Methods for controlling the crystal form of natural gas hydrates

Abstract

This invention discloses a method for regulating the crystal form of natural gas hydrate formation. A composition consisting of salts, surfactants, water-soluble thermodynamic additives, and water is introduced during the natural gas hydrate formation process. The salts and surfactants also work synergistically with the water-soluble thermodynamic promoter. The addition of the salts and surfactants alters the local solubility of the water-soluble thermodynamic additive in water, enabling the regulation and control of the hydrate crystal formation process, thereby increasing the gas storage capacity of the hydrate and solving the problem of low natural gas storage in natural gas hydrates generated by water-soluble thermodynamic additive systems.

Description

Technical fields:

[0001] This invention relates to the field of gas hydrate technology, and more specifically to a method for regulating the crystal form of natural gas hydrate formation. Background technology:

[0002] Gas hydrates, also known as gas cage-like inclusions, are non-stoichiometric crystalline inclusions. In hydrates, the main water molecules are spatially linked by hydrogen bonds, forming a series of polyhedral cavities filled with gas. When the crystal lattice is disrupted, for example by increasing the storage temperature of the gas hydrate, the gas is released. Due to these unique physicochemical properties, gas hydrate technology is widely used for the separation, capture, storage, and transport of gases.

[0003] Natural gas storage and transportation have always been major challenges in international natural gas trade and the development of marginal oil and gas fields. Current main natural gas transportation methods, such as pipeline natural gas, compressed natural gas (CNG), and liquefied natural gas (LNG), suffer from drawbacks including high investment and operating costs, long process flows, and low safety. Natural gas hydrate solidification storage and transportation technology, as a new method, offers advantages such as low cost, high safety, and a short process flow. In recent years, this technology has developed rapidly thanks to the efforts of researchers worldwide, but its industrialization still faces the core challenge of low hydrate storage capacity. This low storage capacity becomes even more pronounced after using thermodynamic additives to address the high hydrate formation conditions. This is mainly because the additive molecules themselves participate in the construction of hydrate cages, occupying a portion of the cages, thus leaving fewer cages for methane molecules to occupy.

[0004] Researchers are exploring various methods to increase the interfacial contact between liquid water or solid ice and methane gas, thereby improving the formation rate and storage density of gas hydrates. These methods include applying high pressure, vigorous stirring, using dry water, using surfactants such as sodium dodecyl sulfate (SDS), and using carriers such as porous silica or polymers.

[0005] Although these methods can all increase the gas storage capacity of gas hydrates to some extent, they are all achieved by forming more natural gas hydrates.

[0006] There is an urgent need to solve the problem of low gas storage capacity of natural gas hydrates without generating more natural gas hydrates.

[0007] 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, while their 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 of type I hydrates are the largest.

[0008] Previous studies (1. Yu YS, Zhang QZ, Lv QN, et al. A kinetic study of methanehydrate formation in the corn Cobs+Tetrahydrofuran solution system[J]. Fuel, 2021, 302: 121-143.; 2. Kim DY, Park J, Lee J, et al. Critical guest concentration and complete tuning pattern appearing in the binary clathrate hydrates[J]. Journal of the American Chemical Society, 2006, 128(48): 15360-15361.) have shown that the hydrate formed by methane gas in tetrahydrofuran aqueous solution is only a II-type THF / CH4 mixed hydrate (16(CH4)·8(THF+CH4)·136H2O).

[0009] Therefore, there is an urgent need to develop a method for controlling the crystal form of natural gas hydrate formation, which can generate type I hydrates in a system with added water-soluble thermodynamic additives to reduce the formation conditions of natural gas hydrates without generating more natural gas hydrates, thus fundamentally solving the problem of low gas storage capacity of natural gas hydrates. Summary of the Invention:

[0010] The purpose of this invention is to provide a method for regulating the crystal form of natural gas hydrate formation, which solves the problem of low natural gas gas storage in the formation of natural gas hydrate in water-soluble thermodynamic additive systems.

