CONVERSION OF METHANE TO CARBONATE ON MOLYBDENUM AND ZIRCONIUM CATALYSTS

ABSTRACT

The article describes thermodynamic calculations and the main characteristics of the conversion of methane to steam-carbon. The conversion of methane to carbon dioxide in the range of H2:CO ratio from 1:1 to 2:1 was used to obtain gas synthesis and its further research. This ratio is preferred for the production of hydrocarbons using the Fischer-Tropsch method. An experimental scheme of steam-carbon dioxide methane conversion with a flow system has been developed. Installation parameters are specified to allow operation at temperatures up to 800 ℃. Comparison of the calculated and experimentally obtained compositions was carried out using indicators of a thermocouple recording the temperature at the upper limit of the catalyst. The graphical dependence of the synthesis gas module on the concentration of carbon dioxide at different H2O:CH4 ratios was studied by comparison with the dependences corresponding to thermodynamic equilibrium at P = 5 atm and temperatures of 700 and 800 ℃.

АННОТАЦИЯ

В статье описаны термодинамические расчеты и основные характеристики превращения метана в пароуглерод. Конверсию метана в углекислый газ в диапазоне соотношения H2:CO от 1:1 до 2:1 использовали для получения газового синтеза и его дальнейших исследований. Это соотношение является предпочтительным для добычи углеводородов методом Фишера-Тропша. Разработана экспериментальная схема паро-углекислотной конверсии метана с проточной системой. Параметры установки указаны для обеспечения работы при температуре до 800 ℃. Сравнение расчетного и экспериментально полученного составов осуществляли с помощью индикаторов термопары, регистрирующей температуру на верхнем пределе катализатора. Графическую зависимость модуля синтез-газа от концентрации углекислого газа при различных соотношениях H2O:CH4 изучали путем сравнения с зависимостями, соответствующими термодинамическому равновесию при P = 5 атм и температурах 700 и 800 ℃.

Keywords: methane, carbon dioxide, conversion, molybdenum, zirconium.

Ключевые слова: метан, диоксид углерода, конверсия, молибден, цирконий.

1. Introduction

Synthesis gas is a mixture of carbon monoxide and hydrogen, the ratio of CO:H₂ varies from 1:1 to 1:3, depending on the method of synthesis gas production. Synthesis gas production was considered one of the most important tasks of modern gas chemistry. Different H₂/CO ratios of synthesis gas can produce different valuable products.

There are four ways to get synthesis gas from methane:

• Steam conversion: СH₄ + H₂O ↔ CO + 3H₂∆H=+206 Kj/mol                          (1)
• Partial oxidation with oxygen: CH₄ + 1/2O₂ ↔ CO + 2H₂∆H=+35.6 Kj/mol     (2)
• Carbon dioxide conversion: CH4 + CO2 ↔ 2CO + 2H₂∆H=+247 Kj/mol        (3)
• Autothermal conversion: CH4 + 2O2 ↔ 2CO2 + 2H₂∆H=+802 Kj/mol            (4)

The steam conversion method is the main process of synthesis gas production. Figure 1.1 shows the stages used in hydrogen production plants based on the steam conversion method.

Figure 1. Scheme of obtaining hydrogen from natural gas

Steam conversion involves endothermic processes to produce hydrogen and carbon monoxide from methane and water vapor (reaction 1). This process takes place at a temperature of 700-850 °C, at a pressure of 3-25 atm, and Ni-based catalysts are used [1-4].

Steam conversion results in a ratio of H2/CO of 3:1, which is higher than that required for synthesis via the Fischer-Tropsch reaction from substances such as methanol or hydrocarbons [5-7].

2. Methods of research

The study of the process of conversion of methane to carbon dioxide was carried out using a laboratory flow reactor.

