Facility and method for liquefying a fluid flow

Abstract

The invention relates to a facility for liquefying a fluid, the facility comprising a fluid circuit (2) to be cooled, a cryogenic refrigerator (6) having a cycle fluid, the refrigerator comprising at least one cooling exchanger (8) which carries out heat exchange between the fluid to be cooled and the cycle fluid, the facility further comprising an expansion turbine (7) for expanding the gas flow to be cooled, the expansion turbine (7) being mounted on a rotating shaft supported by at least one static gas bearing (10), the facility comprising a pipe (11) for injecting pressurised gas having an upstream end intended to receive pressurised fluid to be liquefied and a downstream end connected to the bearing (10) in order to support the rotating shaft, the facility (1) comprising a pipe (13) for recovering the gas that has been used in the bearing (10), the pipe (13) comprising a downstream end connected to the downstream end (22) of the fluid circuit (2) to be cooled and / or a recovery system, the expansion turbine (7) for expanding the gas flow to be cooled being arranged upstream of the cryogenic refrigerator (6).

Description

Installation and process for liquefying a fluid flow

[0001] The invention relates to an installation and a method for liquefying a fluid flow, for example natural gas,

[0002] The invention relates more particularly to an installation for liquefying a fluid flow such as natural gas, the installation comprising a circuit of fluid to be cooled provided with an upstream end intended to be connected to a source of gas to be liquefied under pressure and a downstream end intended to be connected to a receiving or user member of the liquefied gas, the installation comprising, between the upstream and downstream ends, a set of members intended to cool and liquefy said fluid to be cooled and in particular a cryogenic refrigerator, for example of the cycle fluid type comprising at least one cooling exchanger ensuring a heat exchange between the fluid to be cooled and the cycle fluid, the circuit of fluid to be cooled further comprising a turbine for expanding the flow of gas to be cooled, the expansion turbine being mounted on a rotating shaft supported by at least one static gas bearing,the circuit of fluid to be cooled comprising a pressurized gas injection pipe having an upstream end intended to receive pressurized fluid supplied by the source and a downstream end connected to the bearing to ensure the support of the rotating axis, the installation comprising a pipe for recovering the gas having been used in the bearing, the recovery pipe comprising an upstream end connected to the bearing, the recovery pipe comprising a downstream end connected to the downstream end of the circuit of fluid to be cooled and / or a recovery system.,

[0003] In cryogenic liquefaction installations the flow of fluid to be liquefied (inlet flow or "feed") is sometimes available at a relatively high pressure, for example 30 to 80 barg, in particular when the fluid to be liquefied comes from a pressurized network.

[0004] This may apply in particular to the liquefaction of natural gas from distribution pipe networks.

[0005] In some cases, the desired liquefaction capacity is greater than the liquefier's usual capacity. One way to increase the liquefier's capacity is to increase the pressure of the fluid to be liquefied upstream of the liquefier via compressors. This can be applied to biogas liquefaction, for example. This gas generally comes from a digester at a pressure close to atmospheric pressure.

[0006] However, available liquefaction machines do not always allow operation at such high pressure if this pressure is higher than the maximum allowable pressure (also called design pressure) of the liquefier or any other unit required for the liquefaction process. For example, the design pressure of some liquefiers or refrigerators may be around 50 barg.

[0007] High liquefaction pressure improves production capacity as it corresponds to a liquefaction plateau (zone of gas-to-liquid phase change) of the fluid at a higher temperature. Liquefying gases at a higher temperature theoretically makes it possible to provide cold power at a less cold temperature and therefore improve the energy efficiency of the liquefier.

[0008] Furthermore, the available liquefaction turbomachines do not always allow this pressure to be exploited for technical reasons (impeller size, engine rotation speed, excessively high cycle pressure, for example). The optimum operating temperature may be at a different pressure / liquefaction stage temperature pair, sometimes lower than the available and initial pair. It is therefore necessary to exploit the pressure drop towards this more appropriate pair by optimizing the overall chain.

[0009] A simple and intuitive solution is to reduce the pressure upstream of the liquefier and downstream of the pre-cooler via an expansion valve. This pressure reduction would allow for most fluids to reduce the temperature at the liquefier inlet through the Joule-Thompson effect. However, this isenthalpic device does not remove cooling power from the flow, which does not appear to be sufficient optimization for a constant production train.

