A SINGLE PARTICLE VIEW OF FLUIDIZATION

Radiation-based single particle tracking approaches have distinct advantages in investigating opaque particle systems such as fluidized beds. The principles of one of these – positron emission particle tracking (PEPT) – are summarised here, together with recent developments in the use of the technique. Applications in bubbling beds, circulating beds, in heat transfer and in coating are illustrated. PEPT is beginning to be used in validation of computational methods for simulating fluidized beds, such as discrete element methods

THE TRANSPORT DISENGAGEMENT HEIGHT (TDH) IN A BUBBLING FLUIDIZED BED (BFB)

Bubbles bursting at the surface of a BFB project particles into the freeboard. Coarser particles fall back, the solids loading declines with height in the freeboard, and fines are ultimately carried over. The height of declining solids loading is the TDH, measured in this research by Positron Emission Particle Tracking, and modeled from a balance of forces on ejected particles. Model predictions and PEPT-data are in good agreement. Empirical equations overestimate the TDH

THE SOLIDS FLOW IN THE RISER OF A CFB VIEWED BY POSITRON EMISSION PARTICLE TRACKING (PEPT)

The PEPT study of the riser of a CFB determines (i) the acceleration length and time, (ii) the upwards and downwards velocities; (ii) the population densities; (iii) the flow pattern at the riser bottom; and (iv) the existence of different flow regimes for associated operating conditions of gas flow rate (U) and solids circulation flux (G)

Supercritical fluid coating of API on excipient enhances drug release

A process to coat particles of active pharmaceutical ingredient (API) onto microcrystalline cellulose (MCC) excipient shows promise as a new way to dosage forms showing enhanced drug release. The process consists of a fluidized bed operated at elevated pressure in which API particles are precipitated from a Supercritical Anti-Solvent process (SAS). MCC particles were used as an excipient in the fluidized bed and collect the SAS-generated API particles. Naringin was selected as the model API to coat onto MCC. A number of operational parameters of the process were investigated: fluidization velocity, coating pressure, temperature, concentration of drug solution, drug solution flow rate, drug mass, organic solvent, MCC mass and size and CO2-to-organic solution ratio. SEM and SPM analyses showed that the MCC particle surfaces were covered with near-spherical nanoparticles with a diameter of approximately 100–200 nm, substantially smaller than the as-received API material. XRD showed that naringin changed from crystalline to amorphous during processing. The coated particles resulting from the SAS fluidized bed process have a higher loading of API, gave faster release rates and higher release ratios in comparison with those produced using a conventional fluidized bed coating process. The approach could be transferred to other industries where release is important such as agrochemical, cosmetic and food

Investigation of the Sources of Variability in the Wurster Coater: Analysis of Particle Cycle Times using PEPT

Positron Emission Particle Tracking (PEPT) has been used successfully to study pellet motion within the Wurster coater. PEPT experiments were undertaken to understand how the parameters of batch size and partition gap interact with each other; and to determine their effects on the particle cycle time and the components of the particle cycle time: the flight and annulus times and their respective distributions. This enabled the determination of optimum operating conditions for a given set of process conditions

Effect of surface energy on the transition from fixed to bubbling gas-fluidised beds

AbstractTwo-dimensional DEM–CFD simulations have been performed in order to examine the effect of surface energy on the transitional behaviour from fixed bed to bubbling bed for Geldart Type A particles. The results of the simulations presented in the paper show that any effect of surface energy on the magnitude of Umf is not due to increasing bed resistance as a result of increasing the interparticle bond strength. It is demonstrated that Umf corresponds to a deterministic (isostatic) state that is in effect the initiation of the transition from solid-like to fluid-like behaviour. It is also shown that the so-called ‘homogeneous expansion' regime is not in fact homogeneous. This is because the system, when U>Umf, consists of agglomerates. Consequently, the idea that bed expansion is due to the ‘elasticity’ of the bed is not tenable. In order to break up the agglomerates and create a fully fluidised bed that will allow bubbling to occur, higher superficial gas velocities are required for higher values of surface energy. Once the bed is fully fluidised and bubbling occurs the effect of surface energy becomes insignificant

A DEM investigation of transitional behaviour in gas-fluidised beds

AbstractUsing DEM simulations, the paper examines the different types of behaviour as the gas velocity is increased to cover the complete range from fixed bed to homogeneous expansion, bubbling, turbulent and fast fluidisation. The paper highlights the transitions between the various regimes. At minimum fluidisation velocity, Umf, the structure of the bed is isostatic. When the gas velocity U is increased the system immediately breaks up into large clusters of contacting particles which gradually disintegrate with further increases in gas velocity until, at minimum bubbling velocity, Umb, the first bubbles start to appear. Conventionally, the regime Umf<U<Umb is referred to as homogeneous expansion. However, it is shown that the expansion is not homogeneous. Above Umb, the amplitude of the pressure drop fluctuations increases to a maximum when U=Uc, which marks the transition from bubbling to turbulent behaviour. The simulations also show that in the turbulent regime the average pressure drop increases with increasing gas velocity. This aspect appears not to have been reported previously in the literature. Finally, when U>Uk, corresponding to “fast fluidisation”, the particle system behaves as a granular gas. A new criterion is suggested to define the transition from turbulent fluidisation to fast fluidisation, defined by Uk