Wet Coating of Geldart-C Type Particles in a Rotating Fluidized Bed in a Static Geometry

Fine particle coating has important applications in the pharmaceutical and food/feed industry. Two main routes can be used: wet- and dry coating. Fluidized bed technology is widely used for wet particle coating. The performance of conventional (i.e. gravitational) fluidized beds is, however, limited when coating cohesive Geldart-C type particles. In this work, a rotating fluidized bed in a static geometry (RFB-SG) [1] was used for coating fine particles with an aqueous polymer solution. Proof of concept was given and it was shown that high-G operation allows removing important limitations of conventional fluidized beds.

A 24 cm diameter, 5 cm length chamber, equipped with 72, 0.2 mm gas inlet slots was used. The particles to be coated were fed via the front side of the chamber, whereas the liquid solution was injected using a 15° spray mounted centrally in the chamber and directing toward the outer wall of the reactor (Figure 1). The particles with a mean diameter of 70 micron and a density of 260 kg/m³ were fed at 2 g/s. The liquid solution was heated up to 90°C prior to injection and the liquid droplets leaving the spray nozzle were on average 65 micron. The liquid flow rate and liquid-solid contact time were varied. The liquid solution was injected after establishing a stable rotating particle bed. Experiments were carried out at different air flow rates, 250 and 400 Nm3/h. The air fed was heated by means of an electric resistance to average feed temperatures between 55°C and 70°C. Batch-wise and continuous particle coating was studied.

The operating conditions were found to be critical for both the particle bed stability and the quality of the particle coating process. Within a range of operating conditions, coated particles could be produced in a reproducible way. Figure 2 shows an example of coated powder that could be produced and its particle size distribution. A comparison with the original powder is made. Some agglomeration was observed, but the agglomeration could be controlled by means of the gas, liquid and solids flow rates, as illustrated in Figure 3. The role of the operating conditions and the chamber geometry are discussed in detail.

Figure 1. Schematic representation of the RFB-SG particle coater, top and front view.

Figure 2. (a) Visual comparison of the uncoated (left) and coated (right) powder; (b) Particle size distribution of the uncoated and coated powder. Solids feeding rate: 2 g/s, liquid solution feeding rate: 1 g/s, air flow rate: 250 Nm3/h at 55°C.

Figure 3. (a) Uncoated powder; (b)-(d) Coated particles of different granulometry produced with decreasing liquid-solid contact time.

[1] J. De Wilde and A.de Broqueville: Rotating fluidized beds in a static geometry: Experimental proof of concept. AIChE Journal, 53, p. 793–810, 2007.