JP4838014B2 - Gas hydrate decomposition amount estimation method and decomposition gas utilization system - Google Patents

Description

  The present invention relates to a method for estimating the amount of decomposition of gas hydrate and a system for using cracked gas by an inclusion compound of water and a gaseous hydrate forming substance that forms a gas hydrate such as natural gas, methane, ethane, and carbon dioxide. .

A gas hydrate is an ice-like solid crystal composed of water molecules and gas molecules. The clathrate is formed by gas molecules being taken into the cage of a three-dimensional structure created by water molecules. ) A general term for hydrates. The amount of gas that can be contained in 1 m ³ gas hydrate is as large as 165 Nm ³ . Therefore, a system for generating, storing and transporting natural gas as hydrate (NGH system: Natural Gas Hydrate System) has been studied.

  In storing gas hydrates at atmospheric pressure, the key to minimizing the amount of gas decomposition is to use the natural gas hydrate (NGH) self-preserving property.

  FIG. 5 is a known hydrate equilibrium diagram (example of methane hydrate). In FIG. 5, the upper left region of the equilibrium line 21 is a hydrate generation region, and the lower right region of the equilibrium line 21 is outside the hydrate generation region. In addition, H represents hydrate, G represents gas, I represents ice, and LW represents liquid water.

  Natural gas and water at a low temperature and high pressure (for example, about 0.1 to 3 ° C. at 5 MPa on the high pressure and low temperature side of the hydrate equilibrium condition) in the hydrate formation region in the hydrate formation reaction Natural gas hydrate is produced when reacted under.

  When the produced natural gas hydrate is cooled to below the freezing point (0 ° C. to −40 ° C., point B in FIG. 5) at an equal pressure, it freezes. The frozen natural gas hydrate is depressurized to a storage pressure (near atmospheric pressure of 0.1 MPa, point C in FIG. 5) and stored in a storage tank.

  Usually, in the storage tank, the storage pressure is set slightly higher than the atmospheric pressure from the viewpoint of preventing inadvertent entry of air into the storage tank. Although the temperature and pressure of the storage tank are located outside the hydrate generation region, decomposition of the gas hydrate is suppressed below the freezing point and is in a metastable state. This phenomenon of metastable state is known as self-preserving property.

  Gas hydrate is compressed and stored in the form of pellets in order to improve the filling rate of the storage tank, safety during transportation and storage, ease of handling during cargo handling, etc. Is done. Usually, the pellet size is about 5 mm to 100 mm. Furthermore, the filling rate of the gas hydrate stored in the storage facility can be improved by mixing and storing two or more kinds of gas hydrate pellets having different dimensions (Patent Document 1: JP-A-2002-220353). Issue gazette).

  As for the technology for storing gas hydrate, the decomposition of the gas hydrate is suppressed by controlling the temperature of the storage tank to a temperature at which the gas hydrate exhibits self-preserving properties. A storage method has been studied (Patent Document 2: Japanese Patent Application Laid-Open No. 2005-201286).

JP 2002-220353 A Japanese Patent Laid-Open No. 2005-201286

  When part of the gas hydrate particles and gas hydrate pellets stored in the storage tank is decomposed, gasified gas molecules are generated as a decomposition gas. When the cracked gas is generated, the pressure in the storage tank rises, but usually a safety valve is provided in the storage tank, and when the pressure in the storage tank becomes higher than a certain pressure, the safety valve operates to release the cracked gas, It is configured to be kept at a safe pressure.

  In addition, the cracked gas is effectively used without being wasted. When a large amount of gas hydrate particles and pellets are transported by ship, they can be used as fuel for the main engine or generator. The cracked gas can be compressed and stored again, and then used.

Previous studies have shown that the stability of self-preserving gas hydrate particles and pellets varies with storage temperature range. Further, it is qualitatively shown that the stability varies greatly depending on properties such as the density and particle size of the gas hydrate particles and pellets, and the surface conditions of the gas hydrate particles and pellets.
However, there was no means for quantitatively estimating the amount of decomposition of the gas hydrate pellets when the gas hydrate pellets having a fixed property were stored in the storage tank.

  If the amount of decomposition of gas hydrate cannot be estimated, it will be difficult to determine the specifications of safety valves provided in the storage tank and the specifications of equipment for using the cracked gas. When the decomposition amount of the gas hydrate is larger than the processing capacity of the selected facility, the decomposition gas cannot be processed and the gas is wasted. Also, if the gas hydrate decomposition amount is less than the processing capacity of the selected equipment, the equipment will be overdesigned and cost will increase.

  An object of the present invention is to suppress the amount of decomposition of gas hydrate when storing gas hydrate, predict the amount of decomposition, and effectively use the decomposition gas generated by decomposition of gas hydrate without waste. It is an object of the present invention to provide a method for estimating the amount of decomposition of gas hydrate and a system for using cracked gas, which enables the above.