[0010] WO2021063429A1 describes a natural gas liquefaction system whose expansion is used to cool a refrigerant cycle. This solution uses a nitrogen loop whose leaks must be compensated.

[0011] Document WO20259990A1 describes the use of a turbine at the outlet of the liquefier. This does not resolve the pressure management problems in the upstream part of the installation.

[0012] An aim of the present invention is to overcome all or part of the drawbacks of the prior art noted above.

[0013] To this end, the installation according to the invention, which also conforms to the generic definition given in the preamble above, is essentially characterized in that the expansion turbine for the flow of gas to be cooled is arranged upstream of the cryogenic refrigerator.

[0014] Furthermore, embodiments of the invention may comprise one or more of the following features: the recovery pipe comprises a portion in heat exchange with the cycle fluid in the at least one cooling exchanger of the cryogenic refrigerator, the recovery pipe is in heat exchange with the at least one cooling exchanger of the cryogenic refrigerator via one or more passages in the exchanger(s) which are distinct from the passages provided for the circuit of fluid to be cooled, i.e. the fluid to be cooled and the gas circulating in the recovery pipe are cooled in parallel and separately before being mixed downstream, the gas injection pipe comprises a regulating and / or expansion member, for example a valve, the regulating and / or expansion member is configured to maintain the flow rate in the injection pipe at a value between 0,5% and 10% of the flow rate of the pressurized fluid supplied by the source, the circuit of fluid to be cooled comprises a pre-cooling device arranged upstream of the expansion turbine and configured to ensure pre-cooling of the gas flow to be cooled before its expansion in said turbine, the installation is configured to supply a flow of fluid at the inlet of the expansion turbine at a first determined pressure and to expand this flow at the outlet of the expansion turbine to a second determined pressure, the pre-cooling device being configured to cool the flow of fluid at the inlet of the expansion turbine to a determined temperature bringing said fluid into a state of saturation or close to saturation at the outlet of the expansion turbine,the rotating shaft carrying the expansion turbine also carries a compressor configured to recover the work provided by the expansion turbine in a fluid circuit for dissipating this work and / or an alternator and / or an eddy current type brake.,

[0015] The invention also relates to a method for liquefying a fluid flow, for example natural gas, using an installation comprising a circuit of fluid to be cooled provided with an upstream end connected to a source of gas to be liquefied under pressure and a downstream end connected to a member receiving or using the liquefied gas, the installation comprising, between the upstream and downstream ends, a set of members intended to cool and liquefy said fluid to be cooled and in particular a cryogenic refrigerator, for example a cycle fluid comprising at least one cooling exchanger ensuring a heat exchange between the fluid to be cooled and the cycle fluid, the circuit of fluid to be cooled further comprising a turbine for expanding the flow of gas to be cooled arranged upstream of the cryogenic refrigerator, the expansion turbine being mounted on a rotating shaft supported by at least one static gas bearing,the circuit of fluid to be cooled comprising a pressurized gas injection pipe having an upstream end intended to receive pressurized fluid supplied by the source and a downstream end connected to the bearing to ensure the support of the rotating axis, the installation comprising a pipe for recovering the gas having been used in the bearing), the recovery pipe comprising an upstream end connected to the bearing, a portion in heat exchange with the cycle fluid in the at least one cooling exchanger of the cryogenic refrigerator, the recovery pipe comprising a downstream end connected to the downstream end of the circuit of fluid to be cooled, the method comprising a step of expanding the majority of the flow of fluid to be cooled supplied by the source in the expansion turbine upstream of the cooling exchanger of the cryogenic refrigerator and, simultaneously,a step of taking a fraction of the flow of fluid to be cooled supplied by the source to supply the bearing and ensure the support of the rotating axis.,

[0016] According to other possible features: the pressure of the fluid flow at the inlet of the turbine is at a first determined pressure of between 100 and 50 barg, the turbine being configured to expand this flow at the outlet of the expansion turbine to a second determined pressure of between 50 and 20 barg, the method comprising a step of cooling the fluid flow at the inlet of the expansion turbine to a determined temperature to bring the fluid into a state of saturation or close to saturation at the outlet of the expansion turbine, during the sampling step, the fraction of the fluid flow to be cooled supplied by the source used to supply the bearing is between 0.5 and 10% of the fluid flow to be cooled supplied by the source, the rotating shaft carrying the expansion turbine also carries a compressor configured to recover the work supplied by the expansion turbine in a fluid circuit for dissipating this work,the method comprising transmitting the power extracted by the expansion turbine to the compressor and dissipating this power by the compressor by compressing a fluid in a circuit while producing heat or electricity and recovering this heat or electricity.,

[0017] The invention may also relate to any alternative device or method comprising any combination of the above or below features within the scope of the claims.

