (small and remote gas fields).

  The term “inland” as used herein refers to what is on land.

  The term “offshore” as used herein refers to what is in the sea away from the coast and is obtained out of the sea, not the coastline.

  As an alternative embodiment, the system may be located at two separate locations (two different fixed structures, two different movable structures or one fixed structure and one movable structure). One location may be dedicated to the gas pretreatment unit and the NGL extraction facility, and the other location may be dedicated to the liquefaction unit.

  Accordingly, the third aspect of the present invention refers to the use of the system described above with respect to natural gas fields and preferably to stranded natural gas fields.

  The term “Stranded Natural Gas Field”, as used herein, refers to a natural gas field that has been discovered but remains unsuitable for use for physical or economic reasons. Economically, the reserves are too far from the natural gas market, and physically, the gas fields are too deep to be drilled or under obstacles.

  Another aspect of the invention relates to the use of the system described above for producing LNG of at least 0.1 MTA (1 million tons / year), preferably the production of LNG is within the range of 0.5-3 MTA. is there.

  Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Methods and materials similar or equivalent to those described herein can be used in the practice of the present invention. Throughout the specification and claims, the word “comprising” and variations thereof are not intended to exclude other technical features, additives, ingredients or steps. Additional objects, advantages and features of the present invention will become apparent to those skilled in the art upon review of this specification or may be learned by practice of the present invention. The following examples and figures are provided by way of illustration and are not intended to limit the invention. Various necessary subsystems such as valves, control systems, sensors, clamps and riser support structures have been omitted from the figure for simplicity and clarity.

1 is a schematic diagram of a closed refrigerant cycle according to the present invention. FIG. 1 is a schematic diagram of an open-type refrigerant cycle according to the present invention. 1 is a schematic diagram of a closed refrigerant cycle according to the present invention. FIG.

  The following examples show some details of possible process schemes, operating states that are not exhaustive of all possible schemes, and states detailed in the list of claims given below. give.

  FIG. 1 illustrates one embodiment of the present invention as applied to the liquefaction of a natural gas feed stream using air as the refrigerant. The natural gas feed stream 1 is processed in a conventional pretreatment plant A to remove CO2, H2S, water and mercury contaminants.

  The treated gas stream 2 corresponds to a desulfurized dry natural gas stream at 15 ° C. and 30 bar. Stream 2 has a molar composition as given in Table 1 below.

  Stream 2 enters a liquefaction plant that passes through two heat exchangers 100, 101 to obtain a supercooled high pressure liquid stream 6. In the first heat exchanger 100, the natural gas is pre-cooled to an intermediate temperature of about −69 ° C. (stream 3) in order to condense the natural gas liquid. Stream 3 enters column B, natural gas liquid is extracted as stream 4 at the bottom, and lean gas stream 5 exits the column at the top. When specific products such as propane and butane are required, stream 4 is directed to the fractionation section.

  The lean natural gas (stream 5) enters the second heat exchanger 101 and is cooled to a temperature of about −130 ° C. to obtain a supercooled high pressure liquid stream (stream 6) that is directed to the JT-valve 102. It is done. Stream 6 is adiabatically expanded to 1.1 bar through JT-valve 102, and finally directed to end flash vessel 103, separated into liquid and vapor, and LNG to stream (stream 8) And end flash gas (stream 9). Both are about -160 ° C and 1.1 bar.

  Each stream 9 is returned to both heat exchangers 101 and 100 and the cryogenic energy of this stream is recovered. Thus, stream 9 exits heat exchanger 101 at −91 ° C. to obtain stream 10 and is further heated by heat exchanger 100 to a temperature of 15 ° C. (stream 11). This steam stream 11 can be used as fuel in the plant. When this fuel is supplied to a gas turbine, the stream 11 typically needs to be pressurized by a compressor before being introduced into the gas turbine.

  In this embodiment, the heat exchanger on the natural gas side is a plate fin heat exchanger.

