Modeling and Simulation of a Wurster Column for Particulate Coating Using Population Balance Moldels and Thermodynamics

The Wurster column coating process of pharmaceutical solids has been studied for many years and a considerable body of literature has been produced covering various aspects of the process.  To achieve uniform active coating content is one of the key interests for the pharmaceutical manufacturing industry.  Thus, a process model for predicting the evolution of a granule size distribution from first principles is desired. 

In this work, a compartment based population balance model, combined with thermodynamic coating model has been developed to model a pharmaceutical coating process in the Wurster column coater.  The model can predict certain key process parameters and the dynamic particle size distribution, and it can also be used to simulate responses of the system to a change in the working conditions. 

In many industrial practices, the coating equipment has been identified into coating/non-spray regions.  It is believed that both the variation of residence time in the spray region and the variation of cycle time lead to the non-uniform coating mass distribution among individual particles.  Crites and Turton (2005) have developed a compartment based model to incorporate the cycle time distribution feature for the coating process in the Wurster column coater.  The model was composed of fundamental flow types such as plug flow and well-mixed flow regions to describe phenomena including dead zone and side pass.  Other instances include a series of well-mixed tanks model structure for a top-spray fluidized bed coater (Ronsse 2005), a twin region compartmental model for a rotating drum coater (Dennis et al. 2001).

In this work we use a population balance (PB) approach to model the particle size distribution of the coated particulates as a function of time.  While the particle growth due to coating strongly relates to the number and duration particles passing through the spray region, a practical way to constructing PB models is based on the decomposition of the coater into spray/non-spray compartments. Thermodynamics of the spray are evaluated with a simple energy balance (see am Ende et al. 2005)

In terms of the flow of particles across the unite, the Wurster column coater is divided into three compartments  including an ideal plug flow region, an ideal fountain CSTR, and a non-ideal annulus CSTR (in a similar way to that proposed by Crites and Turton (2005)).

1.            The plug flow compartment refers to the region enclosed by the draft-tube, where particles are highly fluidized and heated by the upflow hot air.  The coating solution is assumed to be delivered on to these particles from the spray nozzle centered in the middle of the distribute plate.

2.  The ideal CSTR compartment represents the fountain region, in which particles enter into the chamber and continuously receive spray. 

3.   Particles are then settled down towards the bottom of the coater in the annulus region, described by the non-ideal CSTR.  Particles spend most of the time in this region.  The non-ideality refers to the factor that smaller particles have longer recycle time  compared to larger particles (Liu and Litster 1993), so the characteristic time of the CSTR for the annulus region is a function of particle size.

Experimental data is used to determine the thermal properties of the column as well as growth kinetics. Predicted values of particle size as a function of time were found in good agreement with particle size distribution measurements (by laser diffraction) of samples taken throughout the process.

References

am Ende, M.T. and Berchielli, A., 2005. A thermodynamic model for organic and aqueous tablet film coating.  Pharmaceutical Development and Technology 1, 47–58.

Crites, T and Turton, R., 2005. Mathematical model for the prediction of cycle-time distributions for the Wurster column-coating process. Ind. Eng. Chem. Res. 44, 5397-5402.

Denis, C., Hemati, A., Chulia, D., Lanne, J.Y., Buisson, B., Daste, G., Elbaz, F., 2003. A model of surface renewal with application to the coating of pharmaceutical tablets in rotary drums. Powder Technology 130,174-180.

Liu, L. X.,  Litster, J. D., 1993. Coating mass-distribution from a spouted bed seed coater - experimental and modeling studies. Powder Technology 74, 259-270.

Ronsse, F., Pieters, J.G., Dewettinck, K., 2007. Combined population balance and thermodynamic modelling of the batch top-spray fluidised bed coating process. Part I—Model development and validation. Journal of Food Engineering 78, 296–307.