Diffusion Complications in the Synthesis of Organic Compounds from Carbon Monoxide and Water Vapor under Medium Pressure
Abstract Generated abstract
This paper examines how diffusion limitations affect the synthesis of organic compounds from carbon monoxide and water vapor at elevated pressure. Using an iron, copper precipitated catalyst, the authors compare experiments at atmospheric pressure and 10 atm, evaluating carbon monoxide conversion to hydrocarbons and hydrogen as functions of fictitious contact time, temperature, and selectivity. The analysis of apparent activation energies and temperature dependence of selectivity indicates that, while the atmospheric pressure process is kinetic at lower temperatures and diffusion-influenced at higher temperatures, operation at 10 atm shows stronger diffusional inhibition across the studied range. The study concludes that increasing pressure enhances the role of diffusion transport in controlling reaction rates and selectivity.
Full Text
DIFFUSION COMPLICATIONS IN THE PROCESS OF SYNTHESIS OF ORGANIC COMPOUNDS FROM CARBON MONOXIDE AND WATER VAPOR UNDER MEDIUM PRESSURE
S. I. Smolyaninov, V. M. Mironov, V. A. Kornyakov
(Presented by the organic section of the KhTF seminar)
An increase in the total pressure favorably affects the yield and composition of the products of synthesis from carbon monoxide and water vapor. The very first studies of this process showed [1, 2, 3] that increasing the working pressure in the range from 0 to 100 atm leads to an increase in the total yield of reaction products from 180 to 216 g/nm³ of processed carbon monoxide. In addition, increasing the pressure in the indicated region raises the yield of oxygen-containing organic compounds from 0.1 to 30%. In this connection, the question of the influence of the diffusion rate on the activity and selectivity of the synthesis process from carbon monoxide and water vapor at elevated pressures is of great interest.
Investigation of the influence of diffusion retardation on the synthesis at atmospheric pressure [4] revealed the existence of this influence on the overall rate of conversion of carbon monoxide. It is known that increasing the pressure decreases the coefficient of molecular diffusion of gases [5]; therefore it may be assumed that an increase in the total pressure in the synthesis process will lead to a higher degree of influence of the conditions of diffusion transport of the reactants on the rate of the process. However, in the work of G. Kölbel and G. Hammer [6], on the basis of insufficiently substantiated evidence, the influence of diffusion conditions of the initial substances on the reaction rate is denied. In this connection we have processed the results available to us from studies on synthesis from carbon monoxide and water vapor in light of a possible determination of the influence of the rate of diffusion of the reactants on the overall indices of the indicated process.
The studies were carried out on an iron–copper precipitated catalyst; the bed length was 32 cm. The catalyst was reduced and conditioned according to the known procedure [7]. After conditioning of the catalyst, two series of experiments were conducted: at atmospheric pressure and at a pressure of 10 ata.
As the initial feedstock, a gas containing not less than 96% carbon monoxide was used. Analysis of the initial and final gas was carried out on a KhT-2M chromatograph (adsorbent—AG-3 charcoal, fraction 0.25–0.5 mm; column length 3 m, diameter—6 mm; carrier-gas flow rate—100 ml/min). From the gas-analysis data and the gas flow rate at the inlet and outlet, the degrees of conversion of carbon monoxide to hydrocarbons \((Y_{\mathrm{CH}_2})\) and hydrogen \((Y_{\mathrm{H}_2})\) were calculated.
Kinetic plots, one of which is presented in Fig. 1, were constructed from the dependence of the degree of conversion of carbon monoxide on the fictitious reaction (contact) time. This quantity \((\omega)\) characterizes the residence time of the reactants in the catalysis zone under the condition that there is no diffusive removal of components of the reaction mixture. Such an assumption is quite possible in our case, since according to J. Carberry [8]
\[ N=\frac{l}{d}=\frac{32}{0.3}=107>20, \]
where
\(l\) — length of the catalyst bed, cm;
\(d\) — largest diameter of a catalyst grain, cm.
Then
\[ \omega=\frac{3600}{v_0}\cdot p, \]
where
\(\omega\) — fictitious reaction time, sec;
\(v_0\) — volumetric velocity, hour\(^{-1}\);
\(p\) — total pressure, atm.
It is seen from Fig. 1 that, over the range of volumetric velocities studied, the degree of conversion of carbon monoxide to hydrocarbons increases linearly with increasing contact time \((\omega)\). The angle of inclination of the straight lines to the abscissa axis is proportional to the rate of the reaction forming organic compounds. Calculating the tangent of this angle gives the reaction rate with the dimension sec\(^{-1}\). From these values a plot was constructed in coordinates \(\ln \omega\)—\(1/T\), where \(\omega\) is the reaction rate and \(T\) is the temperature, shown in Fig. 2. From this figure it is seen that at low temperatures (200–215°C), at atmospheric pressure (solid line), the synthesis reaction proceeds in the kinetic region, while at high temperatures (215–250°C) it passes into the diffusion region. For the reaction under pressure (dashed line), a stronger dependence of the process on diffusion conditions is characteristic, since the degree of dependence of the process rate on temperature in this case is lower than at atmospheric pressure. From the tangent of the angle of inclination of the straight lines to the abscissa axis, the apparent activation energy for hydrocarbon formation could be determined. For atmospheric pressure in the temperature interval 200–215°C it is equal to 52 kcal/mol; for temperatures of 215–250°C, respectively, 8.8 kcal/mol; and for a pressure of 10 atm over the investigated temperature range, 6.6 kcal/mol.
