Novel Solar-Based Photocatalytic Reactor for Degradation of Refractory Pollutants Sanjay P. Kamble, Sudhir B. Sawant, and Vishwas G. Pangarkar Institute of Chemical Technology, University of Mumbai, Matunga, Mumbai 400 019, India DOI 10.1002/aic.10125 Published online in Wiley InterScience (www.interscience.wiley.com). Keywords: photocatalytic degradation, slurry bubble column reactor, concentrated solar radiation Considerable information on the chemical aspects of photo- catalytic degradation (PCD) is available in the literature, which was recently reviewed by Bhatkhande et al. (2001). Notwith- standing the substantial advantages of PCD, this technique is yet to be implemented on large scale for treatment of industrial wastes. The main hurdle in the commercialization is the lack of suitable hardware—that is, a reactor—which gives high space– time yields. The PCD process is discussed in detail by Bhat- khande et al. (2001). The electrons generated in the process are removed by oxygen supplied through air. Thus, the supply of oxygen to the catalyst is of significant importance. Further, it is now established (Mills et al., 1993a; Turchi and Ollis 1990) that PCD occurs by attack of photogenerated OH  on the adsorbed substrate. The latter needs to diffuse from the bulk liquid to the catalyst surface before it is adsorbed. The adsorp- tion process can be considered as relatively rapid and hence at equilibrium. The mechanism outlined above indicates that a commercial reactor should have the following attributes: (1) High catalyst surface area per unit volume (2) Maximum penetration of the incident radiation to all parts of the reaction volume (3) High rates of oxygen and substrate mass transfer from the respective phases to the catalyst surface Degussa P-25, a mixture of anatase (70%) and rutile (30%), is the most widely used photocatalyst. This catalyst has a very small particle size (30 nm), and thus its use in free form creates serious problems of filtration. In view of this, most investiga- tors have used the catalyst in immobilized forms. Two types of immobilized photocatalysts have been used: (1) Catalyst immobilized on fixed surfaces (such as Pyrex glass tubes and optical fibers; Anderson et al., 1993; Hofstadler et al., 1994; Yatmaz et al., 1993; Yue and Puma, 1999). (2) Catalyst immobilized on silica particles (Haarstrick et al., 1996; Ray, 1999). In the first type the drawbacks of poor diffusive transport and low catalyst surface far outweigh the advantage derived by immobilizing the catalyst (that is, no filtration). In the second case the catalyst carrying particles are fluidized, thus allowing turbulent mass transfer. This type has three drawbacks: (1) limited surface area, (2) loss of catalyst resulting from attrition, and (3) low catalyst loading needed to allow good transmission of photons to the interior. One alternative that can overcome this problem lies in the use of candle filters that rely on surface filtration under condi- tions such that there is a zone of high shear near the filter surface, which prevents solid deposition. With this alternative it is possible to use fine photocatalysts without any filtration problem. The high surface area per unit mass of this catalyst allows low catalyst loading (1 wt %). For instance, 0.5 wt % loading of the catalyst in a predominantly aqueous solution affords a surface area of about 200 m 2 /m 3 of the reactor volume, which is far greater than what can be achieved with an immobilized catalyst. Further, the low photocatalyst loading can allow better penetration of the incident photons. Figure 1 shows the configuration of the slurry bubble column used. The air passed into the reactor was presaturated with water. The candle filter is located in the vicinity of the air sparger. In this region there is a relatively high (several m/s) liquid circulation velocity, which prevents deposition of the solid on the filter surface. The liquid hydrostatic head in the column serves as the driving force for filtration, allowing retention of the catalyst in the reactor. The small particle size and catalyst loading allow ease of solid suspension at relatively low (0.03– 0.05 m/s) air sparging rates. Thus, the energy requirement for air sparging can be kept low. In the case of artificial UV radiation, the efficiency of conversion of electricity is another factor that affects the economics of PCD. The efficiency of conversion of electricity to the UV radiation and its subsequent use in PCD is very low (Mills et al., 1993a). Solar radiation, which is abun- dantly available in the equatorial and tropical countries (such as South America, African countries, Middle East countries, In- Correspondence concerning this article should be addressed to V. G. Pangarkar at vgp@udct.org (or, v_pangarkar@hotmail.com). AIChE Journal 1647 July 2004 Vol. 50, No. 7