AU2005236214B2

AU2005236214B2 - Method for the liquefaction of a gas involving a thermo-acoustic cooling apparatus

Info

Publication number: AU2005236214B2
Application number: AU2005236214A
Authority: AU
Inventors: Beatrice Fischer
Original assignee: IFP Energies Nouvelles IFPEN
Current assignee: IFP Energies Nouvelles IFPEN
Priority date: 2004-03-23
Filing date: 2005-02-21
Publication date: 2009-10-08
Anticipated expiration: 2025-02-21
Also published as: AU2005236214A1; MY136422A; WO2005103583A1; FR2868154B1; FR2868154A1

Description

PATENT INSTITUT FRANQAIS DU PETROLE GAS LIQUEFACTION METHOD INCLUDING A THERMOACOUSTIC COOLING DEVICE Inventor: B6atrice FISCHER ABSTRACT The gas circulating in line I is cooled and liquefied under pressure in heat exchanger 11. The gas flowing from heat exchanger 11 through line 2 is fed through line 3 into expansion device 13. The gas is expanded to a pressure close to the atmospheric pressure. The fluid discharged through line 4 constitutes the liquefied gas, for example the liquefied natural gas. The cooling cycle consists of a cooling fluid circulating in compressor 20, condenser 21, heat exchanger 11 and expansion device 22. According to the invention, a thermoacoustic cooling device 12 cools the fluid obtained at the outlet of exchanger 11 through line 3, and a thermoacoustic cooling device 23 cools the fluid obtained at the outlet of exchanger 11 through line 24.

FIELD OF THE INVENTION The present invention relates to the field of gas liquefaction, using a thermoacoustic cooling device. The aim of the method according to the invention is to liquefy gases by cooling, for 5 example neon, hydrogen, helium or natural gas. What is referred to as natural gas is a gaseous, liquid or two-phase mixture comprising at least 50 % methane, and possibly other hydrocarbons and nitrogen. Natural gas is generally produced in gaseous form, and at a high pressure ranging for example between 1 MPa and 15 MPa. 10 Liquefaction of natural gas consists in condensing the gas, then in subcooling it to a sufficiently low temperature so that it can remain liquid at atmospheric pressure. Finally, the liquid natural gas is transported in LNG carriers. BACKGROUND OF THE INVENTION Currently, the international liquid natural gas (LNG) trade is developing fast. 15 However, the whole of the LNG production chain requires considerable investments and operating costs. Document FR-2,778,232 provides a liquefaction method comprising two cooling mixtures circulating in two independent and closed circuits. Each circuit works by means of a compressor supplying the cooling mixture with the power required to cool 20 the natural gas. Each compressor is driven by a gas turbine that is selected from among the standard product line commercially available. However, the power of the gas 2 turbines currently available is limited. The LNG production capacity of a liquefaction unit is thus limited by the power of the gas turbines. The present invention proposes improving the method disclosed by document FR 2,778,232 in order to increase the liquefaction power while keeping the standard 5 compressors. SUMMARY OF THE INVENTION The present invention proposes combining a conventional liquefaction method with cooling operated by a thermoacoustic refrigeration device. In general terms, the present invention relates to a gas liquefaction method, the gas 10 being available at a pressure P1. The gas is subjected to the following stages - condensing the gas under pressure P1 by heat exchange with a cooling fluid so as to obtain a liquid under pressure P1, the cooling fluid being vaporized during heat exchange, and 15 - expanding the liquid under pressure P1 to a pressure P2. The cooling fluid is subjected to the following stages - compressing the vaporized cooling fluid, - cooling the compressed cooling fluid, - expanding the cooled cooling fluid, the expanded cooling fluid condenses the gas 20 under pressure P1 by heat exchange, and at least one of the following two stages is carried out: 3 - prior to expansion, subcooling the liquid under pressure P1 by means of a first thermoacoustic cooling device, - prior to expansion, subcooling the cooled cooling fluid by means of a second thermoacoustic cooling device. 5 According to the invention, the gas can be a natural gas at a pressure ranging between 1 MPa and 15 MPa and at a temperature ranging between 20*C and 60'C. The first thermoacoustic cooling device can lower the temperature of the cooled liquid by a value ranging between PC and 20'C. The second thermoacoustic cooling device can lower the temperature of the cooled cooling fluid by a value ranging between 10 1*C and 20 0 C. According to the invention, the thermoacoustic cooling devices can comprise: - a heat-supplied thermal engine generating an acoustic wave, - a resonator in which the thermoacoustic wave is established, - a refrigerator arranged downstream from the resonator, using the energy of the 15 acoustic wave to produce a cold spot at low temperature. According to the invention, the liquid under pressure P1 can be subcooled by several thermoacoustic cooling devices arranged in parallel. The cooled cooling fluid can be subcooled by several thermoacoustic cooling devices arranged in parallel. The thermoacoustic cooling device allows to increase the production capacity of a 20 conventional liquefaction method. Furthermore, the thermoacoustic cooling device allows to change the production capacity without changing the method of operation of the gas turbines supplying the cooling circuits with energy.

