Countercurrent multistage fluidized bed reactor for immobilized biocatalysts: III. Hydrodynamic aspects
In Parts I and II of this series we described the modelling, design, and operation of a multistage fluidized bed reactor (MFBR) for immobilized biocatalysts. This article deals with those aspects of the MFBR which are different from single‐stage fluidized beds which are operated in batch mode with respect to the solids. The semicontinuous transport of the particles requires perfect mixing of the particles in the reactor compartments, because particles are mainly transported from the bottom of these compartments. A large spread in the physical properties of the biocatalyst particles, especially of both size and density, may cause the particles to segregate into layers with different diameter and/or density. This affects the efficient use of the biocatalyst. The properties of the particles are dependent on the immobilization method. The suitability of different methods for possible future application in the MFBR is therefore compared. Because of segregation, successful use of a biofilm catalyst with a nonuniform thickness of the biofilm is doubtful. Experiments in a small scale reactor (± 0.1 m diameter) demonstrated that perfect particle mixing is possible using commercially available biocatalyst particles of uniform density. Co‐immobilization of the biocatalyst with glass powder in a gel is a simple and effective method of increasing gel density. High density particles allow high liquid flow rates, and thus an improved external mass transfer can be achieved.
Axial dispersion of spheres fluidized with liquids
Solids dispersion due to the simultaneous diffusion (random particle motion) and classification (segregation) of fluidized spheres has been investigated. The model developed permits calculation of the mean concentration of spheres of both sizes in a mixture of two sizes of fluidized spheres as a function of bed length. Fick's law has been applied to the diffusional phenomenon. A hypothesis has been advanced and confirmed which permits calculation [Equation (5)] of the classification velocity for each size of sphere in a fluidized mixture. Calculation of the classification velocities in a mixture of spheres is based on the relation of void fraction to superficial velocity for the individual sizes of spheres. To facilitate carrying out these calculations, a generalized equation has been developed for the relation of void fraction to superficial velocity in terms of the Galileo number (d3gΔρρƒ/μ2) and sphere-to-column diameter ratio. Measurement of the bead size gradients (change in bead size with bed length) at steady state fluidization was used to investigate dispersion. Closely sized glass beads of 0.1 and 0.2 cm. diameters were fluidized with liquids of 1 and 15 cps. at void fractions of 0.5 to 0.8 in 2.5 and 5.0 cm. columns. Lead beads of 0.12 cm. diameter were also used. Further, several experiments were conducted by a method which is analogous to that used in molecular diffusion cells.
Flow Patterns in Particle Beds
THE technique of observing gas-flow patterns in fluidized beds described in a recent communication1 is a useful contribution to the methods available for investigating fluidization behaviour. A comparable technique has been developed here using dye injection for tracing the flow pattern of water passing through particle beds. Though this technique was found useful in the pre-expansion stage, it was the reverse during bed expansion when it actually tended to mask the interesting phenomena shown in Fig. 1. These photographs were taken by transmitted light without the use of dyes, and show the activation of a bed of 100µ glass spheres by water. Local bed expansion or increased voidage is indicated by increase in transmitted light. Photograph C13 shows the approximate steady-state conditions in a bed which, viewed superficially, has all the appearance of uniform and smooth participate fluidization. The series A1–A12 (of which A1, 3, 6 and 12 are reproduced) shows the development of a flow pattern in the same bed at a much lower superficial water velocity. The last of that series again approximates to steady-state conditions, the only changes thereafter being a slow meandering of the flow channels. Not seen in the photographs but observed visually are occasional series of mushroom-shaped low-density discontinuities rising in one or more of the channels.