THE MAIN CHARACTERISTICS OF STEAM-CARBON-DIOXIDE CONVERSION OF METHANE

ABSTRACT

The article discusses the main properties of steam-carbon dioxide conversion of methane and thermodynamic calculations. The study used the conversion of methane to carbon dioxide in a ratio of 1:1 to 2:1. This ratio is preferred for the production of hydrocarbons by the Fischer-Tropsch process. An experimental scheme of steam-carbon dioxide conversion of methane with a flow-through system has been developed. A comparison of the calculated and experimentally obtained compositions was carried out following the indication of the thermocouple registering the temperature at the upper boundary of the catalyst. The graphical dependence of the gaseous synthesis modulus on the concentration of carbon dioxide was studied at various ratios of H₂O:CH₄ in comparison with the dependences with the corresponding thermodynamic equilibrium at P = 5 atm and temperatures of 700 and 800 ℃.

АННОТАЦИЯ

В статье рассматриваются основные свойства пароуглекислотная конверсии метана и термодинамические расчеты. В исследовании использовалось преобразование метана в диоксид углерода в соотношении от 1: 1 до 2: 1. Это соотношение предпочтительно для производства углеводородов по методу Фишера-Тропша. Разработана экспериментальная схема пароуглекислотной конверсии метана с проточной системой. Сравнение расчетных и экспериментально полученных композиций проводилось в соответствии с индикацией термопары регистрирующей температуру на верхней границе катализатора. Графическая зависимость газообразного модуля синтеза из концентрации диоксида углерода изучалась при различных соотношениях Н₂О:СН₄ по сравнению с зависимостями с соответствующим термодинамическим равновесием при P = 5 атм и температурой 700 и 800 ℃.

Keywords: methane, steam-carbon dioxide conversion, synthesis gas, material balance.

Ключевые слова: метан, пароуглекислотная конверсия, синтез-газ, материальный баланс.

Introduction. Currently, synthesis gas plays a key role not only in the production of components for motor fuels (Fischer–Tropsch synthesis) but, above all, in organic synthesis processes to obtain methanol, dimethyl ether, butanol, methyl tert-butyl ether, formaldehyde and others [1, 2, 3].

To obtain synthesis gas in industry, the process of steam reforming of methane is used, however, this type of conversion has several significant disadvantages, such as the need for insufficiently high volumetric velocities (average space velocity for methane 1200 ч^-1), a high Н₂:СО ratio in the resulting synthesis. gas; The most significant drawback of this process is the high economic costs during its implementation, as a result of which the cost of synthesis gas obtained using this technology is approximately two-thirds of the cost of the final products (methanol or dimethyl ether). The process of carbon dioxide conversion of methane (CDCM) makes it possible to obtain synthesis gas with a lower ratio, in the range from 2:1 to 1:1. Currently, many technologies require such a low Н₂:СО ratio. For example, this ratio is preferable for the production of hydrocarbons by the Fischer-Tropsch method, for hydroformylation, for the production of methanol, formaldehyde, dimethyl ether and many other organic compounds, which eliminates the need to regulate the H₂: CO ratio using a water gas shift reaction [4-6].

The conversion of methane to carbon dioxide also allows the use of carbon dioxide in the synthesis, the reserves of which are enormous, and the scale of use in industry is small. In addition, the greenhouse gas is believed to be responsible for global warming on Earth. In the high-temperature pyrolysis of methane, ethylene, acetylene and hydrogen can be formed as the main products. This option for converting methane into ethylene is of great industrial interest since ethylene is the main starting compound for the production of many petrochemical products [7,8,9,10,11]. The authors of [12-15] found that ethylene is formed by the interaction of methane with oxygen on various oxide catalysts at temperatures from 600 to 750 ºС. The main problem of oxidative condensation of methane is [16-19] that the active centres of the catalyst for methane condensation can activate C – H bonds in C₂H₆ and C₂H₄ molecules, which can promote the formation of CO₂ [10, 11, 20, 21]. The formation of carbon dioxide leads not only to a decrease in selectivity but also to an increase in the released heat of this reaction; therefore, heat removal is an important problem [22-24].

Experimental part. The schematic diagram of the laboratory unit for steam-carbon dioxide conversion of methane is shown in Fig. 1.

Figure. 1. Diagram of the block of steam-carbon dioxide conversion of methane:

1 - conversion reactor, 2 - check valve, 3 - flow meter, 4 - water bottle, 5 - refrigerator, 6 - cyclone, 7 - separator, 8, 9 - valves.

