AU2003227266B2 - Gas hydrate and method for production thereof - Google Patents

Description

DESCRIPTION

GAS HYDRATE AND METHOD FOR PRODUCTION THEREOF TECHNICAL FIELD: The present invention relates to a gas hydrate which is an inclusion hydrate of a gas hydrate forming material, such as natural gas, methane gas or carbon dioxide gas, with water, and to a method for producing the gas hydrate.

BACKGROUND ART: A gas hydrate is an ice-like solid material formed from water molecules and molecules of a gas hydrate forming substance and is an inclusion hydrate having a structure in which molecules of the gas hydrate forming substance are included within cage-like structures formed of water molecules. The gas hydrates is formed by reaction of water with the gas hydrate forming substance at predetermined pressure and temperature and is dissociated into water and the gas hydrate forming substance when the temperature and/or pressure are changed. Since gas hydrates have a high gas inclusion efficiency, a large latent heat of formation and dissociation, a property to generate a high pressure upon a slight temperature change and a selectivity of hydrated molecules, studies are being made with a view toward utilizing them for a variety of applications such as transportation and storage of, for example, natural gas, a heat storage system, an actuator, and gas separation and recovery.

For example, one merit of the transportation and storage of natural gas in the form of hydrate is that the natural gas can be transported and stored at considerably milder temperature conditions than the conventionally practiced transportation and storage temperature 163C) for liquefied natural gas (LNG), since the equilibrium temperature of natural gas hydrate (hereinafter occasionally referred to "NGH") is about -80oC (in the case of pure methane) under atmospheric pressure. Thus, it is expected that the transportation and storage equipments can be greatly simplified with respect to the pressure resistance and heat insulation thereof.

The transportation and storage of NGH are contemplated to be performed such that the gas hydrate produced is formed into slurry, powder, pellets, compressed block, etc.

2 It is desirable that the temperature at which gas hydrates are transported and stored is as near to ambient temperature as possible but not higher than 0OC with the consideration of the superiority over the abovedescribed liquefied natural gas (LNG) and economy with respect to installation costs and operation costs. However, the use of a high temperature has a problem because the decomposition of the gas hydrates occurs in a large amount.

For example, in the case of the conventional NGH, about 80 thereof decomposes when stored at -20C under atmospheric pressure for 2 weeks. As the amount of decomposed gas hydrates increases, the amount of the gas included therein per unit weight of the gas hydrate to be transported and stored decreases so that the transportation and storage efficiency is lowered. Thus, in order to maintain the desired transportation and storage efficiency, it is necessary that the transportation and storage equipments be furnished with a device and a tank for recovering the decomposed gases, and a device for again forming dehydrate with the decomposed gases. This causes an increase of the installation costs and operation costs.

With regard of the stabilization of gas hydrates during the transportation and storage, Japanese patent No.3,173,611 proposes and suggests the use of a stabilizing agent for water to increase the gas storage and transportation capacity for the gas hydrates. However, the Japanese patent No.

3,173,611 does not specifically describe what substance the "stabilizing agent for water" is. Thus, the proposal is still a matter of idea.

DISCLOSURE OF THE INVENTION As described in the foregoing, in order to transport and store gas hydrates in the form of slurry, powder, pellets, compressed blocks, etc., it is necessary to suppress the decomposition of the gas hydrates as much as possible.

The objective problem of the present invention is to provide a technique capable of improving the preservability of gas hydrates and of suppressing the decomposition of the gas hydrates during transportation and storage.

In solving the above problem, a gas hydrate according to a first aspect of the present invention is a method of producing a gas hydrate comprising reacting raw material water with a gas hydrate forming material under conditions suitable for forming the gas hydrate, characterized in that the gas 3 hydrate is formed in the presence of a substance having the function of suppressing the decomposition of the gas hydrate. As a consequence of this feature, it is possible to produce a gas hydrate which has excellent selfpreservation effect and which is small in decomposition amount during transportation and storage by forming the gas hydrate in the presence of a substance having the function of suppressing the decomposition of the gas hydrate. Therefore, the gas hydrate produced by the above process is excellent in transportation and storage efficiency and enables to omit or simplify equipment for forming hydrate with the decomposed gases.

A method for producing a gas hydrate according to a second aspect of the present invention comprises reacting raw material water with a gas hydrate forming material under conditions suitable for forming the gas hydrate, and is characterized in that a substance having the function of suppressing the decomposition of the gas hydrate and/or a material which forms the substance in water are added to the raw material water. As a consequence of this feature, the working effect similar to that attained in the above first aspect is obtainable.

A method for producing a gas hydrate according to a third aspect of the present invention is a method for producing a gas hydrate according to the above first or second aspect, characterized in that the substance having the function of suppressing the decomposition of the gas hydrate is an ion formed by dissociation of an electrolyte in a solution. As a consequence of this feature, the working effect similar to that attained in the above first or second aspect is obtainable.

