2982
Ind. Eng. Chem. Res. 2005, 44, 2982-2989
Simulation Model of the Decomposition Process of Phenol in Water by Direct Contact of Gas Corona Discharge in a Cylindrical Reactor Noriaki Sano* and Daisuke Yamamoto Department of Mechanical and System Engineering, Himeji Institute of Technology, University of Hyogo, 2167 Shosha, Himeji, Hyogo, 671-2201 Japan
An empirical model is proposed for the decomposition process of phenol in water by direct contact of gas corona discharge in a cylindrical wetted-wall reactor. The proposed model is based on an assumption that reactive radicals in water, OH, induced by the corona discharge are the main species to decompose phenol and its derivatives. The phenol degradation characteristics calculated by this model are successfully correlated with the experimental data obtained with many varied parameters, such as the discharge current, diameter of the wire cathode, axial length of the discharge zone, radius of the cylindrical anode, and water flow rate. The calculated results can also show the effect of the ion wind. The proposed model will be useful to the design of water purification systems using the relevant method, which has lately been developed as an effective technique to decompose stable organic compounds.
Introduction It is widely known that high-voltage electric discharge can be used for some applications, for example, modification of a polymer surface,1 ozone (O3) generation,2 gas purification,3-5 and water purification. For water purification, traditionally the corona discharge is used to generate O3 in a gas stream and the O3-containing gas is carried into the water, which is contaminated with target contaminants.6,7 In such systems, O3 dissolved in water oxidizes organic compounds. Although O3 is effective in oxidizing some organic contaminants, its reactivity is not strong enough to decompose some stable species. Lately, there is a demand to develop effective water purification methods to degrade some hazardous compounds that have strong toxicities even with extremely low concentrations, such as dioxins and endocrine disruptors. Therefore, it is important to continue efforts to develop effective water purification methods that may be stronger than the O3 oxidation method. In the past few years, some water purification methods that directly use short-lived radicals produced by high-voltage electric discharge have been developed.8-10 For instance, a pulsed discharge is generated in water to supply the reactive radicals to target water from a submerged corona spot.11,12 As an alternative concept, the reactor using the direct contact of gas corona discharge with water has been investigated for an advanced technique.13-21 The reactivity in this reactor is considered to be high because gaseous ionic species and short-lived radicals, which are not used by O3 oxidation for water treatment, can be utilized here. * To whom correspondence should be addressed. Tel.: +81-792-67-4845. Fax: +81-792-67-4845. E-mail:
[email protected].
Recently, it was found that the electron efficiency and the energy efficiency are significantly improved if a cylindrical anode replaced the plate anode.19 Because the cylindrical-anode corona discharge reactor was adopted for water purification, the effects of some operational parameters and reactor structures have been investigated,19,21 for instance, the discharge current, diameter of the wire cathode, axial length of the discharge zone, radius of the cylindrical anode, and water flow rate. Although such qualitative information is important to considering the reactor design, to quantitatively simulate the water purification characteristics of this reactor, a kinetic model is needed in the design of the water purification systems using this type of reactor. In this study, we present a simple model of the decomposition of phenol in water by the relevant reactor to simulate the degradation process and the effects of the operational parameters and reactor structures. Experimental Section The apparatus is described in the previous papers.19-21 To obtain the experimental data, a corona discharge reactor consisting of a cylindrical anode (stainless steel, length ) 200 mm) covered with a falling-water film was used in this study. A stainless steel wire cathode (diameter ) 0.34, 0.68, 1.15, and 1.55 mm) was stretched along the axis of this anode (i.d. ) 20, 30, 34, and 40 mm). A direct current (dc) high voltage of negative polarity (
