Defrosting process for a cryogenic fluid production installation

Abstract

A method for defrosting a cryogenic fluid production installation (1) comprising a gas circuit to be cooled (2), a pre-cooling device (8) comprising a pre-cooling circuit (18), a cryogenic cooling device (9) comprising a cooling circuit (19), at least one first cold box (7) and a second cold box (10), the method comprising the steps: a) shutting down the installation (1); b) depressurizing at least one of the following: the gas circuit to be cooled (2), the pre-cooling circuit (18) and the cooling circuit (19); c) circulating a first defrosting gas through the gas circuit to be cooled (2) to pass through the first cold box (7) and / or the second cold box (10). Abbreviated figure: Fig. 1

Description

Title of the invention: Method for defrosting a cryogenic fluid production installation

[0001] The present invention relates to a method for defrosting a cryogenic fluid production installation.

[0002] Cryogenic fluid production facilities, such as hydrogen liquefaction units, operate at cryogenic temperatures as low as 20 K.

[0003] During the life of one of these units, it may be necessary to defrost certain equipment or circuits, i.e. to raise their temperature to bring them, for example, to an ambient temperature, in order to carry out intrusive maintenance or to eliminate impurities accumulated in the circuits or equipment, for example in the case of an operational incident.

[0004] This type of defrosting is already implemented in cryogenic air separation units. However, the processes implemented in these cryogenic installations operating at 80 K cannot be implemented in hydrogen liquefiers whose operating temperature is much lower (around 20 K).

[0005] Indeed, the injection of hot nitrogen into circuits containing hydrogen at 20 K would cause the nitrogen to crystallize and would generate risks of clogging or even damage to these circuits.

[0006] The present invention aims to effectively overcome these drawbacks by proposing a method for defrosting a cryogenic fluid production installation, such as liquefied hydrogen, the installation comprising: - a circuit for the gas to be cooled having an upstream end intended to be connected to a gas source and a downstream end to deliver the cryogenic fluid; - a set of heat exchangers in heat exchange with the gas circuit to be cooled; - a pre-cooling device in heat exchange with at least a first part of the heat exchanger assembly and configured to pre-cool the gas circuit to be cooled to a first determined temperature, in particular a temperature close to 80 K, the pre-cooling device comprising a pre-cooling circuit with a refrigeration cycle of a pre-cooling fluid, the pre-cooling circuit comprising a fluid compression element pre-cooling and a final expansion device for the pre-cooling fluid; - a cryogenic cooling device in heat exchange with at least a second part of the heat exchanger assembly and configured to cool the gas circuit to be cooled to a second determined temperature lower than the first temperature, in particular a temperature close to 20 K, the cryogenic cooling device comprising a refrigeration cycle cooling circuit of a cycle gas, the cooling circuit comprising a cycle gas compression element, at least one turbine and a final expansion element of the cycle gas; - at least one first cold box, in particular perlite-lined and under nitrogen, in which the first part of the heat exchanger assembly is placed, to cool the gas circuit to be cooled to the first determined temperature; - at least one second cold box, separate from the first cold box and in which the second part of the heat exchanger assembly is placed, to cool the gas circuit to be cooled to the second determined temperature, the second cold box being in particular insulated and under vacuum; The process includes the following steps: - a) shutdown of the installation, in particular by stopping at least one of the following: the pre-cooling fluid compression unit, the final pre-cooling fluid expansion unit, the cycle gas compression unit, the final cycle gas expansion unit and the turbine; - b) depressurization of at least one of the following: the gas circuit to be cooled, the pre-cooling circuit and the cooling circuit, in particular of each of them, for example as long as the flow rate in said circuit is greater than a first determined threshold, such as 500 mbar; - c) circulation of a first defrosting gas through at least part of the circuit of gas to be cooled to pass at least partially through the first cold box and / or the second cold box, in the direction going from the upstream end to the downstream end, the first defrosting gas consisting mainly of H2, the first defrosting gas preferably consisting of less than 10 ppb of O2, the first defrosting gas preferably being at ambient temperature, for example at a temperature above 0°C.

