NITROGEN MANAGEMENT IN AN LNG PLANT

Abstract

Electric, steam, or expansion driven compressors eliminate the ability to process feed gas with higher nitrogen content without a complex nitrogen rejection unit. An LNG slip stream from a high-stage (HS) flash drum vapor, subcooled high pressure LNG from methane economizer, or high pressure methane recycle stream is subcooled in a heat exchanger by low stage (LS) methane refrigerant. The subcooled slip stream is flashed to pressure higher than the LS flash drum pressure in a flash vessel. The vapor stream is recycled and is mixed with the vapor from LS flash drum. The high-nitrogen liquid stream from the side flash drum is sent to the LNG product. This process allows rejecting more light components including nitrogen to an LNG product eliminating the accumulation of light components in the system for the feed gas with high concentrations of light components.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application which claims benefit under 35 USC § 119(e) to U.S. Provisional Application Ser. No. 63/755,178 filed Feb. 6, 2025, entitled “LOW EMISSION LNG NITROGEN REMOVAL,” which is incorporated herein in its entirety.

FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

None.

BACKGROUND OF THE INVENTION

In the process of liquified natural gas (LNG) production there is a need to replace hydrocarbon powered generators with electric, expansion, and steam powered generators that use alternative power sources to drive compressors and other equipment.

An example cascade LNG facility 100 as shown in FIG. 1 (US20210140710) does not contain a Nitrogen Rejection Unit (NRU). The LNG facility 100 generally comprises a first refrigeration cycle 30 (e.g., a propane refrigeration cycle), a second refrigeration cycle 50 (e.g., an ethylene refrigeration cycle), and a third refrigeration cycle 70 (e.g., a methane refrigeration cycle) with an expansion section 80. Those skilled in the art will recognize that FIG. 1 is a schematic only and, therefore, various equipment, apparatuses, or systems that would be needed in a commercial plant for successful operation have been omitted for clarity. Such components might include, for example, compressor controls, flow and level measurements and corresponding controllers, temperature and pressure controls, pumps, motors, filters, additional heat exchangers, valves, and/or the like. Those skilled in the art will recognize such components and how they are integrated into the systems and methods disclosed herein.

Operation of the LNG facility 100 begins with the propane refrigeration cycle 30. Propane is compressed in a multi-stage (e.g., three-stage) propane compressor 31 driven by, for example, a gas turbine driver (not illustrated). The stages of compression may exist in a single unit or a plurality of separate units mechanically coupled to a single driver. Upon compression, the propane is passed through a conduit 300 to a propane cooler 32 where the propane is cooled and liquefied through indirect heat exchange with an external fluid (e.g., air or water). A portion of the stream from the propane cooler 32 can then be passed through conduits 302 and 302 A to a pressure reduction system 36 A, for example, an expansion valve, as illustrated in FIG. 1. At the pressure reduction system 36 A, the pressure of the liquefied propane is reduced, thereby evaporating or flashing a portion of the liquefied propane. A resulting two-phase stream then flows through a conduit 304 A into a high-stage propane chiller 33 A, which cools the natural gas stream in indirect heat exchange 38. A high stage propane chiller 33 A uses the flashed propane refrigerant to cool the incoming natural gas stream in a conduit 110. Another portion of the stream from the propane cooler 32 is routed through a conduit 302 B to another pressure reduction system 36 B, illustrated, for example, in FIG. 1 as an expansion valve. At the pressure reduction system 36 B, the pressure of the liquefied propane is reduced in a stream 304 B.

The cooled natural gas stream from the high-stage propane chiller 33 A flows through a conduit 114 to a separation vessel. At the separation vessel, water and in some cases a portion of the propane and/or heavier components are removed. In some cases where removal is not completed in upstream processing, a treatment system 40 may follow the separation vessel. The treatment system 40 removes moisture, mercury and mercury compounds, particulates, and other contaminants to create a treated stream. The stream exits the treatment system 40 through a conduit 116. The stream 116 then enters the intermediate-stage propane chiller 34. At the intermediate-stage propane chiller 34, the stream is cooled in indirect heat exchange 41 via indirect heat exchange with a propane refrigerant stream. The resulting cooled stream output into a conduit 118 is routed to the low-stage propane chiller 35, where the stream can be further cooled through indirect heat exchange means 42. The resultant cooled stream exits the low-stage propane chiller 35 through a conduit 120. Subsequently, the cooled stream in the conduit 120 is routed to the high-stage ethylene chiller 53.

A vaporized propane refrigerant stream exiting the high-stage propane chillers 33 A and 33 B is returned to a high-stage inlet port of the propane compressor 31 through a conduit 306. An unvaporized propane refrigerant stream exits the high-stage propane chiller 33 B via a conduit 308 and is flashed via a pressure reduction system 43, illustrated in FIG. 1 as an expansion valve, for example. The liquid propane refrigerant in the high-stage propane chiller 33 A provides refrigeration duty for the natural gas stream. A two-phase refrigerant stream enters the intermediate-stage propane chiller 34 through a conduit 310, thereby providing coolant for the natural gas stream (in conduit 116) and the stream entering the intermediate-stage propane chiller 34 through a conduit 204. The vaporized portion of the propane refrigerant exits the intermediate-stage propane chiller 34 through a conduit 312 and enters an intermediate-stage inlet port of the propane compressor 31. The liquefied portion of the propane refrigerant exits the intermediate-stage propane chiller 34 through a conduit 314 and is passed through a pressure-reduction system 44, for example an expansion valve, whereupon the pressure of the liquefied propane refrigerant is reduced to flash or vaporize a portion of the liquefied propane. The resulting vapor-liquid refrigerant stream is routed to the low-stage propane chiller 35 through a conduit 316. At the low-stage propane chiller 35, the refrigerant stream cools the methane-rich stream and an ethylene refrigerant stream entering the low-stage propane chiller 35 through the conduits 118 and 206, respectively. The vaporized propane refrigerant stream exits the low-stage propane chiller 35 and is routed to a low-stage inlet port of the propane compressor 31 through a conduit 318. The vaporized propane refrigerant stream is compressed and recycled at the propane compressor 31 as previously described.

