CN109387030B - System and method for preparing LNG (liquefied Natural gas) from low-concentration coal mine gas by liquefying and concentrating methane - Google Patents

System and method for preparing LNG (liquefied Natural gas) from low-concentration coal mine gas by liquefying and concentrating methane Download PDF

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CN109387030B
CN109387030B CN201811370335.7A CN201811370335A CN109387030B CN 109387030 B CN109387030 B CN 109387030B CN 201811370335 A CN201811370335 A CN 201811370335A CN 109387030 B CN109387030 B CN 109387030B
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pipeline
gas
channel
methane
switching valve
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CN109387030A (en
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范庆虎
周洪达
宋静
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Shanghai Taochuan Energy Technology Co ltd
Hangzhou Hongsheng Zhonghong New Energy Co ltd
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Shanghai Taochuan Energy Technology Co ltd
Hangzhou Hongsheng Zhonghong New Energy Co ltd
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0204—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the feed stream
    • F25J3/0209—Natural gas or substitute natural gas
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0233—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of CnHm with 1 carbon atom or more
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0257—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of nitrogen
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0266—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of carbon dioxide
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0295—Start-up or control of the process; Details of the apparatus used, e.g. sieve plates, packings
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/08—Separating gaseous impurities from gases or gaseous mixtures or from liquefied gases or liquefied gaseous mixtures
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/02—Processes or apparatus using separation by rectification in a single pressure main column system
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/40—Features relating to the provision of boil-up in the bottom of a column
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2205/00—Processes or apparatus using other separation and/or other processing means
    • F25J2205/24—Processes or apparatus using other separation and/or other processing means using regenerators, cold accumulators or reversible heat exchangers
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2205/00—Processes or apparatus using other separation and/or other processing means
    • F25J2205/30—Processes or apparatus using other separation and/or other processing means using a washing, e.g. "scrubbing" or bubble column for purification purposes
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2210/00—Processes characterised by the type or other details of the feed stream
    • F25J2210/40—Air or oxygen enriched air, i.e. generally less than 30mol% of O2
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2215/00—Processes characterised by the type or other details of the product stream
    • F25J2215/04—Recovery of liquid products
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2270/00—Refrigeration techniques used
    • F25J2270/12—External refrigeration with liquid vaporising loop
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2270/00—Refrigeration techniques used
    • F25J2270/40—Quasi-closed internal or closed external air refrigeration cycle

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  • Physics & Mathematics (AREA)
  • Mechanical Engineering (AREA)
  • Thermal Sciences (AREA)
  • General Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Oil, Petroleum & Natural Gas (AREA)
  • Separation By Low-Temperature Treatments (AREA)

Abstract

The invention discloses a system and a method for preparing LNG (liquefied Natural gas) from liquefied and concentrated methane by low-concentration coal mine gas, wherein the system comprises a gas input main pipeline, a supercooling heat exchanger, a rectifying tower, a reboiler, a compressor, a cooler, a cooling medium conveying pipe, a liquid methane output pipeline and an emptying main pipeline; freezing the low-concentration coal mine gas through a precooling heat exchanger and a cryogenic heat exchanger to remove impurities such as water, carbon dioxide and the like, and liquefying methane in the supercooled gas into condensate through the supercooled heat exchanger after cryogenic cooling; the supercooled low-concentration coal mine gas enters a rectifying tower, and a high-concentration liquid natural gas product is obtained from the bottom of the rectifying tower; the system replaces an oxidation combustion method to recycle coal mine gas, so that the resource utilization rate is improved, and the CO is reduced 2 Emission of greenhouse gases.

Description

System and method for preparing LNG (liquefied Natural gas) from low-concentration coal mine gas by liquefying and concentrating methane
Technical Field
The invention relates to the technical field of coal mine gas treatment, in particular to a system and a method for preparing LNG (liquefied Natural gas) from low-concentration coal mine gas by liquefying and concentrating methane.
Background
Coal mine gas is also called Coal-bed gas (Coal-mine gas) and Coal-bed gas, and is mixed gas composed of methane, carbon dioxide, nitrogen and other gases which escape from Coal and surrounding rock. Coalbed methane is a harmful factor in coal mine production, not only pollutes air, but also causes explosion when meeting fire when the gas content in the air is 5% -16%, thereby causing safety accidents.
Coal mine gas is an unconventional natural gas resource symbiotic with coal, and is a potential clean energy with huge reserves. The coal mine gas has good environmental performance and high heat efficiency, and the heat value of the coal mine gas is 14.65-31.40 MJ/m 3 Compared with the fire coal, the ash emission amount of the coal mine gas combustion is 1/148 of that of the fire coal and SO 2 The emission amount is 1/700 of that of the fire coal and CO 2 The emission amount is 3/5 of that of coal, so coal mine gas is the most realistic reliable supplementary or alternative energy source of conventional natural gas.