A method for producing a gas hydrate according to a fourth aspect of the present invention is a method for producing a gas hydrate according to the above third aspect, characterized in that the ion comprises, as a constituent component, one or more element selected from the group consisting of lithium sodium potassium rubidium cesium beryllium (Be), magnesium calcium strontium barium fluorine chlorine bromine iodine carbon sulfur nitrogen oxygen boron phosphorus manganese iron Copper (Cu), zinc cadmium aluminum silicon tin lead (Pb), vanadium chromium molybdenum cobalt (Co) and nickel (Ni).

As a consequence of this feature, the working effect similar to that attained in the above first or second aspect is obtainable.

A method for producing a gas hydrate according to a fifth aspect of the present invention is a method for producing a gas hydrate according to the above first or second aspect, characterized in that the substance having the function of suppressing the decomposition of the gas hydrate is one or more ions selected from the group consisting of a chloride ion a fluoride ion a bromide ion an iodide ion a sodium ion a potassium ion a lithium ion a calcium ion a magnesium ion a carbonate ion (C0 3 2 a phosphate ion (P043 and an ammonium ion As a consequence of this feature, it is possible to produce a gas hydrate which has excellent self-preservation effect and which is small in decomposition amount by using one or more ions selected from the above members as the substance having the function of suppressing the decomposition of the gas hydrate.

A method for producing a gas hydrate according to a sixth aspect of the present invention is a method for producing a gas hydrate according to the above first or second aspect, characterized in that the substance having the function of suppressing the decomposition of the gas hydrate is one or more metals selected from the group consisting of zinc, iron and manganese or an ion of the metal. As a consequence of this feature, it is possible to produce a gas hydrate which has excellent self-preservation effect and which is small in decomposition amount by using one or more ions or metals selected from the above members as the substance having the function of suppressing the decomposition of the gas hydrate.

A gas hydrate according to a seventh aspect of the present invention is characterized in that the hydrate comprises an ion formed by dissociation of an electrolyte in a solution. According to the invention directed to a gas hydrate, the gas hydrate has excellent self-preservation effect and is small in decomposition amount during transportation and storage, because the hydrate comprises an ion formed by dissociation of an electrolyte in a solution.

Incidentally, the term "comprise" as used in this aspect is intended to include a state in which the ion is contained within the crystals as well as a state in which the ion is present around the crystals such that the ion is not separable from the gas hydrate particles.

A gas hydrate according to an eighth aspect of the present invention is a gas hydrate according to the above seventh aspect, characterized in that the ion comprises, as a constituent component, one or more element selected from the group consisting of lithium sodium potassium rubidium (Rb), cesium beryllium magnesium calcium strontium (Sr), barium fluorine chlorine bromine iodine carbon sulfur nitrogen oxygen boron phosphorus manganese (Mn), iron Copper zinc cadmium aluminum silicon tin lead vanadium chromium molybdenum cobalt (Co) and nickel As a consequence of this feature, the working effect similar to that attained in the above seventh aspect is obtainable.

A gas hydrate according to a ninth aspect of the present invention is characterized in that the hydrate comprises one or more ions selected from the group consisting of a chloride ion a fluoride ion a bromide ion an iodide ion a sodium ion a potassium ion a lithium ion a calcium ion (Ca2), a magnesium ion a carbonate ion (CO 3 2 a phosphate ion (PO 4 3 and an ammonium ion According to the invention directed to a. gas hydrate, the gas hydrate has excellent selfpreservation effect and is small in decomposition amount during transportation and storage, because the hydrate comprises one or more ions selected from the above members. Incidentally, the term "comprise" as used in this aspect is intended to include a state in which the ion is contained within the crystals as well as a state in which the ion is present around the crystals such that the ion is not separable from the gas hydrate particles.

A gas hydrate according to a tenth aspect of the present invention is characterized in that the hydrate comprises one or more metals selected from the group consisting of zinc, iron and manganese or an ion of the metal.

According to the invention directed to a gas hydrate, the gas hydrate has excellent self-preservation effect and is small in decomposition amount during transportation and storage, because the hydrate comprises one or more metals or ions thereof selected from the above members. Incidentally, the term "comprise" as used in this aspect is intended to include a state in which the ion is contained within the crystals as well as a state in which the ion is present around the crystals such that the ion is not separable from the gas hydrate particles.

An agent for suppressing the decomposition of a gas hydrate according to an eleventh aspect of the present invention is characterized in that the agent comprises an ion formed by dissociation of an electrolyte in a solution. The use 6 of the agent for suppressing the decomposition of a gas hydrate makes it possible to improve the preservability of the gas hydrate and to suppress the decomposition thereof.