  Here, an air refrigeration cycle for converting a gas stream 2 starting from an air stream 12 into a liquid stream 6 will be described. The air stream 12 uses up all or most of its cooling properties by absorbing heat from the feed gas. The stream 12 at about 34 ° C. is fed to a multi-stage compressor unit 104 with an intermediate cooling stage and a rear cooling stage at the minimum pressure of the cycle (about 2 bar) and recompressed to produce a compressed stream 18. The compressor section includes three compressors 105, 107 and 109, with one heat exchanger 106 between the compressor 105 and the compressor 107, and one between the compressor 107 and the compressor 109. There is a heat exchanger 108 and there is one heat exchanger 110 after the last compressor 109. Intermediate coolers 106 and 108 and rear cooler 110 use water as a refrigerant. The compressed stream 18 exits the compressor unit 104 at 40 ° C. and 30 bar, is directed to the heat exchanger 100 and is pre-cooled to −24 ° C. by countercurrent passage of the air refrigerant stream 21 and the end flash gas 10. Stream 18 emerges from heat exchanger 100 as stream 19 and is fed into expander section 111, reducing the pressure and temperature of air stream 19, resulting in stream 24. The expander section includes two turbo expanders 112 and 113 that are in parallel and is used to provide a portion of the power for the compressor of the compressor unit 104. The air stream 24 (expanded in the expander section 111) is 2.1 bar and is at a temperature of about −135 ° C. Each air stream 24 passes through both heat exchangers 101 and 100. In heat exchanger 101, stream 24 provides sufficient cooling to liquefy natural gas stream 5 to form liquid natural gas (stream 6). Stream 24 emerges from heat exchanger 101 as stream 25 at a temperature of −73 ° C. and enters heat exchanger 100 to precool both natural gas (stream 2) and compressed air (stream 18). Stream 25 exits heat exchanger 100 as stream 12 and begins the cycle again.

  The air cycle has a point to refill to compensate for air loss in the air cycle. Air replenishment must be processed in the processing facility to eliminate the CO2 and water that can be carried.

  Table 2 shows the operating state of the main stream of FIG.

  FIG. 2 shows another embodiment of the present invention. In the embodiment shown in FIG. 2, as a modification with respect to FIG. 1, air used as refrigerant flows in the open loop.

  As in the previous example, natural gas 1 is treated in pretreatment plant A to remove CO2, H2S, water and mercury contaminants (treated gas stream 2 is listed in Table 1). With the indicated composition) and then liquefied by exchange with cold air in two steps. First, it is precooled in the heat exchanger 100 to a temperature of about −69 ° C. (stream 3). Stream 3 passes through column B, liquid is extracted as bottom stream 4, and lean natural gas exits column B at the top (stream 5) and enters the second heat exchanger 101. The liquid stream emerging from the heat exchanger 101 at about −130 ° C. (stream 6) is directed to the expansion section and is adiabatically expanded to 1.1 bar in the JT-valve 102 (stream 7). Finally, stream 7 is directed to end flash vessel 103 and separated into liquid and vapor to produce LNG (stream 8) and end flash gas (stream 9) to the reservoir. Both are about -160 ° C and 1.1 bar.

  Each stream 9 is returned to both heat exchangers 101 and 100 and the cryogenic energy of this stream is recovered in the same manner as in Example 1. Eventually, the end flash gas exits heat exchanger 100 as stream 11 at 15 ° C. and 1 bar. This steam stream 11 can be used as fuel in the plant.

