A process for producing LNG from associated gas in oil fields

Abstract

The present invention provides a process for producing LNG from associated gas in oil fields, which is characterized by including a raw gas buffer tank, a raw gas compressor, a raw gas cooler, etc. The external raw gas enters the raw gas buffer tank through a conveying pipeline. The raw gas buffer tank is connected to a wastewater output pipeline outside the boundary, a raw gas compressor, and a stabilized light hydrocarbon tower through conveying pipelines. The raw gas compressor is connected to a three-phase separator through a conveying pipeline, and a raw gas cooler is installed on this conveying pipeline. The present invention is applicable to associated gas in oil fields, natural gas wellhead gas, natural gas condensate gas, and flash vapor from crude oil tank farms. The present invention adds a stabilized light hydrocarbon tower to remove most of the heavy hydrocarbons, reduce the influence of heavy hydrocarbons on the subsequent decarbonization and desulfurization systems and molecular sieve dryers, solve the problems of oil entrainment and foaming in the amine liquid absorption tower and molecular sieve poisoning and failure, and reduce the loads of the cold box and the refrigeration compressor.

Description

Technical Field

[0001] The present invention relates to the field of oilfield exploitation, and particularly to a process for producing LNG from associated gas in oilfields. Background Art

[0002] During the process of crude oil exploitation, relatively light components such as methane, ethane, propane, n-butane, isobutane, n-pentane, isopentane, and neopentane, which are usually dissolved in crude oil, will precipitate, and are called associated gas in oilfields, which is also another source of natural gas.

[0003] Due to the relatively dispersed associated gas and the difficulty in recycling and utilization, the current associated gas recycling and utilization technology mainly recovers some of the light hydrocarbons, and the remaining dry gas is used for combustion and flaring or power generation, while the process technology for liquefying associated gas into LNG has formed a gap, resulting in serious waste of resources.

[0004] With the rapid development of the petroleum industry in recent years, the government has paid more and more attention to the energy conservation, emission reduction and environmental protection of oil enterprises. As an important part of energy conservation, emission reduction, production expansion and efficiency improvement, if all of these associated gases are recovered, it is not only beneficial to solve environmental problems, but also can increase economic benefits, which is of great significance to the sustainable green development of enterprises and the country.

[0005] Most of the existing reports on associated gas in oilfields only carry out light hydrocarbon recovery, and there is almost no process for further liquefying it into LNG. The process of liquefying natural gas into LNG generally refers to the natural gas from wellheads in gas fields or pipeline gas. For this kind of gas quality composition and gas volume, they are relatively stable and have small fluctuations. The natural gas is directly subjected to decarbonization, desulfurization, dehydration and demercuration treatments, and then enters the cold box for liquefaction. In an environment with a relatively high heavy hydrocarbon content, large changes in gas quality composition and unstable gas volume, a series of problems are likely to occur. They lack a light hydrocarbon stabilization device, and the heavy hydrocarbon removal device will have an impact on the subsequent decarbonization and desulfurization units, the service life of molecular sieves, and the freezing blockage of the cold box.

[0006] For heavy hydrocarbons, they generally adopt adsorption separation methods, isopentane dissolution methods, cryogenic separation methods, etc. The adsorption separation method mainly uses porous solids such as silica gel, molecular sieves, activated carbon, etc. as adsorbents, and separates hydrocarbon gases according to the difference in the adsorption capacity of hydrocarbon components. This method mainly has two-column processes and three-column processes. The adsorption process is divided into three stages: adsorption, heating desorption, and regeneration.

[0007] The isopentane dissolution method is to add a washing tower after the heavy hydrocarbon separation tank, and utilize the principle of similar solubility to absorb and dissolve the heavy hydrocarbons in natural gas.

[0008] The cryogenic separation method mainly uses the different boiling points of hydrocarbon components to achieve the purpose of gas-liquid separation. It cools natural gas to the required low temperature below the hydrocarbon dew point temperature to obtain a part of the condensate rich in C5+ hydrocarbons for heavy hydrocarbon separation and removal. Summary of the Invention

[0009] In view of the above technical problems, the present invention combines stable light hydrocarbons with low-temperature rectification and refrigeration technologies, which can effectively adapt to the working conditions of different qualities of associated gas in oilfields, solve the problems of amine solution oil carry-over in the decarbonization and desulfurization system, foaming and flooding in the absorption tower, molecular sieve poisoning, and cold box freezing during the natural gas treatment process, and provide the necessary conditions for the liquefaction of associated gas in oilfields.