In the coal mine gas, the methane concentration is different along with different geological conditions and extraction methods, the methane concentration in the high-concentration coal bed gas extracted by adopting a ground wellhead can reach more than 90%, and the methane content in the low-concentration coal bed gas recovered by adopting a downhole drainage system is about 30% -50%. According to the evaluation and statistics result of the coalbed methane grades of the key coal mines in China, the total amount of coalbed methane gushes out of the key coal mines in China in 2003 is 62.3 hundred million cubic meters, wherein the low-concentration coalbed methane accounts for 11 percent, and the high-concentration coalbed methane accounts for 48 percent; and the gas emission amount of the local coal mine is 30-50 hundred million cubic meters, wherein the low-concentration coal bed gas accounts for 89.4%, and the high-concentration coal bed gas accounts for 10.5%.
At present, high-concentration gas or purified and enriched high-grade methane gas can be used for replacing natural gas or used as chemical raw material gas to produce high-added-value products such as methanol, formaldehyde and the like; for low-concentration coal mine gas which does not meet the gas utilization requirement, the methane is oxidized and burned into CO by a torch burning technology 2 So as to achieve the aim of safely discharging the coal mine gas. However, the oxidation combustion method treatment of low-concentration coal mine gas not only causes serious waste of coal mine gas resources, but also concentrates CO formed by combustion 2 It is necessary to design a system that can effectively utilize low concentration coal mine gas because the climate in the local area will be warmed.
Disclosure of Invention
The invention aims to overcome the defects in the prior art, provides a system which has reasonable structural design and perfect system and is used for liquefying and extracting LNG (liquefied natural gas) prepared from high-concentration coal mine gas, and provides specific method steps.
The invention solves the problems by adopting the following technical scheme: the utility model provides a system for low concentration colliery gas liquefaction concentration methane system LNG, its characterized in that: the device comprises a gas input main pipeline, a precooling heat exchanger, a cryogenic heat exchanger, a supercooling heat exchanger, a rectifying tower, a reboiler, a compressor, a cooler, a cooling medium conveying pipe, a liquid methane output pipeline and an emptying main pipeline; the precooling heat exchanger is internally provided with a channel A, a channel B, a channel C and a channel D, the cryogenic heat exchanger is internally provided with a channel E, a channel F, a channel G and a channel H, and the supercooling heat exchanger is internally provided with a channel I and a channel J;
a first flow control valve is arranged on the gas input main pipeline, and the tail end of the gas input main pipeline is branched into a first gas input pipeline and a second gas input pipeline; the first gas input pipeline is provided with a first switching valve, the tail end of the first gas input pipeline is communicated with one end of a channel C, the other end of the channel C is communicated with one end of a channel G through a first connecting pipeline, the other end of the channel G is communicated with one end of a channel J through a second connecting pipeline, the other end of the channel J is connected with a third connecting pipeline, and a fifth switching valve is arranged on the third connecting pipeline; the second gas input pipeline is provided with a second switching valve, the tail end of the second gas input pipeline is communicated with one end of a channel D, the other end of the channel D is communicated with one end of a channel H through a fourth connecting pipeline, the other end of the channel H is communicated with one end of a channel I through a fifth connecting pipeline, the other end of the channel I is connected with a sixth connecting pipeline, and the sixth connecting pipeline is provided with a sixth switching valve;
the first gas input pipeline is branched with a first emptying pipeline, the second gas input pipeline is branched with a second emptying pipeline, the first emptying pipeline and the second emptying pipeline are both connected with the main emptying pipeline, the first emptying pipeline is provided with a third switching valve, and the second emptying pipeline is provided with a fourth switching valve;
the reboiler is arranged at the bottom of the rectifying tower, the bottom of the reboiler is connected with a liquid methane output pipeline, and the liquid methane output pipeline is connected with an external LNG storage tank; the top of the rectifying tower is connected with a noncondensable gas output pipeline, the middle part of the rectifying tower is connected with a purified gas input pipeline, one end of a cooling medium conveying pipe is connected with the upper part of the rectifying tower, and the other end of the cooling medium conveying pipe is connected with an external cold source; the third connecting pipeline and the sixth connecting pipeline are connected with the noncondensable gas output pipeline, the third connecting pipeline is branched to form a first branch pipeline, the sixth connecting pipeline is branched to form a second branch pipeline, the first branch pipeline and the second branch pipeline are connected with the purified gas input pipeline, the first branch pipeline is provided with a seventh switching valve, and the second branch pipeline is provided with an eighth switching valve;
the air outlet of the compressor is connected with the air inlet of the cooler through a first circulating air pipeline, the air inlet of the cooler is connected with the air inlet of the channel A through a second circulating air pipeline, the air outlet of the channel A is connected with the air inlet of the channel E through a third circulating air pipeline, the air outlet of the channel E is connected with the air inlet of the reboiler through a fourth circulating air pipeline, the air outlet of the reboiler is connected with the air inlet of the channel F through a fifth circulating air pipeline, the air outlet of the channel F is connected with the air inlet of the channel B through a sixth circulating air pipeline, and the air outlet of the channel B is connected with the air inlet of the compressor through a seventh circulating air pipeline.
