A natural gas high-pressure liquefaction system

Abstract

The present invention discloses a natural gas high-pressure liquefaction system, comprising: a plurality of compressor units connected in series, wherein the natural gas feedstock outputted by the plurality of compressor units sequentially enters a first, second, third, and fourth heat exchangers for cooling; a first delivery pipe with a first throttle valve is connected to the output end of the feedstock channel of the fourth heat exchanger, the first delivery pipe being in communication with a first gas-liquid separator tank; a first liquid output pipe with a second throttle valve on the first gas-liquid separator tank being connected to a second gas-liquid separator tank; a second liquid output pipe on the second gas-liquid separator tank outputs the liquefied natural gas product; the natural gas outputted by the first gas output pipe on the first gas-liquid separator tank sequentially enters the fourth, third, and first heat exchangers to provide cooling; R22 refrigerant outputted by the second condenser enters the R22 refrigerant channel of the second heat exchanger, and R23 refrigerant outputted by the third heat exchanger enters the second condenser. Advantages of the present invention include effectively reducing energy consumption and significantly improving the purity of liquefied natural gas.

Description

Technical Field

[0001] The present invention relates to the technical field of natural gas liquefaction. Background Art

[0002] Liquefied natural gas (LNG) is a low-temperature liquid produced from natural gas extracted from gas fields, which is pre-processed and then cooled through a liquefaction process. Its primary component is methane. LNG's key characteristics are that its volume is only 1 / 625 of the volume of natural gas at the same mass, and its mass is only 45% of the mass of water at the same volume.

[0003] The natural gas liquefaction process is a low-temperature process. The cooling capacity required for natural gas liquefaction usually needs to be provided by an external refrigeration cycle. The equipped refrigeration system needs to enable the heat exchanger to achieve the minimum temperature difference between the cold and hot flows, thereby obtaining higher refrigeration efficiency.

[0004] Traditional natural gas liquefaction can be divided into cascade liquefaction, liquefaction with an expander, and mixed refrigerant liquefaction based on the refrigeration method. Among them, the drawbacks of cascade liquefaction include the need for multiple units, large initial investment, complex connections between equipment and pipelines, and constant maintenance. The drawbacks of liquefaction with an expander are the need for a large heat exchange area, which results in a large heat exchanger, a low liquefaction rate, and high energy consumption. Mixed refrigerant liquefaction has the advantages of a simple process flow, low initial investment, and the ability to liquefy natural gas by allowing components with different boiling points to exert their respective refrigeration effects in different temperature zones. It is currently widely used in base-load natural gas liquefaction plants. However, the mixed refrigerant liquefaction process still has disadvantages such as the significant impact of the mixed refrigerant ratio on process power consumption and the need to reconfigure the mixed refrigerant ratio when the working environment undergoes significant changes. Summary of the Invention

[0005] The purpose of the present invention is to provide a natural gas high-pressure liquefaction system that can greatly improve the purity of liquefied natural gas products and effectively reduce energy consumption, thereby effectively reducing the cost of liquefied natural gas products.

[0006] To achieve the above-mentioned object, the present invention adopts a technical solution: a natural gas high-pressure liquefaction system, comprising: a plurality of compressor units connected in series, each compressor unit including a compressor and an air cooler, the input end and output end of the compressor units connected in series are respectively connected to a natural gas feed inlet pipe and a natural gas feed outlet pipe, and the natural gas feed in each compressor unit first enters the compressor of that stage and then enters the air cooler of that stage;

[0007] The natural gas raw material output from the natural gas raw material output pipe enters the first heat exchanger, the second heat exchanger, the third heat exchanger, and the fourth heat exchanger in sequence for cooling. Each heat exchanger is provided with a raw material channel, and the natural gas raw material flows through the raw material channel of each heat exchanger;

[0008] The output end of the raw material channel of the fourth heat exchanger is connected to the first delivery pipe, the first delivery pipe is provided with a first throttle valve, and the first delivery pipe is connected to the first gas-liquid separation tank;