[0018] Other features and advantages will appear on reading the description below, made with reference to the figures in which: Brief description of the figures

[0019] The invention will be better understood on reading the following description, given solely by way of example and with reference to the appended drawings in which:

[0020] is a schematic and partial view illustrating an example of structure and operation of an installation in accordance with the invention. Detailed description

[0021] In all figures, the same references refer to the same elements.

[0022] In this detailed description, the following embodiments are examples. Although the description refers to one or more embodiments, this does not mean that the features apply only to a single embodiment. Single features of different embodiments may also be combined and / or interchanged to provide other embodiments.

[0023] The illustrated installation 1 for liquefying a flow of fluid such as natural gas comprises a circuit 2 of fluid to be cooled provided with an upstream end 12 intended to be connected to a source 3 of gas to be liquefied under pressure (for example a natural gas network) and a downstream end 22 intended to be connected to a member 4 receiving or using the liquefied gas (for example a liquefied natural gas tank).

[0024] Between the upstream 12 and downstream 22 ends, the installation 1 comprises, arranged in series, a device 5 for pre-cooling the circuit 2 of fluid to be cooled, a turbine 7 for expanding the flow of gas to be cooled and a refrigerator 6, preferably cryogenic. The refrigerator 6 is for example of the type using a cycle fluid but could be of the type using a refrigerant mixing cycle "MRC" or cascade using gas and / or liquid. The pre-cooling device 5 may comprise for example at least one heat exchanger in heat exchange with the circuit 2 of fluid to be cooled and with a reserve of cold fluid (nitrogen or other and / or with a refrigeration loop.

[0025] As illustrated, the cycle fluid is in a separate loop independent of the circuit 2 of fluid to be cooled. In particular, the cycle fluid may be of a separate nature from the fluid to be cooled.

[0026] The downstream cryogenic refrigerator 6 comprises at least one cooling exchanger 8 ensuring a heat exchange between the fluid to be cooled and the cycle fluid.

[0027] The expansion turbine 7 is mounted on a rotating shaft supported by at least one static gas type bearing 10.

[0028] The circuit 2 of fluid to be cooled comprises a pressurized gas injection pipe 11 having an upstream end intended to receive pressurized fluid supplied by the source 3 and a downstream end connected to the bearing 10 to ensure the support of the rotating axis. The expansion turbine 7 arranged upstream of the liquefier levitates thanks to a continuous flow of gas in the bearings 10.

[0029] This gas flow rate for the stage 10 can thus be removed from the upstream flow at ambient temperature and can be expanded in order to minimize its flow rate. For example, the gas injection line 11 comprises a regulating and / or expansion member 14, for example an expansion valve. A pressure reduction reduces the flow rate while maintaining a downstream pressure close to the storage or use pressure. The stage flow rate is preferably kept less than 10% of the total flow rate of the upstream flow.

[0030] The installation 1 comprises a pipe 13 for recovering the gas that has been used in the bearing 10. This recovery pipe 13 has an upstream end connected to the bearing 10 and may comprise a portion in heat exchange with the cycle fluid in the at least one cooling exchanger 8 of the cryogenic refrigerator 6 (for example in one or more dedicated passages). After passing through the cooling exchanger 8, the recovery pipe 13 may comprise a downstream end connected to the downstream end 22 of the circuit 2 of fluid to be cooled. Thus, the flow of gas that has passed through the bearings 10 can be liquefied at a pressure close to the final use or storage pressure.

[0031] Of course, this configuration is not limiting. As a variant or in combination, and as illustrated in dotted lines, the recovery pipe 13 can conduct the gas flow to another recovery system 18, without passing through the cryogenic refrigerator 6, for example to a burner to supply energy to the installation 1.

[0032] The installation 1 is configured to provide a flow of fluid to the inlet of the expansion turbine 7 at a first pressure determined, for example, 70 barg. This pressure is, for example, fixed and determined by the source 3 (network, for example).

