Mathematical model of the efficiency of the catalyst in the synthesis of vinyl acetate

АННОТАЦИЯ

Изучены кинетические законы и кинетика и механизм реакции этилена в паровой фазе. Установлено, что общая скорость реакции пропорциональна размеру неинформированных и модифицированных активных центров палладия (а не кластеров). Доказано, что избыточное количество модификатора (как ацетат калия, так и мис) может снижать эффективность катализатора и блокировать активные центры.

ABSTRACT

The kinetic laws and kinetics and mechanism of the vapor phase reaction of ethylene have been studied differently. It was found that the total rate of the reaction was proportional to the amount of unmodified and modified active centers of palladium (not clusters). Excess amounts of the modifier (both potassium acetate and copper) have been shown to block active sites by reducing catalyst efficiency.

Ключевые слова: этилен, кислород, уксусная кислота, винилацетат, кинетическое уравнение, механизм.

Keywords: ethylene, oxygen, acetic acid, vinyl acetate, kinetic equation, mechanism

Production of vinyl acetate in heterogeneous catalysts began in Germany in the late 1930s, a method that has not lost its relevance today: the reaction is carried out in a catalytic system of zinc acetate in a temperature range of 160-240 ^оС and a carrier  (activated carbon) at a pressure close to atmospheric pressure, is carried out [1-2-3]. To reduce the yield of ethylidendiacetate, the reaction is carried out in excess acetylene (ratio of acetylene to acetic acid = 4-10: 1) and the process is carried out either in tubular reactors, or in fake dilute layer reactors of the catalyst. The volumetric load is 100-500 h^-1, the conversion of acetic acid is about 50%, the yield of vinyl acetate is 40-80 kg/m³ per hour. Due to the accumulation of polymer and resin on the zinc acetate catalyst, it loses its activity over time, forcing the process temperature to gradually increase from 160-180 ^оС to 220-240 ^оС [4-5-6-7].

Mathematical model of catalyst efficiency. In order to create a general mathematical model showing the dependence of the efficiency of the catalyst on its composition, we present the experimental dependencies in Table 1[8-9-10].

Table 1

Conditions for conducting a series of experiments and the mathematical models obtained in them

The total rate of reaction is proportional to the amount of unmodified and modified active centers of palladium (not clusters). Excess amounts of the modifier (both potassium acetate and copper) block the active sites, reducing the efficiency of the catalyst. The physical meaning of the partial mathematical models obtained here consists of fractional-linear functions. The difference in the equations of the rates of formation of vinyl acetate and CO₂ indicates that these reactions occur at different active sites of the catalyst.

Taking into account the above considerations, we have the following functional dependencies:

W_BA=[Pd]∙(C₁+C₂[CH₃COOK]+C₃[Cu])/(1+C₄[Pd]⁴+C₅[CH₃COOK]²+C₆[Cu])

W_CO2 = [Pd]∙(C₁^{`</sup>+C<sub>2</sub><sup>`}[CH₃COOK]+C₃^{`</sup>[Cu])/(1+C<sub>4</sub><sup>`}[Pd]⁴)

The numerical values of the coefficients C1-C6 and C1`-C4` were obtained by comparing the data of the equations with the partial model of the reaction rates obtained earlier and using a number of regression methods. Thus, the generalized mathematical models of catalyst activity, which are defined as the rates of reactions for the formation of vinyl acetate and CO₂, are as follows:

W_BA = ([Pd]∙(0,35 + 0,38[CH₃COOK] + 4,2[Cu]))/((1 + 0,05(1 + 80[Pd]⁴ + 0,01[CH₃COOK]² + 1,1[Cu])))

W_CO2 = ([Pd]∙(0,09 + 0,0244[CH₃COOK] + 0,1[Cu]))/((1 + 0,07(1 + 20[Pd]⁴)))

Among all the experiments, the rates of the formation of vinyl acetate and the values ​​calculated by mathematical models are well matched.

The correlation between the experimental and calculated values ​​for the rates of carbon dioxide formation is two parallel straight lines - for a series of experiments on the effect of [Cu] (curve 1 to the right of Fig. 1). and [Pd] for a series of experiments on the effect of quantity (curve 2 to the right of Fig. 1). The significant increase in the experimental values ​​of the W_CO2 velocities for a series of reactions to study the effects of copper is explained by the use of the above-mentioned reason - the use of catalyst samples prepared in violation of the technology. Thus, the velocities of W_CO2 differ from those calculated from the experimental values ​​by a constant magnitude, indicating that additional reactions of ethylene combustion occur in the holder rather than in the active palladium centers. The latter value used mathematical models to optimize the catalyst composition in order to find the most efficient samples.[11-12-13]

Figure 1. Correlation between experimental and calculated values of reaction rates

Using mathematical models of the dependence of catalyst activity on the components stored in it (Pd, CH₃COOK and Cu), we determined their optimal composition to achieve both high rates and selectivity of vinylacetate formation: 0.4% Rd + 4% Cu + 7% CH₃COOK / VKTs.[3-4-5-7]

Effect of steam-gas mixture volume rate on vinyl acetate yield. BGA volumetric velocity range: 2000 to 10000 h-1 in the middle zone of the reactor at a temperature of 165℃, a pressure of 4 atm, a ratio of ethylene to acetic acid 4: 1 and an oxygen content of 7%. The results of the experiments are presented in Table 2.

The dependence of the reagent conversion on the time the reaction mass remains in the reactor is linear, indicating that the reaction rate is constant during this time due to the insignificant value of the reagent conversion. This is confirmed by maintaining the linearity of the time dependence of vinyl acetate output and CO2 formation.[11-12]

Table 2

The effect of the volumetric velocity of a vapor-gas mixture on the yield of vinyl acetate

Volumetric speed ПГС=2000 hour^-1

Volumetric speed ПГС=4000 hour ^-1

Volumetric speed ПГС=6000 hour ^-1

Volumetric speed ПГС=7500 hour ^-1

CONCLUSION

The process of obtaining ethylene by catalytic oxidation in the vapor phase to obtain vinyl acetate was studied in detail in a catalyst of the order 0.4% Rd + 4% Cu + 7% CH3COOK / VKTs. The overall rate of the reaction was found to be proportional to the amount of unmodified and modified active centers of palladium (not clusters). Excessive amounts of the modifier (both potassium acetate and copper) have been shown to reduce the efficiency of the catalyst and block the active sites. The study selected the following optimal conditions for the reaction: in the middle zone of the reactor at a temperature of 165℃ volumetric speed - 2000 h - 1, 4 atm, the ratio of ethylene to acetic acid is 4: 1 and the oxygen content is 7%. A mechanism for the formation of vinyl acetate from ethylene and acetic acid in the presence of a palladium catalyst has been proposed.

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