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.

Thesis (MScEng)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Can one measure the value of knowledge sharing? How does one assess the effectiveness of a knowledge network with regard to supporting knowledge sharing? These are pertinent questions that knowledge managers are faced with, because geographically dispersed organizations rely on knowledge networks to integrate its dispersed knowledge. These questions must be understood in the context of a knowledge management approach, for the approach determines how one navigates the complex landscape of knowledge sharing. This complexity is driven by the social dynamics that exist between organizational members. If the intricacies of these social dynamics can be highlighted, it will assist management in deciding which interventions to implement to increase a knowledge network’s effectiveness. Firstly, a pragmatic approach to knowledge management proposes that management can switch between a bottom-up and top-down view of knowledge sharing in a knowledge network. Secondly, management requires a vehicle to implement the pragmatic approach. The proposed vehicle is the knowledge network framework, for it plays a central role in the design, implementation and maintenance of a knowledge network. Finally, the embeddedness-measuring framework is developed to analyze the social dynamics between knowledge network members and the content they share amongst one another. The implementation of this framework allows for the analysis of interviews that were conducted in a case study at A Chemicals Company (ACC). Five communities of ACC’s knowledge network are investigated and the results emphasize the dynamics of knowledge sharing between network members. The implications of the results are as follows: • A navigation system is proposed that enables the implementation of the pragmatic approach. • A schedule for the evaluation of a knowledge network enables management to assess the effects of interventions. / AFRIKAANSE OPSOMMING: Kan die waarde van die proses om kennis te deel, gemeet word? Hoe word ‘n kennis-netwerk se doeltreffendheid gemeet met betrekking tot die ondersteuning wat dit bied vir netwerk-lede om hul kennis te deel? Dit is die tipe vrae wat kennis-bestuurders mee gekonfronteer word, want organisasies wat geografies versprei is maak staat op kennis-netwerke om verspreide kennis te integreer. Om hierdie vrae te verstaan, moet dit gesien word in die konteks van 'n kennisbestuur-strategie. Die kennisbestuur-strategie bepaal hoe ‘n mens die kompleksiteit van kennis-skepping en -uitruiling benader. ‘n Groot faktor wat dié kompleksiteit beïnvloed, is die sosiale dinamika tussen netwerk-lede. Daar word dus geredeneer, dat as sin gemaak kan word van dié dinamika, bestuur se vermoë om in te gryp in die kennis-netwerk verbeter sal word. Gevolglik kan die doeltreffendheid van ‘n kennis-netwerk verbeter word. Eerstens word die pragmatiese benadering tot kennisbestuur voorgestel. Dié strategie gee bestuur die vermoë om te wissel tussen ‘n top-down benadering en ‘n bottom-up benadering tot kennisbestuur. Tweedens word die kennis-netwerk raamwerk bespreek. Hierdie raamwerk beskryf die ontwerp, implimentering en instandhouding van ‘n kennis-netwerk. Dit bied bestuur die vermoë om die pragmatiese benadering te volg. Ten slotte word die geïntegreerde metings raamwerk ontwikkel. Hierdie raamwerk stel mens instaat om ‘n analise te doen van die sosiale dinamika tussen netwerk-lede en die inhoud wat hulle met mekaar deel. Die geïntegreerde metings raamwerk word gebruik om onderhoude te analiseer van die lede van ‘n kennis-netwerk gevallestudie. Hierdie gevallestudie is gesentreer rondom die bedryf A Chemical Company (ACC) se kennisnetwerk. Vyf kennis-uitruil gemeenskappe van ACC se kennis-netwerk word ondersoek. Die resultate beklemtoon die dinamika tussen netwerk-lede in die proses om kennis uit te ruil. Die gevolgtrekking van die navorsing is soos volg: • ‘n Strategie word ontwikkel wat die implimentering van die pragmatiese benadering tot kennis-bestuur beskryf. • ‘n Skedule vir die evaluering van ‘n kennis-netwerk word voorgestel wat bestuur die vermoë bied om die gevolge van veranderinge in ‘n kennisnetwerk te assesseer.

Geometrical discontinuities, like sharp notches, appear in constructions and engineering structures and lead to stress concentrations. These technical objects are very dangerous due to the fact that they reduce the structural conformity and can lead to crack initiation. Technical objects are not always designed as homogenous bodies but can consist of two or more materials with sharp notches on the interface of these materials. The influence of free surface on crack initiation conditions is studied and assessed by means of 3D model of sharp and bi-material notches with finite thickness. Stress fields around the singular stress concentrators are calculated with finite element method and the results are evaluated by means of criterion of critical quantity. This approach is easy applicable and can be used in combination with the knowledge of basic material properties and results of finite element analysis of the assessed notches. In order to estimate weather crack will initiate from the middle of the observed notched specimen or from its free surface, the value of averaged critical applied stress was introduced. With this value it’s possible to determine the location of crack initiation thru the sample thickness. Thru the ratio of values of critical applied stress in the middle and on the free surface of the observed specimen it’s possible to quantify the influence of the free surface on the location of crack initiation. With the use of this approach it’s shown, that the location of crack initiation depends on more parameters, loading direction, the notch opening angle and the sample thickness. In case of bi-material notches it depends also on the ratio of young modulus.