Drying kinetics, coalescence and agglomeration of dairy products particles
Doctoral thesis, 2015
Spray drying is used in the food, pharmaceutical, and chemical industries for producing powder from a liquid feed. The unique advantages of spray drying are producing powders of a specific particle size and moisture content, handling heat sensitive products, and high capacity. However, spray drying is a complex process. It involves numerous phenomena, such as feed atomization, airflow, particle heat and mass transfer, and particle interactions, which make the design and scale-up of the process an arduous task. For this purpose, modelling tools can be used to reduce time and cost for designing and scaling up the spray-drying process. This research aims to develop modeling tools and an experimental procedure to quantify and better understand the drying-kinetics, coalescence, and agglomeration of particles of dairy products during spray drying.
The drying-kinetics model consisted of a distributed parameter model able to simultaneously simulate water content and temperature profiles within a particle, and in which physical properties were considered as a function of the local temperature and moisture content. The effective diffusion coefficient of water in skim milk as a function of solid content and temperature, which is one of the most important physical properties of the drying-kinetics model, was evaluated using a new approach that combines Nuclear Magnetic Resonance (NMR) with a parameter estimation method. The effective diffusivity values over a full range of water contents and temperatures were obtained and experimentally validated for dairy products.
The experimental single-particle drying-kinetics device used for model validation allowed the assessment of the effect of the composition of dairy products on drying time and changes in morphology. The higher the fat content, the lower the diffusivity of water and, consequently, the longer the drying time. A higher fat content and a slower drying-rate resulted in less shrivelled particles.
The validated drying model was combined with a CFD simulation model of a pilot spray dryer to investigate the drying-kinetics, and preconditions for the coalescence and agglomeration of skim milk powders. The results showed the need of accurate values for water diffusivity and stickiness conditions to predict the surface properties of the particles and, consequently, the regions of coalescence and agglomeration.
To further understand the coalescence and agglomeration mechanisms of particles during spray drying, inter-particles behaviour during drying was studied by combining experiments and modelling. The drying-kinetics device was adapted to dry two individual particles at the same time and to perform contact tests throughout the drying time. The validated drying-kinetics model was used to predict surface conditions during contact tests. Three major mechanisms were observed; coalescence, stickiness and non-stickiness, which were related to the adhesion and cohesion forces of particles. The physical conditions of coalescence and agglomeration were evaluated, based on the Ohnesorge dimensionless number and the glass transition temperature. The model was then used to simulate the formation of wet and dry shells throughout the drying time, and the results provided a better understanding of particle-particle interactions.
In conclusion, the methodologies developed can be used to optimize, design, and scale-up spray-drying equipment and to understand product properties during spray drying.