WO2007097127A1

WO2007097127A1 - Method of estimating amount of gas hydrate decomposed and decomposition gas utilizing system

Info

Publication number: WO2007097127A1
Application number: PCT/JP2007/000133
Authority: WO
Inventors: Toru Iwasaki; Yuichi Kato; Masahiro Takahashi
Original assignee: Mitsui Engineering & Shipbuilding Co., Ltd.
Priority date: 2006-02-27
Filing date: 2007-02-27
Publication date: 2007-08-30
Also published as: JP2007231053A; JP4838014B2

Abstract

A method of estimating the amount of gas hydrate decomposed, and decomposition gas utilizing system, being capable of realizing not only, at the storage of gas hydrate, restriction of the amount of gas hydrate decomposed but also estimation of the decomposition amount and effective utilization of decomposition gas produced by decomposition of gas hydrate without wasting the same. There is provided a method of estimating the amount of gas hydrate decomposed in the storage of gas hydrate under such conditions that the gas hydrate exerts self-preserving effect, characterized in that the decomposition rate of gas hydrate (β) is calculated according to given mathematical formula, thereby attaining estimation of the amount of gas hydrate decomposed.

Description

Specification

Method for estimating the amount of decomposition of gas hydrate

[0001] The present invention relates to a method for estimating the amount of decomposition of a gas hydrate by using a clathrate compound of water and a gaseous hydrate forming substance that forms a gas hydrate such as natural gas, methane, ethane, and carbon dioxide, and use of the cracked gas About the system.

Background art

[0002] A gas hydrate cage is an ice-like solid crystal composed of water molecules and gas molecules, and the inclusion (formation) formed by the gas molecules being taken into the cage (cage) of the three-dimensional structure created by the water molecules. This is a generic name for clathrate hydrate. The amount of gas that can be stored in a 1 m ³ gas hydrate is about 1 65 Nm ³ . Therefore, a system that generates, stores, and transports natural gas as hydrated straw (NGH system: Natural Gas Hydrate System) is being studied.

[0003] In storing gas hydrates at atmospheric pressure, the key to minimizing the amount of gas decomposition is to use the self-preserving properties of natural gas hydrate (NGH).

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 the hydrate generation region, and the lower right region of the equilibrium line 21 is outside the hydrate generation region. H stands for Hydrate, G stands for Gas, I stands for Ice, and LW stands for Liquid Water.

[0005] Low temperature and high pressure in the hydrate formation region in the hydrate formation reaction of natural gas and water (for example, about 0.1 to 3 ° C at 5MPa on the high pressure and low temperature side of the hydrate equilibrium condition) Natural gas hydrates are produced when the reaction is carried out under (point A).

[0006] The generated natural gas hydrate freezes when cooled to below the freezing point (0 ° C to 40 ° C, point B in Fig. 5) at the same pressure. And the frozen natural gas hydrate The pressure is reduced to the storage pressure (atmospheric pressure near 0.1 MPa, point C in Fig. 5) and stored in the storage tank.

[0007] Usually, in the storage tank, the storage pressure is set to be 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 this storage tank are located outside the hydride production area, decomposition of the gas hydrate is suppressed below the freezing point and is in a metastable state. This metastable phenomenon is known as self-preservation.

[0008] In order to improve the filling rate of the storage tank, the safety during transportation and storage, the ease of handling during cargo handling, etc., the gas hydrate is compressed and molded into a pellet-like shape. Stored in Usually, the pellet size is about 5 mm to 10 O mm. Further, by mixing and storing two or more kinds of gas hydrate pellets 異なる having different dimensions, the filling rate of the gas hydrate 卜 stored in the storage facility can be improved (Patent Document 1: Japanese Patent Application Laid-Open No. 2000-32083). 2—2 2 0 3 5 3).

