JP2002038170A - Method for producing town gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably control a liquid level and raise pressure by a pump without generating cavitation in a method for producing a town gas by fractional distillation. SOLUTION: This method for producing a town gas is to mix an LNG with a natural gas vaporized in a heat exchanger 13 for vaporizing the LNG, separate a low calorie gas in a fractionation vessel 15 to obtain a predetermined calorific value by using a gas-liquid equilibrium state, cool a fractionated liquid to a supercooled state against a pressure considered to be reduced by pressure fluctuation in a heat exchanger 22, as the fraction is in a saturated state, store the supercooled liquid in a vessel 23, raise pressure by a booster pump 18 for the fractionated liquid depending on demands of the town gas, control a flow rate by a flow-controlling valve 19, vaporize and raise temperature with a heat exchanger 20 for vaporizing the fractionated liquid and feed the gas into the town.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing city gas using liquefied natural gas (hereinafter sometimes abbreviated as "LNG") as a main raw material and adjusting the calorific value to a specified value and supplying it to the city. .

[0002]

2. Description of the Related Art Conventionally, LNG has been used as a main raw material in the production of city gas. The LNG terminal has a facility for storing LNG carried in from abroad and sending it out after adjusting the calorific value according to the demand of the city gas. It is manufactured in. At the LNG terminal, LNG as a raw material is stored in an extremely low temperature liquid state at about -160 ° C. in an LNG tank at normal pressure. The city gas supplied from the LNG terminal needs to be transported to the customers via conduits, and the LNG stored in the LNG tank is
When extracting NG as a raw material for city gas, it is necessary to increase the pressure to, for example, 4.9 MPaG. In a city gas production facility using LNG as a main raw material, LNG is pressurized to a pressure required for delivery and vaporized by an LNG vaporizer.

[0003] The vaporized LNG is natural gas (hereinafter referred to as "N
G "), which is adjusted to a prescribed calorific value, for example, 46 MJ per unit volume of 1 m ³ under standard conditions, and sent to the market. Since the specified calorific value is larger than the calorific value of natural gas, it is usually mixed with liquefied petroleum gas (hereinafter abbreviated as “LPG”) that has a large calorific value to adjust the calorific value before the market. Sent out during. However, since LPG is expensive, fractionation technology has been developed as a technology that can reduce LPG as much as possible.

[0004] In fractionation technology, utilizing the vapor-liquid equilibrium relationship,
Light components such as low-calorie methane (NH ₄ ) are extracted from LNG to increase the amount of heat generated in the remaining portion. By using such a fractionation technique, LNG can be separated into a low-calorie gas of 40.2 MJ per unit volume of 1 m ³ and a high-calorie gas of 46 MJ in a standard state. If a heavy component is fractionated from LNG and used for heat increase, or if a component with a prescribed calorific value is obtained and the amount of LPG used is reduced, the cost difference between LNG and LPG leads to a reduction in raw material costs. Down can be planned. However, for this purpose, it is necessary that there be a low-calorie customer for industrial use or the like that supplies the low-calorie gas generated during fractionation without adjusting the calorific value. Equipment that uses a large amount of gaseous fuel.
If designed as MJ, it can be secured as a supply destination of low calorie gas. If such low calorie customers can be secured, LP which is usually more expensive than LNG
LNG where heat must be increased using G
It can be adjusted to a predetermined amount of heat only by the above. In addition, the fractionation increases the amount of LNG used, contributes to LNG inventory adjustment when the demand for city gas is small, and makes it possible to flexibly purchase raw materials.

FIG. 4 shows the outline of a city gas production facility using a fractionation facility. LNG is stored in the LNG tank 1 at, for example, a temperature of −157 ° C. and a pressure of 39.2 kPaG. In order to obtain city gas from stored LNG, first, LNG is boosted to 4.9 MPa by the LNG booster pump 2.
Increase to G. Part of the pressurized LNG is sent to the LNG vaporizing heat exchanger 3. In the LNG vaporizing heat exchanger 3, heat exchange is performed between the LNG and the heat exchange seawater supplied from the seawater pump 4, and the LNG is vaporized by heat from the seawater and heated. Natural gas (NG) having a temperature of 20 ° C. and a pressure of 3.4 MPaG is obtained from the LNG vaporization heat exchanger 3.

