JPS5938407B2 - Operation method for power recovery from LNG - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for recovering cold heat using a Rankine cycle using a multi-component mixed working fluid in regasification of liquefied natural gas (LNG).

More specifically, the present invention relates to a method of continuously recovering cold heat with high efficiency without performing complicated operations even when the temperature of an external high heat source changes.

As a method for effectively utilizing the cold energy of LNG, for example, US Pat.
No. 1, the process principle for liquefying natural gas is operated in reverse, that is, LNG and a mixed working fluid such as C1 to C6 hydrocarbons are passed through a multi-fluid heat exchanger to form a Rankine cycle. A method has been proposed in which power is recovered by driving a turbine with a mixed working fluid while gasifying it.

Currently, this recovered energy is generally recovered as electricity.

The Rankine cycle of this multi-component mixed working fluid is formed using LNG as a cold heat source, but seawater, hot water, exhaust steam, or flue gas can be used as a high heat source. It is common to use

LNG, which serves as a cold heat source, is normally transported and stored at a temperature of around -160° C., and remains at a nearly constant temperature, whereas the temperature of seawater, which is a high heat source, changes depending on the season.

Therefore, in order to efficiently recover cold heat as electricity or the like in both summer and winter, it is necessary to operate the cold heat recovery process in response to temperature changes of this high heat source.

Conventionally, the following three types of methods have been considered as ways to respond to changes in the temperature of this high heat source.

(1) The turbine front pressure is kept almost constant, and the composition of the multi-component mixed working fluid is changed according to the temperature change of the high heat source.

(2) Adjust the turbine front pressure according to the temperature of the high heat source so that the multi-component mixed working fluid is completely vaporized at the turbine inlet.

(3) The pressure in front of the turbine is kept almost constant, and a multi-component mixed working fluid is used that completely vaporizes at the turbine inlet even in winter.

However, all of these methods have the following drawbacks and are not sufficient.

In other words, in the case of method (1), although it is possible to recover cold energy efficiently, continuous operation is not possible because the composition of the multi-component mixed working fluid changes in response to changes in seawater temperature. This also makes the driving operation complicated, which is not desirable.

In the case of method (2), when the turbine front pressure is changed, the inflection point of the temperature curve for the heat load of the high pressure working fluid heated by the low pressure working fluid changes, especially when the turbine front pressure is lowered. In the multi-fluid heat exchanger, the minimum point (pinch point) of the temperature difference for heat transfer becomes too small, which tends to cause heat transfer failure, and it is extremely difficult to ensure stable operation. In addition, energy recovery efficiency is low.

Further, in the case of method (3), although no operational problems occur, the energy recovery efficiency in the summer season is greatly reduced, which is not preferable.

The present inventors have completed the present invention as a result of intensive studies on a method for solving such problems and recovering the cold energy of LNG stably and efficiently even when a temperature change occurs in a high heat source.

That is, the present invention provides a method for regasifying liquefied natural gas (LNG), using LNG as a cold heat source, and using an external high heat source to perform a Rankine cycle on a multi-component mixed working fluid while rotating a turbine and recovering power. , on the flow path from the heat exchanger to the turbine that performs heat exchange between the external high heat source and the multicomponent mixed working fluid, gas-liquid separation of the multicomponent mixed working fluid is performed in response to temperature changes of the external high heat source. This is a unique method for recovering power from LNG.

According to this method, the turbine front pressure can be kept constant and the turbine front pressure does not change as described in (2) above, so self-heat exchange of the working fluid that performs the Rankine cycle in the multifluid heat exchanger becomes impossible. Stable operation can be maintained without any problems.

The method for recovering power from LNG according to the present invention will be specifically explained below with reference to FIG. 1.

LNG is stored in an LNG storage tank 1, and is pressurized from the LNG tank by an LNG supply pump 2 as needed and sent to a regasification process.

Regasification is performed by exchanging heat with the working fluid in the multifluid heat exchanger 3.