[0011] This invention is achieved through the following technical solutions:

[0012] A method for controlling the crystal form of natural gas hydrate formation, the method comprising the following steps: introducing a composition consisting of salts, surfactants, water-soluble thermodynamic additives, and water during the formation of natural gas hydrate, and then controlling the temperature at 274.15K-288.15K and the pressure at 6-8MPa.

[0013] Specifically, the composition is dispersed by ultrasonication.

[0014] The natural gas hydrate is a hydrate formed from methane, a gas with low solubility in water.

[0015] The water-soluble thermodynamic additives mainly include water-soluble hydrate thermodynamic additives such as tetrahydrofuran, tetrabutylammonium bromide, and tetrabutylammonium fluoride.

[0016] The water-soluble thermodynamic additive has a molar fraction of 1.0%-5.6% in water.

[0017] The types and concentrations of the salts and surfactants depend on the types and concentrations of the water-soluble thermodynamic additives used in the experiment. The ratio of the total mass of the salts and surfactants to the mass of the water-soluble thermodynamic additives is between (1 / 9) and (1 / 3); the mass ratio of the surfactants to the salts is between (1 / 2) and (1 / 6).

[0018] Surfactants include commonly used sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), and disodium lauryl sulfosuccinate monoester (DLS). It is worth noting that these surfactants are foaming surfactants, with high-foaming surfactants being particularly effective. Salts include commonly used sodium chloride (NaCl), potassium chloride (KCl), and potassium nitrate (KBr).

[0019] The principle of this invention is as follows: The adjustment and control of methane hydrate crystals mainly includes two steps: First, methane molecules replace additive molecules to occupy the large cage of type II hydrate (5 12 First, it forms type II pure methane hydrate. Second, since type II pure methane hydrate is unstable, by controlling thermodynamic conditions, type II pure methane hydrate can be rapidly transformed into type I methane hydrate. The first step is crucial for adjusting and controlling the hydrate crystal structure. According to classical thermodynamic theory, achieving this first step requires ensuring that methane molecules remain within the large cage of type II hydrate (5...). 12 The percentage of additive molecules in the large cage (5) is higher than that in the large cage (5) 12 The proportion of the additive in the aqueous solution is crucial. This means that the first step requires reducing the solubility of the additive in the aqueous solution. Therefore, changing the solubility of water-soluble thermodynamic additives in water becomes key to the regulation and control of hydrate crystals in soluble / easily soluble and miscible water-soluble thermodynamic additives. The addition of a mixture of salts and surfactants can alter the local solubility of these thermodynamic additives in water, thus enabling the regulation and control of hydrate crystals.

[0020] This invention also protects the application of the above-mentioned method for regulating the crystal form of natural gas hydrates in natural gas storage and transportation.

[0021] The beneficial effects of this invention are as follows:

[0022] 1) The surfactant of the present invention can increase the contact area between gas and liquid, reduce the mass transfer resistance between gas and liquid, and improve the formation rate of gas hydrate. In addition, the salt and surfactant also work synergistically with the water-soluble thermodynamic promoter. The addition of the salt and surfactant can change the local solubility of the water-soluble thermodynamic additive in water, so that the adjustment and control process of hydrate crystal can be realized, thereby increasing the gas storage capacity of hydrate.

[0023] 2) The materials involved in the technical solution provided by this invention are readily available, the production process and industrial chain are mature, and the price is low. There is no need to build a raw material supply ecosystem.

[0024] 3) The method for regulating the crystal form of gas hydrate formation in this invention also provides a method for the storage capacity of water-soluble natural gas hydrate, creatively and fundamentally solving the problem of low natural gas storage capacity in water-soluble thermodynamic additive systems.

[0025] 4) 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. Attached image description:

[0026] Figure 1This is the PXRD pattern of natural gas hydrate obtained in Example 1. Detailed implementation method:

[0027] The following is a further description of the invention, but not a limitation thereof.

