EFFECT OF EXTERNAL CONDITIONS ON THE SELECTIVITY OF THE PROCESS OF SYNTHESIS OF ORGANIC COMPOUNDS FROM CARBON MONOXIDE AND WATER VAPOR

A. V. Kravtsov, S. I. Smolyaninov, V. M. Mironov

(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

The rapid development of the petrochemical industry is accompanied by increased requirements for the purity and cost of the starting raw material. Its principal needs are met by products of secondary petroleum-refining processes, but raw material for petrochemistry can also be obtained—and is obtained, as is known, on an industrial scale—by synthetic methods and, in particular, by the hydrogenation of carbon monoxide with hydrogen or water vapor.

Previous studies, in particular those of I. F. Bogdanov and co-workers [1, 2], have shown that in synthesis from carbon monoxide and water vapor, mainly aliphatic hydrocarbons are formed, predominantly with a straight chain, which are good raw materials for organic synthesis.

The dependence of the selectivity of a chemical process on temperature can be expressed through the temperature coefficients of the stages constituting the chemical process:

\[ \frac{\xi_2}{\xi_1} = \frac{K_t^{0.1\Delta t}}{k_t^{\prime\,0.1\Delta t}}, \tag{1} \]

where \(\xi_2\), \(\xi_1\) are the selectivities of the process at \(t_2\) and \(t_1^\circ\mathrm{C}\).

It follows from expression (1) that an increase in selectivity for the principal reaction with rising temperature occurs only on condition that the temperature coefficient of the target reaction is higher than that of the side reaction.

Calculation of the temperature coefficients of the principal reaction (formation of hydrocarbons) and the side reaction of synthesis from kinetic plots (method of D. I. Orochko) showed that the temperature coefficient of the hydrocarbon-formation reaction for both catalysts is higher than the temperature coefficient of the conversion reaction. The activation energies of the principal synthesis reaction and of the conversion reaction are, respectively: for the cobalt–thorium catalyst in the temperature interval \(200 \div 210^\circ\mathrm{C}\),

\[ 34.4\,\frac{\mathrm{kcal}}{\mathrm{mol}} \quad \text{and} \quad 27.6\,\frac{\mathrm{kcal}}{\mathrm{mol}}, \]

and for the iron–copper catalyst in the temperature interval \(230 \div 240^\circ\mathrm{C}\),

\[ 23.8\,\frac{\mathrm{kcal}}{\mathrm{mol}} \quad \text{and} \quad 22.8\,\frac{\mathrm{kcal}}{\mathrm{mol}}. \]

Thus, in accordance with expression (1), the selectivity of synthesis with respect to the main reaction of hydrocarbon formation should increase in both cases, and on a Co–Th catalyst to a much greater extent than on an iron–copper catalyst. This has been confirmed experimentally (Fig. 1).

The activation energy for the formation of gaseous hydrocarbons, calculated from the kinetic curves, is 36.6 kcal/mol, which is higher than the average activation energy for hydrocarbon formation. Thus, the selectivity with respect to gaseous hydrocarbons should increase as the synthesis temperature is raised, as is evident from the experimental data in Fig. 2.

It is interesting to note that, when a cobalt catalyst is used, raising the synthesis temperature above 230°C leads to a sharp decrease in selectivity with respect to unsaturated hydrocarbons and an increase with respect to paraffinic hydrocarbons. A similar phenomenon is not observed in the case of an iron catalyst, which is apparently explained by the strong hydrogenating properties of cobalt.

Fig. 1. Dependence of selectivity with respect to the main synthesis reaction (hydrocarbon formation) on temperature, in %. Curve 1—cobalt–thorium catalyst bed length 200 cm. Curve 2—cobalt–thorium catalyst bed length 100 cm; curve 3—cobalt–thorium catalyst bed length 26 cm; curve 4—iron–copper catalyst bed length 200 cm.

Changing the contact time of the reacting feed with the catalyst surface can have a significant influence on the selectivity of parallel reactions when their kinetic orders differ [4]. In our case, decreasing the contact time causes a decrease in selectivity with respect to the main reaction.

The selectivity with respect to the sum of gaseous hydrocarbons has a certain minimum at intermediate (among those compared) space velocities. This leads to the assumption that the reactions forming gaseous and higher hydrocarbons are apparently consecutive.

The magnitude of the CO : H₂O ratio, i.e., the mole fraction of carbon monoxide in the initial feed, will affect selectivity only in the case where the orders of the reactions occurring in the process differ. This follows from the expression obtained for parallel reactions:

\[ \xi = \frac{k_1}{k_2} \cdot N_A^{m-n}, \quad \text{where} \tag{3} \]

\(m;\ n\)—the orders of the reactions being compared.

The experimental data for the process under study show that the selectivity with respect to the main synthesis reaction increases with increasing

mole fraction of carbon monoxide in the initial mixture, and for the cobalt–thorium catalyst to a considerably greater extent than for the iron–copper catalyst. This is possible provided that the order of the main reaction is higher than the order of conversion both in the case of a parallel and of a consecutive mechanism of the process.

