JPS5939638B2 - Power recovery method from liquefied natural gas for low load stability - 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 for stably recovering cold heat with high efficiency even when fluctuations occur in the regasification load.

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 C0 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.

Before explaining the power recovery method from liquefied natural gas for the purpose of low load stabilization according to the present invention, in the regasification of LNG, a multi-component mixed working fluid (hereinafter referred to as A system to which the present invention can be applied, which rotates a turbine while causing a Rankine cycle in a working fluid (abbreviated as "working fluid") and recovers LNG cold energy, will be specifically described with reference to FIG. 1, which shows an overview as an example. explain.

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 carried out in multifluid heat exchangers 3 and 3' by exchanging heat with the working fluid.

In this case, the working fluid is a mixture of hydrocarbons, such as methane, ethane, propane, butane, etc.

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 6 to the multi-fluid heat exchangers 3''EL and 3. This is done using the latent heat of the condensing working fluid.

Most of the LNG exiting the multifluid heat exchanger 3' is vaporized, and if necessary, heat exchange with an external heat source such as seawater is performed.
It is heated and completely vaporized by the G heater 5 and sent to the consumer.

Here, the LNG intermediate drum 4 is a gas-liquid separator, and when a plurality of multi-fluid heat exchangers are divided and installed, the gas-liquid distribution in the LNG flow path to each multi-fluid heat exchanger is uniformly distributed. It is necessary to make it work.

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

The high-temperature, low-pressure working fluid coming out of the working fluid turbine 6 is mostly in the gas phase and enters the multi-fluid heat exchangers 3' and 3, where it is mixed with the low-temperature, high-pressure working fluid supplied via the LNG 4 and the working fluid pump 9. The working fluid is cooled while mainly imparting latent heat of condensation, and when it leaves the multi-fluid heat exchanger 3, it is all liquefied and transformed 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 8.
The working fluid pump 9 raises the pressure to a predetermined pressure.

The low-pressure working fluid in the flow path from the working fluid turbine 6 to the working fluid pump 9 via the multi-fluid heat exchangers 3' and 3 is hereinafter abbreviated as low-pressure working fluid.

The low temperature and high pressure working fluid pressurized by the working fluid pump 9 is
The fluid is guided again to the multi-fluid heat exchangers 3 and 3', where it exchanges heat with the LNG while mainly receiving latent heat of condensation from the high-temperature, low-pressure working fluid, and is heated to become a gas-liquid multiphase fluid.

A working fluid intermediate drum 10 located between the multi-fluid heat exchangers 3 and 3' is a gas-liquid separator similar to the LNG intermediate drum 4 in the LNG flow path, and uniformly distributes gas and liquid within the multi-fluid heat exchanger. It was set up to make it possible.

Since the high-pressure working fluid exiting the multi-fluid heat exchanger 3 is a gas-liquid multiphase flow containing unvaporized gas, it is further heated in the working fluid heater 11 by an external high heat source, such as seawater, and is completely vaporized to the dew point. The working fluid becomes a high-pressure, high-temperature working fluid and is led to the working fluid turbine 6 via the turbine inlet gas-liquid separator 12.

Here, the working fluid turbine 6 is rotated to generate power, and at the same time, the pressure and temperature of the working fluid are reduced, and the working fluid becomes a high-temperature, low-pressure working fluid, which is again circulated to the multi-fluid heat exchanger 3' to form a closed cycle.

The high-pressure working fluid in the flow path that exits the working fluid pump 9, passes through the multi-fluid heat exchangers 3, 3' and the heater 11, and reaches the working fluid turbine 6 will be abbreviated as high-pressure working fluid hereinafter.

The major difference between the LNG cold recovery process and the LNG liquefaction process is that the demand for vaporized LNG changes significantly even during the day, as it is supplied to consumers as fuel for power generation or as city gas. .

Normally, the load fluctuation of the LNG regasification process based on this change in demand (fluctuation in the amount of LNG to be regasified) is 1
It reaches the level of 00-25.

Therefore, in order to recover the cold energy of LNG, it is required that the efficiency is sufficient to cope with this large load fluctuation.

When load fluctuations are small, one or more multifluid heat exchangers are installed in series, as in the case of the LNG liquefaction process, so that the lower part of the multifluid heat exchanger is at room temperature and the upper part is at low temperature. In this way, it is possible to form a stable Rankine cycle by exchanging heat between the LNG1 high-pressure working fluid and low-pressure working fluid while flowing through the flow paths of each fluid.