An agent for suppressing the decomposition of a gas hydrate according to a twelfth aspect of the present invention is the above eleventh aspect, characterized in that the ion comprises, as a constituent component, one or more element selected from the group consisting of lithium sodium (Na), potassium rubidium cesium beryllium magnesium (Mg), calcium strontium barium fluorine chlorine bromine iodine carbon sulfur nitrogen oxygen boron phosphorus manganese iron Copper zinc cadmium aluminum silicon tin lead vanadium chromium molybdenum cobalt (Co) and nickel As a consequence of this feature, the working effect similar to that attained in the above eleventh aspect is obtainable.

An agent for suppressing the decomposition of a gas hydrate according to a thirteenth aspect of the present invention is characterized in that the agent comprises one or more ions selected from the group consisting of a chloride ion a fluoride ion a bromide ion an iodide ion a sodium ion a potassium ion a lithium ion a calcium ion a magnesium ion and an ammonium ion (NH 4 An agent for suppressing the decomposition of a gas hydrate according to a fourteenth aspect of the present invention is characterized in that the agent comprises one or more metals selected from the group consisting of zinc, iron and manganese or an ion of the metal.

An agent for suppressing the decomposition of a gas hydrate according to a fifteenth aspect of the present invention is characterized in that the agent comprises a substance capable of forming, when dissociated in water, one or more ions selected from the group consisting of a chloride ion a fluoride ion a bromide ion an iodide ion a sodium ion (Nat), a potassium ion a lithium ion (Lit), a calcium ion a magnesium ion and an ammonium ion (NH4+).

An agent for suppressing the decomposition of a gas hydrate according to a sixteenth aspect of the present invention is characterized in that the agent comprises a substance capable of forming, in water, one or more metals selected 7 from the group consisting of zinc, iron and manganese or an ion of the metal.

The use of the agent for suppressing the decomposition of a gas hydrate according to the above thirteenth to sixteenth aspects makes it possible to improve the preservability of the gas hydrate and to suppress the decomposition thereof.

A seventeenth aspect according to the present invention is characterized in that a use is made of an electrolyte or an ion formed by dissociation of the electrolyte in a solution for the production of a stable gas hydrate. A stable gas hydrate which is difficult to be decomposed is obtainable by the use thereof.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graphical drawing showing a change of decomposition rate with time of methane hydrate of Example 1 and comparative methane hydrate.

FIG. 2 is a graphical drawing showing a change of decomposition rate with time of methane hydrate of Example 2 and comparative methane hydrate.

FIG. 3 is a graphical drawing showing a change of decomposition rate with time of methane hydrate of Example 3 and comparative methane hydrate. FIG. 4 is a graphical drawing showing a change of decomposition rate with time of methane hydrate of Example 4 and comparative methane hydrate. FIG. 5 is a graphical drawing showing a change of decomposition rate with time of methane hydrate of Example 5 and comparative methane hydrate. FIG. 6 is a graphical drawing showing a change of decomposition rate with time of methane hydrate of Example 6 and comparative methane hydrate. FIG. 7 is a graphical drawing showing a change of decomposition rate with time of methane hydrate of Example 7 and comparative methane hydrate. FIG. 8 is a graphical drawing showing a change of decomposition rate with time of methane hydrate of Reference Example. FIG. 9 is an illustration of a principle for explaining a mechanism of self preservation effect of natural gas hydrate.

BEST MODE FOR CARRYING OUT THE INVENTION A method for producing a gas hydrate according to the present invention is performed by reacting raw material water with a gas hydrate forming material in the presence of a substance having the function of suppressing the decomposition of the gas hydrate (hereinafter occasionally referred to as "decomposition suppressing substance").

Raw material water: In general, as raw material water for the production of gas hydrates, demineralized water or purified water free of contaminants that may adversely affect the production of gas hydrates is used. In the present invention, the decomposition suppressing substance may be added to demineralized water or purified water. When a suitable amount of the decomposition suppressing substance is contained in raw material water, it is used as such.

Gas hydrate forming substance: There is no specific limitation in the kind of gas hydrates in the present invention. That is there is no specific limitation in the kind of gas hydrate forming substances, as long as they can form gas hydrates under given pressure and temperature conditions. As the gas hydrate forming substance, there may be mentioned a substance (gas) which is a gas at ambient temperature and ambient pressure, such as methane, natural gas (mixed gas containing methane as a major ingredient and other gases such as ethane, propane and butane) or carbonic acid gas (carbon dioxide).

Conditions for forming gas hydrates: Conditions such as temperature and pressure for forming gas hydrates vary with the kind of the substance and are known per se. In the case of methane hydrate, for example, the conditions shown in Examples described hereinafter may be adopted for the production thereof.

When a gas hydrate forming substance is a gas, the reaction of water with the gas hydrate forming substance may be carried out in a state where the water and the gas are contacted so that the gas hydrate is formed in the gas liquid interface.

Decomposition suppressing substance: Any decomposition suppressing substance may be used without specific limitation as long as it can improve self preservation effect of gas hydrates, though the following substances are preferably used.

