JP2004197006A - Process and apparatus for producing gas hydrate having excellent storage stability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a process and an apparatus for producing a hydrate which can improve the stability of the hydrate while restraining the ratio of ice to a minimum. <P>SOLUTION: The process for producing a gas hydrate composed of a nucleus part which a gas molecule and a water molecule constitute and a shell part which has ice as the component and covers the nucleus part comprises a step of forming a first hydrate 1 of a first gas which is a guest molecule of the hydrate in the above nucleus part, a step of forming a second hydrate 2 having a second gas as the guest molecule around the above hydrate 1 of the first gas under conditions milder than the conditions of allowing the first gas to form the hydrate, and a step of decomposing the above second hydrate by adjusting the temperature and/or the pressure to the conditions which cannot allow the above second gas to form a hydrate and forming an ice shell 3 by freezing the water which has constituted the above second hydrate on the surface of the above first hydrate. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a hydrate used as, for example, a means for transporting and storing natural gas, and particularly to a method for producing a hydrate having excellent stability.
[0002]
[Prior art]
Gas hydrates (sometimes simply referred to as “hydrates”) are gas molecules incorporated into a cage structure composed of water molecules, and can store a large amount of gas per unit volume. Attention has been paid to the application to transportation and storage.
Gas hydrate does not decompose below the temperature and above the pressure indicated by the equilibrium curve specific to each type of gas. However, the temperature and pressure are low and high, for example, pure methane hydrate is about -78 ° C or less at atmospheric pressure.
As described above, although the gas hydrate has an excellent gas storage amount per unit volume, there is a problem that stable conditions are severe.
[0003]
By the way, it is known that, even at −78 ° C. or more under atmospheric pressure, ice can serve as a kind of pressure vessel by dispersing methane hydrate in ice at a temperature below freezing, thereby preventing its dissociation. (Self-preserving effect).
Therefore, it has been proposed to produce hydrates above the freezing point and stabilize the hydrate by cooling the hydrate to below the freezing point with a small amount of water remaining after dehydrating unreacted water and covering the hydrate with ice. (For example, see Patent Document 1).
[0004]
[Patent Document 1]
JP 2000-303083 A
[Problems to be solved by the invention]
In the method of cooling the hydrate to below freezing with a small amount of water remaining after dehydrating unreacted water and covering the hydrate with ice as in the above-mentioned conventional example, the hydrate is unevenly distributed due to the uneven distribution of water after dehydration. In some cases, the generated ice is unevenly distributed, and the ice does not function sufficiently as a pressure vessel.
On the other hand, in order to uniformly infiltrate water throughout the hydrate, a large amount of water is required, the proportion of gas in the entire mixture of hydrate and ice is reduced, and transport and storage efficiency is reduced.
[0006]
The present invention has been made in order to solve such a problem, and provides a method and an apparatus for producing a hydrate which can improve the stability of the hydrate while minimizing the proportion of ice.
[0007]
[Means for Solving the Problems]
The method for producing a gas hydrate according to the present invention produces a gas hydrate composed of a core portion composed of a hydrate composed of gas molecules and water molecules, and a shell portion composed of ice covering the core portion. A way to
Generating a first hydrate of the first gas, which is a guest molecule of the hydrate of the nucleus portion; and forming a first hydrate around the hydrate of the first gas, the conditions for generating the hydrate being lower than the first gas. Decomposing the second hydrate by adjusting a temperature and / or a pressure so that the second gas cannot form a hydrate; and Freezing the water constituting the second hydrate on the surface of the first hydrate.
[0008]
Further, a method for producing a gas hydrate composed of a core portion composed of a hydrate composed of gas molecules and water molecules, and a shell portion composed of ice covering the core portion,
A step of generating the first hydrate of the core portion under the temperature and pressure conditions at which the first gas that is the guest molecule of the hydrate of the core portion generates hydrate; The second gas generates hydrate around the first hydrate with the second gas having a less hydrate generation condition than the first gas while maintaining the condition that the first gas generates hydrate. Forming a second hydrate having a second gas as a guest molecule on the surface of the first hydrate by supplying until the conditions are satisfied; and a partial pressure of the first gas and the second gas is determined by the hydration of each gas. The second hydrate is decomposed by adjusting the temperature condition to a condition in which the second gas cannot form a hydrate while controlling the temperature within the conditions for generating the rate. Configured to have water is obtained and a step of generating a shell portion to ice component on the surface of the first hydrate by freezing on the surface of the first hydrate.