In one implementation, a stream of ethylene refrigerant in a conduit 202 enters the high-stage propane chiller 33 B. At the high-stage propane chiller 33 B, the ethylene stream is cooled through indirect heat exchange 39. The resulting cooled ethylene stream is routed in the conduit 204 from the high-stage propane chiller 33 B to the intermediate-stage propane chiller 34. Upon entering the intermediate-stage propane chiller 34, the ethylene refrigerant stream may be further cooled through indirect heat exchange 45 in the intermediate-stage propane chiller 34. The resulting cooled ethylene stream exits the intermediate-stage propane chiller 34 and is routed through a conduit 206 to enter the low-stage propane chiller 35. In the low-stage propane chiller 35, the ethylene refrigerant stream is at least partially condensed, or condensed in its entirety, through indirect heat exchange 46. The resulting stream exits the low-stage propane chiller 35 through a conduit 208 and may be routed to a separation vessel 47. At the separation vessel 47, a vapor portion of the stream, if present, is removed through a conduit 210, while a liquid portion of the ethylene refrigerant stream exits the separator 47 through a conduit 212. The liquid portion of the ethylene refrigerant stream exiting the separator 47 may have a representative temperature and pressure of about −24° F. (≈−31° C.) and about 285 psig (≈1,965 kPa and 20 bar). However, other temperatures and pressures are contemplated.

Turning now to the ethylene refrigeration cycle 50 in the LNG facility 100, in one implementation, the liquefied ethylene refrigerant stream in the conduit 212 enters an ethylene economizer 56, and the stream is further cooled by an indirect heat exchange 57 at the ethylene economizer 56. The resulting cooled liquid ethylene stream is output into a conduit 214 and routed through a pressure reduction system 58, such as an expansion valve. The pressure reduction system 58 reduces the pressure of the cooled predominantly liquid ethylene stream to flash or vaporize a portion of the stream. The cooled, two-phase stream in a conduit 215 enters the high-stage ethylene chiller 53. In the high-stage ethylene chiller 53, at least a portion of the ethylene refrigerant stream vaporizes to further cool the stream in the conduit 120 entering an indirect heat exchange 59. The vaporized and remaining liquefied ethylene refrigerant exits the high-stage ethylene chiller 53 through conduits 216 and 220, respectively. The vaporized ethylene refrigerant in the conduit 216 may re-enter the ethylene economizer 56, and the ethylene economizer 56 warms the stream through an indirect heat exchange 60 prior to entering a high-stage inlet port of the ethylene compressor 51 through a conduit 218. Ethylene is compressed in multi-stages (e.g., three-stage) at the ethylene compressor 51 driven by, for example, a gas turbine driver (not illustrated). The stages of compression may exist in a single unit or a plurality of separate units mechanically coupled to a single driver.

The cooled stream in the conduit 120 exiting the low-stage propane chiller 35 is routed to the high-stage ethylene chiller 53, where it is cooled via the indirect heat exchange 59 of the high-stage ethylene chiller 53. The remaining liquefied ethylene refrigerant exiting the high-stage ethylene chiller 53 in a conduit 220 may re-enter the ethylene economizer 56 and undergo further sub-cooling by an indirect heat exchange 61 in the ethylene economizer 56. The resulting sub-cooled refrigerant stream exits the ethylene economizer 56 through a conduit 222 and passes a pressure reduction system 62, such as an expansion valve, whereupon the pressure of the refrigerant stream is reduced to vaporize or flash a portion of the refrigerant stream. The resulting, cooled two-phase stream in a conduit 224 enters the low-stage ethylene chiller/condenser 55.

A portion of the cooled natural gas stream exiting the high-stage ethylene chiller 53 is routed through conduit a 122 to enter an indirect heat exchange 63 of the low-stage ethylene chiller/condenser 55. In the low-stage ethylene chiller/condenser 55, the cooled stream is at least partially condensed and, often, subcooled through indirect heat exchange with the ethylene refrigerant entering the low-stage ethylene chiller/condenser 55 through the conduit 224. The vaporized ethylene refrigerant exits the low-stage ethylene chiller/condenser 55 through a conduit 226, which then enters the ethylene economizer 56. In the ethylene economizer 56, vaporized ethylene refrigerant stream is warmed through an indirect heat exchange 64 prior to being fed into a low-stage inlet port of the ethylene compressor 51 through a conduit 230. As shown in FIG. 1, a stream of compressed ethylene refrigerant exits the ethylene compressor 51 through a conduit 236 and subsequently enters the ethylene cooler 52. At the ethylene cooler 52, the compressed ethylene stream is cooled through indirect heat exchange with an external fluid (e.g., water or air). The resulting cooled ethylene stream may be introduced through the conduit 202 into high-stage propane chiller 33 B for additional cooling, as previously described.

The condensed and, often, sub-cooled liquid natural gas stream exiting the low-stage ethylene chiller/condenser 55 in a conduit 124 can also be referred to as a “pressurized LNG-bearing stream.” This pressurized LNG-bearing stream exits the low-stage ethylene chiller/condenser 55 through the conduit 124 prior to entering a main methane economizer 73. In the main methane economizer 73, methane-rich stream in the conduit 124 may be further cooled in an indirect heat exchange 75 through indirect heat exchange with one or more methane refrigerant streams (e.g., 76, 77, 78). The cooled, pressurized LNG-bearing stream exits the main methane economizer 73 through a conduit 134 and is routed to the expansion section 80 of the methane refrigeration cycle 70. In the expansion section 80, the pressurized LNG-bearing stream first passes through a high-stage methane expansion valve or expander 81, whereupon the pressure of this stream is reduced to vaporize or flash a portion thereof. The resulting two-phase methane-rich stream in a conduit 136 enters into a high-stage methane flash drum 82. In the high-stage methane flash drum 82, the vapor and liquid portions of the reduced-pressure stream are separated. The vapor portion of the reduced-pressure stream (also called the high-stage flash gas) exits the high-stage methane flash drum 82 through a conduit 138 and enters into the main methane economizer 73. At the main methane economizer 73, at least a portion of the high-stage flash gas is heated through the indirect heat exchange means 76 of the main methane economizer 73. The resulting warmed vapor stream exits the main methane economizer 73 through the conduit 138 and is routed to a high-stage inlet port of the methane compressor 71, as shown in FIG. 1.