4 BRIEF DESCRIPTION OF THE FIGURES Other features and advantages of the invention will be clear from reading the description hereafter, with reference to the accompanying figures wherein : - Figures 1, 2 and 3 diagrammatically show the method according to the invention, 5 - Figure 4 shows a liquefaction method according to the prior art, - Figures 5, 6 and 7 diagrammatically show particular embodiments of the invention, - Figure 8 shows the principle of a thermoacoustic cooling device. DETAILED DESCRIPTION Figure 1 diagrammatically shows a method of liquefying a gas by compression and 10 expansion of a cooling fluid. The gas flowing in through line 1 is under pressure, for example a pressure ranging between 1 MPa and 15 MPa. For example, the gas has been compressed by a compressor, or, in case of a natural gas, the gas is obtained under pressure at the production well outlet. 15 The gas circulating in line I is cooled and liquefied under pressure in heat exchanger 11. The gas flowing from heat exchanger 11 through line 2 is fed through line 3 into expansion device 13, for example a valve and/or an expansion turbine. The gas is expanded to a pressure close to atmospheric pressure. The fluid discharged through line 4 constitutes the liquefied gas, for example the liquefied natural gas. 20 The cooling cycle consists of a cooling fluid circulating in parts 20, 21, 11 and 22. This cycle is simplified and can be changed and completed without departing from the 5 scope of the invention. Compressor 20, which can be driven by a gas turbine, allows to provide the required cooling power. The cooling fluid is compressed in compressor 20, cooled and partly or totally condensed in heat exchanger 21, for example by heat exchange with water or with the 5 ambient air. Then, the cooling fluid is subcooled in exchanger 11 and discharged through line 24. Advantageously, the cooling fluid obtained at the outlet of exchanger 11 is liquid. Then, the cooling fluid is expanded in expansion device 22 to be cooled, then it is sent to heat exchanger 11 to be heated and vaporized, prior to being sent back into compressor 20. 10 For example, the cooling fluid can be a mixture of nitrogen and hydrocarbons such as methane, ethane and propane. According to the invention, a thermoacoustic cooling device 12 cools the fluid obtained at the outlet of exchanger 11 through line 2. The cooling device lowers the temperature of the fluid by a value ranging between P C and 20*C, preferably between 15 1*C and 5*C. The thermoacoustic cooling device is described in a more complete way hereafter, in connection with Figure 8. Figure 2 shows a variant of the method according to the invention. The reference numbers in Figure 2 identical to those of Figure 1 designate the same elements. 20 The method diagrammatically shown in Figure 2 is identical to the method diagrammatically shown in Figure 1, except that, according to the method of Figure 2, 6 the thermoacoustic cooling device is not arranged on line 3. On the other hand, in Figure 2, cooling device 23 is arranged on line 24. In Figure 2, thermoacoustic cooling device 23 cools the fluid obtained at the outlet of exchanger 11 through line 24. Cooling device 23 lowers the temperature of the fluid 5 by a value ranging between I C and 20*C, preferably between 1* C and 5*C. Figure 3 shows a variant of the method according to the invention. The reference numbers in Figure 3 identical to those of Figure 1 designate the same elements. The method diagrammatically shown in Figure 3 is identical to the method diagrammatically shown in Figure 1, except that, according to the method of Figure 3, 10 an additional thermoacoustic cooling device 23 is arranged on line 24. In Figure 3, thermoacoustic cooling device 12 cools the fluid obtained at the outlet of exchanger 11 through line 2, and thermoacoustic cooling device 23 cools the fluid obtained at the outlet of exchanger 11 through line 24. Devices 11 and 23 lower the temperature by a value ranging between 1*C and 20*C, preferably between I *C