In this case, the working fluid is a mixture of hydrocarbons such as methane, ethane, propane, butane, etc., with a composition such that the temperature difference between its cooling curve at low pressure and the evaporation curve of the LNG to be heated and regasified is as small as possible. It is desirable to use one that has the following properties in order to increase energy recovery efficiency.

The working fluid that serves as a heating source for LNG regasification is a high-temperature, low-pressure working fluid that returns from the working fluid turbine 7 to the multi-fluid heat exchanger 3, and the heating of LNG, that is, the regasification, mainly involves the use of the condensing working fluid. It is done using latent heat.

Most of the LNG exiting the multifluid heat exchanger 3 is vaporized, and if necessary, the LNG can be used for heat exchange with an external heat source, such as seawater.
It is heated and completely vaporized by the heater 4 and sent to the consumer.

The working fluid, on the other hand, is generally circulated forming a closed cycle.

The high-temperature, low-pressure working fluid that comes out of the working fluid turbine γ is mostly in the gas phase and enters the multi-fluid heat exchanger 3, where it is mixed with the aforementioned LNG and the low-temperature, high-pressure working fluid supplied via the working fluid pump 11. It is mainly cooled while imparting latent heat of condensation, and when it leaves the multi-fluid heat exchanger 3, it is all liquefied.
Transforms into a low-temperature, low-pressure working fluid.

The low-temperature, low-pressure working fluid thus liquefied is stored in the working fluid storage tank 1.
The pressure is stored at 0, and the pressure is increased to a predetermined pressure by the pump 11.
The working fluid becomes a low-temperature, high-pressure working fluid, and is guided again to the multi-fluid heat exchanger 3, where it exchanges heat with the LNG while mainly receiving its latent heat of condensation from the previous high-temperature, low-pressure working fluid, and is heated up to become a gas-liquid multiphase fluid. Become.

Since the high-pressure working fluid exiting the multi-fluid heat exchanger 3 is a gas-liquid mixed flow containing unvaporized gas, it is further heated in the working fluid heater 5 by an external high heat source, such as seawater.

As explained above, conventionally, the composition of the multi-component mixed working fluid or the turbine inlet pressure is adjusted or It was set.

In the present invention, since the multi-component mixed working fluid and turbine inlet pressure are kept constant, depending on the temperature change of the external high heat source in the working fluid heater, the temperature may drop below the dew point, resulting in a gas-liquid mixed phase.

It is not appropriate to supply the gas-liquid mixture to the turbine because the turbine blades etc. will be damaged by the liquid splashes.

In the method of the present invention, for example, a gas-liquid separator 6 is installed on the flow path from the working fluid heater 5 to the working fluid turbine 7 to perform gas-liquid separation of the high-pressure working fluid, and the gas phase portion is sent to the working fluid turbine 7. There, the turbine is rotated to generate power, and at the same time, both the pressure and temperature are reduced, and the fluid becomes a high-temperature, low-pressure working fluid, which is circulated again to the multi-fluid heat exchanger 3 to form a closed cycle.

On the other hand, the liquid phase separated by the gas-liquid separator 6 bypasses the turbine, passes through the expansion valve 12, enters the low-pressure working fluid drum 9 installed as required, and then returns to the multifluid heat exchanger 3. This forms a closed cycle similar to the gas phase.

The low-pressure working fluid drum 9 is a gas-liquid separator that combines the gas-liquid mixed-phase low-pressure working fluid from the turbine with the gas-liquid mixed low-pressure working fluid from the expansion valve, and is usually a multi-fluid drum in which a plurality of drums are installed in parallel. This is provided to prevent deterioration of heat exchange performance by uniformly distributing gas and liquid to each low-pressure working fluid flow path in the heat exchanger.

By installing the gas-liquid separator 6, even if there is a temperature change in the external high heat source, such as when the seawater temperature drops in winter and the working fluid exiting the working fluid heater 5 becomes a gas-liquid mixture,
Since only the gas phase portion of the working fluid can be supplied to the turbine, the inlet pressure of the turbine can be kept substantially constant without being substantially changed.

