Hydrodynamics study of liquid-solid micro-circulating fluidised bed

Abstract

Solid-liquid circulating fluidised beds possess many qualities which makes them useful for industrial operations where particle-liquid contact is vital, e.g. improved heat transfer performance, and consequent uniform temperature, limited back mixing, excellent solid-liquid contact, good control of reaction and regeneration of catalysts or bio-solids at the same time. All these characteristics make them suitable for various industrial processes, e.g. waste water treatment, food processing, and bioconversion of agricultural-waste into lactic acid, fermentation, linear alkyl benzene production, and photo-catalytic ozonation. Despite this, they have seen no application in the micro-technology context. Solid-liquid micro circulating fluidised beds (µCFBs), which essentially involve fluidisation of micro-particles in sub-centimetre beds, hold promise of applications in the areas of microfluidics and micro-process technology. This is mostly due to fluidised particles providing enhancement of mixing, mass and heat transfer under the low Reynolds number flows that dominate in micro-devices. Albeit there are few reports on liquid-solid micro-fluidised beds, this thesis presents the first experimental study of a solid-liquid circulating fluidised bed at the microscale. It is well known that particle handling in micro technology devices remains one of the big challenges in the field. Development of a micro circulating fluidised bed is providing one solution to the problem, e.g. for solid catalyst recovery, recycle and regeneration. This thesis reports on the design and study the hydrodynamics of a liquid-solid microcirculating fluidised (LS µCFB) systems for possible applications as novel micro (bio)-reactors and diagnostics device, high-throughput kinetics screening, high-heat flux cooling and others. In order to successfully implement this, it is very important to understand the hydrodynamic parameters such as the influence of surface forces and inevitable wall effect on minimum fluidisation velocity, and bed expansion dynamics as they play a crucial role in the hydrodynamic behaviour and determine the bed performances, as well as dictating the solidliquid contacting. The experimental research was performed in a novel micro-circulating fluidised bed which was made by micro-machining channels of 1mm2 cross section in Perspex. (Polymethylmethacrylate (PMMA) and soda lime glass microspheres particles were used as the fluidised particles and tap water and glycerol of different concentration (5, 10, and 15 vol. % aqueous glycerol) as the fluidising liquid to study the hydrodynamics of solid-liquid ii fluidisation in micro-circulating fluidised bed channel. Furthermore, additive manufacturing technology, digital light processing (Miicraft+ printer) and stereolithography (Form2 printer) were also used to fabricate the novel micro-circulating fluidised bed. This allowed the rapid fabrication of a reliable micro-circulating fluidised bed using low cost material and most importantly, the bed geometry could easily be modified. Two novel measurement techniques, the valve accumulation and digital particle image velocimetry (PIV) methods were developed to measure the particle velocity in the microcirculating fluidised bed system, and the results looks relevant when compared with previous reported studies. As in a macroscopic circulating fluidised bed, the solid flux in a microcirculating fluidised bed increases with liquid velocity in two distinct zones, increasing sharply first then levelling off at higher inlet fluid velocities. The result indicates that fluidisation in a solid-liquid micro circulating fluidised bed system could be categorised in four operating regimes like in macroscopic case: fixed bed, conventional fluidisation, circulating fluidisation, and transport regime. However, the surface forces influence strongly the minimum fluidisation velocity which can be up to 20 times bigger for the smallest PMMA microparticles while the increase is only minor for glass particle (less than 2 times for the same size smallest glass microparticles). The determined critical transition velocity is comparable to the particle terminal velocity, i.e. the normalised transition velocity is approximately 1 in line with previous macroscopic studies. Yet, there was a weak increase in the normalized transition velocity with particle size which is probably due the wall effects (higher particle to bed ratio). In addition, the normalised velocity is slightly higher for PMMA particles due to stronger adhesion and cohesion forces, but influence is minimal in comparison with influence on the minimum fluidisation velocity. Finally, it seems that transition to the transport regime is influenced by cohesion so the relative transition velocity for PMMA particles is around 20 times particle terminal velocity while it is only 5 times for the glass beads. Consequently, the conventional regime is proportionally bigger for the glass beads in comparison with PMMA particles, whist the situation is opposite for circulating fluidisation regime as it is bigger for PMMA particles. The study also confirms that fluidisation behaviour in a liquid-solid micro-circulating fluidised bed system is also influenced by bed geometry such as the size of solid feed pipe cross section and the angle between the riser and solid feed pipe. iii The results also show that solid flux in a micro-circulating fluidised bed is influenced by the viscosity of the fluidised liquid. The minimum superficial liquid velocity at which particles fluidisation is achieve decreases with increasing liquid viscosity. The reduction in the minimum fluidisation velocity with an increase in the liquid viscosity is mostly due to the fact that viscous systems have a lower ratio of adhesion to drag force.