JP2003232498A - Vent gas treating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent heavy content included in vent gas in a tank, from being sucked into boiloff gas piping when recovering the vent gas produced in a facility for treating low-temperature liquefied gas such as LNG, to return the vent gas to a low-temperature liquefied gas storage tank. <P>SOLUTION: In recovering the vent gas produced in the facility for treating low-temperature liquefied gas, to return the vent gas to the low-temperature liquefied gas storage tank 2, the vent gas from vent gas piping 6 is led into an LNG receiving pipe 1, for instance, and the vent gas is returned to the tank 2 in the state of being liquefied or made into low-temperature multiphase flow of liquid and gas using the cold energy of LNG. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vent gas treatment method for recovering vent gas generated in a facility that handles low temperature liquefied gas such as LNG and returning it to a tank for storing low temperature liquefied gas.

[0002]

2. Description of the Related Art FIG. 4 shows an example of a gas treatment flow sheet in a facility handling low-temperature liquefied gas such as LNG. The LNG carried by the LNG tanker is stored in the LNG tank 2 installed in the facility via the receiving pipe 1. An LPG tank 2P is also installed in the facility, and the LPG transported by the LPG tanker is stored in the LPG tank 2P via the receiving pipe 1P. Stored LNG is pumped out by pump and pipe 3
To be sent to the LPG tank 2P
After G is added and the amount of heat is adjusted, it is sent to the vaporizers 4 installed in parallel to be vaporized and sent as city gas.

In the LNG tank 2, boil-off gas (BOG) is generated by heat input. It is necessary to discharge boil-off gas in order to maintain the inside of the tank at a predetermined pressure. Therefore, boil-off gas pipe 11 equipped with a compressor 12 in the middle
Is provided, the boil-off gas is passed therethrough to be boosted in pressure, and after that, the amount of heat is adjusted by the addition of LPG delivered from the LPG tank 2P, and the boil-off gas is delivered as city gas. LNG itself is a multi-component carbide fluid, but most of the boil-off gas is methane because of its boiling point.

Even when there is no LNG supply from the tanker,
In order to maintain the LNG receiving piping system in a predetermined low temperature environment, a cold insulation circulation pipe 5 is provided, and a part of the LNG in the LNG tank 2 pumped up by the pump passes through the cold insulation circulation pipe 5. Is sent to the upstream side of the LNG receiving pipe 1. On the LPG tank 2P side, a similar cold insulation circulation pipe 5P is provided.

As described above, in the LNG facility, a small amount of gas is generated for convenience of operation or in the process of purging various facilities and composition analysis (in this specification, this gas is generically referred to as "vent gas"). "). It is desirable from the environmental and economic standpoint to collect this vent gas without discarding it and to return it to the appropriate tank 2 for LNG storage. For example, when a part of a plurality of carburetors 4 installed is stopped due to load fluctuation or for the purpose of equipment inspection, the gas accumulated in the piping system downstream from the shutoff valve is normally used as a vent gas at room temperature. As shown in FIG. 4, the gas is returned to the low temperature gas layer in the LNG storage tank 2 via the vent gas pipe 6. In an actual LNG facility,
Vent gas piping is installed in more places,
Everything is open to tank 2.

[0006]

FIG. 5 shows a conventional LNG.
It is a conceptual diagram showing a main part of the facility, and the LNG tank 2 is shown in a vertical cross section. As shown in the figure, in the gas phase portion of the tank 2, there are an inlet 11a into which the vent gas flows into the boil-off gas pipe 11 by suction by the compressor 12, and an inlet 6a for the vent gas from the vent gas pipe 6. It is open. For the above reason, the vent gas returned from the vent gas inlet 6a into the gas phase of the LNG tank 2 is not a single component of methane but a multi-component system of C2H6.
It contains the above heavy objects.

If the room temperature vent gas from the vent gas inlet 6a is sent to the low temperature gas layer of the LNG storage tank 2, the heavy component is cooled and becomes a mist component. When these are sucked into the boil-off gas inflow port 11a, the vent gas containing the mist component is further cooled in the boil-off gas pipe 11 and its heavy component is condensed and accumulated,
This may be an obstacle in operating the compressor 12 for boil-off gas treatment. For this reason, it is necessary to periodically withdraw the heavy components accumulated in the boil-off gas pipe 11. In particular, in the equipment shown in FIG. 4, since the heat quantity is adjusted by the LPG before entering the vaporizer, the vent gas generated there may contain a larger amount of heavy components than the pure methane component. Conceivable.

On the other hand, if the vent gas inlet 6a opened in the tank 2 and the boil-off gas inlet 11a are arranged so as to be sufficiently infinite, the intake of the vent gas into the boil-off gas pipe 11 can be suppressed. I can, but
In many cases, it is difficult to place the two in a limited facility so that they are sufficiently separated from each other, and if the vent gas floats in the gas phase, it may be sucked into the boil-off gas pipe 11 even if the distance is sufficiently separated. However, it is hard to say that it is a desirable solution.

