On Some Secondary Reactions Occurring in Synthesis from Carbon Monoxide and Steam

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

(Recommended by the Scientific-Methodological Seminar of the CTF)

The accelerated development of the organic-synthesis industry imposes special requirements on the purity of the feedstock used. These requirements are most stringent with respect to the sulfur content, since sulfur is known to be the most active contact poison.

At present, the most promising feedstock for organic synthesis is petroleum-refining products. However, despite their low cost and large reserves, petroleum products have a number of disadvantages. These include: an increase in cost due to sulfur removal and the difficulty of isolating individual compounds from the mixture (especially the separation of normal and iso compounds).

In addition, petroleum products cannot supply the organic-synthesis industry with inexpensive acyclic oxygen-containing compounds.

From the standpoint of eliminating some of the disadvantages described, it is of considerable interest to consider the process of synthesizing organic compounds from carbon monoxide and steam. This synthesis, discovered a little more than ten years ago on the basis of a body of work on synthesis from carbon monoxide and hydrogen [1], differs from the latter by the absence of costly hydrogen in the process, by a relatively high yield of unsaturated hydrocarbons (up to 80% α-olefins) and oxygen-containing organic compounds.

The process proceeds rather smoothly and in one stage according to the general hypothetical equation

\[ 3\mathrm{CO} + \mathrm{H}_2\mathrm{O} = (-\mathrm{CH}_2-) + 2\mathrm{CO}_2 \tag{1} \]

with an overall degree of conversion of carbon monoxide of 97–98%, at temperatures of about 210–260°C, with a maximum product yield of up to 250 g/nm\(^3\) CO. Increasing the pressure has a favorable effect on the yield of oxygen-containing organic compounds and suppresses the side reaction of methane formation [2]. Until now, all studies of this synthesis have been carried out on standard catalysts for synthesis from carbon monoxide and hydrogen.

The ratio of the yields of the various classes of organic compounds depends on the conditions under which the process is carried out and on the catalyst used [1, 3, 4].

The most characteristic advantage of synthesis from carbon monoxide and steam is the independence of the quantitative yield of pro-

products on the content of inert impurities in the synthesis gas. This makes it possible to use, as feedstock for the synthesis, waste gases from industrial plants and gases with a low carbon monoxide content, for example blast-furnace gas [5], underground coal-gasification gas [6], gas from chemical-metallurgical processes, etc. When waste gases from metallurgical plants are used, the final product contains no sulfur compounds.

The most appreciable drawback of the process described is the multicomponent nature of its products. In the course of synthesis the following normally structured organic compounds are obtained: saturated and unsaturated hydrocarbons, alcohols, aldehydes, ketones, carboxylic acids, and esters.

Effective separation of such a spectrum of products is difficult and, possibly, will not be economically justified. It should be noted that all compounds of the classes described are represented in the synthesis products from \(C_1\) to \(C_{20+}\).

At present there is only one way to eliminate this drawback: to determine the conditions for directed synthesis from carbon monoxide and steam, with the aim of obtaining individual classes of organic compounds.

Some conclusions concerning this direction of work can be drawn from a study of the mechanism of the process.

The mechanism proposed by G. Kölbel and coworkers [7] and by I. F. Bogdanov [8] for the formation of the methylene radical in synthesis from carbon monoxide and steam is based on the formal summation of consecutive reactions according to the scheme

\[ \mathrm{CO} + \mathrm{H_2O} = \mathrm{CO_2} + \mathrm{H_2} \tag{2} \]

\[ + \]

\[ 2\mathrm{CO} + \mathrm{H_2} = (-\mathrm{CH_2}-) + 2\mathrm{CO_2} \tag{3} \]

\[ \overline{3\mathrm{CO} + \mathrm{H_2O} = (-\mathrm{CH_2}-) + 2\mathrm{CO_2}} \tag{1} \]

It is assumed here that the rate of reaction 2 is much lower than the rate of reaction 3; however, the yield of hydrogen as a by-product in the synthesis is attributed to reaction 2.

In investigating the influence of hydrodynamic conditions on the synthesis process proceeding in the diffusion region [9], it is found that the amount of hydrogen in the final gas decreases, while the overall contraction of carbon monoxide increases as the diffusion complications are removed. From the standpoint of diffusion kinetics [10], such an effect could be explained by a sequence in the course of the reactions for the formation of hydrocarbons and hydrogen according to the scheme: \(\mathrm{CO} + \mathrm{H_2O} \rightarrow (-\mathrm{CH_2}-) \rightarrow\) hydrocarbons \(\rightarrow\) hydrogen. On this basis, one may assume a “complex” course of the reaction of methylene-radical formation according to equation 1, followed by conversion of the hydrocarbons with steam according to the reaction

\[ \mathrm{C_nH_{2n}} + 2n\mathrm{H_2O} = n\mathrm{CO_2} + (4n + 2)\mathrm{H_2}. \tag{4} \]

It may also be assumed that, in the synthesis process, a reaction of “conversion dehydrogenation” of hydrocarbons proceeds according to the scheme

\[ \mathrm{C_nH_{2n+2}} + 2\mathrm{H_2O} = \mathrm{CO_2} + \mathrm{C_{n-1}H_{2n}} + 3\mathrm{H_2}. \tag{5} \]

The conversion of hydrocarbons with steam at high temperatures (800–1000°C) has long been known [11]. However, it was of interest to find out how this reaction can proceed under the conditions of synthesis from carbon monoxide and steam.

