INSTALLATION AND METHOD FOR STORING LIQUEFIED GAS

An installation and method for storing liquefied gas, comprising a plurality of separate storage tanks each configured to contain liquefied gas, comprising a cooling circuit provided with a refrigeration device, a withdrawal pipe and a plurality of first injection pipes towards each of the tanks in order to cool the withdrawn fluid flow, the tanks comprising a degassing line, the installation comprising a set of controlled valves situated at least in the cooling circuit, wherein the first tank contains liquid and at least one other tank is empty, containing essentially boil-off gas, that is to say containing little or no liquid, wherein there is cooling of the fluid contained in the first tank, re-injection into the first tank, injection of liquid cooled by the refrigeration device into at least one other empty tank and transfer of boil-off gas generated in the or the other tanks to the first tank

FIELD OF THE INVENTION

The invention relates to an installation and a method for storing liquefied gas, for example liquefied natural gas.

The invention may relate in particular to the storage and transportation of liquefied natural gas on-board methane tankers, for example.

SUMMARY OF THE INVENTION

In certain embodiments, the invention may relate more specifically to a storage installation for liquefied gas, for example liquefied natural gas, in particular on board a ship, comprising several separate storage tanks each designed to contain the liquefied gas, each tank comprising a lower portion intended to contain the liquefied gas in liquid state, and an upper portion intended to contain the vapors from the liquefied gas, the installation comprising a cooling circuit provided with a refrigeration device, the cooling circuit comprising a withdrawal line comprising a first end opening into the lower portion of a first tank and a second end connected to an inlet of the refrigeration device, the cooling circuit comprising a plurality of first injection lines connecting an outlet of the refrigeration device to each of the tanks respectively, the refrigeration device being designed to cool the fluid flow circulating between the inlet and outlet thereof.

In certain embodiments, the invention may also relate in particular to the operation of cooling a methane tanker (with several storage tanks) during liquefied natural gas loading operations (filling).

The invention is intended to reduce or prevent the generation of boil-off gas during this operation.

A known solution involves injecting empty tanks with liquefied gas pumped from a full tank or a tank containing residual liquid. However, this generates boil-off gases in the empty tank to be cooled. These boil-off gases are either lost (discharged through vents or used to propel the ship or burned in a flare) or have to be conveyed to a liquefaction unit to be reinjected into one of the tanks.

The first solution results in a loss of product, while the second solution is costly and notably requires boil-off gas compressors to reliquefy said gases.

One objective of this invention is to mitigate some or all of the drawbacks of the prior art as set out above.

For this purpose, the installation according to certain embodiments of the invention, which otherwise corresponds to the general definition given in the preamble above, may include a plurality of storage tanks, with each storage tank comprising a degassing line having a first end connected to the upper portion of the tank and a second end connected to at least one recovery zone, the degassing line of each tank other than the degassing line of the first tank comprising a third end connected to the upper portion of the first tank, the installation comprising a set of controlled valves located at least in the cooling circuit and an electronic control member for said valves, the control member being configured to command the opening and closing of the valves to cool the fluid contained in the first tank via the cooling circuit by drawing liquid from the first tank, cooling this drawn liquid in the refrigeration device and reinjecting this liquid into the first tank, the control member being further configured to inject the liquid cooled in the refrigeration device into at least one other empty tank via the first injection line or lines and to transfer the boil-off gas generated in this or these other empty tanks to the first tank via the third end or ends of the degassing line.

Furthermore, the embodiments of the invention may have one or more of the following features:each first injection line comprises a first end connected to the outlet of the refrigeration device and a second end opening into an upper portion of a tank,the cooling circuit comprises a second injection line connecting the outlet of the refrigeration device to the lower portion of the first tank,the control member is configured to command the opening and closing of the set of valves to cool the fluid contained in the first tank by reinjecting the cooled liquid via the first injection line and/or the second injection line,the installation comprises a set of pressure sensors in the tanks, the control member being configured to command the opening and closing of the valves to distribute the cooled liquid flow reinjected into the first tank between the first injection line and the second injection line as a function of the pressure level measured by the set of pressure sensors,the installation comprises a set of temperature sensors for the fluid in the tanks, the control member being configured to command the opening and closing of the valves as a function of the temperature level measured by the set of temperature sensors.

