JP2007155060A - Boiled-off gas re-liquefying method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To re-liquefy BOG by using LNG even in the night time requiring less gas consumption. <P>SOLUTION: This method is provided for re-liquefying boiled-off gas (b) which is generated from liquefied natural gas (a) in a storage tank 1. Herein, the boiled-off gas (b) is compressed by a compressor 2, and the boiled-off gas (b) after compressed is cooled by a pre-cooler 3 and a main cooler 4 and supplied into a mixing drum 8. On the other hand, the liquefied natural gas (a) is boosted by a first boost pump 5 and supplied to a vaporizer 7, part thereof is injected through a pressure reducing valve 6 into the mixing drum 8, and the liquefied natural gas (a) atomized by the injection and the boiled-off gas (b) in the mixing drum 8 are put in gas-liquid contact with each other to re-liquefy the boiled-off gas (b) in the mixing drum 8. Furthermore, the re-liquefied boiled-off gas (b) is boosted by a second boost pump 21 and then supplied to the pre-cooler 3 to pre-cool the boiled-off gas (b) after compressed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for re-liquefying a boil-off gas in which boil-off gas (hereinafter referred to as BOG) generated by the penetration of heat into liquefied natural gas (hereinafter referred to as LNG) is liquefied again.

  When using BOG generated by the heat intrusion into LNG as a raw material for city gas, it is necessary to increase the BOG to a pressure higher than the delivery pressure of city gas (up to about 6.9 MPa). As this delivery pressure increases, there is a problem that the power for boosting increases and the cost increases.

Therefore, in a general LNG base, cost reduction is attempted by the method shown in FIG.
That is, in the conventional BOG reliquefaction method (conventional example 1), the BOG (b) generated in the LNG tank 101 is boosted to a reliquefaction pressure (about 0.5 MPa) by the compressor 102, and the boosted BOG (b) is increased. Precooled by the precooling heat exchanger 103 and reliquefied by the plate fin heat exchanger 104 or the like. The re-liquefied BOG (b) is once stored in a condensate drum (condensing drum) 105, and then the pressure is increased to the LNG discharge pressure (about 3.5 MPa) by the boost pump 106, and the re-liquefied BOG (b) is increased. It is mixed with the cooling LNG (a) and supplied to the vaporizer 107, and the mixture c of the reliquefied BOG (b) and the cooling LNG (a) supplied to the vaporizer 107 is heated with seawater to produce a high-pressure city. Gas (about 3.0 MPa) d is manufactured.

On the other hand, as in Conventional Example 2 shown in FIG. 7, the BOG (b) extracted from the LNG tank 201 is compressed by the first compressor 202 to the first pressure (about 0.58 to 0.87 MPa). BOG (b) is pre-cooled by the heat exchanger 203 using the cold heat of LNG, and the compressed BOG (b) after pre-cooling is mixed with LNG (a) extracted from the LNG tank 201 in the mixer 204 to at least A part of the mixture is liquefied, and the mixed fluid c is gas-liquid separated by the gas-liquid separator 205, and the pressure of the liquid component is increased by the second pump 206, and is further led to the vaporizer 207 via the heat exchanger 203. On the other hand, a boil-off gas treatment method has been proposed in which the gas component is compressed to a second pressure (6.9 MPa) higher than the first pressure by the second compressor 208 (for example, Patent Document 1). Ether.).
JP-A-9-59657

  As mentioned above, when boosting the BOG above the city gas delivery pressure, the reason for boosting after re-liquefying the BOG is to boost the pressure after re-liquefying the BOG rather than boosting the BOG in the gas phase. This is because the pump power can be reduced. That is, if the 1t / h BOG is increased to the city gas delivery pressure in the gas phase, the power of the compressor becomes 192 kW. This is because 90.2 kW is sufficient.

  Here, the difference between the gas phase compression power of the compressor and the liquid phase power after BOG reliquefaction will be described as follows.

