Mechanistic optimization of a dual frequency sonochemical reactor

The present study addresses the problem of optimization of the dual frequency ultrasonic processor with a two-fold approach, viz. optimization of the resultant acoustic field in the processor, and assessment of production of radicals from cavitation bubbles. The resultant acoustic field in the processor has been optimized using two conditions, viz. highest total energy dissipation and most uniform energy dissipation in the processor. The optimization parameters are the frequency ratio (alpha) of and phase difference (phi) between the two ultrasound sources. Set of combinations of alpha and phi have been found corresponding to the two conditions mentioned above, and simulations of radial bubble motion have been carried out for these sets at three locations in the processor. Results of simulations indicate that the extent of radical production for the condition of most uniform energy dissipation is at least two fold higher than that for the highest total energy dissipation. Among the two optimization parameters, phi has been found to have greater influence on production of radicals. On the other hand, a has only a marginal impact on the sonochemical effect. A variation of 3-4 fold in alpha causes only marginal change in radical production. The effect of interference between two ultrasound waves on radical production has also been assessed. It is revealed that if the compression phase of the coupling frequency coincides with the transient collapse of the bubble initiated by the base frequency, the intensity of the collapse receives an additional boost resulting in greater production of radicals. Some recommendations for the design of the dual frequency processor based on the results of simulations have also been given. (C) 2009 Elsevier Ltd. All rights reserved.