[0009]
Further, a method for producing a hydrate composed of a core portion composed of a hydrate composed of gas molecules and water molecules, and a shell portion composed of ice covering the same, using a mixed gas as a raw material,
A step of generating a first hydrate under a temperature and pressure condition at which a hydrate having a first gas in the mixed gas as a guest molecule can be generated, and a first gas which is a guest molecule of the first hydrate generates a hydrate While maintaining the condition that the second gas in the mixed gas is less hydrate-generating than the first gas, the second gas in the mixed gas is set to the condition that produces the hydrate, thereby keeping the second gas on the surface of the first hydrate. Forming a second hydrate using as a guest molecule, and adjusting the temperature and / or pressure conditions to a region where the second gas cannot form a hydrate, thereby decomposing the second hydrate, Water forming the hydrate is frozen on the surface of the first hydrate to form a shell portion containing ice as a component on the surface of the first hydrate. Those were.
[0010]
In the above-mentioned invention, the terms “first gas” and “second gas” include a case where each of the first gas and the second gas is composed of a single component and a case where each of them is composed of a plurality of components.
For example, when the first gas and the second gas are composed of a plurality of components, the first gas and the second gas mean all component gases of the plurality of components.
[0011]
The apparatus for producing a hydrate according to the present invention is an apparatus for producing a hydrate composed of a core portion composed of a hydrate composed of gas molecules and water molecules, and a shell portion covering the core with ice. And
A storage tank in which the first hydrate is stored, first gas supply means for supplying the same first gas as the first hydrate guest molecules to the storage tank, and a first hydrate of the first hydrate in the storage tank. Second gas supply means for supplying the same second gas as the guest molecules of the second hydrate formed on the surface, temperature adjustment means for adjusting the temperature of the storage tank, and the amount of the first gas in the storage tank. It comprises a first partial pressure measuring means for measuring the pressure and a second partial pressure measuring means for measuring the partial pressure of the second gas in the storage tank.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiment 1 FIG.
FIG. 1 is an explanatory view schematically showing a hydrate production process which is an embodiment of the present invention and has excellent storage stability, and shows the hydrate structure in each process. FIG. 2 is a hydrate generation equilibrium curve of each gas.
First, the hydrate manufacturing method of the present embodiment will be described with reference to FIGS.
[0013]
The hydrate production method of the present embodiment produces a gas hydrate composed of a core part composed of methane hydrate composed of methane molecules and water molecules, and a shell part composed of ice covering the core part. How to
First, methane hydrate 1 constituting a core portion is generated (FIG. 1A). The conditions for generating the methane hydrate 1 are in the region (a) in FIG. In addition, a method for generating methane hydrate is a known method, for example, methane gas as a raw material is supplied in the form of fine bubbles to water stored in a storage tank, and the conditions (temperature and pressure) in the tank are shown in FIG. It depends on the method set in the area of a).
[0014]
Next, butane hydrate 2 is generated around methane hydrate 1 using a second gas, for example, butane gas, whose guest molecules are less hydrate-generating than methane gas, as guest molecules (FIG. 1B).
The conditions for generating this butane hydrate 2 are the region (b) in FIG.
[0015]
Next, the temperature condition is adjusted to a region where butane cannot form a hydrate. The temperature condition at this time is a region (c) in FIG. As the adjustment of the temperature condition, the temperature is raised from below freezing to 4 ° C. Thus, by adjusting the temperature condition, butane hydrate 2 is decomposed, and water constituting the butane hydrate 3 is frozen on the surface of the methane hydrate 1 to form an ice crust 3 ( FIG. 1 (c)).
[0016]
As described above, according to the method of the present embodiment, it is possible to improve the stability of the hydrate by forming an ice shell on the surface of the methane hydrate while minimizing the proportion of ice.
[0017]
Embodiment 2 FIG.
FIG. 3 is an explanatory view of an apparatus for producing gas hydrate having excellent storage stability according to an embodiment of the present invention.
The apparatus according to the present embodiment includes two ice-shell manufacturing apparatuses A and B connected to a known methane hydrate manufacturing apparatus 5 via a hydrate extraction pipe 7 and valves 9a and 9b.