The liquid portion of the reduced-pressure stream exits the high-stage methane flash drum 82 through a conduit 142 and re-enters the main methane economizer 73. The main methane economizer 73 cools the liquid stream through indirect heat exchange 74 of the main methane economizer 73. The resulting cooled stream exits the main methane economizer 73 through a conduit 144 and is routed to a second expansion stage, illustrated in FIG. 1 as intermediate-stage expansion valve 83 and/or expander, as an example. The intermediate-stage expansion valve 83 further reduces the pressure of the cooled methane stream, which reduces the temperature of the stream by vaporizing or flashing a portion of the stream. The resulting two-phase methane-rich stream output in a conduit 146 enters an intermediate-stage methane flash drum 84. Liquid and vapor portions of the stream are separated in the intermediate-stage flash drum 84 and output through conduits 148 and 150, respectively. The vapor portion (also called the intermediate-stage flash gas) in the conduit 150 re-enters the methane economizer 73, wherein the vapor portion is heated through an indirect heat exchange 77 of the main methane economizer 73. The resulting warmed stream is routed through a conduit 154 to the intermediate-stage inlet port of methane compressor 71.

The liquid stream exiting the intermediate-stage methane flash drum 84 through the conduit 148 passes through a low-stage expansion valve 85 and/or expander, whereupon the pressure of the liquefied methane-rich stream is further reduced to vaporize or flash a portion of the stream. The resulting cooled two-phase stream is output in a conduit 156 and enters a low-stage methane flash drum 86, which separates the vapor and liquid phases. The liquid stream exiting the low-stage methane flash drum 86 through a conduit 158 comprises the liquefied natural gas (LNG) product at near atmospheric pressure. This LNG product may be routed downstream for subsequent storage, transportation, and/or use.

A vapor stream exiting the low-stage methane flash drum 86 (also called the low-stage methane flash gas) in a conduit 160 is routed to the methane economizer 73. The methane economizer 73 warms the low-stage methane flash gas through an indirect heat exchange 78 of the main methane economizer 73. The resulting stream exits the methane economizer 73 through a conduit 164. The stream is then routed to a low-stage inlet port of the methane compressor 71.

The methane compressor 71 comprises one or more compression stages. In one implementation, the methane compressor 71 comprises three compression stages in a single module. In another implementation, one or more of the compression modules are separate but mechanically coupled to a common driver. Generally, one or more intercoolers (not shown) are provided between subsequent compression stages.

As shown in FIG. 1, a compressed methane refrigerant stream exiting the methane compressor 71 is discharged into a conduit 166. The compressed methane refrigerant is routed to the methane cooler 72, and the stream is cooled through indirect heat exchange with an external fluid (e.g., air or water) in the methane cooler 72. The resulting cooled methane refrigerant stream exits the methane cooler 72 through a conduit 112 and is directed to and further cooled in the propane refrigeration cycle 30. Upon cooling in the propane refrigeration cycle 30 through a heat exchanger 37, the methane refrigerant stream is discharged into s conduit 130 and subsequently routed to the main methane economizer 73, and the stream is further cooled through indirect heat exchange 79. The resulting sub-cooled stream exits the main methane economizer 73 through a conduit 168 and then combined with the stream in the conduit 122 exiting the high-stage ethylene chiller 53 prior to entering the low-stage ethylene chiller/condenser 55, as previously discussed.

By converting to an electric, steam, or expansion drive system, the need for fuel gas may be reduced or removed, thereby considerably decreasing the rejection of nitrogen into the fuel gas. What is needed is an efficient method to remove nitrogen without requiring complex facilities.

SUMMARY OF THE INVENTION

The invention aims to reduce excessive nitrogen accumulation in LNG liquefaction systems without the need for a complex, multi-component nitrogen rejection unit. The present invention which provides a novel LNG side liquefaction process to reject nitrogen to LNG product has been developed. In one embodiment, the invention allows an LNG plant to process feed gas with low nitrogen content without requiring a nitrogen rejection unit. It rejects nitrogen to LNG product while ensuring that the nitrogen content in the LNG product remains below the specification which is typically 1 mole %. In another embodiment, the invention allows an LNG plant to process feed gas with high nitrogen content utilizing a simple single column nitrogen rejection unit to meet nitrogen specification of LNG product and to improve process efficiency.

The invention more particularly includes an apparatus for liquified natural gas (LNG) production with a propane cycle, an ethylene cycle, a methane cycle, including a side draw chiller said side draw chiller receiving a nitrogen rich feed stream and cooling the nitrogen rich feed stream with an intermediate stage methane refrigerant and / or a low stage methane refrigerant to produce a condensed and subcooled nitrogen rich stream; and flashing said subcooled nitrogen rich stream in a flash drum to produce a nitrogen rich LNG product and a recycle flash gas.

In another embodiment, a process for producing liquified natural gas (LNG) includes receiving a LNG feed comprising nitrogen, chilling said LNG feed comprising nitrogen in an LNG facility to liquid form for non-pressurized storage and transport, where the LNG facility has a side draw chiller said side draw chiller receiving a nitrogen rich feed stream and cooling the nitrogen rich feed stream with an intermediate stage methane refrigerant and/or a low stage methane refrigerant or a rundown LNG to produce a condensed and subcooled nitrogen rich stream; and flashing said chilled nitrogen rich stream in a flash drum to produce a nitrogen rich LNG product and a recycle flash gas.

In one embodiment, the LNG facility may incorporate a methane refrigeration cycle having a high-stage methane flash drum, an intermediate-stage methane flash drum and a low-stage methane flash drum.

