Model-based measurement and control of fluidised bed spray granulation processes

Particulate products play an important role in many industr ie and applications, for instance pharmaceuticals, fertilisers, or foods. Compared to the often l iquid state of the raw materials, they are easier to transport, store and post-process. Furthermore, the transformation from liquid to solid allows for particle formulation, i.e. the production of par ticles with defined characteristics. In general, the particles are not uniform, i.e. they possess a distribution in their properties, for instance size, shape, moisture content, or composition. Th is distribution also characterises the quality of the product. Increasing customer requirements g ive rise to the need to realise predefined, or required, property distributions of the particl e in the process. Widely used processes for the production of dustless, freeflowing powders from liquid raw materials are crystallisation and spray granulation in fluidise b ds. In fluidised bed spray granulation, which can be run batch-wi se as well as in continuous mode, a solution, suspension or melt is sprayed onto a particle bed w hich is fluidised by heated air. Due to evaporation of the liquid in the spray, a particle growth c an be observed as the solid in the spray solidifies on the particle surface. The combination of heat and mass transfer with particulate processes renders fluidised bed spray granulation a complex ultiphase process. The realisation of a desired property distribution therefore necessitates th use of process control. In this thesis a model-based scheme for the feedback control of particle size distributions as well as the particle temperature and moisture content in fluidise d bed spray granulation is devised. The focus lies on the control of the particle properties in batch spray granulation and in continuous spray granulation with external classification by sieves an d mills. Depending on the configuration of the external classification, i.e. the parametrisati on of the sieves and the mill, a di fferent dynamic behaviour is obtained: Whereas for some configurati ons a stable steady-state is attained, the steady-state is unstable for others. To achieve a steady -st te with constant product mass flow for these cases, the unstable steady-states have to be stabi lised y feedback control. As a basis, mathematical process models for particle format ion and heat and mass transfer in fluidised bed spray granulation are derived. In order to acco unt for the distributed character of the particle properties the population balance approach is uti li ed yielding non-linear partial-integro differential equations for the temporal evolution of the partic le size distribution. The models allows to analyse the dynamic behaviour of the process and build the core of the model-based feedback control scheme. The implementation of feedback control requires informati on on the state of the process, for instance the particle size distribution. As this knowledge is not always available through direct measurements, model-based measurement systems, also know n as state observers or estimators, are designed to reconstruct the particle size distribution fr m other measurements utilising the mathematical process models. It is demonstrated that the si ze distribution can be successfully reconstructed, for instance from measurements of the mean p article diameter.