Suspension of solids in mechanically agitated vessels is important industrially for many solid-liquid processes such as dissolution and leaching. This becomes especially important for process intensification in agitated vessels which involves processing high solids concentration slurry for the purpose of increased throughput per unit volume without major changes in the geometry of the existing infrastructure. To achieve the off-bottom suspension of high concentration slurry, impeller speed and power draw need to be increased substantially but there is no guarantee this will lead to improved solid-liquid mass transfer. Solid-liquid mass transfer in an agitated vessel has been studied extensively during the last few decades but the knowledge is limited to low solids concentration systems. Mass transfer in systems with high solids loading is not fully understood yet. Also, there have been very few attempts to investigate the effects of active particle mass fraction on solid-liquid mass transfer in agitated vessels. This study investigates the effects of solids loading and the active particle mass fraction on solid-liquid mass transfer in an agitated dissolution system. Experiments were carried out in a 0.2 m diameter cylindrical perspex vessel equipped with four equally spaced baffles. A six-bladed Rushton turbine and 45°pitched blade impeller were used as the impellers. Glass particles coated with benzoic acid (active particles) and water were used as the solid and liquid phases, respectively. Total solids concentration Cv was varied from 3 to 30% (v/v) and the concentration of active particles (benzoic acid coated particles) in the total solids was varied from 3 to 10% (v/v). The critical impeller Suspension of solids in mechanically agitated vessels is important industrially for many solid-liquid processes such as dissolution and leaching. This becomes especially important for process intensification in agitated vessels which involves processing high solids concentration slurry for the purpose of increased throughput per unit volume without major changes in the geometry of the existing infrastructure. To achieve the off-bottom suspension of high concentration slurry, impeller speed and power draw need to be increased substantially but there is no guarantee this will lead to improved solid-liquid mass transfer. Solid-liquid mass transfer in an agitated vessel has been studied extensively during the last few decades but the knowledge is limited to low solids concentration systems. Mass transfer in systems with high solids loading is not fully understood yet. Also, there have been very few attempts to investigate the effects of active particle mass fraction on solid-liquid mass transfer in agitated vessels. This study investigates the effects of solids loading and the active particle mass fraction on solid-liquid mass transfer in an agitated dissolution system. Experiments were carried out in a 0.2 m diameter cylindrical perspex vessel equipped with four equally spaced baffles. A six-bladed Rushton turbine and 45°pitched blade impeller were used as the impellers. Glass particles coated with benzoic acid (active particles) and water were used as the solid and liquid phases, respectively. Total solids concentration Cv was varied from 3 to 30% (v/v) and the concentration of active particles (benzoic acid coated particles) in the total solids was varied from 3 to 10% (v/v). The critical impeller speed Njs required to ‘just suspend’ the solids off the tank bottom was determined by measuring the sedimentation bed height (HB) visually and defining the impeller speed at which HB becomes zero as Njs. The changes in the conductivity values of water due to the dissolution of benzoic acid from solid surface were measured as a function of time and used in determining the volumetric solid-liquid mass transfer coefficient kSLap. Experimental results show that, regardless of Cv used, kSLap increases with the impeller speed gradually up to Njs and remains more or less constant beyond that. It has been also found that kSLap increases rapidly with an increase in Cv from 3 to 10 % (v/v) and remains more or less constant beyond that, regardless of the active particle concentration used. It was interesting to note that kSLap values increases for solids with higher active particle mass fraction for all Cv used. These results suggest that higher values of kSLap can be achieved in agitated vessels by operating them at higher total solids loadings with higher active particle concentration than hitherto thought. When these results are considered in conjunction with the specific impeller power input results, it can be concluded that operating the agitated dissolution vessels with a higher solids concentration will lead to higher dissolution rate but with lower impeller specific power input or higher energy efficiency. Results from this work will provide a framework for achieving process intensification in industrial agitated solid-liquid systems and a further understanding of the design of solid-liquid agitated vessel handling high concentration solids.
展开▼