In this connection, the thermodynamic probability of conversion of certain hydrocarbons under synthesis conditions was calculated. Since in the work of N. V. Lavrov and co-workers [12] the values of the \(\Phi^*\)-potentials and the standard changes in heat content for gasification reactions and synthesis from gases were calculated and reported, we used, in calculating the equilibrium constants of the reactions, the formula

\[ RT \lg K_p = -\Delta Z_T^0 = T \cdot \Delta \Phi^* - \Delta H_0^0 \]

\[ \lg K_p = \left( \Delta \Phi^* - \frac{\Delta H_0^0}{T} \right) \cdot \frac{1}{4.5756}, \]

where \(R\) is the gas constant;
\(T\) is the thermodynamic temperature, \({}^\circ\mathrm{K}\);
\(K_p\) is the equilibrium constant of the reaction;
\(\Delta H_0^0\) is the change in heat content;
\(\Delta \Phi^*\) is the change in the statistical thermodynamic function

\[ \Phi^* = - \frac{Z^0 - H_0^0}{T}. \]

The reactions for which the logarithm of the equilibrium constant was calculated are given in Table 1. The values of the logarithms of the equilibrium constants of these reactions are presented in Table 2.

The data of Table 2 show that, at synthesis temperatures, the thermodynamic probability of the occurrence of hydrocarbon conversion reactions with steam is insignificant and increases with an increase in the number of carbon atoms in the molecule of the compound; the reaction of complete conversion of hydrocarbons should proceed much more readily than partial conversion (“conversion dehydrogenation reaction”); the probability of the occurrence of the reaction of hydrocarbon conversion with steam through the successive interaction of water molecules with \(\mathrm{CH}_2\) groups is higher than such interaction as a result of rupture of the hydrocarbon chain at several points.

For experimental verification, experiments were carried out on the conversion of normal octane over an iron–copper catalyst for synthesis from carbon monoxide and hydrogen under the conditions of synthesis of organic compounds from carbon monoxide and steam. Nitrogen purified from traces of oxygen was used as the carrier gas. The process indicators were the yield of carbon dioxide, the yield of hydrogen, and the change in the iodine and acid numbers of the liquid condensation products.

The results of the experiments, presented in Figs. 1, 2, and 3, showed that when the process temperature is raised, the yield of carbon dioxide and hydrogen increases and the iodine number of the condensate rises, which is in full agreement with calculations of thermodynamic parameters. The same effect is also observed when the space velocity of the carrier gas is decreased. This can be explained by an increase in the depth of hydrocarbon decomposition with increasing contact time. The maximum value of unsaturation of the oil layer of the condensate, calculated as octene, was slightly more than 3%. The acid number of the aqueous layer of the condensate remained practically constant under all changes in the reaction conditions.

Conclusions

1. The thermodynamic probability of the occurrence of certain reactions of hydrocarbon conversion with steam at various temperatures has been calculated theoretically.

