Study of methane carbonate conversion process in fixed catalyst layer in different membrane reactors

. The work studied the characteristics of membrane catalysts in the reaction of methane carbonate conversion as a model reaction. This process produces synthesis gas (a 1:1 mixture of Н 2 and СО), which can be used to make many products, including synthetic fuels (in the Fischer-Tropsch process). One of the serious problems of methane carbonate conversion is the presence of side reactions that prevent the production of synthesis gas with the required ratio of products (H 2 :CO=1:1).Membrane catalysts were investigated in carbonate conversion of methane and membrane catalytic reactors: extractor, contactor, and distributor. Carbonate conversion of methane was compared with their material performance characteristics in the ground state in a conventional catalytic reactor with a fixed catalyst bed on membrane catalysts in the contactor and distributor. As the main parameters of the process of catalytic methane carbonate conversion, the change of the main parameters of the process - the concentration of the components in the products, the degree of change of the initial reagents and the ratio of the resulting synthesis gas components, depending on the contact time - were considered. The work aims to study methane's carbonate conversion process in a fixed catalyst layer in different membrane reactors.


Introduction
The high dependence of energy supply on fossil fuels leads the world to serious problems such as the emission of greenhouse gases and problems related to the depletion of energy sources [1,2].According to the latest report from the International Energy Agency [3], carbon emissions increased by 1.5% to a record 33.1 billion tons in 2018, with fossil fuels still accounting for 70% of the increase.By 2050, it seems clear that international regulations and policies for a low carbon footprint will be firm [4].
In recent years, much attention has been paid to the use of biomass as a source of renewable energy, which has been considered in terms of general strategies for biofuel production [5][6][7].
The main products of reforming are hydrogen, carbon monoxide and carbon dioxide.The main task to be solved is the choice of a catalyst, the technology of its application and the creation of a reformer-reactor.Currently, this is done by burning part of the methane in the intertube space of the reactor, and the steam conversion process is carried out in tubes filled with pellets.The most economical is the nickel catalyst.
The development of heat exchangers -the creation of mini-and microchannel heat exchangers gave impetus to the development of new possible devices for the production of hydrogen and synthesis gas [13,15,17].To date, obtaining methanol, and dimethyl ethers based on synthesis gas and synthesizing lower molecular unsaturated hydrocarbons from them, as well as obtaining ethylene and propylene in one barrel from methane, are becoming important.Currently, the processes of obtaining mesoporous carbon and hydrogen from natural gas, petroleum satellite gases, and propane-butane fractions are of great interest to world scientists [21][22][23][24][25].

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Experimental part

Method of chromatographic analysis of gaseous products
The analysis of the mixture of gaseous products was carried out by the method of gas adsorption chromatography on the "Krystallyuks-4000M" device.Detector scatterometer, gas carrier helium (gas flow rate 20 ml/min).Two chromatographic columns are used to analyse gaseous products.Chromatography columns were calibrated using the absolute calibration method.The assay conditions are listed in Table 1.A chromatographic column with CaA molecular sieves (3m x 3mm) was used to separate CO and CH4, and the analysis mode was isothermal (80 ℃).Carrier gas -helium (gas consumption -20 ml/min).A schematic diagram of the steam-carbon dioxide conversion of methane is shown in Fig. 1.The scheme of the membrane reactor for obtaining synthesis gas by steam-carbonate conversion of methane is presented in Figure 2 below.
Syngas can be converted into fuel using the Fischer-Tropsch process.Where p = 2 mm is the radius of the pipe through which the gas mixture is supplied.

Calculation of water vapour supply
Water vapour supply is carried out at the same pressure and temperature values.To achieve maximum mixing of the mixture of CH4 + CO2 and water vapour, the product of the flow rate and its volume flow in both streams must be the same.Therefore, the cross-section of the channel through which water vapour is delivered must be related to the cross-section through which H2O and (CH4 + CO2) are delivered as the corresponding volumetric flow rate.
Water supply is calculated according to the formula;

Calculation of the velocity of the gas mixture in the catalyst bed
The movement speed of the gas mixture in the catalyst layer was determined by the formula derived from the Boyle-Marriott and Gay-Lussac laws, as well as the geometric ratios of the cross-sectional area and volume: Vk -is the volumetric consumption of all gas being converted (under normal conditions): where p= 16,5 mm is the radius of the conversion reactor body; C = 273 K / atm (constant); K = 0,7 -is the mass filling coefficient.
Based on the material balance, the indicators of the process were calculated: where: mCH4 mass of methane sent to the system (g); m'CH4-unaffected methane mass (g); ∑mx-a mass of oxygen or CO2 supplied to the system (g); ∑m1-total mass of gaseous products of reactions (g);  − imbalance associated with the formation of water and coke.
The conversion of the initial reagents was calculated according to the following formula: The amounts of the initial components and reaction products, their known concentrations and molecular masses in the mixture, the volumetric rate of injection of the mixture and the time of the experiment were calculated.During the oxidative conversion of methane, the volume change can be neglected, since the process is carried out in an abundant amount of nitrogen (~8…10).To calculate initial flows in the case of carbonate reforming, it is necessary to carry out an elemental balance at comparable concentrations of CH4 and CO2.The amount of products obtained was calculated according to the following formula: Where: Ci-the concentration of product i in the mixture, volume %; ω-the volumetric rate of the mixture at the exit from the reactor, l/ hour; t -time of the experiment, hours; mi-is the molecular mass of the i product.The productivity of the reactor in terms of products was determined according to the following formula: , /(hour •  .

)
Where V(memb) -membrane volume, dm 3 ; The contact time was determined based on the following formula: S(membrane side surface) -side surface of the membrane, m 2 ; ∆P-pressure difference, atm.Permeability coefficient: hmemb.-thickness of membrane walls, sm.