[0009] In addition, with regard to the technology for storing gas hydrates, the temperature of the storage tank is controlled to a temperature at which the gas hydrates exhibit self-preserving properties, thereby suppressing the decomposition of the gas hydrates and efficiently (Patent Document 2: Japanese Patent Laid-Open No. 2 0205-2 086) has been studied.

Patent Document 1: Japanese Patent Application Laid-Open No. 2 0 0 2-2 2 0 3 5 3

Patent Document 2: Japanese Patent Laid-Open No. 2 0 0 5 _ 2 0 1 2 8 6

Disclosure of the invention

Problems to be solved by the invention

[001 1] When part of the gas hydrate particles or gas hydrate pellets stored in the storage tank is decomposed, gasified gas molecules are generated as a decomposition gas. The generation of cracked gas increases the pressure in the storage tank. Normally, a safety valve is provided in the storage tank, and when the pressure in the storage tank reaches a certain level or higher, the safety valve operates to release the cracked gas, Configured to maintain a safe pressure [0012] Further, the cracked gas is effectively used without being wasted. When a large amount of gas hydrate particles or pellets is transported by ship, it can be used as fuel for the main engine or generator. The cracked gas can be compressed and stored again, and then used.

[0013] Previous studies have shown that the stability of self-preserving gas hydrate particles and pellets varies with the storage temperature range. Further, it is qualitatively shown that the stability varies greatly depending on properties such as the density and particle size of gas hydrate particles and pellets, and the surface conditions of gas hydrate particles and pellets.

However, there was no means for quantitatively estimating the amount of decomposition of gas hydrate pellets when storing gas hydrate pellets with a fixed property in a storage tank.

[0014] If the amount of decomposition of the gas hydrate tank cannot be estimated, it will be difficult to determine the specifications of the safety valve provided in the storage tank and the specifications of the equipment for using the cracked gas. If the amount of gas hydrate decomposition is greater than the treatment capacity of the selected equipment, the cracked gas cannot be processed and the gas is wasted. In addition, if the amount of gas hydrate decomposition is less than the treatment capacity of the selected facility, the facility will be overdesigned, increasing costs.

[0015] An object of the present invention is to suppress the decomposition amount of the gas hydrate when storing the gas hydrate, predict the decomposition amount, and efficiently use the decomposition gas generated by the decomposition of the gas hydrate. The object of the present invention is to provide a method for estimating the amount of decomposition of a gas hydride and a system for using cracked gas that can be used effectively.

Means for solving the problem

[0016] In order to solve the above-described problem, the method for estimating the decomposition amount of a gas hydrate according to the first aspect of the present invention includes a gas hydrate when storing the gas hydrate under conditions where the gas hydrate exhibits a self-preserving effect. The amount of decomposition of the gas hydrate is determined on the basis of the following formula (1), and the amount of decomposition of the gas hydrate is estimated. [0017] [Equation 3] β = 1-(1 1 Π ^ / r ₀ ) ³ · · · (1)

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)

[0018] According to the present invention, when gas hydrate particles and pellets are stored under conditions that exhibit a self-preserving effect, the amount of decomposed gas generated by the decomposition of the gas hydrate cake is accurately estimated. Can do.

[0019] A cracked gas utilization system for a gas hydrate tank according to a second aspect of the present invention includes a storage tank in which a gas hydrate is stored, and a cracked gas that uses the cracked gas of the gas hydrate tank generated from the storage tank. A gas hydrate cracking gas utilization system comprising: a gas hydrate cracking system, wherein the cracking gas utilization facility is estimated using a gas hydrate cracking rate (S) determined based on the following equation (1): It is characterized in that it is formed on a scale corresponding to the amount of dissolved gas.