[0006] Of the LNG pressurized by the LNG pressurizing pump 2, the remaining portion excluding the portion vaporized by the LNG vaporizing heat exchanger 3 is mixed with the vaporized NG and sent to the fractionator 5. . The flow rate of LNG is 43 t by the flow control valve 6.
/ H, mixed with NG adjusted to 27 t / h by the flow control valve 7, and 2.5MP at a temperature of -95 ° C.
It is in a gas-liquid mixed state at the pressure of aG. When this gas-liquid mixed state is supplied to the fractionator 5, a gas having a total heating value of about 40.2 MJ is emitted from the tower top 5a at 23 t / h, and a liquid having a total heating value of about 46 MJ is emitted from the tower bottom 5b. Is generated at a flow rate of 47 t / h, respectively. Thereafter, the fractionated gas is taken by low-calorie customers such as industrial customers, and the fractionated liquid is vaporized using seawater as a heat source and sent out into the city as city gas. A fractionated liquid booster pump 8 is provided for increasing the pressure of the fractionated liquid, and a flow rate adjusting valve 9 is provided on the discharge side of the fractionated liquid booster pump 8.

[0007] Regarding the LNG fractionation technology, the applicant of the present invention has disclosed, for example, JP-A-60-262890, JP-A-8-269468 and JP-A-10-1954.
No. 64, for example.

[0008]

In the apparatus for producing city gas as shown in FIG. 4, the pressure of the liquid fractionated by the fractionator 5 is lower than the required delivery pressure of city gas, so that the fractionated liquid is supplied. It is necessary to raise the pressure to the pressure. As a means for increasing the pressure of the fraction, a fraction pump 5 is provided downstream of the fractionator 5. At this time, it is necessary that the fractionated liquid pressure pump 8 sucks only the fractionated liquid and not the fractionated gas. For this reason, so that the liquid level of the fractionator 5 becomes constant,
A flow regulating valve 9 is provided on the outlet side of the fractionated liquid pump 8 to control the flow rate of the fractionated liquid to be sent out. If the fractionated gas is sucked into the fractionated pressure pump 8, the fractionated pressure pump 8 may be damaged by a phenomenon called cavitation. A method is also conceivable in which a container for storing the fractionated liquid is provided between the fractionator 5 and the fractionated liquid pressure pump 8 and the liquid level of the container is controlled by the flow rate adjusting valve 9 to be constant.

[0009] The principle that a liquid having a predetermined calorie can be obtained by the fractionator 5 is based on the fact that the liquid is saturated at a concentration corresponding to the predetermined calorie. Therefore, in the liquid fractionated by the fractionator 5, LNG contains a heavy component of NG at a saturated concentration. At this time, when the pressure in the fractionator 5 or in the container provided between the fractionator 5 and the fractionated liquid pressure pump 8 fluctuates and decreases, the saturated liquid LNG becomes saturated with the reduced pressure. Evaporates rapidly to reach a concentration. For this reason, a large fluctuation occurs in the liquid level, and control becomes difficult. Further, since the fractionated liquid is a saturated liquid, if heat is input to the piping route to the fractionated liquid pump 8, the fractionated liquid evaporates and gas is generated. When the fractionated liquid in which the generated gas is mixed is introduced into the fractionated pressure pump 8, cavitation occurs in the fractionated pressure pump 8. When cavitation occurs in the fractionated liquid pump 8, bubbles are generated in the fractionated liquid, the pressure is reduced, and the pressure of the fractionated liquid cannot be increased to a target pressure.

[0010] An object of the present invention is to provide a method for producing city gas which can stably handle the fractionated liquid when producing city gas by fractionating liquefied natural gas.

[0011]

SUMMARY OF THE INVENTION The present invention relates to a method for producing a city gas which uses liquefied natural gas as a main raw material, adjusts the calorific value to a specified calorific value larger than the calorific value of the natural gas and sends it out to the city. Mix natural gas and liquefied natural gas, adjust the temperature of the mixed liquid to separate gas and liquid so that a saturated liquid having a predetermined calorific value is obtained, and separate the distillate with the cold heat of liquefied natural gas. This is a method for producing city gas, wherein the pressure is increased by a pump after cooling.