The invention also relates to a method for cooling tanks in an installation for storing liquefied gas comprising several separate storage tanks each designed to contain the liquefied gas, each tank comprising at least a lower portion intended to contain the liquefied gas in liquid state, and at least an upper portion intended to contain the vapors from the liquefied gas, the installation comprising a cooling circuit provided with a refrigeration device, the cooling circuit comprising a withdrawal line comprising a first end opening into the lower portion of a first tank and a second end connected to an inlet of the refrigeration device, the cooling circuit comprising a plurality of first injection lines connecting an outlet of the refrigeration device respectively to each of the tanks, the refrigeration device being designed to cool the fluid flow circulating between the inlet and outlet thereof, each of the tanks comprising a degassing line comprising a first end connected to the upper portion of the tank and a second end connected to at least one recovery zone, the degassing line of each tank other than the degassing line of the first tank comprising a third end connected to the upper portion of the first tank, the installation comprising a set of controlled valves located at least in the cooling circuit, in which the first tank contains liquid and at least one other tank is empty, essentially containing the boil-off gas, i.e. containing little or no liquid, the method comprising: a step of cooling the fluid contained in the first tank via the cooling circuit by drawing liquid from the first tank, cooling this liquid in the refrigeration device and reinjecting this liquid into the first tank, a step of injecting liquid cooled in the refrigeration device into at least one other empty tank, and a step of transferring boil-off gas generated in the other tank or tanks into the first tank.

According to possible particular features:during the cooling step, the fluid contained in the first tank is cooled to a temperature equal to or less than the saturation temperature of the fluid at the pressure in the tank,the step of transferring boil-off gas to the first tank is carried out by pressure balancing and/or via a pumping member such as a compressor,the step of injecting cooled liquid into at least one other empty tank and the step of transferring boil-off gas generated in the other tank or tanks into the first tank are carried out at least partially simultaneously,the step of injecting cooled liquid into at least one other empty tank is carried out until the fluid in the other tank or tanks reaches a given temperature, for example a temperature that may be as low as a temperature equal to or less than the saturation temperature of the fluid at the pressure in the tank,the method comprises, following the step of injecting cooled liquid into at least one other empty tank, a step of filling at least one of the other tanks with the liquefied gas,the fluid is liquefied natural gas.

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

DETAILED DESCRIPTION OF THE INVENTION

The illustrated liquefied gas storage installation1comprises several separate storage tanks2,3,4,5each designed to contain the liquefied gas, in particular the liquefied natural gas (four in this nonlimiting example).

Each tank2,3,4,5conventionally comprises a lower portion intended to contain the liquefied gas in liquid state and an upper portion intended to accommodate the vapors from the liquefied gas.

The installation1further comprises a cooling circuit provided with a refrigeration device7. The cooling circuit comprises at least one withdrawal line6comprising a first end opening into the lower portion of a first tank2and a second end connected to an inlet of the refrigeration device7.

The withdrawal line6may in particular include a pump26intended to aspirate the liquid in the first tank2.

The cooling circuit comprises cooled-fluid injection lines8connecting an outlet of the refrigeration device7to each of the tanks2,3,4,5respectively.

The refrigeration device7is designed to cool, and notably to subcool where applicable, the fluid flow circulating between the inlet and outlet thereof.