and 15 5 0 C. Figure 4 shows a natural gas liquefaction method according to the prior art. This method is notably described by document FR-2,778,232. According to the natural gas liquefaction method described in Figure 4, the natural gas flowing in through line 101 is cooled by indirect heat exchange with two cooling 20 mixtures, each cooling mixture circulating in a closed and independent circuit. The natural gas flows in through line 101, for example at a pressure ranging between 1 MPa and 15 MPa, preferably between 4 MPa and 7 MPa, and at a temperature ranging 7 between 20*C and 60*C. The gas circulating in line 101, the first cooling mixture circulating in line 135 and the second cooling mixture circulating in line 123 flow into heat exchanger 111 where they circulate in parallel directions and in a cocurrent flow. The natural gas flows from exchanger 111 through line 102, for example at a 5 temperature ranging between -35*C and -70*C. The second cooling mixture leaves exchanger 111 totally condensed through line 124, for example at a temperature ranging between -35'C and -70*C. In exchanger 111, three fractions of the first cooling mixture in liquid phase are successively discharged. The fractions are expanded through expansion valves 132, 133 10 and 134 to three different pressure levels, then vaporized in exchanger 111 by heat exchange with the natural gas, the second cooling mixture and part of the first cooling mixture. The three vaporized fractions are sent to various stages of compressor 130. The vaporized fractions are compressed in compressor 130, then condensed in condenser 131 by heat exchange with an outside cooling fluid, water or air for example. The first 15 cooling mixture from condenser 131 is sent to exchanger 111 through line 135. The pressure of the first cooling mixture at the outlet of compressor 130 can range between 2 MPa and 4 MPa. The temperature of the first cooling mixture at the outlet of condenser 131 can range between 30*C and 55 0 C. The first cooling mixture can consist of a mixture of hydrocarbons such as a 20 mixture of ethane and propane, but it can also contain methane, butane and/or pentane. The proportions in molar fractions (%) of the constituents of the first cooling mixture can be as follows: Ethane: 30 % to 70 % Propane: 30 % to 70 % 8 Butane: 0% to 10%. The natural gas flowing from exchanger 111 through line 102 can be fractionated, i.e. part of the C 2 . hydrocarbons containing at least two carbon atoms is separated from the natural gas, using a device known to the man skilled in the art. The fractionated 5 natural gas is sent through line 102 into exchanger 112. The C 2 . hydrocarbons collected are sent to fractionating columns comprising a deethanizer. The light fraction collected at the top of the deethanizer can be mixed with the natural gas circulating in line 102. The liquid fraction collected at the bottom of the deethanizer is sent to a depropanizer. The gas circulating in line 102 and the second cooling mixture circulating in line 10 124 flow into exchanger 112 where they circulate in parallel directions and in cocurrent flows. The second cooling mixture flowing from exchanger 112 through line 125 is expanded by expansion device 122. Expansion device 122 can be a turbine, a valve or a combination of a turbine and of a valve. The expanded second cooling mixture from 15 expansion device 122 is sent to exchanger 112 to be vaporized by circulating countercurrent to the natural gas and the second cooling mixture. At the outlet of exchanger 112, the vaporized second cooling mixture is compressed by compressor 120, then cooled in indirect heat exchanger 121 by heat exchange with an outside cooling fluid, water or air for example. The second cooling mixture from exchanger 121 is sent 20 to exchanger 111 through line 123. The pressure of the second cooling mixture at the outlet of compressor 120 can range between 2 MPa and 6 MPa. The temperature of the second cooling mixture at the outlet of exchanger 121 can range between 30'C and 55 0

C.

In the method described in connection with Figure 4, the second cooling mixture is not divided into separate fractions but, in order to optimize the approach in exchanger 112, the second cooling mixture can also be separated into two or three fractions, each fraction being expanded to a different pressure level, then sent to different stages of 5 compressor 120. The second cooling mixture consists for example of a mixture of hydrocarbons and nitrogen such as a mixture of methane, ethane and nitrogen, but it can also contain propane and/or butane. The proportions in molar fractions (%) of the constituents of the second cooling mixture can be as follows: 10 Nitrogen: 0%to 10% Methane: 30 % to 70 % Ethane : 30 % to 70 % Propane: 0 %to 10 %. The natural gas flows from heat exchanger 112 liquefied through line 103, at a 15 pressure identical to the natural gas inlet pressure, except for the pressure drops. For example, the natural gas leaves exchanger 112 at a pressure ranging between I MPa and 15 MPa, preferably between 4 MPa and 7 MPa. This liquefied natural gas under pressure is expanded by expansion device 113 to a pressure close to the atmospheric pressure. The liquid natural gas obtained is discharged through line 104. 20 The method described in Figure 5 is a particular embodiment of the invention. The method shown in Figure 5 is identical to the method described in Figure 4, except that, according to the method of Figure 5, an additional thermoacoustic cooling device is arranged on line 103.

10 In Figure 5, thermoacoustic cooling device 105 cools the fluid obtained at the outlet of exchanger 112 through line 103. Cooling device 105 provides cooling of the fluid by a value ranging between I*C and 20*C, preferably between 1*C and 5*C. The method described in Figure 6 is a particular embodiment of the invention. 5 The method shown in Figure 6 is identical to the method described in Figure 4, except that, according to the method of Figure 6, an additional thermoacoustic cooling device 126 is arranged on line 125. In Figure 6, thermoacoustic cooling device 126 cools the second cooling mixture obtained at the outlet of exchanger 112 through line 125. Cooling device 126 provides 10 cooling of the fluid by a value ranging between 1 0 C and 20*C, preferably between 1*C and 5*C. The method described in Figure 7 is a particular embodiment of the invention. The method shown in Figure 7 is identical to the method described in Figure 4, except that, according to the method of Figure 7, an additional thermoacoustic cooling 15 device 105 is arranged on line 103, and an additional thermoacoustic cooling device 126 is arranged on line 125. In Figure 7, thermoacoustic cooling device 105 cools the fluid obtained at the outlet of exchanger 112 through line 103, thermoacoustic cooling device 126 cools the second cooling mixture obtained at the outlet of exchanger 112 through line 125. Devices 105 20 and 112 operate cooling by a value ranging between 1*C and 20*C, preferably between I*C and 5'C.

11 Operation of the methods described in connection with Figures 4, 5, 6 and 7 is illustrated by the numerical examples hereafter. For the methods of Figures 4 to 7, the natural gas flows in through line 101 at a flow rate of 40 000 Kmol/hour (695 tons/hour) at a temperature of 30*C and at a 5 pressure of 5 MPa. The volume composition of the gas is 92 % methane, 6 % ethane, 1.5 % propane and 0.5 % nitrogen. According to the method diagrammatically shown in Figure 4, the gas obtained in line 103 is liquefied and cooled to a pressure of 4.85 MPa and to a temperature of -163.5*C so as to be totally liquid after expansion to the storage pressure. The two 10 cascade cooling mixture cycles allow this temperature to be obtained. The total compression power required is 194.4 MW (97.2 MW on 120 and 130). For example, for each cooling mixture cycle, the compressors are driven by a 82-MW gas turbine and a 15.2-MW connected engine. In relation to the method according to Figure 4, according to the method shown in 15 Figure 5, a thermoacoustic cooling device 105 is arranged on line 103 before expansion device 113. This device 105 allows to provide a refrigeration power of 2 MW, thus allowing to cool the 695 t/h natural gas from -160.5*C to -163.5*C. Thus, if the same natural gas flow rate as in the example of Figure 4 is maintained, the temperature of the natural gas 20 at the outlet of exchanger 112 can be increased to -160.5*C. Consequently, thermoacoustic device 105 allows to decrease the compression power required by the two cooling mixture cycles to 186.4 MW.

12 Alternatively, if the same compression power as in the example illustrating the method of Figure 4 is maintained, the liquefied natural gas flow obtained in line 104 is increased by 4.3 %. In relation to the method according to Figure 4, according to the method shown in 5 Figure 6, a thermoacoustic cooling device 126 is arranged on line 125 before expansion device 122. This device 126 allows to lower the temperature of the second cooling mixture by a value of 2.4*C by means of a calorific value of 2 MW. Thus, if the same natural gas flow rate as in the example of Figure 4 is maintained, the pressure of the second cooling 10 fluid at the outlet of expansion device 122, i.e. the pressure at the inlet of compressor 120, can be increased by 0.03 MPa. Consequently, device 126 allows to reduce the compression power to 186.7 MW. Alternatively, if the same compression power as in the example illustrating the method of Figure 4 is maintained, the natural gas flow rate obtained in line 104 is 15 increased by 4.2 %. In relation to the method according to Figure 4, according to the method shown in Figure 7, a thermoacoustic cooling device 126 is arranged on line 125 before expansion device 122, and a thermoacoustic cooling device 105 is arranged on line 103 before expansion device 113. 20 Device 126 allows to lower the temperature of the second cooling mixture by 2.4*C, by means of a calorific value of 2 MW. Thus, if the same natural gas flow as in the example of Figure 4 is maintained, the pressure of the second cooling fluid at the outlet 13 of expansion device 122, i.e. the pressure at the inlet of compressor 120, can be increased by 0.03 MPa. Device 105 allows to provide a refrigeration power of 2 MW, thus allowing to cool the 695 t/h natural gas from -160.5*C to -163.5'C. Thus, the temperature of the natural gas at the outlet of exchanger 112 can be increased to 5 -160.5*C. Consequently, thermoacoustic devices 105 and 126 allow to decrease the compression power required by the two cooling mixture cycles to 178.7 MW. Alternatively, if the same compression power as in the example illustrating the method of Figure 4 is maintained, the natural gas flow rate obtained in line 104 is increased by 8.8 %. 61 t/h additional LNG is produced in relation to the method 10 according to Figure 4. The numerical example illustrating the method according to Figure 7 shows the advantage of the invention in relation to the two liquefaction modes which are the conventional liquefaction by heat exchange with two cooling fluids (method described in connection with Figure 4) and the thermoacoustic liquefaction. In fact, a 4-MW 15 thermoacoustic liquefaction, for example two 2-MW modules, allows to produce approximately 17 t/h LNG. On the other hand, 4-MW thermoacoustic liquefaction (two 2-MW modules) combined with a conventional liquefaction unit allows an additional 61 t/h production. Conventional natural gas liquefaction units have good efficiencies, due to the 20 optimizations achieved for many years. However, there are not many different sizes for the gas turbines intended to drive the compressors, and the LNG production capacity of the liquefaction unit is often set according to these gas turbines. Furthermore, there are not many axial compressors and the size of the centrifugal compressors is limited by the 14 allowable speed at the blade end. Because of these limitations, the liquefaction units have very low-flexibility production capacities, i.e. it is difficult to vary the flow of natural gas produced in the course of time. Besides, the temperature variations between summer and winder can generate an uncontrollable production capacity variation, which 5 can pose seasonal problems in supplying customers with LNG. The method according to the invention allows to easily adjust the production capacity of a natural gas liquefaction unit. In fact, the thermoacoustic cooling devices being installed downstream from a valve, it is easy to have several of them, arranged in parallel for example, with valves also arranged in parallel. For example, in connection 10 with Figure 1, the fluid flowing in through line 2 is divided into several streams. Each one of these streams is cooled by a thermoacoustic cooling device. Each one of the cooled streams is expanded by a valve. Finally, the expanded streams are combined to form the fluid circulating in line 4, i.e. the liquefied natural gas at atmospheric pressure. Currently, thermoacoustic cooling devices have relatively low efficiencies. 15 However, the efficiency of these equipments, not yet used in the field of natural gas liquefaction, can possibly be considerably improved as they are developed for the present invention. A low efficiency, therefore a high gas consumption in a small part of the liquefaction unit, by the thermoacoustic cooling device, is acceptable in a production 20 site where the gas price is not very high. This additional gas consumption is all the more acceptable as the improvement in the total yield of the method according to the invention in relation to a conventional liquefaction unit compensates for the low efficiency of the thermoacoustic device.

