JPH02138600A - Operating method of liquefied gas storage tank - Google Patents

Abstract

PURPOSE:To operate a storage tank efficiently by providing such a constitution as selecting the operation based on the observation of the situation of a stored liquid in a liquefied gas storage tank, the forecast of the future situation of the stored liquid from the future receipt and consumption conditions of the liquefied gas, and the forecast of roll over phenomenon to the layering. CONSTITUTION:A situation observation process P1 of the composition, quantity of state, and liquid amount of a stored liquid in a liquefied gas storage tank, a process P2 of judging a layering from the observed situation of the stored liquid, a process P3 of forecasting the future situation of the stored liquid in the storage tank, and a process P4 of forecasting the layering based on the forecasted situation of the stored liquid are provided. Also, a process P5 of forecasting the occurrence time of roll over phenomenon from the composition, quantity of stated, and liquid amount of each of the upper and lower layer solutions corresponding to each of the layering judgment or layering forecast in the process P2 or P4 and the BOG yield at that time and an operation selecting process P9 of selecting any of operation processes P7, P8 based on the process P5 are included.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention comprises 1. 1. This relates to the method of operating liquefied gas storage tanks such as NG storage tanks at NG receiving terminals.

[Conventional technology and its problems]

For example, when newly receiving liquid in a liquefied gas storage tank such as LNG storage ffI at an LNG receiving terminal, if the density of the stored liquid remaining in the storage tank and the newly received liquid are different, , as shown in Figure 1(a), the amount of liquid in storage tank 1
In the liquid, the liquid may be stratified into two layers: the lower layer liquid 2[, which has a higher density, and the upper layer liquid 2U].

When stratified in this way, a severe 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 form a severe temperature gradient.
As shown in FIG. 1(b), independent thermal convection 3U and 3L occur within 2L, and the two layers do not mix easily. Therefore, although the upper layer liquid 2U receives a cooling effect due to evaporation from the surface, the temperature of the lower layer liquid 2[, increases due to heat input from the bottom wall and side wall of the storage tank 1, and its density d[ becomes 2
It gradually decreases as shown in Figure (b). Thus, above,
When there is no difference in the densities dU and dL of the lower liquids 2U and 2L, the upper and lower liquids 2U and 2 [in the storage tank 1, as shown in FIG. A mixing phenomenon occurs, which is referred to as a rollover phenomenon. When such a rollover phenomenon occurs, the amount of BOG (boil-off gas) increases rapidly as shown in FIG. 2, and it is dangerous if the amount exceeds the processing capacity.

For this reason, in the past, at LNG receiving terminals, etc., storage tanks were provided for each type of oil, or the storage tanks were divided into receiving tanks and stockpiling tanks so that the liquid level could be lowered to allow for the rollover. However, this method has the problem of requiring a large number of storage tanks because storage tanks are required for each oil type and application.

Incidentally, many studies have recently been reported on the destabilization or mixing process of stratified liquids, the mechanism of rollover phenomenon, and the like.

Among them, for example, experimental study of rollover phenomenon in rLNG storage tank, "Mitsubishi Heavy Industries Technical Report", Vol. 21
, k2 reprint, 1984, P, 1-P.

No. 11 discloses the results of research aimed at improving the accuracy of predicting the occurrence of rollover phenomena by creating a simulation model that incorporates experimental results.

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

In this method, as shown in Fig. 3, an analytical model is assumed in which the liquid stored in the storage tank is stratified into two layers, an upper layer and a lower layer, and receives heat load from the side wall and bottom surface. Upper and lower liquids 2L and 1.2L at a certain moment
Determine the heat and mass transfer II from the state of
When the difference in density disappears or the interface position reaches the bottom or surface of the liquid, it is determined that the upper and lower interfaces have disappeared and a complete mixing state has been achieved, that is, a rollover phenomenon has occurred, In this way, the time point at which the rollover phenomenon occurs and the evaporation point at that time, that is, the occurrence of BOG, etc. are predicted. This method is actually 1. When applied to N G storage tanks, L N G can be used to store methane (CHa), ethane (C2He), propane (C3Hs), butane (
04H1o), pentane (C5H12) and nitrogen (N2
) as a six-component system, and based on the calculation formula in the numerical table < if
This method predicts rollover phenomena by calculating
Then, as a result of performing the above calculations on the actual measurement example of L N GIFi'l'l and comparing it with the actual measurement value,
There is a disclosure that it has been confirmed that 1 minute is effective in predicting actual rollover phenomena.

