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The effect of packing on the hydrodynamic and mass transfer characteristics of a co-current downflow contactor (the CDC) is studied in the present work in order to investigate both the viability of this bubble column as a fixed bed catalytic reactor, and the fundamentals of the CDC fixed bed mode. In order to achieve these objective a comparison between the hydrodynamics and the performance of the CDC unpacked and fixed bed modes are made for the coalescence system of oxygen-water. Indirectly, qualitative information is also obtained about the hydrodynamic response of the downflow co-current operation mode to the presence of packing. This response differs from that reported in the literature for co-current up-flow operation mode in bubble column. Commercial packing such as Pall Rings, Raschig Rings, Intalox Saddles and Bert Saddles of various sizes and void fractions are tested. Packings are classified as packings of low voidage (52-72%) and packings of high voidage ( 72-98%) according to the nature of the disturbance cause by the packings on the bubble dispersion and co-current downtlow operation. Maximum disturbance on the co-current downflow bubble dispersion is observed with packing of low voidage. For coalescing system, the nature of the bubble dispersion is affected by the addition of high voidage packing in the system since the balance between the coalescence and break-up rates changes in the packed section in the packed section. Depending on the type of the packing and the properties of the packed section such as packing arrangement, ratio between the column diameter and packing equivalent diameter, flow instability in the packed section may be established. For systems where flow instability is established the amount of packing wetted decreases, also a higher requirement of energy input than that for unpacked mode for similar operation condition, is observed. The 2Smm Pall Ring stacked show less capacity to disturb the bubble dispersion in the packed section. The coalescence properties of the CDC packed section are investigated. From the studies emerged the possibility of attaing co-current downflow stable operation in the CDC when high voidage packing (dumped) is used, by controlling the rate of the coalescence-beak-up inthe packed section by lowering the surface tension. The bubble dispersion of the CDC unpacked mode can not be called homogeneous since two different regions of different mean bubble sizes are observed along the CDC length (the inlet and bulk regions). Due to the existence of these zones, the interfacial area per unit volume, the volumetric liquid-side mass transfer coefficient and liquid-side intrinsic mass transfer coefficient, are higher in the inlet region than in the bulk region. The presence of packing affects the gas hold-up values for a given operation condition, A maximum value of 78% was found for the 25mm Bed Saddles and 25mm Raschig Ring dumped. The volumetric liquid-side mass transfer coefficient is affected by packing size, type, arrangement mode. A maximum value of 0.294 S·l was found for the 25mm Berl Saddles. A model to predict the mass transfer coefficient is proposed.

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In this Thesis we study theoretically some aspects of the dynamics of soft active suspensions. Following a bottom-up approach, we derive the continuum equations for a suspension starting from underlying models for the microscopic dynamics, where the microscopic objects are constructed out of spheres subjected to forces at low Reynolds number. Examples of systems examined here are collections of rotating objects in two dimensions, swimmers and a chain of oscillators. Their macroscopic properties are described by means of scalar and pseudo-scalar or vector and tensor or complex-valued quantities. In our work we explicitly address the fast, periodic nature of the microscopic dynamics and devise a general framework, that is based on a procedure of averaging, to incorporate these dynamical features and study their effects on the long-wavelength, low-frequency description of the system. There, all the resulting parameters are determined by time-averages of the microscopic properties. By exploiting a superposition of rotations we introduce a novel, minimal model swimmer at low Reynolds number that can both translate and rotate in a plane. We examine its dynamics by extending the analysis of the internal motions to the regime of strong deformation and compare to the well-known three-bead swimmer under the same conditions. For swimmers suspensions we consider a model where it is possible to generate spatially homogeneous symmetry breaking and examine its stability to small fluctuations. We find that the hydrodynamic interactions are enough to promote order; this, however, is unstable to small perturbations that exceed a critical angle from the ordering direction. Finally we introduce a new generic model of self-sustained oscillator to study hydrodynamic synchronisation. Here we find an interplay between hydrodynamic interactions and the oscillators features that make them good synchronisers. We then investigate the long-wavelengths dynamics of an array.