A method for increasing the gas storage capacity of natural gas hydrate based on crystal regulation principle

Abstract

The application relates to the field of natural gas storage and transportation technology, in particular to a method for increasing the storage capacity of natural gas hydrate based on the crystal regulation principle, which adds a micro-soluble or insoluble thermodynamic additive in a hydrate generating system to reduce the hydrate generating conditions, forms a II structure, and then controls the temperature and pressure to adjust and control the crystal structure of the generated hydrate in the system to be I type methane hydrate, so that the method for increasing the storage capacity of natural gas hydrate is provided, and the problem that the gas storage capacity in the thermodynamic additive system is not high is fundamentally solved creatively.

Description

Technical fields:

[0001] This invention relates to the field of natural gas storage and transportation technology, specifically to a method for increasing the storage capacity of natural gas hydrates based on the principle of crystal regulation. Background technology:

[0002] Natural gas solidification storage and transportation technology based on hydrates is a new natural gas storage and transportation technology developed in recent years. It mainly involves fixing natural gas on a large scale within a solid hydrate phase and storing and transporting it in the form of solid natural gas hydrates. Traditionally, under standard conditions, 1m... 3 Hydrates can store 160-180m³ 3 Natural gas. Compared to traditional commercially available natural gas storage and transportation technologies such as liquefied natural gas (LNG) and compressed natural gas (CNG), natural gas hydrate solidification storage and transportation technology has advantages such as a short process flow, low cost, and safety without pollution. However, the core issues restricting the further industrialization of this technology mainly include the following two aspects: First, the formation conditions of natural gas hydrates are too high; second, the gas storage capacity of natural gas hydrates is relatively low. To address the first issue, researchers mainly introduce thermodynamic additives to reduce the formation conditions of natural gas hydrates. It is worth noting that current research indicates that adding thermodynamic additives may be the only effective and currently available way to reduce hydrate formation conditions. However, because thermodynamic additives themselves occupy some of the cage space of hydrates, they reduce the hydrate cages available for gas molecules, thereby reducing the gas storage capacity of the hydrates.

[0003] Some researchers have proposed using kinetic promoters, dispersants, and methods such as stirring, bubbling, and spraying to enhance the gas-liquid mass transfer process in order to increase the apparent total gas storage capacity. The patent "Hydrate promoter and its application in the preparation of high gas storage density gas hydrates" (Publication (Announcement) No. CN104974713A) proposes using aqueous solutions of amino acids of different concentrations to promote the formation of gas hydrates. The results show that it can increase the gas storage capacity and improve the gas storage density to a certain extent. The patent "A device and method for low-energy hydrate-air separation" (Publication (Announcement) No. CN104841237B) proposes using one or more of the following kinetic promoters and thermodynamic promoters in synergy to promote the formation of gas hydrates. The article "Research progress on enhanced natural gas hydrate formation technology and methods [J]. Oil & Gas Storage and Transportation, 2012, 31(10):725-732" reviews the promoting effect of stirring, bubbling, and spraying on the formation of gas hydrates and the influence of gas storage capacity. While these methods can all increase the gas storage capacity of gas hydrates to some extent, they all achieve this by forming more natural gas hydrates. Therefore, there is an urgent need to develop a method that can fundamentally solve the problem of low gas storage capacity of natural gas hydrates in thermodynamic additive systems by adjusting the crystal structure of natural gas hydrates without generating more natural gas hydrates.

[0004] Natural gas hydrates are cage-like inclusion compounds, with water molecules forming a spatial lattice structure. Gas molecules, as guests, fill the cavities between the lattice points. There is no stoichiometric relationship between the gas and water. The water molecules forming the lattice are bonded by strong hydrogen bonds, while the forces between gas and water molecules are van der Waals forces. Currently, four types of hydrate structures have been discovered: Type I, Type II, Type H, and Type T. Type I hydrates have a cubic crystal structure. Due to the small volume of their internal cavities (average cavity diameter 0.78 nm), they can only accommodate small molecules such as methane, ethane, nitrogen, carbon dioxide, and hydrogen sulfide. Type I hydrates are the most widely distributed in nature; pure methane and pure ethane hydrates are Type I. The general composition of this type of methane hydrate is CH4.5.75H2O. Type II hydrates have a rhombic crystal structure, which can accommodate C1 and C2 small molecules, and its larger cavities tend to accommodate hydrocarbon molecules such as propane (C3) and isobutane (i-C4). Type H hydrates have a hexagonal crystal structure, and their cavities can even accommodate i-C5 molecules and other molecules with diameters between 0.75-0.86 nm. Analysis of the four structural characteristics of natural gas hydrates shows that the ratio of small cavities to large cavities in Type I, Type II, Type H, and Type T hydrates is 1:3, 2:1, 5:1, and 1:4, respectively. If methane were to occupy all five cavities of Type I... 12 and 5 12 6 2 The methane reserves in type I hydrates are the largest. Summary of the Invention:

[0005] The purpose of this invention is to provide a method for increasing the gas storage capacity of natural gas hydrate based on the principle of crystal regulation, which solves the problem of low gas storage capacity of natural gas hydrate in thermodynamic additive systems.