According to the present invention, when liquefied natural gas is used as a main raw material, and the calorific value is adjusted to a specified calorific value larger than the calorific value of natural gas and is sent out to the city as city gas, a part of liquefied natural gas is used. Is vaporized, the vaporized natural gas and the liquefied natural gas are mixed, the temperature is adjusted so as to obtain a saturated liquid having a predetermined calorific value, and fractionation is performed. Since the fractionated liquid is a saturated liquid, it tends to boil due to pressure fluctuations and the control tends to be unstable, but since the saturated liquid is cooled by the cold heat of the liquefied natural gas, the fractionated liquid is in a supercooled state and the pressure is reduced. Even if the drop occurs, it is difficult to boil, and stable liquid level control and the like can be performed. Even when the fractionated liquid is pressurized by a pump, the liquid evaporates due to the heat input to the piping, and cavitation can be less likely to occur in the pump.

Further, in the present invention, the predetermined heat generation amount is equal to the predetermined heat generation amount, and the cooling is performed by a heat exchanger.

According to the present invention, the fractionated liquid is generated at the temperature of the saturated liquid corresponding to the prescribed heat value, and then cooled with the heat from the liquefied natural gas in the heat exchanger to be in a supercooled state. In addition, it is possible to stably control the fractionated liquid having a specified calorific value and increase the pressure without generating cavitation.

Further, in the present invention, the predetermined heat value is set larger than the specified heat value, and the cooling is performed by direct mixing.

According to the present invention, the calorific value of the fractionated liquid is made larger than the prescribed calorific value required for city gas, so that the fractionated liquid contains less low calorie components. When the liquefied natural gas is directly mixed with the fractionated liquid, the calorific value decreases, but the calorific value of the fractionated liquid before mixing is larger than the prescribed calorific value. Can be Since low-temperature liquefied natural gas is mixed with the fractionated liquid, it is supercooled after the fractionated liquid and the liquefied natural gas are mixed, so that it can be handled stably and cavitation does not occur with pumps etc. it can.

[0017]

FIG. 1 shows a city gas production facility 1 to which a city gas production method according to an embodiment of the present invention is applied.
0 shows a schematic configuration. In the present embodiment, L before fractionation
Since the NG and the fractionated liquid after the fractionation are heat-exchanged, the fractionated liquid can be brought into a supercooled state, so that the fractionated liquid does not boil due to pressure fluctuation, and stable liquid level control can be achieved. It becomes possible.
Further, even with respect to heat input from the pipe, it is possible to prevent cavitation from occurring in the pump without evaporating a part of the fractionated liquid.

LNG serving as a raw material for city gas is stored in an LNG tank 11. When LNG is used as city gas, LNG is boosted by the LNG boosting pump 12 to a pressure required for delivery. LNG is supplied to the manufacturing equipment from the LNG booster pump 12 in a state N1 in which the temperature is -155 ° C., the pressure is 4.9 MPaG, and the calorific value is 44 MJ per unit volume of 1 m ^{3 in} a standard state. Part of the LNG in the state of N1 is supplied to the LNG vaporizing heat exchanger 13, where it exchanges heat with seawater supplied from the seawater pump 14 to be vaporized and heated. The remaining portion of the LNG in the state of N1 is supplied to the fractionator 15 and is subjected to heat adjustment by fractionation. LNG supplied to the fractionator 15 is supplied to the flow control valve 16.
, And the temperature is adjusted to a state N2 at -154 ° C. and a flow rate of 45 t / h. The NG vaporized by the LNG vaporization heat exchanger 13 is also supplied to the fractionator 15 via the flow control valve 17. The temperature after mixing LNG and NG changes according to the mixing ratio, and a saturated liquid that is in a gas-liquid equilibrium state at that temperature is obtained. The mixing ratio is adjusted so that the calorific value of the saturated liquid becomes a specified value for city gas. In the fractionator 15, a gas having a low calorie content is separated from the top 15a, and a fraction having a predetermined calorific value is separated from the bottom 15b. The fractionated liquid having a predetermined calorific value is pressurized by the fractionated liquid pressure pump 18 and the flow rate is adjusted by the flow rate regulating valve 19, and then heated and vaporized by the fractionated liquid vaporizing heat exchanger 20 to be converted into city gas. Sent out to the city. The NG vaporized in the LNG vaporization heat exchanger 13 is mixed with LPG in the LPG mixer 21 to adjust the calorific value, and is supplied to the city as a city gas together with the gas obtained from the fractionated liquid vaporization heat exchanger 20. Sent out.

In the city gas production facility 10 of the present embodiment,
Before being supplied to the fractionator 15, the LNG vaporizing heat exchanger 1
The cold heat of the LNG before mixing with the NG vaporized in 3 is used for cooling the fractionated liquid obtained from the bottom 15b of the fractionator 15 using the heat exchanger 22. Further, the fractionated liquid in the supercooled state in the heat exchanger 22 is temporarily stored in the container 23 and then the pressure thereof is increased by the fractionated liquid pressure pump 18.