Selectivity with respect to individual hydrocarbons is optimal at the stoichiometric ratio $\mathrm{CO}:\mathrm{H_2O}=3:1$. Study of the effect of dilution of the feedstock with inert additives is of great importance in view of the prospective use, for synthesis purposes, of producer gas containing a high percentage of nitrogen and carbon dioxide. As with the effect of the $\mathrm{CO}:\mathrm{H_2O}$ ratio, in this case the selectivity will depend on the difference in the orders of the reactions being compared and on the partial pressures of the reactants.

The experimental data obtained indicate that, as the dilution of the feedstock with an inert gas is increased, the selectivity with respect to the main reaction decreases and the selectivity with respect to gaseous hydrocarbons increases.

Fig. 2. Dependence of the selectivity of the synthesis process with respect to individual hydrocarbons on temperature. Curve 1 — ethylene, curve 2 — propylene, curve 3 — ethane, curve 4 — propane, curve 5 — for the water-formation reaction, curve 6 — for the sum of gaseous hydrocarbons. Space velocity — 25 h; catalyst bed length — 200 cm.

The influence of diffusion complications on the change in the selectivity of the process should in all probability be significant. Let us assume that both reactions (irrespective of whether they proceed in parallel or consecutively) take place in the external-diffusion region; then the dependence of the selectivity of the process on temperature will be very weak, since in this case $\Delta \varepsilon = \varepsilon_1 - \varepsilon_2 \approx 0$, and the value $e^{\Delta \varepsilon/RT} \approx 1$.

Calculation of the activation energy confirms this conclusion. Returning to Fig. 1, we see that, when the linear velocity is decreased below a certain limit ($l_{\text{reactor}} = 26$ cm), when diffusion retardation is possible, the increase in selectivity with temperature is insignificant (curve 3).

The selectivity with respect to individual hydrocarbons decreases with increasing diffusion complications for both catalysts.

The nature of the catalyst has a substantial effect on the magnitude of the selectivity. The selectivity with respect to gaseous hydrocarbons, for identical degrees of CO conversion in the main reaction, is significantly higher for the iron–copper catalyst. The hydrocarbon synthesis products were analyzed by gas–liquid chromatography. It was shown that the gaseous hydrocarbons consist of 40–50% (by weight) ethylene. The ratio of the amount of olefin

Table 1

Optimal regimes and selected synthesis parameters for obtaining the highest relative yield of gaseous hydrocarbons and ethylene

to paraffin rapidly decreases with an increase in the number of carbon atoms in the molecule.

The composition of the synthesis product on the iron–copper catalyst is practically identical to that obtained on the cobalt–thorium catalyst, as confirmed by chromatographic analysis. An analysis was carried out of the “condensate oil” from the synthesis up to $C_{19}H_{40}$, and it was shown that the product consists mainly of saturated hydrocarbons with a straight chain. The admixture of iso-hydrocarbons is appreciable.

On the basis of the results obtained, the optimal conditions for the selectivity of the synthesis with respect to the main reaction and to individual hydrocarbons are given.

Practical recommendations for selecting conditions that ensure the maximum yield of the desired product (for example, ethylene and the sum of gaseous hydrocarbons) were made on the basis of comparing the values of the integral selectivity for the given component with the overall degree of conversion of carbon monoxide and are presented in the final Table 1.

The total yield of the synthesis product in $g/nm^3$ of CO can be calculated for all experiments using the method proposed by I. F. Bogdanov and co-workers [2]:

\[ A=\frac{208 \times C_{\text{t.pr.}} \times \xi_{\text{main r.}}}{100}, \quad \text{where} \]

208 is the theoretical yield of hydrocarbons in the synthesis, $g/nm^3$ CO.

$C_{\text{t.pr.}}$ is the overall degree of conversion of CO in fractions of unity.

$\xi_{\text{main r.}}$ is the selectivity of synthesis with respect to the main reaction (formation of hydrocarbons), %.

Conclusions

1. The optimal conditions for obtaining the target synthesis products have been determined.

2. It has been shown that external conditions exert a substantial influence on the selectivity of the synthesis process from carbon monoxide and water vapor.

Literature

1. I. F. Bogdanov, A. M. Mosin, M. A. Samarskaya. Proceedings of the IGI, Academy of Sciences of the USSR, Moscow, 1959.

2. I. F. Bogdanov. Proceedings of the IGI, Academy of Sciences of the USSR, XVIII, 1962.

3. S. Yu. Elovich. Problems of Kinetics and Catalysis. Publishing House of the Academy of Sciences of the USSR, Moscow, 375–404, 1949.

4. S. L. Kiperman. Introduction to the Kinetics of Heterogeneous Catalytic Reactions. Nauka Publishing House, Moscow, 1964.