QUASI-STATIONARY MATHEMATICAL MODEL OF A REACTOR FOR VINYL ACETATE SYNTHESIS VIA ACETYLENE VINYLATION OF ACETIC ACID

ABSTRACT

Acetylene and ethylene are used as raw materials for the production of vinyl acetate. The competitiveness of a particular process is mainly determined by the availability and cost of the initial reactants. The influence of the above-mentioned factors can be regarded as a decrease in the surface of the catalytic system occupied by active sites. The results show that temperature, reactant ratio, and catalyst activity significantly affect process efficiency and selectivity. The proposed model can be used for reactor optimization, scale-up, and process control in industrial vinyl acetate production.

АННОТАЦИЯ

Ацетилен и этилен используются в качестве сырья для производства винилацетата. Конкурентоспособность конкретного процесса в основном определяется доступностью и стоимостью исходных реагентов. Влияние вышеуказанных факторов можно рассматривать как уменьшение поверхности каталитической системы, занятой активными центрами. Результаты показывают, что температура, соотношение реагентов и активность катализатора существенно влияют на эффективность процесса и селективность. Предложенная модель может быть использована для оптимизации работы реактора, масштабирования процесса и управления им в промышленном производстве винилацетата.

Keywords: vinyl acetate, acetylene, acetic acid, quasi-stationary model, reactor, catalytic system, reaction kinetics, process optimization

Ключевые слова: винилацетат, ацетилен, уксусная кислота, квазистационарная модель, реактор, каталитическая система, кинетика реакции, оптимизация процесса

INTRODUCTION

Acetylene and ethylene serve as the primary raw materials for vinyl acetate production. The competitiveness of a given method largely depends on the availability and cost of the starting reagents.

In 1938, Kuskovskiy first produced vinyl acetate on an industrial scale using a liquid-phase process from acetylene and acetic acid. This method involved passing acetylene through acetic acid in the presence of a catalyst, which consisted of inorganic (H₂SO₄, H₃PO₄) and organic acids (sulfonic acids), or mercury salts in a zinc and/or cadmium acetate medium, supported on silica gel, pumice, or alumina. The reaction was conducted at 60–66 °C. In addition to the target product, ethyl diacetate was formed as a by-product, and the yield of vinyl acetate per batch was only 3–5%. The method had limited practical application due to low profitability, severe corrosion of the catalytic system, and mercury salt toxicity [1–2].

Later, in 1953, vinyl acetate was synthesized in the liquid phase at elevated temperatures from acetic anhydride and acetaldehyde using 0.4% sulfuric acid as a catalyst. Benzene or toluenesulfonic acid could also serve as catalysts. Along with vinyl acetate, acetic acid was produced [3–5]. Since the starting reagents (acetaldehyde and acetic anhydride) were themselves synthesized from ethylene and oxygen, the overall process was multi-step, which limited its practical application.

EXPERIMENTAL RESULTS AND DISCUSSION

The decrease in catalyst activity is mainly attributed to the following factors:

1. Removal of metal acetates from the catalyst surface by the flow of reactant gases;

2. Polymerization of reaction products on the catalyst surface;

3. Partial volatilization of products at elevated temperatures.

The influence of these factors can be considered as a reduction in the surface area of the catalytic system occupied by active sites. The catalyst activity at time τττ can be determined using the activity parameter θ(τ)\theta(τ)θ(τ), which represents the ratio of the surface of active sites at time τττ to that at the initial moment [6–7].

Table 1.

Dependence of the composition of the reaction mixture at the outlet of a reactor designed for vinyl acetate production on the molar ratio of acetylene to acetic acid in ZnO∙CdO∙ZrO₂/ceramsite catalyst, with a volumetric flow rate of the reaction mixture of 200 L/(cat·h).

Table 2.

Dependence of the reaction mixture at the outlet of a reactor designed for vinyl acetate production on the volumetric flow rate and 3:1 molar ratio of acetylene to acetic acid using a ZnO∙CdO∙ZrO₂/ceramsite catalyst 

Table 3.

Dependence of the composition of the reaction mixture at the reactor outlet during vinyl acetate production on the temperature at a volumetric flow rate of 1500 L/(L·cat·h) and a 5:1 molar ratio of acetylene to acetic acid.

 Reaction rate of the reaction mixture

 Deactivation rate constant given by

 the pre-exponential factor;

 activation energy;

R- universal gaz constant

T-absolute temperature

To provide a detailed description of the deactivation equation parameters, long-term experimental data obtained using a mixing apparatus with a reaction mixture of 38 mm diameter and 2.65 m length at various linear flow rates and synthesis temperatures are presented [8–9]. The selection of the rate constants was carried out by searching for values that best approximated the calculated concentrations of the reaction gas components at the reactor outlet over the entire operating time of the tube and the calculated temperatures at three control points to the corresponding experimental data. In this case, the mathematical model of the catalytic tube was employed:

C₁-C₈ - Molar concentrations of acetylene, acetic acid, vinyl acetate, acetaldehyde, acetone, crotonaldehyde, water, and carbonic anhydride, mol/mol

 consumption rate of component iii in the reaction mixture (mol/cm³·s)

  component of the reaction mixture

 molar mass and density of the reaction mixture, respectively (kg/mol, kg/m³)

 pressure in the tube, Pa

 heat effect of the reaction

 reaction number; ΔHj\Delta H_jΔHj​ in kJ/mol

 tube diameter, m

heat transfer coefficient, W/cm²·s

 cooling temperature, K

The calculation results are as follows:

Figure 1 shows the experimental and calculated curves of the variation in vinyl acetate (VA) removal over time at two linear flow rates of the reaction mixture. The average deviation of the calculated curves from the experimental data is 6.6%, with a maximum deviation of 8.9%, which is within the measurement error range [10]. One of the key indicators in VA synthesis is the characteristics of the catalyst used, with the most important being activity, selectivity, and service life. By considering the quantitative dependence of each of these indicators on the composition, the optimal chemical composition of the catalyst can be selected through mathematical modeling.

The developed mathematical model can be used to optimize the carrier selection for the catalyst in ethylene acetoxylation synthesis and to determine its operational lifetime. Overall, the minimal selectivity of the catalyst limits its service life, and optimizing the chemical composition, including the content of valuable metals, allows maintaining its efficiency while minimizing deactivation.

Figure 1. Variation of vinyl acetate (VA) removal (experimental and calculated) and the corresponding changes in synthesis temperature at linear flow rates of U = 0.2 m/s (a) and U = 0.3 m/s (b).

Conclusion

A quasi-stationary mathematical model of a reactor for vinyl acetate synthesis via acetylene vinylation of acetic acid was developed. The model adequately describes the influence of key operating parameters on reactant conversion and product yield under steady-state conditions. The results show that temperature, reactant ratio, and catalyst activity significantly affect process efficiency and selectivity. The proposed model can be used for reactor optimization, scale-up, and process control in industrial vinyl acetate production.

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