STUDY OF KINETIC LAWS OF METHANE OXYCHLORINATION REACTION AND ASSESSMENT OF ACTIVATION ENERGY

ABSTRACT

The kinetics of the methane oxychlorination reaction were studied and the activation energy was estimated. The experiments were conducted in a 20 mm flow reactor. The kinetics of the methane oxychlorination reaction were studied under the following optimal conditions: the composition of the mesoporous catalyst with high catalytic activity, (CuCl₂)x∙(KCl)y∙(ZnCl₂)z∙(MnCl₂)k, the size of the mesoporous catalyst fractions with high catalytic activity 0.7÷1.2 mm, P = 0.1 MPa, the gas flow rate 17.2 l/h, the contact time 0.1 sec, the linear flow velocity 10.2 cm/sec. The rate of adsorption of methane, hydrogen chloride and oxygen on the surface of a mesoporous catalyst with high catalytic activity was taken as a decisive (limiting) step in deriving the kinetic equation of the catalytic oxychlorination reaction of methane. The aim of the work is to study the kinetic laws of the methane oxychlorination reaction and estimate the activation energy.

АННОТАЦИЯ

Изучена кинетика реакции оксихлорирования метана и оценена энергия активации. Эксперименты проводились в проточном реакторе диаметром 20 мм. Кинетика реакции оксихлорирования метана изучалась при следующих оптимальных условиях: состав мезопористого катализатора с высокой каталитической активностью (CuCl₂)x∙(KCl)y∙(ZnCl₂)z∙(MnCl₂)k, размер фракций мезопористого катализатора с высокой каталитической активностью 0,7÷1,2 мм, P = 0,1 МПа, расход газа 17,2 л/ч, время контакта 0,1 с, линейная скорость потока 10,2 см/с. Скорость адсорбции метана, хлористого водорода и кислорода на поверхности мезопористого катализатора с высокой каталитической активностью была принята в качестве решающего (лимитирующего) шага при выводе кинетического уравнения реакции каталитического оксихлорирования метана. Цель работы – изучение кинетических закономерностей реакции оксихлорирования метана и оценка энергии активации.

Keywords. Monochloromethane, oxychlorination, methane, ethylene, propylene, pyrolysis, catalyst, porosity, oxygen, nitrogen, hydrogen chloride.

Ключевые слова: Монохлорметан, оксихлорирование, метан, этилен, пропилен, пиролиз, катализатор, пористость, кислород, азот, хлористый водород.

Introduction.

The results of the study of the catalytic oxychlorination of methane carried out using LaCl₃, a selected reaction rate enhancer for the oxychlorination of methane with hydrogen chloride in the presence of air oxygen to obtain monochloromethane and di- and tri-chloro derivatives of methane [1; 9; 12; 13; 18; 21], indicate the high stability of this reaction rate enhancer and the high selectivity of the formation of monochloromethane [20].

The catalytic oxychlorination of methane with LaCl₃ is carried out by the formation of hypochlorite on the surface of the reaction rate enhancer for the oxychlorination of methane with hydrogen chloride in the presence of air oxygen to obtain monochloromethane and di- and tri-chloro derivatives of methane. Methane reacts with hypochlorite to form a carbonium ion, resulting in the formation of monochloromethane and a hydroxyl group. During the reaction of hydrogen chloride with the hydroxyl group, Cl is released, and then methane is oxychlorinated with hydrogen chloride in the presence of air oxygen to produce monochloromethane and the di- and tri-chloro derivatives of methane, resulting in the formation of hypochlorite with oxygen on the surface of a selected reaction rate-increasing substance [2; 8; 10; 11; 14].

Many studies [15; 16] have shown that when high-silica zeolites are modified with metals such as Co, Fe, Mn, and Mg, the operating life of the selected reaction rate enhancer for the oxychlorolysis of methane with hydrogen chloride in the presence of air oxygen to obtain monochloromethane and di- and trichloromethane derivatives increases, which is explained by the fact that these metals reduce the degree of coking of the selected reaction rate enhancer for the oxychlorolysis of methane with hydrogen chloride in the presence of air oxygen to obtain monochloromethane and di- and trichloromethane derivatives. Currently, a number of research works are being conducted on the production of motor fuels and aromatic hydrocarbons from natural gas, oil tailings, and gas condensates in the presence of selected reaction rate-increasing agents to obtain monochloromethane and di- and trichloromethane derivatives of methane by oxychlorination of zeolite methane with hydrogen chloride in the presence of atmospheric oxygen [3—7; 17; 19].

