Fluidized catalyst beds flow properties

First, it will be shown that flow properties of the fluidized catalyst bed (FCB) are clearly different from those of other conventional fluidized beds. The different treatment required is very significant for research and development on fluidized catalytic beds. Next, factors affecting the flow properties are discussed, especially particle size distribution, and also heat and mass transfer, and mixing properties. 

In the preceding section, the flow properties of fluidized catalyst beds have been clarified mainly on the basis of experimental observations. In the case of FCC catalyst, the apparent viscosity of the emulsion is usually very small, and the emulsion shows good fluidity. Catalyst particles once charged into a fluidized bed reactor are usually in service for several months. Hence it is justifiable to prepare the particles very carefully, so that the fluidized bed shows the best fluidization possible. This kind of careful preparation is usually impractical in the case of single-pass particles such as coal, mineral ores, or grain. 

Currently available data for the flow properties of the fluidized catalyst bed are fragmentary, since the local motion of the emulsion phase is diflicult to measure experimentally. Therefore, it is useful to clarify the flow properties of the bed in terms of our knowledge of bubble columns. First, the fluid-dynamic properties of the bubble columns will be explained then, the available data will be adapted to apply to fluid catalyst beds. The reader will be able to picture an emulsion phase of carefully prepared catalyst particles operating in intense turbulence for fluidized beds under conditions of practical interest. This turbulence distinguishes the flow properties of fluid catalyst beds from those of widely studied teeter beds. 

Longitudinal dispersion in the continuous phase (the liquid phase for a bubble column, and the emulsion phase for a fluidized catalyst bed) is closely related to flow properties of the equipment. Here, we wish to describe the longitudinal dispersion phenomena in terms of the fluid-dynamic properties of the equipment. The prime purpose is to test whether the fluid-dynamic analysis developed earlier is sound, but lon-... 

A gas bubble column is taken here as a model equipment undergoing longitudinal dispersion of the continuous phase. The theory obtained is equally applicable to a fluidized catalyst bed of good fluidity exhibiting similar flow properties. The following procedure is from Miyauchi (M27). 

Heat and mass transfer constitute fundamentally important transport properties for design of a fluidized catalyst bed. Intense mixing of emulsion phase with a large heat capacity results in uniform temperature at a level determined by the balance between the rates of heat generation from reaction and heat removal through wall heat transfer, and by the heat capacity of feed gas. However, thermal stability of the dilute phase depends also on the heat-diffusive power of the phase (Section IX). The mechanism by which a reactant gas is transferred from the bubble phase to the emulsion phase is part of the basic information needed to formulate the design equation for the bed (Sections VII-IX). These properties are closely related to the flow behavior of the bed (Sections II-V) and to the bubble dynamics. 

The object of the following treatment is to establish a physically sound reactor model to obtain A or> based on the flow and transport properties of fluidized catalyst beds. Bed performance for chemical kinetics other than the first-order reaction may be computed after a sound bed performance has been established. 

A reactor model is developed to include reaction taking place in the dilute phase, and to be reasonably consistent with the known flow properties of fluidized catalyst beds operated under relatively high gas velocity. According to this model, reaction proceeds successively in the dense phase and in the dilute phase. 

Flow properties. Slugging tendency of a fluidized bed increases with increased Stormer viscosity (207). The modified Stormer viscometer used ill this work is provided with a paddle that is rotated in the fluidized bed (by means of a weight attached to a string passing over a system of pulleys). Viscosity is measured by determining the weight required to spin the paddle at 200 r.p.m. Stormer viscosity of the bed increases with decreased gas velocity and with increased size and density of particles. Viscosity of coarse catalyst is decreased, within limits, by addition of fines. 

The intense research effort carried out into the study of catalyst properties for the conversion of plastic wastes is in contrast with the few studies that have addressed reactor design. Thus, most of the studies use batch or simple fixed bed reactors despite the heat transfer and flow problems associated with the low thermal conductivity and high viscosity of the molten plastics. Various alternatives have been proposed to solve these problems the use of fluidized bed reactors, dissolution of the plastics in heavy oil fractions previously fed into the reactor, and a combination of thermal and catalytic treatments. However, all these processes present a number of difficulties, which makes further work on the reactor design necessary. 

Various types of industrial reactors may occur in different phases as applications and desired properties of the final product, for example, the fixed bed, fluidized bed, slurry bed, and bed phase reactors. In fluidized bed reactors as in slurry bed, the solid (catalyst) is composed of very small particles and moving along the reactor. The fluid flow over these reactors is complex. In these systems, the flow of the fluid phase is not homogeneous and there are large deviations from the ideal behavior of a CSTR or plug flow reactor (PFR), characterizing them in nonideal reactors. 

The most significant development has been the introduction of fluidized bed conversion. One fluid bed replaces the three beds before intermediate absorption in the double absorption set-up. In a fluidized bed converter the gas flow is reversed and the catalyst becomes fluid or movable. The heat transfer properties are then much greater than for fixed beds and internal boiler tubes can control the temperature to within a degree in the 460-500 °C range. This technique has not been used except as a prototype because of the parlous state of the industry following recession. 

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