  In order to solve the above-described problem, the gas hydrate decomposition amount estimation method according to the first aspect of the present invention provides a gas hydrate when storing the gas hydrate under conditions where the gas hydrate exhibits a self-preserving effect. The decomposition amount estimation method for the gas hydrate is characterized in that the decomposition rate β of the gas hydrate is obtained based on the following formula (1) and the decomposition amount of the gas hydrate is estimated.

Ｋ ：貯蔵圧力、貯蔵温度、ガスハイドレートの密度、及びガス組成に応じて実験によって決まる分解速度定数
ｒ_０：ガスハイドレート半径（ｍ）
ｔ ：貯蔵時間（ｓ）

K: Decomposition rate constant determined by experiment depending on storage pressure, storage temperature, gas hydrate density, and gas composition r ₀ : gas hydrate radius (m)
t: Storage time (s)

  According to the present invention, when the gas hydrate particles and pellets are stored under conditions that exhibit a self-preserving effect, the amount of cracked gas generated by the decomposition of the gas hydrate can be accurately estimated.

  The gas hydrate cracking gas utilization system according to the second aspect of the present invention includes a storage tank in which the gas hydrate is stored, and a cracked gas utilization facility that uses the cracked gas hydrate gas generated from the tank. The cracked gas utilization system of the gas hydrate provided with the cracked gas utilization equipment, wherein the cracked gas utilization facility is estimated by using the cracked rate β of the gas hydrate determined based on the following formula (1) It is characterized by being formed on a scale corresponding to the above.

Ｋ ：貯蔵圧力、貯蔵温度、ガスハイドレートの密度、及びガス組成に応じて実験によって決まる分解速度定数
ｒ_０：ガスハイドレート半径（ｍ）
ｔ ：貯蔵時間（ｓ）

K: Decomposition rate constant determined by experiment depending on storage pressure, storage temperature, gas hydrate density, and gas composition r ₀ : gas hydrate radius (m)
t: Storage time (s)

  According to the present invention, it is possible to accurately estimate the cracked gas amount of gas hydrate stored under conditions that exhibit a self-preserving effect, and therefore, based on the pressure design value of the hydrate storage tank, Equipment can be provided appropriately, the inside of the storage tank does not become higher than the pressure design value, or the pressure is not reduced, and safety is improved.

  In addition, it is possible to introduce equipment of an appropriate scale according to the estimated amount of cracked gas. That is, a facility that is too small relative to the amount of cracked gas hydrate is introduced, and the cracked gas is not dissipated unnecessarily, thereby improving economy. Furthermore, it is possible to reduce the construction cost without introducing excessive facilities with respect to the amount of gas hydrate decomposition gas.

  According to the present invention, when storing gas hydrate particles and pellets, it is possible to predict the decomposition amount of the gas hydrate and effectively use the decomposition gas generated by the decomposition without waste.

  The gas hydrate decomposition amount estimation method and decomposition gas utilization system according to the present invention will be described below. In the present invention, the type of hydrate is not particularly limited. That is, the kind of the gaseous hydrate forming substance that forms hydrate may be any substance that forms hydrate under predetermined temperature and pressure conditions. And methane gas, ethane gas, carbon dioxide gas (carbon dioxide gas), and the like. The gas hydrate particles and pellets are stored in a state that exhibits a self-preserving effect. It is practical to manufacture the gas hydrate pellets usually in the range of 5 mm to 100 mm.

[Method of estimating the amount of decomposition of gas hydrate]
(1) gas hydrate particles, the temperature dependency of Figure 1 the rate of degradation of the pellets, methane hydrate pellets reduction ratio of the guest molecules occluded index alpha _H degradation rate in each storage temperature (hereinafter, referred to as MGHP) Δα _H / Δt (s ⁻¹ ).
The definition of the guest molecule inclusion rate α _H is shown in the following formula (2).

水分子が作る全てのケージにゲスト分子が包摂した場合は、α_Ｈ＝１．０となる。水和数とは、ガス分子に対する水分子数の割合である。本実施例においては、Ｉ型構造のガスハイドレートの理論水和数である５．７５を値として用いた。

When guest molecules are included in all cages formed by water molecules, α _H = 1.0. Hydration number is the ratio of the number of water molecules to gas molecules. In this example, 5.75, which is the theoretical hydration number of the gas hydrate having the I-type structure, was used as the value.

In the case of MGHP, the decomposition rate was high at a storage temperature of 268K, but the decomposition rate was reduced as the storage temperature was lowered, and decreased to 3 × 10 ⁻⁸ s ⁻¹ at 253K. The decomposition rate increased again when the storage temperature was lower than 253K, and the decomposition rate was maximized at 210K, but the decomposition rate decreased at a lower temperature, and the decomposition rate became zero at 168K. The self-preserving property of MGHP was expressed in a limited temperature range of 226K to 268K, and it was confirmed that the stability was highest around 253K within this temperature range.