[0009] The liquid phase outlet and the gas phase outlet on the first gas-liquid separation tank are respectively provided with a first liquid output pipe and a first gas output pipe, a second throttle valve is provided on the first liquid output pipe, and the first liquid output pipe is connected to the second gas-liquid separation tank;

[0010] The liquid phase outlet and the gas phase outlet of the second gas-liquid separation tank are respectively provided with a second liquid output pipe and a second gas output pipe, and the second liquid output pipe outputs the liquid natural gas product;

[0011] The fourth heat exchanger, the third heat exchanger, and the first heat exchanger are each provided with a natural gas channel. The first gas output pipe is connected to the natural gas channel inlet of the fourth heat exchanger. The natural gas channel outlet of the fourth heat exchanger is connected to the natural gas channel inlet of the third heat exchanger. The natural gas channel outlet of the third heat exchanger is connected to the natural gas channel inlet of the first heat exchanger.

[0012] The second heat exchanger and the second condenser serve as evaporator components of the R22 refrigeration unit. The second condenser and the second heat exchanger are both provided with R22 refrigerant channels. The R22 refrigerant output by the second condenser enters the R22 refrigerant channel of the second heat exchanger. The second condenser is also provided with an R23 refrigerant channel.

[0013] The third heat exchanger serves as an evaporator component of the R23 refrigeration unit. An R23 refrigerant channel is provided in the third heat exchanger. The outlet of the R23 refrigerant channel is connected to the inlet of the R23 refrigerant channel of the second condenser. The R23 refrigerant output by the third heat exchanger enters the second condenser to provide cooling capacity.

[0014] Furthermore, in the aforementioned natural gas high-pressure liquefaction system, the compressor unit is provided with four stages, namely, a first-stage compressor unit, a second-stage compressor unit, a third-stage compressor unit, and a fourth-stage compressor unit; the natural gas raw material inlet pipe is connected to the compressor input end of the first-stage compressor unit, and the air cooler output end of the fourth-stage compressor unit is connected to the natural gas raw material output pipe.

[0015] Furthermore, in the aforementioned natural gas high-pressure liquefaction system, the second gas output pipe is connected to the natural gas compressor, the output end of the natural gas compressor is connected to the natural gas input end of the first condenser, the natural gas output end of the first condenser is connected to the condenser natural gas output pipe, and the condenser natural gas output pipe is connected to the mixer; the first delivery pipe is first connected to the mixer, the first delivery pipe is connected to the first gas-liquid separation tank through the mixer, the natural gas delivered by the first delivery pipe in the mixer is mixed with the natural gas input by the condenser natural gas output pipe, and the output end of the mixer is connected to the first gas-liquid separation tank.

[0016] Furthermore, in the aforementioned natural gas high-pressure liquefaction system, the first heat exchanger, the second heat exchanger, the third heat exchanger and the fourth heat exchanger are all printed circuit board type heat exchangers.

[0017] Furthermore, in the aforementioned natural gas high-pressure liquefaction system, the temperature of the natural gas raw material output from the output end of the compressor units connected in series is not higher than 50°C, and the pressure is 2.4×10 4 kPa.

[0018] Furthermore, in the aforementioned natural gas high-pressure liquefaction system, the temperature of the natural gas feedstock output by the fourth heat exchanger is -75.95°C.

[0019] The advantages of the present invention are as follows: 1. A multi-stage compressor unit is used to pressurize the natural gas raw material, and then the pressurized natural gas raw material enters the first heat exchanger, the second heat exchanger, the third heat exchanger, and the fourth heat exchanger in sequence for step-by-step cooling. Among them, the second heat exchanger and the second condenser serve as the evaporator components of the R22 refrigeration unit, and the R23 refrigerant output by the third heat exchanger enters the second condenser to provide cooling. This cooling method effectively reduces energy consumption, the process is simple and easy to operate, and the investment cost is low. 2. A first gas-liquid separation tank and a second gas-liquid separation tank are used to perform two gas-liquid separations, thereby greatly improving the purity of the liquefied natural gas product. 3. The vaporized natural gas produced after the second gas-liquid separation is pressurized and condensed, and then mixed with the natural gas output by the fourth heat exchanger before entering the first vaporization separation tank, which greatly improves the gas-liquid separation effect, thereby effectively improving the purity of the liquefied natural gas product. Using this high-pressure liquefaction system, the purity of liquefied natural gas can reach more than 95%. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the principle structure of a natural gas high-pressure liquefaction system described in the present invention. DETAILED DESCRIPTION

[0021] The present invention will be further described in detail below with reference to the accompanying drawings and preferred embodiments.

[0022] like Figure 1The illustrated system for high-pressure natural gas liquefaction includes several stages of compressor units connected in series, each including a compressor and an air cooler. The input and output ends of the series-connected compressor units are connected to a natural gas feed inlet pipe 20 and a natural gas feed outlet pipe 5, respectively. In each compressor unit, the natural gas feed first enters the compressor of that stage and then enters the air cooler of that stage. To effectively increase the pressure, the compressor units in this embodiment are configured in four stages: a first-stage compressor unit 1, a second-stage compressor unit 2, a third-stage compressor unit 3, and a fourth-stage compressor unit 4. The first-stage compressor unit 1 includes a first compressor 11 and a first air cooler 12; the second-stage compressor unit 2 includes a second compressor 21 and a second air cooler 22; the third-stage compressor unit 3 includes a third compressor 31 and a third air cooler 32; and the fourth-stage compressor unit 4 includes a fourth compressor 41 and a fourth air cooler 42. The natural gas feed inlet pipe 20 is connected to the input of the first compressor 11 in the first-stage compressor unit 1, and the output of the fourth air cooler 42 of the fourth-stage compressor unit 4 is connected to the natural gas feed outlet pipe 5. The temperature of the natural gas raw material output from the output end of the compressor units connected in series is not higher than 50°C and the pressure is 2.4×10 4 kPa.

[0023] The natural gas feedstock output from the natural gas feedstock output pipe 5 sequentially enters the first heat exchanger 6, the second heat exchanger 7, the third heat exchanger 8, and the fourth heat exchanger 9 for cooling. Each heat exchanger is provided with a feedstock channel, through which the natural gas feedstock flows. Specifically, the first heat exchanger 6 is provided with a first feedstock channel 62, the second heat exchanger 7 is provided with a second feedstock channel 72, the third heat exchanger 8 is provided with a third feedstock channel 82, and the fourth heat exchanger 9 is provided with a fourth feedstock channel 92. To improve heat exchange efficiency, in this embodiment, the first heat exchanger 6, the second heat exchanger 7, the third heat exchanger 8, and the fourth heat exchanger 9 all utilize printed circuit board heat exchangers.

[0024] The output end of the raw material channel of the fourth heat exchanger 9 is connected to the first delivery pipe 10. The temperature of the natural gas raw material output by the fourth heat exchanger 9 is -75.95°C.

[0025] A first throttle valve 101 is provided on the first delivery pipe 10, and the first delivery pipe 10 is connected to the first gas-liquid separation tank 15; the liquid phase outlet and the gas phase outlet of the first gas-liquid separation tank 15 are respectively provided with a first liquid output pipe 16 and a first gas output pipe 151, and a second throttle valve 161 is provided on the first liquid output pipe 16, and the first liquid output pipe 16 is connected to the second gas-liquid separation tank 17.

[0026] The liquid phase outlet and the gas phase outlet of the second gas-liquid separation tank 17 are respectively provided with a second liquid output pipe 171 and a second gas output pipe 172 . The second liquid output pipe 171 outputs liquid natural gas products.

[0027] Natural gas channels are provided in the fourth heat exchanger 9, the third heat exchanger 8, and the first heat exchanger 6. Specifically, a fourth natural gas channel 91 is provided in the fourth heat exchanger 9, a third natural gas channel 81 is provided in the third heat exchanger 8, and a first natural gas channel 61 is provided in the first heat exchanger 6. A first gas output pipe 151 is connected to the natural gas channel inlet of the fourth heat exchanger 9, the natural gas channel outlet of the fourth heat exchanger 9 is connected to the natural gas channel inlet of the third heat exchanger 8, and the natural gas channel outlet of the third heat exchanger 8 is connected to the natural gas channel inlet of the first heat exchanger 6. The vaporized natural gas in the first gas-liquid separation tank 15 enters the natural gas channels of the fourth heat exchanger 9, the third heat exchanger 8, and the first heat exchanger 6, thereby providing cooling for the fourth heat exchanger 9, the third heat exchanger 8, and the first heat exchanger 6. This is done to reduce energy consumption and thus costs.

