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.
On the relative motion of a particle in a swarm of different particles
Abstract The classification of dissimilar particles in a liquid suspension is exploited in many industrial unit operations. To understand and predict fluidization phenomena in general and particle classification in particular, adequate expressions for each of the governing forces—gravity, buoyancy and drag forces—are required. A possible formulation of the effective driving force and friction in multicomponent mixtures is discussed in this paper. A new expression is derived, which is based on a steady-state force balance for the classifying particle. The expression requires only the usual fluidization data of a single particle species such as the terminal velocity and the Richardson and Zaki index. The new expression is compared with some existing classification models with respect to their predictive value for a wide range of classification data. The new expression proved to be accurate over the entire range of known experimental data and can be extended easily to multicomponent fluidized suspensions. We have found strong evidence that the exact formulation of the buoyancy acting on the classifying particle originates from the bulk density of the fluidized suspension, regardless of the ratio of particle sizes. To clarify the relation between the “overall” drag force and the ratio of particle sizes, the “overall” friction on the classifying particle is decomposed into a solid-liquid and a solid-solid component. The particle-particle interaction force is proportional to the product of the hold-up of the fluidized particles and the slip velocity of dense and fluidized particles. In agreement with other authors, we found an increased friction with a decreasing ratio of classifying and fluidized particles. The particle-particle interaction coefficient was correlated with a modified Stokes-Einstein equation.
Spin label motion in fatty acids.
Spin labels dissolved in highly purified fatty acid systems exhibit nearly identical tumbling rates in liquid and solid phases. Even though the spin labels do not have the same molecular geometry as the lipid matrix the melting point of the matrix can be inferred by measurements of the temperature dependency of molecular motion.