[0006] This invention is achieved through the following technical solutions:

[0007] A method for increasing the gas storage capacity of natural gas hydrates based on the principle of crystal regulation includes the following steps: introducing a slightly soluble or insoluble thermodynamic additive to allow methane molecules to replace the additive molecules and occupy the large cages (5) in type II hydrates. 12 6 4 ) generated type II pure methane hydrate (16(5) 12 )·8(5 12 6 4 )·136H2O); By controlling the temperature to 274.15K-288.15K and the pressure to 5-9MPa, the unstable type II pure methane hydrate (16(5)) was hydrated. 12 )·8(5 12 6 4 )·136H2O) is rapidly converted into type I pure methane hydrate (2(5) 12 )·6(5 12 6 2 )·46H2O).

[0008] The thermodynamic additives that are slightly soluble or insoluble in water as described in this invention refer to promoters that can typically form type II hydrates with methane. For example, traditional thermodynamic additives such as cyclopentane, propane, and trimethylsulfide can meet this requirement.

[0009] The thermodynamic additives that are slightly soluble or insoluble in water have low solubility in water. Therefore, regardless of the amount added, their solubility change in aqueous solution is minimal. Consequently, the effect of these additives on reducing hydrate formation conditions in the overall system is less affected by the amount added. Therefore, this adjustment scheme can meet the requirements of large-scale crystal regulation and control, and is suitable for the development requirements of large-scale industrial applications.

[0010] The principle of crystal adjustment and control of the hydrate is as follows: According to the traditional van der Waals and Platteeuw theory, when gas molecules are at 5... 12 6 4 Hole occupancy rate in the cage (θ) L (Gas) is higher than the additive molecules at 5 12 6 4 Hole occupancy rate in the cage (θ)L When (Promoter) is activated, gas molecules can replace additive molecules and occupy the large cage of type II hydrate. Hole occupancy rate (θ) L ) through formula θ Li =C Li p i / (1+C L1 p1+C L2 p2) is used for calculation, where C Li The Langmuir adsorption coefficient is p. i This refers to the partial pressure of the guest molecules (gas or additives). To satisfy θ L (Gas)>θ L (Promoter) This requires either increasing the system pressure or reducing the solubility of the additive in water around the hydrate growth point. Therefore, selecting slightly soluble or insoluble thermodynamic additives is an effective way to achieve the substitution of additive molecules by methane molecules to form methane hydrates. Furthermore, studies have shown that type II CH4 hydrates are inherently unstable under medium to low pressure conditions and spontaneously transform into stable type I hydrates, with the rate of this process influenced by the system's supercooling. Adjusting the supercooling can enable the rapid conversion of type II methane hydrates into type I methane hydrates.

[0011] It is worth mentioning that the thermodynamic additive molecules of this invention have a large molecular diameter and are unable to occupy the cage of type I hydrate.

[0012] Therefore, the present invention selects thermodynamic additives that are slightly soluble or insoluble in water and capable of forming a type II structure with methane.

[0013] Preferably, the volume ratio of the slightly soluble or insoluble thermodynamic additive to water is (15-24):(76-85).

[0014] The crystal conditioning and control process of the hydrate described above has universal applicability and does not require other auxiliary equipment.

[0015] The pressure and temperature control process in step two is achieved by refrigeration and heating devices, and the specific process depends on the thermodynamic additives used.

[0016] When the thermodynamic additive that is slightly soluble or insoluble in water is the commonly used cyclopentane, the pressure is 5-9 MPa and the reaction temperature is 274.15 K-288.15 K.

[0017] When the thermodynamic additive that is slightly soluble or insoluble in water is commonly used propane, the pressure is 5-7 MPa and the reaction temperature is 276.15 K-283.15 K.

[0018] The method for increasing the storage capacity of natural gas hydrates provided by this invention is mainly achieved by adjusting and controlling the hydrate crystal structure generated in the system. By adjusting and controlling the hydrate crystal structure, the problem of low hydrate gas storage capacity in thermodynamic additive systems can be fundamentally solved.

