Process Intensification through Acoustic and Hydrodynamic Cavitation

Abstract: Process industries are cornerstones in today’s industrialized world. They contribute significantly to the development of diverse commodities and materials that are used in our daily lives. Process intensification is an approach implemented to boost manufacturing efficiency and capacities in a more sustainable and energy efficient way. The focus of this thesis is to utilize the concept and advantages of hydrodynamic and acoustic cavitation in the ultrasonic range. High-intensity cavitation can improve the physical and chemical properties of a wide range of materials and provides a sustainable alternative for process intensification. Although the use of hydrodynamic and acoustic cavitation techniques have become advantageous, applications in process industry are still limited, as the approach needs thorough refinement based on several process parameters and complications encountered in a large-scale implementation. In order to address challenges such as stability and robustness as well as energy conservation and high flow speeds, scalable reactor designs are essential for industrial applications.  This research focuses on the methods to develop and maximize cavitation activity in a flow-through reactor. The application comprises of hydrodynamic activation of tiny gas bubbles in the fluid to be excited and collapsed by high intensity ultrasound. The transient collapse of cavitation bubbles and clouds of bubbles generates high temperatures, extreme pressures and shockwaves in a microscale, leading to both a physical and chemical impact. To achieve an efficient energy transfer and conversion optimization with respect to physical and process related parameters are needed. The optimization of the reactor design requires both experimental and numerical investigations. Numerical simulations have been carried out with the help of a commercially accessible multiphysics simulation software that incorporates acoustics, structural dynamics, fluid dynamics and piezoelectrics. The reactor design methodology is validated by measurements of impedance and acoustic pressure as well as aluminum foil erosion and calorimetric tests. The developed cavitation reactors have been implemented in two case studies: I) Modification of cellulose fiber properties and II) Leaching of metals from mineral concentrates. In case study I, the developed method for fibrillation of cellulose fibers enables an energy-efficient change in mechanical properties of the fiber wall. As a consequence of cavitation, fibers are exposed to shear forces and micro-jets, inducing peeling, swelling, delamination and external and internal fibrillation. The parameters of significance are excitation frequency, electrical power, flow characteristics, concentration (viscosity), static pressure and temperature.  The maximum flow rate of the reactor is 80 l/min and power density is 0.45 W/cm3. The developed reactor has a 36 % power conversion efficiency and is well adapted for scale-up. The critical aspect is to balance the contribution of hydrodynamic and acoustic cavitation to the pulp properties. For high temperature chemi–thermomechanical pulp (HT-CTMP) from spruce, the best quality of fiber properties was obtained at 1.5 % concentration and 60° C using an electrical energy supply of 386 kWh/bdt.  In case study II, the aim of the investigation was to explore the impact of hydrodynamic and acoustic cavitation (HAC) on the leaching ability of tungsten. The objectives were to minimize leaching time, reduce energy usage and increase the recovery rate. Various experimental conditions such as dual-frequency excitation and various orifice geometries have been explored during this investigation. The reactor was excited by 23 kHz and 39 - 43 kHz frequencies in different flow settings. The effects of leaching time, temperature, acoustic pressure and geometry of the orifice plate have been studied. The leaching temperature varied from 40°C to 80°C. The concentration of sodium hydroxide (NaOH) leaching agent was 10 mol/L. The results has been compared to traditional chemical and laboratory autoclave leaching. The energy enhancement of 130 kWh/kg concentrates acoustic cavitation resulting in a 71.5 % leaching recovery of tungsten (WO3), relative to 36.7 % obtained in the absence of ultrasound. The developed method is found to be energy effective and provides a higher recovery rate than current chemical methods at lower temperature and static pressure.Energy efficient process intensification requires hydrodynamic initiation of cavitation bubbles, high acoustic cavitation strength by several excitation frequencies tailored to the reactor's optimized design and optimum process pressure and temperature concerning the materials to be processed. The cavitation effect improves extensively in the flow-through mode and offers stable results. The effect of flow conditions and hydrodynamic cavitation at the same ultrasonic power input is essential and nearly doubles the yield.