RU2500661C1 - Method of extracting methane from gas mixtures - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to a method of extracting methane from gas mixtures by bringing the mixture into contact with an aqueous solution of a cyclic ether with concentration of not more than 20 mol % at temperature not higher than 20°C and pressure of up to 3.0 MPa to obtain a condensed phase containing mixed hydrates of methane and the cyclic ether and a gas phase, separating the gas phase, extracting methane from the condensed phase and recycling the aqueous solution of the cyclic ether for extraction.

EFFECT: method enables to efficiently extract methane from gas mixtures by simplifying the process, particularly lowering the separation pressure.

2 cl, 8 tbl, 8 ex, 1 dwg

Description

The invention relates to the chemical, oil and gas industry and can be used in the processes of separation and purification of hydrocarbon gas mixtures, as well as for the utilization of mine methane released during mining of gas-bearing mineral deposits.

A known method for the separation of methane from gas mixtures is the preparation of hydrocarbon gas (RU 2460759, 2012), including stepwise separation, cooling the gas between the stages of separation, separation of hydrocarbon condensate from the initial stages of separation, cooling it with condensate of the last low-temperature stage of separation and using it as an absorbent. The disadvantages of this method are the multi-stage process of methane evolution, increased energy costs associated with maintaining low temperatures, the need for preliminary drying of the gas mixture and the use of a thermodynamic inhibitor - triethylene glycol to prevent the formation of hydrates.

A known method of preparing gas for pipeless transportation (RU 2277121, 2006), including preliminary purification of gas from heavy hydrocarbons, obtaining gas hydrates by mixing the gas to be purified with water in the reactor, continuous cooling and maintaining the required temperature of the resulting mixture while maintaining the pressure not lower than equilibrium, necessary for hydrate formation, the supply of natural gas to the hydrate reactor is carried out from the high-pressure gas transport line, and continuous cooling is uschestvlyayut due to reduced temperature of the gas passing rate reduction, which is recycled after the heat transfer in the low pressure line. The main disadvantage of this method is the need to maintain high pressures ≥3 MPa in the reactor for hydrate formation to occur. In this regard, this method is unacceptable for the extraction of methane from low-pressure gases.

A known method of purification of natural gas (RU 2288774, 2006). A method of purifying natural gas from impurities involves contacting the purified natural gas in the reactor with an aqueous medium under initial thermobaric conditions characterized by a pressure that ensures the formation of hydrates of the main component of natural gas - methane and mixed natural gas hydrates enriched in impurity hydrocarbon components. After precipitation of the first hydrates, the initial pressure in the reactor is reduced to a value below the equilibrium pressure of hydration of methane, but above the equilibrium pressure of hydration for the purified natural gas. The disadvantages of this method are the technical difficulties associated with the creation of high pressures in the reactor to initiate the hydrate formation process, as well as the low selectivity of the purification process associated with the formation of mixed hydrates, which include not only impurity components (C ₂ H ₆ , C ₃ H ₈ , iC ₄ H ₁₀ nC ₄ H ₁₀ ), but the target component is CH ₄ .

Closest to the claimed method is a method for extracting methane from a methane-air mixture (RU 2302401, 2007), in which a compressed methane-air mixture is passed through an aqueous hydroquinone solution at a pressure of at least 3 MPa and a temperature of no higher than + 2 ° C, where air is separated to form clathrates methane with hydroquinone, which is then heated, after which the methane released from them is sent for recycling, and the aqueous hydroquinone solution is reused in a cycle. The disadvantage of this method is the need to maintain high blood pressure. Thus, this method is not effective enough.

The objective of the invention is to increase the efficiency of the method of separation of methane from gas mixtures.

The problem is solved by the described method for the separation of methane from gas mixtures by contacting the mixture with an aqueous solution of cyclic ether (CPE) with a concentration of not more than 20 mol%. at a temperature not exceeding 20 ° C and a pressure of up to 3.0 MPa to obtain a condensed phase containing mixed hydrates of methane and a cyclic ether, and a gas phase, separation of the gas phase, extraction of methane from the condensed phase, followed by recirculation of the aqueous cyclic ether solution to selection.

Preferably, tetrahydrofuran, furan, oxetane (trimethylene oxide), 1,3-dioxane, 1,4-dioxane, pyran, tetrahydropyran, 1,3,5-trioxane are used as the cyclic ether.

The technical result achieved is to increase the efficiency of the method by simplifying the process technology, in particular, reducing the separation pressure.

The method is as follows.