(1) Power to compress BOG (gas) 1t / h in the gas phase
BOG compressor
(A) Suction pressure: 0.1 MPaA
(B) Discharge pressure: 5.29 MPaA
(C) Power: 192kW
(2) Power to boost BOG (liquid) 1t / h by pump ・ Low pressure power for re-liquefaction
BOG compressor
(A) Suction pressure: 0.1 MPaA
(B) Discharge pressure: 1.08 MPaA
(C) Power: 87kW
(D) LNG pump total head (*): 977m
(*) Total pump head from the discharge pressure (1.08 MPaA) to the equivalent pressure (5.29 MPaA) (liquid density = 0.44 t / m ³ )
(E) Power: 3.3kW
(F) Total power: 90.2 kW

  However, in the case of the former (conventional example 1), the temperature pattern (also referred to as a heat flow diagram) in the precooling heat exchanger 103 is similar to that shown in FIG. ) Has a minimum (referred to as pinch temperature). It is said that ΔT is preferably not lower than 5 ° C. in order not to make the heat transfer area of the precooling heat exchanger 103 excessive.

  Considering the pinch temperature and the like, there was a limit to the amount of LNG passing as cold heat. For example, when an entire amount of 10.2 t / h BOG is liquefied, the supply amount G of LNG necessary for the liquefaction of BOG is 55.8 t / h (see “Table 1”).

  Therefore, since the supply amount G of LNG is about 5.5 times the supply amount of BOG (= 55.8 / 10.2), when the gas consumption is small, such as at night, the liquefaction of BOG There was a problem that the LNG required for the system could not be secured.

  In FIG. 6, the enthalpy ix at the outlet of the precooling heat exchanger is 81.9 kcal / kg, and the temperature at the outlet of the precooling heat exchanger is −118 ° C.

  On the other hand, in the case of the latter (conventional example 2), when the supply amount G of LNG necessary to reliquefy the total amount of 9.4 t / h BOG is calculated under the conditions shown in FIG. / H.

That is,
(A) Heat balance of mixer 204 (assuming a static mixer) 47 × (G + i1) × 9.4 = 13 × (9.4 + G) (1)
(B) Thermal balance of precooler (266-i1) × 9.4 = (i2-58) × (9.4 + G) (2)
(C) Thermal balance of separation drum i3 × (9.4 + G) = 170 × (G1 + 58) × G2 (3)
here,
G1 + G2 = 9.4 + G (4)
It is.

  Therefore, i1 (precooler outlet enthalpy), i2 (precooler outlet enthalpy), i3 (enthalpy between mixer and gas-liquid separator), G (LNG supply amount), G1 (gas-liquid separator outlet The liquid component amount) and G2 (gas component amount at the gas-liquid separator outlet) are as follows.

i1 = 140 kcal / kg * (wet steam) -125 ° C.
i2 = 75.6 kcal / kg * -125 ° C
i3 = 60.6 kcal / kg * (wet steam) -143 ° C
G = 57.8t / h
G1 = 1.2t / h
G2 = 66.0t / h
That is, the supply amount G of LNG necessary for reliquefaction of the total amount of 9.4 t / h BOG is 57.8 t / h.

  Therefore, since the supply amount G of LNG is about 6.2 times the supply amount of BOG (= 57.8 / 9.4), it is the same as the former when the gas consumption is small, such as at night. In addition, there is a problem that LNG necessary for reliquefaction of BOG cannot be secured.

  The present invention has been made in order to solve such problems. The object of the present invention is to use BOG as a raw material for city gas, even when the gas consumption is small such as at night. It is an object of the present invention to provide a boil-off gas reliquefaction method that can reliquefy the total amount of LNG using LNG.