An ion formed by dissociation of an electrolyte in a solution may be suitably used as the decomposition suppressing substance. As such an ion, there may be mentioned, for example, an ion which comprises, as a constituent component, an alkali metal element, an alkaline earth metal element, a halogen element, a non-metallic element and a metallic element (except the abovedescribed alkali metal element and alkaline earth metal element). As the electrolyte, any electrolyte is usable without specific limitation as long as it has the above element as the constituent component, though an electrolyte which does not form a gaseous substance by decomposition in raw material water is preferable as described hereinafter.

As the alkali metal element, there may be mentioned, for example, lithium sodium potassium rubidium (Rb) and cesium As the alkaline earth element, there may be mentioned, for example, beryllium magnesium calcium strontium (Sr) and barium As the halogen element, there may be mentioned, for example, fluorine chlorine bromine (Br) and iodine As the non-metallic element, there may be mentioned, for example, carbon sulfur nitrogen oxygen boron and phosphorus As the metallic element, there may be mentioned, for example, manganese iron Copper zinc cadmium (Cd), aluminum silicon tin lead vanadium chromium (Cr), molybdenum cobalt (Co) and nickel (Ni).

Specific examples of the suitable ion as the decomposition suppressing substance include those exemplified in below: ions such as chloride ion fluoride ion bromide ion iodide ion sodium ion potassium ion lithium ion calcium ion magnesium ion (Mg2 and ammonium ion (NH4+).

Specific examples of the suitable metal or metal ion as the decomposition suppressing substance include those exemplified in below: metals such as zinc, iron and manganese and ions thereof.

The above decomposition suppressing substances may be used singly or in combination of two or more thereof. There are cases where the use of a plurality of the decomposition suppressing substances in combination is preferable as shown in Examples which will be described hereinafter.

The amount (amount present, amount added) of the decomposition suppressing substance is preferably such that it is contained in the raw material water in an amount of not smaller than 0.1 ppm but not greater than 10,000 ppm (1 by weight), more preferably not smaller than 1 ppm but not greater than 1,000 ppm, in the case of the ions produced by dissociation of an electrolyte in a solution or the ions above. In the case of the metal or ion thereof shown in above, the amount of the decomposition suppressing substance is preferably such that it is contained in the raw material water in an amount of not less than 0.01 ppm but not greater than 1,000 ppm (0.1 by weight), more preferably not smaller than 0.01 ppm but not greater than 100 ppm.

When the electrolyte is added to raw material water, the electrolyte may be added in an amount so that the ion be contained in the raw material water in an amount as described above. When the substance or as described below is used as the decomposition suppressing substance, such as substance is may be added in an amount so that the substance or be contained in the raw material water in an amount as described above.

Incidentally, the decomposition suppressing substances include those (such as Na and C1 which are known to hinder the formation of gas hydrates.

The hindrance by these substances is generally caused under conditions where these substances are present in an amount of several by weight. On the other hand, since the decomposition suppressing substance takes effect in a significantly lower concentration level, no hindrance of the formation of hydrates is caused or, even when hindrance is caused, the influence is restricted at a slight level. In case where an improvement in self preservation is a primary matter, the decomposition suppressing substance may be present in an amount beyond the above-described range, although a hindrance of the formation of hydrate is caused.

The thus obtained gas hydrates according to the present invention has an enhanced self preservation effect by virtue of the decomposition suppressing substance and has an improved transportation and storage efficiency under gas hydrate decomposition conditions. While the pressure and temperature during the transportation and storage are desirably atmospheric pressure and to 0OC, respectively, from the standpoint of reduction of energy consumption and installation load, the gas hydrates produced in the presence of the decomposition suppressing substance permit the transportation and storage at milder conditions, for example, at 15'C under atmospheric pressure.

More specifically, in the case of NGH, for example, the following effects may be obtained.

In the ordinary NGH decomposition range (at about 15 0 C under atmospheric pressure, for example), a sufficient level of self preservation effect of NGH can be obtained, so that the transportation and storage can be performed with a small amount of decomposed gases. Therefore, the amount of natural gas transported and the amount of natural gas stored can be increased, ensuring high transportation and storage efficiencies.

Since the temperature at which the transportation and storage are performed can be increased to about -15'C, it is possible to reduce an energy required for cooling and to simplify the equipment.

Since the temperature at which the transportation and storage are performed can be increased to about -15'C, it is possible to reduce an energy required for re-gasifying and to simplify the equipment.

Since heat penetration is reduced, the thickness of a heat insulating material (amount of the heat insulating material) may be reduced. Therefore, with a vessel having the same volume, it is possible to reduce the external dimensions thereof, so that the volume efficiency is improved.

Since the amount of decomposed gases can be reduced, a device for recovering the decomposed gases and an equipment for re-hydrating the decomposed gases are not needed.