The present invention has been made in view of the above circumstances, and its main purpose is to almost completely prevent the heavy components contained in the vent gas from being sucked into the boil-off gas pipe inside the tank. It is to provide a vent gas treatment method that can be suppressed.

[0010]

In order to achieve the above object, in the present invention, basically, vent gas generated in a facility handling low temperature liquefied gas is recovered and returned to a tank for storing low temperature liquefied gas. In this case, the vent gas is returned to the storage tank in a state of being liquefied by using the cold heat of the low temperature liquefied gas or in a low temperature mixed phase flow of liquid and gas.

According to this, since the vent gas returned to the storage tank is returned to the tank as a low temperature liquid phase flow or mixed phase flow, not as a room temperature gas phase flow, the low temperature gas in the LNG storage tank is discharged. No mist is generated in the bed, and vent gas containing mist is almost completely prevented from being sucked into the boil-off gas pipe. Therefore, the heavy components contained in the vent gas do not enter the boil-off gas pipe, and the heavy components condense in the boil-off gas pipe, causing an obstacle in operating the compressor for boil-off gas treatment. Things can be avoided. Further, the work of processing heavy components can be omitted.

There is no particular limitation on the mode of utilizing the cold heat of the low-temperature liquefied gas to liquefy the vent gas or to make it into a low-temperature mixed-phase flow of liquid and gas. May be Typically, the vent gas may be cooled by allowing the vent gas to flow into the low temperature liquefied gas receiving pipe and directly contacting the vent gas with the low temperature liquefied gas flowing through the receiving pipe. A heat exchanger may be arranged between the pipe and / or the delivery pipe and the vent gas pipe to indirectly exchange heat between the low temperature liquefied gas and the vent gas to cool the vent gas.

Further, an open branch pipe is branched into a storage tank from the low-temperature liquefied gas receiving pipe and / or the discharge pipe, and a heat exchanger is arranged between the branch pipe and the vent gas pipe. You may make it perform a required heat exchange. Further, a vent gas pipe of a required length is arranged in the tank for storing the low temperature liquefied gas, and heat exchange is performed between the vent gas flowing in the pipe and the low temperature atmosphere caused by the cold heat of the low temperature liquefied gas in the tank. It may be performed.

In the present invention, it is the most preferable mode that the vent gas is returned to the tank in a completely liquefied state by being cooled by the cold heat of the low temperature liquefied gas such as LNG. Even if a part of the gas exists in the gas phase, that is, in a low-temperature mixed-phase flow of liquid and gas, it is possible to significantly suppress the heavy components contained in the vent gas from entering the boil-off gas pipe. confirmed.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Referring to the accompanying drawings,
The vent gas treatment method according to the present invention will be described in detail. FIG. 1 is a first embodiment, and in this embodiment, a vent gas is caused to flow into a low temperature liquefied gas receiving pipe 1 and the low temperature liquefied gas flowing through the receiving pipe 1 is directly contacted with the vent gas to cool the vent gas. I am trying. The tank 2 is a tank for storing LNG, and the LNG carried by the LNG tanker is sent through the receiving pipe 1. The gas phase portion of the tank 2 is provided with an inflow port 11a through which boil-off gas flows into the boil-off gas pipe 11. Even when there is no LNG supply from the tanker, the LNG receiving pipe system is maintained in a predetermined low temperature environment. To keep
A cold circulation pipe 5 is provided. Note that this configuration is the same as that described in FIG. Further, in FIG. 1, the delivery pipe 3 system is omitted.

In the above embodiment, the vent gas inlet side 61 of the vent gas pipe 6 described with reference to FIG. 5 is connected not to the gas phase portion of the tank 2 but to the receiving pipe 1. As a result, the vent gas sent from the vent gas pipe 6 flows into the receiving pipe 1 and L flows there.
It is cooled while becoming a mixed phase flow with NG and completely liquefied, or becomes a low temperature mixed phase flow state of liquid and gas and flows into the gas phase part of the tank 2. Therefore, it is possible to effectively prevent the vent gas containing the heavy mist component from flowing into the boil-off gas pipe 11 from the inlet 11a, and even if the vent gas contains a large amount of the heavy component, Heavy matters do not condense or accumulate in the piping system of the boil-off gas piping 11.

In the illustrated example, a check valve 62 is provided in the vent gas pipe 6 to avoid the risk of LNG entering the vent gas pipe 6 when the vent gas is not fed. Further, it is preferable to connect the vent gas inlet side 61 of the vent gas pipe 6 at a location where the receiving pipe 1 is running in the horizontal direction, and it has been experimentally confirmed that the liquefaction of the vent gas reliably proceeds. . FIG. 2 shows a second embodiment. In this embodiment, the LNG gas and the vent gas are passed through the heat exchanger 40 arranged between the branch pipe 31 branched from the low temperature liquefied gas delivery pipe 3 and the vent gas pipe 6. Indirect heat exchange is carried out between and to cool the vent gas.

