Gas cooling system

Abstract

This gas cooling system 100 comprises: a cooling device 120 that cools a gas to be cooled, said gas having methane as a main component; a first depressurization unit 130 that depressurizes the gas being cooled that is cooled by the cooling device 120; a nitrogen adjustment unit 150 that makes an adjustment such that the nitrogen concentration of a liquefied gas obtained due to depressurization by the first depressurization unit 130 remains within a prescribed range; a second depressurization unit 160 that depressurizes the liquefied gas of which the nitrogen concentration has been adjusted by the nitrogen adjustment unit 150; a gas-liquid separation drum (second gas-liquid separation drum 170) that conducts gas-liquid separation on the gas / liquid mixture obtained due to depressurization by the second depressurization unit 160; and a pump 180 that pressurizes the liquefied gas separated by the gas-liquid separation drum. The cooling device 120 cools the gas to be cooled using the liquefied gas pressurized by the pump 180.

Description

recovery unit 154 brings the liquefied natural gas LNG and the separated gas NR into contact with each other (performs washing) so as to cause the liquefied natural gas LNG to absorb the methane contained an the separated gas NR. A methane-enriched liquefied gas LM (liquefied natural gas LNG absorbing methane) generated an the methane recovery unit 154 is supplied to the second gas-liquid separation drum 170 to be described later. The methane-enriched liquefied gas LM is, for example, -135°C and 0.6 MPaG.

[0031] Meanwhile, in the methane recovery unit 154, the separated gas NR from which the methane has been removed is exhausted to the outside as the exhaust gas EX. The exhaust gas EX is used as, for example, a fuel of a plant in which the gas cooling system 100 is provided.

[0032] A central control unit (not shown) controls a flow rate of the compressor 152 so that the nitrogen concentration of the liquefied gas LA falls within a predetermined range (for example, 0.5 mol% or more and 2 mol% or less). As a result, the central control unit controls the pressure of the first gas-liquid separation drum 140, and further controls the flow rate of the liquefied natural gas LNG to be supplied to the methane recovery unit 154.

[0033] The central control unit includes a semiconductor integrated circuit including a central processing unit (CPU). The central control unit reads out, for example, a program or parameters for operating the CPU itself from a ROM. The central control unit manages and controls the entire gas cooling system 100 in cooperation with a RAM serving as a working area and other electronic circuits.

[0034] Referring back to FIG. 1, the second pressure reducing unit 160 reduces the pressure of the liquefied gas LA whose nitrogen concentration has been adjusted by the nitrogen adjusting unit 150. In this embodiment, the second pressure-reducing unit 160 is a pressure reducing valve. The second pressure-reducing unit 160 reduces the pressure of the liquefied gas LA (performs flash cooling (evaporative cooling)), so as to obtain a gas-liquid mixture MZ of, for example, -155°C and 0.05 MPaG.

[0035] The second gas-liquid separation drum 170 (gas liquid separation drum) performs gas-liquid separation of the gas-liquid mixture MZ.

[0036] The pump 180 boosts the pressure of the liquefied natural gas LNG (liquefied gas) which has been separated from the gas-liquid mixture MZ by the second gas-liquid separation drum 170. In this embodiment, a discharge pressure of the pump 180 is a pressure at which, inside of the heat exchanger 124, at least a part of the liquefied natural gas LNG (liquefied gas) performs a phase change (evaporation). The discharge pressure of the pump 180 falls within a range of a predetermined lower limit pressure or more and a predetermined upper limit pressure or less. The lower limit pressure is, for example, 1.0 MPaG. The upper limit pressure is, for example, 2.0 MPaG. For example, the pump 180 boosts the pressure of the liquefied natural gas LNG to 1.6 MPaG. The pump 180 boosts the pressure of the liquefied natural gas LNG so as to shift the temperature for causing evaporation at the time of exchanging heat in the heat exchanger 124. The discharge pressure of the pump 180 can be measured by a mechanical pressure gauge or a pressure gauge including a pressure receiving element.