The experimental scheme was a flow-through system, the central part of which was a tube made of a heat-resistant alloy 16×2 mm in diameter and 300 mm in length. In the upper zone of the tube was placed 5 ml of the catalyst, previously crushed to granules with a size of 0.4-0.5 mm. A CH₄: CO₂ gas mixture with a methane spatial velocity of 800 h ^-1 was fed into the catalytic bed. To analyse a mixture of gaseous products, we used gas adsorption chromatography and a Kristalluks-4000M device. Detector-katharometer, carrier gas-helium (gas flow rate - 20 ml/min). For the analysis of gaseous products, two chromatographic columns are used. The chromatographic columns are calibrated using the absolute calibration method. The conditions for the analysis are given in the table. 1.

Table. 1.

Analysis of a mixture of gaseous products

To separate CO and CH₄, a chromatographic column with Ca A molecular sieves (3m x 3mm) was used, the analysis mode was isothermal (80 ℃). Carrier gas - helium (gas flow rate - 20 m). The main methane conversion rates were calculated using the following formulas:

Methane conversion:

Hydrogen output:

Carbon monoxide output:

Selectivity for hydrogen:

Selectivity for hydrogen:

Where,  - the amount of methane in the loading of the molar conversion reactor;

 - the amount of methane at the exit from the molar conversion reactor;

- the amount of formed hydrogen mol;

 is the amount of water in the reactor charge, mol;

- the amount of formed carbon monoxide, mol;

 - the amount of carbon dioxide in the reactor charge, mol.

Results and discussion. The steam-carbon dioxide conversion of methane is a strongly endothermic reaction, as a result of which the process temperature in the lower zone of the catalytic layer is significantly lower than the temperature of the upper layer of the catalyst. The process was controlled by the temperature of the upper point of the catalytic layer with a temperature of 750 ℃. Thus, the temperature on the entire catalytic layer was in the range of 700-750 ℃. The results obtained during the experiment in comparison with the parameters corresponding to the thermodynamic equilibrium at a temperature of 750 ℃ are shown in Table 2.

Table 2.

Comparison of experimental data with the parameters of the process of steam-carbon dioxide conversion of methane under conditions of thermodynamic equilibrium at 750 ℃

From table 2 it can be seen that the degree of conversion of methane and carbon dioxide is significantly lower than the thermodynamically calculated one. This may be due to the insufficient activity of the catalyst, the influence of the water shift reaction, the presence of the coke formation process, as well as the possible inaccuracy of the problem of reagent consumption and chromatographic analysis. The results of the comparison in the temperature range 700-750 ℃ are shown in Figure 2.

Figure. 2. Dependence of the modulus of synthesis gas on the concentration of carbon dioxide at various ratios of H₂O:CH₄ in comparison with the dependences corresponding to thermodynamic equilibrium at a pressure of 5 atm for temperatures of 700 and 750 ℃.

At the same time, during the experiments with the ratio CH₄:H₂O=1:0.9, coke formation was observed, which intensified with a decrease in the concentration of carbon dioxide in the initial mixture. A decrease in the concentration of water vapour in the raw mixture leads to a decrease in the synthesis gas modulus, bringing the indicators closer to those corresponding to thermodynamic equilibrium, which indicates a decrease in the effect of the water shift reaction.

When the concentration of water vapour was not lower than the ratio CH₄:H₂O = 1: 1.05, no coke formation on the catalyst was observed. The estimation of the parameters of the process of steam-carbon dioxide conversion of methane was carried out according to thermodynamic calculations, the results of which are presented in Table. 3 and 4.

Table 3.

The data of thermodynamic calculation of the composition of the mixture obtained in the process of steam-carbon dioxide conversion of methane at a pressure of 5 atm and the composition of the feed gas CH₄: H₂O: CO₂ = 1: 1: 0.4.

From table 3, it can be seen that the minimum value of the conversion temperature for obtaining synthesis gas acceptable for the synthesis of liquid hydrocarbons is 700 ℃. Table 4 shows the indicators of the process of steam-carbon dioxide conversion of methane, at various ratios of СН₄, СО₂ and Н₂О in the initial gas mixture.

The purpose of these experiments was to determine the composition of the initial gas mixture at which, as a result of the conversion, synthesis gas with a ratio of Н₂ to CO in the range of 2 - 2.2 will be obtained.

Table 4.