However, when load fluctuations are large, the following problems occur with the above method.

In other words, the width (area) of the flow paths for LNG, high pressure working fluid, and low pressure working fluid in a multifluid heat exchanger is usually constant for manufacturing reasons, but during low load operation, the width (area) of the flow paths for LNG, high pressure working fluid, and low pressure working fluid is constant. The flow velocity becomes slower.

For this reason, in the vertical upward flow of LNG or each working fluid, a slip occurs in the flow velocity between gas and liquid due to the influence of gravity, and the liquid portion causes a downward flow, causing back mixing. The temperature curve for the heat load of the LNG or working fluid changes, and the outlet of the multifluid heat exchanger,
Significant changes occur in the temperature and pressure at the inlet and inside, making it impossible to maintain the temperature difference necessary for heat transfer; this makes operation impossible.

On the other hand, if the flow path width is reduced to prevent the fluid flow velocity from decreasing too much during low-load operation, the flow path width in the multifluid heat exchanger is constant, so the area where these fluids evaporate and increase in volume increases. The flow velocity becomes large, especially during high-load operation, which becomes extremely fast.

In such a case, the pressure loss of the fluid within the multi-fluid heat exchanger becomes large, which leads to a decrease in the target power recovery, which is not preferable.

The present inventors have overcome these problems and have achieved stable operation even at low loads, while achieving efficiency < L at high loads.
As a result of various studies on regasification methods that can recover the cold energy of NG, the present invention was completed.

In other words, 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 cause the working fluid to undergo a Rankine cycle while rotating a turbine and recovering power. The heat exchange between the LNG and the working fluid is carried out in a multi-fluid heat exchanger divided into a room temperature section and a low temperature section, and in the low temperature section of the multi-fluid heat exchanger, LNG and the high pressure working fluid are This is a method for recovering power from liquefied natural gas, characterized in that the vertical upward flow and the low pressure working fluid are vertical downward flows.

The multi-component mixed working fluid used in the present invention is a mixture mainly containing C1 to C6 hydrocarbons and has a composition such that the evaporation curve of LNG and the cooling curve of the working fluid match with the smallest possible temperature difference. It is preferable to do so in order to improve energy recovery efficiency.

Furthermore, from the viewpoint of safety, a halogenated hydrocarbon mixture having a boiling point similar to the above-mentioned hydrocarbon mixture can also be used.

Further, as the external high heat source, seawater, hot water, exhaust steam, exhaust gas, etc. can be used, but seawater is easy to obtain and is excellent in that it is difficult to freeze.

From the viewpoint of increasing recovered energy, it is preferable that the temperature of the high heat source is high, and therefore it is also possible to use seawater heated by exhaust steam or the like.

In the present invention, heat exchange between LNG and the working fluid is performed in a multifluid heat exchanger that is divided into a room temperature section and a low temperature section.

In the present invention, a multifluid heat exchanger that is divided into a room temperature section and a low temperature section is a structure in which the room temperature section and the low temperature section are connected in series, and the connecting sections of the two do not touch each other. However, this does not necessarily mean a two-stage multi-fluid heat exchanger, and may be composed of three or more stages of multi-fluid heat exchangers.

The method of the present invention is such that the flow direction of the fluid in the low temperature section of the multifluid heat exchanger is a vertical upward flow for LNG and high pressure working fluid, while a vertical downward flow for low pressure working fluid.

A large volume change, that is, a change in flow rate, occurs in a multifluid heat exchanger when the fluid changes from gas to liquid or from liquid to gas. Among them, the one that shows the greatest change is the low-pressure working fluid due to its pressure.

As a result of our studies focusing on this point, the present inventors found that the low-pressure working fluid should flow vertically downward in the low-temperature section of the multi-fluid heat exchanger where the low-pressure working fluid condenses and the flow velocity is slowest. This method was found to be most effective in preventing back mixing due to the influence of gravity and maintaining stable operation at low loads.

Therefore, the LNG and high pressure working fluid in the low temperature section inevitably flow upward.

Although these fluids are not as bad as the low-pressure working fluid, there is a possibility that back-mixing may occur, so even under low load, the vertical upward flow rate of LNG and high-pressure working fluid at the inlet of the low-temperature section of the multifluid heat exchanger should be reduced to 4 crrL. It is preferable to keep the time at least 1/sec.