For example, the refrigeration device7comprises a cryogenic refrigerator with a refrigeration cycle for a cycle gas in a working circuit. The cycle gas for example comprises a pure gas or a gas mixture (for example nitrogen, neon, argon, helium, hydrogen or a mixture thereof). The working circuit of the refrigerator7comprises a member for compressing the cycle gas (for example one or more motorized compressors), a member for cooling the cycle gas (one or more cooling heat exchangers, for example), a member for expanding the cycle gas (one or more turbines and/or expansion valves) and a member for heating the cycle gas (one or more heat exchangers). The work of at least one of the turbines is preferably transferred to a compressor via a shared drive shaft. Heating and cooling can especially be effected at least in part by one or more countercurrent exchangers in which two separate portions of the cycle gas circulate under different thermodynamic conditions (especially temperature).

In other words, the working circuit of the refrigerator may be configured to subject the working gas to a thermodynamic cycle that produces, at one end of the working circuit, a cooling capacity that is transferred to the fluid circuit to be cooled via one or more heat exchangers. The refrigeration device7may notably be a “Turbo-Brayton” device marketed by the applicant.

As illustrated, each of the tanks2,3,4,5comprises a degassing line10comprising a first end connected to the upper portion of the tank and a second end connected to at least one recovery zone11(for example a vent to atmosphere and/or to an inlet of an engine or a gas combustion member).

The degassing line10of each tank3,4,5other than the degassing line of the first tank2also being connected (via a third end) to the upper portion of the first tank2.

As illustrated, the degassing lines10of each tank2,3,4,5may be connected in parallel via a shared line to a recovery zone11. This also creates a fluidic link between upper portions of all the tanks2,3,4,5via the degassing lines10(enabling the transfer of boil-off gas from one tank to the other, as required and as described below).

The installation1comprises a set of valves16,18, for example controlled valves, located at least in the cooling circuit. The set of valves is in particular designed to enable the targeted cooling of the tank or tanks2,3,4. The set of valves is preferably controlled (opened/closed) by an electronic control member12, for example comprising a microprocessor.

The control member12may in particular be configured to command the opening and closing of the valves to cool the fluid contained in the first tank2via the cooling circuit.

This cooling may be carried out by drawing liquid from the first tank2via the withdrawal line6, then cooling this drawn liquid by means of a heat exchange with a cold portion of the refrigerator7, then reinjecting this cooled liquid into the first tank2via the injection line8. As illustrated, the injection line8comprises a first end connected to the outlet of the refrigeration device7and a second end opening into an upper portion of the tank2, for example in the form of one or more nozzles opening into the upper portion of the tank. In other words, the injection member injects the subcooled liquefied gas into the vapor phase, i.e. above the level of the liquefied gas in liquid state.

Some or all of the other tanks3,4,5preferably also comprise such an injection line8connected to the outlet of the refrigeration device7and a second end opening into an upper portion of the tank2.

In the arrangement shown inFIG.1, the first tank2still contains liquid, but the other tanks3,4,5are empty and relatively hotter (little or no liquid). In the case of liquefied natural gas, the temperature of the gas in these other empty tanks3,4,5may be greater than −130° C. and for example between −130° C. and +30° C. In each of these tanks2,3,4,5, the pressure is lower than the set pressure of the safety valve (not shown, for the sake of simplicity). Typically, the pressure in each of the tanks is maintained below a given value in the order of several hundred millibars (gauge pressure) above atmospheric pressure or several bars (depending on the nature of the tanks).

In a first phase or step, the content of the first tank2may be cooled or subcooled as mentioned above.

Liquid may be pumped from the first tank2via the withdrawal line6then cooled by the refrigerator7before being reinjected into the first tank2via the injection line8(corresponding valves16,18open).

This means that, preferably, before cooling of the other empty tanks3,4,5begins, the first tank2, which still contains liquid, is completely cooled, preferably to or below the saturation temperature. For example, the refrigeration device7may be designed to cool the liquefied gas coming from the tank2to a temperature of between 35 K and 150 K, for example to 110 K or 80 K (and for example at a rate of between 5 m3/h et 50 m3/h).