The nitrogen rich feed stream may be a slip stream from a high stage (HS) flash drum vapor, high pressure methane recycle stream, the intermediate-stage flash drum vapor stream, the subcooled high pressure LNG from methane economizer, the high stage methane compressor discharge, the high stage methane compressor suction, the subcooled high pressure LNG from the methane economizer, the slightly subcooled high pressure LNG from the low-stage ethylene chiller/condenser, the subcooled intermediate pressure LNG stream from the methane economizer, the high stage methane flash drum liquid stream, or combinations thereof. The nitrogen rich feed stream may be a slip stream from high stage (HS) methane compressor suction or HS methane compressor discharge cooler outlet. The side vapor draw may be compressed in a compressor. In another embodiment, the side-draw chiller may receive both an intermediate-stage methane liquid or two-phase refrigerant stream and a low-stage methane liquid or two-phase refrigerant stream for cooling the nitrogen-rich feed stream. The nitrogen-rich feed stream may be a vapor side-draw from a methane flash drum and be supplied to the side-draw chiller without compression. The nitrogen-rich feed stream may be withdrawn from a suction line of a high-stage methane compressor and may be compressed in a side-draw compressor upstream of the side-draw chiller. The nitrogen-rich feed stream may be withdrawn from a discharge line of a high-stage methane compressor and may be cooled in an ethylene chiller prior to subcooling in the side-draw chiller. In some embodiments, a side draw compressor may be configured to compress the nitrogen rich feed stream upstream of the side draw chiller. The compressed nitrogen rich feed stream may be cooled in an aftercooler prior to being introduced into the side draw chiller. In some embodiments the nitrogen-rich feed stream may be partially condensed in a low-stage ethylene chiller prior to being subcooled in the side-draw chiller. The nitrogen-rich feed stream may be further subcooled in a main methane economizer prior to entering the side-draw chiller.

In another embodiment, the side-draw flash drum may be operated at a pressure higher than a pressure of the low-stage methane flash drum. The flash gas from the side-draw flash drum may be combined with flash gas from the low-stage methane flash drum and routed to a methane economizer. In another embodiment, the recycle flash-gas stream may be routed to a suction port of a methane compressor.

The nitrogen-rich LNG product stream may be routed to an LNG storage tank. The nitrogen-rich LNG product stream may be mixed with an LNG product stream of lower nitrogen content. The nitrogen content of the mixed LNG product stream may be controlled by mixing nitrogen-rich LNG product stream and lower-nitrogen LNG product stream to achieve a consistent LNG product with a known nitrogen content. The LNG content of the mixed LNG product stream may be controlled to achieve a nitrogen content lower than 1 mole %.

As used herein, liquified natural gas (LNG) refers to natural gas stream that has been cooled to liquid form for non-pressurized storage and transport.

A natural gas stream is any stream principally comprised of methane which originates in major portion from a natural gas feed stream, such feed stream for example containing at least 85 mole percent methane, with the balance being undesirable components such as ethane, higher hydrocarbons, nitrogen, carbon dioxide, and a minor amount of other contaminants such as mercury, water, hydrogen sulfide, and mercaptan.

As used herein the OPTIMIZED CASCADE® Process (OCP) is based on three multi-staged, cascaded refrigerant circuits using pure refrigerants. Refrigerants may include propane, ethane, ethylene, or methane.

BRIEF DESCRIPTION OF DRAWINGS

A more complete understanding of the present invention and benefits thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings.

FIG. 1 illustrates an example simplified flow diagram of a cascade refrigeration process.

FIG. 2 illustrates an example simplified flow diagram of a cascade refrigeration process with one embodiment of a side draw supply from a high stage (HS) methane flash drum.

FIG. 3 illustrates an example simplified flow diagram of a cascade refrigeration process with one embodiment of a side draw supply from HS methane compressor suction with side draw compressed by a compressor.

FIG. 4 illustrates an example simplified flow diagram of a cascade refrigeration process with one embodiment of a side draw supply from HS methane compressor discharge cooler outlet with side draw compressed by a compressor.

FIG. 5 is a schematic flow diagram of a cascade refrigeration process with one embodiment of a simple single column nitrogen rejection unit to remove nitrogen from a natural gas stream.

FIG. 6A is a simplified sketch of a side draw liquefier system integrated with methane refrigerant system.

FIG. 6B is a simplified sketch of a simple single column nitrogen rejection unit integrated with methane refrigerant system.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow.

The presently disclosed technology can be implemented in a facility used to cool natural gas to its liquefaction temperature, and thereby produce LNG. The LNG facility generally employs one or more refrigerants to extract heat from the natural gas and reject to the environment. Numerous configurations of LNG systems exist and may be used in conjunction with the nitrogen rejection units and the processes of using the nitrogen rejection units disclosed herein.

A carbon reduction opportunity has provided using large electric motors to drive the refrigerant compressors on an LNG liquefaction facility in place of gas turbines and consequently eliminating the onsite carbon emissions associated with gas turbines. The e-drive LNG facility may significantly reduce or eliminate fuel gas demand. Loss of fuel gas demand and recycling the end flash gas (EFG) and boil off gas (BOG) to the LNG liquefaction system resulting in the accumulation of nitrogen in the liquefaction system.

OCP uses a methane refrigeration cycle as the final refrigeration cycle for liquefying natural gas. In an open loop methane refrigeration cycle, the predominately methane refrigerant is derived from the natural gas undergoing liquefaction and at least part of the predominately methane refrigerant is recombined with the natural gas undergoing liquefaction. Due to multiple stage flash in the open loop process, the components lighter than methane (such as N2, H2, He, and/or Ar) accumulate in the methane recycle stream. With the feed gas N2 content of 0.5 mole %, the methane compressor discharge stream may have a N2 content that is more than 10 times higher than the feed gas N2 content for the open loop process. The compression power increases significantly, and the process efficiency decreases for the open loop process when the light component concentration in the feed gas increases.