[0019] Therefore, this invention also protects the application of the method for increasing the storage capacity of natural gas hydrates based on the crystal regulation principle in natural gas storage and transportation.

[0020] The beneficial effects of this invention are as follows:

[0021] 1) This invention forms a type II structure by adding slightly soluble or insoluble thermodynamic additives to the hydrate formation system to reduce the hydrate formation conditions. Then, by controlling the temperature and pressure, the crystal structure of the hydrate formed in the system is adjusted and controlled to be type I methane hydrate. This invention provides a method to increase the gas storage capacity of natural gas hydrates and creatively solves the problem of low gas storage capacity in thermodynamic additive systems.

[0022] 2) This invention is applicable to the large-scale generation of gas hydrates and can meet the industrial development requirements of natural gas solidification storage and transportation technology based on hydrate method.

[0023] 2) This invention does not require the introduction of other equipment and auxiliary devices, does not reduce the effect of additives on reducing hydrate formation conditions, and does not cause a significant increase in cost, thus having wide applicability. Attached image description:

[0024] Figure 1 This is the PXRD pattern of natural gas hydrate obtained in Example 1;

[0025] Figure 2 This is the PXRD pattern of natural gas hydrate obtained in Example 3. Detailed implementation method:

[0026] The following is a further description of the invention, but not a limitation thereof.

[0027] Example 1:

[0028] 76 ml of water and 24 ml of cyclopentane, with a total volume of 100 ml, were measured using a graduated cylinder and placed in a 400 ml high-pressure gas hydrate reactor. After completion, methane gas was introduced to purge the hydrate reactor to remove air. Subsequently, methane was introduced as a reactant gas into the system and pressurized to 8.0 MPa. The reaction temperature was cyclically oscillated between 274.15 K and 288.15 K as required, with a single cycle time of 1.0 h. After the hydrate reaction continued for 5.0 h, the obtained hydrate gas storage capacity reached 152 V / V. X-ray powder diffraction results showed that the generated hydrate system contained both type II pure methane hydrate and type I pure methane hydrate.

[0029] Example 2:

[0030] Based on a total volume of 100 ml, 85 ml of water and 15 ml of cyclopentane were measured using a graduated cylinder and placed in a high-pressure gas hydrate reactor (400 ml). After completion, methane gas was introduced to purge the hydrate reactor to remove air. Subsequently, methane was introduced as a reactant gas into the system and pressurized to 8.0 MPa. The reaction temperature was cyclically oscillated between 276.15 K and 283.15 K as required, with a single cycle time of 1.0 h. After the hydrate reaction continued for 5.0 h, the obtained hydrate gas storage capacity reached 124 V / V. X-ray powder diffraction results showed that the generated hydrate system contained both type II pure methane hydrate and type I pure methane hydrate.

[0031] Example 3:

[0032] 20 ml of water (out of a total volume of 100 ml) was measured using a graduated cylinder and placed in a high-pressure gas hydrate reactor (400 ml). After completion, methane gas was introduced to purge the hydrate reactor to remove air. Subsequently, a methane + propane mixture was introduced into the system as the reactant gas and pressurized to 6.0 MPa. The reaction temperature was cyclically oscillated between 276.15 K and 283.15 K as required, with a single cycle time of 1.0 h. After the hydrate reaction continued for 5.0 h, the obtained hydrate gas storage capacity reached 124 V / V. It is worth noting that since the molar fraction of propane in commercial natural gas is approximately 0.72 mol%, the propane component in the methane + propane mixture in this embodiment is also 0.72 mol%. X-ray powder diffraction results showed that the generated hydrate system contained both type II pure methane hydrate and type I pure methane hydrate.

Claims

1. A method for increasing the storage capacity of natural gas hydrates based on the principle of crystal regulation, characterized in that, The method includes the following steps: introducing a slightly soluble or water-insoluble thermodynamic additive to generate type II pure methane hydrate; controlling the temperature at 274.15K-288.15K and the pressure at 5-9MPa to rapidly convert the unstable type II pure methane hydrate into type I pure methane hydrate; the slightly soluble or water-insoluble thermodynamic additive is selected from either cyclopentane or propane; when the thermodynamic additive is cyclopentane, the pressure is 7-9MPa and the temperature is 274.15K-288.15K; when the thermodynamic additive is propane, the pressure is 5-7MPa and the temperature is 276.15K-283.15K.

2. The method according to claim 1, characterized in that, The volume ratio of slightly soluble or insoluble thermodynamic additives to water is (15-24):(76-85).

3. The application of the method of claim 1 in natural gas storage and transportation.

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