CN101899342A - A process for producing liquefied natural gas from coalbed methane in a coal mining area - Google Patents

Abstract

The invention discloses a process for producing liquefied natural gas from coalbed methane in a coal mining area. After deoxygenation, pressurization, desulfurization, decarburization and drying treatment, the coalbed methane in a coal mining area enters a low-temperature device as a raw material gas for low-temperature separation. After separation, the Waste nitrogen and gas methane are obtained, and then the gas methane obtained by low-temperature separation enters a refrigeration liquefaction device for refrigeration and liquefaction, thereby obtaining liquefied natural gas. By adopting the process of the invention, the coalbed methane in the coal mining area with a relatively low methane content can be used as raw material gas to produce liquefied natural gas (LNG) with a methane content of more than 99%. Moreover, the technological process is simple, the equipment investment and maintenance cost are small, and the power consumption of cycle compression can be reduced.

Description

A process for producing liquefied natural gas from coalbed methane in a coal mining area

technical field

The invention belongs to the technical field of gas separation and liquefaction using a cryogenic method, and in particular relates to a process for producing liquefied natural gas from coalbed methane in a coal mining area.

Background technique

Coalbed methane is a methane-rich gas and a clean fuel. At present, my country's coalbed methane mainly comes from underground drainage, which is used to ensure the safety of coal mines. Due to the constraints of drainage technology and mining technology, the methane content of the obtained coalbed methane (coal bed methane, CMM) is about 30% to 60% (volume, the same below), and the concentration is not high, and it will occur with changes in the mining environment. Variety. Coalbed methane with such a content can usually be used as domestic fuel, utility gas and power generation when utilized. However, due to limited civil use and low power generation efficiency, most of the extracted coalbed methane is directly discharged into the atmosphere, which not only wastes resources, but also pollutes the environment and aggravates the greenhouse effect. In order to make full use of such coalbed methane (CMM) resources, according to the national standard "Natural Gas" GB 17820-1999, the methane in such coalbed methane must be enriched to a methane content of more than 85% and exported as natural gas products. For the convenience of long-distance transportation, the production of liquefied natural gas (LNG) is a better choice. LNG requires a higher purity of methane, usually over 99%, and a nitrogen content of less than 1%. The higher the nitrogen content, the lower the LNG temperature. According to the equilibrium composition data of the methane-nitrogen system vapor-liquid two-phase provided on page 277 of "Cryogenic Handbook" (Volume 1) (Edited by the Fourth Design Institute of the Ministry of Chemical Industry, Fuel Press, published in 1973), at 0.2026MPa (2atm) Under pressure, when nitrogen is 0, its equilibrium temperature is 120.2K (-152.95°C); when nitrogen content is 1%, its equilibrium temperature is 115.2K (-157.95°C), the latter is 5°C lower than the former, and the temperature is lower, The more cooling capacity required, the greater the energy consumption.

In order to obtain high-purity methane, the inventor has devoted himself to this research for many years, and has continuously improved the process of producing high-concentration methane from various coalbed methane. The Chinese patent ZL 200410040155.4 filed by the applicant in July 2004 discloses a "coal bed methane low-temperature separation and enrichment process for methane". Auxiliary circulation system, the cooled and liquefied methane in the auxiliary circulation flows into the condenser as a cold source for condensation, which can ensure that the purity of methane is increased to 95% to 99%; however, methane in this concentration range cannot be used as liquefied natural gas. (LNG) requirements.

At present, there is only an industrial device in the world that can separate methane with a purity of about 96% from coalbed methane with a relatively high methane content (~67%), and there is no separation from coalbed methane with a low methane content (above 35%) According to reports on the industrialization of liquefied natural gas products (LNG) with a methane content of more than 99% and related processes, according to the existing technology, it is impossible to separate liquefied methane products (LNG) from low-quality coalbed methane with low methane content.

Contents of the invention

The main purpose of the present invention is to solve the problems in the above-mentioned prior art that liquefied methane products (LNG) with higher methane content cannot be separated from low-quality coal bed methane in coal mine areas with lower methane content, etc., and provide a The process of producing liquefied natural gas by using low-quality coalbed methane in coal mining areas with low methane content as raw material can produce liquefied natural gas LNG with a methane content of more than 99%.