[0010] The present invention provides a process for producing LNG from associated gas in oilfields. The specific process flow is as follows: The raw gas is pressurized to 40 - 50 bar by a three-stage booster unit, cooled to 45 °C by an air cooler, then further cooled to 15 - 20 °C in a pre-cooling heat exchanger, and enters a three-phase separator for separation. The gas phase enters the gas treatment unit, and the oil phase enters the stable light hydrocarbon tower. A reboiler is provided at the bottom of the tower, with heat transfer oil or steam as the heat source, and the temperature is controlled at 100 °C - 140 °C. Through heating, the light components are evaporated from the top of the tower and enter the raw gas tank, and the water phase enters the sewage tank;

[0011] The gas phase coming from the three-phase separator sequentially passes through the decarbonization and desulfurization system, the molecular sieve dryer, and the mercury removal tank;

[0012] The treated natural gas first enters the middle part of the heavy hydrocarbon tower. Through rectification, the light components methane and ethane are separated out. A reboiler is provided in the heavy hydrocarbon tower, with heat transfer oil or steam as the heat source, and the temperature is controlled at 100 °C - 140 °C. The gas phase at the top of the heavy hydrocarbon tower then enters the cold box and is cooled to -40 °C - -75 °C, and enters the heavy hydrocarbon separation tank. The gas phase coming out of the heavy hydrocarbon tank continues to be cryogenically cooled to about -155 °C, throttled and depressurized to 1.5 bar - 2 bar and enters the LNG storage tank; The liquid phase coming out of the heavy hydrocarbon tank, through the heavy hydrocarbon pump, returns as reflux to the top of the heavy hydrocarbon tower; The bottom materials of the heavy hydrocarbon tower and the bottom materials of the stable tower are depressurized and cooled to 45 °C, and then sent to the mixed hydrocarbon storage tank, or further processed.

[0013] A process method for producing LNG from associated gas in oil fields. Its process system includes a raw gas buffer tank, a raw gas compressor, a raw gas cooler, a three-phase separator, a stabilized light hydrocarbon tower, a stabilized light hydrocarbon tower reboiler, a decarbonization and desulfurization system, a molecular sieve dryer, a mercury removal tank, a heavy hydrocarbon tower, a reboiler, a low-temperature gas-liquid separator, a heavy hydrocarbon reflux pump, a cold box, a pre-cooling compressor unit, a pre-cooling compressor unit condenser, a main cooling compressor unit, and a main cooling compressor unit condenser. External raw gas enters the raw gas buffer tank through a conveying pipeline. The raw gas buffer tank is connected to a wastewater output pipeline outside the boundary, a raw gas compressor, and a stabilized light hydrocarbon tower through conveying pipelines. The raw gas compressor is connected to the three-phase separator through a conveying pipeline, and a raw gas cooler is installed on this conveying pipeline. The three-phase separator is connected to the decarbonization and desulfurization system, a wastewater output pipeline outside the boundary, and a stabilized light hydrocarbon tower through conveying pipelines. The stabilized light hydrocarbon tower is also connected to a mixed hydrocarbon conveying pipeline and a stabilized light hydrocarbon tower reboiler respectively. The decarbonization and desulfurization system is connected to the molecular sieve dryer through a conveying pipeline. The molecular sieve dryer is connected to the mercury removal tank through a conveying pipeline. The mercury removal tank is connected to the heavy hydrocarbon tower through a conveying pipeline. The heavy hydrocarbon tower is connected to the cold box, a heavy hydrocarbon reflux pump, and a reboiler through conveying pipelines respectively. The heavy hydrocarbon reflux pump is connected to the low-temperature gas-liquid separator. The low-temperature gas-liquid separator is connected to the cold box. The cold box is also connected to the pre-cooling compressor unit, the pre-cooling compressor unit condenser, the main cooling compressor unit, and the main cooling compressor unit condenser. The cold box is connected to an LNG conveying pipeline, and a valve is installed between the cold box and the LNG conveying pipeline.

