JPH02240499A - Evaporated gas processing for liquefied natural gas storage facility - Google Patents

Abstract

PURPOSE:To permit effective liquefaction in a easy-to-operate and low-cost facility by having at least one type of C2-C4 saturated hydrocarbon added to pressurized evaporated-gas and then heat-exchanged with liquefied natural gas from a storage facility to liquefy the evaporated gas. CONSTITUTION:BOG which is given out of a LNG storage tank 1 is pressurized by a BOG compressor 2 to add thereto at least one type of saturated hydrocarbon, such as ethane, propane or butane, led through a heavy-gravity lead line 9 and sent to a heat exchanger 3. On the other hand, LNG which is pressurized up by a LNG pump 6 through the LNG storage tank 1 is led into the heat exchanger 3. Then, heavy-gravity added BOG is cooled and liquefied for storage in a drum 4 and, after returned to the tank 1 or pressurized up by a pump 7, jointed to the LNG which is extracted through the heat exchanger 3, to be sent to consumers through a sending gas pipe system via a LNG carbureter 5. As a result, it is possible to perform effective liquefaction in a easy-to-operate and low-cost facility.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to liquefied natural gas (hereinafter referred to as LNG).

) evaporated gas (hereinafter referred to as BOG) in a storage facility.

). More specifically, the present invention relates to efficiently liquefying BOG by adjusting BOG components.

[Conventional technology]

LNG is currently being used in extremely large quantities as a clean energy source. In Japan, most of the LNG is imported, and is stored in insulated low-temperature storage tanks at receiving LNG terminals. In this case LNG
Because the composition consists of low boiling point components such as methane, evaporation due to external heat input during storage is unavoidable, and a large amount of BOG is generated. This BOG is normally treated by mixing it into the gasified LNG delivery gas piping system and sending it to the demand destination.

As the delivery gas pressure increases, the power consumption of the BOG compressor increases. Therefore, various methods have been proposed for processing BOG in order to reduce power consumption, and there are two main methods: direct contact with LNG and absorption and liquefaction, and liquefaction by heat exchange with LNG through a heat exchanger. are categorized.

For example, Japanese Patent Application Laid-Open No. 55-145897 describes BOG as BOG.
After increasing the pressure in a compressor, it exchanges heat with LNG in a heat exchanger and liquefies it, then merges with LNG and sends it to a vaporizer. In addition, JP-A No. 57-43095 discloses that BOG is depressurized by a BOG compressor, supplied into a pressurized holder, and LNG is sprayed into the holder to bring the BOG into direct contact with the LNG, thereby cooling and liquefying the BOG. and stored in the pressurized holder.

Furthermore, JP-A-62-142980 discloses a method in which BOC is liquefied by a combination of two-stage compression using a compressor, a heat exchanger using LN (1) as a refrigerant, and an expansion valve, and then returned to a storage tank.

[Problem to be solved by the invention]

All of the above-mentioned conventional methods liquefy BOG, but the liquefaction system becomes complicated and the number of equipment increases, making operations less easy and increasing equipment costs. For this reason, the inventors conducted a new study on BOG processing, which is easy to operate and does not require high equipment costs, from a completely different perspective from conventional methods, and arrived at the present invention.

That is, LNG is the most effective refrigerant for liquefying BOG at LNG terminals.

As mentioned above, in order to liquefy BOG, it is necessary to pressurize BOG to some extent with a compressor and then apply latent heat of liquefaction to BOG with a heat exchanger. On the other hand, LNG itself, which serves as a refrigerant, exchanges heat in the heat exchanger mostly in the form of sensible heat, so the amount of LNG required to liquefy BOG is usually several times the amount of BOG processed.
It becomes 10 times.

The LNG used for this BOG liquefaction will ultimately be vaporized in a vaporizer and sent to the delivery gas piping system. However, the demand for LNG gas fluctuates depending on various factors such as time and season, and an increase in the amount of LNG extracted for BOG liquefaction may hinder the operation of LNG terminals.
It is desirable to keep the amount of LNG for BOG liquefaction as small as possible.

The present invention adds a higher boiling point component, that is, a heavier component than the BOG component to pressurized BOG to more efficiently utilize the cold energy of LNG,
This is the simplest conventional BOG liquefaction method that uses an OG compressor and a heat exchanger, which reduces the amount of LNG used as a refrigerant and provides effective liquefaction.

[Means to solve the problem]

According to the present invention, in the treatment of evaporative gas generated from a liquefied natural gas storage facility, the evaporative gas is pressurized, at least one of 02 to C4 saturated hydrocarbons is added to the pressurized evaporative gas, and then the evaporated gas is stored. Provided is a method for treating evaporated gas from a liquefied natural gas storage facility, which comprises liquefying the evaporated gas by exchanging heat with liquefied natural gas from the facility.

〔Example〕

The present invention will be explained in detail by way of examples based on the drawings.