[0018]
Since the ice-shell manufacturing apparatuses A and B have the same configuration, the ice-shell manufacturing apparatus A will be described below. In addition, when indicating the ice-shell manufacturing apparatus A, a suffix a is added after the code, and when indicating the ice-shell manufacturing apparatus B, the suffix b is added after the code.
The ice crust manufacturing apparatus A according to the present embodiment includes a storage container 11a for storing methane hydrate supplied from the hydrate extraction pipe 7, and a temperature control provided in the storage container 11a to adjust the temperature of the storage container 11a. Device 13a.
[0019]
Further, a methane gas cylinder 19a for supplying methane gas to the storage container 11a via a methane gas supply pipe 15a and a flow control valve 17a, and a butane gas cylinder 25a for supplying butane gas to the storage container 11a via a butane gas supply pipe 21a and a flow control valve 23a. And The partial pressures of the methane gas and the butane gas in the storage container 11a can be measured by the methane partial pressure measuring means 27a and the butane partial pressure measuring means 29a, and the flow regulating valves 17a and 23a are controlled based on the measured values. Is done.
[0020]
An ice-shell hydrate extraction pipe 31a is connected to the bottom of the storage container 11a, and an ice-shell hydrate storage tank 33 for storing the ice-shell hydrate extracted from the storage container 11a is provided at an end thereof. The ice-shell hydrate extraction pipe 31a is provided with an ice-shell hydrate discharge valve 36a.
[0021]
A method for producing an ice-shell hydrate having excellent preservability by the above-configured apparatus will be described below.
The methane hydrate production apparatus 5 produces methane hydrate by a known technique by setting the conditions in the region on the left side of the methane hydrate production equilibrium curve shown in FIG. FIG. 1A is a sectional view of the hydrate at this time.
[0022]
The generated methane hydrate is supplied to the storage container 11a by opening the hydrate supply valve 9a. At this time, the valve 9b on the ice shell manufacturing apparatus B side is closed.
In the storage container 11a, the temperature adjusting device 13a is set so that the temperature, the methane partial pressure, and the butane partial pressure are in the regions (a) and (b) shown in FIG. It is controlled by butane partial pressure measuring means 29a, methane partial pressure measuring means 27a, and flow control valves 17a and 23a.
[0023]
As a partial pressure adjusting procedure, first, only methane is supplied to the storage container 11a up to a pressure of 3 MPa while being measured by the methane partial pressure measuring means 27a, and then butane is measured by the butane partial pressure measuring means 29a. Until the internal pressure becomes 6 MPa. Thereby, the partial pressures of methane and butane are adjusted to 3 MPa, respectively.
[0024]
Since the conditions in the storage container 11a are in the regions (a) and (b) shown in FIG. 2 and are conditions under which both methane hydrate and butane hydrate can be produced, the methane hydrate production device 5 produces the methane hydrate. Methane hydrate is supplied to the storage container 11a in a stable state without decomposition.
[0025]
By the way, as is clear from FIG. 2, at the same temperature, butane hydrates under a pressure condition of about 2 MPa lower than that of methane, so that the storage vessel 11a which keeps methane and butane at the same partial pressure is used. Butane hydrate is easier to produce than methane hydrate.
Therefore, when the methane hydrate 34 dehydrated to a degree that the surface is wet is charged into the storage container 11a, the water on the surface preferentially forms butane hydrate for the reason described above. In addition, even if the dehydration is completely performed in the methane hydrate production device 5 and there is no moisture on the methane hydrate surface, the surface portion of the methane hydrate is replaced with butane hydrate for the above-described reason.
[0026]
When butane hydrate is produced or methane hydrate is replaced with butane hydrate, the temperature in the storage container 11a changes. This temperature change is reduced to about 0 ° C. by heating and cooling with the temperature control device 13a. Is kept.
Further, the pressure in the storage container 11a also changes due to butane hydrate generation or methane hydrate replacement with butane hydrate, and this pressure change is adjusted by the flow control valves 17a and 23a.
That is, when the water on the methane hydrate surface is converted to butane hydrate, the pressure in the storage container 11a decreases. However, since the gas consumed at this time is butane, the pressure in the storage container 11a becomes 6 MPa. Is controlled by a butane partial pressure measuring means 29a and a flow regulating valve 23a.