Table 1

Table 2

| Reaction No. | \multicolumn{8}{c}{Values of \(K_p\) at temperatures, °K} |
|---:|---:|---:|---:|---:|---:|---:|---:|---:|
| | 300 | 400 | 500 | 600 | 700 | 900 | 1100 | 1300 |
| 1 | −24,66 | −14,22 | −7,87 | −3,90 | −0,72 | 3,69 | 6,39 | 8,24 |
| 2 | −61,99 | −31,90 | −11,79 | −1,43 | 8,00 | 20,18 | 28,28 | 34,08 |
| 3 | −106,69 | −54,72 | −28,82 | 1,27 | 17,63 | 23,61 | 55,20 | 65,15 |
| 4 | −154,21 | −77,69 | −27,92 | 4,07 | 25,73 | 61,55 | 83,21 | 98,27 |
| 5 | −195,40 | −95,52 | −37,91 | 6,28 | 36,94 | 77,35 | 106,95 | 124,10 |
| 6 | −24,50 | −14,40 | −7,90 | −3,80 | −0,87 | 3,70 | 6,40 | 8,30 |
| 7 | −63,80 | −31,90 | −14,00 | −1,30 | 8,03 | 21,30 | 28,30 | 34,00 |
| 8 | −107,00 | −53,60 | −20,80 | 1,96 | 17,70 | 41,50 | 56,30 | 66,00 |
| 9 | −142,10 | −71,50 | −28,30 | 1,53 | 19,50 | 51,10 | 71,20 | 83,40 |
| 10 | −150,00 | −77,00 | −28,80 | 2,20 | 23,60 | 58,00 | 85,30 | 93,40 |
| 11 | −26,58 | −15,67 | −9,97 | −4,52 | −1,48 | 3,10 | 5,23 | 7,81 |
| 12 | −45,00 | −24,30 | −11,80 | −3,10 | 3,10 | 11,60 | 16,00 | 20,70 |
| 13 | −71,50 | −37,00 | −16,00 | −1,50 | 18,40 | 24,00 | 33,60 | 39,80 |
| 14 | −95,00 | −52,00 | −21,40 | −2,60 | 11,00 | 30,00 | 43,00 | 50,50 |
| 15 | −103,00 | −52,00 | −21,20 | 0,44 | 15,70 | 37,60 | 49,00 | 61,00 |
| 16 | −34,60 | −13,00 | −7,90 | −3,50 | −0,66 | 3,70 | 6,60 | 8,30 |
| 17 | −42,70 | −22,30 | −10,50 | −1,97 | 3,70 | 12,20 | 15,70 | 21,40 |
| 18 | −62,60 | −32,00 | −14,00 | −1,30 | 7,40 | 20,00 | 28,40 | 33,70 |
| 19 | −86,50 | −45,50 | −19,00 | −1,97 | 11,00 | 28,60 | 40,00 | 48,00 |
| 20 | −24,40 | −16,00 | −7,95 | −3,60 | −0,76 | 3,60 | 6,40 | 8,30 |
| 21 | −34,00 | −18,70 | −11,30 | −3,06 | 1,50 | 7,60 | 12,00 | 15,50 |
| 22 | −47,00 | −26,20 | −13,80 | −4,37 | −2,00 | 10,60 | 16,00 | 20,00 |
| 23 | −56,00 | −29,00 | −12,90 | −1,20 | 6,50 | 17,20 | 24,50 | 29,50 |
| 24 | −27,50 | −16,50 | −9,60 | −5,15 | −2,20 | 2,56 | 5,50 | 7,40 |
| 25 | −37,60 | −20,20 | −10,10 | −3,02 | 1,78 | 8,50 | 12,90 | 15,90 |
| 26 | −27,50 | −15,80 | −8,50 | −3,63 | 0,00 | 5,00 | 8,40 | 10,40 |
| 27 | −19,00 | −12,50 | −7,90 | −4,85 | −2,86 | 0,46 | 2,44 | 3,84 |
| 28 | −40,32 | −21,59 | −9,88 | −2,45 | 3,27 | 12,59 | 15,81 | 19,72 |
| 29 | −69,85 | −33,81 | −14,48 | 2,73 | 12,43 | 28,45 | 37,72 | 53,65 |
| 30 | −111,58 | −64,10 | −31,40 | −9,23 | 6,26 | 27,28 | 41,35 | 50,10 |
| 31 | −56,10 | −29,60 | −12,70 | −1,30 | 6,90 | 17,50 | 24,60 | 29,30 |
| 32 | −31,30 | −17,00 | −8,50 | −2,80 | −0,60 | 6,60 | 10,30 | 12,70 |
| 33 | −81,00 | −45,30 | −24,00 | −10,00 | 0,22 | 13,80 | 22,00 | 28,00 |
| 34 | −40,50 | −21,80 | −10,50 | −2,60 | 3,00 | 10,70 | 15,70 | 20,80 |
| 35 | −63,00 | −35,00 | −18,50 | −6,80 | 1,30 | 12,00 | 19,00 | 25,00 |
| 36 | −36,00 | −21,00 | −11,80 | −5,70 | −1,50 | 4,60 | 8,40 | 11,00 |

Fig. 1. Dependence of the carbon dioxide yield on the space velocity of the synthesis gas at different process temperatures.
Curve 1 — 280°C,
Curve 2 — 240°C,
Curve 3 — 220°C

Fig. 2. Dependence of the iodine number of the oily condensate layer on the synthesis temperature.
Curve 1 — synthesis-gas space velocity 16 h\(^{-1}\),
Curve 2 — synthesis-gas space velocity 30 h\(^{-1}\),
Curve 3 — synthesis-gas space velocity 50 h\(^{-1}\),

Fig. 3. Dependence of the hydrogen yield on the space velocity of the synthesis gas at different process temperatures.
Curve 1 — 210°C,
Curve 2 — 220°C,
Curve 3 — 230°C,
Curve 4 — 250°C

1. The possibility of converting hydrocarbons with steam under the conditions of synthesis from carbon monoxide and steam into carbon dioxide, hydrogen, and unsaturated hydrocarbons has been experimentally confirmed.

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