- Posted by Okechukwu Anosike on December 17, 2009
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There is little doubt that cavitation does influence the chemical acceleration process and so a little in sight of the mechanism and unanswered question is necessary. The subject has been and will continue to be covered by people in this field and there is much detailed published work. Some important references considering much of the details are given but the following gives a brief general explanation.
Cavitation may occur when applying high intensity ultrasound to liquids. In generating cavitation a sinusoidal pressure is superimposed on the steady ambient pressure. All liquids contain small gas bubbles in suspension or gas entrained in colloidal solids. Their response to the alternating sound field may be violent or rather gentle, depending on pressure levels, frequency of the alternating pressure and other ambient conditions. The violent form is known as transient cavitation and the gentle is known as stable cavitation. Stable cavities are often non linear bubbles that oscillate around some equilibrium size. They are relatively permanent and can continue to oscillate for many cycles of acoustic pressure. On the other hand, transient cavities usually exist for less than one cycle. If the acoustic pressure is great enough for the liquid to go into tension during the negative half cycle, then the cavities will expand rapidly often to many times their original size. Following this on the positive half-cycle, they collapse violently often breaking up into many smaller bubbles. It is during this phase that the well known disruptive effects of cavitation occur.
Now in my experience, for this phenomena to occur at it’s optimum and have the desired effect within a reactor one must determine energy inputs with reference to frequencies which generate this effect. It has been seen that during reactions which should have defined behavioural patterns, reactions that should generate phase changes tend to occur at a pace of twice the reaction time or less than what was estimated. It is therefore necessary that if we are looking at J/litre energy input within a transducer of 20KHz, one must determine the optimum reaction corridor for which your reaction will occur within reasonable kinetics. It should be noted that this cavitation sequence normally present what I call an energy field within which the reaction within the reactor is fastest. When loads of energy is normally jammed into a reaction it is expected that the reaction will run twice faster but in the case of ultrasound, I have found out above a certain threshold, you have what I refer to as a field block effect where the original and optimal field effect is not longer achieved and the waves form a blocked ring that does not allow the ultrasound have it’s optimal field effect. Studies are still be carried out to determine why this happens and how much of kinetics is responsible in facilitating this phenomenon. Experimental work is still being carried out and I will update as I get results towards the effect in reaction engineering.
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