[0007] The invention thus enables the defrosting of a cryogenic installation operating at temperatures below 80 K, in particular at temperatures up to about 20 K.

[0008] The invention also allows defrosting of a cryogenic installation such as a hydrogen liquefier, without risk of causing blockage of its circuits.

[0009] According to one embodiment, in step c), the circulation of the first defrosting gas takes place in the direction going from a hot end of the gas circuit to be cooled to a cold end of the gas circuit to be cooled.

[0010] The hot end and cold end of a circuit are understood to be portions of said circuit at distinct temperatures, the hot end having a temperature higher than that of the cold end, in particular higher by at least 50 °C.

[0011] According to one embodiment, step c) includes a step of reducing the flow of the first defrosting gas, between the upstream end and the downstream end, in particular by venting part of the first defrosting gas, for example at a portion located between the first cold box and the second cold box.

[0012] Thus, the flow rate of the first defrosting gas entering the circuit of gas to be cooled is greater than the flow rate of the first defrosting gas leaving the circuit of gas to be cooled.

[0013] According to one embodiment, the first defrosting gas is drawn from the gas source.

[0014] According to one embodiment, step c) is carried out as long as the temperature of the first gas of defrosting at the downstream end is less than a second determined threshold, the second threshold being for example equal to 80 K.

[0015] According to one embodiment, step c) is carried out as long as the temperature of the first defrosting gas between the first cold box and the second cold box is below a third determined threshold, the third threshold being for example equal to -20 °C.

[0016] According to one embodiment, step c) includes a step for controlling the increase in temperature of the first defrosting gas in the circuit of gas to be cooled, the increase remaining less than 5 degrees per minute, for example less than or equal to 1 degree per minute.

[0017] According to one embodiment, step c) implements successive cycles of pressurization and depressurization, in particular until complete evaporation of hydrogen in liquid form in the gas circuit to be cooled.

[0018] According to one embodiment, in step c), the first defrosting gas flows from a first connection point of the gas circuit to be cooled upstream of the first cold box to a second connection point of the gas circuit to be cooled downstream of the first cold box and upstream of the second cold box, or in step c), the first defrosting gas flows from the first connection point to a third connection point of the gas circuit to be cooled downstream of the second cold box, or in step c), the first defrosting gas flows from the second connection point to the third connection point.

[0019] According to one embodiment, the process includes a step d) of circulating a second defrosting gas through at least a portion of the cooling circuit to pass through at least part of the first cold box and / or the second cold box, in particular in the direction from a hot end of the cooling circuit to a cold end of the cooling circuit, the second defrosting gas consisting mainly of H2, the second defrosting gas preferably consisting of less than 10 ppb of O2, the second defrosting gas preferably being at ambient temperature, for example at a temperature above 0°C.

[0020] According to one embodiment, step d) includes a step of defrosting the turbine with the second defrosting gas, in particular with a flow rate of the second defrosting gas, lower than that circulating in the cooling circuit before it passes through the turbine.

[0021] According to one embodiment, step d) includes a de-icing step of an expansion member of a turbine bypass, with the second de-icing gas.

[0022] According to one embodiment, step d) includes a defrosting step of the final expansion device of the cycle gas, with the second defrosting gas.

[0023] According to one embodiment, step d) includes a step dl) of circulating the second defrosting gas in a high-pressure portion of the cooling circuit, in particular located between the outlet of the cycle gas compression member and the inlet of the turbine or between the outlet of the cycle gas compression member upstream of an expansion member of a turbine bypass, the second defrosting gas circulating through the first cold box without passing through the second cold box, or circulating through the first cold box and through the second cold box, or circulating through the second cold box without passing through the first cold box, the second defrosting gas being in particular at least partially discharged upstream of the turbine or upstream of the final expansion member of the cycle gas.

