Support fluidized beds

The basic fluid-bed unit consists of a refractory-lined vessel, a perforated plate that supports a bed of granular material and distributes air, a section above the fluid bed referred to as freeboard, an air blower to move air through the unit, a cyclone to remove all but the smallest particulates and return them to the fluid bed, an air preheater for thermal economy, an auxiUary heater for start-up, and a system to move and distribute the feed in the bed. Air is distributed across the cross section of the bed by a distributor to fluidize the granular soflds. Over a proper range of airflow velocities, usually 0.8-3.0 m/s, the sohds become suspended in the air and move freely through the bed. 

Heterogeneous hydrogenation catalysts can be used in either a supported or an unsupported form. The most common supports are based on alurnina, carbon, and siUca. Supports are usually used with the more expensive metals and serve several purposes. Most importandy, they increase the efficiency of the catalyst based on the weight of metal used and they aid in the recovery of the catalyst, both of which help to keep costs low. When supported catalysts are employed, they can be used as a fixed bed or as a slurry (Uquid phase) or a fluidized bed (vapor phase). In a fixed-bed process, the amine or amine solution flows over the immobile catalyst. This eliminates the need for an elaborate catalyst recovery system and minimizes catalyst loss. When a slurry or fluidized bed is used, the catalyst must be separated from the amine by gravity (settling), filtration, or other means. 

Activated alumina and phosphoric acid on a suitable support have become the choices for an iadustrial process. Ziac oxide with alumina has also been claimed to be a good catalyst. The actual mechanism of dehydration is not known. In iadustrial production, the ethylene yield is 94 to 99% of the theoretical value depending on the processiag scheme. Traces of aldehyde, acids, higher hydrocarbons, and carbon oxides, as well as water, have to be removed. Fixed-bed processes developed at the beginning of this century have been commercialized in many countries, and small-scale industries are still in operation in Brazil and India. New fluid-bed processes have been developed to reduce the plant investment and operating costs (102,103). Commercially available processes include the Lummus processes (fixed and fluidized-bed processes), Halcon/Scientific Design process, NIKK/JGC process, and the Petrobras process. In all these processes, typical ethylene yield is between 94 and 99%. 

The violent motion of a fluidized bed requires ample foundations and sturdy supporting struc ture for the reactor. Even a relatively small differential movement of the reactor shell with the lining will materially shorten refractoiy life. The lining and shell must be designed as a unit. Struc tural steel should not be supported from a vessel that is sub-jec t to severe vibration. 

Recently, a new concept in fixed film reac tors that uses an expanded or fluidized bed of particdes as the biomass support medium has been introduced. This reactor type can easily handle both low- and high-strength wastes with most electron acceptors. It will be discussed in detail in a later section. 

The results of Massimilla et al., 0stergaard, and Adlington and Thompson are in substantial agreement on the fact that gas-liquid fluidized beds are characterized by higher rates of bubble coalescence and, as a consequence, lower gas-liquid interfacial areas than those observed in equivalent gas-liquid systems with no solid particles present. This supports the observations of gas absorption rate by Massimilla et al. It may be assumed that the absorption rate depends upon the interfacial area, the gas residence-time, and a mass-transfer coefficient. The last of these factors is probably higher in a gas-liquid fluidized bed because the bubble Reynolds number is higher, but the interfacial area is lower and the gas residence-time is also lower, as will be further discussed in Section V,E,3. 

Of these, fixed-bed adiabatic reactors are the cheapest in terms of capital cost. Tubular reactors are more expensive than fixed-bed adiabatic reactors, with the highest capital costs associated with moving and fluidized beds. The choice of reactor configuration for reactions involving a solid supported catalyst is often dominated by the deactivation characteristics of the catalyst. 

In gas phase reactors, the monomer is introduced to the bottom of reactor where it percolates up through a fluidized bed of polymer granules and inert-media supported catalyst. A fraction of the monomer reacts to form more polymer granules, the remaining monomer being drawn from the top of the reactor, cooled, and recycled. Polymer granules are continuously wthdrawn from the bottom of the fluidized bed and the catalyst is replenished. 

Questions as to the validity of the theory supporting Horio s development are based on deficiencies of the CAFM to fully describe circulating fluidized bed flow and the lack of justification for the premise that the five characteristics comprise all that is of interest in a circulating fluidized bed. In particular, the assumption of pressure drop minimization, although convenient, is not justified physically. 

Grishin, S. I., and Tuovinen, O. H., Scanning Electron Microscopic Examination of Thiobacillus ferrooxidans on Different Support Matrix Materials in Packed Bed and Fluidized Bed Bioreactors, Appl. Microbiol. Biotechnol., 31 505 (1989)... 

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