RU2680285C2 - Station for reducing gas pressure and liquefying gas - Google Patents

Abstract

FIELD: oil and gas industry.SUBSTANCE: invention relates to the treatment of natural gases. In the method, the station for reducing gas pressure and for liquefying natural gas contains expansion turbine (12), a means for recovering mechanical work produced in the process of gas pressure reduction, a cooling system containing compression means (C1, C2, C3), condensation means (14) for liquefying gas, equipped with branch pipe (09) downstream of expansion turbine (12), device for heat recovery (Q) produced by compression means (C1, C2, C3; C) of cooling systems that are connected to devices (10; 40; 110) for heating the gas upstream of expansion turbine (12). Said cooling system contains compressors and/or expanders with radial gas flow.EFFECT: utilization of the energy of gas expansion and the prevention of ice formation inside the pipes of the stations.9 cl, 5 dwg

Description

FIELD OF TECHNOLOGY

The present invention relates to a station for reducing gas pressure and for liquefying gas, in particular natural gas. Thus, the technical field to which the present invention relates is the treatment of gases, in particular natural gases, for the production of liquefied natural gas.

BACKGROUND

Liquefied natural gas is used in various fields. It is mainly used as fuel for vehicles, in particular trucks. The diesel fuel commonly used for such vehicles can indeed be replaced by compressed gas or liquefied natural gas. Compared with the use of compressed gas cylinders, the use of liquefied gas has an advantage in terms of volume and weight, since, on the one hand, natural gas liquefied by cooling takes up a much smaller volume than the same amount of gaseous natural gas, and on the other on the other hand, the thermal insulation of cryogenic reservoirs is much lighter than the shell of gas cylinders. Consequently, vehicles gain much more autonomy. In addition, liquefied natural gas is a clean source of energy that limits emissions of fine particles such as soot and so on.

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

Gas pipelines are pipelines designed to transport gaseous materials under pressure. Most pipelines transfer natural gas between areas of its production and consumption or export. From gas field purification plants, gas is transported under high pressure (from 16 bar to more than 100 bar) to gas supply points, where it must be brought to much lower pressure for subsequent use.

To this end, gas passes through gas pressure reducing stations in which the pressure is reduced by expansion by means of a pressure reducing valve or expander. The reduction in pressure thus achieved is accompanied by the release of energy, which is lost in the case of a reducing valve.

Known gas expansion systems using natural gas as a refrigerant entering the stations to reduce pressure, which can be described as an open cycle (Linde, Solve or Claude cycles). These systems use the fact that natural gas is under high pressure. Natural gas expands in a reduction valve and during this expansion a small portion of the gas liquefies. The resulting liquid is collected, and cold low-pressure natural gas exiting the pressure-reducing valve is transferred to the low-pressure pipe of the station to lower the pressure. Such systems have the advantage of comparative simplicity, but since the temperature obtained at the outlet of the reducing valve depends on the composition of the gas and natural gas has a variable composition, the gases liquefied by these systems are mainly heavy gases such as propane and butane but not methane. This gas liquefaction method is also known as flushing.

All gas entering the pressure reducing station and passing through a pressure reducing valve or expander is cooled during a pressure drop. In this case, the gas continues to contain water and carbon dioxide in an amount of about one hundred mg / m ³ or one percent. At the expansion stage, a condensation phenomenon may occur that can cause the formation of ice (crystalline hydrates), which can clog the pipes. Therefore, it is necessary to purify the gas stream from water and carbon dioxide contained in natural gas in order to prevent them from turning into ice in pipes, which causes problems in the transportation of natural gas during its processing at stations to reduce gas pressure.

SUMMARY OF THE INVENTION

The aim of the present invention is, in particular, to provide devices for liquefying gas, in particular natural gas, at the station site to reduce gas pressure by controlling the composition of the resulting liquefied gas. Preferably, the station according to the invention allows to utilize the gas expansion energy resulting from the difference in gas pressure between the inlet and outlet of the station to reduce gas pressure, in order to produce a fraction of the liquefied natural gas, avoiding the formation of ice inside the pipes of these stations. Even more preferably, the station is easy to use and has a simple structure.