[0028] The second heat exchanger 7 and the second condenser 30 serve as the evaporator components of the R22 refrigeration unit. Both the second condenser 30 and the second heat exchanger 7 are provided with R22 refrigerant channels. The R22 refrigerant output by the second condenser 30 enters the R22 refrigerant channels of the second heat exchanger 7. The second condenser 30 is also provided with R23 refrigerant channels. Specifically, the second condenser 30 is provided with a second condenser R22 refrigerant channel 301 and a second condenser R23 refrigerant channel 302, and the second heat exchanger 7 is also provided with a second heat exchanger R22 refrigerant channel 73.

[0029] The third heat exchanger 8 serves as an evaporator component of the R23 refrigeration unit. A third heat exchanger R23 refrigerant channel 83 is provided in the third heat exchanger 8. The outlet of the third heat exchanger R23 refrigerant channel 83 is connected to the inlet of the second condenser R23 refrigerant channel 302 of the second condenser. The R23 refrigerant output by the third heat exchanger 8 enters the second condenser 30 to provide cooling.

[0030] In this embodiment, the second gas output pipe 172 is connected to the natural gas compressor 18. The output of the natural gas compressor 18 is connected to the natural gas input of the first condenser 14. The natural gas output of the first condenser 14 is connected to the condenser natural gas output pipe 141, which is then connected to the mixer 13. The first delivery pipe 10 is first connected to the mixer 13, and then communicates with the first gas-liquid separator 15 through the mixer 13. The natural gas delivered by the first delivery pipe 10 in the mixer 13 is mixed with the natural gas input from the condenser natural gas output pipe 141. The output of the mixer 13 is then connected to the first gas-liquid separator 15. The first throttle valve 101 has a low liquefaction rate after throttling and reducing the pressure of the high-pressure, low-temperature natural gas. By slightly pressurizing and pre-cooling the second gas output pipe 172, the low-temperature natural gas with slightly different temperatures and pressures is mixed through the mixer 13, and a sufficient heat and mass transfer homogeneous process is achieved in the mixer. The outlet of the mixer 13 has achieved low-temperature natural gas with uniform temperature and composition. In addition, the mixer 13 can be provided to ensure that the flow rate entering the first gas-liquid separation tank 15 has no pulse fluctuation, thereby stabilizing the flow rate.

[0031] The specific liquefaction process is given below: the molar composition of the natural gas feedstock is 87.37% methane + 0.93% ethane + 0.28% propane + 0.28% isobutane + 0.07% n-butane + 0.07% isopentane + 11% nitrogen, the temperature is 31.16°C, the pressure is 530.0 kPa, and the molar flow rate is 105.7 kmol / h.

[0032] The natural gas feedstock, delivered through the natural gas feedstock inlet pipe 20, enters the first compressor 11 and first air cooler 12 of the first-stage compressor unit 1, the second compressor 21 and second air cooler 22 of the second-stage compressor unit 2, the third compressor 31 and third air cooler 32 of the third-stage compressor unit 3, and the fourth compressor 41 and fourth air cooler 42 of the fourth-stage compressor unit 4. The natural gas feedstock is pressurized in each compressor stage and then cooled in each cooler stage. The natural gas feedstock output through the natural gas feedstock output pipe 5 has a temperature of 50°C and a pressure of 2.4×10 4 kPa, the molar flow rate is 105.7 kmol / h, and its composition has not changed.

[0033] The raw natural gas in the natural gas raw material output pipe 5 enters the first heat exchanger 6, the second heat exchanger 7, the third heat exchanger 8, and the fourth heat exchanger 9 in sequence for cooling. The temperature of the natural gas raw material delivered to the first delivery pipe 10 is -75.95°C and the pressure is 2.39×10 4 kPa, the molar flow rate is 105.7 kmol / h, and its composition has not changed.

