(22h) Design Rules for Multiphase Photoreactors

Photocatalysis has various applications including degrading organic contaminants in aqueous solutions, oxidizing liquid hydrocarbons, and reduction of carbon dioxide into valuable hydrocarbons. In a photocatalytic reactor, the absorbed photons excite electrons, creating electron-hole pairs that subsequently enter a set of redox reactions, referred to as a photoreaction. The rate of photoreactions, therefore, depends not only on the concentration of reactants, but also on the rate of photon absorption. The gradient in the profile of photon absorption rate in slurry photocatalytic reactors results in a gradient in the photoreaction rate, even if the reactor is ideally mixed. In the absence of accurate data on the local rate of photon absorption, the measured kinetic data at lab scale cannot be interpreted accurately to derive an intrinsic kinetic rate. Thus, such reported kinetic rate expressions cannot be used for the design of industrial scale photoreactors. The goal of our research is to evaluate the photon absorption rate and in combination with a meaningful kinetic rate expression (e.g., [1]), quantify photonic efficiencies, study the diffusion limitation in photocatalytic reactors, and generate design rules for multiphase slurry photocatalytic reactors.

We will illustrate the mathematical models we have developed to generate guidelines and design rules for slurry [2] and bubbly slurry photocatalytic reactors [3]. Our main objective in setting up these mathematical models was to develop simple analytical expressions based on logical simplifications, avoiding complex numerical simulations. To achieve this, we used a 1D description of the reactor, considering both low and high light intensities, leading to a linear and square root dependence of reaction rate on the local volumetric rate of photon absorption, respectively. We will present analytical expressions that can be used to predict the quantum efficiency, quantify photon losses at different steps of the chain of photocatalytic events, and calculate the relevant optical thickness for kinetic measurement studies in slurry photocatalytic reactors. Further, we will discuss the effect of introducing bubbles in slurry photocatalytic reactors on the scattering of photons and the eventual rate of photon absorption by photocatalytic particles. We conclude that for typical operating conditions the distribution of light inside a photoreactor is hardly affected by the presence of bubbles.

References

[1] Orlando M. Alfano, Marı́a I. Cabrera, Alberto E. Cassano, Photocatalytic Reactions Involving Hydroxyl Radical Attack, Journal of Catalysis 172 (1997) 370-379

[2] Mahsa Motegh, Jiajun Cen, Peter W. Appel, J. Ruud van Ommen, Michiel T. Kreutzer, Photocatalytic-reactor efficiencies and simplified expressions to assess their relevance in kinetic experiments, Chemical Engineering Journal 207–208 (2012)  607-615

[3] Mahsa Motegh, J. Ruud van Ommen, Peter W. Appel, Robert F. Mudde, Michiel T. Kreutzer, Bubbles scatter light, yet that does not hurt the performance of bubbly slurry photocatalytic reactors, Chemical Engineering Science, Available online 24 February 2013, DOI: 10.1016/j.ces.2013.02.022.