The LNG adjusted to the state of N2 by the flow control valve 16 exchanges heat with the fractionated liquid in the heat exchanger 22.
The temperature is raised to 147 ° C. state N3. LNG in N3 state
The NG mixed with LNG is generated from the LNG vaporization heat exchanger 13 at a normal temperature, for example, at a temperature N20 at 20 ° C., and the flow rate is adjusted by the flow control valve 17, and at a temperature N15 at a flow rate of 26 t / h at 15 ° C. Mix with LNG in N3 state. Mixing gives a state N6 at a temperature of -95 ° C. and a flow rate of 70 t / h. The liquid of this gas-liquid mixture is 2.65MP
aG pressure. The liquid of the gas-liquid mixture in the state N6 is separated by the fractionator 15 into a gas and a fractionated liquid. From the top 15a, low calorie components are taken out, and from the bottom 15b, the total calorific value per unit volume of 1 m ^{3 in the} standard state is 46 MJ,
A fraction at a temperature of −95 ° C. in the state N7 is obtained at a flow rate of 47 t / h. From the tower top 15a of the fractionator 15, a gas in a state N8 having a total heating value of about 40.2 MJ per unit volume of 1 m ³ in a standard state is obtained at a flow rate of 23 t / h.

In the heat exchanger 22, the fractionated liquid in the state N7 at -95 ° C exchanges heat with the LNG in the state N2, and is cooled by the cold heat of the LNG to be in the state N9 at -100 ° C. The fractionated liquid is supercooled by the heat exchanger and stored in the container 23. The liquid level in the container 23 is the
The flow control valve 19 provided on the outlet side of the control unit 8 controls the flow to a constant height. The fractionated liquid guided from the container 23 to the fractionated pressure booster pump 18 is
At a pressure of 4.2 MPaG and a temperature of −98 ° C. N10
Pressure. The liquid in the state N10 is sent to the heat exchanger for vaporizing a fractionated liquid 20, and after being vaporized, is sent out as city gas.

FIG. 2 shows a partial configuration of a production facility to which a city gas production method as another embodiment of the present invention is applied. The city gas production equipment which is a target of the city gas production method of the present embodiment is almost the same as the city gas production equipment 10 of the embodiment shown in FIG. 1, and a tray 24 is provided inside instead of the fractionator 15. The difference is that a fractionator 25 provided is used. A demister 26 is provided in the upper part of the fractionator 25 to remove mist-like liquid components from low-calorie gas flowing to the top 25a. Such a demister 26 can be similarly provided in the fractionator 15 shown in FIG. The tray 24 in the fractionator 25 is provided at a position above the liquid level 27 of the fractionated liquid stored in the fractionator 25. A saturated liquid is obtained in the tray 24, the saturated liquid is extracted, and heat exchange with LNG is performed in the heat exchanger 22 shown in FIG.
The supercooled liquid is returned to the liquid level 27 side of the fractionator 25, and the supercooled fraction is stored therein.
8

FIG. 3 shows a schematic configuration of a city gas production facility 30 which is a target of a city gas production method as still another embodiment of the present invention. In the present embodiment, portions corresponding to the city gas production facility 10 of the embodiment shown in FIG. 1 are denoted by the same reference numerals, and redundant description will be omitted. In the present embodiment, the fractionated liquid is directly mixed with LNG supplied through the flow rate regulating valve 36 by using the fractionator 35 to be in a supercooled state.
LNG in state N1 boosted by LNG boosting pump 12
Is supplied to the fractionator 35 via the flow control valve 16.
The NG mixed with this LNG is the LNG vaporizing heat exchanger 1
A part of the NG vaporized in 3 is cooled to 1 by the flow control valve 17.
State N1 adjusted so that the flow rate is 19 t / h at 5 ° C.
It is one. LNG mixed with NG in state N11 is −
The state N12 is 154 ° C. and the flow rate is 21 t / h. On the other hand, LNG in the state N2 from the LNG booster pump 12 is:
The flow rate is adjusted by the flow rate adjusting valve 36, and the temperature is −154 ° C., the flow rate is 30 t / h, and the total calorific value per unit volume of 1 m ³ is 44 MJ in a standard state. After mixing the NG of the state N11 and the LNG of the state N12, the temperature is −93 ° C. and the pressure is 2.56 MPa.
G, a gas-liquid mixed liquid having a flow rate of 40 t / h in the state N14 is introduced into the fractionator 35, and is separated into a gas and a fractionated liquid. From the tower top 35a of the fractionator 35, gas in the state N15 having a total heating value of about 40 MJ per unit volume of 1 m ^{3 in the} standard state is discharged at a flow rate of 20 t / h. The total calorific value per unit volume of 1 m ³ in the standard state separated in the fractionator 35 is 4
The 7.4 MJ saturated liquid accumulates in the lower part of the fractionator 35.
When LNG condition N13 is introduced into the liquid, after mixing with fractionation liquid, it becomes subcooled with respect to the pressure in the fractionator 35, unit volume 1 m ³ per gross calorific value standard state 46
A liquid that becomes MJ is generated. In this case as well, the liquid in the fractionator 35 is supercooled, and stable liquid level control is possible. At the same time, the liquid enters the state N16 at a temperature of about −130 ° C. Even if there is, it is possible to prevent a part from being vaporized, and it is possible to prevent the occurrence of cavitation in the fractionated liquid booster pump 18.