[0037] As a result, the liquefied natural gas LNG whose pressure has been boosted is shipped as a product or supplied to an LNG using destination. Further, a part of the liquefied natural gas LNG whose pressure has been boosted is supplied to the heat exchanger 124 of the cooling apparatus 120, and is then supplied to the precooling unit 122 so as to be used for cold heat recovery as a part of the refrigerant for cooling the natural gas NG in the cooling apparatus 120.

[0038] The flash gas compressor 190 boosts a pressure of a flash gas FG which has been separated in the second gasliquid separation drum 170. For example, the flash gas compressor 190 boosts the pressure of the flash gas FG to 1.5 MPaG. The flash gas FG whose pressure has been boosted by the flash gas compressor 190 is supplied to the precooling unit 122 so as to be used for cold heat recovery as a part of the refrigerant.

[0039] As described above, the gas cooling system 100 according to this embodiment uses the produced liquefied natural gas LNG as the refrigerant for cooling the natural gas NG. As a result, the gas cooling system 100 can partially omit a compressor and a cooler which are dedicated to the refrigerant, as compared to the dual-expander process of the related art. Thus, the gas cooling system 100 can have a simplified equipment configuration, and can reduce the cost for liquefaction.

[0040] Further, the precooling unit 122 of the gas cooling system 100 cools the supercritical fluid SF through use of the non-flammable refrigerant. Accordingly, when the gas cooling system 100 is used together with the precooling unit 122 using the non-flammable refrigerant, the gas cooling system 100 can produce the liquefied natural gas LNG at low cost.

[0041] Further, as described above, the natural gas NG contains a very small amount of nitrogen in addition to methane. The boiling point of nitrogen is lower than the boiling point of methane, and hence the flash gas FG generated when the pressure is reduced by the second pressure-reducing unit 160 contains nitrogen in priority to methane. Accordingly, in a case in which no nitrogen adjusting unit 150 is included, when the flash gas FG is returned to the booster compressor 110, along with an elapse of an operation time of the gas cooling system 100, the nitrogen concentration of the natural gas NG serving as the cooling target gas is increased (nitrogen is concentrated in the natural gas NG), and thus the amount of the flash gas FG is increased. Further, when the nitrogen concentration of the natural gas NG is excessively increased, the liquefaction efficiency is reduced. Accordingly, in the case in which no nitrogen adjusting unit 150 is included, the flash gas FG is needed to be discarded, and there arises a problem in that methane contained in the flash gas FG is also discarded.

[0042] In view of the above, as described above, the gas cooling system 100 includes the nitrogen adjusting unit 150 for removing nitrogen from the gas-liquid mixture MY. As a result, the gas cooling systen 100 can prevent the flash gas FG from increasing in amount, and thus can maintain the flash gas FG in a predetermined target amount. The target amount is an amount of the flash gas FG at the time when, in a case in which the flash gas FG is returned to the booster compressor 110, the nitrogen concentration of the natural gas NG obtained after the pressure is boosted by the booster compressor 110 becomes a nitrogen concentration that allows efficient liquefaction. Thus, the gas cooling system 100 can maintain the nitrogen concentration of the natural gas NG to a concentration that allows efficient liquefaction even when the flash gas FG is returned to the booster compressor 110 (precooling unit 122). Accordingly, the gas cooling system 100 can avoid a situation in which methane contained in the flash gas FG is discarded unnecessarily. Further, the flash gas FG can be maintained to the target amount, and hence the gas cooling system 100 can prevent the power of the flash gas compressor 190 from increasing.

[0043] When the nitrogen concentration of the liquefied gas LA is less than the lower limit value (for example, 0.5 mol%) of the above-mentioned predetermined range, the cooling efficiency to be obtained by the second pressure-reducing unit 160 is reduced. Meanwhile, when the nitrogen concentration of the liquefied gas LA exceeds the upper limit value (for example, 2 mol%) of the above-mentioned predetermined range, the flash gas FG is provided more than the target amount.

