Unraveling the droplet drying characteristics of crystallization‐prone mannitol – experiments and modeling

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

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.

Unraveling Particle Formation: From Single Droplet Drying to Spray Drying and Electrospraying

Spray drying and electrospraying are well-established drying processes that already have proven their value in the pharmaceutical field. However, there is currently still a lack of knowledge on the fundamentals of the particle formation process, thereby hampering fast and cost-effective particle engineering. To get a better understanding of how functional particles are formed with respect to process and formulation parameters, it is indispensable to offer a comprehensive overview of critical aspects of the droplet drying and particle formation process. This review therefore closely relates single droplet drying to pharmaceutical applications. Although excellent reviews exist of the different aspects, there is, to the best of our knowledge, no single review that describes all steps that one should consider when trying to engineer a certain type of particle morphology. The findings presented in this article have strengthened the predictive value of single droplet drying for pharmaceutical drying applications like spray drying and electrospraying. Continuous follow-up of the particle formation process in single droplet drying experiments hence allows optimization of manufacturing processes and particle engineering approaches and acceleration of process development.

Time scale based analysis of in-situ crystal formation in droplet undergoing rapid dehydration

The surface structure of crystalline particles affects the functionality of the particles in drug delivery. Prediction of the final structure of particles that crystallize easily within the spray drying process is of interests for many applications. A theoretical framework was developed for the prediction of crystal structure precipitating on the surface of the particle. This model was based on the dimensionless Damkohler number (Da), to be an indicator of final particle morphology. Timescales of evaporation and reaction were required for calculation of the Damkohler number. The modified evaporation time scale was estimated based on the time that is available for the crystal to precipitate after supersaturation. The reaction time scale was estimated based on the time scale for induction time. Mannitol was produced under different processing conditions in order to validate the theoretical model. Results showed for the high Damkohler numbers, the surface structure of the particle was rough, while smaller Damkohler numbers led to relatively smooth particle surfaces. Additionally, although the beta polymorph was dominant in all of the experiments, alpha polymorph was precipitated in the experiments with a large Damkohler number. The theoretical framework developed will be a useful predictive tool to guide the manipulation of particle crystallization in spray dryers.