FIG. 1 is a system diagram of an embodiment applied to an LNG terminal. The BOG generated in the LNG storage tank is pressurized to approximately 5 Kg/cm" by the BOG compressor 2, and then compressed to at least one of saturated hydrocarbons such as ethane, propane, and butane.
After the seeds are introduced through the heavy fraction introduction line 9, they are sent to the heat exchanger 3. In this case, the amount of these hydrocarbons added is LN
The composition of BOG changes depending on the composition of G, the LNG storage tank, the LN'G base environment, etc., so it cannot be absolutely specified, but - for boat fishing, the ratio of heavy components in BOC after adding heavy components. is 1 to 3% by volume (hereinafter, % is % by volume).

) is preferably added. In this case, the composition of the heavy component added is LN used as a refrigerant.
Although it is selected appropriately depending on the G composition, pressure, size and structure of the heat exchanger, etc., liquefied petroleum gas (LPG) is usually selected.
) is used. Further, the heavy component to be added may be either liquid or gaseous, but it is desirable that the entire amount be evaporated into BOC at the inlet of the heat exchanger. The heavy content added BOG sent to the heat exchanger 3 is extracted from the tank 1 and converted into LNG.
After being liquefied by heat exchange with LNG pressurized by the pump 6, it is stored in the drum 4.

The stored liquefied BOG is returned to the tank 1, or after being pressurized by the liquefied BOG pump 7, it joins the extracted LNG through the heat exchanger 3, and is sent to the LNG vaporizer 5 where it is vaporized, and then converted into the delivery gas. The gas is sent through a piping system 8 to power plants and other gas demand destinations.

In the above example, the LNG required when 55 Kg/hr of propane was added as a heavy component to liquefy 137 ONm3/hr of BOG to obtain 1 ton of liquefied BOG
was 4.4 tons. On the other hand, when propane was not added, 5.4 tons of LNG were required to similarly obtain 1 ton of liquefied BOG under the same conditions.

The operation of the method of the present invention configured as in the above embodiment will be explained with reference to the BOG cooling curve and LNG temperature rising curve in the heat exchanger shown in FIG.

In Figure 2, BO consists of 97% methane and 3% nitrogen.
G is cooled along the BOG cooling curve of A'-B'-C', and liquefaction begins at point B' and ends at point C'. BOG is composed of methane and nitrogen, which are the heavy components of LNG, although there are some differences depending on the composition of the stored LNG. Most of the BOG is methane paper containing about 2 to 10% nitrogen, and it is a methane paper that contains methane and nitrogen, which are the heavy components of LNG such as ethane and propane. Only a small amount may be included.

On the other hand, LNG used to cool BOG is generally nitrogen 0°04~
0.15%, methane 82-99.8%, ethane 0.07
~16%, propane O~4%, and butane 0~1.5%, while giving cold heat to BOG, temperature rise curve D''-E in Figure 2
The temperature is increased along '-F'.

In this case, in order to liquefy BOG, the BOG cooling curve and L
The amount of LNG supplied to the heat exchanger 3 is adjusted according to the amount of BOG generated so that the NG temperature rise curve does not intersect.

In the method of the present invention, propane is added to BOG as in the above example, nitrogen is 2.9%, methane is 95.1%,
In the case of BOG with 2% propane, the cooling curve A-B
-C, liquefaction starts at point B and ends at point 0.

When LNG for refrigerant is supplied with the same composition as above,
As is clear from FIG. 2, the temperature difference between D and F in the temperature rise curve D-E-F can be larger than the temperature difference between D' and F, and the method of the present invention efficiently utilizes the cooling energy of LNG. It turns out that it is available.

(Effects of the Invention) The present invention does not send BOG generated from LNG storage equipment directly to the delivery gas piping system, but instead liquefies it through a heat exchanger. There is no increase in BOG compressor power due to an increase in delivery gas pressure, and the amount of LNG required for BOG liquefaction can be reduced, and the minimum delivery gas to the delivery gas piping system can be reduced. Furthermore, it requires less equipment and equipment, reducing equipment costs, and the processing operation is simple, making it extremely useful industrially.

[Brief explanation of drawings]

FIG. 1 is a system diagram showing an example of the present invention, and FIG. 2 is a BOG cooling curve and an LNG temperature increase curve within a heat exchanger in the example and the comparative example. 1・-LNG storage tank 2・-B OG compressor 3...heat exchanger 4...drum 5-LNG
Vaporizer 6・-L N G pump 7...Liquefied BO'G
Pump Figure 1 Figure 2

Claims (1)

[Claims] (1) In the treatment of evaporative gas generated from liquefied natural gas storage equipment, the evaporative gas is pressurized and converted into pressurized evaporative gas.
A method for treating evaporated gas in a liquefied natural gas storage facility, which comprises adding at least one saturated hydrocarbon of 2 to C_4, and then liquefying the evaporated gas by exchanging heat with the liquefied natural gas from the storage facility. .