(158b) Tracking pellet motion in a Wurster coater using positron emission

Fluidised beds with internal draft tubes ? commonly known as Wurster coaters ? are widely used in the pharmaceutical industry to apply coatings to pellets, tablets and other particulate dosage forms. In such devices, a gas flow is arranged such that the particles to be coated are projected up through the axial draft tube and into the void space above the bed, falling back onto the surrounding annulus, in which they move slowly downwards to complete their cycle. The coating liquid is normally applied upwards through the gas distributor and close to the base of the draft tube (Figure1).

Figure1. Schematic Diagram of the Wurster Coating Unit

The quality of the coating is of considerable importance, whatever its function, and it is desirable to minimise variation in coating thickness, both on a particle-to-particle basis and from point-to-point on each single particle. Among the principal factors which adversely affect such uniformity are (a) the distribution in the particle cycle time within the device, and (b) the variation in particle entry point into the spray region. It is known that both factors are affected by

• the amount of gas flow delivered to the bed and the distribution of this flow between the centre and the annulus of the bed
• the magnitude of the coating spray rate, gas flow and nozzle design
• the dimensions of the equipment, particularly the gap between the bottom of the draft tube and the gas distributor, through which the particles must pass prior to entering the spray region

Analysis of the problem is complicated by interrelations between these and other variables, e.g. increasing the batch size changes the gas flow distribution between the draft tube and the annulus.

Particle motion is difficult to study directly, because of the opaque nature of the bed (and usually the walls of the equipment). In the present study, the motion of a single pellet of microcrystalline cellulose is followed, within a bed of identical particles, using the technique of positron emission particle tracking (PEPT), which enables particles to be located at up to about 100 times/s, at speeds of up to 10 m/s and to an precision of about ±1 mm.

Figure 2: PEPT  Processed Data a) Typical Velocity Distribution b) Typical Occupancy Distribution (The colours represent the occupancy distribution, identifying the fractions of time the tracer spent in the differing sections of the Wurster Coater) c) Typical Velocity and Occupancy Distribution combined.

Experiments were carried out both with and without spray addition, varying the gas flows, draft tube position and batch size. Use was also made of a novel segmented gas distributor, which allows the gas supplies to the draft tube and annulus to be separately controlled. Results reveal the overall particle motion and quantify the velocity distributions (Figure 2).  They also enable distributions of cycle time and entry point into the spray to be obtained. The effects of operational variables on the width of the cycle time distribution reveal optimal conditions for minimisation of coating thickness variations. Interestingly, particle trajectories and radial profiles of vertical particle velocities indicate internal circulation under certain conditions within the draft tube, which could have a significant impact on pellet coating thickness and uniformity.