In addition, the flow velocity ratio of LNG and high-pressure working fluid at the outlet and inlet of the low temperature section of the multifluid heat exchanger should not exceed 10, respectively.
Keeping the outlet temperature of the multi-fluid heat exchanger low means that the flow path of the multi-fluid heat exchanger is constant from the inlet to the outlet, resulting in stable operation without back mixing due to the influence of gravity at low loads and at high loads. This is desirable in order to prevent an increase in pressure drop and maintain a high power recovery rate.

On the other hand, in the normal temperature section of the multifluid heat exchanger, it is preferable that the LNG and high-pressure working fluid flow vertically downward, and the low-pressure working fluid flow vertically upward.

In a multi-fluid heat exchanger, two or more fluids to be heated can be simultaneously heated with one heating fluid.

In the method of the present invention, in the multifluid heat exchanger, both the LNG and the high pressure working fluid are heated by the low pressure working fluid, and the low pressure working fluid is in turn cooled by these fluids, as described above.

Figure 2 shows the relationship between the heat exchange amount Q and temperature T of each fluid in the room temperature section 3' of this multi-fluid heat exchanger, where a is the heating curve of LNG, and b is the heating curve of the high-pressure working fluid. , C is a and b
d is the cooling curve of the low-pressure working fluid, and b' and C' are the curves of the high-pressure working fluid when slip occurs in the liquid part near the pinch point. A curve showing each change in is shown.

Paying attention to the curves and curve C in Figure 2, the evaporation start point (boiling point) BP on the curve forms an inflection point, and the corresponding point on curve C is the point in the room temperature section of the multifluid heat exchanger. Pinch point in heat exchange (minimum point of temperature difference for heat transfer) P
It becomes.

This pinch point P normally exists in the normal temperature section of the multifluid heat exchanger according to the present invention.

Assuming that the high-pressure working fluid has a vertical upward flow, if a slip occurs in the liquid part relative to the gas part near the evaporation start point BP of the high-pressure working fluid, heavy components (high boiling point components) in the working fluid near the pinch point As the ratio increases, the boiling point temperature rises, and the evaporation start point BP rises to BPI, and accordingly, the pinch point (P) on the synthetic heating curve of LNG and high-pressure working fluid also rises to Pl.

For this reason, the pinch point in heat exchange approaches the cooling curve d of the low-pressure working fluid, making it impossible to secure the temperature difference necessary for heat transfer, and causing a serious problem in the heat exchange process.

Furthermore, due to the above slip, the combined heating curve of LNG and high-pressure working fluid and the cooling curve of low-pressure working fluid may intersect (temperature reversal), and in this case, the specified heat exchange in the normal temperature section of the multifluid heat exchanger cannot be performed. becomes.

Therefore, the starting point B of evaporation of high pressure working fluid due to the influence of gravity
In order to reduce the temperature rise of P, in the normal temperature section of the multifluid heat exchanger, it is preferable that the LNG and high pressure working fluid flow vertically downward, and the low pressure working fluid flow upward.

However, the flow direction of each fluid may be arbitrarily selected if the above-mentioned slip does not occur, or even if it occurs, there is no problem with heat exchange due to the composition of the LNG and working fluid, the relationship between the heat transfer area and the load amount, etc. .

Generally, when there is a large amount of heavy fraction in LNG and working fluid, LNG is
Preferably, the high pressure working fluid is in a vertical downward flow.

It is desirable that the flow path width of the LNG and high-pressure working fluid in the room temperature section of the multifluid heat exchanger be wider than that in the low temperature section so that pressure loss does not become large even during high load operation.

LN between the normal temperature section and the low temperature section of the multifluid heat exchanger
The gas-liquid ratio of each of the G1 high-pressure working fluid and low-pressure working fluid fluctuates significantly in response to load fluctuations.

Normally, in industrial-scale regasification processes, heat exchange is carried out by installing a large number of multi-fluid heat exchangers in parallel.
When the amount of liquid phase in these fluids fluctuates significantly, these fluids tend to undergo gas-liquid separation at the inlet of the room-temperature section or the inlet of the low-temperature section, and as a result, each fluid is re-entered into its own numerous channels. During distribution, the ratio of the gas-liquid phase will be distributed in a non-uniform manner, resulting in a decrease in heat exchange performance.

