EFFECT OF TEMPERATURE AND SPACE VELOCITY OF SYNTHESIS GAS ON THE YIELD OF SOME HYDROCARBONS IN THE SYNTHESIS FROM CARBON MONOXIDE AND WATER VAPOR

S. I. Smolyaninov, A. V. Kravtsov

(Recommended by the scientific-methodological seminar of the Chemical Technology Faculty)

The rapid progress of technology poses for chemistry a task of paramount importance—the creation of new chemical materials with predetermined properties that could successfully replace products made of metal and natural materials.

Such materials can be obtained only from homogeneous and pure feedstock; therefore, the question of the homogeneity and purity of the feedstock comes to the fore.

In synthesis from CO and water vapor, hydrocarbons of normal structure are formed (predominantly olefins), with a small proportion of oxygen-containing compounds and without admixtures of isomeric, naphthenic, aromatic, and other compounds. Therefore, the synthesis products can be successfully used as raw material for fine organic synthesis.

The work we carried out had the aim of investigating the effect of various conditions on the synthesis from carbon monoxide and water vapor over an industrial cobalt catalyst used at Plant No. 17 in Novocherkassk for the synthesis of organic compounds from CO + H₂.

As Kölbel and co-workers [1] and I. F. Bogdanov and co-workers [2] showed, under the given synthesis conditions a mixture of hydrocarbons of normal structure is formed practically exclusively, without admixture of other organic compounds. These authors did not investigate the yield of individual hydrocarbons.

In our work, we determined the dependence of the degree of conversion of carbon monoxide, the yield of hydrogen, carbon dioxide, methane, and gaseous gasoline in the final synthesis gas, and the change in the yield of individual hydrocarbons from C₁ to C₁₀ with changes in the synthesis conditions.

Experimental Part

The synthesis from carbon monoxide and water vapor was carried out over a cobalt–thorium catalyst in a reactor 1 m long, with taps along the length of the catalyst bed for taking gas samples for analysis and measuring the temperature. The analysis of the initial and final gases and of the reaction products was carried out by the chromatographic method.

In the synthesis from carbon monoxide and water vapor, the contact time of the feedstock with the catalyst has a substantial influence. We investigated the influence of space velocity on the course of the synthesis. Contraction,

the degree of conversion of carbon monoxide, the yield of carbon dioxide, gas gasoline, individualized hydrocarbons: ethylene, ethane, propylene, propane, butylene, butane, amylene, pentane, and the sum of hexene with hexane—falls rather rapidly with increasing space velocity, i.e., with decreasing contact time.

The calculated rate of the synthesis reaction \(r\left[\dfrac{\text{ml CO}}{\text{h},\,\text{l}}\right]\) also falls rapidly with increasing space velocity.

Temperature has an especially large effect on the synthesis. The synthesis was carried out in the temperature range 200–240°C. The lower boundary of the synthesis was determined by the low degree of conversion of carbon monoxide; the upper boundary, by the onset of vigorous methane formation. A change in synthesis temperature has a strong effect on all synthesis indices—the contraction and degree of conversion of carbon monoxide increase, the yield of carbon dioxide and gas gasoline rises, i.e., the yield of individual hydrocarbons with the number of carbon atoms from 1 to 10 increases (Fig. 1).

The reaction rate \(r\left[\dfrac{\text{ml CH}}{\text{h},\,\text{l}}\right]\) also increases with increasing reaction temperature, other conditions being equal.

When considering the yield of individual hydrocarbons, it becomes evident that, for gas gasoline, the yield of ethylene is predominant under all process conditions. The yield of ethylene is 3–4 times greater than the yield of any other individual hydrocarbon.

It should be noted that under all conditions of carrying out the synthesis, more olefins are always formed than paraffinic hydrocarbons, and

\[ K=\frac{\%\,\text{olef.}}{\%\,\text{paraff.}} \]

for a hydrocarbon with 2 carbon atoms is equal to 5–7. For subsequent hydrocarbons it decreases, becomes minimal for hydrocarbons with 5 or 7 carbon atoms, and then increases again. We shall try to clarify the reason for this below when considering the synthesis mechanism. The proportional ratio among the hydrocarbons formed usually remains constant for all cases. In studying the liquid synthesis products in the range of carbon atoms from 6 to 10 inclusive, the rule is an increase in the absolute percentage content with increasing number of carbon atoms up to \(C_8\) (both olefins and paraffins).