[0020] [Equation 4] 3 1-(1 Γ Τ / τ ₀ ) ³ .. (1)

：: 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)

[0021] According to the present invention, it is possible to accurately estimate the cracked gas amount of a gas hydrate tank stored under conditions that exhibit a self-preserving effect. Therefore, based on the pressure design value of the hydrate tank, Equipment such as safety valves can be properly installed, and the storage tank will not be higher than the pressure design value or depressurized, improving safety. [0022] In addition, it is possible to introduce equipment of an appropriate scale according to the estimated amount of cracked gas. That is, an installation that is too small relative to the amount of gas hydrate cracked gas is introduced, and the cracked gas is not wasted and the economy is improved. Furthermore, construction costs can be reduced without introducing excessive facilities for the amount of gas hydrate cracked gas.

The invention's effect

[0023] According to the present invention, when storing gas hydrate particles and pellets, it is possible to predict the amount of decomposition of the gas hydrate soot and effectively use the decomposition gas generated by the decomposition without waste. .

BEST MODE FOR CARRYING OUT THE INVENTION

[0024] Hereinafter, a method for estimating the amount of decomposition of a gas hydrate soot and a system for using cracked gas according to the present invention will be described. In the present invention, the type of hydrated rice cake is not particularly limited. In other words, the type of gaseous hydrate-forming substance that forms hydrates may be any material that can form hydrates at a predetermined temperature and pressure. For example, natural gas (methane as the main component and subcomponents). And methane gas, ethane gas, carbon dioxide gas (carbon dioxide gas), and the like. Gas hydrate particles and pellets are stored in a state of self-preserving effect. It is practical to manufacture the gas hydrate and pellets in the range of 5 mm to 10 O mm.

[0025] [Method of estimating the amount of decomposition of gas hydrate]

(1) Temperature dependence of decomposition rate of gas hydrate particles and pellets

Fig. 1 shows the decomposition rate of methane gas hydrate pellets (hereinafter referred to as MGHP) at each storage temperature in terms of the reduction rate of the gas 卜 molecular inclusion rate Qf H, Δ Of _H Z 厶 t (s _ ¹ ).

The definition of guest molecule inclusion rate Qf H is shown in the following formula (2).

[0026] [Equation 5] Hydrate's stored gas volume (mo 1)

• (2) Raw water (m o 1) Hydration number

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

[0027] For MGH P, is large decomposition rate in storage temperature 268 K, and the decomposition rate of the storage temperature is lower small and Li was reduced to 253 in ^{^{K 3 X 1 0_ 8 s_ 1}} . The decomposition rate increased again when the storage temperature was lower than 253 K, and the decomposition rate reached the maximum at 2 1 OK, but decreased at a lower temperature, and the decomposition rate became zero at 1 68 K. The self-preserving property of MG HP was exhibited in a limited temperature range of 226 K to 268 K, and it was confirmed that the highest stability was observed at around 253 mm within this temperature range.

[0028] Also in the case of a mixed gas hydrate pellet (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. Yes. The temperature dependence of the mixed GHP decomposition rate shows the same tendency as MGH P regardless of the composition and concentration.

[0029] (2) Estimation formula for decomposition rate of gas hydrate particles and pellets

Next, when the surface state of the gas hydrate and pellets was observed using a scanning confocal microscope, the surface of the sample heated up to 253 K, the temperature showing the strongest self-preserving property, was glossy as a whole. A state of being covered in a film was observed

[0030] Fig. 2 is a decomposition reaction model of spherical gas hydrate particles and pellets. The meaning of the symbols is shown below r: Hydrate particle, Perez radius (m)

V: Hydrate soot particle, Perez soot volume (m ³ )

X: Decomposed hydrate layer thickness (m)

The subscript 0 indicates the initial state.

[0031] By using the decomposition reaction model of the spherical gas hydrate particles and pellets, the gas hydrate pellets in a self-preserving state in which the MGHP is in the temperature range of 2 26 K to 2 68 K, particularly around 2 53 3 Estimate the amount of soot decomposition.