  The air refrigeration cycle in FIG. 2 is an open loop. In this cycle, air is continuously taken from the atmosphere at ambient conditions (stream 12 '). Stream 12 'enters processing plant C, which is responsible for the removal of CO2 and water capable of carrying air, and exits the plant as stream 12 (15 ° C, 1 bar). Stream 12 is compressed in a multi-stage compressor unit 104 with an intermediate cooling stage and a rear cooling stage to produce a compressed stream 18 and exits compressor unit 104 at 40 ° C. and 16 bar. The compressed stream 18 is directed to the heat exchanger 100 and pre-cooled to −27 ° C. by countercurrent passage of the air refrigerant stream 21 and the end flash gas 10. Stream 18 emerges from heat exchanger 100 as stream 19 and is fed into expander section 111 where the pressure and temperature are reduced to about 1.2 bar and −133 ° C., respectively (stream 24). Each stream 24 passes through both heat exchangers 101 and 100. In heat exchanger 101, stream 24 provides sufficient cooling to liquefy natural gas stream 5 to form liquid natural gas (stream 6). Stream 24 emerges from heat exchanger 101 as stream 25 at a temperature of about −74 ° C. and enters heat exchanger 100 to precool both natural gas (stream 2) and compressed air (stream 18). Stream 25 exits heat exchanger 100 at about 33 ° C. and 1 bar and is released directly to atmosphere 26.

  Table 3 shows the operating state of the main stream of FIG.

  FIG. 3 shows another embodiment of the present invention.

  As in the previous example, natural gas 1 is processed in pretreatment plant A (processed gas stream 2 has the composition shown in Table 1), and then heat in two steps. And liquefied by exchange with cold air. First, it is pre-cooled in the heat exchanger 120 to a temperature of about −69 ° C. (stream 3). Stream 3 passes through column B, liquid is extracted as bottom stream 4, and lean natural gas exits column B at the top (stream 5) and enters the second heat exchanger 101. The liquid stream emerging from the heat exchanger 101 at about −131 ° C. (stream 6) is directed to the expansion section and is adiabatically expanded to 1.1 bar in the JT-valve 102 (stream 7). Finally, stream 7 is directed to end flash vessel 103 and separated into liquid and vapor to produce LNG (stream 8) and end flash gas (stream 9) to the reservoir. Both are about -160 ° C and 1.1 bar.

  Stream 9 is returned to both heat exchangers 101 and 100, respectively, and the cryogenic energy of this stream is recovered in the same manner as in Examples 1 and 2. Eventually, the end flash gas exits heat exchanger 120 as stream 11 at 15 ° C. and 1 bar. This steam stream 11 can be used as fuel in the plant.

  Here, the air refrigeration cycle for converting the gas stream 2 to the liquid stream 6 will be described from the air stream 12. Stream 12, which is approximately 36 ° C. and 3.6 bar, is compressed in a multistage compressor unit 104 with an intermediate cooling stage and a rear cooling stage to produce a compressed stream 18, at 40 ° C. and 43 bar, compressor unit 104. Exit. The compressed stream 18 is directed to a further heat exchanger 114 and precooled to −33 ° C. by countercurrent passage of the air refrigerant stream 121. Stream 18 emerges from heat exchanger 114 as stream 19 and is fed into expander section 111 where the pressure and temperature are reduced to approximately 4 bar and -135 ° C., respectively (stream 24). Each stream 24 passes through both heat exchangers 101 and 120. In heat exchanger 101, stream 24 provides sufficient cooling to liquefy natural gas stream 5 to form liquid natural gas (stream 6). Stream 24 emerges from heat exchanger 101 as stream 25 at a temperature of about −81 ° C. and enters heat exchanger 120 to precool natural gas (stream 2). Stream 25 exits heat exchanger 120 as stream 121 and is directed to heat exchanger 114 at approximately −43 ° C. and 3.7 bar. This stream precools the countercurrent air stream 18. Stream 121 exits heat exchanger 114 and begins the cycle again.

  The air cycle has a point to refill to compensate for air loss in the air cycle. Air replenishment must be processed in the processing facility to eliminate the CO2 and water that can be carried.

  Table 4 shows the operating state of the main stream of FIG.

