Reactor design transport-limited operation

Although the hydrogenation activity of metal sulfides is lower by several orders of magnitude than that of metal catalysts, sulfides allow operations under conditions that are impractical for metals. They are generally used as highly dispersed materials on a high surface area support, such as y-alumina, in fixed bed operation. Most important is catalyst design to minimize deactivation due to the deposition of metals (V, Ni) in the feed and of coke at the mouths of the pores. Metal sulfides can also be used as finely dispersed phases in continuous slurry reactors to reduce the mass transport limitations of heavy oils. 

Low coke concentration is desirable in order to maintain satisfactory activity in the reactor and to facilitate operation of the regenerator. Unfortunately, however, catalyst circulation in the TCC units was limited by the bucket elevators. This drawback was not overcome until the development of the gas-lift system for transporting catalyst, as adopted in the Houdriflow and air-lift TCC designs. 

The other common reactor type is a cold-wall reactor. Here only the substrate is heated, and the gas in the forced convection region as well as the reactor walls are considerably colder than the substrate. This design has limited capacity the substrate is usually coated on one face only and although uniform gas ffow to the substrate is easier to control, heating the substrate is relatively difficult. These reactors are usually operated under mass transport limited conditions. Heating is accomplished in one of four ways ... 

The kinetics of the reaction and the properties of the catalyst, especially the thermal stability, will further narrow the range of possible reaction conditions and define a "window" of possible operating parameters. Process optimization, energy efficiency, and safety aspects will then determine at what conditions within the "window" the reactor should operate to give the optimum result. And then mathematical models are used to determine how big the reactor must be to obtain the performance (conversion and pressure drop) determined by the process optimization. Instrumentation is then considered, proper materials of construction are selected, catalyst loading and unloading is considered, possible transport limitations are determined, workshop manufacture is considered, and at last the design of the reactor is completed. The procedure is, of course, iterative since the reactor cost is one of the parameters in the economical optimization, but, as mentioned above, often a factor of minor importance for the overall result. 

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