Preferably, a second flow control valve is arranged on the liquid methane output pipeline, a third flow control valve is arranged on the cooling medium conveying pipe, a fourth flow control valve is arranged on the emptying main pipeline, and a pressure control valve is arranged on the fifth circulating pipeline.
Preferably, a circulating gas regulating bypass is communicated between the third circulating gas pipeline and the fourth circulating gas pipeline, and a fifth flow control valve is arranged on the circulating gas regulating bypass.
Preferably, the gas conveyed in the gas input main pipeline is coal mine gas with methane volume concentration of 20% -50% and air volume content of 50% -70%, the pressure is controlled within 0.1MPaG, and the temperature is controlled within 45 ℃.
Preferably, the temperature of the gas output by the pre-cooling heat exchanger is-60 ℃ to-70 ℃, the temperature of the gas output by the deep cooling heat exchanger is-150 ℃ to-160 ℃, and the temperature of the gas output by the supercooling heat exchanger is-168 ℃ to-173 ℃.
Preferably, the cooling medium conveyed by the cooling medium conveying pipe is liquid nitrogen.
The invention provides another technical scheme for solving the technical problems: a method for preparing LNG from low-concentration coal mine gas by liquefying and concentrating methane comprises the following steps:
the first step: switching valves I, IV, seventh and sixth are opened, switching valves II, III, V and eighth are closed;
and a second step of: the flow rate of the gas conveyed by the gas input main pipeline is regulated through a first flow control valve, the gas enters a channel C of a precooling heat exchanger through a first gas input pipeline for cooling and drying, so that trace water impurities in the gas are frozen on the surface of the channel C, then the gas is output from the channel C, the temperature of the gas in output is-60 ℃ to-70 ℃, the gas enters a channel G of a cryogenic heat exchanger through a first connecting pipeline for removing and purifying carbon dioxide impurities, the carbon dioxide impurities in the gas are frozen on the surface of the channel G, then the gas is output from the channel G, the temperature of the gas in output is-150 ℃ to-160 ℃, the gas enters a channel J of a second connecting pipeline for further cooling, methane in the gas is condensed into liquid, then the gas containing liquid methane is output from the channel J, the temperature of the gas in output is-168 ℃ to-173 ℃, and the gas enters a first branch pipeline of a third connecting pipeline for purifying gas input pipeline and then enters the middle part of a rectification tower;
and a third step of: separating impurities such as nitrogen, oxygen and the like in gas in a rectifying tower, enabling liquid methane in the gas to enter a reboiler at the bottom of the rectifying tower, outputting and storing one part of liquid methane in an external LNG storage tank through a liquid methane output pipeline, enabling the other part of liquid methane to enter a channel F as circulating methane after passing through a fifth circulating gas pipeline and controlling pressure through a pressure control valve, enabling the liquid methane to enter a compressor for pressurization sequentially through a sixth circulating gas pipeline, a channel B and a seventh circulating gas pipeline, enabling the liquid methane to enter a cooler for cooling through a first circulating gas pipeline, enabling the liquid methane to sequentially enter a channel A, a third circulating gas pipeline, a channel E and a fourth circulating gas pipeline through a second circulating gas pipeline after cooling, and enabling the reboiler to serve as a heat source for realizing a circulating process; the non-condensable gas sequentially passes through a non-condensable gas output pipeline, a six-connecting pipeline, a channel I, a five-connecting pipeline, a channel H, a four-connecting pipeline, a channel D and a second emptying pipeline branched on a second gas input pipeline, enters an emptying main pipeline to be rewarmed to normal temperature and is then intensively discharged into the atmosphere;
fourth step: liquid nitrogen is input from the top of the rectifying tower through a cooling medium conveying pipe as reflux liquid of the rectifying tower, and heat transfer is carried out with ascending steam stripping provided by a reboiler, so that the condensation and liquefaction of methane components in the rectifying tower are realized, and the concentration of methane in non-condensable gas discharged from the top of the rectifying tower is controlled;
fifth step: when the freezing amount of water in the channel C and the freezing amount of carbon dioxide in the channel G reach a certain amount, switching the gas channel and the non-condensable gas discharging channel, closing a first switching valve, a fourth switching valve, a seventh switching valve and a sixth switching valve, and opening a second switching valve, a third switching valve, a fifth switching valve and a eighth switching valve; then, the gas sequentially passes through a second gas input pipeline, a channel D, a fourth connecting pipeline, a channel H, a fifth connecting pipeline, a channel I and a second branch pipeline branched from a sixth connecting pipeline, and a purified gas input pipeline, and enters the middle part of the rectifying tower for gas separation, the non-condensable gas of the rectifying tower sequentially passes through a non-condensable gas output pipeline, a third connecting pipeline, a channel J, a second connecting pipeline, a channel G, a first connecting pipeline, a channel C and a first emptying pipeline branched from a first gas input pipeline, and enters the main emptying pipeline for re-warming to normal temperature and then is intensively discharged into the atmosphere, in the process, the non-condensable gas thaws frozen water in the channel C and frozen carbon dioxide in the channel G to take out of the heat exchanger, and after the freezing amount of the water in the channel D and the freezing amount of the carbon dioxide in the channel H reach a certain amount, the gas channel and the non-condensable gas discharging air channel are switched again, and sequentially and circularly works.