It has been established that the content of hydrocarbon \(C_{10}H_{22}\) on average corresponds to 6.2% of the total amount of liquid product, hydrocarbon \(C_9H_{20}\)—6.5%, hydrocarbon \(C_8H_{18}\)—5.0%, \(C_7H_{16}\)—1%, and \(C_6H_{14}\)—0.2%, likewise of the total amount of liquid product.

On the basis of the data we have obtained, one may make an assumption about the probable mechanism of the synthesis of organic compounds from

Fig. 1. Hydrocarbon content in the final gas as a function of synthesis temperature.
S.V. \(=25\ \text{h}^{-1}\).

—○—○— ethylene
—○—○— ethane
—×—×— amylene
—×—×— pentane
—△—△— butylene
—△—△— butane
—— hexane + hexene

carbon monoxide and water vapor. Unlike Kölbel [3], who believes that the reaction proceeds in two stages:

\[ \mathrm{CO}+\mathrm{H_2O}\to \mathrm{CO_2}+\mathrm{H_2}, \]

\[ \mathrm{CO}+2\mathrm{H_2}\to {<}\mathrm{CH_2}+\mathrm{H_2O}, \]

we assume that hydrocarbons are formed in a single step according to the equation:

\[ 3\mathrm{CO}+\mathrm{H_2O}\to {<}\mathrm{CH_2}+2\mathrm{CO_2}, \]

while the conversion reaction of carbon monoxide is a parallel one.

Experiments show that hydrocarbons are already formed at the first point after entering the reactor.

The mechanism of chain growth evidently proceeds by the gradual buildup of a chain of the methylene radical according to the scheme

\[ n\mathrm{CH_2}{<}\to 2\mathrm{C}_{n/2}\to \mathrm{C}_n, \]

and not according to the Crawford scheme

\[ n\mathrm{CH_2}{<}\to \mathrm{C}_n\to 2\mathrm{C}_{n/2}. \]

In favor of the first scheme is the increase in the yield of light hydrocarbons

\[ \frac{\mathrm{C_2}+\mathrm{C_3}+\mathrm{C_4}}{\sum \mathrm{C}_n} \]

with increasing space velocity. Moreover, the growth of the hydrocarbon chain most probably occurs by the addition not of a single methylene radical, whose lifetime is too short (owing to its high radiation capacity) for it to collide with this chain and react with it, but by the addition of the group \((-\mathrm{CH_2}-\mathrm{CH_2}-)\). This is why more hydrocarbons with 2, 4, and 6 carbon atoms are formed than hydrocarbons with 1, 3, and 5 carbon atoms.

We have already noted earlier that the olefin-to-paraffin ratio is highest for \(\mathrm{C_2}\), then decreases as the number of carbon atoms increases to 6 or 7, and then begins to rise again. This is apparently explained by the fact that ethylene formation is the most readily realized variant of the combination of methylene groups. Since hydrogen is present in the reaction mixture, there is a possibility for hydrogenation of the olefins formed into paraffins, and the longer the hydrocarbon chain, i.e., the more difficult it is for it to desorb from the catalyst, the greater this possibility. But starting with \(\mathrm{C_8}\) and onward, with ever-increasing sizes, olefin formation begins, i.e.,

\[ K=\frac{\%\ \text{olef.}}{\%\ \text{paraff.}}. \]

(Fig. 2).

According to thermodynamic calculations we carried out earlier, at synthesis temperatures, beginning with the hydrocarbon \(\mathrm{C_7H_{16}}\), the conversion dehydrogenation reaction of paraffins becomes probable; for example,

\[ \mathrm{C_8H_{18}}+2\mathrm{H_2O}\to \mathrm{C_7H_{14}}+\mathrm{CO_2}+4\mathrm{H_2}. \]

The occurrence of this reaction has been confirmed by us [5].

