RU2796115C1 - Device and method for three-cycle natural gas liquefaction suitable for ultra-large scale - Google Patents

Abstract

FIELD: liquefaction of natural gas.

SUBSTANCE: device and method for three-cycle liquefaction of natural gas suitable for ultra-large scale. Device is comprised of natural gas liquefaction pipeline, a propane pre-cooling cycle system, a warm mixed refrigerant (WMR) refrigeration cycle system, and a cold mixed refrigerant (CMR) refrigeration cycle system. This device is based on three cycles to distribute the cooling load of each stage. The natural gas is cooled by one finned plate heat exchanger and two coil type heat exchangers in series until the natural gas is liquefied and subcooled. The cooling capacity required for the liquefaction process is provided by three systems, i.e., propane, WMR mixed refrigerant and CMR mixed refrigerant.

EFFECT: simplification of technology and reduction of energy costs.

10 cl, 1 dwg

Description

FIELD OF TECHNOLOGY

The present invention relates to the field of cryogenic liquefaction of natural gas, in particular to an apparatus and method for three-cycle liquefaction of natural gas, suitable for ultra-large scale, while the loads of pre-cooling, liquefaction and sub-cooling are distributed over a three-stage refrigeration cycle, high-efficiency equipment is used, and the corresponding sequence of operations is simplified , thereby satisfying the requirements of ultra-large-scale natural gas liquefaction apparatus design, with a single production line capacity of 6-8 million tons per year.

BACKGROUND OF THE INVENTION

With the continuous development of technology and equipment for liquefied natural gas (LNG) in the construction of LNG plants around the world, large-scale plants with a production capacity of one process line reaching in most cases 3-5.5 million metric tons per year and up to 7, 8 million metric tons per year. The liquefaction technologies used worldwide in large scale LNG production units mainly include APCI's C3MR, DMR and AP-X processes, ConocoPhillips' Optimized Cascade process, Linde Group's MFC process and Shell's DMR process. The use of large-scale LNG production machines can reduce the production cost per unit of LNG production to realize economies of scale.

The maximum production capacity of one LNG production line in China is 1.2 million metric tons per year, which uses the multi-stage single-component refrigeration and liquefaction (MSC) process. The LNG liquefaction plant with a single line production capacity of 3.5 million metric tons per year was developed based on the MSC process. However, the process does not meet the requirements for building an ultra-large-scale LNG production apparatus with a single production line capacity of 6-8 million metric tons per year. Therefore, it is necessary to develop a natural gas liquefaction process suitable for an ultra-large scale natural gas liquefaction apparatus with a production capacity of 68 million metric tons per year.

SUMMARY OF THE INVENTION

To overcome the shortcomings of existing technologies, the present invention provides a three-cycle natural gas liquefaction apparatus and method suitable for ultra-large scale, which has the advantages of simple operation sequence and energy saving.

The technical scheme adopted in the present invention is a three-cycle natural gas liquefaction apparatus suitable for ultra-large scale, comprising a natural gas liquefaction pipe system, a propane pre-cooling cycle system, a warm mixed refrigeration (hereinafter referred to as WMR) cycle system, and a cold mixed cycle system. cooling (hereinafter referred to as CMR), while:

the piping system for liquefying natural gas comprises a propane heat exchanger, a WMR heat exchanger, a CMR heat exchanger, an LNG expander, an LNG expansion tank, and an LNG make-up pump, which are connected in series;

the propane pre-cooling cycle system comprises a propane heat exchanger, a propane compressor, a propane compressor aftercooler, a high pressure propane decoupler, a medium pressure propane decoupler and a low pressure propane decoupler, which are used for pre-cooling natural gas, WMR refrigerant and CMR refrigerant;

the WMR refrigeration cycle system comprises a WMR heat exchanger, a WMR compressor inlet separator, a WMR compressor, a WMR compressor aftercooler, and a WMR expander, which are used for post-cooling and partial liquefaction of natural gas and CMR refrigerant; And

The CMR refrigeration cycle system comprises a CMR heat exchanger, a CMR compressor inlet separator, a CMR compressor, a CMR compressor aftercooler, and a CMR expander that are used to provide cooling capacity for the total liquefaction and subcooling of natural gas and CMR refrigerant.

