Fluid Bed Granulation Articles

The fluidized bed is a process, which is difficult to manipulate: it either works or it doesn't. How it works has been the subject of much research, o­nly a few studies have considered how the operation of a fluidized process may be influenced. This article describes how fluidized beds can be manipulated to some extent by using internals: the natural tendency for particle segregation in the bed can be enhanced (e.g. for particle classification), and the application range of fluidization can be extended to finer or more sticky powders.


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A Molecular Dynamics (MD) simulation technique is used to investigate the effect of moisture o­n interparticle forces. This is carried out by simulating two opposing moist quartz surfaces at different separations. The spontaneous formation of a liquid bridge, was observed with o­nly three and a half monolayers o­n the surfaces, corresponding to a relative humidity of about 70% according to the BET-isotherm. In the article a brief account of the methodology is given. Results are shown in the form of snapshots of the position of the molecules at a given instant in time and mean forces between the quartz plates as a function of surface separation. The relation of the results to the macroscopic theory of liquid bridging is also mentioned, as are the density and diffusivity profiles of the adsorbed water layers.

A stochastic model describing the axial particle transport and the particle residence time distribution (RTD) in continuous fluidized beds is presented. This model has the advantage of clarity and intuitive appeal. Moreover, the method of solution of the model is far simpler and more powerful than for the traditional modelling approach based o­n conservation equations. After presentation of the model and the method of solution, predictions for the transient axial distribution of tracer particles in a fluidized bed are shown graphically. Predicted RTD curves are also shown graphically and compared with experimental data from the literature. Agreement is good, although some discrepancy between prediction and experiment remains. A discussion is provided.


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Olav Bolland and Rainer Nicolai

Dilip M. Parikh,

With a view to investigating the potential of a four-compartment interconnected fluidized bed for the combustion of biomass, the motion of relatively large pellets in the bed has been examined. Different experimental techniques were used to look into the following aspects: the pellet distribution between the beds, the pellet circulation rate and the pattern of movement of individual pellets in the compartments. The results from the different types of experiments are shown and related to each other. Emphasis is laid o­n the quality and rate of pellet circulation and the dynamics and mode of transport of the individual pellets in the compartments. Pellet circulation was found to be unsatisfactory when the slow compartments were aerated at or above the rate required to fluidize the material. The pattern of axial movement is shown to be consistent with upwards motion mainly in the wakes of fluidization bubbles, and downwards motion in the bulk.

HERCULES : Pharmaceutical Technology Report