In order to achieve the above object, the present invention is configured as follows.
The boil-off gas re-liquefaction method according to the invention of claim 1 is a boil-off gas re-liquefaction method for re-liquefying boil-off gas generated from liquefied natural gas in a storage tank, wherein the boil-off gas is compressed by a compressor, The compressed boil-off gas is cooled by the precooler and the main cooler and supplied into the mixing drum, while the liquefied natural gas is pressurized by the first booster pump and supplied to the vaporizer, and part of it is supplied. The liquefied natural gas sprayed into the mixing drum through the pressure reducing valve is brought into gas-liquid contact with the liquefied natural gas atomized by the injection and the boil-off gas in the mixing drum to re-liquefy the boil-off gas in the mixing drum, The boosted pressure of the reliquefied boil-off gas by a second booster pump is supplied to the precooler to precool the boil-off gas after compression. By that.

  The boil-off gas re-liquefaction method according to the invention of claim 2 is sprayed from a liquefied natural gas atomized by a spray nozzle provided at the top of the mixing drum and a spray nozzle provided at a body portion of the mixing drum. The boil-off gas re-liquefaction method according to claim 1, wherein the boil-off gas is brought into gas-liquid contact, and the boil-off gas is re-liquefied in the process of gas-liquid contact.

  According to a third aspect of the present invention, there is provided a method for re-liquefying boil-off gas, wherein the boil-off gas after pressurization is pre-cooled with a mixed liquid of boil-off gas and liquefied natural gas after re-liquefaction by the mixing drum, and boil-off after pre-cooling is performed. The boil-off gas reliquefaction method according to claim 1, wherein the gas is further cooled by liquefied natural gas supplied to the main cooler.

  As described above, the present invention relates to a boil-off gas re-liquefaction method for re-liquefying boil-off gas generated from liquefied natural gas in a storage tank. The boil-off gas is compressed by a compressor, and the boil-off gas after compression is precooled. The liquefied natural gas is pressurized by a first booster pump and supplied to the vaporizer while being partially cooled by a condenser and a main cooler, and a part of the liquefied natural gas passes through a pressure reducing valve. The liquefied natural gas atomized by the injection is brought into gas-liquid contact with the boil-off gas in the mixing drum to re-liquefy the boil-off gas in the mixing drum, and the re-liquefied boil-off gas is After the pressure is raised by the pressure booster pump, the boil-off gas after being compressed is supplied to the pre-cooler to pre-cool the boil-off gas. It has become possible to re-liquefied natural gas.

  That is, according to the present invention, the ratio (LNG / BOG) of liquefied natural gas (LNG) to boil-off gas (BOG) is 4.47 (= 54.1 / 12.1). Even when the gas consumption is small, it is possible to secure liquefied natural gas necessary for reliquefaction of boil-off gas, which is an industrially useful invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
In FIG. 1, reference numeral 1 denotes an LNG tank, and LNG (liquefied natural gas) a stored therein generates BOG (boil-off gas) b due to heat intrusion. This BOG (b) is extracted from the LNG tank 1 and compressed by the compressor 2. The BOG (b) compressed to a predetermined pressure by the compressor 2 is pre-cooled by the pre-cooler 3 using the cold of the mixed liquid (c) of the reliquefied BOG and LNG, and then further LNG (a ) Is cooled to a predetermined temperature by the main cooler 4 using the cold heat.

  On the other hand, the LNG (a) extracted from the LNG tank 1 is boosted by an LNG pump (first booster pump) 5. The LNG (a) whose pressure has been increased to a predetermined pressure (for example, 20 ata (1.86 MPa)) by the first booster pump 5 is supplied to the vaporizer 7, and a part of the LNG (a) is depressurized via the main cooler 4. Supplied to the valve 6.

  In the present invention, the LNG (a) decompressed by the pressure reducing valve 6 and the BOG (b) cooled by the main cooler 4 are jetted into the mixing drum 8 for gas-liquid mixing. It is characterized in that the BOG (b) is reliquefied in the process of contact.

  Specifically, as shown in FIG. 2, the mixing drum 8 is mainly composed of a pressure-resistant tank 10, and a spray nozzle 11 for LNG injection is provided downward at the top 10a.

