Different aspects that influence the fluidked bedparticulate coating process have been investigated. An easy touse procedure for creating temperature and humidity profilesfor the gas inside the bed was developed. The procedureinvolves measuring a limited number of points inside the bedand using these points generate more data points used for thecreation of the profiles. The profiles revealed that differentparts of the bed have a different involvement in the coatingprocess. Apart from showing the hydrodynamic conditions insidethe bed, the profiles can be used to optimize the size of thebed, to map the net migration of particles and to set thespraying rate limits.
The three-domain representation of the bed was used to modelthe coating process. The model showed that large beds and theexistence of stagnant region within the bed widen the toatingdistribution. Increasing the rate of transfer to the sprayingdomain or increasing the overall mixing of the bed Will havethe opposite effect of narrowing the coating distribution. Theunequal transfer between domains can be used to manipulate thetoating distribution. The combination of bed ske, differentrates of transfer between domains, size of spraying domain andthe rate of spraying can be used to obtain particles that arecoated to a particular distribution.The mechanism and kinetics of growth in a top sprayingprocess were determined by coating two poly-distributed seedparticles with a cellulose under different operatingconditions. The resulting particle distribution reveals thatnot all particles in the bed are equally coated. For the topspraying process, smaller particles Capture more toating thanlarger particles. A narrowing of the seed distribution wasfound to increase the chance of toating the large particles.The results also confirmed that particles are coated only aftervisiting the spraying region, which is small compared to therest of the bed. A growth model developed using theexperimental results introduces a segregation factor whichrepresent the probability of different particle sizes beingcoated. For the top spraying coating of lactose particle with acellulose, the segregation factor was found to be anexponential decaying function of the particle weight.
Keywords:granulation, coating, surface layering,segregation, coating uniformity, temperature protile, humidityprofile, growth kinetics, particle coating.
Stockholm: Kemiteknik , 1998. , 69 p.