Preferably, in the third step, a part of the circulating methane enters a circulating gas regulating bypass, the circulating methane is used as a heat source of a reboiler, the temperature of the circulating methane entering the reboiler is controlled by regulating a fifth flow control valve, and the rising stripping amount generated by the reboiler is further controlled, so that the concentration of liquid methane at the bottom of the reboiler is controlled.
Compared with the prior art, the invention has the following advantages and effects: 1. compared with the traditional low-concentration coal mine gas treatment mode, the system can fully extract methane in the low-concentration coal mine gas to prepare an LNG product; 2. the gas channel and the noncondensable gas air release channel can be switched, and the noncondensable gas is used as a heat source to defrost and bring out water impurities and carbon dioxide impurities frozen in the pre-cooling heat exchanger and the cryogenic heat exchanger for centralized discharge, so that the self-cleaning of the pre-cooling heat exchanger and the cryogenic heat exchanger is realized; 3. and designing a circulating methane channel, wherein the circulating methane provides a heat source for the reboiler, arranging a circulating gas adjusting bypass beside the cryogenic heat exchanger, controlling the temperature of the circulating methane entering the reboiler, and further controlling the rising stripping amount generated by the reboiler, so as to control the concentration of liquid methane at the bottom of the reboiler.
Drawings
Fig. 1 is a schematic structural view of an embodiment of the present invention.
Reference numerals illustrate: the gas input main line 1, the first gas input line 101, the second gas input line 102, the first evacuation line 103, the second evacuation line 104, the evacuation main line 9, the cooling medium delivery line 10, the purge gas input line 4, the noncondensable gas output line 6, the liquid methane output line 5, the first connection line 2, the second connection line 3, the third connection line 21, the fourth connection line 8, the fifth connection line 7, the sixth connection line 22, the first branch line 211, the second branch line 221, the precooling heat exchanger 31, the cryogenic heat exchanger 32, the supercooling heat exchanger 33, the rectifying column 34, the reboiler 35, the compressor 36, the cooler 37, the first flow control valve 41, the second flow control valve 42, the third flow control valve 43, the fourth flow control valve 44, the fifth flow control valve 45, the pressure control valve 46, the first switching valve 47, the third switching valve 48, the second switching valve 49, the fourth switching valve 50, the sixth switching valve 51, the eighth switching valve 52, the fifth switching valve 53, the seventh switching valve 54, the first switching valve 12, the seventh switching valve 16, the fourth circulation gas circulation line 13, the fourth circulation gas circulation line 11, the fourth circulation gas circulation line 15.
Detailed Description
The present invention will be described in further detail by way of examples with reference to the accompanying drawings, which are illustrative of the present invention and not limited to the following examples.
Referring to fig. 1, an embodiment of the invention is a system for preparing LNG by liquefying and concentrating low-concentration coal mine gas, which comprises a gas input main pipeline 1, a pre-cooling heat exchanger 31, a cryogenic heat exchanger 32, a supercooling heat exchanger 33, a rectifying tower 34, a reboiler 35, a compressor 36, a cooler 37, a cooling medium conveying pipe 10, a liquid methane output pipeline 5 and an emptying main pipeline 9.
In this embodiment, a channel a, a channel B, a channel C, and a channel D are provided in the pre-cooling heat exchanger 31, a channel E, a channel F, a channel G, and a channel H are provided in the cryogenic heat exchanger 32, and a channel I and a channel J are provided in the supercooling heat exchanger 33. The temperature of the gas output by the precooling heat exchanger 31 is minus 60 ℃ to minus 70 ℃, the temperature of the gas output by the cryogenic heat exchanger 32 is minus 150 ℃ to minus 160 ℃, and the temperature of the gas output by the supercooling heat exchanger 33 is minus 168 ℃ to minus 173 ℃.
In this embodiment, the first flow control valve 41 is provided on the gas input main pipe 1, and the end of the gas input main pipe 1 is branched into a first gas input pipe 101 and a second gas input pipe 102. The gas conveyed in the gas input main pipeline 1 is coal mine gas with methane volume concentration of 20% -50% and air volume content of 50% -70%, the pressure is controlled within 0.1MPaG, and the temperature is controlled within 45 ℃.
In this embodiment, the first gas input pipeline 101 is provided with a first switching valve 47, the end of the first gas input pipeline 101 is connected with one end of a channel C, the other end of the channel C is connected with one end of a channel G through a first connecting pipeline 2, the other end of the channel G is connected with one end of a channel J through a second connecting pipeline 3, the other end of the channel J is connected with a third connecting pipeline 21, and a fifth switching valve 53 is disposed on the third connecting pipeline 21.