  A spray nozzle 12 for spraying BOG is provided in the tank 10 so as to be positioned below the spray nozzle 11. This spray nozzle 12 is provided with a large number of minute spray holes (not shown) at equal intervals below the horizontal spray pipe 13 penetrating the body 10b of the tank 10. It is not limited to. In short, it is sufficient if BOG (b) can be dispersed.

  The mixing drum 8 is used for respreading the liquid accumulated in the bottom of the tank 10, that is, a part of the mixed liquid c of LNG (a) and re-liquefied BOG (b) into the tank 10. A reduction path 14 is provided. The reduction path 14 is provided at a branch pipe 16 branched from a pipe 15 connected to the bottom of the tank 10, a reflux pump 20 that conveys the mixed liquid c, and a tip of the branch pipe 16. The spray nozzle 11 is composed of a second spray nozzle 17 provided between the spray nozzle 11 and the spray nozzle 12.

  The tank 10 has a blow line 18 so that the non-reliquefied BOG in the tank 10 is returned to the suction side of the BOG compressor 2. Further, a delivery pump 19 is provided in the pipe 15 connected to the bottom of the tank 10.

  Accordingly, the spray nozzle 11 flushes into the tank 10 when it passes through the pressure reducing valve 6 to inject LNG (a) having a reduced pressure and temperature, and from the spray nozzle 12 into the tank 10. When BOG (b) adjusted to a predetermined pressure and temperature is sprayed (spouted), LNG (a), which is atomized or formed into fine droplets, and gaseous BOG (b) come into gas-liquid contact. The BOG (b) is liquefied again.

  As shown in FIG. 1, the mixed liquid c in which the reliquefied BOG (b) and LNG (a) are mixed is supplied to the precooler 3 by a reliquefier pump (second booster pump) 21. At that time, the mixed liquid c is made the same pressure as the LNG (a) supplied to the vaporizer 7 by the reliquefaction pump (second booster pump) 21. The mixed liquid c supplied to the precooler 3 precools the BOG (b) compressed by the compressor 2. The mixed liquid c in which BOG (b) is pre-cooled while passing through the pre-cooler 3 joins with LNG (a) supplied to the vaporizer 7 and is supplied to the vaporizer 7.

  The mixed liquid c and LNG (a) supplied to the vaporizer 7 are heated and gasified by seawater e which is a heat source, and become high-pressure (eg, 2.35 MPa) city gas d.

Next, the required amount G of LNG was calculated based on the “heat flow diagram” in FIG. 3 and the “heat medium balance diagram” in FIG.
(A) Thermal balance The thermal balance of each device is as follows.

(A) Thermal balance of precooler (266-i1) × 12.1 = (87-74) × (12.1 + G) (1)
(B) Heat balance of main cooler G × (i3-47) = 12.1 × (i1-i2) (2)
(C) Heat balance of mixing drum i2 × 12.1 + i3 × G = 74 × (12.1 + G) (3)
here,
i3 = 65 kcal / kg * −135 ° C.
It is.

In addition,
i1: Enthalpy (kcal / kg) between precooler and main cooler
i2: enthalpy of main cooler outlet (kcal / kg)
i3: Enthalpy between main cooler and mixing drum (kcal / kg)
It is.

The above (1) to (3) are converted as follows.
12.1 * i1 + 13 * G = (266 * 12.1-13 * 12.1) = 3061
(1 ')
12.1 * i1-12.1 * i2-G * i3 = -47 * G (2 ')
12.1 × i2 + G × i3 = 74 × 12.1 + 74 × G (3 ′)
Therefore, (2 ′) + (3 ′) is as follows.
12.1 × i1 = 74 × 12.1 + 27 × G (4 ′)
Next, (4 ′) is substituted into (1 ′).
74 × 12.1 + 27 × G + 13 × G = 3061 (5 ′)
Therefore,
G = 54.1t / h
i1 = 194.8 kcal / kg * −89 ° C.
i2 = 114.3 kcal / kg * -125 ° C
Therefore,
LNG + BOG = 12.1 + 54.1 = 66.2 t / h
It becomes.