Fig. 1. Dependence of the degree of conversion of carbon monoxide to hydrocarbons on the fictitious reaction time.
—— atmospheric pressure,
- - - pressure 10 atm, 1—200°C,
2—215°C, 3—225°C, 4—230°C, 5—250°C.
The degree of conversion of carbon monoxide to hydrogen reaches a maximum as a function of the fictitious reaction time. This dependence basically does not differ from that obtained earlier [6]. More interesting, from the standpoint of proving the transition of the reaction into the diffusion region, is...
with increasing pressure, the dependence of the selectivity of the process with respect to the principal reaction
\[ 3\mathrm{CO}+\mathrm{H}_2\mathrm{O}=(-\mathrm{CH}_2-)+\mathrm{CO}_2 \tag{1} \]
relative to the hydrogen-formation reaction
\[ \mathrm{CO}+\mathrm{H}_2\mathrm{O}=\mathrm{H}_2+\mathrm{CO}_2. \tag{2} \]
In this case the selectivity is expressed as follows:
Fig. 2. Dependence of the rate of hydrocarbon formation on temperature.
— atmospheric pressure; – – – pressure 10 ata
\[ \xi=\frac{Y_{\mathrm{CH}_2}}{Y_{\mathrm{H}_2}}. \]
At the same time
\[ \xi=\frac{Z_1}{Z_2}\cdot e^{\frac{\Delta E}{RT}}, \tag{3} \]
where
\[ \Delta E=E_2-E_1, \qquad \text{and} \]
\(E_1\) is the activation energy of the principal reaction;
\(E_2\) is the activation energy of the side reaction.
When \(\Delta E<0\), selectivity increases with increasing temperature, while the rate of its increase is determined by the degree to which diffusion processes affect the reactions being compared: the greater it is, the slower the increase in selectivity with rising temperature.
Taking the logarithm of (3):
\[ \ln \xi=\ln Z_1-\ln Z_2+\frac{\Delta E}{R}\cdot\frac{1}{T}. \]
Figure 3 presents plots of the dependence of $\ln \xi$ on $1/T$, where $\xi$ is the selectivity. It is seen from the plots that, at elevated pressure, the selectivity depends only very weakly on temperature; that is, the difference between the activation energies of the main and side reactions, determined by the tangent of the angle of inclination of the straight lines to the abscissa axis, is small. At atmospheric pressure
Fig. 3. Dependence of the selectivity of the synthesis process with respect to the main reaction on temperature.
— atmospheric pressure, ------ pressure 10 ata. Fictitious reaction time:
1—100 sec; 2—200 sec; 3—300 sec
this difference is much greater. Thus, with increasing pressure, the process of synthesis from carbon monoxide and water vapor passes into a region where the rates of both reactions being compared are largely determined by the rate of diffusion of the reactants and reaction products.
Thus, the experimental data on the synthesis of organic compounds from carbon monoxide and water vapor at elevated pressure make it possible to conclude that increasing the pressure increases the degree of diffusional inhibition of the rate of the process under investigation.
REFERENCES
- H. Kölbel, F. Engelhardt. Synthesis of hydrocarbons and oxygen-containing compounds from water and carbon monoxide. Erdöl und Kohle, 5, No. 1, 1–9, 1952.
- H. Kölbel, F. Engelhardt. Synthesis of hydrocarbons and oxygen-containing compounds from carbon monoxide and water vapor. Brennstoff-Chem., 32, 150, 1951.
[3] H. Kölbel, F. Engelhardt. Synthesis of hydrocarbons from carbon monoxide and water. Angew. Chem., 64, No. 2, 54–58, 1952.
-
S. I. Smolyaninov, V. M. Mironov, A. V. Kravtsov. The influence of hydrodynamic conditions on the synthesis of organic compounds from carbon monoxide and water vapor. Khimiya i tekhnol. topliv i masel, No. 8, 12–16, 1962.
-
E. Wheeler. Reaction rates and selectivity in catalyst pores. In: Catalysis. Problems of Theory and Methods of Investigation, pp. 479–563, Izd. IL, Moscow, 1955.
-
H. Kölbel, H. Hammer. Synthesis of hydrocarbons from carbon monoxide and water vapor. Communication V. Synthesis on cobalt and iron catalysts. Z. Elektrochem. Ber. Bunsenges. Phys. Chem., 64, 224–234, 1960.
-
N. V. Lavrov, M. A. Samarskaya. Organic synthesis from carbon monoxide and water vapor. Trudy IKHI AN SSSR, 11, 100–104, 1959.
-
J. Carberry. Transfer processes and heterogeneous catalysis. 44, No. 6, 306–315, 1963.