To this end, the present invention provides a station for reducing pressure and liquefying a gas, in particular natural gas, comprising:

- turbo expander,

- a device for the disposal of mechanical work done while reducing gas pressure,

a cooling system including compression devices, and

- a device for condensation of liquefied gas.

In addition, according to the invention, this station contains a device for heat recovery, which is produced by devices for compressing a cooling system, which is connected with a device for heating a gas stream before it enters a turbo-expander.

Therefore, such a station provides the integration of heating natural gas before its expansion and cooling the refrigerant with the saving of a significant amount of energy and / or gas for the production of liquefied (natural) gas.

The flow of (natural) gas in gaseous form is always contained between the high pressure pipe and the low pressure pipe, which are connected to the station to reduce pressure. Based on the calculation of the volume of natural gas, 100 m ³ entering the station, for example, from 5 m ³ to 15 m ³ , are converted into liquefied natural gas. In this case, energy can be extracted during the expansion of the gas between two pressure levels for the subsequent use to convert a small part (5% to 15%) of (natural) gas into liquefied (natural) gas.

The gas is heated, for example, at the entrance to the station to reduce the pressure (in other words, in the gas stream before entering the turbo expander) with heat, which is released by compression devices used to liquefy the gas. The gas flowing from the high pressure pipe to the low pressure pipe is heated before entering the station to reduce the pressure so that it is at the outlet of the station with a temperature higher than the pour point of water.

To optimize the station described in this document and extract the maximum amount of energy, it is provided that first the high-pressure gas is sent to the turbo-expander, and then downstream of the turbo-expander, and part of the expanded gas is removed to be directed to the condensation devices. It is contemplated that these condensation devices are supplied with gas by means of a branch pipe located downstream of the turboexpander.

According to a first embodiment of the invention, the station comprises a closed loop between a condensation device, compression devices and devices for heating natural gas. This closed loop allows you to combine a cooling system (compressor and cooler) for liquefying gas with a heat exchanger, leading to thermal integration between lowering the gas pressure and generating liquefied gas.

According to a second embodiment of the invention, the station comprises a first closed circuit between compression devices, a condensation device and at least one intermediate heat exchanger, as well as a second closed circuit, possibly using a coolant different from that used in the first closed circuit, between at least one intermediate heat exchanger and a device for heating gas.

The invention proposed in this document, including these two options for its implementation, is a station with an intermediate system, which can be equated to a closed loop, possibly double, making it possible to cool a fraction of the gas before liquefaction. The advantage of an independent closed loop system is that it allows you to achieve significantly lower temperatures, since it is not associated with a decrease in pressure in the station to reduce gas pressure. Thanks to this system, the composition of the liquefied gas with respect to the incoming gas is almost unchanged due to the fact that the change in state is achieved by direct cooling inside the heat exchanger, designed to perform this operation instead of the traditional "flashing" system.

In the particular case of an embodiment of the invention, a device for utilizing mechanical work that has been done while reducing gas pressure is associated with a device for converting mechanical work into electrical energy. In this embodiment, a device for disposing of mechanical work done while lowering the gas pressure can be mechanically connected to an electric generator, and the compression devices are preferably driven by an electric motor powered by an electric generator.

In another embodiment of the invention, stations for reducing pressure and liquefying gas, a device for utilizing the mechanical work done while lowering the gas pressure is mechanically connected to compression devices. An auxiliary engine may be additionally provided to drive the gas compression devices.

Therefore, within such a station there is an integration of the cooling circuit of the liquefied gas and the pre-heating of the gas at the inlet to the turbo expander.

According to the invention, liquefied natural gas can be produced at the station by means of a cooling unit containing a refrigerant system using interchangeable nitrogen and / or a mixture of hydrocarbons.

According to the invention, the cooling system used in the station may, for example, comprise a heat exchanger and / or an evaporator such as an aluminum plate-fin heat exchanger.

