RU2568731C1 - Condensation method of obtaining gas hydrates - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: gas hydrates, for example methane hydrate, are obtained by low temperature condensation of molecular beams of rarefied steam and gas. Molecular beams are supplied into vacuum chamber in spraying zone by separate steam-conducts through Laval nozzles, placed on the output from steam-conducts, and have temperature lower 100 K on the output from nozzles.

EFFECT: increase of rate and economical efficiency of obtaining gas hydrates.

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Description

The invention relates to the production of gas hydrates with the aim of applying hydrated technologies for storing and transporting gas in the energy and gas industries. Storage and transportation of natural gas in the form of hydrates is currently being considered as an alternative to storage and transport technologies for liquefied and compressed gas. According to available estimates, for the development of small and medium-sized gas fields, hydrated technology for storing and transporting natural gas is economically more profitable and safe. About 80% of the world's natural gas reserves are located in such fields. In this regard, the development of cost-effective methods for producing gas hydrates and the intensification of the hydration process are relevant.

FIELD OF TECHNOLOGY

Currently known methods for producing gas hydrates are associated with the use of high pressures in the range from 30 to 250 bar in laboratory and technological equipment at temperatures below the equilibrium temperature of hydrate formation. For example, the pressure corresponding to the conditions for the formation of methane hydrate at temperatures close to 0 ° C is tens of bars. The formation of hydrates in this case requires a long and intensive mixing of the water-gas mixture. Such conditions are used in most known and patented methods for producing gas hydrates. To intensify the hydrate formation process, various methods are proposed, among which are finely dispersed spraying of a water-gas mixture in a gas atmosphere, impact by shock waves on an aqueous medium saturated with gas, vibration and ultrasound exposure. In a number of Western countries, pilot plants for the production of natural gas hydrates are being developed and put into operation. Active research is being carried out on the possibility of using gas hydrate technology in connection with the development of hydrogen energy. The projects of converting greenhouse gases (mainly carbon dioxide) to a gas hydrate state and their disposal on the bottom of the oceans are discussed.

The proposed method for the production of gas hydrates ensures the continuity of the process and contains a number of obvious technological advantages (primarily in terms of productivity and energy costs) over known methods.

Currently, a number of methods for producing gas hydrates are known.

A known method of producing gas hydrates in gas hydrate desalination and purification of sea and mineralized water (patent RU 2405740 C2, 02.24.2009, IPC C02F 1/00, B01F 3/04), according to which hydrate formation occurs in a reactor under gas-liquid compression and cooling the mixture is below the equilibrium temperature of hydrate formation when shock waves act on the mixture with increasing pressure and with the occurrence of crushing drops of liquefied gas and gas hydrate shells on the surface of liquid drops. However, the practical implementation of the method is associated with high energy costs and structural complexity of the technological equipment.

A known method of producing methane hydrate or other gas (patent GB 2347938 A, 09/20/2000, IPC C07C 7/152), in which the interaction of gas with water occurs in a reactor under thermobaric conditions corresponding to the formation of a hydrate. The flow of water into the reactor, filled with gas, occurs through the nozzles in atomized form. To intensify hydrate formation, an ultrasonic emitter is used, which should destroy the hydration shells on the surface of large drops of water. However, the impossibility of obtaining sufficiently large pressure amplitudes due to the large compressibility of the gas-liquid medium and strong radiation attenuation with increasing distance from the emitter does not allow to provide the necessary increase in the interphase surface and the number of centers of gas hydrate nucleation and, as a consequence, the high efficiency of the process.

A known method (patent RU 2293907 C2, 08/24/2004, IPC F17C 11/00) converting natural gas and other hydrate-forming gases into a hydrated state for the purpose of storage. When storing natural gas in containers, an aqueous solution of surfactants is used as an aqueous hydrate-forming medium. The solution is maintained at a pressure of 20-30% above the equilibrium value corresponding to the formation of hydrate at a given temperature. Using the method is expected to increase the mass of stored gas per unit volume of the storage tank and simplify the storage method. However, the low rate of hydrate formation under such conditions does not provide the necessary efficiency of using the method in practice.

The closest in technical solution to the claimed invention should be considered a method of producing gas hydrates (patent RU No. 2457010, 2010, B01D 9/00), in which molecular beams of water and gas are deposited in vacuum on a cooled substrate. Crystallization of the resulting nonequilibrium condensates leads to the formation of gas hydrate. The specified method solves the problem of obtaining gas hydrates in thermobaric conditions, allowing to do without the use of high pressure technology and compression of the water-gas medium. However, it is not possible to achieve higher rates of formation of gas hydrates due to limitations associated with the removal of heat from the substrate released during the condensation of the water-gas mixture.

SUMMARY OF THE INVENTION

The present invention is devoid of the aforementioned disadvantage associated with the need to remove the heat of condensation, and allows us to solve the problem of not only significantly increasing the rate of formation of gas hydrate, but also significantly reducing the flow rate of the refrigerant needed to cool the substrate.

The problem is solved in that the molecular beams of rarefied steam and a hydrate forming gas, for example methane, are fed into the vacuum chamber through Laval nozzles, which allow the molecular beams to be accelerated to supersonic speeds. The adiabatic expansion of supersonic molecular flows leads to a drop in temperature at the exit of the nozzles below 100 K.

When studying methods for producing gas hydrates, no hydrate synthesis options were found from the amorphous solid state of a water-gas mixture with its preliminary cooling due to the adiabatic expansion of supersonic molecular flows.

The invention solves the problem of increasing the speed and cost-effectiveness of producing gas hydrates without the use of high-pressure technology, requiring significant energy costs for its generation and complex technical solutions in the development and manufacture of technological equipment.

