Study of the Kinetics of Low-Molecular-Weight Hydrocarbon Synthesis from Carbon Monoxide and Steam
Abstract Generated abstract
This paper examines the kinetics of low molecular weight hydrocarbon synthesis, mainly ethylene and other C2 products, from carbon monoxide and water vapor over an iron copper catalyst. It applies a pulsed chromatographic microreactor method in which carbon monoxide serves as the carrier gas and water vapor is introduced in pulses, and relates product yield to a fictitious reaction time at several temperatures. The observed linear dependence of C2 hydrocarbon yield on reaction time is interpreted as indicating low conversion and approximate zero order behavior, allowing estimation of rate constants and a true activation energy of 190 kJ/mol. The study argues that the method can be used to investigate the true kinetics of this bimolecular catalytic process and compares its results with earlier classical kinetic measurements.
Full Text
STUDY OF THE KINETICS OF THE SYNTHESIS OF LOW-MOLECULAR-WEIGHT HYDROCARBONS FROM CARBON MONOXIDE AND WATER VAPOR
S. I. Smolyaninov, A. V. Kravtsov, G. G. Blagopoluchnaya
(Presented by the scientific-methodological seminar of the CTF)
In connection with the need to study the rapidly developing processes of petrochemistry and petroleum refining, investigation of the kinetics of bimolecular processes is becoming of great importance; a typical example of such processes may be the synthesis of organic compounds from carbon monoxide and water vapor using various methods [1—8].
In the present work, various methods for studying chemical reactions are considered in sufficient detail, with particular emphasis on the modern pulsed chromatographic method [2—8], one of whose distinctive features is that one of the reaction components serves as the carrier gas. In our study this component is carbon monoxide, while the second component, water vapor, was introduced in pulses into the stream of the former. We attempted to apply this method to the study of the kinetics of the synthesis process from carbon monoxide and water vapor. When this process was carried out in a pulsed chromatographic regime with a microreactor, a rectilinear dependence was obtained for the yield of the sum of hydrocarbons C₂ on the fictitious reaction time (Fig. 1). On the basis of these results it may apparently be stated that the reaction proceeds with an insignificant depth of conversion and, consequently, in a first approximation, as a zero-order reaction [9].
The values of the rate constants of this reaction (mainly the formation of C₂H₄) at various temperatures, determined from the tangent of the angle of inclination of the above-mentioned linear dependence, have the following values:
\[ K_t = 0.28 \ \text{mol}/\text{l}\cdot\text{s}; \quad t = 240^\circ\text{C}; \]
\[ K_t = 1.4 \ \text{mol}/\text{l}\cdot\text{s}; \quad t = 250^\circ\text{C}; \]
\[ K_t = 3.4 \ \text{mol}/\text{l}\cdot\text{s}; \quad t = 260^\circ\text{C}; \]
\[ K_t = 4.7 \ \text{mol}/\text{l}\cdot\text{s}; \quad t = 270^\circ\text{C}. \]
Taking into account that for a zero-order reaction
\[ E_{\text{app}} = E_{\text{true}}, \]
from the Arrhenius dependence \(\lg K = f\left(\frac{1}{T}\right)\) one can determine the true activation energy of hydrocarbon formation in the process under study:
\[ E_{\text{true}} = 190\,000 \ \text{J/mol} \ (45\,000 \ \text{cal/mol}). \]
And the intercept cut off on the ordinate axis will be equal to \(K_0\). Proceeding from
Fig. 1. Dependence of the yield of hydrocarbons \((\Sigma C_2)\) on the fictitious reaction time at various temperatures (iron–copper catalyst).
From the data obtained \((K_0, K, E)\), one can obtain an equation describing the specific rate of formation of \(C_2\) hydrocarbons.
\[ K = 16{,}85 \cdot l^{\frac{45000}{RT}} \]
A comparison was made of the experimental data obtained in studying the kinetics of synthesis of the sum of \(C_2\) hydrocarbons (mainly ethylene) from carbon monoxide and water vapor in a chromatographic regime with a microreactor, with kinetic constants obtained earlier in studying the same process by the classical method [1].
It is shown that, while a satisfactory uniqueness of the values of the specific rates of this process is retained, the magnitude of the apparent activation energy obtained in studying the kinetics of synthesis from carbon monoxide and water vapor in a pulsed chromatographic regime decreases with increasing depth of process progress, which is apparently explained by an increase in the degree of diffusion inhibition [8].
Conclusions
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The possibility has been shown of studying the true kinetics of a bimolecular process in a chromatographic regime with a microreactor, using one of the components—carbon monoxide—as the carrier gas.
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The kinetic constants of the bimolecular reaction (synthesis of ethylene from carbon monoxide and water vapor) have been determined: the specific rate
\[ K = 16{,}85 \cdot l^{\frac{45000}{RT}}; \]
the reaction order \(P = 0\); the true activation energy \(E_{\text{true}} = 190\,000\) J/mol \((45\,000\ \text{cal/mol})\).
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