As described above, according to the present invention, the vapor-liquid equilibrium relationship in the fractionators 15, 25, 35 is utilized to
When LNG is separated into a low calorie gas and a high calorie liquid, the liquid after fractionation is cooled to a supercooled state by LNG before fractionation, and is stabilized against a pressure that is considered to decrease due to pressure fluctuation. To control the liquid level, and also to prevent cavitation in the booster pump. LNG
It is possible to fractionate heavy components from inside and use them for increasing the heat, or fractionate so as to directly obtain a component having a predetermined calorific value as city gas. Material cost can be reduced. Also,
Since fractionation increases the amount of LNG used,
It also contributes to inventory adjustment of NG and makes it possible to increase the flexibility of purchasing raw materials.

[0025]

As described above, according to the present invention, even if a liquefied natural gas is fractionated to produce a saturated fraction, the fractionated liquid is cooled by the cold heat of the liquefied natural gas, and then the pump is cooled. , And can be stably handled in a super-cooled state in which it is difficult to boil due to a pressure drop or the like, and can be pressurized without generating cavitation or the like by a pump.

Further, according to the present invention, after the liquefied natural gas is fractionated and adjusted to a specified calorific value, it is supercooled by heat exchange with the liquefied natural gas so that it can be stably handled. .

According to the present invention, the liquefied natural gas is fractionated to obtain a heating value larger than the specified heating value of the city gas. Therefore, the liquefied natural gas is directly mixed and supercooled. It is possible to stably handle it by adjusting it to a state and a specified heat value.

[Brief description of the drawings]

FIG. 1 is a piping diagram showing a schematic configuration of a city gas production facility 10 to which a city gas production method as one embodiment of the present invention is applied.

FIG. 2 is a piping diagram showing a partial configuration of a city gas production facility to which a city gas production method is applied as another embodiment of the present invention.

FIG. 3 is a piping diagram showing a schematic configuration of a city gas production facility 30 to which a city gas production method as still another embodiment of the present invention is applied.

FIG. 4 is a piping diagram showing a schematic configuration of a city gas production facility provided with an LNG fractionation facility.

[Explanation of symbols]

 10, 30 city gas production equipment 11 LNG tank 12 LNG booster pump 13 LNG vaporization heat exchanger 15, 25, 35 fractionator 15a, 25a, 35a tower top 15b, 25b, 35b tower bottom 16, 17, 19, 36 Flow control valve 18 min distillate booster pump 20 min distillate vaporizer heat exchanger 22 heat exchanger 23 container 24 tray 27 liquid level

Claims (3)

[Claims] Claims: 1. A method for producing a city gas in which liquefied natural gas is used as a main raw material and adjusted to a specified calorific value larger than the calorific value of the natural gas and sent to the city, wherein the vaporized natural gas and the liquefied natural gas are mixed. Adjust the temperature of the mixed liquid to separate gas and liquid so that a saturated liquid having a predetermined calorific value is obtained, cool the fractionated liquid with the cold heat of liquefied natural gas, and then increase the pressure with a pump. A method for producing city gas. 2. The heat generation amount according to claim 1, wherein the predetermined heat generation amount matches the predetermined heat generation amount, and the cooling is performed by a heat exchanger.
The city gas production method described. 3. The method for producing a city gas according to claim 1, wherein the predetermined heat value is larger than the specified heat value, and the cooling is performed by direct mixing.