[0014] The cold box, the pre-cooling compressor unit, and the pre-cooling compressor unit condenser are connected through pipelines to form a closed pre-cooling system. The cold box, the main cooling compressor unit, and the main cooling compressor unit condenser are connected through pipelines to form a closed deep-cooling system. A valve is installed between the cold box and the pre-cooling compressor unit condenser, and a valve is installed between the cold box and the main cooling compressor unit condenser. The pre-cooling compressor unit provides the cold source for the high-temperature section with a single refrigerant medium, including propane, ammonia, R134a, R22, R23, and the refrigeration temperature range is (-10°C to -40°C). The main cooling compressor unit provides the cold source for the deep-cooling section with a mixed refrigerant medium, including a combination of nitrogen, methane, propane, ethylene, isobutane, and isopentane in different ratios, and the refrigeration temperature range is -80°C to -180°C.

[0015] The cooling capacity of the cold box is provided by the pre-cooling compressor and the main cooling compressor.

[0016] The pre-cooling compressor provides the cold source for the high-temperature section with a single refrigerant medium, and the cold source includes propane, ammonia, R134a, R22, R23, and the refrigeration temperature range is -10°C to -40°C.

[0017] The main cold compressor provides a cryogenic section cold source for the mixed refrigerant medium. The cold source includes a combination of different ratios of nitrogen, methane, propane, ethylene, isobutane, and isopentane, and the refrigeration temperature range is -80°C to -180°C.

[0018] The gas sequentially passes through a decarbonization and desulfurization system, a molecular sieve dryer, and a mercury removal tank, reducing the content of CO2 in the natural gas to less than 30 PPM, the sulfur content to less than 3 PPM, the water content to less than 1 PPM, and the mercury content to less than 0.01 μg / m3, preventing the cold box from freezing and plugging, and preventing mercury from corroding the aluminum plate fin heat exchanger in the cold box, resulting in damage to the cold box.

[0019] The beneficial effects of the present invention are as follows: The present invention is applicable to associated gas in oil fields, natural gas wellhead gas, natural gas condensate gas, and flash vapor in crude oil tank farms. The present invention adds a stabilized light hydrocarbon tower to remove most of the heavy hydrocarbons, reducing the impact of heavy hydrocarbons on the subsequent decarbonization and desulfurization system and molecular sieve dryer, solving the problems of oil foam in the amine liquid absorption tower and molecular sieve poisoning and failure, and reducing the load on the cold box and refrigeration compressor.

[0020] The heavy hydrocarbon tower of the present invention is a rectification tower, which is divided into two parts: a rectification section and a stripping section. The treated associated gas in the oil field first enters the middle of the heavy hydrocarbon tower. The top of the tower relies on the reflux of low-temperature liquid to gather the heavy hydrocarbons at the bottom of the tower and is taken out from the tower kettle, avoiding the heavy hydrocarbons in the associated gas from entering the cold box again and causing the cold box to freeze and plug; the bottom of the tower is heated by a reboiler to separate methane and ethane. The gas phase at the top of the tower contains almost no components above C3, and the bottom of the tower contains a trace amount of ethane.

[0021] The present invention is provided with two sets of refrigeration units. The cooling capacity is provided by two sets of refrigeration units with different grades. The pre-cooling compressor unit is a single refrigerant medium that provides a cold source for the high-temperature section, including propane, ammonia, R134a, R22, R23, etc., and the refrigeration temperature range is -10°C to -40°C. The main cold compressor unit is a mixed refrigerant medium that provides a cold source for the cryogenic section, including a combination of different ratios of nitrogen, methane, propane, ethylene, isobutane, isopentane, etc., and the refrigeration temperature range is -80°C to -180°C. The two sets of refrigeration units can handle raw materials with different compositions, have stable operation, and high adaptability.