Therefore, LN is installed between the room temperature section and the low temperature section of the multifluid heat exchanger.
A gas-liquid separator is installed for each of the G1 high-pressure working fluid and low-pressure working fluid to perform gas-liquid separation, and for all flow paths at the downstream multifluid heat exchanger inlet corresponding to the fluids. By uniformly redistributing the gas phase and the liquid phase, it is possible to prevent the heat exchange performance from deteriorating due to non-uniform distribution of gas and liquid due to load fluctuations, which is a further improved method of the present invention.

As described above, the method of the present invention provides a method for recovering cold energy while regasifying LNG, which enables stable operation even when the load fluctuates, especially when the load becomes low. Efficiency during load <LNG? It is possible to recover e-heat, and it is extremely effective in industrial implementation.

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

Example 1 The LNG regasification power recovery process shown in FIG. 1 was operated.

LNG is stored in an LNG storage tank 1 at -160°C, and is pressurized to 26 ata by an LNG supply pump 2 and sent to the regasification process.

The delivery flow rate at this time was varied from a maximum of 100 t/h to a minimum of 251 t/h depending on demand.

Regasification is performed by exchanging heat with the working fluid in the multifluid heat exchanger low temperature section 3 and the multifluid heat exchanger normal temperature section 3'.

The multifluid heat exchanger low temperature section 3 has a length of 1.000 mm and a width of 800 mm.
6 plate fin heat exchangers with a width of 5,000 mm in height
They are connected in parallel, and are installed so that the bottom direction in the drawing is the vertical direction.

Multi-fluid fluid exchanger room temperature section 3' is 1,100 mm long x 9 wide
It is constructed by connecting around 100 plate-fin heat exchangers with a height of 4+ OOO mm, and these are also installed so that the bottom direction in the drawing is the vertical direction.

The working fluids are methane, ethane, propane, i-butane and n-butane at 28.0.45.0.16.0, respectively.
It is a mixture having a composition of 4.4 and 6.6 molar ratios.

When LNG leaves the low-temperature section 3 of the multifluid heat exchanger, it is in a gas-liquid mixed phase, and its temperature and gas-liquid volume ratio are such that the flow rate is 100 t/h.
When the flow rate was 251/h, the temperature was -85°C and 90:10, and when the flow rate was 251/h, the temperature was -53°C and 99:1.Then, it entered the LNG intermediate drum 4 and was separated into gas and liquid.

The drum 4 contains 10 LNGs in the normal temperature section 3' of the multifluid heat exchanger.
It is installed to uniformly distribute gas and liquid in the flow path.

Most of the LNG leaving the normal temperature section 3' was vaporized, and further heated to 12.5° C. in a heat exchanger 5 with seawater to completely stifle it before being sent to consumers.

On the other hand, the working fluid is circulated forming a closed cycle, and the high-temperature, low-pressure working fluid that comes out of the working fluid turbine 6 is mostly in the gas phase, with a pressure of 3.1 ata and a temperature of -2 ,0 to -25,0°C.

The working fluid then enters the room temperature section 3' of the multi-fluid heat exchanger, where it is cooled to -72 to -50°C while imparting latent heat of condensation to the LNG and the low-temperature, high-pressure working fluid. ', the low-pressure working fluid enters the intermediate drum 13 for gas-liquid separation, and is then uniformly distributed to the six channels of the low-temperature section 3' of the multi-fluid heat exchanger.

The low-pressure working fluid that entered the low-temperature section 3 exchanged heat with the LNG and high-pressure working fluid again, and when it left the low-temperature section, it was all liquefied and the temperature was -129°C.

The low-temperature, low-pressure working fluid thus liquefied is stored in the working fluid storage tank 8.
is stored at a predetermined pressure 14.8a by the working fluid pump 9.
The fluid is pressurized to ta, becomes a low-temperature, high-pressure working fluid, and is led again to the multi-fluid heat exchangers 3 and 3'X, where it is heated together with LNG by the previous low-pressure working fluid and becomes a gas-liquid multiphase fluid.

At this time, the temperature at which the high-pressure working fluid exits the low-temperature section 3 is approximately equal to the temperature at which LNG exits the low-temperature section 3, and the gas-liquid volume ratio is -85°C when the flow rate is 1oot, 'h, O: 100.
When the flow rate is 25t/h, it is -53°G, 88:12
The high-pressure working fluid then entered the intermediate drum 10 and was separated into gas and liquid.