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

Cp: Specific heat (kcal/#'C)F
: Buoyancy (N) Q : Acceleration of gravity (m/52) H : Liquid depth
(m) HT: Total liquid depth (HT - HL + Hu) (m
) Pr Nybrantl number qo: horizontal fluid layer heat flux (kcal/Tdh) bottom heat flux (kcal/situation h) evaporation heat flux
(kcal/butt h) Two-layer interfacial heat flux (kcal
l/TIth) side wall heat flux (kcal/Td
h) Interfacial heat flux (kcal/Tdh) stability factor due to mixing a Q 0M 0w x R γ ■ au α β γ λ ν φC Latent heat of vaporization (kcal/Kg) Impurity concentration
(Kg/Ky) Temperature (”
C) Saturation temperature ('C) Body expansion coefficient due to impurity 11 degrees Body expansion coefficient due to temperature (1/'C) Specific V amount (Kg/#+3) Thermal conductivity
(kcal/mh'c) Kinematic viscosity coefficient
(GO/S) Mass transfer flux due to evaporation (##h) φS: Mass transfer flux at the interface due to diffusion (phantom/ydi) φX: Mass transfer flux at the interface due to mixing (Kg/Td
h) φS book: Dimensional mass transfer m [αcpφS/βqM) φ So book: φS 1 book: φST: [Object of the invention] The present invention was created in view of the above points, that is, the previous)
By operating liquefied gas storage tanks by rationally applying methods for predicting rollover phenomena based on simulation models incorporating experimental results, such as the method disclosed in the published literature, we can safely prevent rollover phenomena. The purpose of this system is to manage storage tanks, thereby efficiently operating a small number of storage tanks so that a large amount of liquefied gas can be handled.

[Structure of the invention]

The operating method of the present invention to achieve this object will be explained with reference to the flowcharts of FIGS. 4 and 5 corresponding to the embodiment. A situation observation process P1 that observes the composition, state quantity, and liquid volume of the stored liquid 2; a stratification determination process P2 that determines stratification from the observed situation of the stored liquid 2; A situation prediction process P3 that predicts the future situation of the stored liquid 2 in the storage tank 1 based on consumption conditions, and a stratification prediction process P4 that predicts stratification based on the predicted situation of the stored liquid
and the stratification judgment error NP2 or the stratification prediction process P4.
Stratification judgment in (corresponding to each prediction of higher stratification, upper and lower layer liquid 2U.

2. An O-rollover prediction process P5 that predicts the time point at which a rollover phenomenon occurs and the amount of BOG generated at that time from the composition of L, the state ffHit, and [, and the occurrence of a rollover phenomenon predicted by the rollover prediction process P5. Time and B
Based on the amount of OG generated, an operation process P6 that eliminates stratification by consuming the lower layer liquid before the occurrence point, an operation process P7 that allows the occurrence of a rollover phenomenon, or an operation process that operates the stratification active elimination means 4. The gist thereof is to include an operation selection process P9 for selecting one of P8.

[Function and Examples]

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

(I) Situation Observation Process P1 First, in the situation observation process P1, the situation of the stored liquid 2 is observed by a predetermined observation means 5. The observation means 5 is for observing the composition, state quantity, and liquid level of the stored liquid 2, which are necessary for stratification determination in the stratification determination process P2 and rollover prediction in the rollover prediction process P5. In the embodiment shown in FIG. 4, a temperature distribution measuring means 5a for measuring the temperature distribution in the height direction using a large number of temperature sensors 6 disposed in the height direction in the storage tank 1, and It consists of a composition measuring means 5b such as a gas chromatograph which measures the composition in the upper and lower parts of the liquid 2 and the gas layer four times, and a pressure measuring means 5C which measures the pressure in the gas layer four times.