EXPERIENCE PART

X-ray diffraction (XRD) analysis of the samples was performed on an XRD-6100 (Shimadzu, Japan) diffractometer. Phase identification was performed based on the literature data of individual component X-ray diffraction patterns (JSPDS file). The intensity of the lines (stripes) was measured on a 100-point scale based on the peak height. The interplanar spacing (d, nm) in the crystal lattice was determined according to the Wolf-Bragg formula:

- return rate; -x-ray wavelength, -shift bregg angle, grad.

The average size of crystallites was estimated according to the Selyakov-Scherrer equation:

The smaller the size of the catalyst particles, the lower its diffusion and thermal resistance and the higher its activity.

Kinetic laws of methane oxychlorination reaction

Computation of hierarchical kinetic models. Mole concentrations of substances were used to calculate kinetic relationships. The mole concentrations of the products were calculated according to the following formula:

where -i is the mole fraction of the component, τ is the experiment time (hours), ω is the gas flow rate under reaction conditions (l/hour), and  is Avogadro's number. To calculate the molar concentrations of the initial reactants

the average amount of substances calculated according to the formula was obtained. When a mixture of gases is converted to liters

The target function is the mole concentration of monochloromethane. thus, the formula expressing the rate of the methane oxychlorination reaction and the reaction order in terms of reagents for this process can be expressed as:

where  is the reaction rate constant,  is the concentration of the reactants.

The adequacy of the equations was checked based on the standard deviation (S) of the difference between the experimental and theoretical calculation results.

To calculate the activation energy

formula was used.

The table below gives data for calculating the activation energy under kinetic conditions.

Table 1.

Data for calculating the activation energy of the methane oxychlorination process (contact time 0.03 sec, P=0.1 MPa, CH₄:HCl:O₂:N₂=13:1:0.5:5)

was calculated using the following formula:

We did not take into account the concentration of HCl in the calculation, since its value is approximately 1. Thus, the activation energy is 96.74 kJ/mol.

Based on the experimental results obtained, it can be concluded that the limiting step is a chemical reaction on the entire surface of the mesoporous catalyst with high catalytic activity. This, in turn, gives grounds for concluding that the reaction of formation of unsaturated hydrocarbons: ethylene, propylene and butylenes from monochloromethane proceeds in the kinetic domain.

The calculation of the value of the rate constant (k) was carried out at temperatures of 400^oC, 420 ^oC, 440 ^oC and 460 ^oC. The data obtained on the value of logk allowed us to find the activation energy and the values ​​of the old exponential multiplication. The values ​​were determined using the Arrhenius equation:

When determining the activation energy experimentally, knowing k, the integral form of the Arrhenius equation was used:

lgk = lgA -

The inverse of temperature has a linear relationship with ln k. The activation energy is found by the value of the slope tangent:

;        

If the rate constant of the reaction at two different temperatures is given, then the activation energy of the process is found as follows:

 from

the figure shows that the resulting correlations lie satisfactorily on a straight line.

Figure 1. Decimal logarithmic inverse temperature dependence of the reaction rate constant

As a result of the research, the activation energy (E_a), the exponential product (A) was determined:

Е_а =  51682 ± 56000Ж/моль = 51,682 ± 56кЖ/моль

А = 107682±1,0

CONCLUSION

Based on the ideas about the reaction mechanism, it is possible to derive a kinetic equation that includes the rate constants and adsorption coefficients of the elementary stages of the reactions.

In deriving the kinetic equation of the catalytic oxychlorination reaction of methane, the rate of adsorption of methane, hydrogen chloride and oxygen on the surface of a mesoporous catalyst with high catalytic activity was taken as the decisive (limiting) step. The adsorption process occurring on the surface of a mesoporous catalyst with high catalytic activity is monomolecular, and methane, hydrogen chloride and oxygen are adsorbed separately on active centers.

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