The tank 2 is the same as that described with reference to FIG. 5, and the storage LNG is sent to the vaporizer 4 described with reference to FIG. The gas phase portion of the tank 2 is provided with an inlet 11a through which boil-off gas flows into the boil-off gas pipe 11, and although not shown, a receiving pipe 1 and a cold circulation pipe 5 are also provided. In this aspect, the payout pipe 3 to the branch pipe 3
1 is branched, and the branch pipe 31 is open to the gas phase portion of the tank 2. The branch pipe 31 passes through an appropriate heat exchanger 40 as a heat radiation side, and the vent gas pipe 6 passes through the heat receiving side of the heat exchanger 40. The vent gas inlet end 61 of the vent gas pipe 6 is open to the gas phase part of the tank 2, and the vent gas cooled by heat exchange becomes a liquefied or low-temperature mixed phase flow of liquid and gas to form a gas phase part of the tank 2. Flow into.

Although not shown, the heat radiation side of the heat exchanger 40 is
It is not limited to the branch pipe 31 branched from the delivery pipe 3 described above, and any pipe system through which LNG passes can be used. A branch pipe may be branched from the receiving pipe 1, and in some cases, the receiving pipe 1 or the payout pipe 3 itself may be used.

FIG. 3 shows a third embodiment. In this embodiment, heat exchange is performed between the vent gas flowing in the pipe 62 arranged in the tank 2 and the low temperature atmosphere in the tank 2, The vent gas is cooled. In this mode, the tank 2 is the same as the conventional one described in FIG. 5, but the vent gas inlet side 61 of the vent gas pipe 6 is
Is connected to a pipe 62 for heat exchange, which is arranged in the gas phase portion of the tank 2 and has an appropriate heat transfer area capable of lowering the temperature of the ambient temperature vent gas. The vent gas from the vent gas pipe 6 is cooled by the cold heat in the tank 2 by passing through the heat exchange pipe 62, and becomes a liquid single-phase flow or a low-temperature mixed-phase flow and flows into the gas phase part of the tank 2. To do. In addition, in the tank 2 shown in FIG.
Illustration of the system and the cold circulation circuit 5 system is omitted.

[0021]

As described above, according to the present invention, when the vent gas generated in the facility handling the low temperature liquefied gas is recovered and returned to the tank for storing the low temperature liquefied gas, the heavy gas contained in the vent gas inside the tank is recovered. It is possible to almost completely prevent the minute from being sucked into the boil-off gas pipe.
As a result, a situation where the operation of the compressor for boil-off gas treatment is impaired is effectively avoided.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing a main part of an LNG facility to which an aspect of a vent gas treatment method according to the present invention is applied.

FIG. 2 is a conceptual diagram showing a main part of an LNG facility to which another aspect of the vent gas treatment method according to the present invention is applied.

FIG. 3 is a conceptual diagram showing a main part of an LNG facility to which still another aspect of the vent gas treatment method according to the present invention is applied.

FIG. 4 is a diagram showing an example of a gas treatment flow sheet in a facility handling low-temperature liquefied gas such as LNG.

FIG. 5 is a conceptual diagram showing a main part of a conventional LNG facility.

[Explanation of symbols]

1 ... LNG receiving pipe, 2 ... LNG storage tank, 3
... Delivery pipe, 5 ... Cooling circulation pipe, 6 ... Vent gas pipe, 61 ... Vent gas inlet side, 11 ... Boil-off gas pipe, 11a ... Boil-off gas inlet

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Claims (5)

[Claims] 1. A vent gas treatment method for recovering vent gas generated in a facility handling low temperature liquefied gas and returning it to a tank for storing low temperature liquefied gas, wherein the vent gas is liquefied or liquefied by utilizing cold heat of the low temperature liquefied gas. A method for treating vent gas, characterized in that the mixed gas is returned to a storage tank in a low-temperature multiphase flow. 2. The vent gas according to claim 1, wherein the cold heat of the low-temperature liquefied gas is utilized by flowing a vent gas into a pipe for receiving the low-temperature liquefied gas and directly contacting the low-temperature liquefied gas with the vent gas. Processing method. 3. The cold heat of the low temperature liquefied gas is utilized through a heat exchanger arranged between the low temperature liquefied gas receiving pipe and / or the discharge pipe and the vent gas pipe. Vent gas treatment method. 4. A heat exchanger in which the utilization of cold heat of low-temperature liquefied gas is arranged between a branch pipe opened to a storage tank branched from a low-temperature liquefied gas receiving pipe and / or a discharge pipe and a vent gas pipe. The vent gas treatment method according to claim 1, wherein the vent gas treatment method is performed via 5. The cold heat of the low temperature liquefied gas is utilized by heat exchange between a vent gas flowing in a pipe arranged in a tank for storage and a low temperature atmosphere in the tank. The described vent gas treatment method.