In such a configuration, the temperature distribution in the height direction measured by the temperature distribution measuring means 5a shows that, in addition to the depth of the stored liquid 2, if the storage liquid 2 is layered, there is a sudden rise near the interface between the upper and lower liquids. Since a temperature gradient is formed, the depth of the interface can be known, and based on these, the composition measured by the composition measuring means 5b, the gas layer pressure measured by the pressure measuring means 5G, and the known inner diameter of the storage tank 1, storage The composition, state quantity, and liquid volume of the liquid 2 can be observed. At this time, directly 4
Don't measure the necessary stateff! Look, it's ratio! P γ, specific heat c
p, thermal conductivity λ, static viscosity coefficient μ, and Prandtl number PrW
is the stored liquid 2 measured by the temperature distribution measuring means 5a.
temperature, the composition K [composition determined by the measuring means 5b, and the pressure J11 can be derived based on the gas layer pressure determined by the measuring means 5c. Of course, these states a are different from each other when the stored liquid 2 is stratified. can be derived for layers.

Then, the required ships such as liquid IW (/(y), liquid depth H (m), and contained heat mQ (kcal) are (W−πD2H
γ/4), (H=・4W/πD2γ] and (Q=Cp
WT) formula, the above derived state! 1ffi.

(II) Stratification determination process P2 In the stratification determination process P2, the situation observation process P1
The stratification is determined based on the condition of the stored liquid 2 observed by. That is, as described above, stratification can be determined by comparing the presence or absence of a rapid temperature gradient formed near the interface between the upper and lower liquids and the ratio 1ffi derived by calculation from the measured values.

In addition, the storage tank 1 may be provided with a measuring means for measuring the density distribution in the height direction, and stratification may be determined based on the difference in density between the upper and lower liquids.

(I [I) Situation prediction process P3 and stratification prediction process P4 The situation prediction process P3 is based on the future liquefied gas reception and consumption conditions and the situation of the storage liquid 2 observed in the situation observation process P1. The stratification prediction process P4 predicts the future state of the storage liquid 2 in the storage liquid 1 and predicts the stratification based on the future state. These processes P
3. Even when the storage liquid 2 is not currently stratified, 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 or predicted, the previous operation is continued. If stratification is determined or predicted, in the next rollover prediction process P5, the time point at which the rollover phenomenon occurs and the amount of BOG generated at that time are predicted.

(IV) Rollover prediction process P5 In the 0-rollover prediction process P5, the situation observation process P
Based on the observations made in Step 1 or predicted in the situation prediction process P3, the composition, state quantity, and liquid m of each lower layer liquid 2tJ, 2L, and the assumed intrusion heat flux from the bottom, side, and liquid surface. From the point of occurrence of the rollover phenomenon and B at that time
Measure the OG occurrence.

For example, as disclosed in the above-mentioned literature, the rollover prediction is based on a state in which the stored liquid 2 in the storage tank 1 is stratified into two layers, an upper layer and a lower layer, and is subjected to heat load from the side wall and bottom surface, as shown in FIG. is assumed as an analytical model, and for this analytical model, upper and lower liquids 2U, 2L at a certain moment
The amount of heat and mass transfer is determined from the state of , and the state of the liquid after a minute time is calculated from the mass balance and heat balance.The process of change of the liquid state is calculated sequentially in this way, and the upper and lower liquids are 2U.

2. When the difference in density disappears or the interface position reaches the bottom or surface of the liquid, it is determined that the upper and lower interfaces have disappeared and a complete mixing state has been achieved, that is, a rollover phenomenon has occurred. However, in this way, it is possible to predict the time point at which the rollover phenomenon occurs and the amount of evaporation at that time, that is, the amount of BOG generated, etc., and the above-mentioned formula can be used as a specific calculation formula.

In addition, in the storage tank 1, an intermediate layer may occur between the upper and lower 2U and 21, and in this case, 31 cylinders may be formed based on an experimental formula that takes into account the existence of such an intermediate layer. The existence of such a layer (l!1) can be known along with the above-mentioned determination or prediction of stratification.

