Butene dehydrogenation, coking

Figure I Butene dehydrogenation. Coke content of catalyst as a fimction of time in thermobalance experiment.

Fig 13. Evolution of the coke profile in an adiabatic reactor for butene dehydrogenation [Ref 331 ... 

Figure I Butene dehydrogenation. Partial pressure and coke profiles inside a catalyst particle. Parallel-consecutive coking mechanism and inhibition by hydrogen.

Figure 2 Coke content profiles in ]-butene dehydrogenation reactor.

The data required for a kinetic formulation of the deactivation of the main reaction are probably best collected in a differential reactor. Some extrapolation to zero time is required when the reaction rate of the main reaction cannot be observed at zero coke content. The procedure can be hazardous with very fast coking, of course. In their study of butene dehydrogenation, Dumez and Froment [1976] were able to take samples of the exit stream of stabilized operation of the fixed bed reactor after 2 minutes, while the observations extended over more than 30 minutes. 

Butene dehydrogenation. Partial pressure, rate, and coke profiles inside a catalyst particle located at the reactor inlet for zero process time and after 0.25 h. Parallel-consecutive coking mechanism and inhibition by hydrogen. From Dumez and Froment [1976]. 

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. 

Marin GB, Beeckman JW, Froment GF Rigorous kinetic-models for catalyst deactivation by coke deposition—apphcation to butene dehydrogenation, J Catal 97(2) 416—426, 1986. 

Pena JA, Monzon A, Santamaria J Deactivation by coke of a Cr203/A1203 catalyst during butene dehydrogenation, J Catal 142(l) 59-69, 1993. 

Dumez, F.J. and G.F. Froment, "Dehydrogenation of 1-Butene into Butadiene. Kinetics, Catalyst Coking, and Reactor Design", Ind Eng. Chem. Proc. Des. Devt., 15,291-301 (1976). 

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%. 

Table 8-6 lists poisons for various catalysts and reactions. The materials that are added to reactant streams to improve the performance of a catalyst are called accelerators. They are the counterparts of poisons. For example, steam added to the butene feed of a dehydrogenation reactor appeared to reduce the amount of coke formed and increase the yield of butadiene. The catalyst in this case was iron. ... 

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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