15 Moreover, the financial investment corresponding to a thermoacoustic cooling device is not very high in relation to the considerable price of a natural gas liquefaction unit. Furthermore, a thermoacoustic cooling device is very compact, and thus hardly increases the significant space requirements linked with the installation of a 5 conventional liquefaction unit; for example, a 2-MW thermoacoustic device can fit in a 5-m diameter circle. The working principle of a thermoacoustic cooling device as used in the invention is diagrammatically shown in Figure 8. The principle of a thermoacoustic device consists in producing cold from "raw heat" using the properties of phenomena referred 10 to as "thermoacoustic", which are phenomena of heat exchange and energy conversion in contact zones, also referred to as thermal boundary layers, between a solid and a liquid. In general, the thermoacoustic device comprises three parts. Thermal engine 43 is used to generate an acoustic wave. Thermal engine 43 is supplied with primary energy in form of heat admitted through stream 51. A significant 15 thermal gradient is created between a zone heated by stream 51 and a zone cooled by a fluid at ambient temperature flowing in through line 63 and discharged through line 64. The cooling fluid can be water. This thermal gradient allows to generate, by means of a thermoacoustic phenomenon, an acoustic wave which is transmitted to refrigerator 41 by means of a resonator 42. 20 Resonator 42 consists for example of one or more closed tubes containing, for example, helium at medium pressure. The acoustic wave is established within the tube(s).

16 A refrigerator 41 is arranged downstream from resonator 42. Refrigerator 41 uses the energy of the acoustic wave to produce a cold spot at low temperature according to a thermoacoustic conversion process. The fluid to be cooled to a low temperature is fed into refrigerator 41 through line 71, then discharged at a lower temperature through line 5 72. Refrigerator 41 has to release heat by means of a fluid at ambient temperature flowing in through line 61 and discharged through line 62. This fluid can be water. This fluid can also advantageously be a fluid obtained at a temperature close to or lower than 0*C upon purification of the natural gas, or of the cooling mixture of the first cycle of the liquefaction process. 10 The thermoacoustic cooling devices used in the present invention can be developed from the prototypes described in the documents mentioned hereunder: - "Thermoacoustic Natural Gas Liquefier" by R.J. Hanold and G.W. Swift, Journal US DOE et al., Natur. Gas RDD Contract. Rev. Mtg. (Baton Rouge, La, 4/4-6/95) Proc. 2 506-511 (April 1995) Petroleum Abstracts ABSTR. No. 637,273 V36 N.48 (11/30/96) 15 ISSN: 0031-6423, - "Development of a thermoacoustic natural gas liquefaction" by G.W. Swift (Second topical conference on Natural Gas Utilization - AIChE Spring Meeting March 10-14, 2002), - "A thermoacoustically driven pulse tube refrigerator capable of working below 120 K" 20 by T. Jin, G.B. Chen, Y. Shen, Journal of Cryogenics V41, No.8 2001, p. 595-601, - US-4,953,366.

- 16A It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any 5 other country. In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary 10 implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 15 Numerous variations and modifications will suggest themselves to persons skilled in the relevant art, in addition to those already described, without departing from the basic inventive concepts. All such variations 20 and modifications are to be considered within the scope of the present invention, the nature of which is to be determined from the above description. 17598981 (GHMatters) 10109/09