To prevent or limit the possible generation of boil-off gas in the first tank2during this step, the flow of (sub) cooled liquid reinjected may be distributed between the upper part and/or the lower part of the tank2.

For this purpose and as illustrated, the installation1may have (at least for the first tank2) a second injection line9connecting the outlet of the refrigeration device7to the inside of the first tank2, for example opening into the lower portion of the first tank2. Furthermore, the set of valves comprises at least one valve designed to control the reinjection of the cooled liquid via the first injection line8and/or the second injection line9(seeFIG.1).

It should be noted that the second injection line9may be designed (one or several other separate second injection lines9may be provided) to enable the cooled (or supercooled) liquid to be injected into this or these other tanks.

If the pressure in the tank2increases excessively (for example above a given level), the cooled liquid is reinjected mainly or exclusively (seeFIG.2) into the upper part of the tank2(via the first injection line8). Conversely, if the pressure drops (for example below a given level), the cooled liquid is reinjected for example mainly or exclusively into the lower part of the tank2(via the second injection line9).

Preferably, at the beginning of this cooling phase of the first tank2, approximately 10% of the cooled liquid flow is reinjected via the first injection line8.

This detection of the pressure level and/or variations therein may be carried out by at least one sensor20measuring the pressure in the tank2, for example in the degassing line10.

Once the first tank2has been sufficiently cooled (saturation temperature or below), the (sub) cooled liquid contained in the first tank2may be injected into the other empty tank or tanks to cool said tanks (simultaneously or successively).

As illustrated inFIG.3, the cooled liquid coming out of the refrigeration device7may be injected simultaneously into the first tank2and into some or all of the other tanks via the respective second injection lines8(corresponding valves18open).

Thus, the installation1enables the empty tanks3,4,5to be injected with the subcooled liquid drawn from the first tank2, which is not empty. The boil-off gases generated when the cold liquid is injected into the empty tanks are aspirated and returned to the first tank, which contains liquid and which is subcooled. The solution provides for the option of returning the subcooled liquid into each of the tanks3,4,5separately. This may be done using a distribution valve for each tank, or any other suitable means.

Injecting cold liquid into the empty tanks3,4,5generates boil-off gases in the relatively hotter tanks3,4,5. These boil-off gases are returned to the cold first tank2via the degassing lines10connected to the first tank2. These boil-off gases are aspirated by the first tank2by pressure differential (or are forced where necessary). These boil-off gases are cooled and recondensed at least partially in the cold environment of the first tank2.

The cooled liquid flow injected into the hot tanks3,4,5may be regulated as a function of the pressure in these tanks.

For example, in the event of an excessive pressure increase (for example above a given level), the liquid flow injected may be relatively reduced. In the event of relatively low or diminishing pressure, the liquid flow injected may be relatively increased.

This process of cooling the tanks3,4,5may be continued until the empty tanks to be cooled are brought to a sufficiently low temperature, for example equal to or less than the saturation temperature, for example equal to or less than −130° C.

This detection of the temperature level and/or variations therein may be carried out by at least one sensor21measuring the temperature in the tank or tanks, for example in the degassing line10.

Once the empty tanks have been sufficiently cooled, injecting additional liquid therein causes liquid to appear or increase in quantity in these tanks (seeFIG.4).

Advantageously, this step of cooling the empty tanks3,4,5is carried out after a discharge of liquefied gas and before subsequent filling of the tank or tanks in the installation1.

Some or all of the valves may be valves controlled by the control member12in response to the signals from the pressure sensor20and/or the temperature sensor21in the installation.

The invention may be used in installations for storing liquefied natural gas or any other cryogenic gas that for example liquefies below −100° C. or an appropriate mixture. For example, biomethane, nitrogen, oxygen, argon and mixtures thereof.

The invention also relates to a transport vehicle, for example a transport ship, for transporting a liquefied gas, for example liquefied natural gas, the transport vehicle comprising such an installation.