YIELD AND COMPOSITION OF SYNTHESIS PRODUCTS FROM CARBON MONOXIDE AND WATER VAPOR

S. I. SMOLYANINOV, A. V. KRAVTSOV

(Presented by the Scientific-Methodological Seminar of the KhTF)

The composition of the synthesis products is determined by the nature of the catalyst used and by the external conditions under which the process is carried out. According to I. F. Bogdanov [1], the synthesis product is characterized by the following specific features:

1. The carbon chain of the hydrocarbons obtained has a normal structure. Hydrocarbons of isomeric structure are practically not formed;

2. The hydrocarbon mixture obtained contains a large amount of unsaturated hydrocarbons. In synthesis with an iron catalyst, the content of unsaturated compounds reaches 70–80%;

3. The double bond in these ethylenic hydrocarbons is found only in the $\alpha$-position, i.e., at the end of the chain;

4. At atmospheric pressure the synthesis product on a cobalt catalyst contains practically no oxygen-containing compounds; on an iron catalyst it contains mainly alcohols, of which ethyl alcohol predominates, followed by n-butyl and n-octyl alcohols.

The author notes that the yield of light hydrocarbons at atmospheric pressure on an iron-copper catalyst was considerably greater than that of fractions above 200°C. Synthesis at a medium pressure of 10 atm., on the contrary, gave a higher yield of heavy hydrocarbons.

H. Kölbel and G. Hammer [2] studied the synthesis product as a function of the composition of the iron catalyst, carrying out the reaction on the basis of producer gas in the liquid phase. They showed that, by modifying the catalyst, the process can be directed either toward the predominant formation of gasoline or toward paraffin and wax, while maintaining the total hydrocarbon yield within the range of 175–185 g/nm³ CO.

Studying the synthesis process over a stationary iron catalyst, H. Kölbel and F. Engelhardt [3] found that the oxygen-containing organic compounds of the synthesis product consist mainly of higher alcohols, the yield of which increases with increasing pressure.

H. Kölbel, I. Kuschel, and G. Hammer [4], in carrying out the synthesis process over a cobalt-thorium-magnesium catalyst at 30 atm., showed that, in addition to aliphatic hydrocarbons, the synthesis product contains significant amounts of oxygen-containing compounds, and they carried out a fairly complete identification of them.

The work carried out by us had the aim of determining the yield of the synthesis product under various external conditions and determining its individual composition.

The experimental part of the work was performed on a flow-through installation

with a fixed bed of the catalysts tested—cobalt–thorium and iron–copper—at atmospheric pressure.

In the synthesis of organic compounds from carbon monoxide and water vapor on both catalysts, a considerable amount of gaseous products was formed, consisting of carbon dioxide, hydrogen, and aliphatic hydrocarbons from C₁ to C₅. The predominant share among the gaseous hydrocarbons is ethylene (40–60 wt %) and propylene (about 15–20%). It should be noted that the ratio of olefin yield to paraffin yield decreases very rapidly with an increase in the number of carbon atoms in the molecule on both catalysts. If methane and ethane at low and medium synthesis temperatures are formed in very small amounts, then the yield of the next paraffins in the homologous series increases rapidly, and, correspondingly, the yield of olefins decreases.

A comparison of synthesis productivity with respect to gaseous hydrocarbons on cobalt–thorium and iron–copper catalysts at the same degree of conversion and other equal conditions shows that it is higher on the Fe–Cu catalyst than on the Co–Th catalyst. For example, at an overall degree of conversion of carbon monoxide of 62%, the productivity with respect to the sum of gaseous hydrocarbons on the iron catalyst is 1.40 times greater, and with respect to ethylene 1.05 times greater, than on the cobalt catalyst.

The maximum ethylene content in the final synthesis gas does not exceed 5% (by weight).

Chromatographic analysis of the condensate oil showed that the gaseous and liquid synthesis product obtained both on the cobalt–thorium and on the iron–copper catalysts is identical in its individual composition. A chromatogram of a complete analysis of the condensate oil, recorded on a chromatograph with programmed heating (from 70 to 220°C), (stationary phase—SE-30), indicates that the liquid synthesis product on the cobalt–thorium catalyst consists mainly of saturated and unsaturated aliphatic hydrocarbons from C₅ to C₂₀, with the content of paraffins from C₉ to C₁₆ approximately the same (in wt %), while the content of the corresponding olefins decreases (almost to zero) with an increase in the number of carbon atoms in the hydrocarbon molecule. As is seen from the chromatogram, the content of iso-hydrocarbons is appreciable, which confirms thermodynamic calculations on the equal probability of formation during synthesis of hydrocarbons both with a straight chain and with an iso-structure [5].

The overall yield of hydrocarbons in the synthesis can be determined using the method of I. F. Bogdanov [1]. In our case, the yield of any hydrocarbon, group of hydrocarbons, or the overall yield of hydrocarbons in the synthesis is equal to the product of the theoretically possible yield (208 g/Nm³ CO) by the degree of conversion of carbon monoxide in the synthesis (as a fraction of unity) and by the corresponding integral selectivity ($S\%$)

\[ A=\frac{208\times C_{\mathrm{st.\,conv.}}\times S}{100}\ \mathrm{g/Nm^3\ CO}. \]

In our experiments, the maximum overall yield of hydrocarbons reached 190–200 g/Nm³ CO on the cobalt–thorium catalyst, and 160–170 g/Nm³ CO on the iron–copper catalyst. The conditions for obtaining the highest yield of gaseous hydrocarbons and ethylene are presented in Table 1.

Conclusions

1. The gaseous synthesis hydrocarbons consist mainly of ethylene (40–60 wt %) and propylene (15–20 wt %).

2. At the same degree of conversion of carbon monoxide, the productivity of the iron–copper catalyst with respect to gaseous hydrocarbons

Table 1

Optimal synthesis conditions for obtaining the largest relative yield of gaseous hydrocarbons and ethylene

approximately one and a half times greater than that of the cobalt–thorium catalyst.

1. The ratio of the yields of olefinic hydrocarbons to paraffinic hydrocarbons for the catalysts tested falls very rapidly as the number of atoms in the hydrocarbon molecule increases.

2. The liquid synthesis product is practically identical for both catalysts and consists mainly of saturated and unsaturated aliphatic hydrocarbons.

REFERENCES

1. I. F. Bogdanov. Proceedings of the Institute of Fossil Fuels, Academy of Sciences of the USSR, XVIII, 1962.

2. H. Kölbel, H. Hammer, “Chemical and Process Engineering,” 42, 3, 1961.

3. H. Kölbel and Engelgardt, “Erdöl und Kohle,” 5, 1, 1952.

4. H. Kölbel, J. Kuschel and H. Hammer, “Liebigs. Ann. Chemie,” 632, 8, 1960.

5. N. V. Lavrov, V. V. Korobov, V. I. Filippova. Thermodynamics of Gasification and Gas-Synthesis Reactions. Publishing House of the Academy of Sciences of the USSR, Moscow, 1960.