Claims (21)

A movable structure;
A natural gas liquefaction plant installed in the movable structure;
A liquefied natural gas (LNG) production system comprising:
The natural gas liquefaction plant is
At least one compressor (105, 107, 109) in which air (12) is compressed to obtain compressed air (18);
At least one expander (112, 113), wherein the cooled compressed air (19) is expanded to obtain expanded air (24, 25);
At least one heat exchange wherein natural gas (2,5) is cooled using said expanded air (24,25) and end flash gas (9,10) to obtain cooled natural gas (3,6) Vessel (100, 120, 101),
An expansion device (102) in which the cooled natural gas (6) is expanded to obtain an expanded cooled natural gas (7);
Liquefied natural gas (LNG) comprising an end flash vessel (103) in which the expanded cooled natural gas (7) is separated into a liquefied natural gas LNG (8) and the end flash gas (9). Manufacturing system. The heat exchanger (100) further
An inlet for compressed air (18);
An outlet for cooled compressed air (19);
An outlet for air (12) further connected to the compressor (105, 107, 109) in the closed refrigeration cycle or to the atmosphere (26) in the open refrigeration cycle;
The manufacturing system of the liquefied natural gas (LNG) of Claim 1 containing this.   The compressed air (18) comprises a further heat exchanger (114) that is cooled with air (121) from the heat exchanger (120) to obtain cooled compressed air (19). Liquefied natural gas (LNG) production system.   Fractionation located after the heat exchanger (100, 120), into which the cooled natural gas (3) enters, the natural gas liquid rich stream (4) is extracted, and the lean natural gas (5) is extracted The liquefied natural gas (LNG) manufacturing system according to claim 1, further comprising a unit B.   Lean natural gas (5) is cooled using expanded air (24) and end flash gas (9), and after fractionation unit B to obtain cooled natural gas (6), a heat exchanger (101) The liquefied natural gas (LNG) production system according to claim 4, which is located.   The liquefied natural of claim 1, further comprising a pretreatment plant A located in front of the heat exchanger (100, 120) and from which unwanted contaminants or impurities present in the natural gas feed (1) are removed. Gas (LNG) production system.   2. The processing plant C of claim 1 further comprising CO2 and water removed from air (12 ') taken from the atmosphere to obtain the air (12) directed to a compressor (105, 107, 109). Liquefied natural gas (LNG) production system.   The liquefied natural gas (LNG) manufacturing system according to claim 1, wherein the movable structure includes a barge.   The liquefied natural gas (LNG) manufacturing system according to claim 1, wherein the movable structure includes a ship. a. Compressing air (12) to obtain compressed air (18);
b. Cooling the compressed air (18) to obtain cooled compressed air (19);
c. Expanding the cooled compressed air (19) to obtain expanded air (24, 25);
d. Using the expanded air (24, 25) to cool the natural gas stream (2, 5) to obtain a cooled natural gas (6);
e. The cooled natural gas (6) is expanded to obtain expanded natural gas (7), and the expanded natural gas is separated in liquid (LNG) (8) and end flash gas (9). Process,
A method for producing liquefied natural gas (LNG) containing
The method wherein the end flash gas (9) is used to cool the natural gas stream (2, 5).   f. The method of claim 10, further comprising incorporating air replenishment into the air circuit to compensate for air loss.   The method according to claim 10, wherein the step of compressing air (12) is performed in two or more stages.   11. The method according to claim 10, wherein the expanded air (25) used to cool the natural gas is returned to the beginning of the process as further compressed air (12).   11. A method according to claim 10, wherein the expanded air (25) once used to cool the natural gas is returned to the atmosphere (26).   g. 11. The method of claim 10, further comprising separating the natural gas into a natural gas liquid rich stream (NGL) (4) and a lean gas (5) prior to liquefaction of the natural gas.   The method of claim 10, wherein the natural gas stream is fed to the process at a pressure of at least 1 bar.   The method of claim 10, wherein the natural gas is cooled to liquefy in two stages.   18. The method according to claim 17, wherein the natural gas stream is cooled in a first stage of cooling to a temperature that allows extraction of the natural gas liquid rich stream (NGL) (4).   18. The method of claim 17, wherein the natural gas stream is cooled in a second stage of cooling to a temperature that allows liquefaction of the natural gas stream.   The method of claim 10, wherein the natural gas is pretreated before cooling.   21. The method of claim 20, wherein the natural gas pretreatment comprises removal of CO2, H2S, H2O or Hg.

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