[0024] According to one embodiment, step d) includes a step d2) of circulating the second defrosting gas in a low pressure portion of the cooling circuit, in particular located between the inlet of the cycle gas compression member and the downstream of the final expansion member of the cycle gas, the second defrosting gas circulating through the first cold box without passing through the second cold box, or circulating through the first cold box and through the second cold box, or circulating through the second cold box without passing through the first cold box, the second defrosting gas being in particular discharged downstream of the final expansion member of the cycle gas.

[0025] According to one embodiment, step d) includes a step d3) of circulating the second defrosting gas in a medium pressure portion of the cooling circuit, in particular located downstream of the turbine and at the level of a bypass of the final expansion device of the cycle gas.

[0026] According to one embodiment, the process includes a step e) of circulating a third defrosting gas through the pre-cooling circuit, in particular in the direction from a hot end of the pre-cooling circuit to a cold end of the pre-cooling circuit, the third defrosting gas consisting mainly of an inert gas, for example N2, in particular at an ambient temperature such as a temperature above 0 °C.

[0027] According to one embodiment, step e) is accompanied by a step of fluidic isolation of the pre-cooling fluid compression element from the rest of the pre-cooling circuit so that the third de-icing gas cannot circulate through the pre-cooling fluid compression element.

[0028] According to one embodiment, the process includes a step f) of circulating a fourth de-icing gas through the turbine, the turbine being fluidly isolated from the rest of the cooling circuit, the fourth de-icing gas being notably derived from the second de-icing gas.

[0029] According to one embodiment, in step dl), the second defrosting gas circulating through the second cold box, the process includes a step of circulating the second defrosting gas through the turbine, in particular with a reduction of the flow rate of the second defrosting gas before entering the turbine.

[0030] According to one embodiment, step e) is accompanied by a step of injecting the third defrosting gas upstream of the pre-cooling fluid compression element and a step of evacuating the third defrosting gas downstream of the final expansion element of the pre-cooling fluid.

[0031] According to one embodiment, at step dl), the process includes a step of circulating the second defrosting gas through the final expansion element of the cycle gas.

[0032] According to one embodiment, at step dl), the second defrosting gas circulates from the eighth connection point to the sixth connection point of the cooling circuit, passing through both the first part, the second part of the heat exchanger assembly and the final expansion element of the cycle gas.

[0033] According to one embodiment, in step dl), the second de-icing gas flows from the seventh connection point or from the eighth connection point, to a thirteenth connection point of the cooling circuit located downstream of the turbine, the flow rate of the second de-icing gas being, for example, lowered before it passes through the turbine.

[0034] According to one embodiment, in step d), the second de-icing gas circulates through a bypass of the turbine without passing through the turbine, in particular by circulating through the medium pressure portion.

[0035] According to one embodiment, in step d) the second defrosting gas circulates through the first part of the heat exchanger assembly and / or the second part of the entire heat exchanger assembly, passing through the turbine and without passing through the final expansion unit of the cycle gas or the compression unit of the cycle gas.

[0036] According to one embodiment, step d) is accompanied by a reduction in the flow rate of the second de-icing gas between the upstream and downstream of the turbine, in particular by an evacuation of the second de-icing gas upstream of the turbine.

[0037] According to one embodiment, in step d), the process includes a step of controlling the increase in the temperature of the cooling circuit, the increase remaining less than 5 degrees per minute, for example less than or equal to 1 degree per minute.

[0038] According to one embodiment, the second defrosting gas is derived from the gas source.

[0039] According to one embodiment, step d) is carried out as long as the fluid temperature circulating in the second cold box is less than 80 K.

[0040] According to one embodiment, step a) is carried out before step b).

[0041] According to one embodiment, steps c), d) and e) are carried out after step b).

[0042] According to one embodiment, steps c) and e) are started before the start of step d).

[0043] The invention may also relate to any alternative device or method comprising any combination of the above or below features.

