Ultrasonic propagation in high-concentration TiO2 slurries for photocatalytic reactors

This work reports measurements of the speed of sound and ultrasonic attenuation in high concentration (up to 3 g center dot L-1) aqueous TiO2 slurries close to neutral pH. Measurements were made at frequencies of 1675, 5545, and 9340 kHz using ultrasonic transducers submerged in the slurry. The experimental setup may be applied to slurries in practical reactors while in operation. Within an experimental uncertainty of +/- 0.5 m center dot s(-1), the speed of sound in the slurry was the same as in pure water, at all the studied catalyst concentrations and frequencies. The excess ultrasonic attenuation with respect to pure water increases linearly with the catalyst concentration and is inversely proportional to the square of the excitation frequency. This indicates that viscous and thermal acoustic losses predominate over scattering. For TiO2 concentrations in the range of 0.15-3 g center dot L-1, the catalyst concentration in the slurry may be estimated as a linear function of the excess ultrasonic attenuation, with an uncertainty of 0.1 g center dot L-1.