JPH0650086B2 - Boil-off gas compressor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a compressor for supplying boil-off gas, particularly gas boiled-off from a liquefied natural gas tank, to a diesel engine or the like.

[Conventional technology]

An example of the prior art is shown in FIG. In the figure, the gas 21 boiled off from the liquefied natural gas tank 1 is introduced into the compressor 10 through a pipe 22, and the pressure is increased by the compressor 10.
It is cooled to normal temperature by seawater in the heat exchanger 5 and supplied to the diesel engine 6 as fuel. The compressor 10 is 1
As an example, it has a reciprocating type four-stage compression mechanism 11, 12, 13, and 14 and is driven by an electric motor 15. Each compression mechanism is a connecting pipe
It is connected by 23, and is equipped with an intake valve and a discharge valve, which are not shown in the drawing, but are required for gas suction and compression. An intergas cooler 20 for cooling with seawater is provided between the third-stage compression mechanism 13 and the fourth-stage compression mechanism 14.

Incidentally, this type of conventional technology is described in, for example, Journal of the Japan Society for Marine Engines, Vol. 19, No. 10 (October 1984), "Gas Induction Diesel Engine and Its Application to LNG Ships".

[Problems to be Solved by the Invention] Power is required to pressurize the boil-off gas. The higher the pressure to reach, the more power is needed.

As an example of one calculation, the capacity of a liquefied natural gas tank is 125,
When 0.1% of 000 m ^{3 is} boiled off per day, the compressor power required to pressurize it to 250 bar to supply it as engine fuel is about 700 kW.

[Means for solving problems]

(1) The compressor is provided with an injection port for adding liquefied natural gas to the boil-off gas being compressed, and the liquefied natural gas is added to the boil-off gas being compressed.

(2) In the compressor, the addition of liquefied natural gas is controlled so that the boil-off gas changes along the saturated vapor line of the TS diagram (see FIG. 2).

[Work]

(1) The temperature at the inlet of the boil-off gas to the compressor is lowered, and the rise in gas temperature during the compression process is suppressed to a minimum.

(2) There is almost no mixed-phase flow state consisting of the vapor phase of the boil-off gas to be pressurized and the liquid phase of the added liquefied gas.

〔Example〕

Figure 1 shows boil-off gas 21 from liquefied natural gas tank 1.
A system diagram for pressurizing and supplying the diesel engine 6 is shown. FIG. 2 is a T-S diagram of methane for explaining the state in the pressurizing step of the boil-off gas.

In FIG. 1, 1 is a liquefied natural gas tank, 2 is a liquefied natural gas transfer pump, 3 is a service tank, 4 is a liquefied natural gas pressurizing pump, 5 is a heat exchanger, 6 is a diesel engine, and 10 is a compressor. Indicates. Further, 11, 12, 13, 14 are compression mechanisms, 15 is an electric motor, 16, 17, 18, 19 are liquefied natural gas injection ports, 21 is boil-off gas, 22 is a pipe, and 23 is a connecting pipe.

The liquefied natural gas in the tank 1 is always evaporated by heat invasion from the outside, and boil-off gas 21 is generated. The boil-off gas 21 is pressurized in the compressor 10, and the liquefied gas is added to the boil-off gas 21 in the compressor 10. Liquefied natural gas to be added is liquefied gas transfer pump 2 from tank 1.
Is transferred to the service tank 3 through the transfer pipe 26, and the liquefied natural gas pressure pump 4 is used to liquefy the liquefied natural gas pipe 27.
After that, the liquid is injected into the cylinders from the liquefied gas injection ports 16, 17, 18, 19 provided in the cylinders of the compression mechanisms 11, 12, 13, 14 of the compressor 10.

The pressurized boil-off gas is cooled or heated by seawater in the heat exchanger 5 and then supplied as fuel to the diesel engine. In order to explain the state change of each process of boil-off gas with the TS diagram of FIG. 2, the following conditions are assumed. That is, the liquefied natural gas component is pure methane, the temperature of the boil-off gas at the compressor inlet supplied through the connecting pipe 22 is 130 ° K, and the pressure is 1 bar.
° K), a pressurized gas with a pressure of 250 bar shall be obtained. It is assumed that the liquefied natural gas droplets injected from the injection ports 16, 17, 18, 19 are ideally minute, and are mixed homogeneously in the boil-off gas. In addition, heat penetration from the outside and mechanical friction, etc. Shall be ignored.

In FIG. 2, the vertical axis represents temperature, the horizontal axis represents entropy, AB is a saturated liquid line, I-J-K-F is a saturated vapor line,
MH indicates a 250 bar isobar and AIL indicates a 1 bar isobar.

The state of the boil-off gas at the compressor inlet is indicated by point L.
If this is simply compressed in 4 stages and pressurized to 250 bar, it will reach point M. The point is reached by cooling this to room temperature (approximately 300 ° K) with 250 bar of constant pressure seawater. This is the condition that is supplied to the diesel engine. LM
The enthalpy difference between the two is about 170 Kcal / kg, and the enthalpy difference between M and H is about -130 Kcal. It requires kinetic energy equivalent to about 170 Kcal per kg of boil-off gas, which means that most of about 130 Kcal is wasted into seawater as heat energy.

Next, a case where the liquefied gas is injected and added to the boil-off gas will be described. At the starting point of compressing 0.5 kg of liquefied gas at A point to 1 kg of boil-off gas at L point, if an example is given in which the entire amount of liquefied gas is injected at 1 o'clock, the state after injection is a two-phase indicated by C point. Become a fluid. When this is compressed to 250 bar in four stages, it becomes the H point. The temperature at point H is about 300 ° K.
The enthalpy difference between O and H is about 85 Kcal / kg. L
0.5 kg of liquefied gas at point A is added to 1 kg of boil-off gas at point to make 1.5 kg of two-phase fluid at point C. The power energy required to compress this into 1.5 kg of pressurized gas at point H is 8
This is equivalent to 5 Kcal / kg × 1.5 kg ≈ 130 Kcal. Compared to the case of compressing the boil-off gas at point L, the power energy required is less, and the amount of gas at point H obtained is 1.5 times. If the amount of liquefied gas added to 1 kg of boil-off gas is more than 0.5 kg, the obtained two-phase flow state will be at the point A side from point C, for example C ', and if it is less, it will be C ″, up to 250 bar. When compressed, H ',
The temperature of the pressurized gas that can become H ″ changes. If this temperature exceeds the range allowed for supplying to the diesel engine 6, it is heated or cooled by seawater in the heat exchanger 5. Boil-off beforehand If the ratio of the liquefied gas to be added to the gas can be set within a certain limited range, that is, the compressor
10 If the gas temperature at the outlet can be set within the allowable range, heat exchanger 5
Can be omitted.

The state indicated by the point C is a two-phase flow which is a mixture of the liquid phase indicated by the point A and the gas phase indicated by the point I.
In the compression process up to the point, the compression from the point C to the point F is a two-phase flow. Explaining each compression mechanism of the compressor 10, the first compression mechanism 11 compresses the two-phase flow from point C to point D, and the second compression mechanism 12 compresses the two-phase flow from point D to point E. The phase flow is compressed, and the third compression mechanism 13 starts compressing the two-phase flow at point E. At point F in the middle of the compression process, the liquid phase is completely evaporated and the liquid phase is lost. , From point F to point G, only the gas phase is compressed.

The above explanation is based on the assumption that the entire amount of the liquefied gas to be added is first added at one time and that the liquid phase component and the gas phase component are ideally and homogeneously mixed. In the actual handling of two-phase flow, for example, the liquid phase component adheres to the surface of the device, impedes the smooth operation of the device, exchanges heat between the two phases, and the state conversion does not proceed ideally. There is. If the compression is performed outside the range of the two-phase flow, the gas temperature to be handled increases and the required power increases. Therefore, the addition of the liquefied gas in the present invention is performed so that the boil-off gas in the compression process changes along the saturated vapor line in the TS diagram. In the case of adding 0.5 kg of liquefied gas in total to 1 kg of boil-off gas, in the first compression mechanism 11 of the compressor 10, first, in the process of sucking the wheel-off gas into the cylinder, The part is injected from the injection port 16 to change the state of the boil-off gas from the L point to the I point. Then, when an appropriate amount of liquefied gas is injected corresponding to the compression process, the state of the gas reaches from point I to point J. Similarly, in the second compression mechanism 12, an appropriate amount of liquefied gas is injected from the injection port 17 corresponding to the compression process, and the state of the gas reaches the point K from the point J. Third compression mechanism 13
At this time, the liquefied gas corresponding to the compression process
However, since the amount of gas is small, the state of the gas changes from point K to point F, where it reaches point G apart from the saturated steam. In the fourth compressor 14, there is no injection of liquefied gas, and pressure is applied from point G to point H. Boil-off gas 1kg
On the other hand, in this example in which 0.5 kg of liquefied gas is added, the injection port 19 is not used and the heat exchanger 5 is unnecessary.

The injection addition of liquefied gas in each compression process is performed so that the gas changes along the saturated vapor line, but it takes a little time for the injected liquefied gas to mix and vaporize, so this should be taken into consideration. Then, the injection is performed earlier than the theoretical value on the T-S line.

〔The invention's effect〕

(1) Not only can the temperature before pressurizing the boil-off gas be lowered, but also the temperature rise during the compression process can be suppressed to the lowest level, which can reduce the pressurizing power of the boil-off gas. .

(2) The amount of high-pressure gas increases by the amount of liquefied gas added to the compression mechanism.

(3) Since the addition of liquefied gas is properly controlled, the problems that occur when handling a two-phase flow do not occur.

[Brief description of drawings]

FIG. 1 is a system diagram of a boil-off gas compressor and its front and rear as one embodiment of the present invention, FIG. 2 is a T-S diagram for explaining the state of gas in FIG. 1, and FIG. 2 is a system diagram of the boil-off gas compressor of FIG. 1 liquefied natural gas tank 5 ...... heat exchanger 6 …… Diesel engine 10 …… compressor 11, 12, 13, 14 …… compression mechanism 16, 17, 18, 19 …… liquefied natural gas injection port 21 …… Boil-off gas 27 …… Liquefied natural gas pipe

Claims (1)

[Claims] 1. A liquefied natural gas for injecting liquefied natural gas into the compressor in a boil-off gas compression device that guides the gas boiled off from a liquefied natural gas tank to a compressor and supplies it to a prime mover via a heat exchanger after pressurization. A boil-off gas compressor comprising: a supply means; and a control means for adjusting the injection amount of the liquefied natural gas so that the boil-off gas changes along the saturated vapor line of the TS diagram.