STATION FOR REDUCING GAS PRESSURE AND LIQUEFYING GAS

Abstract

The invention relates to a station comprising an expansion turbine 12; means for recovering mechanical work (G) produced during the gas pressure reduction in the expansion turbine; a cooling system (6) comprising compression means (C1, C2, C3), condensation means (14) for liquefying gas (G11) using the cold provided by the cooling system, means for recovering heat produced by the compression means (C1, C2, C3) of the cooling system and means (10) for heating the gas upstream of the expansion turbine that are associated with the heat-recovery means.

Description

The present invention relates to a station for reducing the pressure of a gas and for liquefying the gas, particularly natural gas.

Thus, the field of the present invention is that of treatment of gases, particularly of natural gases, for the production of liquefied natural gas.

Liquefied natural gas is used in different applications, It is mainly used as fuel for vehicles, particularly transport trucks. The fuel oil generally used for such vehicles can indeed be replaced by pressurized gas or liquefied natural gas, In comparison with the use of bottles of pressurized gas, the use of liquefied gas offers an advantage in terms of volume and weight inasmuch as, on one hand, natural gas liquefied by cooling occupies much less volume than the same quantity of gaseous natural gas, and on the other hand, the thermal insulation of the cryogenic tanks is much less heavy than the jacket of the gas bottles. The vehicles therefore have much more autonomy. Liquefied natural gas is moreover a clean energy source, which limits the discharge of fine particles such as soot, etc.

Liquefied natural gas can also be used for supplying small gas power plants or for supplying small networks in villages.

Gas pipelines, or pipelines, are pipelines intended for transporting gaseous materials under pressure. The majority of gas pipelines convey natural gas between extraction zones and consumption or exportation zones. From treatment sites of the gas fields or storage sites, the gas is transported at high pressure (from 16 to more than 100 bars) to delivery sites where it must be brought to a much lower pressure so that it can be used.

For this purpose, the gas passes through pressure reduction stations in which the pressure of the gas is reduced by expansion through a valve or a turbine. The pressure reduction thus achieved produces energy which, in the case of a valve, is lost.

There are known gas expansion systems that use the natural gas entering the pressure reduction stations as refrigerant in a system that can be described as open loop (Linde, Solvay or Claude cycles). In these systems, one uses the fact that the natural gas is present under high pressure. The natural gas is expanded in a valve, and during this expansion a small portion of the gas is liquefied. The liquid obtained is collected, and the cold low pressure natural gas coming out of the valve is conveyed to the low pressure pipe of the reduction station. These systems have the advantage of being relatively simple, but since the temperature obtained coming out of the valve depends on the composition of the gas and since the composition of natural gas is variable, the gases liquefied with these systems are mainly heavy gases such as butane or propane but not methane. This gas liquefaction method is also known as flashing.

All of the gas entering the pressure reduction station and passing through the valve or the turbine is cooled during the pressure drop that occurs. The gas still contains water and carbon dioxide at contents on the order of around one hundred ppm or one percent. A condensation phenomenon can then occur during this expansion step, which is capable of causing the formation of ice (hydrates) that can block the pipes. It is therefore necessary to treat the gas flow in order to prevent the water and carbon dioxide contained in the natural gas from being transformed into ice in the pipes and thus causing problems in conveying the natural gas during its treatment in the pressure reduction stations.

The present invention aims in particular to provide means making it possible, at the site of a pressure reduction station, to liquefy gas, particularly natural gas, by controlling the composition of the liquefied gas obtained. Advantageously, a device according to the invention will make it possible. to recover energy of expansion resulting from the difference in pressure of the gas between the inlet and the outlet of the pressure reduction station in order to produce a liquefied natural gas fraction while avoiding the formation of ice inside the pipes of these stations. The device will also preferably be easy to use and have a simple design.

For this purpose, the present invention proposes a station for reducing the pressure of a gas and for liquefying gas, particularly natural gas, comprising:

• • an expansion turbine,
  • means for recovering mechanical work produced during the reduction of the pressure of the gas,
  • a refrigeration system comprising compression means, and
  • condensation means for liquefying gas.

According to the invention, this station moreover comprises means for recovering heat produced by the compression means of the refrigeration system, which are associated with means for heating the gas upstream of the expansion turbine.

Such a station thus provides for integrating the heating of the natural gas before its expansion and the cooling of the refrigerant while saving a significant amount of energy and/or gas for the manufacture of the liquefied (natural) gas.

