Photocatalytic wall reactor

SCALING-UP OF A PHOTOCATALYTIC WALL REACTOR WITH RADIATION ABSORPTION AND REFLECTION... 

Imoberdorf GE, Cassano AE, hazoqui HA, Alfano OM (2007) Optimal design and modeling of annular photocatalytic wall reactors. Catal Today 129 118-126... 

Photoreactors for dissolved pollutants operate by bubbling a carrier gas (usually air) through a reservoir of liquids (usually water). The pollutant-loaded gas is then passed though the photocatalytic reactor and released. Several t)q)es of reactors have been connected with this application, including PBRs and coated wall reactors. 

The advantages of microreactors, for example, well-defined control of the gas-liquid distributions, also hold for photocatalytic conversions. Furthermore, the distance between the light source and the catalyst is small, with the catalyst immobilized on the walls of the microchannels. It was demonstrated for the photodegradation of 4-chlorophenol in a microreactor that the reaction was truly kinetically controlled, and performed with high efficiency [32]. The latter was explained by the illuminated area, which exceeds conventional reactor types by a factor of 4-400, depending on the reactor type. Even further reduction of the distance between the light source and the catalytically active site might be possible by the use of electroluminescent materials [19]. The benefits of this concept have still to be proven. 

Tubular reactors are probably the most common photocatalytic reactors. Their popularity stems, most likely, from their simplicity. They are characterized by a gas flow along the axis of a tube, which contains the photocatalyst in many possible forms such as a thin coated film on its wall, fluidized particles, a coated monolith, or even as a free powder resting on an appropriate support. The light sources are located, in most cases, externally to the tube, in a parallel configuration relative to its axis. Reflecting surfaces encompass the lamps array, assuring that the only absorbance of photons would be that of the photocatalyst (Figure 7). 

The number of carriers collected (in an external circuit, for example) versus those optically generated defines the quantum yield (C>), a parameter of considerable interest to photochemists. The difficulty here is to quantify the amount of light actually absorbed by the semiconductor since the cell walls, the electrolyte and other components of the assembly are all capable of either absorbing or scattering some of the incident light. Unfortunately, this problem has not been comprehensively tackled, unlike in the situation with photocatalytic reactors involving semiconductor particulate suspensions where such analyses are available [204-207]. Pending these, an effective quantum yield can still be defined. 

These results highlight a) Photodegradation reaction rates should be defined on the basis of phenomenologically meaningful parameters, case of W rr, b) Reaction rate evaluation is a task that should be developed carefully, accounting for possible nonidealities in the photocatalytic reactor such as particle wall fouling. 

In typical fixed photocatalytic reactors, the photocatalyst can be coated or anchored on the reactor walls around the light source casing or attached to a solid matrix. Since Ti02 is not present in the water or air streams at any time, these reactors have the intrinsic advantage of not requiring a catalyst recovery operation. Relevant examples of this type of reactor are the coated fiber optic cable reactor and the multiple tube reactor (Peill and Hoffmann, 1995, 1996, 1997, 1998, Ray and Beenackers, 1998a). 

FIGURE 4.1. Macroscopic radiation balances around the catalyst suspension and the reactor inner wall. (Reprinted from Chem. Eng. ScL, 59, M. Salaices, B. Serrano and H.l. de Lasa, Photocatalytic conversion of phenolic compounds in slurry reactor s, 3-15, Copyright 2004 with permission from Elsevier). 

A low pressure UV lamp (11W, Amax = 253.7 mn) was positioned vertically inside the quartz glass cylinder in the middle of the photocatalytic zone. Air was supplied from a porous titanium plate directly below the membrane module. The purpose of the aeration was to provide dissolved oxygen for photoreaction, to fluidize the IIO2 particles and to create sufficient turbulence along the membrane surface. The reaction temperature was controlled by using cooling water. Permeate was withdrawn from the system with the help of a suction pump. A water level sensor was used to maintain a constant level of solution in the reactor. Additionally, the exterior wall of the reactor was covered with a reflecting aluminum foil to improve the efficiency of UV utilization. 

H. Nakamura, X. li, H. Wang, M. Uehara, M. MiyazakL A simple method of self assembled nano-particles deposition on the micro-capillary inner walls and the reactor application for photocatalytic and enzyme reactions, Chem EngJ., 2004, 101, 261-268. 

Honeycomb monolith reactors are commonly used in automobile exhaust emission control and for NOx reduction in power-plant flue gases by catalytic reduction, but they also can be used for photocatalytic reactions in the gas phase (see Fig. 7.2c). This type of reactors contains certain number of channels of circular or square cross section. The photocatalyst is coated onto the inner walls of channels... 

Gas phase photoreactors Relatively low levels of radiation intensity needed to perform reaction Small amount of photons adsorbed by air and sufficient electron scavengers Photocatalyst in a form of thin layer -there is no separation of product and photocatalyst Higher quantum yield of photocatalytic reaction (comparing to liquid phase) Pollutant adsorption on the walls of the reactor... 

Van Grieken R, Marugan J, Soldo C, Pablos C (2009) Comparison of the photocatalytic disinfection of E. coli suspensions in slurry, wall and fixed-bed reactors. Catal Today 144 48-54... 

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