The problem is solved in that in the proposed method for organizing the dynamics of molecular beams using Laval nozzles. The adiabatic expansion of supersonic molecular flows leads to a decrease in the temperature at the exit from the nozzles below 100 K and provides a 4-fold increase in the productivity of the hydrate formation process and the saving of refrigerant.

EXAMPLES OF SPECIFIC IMPLEMENTATION

The inventive method of producing gas hydrates is implemented for methane, ethane, propane and carbon dioxide in the laboratory conditions of the Federal State Budgetary Institution of Science, Institute of Thermophysics, Ural Branch of the Russian Academy of Sciences (Yekaterinburg) using equipment and devices manufactured by domestic enterprises or purchased from foreign manufacturers.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1. Scheme of a method for producing gas hydrate in a vacuum cryostat. 1 - a vacuum chamber, 2 - a copper substrate, 3 - a window, 4 - a mask, 5 - a screen, 6 - liquid nitrogen, 7 - a capacitive sensor, 8 - water-gas condensate, 9 - steam pipelines, 10 - Laval nozzles.

The description of the method for producing gas hydrate using methane hydrate as an example is as follows. The condensate of amorphous ice saturated with gas is formed in the vacuum chamber (1) of the cryostat (Fig. 1) by the condensation of molecular beams of rarefied steam and gas on the surface of a copper substrate cooled by liquid nitrogen (2). The water and gas tanks are located at room temperature outside the vacuum chamber. Molecular bundles of components enter the spray zone via separate steam lines (9). Precipitation at fixed flow rates of water and methane allows to obtain condensates of constant composition. To organize the dynamics of molecular beams, Laval nozzles (10) are used, which are located at the outlet of the steam pipelines. The adiabatic expansion of supersonic molecular flows leads to a drop in temperature at the exit of the nozzles below 100 K. The temperature in the molecular beam of water vapor at the exit of the nozzle is determined by the ideal gas equation

pV = RT,

where p is pressure, V is specific volume, T is temperature, R is the universal gas constant, and the Hugoniot equation

, $$p_{\mathrm{one}}{V}_{\mathrm{one}}^{\gamma }=p_2{\mathrm{<figure-callout\; id="2"\; label="V"\; filenames="00000003.png"\; state="\{\{state\}\}">V</figure-callout>}}_2^{\gamma }$$

,

where p ₁ and V ₁ - pressure and specific volume of steam at the inlet to the nozzle, p ₂ and V ₂ - pressure and specific volume at the exit of the nozzle, γ - adiabatic index equal to the ratio of isobaric and isochoric heat capacities of steam: γ = c _p / c _V. At room temperature, for water, γ = 1.33 (for methane, γ = 1.32). Hence, for the temperature of the steam at the exit of the nozzle we have:

. $${\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="00000003.png"\; state="\{\{state\}\}">T</figure-callout>}}_2=T_{\mathrm{one}}{\left(\frac{p_{\mathrm{one}}}{p_2}\right)}^{\frac{\mathrm{one}-\gamma }{\gamma }}$$

.

At T ₁ = 298 K, p ₁ = 24 mmHg in the tank and p ₂ = 0.1 mm Hg in a molecular beam for the temperature at the exit of the nozzle we get: T ₂ = 77 K.

The formation of an amorphous state is ensured by a high cooling rate, which reaches 10 ¹⁵ K / s under such conditions. A vacuum of no worse than 10 ^-5 mm Hg is maintained in the cryostat chamber. The temperature on the surface of the substrate is controlled by a copper-constantan thermocouple with an accuracy of 0.5 K. To monitor the condensate, a capacitive sensor (7) is used, which is placed on the substrate and is a film capacitor made by photolithography. The capacitor plates are thin copper strips (0.1 mm wide, 1-3 microns high) deposited on a dielectric plate with a surface of 20 × 20 mm and a thickness of 0.5 mm. By changing the dielectric properties with a change in temperature, one can follow the transformations in the condensate — the transition from a solid amorphous state to a liquid viscous fluid and subsequent crystallization.

Crystallization of amorphous condensates under conditions of strong metastability leads to the formation of gas hydrate. The avalanche-like nucleation of crystallization centers freezes gas molecules and does not lead to their displacement by the crystallization front. The concentration of methane in the crystallized condensate reaches 15 mass percent. This corresponds to the complete filling of the cavities of the formed clathrate framework with methane molecules. A single volume of the obtained gas hydrate contains 160-170 volumes of methane gas.

The proposed method is suitable for producing a hydrate of any gas with a condensation temperature above the temperature of the substrate. An industrial version of a plant for the production of gas hydrate can be implemented with an increase in the volume of the vacuum chamber, the amount of incoming water-gas mixture and an increase in the cooled surface on which condensate is deposited and subsequent hydrate synthesis. Further, after removing it from the plant and granulating, it is possible to obtain a product suitable for storage and transportation.

INFORMATION SOURCES

1. Patent RU 2405740 C2, 02.24.2009, IPC C02F 1/00, B01F 3/04.

2. Patent GB 2347938 A, 09/20/2000, IPC C07C 7/152.

3. Patent RU 2293907 C2, 08.24.2004, IPC F17C 11/00.

4. Patent RU No. 2457010, 2010, B01D 9/00.

Claims (1)

A method of producing gas hydrates, for example methane hydrate, for their storage and transportation, obtained by low-temperature condensation of molecular beams of rarefied steam and gas, characterized in that the molecular beams enter the vacuum chamber into the spray zone through separate steam pipelines through Laval nozzles located at the outlet of the steam pipelines , and have a temperature below 100 K.

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