Thus, it becomes clear that the gradual increase in the yield of olefin relative to paraffin, starting with the hydrocarbon \(\mathrm{C_8}\), occurs as a result of the conversion dehydrogenation of the \(n\)-paraffins formed.

The rate of the carbon conversion reactions was calculated by the method of Kölbel [4]:

\[ r_{\mathrm{co}} = F_{\mathrm{co}}\cdot \frac{du}{dv}, \]

\[ \text{O.S.}=\frac{F_{\mathrm{co}}}{v}, \qquad \tau=\frac{1}{\text{O.S.}}, \]

\[ r_{\mathrm{co}}=\frac{du_{\mathrm{co}}}{d\tau}\left[\frac{\text{nl CO}}{\text{h}\cdot\text{l}}\right]. \]

To determine the reaction rate by this method, we plotted graphs of the dependence of the degree of conversion of carbon monoxide on contact time, \(U_{\mathrm{co}}=f(\tau)\), along the length of the catalyst bed. The reaction rate was determined as the tangent of the angle of inclination of the \(U_{\mathrm{co}}\) axis and the \(\tau\) axis. Since the slope of the straight line \(U_{\mathrm{co}}=f(\tau)\) for the given conditions remains constant along the length of the catalyst bed, the reaction rate under these conditions is also constant. This dependence remains constant for all operating temperatures and space velocities.

The value calculated in this way

\[ \left[\frac{\text{nl CO}}{\text{h}\cdot\text{l}}\right] \]

is a function of the space velocity and the synthesis temperature.

The graph \(r=f(\text{O.S.})\) at different temperatures shows that the rate of the carbon conversion reaction falls rapidly with increasing space velocity and increases by a factor of 1.5–2.5 when the temperature is raised by \(10^\circ\)C.

It was of interest to determine which stages in the course of the reaction are limiting.

Fig. 2. Change in \(K=\dfrac{\% \text{ olefin}}{\% \text{ paraffin}}\) as a function of the number of carbon atoms in the hydrocarbon molecule. Temp. \(240^\circ\)C, O.S. \(=25\ \text{h}^{-1}\).

We calculated the degree of conversion of carbon monoxide at constant linear velocities and plotted the dependence \(U_{\mathrm{co}}=f(\text{O.S.})\) at different linear velocities.

The points for all linear velocities fell on a single curve. This indicates that, for the investigated range of carbon conversion (from 0 to 40%), the reaction rate is determined by the kinetics on the catalyst surface and does not depend on the rate at which the initial feedstock is supplied.

Conclusions

1. The influence of space velocity and temperature on the yield of individualized hydrocarbons with the number of carbon atoms from 1 to 10 has been determined.

2. It has been shown that their yield increases with decreasing O.S. and with increasing temperature.

3. On the basis of analysis of the reaction products, an assumption has been made concerning the mechanism of polymerization of the methylene radical.

1. It has been explained that the gradual increase in the yield of olefin relative to paraffin, beginning with the hydrocarbon $C_8H_{18}$, occurs as a result of the conversion dehydrogenation of the n-paraffins formed.

REFERENCES

1. G. Kölbel, E. Vorwerk. Synthesis of hydrocarbons from carbon monoxide and water vapor. Communication 3. Chemistry and Chemical Technology, 11, 122—139, 1957.

2. I. F. Bogdanov, M. A. Samarskaya. Synthesis of organic compounds from carbon monoxide and water vapor on a cobalt catalyst. Transactions of the Institute of Fossil Fuels, Academy of Sciences of the USSR, XI, 91—99, 1959.

3. G. Kölbel, Fr. Engelhardt, G. Hammer. Synthesis of hydrocarbons from carbon monoxide and water vapor. Communication 7. Brennstoffchemie, 42, 3, 65—72, 1961.

4. G. Kölbel, G. Hammer. Synthesis of hydrocarbons from carbon monoxide and water vapor. Communication 5. Zeitschrift für Elektrochemie, 64.2, 224—234, 1960.

5. S. I. Smolyaninov, V. M. Mironov, A. V. Kravtsov, B. I. Odintsov. On certain secondary reactions occurring in synthesis from carbon monoxide and water vapor. Izv. TPI, 1965 (in press).