Method and apparatus for optimizing control of an immobilized film photoreactor

#This invention relates to photocatalysis and, more particularly, to the control of photoreactors that catalyze the oxidation of organic compounds.

#Photocatalysis is a process that involves illuminating a wide bandgap semiconductor with light having an energy greater than the semiconductor bandgap and then exposing the surface of the semiconductor material to organic compounds. In many cases, the result is oxidation of the organic compound to harmless components, e.g., H.sub.2 O and CO.sub.2. Many types of semiconducting compounds and mixtures thereof have been found to have varying degrees of efficiency. For example, TiO.sub.2, SnO.sub.2, ZNO, WO.sub.3, and SrTiO.sub.3 have been found to be effective photocatalysts. TiO.sub.2 has been found to be the most effective in terms of photoactivity, photostability and cost, but the following discussion is not limited to TiO.sub.2.

#As used herein, the use of the term "photocatalyst" means any compound in which irradiation of the compound with electromagnetic radiation of visible or ultraviolet wavelength will result in conduction band electrons and valence band holes that can undergo redox reactions at the catalyst surface with species such as water or inorganic and organic compounds. The illuminating light has an energy higher than the band gap of the semiconductor (e.g., generally in the ultraviolet (UV), i.e., <385 nm for anatase TiO.sub.2).

#The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:

#FIG. 1 is a schematic of a photoreactor control system according to one embodiment of the present invention.

#FIG. 2 is a pictorial illustration of a photoreactor according to one embodiment of the present invention.

#The process parameters needed to achieve an optimized energy efficient photoreactor for photocatalysis will vary from system to system and cannot be predicted. For example, light intensity, mass transfer rates of reactants (influenced by flow rates and reactant concentrations), or surface adsorption kinetics (influenced by type of reactant and reactant concentration) will affect in a complex interrelated manner the electric field within a photocatalytic semiconductor needed to achieve maximum degradation rates. Note that the use of the term "reactants" refers to both oxidants and reductants. In accordance with the present invention, an integrated photoreactor control system is provided for controlling one or more process parameters, e.g., flow rates, oxidant and reductant feed rates, applied voltage bias levels, light intensity, etc.. A processor, which is preferably a digital computer, monitors photocurrent or open-circuit photovoltage values and varies flow rates, oxidant and reductant feed rates, and light intensities, to maximize degradation rates of target organic reactant material.

#A suitable semiconductor material is placed in contact with an electrical conductor wherein a voltage bias applied to the conductor produces an electric field along the semiconductor (e.g., parallel to a UV transparent substrate that supports the semiconductor) or through the semiconductor (e.g., perpendicular to a UV transparent substrate that supports the semiconductor). The processor increases or decreases the applied voltage bias as a function of measured photocurrent or photovoltage produced by the semiconductor in contact with reactants under an applied light source. A maximum photocurrent is sought at a minimum applied voltage bias to optimize the removal of excess electrical charge from the semiconductor it should be noted that for electrode configurations such as shown in FIG. 1, an overly high applied bias could inhibit hole migration to the semiconductor solution interface and inhibit the degradation rate.

#FIG. 1 is a schematic illustration of one embodiment of a photoreactor control system according to the invention. Photoreactor 10 can be configured to operate as an isolated film device, such as shown in FIG. 3, and referred to herein as a "type I" system, or as an photoreactor cell configuration, referred to herein as a 637 type II" system. In a type I system an electric field is applied across or through at least one semiconductor electrode, but not across or through a reactant solution. The optimum photocurrent from the semiconductor is a minimum current; in a type II system, the optimum photocurrent is a maximum photocurrent.