[0022] The stable light hydrocarbon tower of the present invention is respectively connected to the heavy hydrocarbon tower, the mixed hydrocarbon conveying pipeline, and the reboiler of the stable light hydrocarbon tower. The design of the stable light hydrocarbon tower can handle raw materials rich in components C3 and above, remove most of the heavy hydrocarbons, effectively reduce the load of the refrigeration device and the treatment load of the heavy hydrocarbon tower. It can also avoid the problem of amine solution carrying oil and amine solution foaming in the absorption tower; the stable light hydrocarbon tower of the present invention removes benzene and aromatic hydrocarbons in advance, avoiding the problem of poisoning of the molecular sieve drying tower and extending the service life of the molecular sieve. The purified natural gas of the present invention first enters the heavy hydrocarbon separation tower to remove heavy hydrocarbons, and then the gas phase enters the liquefaction cold box, which can effectively prevent the blockage of the liquefaction cold box. Further remove heavy hydrocarbons to reduce the risk of cold box freezing. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic structural diagram of the present invention.

[0024] As shown in the figure: 1 - raw gas buffer tank; 2 - raw gas compressor; 3 - raw gas cooler; 4 - three-phase separation tank; 5 - stable light hydrocarbon tower; 6 - reboiler of the stable light hydrocarbon tower; 7 - decarbonization and desulfurization system; 8 - molecular sieve dryer; 9 - mercury removal tank; 10 - heavy hydrocarbon tower; 11 - reboiler; 12 - low-temperature gas-liquid separator; 13 - heavy hydrocarbon reflux pump; 14 - cold box; 15 - pre-cooling compressor unit; 16 - condenser of the pre-cooling compressor unit; 17 - main cooling compressor unit; 18 - condenser of the main cooling compressor unit. DETAILED DESCRIPTION OF THE INVENTION

[0025] Example 1

[0026] The present invention provides a process for producing LNG from associated gas in oil fields, which includes a raw gas buffer tank, a raw gas compressor, a raw gas cooler, a three-phase separator, a stabilized light hydrocarbon column, a reboiler for the stabilized light hydrocarbon column, a decarbonization and desulfurization system, a molecular sieve dryer, a mercury removal tank, a heavy hydrocarbon column, a reboiler, a low-temperature gas-liquid separator, a heavy hydrocarbon reflux pump, a cold box, a pre-cooling compressor unit, a condenser for the pre-cooling compressor unit, a main cooling compressor unit, and a condenser for the main cooling compressor unit. The external raw gas enters the raw gas buffer tank through a conveying pipeline. The raw gas buffer tank is connected to a wastewater outlet pipeline outside the boundary, the raw gas compressor, and the stabilized light hydrocarbon column through conveying pipelines respectively. The raw gas compressor is connected to the three-phase separator through a conveying pipeline, and a raw gas cooler is installed on this conveying pipeline. The three-phase separator is connected to the decarbonization and desulfurization system, the wastewater outlet pipeline outside the boundary, and the stabilized light hydrocarbon column through conveying pipelines. The stabilized light hydrocarbon column is also connected to a mixed hydrocarbon conveying pipeline and a reboiler for the stabilized light hydrocarbon column respectively. The decarbonization and desulfurization system is connected to the molecular sieve dryer through a conveying pipeline. The molecular sieve dryer is connected to the mercury removal tank through a conveying pipeline. The mercury removal tank is connected to the heavy hydrocarbon column through a conveying pipeline. The heavy hydrocarbon column is connected to the cold box, the heavy hydrocarbon reflux pump, and the reboiler through conveying pipelines respectively. The heavy hydrocarbon reflux pump is connected to the low-temperature gas-liquid separator, and the low-temperature gas-liquid separator is connected to the cold box. The cold box is also connected to the pre-cooling compressor unit, the condenser for the pre-cooling compressor unit, the main cooling compressor unit, and the condenser for the main cooling compressor unit. The cold box is connected to an LNG conveying pipeline, and a valve is installed between the cold box and the LNG conveying pipeline. The cold box, the pre-cooling compressor unit, and the condenser for the pre-cooling compressor unit are connected through pipelines to form a closed pre-cooling system. The cold box, the main cooling compressor unit, and the condenser for the main cooling compressor unit are connected through pipelines to form a closed deep-cooling system. Valves are installed between the cold box and the condenser for the pre-cooling compressor unit, and between the cold box and the condenser for the main cooling compressor unit. The pre-cooling compressor unit provides the cold source for the high-temperature section with a single refrigerant medium, including propane, ammonia, R134a, R22, R23, and the refrigeration temperature range is (-10°C to -40°C). The main cooling compressor unit provides the cold source for the deep-cooling section with a mixed refrigerant medium, including a combination of different ratios of nitrogen, methane, propane, ethylene, isobutane, and isopentane, and the refrigeration temperature range is -80°C to -180°C.

[0027] Example 2

[0028] The specific usage method of the process for producing LNG from associated gas in oil fields of the present invention is as follows:

[0029] The raw material gas is pressurized to 40 - 50 bar by a three - stage booster unit. After being cooled to 45°C by an air cooler, it then enters a pre - cooler heat exchanger and is cooled to 15 - 20°C, and then enters a three - phase separator for separation. The gas phase enters the gas - phase treatment unit, the oil phase enters the stabilized light hydrocarbon tower. A reboiler is installed at the bottom of the tower, with heat transfer oil or steam as the heat source, and the temperature is controlled at 100°C - 140°C. Through heating, the light components are evaporated from the top of the tower and enter the raw material gas tank, and the water phase enters the sewage tank; The gas phase coming in from the three - phase separator passes through a decarbonization and desulfurization system, a molecular sieve dryer, and a mercury removal tank in sequence; The treated natural gas first enters the middle of the heavy hydrocarbon tower. Through rectification, the light components methane and ethane are separated out. A reboiler is installed in the heavy hydrocarbon tower, with heat transfer oil or steam as the heat source, and the temperature is controlled at 100°C - 140°C. The gas phase at the top of the heavy hydrocarbon tower then enters a cold box and is cooled to - 40°C - - 75°C, and enters a heavy hydrocarbon separation tank. The gas phase coming out of the heavy hydrocarbon tank continues to be cryogenically cooled to about - 155°C, throttled and depressurized to 1.5 bar - 2 bar and enters the LNG storage tank; The liquid phase coming out of the heavy hydrocarbon tank, through a heavy hydrocarbon pump, returns as reflux to the top of the heavy hydrocarbon tower; The bottom materials of the heavy hydrocarbon tower and the bottom materials of the stabilization tower are depressurized and then cooled to 45°C, and then sent to a mixed hydrocarbon storage tank, or further processed.

[0030] The above shows and describes the basic principles, main features and advantages of the present invention. Each component mentioned in the present invention is a common technology in the existing field. Those skilled in the art should understand that the present invention is not limited by the above - mentioned embodiments. What is described in the above - mentioned embodiments and the specification only illustrates the principle of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A process for producing LNG from associated gas in oilfields, characterized in that: The feed gas is pressurized to 40 - 50 bar by a three-stage booster unit, cooled to 45°C by an air cooler, then further cooled to 15 - 20°C in a pre-cooling heat exchanger, and enters a three-phase separator for separation. The gas phase enters the gas treatment unit, and the oil phase enters the stabilized light hydrocarbon column. A reboiler for the stabilized light hydrocarbon column is installed at the bottom of the column, with heat transfer oil or steam as the heat source, and the temperature is controlled at 100°C - 140°C. Through heating, the light components are evaporated from the top of the column and enter the feed gas buffer tank, while the water phase enters the sewage tank. The gas phase coming out of the three-phase separator sequentially passes through a decarbonization and desulfurization system, a molecular sieve dryer, and a mercury removal tank. After treatment, it first enters the middle part of the heavy hydrocarbon column. Through rectification, the light components methane and ethane are separated. A reboiler is installed in the heavy hydrocarbon column, with heat transfer oil or steam as the heat source, and the temperature is controlled at 100°C - 140°C. The gas phase at the top of the heavy hydrocarbon column then enters a cold box and is cooled to -40°C - -75°C, and enters a heavy hydrocarbon separation tank. The gas phase coming out of the heavy hydrocarbon separation tank continues to be cryogenically cooled to -155°C, throttled and depressurized to 1.5 bar - 2 bar, and then enters the LNG storage tank. The liquid phase coming out of the heavy hydrocarbon separation tank, through a heavy hydrocarbon reflux pump, is returned as reflux to the top of the heavy hydrocarbon column. The bottom materials of the heavy hydrocarbon column and the bottom materials of the stabilized light hydrocarbon column are depressurized and cooled to 45°C, and then sent to a mixed hydrocarbon storage tank or further processed. The specific process system includes a feed gas buffer tank, a feed gas compressor, a feed gas cooler, a three-phase separation tank, a stabilized light hydrocarbon column, a reboiler for the stabilized light hydrocarbon column, a decarbonization and desulfurization system, a molecular sieve dryer, a mercury removal tank, a heavy hydrocarbon column, a reboiler, a heavy hydrocarbon separation tank, a heavy hydrocarbon reflux pump, a cold box, a pre-cooling compressor unit, a condenser for the pre-cooling compressor unit, a main cooling compressor unit, and a condenser for the main cooling compressor unit. The external feed gas enters the feed gas buffer tank through a conveying pipeline. The feed gas buffer tank is connected to a wastewater output pipeline outside the boundary, a feed gas compressor, and a stabilized light hydrocarbon column through conveying pipelines respectively. The feed gas compressor is connected to the three-phase separation tank through a conveying pipeline, and a feed gas cooler is installed on this conveying pipeline. The three-phase separation tank is connected to the decarbonization and desulfurization system, a wastewater output pipeline outside the boundary, and a stabilized light hydrocarbon column through conveying pipelines respectively. The stabilized light hydrocarbon column is also connected to a mixed hydrocarbon conveying pipeline and a reboiler for the stabilized light hydrocarbon column respectively. The decarbonization and desulfurization system is connected to the molecular sieve dryer through a conveying pipeline. The molecular sieve dryer is connected to the mercury removal tank through a conveying pipeline. The mercury removal tank is connected to the heavy hydrocarbon column through a conveying pipeline. The heavy hydrocarbon column is connected to the cold box, a heavy hydrocarbon reflux pump, and a reboiler through conveying pipelines respectively. The heavy hydrocarbon reflux pump is connected to the heavy hydrocarbon separation tank. The heavy hydrocarbon separation tank is connected to the cold box. The cold box is also connected to the pre-cooling compressor unit, a condenser for the pre-cooling compressor unit, a main cooling compressor unit, and a condenser for the main cooling compressor unit. The cold box is connected to an LNG conveying pipeline, and a valve is installed between the cold box and the LNG conveying pipeline.

2. The process for producing LNG from associated gas in oilfields according to claim 1, characterized in that The gas passes through the decarbonization and desulfurization system, molecular sieve dryer, and mercury removal tank in sequence, reducing the content of CO2 in the gas to less than 30 ppm, the sulfur content to less than 3 ppm, the water content to less than 1 ppm, and the mercury content to less than 0.01 μg / m 3 to prevent the cold box from freezing and mercury from corroding the aluminum plate fin heat exchanger in the cold box, causing damage to the cold box.

3. The process for producing LNG from associated gas in oilfields according to claim 1, characterized in that The cold box, the pre-cooling compressor unit, and the pre-cooling compressor unit condenser are connected by pipelines to form a closed pre-cooling system. The cold box, the main cooling compressor unit, and the main cooling compressor unit condenser are connected by pipelines to form a closed cryogenic system. Valves are installed between the cold box and the pre-cooling compressor unit condenser, and valves are installed between the cold box and the main cooling compressor unit condenser. The pre-cooling compressor unit provides the cold source for the high-temperature section with a single refrigerant medium, and the single refrigerant medium includes propane, ammonia, R134a, R22, and R23. The refrigeration temperature range is -10°C to -40°C. The main cooling compressor unit provides the cold source for the cryogenic section with a mixed refrigerant medium, and the mixed refrigerant medium includes combinations of different ratios of nitrogen, methane, propane, ethylene, isobutane, and isopentane. The refrigeration temperature range is -80°C to -180°C.

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