[0044] The invention will be better understood upon reading the following description and examining the accompanying figures. These figures are given only to illustrate, but in no way limit, the invention.

[0045] [Fig-1] Fig. 1 is a schematic representation of an installation in which is implemented the process according to the invention; and

[0046] [Fig.2] Fig.2 is a schematic representation of the process according to the invention.

[0047] With reference to [Fig.1], a cryogenic fluid production installation 1, such as liquefied hydrogen, has been shown.

[0048] Installation 1 comprises: - a circuit for the gas to be cooled 2 having an upstream end 21 intended to be connected to a gas source and a downstream end 22 to deliver the cryogenic fluid; - a set of heat exchangers 5, 6 in thermal exchange with the circuit 2 of gas to be cooled; - a pre-cooling device 8 in heat exchange with at least a first part 5 of the heat exchanger assembly 5, 6 and configured to pre-cool the gas circuit to be cooled 2 to a first determined temperature, in particular a temperature close to 80 K, the pre-cooling device 8 comprising a pre-cooling circuit 18 with a refrigeration cycle of a pre-cooling fluid, the pre-cooling circuit 18 comprising a fluid compression element pre-cooling 28 and a final expansion device for the pre-cooling fluid 38; - a cryogenic cooling device 9 in heat exchange with at least a second part 6 of the heat exchanger assembly 5, 6 and configured to cool the circuit 2 of gas to be cooled to a second determined temperature lower than the first temperature, in particular a temperature close to 20 K, the cryogenic cooling device 9 comprising a cooling circuit 19 with a cycle refrigeration of a cycle gas, the cooling circuit 19 comprising a cycle gas compression element 29, at least one turbine 49 and a final expansion element of the cycle gas 39; - at least one first cold box 7, in particular perlite-lined and under nitrogen, in which is placed the first part 5 of the heat exchanger assembly, to cool the gas circuit to be cooled 2 to the first determined temperature; - at least one second cold box 10, separate from the first cold box and in which is arranged the second part 6 of the heat exchanger assembly, to cool the gas circuit to be cooled 2 to the second determined temperature, the second cold box being in particular insulated and under vacuum.

[0049] As illustrated in [Fig.2], to defrost the installation 1, a defrosting process is implemented comprising the following steps: - a) shutdown of the installation 1, in particular by stopping at least one of the following: the pre-cooling fluid compression unit 28, the pre-cooling fluid final expansion unit 38, the cycle gas compression unit 29, the cycle gas final expansion unit 39 and the turbine 49; - b) depressurization of at least one of the following: the gas circuit to be cooled 2, the pre-cooling circuit 18 and the cooling circuit 19, in particular of each of them, for example as long as the flow rate in said circuit is greater than a first determined threshold, such as 500 mbar; - c) circulation of a first defrosting gas through at least a part of the circuit of gas to be cooled 2 to pass at least partially through the first cold box 7 and / or the second cold box 10, in the direction from the upstream end 21 to the downstream end 22, the first defrosting gas consisting mainly of H2, the first defrosting gas preferably containing less than 10 ppb of O2, the first defrosting gas being of preferably at room temperature, for example at a temperature above 0°C.

[0050] In step c), the circulation of the first defrosting gas takes place in the direction going from a hot end of the gas circuit to be cooled 2 to a cold end of the gas circuit to be cooled 2.

[0051] The hot end and cold end of one of the circuits (namely, of the gas circuit to be cooled 2, of the pre-cooling circuit 18 and / or of the cooling circuit 19) are understood to be portions of said circuit at distinct temperatures, the hot end having a temperature higher than that of the cold end, in particular higher by at least 50 °C.

[0052] Step c) may include a step of reducing the flow of the first defrosting gas, between the upstream end 21 and the downstream end 22, in particular by venting part of the first defrosting gas, for example at a portion located between the first cold box 7 and the second cold box 10.

[0053] In one embodiment, the first defrosting gas is from the gas source.

[0054] In one embodiment, step c) is carried out as long as the temperature of the the first de-icing gas at the downstream end 22 is less than a second determined threshold, the second threshold being for example equal to 80 K.

[0055] In one embodiment, step c) is carried out as long as the temperature of the first defrosting gas between the first cold box 7 and the second cold box 10 is below a third determined threshold, the third threshold being for example equal to -20 °C.

[0056] Step c) may include a step for controlling the increase in temperature of the first defrosting gas in the gas circuit to be cooled 2, the increase remaining less than 5 degrees per minute, for example less than or equal to 1 degree per minute.

[0057] In one embodiment, step c) implements successive cycles of pressurization and depressurization, in particular until complete evaporation of hydrogen in liquid form in the gas circuit to be cooled 2.

[0058] In one embodiment, at step c), the first defrosting gas flows from a first connection point 61 of the gas circuit to be cooled 2 upstream of the first cold box 7 to a second connection point 62 of the gas circuit to be cooled 2 downstream of the first cold box 7 and upstream of the second cold box 10.

[0059] Alternatively, in step c), the first defrosting gas flows from the first connection point 61 to a third connection point 63 of the gas circuit to be cooled 2 downstream of the second cold box 10.

[0060] Alternatively, in step c), the first defrosting gas flows from the second connection point 62 to the third connection point 63.

[0061] The process may include a step d) of circulating a second defrosting gas through at least a part of the cooling circuit 19 to pass through at least a part of the first cold box 7 and / or the second cold box 10, in particular in the direction from a hot end of the cooling circuit 19 to a cold end of the cooling circuit 19, the second defrosting gas consisting mainly of H2, the second defrosting gas preferably consisting of less than 10 ppb of O2, the second defrosting gas preferably being at ambient temperature, for example at a temperature above 0°C.

[0062] In one embodiment, step d) includes a step of defrosting the turbine 49 with the second defrosting gas, in particular with a flow rate of the second defrosting gas, lower than that circulating in the cooling circuit 19 before it passes through the turbine 49.

[0063] In one embodiment, step d) includes a de-icing step of an expansion member of a bypass 59 of the turbine 49, with the second de-icing gas.

[0064] In one embodiment, step d) includes a defrosting step of the final expansion device of the cycle gas 39, with the second defrosting gas.

[0065] In one embodiment, step d) includes a step dl) of circulating the second defrosting gas in a high-pressure portion of the cooling circuit 19, in particular located between the outlet of the cycle gas compression member 29 and the inlet of the turbine 49 or between the outlet of the cycle gas compression member 29 upstream of an expansion member of a bypass of the turbine 49, the second defrosting gas circulating through the first cold box 7 without passing through the second cold box, or circulating through the first cold box 7 and through the second cold box, or circulating through the second cold box 10 without passing through the first cold box 7, the second defrosting gas being in particular at least partially discharged upstream of the turbine 49 or upstream of the final expansion member of the cycle gas 39.

[0066] In one embodiment, step d) includes a step d2) of circulating the second defrosting gas in a low-pressure portion of the cooling circuit 19, in particular located between the inlet of the cycle gas compression member 29 and the downstream of the final expansion member of the cycle gas 39, the second defrosting gas circulating through the first cold box 7 without passing through the second cold box, or circulating through the first cold box 7 and through the second cold box, or circulating through the second cold box 10 without passing through the first cold box 7, the second defrosting gas being in particular discharged downstream of the final expansion member of the cycle gas.

[0067] In one embodiment, step d) includes a step d3) of circulating the second de-icing gas in a medium-pressure portion of the circuit of cooling 19, in particular located downstream of turbine 49 and at the level of a bypass of the final expansion unit of the cycle gas.

[0068] In one embodiment, at step dl), the second defrosting gas circulating through the second 10 cold box, the process includes a step of circulating the second defrosting gas through the turbine 49, in particular with a reduction of the flow rate of the second defrosting gas before entering the turbine 49.

[0069] In one embodiment, step e) is accompanied by a step of injecting the third defrosting gas upstream of the pre-cooling fluid compression element 28 and a step of evacuating the third defrosting gas downstream of the final expansion element of the pre-cooling fluid 38.

[0070] In one embodiment, at step dl), the process includes a step of circulating the second defrosting gas through the final expansion member of the cycle gas 39.

[0071] In one embodiment, at step d2) the second defrosting gas flows from a fourth connection point 64 of the cooling circuit 19 located between the first part 5 of the heat exchanger assembly 5, 6 and the inlet of the cycle gas compression element 29, to a fifth connection point 65 of the gas circuit to be cooled 2 located downstream of the first cold box 7 and upstream of the second cold box 10 by passing through the first part 5 of the heat exchanger assembly 5, 6 without passing through the second part 6 of the heat exchanger assembly 5, 6.

[0072] In one embodiment, at step d2), the second defrosting gas flows from the fourth connection point 64 to a sixth connection point 66 of the cooling circuit 19 located between the second part 6 of the heat exchanger assembly 5, 6 and the outlet of the final expansion member of the cycle gas 39, passing through the first part 5 and the second part 6 of the heat exchanger assembly 5, 6 without passing through the final expansion member of the cycle gas 39.

[0073] In one embodiment, at step d2), the second defrosting gas flows from the fifth connection point 65 to the sixth connection point 66 through the second part 6 of the heat exchanger assembly 5, 6 without passing through the first part 5 of the heat exchanger assembly or the final expansion element of the cycle gas 39.

[0074] In one embodiment, at step d1), the second defrosting gas flows from a seventh connection point 67 of the cooling circuit 19 located between the first part 5 of the heat exchanger assembly 5, 6 and the outlet of the cycle gas compression unit 29, to an eighth connection point 68 of the gas circuit to be cooled 2 located downstream of the first cold box 7 and in upstream of the second cold box 10 by passing through the first part 5 of the heat exchanger assembly 5, 6 without passing through the second part 6 of the heat exchanger assembly.

[0075] In one embodiment, at step d1), the second defrosting gas flows from the seventh connection point 67 to the sixth connection point 66 of the cooling circuit 19, passing through both the first part 5, the second part 6 of the heat exchanger assembly 5, 6 and the final expansion element of the cycle gas 39.

[0076] In one embodiment, at step d1), the second defrosting gas flows from the eighth connection point 68 to the sixth connection point 66 of the cooling circuit 19, passing through both the first part 5, the second part 6 of the heat exchanger assembly 5, 6 and the final expansion element of the cycle gas 39.

[0077] In one embodiment, at step dl), the second de-icing gas flows from the seventh connection point 67 or from the eighth connection point 68, to a thirteenth connection point 73 of the cooling circuit 19 located downstream of the turbine 49, the flow rate of the second de-icing gas being lowered for example before it passes through the turbine 49.

[0078] In one embodiment, in step d), the second de-icing gas flows through a bypass 59 of the turbine 49 without passing through the turbine 49, in particular by flowing through the medium pressure portion.

[0079] In one embodiment, at step d) the second defrosting gas circulates through the first part 5 of the heat exchanger assembly 5, 6 and / or the second part 6 of the heat exchanger assembly 5, 6, also passing through the turbine 49 and without passing through the final expansion member of the cycle gas 39 or the compression member of the cycle gas 29.

[0080] In one embodiment, step d) is accompanied by a reduction in the flow rate of the second de-icing gas between the upstream and downstream of the turbine 49, in particular by an evacuation of the second de-icing gas upstream of the turbine 49.

[0081] In one embodiment, in step d), the process includes a step of controlling the increase in the temperature of the cooling circuit 19, the increase remaining less than 5 degrees per minute, for example less than or equal to 1 degree per minute.

[0082] In one embodiment, the second defrosting gas is derived from the gas source.

[0083] In one embodiment, step d) is carried out as long as the temperature of the fluid circulating in the second cold box is less than 80 K.

[0084] The process may include a step e) of circulating a third defrosting gas through the pre-cooling circuit 18, in particular in the direction from a hot end of the pre-cooling circuit 18 to a cold end of the pre-cooling circuit 18, the third defrosting gas consisting mainly of an inert gas, for example N2, in particular at an ambient temperature such as a temperature above 0 °C.

[0085] In one embodiment, step e) is accompanied by a fluidic isolation step of the pre-cooling fluid compression element 28 from the rest of the pre-cooling circuit 18 so that the third de-icing gas cannot circulate through the pre-cooling fluid compression element 28.

[0086] In one embodiment, at step e), the third defrosting gas flows from a ninth connection point 69 of the pre-cooling circuit 18, located upstream of the pre-cooling fluid compression element 28, to a tenth connection point 70 of the pre-cooling circuit 18, located downstream of the final expansion element of the pre-cooling fluid 38.

[0087] In one embodiment, at step e), the precooling fluid compression element 28 is fluidically isolated from the rest of the precooling circuit 18.

[0088] In one embodiment, at step e), the final expansion member of the precooling fluid 38 is fluidically isolated from the rest of the precooling circuit 18.

[0089] The process may include a step f) of circulating a fourth de-icing gas through the turbine 49, the turbine 49 being fluidly isolated from the rest of the cooling circuit 19, the fourth de-icing gas being in particular derived from the second de-icing gas.

[0090] In one embodiment, step a) is performed before step b).

[0091] In one embodiment, steps c), d), e) and f) are carried out after step b).

[0092] In one embodiment, steps c) and e) are started before the start of step d).

Claims

1. Demands De-icing method for an installation (1) for the production of a cryogenic fluid, such as liquefied hydrogen, the installation comprising: - a circuit for the gas to be cooled (2) having an upstream end (21) intended to be connected to a gas source and a downstream end (22) for delivering the cryogenic fluid; - a set of heat exchangers (5, 6) in thermal exchange with the circuit (2) of gas to be cooled; - a pre-cooling device (8) in heat exchange with at least a first part (5) of the heat exchanger assembly (5, 6) and configured to pre-cool the gas circuit to be cooled (2) to a first determined temperature, in particular a temperature close to 80 K, the pre-cooling device (8) comprising a pre-cooling circuit (18) with a refrigeration cycle of a pre-cooling fluid, the pre-cooling circuit (18) comprising a pre-cooling fluid compression element (28) and a final pre-cooling fluid expansion element (38); - a cryogenic cooling device (9) in heat exchange with at least a second part (6) of the heat exchanger assembly (5, 6) and configured to cool the circuit (2) of gas to be cooled to a second determined temperature lower than the first temperature, in particular a temperature close to 20 K, the cryogenic cooling device (9) comprising a cooling circuit (19) with a cycle refrigeration of a cycle gas, the cooling circuit (19) comprising a cycle gas compression element (29), at least one turbine (49) and a final cycle gas expansion element (39); - at least one first cold box (7), in particular perlite-filled and under nitrogen, in which the first part (5) of the heat exchanger assembly is placed, for cooling