A flow of (natural) gas in gaseous form is always maintained between a high pressure pipe and a low pressure pipe which are associated with a pressure reduction station. Based on a volume of 100 m³ of natural gas, 5 to 15 m³, for example, are transformed into liquefied natural gas. Work can here be recovered during the expansion between the two pressure levels in order to be used later for transformation of a small portion (5 to 15%) of the (natural) gas into liquefied (natural) gas.

The heating of the gas occurs, for example, at the inlet of the pressure reduction station (that is to say upstream of the expansion turbine) by recovery of the heat emitted by the compression means used for liquefying the gas. The gas going from the high pressure pipe to the low pressure pipe is thus heated before entering the pressure reduction station so that it is present at the outlet of said station with a temperature higher than the solidification point of the water.

In order to optimize the station described here and to recover a maximum amount of energy, it is provided that the gas under high pressure is first of all run into the expansion turbine and that subsequently, downstream of this turbine, a portion of the expanded gas is removed in order to be sent to the condensation means. it is thus provided that these condensation means are supplied by a branch pipeline downstream of the expansion turbine.

According to a first embodiment, the station comprises a closed loop between the condensation means, the compression means and the means for heating the natural gas. This closed loop makes it possible to combine a refrigeration system (compressor and condenser) for the liquefaction of the gas with a heat exchanger, bringing about the thermal integration between the reduction of the pressure of the gas and the production of liquefied gas.

According to a second embodiment, the station comprises a first closed loop between the compression means, the condensation means and at least one intermediate heat exchanger, as well as a second closed loop, possibly using a different heat transfer fluid from a heat transfer fluid used in the first loop, between at least one intermediate heat exchanger and the means for heating the gas.

Proposed here, with these two embodiments, is a station with an intermediate system that can be likened to a closed loop, possibly double, making it possible to cool a fraction of the gas until its liquefaction. The advantage of an independent closed loop system is that it allows one to reach significantly lower temperatures inasmuch as it is not connected with the lowering of pressure achieved within the pressure reduction station. Thanks to this system, the composition of the liquefied gas hardly varies with respect to the entering gas, given that the change in state is obtained by direct cooling inside of a heat exchanger reserved for this operation, instead of the conventional flashing system.

In a particular embodiment of a pressure reduction and liquefaction station, the means for recovering mechanical work produced during the reduction of the pressure of the gas are associated with means for converting mechanical work into electrical energy. In this embodiment, the means for recovering mechanical work produced during the reduction of the pressure of the gas can be mechanically coupled to an electric generator, and the compression means are advantageously driven by a motor supplied with electrical energy by the electric generator.

In another embodiment of a pressure reduction and. liquefaction station, the means for recovering mechanical work produced during the reduction of the pressure of the gas are mechanically associated with the compression means. An auxiliary motor can optionally be provided for driving the compression means.

Within such a station, one therefore has the integration of a refrigeration loop for liquefying gas and of preheating of the inlet of the expansion turbine.

The liquefied natural gas can be produced within a station according to the invention from a refrigeration unit involving a refrigerant system using interchangeably nitrogen and/or a mixture of hydrocarbons.

A refrigeration system used in a station according to the invention can for example comprise a heat exchanger and/or a condenser of the aluminum PFHE type.

In a particular embodiment, the refrigeration system comprises compressors and/or radial flow expanders.

In another embodiment, the station according to the invention comprises means for treatment of the water and carbon dioxide of the low pressure natural gas by adsorption and/or absorption arranged upstream of the means for condensation of the gas.

Details and advantages of the present invention will appear more clearly from the description that follows given in reference to the appended diagrammatic drawing in which:

FIG. 1 is a very diagrammatic overview illustrating a station according to the present invention,

FIG. 2 is a more detailed diagrammatic view showing a first embodiment of the present invention,

FIG. 3 is a view similar to the view of FIG. 2, illustrating a second embodiment of the invention,

FIG. 4 is a view similar to that of FIGS. 2 and 3, in the case of a third embodiment of the present invention, and

FIG. 5 is a view similar to that of FIGS. 2 to 4, in the case of a fourth embodiment of the present invention.

FIG. 1 diagrammatically represents a gas pipeline 2 conveying a gas, for example, natural gas composed of mostly methane, under high pressure, for example, on the order of 60 to 100 bars (generally in the present application, the examples and the numerical values are illustrative and non-limiting). A gas pressure reduction station, called PLD (English acronym for Pressure Let Down, or in French “baisse de pression” [lowering of pressure]) in FIG. 1, makes it possible to supply a pipe 4 intended for supplying a domestic network or the like with gas (natural gas, to re-use the preceding example) under low pressure, generally on the order of a few bars.

A liquefied gas production unit 6 is associated with the pressure reduction station PLD. It is supplied with gas from the gas pipeline 2, downstream of the pressure reduction station PLD, goes through a treatment unit 8 that performs a treatment of the gas before it enters the production unit 6 in order rid the gas of the impurities that are generally found in “raw” gas. Leaving the production unit 6, a liquefied natural gas LNG is obtained which, for example, is stored in a storage unit (not illustrated in FIG. 1).

When gas is expanded in the pressure reduction station PLD, the gas gives up mechanical work WM. Proposed here is the recovery of all or part of this work in some form, mechanical or electrical, for example, in order to supply the production unit 6 which requires energy in order to switch the gas from its gaseous state to a liquid state. Inasmuch as the energy recovered is not sufficient for the production of liquefied gas, it is possible to supply the production unit with a supplementary source of energy, for example, electrical energy diagrammatically represented by “WE” in FIG. 1. Finally, in the production unit 6, one generally has a compressor (not represented in FIG. 1) or another device that releases heat, represented simply by Q in FIG. 1. Proposed in an original manner is the recovery of this quantity of heat Q in order to heat the gas entering the pressure reduction station PLD. Indeed, in the course of expansion, the expanded gas is cooled. It risks falling below the temperature of solidification of the water and thus leading to formation of ice which can lead to partial or complete obstruction of the corresponding pipeline. By heating the gas before the expansion, it is thus possible to limit the risks of icing and obstruction.

FIG. 2 shows in more detail a first embodiment of the invention implementing the overall scheme of FIG. 1.

In FIG. 2, as well as in the following figures, the references of FIG. 1 are used again to designate similar elements.

One thus finds again in FIG. 2 a gas pipeline 2 which supplies a pressure reduction station PLD in order to provide gas under lower pressure in a pipe 4. Furthermore, a production unit 6 provides liquefied gas LNG.

In the pressure reduction station PLD, gas coming from the gas pipeline 2 passes through pipes G2 and G3. It is heated in each of these pipes by a preheating device 10. Leaving these preheating devices, pipes G4 and G5 are collected in a pipe G6 which supplies an expansion turbine 12. Leaving the turbine, the gas is expanded and can rejoin the pipe 4 directly through a pipe G7.

The production unit 6 essentially comprises a condenser 14. The gas supplying the production unit 6 is supplied from a branch G9 of the pipe G7 before coming to a valve 16 where an additional pressure reduction is achieved. The gas is conveyed through a pipe G10 to the treatment unit 8 which performs a purification of the gas, for example, by absorption or preferably by adsorption. The purified gas is conveyed through G11 to the desuperheater 18 before being introduced through G12 into the condenser 14. Leaving the latter, liquefied gas is obtained, which passes through a pipe L1 to a control valve 20 and then through L2 in order to arrive at a storage device for liquefied natural gas LNG.

An interaction between the expansion turbine 12 of the pressure reduction station PLD and the production unit 6 is achieved here. In this embodiment of FIG. 2, the energy recovered during the expansion in the station PLD is used in the form of electrical energy in the production unit 6, and the heat produced in the production unit 6 is used for heating the gas entering the station PLD, that is to say upstream of the expansion turbine 12.

It is noted in FIG. 2 first that the turbine 12 is coupled to a generator G. Thus, mechanical energy is recovered at the turbine 12 in order to be converted into electrical energy. The electricity thus recovered then supplies a motor M which drives three compressors C1, C2 and C3 each forming a stage of a compression unit. In this way, an electrical coupling is produced between the pressure reduction station and the production unit.

In order to optimize the quantity of mechanical energy recovered at the turbine 12, both the gas intended for supplying the low pressure pipe 4 and the gas intended for supplying the production unit 6, that is to say the gas to be liquefied, are run into this turbine 12.

The thermal integration is achieved by a closed loop circuit which is described hereafter. For this description, we propose then to follow the refrigerant fluid moving in this circuit. As a non-limiting example, the fluid used can be nitrogen or else a mixture of hydrocarbons.

The refrigerant fluid arrives in the compressor C1 through a pipe R1 and leaves it through a pipe R2. It then arrives in a first preheating device 10 in order to heat gas that comes from the gas pipeline 2 and that is intended for supplying the turbine 12 of the pressure reduction station PLD. The fluid is then led through a pipe R3 to a cooler 22 in order to achieve a control of the temperature of the refrigerant fluid before being sent to the compression unit through a pipe R4. The fluid is then compressed by the second compressor C2 and is then led through R5 to the second preheating device 10 before being conveyed through R6 to a second cooler 22 and reaching, through R7, a third compression stage of the compression unit. A third cooler 22, connected to the third compressor C3 through a pipe R8, makes it possible to control the temperature of the fluid leaving the compression unit.

A pipe R9 brings the refrigerant fluid to a counterflow heat exchanger 24, and is then led through R10 to an expander 26. The latter is mechanically connected to the motor M and to the compression unit. Leaving the expander 26, the fluid is then led (R11) to the condenser 14 of the production unit 6 where it absorbs calories from the natural gas portion that one wishes to liquefy in order to obtain liquefied natural gas (LNG). Leaving the condenser 14, the fluid is conveyed (R12) to the desuperheater 18 before reaching, through R13, the counterflow heat exchanger 24 which is connected downstream to the first compressor C1 of the compression unit.

As emerges from this description, the refrigerant fluid is used to achieve a thermal integration between the production unit and the pressure reduction station by recovering in particular the calories released during the compression of the fluid in order to use them for heating the natural gas entering the pressure reduction station PLD.

Accessory elements of the refrigerant circuit are not described in detail here. One thus finds, for example, a tank 28 which is used in a conventional manner as expansion vessel for the refrigerant fluid.

FIG. 3 illustrates an embodiment variant which re-uses certain references of the preceding figures in order to designate similar elements. Compared to the embodiment of FIG. 2, another form of thermal integration is achieved. It is proposed to have a closed loop of pressurized water (or of another heat transfer fluid such as, for example, a thermal oil) in order to recover the heat of compression and to transfer it upstream of the expansion turbine. An air cooler, for example, can be placed on this line in order to adjust the cooling capacity to the demand of the compression loop. A volumetric pump is used in order to allow the circulation of the heat transfer fluid (pressurized water), and an expansion vessel can in a conventional manner be integrated in this circuit.

One thus recognizes in FIG. 3 a refrigerant circuit between the compression unit and its three compressors C1, C2 and C3 and the production unit 6 with its condenser 14. This circuit is simplified. It passes successively through the three stages of the compression unit, and after each stage, passes through a preheating device 10. The refrigerant circuit then passes through the counterflow heat exchanger 24 before going into the expander 26 and then into the condenser 14, again passing through the counterflow heat exchanger 24 and coming back to the first compression stage and its compressor C1.

The main difference from the first embodiment of FIG. 2 is that the preheating devices 10 do not directly transfer the calories extracted from the compression stages to the natural gas but rather they transfer them to another heat transfer fluid such as, for example, pressurized water. A second refrigerant circuit is thus produced, which passes in parallel through the three preheating devices 10 in order to supply a preheating device 110 which transfers the calories coming from the compression stages to the natural gas entering the station PLD. These preheating devices 10 thus form intermediate heat exchangers. Between the preheating devices 10 and the preheating device 110, one notes the presence of a volumetric pump 142 making it possible to circulate the heat transfer fluid in the corresponding circuit, as well as a cooler 122 for controlling the temperature of the heat transfer fluid in this circuit. In a conventional manner for the person skilled in the art, an expansion vessel 144 is advantageously integrated in this refrigerant circuit.

As for FIG. 4, it illustrates a simplified version of the first embodiment illustrated in FIG. 2. Here also, as is generally the case in the present application, the references already used are re-used for designating similar elements in order to simplify reading comprehension.

In this simplified embodiment, one notes that the compression unit only has a single stage with a single compressor C. The natural gas is then heated within a single preheating device 10 which allows a direct exchange of the calories coming from the compressor with the natural gas entering the station PLD, upstream of the expansion turbine 12.

In this embodiment, the refrigerant circuit uses, for example, a mixture of hydrocarbons and nitrogen as heat transfer fluid. The latter is compressed by the compressor C driven by the electric motor M (electrically coupled to the generator G of the turbine 12 of the station PLD. The fluid then cooled in contact with the natural gas in the preheating device 10 at the entrance of the turbine 12 (it is appropriate to note that one could here also provide another refrigerant circuit between the preheating device 10 and the natural gas as in the preceding figure).

A cooler 22 or (air cooler) can be introduced into the circuit in order to adjust the cooling capacity to the demand of the compression loop. The heat transfer fluid is then sent through a heat exchanger 214, for example, of type PHFE (English acronym for Plate Fin Heat Exchanger or in French “échangeur de chaleur à plaques et ailettes” [plate and fin heat exchanger]), where it is cooled and condensed during a first pass. It is then expanded through a valve 246 where, by Joule-Thompson effect, it partially vaporizes, again causing a lowering of its temperature. It again passes (2^nd pass) through the heat exchanger 214 and vaporizes and is heated in contact with the natural gas to be liquefied and with the refrigerant mixture to be condensed. After this second pass, leaving the heat exchanger 214, the heat transfer fluid (mixture of hydrocarbons and nitrogen, for example) returns to the compressor C.

In the embodiment of FIG. 5, compared to the embodiments of the preceding figures, between the pressure reduction station and the production unit, a mechanical integration (FIG. 5) is achieved instead of an electrical integration (FIGS. 2 to 4).

Indeed, whereas in the embodiment of FIG. 2, the turbine 12 drives a generator G which produces electricity which is consumed in a motor M, it is proposed in FIG. 5 to mechanically connect the turbine 12 with the compressors C1, C2 and C3 of the compression unit of the production unit 6.

It seems unnecessary to describe here the different elements of the pressure reduction station that are similar to those represented in FIG. 2. Likewise, one finds again a similar refrigerant circuit for producing both the liquefied gas production unit and the thermal integration of this production unit with the pressure reduction station.

Also represented in this FIG. 5 is a motor M which is used here as additional energy source (corresponds to WE in FIG. 1) in order to adjust the power necessary for the liquefied gas production unit with the power delivered at the site of the pressure reduction station.

As a purely illustrative example, it is possible to provide, for example, in the various embodiments described, that the quantity (weight) of gas passing through the liquefied gas production unit 6 is on the order of 5 to 20% of the quantity (weight) of gas passing through the pressure reduction station PLD, the rest of the gas (80 to 95%) supplying the pipe 4.

The systems described above allow complete control of the production of liquefied natural gas. The composition of this gas can be controlled. It does not depend on the difference in pressure within the pressure reduction station.

Furthermore, the preheating of the gas entering the pressure reduction station makes it possible to prevent problems of icing and obstruction of the pipeline.

Energy recovery takes place at the pressure reduction station, and more precisely at its expansion turbine. This recovery is optimized by having the whole gas flow pass through this turbine, that is to say the gas which is intended for being expanded in gaseous form as well as the gas intended for being liquefied.

The present invention is not limited to the preferred embodiments described above as non-limiting examples. It also relates to the embodiment variants accessible to the person skilled in the art within the scope of the claims hereafter.

Claims

1-11. (canceled)

12. A station for reducing the pressure (PLD) of a gas and for liquefying the gas, comprising: an expansion turbine (12);means for recovering mechanical work (WM) produced during reduction of pressure of the gas;a refrigeration system comprising compression means (C1, C2, C3);condensation means (14) for liquefying the gas; andmeans for recovering heat (Q) produced by the compression means (C1, C2, C3; C) of the refrigeration system; the compression means associated with means (10; 110) for heating the gas upstream of the expansion turbine (12).

13. The station according to claim 1, further comprising a branch pipeline (G9) downstream of the expansion turbine (12) for supplying the condensation means (14).

14. The station according to claim 1, wherein said station comprises a closed loop between the condensation means (14), the compression means (C1, C2, C3; C) and the heating means (10) for the gas.

15. The station according to claim 1, wherein said station comprises a first closed loop between the compression means (C1, C2, C3), the condensation means (14) and at least one intermediate heat exchanger (10); and a second closed loop, between the at least one intermediate heat exchanger (10) and the heating means (110) for the gas.

16. The station according to claim 15, wherein the first closed loop comprises a first heat transfer fluid, and the second closed loop comprises a second heat transfer fluid different from said first heat transfer fluid.

17. The station according to claim 1, further comprising means for converting (G) mechanical work into electrical energy, said converting means associated with the means for recovering mechanical work (WM) produced during the reduction of the pressure of the gas.

18. The station according to claim 17, wherein the converting means (G) comprises an electrical generator mechanically coupled to the means for recovering mechanical work (WM), and further comprising a motor (M) supplied with electrical energy by the electric generator for driving the compression means (C1, C2, C3).

19. The station according to claim 1, wherein the means for recovering mechanical work (WM) produced during the reduction of the pressure of the gas is mechanically connected to the compression means (C1, C2, C3; C).

20. The station according to claim 19, further comprising an auxiliary motor (M) for driving the compression means (C1, C2, C3).

21. The station according to claim 1, wherein the refrigeration system comprises a refrigerant selected from the group consisting of nitrogen, a mixture of hydrocarbons, and nitrogen and a mixture of hydrocarbons.

22. The station according to claim 1, further comprising compressors and/or radial flow expanders.

23. The station according to claim 1, further comprising means for treatment (8, 36) of the gas by at least one of adsorption, adsorption arranged upstream of the condensation means (14), and adsorption and absorption arranged upstream of the condensation means.

24. The station according to claim 1, wherein the gas comprises natural gas.