In the particular case of an embodiment of the invention, the cooling system comprises compressors and / or expanders with a radial gas flow.

According to another embodiment of the invention, the station comprises a device for purifying low-pressure natural gas from water and carbon dioxide by adsorption and / or absorption. The specified device is installed before entering the device for condensation of gas.

BRIEF DESCRIPTION OF GRAPHIC MATERIALS

The details and advantages of the present invention will become clearer from the following description of the schematic graphics in the footnotes, in which:

In FIG. 1 illustrates the most general schematic view of a station according to the present invention,

In FIG. 2 illustrates a more detailed schematic view showing the first embodiment of the invention,

In FIG. 3 illustrates a view of a second embodiment of the invention, similar to the view illustrated in FIG. 2

In FIG. 4 illustrates a view of a third embodiment of the present invention, similar to the views illustrated in FIG. 2 and 3, and

In FIG. 5 illustrates a view of a fourth embodiment of the present invention similar to the views illustrated in FIG. 2-4.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In FIG. 1 schematically illustrates a gas pipeline 2 transporting gas, for example, natural gas, consisting mainly of methane under high pressure, for example, of the order of 60 to 100 bar (throughout the present application examples and numerical values are illustrative and not restrictive character). A gas pressure reducing station called PLD (acronym for "Pressure Let Down" or French "baisse de pression"), illustrated in FIG. 1 makes it possible to supply a pipe 4 intended to supply a public network, or the like, with gas (natural gas to use the previous example again), usually under a pressure of several bar.

Node for the production of liquefied gas 6 is connected with the station to reduce the pressure PLD gas. The station is supplied from the gas pipeline 2 in the direction of the gas flow in the station to reduce the pressure PLD of the gas that passes through the purification unit 8. It purifies the gas before it enters the unit for the production of liquefied gas 6 in order to free gas from extraneous impurities that are usually found in the "raw" gas. At the outlet of the liquefied gas production unit 6, liquefied natural gas LNG is obtained, which, for example, is stored in a storage facility (not illustrated in FIG. 1).

When gas expands in the station to reduce the pressure of the PLD gas, it performs the mechanical work of WM. The solution proposed in the present invention is the utilization of all or part of this work in any form, mechanical or electrical, for example, to supply a unit for the production of liquefied gas 6, which requires energy to transfer gas from a gaseous state to a liquid one. Since the utilized energy is insufficient for the production of liquefied gas, it is possible to supply the unit for producing liquefied gas from an additional energy source, for example, electric energy, schematically represented as “WE” in FIG. 1. Finally, the liquefied gas production unit 6 typically has a compressor (not illustrated in FIG. 1) or another device that generates heat, simply represented as Q in FIG. 1. The proposed original solution is to use this amount of heat Q to heat the gas entering the station to reduce the PLD pressure of the gas. Indeed, with expansion, the gas cools. At the same time, there is a risk of its temperature falling below the pour point of water and, consequently, the formation of ice, which can lead to partial or complete blockage of the corresponding pipeline. The risk of ice formation and plugging can be limited by heating the gas before expanding it.

In FIG. 2, the first embodiment of the invention is illustrated in more detail by implementing the general scheme illustrated in FIG. one.

In FIG. 2, as well as in the following figures, the designations of FIG. 1 is reused to denote such elements.

Thus, it can again be found in FIG. 2, a gas line 2, which supplies a gas pressure reducing station PLD to supply low pressure gas to the pipe 4. In addition, the liquefied gas production unit 6 supplies the liquefied natural gas LNG.