  Also in the case of mixed gas hydrate pellets (hereinafter referred to as mixed GHP) containing methane as the main component and containing ethane and propane, the decomposition rate is the lowest at 253 K within the measurement range. The temperature dependence of the mixed GHP decomposition rate shows the same tendency as MGHP regardless of the composition and concentration.

(2) Estimation formula of decomposition rate of gas hydrate particles and pellets Next, when the surface state of the gas hydrate pellets was observed using a scanning confocal microscope, it increased to 253 K, which is the temperature that most strongly shows self-preserving properties. It was observed that the entire surface of the heated sample was covered with a glossy film.

FIG. 2 is a decomposition reaction model of spherical gas hydrate particles and pellets.
The meaning of the symbols is shown below.
r: hydrate particles, pellet radius (m)
V: Hydrate particles, pellet volume (m ³ )
x: thickness of decomposed hydrate layer (m)
The subscript 0 indicates the initial state.

  Using the spherical gas hydrate particle and pellet decomposition reaction model, the amount of decomposition of the gas hydrate pellet in a self-preserving state when the MGHP is in the temperature range of 226K to 268K, particularly around 253K, is estimated.

  In the decomposition process of the gas hydrate particles and pellets 1, it is assumed that ice 2 generated by the decomposition grows from the surface of the gas hydrate particles and pellets 1 toward the inside. If the ice is porous, the gas generated by the decomposition can quickly pass through the ice layer, but in the case of dense film-like ice, it cannot be easily passed and must diffuse through the ice layer.

  Therefore, the gas hydrate particles in the vicinity of 253K, which have a very low decomposition rate, and the state of the entire surface covered with a glossy film observed on the surface of the pellet 1 are shown in FIG. Assuming that the gas is produced, a diffusion-controlled / interface-decreasing reaction rate equation (Jander equation) is applied to the decomposition rate equation of gas hydrate particles and pellets. This is a feature of the present invention.

  The relationship between the decomposition rate β of the gas hydrate particles and pellets 1 and the thickness x of the ice 2 as the decomposition layer is obtained by the following equation. t represents time (s).

  The volume V when the spherical gas hydrate particles and pellets 1 are decomposed to the thickness x of the decomposition layer can be expressed by equation (3).

球状のガスハイドレート粒子、ペレット１が分解し、分解層の厚さｘとなったときの分解率βを用いて体積Ｖを表すと、以下の（４）式になる。

When the volume V is expressed using the decomposition rate β when the spherical gas hydrate particles and pellets 1 are decomposed to the thickness x of the decomposition layer, the following equation (4) is obtained.

上記（３）式と（４）式から、分解層の厚さｘは（５）式で表せる。

From the above equations (3) and (4), the thickness x of the decomposition layer can be expressed by equation (5).

また、前記拡散律速の仮定より、分解ガスが分解層中を拡散するのに要する時間は、分解層の厚さが増すほど増大するので、分解層の成長速度はその厚さに反比例するものとする。分解速度ｄｘ／ｄｔは以下のように表せる。

Further, from the assumption of the diffusion rate control, the time required for the decomposition gas to diffuse in the decomposition layer increases as the decomposition layer thickness increases, so the growth rate of the decomposition layer is inversely proportional to the thickness. To do. The decomposition rate dx / dt can be expressed as follows.

ｔ＝０でｘ＝０という初期条件を使って（６）式を積分すると、（７）式が得られる。

When the equation (6) is integrated using the initial condition of t = 0 and x = 0, the equation (7) is obtained.

（５）式と（７）式から（８）式が得られる。

Expression (8) is obtained from Expression (5) and Expression (7).

（８）式から分解率βは、下記の（１）式となる。

From the equation (8), the decomposition rate β is the following equation (1).

ゲスト分子包蔵率α_Ｈは（９）式によって求められる。

The guest molecule inclusion rate α _H is obtained by the equation (9).

(3) Decomposition rate constant K
The decomposition rate constant K is a constant determined according to the pellet density (ρ) and gas composition, which are the properties of the gas hydrate pellets, and the storage pressure (P) and storage temperature (T) as storage conditions. The gas hydrate pellets stored in a storage tank have a self-preserving effect by measuring the decomposition rate when the gas hydrate pellets are stored under constant storage conditions. A decomposition rate constant K is determined.

As an example, when methane gas hydrate pellets (MGHP) having a pellet density of 880 to 914 kg / m ³ at 1 atmosphere and 253 K are given, the decomposition rate constant K is 1 × 10 ⁻¹⁶ m ² / s from the experimental results. a ^{~1 × 10 -14 m 2 / s} . The decomposition amount is estimated using K = 3.5 × 10 ⁻¹⁵ m ² /s±2.5×10 ⁻¹⁵ m ² / s as a more preferable range.

