Some modeling aspects of (batch) fluid-bed drying of life-science products

Some specific properties of life-science products, and consequences for fluid-bed drying are discussed. The sorption isotherm and maximum temperature are related to limits in process conditions, and capacity. The occurrence of stickyness is combined with the sorption isotherm and the psychrometric chart, resulting in an extended stickyness diagram, from which operating limits for air temperature and humidity can easily be deduced. Models for the air-side mass transfer are considered. The expanded-bed model, with possible bypass, is then combined with a model for concentration- and temperature-dependent diffusion inside the particles. Also a model thermal degradation reaction is included to illustrate possible quality changes in the product. Simulations show the differences between laboratory and large-scale operations, and illustrate the effect of some process conditions on drying rate, product temperature and quality. For the often implicitly used time-averaged air temperature and humidity to calculate particle drying a theoretical basis is provided. It is also shown that for quality aspects this assumption probably is too much simplified