Decomposition suppressing agent: The above-described decomposition suppressing substances (ions formed by dissociation of an electrolyte in a solution, ions described above, and metals or ions described above) may be used as an agent for suppressing decomposition of gas hydrates. A substance capable of generating a decomposition suppressing substance in water may also be used as an agent for suppressing decomposition of gas hydrates. As "the substance capable of generating a decomposition suppressing substance in water", there may be mentioned, for example, substances generating ions described above when dissociated in water, and substances generating metals or ions thereof described above in water. These substances include electrolytes which are preferable.

As the "substances generating ions described above when dissociated in water", there may be mentioned, for example, sodium chloride, calcium chloride, magnesium chloride, sodium fluoride, calcium bromide and ammonia.

As the "substances generating metals or ions thereof described above in water", there may be mentioned, for example, iron chloride (FeC12), zinc chloride (ZnC12) and manganese chloride (MnCl2).

It is preferred that the decomposition suppressing agent is added to raw material water or the like for gas hydrates so that the amount thereof is as described above. By using the decomposition suppressing agent, it is possible to improve the self preservation property of the gas hydrates.

It is known that a gas hydrate has an effect of suppressing decomposition thereof, called self preservation effect, at a temperature not lower than the equilibrium temperature. Whilst the self preservation effect has many points that have not yet been clarified, the following explanation is given (Hiroshi KANEKO, Journal of Marine Science and Technology, No. 842, p38- 48).

FIG. 9 is a schematic illustration of a cross-section of a gas hydrate particle. When a gas hydrate 50 formed at a low temperature and a high pressure (FIG. is exposed under decomposition conditions such as under atmospheric pressure, the decomposition starts to proceed partially from a surface thereof. Thus, the gas hydrate forming substance is gasified with the simultaneous coverage of the surface of the gas hydrate with a water film 51 (FIG. When a heat is lost by the decomposition of the gas hydrate at the surface thereof, the water film 51 on the surface of the gas hydrate is frozen to form an ice film 52 which covers the surface of the gas hydrate (FIG. When the ice film 52 grows to have a thickness beyond a certain level, heat exchange between the inside (gas hydrate) and outside of the film is inhibited, so that the gas hydrate thereinside is stabilized even when subjected to decomposing conditions such as atmospheric pressure. That is when the ice film 52 acquires mechanical strengths sufficient to resist the pressure from the gas hydrate which is about to decompose (gasify), the gas hydrate is stabilized and exhibits self preservation effect to inhibit further decomposition thereof.

The present invention has been made on the basis of the finding that gas hydrates produced in the presence of a decomposition suppressing substance show improved self preservation effect and stability. The decomposition suppressing substances include those substances which have been hitherto pointed out to adversely affect the formation of gas hydrates.

That such substances have properties to improve the self preservation effect is quite an unexpected result. Although the working mechanism for improving the self preservation properties by the decomposition suppressing substance has not yet been clarified, the following consideration may rationally explain the mechanism.

Among the above decomposition suppressing substances, substances such as a chloride ion a fluoride ion and an ammonium ion (NH 4 are considered to be incorporated, in a small amount, into lattice points through substitution with water molecules of ice crystals (for example, Norikazu MAENO and Masami FUKUDA, Basic Glaciology Course, "Structures and Crystals of Ice and Snow"). Although there are no reports describing the substitution of these substances for water molecules in crystals of a cage structure of a gas hydrate, it is inferred that these substances are incorporated into the crystals in the same manner as that in ice. Once these substances enter the crystals, crystal defects results at the lattice points into which the substances are incorporated.

In general, in the field of metallic materials, it is known that when crystals have some defects and when the amount of the defects is small, dislocation thereof is prevented and the strength thereof is increased. Although no reports concerning ice or gas hydrates are found which describe that lattice defects have an influence upon the strength of ice or gas hydrates, it is inferred that the above-described substances are incorporated into lattice points of ice or gas hydrates to cause lattice defects, resulting in an increase of the mechanical strengths of the ice and gas hydrates in the same manner as that in the case of metals.

In the case of decomposition suppressing substances other than the above-described ions, it is inferred that they can present as impurities in crystal grain boundaries of gas hydrates or ice, although they are not directly incorporated into lattice points of the cage structure of gas hydrates or ice crystals. It is inferred that the impurities in those crystals also have the function to prevent dislocation of crystal and to increase the mechanical strength of ice and gas hydrate.

Under conditions suitable for forming a gas hydrate, the decomposition suppressing substance is present in ice which deposits around the gas hydrate and serves to increase the strength of ice surrounding the gas hydrate. When the gas hydrate is subjected to slowly decomposing conditions, the ice surrounding the gas hydrate temporarily melts. When an ice film (FIG. 9(c)) covering the gas hydrate particle is formed, the decomposition suppressing substance is incorporated into the ice film. Because of the presence of the decomposition suppressing substance, the mechanical strengths of the ice film are considered to be improved so that the resistance to pressure from the inside gas hydrate is improved and, therefore, the self preservation effect is improved.