In order to realize the foregoing invention object, the technical scheme that the present invention adopts is as follows:

A process for producing liquefied natural gas from coalbed methane in a coal mining area:

Coalbed methane in coal mining areas is deoxidized, pressurized, desulfurized, decarbonized and dried, and then enters the low-temperature device as raw material gas for low-temperature separation; after cooling in the first heat exchanger, it enters the reboiler of the rectification tower as an evaporation heat source , and cooled, then enter the second heat exchanger for further cooling and throttling, and then enter the rectification tower. The liquid methane obtained from the tower kettle is throttled into the tower top condenser as a cold source; the evaporated methane is recovered by the heat exchanger and reheated, and then exported to the cryogenic device as gas methane;

in:

In the low-temperature separation process, an auxiliary cycle is set to increase the evaporation capacity of the rectification tower reboiler, and the high-pressure methane in the refrigeration system is throttled to a higher intermediate pressure, and a certain amount of unliquefied methane is drawn into the rectification tower reboiler As an evaporation heat source, methane is cooled and liquefied, then throttled to a lower intermediate pressure, enters the first heat exchanger to cool the coalbed gas entering the tower, and then returns to the corresponding position of the refrigeration system; after evaporation at the top of the tower, and reheating The gas methane is sent to the refrigeration system for refrigeration and liquefaction to obtain liquid methane, which can be exported as liquefied natural gas.

The higher intermediate pressure in the above-mentioned low-temperature separation process is 0.2 MPa higher than the pressure of the rectification column; the lower intermediate pressure is 0.3 MPa to 0.45 MPa.

In the low-temperature separation process, the liquid methane obtained from the bottom of the rectification tower can be subcooled by the methane evaporated from the top of the tower, and then flow into the top condenser.

In the refrigeration liquefaction process, a liquefaction cycle with pre-cooled high-pressure methane secondary throttling can be used, or a post-mounted methane expansion cycle with pre-cooled, and the refrigerant is methane.

In cryogenic devices, the refrigeration system can provide 0.3MPa ~ 0.45MPa liquid methane as a cold source.

As a more specific preferred solution, in the above-mentioned process of the present invention,

The cryogenic separation process includes:

A. The raw material gas enters the cryogenic device and is cooled by the first heat exchanger;

B, enter the 4th heat exchanger in the reboiler of rectifying tower (low-temperature rectifying tower), as the evaporation heat source of tower still liquid, self is cooled while heating tower still liquid;

C, then enter the second heat exchanger for further heat exchange and cooling;

D, through throttling and depressurization, enter rectifying tower then, separate under the rectifying action of rectifying tower, tower top obtains waste nitrogen, and tower still obtains liquid methane (in volume percentage, the purity of liquid methane can be reach more than 99.0%);

E. The waste nitrogen gas obtained at the top of the rectification tower is discharged from the device after being reheated to normal temperature through the second heat exchanger and the first heat exchanger respectively;

F. The liquid methane obtained from the bottom of the rectification tower is supercooled by the third heat exchanger, throttled and depressurized, and then enters the condenser at the top of the rectification tower as a cold source, which can increase the cooling capacity of the condenser and increase reflux The evaporated methane gas is reheated to normal temperature through the third heat exchanger and the first heat exchanger to recover the cooling capacity, and then output the low-temperature device as gas methane, and then enter the refrigeration system for the next step of refrigeration and liquefaction process.

In step E, the waste nitrogen gas obtained from the top of the rectification tower can also be expanded and depressurized by the expander after being heat-exchanged and recovered by the second heat exchanger, and then further heat-exchanged by the first heat exchanger. The recovered cooling capacity is reheated to normal temperature and discharged from the device.

Refrigerated liquefaction process includes:

G. The gas methane output from the cryogenic device enters the refrigeration device and is compressed in 3-5 stages by the compressor;

H. After exiting the compressor, heat exchange and cooling are carried out by the seventh heat exchanger, the sixth heat exchanger and the eighth heat exchanger respectively;

1, through throttling and step-down, then enter the first gas-liquid separator and carry out gas-liquid separation for the first time, obtain unliquefied gas and liquid methane respectively;

J, dividing the unliquefied gas obtained by the first gas-liquid separator into two parts;

J1. The first part enters the low-temperature device, and in the fifth heat exchanger in the rectification tower reboiler of the low-temperature device, it is used as the evaporation heat source of the tower still liquid, and is cooled and liquefied by itself while heating the tower still liquid; Then, it returns to the refrigeration system, mixes with the vaporized methane from step L that has been reheated by the ninth heat exchanger, and then passes through the eighth heat exchanger and the second heat exchanger respectively. Seven heat exchangers exchange heat, reheat to normal temperature, and then enter the compressor, mix with gas methane from the low-temperature device, and enter the next stage of compression together; the circulation of this part of unliquefied gas can reduce the compression ratio, thereby saving compression power consumption ;

