Process for treating evaporated gas from a cryogenic liquid storage

Abstract

Title: Process and apparatus for treating evaporated gas from a cryogenic liquid storage. In a process for treating evaporated gas resulting from the vaporization of a cryogenic liquid, during a first period, a first portion (5) of a subcooled cryogenic liquid (3) is sent to at least one storage (S) where a portion of the liquid vaporizes, forming the evaporated gas (1). During a second period, the evaporated gas from at least one storage is sent to the tank of a scrubbing column (C), a second portion (7) of the subcooled cryogenic liquid is sent to the top of the scrubbing column as scrubbing liquid, a liquid (13) is withdrawn from the column tank, and the liquid is sent to a storage through a loading line (21). Abstract figure: Fig 1

Description

Title of the invention: Method for treating a gas evaporated from a cryogenic liquid storage

[0001] The present invention relates to a method for treating a gas evaporated from a cryogenic liquid storage. The cryogenic liquid can be carbon dioxide, carbon monoxide, LPG, LNG, nitrogen, oxygen, helium or hydrogen, for example.

[0002] Boil-off gas (BOG) can be generated in a cryogenic liquid storage in various ways: - Heat inputs generate vapor if the liquid is stored at a temperature below ambient temperature - If no liquid is withdrawn from the storage tank, the liquid entering it will pressurize the vapor phase within the tank due to its volume. In order to maintain constant pressure within the tank, vapor will need to be removed. - Depending on the pressure and temperature of the liquid, the expansion of the incoming liquid can lead to its partial vaporization, which generates additional vapor in the storage. When the stored liquid is transferred to a means of transport (truck, ship, train, etc.), specific pipes and equipment must be used, such as pumps and liquid loading lines. Since this equipment is not in continuous use, it must be kept cool by a cryogenic liquid circulating in at least one pump and / or at least one line to maintain its temperature or to cool it before loading the means of transport. In both cases, gases are generated. Furthermore, the means of transport themselves must be cooled before loading or after they have warmed up. For expensive fluids such as hydrogen, or those whose loss poses health or environmental problems such as CO2, steam present in a transport vehicle can be recycled to cryogenic liquid storage to compensate for liquid withdrawal. For example, a truck returning from delivering a load of liquid may still contain vaporized liquid. However, this steam is rarely available at equilibrium temperature. Injecting hot steam from the transport vehicles into the storage facility generates evaporated gases and can damage the equipment.

[0003] The management of evaporated gases from a cryogenic liquid storage poses several problems: • their flow rate and / or composition are highly variable and make their liquefaction complicated. • They are available at low pressure and / or low temperature, below the feed pressure of the liquefier from which the stored liquid originates. A dedicated compressor is expensive, especially at low temperatures, as the compressor materials must be high-grade. The compressor must also be able to handle variable flow rates.

[0004] Simply subcooling the liquid to be sent to storage is not a sufficient solution (for example, CO2 produced by a liquefier is generally subcooled by 2°C). This can cause stratification and reduce heat exchange. Furthermore, most of the sensible heat of the fluid is transferred to the liquid phase rather than the gaseous phase. In this case, if a thermal bridge exists in the storage, local vaporization will not be prevented by spraying liquid into the liquid phase.

[0005] Several causes of evaporation are independent of the arrival of liquid (for example, heat inputs into storage), so subcooling does not solve this problem.

[0006] The solution of the present invention has at least one of the following advantages: • It increases the efficiency of subcooling. • It deals with evaporations occurring outside of storage. • It provides sufficient cooling to the storage area to eliminate evaporation gases and avoid costly and complex treatment or recycling.

[0007] WO2016 / 153334 describes a process in which gas evaporated from a storage is sent to the washing column tank (C) containing means for transferring heat and matter, subcooled cryogenic liquid is sent to the top of the washing column as washing liquid and gas is withdrawn from the top of the column.

[0008] According to one object of the invention, a process is provided for treating an evaporated gas resulting from the vaporization of a cryogenic liquid in which:

[0009] i) During a first period, a first portion of a subcooled cryogenic liquid is sent to at least one storage unit where a portion of the liquid vaporizes, forming the evaporated gas; no portion of the subcooled liquid is sent to a scrubbing column, and no gas is sent to the scrubbing column tank.

[0010] And during a second period

[0011] ii) Gas evaporated from at least one storage and / or gas evaporated resulting from the vaporization of a cryogenic liquid from another source is sent to the tank of the washing column containing means for transferring heat and matter without having compressed the gas,

[0012] iii) A second part of the subcooled cryogenic liquid is sent to the top of the washing column as a washing liquid

[0013] iv) No gas is withdrawn from the top of the column

[0014] v) A liquid is drawn from the column tank

[0015] And during the second or a third period, liquid is sent from the column tank to a storage and / or to at least one means of transport through a loading line, where appropriate, the other source of step ii) when present being the at least one means of transport or the loading line.