[0044] In view of the above, as described above, the nitrogen adjusting unit 150 sets the nitrogen concentration of the liquefied gas LA to be supplied to the second pressure-reducing unit 160 to be the above-mentioned lower limit value or more. As a result, the gas cooling system 100 can improve the cooling efficiency to be obtained by the second pressure-reducing unit 160. Thus, the temperature of the liquefied natural gas LNG to be generated when the pressure is reduced by the second pressure-reducing unit 160 can be reduced. That is, the gas cooling system 100 can reduce the temperature of the liquefied natural gas LNG to be supplied to the heat exchanger 124 by the pump 180, and can thus efficiently perform the cooling of the supercritical fluid SF by the heat exchanger 124.

[0045] Further, as described above, the nitrogen adjusting unit 150 sets the nitrogen concentration of the liquefied gas LA to be supplied to the second pressurereducing unit 160 to be the above-mentioned upper limit value or less. As a result, the gas cooling system 100 can maintain the flash gas FG to be generated in the second gasliquid separation drum 170 to the target amount.

[0046] Further, as described above, the heat exchanger 124 cools the supercritical fluid SF through use of the liquefied natural gas LNG. In the heat exchanger 124, it is desired that the flow rate of the liquefied natural gas LNG be as low as possible as compared to the flow rate of the supercritical fluid SF. In view of the above, the heat exchanger 124 uses latent heat obtained by the phase change of the liquefied natural gas LNG so as to cool the supercritical fluid SF through use of the liquefied natural gas LNG in a lower flow rate. In order to achieve a lower flow rate of the liquefied natural gas LNG, it is needed to reduce the pressure of the liquefied natural gas LNG to be supplied to the heat exchanger 124 so that a phase change temperature is reduced. Meanwhile, when the pressure of the liquefied natural gas LNG to be supplied to the heat exchanger 124 is excessively reduced, the pressure of the circulated natural gas CG to be supplied to the booster compressor 110 is reduced, resulting in increasing the power of the booster compressor 110. Further, when the pressure of the liquefied natural gas LNG to be supplied to the heat exchanger 124 is excessively increased, the phase change temperature is increased, resulting in no phase change of the liquefied natural gas LNG in the heat exchanger 124.

[0047] In view of the above, it is needed to appropriately set the phase change temperature of the liquefied natural gas LNG to be supplied to the heat exchanger 124. Accordingly, the discharge pressure of the pump 180 for boosting the pressure of the liquefied natural gas LNG to be supplied to the heat exchanger 124 is set within a range of a predetermined lower limit pressure or more and a predetermined upper limit pressure or less. The lower limit pressure is a pressure at which the power of the booster compressor 110 is prevented from excessively increasing. As a result, the pump 180 can prevent the power of the booster compressor 110 from excessively increasing.

[0048] Further, the upper limit pressure is a pressure at which, inside of the heat exchanger 124, at least a part of the liquefied natural gas LNG performs a phase change. As a result, the heat exchanger 124 can cool the supercritical fluid SF through use of the latent heat of the liquefied natural gas LNG. Thus, the heat exchanger 124 can cool the supercritical fluid SF through use of the liquefied natural gas LNG in an amount smaller than that of the supercritical fluid SF.

[0049] Further, as described above, the first pressure reducing unit 130 causes the supercritical fluid SF to perform a phase change to the gas-liquid mixture MY. Thus, the first pressure-reducing unit 130 can efficiently cool the supercritical fluid SF. Further, the first pressure-reducing unit 130 can cause the nitrogen to move to the separated gas NR. As a result, the nitrogen adjusting unit 150 can efficiently remove the nitrogen.

[0050] Further, as described above, the nitrogen adjusting unit 150 includes the methane recovery unit 154. As a result, the methane recovery unit 154 can increase the methane concentration of the liquefied natural gas LNG (liquefied gas LA).

[0051] [Modification Example] In the above-mentioned embodiment, a case in which the gas cooling system 100 cools the natural gas NG collected from the gas field, that is, a gas containing nitrogen in addition to methane is described as an example. Accordingly, the nitrogen adjusting unit 150 removes the nitrogen from the gas-liquid mixture MY so as to adjust the nitrogen concentration of the liquefied gas LA to be supplied to the second pressure-reducing unit 160.

[0052] However, when the gas cooling system 100 cools a cooling target gas containing almost no nitrogen, such as a renewable natural gas (renewable NG, RNG) or a synthetic natural gas (synthetic NG, SNG), the nitrogen adjusting unit 150 adds nitrogen to the liquefied gas LA so as to optimize the nitrogen concentration of the liquefied gas LA.