In one embodiment, the invention takes a slip stream from a high stage (HS) flash drum vapor, high pressure methane recycle stream, the intermediate-stage flash drum vapor stream, the subcooled high pressure LNG from methane economizer, the high stage methane compressor discharge, the high stage methane compressor suction, the subcooled high pressure LNG from the methane economizer, the slightly subcooled high pressure LNG from the low-stage ethylene chiller/condenser, the subcooled intermediate pressure LNG stream from the methane economizer, or the high stage methane flash drum liquid stream. The slip stream is subcooled in a heat exchanger by low stage (LS) methane refrigerant and optionally IS methane refrigerant. The subcooled slip stream is flashed to pressure higher than LS flash drum pressure in a flash vessel. The vapor stream is recycled and is mixed with the vapor from LS flash drum. The liquid stream from the side flash drum is sent to the LNG product. This process allows rejecting more nitrogen to LNG product. This will eliminate the accumulation of light components in the system for the feed gas with high concentrations of light components (such as N2, H2, He, and/or Ar).

As shown in FIG. 1, the cascade LNG system employs a cascade-type refrigeration process using one or more predominately pure component refrigerants. The refrigerants utilized in cascade-type refrigeration processes can have successively lower boiling points in order to facilitate heat removal from the natural gas stream being liquefied. Additionally, cascade-type refrigeration processes can include some level of heat integration. For example, a cascade-type refrigeration process can cool one or more refrigerants having a higher volatility through indirect heat exchange with one or more refrigerants having a lower volatility. Examples of such cascades include using propane as the first refrigerant of the cascade, ethylene as the second refrigerant and methane as the third refrigerant. In addition to cooling the natural gas stream through indirect heat exchange with one or more refrigerants, LNG system can employ one or more expansion cooling stages to simultaneously cool the LNG, while reducing its pressure.

When the refrigerant compressors of the e-LNG OCP design are driven with electric motors (eDrive), thereby, eliminating the need for a compressor fuel gas stream. To control the methane refrigeration cycle nitrogen content, a side draw stream of the flash vapors are sent to a methane condenser that allows nitrogen to purge through an LNG Stream. Boil off gas from the LNG tank is recompressed and combined with the flash vapors in this open loop where the cycle begins again. The following examples of certain embodiments of the invention are given. Each example is provided by way of explanation of the invention, one of many embodiments of the invention, and the following examples should not be read to limit, or define, the scope of the invention.

Example 1: Nitrogen-Rich Vapor From a Methane Flash Drum

As shown in FIG. 2, a simple heat exchanger/chiller 471 is added to cool nitrogen rich flash products 470 from the methane flash drum 82 with the two-phase intermediate stage methane rich stream 480 from the high-stage flash drum 82 and the two-phase low stage methane rich stream 484 from the intermediate-stage flash drum 84. Nitrogen rich side draw stream 470 can also be taken from high pressure methane recycle stream 168, the intermediate-stage flash drum 84 vapor stream 150, subcooled high pressure LNG stream 142 from methane economizer 73, the high stage methane stream 112 from the methane compressor discharge, the high stage flash gas stream 138 from the high stage methane compressor suction, the subcooled high pressure LNG stream 134 from the methane economizer 73, the slightly subcooled high pressure LNG stream 124 from the low-stage ethylene chiller/condenser 55, the subcooled intermediate pressure LNG stream 144 from the methane economizer 73, the high stage methane flash drum 82 liquid stream 142 or other locations. In the high-stage methane flash drum 82, the vapor and liquid portions of the reduced-pressure stream are separated. The vapor portion of the reduced-pressure stream (also called the high-stage flash gas) exits the high-stage methane flash drum 82 through a conduit 138 and enters into the main methane economizer 73. A portion of stream 138 may be diverted to the side draw chiller 471. The side draw stream 470 is condensed and subcooled in the side draw chiller 471 with a methane refrigerant stream from the high stage methane flash drum 82 and a methane refrigerant stream from the intermediate stage methane flash drum 84. An intermediate-stage expansion valve 83 and/or expander, as an example further reduces the pressure of the cooled methane stream, which reduces the temperature of the stream by vaporizing or flashing a portion of the stream. The resulting two-phase methane-rich stream output in a conduit 146 enters the intermediate-stage methane flash drum 84. Liquid and vapor portions of the stream are separated in the intermediate-stage flash drum 84. A portion of subcooled liquid methane stream 144 from the methane econimizer 73 is output into a conduit 478 and routed through a pressure reduction system 479, such as an expansion valve. The resulting two phase intermediate stage methane refrigerant is output into a conduit 480 and is routed to the side draw chiller 471 to chill the nitrogen rich side draw stream 470. A portion of liquid methane stream from the intermediate-stage flash drum 84 is output into a conduit 482 and routed through a pressure reduction system 483, such as an expansion valve. The resulting two phase low stage methane refrigerant is output into a conduit 484 and is routed to the side draw chiller 471 to chill the nitrogen rich side draw stream 470. The low stage methane refrigerant stream 482 can be taken from the LNG rundown pumps discharge. The subcooled nitrogen rich side draw stream 472 can be reduced in the pressure through a valve 473 and the resulting two phase stream 474 can be flashed in a side draw flash drum 475. The liquid stream 477 from the side draw flash drum is a nitrogen rich LNG feed stream which contains increased nitrogen and other light products that do not interfere with LNG quality. The liquid stream 477 is routed to LNG storage tank. The flash gas 476 from the side draw flash drum is combined with the low stage methane flash drum 86 flash gas creating a mixed gas. The side draw flash drum 475 may not be required and the stream 474 can be routed directly to LNG storage tank. The vapor stream exiting the low-stage methane flash drum 86 (also called the low-stage methane flash gas) mixed with the side draw flash gas in a conduit 160 is routed to the methane economizer 73. The methane economizer 73 warms the low-stage methane flash gas mixture through an indirect heat exchange 78 of the main methane economizer 73. The resulting stream exits the methane economizer 73 through a conduit 164. The stream is then routed to a low-stage inlet port of the methane compressor 71.