The ratio of LNG to BOG is
LNG / BOG = 54.1 / 12.1 = 4.47
It becomes.

  Therefore, since the supply amount G of LNG is about 4.5 times the supply amount of BOG, it is possible to secure LNG necessary for re-liquefaction of BOG even when the amount of gas consumption is small such as at night. It is.

The specifications of the heat load, drum capacity, reliquefaction pump (second pump), and circulation pump are as follows.
(B) Thermal load (a) Precooler: 861.2 × 10 ³ kcal / h
(B) Main cooler: 974.7 × 10 ³ kcal / h
(C) Drum capacity 2.5m ³
(D) Reliquefaction pump 76.2t / h × s0.9MPa, d5.3MPa
(E) Circulation pump 81.2t / h × s0.9MPa, d1.2MPa
here,
d: Discharge pressure s: Suction pressure.

(Example)
Regarding the ratio of LNG to BOG (LNG / BOG), the present invention (see FIG. 1) was compared with Conventional Example 1 (see FIG. 5) and Conventional Example 2 (see FIG. 7). As a result, the ratio of LNG to BOG was found to be the smallest in the present invention (see “Table 1”).

  Therefore, according to the present invention, it is possible to secure LNG necessary for re-liquefaction of BOG even when the gas consumption is small such as at night.

It is a schematic block diagram of the equipment which implements the boil off gas reliquefaction method concerning this invention. It is sectional drawing of a mixing drum. It is a heat flow figure of the reliquefaction installation of FIG. It is a heat balance figure of the reliquefaction installation of FIG. It is a schematic block diagram of the conventional boil-off gas reliquefaction equipment (conventional example 1). It is a heat flow figure of the reliquefaction installation of FIG. It is a schematic block diagram of another example of the conventional boil-off gas reliquefaction equipment (conventional example 2). It is a heat flow figure of the reliquefaction installation of FIG. It is a heat balance figure of the reliquefaction installation of FIG.

Explanation of symbols

a Liquefied natural gas (LNG)
b Boil-off gas (BOG)
DESCRIPTION OF SYMBOLS 1 Storage tank 2 Compressor 3 Precooler 4 Main cooler 5 1st pressure | voltage rise pump 6 Pressure reducing valve 7 Vaporizer 8 Mixing drum 21 2nd pressure | voltage rise pump 21

Claims (3)

In the boil-off gas re-liquefaction method for re-liquefying boil-off gas generated from liquefied natural gas in the storage tank, the boil-off gas is compressed by a compressor, and the boil-off gas after compression is cooled by a precooler and a main cooler. While supplying into the mixing drum, the liquefied natural gas is pressurized with a first booster pump and supplied to the vaporizer, and a part of the liquefied natural gas is injected into the mixing drum through a pressure reducing valve and atomized by the injection. The liquefied natural gas and the boil-off gas in the mixing drum are brought into gas-liquid contact to re-liquefy the boil-off gas in the mixing drum, and further the pressure of the re-liquefied boil-off gas is increased by a second booster pump, A boil-off gas reliquefaction method, wherein the boil-off gas is supplied to the precooler to precool the compressed boiloff gas. The liquefied natural gas atomized by the spray nozzle provided on the top of the mixing drum and the boil-off gas sprayed from the spray nozzle provided on the body of the mixing drum are brought into gas-liquid contact, and in the process of this gas-liquid contact The boil-off gas reliquefaction method according to claim 1, wherein the boil-off gas is reliquefied. Pre-cooling the boil-off gas after being pressurized by the mixed liquid of the boil-off gas and the liquefied natural gas after being liquefied again by the mixing drum, and further cooling the boil-off gas after the pre-cooling by the liquefied natural gas supplied to the main cooler The boil-off gas reliquefaction method according to claim 1.

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