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.

Spray drying is a widely used unit operation for producing particulate products directly from a liquid feed. Important processes that occur inside the spray dryer are droplet formation, droplet drying and interactions between droplets and recycled fines. Various studies have looked at the first two processes, but the latter phenomenon has received less attention. Literature on droplet-particle interaction which aims at quantitatively describing agglomeration in spray drying is scarce and mainly qualitative. For product quality the formation of agglomerates is often desirable. This thesis models and investigates the collisions of individual particles with single droplets. The surface tack of drying droplets has been identified as an important variable for the formation of agglomerates. In this thesis a novel method for measuring tack from the liquid phase has been further improved. The improvements are a more accurate load measurement, an automated control of the tack probe and an improved layout of the sample holder and probe. The key feature of the device is its ability to measure tack of drying droplets, whereas other devices measure tack by wetting a powder. Using our method the tack of a commonly spray dried product, yeast extract, has been measured. From these experiments it was found that with decreasing average moisture content the surface tack increases to a maximum. Below a critical average moisture content the surface of the droplet is dry and the tack rapidly decreases upon further drying. Another important parameter in determining the degree of agglomeration is the degree of penetration. If the particle penetrates the droplet too deeply, the agglomerate structure becomes too dense. To predict the penetration depth, a non-dimensional model has been developed. It describes the penetration of a particle into a liquid droplet during a head-on collision. It is based on a force balance and incorporates surface tension force, viscous force and capillary pressure force. The important parameters determining the collision outcome are the contact angle, the size of the droplet relative to the particle, the Reynolds and Weber numbers. For each contact angle an equilibrium penetration position exists, at this point the surface tension force vector is perpendicular to the penetration direction. Five different penetrations regimes are identified. At low Reynolds numbers, viscous forces dominate and the particle asymptotically travels towards the equilibrium position. Reducing the viscous drag force by increasing the Reynolds number results in initially overshooting the equilibrium position, but the surface tension force pulls the particle back, to attain the equilibrium in an oscillating motion. At even higher Reynolds numbers the particle fully penetrates the droplet, and reaches the centre of the droplet for even higher values for the Reynolds number. The ejection regime is found at high Reynolds number and low Weber numbers and the liquid should be non-wetting. Using the regime maps one is able to identify in which region a spray dryer is operating. Although the full penetration regimes are useful for capturing fines, it should be avoided when agglomeration is desired. The ejection regime should be avoided as well. To validate the model, impact experiments were carried out by dropping glass spheres on the surface of different liquids. These validation experiments were the first attempt to experimentally validate the collision of a single particle with a liquid surface. Besides yeast extract, which has non-Newtonian rheological properties, silicone oils with constant viscosities of 100 mPa•s and 1 Pa•s have been tested. The penetration over time for different impact velocities was determined by analysing high speed camera recordings. The typical penetration times ranged from 0.2 s to 2 s. To obtain accurate location data was recorded at frame rates up to 38 000 frames per second. Glass spheres, with a size of 2 mm were used to allow the visual tracking. Modelling the impacts showed that the model consistently predicted faster penetration times than were observed experimentally. The relative difference increased with increasing viscosity. A parameter fitting exercise showed that better agreement could be obtained by using a higher viscosity and a higher contact angle in the model. With this knowledge the most likely factor influencing the model-experiment mismatch was identified as being the dynamics of wetting of the particle surface. It was also found that using the dynamic contact angle in the model would improve its results. The non-Newtonian characteristics of the yeast extract resulted in the particle rebound and the formation of an air cavity upon impact. The tack measurement technique and penetration model presented in this thesis will be useful tools for the design of spray dryers. Recommendations are made for further model improvement. The experimental validation is the first attempt to validate the presented model. Future improvements are recommended and suggestions are presented.

La thèse porte sur une nouvelle classe de polymères à architecture en brosse, synthétisés à l'échelle industrielle et utilisés comme additifs dans les suspensions colloïdales pour le couchage du papier. Le polymère en brosse contrôle la déshydratation et le séchage des suspensions ce qui permet d'obtenir des revêtements d'une qualité incomparable. L'objectif est de modéliser sur un plan fondamental le rôle fonctionnel des polymères en relation avec les performances en application. Dans une première partie nous étudions leurs propriétés physicochimiques, structurales et rhéologiques en solution en discutant les spécificités dues à l'architecture en brosse. Dans une seconde partie nous analysons à l'aide d'un dispositif expérimental original les propriétés de rétention d'eau apportées par les polymères. Dans une troisième partie nous étudions le séchage de suspensions de carbonate de calcium sur des substrats solides, qui conduit en général à des motifs hétérogènes dits en " anneau de café ". Nous avons découvert que les polymères en brosse à très faible concentration suppriment ces défauts de séchage de façon remarquable. Le nouveau mécanisme physique à l'¿uvre, que nous appelons effet Marangoni auto-induit, résulte des propriétés interfaciales des polymères et de leurs interactions spécifiques avec les particules de carbonate de calcium. Pour conclure nous établissons un lien entre les propriétés de rétention d'eau et l'inhibition des défauts de séchage. Nous démontrons alors le caractère générique de nos résultats en les transposant à une suspension biologique, le sang, où les polymères pourraient présenter un intérêt dans le traitement de pathologies cardiovasculaires. / This work focuses on a new class of bottlebrush polymers, synthesized on an industrial scale and used as additives in colloidal suspensions for paper coatings. The bottlebrush polymer controls the dehydration and drying of the suspensions, and leads to coatings of outstanding quality. Our aim is to model the functional role of these polymers from a fundamental perspective in relation with applicative performances. In a first part we study their physicochemical, structural and rheological properties in solution, emphasizing the specificities due to the brush architecture. In a second part we implement an original experimental setup to analyze the water retention properties brought by the polymers. In a third part we study the drying of calcium carbonate suspensions on solid substrates, which in general forms to heterogeneous patterns called “coffee-rings”. We discovered that at very low concentration, bottlebrush polymers remarkably suppress these defects. We call auto-induced Marangoni effect the new physical mechanism at work. It results from the interfacial properties of the polymers and their specific interactions with calcium carbonate particles. To conclude, we establish a link between water retention properties and drying defect inhibition. We demonstrate the generic character of our results which can be transposed to a biological suspension, blood, where these polymers could be of interest for cardiovascular disease treatment.