[0053] In this case, the nitrogen adjusting unit 150 adds the nitrogen to the natural gas NG at the inlet of the booster compressor 110 so as to set the nitrogen concentration of the liquefied gas LA to be supplied to the second pressure-reducing unit 160 to be the above-mentioned lower limit value or more and the above-mentioned upper limit value or less.

[0054] As a result, the gas cooling system 100 according to the modification example can improve the cooling efficiency to be obtained by the second pressure-reducing unit 160.

[0055] The embodiment has been described above with reference to the attached drawings, but, needless to say, the present disclosure is not limited to the embodiment. It is apparent that those skilled in the art may arrive at various alternations and modifications within the scope of claims, and those examples are construed as naturally falling within the technical scope of the present disclosure.

[0056] For example, in the above-mentioned embodiment, a configuration in which the nitrogen adjusting unit 150 includes the methane recovery unit 154 is described as an example. However, the nitrogen adjusting unit 150 is not needed to include the methane recovery unit 154. The nitrogen adjusting unit 150 is only needed to be at least capable of adjusting the nitrogen concentration of the liquefied gas LA obtained when the pressure is reduced by the first pressure-reducing unit 130.

[0057] Further, in the above-mentioned embodiment, as an example, there is described a case in which, in order to cause the nitrogen concentration of the liquefied gas LA to fall within a predetermined range, the central control unit controls the flow rate of the compressor 152 so as to control the pressure of the first gas-liquid separation drum 140, and further controls the flow rate of the liquefied natural gas LNG to be supplied to the methane recovery unit 154. However, when the nitrogen concentration of the natural gas NG is substantially constant, the central control unit is not needed to control the flow rate of the compressor 152 (not needed to control the pressure of the first gas-liquid separation drum 140), and is further not needed to control the flow rate of the liquefied natural gas LNG to be supplied to the methane recovery unit 154. In this case, the flow rate of the compressor 152 and the flow rate of the liquefied natural gas LNG to be supplied to the methane recovery unit 154 are designed so that the nitrogen concentration of the liquefied gas LA becomes 0.5 noli or more and 2 mol% or less.

[0058] Further, in the above-mentioned embodiment and modification example, a configuration in which the gas cooling system 100 includes the nitrogen adjusting unit 150 is described as an example. However, the gas cooling system 100 is not needed to include the nitrogen adjusting unit 150. That is, the gas cooling system includes: the booster compressor 110 which boosts the pressure of the cooling target gas containing methane as a main component so as to obtain the supercritical fluid SF; the precooling unit 122 which cools the supercritical fluid SF; the heat exchanger 124 which performs heat exchange between the supercritical fluid SF and the refrigerant; the first pressure-reducing unit 130 which reduces the pressure of the supercritical fluid SF which has been cooled by the heat exchanger 124; the second pressure-reducing unit 160 which reduces the pressure of the liquefied gas obtained when the pressure is reduced by the first pressure-reducing unit 130; the gas-liquid separation drum (second gas-loquid separation drum 170) which performs gas-liquid separation of the gas-liquid mixture obtained when the pressure is reduced by the second pressurereducing unit 160; and the punp 180 which boosts the pressure of the liquefied gas which has been separated by the gasliquid separation drum. The heat exchanger 124 cools the supercritical fluid SF through use of, as the refrigerant, the liquefied gas having the pressure boosted by the pump 180. The discharge pressure of the pump is a pressure at which, inside of the heat exchanger 124, at least a part of the liquefied gas performs a phase change.

[0059] The pump 180 of the gas cooling system has a discharge pressure which is set to be the above-mentioned lower limit pressure or more and the above-mentioned upper limit pressure or less. As described above, the lower limit pressure is a pressure at which the power of the booster compressor 110 is prevented from excessively increasing. As a result, the pump 180 can prevent the power of the booster compressor 110 from excessively increasing.

