Concept, design, and measurement methodology of a highly dynamic thin film dryer for experimental in situ drying...

This paper describes the technical setup and validation of a thin film drying device in which the conditions such as air temperature, relative humidity, and velocity can be controlled in the range of 20-200 °C, 0-1.5 m s^-1, and vapor mass flows up to 1200 g min^-1. For the first time, it is possible to perform in situ manipulation of the drying kinetic in a highly dynamic manner. The setup allows for a precise online determination of drying and rehydration kinetics; the recording of relevant process parameters such as air humidity, air and surface temperature; and the monitoring of optically observable material properties. The concept for rapid real-time changes in drying conditions is explained, and the constructive details are elucidated. Highly precise gravimetry by a proposed measurement methodology and simultaneously avoiding condensation during rapid parameter changes is accomplished. This is achieved by the combination of minimized thermal inertia of the system, air conditioning via carrier gas-free evaporation, and a highly responsive active insulation. The influence on the drying kinetics by temperature, humidity, and air velocity is shown with experiments on aqueous maltose solution, and the experimental precision is validated. The validation showed a high grade of accuracy regarding gravimetric determination with a maximum observed mass difference of 0.21% referring to the dried product. The dynamics of the setup under in situ changes in drying conditions is exemplified by further experiments. In addition, the presented setup for the first time enables the real time manipulation and observation of kinetically coupled processes such as crystallization behavior, morphology formation, or material degradation during drying. Therefore, it has important practical value for the development of efficient and energy-saving drying methods and products with specific, tailor-made properties.