(680b) Modeling the Sonochemical Removal of Nitric Oxides Using the Rayleigh-Plesset-Noltingk-Nappiras-Poritsky (RPNNP) Bubble Dynamics Equation
NOx (NO & NO2) and SO2 ? mainly generated from combustion of fossil fuel in power plants and automobiles ? have severe detrimental effects on plants, animals and humans. Numerous environmental and health effects such as acid rain, formation of ground-level ozone, eutrophication, respiratory and heart diseases in humans are caused by NOx. Although emissions of NO2 and SO2 can be controlled easily by scrubbing, nitric oxide (NO) is not easily removed by aqueous scrubbing due to its very low solubility in such solutions. We have previously demonstrated experimentally and reported the effectiveness of sonochemical methods for the removal of NO in a bubble column reactor. Sonochemical techniques utilize ultrasound to produce an oxidative environment via acoustic cavitation due to the formation and subsequent collapse of microbubbles from acoustical wave-induced compression/rarefaction. Ultrasound creates alternating compression and rarefaction pressure fields in a liquid. This causes tiny gas bubbles trapped in the liquid or in the crevices of surfaces to expand and then violently collapse. During the expansion phase water evaporates into the bubble and condenses during the collapse. But since the collapse is extremely fast and almost adiabatic in nature most of the vapor trapped inside the bubble cannot escape and is disintegrated due to the very high temperature reached inside the bubble (≈ 3000-5000 K) to form reactive species such as hydroxyl (∙OH), hydroperoxyl (∙HO2) radicals or oxygen (O) and hydrogen (H) atoms and hydrogen peroxide (H2O2). These species diffuse out of the bubble into a thin interfacial layer where they either recombine or react with compounds present in the liquid.
This paper will present the results of a detailed model developed to investigate the actual cavitation phenomena taking place during the absorption of NO. The development of a mathematical model for such a system has never been undertaken before now. In this study we modeled the expansion and subsequent collapse of the cavitation bubble according to the theory of cavity collapse ? initially developed by Lord Rayleigh and then improved upon by many researchers. The model utilizes the complex Rayleigh, Plesset, Noltingk, Neppiras, and Poritsky (RPNNP) equation coupled with an energy balance of the bubble and the chemical reactions taking place inside the cavity to calculate the composition of the different species formed during the collapse. The model takes into consideration (i) dynamics of cavitation, (ii) generation of oxidizing species from bubble collapse through reaction kinetics, (iii) transfer of NO from from gas to liquid, (iv) chemical reactions of oxidizing species with dissolved NO. The results of the simulations indicate surprisingly that the chemistry induced by ultrasonic cavitation cannot explain the absorption of NO beyond about 30% of the inlet concentration if the mass transfer is assumed to be the same as that in a bubble column without ultrasound. When experimental values of mass-transfer coefficients, calculated in the studies by other researchers (which are in the range of about 10 times the physical mass-transfer coefficient in a bubble column), are used, absorption up to 80% are calculated in the simulations consistent with experimental results obtained from a sonochemical bubble column reactor.
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