The present invention further discloses a three-cycle natural gas liquefaction process suitable for ultra-large scale, comprising the following steps:

1) Propane refrigeration cycle system provides cooling capacity for pre-cooling natural gas, WMR refrigerant and CMR refrigerant:

propane, which is pressurized by the propane compressor and cooled by the propane aftercooler, is divided into two streams, one liquid phase propane stream enters the propane heat exchanger after being throttled and depressurized by the first high pressure propane throttling valve, thereby providing cooling capacity for natural gas cooling , WMR refrigerant and CMR refrigerant, and gas-phase propane leaving the propane heat exchanger is returned to the inlet high pressure separator of the propane compressor; another flow of liquid propane enters the high pressure propane impact separator to separate gas and liquid after throttling and depressurization by the second high pressure propane throttling valve, while: gas-phase propane leaving the top of the impact separator enters the propane high pressure suction inlet of the compressor, and the liquid phase leaving the bottom of the baffle separator is divided into two streams, one liquid phase propane stream enters the propane heat exchanger after throttling and depressurizing by the first medium pressure propane throttling valve, thereby providing cooling capacity for continuous cooling of natural gas, refrigerant WMR and CMR refrigerant, and gas-phase propane leaving the propane heat exchanger is returned to the medium pressure suction inlet of the propane compressor; another flow of liquid phase propane enters the medium pressure propane impactor to separate gas and liquid after being throttled and depressurized by the second medium pressure propane throttling valve, the gas phase propane exiting the top of the impactor enters the medium pressure suction inlet of the propane compressor, liquid phase propane leaving the bottom of the fender separator enters the propane heat exchanger after being throttled and depressurized by the low pressure propane throttling valve, thereby providing cooling capacity for continuous cooling of natural gas, WMR refrigerant and CMR refrigerant, and gas-phase propane leaving the propane heat exchanger, enters the low pressure suction inlet of the propane compressor through the low pressure propane baffle separator; and the low pressure propane, medium pressure propane and high pressure propane are pressurized by the propane compressor for the next cycle;

2) The WMR refrigeration system provides the cooling capacity for post-cooling and partial liquefaction of natural gas and CMR refrigerant:

the WMR refrigerant leaving the bottom of the WMR heat exchanger enters the WMR compressor inlet separator and then enters the propane heat exchanger for cooling and liquefaction after being pressurized by the WMR compressor and cooled by the WMR chiller; liquid WMR enters the WMR heat exchanger for additional cooling, then exits the top of it, then expands to low pressure with the WMR expander and enters the WMR heat exchanger to provide cooling capacity for additional cooling and partial liquefaction of natural gas, WMR refrigerant and CMR refrigerant; and the low pressure WMR refrigerant leaving the bottom of the WMR heat exchanger is returned to the inlet separator of the WMR compressor for the next cycle; And

3) CMR refrigeration system provides cooling capacity for general liquefaction and subcooling of natural gas and CMR refrigerant;

CMR refrigerant leaves the top of the CMR heat exchanger after being pressurized by the CMR compressor and cooled by the CMR refrigerant, propane heat exchanger, WMR heat exchanger and CMR heat exchanger in series, then expanded to low pressure by the CMR expander and enters the CMR heat exchanger to provide cooling capacity for liquefaction and subcooling of natural gas and the CMR refrigerant itself; and the low pressure CMR refrigerant exiting the bottom of the CMR heat exchanger is returned to the CMR compressor inlet separator for the next cycle.

Compared with the prior art, the present invention has the following positive effects:

based on three cycles, the present invention distributes the cooling load of each stage; the natural gas is sequentially cooled by one finned plate heat exchanger and two coil-type heat exchangers until the natural gas is liquefied and subcooled; and the natural gas liquefaction apparatus can realize the production capacity of one process line of 6-8 million metric tons per year with a small amount of processing equipment; the cooling capacity required in the liquefaction process is provided by three systems of propane, WMR refrigerant and CMR refrigerant; and the system supports various means of adjustment to ensure the required cooling capacity, flexible operation and high adaptability to different feed gas compositions. Compared with known technologies, the present invention is characterized in that:

1. The present invention is an innovation and improvement based on a multi-stage one-component liquefaction process (MSC process), which has a practical technical application, and the process technology has continuity. Compared with the MSC process, the present invention has the following advantages:

1) The MSC process uses propylene pre-cooling + ethylene refrigeration + liquefaction and subcooling with a mixed refrigerant (mainly methane and nitrogen). In the present invention, propane is used in the pre-cooling cycle, and a mixed refrigerant mainly containing ethane followed by methane and propane is used in the refrigeration cycle; and for the liquefaction and subcooling cycle, a mixed refrigerant containing nitrogen, methane and ethane is used as a refrigerant.