(V) Driving selection process P9 In the driving selection process P9, the following driving selections are made based on the time of occurrence of the rollover phenomenon predicted by the rollover prediction process P5 and the amount of BOG generated at that time.

First, it is determined whether or not the lower layer liquid 2 is to be consumed before the predicted occurrence of the rollover phenomenon, and if it is to be consumed, the operating process P6 is selected. Therefore, in the operation process P6, the lower layer liquid 2L is released before the predicted occurrence of the rollover phenomenon.
stratification is eliminated.

Next, at the predicted occurrence point of the rollover phenomenon (Puru, B
The amount of OG that can be safely processed by the OG processing means 7 and the predicted BOG generation mountain are compared, and BOG is higher than BOG generation.
If the OG processing capacity is large, the operating process P7 is selected, and if it is small, the operating process P8 is selected.

Therefore, even if a rollover phenomenon occurs in the driving process P7, this is caused by the above-mentioned BOG! 1 tsubo means 7 allows safe processing. In addition, in the operation process P8, the stratification active elimination means 4 such as a jet nozzle is operated to eliminate stratification, so it is possible to prevent the rollover phenomenon that generates 80G exceeding the BOG processable level 1. .

By repeating the above process and operating the storage tank 1, the present invention can 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 this. can.

〔Effect of the invention〕

As described above, the present invention is capable of observing the condition of the stored liquid in the liquefied gas storage tank, predicting the future condition of the stored liquid from the future liquefied gas reception and consumption conditions, and preventing rollover phenomena due to stratification. Since operation is selected based on predictions, it is possible to always operate on the safe side against stratification of the liquid stored in the storage tank and the rollover phenomenon caused by this. Storage tanks can be operated efficiently without having to separate storage tanks into receiving tanks and stockpiling tanks so that liquids can be received at lower liquid levels. It has the advantage of being able to handle liquefied gas.

[Brief explanation of the drawing]

Figures 1 (a), (b), and (c) are explanatory diagrams schematically showing the process from the stratification of the stored liquid in the storage tank to the occurrence of the rollover phenomenon, and Figures 2 (a) and (b) are Corresponding to the state of FIG. 1, an explanatory diagram schematically showing changes in BOGffi and liquid density over time, FIG. 3 is a schematic explanatory diagram showing an example of an analytical model used for rollover prediction, Fourth
The figure is an explanatory diagram 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 flowchart to which the operating method of the present invention is applied. Sign @1... Storage tank, 2 (2U, 2L)... Storage liquid,
3 (3tJ, 3L)... Heat convection, 4... Stratification active elimination means, 5... Observation means, 5a... Temperature 1!1 distribution measuring means, 5b... Composition 4 measuring means, 5C...Pressure measurement means, 6...Temperature sensor, 7...BOG processing means, Pl...Situation observation process, P2...Stratification determination process, P3...Situation prediction process, P4... Layering prediction process, P5... Rollover prediction process, P6. P7.
P8...Driving process, P9...Driving selection process. ~ ■

Claims (1)

[Claims] A situation observation process in which the composition, state quantity, and liquid volume of the stored liquid in the liquefied gas storage tank is observed; a stratification judgment process in which stratification is determined from the observed condition of the stored liquid; A situation prediction process that predicts the future situation of the stored liquid in the storage tank based on consumption conditions, a stratification prediction process that predicts stratification based on the predicted situation of the stored liquid, and the stratification determination process or stratification prediction. The composition of the upper and lower layer liquids, depending on the stratification judgment or stratification prediction in the process,
A rollover prediction process that predicts the time point at which a rollover phenomenon occurs and the amount of BOG generation at that time based on the state quantity and the liquid amount, and the time point at which the rollover phenomenon occurs and the amount of BOG generation predicted by the rollover prediction process, An operation selection process that selects one of an operation process that eliminates stratification by consuming the lower liquid before the occurrence, an operation process that allows the occurrence of a rollover phenomenon, or an operation process that operates an active stratification elimination means. A method of operating a liquefied gas storage tank, comprising: