JPH0633868B2 - Operation method of liquefied gas storage tank - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention relates to a method for operating a liquefied gas storage tank such as an LNG storage tank at an LNG receiving terminal.

[Conventional technology and its problems]

For example, when a new liquid is received in a liquefied gas storage tank such as an LNG storage tank at an LNG receiving terminal, if the storage liquid remaining in the storage tank and the newly received reception liquid have different densities, these liquids As shown in FIG. 1 (a), the storage tank 1 may be layered into two layers, a lower-layer liquid 2L and an upper-layer liquid 2U having a high density.

When layered in this manner, a sharp temperature gradient is formed near the interface between the upper and lower liquids 2U and 2L, and the upper and lower liquids 2U and 2L
As shown in Fig. 1 (b), independent thermal convections 3 in 2L
U and 3L occur and both layers do not mix easily. Although the upper layer liquid 2U is cooled by evaporation from the surface, the lower layer liquid 2L is heated by the heat input from the bottom wall and the side wall of the storage tank 1 and its temperature rises, and its density dL is shown in FIG. It gradually decreases as shown in (b). Thus, the upper and lower layer liquid 2
When there is no difference in the densities dU and dL of U and 2L, in the storage tank 1, as shown in FIG. 1 (c), the development in which the upper and lower layer liquids 2U and 2L are rapidly mixed in an extremely short time is developed. Occurs and this is called the rollover phenomenon. When such a rollover phenomenon occurs, the amount of BOG (boil-off gas) rapidly increases as shown in FIG. 2, and it is dangerous if the processing capacity is exceeded.

For this reason, conventionally, at LNG receiving terminals, etc., a roll-over phenomenon is prevented by providing a storage tank for each liquid type or dividing the storage tank into a receiving tank and a stock tank so that the liquid level can be received at a low level. However, this method has a problem that the number of storage tanks increases because different storage tanks are required for each liquid type and application.

By the way, many studies have recently been reported on the process of destabilization or mixing of stratified liquid, the mechanism of rollover phenomenon, and the like. Among them, for example, “Experimental research on rollover phenomenon in LNG storage tank”, “Mitsubishi Heavy Industries Technical Report”, vol. 21, No.2 Reprint, 19
1984, P. 1-P. 11 discloses a research result in which the accuracy of the prediction of the occurrence of the rollover phenomenon is improved by creating a simulation model in which the experimental result is folded.

Therefore, a method of predicting the occurrence of the rollover phenomenon disclosed in this document will be described.

In this method, as shown in FIG. 3, the storage liquid in the storage tank is stratified into upper and lower two layers, and a state in which a heat load is applied from the side wall and the bottom is assumed as an analytical model. Upper and lower layer liquid 2U, 2L at a certain moment
The amount of heat and the amount of transfer of the substance are calculated from the state of the above, and the state of the liquid after a minute time is calculated from the mass balance and the heat balance. In this way, the process of changing the liquid state is sequentially calculated, and the upper and lower layer liquids 2U and 2L are calculated. The difference between the densities of the liquids disappears, or when the interface position reaches the bottom or surface of the liquid, it is judged that the lower layer interface has disappeared and a complete mixed state has occurred, that is, the rollover phenomenon has occurred. The time when the rollover phenomenon occurs and the amount of evaporation at that time, that is, the amount of BOG generated is predicted. When such a method is applied to an actual LNG storage tank, LNG is added to methane (CH ₄ ), ethane, (C
₂ H ₆ ), propane (C ₃ H ₈ ) butane, (C
₄ H ₁₀ ), pentane (C ₅ H ₁₂ ) and nitrogen (N ₂ ).
It is treated as a 6-component system, and the rollover phenomenon is predicted by the calculation based on the calculation formula in the following table. The above calculation is performed for the actual measurement example in the LNG storage tank, and the result is compared with the actual measurement value. It has been disclosed that it has been confirmed that it is sufficiently effective in predicting the rollover phenomenon.

The symbols in the above table and FIG. 3 are as follows.

Cp: specific heat (kcal / kg ° C.) F: buoyancy (N) g: acceleration of gravity (m / S ² ) H: liquid depth (m) H _T : total layer liquid depth (H _T = H _L + Hu) (m ) Pr: Prandtl number q _O : Horizontal fluid layer heat flux (kcal / m ² h) q _B : Bottom heat flux (kcal / m ² h) q _G : Evaporation heat flux (kcal / m ² h) q _M : Two Layer interface heat flux (kcal / m ² h) q _W : Side wall heat flux (kcal / m ² h) q _X : Interface heat flux associated with mixing (kcal / m ² h) R: Stability factor R _SCR : γ: Latent heat of vaporization (kcal / kg) S: Impurity concentration (kg / kg) T: Temperature (° C) T _SAT : Saturation temperature (° C) α: Body expansion coefficient by impurity concentration β: Body expansion coefficient by temperature (1 / ° C) γ: Specific weight (kg / m ³ ) λ: Thermal conductivity (kcal / mh ° C) ν: Kinetic viscosity coefficient (m ² / S) φ _G : Mass transfer flux due to evaporation (kg / m ² h) φS: By diffusion Mass transfer flux of the surface ^{(kg / m 2 h) φ} X: mass transfer flux interface with a mixed ^{(kg / m 2 h) φS} *: dimensionless mass transfer amount [αCpφS / βq _M] .phi.S ₀ *: .phi.S ₁ *: φST: [Object of the Invention] The present invention was devised in view of the above points, that is, the prediction of the rollover phenomenon by a simulation model including experimental results such as the method disclosed in the above-mentioned literature. By operating the liquefied gas storage tank by applying the method rationally, the storage tank can be managed safely against the rollover phenomenon, so that the small storage tank can be operated efficiently and a large amount of liquefied gas can be handled. The purpose is to make it possible.

[Structure of Invention]

The operation method of the present invention for achieving such an object will be described with reference to the flow charts of FIGS. 4 and 5 corresponding to the embodiment. The operation method of the liquefied gas storage tank of the present invention is as follows. Situation observation process P1 for observing the composition, state quantity, and situation of the liquid quantity of the stored liquid 2, a stratification determination process P2 for determining stratification from the observed situation of the stored liquid 2, and acceptance of future liquefied gas and A situation prediction process P3 that predicts the future situation of the storage liquid 2 in the storage tank 1 from the consumption condition, and a stratification determination process P4 that predicts stratification based on the predicted storage liquid condition.
And the stratification determination process P2 or the stratification prediction process P4
Corresponding to the stratification judgment or stratification prediction in each case, the rollover phenomenon occurrence time and the BOG generation amount at that time are predicted from the composition, state quantity and liquid quantity of the upper and lower layer liquids 2U and 2L. Based on the rollover prediction process P5 and the rollover phenomenon occurrence time and the BOG generation amount predicted by the rollover prediction process P5, the stratification is eliminated by the consumption of the lower layer liquid before the occurrence time.
6, the operation selection process P9 that selects either the operation process P7 that allows the occurrence of the rollover phenomenon or the operation process P8 that operates the stratified positive elimination means 4 is summarized.

[Operation and Example]

 Next, the operation of the present invention will be described based on examples.

[I] Situation Observing Process P1 First, in the situation observing process P1, the situation of the stored liquefied gas storage tank 2 is observed by a predetermined observing means 5. Observation means 5
Is for observing the composition, state quantity, and liquid quantity of the stored liquefaction 2 necessary for the stratification judgment process in the stratification judgment process P2 and the rollover prediction in the rollover prediction process P5. In the embodiment shown in FIG. 4, the temperature distribution measuring means 5a for measuring the temperature distribution in the height direction by the temperature sensors 6 arranged in the height direction in the storage tank 1 and the storage tank 1 are used.
It comprises a composition measuring means 5b such as a gas chromatograph for measuring the composition in the upper and lower parts of the stored liquid 2 and the gas layer, and a pressure measuring means 5c for measuring the pressure of the gas layer.

In such a configuration, from the temperature distribution in the height direction measured by the temperature distribution measuring means 5a, the depth of the storage liquid 2 and the steep temperature gradient near the interface between the upper and lower liquids when stratified As a result, the depth of such an interface can be known, and the stored liquid 2 can be obtained from these, the composition by the composition measuring means 5b, the gas layer pressure by the pressure measuring means 5c, and the known inner diameter D of the storage tank 1. It is possible to observe the composition, state quantity, and liquid quantity situation of. At this time, necessary state quantities that are not directly measured, that is, the specific weight γ, the specific heat Cp, the thermal conductivity λ, the static viscosity coefficient μ, the Prandtl number Pr, etc., are the temperature of the stored liquid 2 measured by the temperature distribution measuring means 5a, It can be derived based on the composition by the composition measuring means 5b and the gas layer pressure by the pressure measuring means 5c. Of course, when the storage liquid 2 is layered, these state quantities can be derived for each layer. it can. And the liquid volume W
(Kg), liquid depth H (m), heat content Q (kcal), etc. are [W = πD ² Hγ / 4] and [H = 4W, respectively]
/ [Pi] D < ² > [gamma]] and [Q = CpWT] equation can be derived from the derived state quantity.

[II] Stratification determination process P2 In the stratification determination process P2, stratification is determined based on the situation of the stock solution 2 observed in the conventional situation observation process P1. That is, as described above, the determination of stratification can be made by comparing the presence or absence of a rapid temperature gradient formed near the interface between the upper and lower layer liquids and the specific weight calculated from the measured value. In addition to this, the storage tank 1 may be provided with a measuring means for measuring the density distribution in the height direction, and the stratification can be determined by the density difference between the upper and lower layer liquids.

[III] Situation forecasting process P3 and stratification forecasting process P4 Situation forecasting process P3 is based on the conditions of future reception and consumption of liquefied gas and the situation of stored liquid 2 observed in the situation forecasting process P1. The future situation of the stock solution 2 of 1. is predicted, and the stratification prediction process P4 predicts stratification based on the future situation. Even if the stock solution 2 is not currently stratified, these processes P3 and P4 can appropriately respond to stratification by predicting stratification due to future reception.

In the above processes P1, P2, P3, and P4, if stratification is not determined and predicted, the previous operation is continued. Then, if stratification is determined or predicted, in the next rollover prediction process P5, the time when the rollover phenomenon occurs and the amount of BOG generation at that time are predicted.

[IV] Rollover prediction process P5 In the rollover prediction process P5, the situation observation process P
1) or predicted in the situation prediction process P3, based on the composition, state quantity and liquid quantity of the lower layer liquids 2U and 2L, and the assumed heat flux from the bottom surface, side surface and liquid surface Point of occurrence of rollover phenomenon and BOG at that time
Predict the amount generated.

The rollover prediction is, for example, as disclosed in the above-mentioned document, as shown in FIG. 3, the storage liquid 2 in the storage tank 1 is stratified into upper and lower two layers and is subjected to a heat load from the side wall and the bottom surface. Is assumed as an analytical model, and the upper and lower liquids 2U and 2L are
The amount of heat and the amount of transfer of the substance are calculated from the state of the above, and the state of the liquid after a minute time is calculated from the mass balance and the heat balance. In this way, the process of changing the liquid state is sequentially calculated, and the upper and lower liquids 2U and 2L are calculated. The difference between the densities of the liquids disappears, or when the interface position reaches the bottom or surface of the liquid, it is judged that the lower layer interface has disappeared and a complete mixed state has occurred, that is, the rollover phenomenon has occurred. When the rollover phenomenon occurs,
The amount of evaporation at that time, that is, the amount of BOG generated can be predicted, and the above-mentioned formula can be used as a specific calculation formula.

In addition, in the storage tank 1, an intermediate layer may be formed between the upper and lower portions 2U and 2L, and in this case, the calculation may be performed based on an empirical formula considering the existence of the intermediate layer.
The presence of such an intermediate layer can be known along with the determination or prediction of the stratification.

[V] Operation selection process P9 In the operation selection process P9, the following operation selection is performed based on the time of occurrence of the rollover phenomenon predicted by the rollover prediction process P5 and the BOG generation amount at that time.

First, it is determined whether or not the lower layer liquid 2L is consumed before the predicted occurrence of the rollover phenomenon, and when it is consumed, the operation process P6 is selected. Therefore, in the operation process P6, the lower layer liquid 2L is generated before the predicted occurrence of the rollover phenomenon.
Is consumed, so the stratification is eliminated.