The subcooled side stream 472 is flashed at higher pressure in the new flash drum 473 compared to LS methane flash drum. Nitrogen rich stream 477 from the flash drum 475 comprises the liquefied natural gas (LNG) and is routed downstream for subsequent storage, transportation, and/or use. Nitrogen rich LNG stream 477 and LNG stream 158 can be mixed in an LNG storage tank. Stream 477 has higher nitrogen content compared to stream 158. Extra nitrogen is retained in the nitrogen rich LNG. This is achieved through a side draw condenser where a side draw stream of the flashed vapors is condensed and subcooled with methane refrigerant and then sent to the LNG storage. This ultimately improves the overall liquefaction unit design and reduces ethylene refrigeration power consumption.

Example 2: Nitrogen-Rich Vapor From a High Stage Methane Compressor Suction With a Side Draw Compressor

As shown in FIG. 3, the nitrogen rich side draw 570 from the suction of the high stage methane compressor 71 is compressed in the side draw compressor 571 and cooled in the aftercooler 573 using coolant such as, for example, air or cooling water. The cooled vapor 574 is further cooled in the main methane economizer 73 before it is routed through a conduit 575 to a simple heat exchanger/chiller 576 that condenses and subcools the nitrogen rich LNG stream from the methane economizer 73 with the two phase intermediate stage methane refrigerant stream 585 from the high stage methane flash drum 82 and the two-phase low stage methane refrigerant stream 589 from the intermediate-stage flash drum 84. In the high-stage methane flash drum 82, the vapor and liquid portions of the reduced-pressure stream are separated. A portion of the high stage methane compressor suction stream 138 may be compressed and be diverted to the side draw chiller 576. The side draw 575 is condensed and subcooled in the side draw chiller 576 with a methane refrigerant stream from the high stage methane flash drum 82 and a methane refrigerant stream from the intermediate stage methane flash drum 84. An intermediate-stage expansion valve 83 and/or expander, as an example further reduces the pressure of the cooled methane stream, which reduces the temperature of the stream by vaporizing or flashing a portion of the stream. The resulting two-phase methane-rich stream output in a conduit 146 enters the intermediate-stage methane flash drum 84. Liquid and vapor portions of the stream are separated in the intermediate-stage flash drum 84. A portion of subcooled liquid methane stream 144 from the methane economizer 73 is output into a conduit 583 and routed through a pressure reduction system 584, such as an expansion valve. The resulting two phase intermediate stage methane refrigerant is output into a conduit 585 and is routed to the side draw chiller 576 to chill the nitrogen rich side draw stream 575. A portion of liquid methane stream from the intermediate-stage flash drum 84 is output into a conduit 587 and routed through a pressure reduction system 588, such as an expansion valve. The resulting two phase low stage methane refrigerant is output into a conduit 589 and is routed to the side draw chiller 576 to chill the nitrogen rich side draw stream 575. The low stage methane refrigerant stream 587 can be taken from the LNG rundown pumps discharge. The subcooled nitrogen rich side draw stream 577 can be reduced in the pressure through a valve 578 and the resulting two phase stream 579 can be flashed in a side draw flash drum 580. The liquid stream 582 from the side draw flash drum is a nitrogen rich LNG feed stream which contains increased nitrogen and other light products that do not interfere with LNG quality. The liquid stream 582 is routed to LNG storage tank. The flash gas 581 from the side draw flash drum is combined with the low stage methane flash drum 86 flash gas creating a mixed gas. The vapor stream exiting the low-stage methane flash drum 86 (also called the low-stage methane flash gas) mixed with the low nitrogen LNG gas in a conduit 160 is routed to the methane economizer 73. The side draw flash drum 580 may not be required and the stream 579 can be routed directly to LNG storage tank. The methane economizer 73 warms the low-stage methane flash gas mixture through an indirect heat exchange 78 of the main methane economizer 73. The resulting stream exits the methane economizer 73 through a conduit 164. The stream is then routed to a low-stage inlet port of the methane compressor 71.

The subcooled side stream 577 is flashed at higher pressure in the new flash drum 580 compared to LS methane flash drum. Nitrogen rich stream 582 from the flash drum 580 comprises the liquefied natural gas (LNG) and is routed downstream for subsequent storage, transportation, and/or use. Nitrogen rich LNG stream 582 and LNG stream 158 can be mixed in an LNG storage tank. Stream 582 has higher nitrogen content compared to stream 158. Extra nitrogen is retained in the nitrogen rich LNG. This is achieved through a side draw condenser where a side draw stream of the high stage methane compressor suction is compressed, cooled, condensed and subcooled with methane refrigerant and then sent to the LNG storage. This ultimately improves the overall liquefaction unit design and reduces ethylene refrigeration power consumption.

Example 3: Nitrogen-Rich Vapor from a High Stage Methane Compressor Discharge With a Compressor

