A novel micro-fluidic-jet-spray-dryer equipped with a micro-fluidic-aerosol-nozzle : equipment development and applications in making functional particles

Spray drying is a continuous operation that converts atomized liquid droplets into dry particles usually in a high temperature environment. The concept of spray drying was firstly reported over 150 years ago. Due to its scalable feeding capacity and simple mechanical device manipulation spray drying has become one of the most important powder manufacturing methods in both research laboratory and industry. The applications of spray drying technology has been expanded to an increasingly wide range, covering food, pharmaceutical, agrochemical, detergent, pigment, ceramic, chemicals, etc. In the last three decades significant amount of research and development has been conducted in the area of spray drying in order to handle various precursor materials, to obtain desirable product characteristics, and to reduce materials waste and energy cost. For particle engineering/science research, spray drying, compared to wet-chemistry process, avoids extensive conjugation or purification steps and offers a more efficient means for obtaining powders in one single step to complete encapsulation, complex formation and polymerization. In terms of product quality, there are always specific requirements on particle properties to realize optimal applications. For example, in drug delivery research, there exists desirable particle size range that is dependent on the targeted organs/cells and the routes of administration. For pharmaceutical controlled release applications, the uniformity of particle size and of morphology can be highly beneficial since wide distribution of particle size could lead to more undesirable ‘burst’ effects. For synthesizing functional inorganic products, e.g. photoluminescence, catalysis, and ceramic materials, non-agglomerated spherical particles are usually desirable to increase the packing density and to elevate PL yield or catalysis efficiency. In terms of spray drying kinetics and particle formation studies, the validation of various mathematical models can be facilitated by the narrow size distribution of droplets/particles. Conventional spray dryers, however, suffer from uneasy controllability on particle characteristics (e.g. particle size, size distribution, density, porosity, shape, morphology, moisture content, etc.) and functionalities (e.g. solubility, dispersibility, flowability, hygroscopicity, etc.). It is mostly ascribed to the size polydispersity of the atomized droplets and complicated air flow pattern making the droplets undergo different thermal-mass transfer histories and evaporation processes despite within the same dryer chamber. In addition, complex drop/particle travelling trajectories may result in unwanted particle agglomeration. In this thesis, a specially designed monodisperse droplet generator, referred as micro-fluidic-aerosol-nozzle (MFAN), is presented. Moreover, design and fabrication is introduced for a novel micro-fluidic-jet-spray-dryer equipped with MFAN as the atomizer, capable of synthesizing particles with uniform characteristics. Numerous trials using commercially available monodisperse droplet generators encountered several challenges, such as complicated operating procedures, limited ability to handle viscous liquid, vulnerability to blockage and low yield in droplet production, etc. Thus special designs of the MFAN have been motivated with a few improved features making it an excellent option as the spray dryer atomizer. A cost-efficient visualization means of using a digital SLR camera with a strobe and a micro-lens has been shown to be sufficiently effective to monitor the droplet generation process and to measure the droplet size. The MFAN operating mechanism has been investigated to construct an operability diagram for monodisperse droplet generation. It has been found that a wide frequency range is able to produce monodisperse droplets. The size of monodisperse droplet formed by the MFAN is ranged from 15 to 600 μm. A range of different materials have been tested using the MFJSD, such as functional food and pharmaceutical carrier particles, ceramic and catalysis composites. The size of uniform particle can be made from 10 to 300 μm. Two experimental case studies using the MFJSD are introduced. Firstly, non-agglomerated and uniform microcomposites containing alpha-D-lactose monohydrate, silica nanoparticle, and Eu(III) have been synthesized by the MFJSD-II. The as-prepared particles have been characterized using the light microscope, field emission scanning electron microscope and energy dispersive X-ray photoelectron spectrometer. The impact of the precursor composition, ingredient concentration, and drying temperature profile on the particle size, morphology, structure, and element distribution in the composite matrix has been investigated. Distinct from the spherical lactose microparticles with smooth surface, both silica nanoparticle/lactose and Eu(III)/silica nanoparticle/lactose microcomposites are of a bowl-like shape with smooth top and wrinkled bottom. Two possible mechanisms induced by the presence of colloidal silica nanoparticles have been proposed for the formation of such composite structure. X-ray diffractometer and Fourier transform infrared spectra have been carried out to identify the physical state and surface chemistry of the particles. Transmission electron microscope images of the calcined spray-dried Eu(III)/silica nanoparticle/lactose microcomposites shows the homogeneous distribution of Eu2O3 nanoparticles. Photoluminescence spectra indicate that high Eu(III)-doped concentration has led to the photoluminescence intensity enhancement, whereas spray drying temperature has shown little impact on the photoluminescence property. Secondly, uniform-sized vitamin B12-loaded silica microencapsulates have been synthesized via the MFJSD-I. Particle size and morphology have been characterized by field-emission scanning electron microscope. With similar initial droplet size and spray drying conditions, added of either alpha-D-lactose monohydrate or Na-alginate generally results in larger particle size. Furthermore, additive lactose facilitates the formation of spherical particles with relatively smooth surface, whereas the presence of Na-alginate renders the particles buckled with either smooth or rough surface, depending on additive amount. In vitro vitamin B12 release and silica-matrix degradation profiles have shown significant dependence with the additive type and content. In general, lactose accelerates both of the drug release and matrix degradation, likely due to the relatively fast lactose dissolution facilitating the buffer penetration. On the contrary, Na-alginate appears to serve as an additional barrier decelerating the matrix degradation and prolonging the drug release. Increasing drug loading accelerates the drug release, especially in first few hours of the process, probably due to the drug enrichments near or on the particle surface increasing the driving force for mass transfer. However, the drug loading amount tested in this study has shown no obvious impact on the silica-matrix degradation rate. The drug release kinetics studies indicate the drug diffusion-dependant release mechanism regardless the additive type and amount.