Butene dehydrogenation coke formation

Coke formation during the catalytic dehydrogenation of butene-1 has been studied in the temperature range 525-600 °C at butene-1 partial pressures of 0.05 to 0.25 bars. Moderate levels of coke deposits led to blocking of the catalyst mesopores and a hyperbolic deactivation function was found to provide the best fit to the data. Increase of temperature caused the deactivation to change from a parallel to a series coking process. 

However, the addition of an oxidant such as oxygen is not without some trade-off. To help solve the problem of catalyst deactivation due to carbon deposit in an alumina membrane reactor for dehydrogenation of butane, oxygen is introduced to the sweep gas, helium, on the permeate side at a concentration of 8% by volume. The catalyst service life increa.scs from one to four or five hours, but the selectivity to butene decreases from 60 to 40% at 480 C [Zaspalis et al., 1991b]. If oxygen is added to the feed stream entering the membrane reactor in order to inhibit coke formation, the butene selectivity decreases even more down to 5%. 

In Fig. 9 we sketch two process routes to isobutene. Starting from n-butane, isomerisation to isobutane is followed by dehydrogenation to isobutene. Both process steps have been widely operated commercially. The second step involves a capital intensive dehydrogenation reaction. Because of the intensity large plants are called for (economy of scale). An alternative and in principle less capital-intensive route is the skeletal isomerisation of n-butenes to isobutene. Previously this was not economically attractive because of the limited lifetime of the catalyst and the limited yield of isobutene. The latter drawbacks were due to the high temperatures applied previously (>400°C) and the related fast coke formation. 

COKE FORMATION IN THE DEHYDROGENATION OF 1-BUTENE INTO BUTADIENE... 

Dumez and Froment [1976] studied the dehydrogenation of 1-butene into butadiene in the temperature range of 480° to 630°C on a chromia/alumina catalyst containing 20 wt-% Cr20s and having a surface area of 57 mVg. The investigation dealt with the kinetics of both the main reaction and the coke formation. 

Gottifredi and Froment [1997] presented a straightforward and accurate semi-analytical solution for the concentration profiles inside a catalyst particle in the presence of coke formation. They applied the solution to the butene dehydrogenation dealt with here and obtained an excellent agreement with the profiles shown in Fig. 5.3.3.A-3. The method significantly simplifies and reduces the computational effort involved in reactor simulation and kinetic analysis. 

Balandin et al. (104) found that the chromia catalyst for the dehydrogenation of butene does not diminish its activity for a rather long time, in spite of the formation of coke. Since not dendrites but tar films are formed on the oxides, it was concluded that the molecules of the decomposition products migrate on the surface, setting free the active centers and accumulating on the inactive sites of the catalyst. 

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