Accordingly, it is considered that not only the decomposition suppressing substances exemplified above but also those substances which can serve to improve the mechanical strength of ice, particularly those substances which can be substituted for water molecules in the crystal lattice, may be used as the decomposition suppressing substance.

Further, as the function in consistent with the above-described inference that lattice defects in the crystals serve to improve the mechanical strengths, there may be made the following explanation based on an ionic effect.

That is, an electrolyte serving as the decomposition suppressing substance has a positive ion and a negative ion with an equal amount of charges. Since the positive and negative ions attract each other, the electrical properties of a gas hydrate change when a trace amount of ions are incorporated into the crystal structure of the gas hydrate or are present between the crystal structures thereof. It has been confirmed that the gas hydrate into which ions are incorporated is apt to be charged with static electricity and is apparently different from the ordinary gas hydrate with respect to the electrical properties. Because of the property of ions, an electric binding force is acted on the structure of the hydrate. It is inferred that the binding force can strengthen the cage structure of water molecules including the hydrate therein so that the discharge of gases contained therein is inhibited to contribute to an improvement of the preservability.

Examples will be next given to describe the present invention in more detail. The present invention is not restricted to the examples in any way.

Example 1 In 200 g of distilled water (raw material water), calcium chloride (CaC12-2H20, molecular weight M 147.02) was dissolved to obtain a calcium chloride solution. The solution was placed in a stainless steel vessel. After hermetically sealing the stainless vessel, methane gas (purity: 99 or higher) was charged therein to a pressure of 8 MPa.

With stirring using a stirrer, a gas hydrate was allowed to form while maintaining the vessel at 4'C. Methane was fed so that the pressure was maintained constant, since otherwise the gas pressure would have decreased with the formation of methane hydrate.

After the formation of methane hydrate, the vessel was cooled to 20 0

C

(0 0 C or lower suffices) to freeze excess water within the vessel. The pressure within the vessel was then released to atmospheric pressure and the methane hydrate at -20"C obtained was taken out of the vessel.

The methane hydrate was sampled and placed in a vessel provided with a small hole for discharging decomposed gases therethrough. The weight of the sample was measured to determine the rate of the decomposition of the gas hydrate from a change of the weight according to the following measuring method.

Measurement of decomposition rate: A methane hydrate sample is placed in the vessel and the vessel is weighed to determine the weight (W1) of the sample.

While maintaining at -20 0 C, the weight of the sample containing vessel is weighed at predetermined time points to determine the weight of the sample.

After a predetermined period of time, the sample is allowed to decompose completely and the weight (W2) of the remaining water (ice) is determined.

A hydration efficiency and a decomposition rate are calculated from the following equations: Hydration Efficiency (Weight of methane hydrate)/(Weight of sample) (W1 W2) (WI W2)/16x5.75x18 Wi Decomposition rate (Initial hydration efficiency Hydration efficiency at a measurement)/(Initial hydration efficiency) x 100 The initial hydration efficiency of the methane hydrate in the present example was found to be 90 The amounts of chloride ion and calcium ion as decomposition suppressing substances were 100 ppm and 57 ppm, respectively.

The results of the measurement of the decomposition rate of the gas hydrate (particle diameter: 1 mm) in the present example were as shown in FIG. 1.

Example 2 In 200 g of distilled water (raw material water), 0.016 g of sodium chloride (NaC1, molecular weight M 58.44) was dissolved to obtain a sodium chloride solution. The procedures of Example 1 were repeated in the same manner as described except that the sodium chloride solution was used, thereby obtaining methane hydrate. The decomposition rate was also measured. The amounts of chloride ion and sodium ion as decomposition suppressing substances were 50 ppm and 32 ppm, respectively. The results of the measurement of the decomposition rate of the gas hydrate (particle diameter: 1 mm) in the present example were as shown in FIG. 2.

Example 3 In 200 g of distilled water (raw material water), 0.0573 g of magnesium chloride (MgCl 6H 2 0, molecular weight M 203.3) was dissolved to obtain a magnesium chloride solution. The procedures of Example 1 were repeated in the same manner as described except that the magnesium chloride solution was used, thereby obtaining methane hydrate. The decomposition rate was also measured. The amounts of chloride ion and magnesium ion as decomposition suppressing substances were 100 ppm and 34 ppm, respectively. The results of the measurement of the decomposition rate of the gas hydrate (particle diameter: 1 mm) in the present example were as shown in FIG. 3.

Example 4 In 200 g of distilled water (raw material water), sodium chloride, calcium, magnesium, zinc, iron and manganese (sodium ion: 20 ppm, chloride ion: 20 ppm, calcium 50 ppm, magnesium: 50 ppm, zinc: 0.012 ppm, iron: 0.08 ppm, manganese: 0.043 ppm) were dissolved. Except that the thus obtained solution was used, the procedures of Example 1 were repeated in the same manner as described to obtain methane hydrate. The decomposition rate was also measured. The results of the measurement of the decomposition rate of the gas hydrate (particle diameter: 4 mm) in the present example were as shown in FIG. 4.