[0033] The expansion turbine 7 is configured to expand this flow at the outlet of the expansion turbine 7 to a second determined pressure, for example to 50 barg (pressure reduction of 20 bar).

[0034] The pre-cooling device 5, for its part, can be configured to cool the fluid flow at the inlet of the expansion turbine 7 to a determined temperature so that the fluid is in a saturation state or close to saturation at the outlet of the expansion turbine 7.

[0035] Close to saturation means that the temperature of the fluid at the outlet of the turbine 7 is plus or minus 10 K from the saturated vapor (saturation) temperature at the liquefaction pressure, for example plus or minus 5K.

[0036] That is to say, the temperature of the fluid can be regulated / controlled or fixed at the inlet of the expansion turbine 7 so that the temperature of the fluid at the outlet of the turbine 7 at the corresponding pressure is equal to the temperature at which the first liquid droplets appear.

[0037] An intermediate pressure, also called liquefaction pressure, can thus be determined.

[0038] The temperature of the fluid liquefaction stage varies under pressure and temperature conditions depending on the composition of the gas to be liquefied. For example, for the liquefaction of natural gas, the composition can vary depending on the gas source but also on the upstream purification process used.

[0039] The upstream location of the expansion turbine 7 provides advantages. The installation makes it possible to avoid having to manage instabilities or wear due to the generation of liquid drops in the turbine during expansion. This liquefaction temperature is significantly lower than the outlet temperature of the pre-cooling system 5. The cold power extracted by the expansion turbine 7 can advantageously be transmitted on a common shaft to at least one compressor 15 at the other end. Alternatively or in combination, a braking system for the expansion turbine 7 could comprise an alternator and / or an eddy current type brake.

[0040] As illustrated, this energy can be dissipated by the compressor in a closed cycle 16 filled with a fluid. The heat produced can be evacuated, for example, by a cooling network of the installation 1. This energy can also be used for the compression of an additional fluid.

[0041] An expansion valve 17 may be provided downstream of the liquefier 6 to expand liquefied gas produced in the liquefier. The temperature is almost stable during the expansion operation (e.g. + / - 2 K). As illustrated, the main stream from the expansion valve 17 and the recovered and cooled bearing gas stream may be mixed before storage or use. The process has no loss of gas supplied by the source 3.

[0042] The use of the expansion turbine 7 upstream of the liquefaction allows work extraction until an optimum liquefaction level for the fluid is approached. Cold power is extracted at relatively low temperatures (between the outlet temperature of the pre-cooling system 5 and down to minus 100°C for example).

[0043] This makes it possible to comply with the pressure constraints of the liquefaction train equipment downstream of the expansion turbine 7.

[0044] The whole unit has high energy efficiency.

[0045] Another approach for a very low bearing gas flow rate (less than 2% of the initial flow rate upstream) could consist of “bubbling” this gas directly into the liquefied gas flow leaving liquefier 6. This configuration would reduce the overall energy efficiency of the train but would be very economical in terms of material investment.