In the PLD pressure reducing station, the gas from gas line 2 passes through pipes G2 and G3. It is heated in each of these pipes by a device for preheating gas 10. After these devices for preheating, pipes G4 and G5 are assembled in one pipe G6, which supplies gas to the turbo-expander 12. When leaving the turbo-expander, the gas expands and can return to pipe 4 directly through the pipe G7

The liquefied gas production unit 6 advantageously comprises an evaporator 14. The gas supplying the liquefied gas production unit 6 is supplied from branch G9 of the pipe G7 before entering valve 16, where an additional reduction in gas pressure is achieved. The gas is passed through a pipe G10 to a purification unit 8, which purifies the gas, for example, by absorption or preferably by adsorption. The purified gas is transported through G11 to the pre-condenser 18 before being introduced through G12 to the evaporator 14. After the evaporator, liquefied gas is obtained, which passes through the pipe L1 to the control valve 20 and then through the pipe L2 to enter the LNG liquefied natural gas storage.

In this case, interaction is achieved between the turbine expander 12 of the station to reduce the gas pressure PLD and the liquefied gas production unit 6. According to this embodiment of the invention illustrated in FIG. 2, the energy extracted during the gas expansion process in the PLD station is used in the form of electric energy in the liquefied gas production unit 6. The heat generated in the liquefied gas production unit 6 is used to heat the gas entering the PLD station, in other words, gas in the stream to the entrance of the turboexpander 12.

In FIG. 2, it is illustrated that the turboexpander 12 is primarily associated with the generator G. Therefore, mechanical energy is extracted in the turboexpander 12 to be further converted into electrical energy. The electricity thus obtained then feeds the engine M, which drives the compressors C1, C2 and C3, each of which constitutes a stage of the compression device. In this way, electrical interaction between the station to reduce pressure and the liquefied gas production unit is carried out.

In order to optimize the amount of mechanical energy extracted in the turboexpander 12, the gas intended to supply the low pressure pipe 4 and the gas intended to supply the unit for the production of liquefied gas 6, in other words, the gases to be liquefied enter this turbine expander 12.

Thermal integration is achieved using a closed loop control system with feedback, which is described below. For this description, we propose further to follow the movement of the refrigerant in this circuit. By way of non-limiting example, the fluid used may be nitrogen or a mixture of hydrocarbons.

Liquid refrigerant enters compressor C1 through pipe R1 and leaves it through pipe R2. Then it enters the first preheater 10 to heat the gas that comes from the gas line 2 and which is designed to supply the turbine expander 12 of the station to reduce the pressure PLD of the gas. Then, the liquid refrigerant is directed through the pipe R3 to the cooler 22 to achieve control of the temperature of the liquid refrigerant before it is sent to the compression unit through the pipe R4. Then, the liquid refrigerant is compressed by the second compressor C2 and then sent through R5 to the second preheater 10 before being transported through R6 to the second cooler 22, and through R7 it reaches the third stage of compression of the compressor unit. The third cooler 22, connected to the third compressor C3 through the pipe R8, allows you to control the temperature of the liquid leaving the compression unit.

The pipe R9 brings the liquid refrigerant to the counterflow heat exchanger 24, and then the liquid refrigerant is directed through R10 to the expander 26. The expander is mechanically connected to the engine M and to the compression unit. Leaving expander 26, the liquid is then directed (R11) to the evaporator 14 of the liquefied gas production unit 6, where it absorbs calories from that part of the natural gas that needs to be liquefied to produce liquefied natural gas (LNG). Leaving the evaporator 14, the liquid is transported (R12) to the pre-condenser 18 before reaching, through R13, a counter-current heat exchanger 24, which in the direction of flow is connected to the first compressor C1 of the compression unit.

As it turns out from this description, it is used in order to achieve thermal integration between the unit for the production of liquefied gas and the station to reduce gas pressure. This is achieved, in particular, by utilizing the calories recovered by the liquid refrigerant during the compression of the liquid, and then to use them to heat the natural gas entering the station to reduce the pressure of the PLD gas.

Auxiliary elements of the refrigerant cycle are not described in detail in this document. For example, a container 28 can be detected, which is used in the usual way as an expansion vessel for liquid refrigerant.