Results and discussion
In a membrane reactor, both a chemical process and a membrane process occur simultaneously.In recent years, many methods have been proposed on how membranes can be used in combination with chemical reactions to speed up the process in general.Depending on the function of the membrane in the membrane reactor, membrane reactors are divided into three main types: 1) In the extractor, the membrane selectively extracts one or more reaction products from the reaction mixture; 2) In the distributor, the membrane controls the addition of the reagent to the reaction mixture; 3) The contactor membrane accelerates the reaction between the reagents and the catalyst.
The "Extractor" type is a membrane reactor.The most common type of membrane reactor operation is the extractor principle operation.In one of these reactors, only one of the products can pass through the membrane, and selective separation occurs, and in others, catalyst retention occurs, that is, the membrane is permeable to all components of the reaction mixture, except for the catalyst.

Selective product release
One of the components formed in a chemical reaction is selectively removed from the reaction mixture.If one of the products acts as an inhibitor, removing it can significantly increase reactor performance.Also, this type of membrane reactor allows for increasing the concentration of initial substances.
An additional advantage of the selective removal of the reaction product is that it allows the separation to be performed by bypassing the subsequent steps in the separation of the mixture, thereby reducing the cost of the separation.
In the case of gas-phase reactions, the driving force of the membrane separation process is the difference in partial pressures on different sides of the membrane.This can also be achieved by creating an absolute pressure difference or by diluting the reaction products with an inert gas, using a "blow" gas.
The "Distributor" type is membrane catalysts.The second type of membrane reactors are reactors operating on the distributor principle.One of the reagents is added to the reaction mixture through the membrane in a known manner.The membranes used in this type of membrane reactor can perform various functions.On the one hand, the membrane evenly distributes the reaction rate of the reagent along the length of the reactor, eliminating local overheating and side reactions.On the other hand, the membrane can serve as a separating curtain, through which one of the components of the mixture is dosed.Both tasks can be combined in one device.
The "Contactor" type is a membrane reactor.The use of reactors of this type as a contactor with a forced flow through the membrane, that is, as a flow catalytic membrane reactor and an interphase contactor, is of interest.Interphase contactor.In an interphase contactor, the membrane controls the contact between the different phases of the reaction mixture.The membrane is catalytically active and separates gas and liquid or organic phase and water.

Flow catalytic membrane reactors
In this type of membrane reactor, initially, mixed reagents are passed through a porous nonselective catalytic membrane.The function of the reactor is to provide a reaction space with a short controlled reaction time and high catalytic activity.The goal of using flow catalytic membrane reactors is to achieve complete conversion in minimum time and minimum reaction space or to achieve maximum selectivity due to a narrow distribution of contact times for this reaction.

Selective integrated flow catalytic membrane reactors
The application of this type of membrane reactor is similar to the previous one, but requires high selectivity of the reaction, so the membrane must meet additional requirements.In addition to high catalytic activity, the membrane must ensure a narrow distribution of contact times, which limits the passage of side reactions.That is, the membrane should have an orderly porous microstructure.This type of membrane reactor is used in gas phase reactions, partial oxidation, hydrogenation, oligomerization and dimerization reactions.In the work, the characteristics of membrane catalysts in the reaction of methane carbonate conversion as a model reaction were studied.This process produces synthesis gas (a 1:1 mixture of H2 and CO), which can be used to make many products, including synthetic fuels (in the Fischer-Tropsch process).One of the serious problems of methane carbonate conversion is the presence of side reactions that prevent the production of synthesis gas with the required ratio of products (H2:CO=1:1).
Membrane catalysts were studied in carbonate conversion of methane, membrane catalytic reactors: extractor (Fig. 3, a) and contactor (Fig. 3, b) and distributor (Fig. 3c).Carbonate conversion of methane was compared with their material performance characteristics in the ground state in a conventional catalytic reactor with a fixed catalyst bed on membrane catalysts in the contactor and distributor.As the main parameters of the process of catalytic methane carbonate conversion, the change of the main parameters of the process -the concentration of the components in the products, the degree of change of the initial reagents and the ratio of the resulting synthesis gas components, depending on the contact time -were considered.
As can be seen from Fig. 4, in all described reactors, the conversion of initial compounds (CH4 and CO2) and the mole ratio of products have different values under the same conditions.The largest conversion values are observed in the contactor and distributor, and the smallest values are observed in conventional catalytic reactors.These sizes differ by several times.In this case, the structure of the membrane catalyst affects the degree of change and selectivity of the process (Fig. 5).

Conclusion
Thus, it was shown experimentally that the use of a membrane catalytic reactor significantly accelerates the process of methane carbonate conversion during the forced migration of reagents, probably due to the increase in the surface area of the catalyst in the case of the contactor and the increase in the selectivity of the reaction when the adjacent interactions are added in the distributor.Carbonate conversion of methane was compared with their material performance characteristics in the ground state in a conventional catalytic reactor with a fixed catalyst bed on membrane catalysts in the contactor and distributor.As the main parameters of the process of catalytic methane carbonate conversion, the change of the main parameters of the process -the concentration of the components in the products, the degree of change of the initial reagents and the ratio of the resulting synthesis gas components, depending on the contact time -were considered.

E3SFig. 3 .
Fig. 3. General scheme and mechanism of operation of membrane reactors

Fig. 4 .
Fig. 4. Comparison of membrane catalytic reactors with the conventional reactor at 900 ℃.Membrane catalyst type -has a damped catalyst layer

Fig. 5 .
Fig. 5. Comparison of Membrane Catalysts at 900 ℃ in a Distributor with a Massive and a Shallowed Catalyst Bed

Table 1 .
Chromatographic analysis conditions

Table 2 .
Properties of membrane catalysts