Example In 200 g of distilled water (raw material water), 0.0044 g of sodium fluoride (NaF, molecular weight M 42) was dissolved to obtain a sodium fluoride solution. The procedures of Example 1 were repeated in the same manner as described except that the sodium fluoride solution was used, thereby obtaining methane hydrate. The decomposition rate was also measured. The amounts of fluoride ion and sodium ion as decomposition suppressing substances were 10 ppm and 12 ppm, respectively. The results of the measurement of the decomposition rate of the gas hydrate (particle diameter: 1 mm) in the present example were as shown in FIG. Example 6 In 200 g of distilled water (raw material water), 0.03 g of calcium bromide (CaBr2) was dissolved to obtain a calcium bromide solution. The procedures of Example 1 were repeated in the same manner as described except that the calcium bromide solution was used, thereby obtaining methane hydrate. The decomposition rate was also measured. The amounts of bromide ion and calcium ion as decomposition suppressing substances were 100 ppm and 50 ppm, respectively. The results of the measurement of the decomposition rate of the gas hydrate (particle diameter: 0.5 mm) in the present example were as shown in FIG. 6.

Example 7 In 200 g of distilled water (raw material water), 0.088 g of ammonium sulfate ((NH4)2SO 4 molecular weight 132) was dissolved to obtain a ammonium sulfate solution. The procedures of Example 1 were repeated in the same manner as described except that the ammonium sulfate solution was used, thereby obtaining methane hydrate. The decomposition rate was also measured.

The amounts of ammonium ion NH4+ and sulfate ion SO 4 2 as decomposition suppressing substances were 120 ppm and 320 ppm, respectively.

The results of the measurement of the decomposition rate of the gas hydrate (particle diameter: 1 mm) in the present example were as shown in FIG. 7.

Comparative Examples 1-7 As comparative examples, Examples 1-7 were each repeated in the same manner as described except that distilled water containing no decomposition suppressing substance was used to obtain methane hydrate. The results of the measurement of the decomposition rates of the comparative gas hydrates (particle diameters were the same as respective corresponding Examples) are shown in FIGS. 1-7, respectively.

Example 8 In 450 g of distilled water (raw material water), 0.055 g of magnesium carbonate (MgCO3, molecular weight: 84) was dissolved to obtain a magnesium carbonate solution. The procedures of Example 1 were repeated in the same manner as described except that the magnesium carbonate solution was used, thereby obtaining methane hydrate. The decomposition rate was also measured.

The amounts of carbonate ion C0 3 2 and magnesium ion Mg 2 t as decomposition suppressing substances were 90 ppm and 35 ppm, respectively.

The results of the measurement of the decomposition rate of the gas hydrate (particle diameter: 1 mm) in the present example reveal that the decomposition was suppressed as compared with the case where no decomposition suppressing substance was added.

Example 9 In 200 g of distilled water (raw material water), 0.060 g of dipotassium hydrogen phosphate (K 2

HPO

4 molecular weight: 133) was dissolved to obtain a dipotassium hydrogen phosphate solution. The procedures of Example 1 were repeated in the same manner as described except that the phosphate solution was used, thereby obtaining methane hydrate. The decomposition rate was also measured.

The amounts of phosphate ion PO43 and potassium ion K- as decomposition suppressing substances were 200 ppm and 80 ppm, respectively.

The results of the measurement of the decomposition rate of the gas hydrate in the present example reveal that the decomposition was suppressed as compared with the case where no decomposition suppressing substance was added.

Example In 200 g of distilled water (raw material water), 0.140 g of lithium chloride (LiC1, molecular weight: 41) was dissolved to obtain a lithium chloride solution. The procedures of Example 1 were repeated in the same manner as 19 described except that the lithium chloride solution was used, thereby obtaining methane hydrate. The decomposition rate was also measured.

The amounts of lithium ion Li and chloride ion Cl as decomposition suppressing substances were 100 ppm and 580 ppm, respectively. The results of the measurement of the decomposition rate of the gas hydrate in the present example reveal that the decomposition was suppressed as compared with the case where no decomposition suppressing substance was added.

Example 11 In 200 g of distilled water (raw material water), 0.0236 g of sodium iodide (Nal, molecular weight: 150) was dissolved to obtain a sodium iodide solution. The procedures of Example 1 were repeated in the same manner as described except that the sodium iodide solution was used, thereby obtaining methane hydrate. The decomposition rate was also measured.

The amounts of iodide ion I- and sodium ion Na as decomposition suppressing substances were 100 ppm and 200 ppm, respectively. The results of the measurement of the decomposition rate of the gas hydrate in the present example reveal that the decomposition was suppressed as compared with the case where no decomposition suppressing substance was added.