[0060] Similarly, the upper limit pressure is a pressure at which, inside of the heat exchanger 124, at least a part of the liquefied natural gas LNG performs a phase change. As a result, the heat exchanger 124 can cool the supercritical fluid SF through use of the latent heat of the liquefied natural gas LNG. Thus, the heat exchanger 124 can cool the supercritical fluid SF through use of the liquefied natural gas LNG in an amount smaller than that of the supercritical fluid SF.

[0061] The present disclosure allows efficient liquefaction of a renewable natural gas having a low supply pressure, and hence can contribute to, for example, Goal 7 of 5 "Ensure access to affordable, reliable, sustainable and modern energy for all" and Goal 13 of "Take urgent action to combat climate change and its impacts" of the sustainable development goals (SDGs). 10 Reference Signs List

[0062] 100: gas cooling system, 110: booster compressor (compressor), 120: cooling apparatus, 122: precooling unit 124: heat exchanger, 130: first pressure-reducing unit, 150: nitrogen adjusting unit, 160: second pressure-reducing unit, 15  170: second gas-liquid separation drum (gas-liquid separation drum), 180: pump

Claims

1. A gas cooling system, comprising:a cooling apparatus which cools a cooling target gas containing methane as a main component;a first pressure-reducing unit which reduces a pressure of the cooling target gas which has been cooled by the cooling apparatus;a nitrogen adjusting unit which adjusts a nitrogen concentration of a liquefied gas obtained when the pressure is reduced by the first pressure-reducing unit so as to fall within a predetermined range;a second pressure-reducing unit which reduces a pressure of the liquefied gas having the nitrogen concentration adjusted by the nitrogen adjusting unit;a gas-liquid separation drum which performs gas-liquid separation of a gas-liquid mixture obtained when the pressure is reduced by the second pressure-reducing unit; anda pump which boosts a pressure of a liquefied gas which has been separated by the gas-liquid separation drum, wherein the cooling apparatus cools the cooling target gas through use of the liquefied gas having the pressure boosted by the pump.

2. The gas cooling system according to claim 1, further comprising a compressor which boosts the pressure of the cooling target gas so as to obtain a supercritical fluid, wherein the cooling apparatus includes:a precooling unit which cools the supercritical fluid; anda heat exchanger which performs heat exchange between the supercritical fluid and the liquefied gas having the pressure boosted by the pump, andwherein a discharge pressure of the pump is a pressure at which, inside of the heat exchanger, at least a part of the liquefied gas performs a phase change.

3. The gas cooling system according to claim 2, wherein the first pressure-reducing unit causes the supercritical fluid to perform a phase change to a gas-liquid mixture .

4. The gas cooling system according to claim 3, wherein the nitrogen adjusting unit removes nitrogen from the gas-liquid mixture obtained when the pressure is reduced by the first pressure-reducing unit so as to adjust the nitrogen concentration of the liquefied gas to fall within the predetermined range.

5. The gas cooling system according to claim 4, wherein the nitrogen adjusting unit recovers methane from a separated gas forming the gas-liquid mixture obtained when the pressure is reduced by the first pressure-reducing unit.

6. The gas cooling system according to any one ofclaims 2 to 5, wherein the precooling unit cools the supercritical fluid through use of a non-flammable refrigerant.

7. A gas cooling system, comprising:a compressor which boosts a pressure of a cooling target gas containing methane as a main component so as to obtain a supercritical fluid;a precooling unit which cools the supercritical fluid;a heat exchanger which performs heat exchange between the supercritical fluid and a refrigerant;a first pressure-reducing unit which reduces a pressure of the supercritical fluid which has been cooled by the heat exchanger;a second pressure-reducing unit which reduces a pressure of a liquefied gas obtained when the pressure is reduced by the first pressure-reducing unit;a gas-liquid separation drum which performs gas-liquid separation of a gas-liquid mixture obtained when the pressure is reduced by the second pressure-reducing unit; anda pump which boosts a pressure of a liquefied gas which has been separated by the gas-liquid separation drum,wherein the heat exchanger cools the supercritical fluid through use of, as the refrigerant, the liquefied gas having the pressure boosted by the pump, andwherein a discharge pressure of the pump is a pressure at which, inside of the heat exchanger, at least a part ofthe liquefied gas performs a phase change.

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