J2. The second part respectively passes through the eighth heat exchanger and the seventh heat exchanger to exchange heat and reheat to normal temperature, then enters the compressor, mixes with other gases in the compressor, and enters the next stage of compression together;

K, the liquid methane obtained by the first gas-liquid separator is supercooled through the ninth heat exchanger; then throttled and depressurized; then enters the second gas-liquid separator for the second gas-liquid separation to obtain gas methane (in small quantities) and liquid methane;

L, the vaporized methane obtained by the second gas-liquid separator, heat exchange through the ninth heat exchanger, while supercooling the liquid methane obtained by the first gas-liquid separator, self is reheated; After heat exchange through the second heat exchanger, the cooled gas is mixed, then heat exchanged through the eighth heat exchanger and the seventh heat exchanger respectively, reheated to normal temperature, and then enters the compressor;

M. The purity of the liquid methane obtained by the second gas-liquid separator can reach more than 99%, which can be exported as liquefied natural gas (LNG).

In step J2, the second part of the unliquefied gas obtained from the first gas-liquid separator can also be expanded and depressurized by an expander after being heat-exchanged by the eighth heat exchanger, and then passed through the The gas after heat exchange in the second heat exchanger is mixed with the vaporized methane reheated by the ninth heat exchanger from step L, and then heat exchanged and reheated by the eighth heat exchanger and the seventh heat exchanger respectively. Heat to normal temperature, and then enter the compressor.

In step K, after the second gas-liquid separation in the second gas-liquid separator, a part of the obtained liquid methane can be extracted, and after throttling, it can be heat exchanged by the second heat exchanger of the cryogenic device for replenishing Cooling capacity, further cooling the feed gas from the fourth heat exchanger in the rectification column reboiler, while being heated; then return to the refrigeration system, and exchange heat with the cooled gas from step J1 through the second heat exchanger, and The vaporized methane from step L after being reheated by the ninth heat exchanger is mixed, then heat exchanged by the eighth heat exchanger and the seventh heat exchanger respectively, reheated to normal temperature, and then enters the compressor.

In the process of the present invention, in the refrigeration cycle, the high-pressure methane is throttled to a higher intermediate pressure (more than 0.2 MPa higher than the rectification tower pressure), and a stream of unliquefied gas methane is taken out to the rectification tower kettle reboiler for evaporation The heat source itself is liquefied, and then throttled to a lower intermediate pressure (0.30MPa ~ 0.45Mpa) to cool the high-pressure CMM, itself is heated, and then returns to the corresponding position of the refrigeration system. In this way, the compression ratio can be reduced, thereby saving compression power consumption; secondly, the methane in the tower tank is subcooled and then throttled into the tower condenser, which can increase the cooling capacity of the tower condenser and increase reflux. In addition, when waste nitrogen expansion is not used, a part of LNG is used as supplementary cooling capacity, because the separation cooling capacity needs to be small, which can simplify the process.

Compared with prior art, the beneficial effect of the present invention is:

By adopting the process of the invention, the coalbed methane in the coal mining area with a relatively low methane content can be used as raw material gas to produce liquefied natural gas (LNG) with a methane content of more than 99%. Moreover, in this process, a part of the methane gas with relatively high intermediate pressure is separated from the unliquefied gas separated by the first gas-liquid separator of the refrigeration liquefaction device and sent to the rectification tower of the cryogenic device as a heat source for the reboiler Heating the tower liquid, and then throttling to 0.30MPa ~ 0.45MPa pressure to cool the high-pressure feed gas, which can save the special auxiliary circulation device usually required for rectification tower heating, as well as the special refrigeration device and refrigeration device usually required in low temperature devices. Agents, etc., and there is no moving machinery in the low-temperature separation part, which makes the whole process simple, reduces equipment investment and maintenance costs, and can also reduce cycle compression power consumption and energy consumption.

Description of drawings

Fig. 1 is the technological process schematic diagram of embodiment 1 of the present invention;

Fig. 2 is the technological process schematic diagram of embodiment 2 of the present invention;

Fig. 3 is a schematic process flow diagram of embodiment 3 of the present invention.

In Fig. 1-3, the marks respectively indicate: E1～E9 are the first to ninth heat exchangers respectively, T is the rectification tower, C is the condenser at the top of the rectification tower, R is the evaporator, EXP is the expansion machine, COP is the compressor, V1 is the first gas-liquid separator, V2 is the second gas-liquid separator, and 1-17, 21-44 are pipelines of different sections.

Detailed ways

The above content of the invention of the present invention will be further described in detail below in conjunction with specific embodiments.

However, it should not be construed that the scope of the above-mentioned subject matter of the present invention is limited to the following examples. Without departing from the above-mentioned technical idea of the present invention, various replacements and changes made according to common technical knowledge and customary means in this field shall be included in the scope of the present invention.