[0028] Example 1:

[0029] Based on a total mass of 100.00 g, 1.3 g of sodium chloride (NaCl), 0.5 g of sodium dodecyl sulfate (SDS), and 15.00 g of tetrahydrofuran (THF) were weighed using a balance, with the remainder being water. First, the weighed sodium chloride (NaCl), sodium dodecyl sulfate (SDS), and tetrahydrofuran (THF) were placed in a closed conical flask and ultrasonically dispersed for 3.5 hours. After dispersion, the weighed water was added to the liquid, and ultrasonic dispersion was continued for another 1.5 hours. Following this, the reaction solution was used to induce the formation of methane hydrate. Under initial pressure of 7 MPa and initial temperature of 274.15 K, the methane storage capacity obtained was 71.43 V / V. Pure type I methane hydrate was also found in the generated hydrate.

[0030] Comparative Example 1:

[0031] Referring to Example 1, the difference is that sodium chloride and sodium dodecyl sulfate were not added, and it was found that the hydrate generated was only a type II THF / CH4 mixed hydrate (16(CH4)·8(THF)·136H2O or 16(CH4)·8(THF+CH4)·136H2O).

[0032] Example 2:

[0033] Based on a total mass of 100.00 g, 2.4 g of sodium chloride (NaCl), 0.6 g of sodium dodecylbenzenesulfonate (SDBS), and 17.00 g of tetrahydrofuran (THF) were weighed using a balance, with the remainder being water. First, the weighed sodium chloride (NaCl), sodium dodecyl sulfate (SDS), and tetrahydrofuran (THF) were placed in a closed conical flask and ultrasonically dispersed for 5 hours. After dispersion, weighed water was added to the liquid, and ultrasonic dispersion was continued for another 2 hours. Following this, the reaction solution was used to induce methane hydrate formation. Under initial pressure of 7 MPa and initial temperature of 274.15 K, the methane storage capacity obtained was 90.84 V / V. Pure type I methane hydrate was also found in the generated hydrate.

[0034] Example 3:

[0035] Based on a total mass of 100.00 g, 4.30 g of sodium chloride (NaCl), 0.80 g of sodium dodecyl sulfate (SDS), and 18.00 g of tetrahydrofuran (THF) were weighed using a balance, with the remainder being water. First, the weighed sodium chloride (NaCl), sodium dodecyl sulfate (SDS), and tetrahydrofuran (THF) were placed in a closed conical flask and ultrasonically dispersed for 4 hours. After dispersion, weighed water was added to the dispersed liquid, and ultrasonic dispersion was continued for another 1.5 hours. Following this, the reaction solution was used to induce the formation of methane hydrate. Under initial pressure of 7 MPa and initial temperature of 274.15 K, the methane storage capacity obtained was 121.81 V / V. Simultaneously, pure type I methane hydrate was found in the generated hydrate, indicating that the scheme provided by this invention allows for the successful adjustment and control of hydrate crystal formation even when the THF molar concentration is close to 5.60 mol%.

Claims

1. A method for controlling the crystal form of natural gas hydrate formation, characterized in that, The method includes the following steps: introducing a composition consisting of salts, surfactants, water-soluble thermodynamic additives, and water during the formation of natural gas hydrates, and then controlling the temperature at 274.15K-288.15K and the pressure at 6-8MPa; the salts are selected from any one of sodium chloride, potassium chloride, and potassium nitrate; the surfactants are selected from any one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and disodium lauryl sulfosuccinate monoester; the water-soluble thermodynamic additives are selected from any one of tetrahydrofuran, tetrabutylammonium bromide, and tetrabutylammonium fluoride; the ratio of the total mass of the salts and surfactants to the mass of the water-soluble thermodynamic additives is between 1 / 9 and 1 / 3, and the ratio of the mass of the surfactants to the salts is between 1 / 2 and 1 / 6.

2. The method according to claim 1, characterized in that, The composition is dispersed by ultrasonic waves.

3. The method according to claim 1, characterized in that, The water-soluble thermodynamic additive has a molar fraction of 1.0%-5.6% in water.

4. The method for controlling the crystal form of natural gas hydrate formation as described in claim 1, applied to natural gas storage and transportation.

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