The method of performing gas-liquid separation and bypassing the turbine with the liquid phase seems to reduce the efficiency of cold heat recovery because the working fluid is seemingly wastedly recycled, but as shown in the examples below, there are other methods. The recovery efficiency is higher than that of the previous method.

The optimum operating method is such that when the seawater temperature is substantially at its maximum in summer, the temperature of the multi-component mixed working fluid at the outlet of the working fluid heater is above the dew point, preferably close to the dew point, and the power recovery The composition of the multi-component mixed working fluid and the pressure at the turbine inlet are selected such that this occurs with high efficiency.

When the seawater temperature drops, such as during winter, the temperature at the outlet of the working fluid heater inevitably falls below the dew point.
The multi-component mixed working fluid exhibits a gas-liquid mixture, but the pressure at the turbine inlet is set to the same as in the summer and is not substantially changed, and the multi-component mixed working fluid exhibits a gas-liquid mixture as described above. This is a separate operation method.

Although the explanation here uses a single-stage multi-fluid heat exchanger, it is also possible to have multiple stages of a multi-fluid heat exchanger, or to perform gas-liquid separation of each fluid between each stage of these multi-fluid heat exchangers. You can choose to install a container as appropriate.

The method of the present invention can recover cold heat stably and with high efficiency without complicated operations even in an LNG regasification power recovery system that uses a heat source with temperature fluctuations such as seawater as an external high heat source. It enables
In industrial implementation, it is extremely effective.

The present invention will be explained below with reference to Examples.

Example 1 LNG having the composition shown in Table 1 was regasified using the LNG regasification power recovery process shown in FIG.

LNG was stored in an LNG storage tank 1 at -160°C, and was pressurized to 26 ata by an LNG supply pump 2 and sent to the regasification process at a flow rate of 100 t/hour.

Regasification is carried out by heat exchange with the working fluid in the multi-fluid heat exchanger 3, and the temperature upon exiting the multi-fluid heat exchanger 3 is -28°C in summer and -38°C in winter, and most of the LNG is vaporized. The heat exchanger 4 with seawater further heats the temperature to 20°C in summer.
In winter, it is heated to 0°C and completely vaporized before being sent to consumers.

Seawater temperature was 28°C in summer and 9°C in winter.

On the other hand, the multi-component mixed working fluid A has the composition shown in Table 1 and circulates in a closed cycle.

The high temperature, low pressure working fluid that comes out of the working fluid turbine is mostly in the gas phase, with a pressure of 3.1 ata and a temperature of -2 in summer.
3℃, low pressure working fluid gas-liquid separation drum 9 at -33℃ in winter
entered in.

The low pressure working fluid then enters the multifluid heat exchanger 3 and enters the LNG
The high-pressure working fluid was liquefied while giving heat, and when it left the multi-fluid heat exchanger 3, it was all liquefied, and the temperature had dropped to -129°C in both summer and winter.

This low-temperature, low-pressure working fluid is stored in the working fluid storage tank 10, and is boosted to 14.9 ata by the pump 11 to become a low-temperature, high-pressure working fluid, which is then led to the multifluid heat exchanger 3 again, where it is heated from the previous low-pressure working fluid. The temperature of the tube was raised and it became a gas-liquid miscible fluid.

This gas-liquid multiphase fluid is then heated in the working fluid heater 5 by seawater, which is an external high heat source (the temperature is the same as in the above case), and in the summer, it is all vaporized to a temperature of 22°C, close to the dew point.
In the winter, the seawater temperature is low, so the seawater is not completely vaporized, but is heated to 3°C and enters the gas-liquid separator 6 in front of the turbine inlet.

The gas-liquid mixed-phase working fluid during winter was separated into gas and liquid, and the θ ratio of gas and liquid at this time was 0.885:0.115.