Using the decomposition rate constant K, the decomposition rate β is obtained by the equation (1), the estimation result of the guest molecule inclusion rate (%) of MGHP obtained by the equation (9), and the actual guest molecule inclusion rate by experiment ( %) And the measurement results are shown in FIG. The guest molecule inclusion rate in FIG. 3 is expressed as a percentage by multiplying α _H defined above by 100.
The estimated value is in good agreement with the experimental results until after 400 hours or more, and it can be said that the decomposition rate of the gas hydrate can be estimated by calculating the decomposition rate of the gas hydrate by the estimation formula (1).

[System for using cracked gas from gas hydrate pellets]
[Example 1]
FIG. 4 is a schematic configuration diagram of a gas hydrate pellet decomposition gas utilization system. Hereinafter, gas hydrate pellets will be described as an example, but the same applies to gas hydrate particles.
Process data of the gas hydrate pellet storage tank 12 [storage temperature T (K), storage pressure (MPa), gas hydrate storage amount (kg), gas composition, pellet density ρ (kg / m ³ ), particle size 2r (mm )] Is set. As the particle size 2r ₀ , an average particle size considering the particle size distribution (%) is used.

  When the minimum diameter and the maximum diameter are different by a factor of two or more, the particle size is divided into several parts, and the decomposition rate β is calculated with the respective average particle sizes. Multiply this by the weight fraction for each particle size and add up to determine the overall decomposition rate. Further, when the gas hydrate particles and pellets are not spherical, the outer diameter may be regarded as the particle diameter. When the outer diameter has a major axis and a minor axis, it is desirable to use the average of the major axis and the minor axis as the particle diameter.

The gas hydrate pellets 11 having the set constant properties (gas composition, particle size 2r ₀ , pellet density ρ) are subjected to storage conditions (storage pressure P, storage temperature T) in which the gas hydrate exhibits a self-preserving effect. ) To determine the decomposition rate constant K of the gas hydrate to be stored based on the estimation formula (1), and to estimate the amount of cracked gas generated when stored in the storage tank 12 Is done.

Next, the operation of this embodiment will be described.
According to the present embodiment, the amount of cracked gas of the gas hydrate pellets 11 stored in the storage tank 12 can be accurately estimated. Therefore, based on the pressure design value of the storage tank 12, facilities such as safety valves are appropriately set. It can be provided, and the inside of the storage tank 12 does not become higher than the pressure design value or is not depressurized, and safety is improved.

  When introducing the cracked gas utilization facility 13 that uses the cracked gas, the cracked gas utilization facility 13 can be designed to have a scale corresponding to the estimated amount of cracked gas. That is, a facility that is too small relative to the amount of cracked gas in the gas hydrate pellet 11 is introduced, and the cracked gas is not wasted and the economy is improved. Furthermore, the construction cost can be reduced without introducing excessive facilities with respect to the cracked gas amount.

  In addition, when the supply amount of cracked gas is insufficient in the cracked gas utilization facility 13, since the amount of cracked gas generated is estimated, the amount of auxiliary fuel for the shortage can be estimated, and an appropriate amount of auxiliary fuel can be estimated. The fuel can be supplied to the cracked gas utilization facility 13.

  The present invention relates to gas hydrate particles and pellets for storing gas hydrate particles, which are clathrate compounds of water and gaseous hydrate forming substances that form gas hydrates such as natural gas, methane, ethane, and carbon dioxide. It can be used for rate cracked gas utilization system.

It is a figure which shows the correlation of the storage temperature of a gas hydrate pellet, and a decomposition rate. It is a figure which shows the decomposition reaction model of spherical gas hydrate particle | grains and a pellet. It is a figure which shows the actual value and estimated value of the gas occlusion rate of a methane gas hydrate pellet. It is a schematic block diagram of the decomposition gas utilization system of the gas hydrate pellet which concerns on this invention. It is a well-known hydrate equilibrium diagram (an example of methane gas hydrate).

Explanation of symbols

1 Gas hydrate particles, pellets, 2 Ice (decomposition layer)
11 Gas Hydrate Pellet, 12 Storage Tank, 13 Cracked Gas Utilization Equipment 21 Equilibrium Line

Claims (1)

A method for estimating the amount of decomposition of gas hydrate when storing the gas hydrate under conditions where the gas hydrate exhibits a self-preserving effect outside the gas hydrate generation region,
A method for estimating the amount of decomposition of a gas hydrate, comprising determining a decomposition rate β of the gas hydrate based on the following formula (1) and estimating the amount of decomposition of the gas hydrate.

K: Decomposition rate constant determined by experiment depending on storage pressure, storage temperature, gas hydrate density, and gas composition r ₀ : gas hydrate radius (m)
t: Storage time (s)

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