2. the gas circuit to be cooled (2) at the first determined temperature; - at least one second cold box (10), separate from the first cold box and in which the second part (6) of the heat exchanger assembly is arranged, to cool the gas circuit to be cooled (2) to the second determined temperature, the second cold box being in particular insulated and under vacuum; The process includes the following steps: - a) shutdown of the installation (1), in particular by stopping at least one of the following: the pre-cooling fluid compression unit (28), the final pre-cooling fluid expansion unit (38), the cycle gas compression unit (29), the final cycle gas expansion unit (39) and the turbine (49); - b) depressurization of at least one of the following: the gas circuit to be cooled (2), the pre-cooling circuit (18) and the cooling circuit (19), in particular of each of them, for example as long as the flow rate in said circuit is greater than a first determined threshold, such as 500 mbar; - c) circulation of a first defrosting gas through at least part of the circuit of gas to be cooled (2) to pass at least partially through the first cold box (7) and / or the second cold box (10), in the direction from the upstream end (21) to the downstream end (22), the first defrosting gas consisting mainly of H2, the first defrosting gas preferably consisting of less than 10 ppb of O2, the first defrosting gas preferably being at ambient temperature, for example at a temperature above 0°C. The method according to claim 1, step c) comprising a step of reducing the flow rate of the first defrosting gas, between the upstream end (21) and the downstream end (22), in particular by venting a portion of the first defrosting gas, for example at a portion located between the first cold box (7) and the second cold box (10).

3. A method according to any one of claims 1 to 2, comprising a step d) of circulating a second defrosting gas through at least a portion of the cooling circuit (19) to pass through at least a portion of the first cold box (7) and / or the second cold box (10), in particular in the direction from a hot end of the cooling circuit (19) to a cold end of the cooling circuit (19), the second defrosting gas comprising predominantly H2, the second defrosting gas preferably comprising less than 10 ppb of O2, the second defrosting gas preferably being at ambient temperature, for example at a temperature above 0°C.

4. Method according to claim 3, step d) comprising a step dl) of circulating the second defrosting gas in a high-pressure portion of the cooling circuit (19), in particular located between the outlet of the cycle gas compression member (29) and the inlet of the turbine (49) or between the outlet of the cycle gas compression member (29) upstream of an expansion member of a turbine bypass (49), the second defrosting gas circulating through the first cold box (7) without passing through the second cold box, or circulating through the first cold box (7) and through the second cold box, or circulating through the second cold box (10) without passing through the first cold box (7), the second defrosting gas being in particular discharged at least partially upstream of the turbine (49) or upstream of the final expansion member of the cycle gas (39).

5. A method according to any one of claims 3 to 4, step d) comprising a step d2) of circulating the second defrosting gas in a low-pressure portion of the cooling circuit (19), in particular located between the inlet of the cycle gas compression member (29) and the downstream of the final expansion member of the cycle gas (39), the second defrosting gas circulating through the first cold box (7) without passing through the second cold box, or circulating through the first cold box (7) and through the second cold box, or circulating through the second cold box (10) without passing through the first cold box (7), the second defrosting gas being in particular discharged downstream of the final expansion member of the cycle gas.

6. A method according to any one of claims 3 to 5, step d) comprising a step d3) of circulating the second de-icing gas in a medium pressure portion of the cooling circuit (19), in particular located downstream of the turbine (49) and at the level of a bypass of the final expansion device of the cycle gas.

7. A method according to any one of claims 1 to 6, comprising a step e) of circulating a third defrosting gas through the pre-cooling circuit (18), in particular in the direction from a hot end of the pre-cooling circuit (18) to a cold end of the pre-cooling circuit (18), the third defrosting gas comprising predominantly an inert gas, for example N2, in particular at an ambient temperature such as a temperature above 0 °C.

8. Method according to claim 7, step e) being accompanied by a step of fluidic isolation of the pre-cooling fluid compression element (28) from the rest of the pre-cooling circuit (18) so that the third de-icing gas cannot circulate through the pre-cooling fluid compression element (28).

9. A method according to any one of claims 1 to 8, comprising a step f) of circulating a fourth de-icing gas through the turbine (49), the turbine (49) being fluidly isolated from the rest of the cooling circuit (19), the fourth de-icing gas being in particular derived from the second de-icing gas.

10. Method according to claim 4, in step dl), the second defrosting gas circulating through the second (10) cold box, the method includes a step of circulating the second defrosting gas through the turbine (49), in particular with a reduction of the flow rate of the second defrosting gas before entering the turbine (49).

Images