The gas phase portion was led to the turbine, and the pressure of the working fluid at the turbine inlet was constant at 11.9 ata in both summer and winter.

The working fluid rotates the turbine, reduces both temperature and pressure, and is led to the low-pressure working fluid drum 9 to form a closed cycle.

On the other hand, the winter liquid phase separated by the gas-liquid separator 6 enters the low-pressure working fluid drum via the expansion valve 12, merges with the working fluid from the turbine, and circulates.

From the generator 8 connected to the working fluid turbine to 4420 in summer! During the winter, 3,200KW of electricity was obtained.

Comparative Example 1 Same as Example 1 except that the expansion valve 12 was closed in the process shown in FIG. 1, and the turbine front pressure was adjusted according to the seawater temperature so that the multi-component mixed working fluid was completely vaporized at the turbine inlet. Similarly, power was recovered while regasifying LNG.

Therefore, the operating conditions during the summer were exactly the same as in Example 1.

In winter, in order to completely vaporize the working fluid at the turbine inlet, the pressure at the turbine inlet must be 8.0 at.
The power recovered by the generator at this time was 2.220 KW.

The temperature and pressure of the working fluid at each location are -23°C and 3.1 ata at the turbine outlet, and -129°C and 2.7 ata at the multifluid heat exchanger outlet (low-pressure flow path).

At the inlet of the multifluid heat exchanger (high pressure flow path) -128℃, 10
．． 4ata, 1.4℃ at the working fluid heater outlet, 8,1
It was ata.

Comparative Example 2 Example except that the expansion valve 12 was closed in the process shown in FIG. 1 and multi-component mixed working fluid B having the composition shown in Table 1 was used so that the working fluid was completely vaporized in winter. Power recovery was performed while regasifying LNG in the same manner as in 1.

In this case, the power recovered by the generator will be 3720 W in the summer,
In winter, it was 3420 KW, and although the recovered power in winter was slightly better than in the case of the method of the present invention, the recovered power in summer was considerably lower.

In addition, the temperature and pressure at each part of the working fluid are -21℃ and 3.1ata at the turbine outlet in summer and -36℃ in winter.
°C, 3.1 ata, multifluid heat exchanger outlet (low pressure flow path)
In summer -130℃, 2.7ata, in winter -130℃,
2.8 ata, multi-fluid heat exchanger inlet (high pressure channel) -129℃ in summer, 13.2 ata, -129℃ in winter, 1
3.0ata1 At the outlet of the working fluid heater, the temperature is 24℃ in summer, 1
1.9 ata, and 11.9 ata at 23°C in winter.

[Brief explanation of drawings]

FIG. 1 is an example of a flow sheet showing an LNG regasification power recovery system used to carry out the method of the present invention. 1: LNG storage tank, 2: LNG supply pump, 3
: Multi-fluid heat exchanger, 4: LNG heater, 5: Working fluid heater, 6: Gas-liquid separator, 7: Working fluid turbine, 8: Generator, 9: Low-pressure working fluid drum, 10: Working fluid storage tank ,
11: working fluid pump, 12: expansion valve.

Claims (1)

[Claims] 1. In the regasification of liquefied natural gas (LNG), L
Heat exchanger uses NG as a cold heat source and performs heat exchange between the external high heat source and the multicomponent mixed working fluid when rotating the turbine and recovering power while performing the Rankine cycle on the multicomponent mixed working fluid using an external high twist source A method for recovering power from LNG, characterized by performing gas-liquid separation of a multi-component mixed working fluid in response to temperature changes of an external high heat source on a flow path from a vessel to a turbine. 2. The method of claim 1, wherein the turbine inlet pressure is not substantially changed. 3. The composition of the multicomponent mixed working fluid and the pressure at the turbine inlet are adjusted so that the temperature of the multicomponent mixed working fluid at the outlet of the heat exchanger is equal to or higher than the dew point when the temperature of the external high heat source is substantially the highest. The method according to claim 1 or 2 for selection.