Reference Example In 200 g of distilled water (raw material water), sodium hypochlorite (NaCIO) was dissolved to obtain a solution having a CO10 concentration of 1 ppm.

The residual chlorine concentration was measured by an orthtoluidin method.

The procedures of Example 1 were repeated in the same manner as described except that the sodium hypochlorite solution was used, thereby obtaining methane hydrate. The decomposition rate was also measured.

The results of the measurement of the decomposition rate of the gas hydrate (particle diameter: 1 mm) in the present example were as shown in FIG.

8. From FIG. 8, it was confirmed that the addition of sodium hypochlorite failed to give decomposition suppressing effect for gas hydrate but, rather, the decomposition rate and the preservability increased as compared with the case where no sodium hypochlorite was used.

Although the reason for the failure to obtain decomposition suppressing effect by addition of sodium hypochlorite has not yet been clarified, the following inference may be made.

Sodium hypochlorite generates a hypochlorite ion (C10 upon dissociation in an aqueous solution. The hypochlorite ion is unstable and is susceptible to be decomposed to finally form molecular oxygen. That is sodium hypochlorite gradually decomposes according to the following reaction formula: 2NaC10 2NaCl 02 Similarly, calcium hypochlorite (Ca(CIO)2) generates molecular oxygen as follows: Ca(CIO)2 CaC12 02 There is a possibility that the oxygen gas produced by the above reactions adversely affects the physical properties of gas hydrate, since the generation thereof continues throughout the course of the formation of the gas hydrate. More specifically, it is inferred that the emanation of the gaseous oxygen results in the formation of fine pores in the gas hydrates and in an increase of the surface area of the gas hydrate, so that the gas hydrate is imparted with properties susceptible to decompose upon a change in temperature and pressure. In view of the foregoing, it is considered that the electrolyte used for the purpose of the present invention is desired to be an electrolyte (gas-formation-free electrolyte) which does not form a gaseous substance such as molecular oxygen in an aqueous solution. When an electrolyte, such as sodium hypochlorite, which is likely to generate a gaseous substance is used, it is preferred that the formation of gas hydrates is performed under such a condition that the generation of a gas is prevented, for example, by controlling the reaction conditions such as pH.

From the results of the above Examples and Comparative Examples, it is seen that the methane hydrate produced in the presence of a decomposition suppressing substance is lower in decomposition rate than that of the comparative methane hydrate when compared under the same conditions.

Accordingly, it has been revealed that by producing a gas hydrate in the presence of a decomposition suppressing substance or with an addition of a decomposition suppressing substance, it is possible to obtain the gas hydrate which is low in decomposition rate during transportation and storage.

INDUSTRIAL APPLICABILITY Stable gas hydrate which is inhibited from decomposing even when 21 subjected to a change in temperature or pressure is useful for application to transportation and storage of, for example, natural gas, a heat storage system, an actuator and gas separation and recovery.

Claims (6)

1. A method for producing a gas hydrate comprising reacting raw material water with a gas hydrate forming material under conditions suitable for forming the gas hydrate,. wherein an electrolyte is added to the raw material water so that the concentration of an ion formed by dissociation of said electrolyte is not lower than 1 ppm but not higher than 10.000 ppm, and the gas hydrate is then formed.

2. A method for prod.ucing a gas hydrate according to claim 1, wherein said ion comprises, as a constituent component, one or more elements selected from the group consisting of lithium sodium potassium rubidium cesium (Cs), beryllium magnesium calcium strontium barium fluorine chlorine bromine iodine carbon sulfur nitrogen oxygen boron phosphorus manganese iron Copper zinc (Zn), cadmium aluminum silicon tin lead vanadium chromium molybdenum cobalt (Co) and nickel (Ni).

3. A method for producing a gas hydrate according to claim 1 or 2, wherein said ion is one or more ions selected from the group consisting of a chloride ion (Cl a fluoride ion (F a bromide ion (Br an iodide ion a sodium ion a potassium ion a lithium ion a calcium ion a magnesium ion (Mg2), a carbonate ion a phosphate ion (P0 4 and an ammonium ion (NH 4 23

4. A method for producing a gas hydrate according to claim 1, wherein said electrolyte is one or more electrolytes selected from the group consisting of sodium chloride, calcium chloride, magnesium chloride, sodium fluoride, calcium bromide, ammonium sulfate, magnesium carbonate, dipotassium hydrogen phosphate, lithium S chloride, sodium iodide, iron chloride, zinc chloride and manganese chloride.

A method for producing a gas hydrate comprising reacting raw material water with a gas hydrate forming material under conditions suitable for forming the gas hydrate, wherein one or more metals selected from the group consisting of zinc, iron and manganese is added to the raw material water in a concentration of not lower than 0.01 ppm but not higher than 1,000 ppm, and the gas hydrate is then formed.

6. A gas hydrate produced by a method for producing a gas hydrate according to any one of claims 1 through