Claims

Installation for liquefying a fluid flow such as natural gas, the installation (1) comprising a circuit (2) of fluid to be cooled provided with an upstream end (12) intended to be connected to a source (3) of gas to be liquefied under pressure and a downstream end (22) intended to be connected to a member (4) receiving or using the liquefied gas, the installation (1) comprising, between the upstream (12) and downstream (22) ends, a set of members (5, 6, 7) intended to cool and liquefy said fluid to be cooled comprising a cryogenic refrigerator (6), of the cycle fluid type comprising at least one cooling exchanger (8) ensuring a heat exchange between the fluid to be cooled and the cycle fluid, the circuit (2) of fluid to be cooled further comprising a turbine (7) for expanding the flow of gas to be cooled, the expansion turbine (7) being mounted on a rotating shaft supported by at least one bearing (10) of the type gas static,the circuit (2) of fluid to be cooled comprising a pressurized gas injection pipe (11) having an upstream end connected to the upstream end of the circuit (2) of fluid to be cooled and intended to receive pressurized fluid supplied by the source (3) and a downstream end connected to the bearing (10) to ensure the support of the rotating axis, the installation (1) comprising a pipe (13) for recovering the gas having been used in the bearing (10), the recovery pipe (13) comprising an upstream end connected to the bearing (10), the recovery pipe (13) comprising a downstream end connected to the downstream end (22) of the circuit (2) of fluid to be cooled and / or a recovery system, the turbine (7) for expanding the flow of gas to be cooled being arranged upstream of the cryogenic refrigerator (6),characterized in that the cycle fluid is in a separate loop independent of the circuit (2) of fluid to be cooled and in that the recovery pipe (13) comprises a portion in heat exchange with the cycle fluid in the at least one cooling exchanger (8) of the cryogenic refrigerator (6), the recovery pipe (13) being in heat exchange with the at least one cooling exchanger (8) of the cryogenic refrigerator (6) via one or more passages in the exchanger(s) (8) which are separate from the passages provided for the circuit (2) of fluid to be cooled, i.e. the fluid to be cooled and the gas circulating in the recovery pipe (13) are cooled in parallel and separately before being mixed downstream., Installation according to claim 1, characterized in that the gas injection pipe (11) comprises a regulating and / or expansion member (14), for example a valve. Installation according to claim 2, characterized in that the regulating and / or expansion member (14) is configured to maintain the flow rate in the injection pipe (11) at a value between 0.5% and 10% of the flow rate of the pressurized fluid supplied by the source (3). Installation according to any one of claims 1 to 3, characterized in that the circuit (2) of fluid to be cooled comprises a pre-cooling device (5) arranged upstream of the expansion turbine (7) and configured to ensure pre-cooling of the flow of gas to be cooled before its expansion in said turbine (7). Installation according to claim 4, characterized in that it is configured to provide a flow of fluid at the inlet of the expansion turbine (7) at a first determined pressure and to expand this flow at the outlet of the expansion turbine (7) to a second determined pressure, the pre-cooling device (5) being configured to cool the flow of fluid at the inlet of the expansion turbine (7) to a determined temperature bringing said fluid into a state of saturation or close to saturation at the outlet of the expansion turbine (7). Installation according to any one of claims 1 to 5, characterized in that the rotating shaft carrying the expansion turbine (7) also carries a compressor (15) configured to recover the work provided by the expansion turbine (7) in a fluid circuit (16) for dissipating this work and / or an alternator and / or a brake of the eddy current type. Installation according to any one of claims 1 to 6, characterized in that the circuit (2) of fluid to be cooled comprises, between the cryogenic refrigerator (6) and the downstream end (22), a member (17) for expanding the fluid to be cooled, for example an expansion valve. Method for liquefying a fluid flow, for example natural gas, using an installation (1) according to any one of claims 1 to 7, the method comprising a step of expanding the majority of the flow of fluid to be cooled supplied by the source (3) in the expansion turbine (7) upstream of the cooling exchanger (8) of the cryogenic refrigerator (6) and, simultaneously, a step of withdrawing a fraction of the flow of fluid to be cooled supplied by the source (3) to supply the bearing and ensure the support of the rotating axis. Method according to claim 8, characterized in that the pressure of the fluid flow at the inlet of the turbine is at a first determined pressure of between 100 and 50 barg, the turbine (7) being configured to expand this flow at the outlet of the expansion turbine (7) to a second determined pressure of between 50 and 20 barg, the method comprising a step of cooling the fluid flow at the inlet of the expansion turbine (7) to a determined temperature to bring the fluid into a state of saturation or close to saturation at the outlet of the expansion turbine (7). Method according to any one of claims 8 to 9, characterized in that during the sampling step, the fraction of the flow of fluid to be cooled supplied by the source (3) used to supply the bearing is between 0.5 and 10% of the flow of fluid to be cooled supplied by the source (3). Method according to any one of claims 8 to 10, characterized in that the rotating shaft carrying the expansion turbine (7) also carries a compressor (15) configured to recover the work provided by the expansion turbine (7) in a fluid circuit (16) for dissipating this work, the method comprising a transmission of the power extracted by the expansion turbine (7) to the compressor (15) and a dissipation of this power by the compressor by compressing a fluid in a circuit producing heat or electricity and a recovery of this heat or electricity. Method according to any one of claims 10 to 11, characterized in that it comprises a step of expanding the flow of liquefied fluid to be cooled downstream of the cooling exchanger (8) of the cryogenic refrigerator (6) and upstream of the downstream end (22) of the circuit (2) of fluid to be cooled, for example via an expansion valve (17).

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