In FIG. 3 illustrates an embodiment of the invention that reuses certain designations of the preceding figures to indicate like elements. In comparison with the embodiment of the invention illustrated in FIG. 2. According to this option, another form of thermal integration is obtained. It is proposed to include a closed loop of pressurized water (or other heat-transfer fluid, such as, for example, thermal oil) in the plant to extract compression heat and direct it upstream of the turboexpander. For example, an air cooler can be installed on this line to regulate the capacity of the cooling device for the needs of the compression circuit. A volumetric pump is used to circulate the heat exchange fluid (pressurized water), and an expansion vessel can be integrated into this circuit in the usual way.

In FIG. 3, a refrigerant circuit can be recognized between the compression unit with its three compressors C1, C2 and C3 and the liquefied gas production unit 6 with its evaporator 14. This circuit is illustrated in a simplified form. The refrigerant successfully passes through the three stages of the compression device, and after each stage passes through the pre-heating device 10. Then the refrigerant circuit passes through the counterflow heat exchanger 24 before entering the expander 26 and then to the evaporator 14, again passes through the counterflow heat exchanger 24 and returns to the first stage of compression and to its compressor C1.

The main difference from the first embodiment of the invention illustrated in FIG. 2, the preheating devices 10 do not directly transfer the calories extracted in the compression stages to natural gas, but rather, they transfer them to another heat exchange fluid, such as pressurized water. In this way, a second refrigerant circuit is formed, which in parallel passes through the three preheaters 10 to supply the preheater 110, which transfers the calories from the compression stages to the natural gas entering the PLD station. Thus, these pre-heaters 10 form intermediate heat exchangers. Between the preheating devices 10 and the preheating device 110, it is possible to note the presence of a volumetric pump 142, which allows the heat exchange fluid to circulate in the corresponding circuit, as well as in the cooler 122 to control the temperature of the heat transfer fluid in this circuit. One skilled in the art will appreciate that expansion tank 144 is successfully integrated into this refrigerant circuit.

With reference to FIG. 4, then a simplified version of the first embodiment of the invention is illustrated, which is illustrated in FIG. 2. Here, as well as in general in this application, already used designations are reused to indicate such elements to simplify reading comprehension.

In this simplified embodiment, it is noticeable that the compression unit has only one stage with a single compressor C. Natural gas is then heated in a single preheater 10, which allows direct exchange of calories from the compressor with natural gas that enters the station PLD upstream of turboexpander 12.

In this embodiment, the refrigerant circuit uses, for example, a mixture of hydrocarbons and nitrogen as a heat exchange fluid. This mixture is compressed by a compressor C driven by an electric motor M (electrically connected to a turboexpander generator 12 of a PLD station G). The liquid is then cooled in contact with natural gas in the preheater 10 at the inlet of the turboexpander 12 (it is appropriate to note that a different refrigerant circuit could also be provided here between the preheater 10 and the natural gas, as in the previous figure).

Cooler 22 (air cooler) can be introduced into the circuit to regulate the cooling capacity for the needs of the compression circuit. The heat transfer fluid is then sent through a heat exchanger 214, for example, type PHFE (acronym for Plate Fin Heat Exchanger - plate-fin heat exchanger, or in French "echangeur de chaleur a plaques et ailettes" [plate and fin heat exchanger - plate-fin heat exchanger] ), where it cools and condenses during the first passage. Then it expands, passing through valve 246, where according to the Joule-Thompson effect (gas cooling during its expansion), it partially evaporates, again causing a decrease in its temperature. It again passes (2nd pass) through heat exchanger 214, evaporates and heats in contact with natural gas, which must be liquefied, and with a mixture of refrigerants, which must be condensed. After the second pass, leaving the heat exchanger 214, the heat exchange fluid (e.g., a mixture of hydrocarbons and nitrogen) is returned to the compressor C.

In the embodiment of the invention illustrated in FIG. 5, in comparison with the embodiments of the invention that were illustrated in the preceding figures, mechanical integration is achieved between the gas pressure reducing station and the liquefied gas production unit (FIG. 5) instead of electrical integration (FIGS. 2-4).