2) The mixed refrigerant is used in the refrigeration section and the liquefaction and subcooling section, respectively. The load of each refrigeration cycle is controlled in conjunction with the optional selection of a compressor driver, thereby flexibly adjusting natural gas refrigeration, liquefaction and subcooling temperatures with improved operational adaptability.

3) In the MSC process, the propylene cooling system and the ethylene cooling system use shell and tube heat exchangers or CIK heat exchangers for heat exchange, and the methane cooling cycle uses a finned plate heat exchanger; in the present invention, a high-efficiency finned plate heat exchanger is used in the propane pre-cooling section, so the heat exchange system is highly integrated and has advantages such as simplified operation sequence and space saving; and the WMR main cryogenic heat exchanger and the CMR main cryogenic heat exchanger used in the present invention are coil-type heat exchangers that can provide a large heat transfer area, thus being useful for realizing a large production capacity of one process line.

2. The two-loop mixed refrigerant liquefaction process (DMR process) implements pre-cooling, liquefaction, and sub-cooling of natural gas through a two-stage mixed refrigerant refrigeration cycle. Compared with the DMR process, the present invention adopts a three-cycle refrigeration cycle to distribute the heat load of each stage, thereby realizing an ultra-large-scale natural gas liquefaction apparatus with a single line production capacity of 6-8 million metric tons per year.

3. The MFC process is a three-cycle refrigeration process in which mixed refrigerants are used for cooling in the pre-cooling section, liquefaction section and sub-cooling section; compared to MFC, the pre-cooling section of the present invention uses pure propane refrigerant.

4. AP-X is another three-cycle refrigeration cycle process that uses propane refrigerant in the pre-cooling section, mixed refrigerant in the liquefaction section, and nitrogen expansion in the sub-cooling section. Compared to the AP-X process, the subcooling section of the present invention uses a mixed refrigerant for cooling.

5. According to the present invention, WMR throttling, CMR throttling, and high pressure LNG throttling are implemented by isentropic expansion hydraulic turboexpanders. Compared with traditional isenthalpic expansion throttling valves, hydraulic turbo expanders have higher expansion efficiency, which can increase the production of LNG product by 1% - 3% and reduce the energy consumption per unit of LNG product by 1% - 3%.

BRIEF DESCRIPTION OF THE ERAPHY MATERIALS

The invention is described in combination with embodiments and accompanying drawings, in which:

the figure shows the sequence of operations according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

The figure shows a three-cycle natural gas liquefaction apparatus suitable for ultra-large scale, comprising a finned plate heat exchanger 1 for propane pre-cooling, a WMR coil type heat exchanger 2, a CMR coil type heat exchanger 3, a hydraulic turbo expander 4 LNG, an expansion tank 5 LNG, a make-up pump 6 LNG , propane compressor 7, propane compressor aftercooler 8, first high pressure propane throttling valve 9, second high pressure propane throttling valve 10, high pressure propane fender 11, first medium pressure propane throttling valve 12, second medium pressure propane throttling valve 13, LP propane separator 14, LP propane choke valve 15, LP propane separator 16, WMR compressor inlet separator 17, WMR compressor 18, WMR compressor aftercooler 19, WMR hydraulic turbo expander 20, CMR compressor inlet separator 21, compressor 22 CMR, aftercooler 23 of the CMR compressor and hydraulic turbo expander 24 CMR, while:

finned plate heat exchanger 1 for propane pre-cooling, coil type WMR heat exchanger 2, coil type CMR heat exchanger 3, hydraulic turbo expander 4 LNG, expansion tank 5 LNG and make-up pump 6 LNG are connected in series;

the propane compressor 7, the propane compressor aftercooler 8, the first high pressure propane throttling valve 9, the propane pre-cooling finned plate heat exchanger 1, and the high pressure suction inlet of the propane compressor 7 are connected in series;

a propane compressor end cooler 8, a second high pressure propane throttling valve 10, a high pressure propane baffle separator 11, a first medium pressure propane throttling valve 12, a propane pre-cooling finned plate heat exchanger 1, and a medium pressure suction inlet of the propane compressor 7 are connected in series;

high pressure propane breaker 11, second medium pressure propane choke valve 13, medium pressure propane breaker 14, LP choke valve 15, finned plate heat exchanger 1 for propane pre-cooling, LP propane breaker 16 and low suction inlet propane compressor pressure 7 connected in series;

the gas phase outlet of the high pressure propane fender 11 is connected to the high pressure suction line of the propane compressor 7; and the gas phase outlet of the medium pressure propane fender 14 is connected to the medium pressure suction line of the propane compressor 7.

WMR compressor inlet separator 17, WMR compressor 18, WMR compressor aftercooler 19, propane pre-cooled finned plate heat exchanger 1, WMR coil type heat exchanger 2 and WMR hydraulic turbo expander 20 are connected in series.

The CMR compressor inlet separator 21, the CMR compressor 22, the CMR compressor aftercooler 23, the propane pre-cooled finned plate heat exchanger 1, the WMR coil type heat exchanger 2, the CMR coil type heat exchanger 3 and the CMR hydraulic turbo expander 24 are connected in series.

The present invention further provides a three-cycle natural gas liquefaction process suitable for ultra-large scale, comprising the following steps:

1) propane pre-cooling system pre-cools natural gas, WMR and CMR to -30°C - -39°C:

the propane pressure is adjusted to 1.5 MPa abs.-2.5 MPa abs. propane compressor 7, and the propane is cooled to 20°C - 50°C by a propane cooler 8 and divided into two streams, one stream of liquid-phase propane (approximately 15% - 25%) enters the finned plate heat exchanger 1 for pre-cooling with propane after throttling and depressurizing to 0.3 MPa abs. - 0.6 MPa abs. the first high pressure propane throttling valve 9, thereby cooling the natural gas, WMR refrigerant and CMR refrigerant (to 20°C - 0°C), and the gas-phase propane exiting the finned plate heat exchanger 1 is returned to the propane high pressure suction inlet compressor 7; another flow of liquid-phase propane (75% - 85%) enters the high-pressure propane fender 11 to separate gas and liquid after throttling and depressurizing to 0.3 MPa abs. - 0.6 MPa abs. the second high pressure propane throttling valve 10, and the gas-phase propane exiting the upper part of the impact separator is returned to the high pressure suction inlet of the propane compressor 7;

the liquid phase leaving the lower part of the high-pressure propane fender 11 is divided into two streams, one stream of liquid-phase propane (45% - 55%) enters the finned plate heat exchanger 1 for pre-cooling with propane after throttling and depressurizing to 0.2 MPa abs. - 0.35 MPa abs. by the first medium pressure propane throttling valve 12, thereby cooling the natural gas, WMR refrigerant and CMR refrigerant to -10°C -20°C, and the gas-phase propane exiting the finned plate heat exchanger is returned to the medium pressure suction inlet of the propane compressor 7 ; another stream of liquid-phase propane (45% - 55%) enters the medium pressure propane impact separator 14 to separate gas and liquid after throttling and depressurizing to 0.2 MPa abs. - 0.35 MPa abs. the second throttling valve 13 of medium pressure propane, and the liquid phase propane exiting the upper part of the baffle separator is returned to the medium pressure suction inlet of the propane compressor 7;

liquid-phase propane leaving the medium pressure propane fender 14 enters the finned plate heat exchanger 1 for pre-cooling with propane after throttling and depressurizing to 0.1 MPa abs. - 0.2 MPa abs. throttling valve 15, thereby cooling natural gas, WMR refrigerant and CMR refrigerant to -30°C to -39°C, and gas-phase propane exiting the finned plate heat exchanger 1 for propane pre-cooling is returned to the propane low pressure suction inlet compressor 7 through a low pressure propane baffle separator 16; And

the pressure of low pressure propane, medium pressure propane and high pressure propane is increased by the propane compressor 7 for the next cycle;

2) WMR refrigeration system cools natural gas and CMR refrigerant to -65°C -95°C:

the WMR refrigerant leaving the bottom of the coil type WMR heat exchanger 2 enters the inlet separator 17 of the WMR compressor and then enters the finned plate heat exchanger 1 for pre-cooling with propane for cooling to -30°C - -39°C and liquefaction after pressurization to 3 MPa abs.- 5 MPa abs. compressor 18 WMR and cooling to 20°C - 50°C cooler 19 WMR; liquid WMR enters the WMR coil type heat exchanger 2 for additional cooling down to -65°С -95°С, then leaves its upper part, expands to 0.2 MPa abs. 0.5 MPa abs. (at -70°С - -100°С) by a hydraulic expander 20 WMR and is included in the heat exchanger 2 of the WMR coil type for cooling natural gas, WMR refrigerant and CMR refrigerant to -65°С - -95°С; and the low pressure WMR refrigerant (0.2 MPa abs. 0.5 MPa abs, -35°C -45°C) leaving the bottom of the WMR coil type heat exchanger 2 is returned to the WMR compressor inlet separator 17 for the next cycle;

3) CMR refrigeration system provides cooling capacity for liquefying and subcooling natural gas and CMR refrigerant;

as a refrigerant of the CMR cooling system, the pressure of the CMR refrigerant is increased to 5 MPa abs. - 8 MPa abs. through the CMR compressor 22, and then cooled to 20°C - 50°C by the aftercooler 23 of the CMR compressor, to -30°C - -39°C by a finned plate heat exchanger 1 for pre-cooling with propane, to -65°C - -95° With heat exchanger 2 coil type WMR and up to -155°C - -160°C in heat exchanger 3 coil type CMR; and the CMR refrigerant exits the top of the CMR coil type heat exchanger 3, then expands to 0.2 MPa abs. 0.5 MPa abs. by means of a CMR hydraulic expander 24 and enters a CMR coil type heat exchanger 3 to provide cooling capacity for liquefying and subcooling natural gas and the CMR refrigerant itself; and low pressure CMR (0.2 MPa abs. - 0.5 MPa abs, -75°C - -95°C) leaving the bottom of the CMR coil type heat exchanger 3 is returned to the inlet separator 21 of the CMR compressor for the next cycle;

4) expansion and vaporization of high pressure LNG.

The natural gas leaving the top of the CMR coil type heat exchanger 3 was liquefied and subcooled to -155°C to -160°C. Subcooled natural gas is expanded to 120 kPa abs. (at -160°C - -163°C) through a hydraulic turboexpander 4 LNG, and it enters the expansion tank 5 LNG for steam generation. The boil-off gas (BOG) is sent to the downstream BOG compression devices, and the LNG at the bottom of the LNG expansion tank 5 is sent to the LNG storage unit via the LNG make-up pump 6.

WMR mixed refrigerant mainly contains ethane and a certain amount of methane and propane, with methane being 5% - 15% (v/v), ethane being 70% - 85% (v/v), and propane being 10% - 20% (about / about). CMR mixed refrigerant contains nitrogen, methane and ethane, with nitrogen being 10% - 25%, ethane being 40% - 65%, and propane being 30% - 45%.

The present invention operates according to the following principle:

The three-cycle liquefaction system based on the high-efficiency equipment provided by the present invention is suitable for ultra-large-scale natural gas liquefaction apparatus with a single-line production capacity of 6-8 million metric tons per year, and adopts a three-stage refrigeration cycle where the refrigerant is propane is used in the pre-cooling section, and the mixed refrigerant is used in the liquefaction section and the sub-cooling section; the pre-cooling cycle, the liquefaction cycle and the subcooling cycle, respectively, employ a set of processes in terms of compression, cooling, condensation, turbo expansion and heat exchange.

According to the present invention, hydraulic turbo expanders are used for WMR, CMR and LNG throttling expansion, thereby achieving relatively high cooling efficiency based on the isentropic expansion principle; and the hydraulic turbo expander is used to recover energy for power generation, thereby reducing the overall power consumption.

The present invention has the following positive effects: 1) the composition and proportion of the mixed refrigerant can be adjusted according to changes in project site environmental conditions and source gas conditions to optimize cooling and liquefaction temperatures in order to minimize the energy consumption of the overall process system and improve local adaptability; 2) three-stage refrigeration cycle, which adopts high efficiency finned plate heat exchanger and coil type heat exchanger to achieve high heat exchange efficiency, thus properly distributing the cooling load in different stages and increasing the production capacity of one process line of the LNG apparatus; 3) WMR refrigerant and CMR refrigerant are throttled by a hydraulic turbo expander, which further reduces the energy consumption per unit of LNG product depending on the high cooling efficiency; and 4) a hydraulic turbo expander is used to choke high pressure LNG, demonstrating efficient throttling and improving the natural gas liquefaction rate.

Claims (20)

1. A three-cycle natural gas liquefaction apparatus suitable for ultra-large scale, characterized in that it comprises a natural gas liquefaction piping system, a propane pre-cooling cycle system, a warm mixed refrigeration (WMR) cycle system, and a cold mixed refrigeration (CMR) cycle system, while this: the piping system for liquefying natural gas comprises a propane heat exchanger, a WMR heat exchanger, a CMR heat exchanger, a liquefied natural gas (LNG) expander, an LNG expansion tank, and an LNG make-up pump, which are connected in series; the propane pre-cooling cycle system comprises a propane heat exchanger, a propane compressor, a propane compressor aftercooler, a high pressure propane decoupler, a medium pressure propane decoupler and a low pressure propane decoupler, which are used for pre-cooling natural gas, WMR refrigerant and CMR refrigerant; the WMR refrigeration cycle system comprises a WMR heat exchanger, a WMR compressor inlet separator, a WMR compressor, a WMR compressor aftercooler, and a WMR expander, which are used for post-cooling and partial liquefaction of natural gas and CMR refrigerant; And The CMR refrigeration cycle system comprises a CMR heat exchanger, a CMR compressor inlet separator, a CMR compressor, a CMR compressor aftercooler, and a CMR expander that are used to provide cooling capacity for the total liquefaction and subcooling of natural gas and CMR refrigerant. 2. The ultra-large three-cycle natural gas liquefaction apparatus of claim 1, wherein in the propane pre-cooling cycle system, a propane compressor, a propane compressor aftercooler, a first high pressure propane throttling valve, a propane heat exchanger, and a suction inlet high pressure propane compressor connected in series; a propane compressor end cooler, a second high-pressure propane throttling valve, a high-pressure propane fender, a first medium-pressure propane throttling valve, a propane heat exchanger, and a medium-pressure suction inlet of the propane compressor are connected in series; the high pressure propane breaker, the second medium pressure propane choke valve, the medium pressure propane breaker, the LP choke valve, the propane heat exchanger, the low pressure propane breaker and the low pressure suction inlet of the propane compressor are connected in series; the gas phase outlet of the high pressure propane fender is connected to the high pressure suction line of the propane compressor; and the gas-phase outlet of the medium pressure propane fender is connected to the medium pressure suction line of the propane compressor. 3. Apparatus for three-cycle liquefaction of natural gas, suitable for ultra-large scale, according to claim 1, characterized in that in the system of the WMR refrigeration cycle, a WMR heat exchanger, a WMR compressor inlet separator, a WMR compressor, a WMR compressor aftercooler, a propane heat exchanger, a WMR heat exchanger, an expander WMR and WMR heat exchanger are connected in series. 4. Apparatus for three-cycle liquefaction of natural gas, suitable for ultra-large scale, according to claim 1, characterized in that in the CMR refrigeration cycle system, a CMR heat exchanger, a CMR compressor inlet separator, a CMR compressor, a CMR compressor aftercooler, a propane heat exchanger, a WMR heat exchanger, a heat exchanger CMR, CMR expander and CMR heat exchanger are connected in series. 5. Apparatus three-cycle liquefaction of natural gas, suitable for ultra-large scale, according to any one of paragraphs. 1-4, characterized in that the LNG expander, the WMR expander and the CMR expander are hydraulic turbo expanders. 6. The ultra-large three-cycle natural gas liquefaction apparatus of claim 5, wherein the propane heat exchanger is a finned plate heat exchanger, and the WMR heat exchanger and the CMR heat exchanger are coil type heat exchangers. 7. A three-cycle natural gas liquefaction process suitable for ultra-large scale, characterized in that it includes the following steps: 1) Propane refrigeration cycle system provides cooling capacity for pre-cooling natural gas, WMR refrigerant and CMR refrigerant: propane, which is pressurized by a propane compressor and cooled by a propane cooler, is divided into two streams, one liquid phase propane stream enters the propane heat exchanger after being throttled and depressurized by the first high pressure propane throttling valve, thereby providing cooling capacity for natural gas cooling, a WMR refrigerant and a CMR refrigerant, and the gas phase propane exiting the propane heat exchanger is returned to the high pressure suction inlet of the propane compressor; another flow of liquid phase propane enters the high pressure propane impact separator to separate gas and liquid after throttling and depressurization by the second high pressure propane throttling valve, wherein: gas phase propane leaving the top of the impact separator is returned to the propane high pressure suction of the compressor, and the liquid phase leaving the bottom of the baffle separator is divided into two streams, one liquid phase propane stream enters the propane heat exchanger after throttling and depressurizing by the first medium pressure propane throttling valve, thereby providing cooling capacity for continuous cooling of natural gas, refrigerant WMR and CMR refrigerant, and gas-phase propane leaving the propane heat exchanger is returned to the medium pressure suction inlet of the propane compressor; another flow of liquid phase propane enters the medium pressure propane impactor to separate gas and liquid after being throttled and depressurized by the second medium pressure propane throttling valve, gas phase propane leaving the top of the impactor is returned to the medium pressure suction inlet of the propane compressor, liquid phase propane leaving the bottom of the fender separator enters the propane heat exchanger after being throttled and depressurized by the low pressure propane throttling valve, thereby providing cooling capacity for continuous cooling of natural gas, WMR refrigerant and CMR refrigerant, and gas-phase propane leaving the propane heat exchanger, returning to the low pressure suction inlet of the propane compressor through the low pressure propane impact separator; and the low pressure propane, medium pressure propane and high pressure propane are pressurized by the propane compressor for the next cycle; 2) WMR refrigeration system provides cooling capacity for additional cooling and partial liquefaction of natural gas and CMR refrigerant: the WMR refrigerant leaving the bottom of the WMR heat exchanger enters the WMR compressor inlet separator and then enters the propane heat exchanger for cooling and liquefaction after being pressurized by the WMR compressor and cooled by the WMR chiller; liquid WMR enters the WMR heat exchanger for additional cooling, then exits from its top, expanded to low pressure by the WMR expander and enters the WMR heat exchanger to provide cooling capacity for additional cooling and partial liquefaction of natural gas, WMR refrigerant and CMR refrigerant; and the low pressure WMR refrigerant leaving the bottom of the WMR heat exchanger is returned to the inlet separator of the WMR compressor for the next cycle; And 3) CMR refrigeration system provides cooling capacity for general liquefaction and subcooling of natural gas and CMR refrigerant; CMR refrigerant leaves the top of the CMR heat exchanger after being pressurized by the CMR compressor and cooled by the CMR refrigerant, propane heat exchanger, WMR heat exchanger and CMR heat exchanger in series, then expanded to low pressure by the CMR expander and enters the CMR heat exchanger to provide cooling capacity for liquefaction and subcooling of natural gas and the CMR refrigerant itself; and the low pressure CMR refrigerant leaving the bottom of the CMR heat exchanger is returned to the inlet separator of the CMR compressor for the next cycle. 8. The method of three-cycle liquefaction of natural gas, suitable for ultra-large scale, according to claim 7, characterized in that the WMR refrigerant contains methane, ethane and propane, while methane is 5% - 15% (vol./vol.), ethane is 70% - 85% (V/V) and propane is 10% - 20% (V/V). 9. The method of three-cycle liquefaction of natural gas suitable for ultra-large scale, according to claim 7, characterized in that the CMR refrigerant contains nitrogen, methane and ethane, while nitrogen is 10% - 25% (vol./vol.), ethane is 40% - 65% (V/V) and propane is 30% - 45% (V/V). 10. The method of three-cycle liquefaction of natural gas, suitable for ultra-large scale, according to claim 7, characterized in that the natural gas exiting the top of the CMR heat exchanger is liquefied and subcooled to -155°C - -160°C before expanding to 120 kPa abs. LNG expander, and then it enters the LNG expansion tank for steam generation; boil-off gas (BOG) is sent to downstream BOG compressors; and LNG at the bottom of the expansion tank The LNG is sent to the LNG storage unit by the LNG make-up pump.

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