In this embodiment, the second gas input pipeline 102 is provided with the second switching valve 49, the end of the second gas input pipeline 102 is connected with one end of the channel D, the other end of the channel D is connected with one end of the channel H through the fourth connecting pipeline 8, the other end of the channel H is connected with one end of the channel I through the fifth connecting pipeline 7, the other end of the channel I is connected with the sixth connecting pipeline 22, and the sixth connecting pipeline 22 is provided with the sixth switching valve 51.
In this embodiment, the first gas input pipeline 101 is branched with a first evacuation pipeline 103, the second gas input pipeline 102 is branched with a second evacuation pipeline 104, the first evacuation pipeline 103 and the second evacuation pipeline 104 are both connected with the evacuation main pipeline 9, the evacuation main pipeline 9 is provided with a fourth flow control valve 44, the first evacuation pipeline 103 is provided with a third switching valve 48, and the second evacuation pipeline 104 is provided with a fourth switching valve 50.
In this embodiment, the reboiler 35 is installed at the bottom of the rectifying tower 34, the bottom of the reboiler 35 is connected with the liquid methane output line 5, the liquid methane output line 5 is connected with the external LNG tank, and the liquid methane output line 5 is provided with the No. two flow control valve 42.
In this embodiment, the top of the rectifying tower 34 is connected with a non-condensable gas output pipeline 6, and the middle of the rectifying tower 34 is connected with a purified gas input pipeline 4. One end of the cooling medium conveying pipe 10 is connected with the upper part of the rectifying tower 34, and the other end of the cooling medium conveying pipe 10 is connected with an external cold source. In this embodiment, the cooling medium conveyed by the cooling medium conveying pipe 10 is liquid nitrogen, and a third flow control valve 43 is arranged on the cooling medium conveying pipe 10 and is used for controlling the amount of liquid nitrogen sprayed into the rectifying tower 34. The third connecting pipeline 21 and the sixth connecting pipeline 22 are connected with the noncondensable gas output pipeline 6, the third connecting pipeline 21 is branched with a first branched pipeline 211, the sixth connecting pipeline 22 is branched with a second branched pipeline 221, the first branched pipeline 211 and the second branched pipeline 221 are connected with the purified gas input pipeline 4, a seventh switching valve 54 is arranged on the first branched pipeline 211, and an eighth switching valve 52 is arranged on the second branched pipeline 221.
In this embodiment, the air outlet of the compressor 36 is connected to the air inlet of the cooler 37 through the first circulating air pipeline 12, the air inlet of the cooler 37 is connected to the air inlet of the channel a through the second circulating air pipeline 13, the air outlet of the channel a is connected to the air inlet of the channel E through the third circulating air pipeline 14, the air outlet of the channel E is connected to the air inlet of the reboiler 35 through the fourth circulating air pipeline 15, the air outlet of the reboiler 35 is connected to the air inlet of the channel F through the fifth circulating air pipeline 16, the air outlet of the channel F is connected to the air inlet of the channel B through the sixth circulating air pipeline 17, and the air outlet of the channel B is connected to the air inlet of the compressor 36 through the seventh circulating air pipeline 11. A pressure control valve 46 is provided in the fifth recycle gas line 16. A circulating gas regulating bypass 18 is communicated between the third circulating gas pipeline 14 and the fourth circulating gas pipeline 15, and a fifth flow control valve 45 is arranged on the circulating gas regulating bypass 18.