This drum, located between the low-temperature section 3 and the normal-temperature section 3' of the multifluid heat exchanger, is also provided to ensure uniform gas-liquid distribution, and the high-pressure working fluid that exits the drum is divided into gas and liquid phases. The liquid phase was uniformly redistributed and introduced into 10 channels in the room temperature section.

Since the high-pressure working fluid leaving the multi-fluid heat exchanger room temperature section 3 is a gas-liquid multiphase flow containing unvaporized gas, the working fluid heater 11
At this time, the water was further heated by seawater as an external high heat source, and was completely vaporized and heated to a temperature of 12.5°C, close to the dew point.

This high-pressure working fluid heated to a high temperature is led to the working fluid turbine 6 via a gas-liquid separation provided at the turbine inlet.
Here, the working fluid turbine 6 is rotated to generate power, and at the same time, the pressure and temperature are reduced, and the working fluid is circulated again as a low-pressure working fluid to the normal temperature section 3' of the multifluid heat exchanger to form a closed cycle.

From a generator 7 connected to a working fluid turbine 6,
Approximately 4,000k when LNG flow rate is 100t/h
At 25 t/h, approximately 600 kW of power was obtained.

Note that the flow rates of LNG, high-pressure working fluid, and low-pressure working fluid in each multifluid heat exchanger are as shown in Table 1, and the flow rate of the upward flow is always maintained at 4 crfL/sec or more.

[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. FIG. 2 is a conceptual diagram showing the relationship between the heat exchange amount Q and temperature T of each fluid in the room temperature section of the multifluid heat exchanger. 1: LNG storage tank, 2: LNG supply pump, 3: Multifluid heat exchanger low temperature section, 3': Multifluid heat exchanger normal temperature section, 4
: LNG intermediate drum, 5: LNG heater, 6: working fluid turbine, γ: generator, 8: working fluid storage tank, 9 two working fluid pumps, 10: high pressure working fluid intermediate drum, 11:
Working fluid heater, 12: Turbine inlet gas-liquid separator, 13
: Low-pressure working fluid intermediate drum, a: LNG heating curve, b
: Heating curve of high pressure working fluid, c: Combined heating curve of LNG and high pressure working fluid, d: Cooling curve of low pressure working fluid, BP:
The starting point of evaporation of high-pressure working fluid, P: pinch point.

Claims (1)

[Claims] 1. In regasification of liquefied natural gas (LNG), LN
G is used as a cold heat source, and when rotating a turbine and recovering power while performing a Rankine cycle on a multi-component mixed working fluid using an external high heat source, heat exchange between LNG and the working fluid is performed between the room temperature part and the working fluid. In the low temperature section of the multifluid heat exchanger, the LNG and high pressure working fluid flow vertically upward, and the low pressure working fluid flows vertically downward. A method for recovering power from liquefied natural gas, characterized by: 2. The method according to claim 1, wherein the velocity of the vertical upward flow of LNG and high-pressure working fluid at the inlet of the low-temperature section of the multifluid heat exchanger is maintained at 4 crrL/sec or more. 3. Maintaining the outlet temperature of the low-temperature section of the multi-fluid heat exchanger so that the flow velocity ratio of the LNG and the high-pressure working fluid between the outlet and the inlet of the low-temperature section of the multi-fluid heat exchanger does not exceed 10. The method described in Scope 1 or 2. 4. The method according to claim 1, 2 or 3, wherein in the normal temperature section of the multi-fluid heat exchanger, the LNG and high pressure working fluid flow vertically downward and the low temperature working fluid flows vertically upward. 5 In the regasification of liquefied natural gas (LNG), LN
When recovering power by rotating a turbine while performing a Rankine cycle on a multi-component mixed working fluid using G as a cold heat source and an external high heat source, the heat exchanger between LNG and the working fluid is set to 0 and room temperature. The method is carried out in a multi-fluid heat exchanger that is divided into a low-temperature section and a low-temperature section, and a gas-liquid separator is installed in each fluid flow path between the normal-temperature section and the low-temperature section of the multi-fluid heat exchanger. so that the gas and liquid phases of each fluid are uniformly distributed, and in the low temperature section of the multi-fluid heat exchanger, the LNG and high pressure working fluid have a vertical upward flow, and the low pressure working fluid has a vertical downward flow. A method for recovering power from liquefied natural gas, characterized by: 6. The method according to claim 5, wherein in the normal temperature section of the multifluid heat exchanger, LNG and high pressure working fluid flow vertically downward, and low pressure working fluid flows vertically upward.