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.
Countercurrent multistage fluidized bed reactor for immobilized biocatalysts: I. Modeling and simulation
For the application of immobilized enzymes, fixed bed reactors are used almost exclusively. Fixed bed reactors have specific disadvantages, especially for processes with a deactivating catalyst. Therefore, we have studied a novel reactor type with continuous transport of the immobilized biocatalyst. Flow of biocatalyst is countercurrent to the substrate solution. Because of a stagewise reactor design, back‐mixing of biocatalyst is very limited and transport is nearly plug flow. The reactor operates at a constant flow rate and conversion, due to constant holdup of catalytic activity. The reactor performance is compared with a configuration of fixed bed reactors. For reactions in the first‐order regime, enzyme requirements in this new reactor are slightly less than for fixed bed processes. The multistage fluidized bed appears to be an attractive reactor design to use biocatalyst to a low residual activity. However, nonuniformity of the particles might affect plug flow transport of the biocatalyst. A laboratory scale reactor and experiments are described in Part II1 of this series. Hydrodynamic design aspects of a multistage fluidized bed are discussed in more detail in Part III.2
Liquid fluidization of binary particle mixtures—II. Bed inversion
Abstract When the size ratio and the density ratio of the two species in a binary solids mixture are on opposite sides of unity, it is possible in some cases for such mixtures to display an inversion phenomenon during liquid fluidization whereby the order in which the two species are vertically segregated is reversed on changing the liquid velocity. This inversion phenomenon, originally reported by Hancock and recently revived in the literature, is investigated experimentally and modelled theoretically. The necessary and sufficient conditions for a bed inversion to occur are derived. Reasonable agreement between theory and experiment is demonstrated. The effect of particle size spread is examined, and the model is also used to rationalize long standing mineral dressing rules on sorting and sizing of particles. Finally some practical implications of the inversion phenomenon are designated.