[0032] In the decomposition process of gas hydrate soot particles and pellets 1, it is assumed that ice 2 generated by the decomposition grows from the surface of gas hydrate soot 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 ice, it cannot diffuse easily and must diffuse through the ice layer.

[0033] Therefore, the degradation rate of the gas hydrate particles in the vicinity of 2 5 3 Κ was observed on the surface of pellet 1 and the entire surface was covered with a glossy film. Assuming that ice-like ice 2 is generated, the diffusion-controlled interface-reduction rate reaction rate equation (J ander equation) is applied to the decomposition rate equation for gas hydrate particles and pellets. This is a feature of the present invention.

[0034] Decomposition rate of gas hydrate particles, pellet 1) S and thickness of ice 2 as decomposition layer

The relationship of X is obtained by the following equation. t represents time (s).

[0035] The volume V when the spherical gas hydrate particles and pellets 1 are decomposed to reach the thickness X of the decomposition layer can be expressed by equation (3).

[0036] [Equation 6]

V = 4 π (r. One X) '3 (3)

When the volume V is expressed using the decomposition rate when the spherical gas hydrate particles and pellet 1 are decomposed and the decomposition layer thickness X is reached, the following equation (4) is obtained. [0037] [Equation 7]

V = 4 π r ₀ ³ (1− β) Z3 (4) From the above equations (3) and (4), the thickness X of the decomposition layer can be expressed by equation (5).

[0038] [Equation 8]

X = r 0 [1 -V (1-/ 3)] · '· (5)

Furthermore, 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. Shall. The decomposition rate d xZd t can be expressed as follows.

[Equation 9] d x / d t = K / (2χ) (6)

Integrating ( ₆ ) using the initial condition of t = ₀ and _χ = ₀ yields (7).

[0040] [Equation 10]

X ² = K t (7)

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

[0041] [Equation 11] The decomposition rate ^ from the equation (8) is the following equation (1).

[0042] [Equation 12] β = 1 — (1-/ r ₀ ) ³ ... (1)

Guest molecule inclusion rate Qf _H can be calculated by equation (9).

[0043] [Equation 13] a = a _H o (l ^— 3)--· (.9)

[0044] (3) Decomposition rate constant K

Decomposition rate constant K is the density of pellets, which is the property of gas hydrate pellets.

(P) and gas composition, storage pressure (P) and storage temperature (storage conditions)

It is a constant determined according to T). By measuring the decomposition rate when the gas hydrate pellets stored in the storage tank are stored under constant storage conditions, the gas hydrate pellets exhibit a self-preserving effect. The decomposition rate constant K of is obtained.

As an example, methane gas hydrate pellets (MGHP) having a pellet density of 880 to 914 kgZm ³ at 1 atmosphere and 253 K are given. The decomposition rate constant K is 1 X 1 0_ ¹⁶ m ² Zs to 1 X 1 0 — ¹⁴ m ² Z s. To estimate the amount of degradation with K = 3. 5 X 1 0_ 15 m 2 Zs ± 2. 5 X 1 0_ 15 m 2 Zs as a more preferable range.

[0046] Using the above decomposition rate constant K, the decomposition rate ^ is obtained by Equation (1), the result of estimation of the guest molecule inclusion rate (%) of MG HP obtained by Equation (9), and the actual result by experiment Figure 3 shows a comparison with the guest molecule inclusion rate (%). The guest molecule inclusion rate in Fig. 3 is expressed as a percentage by multiplying Q? _H defined above by 100.

The estimated value is in good agreement with the experimental results until after 400 hours, and the decomposition rate of the gas hydrate can be obtained by the estimation formula (1). It can be said that the amount of decomposition can be estimated.

[0047] [System for using cracked gas from gas hydrate and pellets]

[Example 1]

Fig. 4 is a schematic configuration diagram of the cracked gas utilization system of gas hydrate and pellets. Hereinafter, the gas hydrate pellets will be described as an example, but the same applies to the gas hydrate particles.