The composition of the synthesis gas and the main indicators of the methane steam-carbon dioxide conversion process (T of the mixing chamber = 450 °C, T of conversion = 750 °C, P = 5 atm, volume velocity 800 h-1, duration of the experiment = 48 h)

The experimental data of this stage show that the proposed methane conversion technology makes it possible to obtain synthesis gas with a Н₂:СО ratio from 1.6 to 4.6.

This range of composition of synthesis gas allows its use in a wide range of gas chemical processes. In the synthesis of liquid hydrocarbons by the Fischer-Tropsch method, the required ratio of H₂ to CO in the synthesis gas is 2-2.2. Synthesis gas of this composition can be obtained with the ratio of the components in the initial gas mixture in the range:

CH₄ : H₂O : CO₂ = 1,0 : 1,16: 0,4. H₂ : CO = 2,5

CH₄ : H₂O : CO₂ = 1,0 : 1,16: 0,7. H₂ : CO = 1,8

Therefore, several experiments were carried out to obtain synthesis gas of the above composition with the ratio of the components of the initial gas mixture CH₄: H₂O: CO₂ = 1.0: 1.0: 0.4. The experimental results are presented in the table. 5.

Table 5.

Experimental data on steam-carbon dioxide conversion of methane

(Т = 750 °С, Р = 5 atm), ( CH₄ : H₂O : CO₂ = 1 : 1: 0,4)

The composition of the resulting synthesis gas, both in terms of the Н₂: CO ratio and the content of CO₂ and CH₄, is acceptable for use in GTL processes without additional composition adjustment. Comparison of the experimentally obtained data with the thermodynamic calculated data shows that the Н₂: CO ratio in the resulting synthesis gas is 4 - 5% lower than the theoretically calculated one. This deviation can be explained by errors in determining the gas composition, as well as by errors in measuring the temperature in the reaction zone. The content of CO₂ and CH₄ in the gas exceeds the theoretically calculated one. At the same time, the total content of these components remains insignificant (from 7.7 to 8.4%), and cannot significantly affect the possibility of using the resulting synthesis gas for laboratory and pilot-industrial installations for the synthesis of liquid hydrocarbons.

Experiments were carried out at conversion temperatures of 700 ℃, 750 ℃ and 800 ℃ in the mode of steam-carbon dioxide conversion of methane, the results of which, including the material balance, are presented in Table 6. The ratio CH₄: H₂O: CO₂ in the initial gas mixture was 1: 1: 0.4.

The material balance was compiled according to the indicators of measurements within 1 hour. The material balance in all experiments is satisfactory.

Table 6.

Material balance of steam-carbon dioxide conversion of methane

(СН₄ : Н₂О : СО₂ = 1 : 1: 0,4)

Table 7 shows the main characteristics of the steam-carbon dioxide conversion of methane.

Table 7.

Main characteristics of steam-carbon dioxide conversion of methane

(СН₄ : Н₂О : СО₂ = 1,00 : 1,00 : 0,4), T = 750 ℃.

As you can see from the table. 7 that with an increase in the conversion temperature from 700 to 800 ℃, the selectivity for H₂ and CO increases by more than 10%, and with an increase in the temperature from 800 to 750 ℃ it remains practically unchanged. The yield of H₂ and CO increases with an increase in temperature from 700 to 800 ℃ and remains practically constant when the temperature rises from 800 to 850 ℃.

Conclusion.

1. A schematic diagram of the laboratory unit for steam-carbon dioxide conversion of methane has been developed.
2. The dependence of the moduli of synthesis gas on the concentration of carbon dioxide at different ratios of Н₂О: СН₄ was studied in comparison with the dependences corresponding to thermodynamic equilibrium at temperatures of 700 and 800 ℃ at Р = const.
3. The material balance of steam-carbon dioxide conversion of methane at various ratios of CH₄: H₂O: CO₂ is presented. The balance in all experimental data is considered satisfactory.