Indeed, in view of the embodiment of the invention illustrated in FIG. 2, where the turboexpander 12 drives a generator G, which produces electricity, which, in turn, is consumed by the engine M, FIG. 5, it is proposed to mechanically connect the turboexpander 12 to the compressors C1, C2 and C3 of the compression device of the unit for the production of liquefied gas 6.

It seems unnecessary to describe here various plant elements for reducing gas pressure, which are similar to those illustrated in FIG. 2. Similarly, you can again find a similar refrigerant circuit of the unit for the production of liquefied gas and the thermal integration of this production unit with the station to reduce gas pressure.

Also in FIG. 5, engine M is illustrated, which is used in this case as an additional energy source (corresponds to WE in FIG. 1) in order to regulate the energy required for the liquefied gas production unit, taking into account the energy produced at the station site to reduce gas pressure.

As a very illustrative example, we can cite, in particular, already described in various embodiments of the invention, that the amount (weight) of gas passing through the unit for the production of liquefied gas 6 is somewhere around 5-20% of the amount (weight) of gas passing through the station to reduce the pressure of the PLD gas, and the remaining gas (from 80% to 95%) supplies the pipe 4.

The systems described above allow full control over the production of liquefied natural gas. The composition of this gas can be controlled. It does not depend on the pressure difference inside the station to reduce gas pressure.

In addition, preheating the gas entering the station to reduce pressure makes it possible to prevent icing and blockage problems in the pipeline.

Energy recovery occurs at the station to reduce gas pressure, and more precisely, in its turboexpander. This utilization is optimized by passing the entire gas stream through a turboexpander, in other words as a gas that is intended to expand in gaseous form, and a gas intended to be liquefied.

The present invention is not limited to the preferred embodiments of the invention, which are described above as non-limiting examples. This also applies to embodiments of the invention available to those skilled in the art within the scope of the claims below.

Claims (14)

1. Station for reducing gas pressure and for liquefying gas, in particular natural gas, containing: - turboexpander (12), - a device for the disposal of mechanical work carried out in the process of reducing gas pressure, - a cooling system containing devices for compression (C1, C2, C3), and a device for condensing (14) liquefied gas provided with a branch pipe (09) downstream of the turbine expander (12), - a device for recovering heat (Q) produced by devices for compressing (C1, C2, C3; C) cooling systems that are associated with devices (10; 40; 110) for heating gas upstream of a turboexpander (12), characterized in that the cooling system contains compressors and / or expanders with a radial gas flow. 2. The station according to claim 1, characterized in that it contains a closed loop between the condensation device (14), compression devices (C1, C2, C3; C) and devices (10; 40) for gas heating. 3. The station according to claim 1, characterized in that it comprises a first closed loop between compression devices (C1, C2, C3), a condensation device (14) and at least one intermediate heat exchanger (10), as well as a second closed loop optionally using a different heat exchange fluid than that used in the first closed loop, between at least one intermediate heat exchanger (10) and gas heating device (110). 4. The station under item 1, characterized in that it contains a device for converting (G) mechanical work into electrical energy, which is associated with a device for utilization of mechanical work produced in the process of reducing gas pressure. 5. The station according to claim 4, characterized in that the device for disposing of mechanical work performed in the process of lowering the gas pressure is mechanically connected to an electric generator (G), and in which the compression devices (C1, C2, C3) are driven an engine (M) supplied with electrical energy by an electric generator (G). 6. The station according to claim 1, characterized in that the device for converting the mechanical work performed in the process of lowering the gas pressure is mechanically connected to compression devices (C1, C2, C3; C). 7. The station according to claim 6, characterized in that an auxiliary engine (M) is provided for driving the compression devices (C1, C2, C3). 8. The station under item 1, characterized in that the cooling system uses a refrigerant selected from nitrogen and / or a mixture of hydrocarbons. 9. A station according to claim 1, characterized in that it comprises a device for purifying (8, 36) natural gas by adsorption and / or absorption, installed in a gas stream before entering the gas condensing device (14).

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