[0016] According to other optional features: • during at least the second period, the liquid withdrawn from the column tank is pressurized by a pump and at least part of the pressurized liquid is returned to the column. • during the second period, liquid from at least one storage is sent to the column tank, preferably to a level below the gas inlet of step ii). • the cryogenic liquid contains at least 90%, preferably 95% or even 99% of a main component and the evaporated gas and / or gas from another source contains at least 80% mol of the main component. • cryogenic liquid is sent directly from a source from which the first part of the cryogenic liquid originates to at least one means of transport without passing through at least one storage or column. • the second part of the liquid arrives at the top of the column without having been stored in at least one storage. • gas from the top of the column is sent to at least one storage if, preferably only if, the pressure of at least one storage is below a threshold. • The flow rate of liquid withdrawn from the column tank is regulated in order to maintain a constant liquid level in the column tank. • the flow rate of the second part of liquid sent to the column is regulated to be substantially proportional to the flow rate of gas sent to the column tank. • the evaporated gas is sent from the storage to the column through a pipe whose pressure is regulated to be below the nominal pressure of at least one storage at least during the second period. • The subcooled cryogenic liquid has a main component which is oxygen, nitrogen, carbon dioxide, carbon monoxide, argon, methane, hydrogen or helium. • The evaporated gas sent to the column has a main component which is the same as the cryogenic liquid and is oxygen, nitrogen, carbon dioxide, carbon monoxide, argon, methane, hydrogen or helium. • Subcooled liquid is sent to the loading line in the absence of a means of transport to load liquid from the column tank. • The subcooled liquid vaporizes in the loading line and is returned to the column tank.

[0017] The invention will be described in more detail with reference to the figure.

[0018] [Fig-1] represents a process for treating evaporated gas according to the invention.

[0019] A liquefier produces a flow rate 3 of subcooled liquefied gas, which may be carbon dioxide, carbon monoxide, LPG, LNG, nitrogen, oxygen, argon, methane, helium or hydrogen.

[0020] A portion 5 of the liquefied gas feeds a storage tank through valve V for a first period. Due to heat inputs and / or the piston effect, evaporated gas forms above the liquid level in the storage tank S.

[0021] During this first period, no liquid and no gas are sent to a washing column C containing mass and heat transfer means, such as trays or packings, for example structured packings.

[0022] According to one embodiment, if the pressure in the storage tank exceeds a threshold, this evaporated gas 1 can be discharged through valve V2. To treat this evaporated gas 1 by reliquefying it, during a second period, the gas 1 is sent to a pipe, the pressure of which varies but cannot exceed the nominal pressure of the storage tank S. The gas 1 passes through valve VI and arrives at the bottom of a scrubbing column C. The gas rises in the column C and is scrubbed by a portion 7 of the liquefied gas 3, which is sent to the top of the column C during the second period as a scrubbing liquid. The column does not contain an overhead condenser or a tank reboiler.

[0023] No gas is withdrawn from the top of column C during the second period. Indeed, the subcooling at the liquefier and the scrubbing flow rate are sufficient to recondense gas 1, or even all the evaporation gases.

[0024] A liquid 13 is drawn from the tank of column C during the second period, is pressurized by a pump P and at least intermittently (during the second or a third period), is sent via pipe 21 and valve V8 to a means of transport L, which may be a train, a ship or a tanker truck, or to a storage facility L. Part 14 of the pumped liquid is sent continuously to column C to maintain pump P at temperature via valve Vil.

[0025] According to another embodiment, column C can be supplied to the tank by a gas 23, which is an evaporated gas resulting from the vaporization of the column's tank liquid 13. Gas 23 can, for example, be a gas produced in the transport or storage means L. Alternatively, gas 23 can result from the vaporization of a cryogenic liquid in a pump and / or a pipe through which the liquid circulates, for example, to cool the pump and / or the pipe before loading the transport means through the pump and / or the pipe. In both cases, gases are generated. Furthermore, the transport means themselves must be cooled before loading or after being heated. Obviously, gas 1 can also be supplied to the column at the same time as gas 23.

[0026] In order to keep the liquid supply lines 21 cold when no transport means L is present, it is desirable to send a fraction 26 of the subcooled liquid 3 through the open valve V9 to these lines to cool them. As the lines cool, the subcooled liquid partially vaporizes, producing evaporated gas which is sent to the column C tank to condense there by passing through line 15, with valve VIO open.

[0027] The pressure in the storage is regulated by three valves: a depressurization valve VI, an air release valve V2 and a repressurization valve V3.

[0028] The setpoint of the depressurization valve V1 is set to the nominal pressure of the storage S and allows gas 1 to be sent to the column C. In this way, it allows the pressure in the storage to be regulated without losing the molecules stored there, since they will be condensed in the column C.

[0029] The vent valve V2 protects the equipment and has a higher setpoint than that of valve V2 to send gas 1 to the air if the opening of valve VI is not sufficient to reduce the pressure in storage S.

[0030] If the pressure in storage S falls below the pressure in the gas line 1 between the storage and the column C, steam 9 is returned to storage S through valve V3. This prevents the line between storage S and column C from being at a higher pressure than storage S.