Next, at the time when the rollover phenomenon is predicted, the BO
The safe BOG processable amount by the G processing means 7 is compared with the predicted BOG generation amount, and the BO amount is higher than the BOG generation amount.
When the G treatable amount is larger, the driving process P7 is selected, and when it is smaller, the driving process P8 is selected.

Even if a rollover phenomenon occurs in the driving process P7, this can be safely processed by the BOG processing means 7. In addition, in the operation process P8, the stratification positive elimination means 4 such as a jet nozzle is operated to eliminate stratification, so that it is possible to prevent the occurrence of a rollover phenomenon that causes BOG exceeding the BOG processable amount. .

According to the present invention, by repeating the above process to operate the storage tank 1, it is possible to always operate on the safe side against the stratification of the stored liquid 2 in the storage tank 1 and the rollover phenomenon caused by the stratification. it can.

〔The invention's effect〕

INDUSTRIAL APPLICABILITY As described above, the present invention observes the state of the stored liquid in the liquefied gas storage tank, predicts the state of the stored liquid in the future from the conditions of receiving and consuming the liquefied gas in the future, and the rollover phenomenon for stratification. Since the operation is selected based on the prediction, it is possible to always operate on the safe side against the stratification of the stored liquid in the storage tank and the rollover phenomenon caused by this. It is possible to efficiently operate the storage tank without dividing the storage tank into a receiving tank and a storage tank so that it can be installed according to the liquid type and to receive at a low liquid level. It has the effect of being able to handle gas.

[Brief description of drawings]

FIGS. 1 (a), (b), and (c) are explanatory views that schematically show the stratification of the stored liquid in the storage tank to the occurrence of the rollover phenomenon,
2 (a) and 2 (b) correspond to the state of FIG. 1, and are explanatory diagrams that schematically show changes in BOG amount and liquid density, respectively, with respect to time, and FIG. 3 is for rollover prediction. FIG. 4 is a schematic explanatory view showing an example of an analytical model used, FIG. 4 is an explanatory view schematically showing an example of the configuration of a storage tank to which the present invention is applied, and FIG. 5 is an example of a flow chart to which the operating method of the present invention is applied. It is a figure. Reference numeral 1 ... storage tank, 2 (2U, 2L) ... storage liquid, 3 (3
U, 3L) ... Heat convection, 4 ... Means for eliminating stratification, 5
...... Observing means, 5a ...... Temperature distribution measuring means, 5b ...... Composition measuring means, 5c ...... Pressure measuring means, 6 ...... Temperature sensor, 7 ...... BOG processing means, P1 ...... Situation observing process, P
2 ... Stratification judgment process, P3 ... Situation prediction process, P4 ...
... Stratification prediction process, P5 ... Rollover prediction process, P
6, P7, P8 ... Driving process, P9 ... Driving selection process.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kaoru Igarashi 12 Nishiki-cho, Naka-ku, Yokohama, Kanagawa Mitsubishi Heavy Industries, Ltd.Yokohama Works (72) Inventor Mikio Tateiwa 12 Nishiki-cho, Naka-ku, Yokohama, Japan Mitsubishi Heavy Industries Yokohama Institute Co., Ltd. (56) Bibliographic reference Sho 59-123798 (JP, U)

Claims (1)

[Claims] 1. A situation observation process for observing the composition, state quantity, and liquid quantity of a storage liquid in a liquefied gas storage tank, a stratification determination process for determining stratification from the observed storage liquid state, and a future From the acceptance and consumption conditions of the liquefied gas, a situation prediction process that predicts the future situation of the storage liquid in the storage tank, and a stratification prediction process that predicts stratification based on the predicted storage liquid condition,
Corresponding to each of the stratification determination or stratification prediction in the stratification determination process or stratification prediction process, the composition of each of the upper and lower layer liquids, the state amount and the amount of liquid from the time of occurrence of the rollover phenomenon and its Based on the rollover prediction process for predicting the BOG generation amount at this time, the rollover phenomenon occurrence time and the BOG generation amount predicted by the rollover prediction process, stratification is achieved by the consumption of the lower layer liquid before the occurrence time. A method for operating a liquefied gas storage tank, comprising: an operation process for eliminating, an operation process for allowing the occurrence of a rollover phenomenon, or an operation selecting process for selecting an operation process for operating the stratified positive elimination means.