Production of uniform particles via single stream drying and new applications of the reaction engineering approach
Creators: name: Patel, Kamleshkumar Chhanabhai
Publisher: name: Monash University
Year: 2017
Subjects: Uncategorized

description: In this thesis investigations are carried out on two research topics in context to spray drying. The first research topic is the production of dried particles having uniform characteristics. The second research topic is the development of new applications of the reaction engineering approach which, in recent times, has emerged as an effective tool to formulate drying kinetics models. The reaction engineering approach is also implemented to simulate the drying of monodisperse droplets corresponding to the experimental work in the first research topic. Manufacturing micron- and nano-sized particles having uniform characteristics has recently become a popular research area due to the unique functionalities of these kinds of particles in biomedical, drug delivery, functional foods, nutraceuticals, cosmetics and other valuable applications. Spray drying has been a common and economical route to produce dried particles. A typical characteristic of spray dried products is the existence of a significant variation in particle properties such as size and morphology. One possible idea to restrict this product non-uniformity is to achieve a good control over the droplet’s behaviour and characteristics inside the drying chamber. The current thesis has investigated an innovative spray drying technique, i.e. a single stream drying approach in order to restrict product non-uniformity. In this drying approach, identical sized droplets having vertical trajectories are dried under controlled gas flow conditions. The piezoelectricity-driven monodisperse droplet generator is used as the atomizer to disperse liquid droplets. A prototype single stream dryer was assembled based on the single stream drying approach using various components designed in the laboratory and several parts purchased from the market. Experiments were carried out using aqueous lactose solutions as a model system in order to check the practicability of manufacturing uniform-sized spherical particles. Preliminary results were found to be positive and reported in this thesis. Mathematical models on the drying of monodisperse droplets were developed in order to predict important droplet and gas parameter profiles during single stream drying. These models serve as a platform for design, optimization and scale-up purposes. Several important advantages and drawbacks of single stream drying are also reported. Problems encountered during the experimental work and future recommendations are presented in detail so that a more robust and effective drying research tool can be developed in future. Recently the reaction engineering approach (REA) has emerged as a simple and reliable technique to characterize the drying of various food and dairy materials. In this thesis two new applications of the REA are described for the first time in context to convective drying of aqueous droplets. The REA is used in this study to formulate the drying kinetics model for the drying of aqueous sucrose and maltodextrin (DE6) droplets. The effect of initial moisture content was explicitly demonstrated. The development of a new ‘composite’ REA which aimed to model the drying of aqueous droplets containing multiple solutes has been described. The composite REA was found to be suitable to characterize the drying behaviour of aqueous sucrose-maltodextrin mixtures of different proportions. The second new application of the REA is the development of a procedure to estimate surface properties of aqueous droplets during drying. In literature various droplet characteristics such as surface moisture contents were normally estimated using the diffusion-based drying kinetics model or the receding interface model. Surface moisture content and surface glass transition temperature profiles were evaluated here using a lumped-parameter model (REA) during the drying of aqueous sucrose, maltodextrin (DE6) and their mixtures. The same experimental data used for the development of the composite REA were used to yield predictions. The procedure was found to be useful in estimating surface moisture contents and understanding the stickiness behaviour of sugar droplets during drying. During the formulation of the REA-based drying kinetics model in this thesis, the assumption of uniform temperature within droplets was used. In most studies published in literature the uniform temperature assumption was justified by calculating the heat-transfer Biot numbers at the beginning and end of drying. However, the conventional Biot number concept does not take into account the evaporation effect and therefore would not be suitable to drying scenarios. In this thesis, a new approximation procedure is developed to estimate surface-centre temperature differences within materials following the entire drying process. This new procedure was helpful to check the extent of temperature non-uniformity within skim milk droplets under isothermal laboratory conditions as well as industrial spray drying conditions. Both conventional and drying-based Biot numbers are calculated and compared. Predictions showed that temperature gradients within the droplets were negligible during the drying of suspended droplets under laboratory drying conditions (slow drying), whilst the gradients were small and existed only for a short drying period for small droplets under industrial spray drying conditions (fast drying). Furthermore, it was observed that the maximum temperature gradient within the droplets did not exist at the starting or end points of the drying process, and therefore the estimation of Biot numbers at the starting and end point does not reflect temperature non-uniformity under drying conditions. This is a significant theoretical development in the area.