Reference:

1. Крылов О. В. Углекислотная конверсия метана в синтез-газ //Российский химический журнал. – 2000. – Т. 44. – №. 1. – С. 19-33.
2. Ghoneim S. A. et al. Review on innovative catalytic reforming of natural gas to syngas //World Journal of Engineering and Technology. – 2016. – Т. 4. – №. 01. – С. 116.
3. Oyama S. T. et al. Dry reforming of methane has no future for hydrogen production: Comparison with steam reforming at high pressure in standard and membrane reactors //International journal of hydrogen energy. – 2012. – Т. 37. – №. 13. – С. 10444-10450.
4. Morgado C. R. V., Esteves V. (ed.). CO2 sequestration and valorization. – BoD–Books on Demand, 2014.
5. Pakhare D., Spivey J. A review of dry (CO 2) reforming of methane over noble metal catalysts //Chemical Society Reviews. – 2014. – Т. 43. – №. 22. – С. 7813-7837.
6. Fayzullayev N. I., Ruziyev I. H. Мetanni karbonatli konversiyalash //ЎзМУ Хабарлари. – 2018. – Т. 3.
7. Файзуллаев Н. и др. Каталитическая дегидроароматизация нефтянного попутного газа //Збірник наукових праць ΛΌГOΣ. – 2020. – С. 122-126.
8. Шоймарданов Т., Жураев А., Файзуллаев Н. Метанни карбонатли конверсиялаш реакциясининг кинетик қонуниятларини ўрганиш //Збірник наукових праць ΛΌГOΣ. – 2020. – С. 106-110.
9. Fayzullayev N. I. et al. Kinetic Laws of Methane Carbonate Conversion Reaction //International Journal of Control and Automation. – 2020. – Т. 13. – №. 4. – С. 268-276.
10. Fayzullaev N. I., Sh S. B. Catalytic aromatization of methane with non-mo-contained catalysts //Austrian journal of technical and natural sciences. – 2018. – №. 7-8.
11. Fayzullaev N. I. et al. Kinetics and Mechanism of the Reaction of Catalytic Dehydroaromatization of Methane //International Journal of Oil, Gas and Coal Engineering. – 2017. – Т. 5. – №. 6. – С. 124.
12. Файзуллаев Н. И., Турсунова Н. С. Получение этилена из метана с использованием марганец содержащего катализатора //Химия и химическая технология. – 2018. – №. 1. – С. 24-28.
13. Файзуллаев Н. И., Турсунова Н. С. Кинетика каталитической реакции димеризации метана с марганец и молибден содержащим катализатором //Главный редактор. – 2019. – Т. 100.
14. Fayzullayev N. I. et al. Kinetics and mechanism of the reaction of the catalytic oxycondensation reaction of methane //Austrian Journal of Technical and Natural Sciences. – 2019. – №. 5-6.
15. Rakhmatov S. B., Fayzullaev N. I. Technology for the production of ethylene by catalytic oxycondensation of methane //European Journal of Technical and Natural Sciences. – 2019. – №. 5-6. – С. 44-49.
16. Rakhmatov S. B., Fayzullayev N. I. Coke Formation of Catalyst on the Ethylene Preparation from the Oxycondensation of Methane and its Regeneration //International Journal of Advanced Science and Technology. – 2020. – Т. 29. – №. 03. – С. 7875-7884.
17. N. S. Tursunova., N. I. Fayzullaev. Kinetics of the Reaction of Oxidative Dimerization of Methane // International Journal of Control and Automation. – 2020, – T. 13, №. 2, – С. 440 – 446
18. Fayzullaev N. I., Tursunova N. S. Thermodynamic Basis of Methane Oxidation Dimerization Reaction and Process Approval //International Journal of Advanced Science and Technology. – 2020. – Т. 29. – №. 5. – С. 6522-6531.
19. Fayzullaev N. I., Raxmatov S. B. Kinetics and Mechanisms of Oxycondensation Reaction in Methane Molybden-Marganets-Zirconium Catalysis //International Journal of Psychosocial Rehabilitation. – 2020. – Т. 24. – №. 04. – С. 1475.
20. Файзуллаев Н.И., Шукуров Б.Ш., Сагинаев А.Т., Холлиев Ш.Х. Каталитическая дегидроароматизация нефтяного попутного газа // Вестник Атырауского университета нефти и газа. – 2020, № 1 (53), стр. 18-25.
21. Shodikulovich S. B. Study of the reaction of catalytic aromatization of methane //ACADEMICIA: An International Multidisciplinary Research Journal. – 2020. – Т. 10. – №. 8. – С. 674-678.
22. Sh S. B. Raxmatov Sh. B., Fayzullayev NI Kaolindan yuqori kremniyli seolitlar olish //СамДУ илмий ахборотномаси N. – 2018. – Т. 5.
23. Fayzullaev N. I., Sh S. B. Synthesis of high silicone zeolites and application of methane in catalytic Synthesis of high silicone zeolites and application of methane in catalytic aromatizing reaction //Journal of critical reviews. – 2020. – Т. 7. – №. 14. – С. 1235-1242.