[0027]
Further, when methane hydrate and butane hydrate on the surface are replaced, the partial pressure of methane increases and the partial pressure of butane decreases, so that the partial pressure of butane cannot be accurately controlled. However, in this case, since only the surface is replaced, the butane partial pressure does not decrease below the pressure that satisfies the conditions for producing butane hydrate. This substitution reaction stops when the entire surface is replaced with butane hydrate.
[0028]
Regardless of the dehydration state of the methane hydrate charged in this way, a two-layered hydrate 35 in which methane hydrate is formed in the core and butane hydrate is formed in the shell is generated in the storage container 11a. A cross-sectional view of the two-layer hydrate 35 is shown in FIG.
[0029]
When a predetermined amount of the two-layered hydrate 35 is stored in the storage container 11a, the valve 9a is closed, and at the same time, the valve 9b is opened to continuously supply methane hydrate to the storage container 11b. In the storage container 11b, the same control as that of the above-described storage container 11a is performed, and the two-layer hydrate 35 is generated (see FIG. 4).
[0030]
On the other hand, in the storage container 11a, after closing the valve 9a, the temperature is increased from 0 ° C. to 4 ° C. by the temperature adjusting device 13a. As a result, the conditions in the storage container 11a become 4 ° C. and about 6 MPa. The condition at this time is shown by (c) in FIG. In this region, methane hydrate exists stably, butane hydrate starts to decompose.
[0031]
Since the decomposition reaction of butane hydrate is an endothermic reaction and the ambient temperature is 4 ° C., water converted into free water by the decomposition of butane hydrate loses heat and forms ice again on the methane hydrate surface. . The two-layered hydrate stored in the storage container 11a in this manner changes to an ice-shell hydrate 37 having an ice-shell on the surface of methane hydrate (see FIG. 4).
[0032]
The ice-shell hydrate 37 thus obtained is supplied to and stored in the ice-shell hydrate storage tank 33 through the ice-shell hydrate take-out pipe 31a by opening the ice-shell hydrate discharge valve 36a.
When the supply of methane hydrate to the storage container 11b reaches a predetermined amount, the methane hydrate is again supplied to the storage container 11a from which all the ice-shell hydrate 37 has been discharged. In the storage container 11b, the same control as that of the storage container 11a described above is performed to manufacture the ice shell hydrate 37 (see FIG. 5).
[0033]
As described above, according to the present embodiment, ice shells can be reliably formed on the surface of methane hydrate, and hydrate stabilization can be achieved while minimizing the proportion of ice.
[0034]
In the above-described embodiment, an example using two batches provided with the storage containers 11a and 11b has been described. However, it goes without saying that ice shell hydrate can be similarly produced by batch processing using one system.
In the above embodiment, methane and butane have the same partial pressure. However, a partial pressure relationship that satisfies the condition that butane hydrate is more easily generated than methane hydrate in the storage container 11a may be used.
[0035]
Embodiment 3 FIG.
In the present embodiment, a case will be described in which a mixed gas such as methane, ethane, propane, and butane is used as a raw material. The manufacturing apparatus may be the same as that shown in FIG.
First, in order to generate a mixture of methane hydrate and ethane hydrate, the condition of FIG. 2C is set. At this time, the temperature must be set to 7 ° C. or higher in order to prevent hydration of propane or butane.
[0036]
Next, when the temperature is lowered to the regions shown in FIGS. 2A and 2B, propane hydrate and butane hydrate are generated on the surface of the mixture of the generated methane hydrate and ethane hydrate. A hydrate having the structure of ()) is generated. Thereafter, by setting the temperature to about 7 ° C. again, propane hydrate and butane hydrate generated on the surface of the mixture of methane hydrate and ethane hydrate begin to decompose. The decomposed propane hydrate and butane hydrate decompose while removing heat, so that water constituting the hydrate forms ice on the surface of the mixture of methane hydrate and ethane hydrate as it is, and FIG. 1 (c) Ice hydrate is produced.
[0037]
As described above, in the case of a mixed gas, for example, natural gas, using a difference in hydrate generation conditions of component gases, for example, only by changing the temperature, a hydrate having an ice shell, for example, methane hydrate and A mixture of ethane hydrates can be produced.
[0038]
【The invention's effect】
As described above, in the present invention, an ice crust can be reliably formed on the hydrate surface, so that the hydrate can be stably stored and transported. Further, since ice shells can be formed only on the surface of the hydrate, it is possible to manufacture a hydrate having a high gas content with a minimum volume of ice.
