Dehydrogenation continuous catalyst regeneration

I. G. Farben also produced butene by butane dehydrogenation. A moving catalyst bed system was used in a tubular reactor. The total catalyst charge was 1.5 tonnes with a residence time of 4 h in the tubes. Yields of 85% at 20-25% conversion were obtained at a liquid space velocity of 2 h and 620°-650°C operating temperature. This was an impressive result for a new reactor design that has now been developed as the continuous catalyst regeneration process and is widely used in refineries. 

Dehydrogenation of /i-Butane. Dehydrogenation of / -butane [106-97-8] via the Houdry process is carried out under partial vacuum, 35—75 kPa (5—11 psi), at about 535—650°C with a fixed-bed catalyst. The catalyst consists of aluminum oxide and chromium oxide as the principal components. The reaction is endothermic and the cycle life of the catalyst is about 10 minutes because of coke buildup. Several parallel reactors are needed in the plant to allow for continuous operation with catalyst regeneration. Thermodynamics limits the conversion to about 30—40% and the ultimate yield is 60—65 wt % (233). 

Only a relatively small fraction of the carbon laydown on the surface can be removed by high temperature dioxygen treatment. After regeneration carbon continues to build up on the catalyst surface in subsequent propane dehydrogenation reactions. 

Entrained catalyst is removed from the product off-gas by means of cyclones. The catalyst circulates continuously from the reactor to the regenerator and vice versa by means of transfer lines. Coke deposited on the catalyst is burnt off in the regenerator however, because the amount of coke is relatively small, additional fuel must be burnt in the regenerator to satisfy the thermal requirements of the endothermic dehydrogenation reaction. However, while this approach is similar to that in the Houdry process, FED does not have a catalyst reduction step with hydrogen before proceeding to the dehydrogenation cycle lack of this step is believed to be somewhat detrimental to the overall performance of the process. 

Design a reactor system to produce styrene by the vapor-phase catalytic dehydrogenation of ethyl benzene. The reaction is endothermic, so that elevated temperatures are necessary to obtain reasonable conversions. The plant capacity is to be 20 tons of crude styrene (styrene, benzene, and toluene) per day. Determine the bulk volume of catalyst and number of tubes in the reactor by the one-dimensional method. Assume that two reactors will be needed for continuous production of 20 tons/day, with one reactor in operation while the catalyst is being regenerated in the other. Also determine the composition of the crude styrene product. 

By temperature compensations, such results can be sustained for periods of a few hundred hours. However, a slow continuing deactivation occurs, and eventually the catalyst must be regenerated by combustion of the deposited coke. After regeneration, reasonable dehydrogenation results are again obtained after a rapid initial deactivation of the catalyst. Steam was found to be a particularly useful diluent with this type of catalyst. In contrast to this, the best results with molybdena-alumina for catalytic reforming are obtained over fresh catalyst and when steps such as dilution with hydrogen are employed to repress the initial rapid deactivation. 

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