The separation of the gas mixture occurs as a result of hydrate formation. An aqueous solution of CPE (for example, tetrahydrofuran, furan, oxetane, 1,3-dioxane, 1,4-dioxane, pyran, tetrahydropyran, 1,3,5-trioxane), which are thermodynamic promoters that are involved in the formation of mixed hydrates, is used. and, which, unlike thermodynamic inhibitors, shift the equilibrium hydrate formation conditions to lower pressures and higher temperatures. In the presence of these compounds, the formation of mixed hydrates with a crystalline structure of KS-II is observed. An elementary cell of such a structure consists of small D-cavities and large H-cavities of molecular size. The maximum degree of filling of cavities in such a structure corresponds to the hydrate formula 8X · 16Y · 136H ₂ O (X is the number of hydrate-forming molecules in large cavities, Y is the number of hydrate-forming molecules in small cavities). In this case, small cavities are occupied by gas molecules with a maximum molecule size of 0.39 to 0.55 nm (CH ₄ ). Large cavities are filled with molecules of thermodynamic promoters, the size of which is in the range from 0.58 to 0.72 nm. If they are insufficiently contained in the aqueous solution, large cavities can be filled with gas molecules (C ₂ H ₆ , C ₃ H ₈ , iC ₄ H ₁₀ ). With a molar fraction in the aqueous solution of auxiliary substances ≥1 / 18 = 0.0556, the H-cavities are almost completely occupied by the latter. Therefore, in spite of the fact that excipients shift the equilibrium towards milder conditions (low pressures, high temperatures), at the same time they are selective inhibitors of hydrate formation with respect to gases with molecular sizes from 0.58 to 0.72 nm in particular to hydrocarbons C ₂ -C ₄ . Thus, in the formation of hydrates in the presence of these oxygen-containing compounds, the mixed hydrate contains exclusively methane molecules and excipient molecules.

The method is as follows.

An aqueous solution of cyclic ether (CPE) is poured into a reactor equipped with a cooling jacket, a pressure, temperature sensor, and a mixing device (for example, a paddle mixer, low-speed compressor, a system for spraying liquid in gas or gas in liquid). In a preferred embodiment of the method, the concentration of CPE in the solution is 5.6 mol%. Depending on the composition of the source gas and the requirements for the final product, the concentration of the auxiliary substance in the solution is up to 20 mol%. Next, the reactor is cooled to a temperature not exceeding 20 ° C, then a shared gas mixture containing hydrate-forming components, for example, gaseous C ₁ -C ₄ hydrocarbons to a pressure of not more than 3.0 MPa, is fed into it. After performing these operations, the gas-liquid medium is mixed using a shovel mixer or by bubbling a shared gas through an aqueous solution. To intensify mass transfer processes and increase the area of the interfacial surface, spraying an aqueous solution of the auxiliary substance into the gas phase through the nozzle can also be used. In this case, the sequence of operations changes: first the gas mixture is fed into the reactor, then the liquid phase is sprayed. A combination of these mixing options may also be used.

At a given temperature, the process of formation of a mixed hydrate of an oxygen-containing compound and a component of the gas phase, methane, begins. Over time, the concentration of CH ₄ in the gas phase decreases. The degree of separation of the gas mixture is controlled by gas chromatography or other physicochemical analysis method. Upon reaching the required degree of separation, the hydrate formation process is stopped, the gas phase is separated, and methane is extracted from the condensed phase containing mixed methane hydrates and CPE. After hydration, the gas mixture is supplied to another container and, if necessary, subjected to a repeated hydration process.

The condensed phase, depending on the process conditions, is either a solid phase — a mixed methane hydrate and CPE, or a liquid phase containing mixed methane hydrates and CPE in suspension in a CPE solution. The extraction of methane from the condensed phase in the case when it is only mixed hydrates or a suspension of the mixed hydrate in a CPE solution is carried out by heating it to a temperature above 20 ° C. Before decomposition of the mixed hydrate by increasing the temperature, the condensed phase, which is a suspension of the mixed hydrate in the THF solution, can be subjected to preliminary separation (without decomposition of the hydrate) to obtain the liquid phase (aqueous solution of THF) and the solid phase (mixed methane hydrate and THF). In this case, only the solid hydrated phase is subjected to subsequent heating.

During the decomposition of the hydrate due to heating into the gas phase, the target component, methane gas, is released, which, after complete decomposition of the hydrate, is transferred from the reactor to a separate tank. The aqueous CPE resulting from the decomposition of the hydrate is reused. In addition to the batch mode described above, the separation process can be implemented in a continuous mode, which involves the continuous supply of separated gas to the reactor and extraction of the hydrate formed from the reactor.