[0031] The second portion 7 of subcooled liquid 3 sent to the top of column C is regulated by a valve V4 which diverts liquid from the storage inlet. The second portion 7 sent to column C is maintained proportional to the flow rate 1 or 23, or to the combined flow rate of 1 and 23, when both flow rates are sent to column C. It is also possible to adjust the flow rate of the second portion 7 according to the pressure of gas 1 to maintain the pressure of the gas line 1 below that of the storage S.

[0032] A V5 vent valve on the gas 11 coming from the head of the column allows gas to be sent to the air in case of malfunction for example in case of incomplete recondensation, or when the upstream liquefier is stopped and cannot supply subcooled liquid to storage.

[0033] Subcooled liquid and recondensed vapor accumulate in the tank of column C and are drawn off as liquid 13 by the pump P which is used to load the transport means L. To maintain the constant liquid level in the tank, a valve V6 can send liquid 17 to the storage S.

[0034] When loading the liquid into a means of transport, the vapor formed in the means of transport or the liquid supply lines can represent a large volume. The liquid from the storage tank must be loaded into the means of transport.

[0035] According to one variant, all liquid from storage S must pass through column C to reach pump P. Pipe 19 brings liquid through valve V7 into tank of column C below gas inlet point 1, 23.

[0036] However, this implies oversizing the column C and therefore at least part of the liquid from the storage can be sent to the pump P without passing through the column.

[0037] The sending of liquid 19 into the tank of column C is regulated by this valve V7 and a valve V8 maintains constant the level of the tank by regulating the flow rate charged.

[0038] Vapor formed by vaporizing the liquid in the transport means L is sent to the line carrying the gas 1 and then to column C. However, not all the vapor formed can be reliquefied in this way. The cooled gas is therefore sent through valve V3 to storage S to compensate for the withdrawn liquid. In this way, the cooled vapor is sent to the storage tank, and the tank is disturbed as little as possible.

[0039] By sending all the evaporated gases to a secondary network including a scrubbing column to recondense them, this process makes it possible to regulate the pressure of the storage.

[0040] This process is well suited to multiple storage systems. All storage units corresponding to storage S can send their evaporated gases into a common pipe. The evaporated gases will then be sent via the common pipe to a common scrubbing column.

Claims

Demands

1. A method for treating an evaporated gas resulting from the vaporization of a cryogenic liquid in which: i) During a first period, a first portion (5) of a subcooled cryogenic liquid (3) is sent to at least one storage (S) where a portion of the liquid vaporizes to form the evaporated gas (1), no portion of the subcooled liquid is sent to a scrubbing column and no gas is sent to the scrubbing column tank. And during a second period, ii) Evaporated gas from the at least one storage and / or evaporated gas (23) resulting from the vaporization of a cryogenic liquid from another source (21, L) is sent to the scrubbing column tank (C) containing heat and mass transfer means without having compressed the gas.iii) A second portion (7) of the subcooled cryogenic liquid is sent to the top of the scrubbing column as scrubbing liquid. iv) No gas (11) is withdrawn from the top of the column. v) A liquid (13) is withdrawn from the column tank. And during the second or third period, liquid (13) is sent from the column tank to a storage and / or to at least one means of transport (L) through a loading line (21), where applicable, the other source from step ii) where present being at least one means of transport or the loading line.

2. A method according to claim 1 wherein during at least the second period, the liquid (13) withdrawn into the tank from the column (C) is pressurized by a pump (P) and at least a part of the pressurized liquid (14) is returned to the column.

3. A method according to claim 1 or 2 wherein during the second period, liquid (19) from at least one storage (S) is sent into the tank of the column (C), preferably at a level below the gas inlet (15, 23) of step ii).

4. A method according to any one of the preceding claims wherein the cryogenic liquid (5,7,19) contains at least 90%, preferably 95% or even 99%, of a principal component and the evaporated gas (1) and / or the gas (15, 23) from another source contains at least 80% mol of the principal component.

5. A method according to any one of the preceding claims wherein cryogenic liquid is sent directly from a source from which the first part (5) of the cryogenic liquid originates to at least one means of transport without passing through at least one storage (S) or column (C).

6. A method according to any one of the preceding claims wherein the second part of the liquid (7) arrives at the top of the column without having been stored in at least one storage (S).

7. A method according to any one of the preceding claims wherein gas from the column head is sent to at least one storage if, preferably only if, the pressure of at least one storage is below a threshold.

8. A method according to any one of the preceding claims wherein the flow rate of liquid (13) withdrawn from the column (C) tank is regulated in order to maintain a constant level of liquid in the column tank.

9. A method according to any one of the preceding claims wherein the flow rate of the second part (7) of liquid sent to the column (C) is regulated to be substantially proportional to the flow rate of gas (1,15,23) sent to the column tank.

10. A method according to any one of the preceding claims wherein the evaporated gas (1) is sent from the storage (S) to the column (C) through a pipe whose pressure is regulated to be lower than the nominal pressure of at least one storage at least during the second period.

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