[Brief description of the drawings]
FIG. 1 is an explanatory view schematically showing a production process of a hydrate having excellent storage stability according to an embodiment of the present invention.
FIG. 2 is a hydrate generation equilibrium curve of each gas.
FIG. 3 is an explanatory diagram of a hydrate manufacturing apparatus according to one embodiment of the present invention.
FIG. 4 is an explanatory diagram of a hydrate production apparatus according to one embodiment of the present invention.
FIG. 5 is an explanatory diagram of a hydrate production apparatus according to one embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Methane hydrate 2 Butane hydrate 3 Ice shell 5 Methane hydrate production equipment 11a, 11b Reservoir 17a, 17b, 23a, 23b Flow control valve 19a, 19b Methane cylinder 25a, 25b Butane cylinder 27a, 27b Methane partial pressure measuring means 29a, 29b Butane partial pressure measuring means

Claims (4)

A method for producing a gas hydrate composed of a core portion composed of a hydrate composed of gas molecules and water molecules, and a shell portion composed of ice covering the core portion,
Generating a first hydrate of a first gas that is a guest molecule of the hydrate of the core portion;
A step of generating a second hydrate around the hydrate of the first gas, the second gas being a guest molecule whose second gas is less hydrate-generating than the first gas;
By adjusting the temperature and / or pressure to such a condition that the second gas cannot form a hydrate, the second hydrate is decomposed, and the water constituting the second hydrate is removed from the first hydrate. A method for producing gas hydrate, comprising: a step of freezing on a surface. A method for producing a gas hydrate composed of a core portion composed of a hydrate composed of gas molecules and water molecules, and a shell portion composed of ice covering the core portion,
Generating a first hydrate of the core under a temperature and pressure condition in which the first gas, which is a guest molecule of the hydrate of the core, generates hydrate;
While maintaining the conditions for the first gas, which is the guest molecule of the first hydrate in the core portion, to generate the hydrate, the second gas whose conditions for generating the hydrate are lower than the first gas is changed to the first gas for the first hydrate. Forming a second hydrate having a second gas as a guest molecule on the surface of the first hydrate by supplying the second gas to the surroundings until the second gas generates hydrate-generating conditions;
By adjusting the temperature condition to a condition under which the second gas cannot form a hydrate, while controlling the partial pressures of the first gas and the second gas within the conditions for generating the hydrate of each gas, the second gas is adjusted. Decomposing the hydrate and freezing the water constituting the second hydrate on the surface of the first hydrate to form a shell portion containing ice as a component on the surface of the first hydrate; A method for producing a hydrate, comprising: Using a mixed gas as a raw material, a method for producing a hydrate composed of a core portion composed of a hydrate composed of gas molecules and water molecules, and a shell portion composed of ice covering the core portion,
Generating a first hydrate under a temperature and pressure condition at which a hydrate having a first gas in the mixed gas as a guest molecule can be generated;
While maintaining the condition that the first gas, which is the guest molecule of the first hydrate, generates the hydrate, the second gas in the mixed gas in which the conditions for generating the hydrate are lower than the first gas generates the hydrate. Forming a second hydrate with a second gas as a guest molecule on the surface of the first hydrate by setting conditions;
By adjusting the temperature and / or pressure conditions to a region where the second gas cannot form a hydrate, the second hydrate is decomposed, and the water constituting the second hydrate is converted into the first hydrate. Producing a shell containing ice as a component on the surface of the first hydrate by freezing the surface of the first hydrate. An apparatus for producing a hydrate composed of a core part composed of hydrate composed of gas molecules and water molecules and a shell part composed of ice covering the core part,
A storage tank in which the first hydrate is stored, first gas supply means for supplying the same first gas as the first hydrate guest molecules to the storage tank, and a first hydrate of the first hydrate in the storage tank. Second gas supply means for supplying the same second gas as the guest molecules of the second hydrate formed on the surface, temperature adjustment means for adjusting the temperature of the storage tank, and the amount of the first gas in the storage tank. An apparatus for producing a hydrate, comprising: a first partial pressure measuring means for measuring a pressure; and a second partial pressure measuring means for measuring a partial pressure of a second gas in the storage tank.

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