[0034] The condensing temperature of the R22 high-temperature refrigeration unit is 44.55°C, the evaporating temperature is -10.35°C, the high-pressure refrigerant flow rate is 2736 kg / h, and the low-pressure refrigerant flow rate is 2321.61 kg / h;

[0035] The condensing temperature of the R23 low-temperature refrigeration unit is -3.28°C, the evaporating temperature is -48.99°C, the high-pressure refrigerant flow rate is 1262.88 kg / h, and the low-pressure refrigerant flow rate is 1080.50 kg / h.

[0036] After passing through the first throttle valve 101, the natural gas feed stream from the first delivery pipe 10 entering the mixer 13 has a temperature of -137.5°C and a pressure of 600 kPa. The natural gas delivered from the condenser natural gas output pipe 141 has a temperature of -136.5°C and a pressure of 600 kPa. The two streams of natural gas are mixed in the mixer 13 and then flow into the first gas-liquid separator 15. The natural gas output from the mixer 13 has a temperature of -137.4°C and a pressure of 600 kPa. Its molar composition is 87.63% methane, 0.87% ethane, 0.26% propane, 0.26% isobutane, 0.07% n-butane, 0.07% isopentane, and 10.85% nitrogen.

[0037] After separation in the first gas-liquid separation tank 15, the natural gas output from the gas phase outlet of the first gas-liquid separation tank 15 flows successively into the fourth heat exchanger 9, the third heat exchanger 8 and the first heat exchanger 6. The gas molar composition of the natural gas discharged from the first heat exchanger 6 is 83.07% methane + 0.02% ethane + 16.91% nitrogen.

[0038] The liquefied natural gas output from the liquid phase outlet of first gas-liquid separator 15 flows through second throttle valve 161 and into second gas-liquid separator 17. The natural gas output after second throttle valve 161 has a temperature of -160.1°C, a pressure of 120 kPa, a molar flow rate of 45.45 kmol / h, and a molar composition of 94.46% methane, 2.13% ethane, 0.65% propane, 0.65% isobutane, 0.16% n-butane, 0.16% isopentane, and 1.78% nitrogen.

[0039] The natural gas volatiles separated from the gas phase outlet of the second gas-liquid separator 17 are pressurized by the natural gas compressor 18 and condensed by the first condenser 14. Before entering the mixer 13, the temperature is -136.5°C and the pressure is 600 kPa. The molar composition is 91.25% methane + 8.75% nitrogen.

[0040] The LNG separated at the liquid phase outlet of the second gas-liquid separation tank 17 has a temperature of -160.1°C, a pressure of 120 kPa, and a molar composition of 95.11% methane + 2.57% ethane + 0.78% propane + 0.78% isobutane + 0.2% n-butane + 0.2% isopentane + 0.36% nitrogen.

[0041] It can be seen that after two separations in the first gas-liquid separation tank 15 and the second gas-liquid separation tank 17 , the purity of the finished liquefied natural gas output from the second gas-liquid separation tank 17 is greatly improved.

[0042] The advantages of the present invention are: 1. A multi-stage compressor unit is used to pressurize the natural gas raw material, and then the pressurized natural gas raw material enters the first heat exchanger, the second heat exchanger, the third heat exchanger, and the fourth heat exchanger in sequence for step-by-step cooling. Among them, the second heat exchanger and the second condenser serve as the evaporator components of the R22 refrigeration unit, and the R23 refrigerant output by the third heat exchanger enters the second condenser to provide cooling. This cooling method effectively reduces energy consumption, the process is simple and easy to operate, and the investment cost is low. 2. A first gas-liquid separation tank and a second gas-liquid separation tank are used to perform two gas-liquid separations, thereby greatly improving the purity of the liquefied natural gas product. 3. The vaporized natural gas produced after the second gas-liquid separation is pressurized and condensed, and then mixed with the natural gas output by the fourth heat exchanger in a mixer before entering the first vaporization separation tank. This further greatly improves the effect of gas-liquid separation, thereby effectively improving the purity of the liquefied natural gas product. Using this high-pressure liquefaction system, the purity of liquefied natural gas can reach more than 95%.