As shown in FIG. 4, the nitrogen rich side draw 670 from the HS Methane Compressor Discharge Cooler Outlet is compressed in the side draw compressor 671 and cooled in the aftercooler 673 using coolant such as, for example, air or cooling water. The cooled vapor 674 is further cooled in the main methane economizer 73 and condensed in the low-stage ethylene chiller/condenser 55. The resulting condensed stream 676 is subcooled in the main methane economizer 73 before it is routed through a conduit 677 to a simple heat exchanger/chiller 678 that subcools the nitrogen rich LNG stream from the methane economizer 73 with the two phase intermediate stage methane refrigerant stream 687 from the high stage methane flash drum 82 and the two-phase low stage methane refrigerant stream 691 from the intermediate-stage flash drum 84. In the high-stage methane flash drum 82, the vapor and liquid portions of the reduced-pressure stream are separated. A portion of the high stage methane compressor discharge stream may be compressed and be diverted to the side draw chiller 678. The side draw 677 is subcooled in the side draw chiller 678 with a methane refrigerant stream from the high stage methane flash drum 82 and a methane refrigerant stream from the intermediate stage methane flash drum 84. An intermediate-stage expansion valve 83 and/or expander, as an example further reduces the pressure of the cooled methane stream, which reduces the temperature of the stream by vaporizing or flashing a portion of the stream. The resulting two-phase methane-rich stream output in a conduit 146 enters the intermediate-stage methane flash drum 84. Liquid and vapor portions of the stream are separated in the intermediate-stage flash drum 84. A portion of subcooled liquid methane stream from the methane economizer 73 is output into a conduit 685 and routed through a pressure reduction system 686, such as an expansion valve. The resulting two phase intermediate stage methane refrigerant is output into a conduit 687 and is routed to the side draw chiller 678 to chill the nitrogen rich side draw stream 677. A portion of liquid methane stream from the intermediate-stage flash drum 84 is output into a conduit 689 and routed through a pressure reduction system 690, such as an expansion valve. The resulting two phase low stage methane refrigerant is output into a conduit 691 and is routed to the side draw chiller 678 to chill the nitrogen rich side draw stream 677. The low stage methane refrigerant stream 689 can be taken from the LNG rundown pumps discharge. The subcooled nitrogen rich side draw stream 679 can be reduced in the pressure through a valve 680 and the resulting two phase stream 681 can be flashed in a side draw flash drum 682. The liquid stream 684 from the side draw flash drum is a nitrogen rich LNG feed stream which contains increased nitrogen and other light products that do not interfere with LNG quality. The liquid stream 684 is routed to LNG storage tank. The flash gas 683 from the side draw flash drum is combined with the low stage methane flash drum 86 flash gas creating a mixed gas. The vapor stream exiting the low-stage methane flash drum 86 (also called the low-stage methane flash gas) mixed with the low nitrogen LNG gas in a conduit 160 is routed to the methane economizer 73. The side draw flash drum 682 may not be required and the stream 681 can be routed directly to LNG storage tank. The methane economizer 73 warms the low-stage methane flash gas mixture through an indirect heat exchange 78 of the main methane economizer 73. The resulting stream exits the methane economizer 73 through a conduit 164. The stream is then routed to a low-stage inlet port of the methane compressor 71.

The subcooled side stream 679 is flashed at higher pressure in the new flash drum 682 compared to LS methane flash drum. Nitrogen rich stream 684 from the flash drum 682 comprises the liquefied natural gas (LNG) and is routed downstream for subsequent storage, transportation, and/or use. Nitrogen rich LNG stream 684 and LNG stream 158 can be mixed in an LNG storage tank. Stream 684 has higher nitrogen content compared to stream 158. Extra nitrogen is retained in the nitrogen rich LNG. This is achieved through a side draw subcooler where a side draw stream of high stage methane compressor discharge is compressed, cooled and condensed in the main methane economizer 73 and the low-stage ethylene chiller/condenser 55 and is subcooled with methane refrigerant and then sent to the LNG storage. This ultimately improves the overall liquefaction unit design and reduces ethylene refrigeration power consumption.

Typically, the LNG specification requires nitrogen to be lower than 1 mole % in LNG product. The side draw liquefaction process is only viable for electric drive LNG plants when the nitrogen content in the feed gas is below 1 mole %. For gas turbine drive LNG plants, this process is viable only when the methane compressor discharge fuel gas draw flow ensures that the nitrogen content in the LNG product remains below 1 mole %. When the nitrogen content in the feed gas is higher than 1 mole %, a simple single column nitrogen rejection unit can be utilized to meet LNG product specifications for electric drive LNG plants. The single column nitrogen rejection unit can also be utilized for both electric drive LNG plants and gas turbine drive LNG plants for low nitrogen feed gas. However, it requires more equipment compared to the side draw liquefaction process. As shown in FIG. 5, the nitrogen rich flash vapor stream 770 from the HS flash drum 82 is condensed in the NRU Feed Chiller 771 by a liquid draw stream 790 and the bottom liquid stream 788 from the NRU column 775. The reboiler duty for the NRU column 775 is provided by vaporizing the stream 790 in the exchanger 771 and the vaporized stream 791 is returned to the NRU column 775 providing the stripping gas for the column. The warmed NRU column bottom stream 789 from the exchanger 771 is sent to the LS flash drum 86. The condensed nitrogen rich stream 772 is sent to a pressure reduction system, such as an expansion valve 773 or work extracting device (e.g. expander). At the pressure reduction system 773, the pressure of the NRU feed stream 772 is reduced, thereby evaporating or flashing a portion of the NRU feed stream. A resulting two-phase stream is fed through a conduit 774 into an intermediate location of the distillation NRU column 775. The NRU column 775 comprises the separation sections with packing and/or trays, which increase contact and thus enhance mass transfer between the upward rising vapor and downward flowing liquid inside the column. Reflux for the NRU column 775 is provided by condensing a portion of the overhead vapor 776 in the NRU Column Condenser 777. Refrigeration for the condenser 777 in this embodiment is provided in two different ways. Some of the refrigeration for cool and subcool the overhead vapor is provided by the cold overhead vapor itself. Some of the refrigeration is provided by methane refrigerant 794 from open loop methane system. The overhead vapor 776 from the NRU column 775 is first warmed in the condenser 777. A portion of the warmed vapor is sent to atmosphere or thermal oxidizer as N2 vent stream 778. The remainder of the warmed vapor stream 779 is compressed in the nitrogen reflux compressor 780 and cooled in the aftercooler 782 using coolant such as, for example, air or cooling water. The cooled vapor stream 783 is further cooled, condensed and subcooled in the condenser 777. The subcooled stream is expanded through an expansion valve 784 and is returned to the top of the NRU column 775 as the reflux. Using the nitrogen reflux compressor and condenser, a nitrogen vent stream 778 with high purity of >99.9 mole % nitrogen can be obtained.

The side draw liquefaction process, illustrated in FIG. 6A, requires minimal equipment and is viable for both electric drive LNG plants and turbine drive LNG plants. For electric drive LNG plant, the nitrogen content in the feed gas shall be below 1 mole % limited the typical nitrogen specification for LNG product. For gas turbine drive LNG plants, this process is viable for a higher nitrogen content in the feed gas as long as the methane compressor discharge fuel gas draw flow ensures that the nitrogen content in the LNG product remains below 1 mole %. However, subcooling the side draw increases refrigeration power consumption. For feed gas with a high nitrogen content, a simple single column nitrogen rejection unit (NRU) shown in FIG. 6B is a more efficient method. The single column nitrogen rejection unit can also be utilized for both electric drive LNG plants and gas turbine drive LNG plants for low nitrogen feed gas. The nitrogen vent stream 778 from the NRU column 775 can be sent to atmosphere or a thermal oxidizer. The high purity methane rich stream 786 from the bottom of the NRU column can be sent back to the LS methane flash drum in methane system. While the single column nitrogen rejection unit efficiently handles a wide range of nitrogen content in feed gas, it requires more equipment compared to the side draw liquefaction process.

Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.

Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as additional embodiments of the present invention.

REFERENCES

In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. Each of the references below is incorporated in their entirety for all purposes.

• • CONOCOPHILLIPS® Liquified Natural Gas, OPTIMIZED CASCADE® Process, 22-0788-lng-brochure.pdf.
  • U.S. Pat. No. 9,335,091 (Qualls, et al.), “Nitrogen Rejection Unit” (2013).
  • U.S. Pat. No. 9,791,209 (Ghandi, et al.), “System and process for liquefying natural gas” (2014).
  • US-2021-0140710 (James, et al.), “Systems and methods for removing nitrogen during liquefaction of natural gas” (2019).

Claims

1. An apparatus for liquified natural gas (LNG) production comprising:

a. a propane refrigeration cycle;

b. an ethylene refrigeration cycle;

c. a methane refrigeration cycle;

d. a side draw chiller said side draw chiller receiving a nitrogen rich feed stream and cooling the nitrogen rich feed stream with an intermediate stage liquid or two-phase chilling stream to produce a chilled and subcooled nitrogen rich stream; and

e. flashing said chilled nitrogen rich stream in a flash drum to produce a nitrogen rich LNG product and a recycle flash gas.

2. The apparatus of claim 1, wherein said methane refrigeration cycle comprising at least a high-stage (HS) methane flash drum, an intermediate-stage (IS) methane flash drum and a low-stage (LS) methane flash drum.

3. The apparatus of claim 1, wherein said nitrogen rich feed stream is a slip stream selected from a HS flash drum vapor, IS flash drum vapor, LS flash drum vapor, a subcooled high pressure LNG from methane economizer, high pressure methane recycle stream, a suction line of a high-stage methane compressor, and a subcooled intermediate pressure LNG from methane economizer.

4. The apparatus of claim 1, wherein the nitrogen-rich feed stream is a vapor side-draw from a methane flash drum and is supplied to the side-draw chiller without compression.

5. The apparatus of claim 1, wherein the nitrogen-rich feed stream is withdrawn from a suction line of a high-stage methane compressor and is compressed in a side-draw compressor upstream of the side-draw chiller.

6. The apparatus of claim 1, wherein the nitrogen-rich feed stream is withdrawn from a discharge line of a high-stage methane compressor and is cooled in an ethylene chiller prior to subcooling in the side-draw chiller.

7. The apparatus of claim 1, further comprising a side draw compressor configured to compress the nitrogen rich feed stream upstream of the side draw chiller.

8. The apparatus of claim 7, wherein the compressed nitrogen rich feed stream is cooled in an aftercooler prior to being introduced into the side draw chiller.

9. The apparatus of claim 1, wherein the nitrogen-rich feed stream is at least partially condensed in a low-stage ethylene chiller, subcooled in a main methane economizer, or at least partially condensed in a low-stage ethylene chiller and subcooled in a main methane economizer prior to being subcooled in the side-draw chiller.

10. The apparatus of claim 1, wherein the nitrogen-rich LNG product stream is routed to an LNG storage tank and is mixed therein with an LNG product stream of lower nitrogen content.

11. A process for producing liquified natural gas (LNG) comprising:

a. receiving an LNG feed comprising nitrogen;

b. chilling said LNG feed comprising nitrogen in an LNG facility to liquid

form for non-pressurized storage and transport;

c. said LNG facility comprising a side draw chiller said side draw chiller receiving a nitrogen rich feed stream and cooling the nitrogen rich feed stream with an intermediate stage liquid or two-phase chilling stream to produce a chilled nitrogen rich stream; and

d. flashing said chilled nitrogen rich stream in a flash drum to produce a nitrogen rich LNG product and a recycle flash gas.

12. The process of claim 11, wherein said methane refrigeration cycle comprising at least a high-stage (HS) methane flash drum, an intermediate-stage (IS) methane flash drum and a low-stage (LS) methane flash drum.

13. The process of claim 11, wherein said nitrogen rich feed stream is a slip stream selected from a HS flash drum vapor, IS flash drum vapor, LS flash drum vapor, a subcooled high pressure LNG from methane economizer, high pressure methane recycle stream, a suction line of a high-stage methane compressor, and a subcooled intermediate pressure LNG from methane economizer.

14. The process of claim 11, wherein the nitrogen-rich feed stream is a vapor side-draw from a methane flash drum and is supplied to the side-draw chiller without compression.

15. The process of claim 11, wherein the nitrogen-rich feed stream is withdrawn from a suction line of a high-stage methane compressor and is compressed in a side-draw compressor upstream of the side-draw chiller.

16. The process of claim 11, wherein the nitrogen-rich feed stream is withdrawn from a discharge line of a high-stage methane compressor and is cooled in an ethylene chiller prior to subcooling in the side-draw chiller.

17. The process of claim 11, further comprising a side draw compressor configured to compress the nitrogen rich feed stream upstream of the side draw chiller.

18. The process of claim 17, wherein the compressed nitrogen rich feed stream is cooled in an aftercooler prior to being introduced into the side draw chiller.

19. The process of claim 11, wherein the nitrogen-rich feed stream is at least partially condensed in a low-stage ethylene chiller, subcooled in a main methane economizer, or at least partially condensed in a low-stage ethylene chiller and subcooled in a main methane economizer prior to being subcooled in the side-draw chiller.

20. The process of claim 11, wherein the nitrogen-rich LNG product stream is routed to an LNG storage tank and is mixed therein with an LNG product stream of lower nitrogen content.