The process of separation of gas mixtures in the presence of CPE can occur at moderate temperatures of 0-20 ° C and pressures up to atmospheric; therefore, it becomes possible to use this process for the single-stage extraction of methane from various hydrocarbon gas mixtures, the utilization of low-pressure gases, and the separation of methane from its mixtures with nitrogen, by air.

Example 1

The figure shows the lines of the three-phase equilibrium gas-liquid-hydrate in the gas mixture system 78.90% CH ₄ , 12.30% C ₂ H ₆ , 7.44% C ₃ H ₈ , 0.93% iC ₄ H ₁₀ , 0 , 46% n-C ₄ H ₁₀ - water - tetrahydrofuran (THF) at various concentrations of ether.

From this figure it follows that the use of THF allows you to shift the three-phase balance of gas-liquid-hydrate in the direction of lower pressures.

In a vacuum reactor having an internal volume of 400 cm ³ serves 200 cm ^{3 an} aqueous solution of THF with a concentration of 5.6 mol%. The reactor is cooled and thermostated at a temperature of 1 ° C and a gas mixture of the above composition is fed into it to an initial pressure of 0.121 MPa. The contents of the reactor are mixed by deflecting it at an angle of ± 45 ° at a speed of 10 min ^-1 , while the formation of a mixed hydrate occurs, which leads to a change in the composition of the gas phase.

As a result of hydrate formation, a methane-depleted gas phase and a condensed phase containing mixed methane hydrate and THF were obtained. The pressure after hydrate formation in the reactor is 0.050 MPa. The condensed phase is separated from the gas and heated to a temperature of 21 ° C. When heated, it decomposes to form methane gas and an aqueous THF solution. The resulting methane gas is placed in another container. The aqueous THF solution is recycled to the methane recovery process.

The results of chromatographic analyzes of the composition of the gas phase are shown in table 1.

Table 1 The composition of the gas phase before and after hydrate formation. Structure The concentration of components in the gas phase,% mol. CH ₄ C ₂ H ₆ C ₃ H ₈ iC ₄ H ₁₀ nC ₄ H ₁₀ Source 78.90 12.30 7.44 0.93 0.46 Finite 49.25 29.54 17.87 2.23 1.10

As can be seen from table 1, the gas phase as a result of separation is enriched in components C ₂ -C ₄ . As a result of the described method, a mixed gas hydrate is formed, the composition of which includes only THF and methane. The methane recovery calculated on the basis of experimental data is 74%.

Example 2

The method is carried out as in example 1 except that the initial pressure in the reactor is 0.559 MPa, the reactor is cooled and thermostated at a temperature of 3 ° C. The pressure in the reactor after hydrate formation is 0.177 MPa. The results of chromatographic analysis of the composition of the gas phase before and after hydrate formation are shown in table 2.

table 2 The composition of the gas phase before and after hydrate formation. Structure The concentration of components in the gas phase,% mol. CH ₄ C ₂ H ₆ C ₃ H ₈ iC ₄ H ₁₀ nC ₄ H ₁₀ Source 78.90 12.30 7.44 0.93 0.46 Finite 32.67 39.19 23.71 2.96 1.47

The methane extraction rate calculated on the basis of experimental data is 87%.

Example 3

The method is carried out as in example 1 except that they use 10.0 mol%. an aqueous solution of tetrahydrofuran, the initial pressure in the reactor is 0.3 MPa, the reactor is cooled and thermostated at a temperature of 3 ° C. The pressure in the reactor after hydrate formation is 0.120 MPa. The results of chromatographic analyzes of the composition of the gas phase are shown in table 3.

Table 3 The composition of the gas phase before and after hydrate formation. Structure The concentration of components in the gas phase,% mol. CH ₄ C ₂ H ₆ C ₃ H ₈ iC ₄ H ₁₀ nC ₄ H ₁₀ Source 78.90 12.30 7.44 0.93 0.46 Finite 47.25 30.71 18.57 2,32 1.15

The methane extraction rate calculated on the basis of experimental data is 76%.

Example 4

The method is carried out as in example 1 except that they use 15 mol%. an aqueous solution of tetrahydrofuran, the initial pressure in the reactor is 1.0 MPa, the reactor is cooled and thermostated at a temperature of 3 ° C. The pressure in the reactor after hydrate formation is 0.357 MPa. The results of chromatographic analyzes of the composition of the gas phase are shown in table 4.

