The Importance and Scale of Fixed Bed Catalytic Processes

The discovery of solid catalysts and their application to chemical processes in the early years of this century has led to the breakthrough of chemical industry. Since these days, this industry has diversified and grown in a spectacular way, through the development of new or the rejuvenation of established processes, mostly based on the use of solid catalysts. 

The major part of these catalytic processes is carried out in fixed bed reactors. Some of the main fixed bed catalytic processes are listed in Table 11.1-1. Except for the catalytic cracking of gas oil, which is carried out in a fluidized bed to enable the continuous regeneration of the catalyst, the main solid catalyzed processes of today s chemical and petroleum refining industry appear in Table 11.1-1. However, there are also fluidized bed alternatives for phthalic anhydride— and ethylene dichloride synthesis. Furthermore, Table 11.1-1 is limited to fixed bed processes with only one fluid phase trickle bed process (e.g., encountered in the hydrodesulfurization of heavier petroleum fractions) are not included in the present discussion. Finally, important processes like ammonia oxidation for nitric acid production or hydrogen cyanide synthesis, in which the catalyst is used in the form of a few layers of gauze are also omitted from Table 11.1-1. 

Ethylene oxide Ethylene didiloride Vinylacetate Butadiene Maleic anhydride Phthalic anhydride Cyclohexane Styrene 

F ure II.I-I Growth curves of reactor capacity in ammonia and phthalic anhydride synthesis from Froment [148] data from [l]and[6]). 

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Fixed-bed reactors are used for testing commercial catalysts of larger particle sizes and to collect data for scale-up (validation of mathematical models, studying the influence of transport processes on overall reactor performance, etc.). Catalyst particles with a size ranging from 1 to 10 mm are tested using reactors of 20 to 100 mm ID. The reactor diameter can be decreased if the catalyst is diluted by fine inert particles the ratio of the reactor diameter to the size of catalyst particles then can be decreased to 3 1 (instead of the 10 to 20 recommended for fixed-bed catalytic reactors). This leads to a lower consumption of reactants. Very important for proper operation of fixed-bed reactors, both in cocurrent and countercurrent mode, is a uniform distribution of both phases over the entire cross-section of the reactor. If this is not the case, reactor performance will be significantly falsified by flow maldistribution. 

The catalyst makes contact with melted MWP. Good contact between plastic particles and the catalyst is one of the key points for process development. Melted plastics can be degraded in a fluidized-bed reactor or a fixed-bed reactor. Since the usage of fixed beds leads to problems of blockage, scale-up to industrial size is not feasible. However the fluidized bed has a number of special advantages for catalytic degradation of plastics, because it is characterized by a good contact between catalyst and plastics as well as an excellent heat and mass transfer [4], In addition to selection of a snitable reactor, the catalyst used is very important in the process. 

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