Claims

1. A natural gas high-pressure liquefaction system, comprising: Several stages of compressor units are connected in series, each stage of the compressor unit includes a compressor and an air cooler, the input end and the output end of the compressor units connected in series are respectively connected to the natural gas raw material inlet pipe and the natural gas raw material outlet pipe, and the natural gas raw material in each stage of the compressor unit first enters the compressor of that stage and then enters the air cooler of that stage; The natural gas raw material output from the natural gas raw material output pipe enters the first heat exchanger, the second heat exchanger, the third heat exchanger, and the fourth heat exchanger in sequence for condensation. Each heat exchanger is provided with a raw material channel, and the natural gas raw material flows through the raw material channel of each heat exchanger; The output end of the raw material channel of the fourth heat exchanger is connected to the first delivery pipe, the first delivery pipe is provided with a first throttle valve, and the first delivery pipe is connected to the first gas-liquid separation tank; The liquid phase outlet and the gas phase outlet on the first gas-liquid separation tank are respectively provided with a first liquid output pipe and a first gas output pipe, a second throttle valve is provided on the first liquid output pipe, and the first liquid output pipe is connected to the second gas-liquid separation tank; The liquid phase outlet and the gas phase outlet of the second gas-liquid separation tank are respectively provided with a second liquid output pipe and a second gas output pipe, and the second liquid output pipe outputs the liquid natural gas product; The fourth heat exchanger, the third heat exchanger, and the first heat exchanger are each provided with a natural gas channel. The first gas output pipe is connected to the natural gas channel inlet of the fourth heat exchanger. The natural gas channel outlet of the fourth heat exchanger is connected to the natural gas channel inlet of the third heat exchanger. The natural gas channel outlet of the third heat exchanger is connected to the natural gas channel inlet of the first heat exchanger. The second heat exchanger and the second condenser serve as evaporator components of the R22 refrigeration unit. The second condenser and the second heat exchanger are both provided with R22 refrigerant channels. The R22 refrigerant output by the second condenser enters the R22 refrigerant channel of the second heat exchanger. The second condenser is also provided with an R23 refrigerant channel. The third heat exchanger serves as the evaporator component of the R23 refrigeration unit. An R23 refrigerant channel is provided in the third heat exchanger. The outlet of the R23 refrigerant channel is connected to the inlet of the R23 refrigerant channel of the second condenser. The R23 refrigerant output by the third heat exchanger enters the second condenser to provide cooling capacity. The compressor unit is provided with four stages, namely a first-stage compressor unit, a second-stage compressor unit, a third-stage compressor unit, and a fourth-stage compressor unit. The natural gas raw material inlet pipe is connected to the compressor input end of the first-stage compressor unit, and the air cooler output end of the fourth-stage compressor unit is connected to the natural gas raw material output pipe. The second gas output pipe is connected to the natural gas compressor, the output end of the natural gas compressor is connected to the natural gas input end of the first condenser, the natural gas output end of the first condenser is connected to the condenser natural gas output pipe, and the condenser natural gas output pipe is connected to the mixer; the first delivery pipe is first connected to the mixer, and the first delivery pipe is connected to the first gas-liquid separation tank through the mixer. The natural gas delivered by the first delivery pipe in the mixer is mixed with the natural gas input by the condenser natural gas output pipe, and the output end of the mixer is connected to the first gas-liquid separation tank; the temperature of the natural gas raw material output by the fourth heat exchanger is -75.95℃.

2. A natural gas high-pressure liquefaction system according to claim 1, characterized in that: The first heat exchanger, the second heat exchanger, the third heat exchanger and the fourth heat exchanger are all printed circuit board type heat exchangers.

3. A natural gas high-pressure liquefaction system according to claim 1, characterized in that: The temperature of the natural gas raw material output from the output end of the compressor units connected in series is not higher than 50°C and the pressure is 2.4×104 kPa.

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