Table 4 The composition of the gas phase before and after hydrate formation. Structure The concentration of components in the gas phase,% mol. CH ₄ C ₂ H ₆ C ₃ H ₈ iC ₄ H ₁₀ nC ₄ H ₁₀ Source 78.90 12.30 7.44 0.93 0.46 Finite 40.18 34.82 21.06 2.63 1.30

The methane extraction rate calculated on the basis of experimental data is 82%.

Example 5

The method is carried out as in example 1 except that they use 5.6 mol%. an aqueous solution of tetrahydropyran, the initial pressure in the reactor is 1.0 MPa, the temperature of thermostating is 3 ° C. The pressure in the reactor after hydrate formation is 0.460 MPa. The results of chromatographic analyzes of the composition of the gas phase are shown in table 5.

Table 5 The composition of the gas phase before and after hydrate formation. Structure The concentration of components in the gas phase,% mol. CH ₄ C ₂ H ₆ C ₃ H ₈ iC ₄ H ₁₀ nC ₄ H ₁₀ Source 78.90 12.30 7.44 0.93 0.46 Finite 53.64 26,99 16.32 2.04 1.01

The methane extraction rate calculated on the basis of experimental data is 69%.

Example 6

The method is carried out as in example 1 except that they use 5.6 mol%. an aqueous solution of 1,3,5-trioxane, the initial pressure in the reactor is 1.5 MPa, the temperature of thermostating is 3 ° C. The pressure in the reactor after hydrate formation is 0.717 MPa. The results of chromatographic analyzes of the composition of the gas phase are shown in table 6.

Table 6 The composition of the gas phase before and after hydrate formation. Structure The concentration of components in the gas phase,% mol. CH ₄ C ₂ H ₆ C ₃ H ₈ iC ₄ H ₁₀ nC ₄ H ₁₀ Source 78.90 12.30 7.44 0.93 0.46 Finite 55.19 26.08 15.78 1.97 0.98

The methane extraction rate calculated on the basis of experimental data is 67%.

Example 7

The method is carried out as in example 1 except that they use 15 mol%. an aqueous solution of tetrahydrofuran, the initial pressure in the reactor is 1.0 MPa, the reactor is cooled and thermostated at a temperature of 5 ° C. The pressure in the reactor after hydrate formation is 0.397 MPa. The results of chromatographic analyzes of the composition of the gas phase are shown in table 7.

Table 7 The composition of the gas phase before and after hydrate formation. Structure The concentration of components in the gas phase,% mol. CH ₄ C ₂ H ₆ C ₃ H ₈ iC ₄ H ₁₀ nC ₄ H ₁₀ Source 78.90 12.30 7.44 0.93 0.46 Finite 46.19 31.32 18.95 2,37 1.17

The methane extraction rate calculated on the basis of experimental data is 77%.

Example 8

Separation is carried out in the same way as in example 1, except that the initial mixture is a gas mixture of 90.01% N ₂ + 9.99% CH ₄ , the initial pressure in the reactor is 0.12 MPa, and the temperature of thermostating is 1 ° C. The pressure in the reactor after hydrate formation is 0.11 MPa. The results of chromatographic analysis of the composition of the gas phase before and after hydrate formation are shown in table 8.

Table 8 The composition of the gas phase before and after hydrate formation Structure The concentration of components in the gas phase,% mol. CH ₄ N ₂ Source 9.99 90.01 Finite 2.04 97.96

The methane extraction rate calculated on the basis of experimental data is 80%.

The use of other CPEs in the described method leads to similar results.

The process in conditions beyond the stated limits, do not lead to the desired results. So, the concentration of the CPE above 20% is ineffective due to the unreasonably high consumption of the CPE, an increase in the contact temperature above 20 ° C leads to an excess of the required pressure at which the hydrate formation process occurs.

Thus, the method according to the invention allows the separation of methane from various gas mixtures at a significantly lower pressure and a high degree of extraction.

Claims (2)

1. The method of separation of methane from gas mixtures by contacting the mixture with an aqueous solution of cyclic ether with a concentration of not higher than 20 mol.% At a temperature not exceeding 20 ° C and a pressure of up to 3.0 MPa to obtain a condensed phase containing mixed methane hydrates and cyclic simple ether, and the gas phase, separation of the gas phase, extraction of methane from the condensed phase, followed by recirculation of the aqueous solution of cyclic ether for isolation. 2. The method according to claim 1, characterized in that tetrahydrofuran, furan, oxetane, 1,3-dioxane, 1,4-dioxane, pyran, tetrahydropyran, 1,3,5-trioxane are used as the cyclic ether.