Process data of gas hydrate pellet storage tank 12 [Storage temperature T (K), storage pressure (MPa), gas hydrate storage volume (kg), gas composition, pellet density iO (kg Zm ³ ), particle size 2 r (mm)] is set. For the particle size 2 r ₀ , an average particle size considering the particle size distribution (%) is used.

[0048] 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 is calculated for each average particle size. Multiply this by the weight fraction for each particle size and add up to obtain the overall decomposition rate. In addition, 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.

[0049] The set constant properties (gas composition, particle size 2 r ₀ , pellet 卜 density gas hydrate pelette 、 1 1, under certain storage conditions (storage pressure where the gas hydrate expresses a self-preserving effect) P, the decomposition rate constant K when storing at storage temperature T) is determined by experiment, and the decomposition rate ^ of the gas hydrate stored based on the estimation formula (1) is obtained and stored in storage tank 1 2 The amount of generated cracked gas is estimated.

[0050] Next, the operation of the present embodiment will be described.

According to this embodiment, the amount of cracked gas stored in the storage tank 1 2 can be accurately estimated. Therefore, based on the pressure design value of the storage tank 1 2 The inside of the storage tank 1 2 does not become higher than the designed pressure value or depressurize, and safety is improved.

[0051] When introducing the cracked gas utilization facility 13 using cracked gas, the scale can be designed according to the amount of cracked gas estimated from the cracked gas utilization facility 13. sand In other words, an installation that is too small relative to the amount of cracked gas hydrate and pellets 11 will be introduced, and the cracked gas will not be dissipated unnecessarily, improving the economy. Furthermore, it is possible to reduce the construction cost without introducing excessive equipment with respect to the amount of cracked gas.

[0052] Also, in the cracked gas utilization facility 13, when the supply amount of cracked gas is insufficient, the amount of cracked gas generated is estimated, so the amount of auxiliary fuel for the shortage can also be estimated. A sufficient amount of auxiliary fuel can be supplied to the cracked gas utilization equipment 13.

Industrial applicability

[0053] The present invention relates to the storage of gas hydrate particles and pellets, which are clathrate compounds of gaseous hydrate forming substances such as natural gas, methane, ethane, carbon dioxide and the like, and water. It can be used in the cracked gas utilization system of the gas hydrate tank.

Brief Description of Drawings

[0054] FIG. 1 is a diagram showing the correlation between storage temperature and decomposition rate of gas hydrate and pellets.

FIG. 2 is a diagram showing a decomposition reaction model of spherical gas hydrate particles and pellets.

FIG. 3 is a diagram showing measured values and estimated values of gas storage rates of methane gas hydrate and pellets.

FIG. 4 is a schematic configuration diagram of a cracked gas utilization system for a gas hydrate pellets according to the present invention.

[Fig. 5] Equilibrium diagram of known hydrate (example of methane gas hydrate)

Claims

The scope of the claims

[1] A method for estimating the amount of decomposition of a gas hydrate when storing the gas hydrate under conditions where the gas hydrate exhibits a self-preserving effect,

A method for estimating the amount of decomposition of a gas hydrate, characterized by calculating a decomposition rate of the gas hydrate based on the following formula (1) 8 and estimating the amount of decomposition of the gas hydrate.

圆

/ S = 1 — (1 1 ^ Π Γ / r ₀ ) ³ · · · (!)

κ: 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)

[2] A storage tank in which the gas hydrate tanks are stored,

A cracked gas utilization system using cracked gas hydrate cracked gas generated from the storage tank, and a cracked gas utilization system of the gas hydrate tank provided with the cracked gas utilization facility based on the following formula (1): A gas hydrate cracked gas utilization system, characterized in that the scale is formed according to the amount of cracked gas estimated using the calculated gas hydrate cracking rate ^.

[Equation 2] β = 1-(1 1 ΛΓ / r ₀ ) ³ ··· (1) κ: 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)