Example 1

The technological process of producing liquefied natural gas from coalbed methane in the coal mining area of this embodiment is shown in FIG. 1 .

Coalbed methane in coal mining area is used as raw material gas after deoxygenation, desulfurization, decarbonization and drying. The flow rate is 6000Nm ³ /h, and the composition (vol%) is: CH ₄ 35%, N ₂ 65%. Pressure 4.3MPa, temperature 30°C.

It enters the first heat exchanger E1 from the pipeline 1, and after cooling, enters the fourth heat exchanger E4 through the pipeline 2 to heat the liquid in the tower, enters the second heat exchanger E2 through the pipeline 3 for further cooling, and exits the second heat exchanger E2 through the pipeline 4. Reduce the pressure to 2.1MPa through throttling, and the temperature is -141°C, and enter the low-temperature rectification tower T through the pipeline 5. Under the rectification of the cryogenic rectification tower T, liquid methane with a purity of 99.5% is obtained in the bottom of the tower at a temperature of -108°C; the top of the tower is waste nitrogen at a temperature of -154°C and contains 2.3% methane.

The waste nitrogen enters the second heat exchanger E2 through the pipeline 6, heats up to -128°C, enters the expander EXP through the pipeline 13, the pressure of the expander is 0.6MPa, enters the pipeline 14 and reduces the pressure to 0.14MPa through throttling, and passes through the pipeline 15 , the second heat exchanger E2, the pipeline 16, and the first heat exchanger E1 are reheated to normal temperature, and the low-temperature device is discharged from the pipeline 17.

The liquid methane in the tower tank enters the third heat exchanger E3 through the pipeline 7, is subcooled to -130°C, is throttled to 0.14MPa through the pipeline 8, and enters the condenser C at the top of the cryogenic rectification tower T through the pipeline 9. The gasified methane is reheated to normal temperature through the pipeline 10, the third heat exchanger E3, the pipeline 11, and the first heat exchanger E1, and the pipeline 12 is output to the cryogenic device with a gas volume of 2017Nm ³ /h, and sent to the refrigeration liquefaction device for refrigeration liquefaction.

The methane to be liquefied is 2017Nm ³ /h, enters the COP of the compressor through the pipeline 21 (same as the pipeline 12), and is compressed by the first stage of the compressor and returned from the pipeline 42 with a gas volume of 2000Nm ³ /h at a lower intermediate pressure (0.3MPa). The gas is mixed, enters the next stage of compression, and is mixed with the gas from pipeline 36 at a relatively high intermediate pressure (2.4MPa) with a gas volume of 1652Nm ³ /h through the outlet of the third stage of compression, enters the next stage of compression, and finally pressurizes to 15.0MPa , gas volume 5669Nm ³ /h.

The compressed gas is pre-cooled to -45°C through the pipeline 22, the seventh heat exchanger E7, the pipeline 23, and the sixth heat exchanger (precooler) E6, and is further cooled to -45°C through the pipeline 24 and the eighth heat exchanger E8. -62°C, into the pipeline 25, throttling to 2.5MPa, and then into the first gas-liquid separator V1 from the pipeline 26, to separate unliquefied gas and liquid methane.

The unliquefied gas from the first gas-liquid separator V1 is divided into two parts through the pipeline 27, the first part has a gas volume of 1125Nm ³ /h, and enters the fifth heat exchanger E5 through the pipeline 29, and the liquid in the heating tower is cooled and liquefied. The pipeline 30 is throttled to about 0.4MPa, and enters the second heat exchanger E2 through the pipeline 31 to cool the raw gas, and the pipeline 32 and the pipeline 41 merge to return to the refrigeration system. The second part is reheated to normal temperature through the pipeline 28, the eighth heat exchanger E8, the pipeline 35, and the seventh heat exchanger E7, and enters the 4th section of the compressor through the pipeline 36 as a return gas with a relatively high intermediate pressure (2.4MPa), and The gas after the three-stage compression of the compressor is mixed and enters the four-stage compression.

The liquid methane coming out of the first gas-liquid separator V1 enters the ninth heat exchanger E9 through the pipeline 37 for subcooling, enters the pipeline 38, throttles to 0.4MPa, enters the second gas-liquid separator V2 through the pipeline 39, and separates to obtain Vaporizing methane and liquefying methane.

The liquefied methane from the second gas-liquid separator V2 is directly exported as the product LNG through the pipeline 40, with a gas volume of 2017Nm ³ /h, a pressure of about 0.4Mpa, and a methane content of 99.5%.

The gasified methane from the second gas-liquid separator V2 is mixed with the methane from the pipeline 32 through the pipeline 41 and then reheated to normal temperature through the pipeline 33, the eighth heat exchanger E8, the pipeline 34, and the seventh heat exchanger E7. The pipeline 42 enters the second stage of the compressor, mixes with the gas compressed in the first stage of the compressor, and enters the second stage of compression.

In this embodiment, the power consumption for liquefying 1 Nm ³ of methane (compressed to 4.3 MPa for feed gas containing CMM) is 0.977 kWh/Nm ³ .

Example 2

The process flow of producing liquefied natural gas from coalbed methane in the coal mining area of this embodiment is shown in FIG. 2 .

Coalbed methane in coal mining area is used as raw material gas after deoxygenation, desulfurization, decarbonization and drying. The flow rate is 6000Nm ³ /h, and the composition (vol%) is: CH ₄ 35%, N ₂ 65%. Pressure 4.3MPa, temperature 30°C.

It enters the first heat exchanger E1 from the pipeline 1, and enters the fourth heat exchanger E4 through the pipeline 2 after cooling, heats the liquid in the tower kettle, enters the second heat exchanger E2 through the pipeline 3 for further cooling, and exits the second heat exchanger E2 by The pressure of pipeline 4 is reduced to 2.1MPa by throttling, and the temperature is -141°C. It enters the low-temperature rectification tower T from pipeline 5. Under the rectification of the low-temperature rectification tower T, liquid methane with a purity of 99.5% is obtained in the tower reactor, and the temperature is - 108°C. The top of the tower is waste nitrogen, the temperature is -154°C, and the methane content is 2.3%.

The waste nitrogen gas is throttled to 0.14 MPa through the pipeline 6 and enters the second heat exchanger E2, then reheated to normal temperature through the pipeline 13 and the first heat exchanger E1, and is discharged from the cryogenic device through the pipeline 14.

The liquid methane in the tower tank enters the third heat exchanger E3 through the pipeline 7, is subcooled to -130°C, is throttled to 0.14MPa through the pipeline 8, and enters the condenser C at the top of the cryogenic rectification tower T through the pipeline 9. The gasified methane is reheated to normal temperature through the pipeline 10, the third heat exchanger E3, the pipeline 11, and the first heat exchanger E1, and the pipeline 12 is output to the cryogenic device with a gas volume of 2017Nm ³ /h, and sent to the refrigeration liquefaction device for refrigeration liquefaction.

The methane to be liquefied is 2017Nm ³ /h, enters the COP of the compressor through the pipeline 21 (same as the pipeline 12), and is compressed by the first stage of the compressor and returns to the lower intermediate pressure (0.3MPa) from the pipeline 44 with a gas volume of 2060Nm ³ /h The gas is mixed and enters the next stage of compression. At the outlet of the third stage of compression, it is mixed with the gas at a relatively high intermediate pressure (2.4MPa) from pipeline 36 with a gas volume of 1807Nm ³ /h, enters the next stage of compression, and is finally pressurized to 15.0MPa , gas volume 5984Nm ³ /h.

The compressed gas is pre-cooled to -45°C through the pipeline 22, the seventh heat exchanger E7, the pipeline 23, and the sixth heat exchanger (precooler) E6, and is further cooled to -45°C through the pipeline 24 and the eighth heat exchanger E8. -62°C, into the pipeline 25, throttling to 2.5MPa, and then into the first gas-liquid separator V1 from the pipeline 26, to separate unliquefied gas and liquid methane.

The unliquefied gas from the first gas-liquid separator V1 is divided into two parts through the pipeline 27, the first part has a gas volume of 1125Nm ³ /h, and enters the fifth heat exchanger E5 through the pipeline 29 to heat the liquid in the tower, which itself is cooled and liquefied, saving energy. When the gas reaches about 0.4MPa, it enters the second heat exchanger E2 through the pipeline 31 to cool the raw gas, and the pipeline 32 merges with the pipeline 41 to return to the refrigeration liquefaction device. The second part is reheated to normal temperature through the pipeline 28, the eighth heat exchanger E8, the pipeline 35, and the seventh heat exchanger E7, and enters the fourth stage of the compressor through the pipeline 36, mixes with the gas compressed by the third stage of the compressor, and enters the fourth stage. level compression.

The liquid methane coming out of the first gas-liquid separator V1 is subcooled by the ninth heat exchanger E9, enters the pipeline 38, throttles to 0.4MPa, enters the second gas-liquid separator V2 through the pipeline 39, and separates to obtain vaporized methane and liquefied methane.

The liquefied methane from the second gas-liquid separator V2 is directly exported as the product LNG through the pipeline 40, with a gas volume of 2017Nm ³ /h, a pressure of about 0.4Mpa, and a methane content of 99.5%.

The gasified methane from the second gas-liquid separator V2 is mixed with the methane from the pipeline 41 and the pipeline 32, and then reheated to normal temperature through the pipeline 42, the eighth heat exchanger E8, the pipeline 43, and the seventh heat exchanger E7. The pipeline 42 enters the second stage of the compressor, mixes with the gas compressed in the first stage of the compressor, and enters the second stage of compression.

In this embodiment, the low-temperature separation part does not adopt the adiabatic expansion refrigeration of waste nitrogen, but the cooling capacity is provided by the refrigeration system, and the ideal refrigeration effect can also be obtained.

In order to supplement the cooling capacity required by the cryogenic separation device, the product liquefied natural gas LNG 112Nm ³ /h (converted into a gaseous state) extracted from the second gas-liquid separator V2 enters the second heat exchanger E2 through the pipeline 33, and cools the raw gas. After being gasified, it is mixed with the pipeline 32 through the pipeline 34, enters the lower pressure refrigeration cycle and returns to the gas system, and is reheated to normal temperature after recovering the cooling capacity, and enters the secondary inlet of the compressor through the pipeline 44, and is compressed with the primary compression of the compressor. The gas mixes and enters the second stage of compression.

In this embodiment, the power consumption for liquefying 1 Nm ³ of methane (compressed to 4.3 MPa for feed gas containing CMM) is 1.01 kWh/Nm ³ .

Example 3

The process flow of producing liquefied natural gas from coalbed methane in the coal mining area of this embodiment is shown in FIG. 3 .

Coalbed methane in coal mining area is used as raw material gas after deoxygenation, desulfurization, decarbonization and drying. The flow rate is 6000Nm ³ /h, and the composition (vol%) is: CH ₄ 35%, N ₂ 65%. Pressure 4.3MPa, temperature 30°C.

It enters the first heat exchanger E1 from the pipeline 1, and enters the fourth heat exchanger E4 through the pipeline 2 after cooling, heats the liquid in the tower kettle, enters the second heat exchanger E2 through the pipeline 3 for further cooling, and exits the second heat exchanger E2 by The pressure of pipeline 4 is reduced to 2.1MPa by throttling, and the temperature is -141°C. It enters the low-temperature rectification tower T from pipeline 5. Under the rectification of the low-temperature rectification tower T, liquid methane with a purity of 99.5% is obtained in the tower reactor, and the temperature is - 108°C. The top of the tower is waste nitrogen, the temperature is -154°C, and the methane content is 2.3%.

The waste nitrogen gas passes through the pipeline 6 and is throttled to 0.14MPa and enters the second heat exchanger E2, passes through the pipeline 13 and the first heat exchanger E1 to reheat to normal temperature, and is discharged from the cryogenic device through the pipeline 14.

The liquid methane in the tower tank enters the third heat exchanger E3 through the pipeline 7, is subcooled to -130°C, is throttled to 0.14MPa through the pipeline 8, and enters the condenser C through the pipeline 9. The gasified methane is reheated to normal temperature through the pipeline 10, the third heat exchanger E3, the pipeline 11, and the first heat exchanger E1, and the pipeline 12 is output to the cryogenic device with a gas volume of 2017Nm ³ /h, and sent to the refrigeration liquefaction device for refrigeration liquefaction.

The methane to be liquefied is 2017Nm ³ /h, enters the COP of the compressor through the pipeline 21 (same as the pipeline 12), and is compressed in the first stage of the compressor and mixed with the return gas from the pipeline 44 with a gas volume of 7913Nm ³ /h, and enters the next stage of compression. Pressurize to 5.0Mpa through three stages of compression.

The compressed gas passes through the pipeline 22, the seventh heat exchanger E7, the pipeline 23, the sixth heat exchanger (precooler) E6, pre-cools to -45°C, enters the eighth heat exchanger E8 for further cooling, and passes through the pipeline 25 Then throttle to 3.0 MPa, enter the first gas-liquid separator V1 through pipeline 26, and separate unliquefied gas and liquid methane.

The unliquefied gas from the first gas-liquid separator V1 is divided into two parts, the first part has a gas volume of 1125Nm ³ /h, and the liquid in the tower is heated through the pipeline 29, the fifth heat exchanger E5, and the pipeline 30, and liquefied, and throttled to 0.4 MPa, enters the second heat exchanger E2 from the pipeline 31, cools the feed gas, and mixes the methane from the pipeline 32 with the pipeline 41 and enters the pipeline 42. The second part 5515Nm ³ /h passes through the pipeline 28, the eighth heat exchanger E8 heats up to -48°C, enters the expander EXP from the pipeline 35, expands to 0.6MPa, and then reduces the pressure to 0.4MPa, passes through the pipeline 36 and pipeline 42 Combined, it enters the eighth heat exchanger E8 as return gas, enters the COP 2 section of the compressor through the pipeline 43, the seventh heat exchanger E7, and the pipeline 44, mixes with the gas compressed in the first stage of the compressor, and enters the second stage of compression.

The V1 liquid methane from the first gas-liquid separator V1 is subcooled through the pipeline 37 and the ninth heat exchanger E9, then throttled to 0.4MPa through the pipeline 38, and then enters the second gas-liquid separator V2 through the pipeline 39 for separation Gasified methane and liquefied methane are obtained.

The liquefied methane from the second gas-liquid separator V2 is directly exported as the product LNG through the pipeline 40, with a gas volume of 2017Nm ³ /h, a pressure of about 0.4Mpa, and a methane content of 99.5%.

The gasified methane from the second gas-liquid separator V2 is mixed with the methane from the pipeline 41 and the pipeline 32, and then reheated to normal temperature through the pipeline 42, the eighth heat exchanger E8, the pipeline 43, and the seventh heat exchanger E7. The pipeline 44 enters the second stage of the compressor, mixes with the gas compressed in the first stage of the compressor, and enters the second stage of compression.

This embodiment adopts a post-installed methane adiabatic expansion refrigeration cycle. In order to supplement the cooling capacity required by the low-temperature separation device, the product liquefied natural gas LNG 112Nm ³ /h (converted into a gaseous state) extracted from the second gas-liquid separator V2 enters the second heat exchanger E2 through the pipeline 33, cools the raw gas, and is itself After gasification, the methane gas is mixed from the pipeline 34 and the pipeline 32, enters the lower-pressure refrigeration cycle return gas system, and is reheated to normal temperature after recovering the cooling capacity, and enters the second-stage inlet of the compressor through the pipeline 44, and is compressed with the first-stage compressor. After the gas is mixed, it enters the second stage of compression.

In this embodiment, the power consumption for liquefying 1 Nm ³ of methane (compressed to 4.3 MPa for feed gas containing CMM) is 1.12 kWh/Nm ³ .

Claims (10)

1. the technology of a producing liquefied natural gas by coal bed gas in mine coal, the coal field coal-seam gas enters cryogenic unit as unstripped gas and carries out low ternperature separation process after deoxygenation, pressurization, desulfurization, decarburization and drying; After the cooling, the reboiler that enters rectifying tower is as the evaporation thermal source, and cooling in first interchanger, enters that second interchanger further cools off, throttling again, enters rectifying tower; Under the refinery distillation of rectifying tower, cat head obtains useless nitrogen, discharger after interchanger reclaims the cold re-heat; The tower still obtains liquid methane, and throttling is gone into overhead condenser as low-temperature receiver; The methane of evaporation is after interchanger reclaims the cold re-heat, as gases methane output cryogenic unit;

It is characterized in that:

In the low ternperature separation process operation, be provided for improving the auxiliary circulation of the steam output of rectifying tower reboiler, be throttled to a higher intermediate pressure by refrigeration system high pressure methane, extract a certain amount of not liquefied methane out and enter the rectifying tower reboiler as the evaporation thermal source, the methane liquefaction that is cooled, the low intermediate pressure of throttling to again, enter second interchanger and cool off into tower coal-seam gas, and then turn back to the corresponding position of refrigeration system; Cat head evaporates, the gases methane after re-heat is sent into refrigeration system again, carries out refrigeration liquefying and obtains liquid methane, promptly can be used as natural gas liquids output.

2. technology according to claim 1 is characterized in that:

Described higher intermediate pressure is for being higher than more than the rectifying tower pressure 0.2MPa;

Described low intermediate pressure is 0.3MPa～0.45MPa.

3. technology according to claim 1 is characterized in that:

The liquid methane that described rectifying Tata still obtains, with the methane of cat head evaporation cross cold after again throttling go into overhead condenser.

4. technology according to claim 1 is characterized in that:

In the described refrigeration liquefying operation, adopt the liquefaction cycle of the high pressure methane second throttle that has precooling, or have the rear methane expansion cycles of precooling, refrigeration agent is a methane.

5. technology according to claim 1 is characterized in that:

In the described cryogenic unit, provide the liquid methane of 0.3MPa～0.45MPa as low-temperature receiver by refrigeration system.

6. technology according to claim 1 is characterized in that:

Described low ternperature separation process operation comprises:

A, unstripped gas enter cryogenic unit, cool off by the first interchanger heat exchange;

B, enter the 4th interchanger in the rectifying tower reboiler,, in the heating tower bottoms, self be cooled as the evaporation thermal source of tower bottoms;

C, enter the further heat exchange of second interchanger cooling again;

D, through the throttling step-down, enter rectifying tower then, under the refinery distillation of rectifying tower, separate, cat head obtains useless nitrogen, the tower still obtains liquid methane;

The useless nitrogen that E, rectifying tower cat head obtain reclaims cold re-heat discharger to the normal temperature through second interchanger, first interchanger respectively;

The liquid methane that F, rectifying Tata still obtain, cold excessively through the 3rd interchanger, again after the throttling step-down, the condenser that enters the rectifying tower cat head is as low-temperature receiver; The methane gas of evaporation reclaims the cold re-heat to normal temperature through the 3rd interchanger, first interchanger respectively, as gases methane output cryogenic unit, enters refrigeration system and carries out next step refrigeration liquefying operation.

7. technology according to claim 6 is characterized in that:

In the described step e, the useless nitrogen that the rectifying tower cat head obtains after reclaiming cold through the second interchanger heat exchange, by the decompressor step-down of expanding, reclaims cold re-heat discharger to the normal temperature by the further heat exchange of first interchanger again.

8. according to claim 6 or 7 described technologies, it is characterized in that:

Described refrigeration liquefying operation comprises:

G, the gases methane of exporting from cryogenic unit enter the refrigeration liquefying device, carry out the compression of 3-5 level by compressor;

H, go out behind the compressor respectively through the 7th interchanger, the 6th interchanger, the 8th interchanger heat exchange cooling;

I, through the throttling step-down, enter first gas-liquid separator then and carry out the gas-liquid separation first time, obtain not liquefied gas and liquid methane respectively;

J, the not liquefied gas that first gas-liquid separator is obtained are divided into two portions;

J1, first part enter cryogenic unit, in the 5th interchanger in the rectifying tower reboiler of cryogenic unit, as the evaporation thermal source of tower bottoms, self are cooled and liquefy in the heating tower bottoms; Through the throttling step-down, again by second interchanger heat exchange gasification; Return refrigeration system then, with from step L after the gasification methane blended after the 9th interchanger re-heat, respectively by the 8th interchanger and the 7th interchanger heat exchange, re-heat to normal temperature, enter compressor again, mix with gases methane, enter the next stage compression jointly from cryogenic unit;

J2, second section respectively by the 8th interchanger and the 7th interchanger heat exchange, re-heat to normal temperature, enter compressor again, mixes with other gas in the compressor, enter next stage jointly and compress;

K, the liquid methane that is obtained by first gas-liquid separator are cold excessively through the 9th interchanger heat exchange; Again through the throttling step-down; Enter second gas-liquid separator then and carry out the gas-liquid separation second time, methane and liquid methane obtain respectively gasifying;

L, the gasification methane that obtains by second gas-liquid separator, through the 9th interchanger heat exchange, when crossing the liquid methane that cold first gas-liquid separator obtains, self is by re-heat; Mix by the cooled gas of the second interchanger heat exchange again with from step J1, pass through the 8th interchanger and the 7th interchanger heat exchange, re-heat then respectively, enter compressor again to normal temperature;

M, by the liquid methane that second gas-liquid separator obtains, export as natural gas liquids.

9. technology according to claim 8 is characterized in that:

In the described step K, after second gas-liquid separator carries out the gas-liquid separation second time, the liquid methane that obtains is extracted a part, after throttling, pass through the second interchanger heat exchange of cryogenic unit, further cooling self is heated from the unstripped gas of the 4th interchanger in the rectifying tower reboiler simultaneously; Return the refrigeration liquefying device then, with from step J1 by the cooled gas of the second interchanger heat exchange and from step L after the 9th interchanger re-heat the gasification methane blended, pass through the 8th interchanger and the 7th interchanger heat exchange, re-heat respectively to normal temperature, enter compressor again.

10. technology according to claim 8 is characterized in that:

Among the described step J2, second section in the not liquefied gas that obtains by first gas-liquid separator, after the 8th interchanger heat exchange, by the decompressor step-down of expanding, again with from step J1 by the cooled gas of the second interchanger heat exchange and from step L after the 9th interchanger re-heat the gasification methane blended, pass through the 8th interchanger and the 7th interchanger heat exchange, re-heat then respectively to normal temperature, enter compressor again.

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