Enhancing energy efficiency and mass transfer in solid-liquid systems using mechanical mixing and cavitation
Creators: name: Stoian, Daniel
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401205
subject: Experimental methods in fluid flow, heat and mass transfer
classificationCode: 401904
subject: Mineral processing/beneficiation
classificationCode: 510301
subject: Acoustics and acoustical devices; waves

description: Solid-liquid agitated vessels are widely used in the mineral process industry where there is a strong demand to intensify existing vessels to process more ore. It has been shown that process intensification can be achieved by increasing solids throughput or by optimising process equipment and conditions. This can be achieved by operating a taller/larger agitated vessel fitted with multiple impellers or by increasing the extraction yield via cavitation. These methods, however, require the addition of extra energy. Therefore, the main objective of this work is to determine the optimum vessel/mixer design and operating condition that will ensure high impeller energy efficiency, increased rates of solid-liquid mass transfer, and high sonochemical reaction yields.<br><br>Impeller power consumption and mass transfer experiments were carried out in 0.2 m diameter (T) cylindrical tanks with liquid height (H) fixed at H=T and H=1.5T for all single- and dual-impeller systems, respectively, to study solids suspension, dispersion, and mass transfer coefficient (kSL). Aqueous NaOH and cationic exchange resins were used as the liquid and solid phases, respectively. Experiments studying the effect of solids concentration, particle type and particle size on cavitation activity were carried out in the 0.2 m agitated vessel irradiated with ultrasound and fitted with an A310 impeller. Cation exchange resins, sand, and glass spherical beads of different sizes were used as the solid phases and aqueous potassium iodide (KI) as the liquid phase. Experiments investigating mass transfer enhancement with cavitation were carried out in the 0.2 m agitated vessel irradiated with ultrasound and fitted with a Rushton turbine. Aqueous NaOH and cation exchange resin were used in the ‘ion exchange’ system whereas polymeric resin saturated with phenol and water were used in the ‘desorption’ system.<br><br>Experimental results show that the Zwietering correlation can be reliably applied to all mixing systems used in this study. It was found that by operating the system at a volumetric solids concentration (CV) at 0.2 (v/v), the specific power consumption is minimised while simultaneously achieving maximum kSL values. Particle dispersion was found to generally increase with increasing CV. The removal of baffles was shown to decrease impeller power requirements for solids suspension and dispersion, but however, its influence on kSL was dependent on impeller type and vessel geometry. Overall, improved energy efficiency and increased solids throughput can be achieved using radial flow impellers under unbaffled conditions at relatively higher CV. Using experimental data, mathematical correlations to estimate impeller power consumption and kSL were derived and estimated values were found to fit experimental data within a ±15% band.<br><br>It was found experimentally that cavitation activity decreases with increasing solids concentration up to 0.1 (v/v) but increases thereafter up to 0.4 (v/v) followed by a further decrease. Cavitation activity increases with increasing particle diameter and surface roughness plays an important role in enhancing sonochemical yields.<br><br>Experimental results suggest that ultrasonic irradiation had no effect on solid-liquid mass transfer rates in the ion exchange system. However, the rate of phenol desorption into water from polymeric resin increased in the presence of ultrasound. The enhancement of mass transfer due to ultrasound was the greatest at CV = 0.1 and 0.15 (v/v). This suggests that increased solid-liquid mass transfer in the presence of ultrasound is dependent on the physical properties of the solid and liquid phases as well as CV. The ultrasonic power consumption and the impeller power consumption are compared and from the results, it is clear that mechanical agitation is more efficient at intensifying solid-liquid agitated vessels.