In this embodiment, the method for preparing LNG from low-concentration coal mine gas by liquefying and concentrating methane includes the following steps:
the first step: switching valve one 47, switching valve four 50, switching valve seven 54 and switching valve six 51 are opened, switching valve two 49, switching valve three 48, switching valve five 53 and switching valve eight 52 are closed;
and a second step of: the flow rate of the gas conveyed by the gas input main pipeline 1 is regulated through the first flow control valve 41, the gas enters a channel C of the precooling heat exchanger 31 through the first gas input pipeline 101 for cooling and drying, so that trace water impurities in the gas are frozen on the surface of the channel C, then the gas is output from the channel C, the temperature of the gas at the output time is minus 60 ℃ to minus 70 ℃, the gas enters a channel G of the cryogenic heat exchanger 32 through the first connecting pipeline 2 for removing and purifying carbon dioxide impurities, the carbon dioxide impurities in the gas are frozen on the surface of the channel G, then the gas is output from the channel G, the temperature of the gas at the output time is minus 150 ℃ to minus 160 ℃, the gas enters a channel J of the supercooling heat exchanger 33 through the second connecting pipeline 3 for further cooling, methane in the gas is condensed into liquid, then the gas containing liquid methane is output from the channel J, the temperature of the gas at the output time is minus 168 ℃ to minus 173 ℃, the gas enters the purifying gas input pipeline 4 through the first branch pipeline 211 on the third connecting pipeline 21, and then enters the middle part of the rectifying tower 34;
and a third step of: separating impurities such as nitrogen, oxygen and the like in the gas in the rectifying tower 34, enabling liquid methane in the gas to enter a reboiler 35 at the bottom of the rectifying tower 34, enabling one part of liquid methane to be output and stored in an external LNG storage tank through a liquid methane output pipeline 5, enabling the other part of liquid methane to enter a channel F as circulating methane after being subjected to pressure control through a pressure control valve 46 through a fifth circulating gas pipeline 16, enabling the liquid methane to enter a compressor 36 for pressurization through a sixth circulating gas pipeline 17, a channel B and a seventh circulating gas pipeline 11 in sequence, enabling the liquid methane to enter a cooler 37 for cooling through a first circulating gas pipeline 12, enabling the liquid methane to enter a channel A, a third circulating gas pipeline 14, a channel E and a fourth circulating gas pipeline 15 in sequence through a second circulating gas pipeline 13 after cooling, and enabling the liquid methane to enter the reboiler 35 to be used as a heat source for realizing a circulating process; the noncondensable gas sequentially passes through a noncondensable gas output pipeline 6, a sixth connecting pipeline 22, a channel I, a fifth connecting pipeline 7, a channel H, a fourth connecting pipeline 8, a channel D and a second emptying pipeline 104 branched on a second gas input pipeline 102, enters an emptying main pipeline 9, is rewarmed to normal temperature, and is then intensively discharged into the atmosphere;
fourth step: liquid nitrogen is input from the top of the rectifying tower 34 through a cooling medium conveying pipe 10 to serve as reflux liquid of the rectifying tower 34, heat transfer is carried out with ascending steam stripping provided by a reboiler 35, condensation and liquefaction of methane components in the rectifying tower 34 are realized, and the concentration of methane in noncondensable gas discharged from the top of the tower is controlled;
fifth step: when the freezing amount of water in the channel C and the freezing amount of carbon dioxide in the channel G reach a certain amount, switching the gas channel and the non-condensable gas discharging channel, closing the first switching valve 47, the fourth switching valve 50, the seventh switching valve 54 and the sixth switching valve 51, and opening the second switching valve 49, the third switching valve 48, the fifth switching valve 53 and the eighth switching valve 52; then, the gas sequentially passes through the second gas input pipeline 102, the channel D, the fourth connecting pipeline 8, the channel H, the fifth connecting pipeline 7, the channel I and the second branch pipeline 221 branched on the sixth connecting pipeline 22, and the purified gas input pipeline 4 enters the middle part of the rectifying tower 34 for gas separation, the non-condensable gas of the rectifying tower 34 sequentially passes through the non-condensable gas output pipeline 6, the third connecting pipeline 21, the channel J, the second connecting pipeline 3, the channel G, the first connecting pipeline 2, the channel C and the first emptying pipeline 103 branched on the first gas input pipeline 101 enter the main emptying pipeline 9 for rewarming to normal temperature and then are intensively discharged into the atmosphere, in the process, the non-condensable gas takes frozen water in the channel C and frozen carbon dioxide in the channel G out of the heat exchanger after the frozen amount of water in the channel D and the frozen amount of carbon dioxide in the channel H reach a certain amount, the gas channel and the non-condensable gas channel is switched again for sequential circulation.
In the third step, a part of the circulating methane enters the circulating gas regulating bypass 18, the circulating methane is used as a heat source of the reboiler 35, the temperature of the circulating methane entering the reboiler 35 is controlled by regulating the fifth flow control valve 45, and the rising stripping amount generated by the reboiler 35 is further controlled, so that the concentration of the liquid methane at the bottom of the reboiler 35 is controlled.
Although the present invention has been described with reference to the above embodiments, it should be understood that the invention is not limited to the embodiments described above, but is capable of modification and variation without departing from the spirit and scope of the present invention.

Claims (8)

1. The utility model provides a system for low concentration colliery gas liquefaction concentration methane system LNG, its characterized in that: the device comprises a gas input main pipeline (1), a precooling heat exchanger (31), a cryogenic heat exchanger (32), a supercooling heat exchanger (33), a rectifying tower (34), a reboiler (35), a compressor (36), a cooler (37), a cooling medium conveying pipe (10), a liquid methane output pipeline (5) and an emptying main pipeline (9); a channel A, a channel B, a channel C and a channel D are arranged in the pre-cooling heat exchanger (31), a channel E, a channel F, a channel G and a channel H are arranged in the deep cooling heat exchanger (32), and a channel I and a channel J are arranged in the sub-cooling heat exchanger (33);
a first flow control valve (41) is arranged on the gas input main pipeline (1), and the tail end of the gas input main pipeline (1) is branched into a first gas input pipeline (101) and a second gas input pipeline (102); the first gas input pipeline (101) is provided with a first switching valve (47), the tail end of the first gas input pipeline (101) is communicated with one end of a channel C, the other end of the channel C is communicated with one end of a channel G through a first connecting pipeline (2), the other end of the channel G is communicated with one end of a channel J through a second connecting pipeline (3), the other end of the channel J is connected with a third connecting pipeline (21), and a fifth switching valve (53) is arranged on the third connecting pipeline (21); the second gas input pipeline (102) is provided with a second switching valve (49), the tail end of the second gas input pipeline (102) is communicated with one end of a channel D, the other end of the channel D is communicated and connected with one end of a channel H through a fourth connecting pipeline (8), the other end of the channel H is communicated and connected with one end of a channel I through a fifth connecting pipeline (7), the other end of the channel I is connected with a sixth connecting pipeline (22), and the sixth connecting pipeline (22) is provided with a sixth switching valve (51);
the first gas input pipeline (101) is branched with a first emptying pipeline (103), the second gas input pipeline (102) is branched with a second emptying pipeline (104), the first emptying pipeline (103) and the second emptying pipeline (104) are connected with the main emptying pipeline (9), the first emptying pipeline (103) is provided with a third switching valve (48), and the second emptying pipeline (104) is provided with a fourth switching valve (50);
the reboiler (35) is arranged at the bottom of the rectifying tower (34), the bottom of the reboiler (35) is connected with the liquid methane output pipeline (5), and the liquid methane output pipeline (5) is connected with an external LNG storage tank; the top of the rectifying tower (34) is connected with a noncondensable gas output pipeline (6), the middle part of the rectifying tower (34) is connected with a purified gas input pipeline (4), one end of the cooling medium conveying pipe (10) is connected with the upper part of the rectifying tower (34), and the other end of the cooling medium conveying pipe (10) is connected with an external cold source; the third connecting pipeline (21) and the sixth connecting pipeline (22) are both connected with the noncondensable gas output pipeline (6), a first branch pipeline (211) is branched from the third connecting pipeline (21), a second branch pipeline (221) is branched from the sixth connecting pipeline (22), the first branch pipeline (211) and the second branch pipeline (221) are both connected with the purified gas input pipeline (4), a seventh switching valve (54) is arranged on the first branch pipeline (211), and an eighth switching valve (52) is arranged on the second branch pipeline (221);
the air outlet of compressor (36) is connected with the air inlet of cooler (37) through circulation air pipeline (12), the air inlet of cooler (37) is connected with the air inlet of passageway A through circulation air pipeline (13) No. two, the air outlet of passageway A is connected with the air inlet of passageway E through circulation air pipeline (14) No. three, the air outlet of passageway E is connected with the air inlet of reboiler (35) through circulation air pipeline (15) No. four, the air outlet of reboiler (35) is connected with the air inlet of passageway F through circulation air pipeline (16) No. five, the air outlet of passageway F is connected with the air inlet of passageway B through circulation air pipeline (17) No. six, the air outlet of passageway B is connected with the air inlet of compressor (36) through circulation air pipeline (11) No. seven.
2. The system for preparing LNG from liquefied and concentrated methane by low-concentration coal mine gas according to claim 1, wherein the system comprises the following components: be provided with No. two flow control valves (42) on liquid methane output pipeline (5), be provided with No. three flow control valves (43) on coolant conveying pipe (10), be provided with No. four flow control valves (44) on evacuation main line (9), be provided with pressure control valve (46) on No. five circulating gas pipelines (16).
3. The system for preparing LNG from liquefied and concentrated methane by low-concentration coal mine gas according to claim 1, wherein the system comprises the following components: a circulating gas regulating bypass (18) is communicated between the third circulating gas pipeline (14) and the fourth circulating gas pipeline (15), and a fifth flow control valve (45) is arranged on the circulating gas regulating bypass (18).
4. The system for preparing LNG from liquefied and concentrated methane by low-concentration coal mine gas according to claim 1, wherein the system comprises the following components: the gas conveyed in the gas conveying main pipeline (1) is coal mine gas with methane volume concentration of 20% -50% and air volume content of 50% -70%, the pressure is controlled within 0.1MPaG, and the temperature is controlled within 45 ℃.
5. The system for preparing LNG from liquefied and concentrated methane by low-concentration coal mine gas according to claim 1, wherein the system comprises the following components: the temperature of the gas output by the pre-cooling heat exchanger (31) is minus 60 ℃ to minus 70 ℃, the temperature of the gas output by the deep cooling heat exchanger (32) is minus 150 ℃ to minus 160 ℃, and the temperature of the gas output by the supercooling heat exchanger (33) is minus 168 ℃ to minus 173 ℃.
6. The system for preparing LNG from liquefied and concentrated methane by low-concentration coal mine gas according to claim 1, wherein the system comprises the following components: the cooling medium conveyed by the cooling medium conveying pipe (10) is liquid nitrogen.
7. A method for preparing LNG from low concentration coal mine gas by liquefying and concentrating methane by using the system of any one of claims 1-6, characterized in that: the method comprises the following steps:
the first step: a first switching valve (47), a fourth switching valve (50), a seventh switching valve (54) and a sixth switching valve (51) are opened, and a second switching valve (49), a third switching valve (48), a fifth switching valve (53) and a eighth switching valve (52) are closed;
and a second step of: the flow rate of the gas conveyed by the gas input main pipeline (1) is regulated through a first flow control valve (41), the gas enters a channel C of a pre-cooling heat exchanger (31) through a first gas input pipeline (101) for cooling and drying, trace water impurities in the gas are frozen on the surface of the channel C, then the gas is output from the channel C, the temperature of the gas at the output time is minus 60 ℃ to minus 70 ℃, the gas enters a channel G of a cryogenic heat exchanger (32) through a first connecting pipeline (2) for removing and purifying carbon dioxide impurities, the carbon dioxide impurities in the gas are frozen on the surface of the channel G, then the gas is output from the channel G, the temperature of the gas at the output time is minus 150 ℃ to minus 160 ℃, the methane in the gas is condensed into liquid through a second connecting pipeline (3) and then the gas containing liquid methane is output from the channel J, the gas temperature at the output time is minus 168 ℃ to minus 173 ℃, and the gas enters a first branch pipeline (211) on a third connecting pipeline (21) for purifying and then enters a middle part (34);
and a third step of: separating impurities such as nitrogen, oxygen and the like in the gas in a rectifying tower (34), enabling liquid methane in the gas to enter a reboiler (35) at the bottom of the rectifying tower (34), enabling one part of liquid methane to be output and stored in an external LNG storage tank through a liquid methane output pipeline (5), enabling the other part of liquid methane to enter a channel F after being used as circulating methane and pressure-controlled through a pressure control valve (46), enabling the liquid methane to enter a channel A, a channel B and a channel B sequentially through a No. six circulating gas pipeline (17), a No. seven circulating gas pipeline (11) to enter a compressor (36) for pressurization, enabling the liquid methane to enter a cooler (37) for cooling through a first circulating gas pipeline (12), enabling the cooled liquid methane to sequentially enter a channel A, a No. three circulating gas pipeline (14), a channel E and a No. four circulating gas pipeline (15) through a No. two circulating gas pipeline (13), and finally enabling the liquid methane to enter the reboiler (35) to serve as a heat source for realizing a circulating process; the noncondensable gas sequentially passes through a noncondensable gas output pipeline (6), a sixth connecting pipeline (22), a channel I, a fifth connecting pipeline (7), a channel H, a fourth connecting pipeline (8), a channel D and a second emptying pipeline (104) branched on a second gas input pipeline (102), enters an emptying main pipeline (9), is rewarmed to normal temperature, and is then intensively discharged into the atmosphere;
fourth step: liquid nitrogen is input from the top of the rectifying tower (34) through a cooling medium conveying pipe (10) as reflux liquid of the rectifying tower (34), heat transfer is carried out with ascending stripping provided by a reboiler (35), condensation and liquefaction of methane components in the rectifying tower (34) are realized, and the concentration of methane in noncondensable gas discharged from the top of the tower is controlled;
fifth step: when the freezing amount of water in the channel C and the freezing amount of carbon dioxide in the channel G reach a certain amount, switching the gas channel and the non-condensable gas discharging channel, closing a first switching valve (47), a fourth switching valve (50), a seventh switching valve (54) and a sixth switching valve (51), and opening a second switching valve (49), a third switching valve (48), a fifth switching valve (53) and a eighth switching valve (52); then, the gas sequentially passes through a second gas input pipeline (102), a channel D, a fourth connecting pipeline (8), a channel H, a fifth connecting pipeline (7), a channel I and a second branch pipeline (221) which are branched on a sixth connecting pipeline (22), the purified gas input pipeline (4) enters the middle part of the rectifying tower (34) to be separated, the non-condensable gas of the rectifying tower (34) sequentially passes through a non-condensable gas output pipeline (6) and a third connecting pipeline (21), a channel J, a second connecting pipeline (3), a channel G, a first connecting pipeline (2), a channel C and a first emptying pipeline (103) which are branched on a first gas input pipeline (101) enter an emptying main pipeline (9) to be rewarmed to normal temperature and then are intensively discharged to the atmosphere, in the process, the non-condensable gas takes frozen water in the channel C and frozen carbon dioxide in the channel G out of the heat exchanger, and after the frozen amount of the water in the channel D and the frozen amount of the carbon dioxide in the channel H reach a certain amount, the gas channel and the non-condensable gas channels are sequentially switched, and the circulating work is performed again.
8. The method for preparing LNG from low-concentration coal mine gas liquefaction and concentration methane according to claim 7, wherein the method comprises the following steps of: in the third step, a part of the circulating methane enters a circulating gas regulating bypass (18), and the temperature of the circulating methane entering a reboiler (35) is controlled by regulating a fifth flow control valve (45).
CN201811370335.7A 2018-11-17 2018-11-17 System and method for preparing LNG (liquefied Natural gas) from low-concentration coal mine gas by liquefying and concentrating methane Active CN109387030B (en)

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