1st faucet; 2-rheometer; 3-gas clock; 4-desulfurization reactor tube; 5-desulfurization reactor furnace; 6-reheating furnace; 7th reactor; 8th reactor furnace; 9-water cooler; 10th receiver; 11-thermocouple; 12-temperature gauge-regulator

Figure 2. Methane Carbonate Conversion Unit

The process was carried out in a 20 mm diameter metal reactor with a catalyst loading of 20 cm3. The reaction zone is located in the middle of the reactor and is provided with a grid on which the catalyst is placed. The reactor-catalyst cell is equipped with a vertical, thermocouple pocket inserted into the catalyst filling zone. The reactor is placed in an electric furnace made of two coaxial quartz tubes. Between them is wrapped nicrospiral, insulated with asbestos. The heating element of the furnace works from the mains -220 V. The temperature in the reactor is maintained with an accuracy of ± 3 ° C. The amount of methane and carbon dioxide supplied to the reactor was monitored using series-connected rheometers and a gas clock. Methane was supplied from the central gas network, carbon dioxide gas from a cylinder. The use of methane in the gas network required its preliminary purification from sulfur compounds. Desulfurization was carried out by passing methane through a layer of copper (II) oxide at 400 °C. Then the methane purified from sulfur compounds was combined with the carbon dioxide stream and bypassed the furnace, where the temperature of the gas stream rose to 500 °C. Then the gas mixture was sent to the reactor, where the temperature is 700 ° C - 900 ° C. The carbon dioxide did not need to be pre-purified and was sent into the reaction immediately after measuring the amount and flow rate. In the reactor, the gas mixture passed through the catalyst bed. After passing through the reactor, the reaction products were cooled to room temperature and separated in a separatory funnel, from which samples were taken. The pressure at the outlet of the reactor was equal to atmosphere. The duration of the experiment exceeded 2 hours in all cases. The interval of measuring consumption of substances at the entrance and exit is 20 minutes, in some cases 10 minutes.

3. Results

When conducting various studies, it is necessary to estimate the magnitude of the experimental error. During the experiment, the main error occurs with various deviations in the supply of raw materials and temperature control, in the analysis of gas products, etc. A number of parallel experiments were conducted to determine the experimental error. The obtained results are shown in Table 1.

Table 1.

Results of parallel experiments

Based on the given data, we estimate the measurement value ẋ, which is defined as the arithmetic mean of all measurements:

The deviation of the results of individual experiments from the arithmetic mean gives the Absolute error of a random measurement:

After a limited number of measurements, an estimate of the variance or sample variance of C is obtained:

Mean squared error of one experiment:

The maximum error of a random distribution can be determined by different methods, for example, using the Student's distribution or the rule of three sigma, almost all errors are between - 3 σ and + 3 σ.

Calculation of quantitative indicators of the reaction. The conversion of substance (a) was determined by the ratio of the difference between the amount of substances in the reaction mixture before and after the reaction to the amount in the initial mixture:

Here, a is the composition of the substance in the initial mixture a is the composition of the substance in the reaction mixture after the experiment. The yield of the product (Y) was determined by the ratio of the sum of the amount of the product after the reaction to the amount in the initial mixture (the amount of starting materials and products are equal):

During the conversion of methane to carbon dioxide, nickel catalysts fail due to the formation of coke as a result of this double reaction and the decomposition of methane. To overcome this problem, a study was conducted on the effect of the catalyst composition on the process. To study coke formation, activated Ni-Co, Ni-Zrva Ni-Fe catalysts were prepared by adding y-Al2O3 to solutions of nickel, cobalt, iron and zirconium nitrates.

The ability of cobalt to reduce coke formation is known from the literature. To evaluate the activity of the Co catalyst, samples of (Ni2O3)x*(Co2O3)y*(ZrO2)z/HZS and (Ni2O3)x*(Co2O3)y*(MoO3)k/HZS (without nickel) were first prepared, which previously converted methane to carbon dioxide conversion process. The experimental results are presented in Table 2.

Table 2.

Results of experiments on conversion of methane to carbon dioxide (CO2/CH4-1.41; voutometric rate of methane = 1000 h-1)

4. Conclusions

At temperatures above 800 ° C, the activity of catalysts turned out to be so high that it was impossible to accurately estimate the amount of residual methane using existing equipment. Calculations were performed using available equipment using software designed to perform thermodynamic calculations to ensure that the residual methane content was high enough for accurate determination. As a result of calculations, the optimal temperature was 700 ° C. The temperature correction for non-equilibrium was calculated using a program designed to calculate steam-carbon dioxide conversion of methane [148]. In this case, the amount of supplied water vapor will be the minimum value (H2O / CH4 is equal to 0.01) to bring the calculation closer to the conditions of conversion to carbon dioxide. Next, a graph of the dependence of the thermodynamic correction on the non-equilibrium and CO2 / CH4 ratio for the prepared catalysts was drawn.

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