Modelling a photocatalytic reaction in a microchannel reactor
Résumé
The present study deals with the photocatalytic degradation of the salicylic acid in a microchannel reactor with a rectangular cross-section and a deposit catalyst at the bottom. Three geometries with a microchannel depth varying between 0.1 to 0.75 mm, have been tested. Our goal was to establish the more important parameters which influence the mass transfer in order to enhance the photocatalytic efficiency.
At room temperature, 21C°, an initial concentration of salicylic acid is injected at a low flow rate into the microchannel reactor under UV illumination of 1.5 mW cm-2. The experimental degradation of salicylic acid during the contact time with the catalyst is measured at the outlet concentration by liquid chromatography HPLC.
We have simulated the reaction by computational fluid dynamics using Comsol Multiphysics software. The kinetic reaction rate was introduced as a flow consumption at the catalyst bottom of the channel. We can choose either a Langmuir Hinshelwood model with two constants or a mass-transfer model with an apparent constant k depending only of the Sherwood number.
The degradation decreases with the flow rate, which is attributed to the reduction of the residence time of the reactants in the microchannels. Furthermore, the degradation performances are also affected by the microchannel height. Under the same experimental conditions, the degradation decreases with the channel depth. In other words, the highest photocatalytic degradation is obtained in the reactor with the lowest channel height (h = 0.1 mm).
In order to confirm that the photocatalytic activity can be correlated to mass-transfer transport of reactant, the mass-transfer characteristics are thoroughly characterized by mass-transfer coefficient, Sherwood and Damköhler numbers Da. For a given geometry, the Damköhler decreases with the flow rate and there is a weak influence of the transfer limitation at high flow rate (above 10 ml/h). The channel depth affects the Da numbers and the data can be divided into two categories depending on the height of the device. On the one hand, for channel depths of 0.75 mm and 0.5 mm, the Da numbers remain larger than 1. It becomes obvious that these configurations suffer from mass-transfer limitations. On the other hand, for channel depth of 0.1 mm, the Da data are lower than or equal to 0.1. These observations reflect that a channel height of 0.1 mm appears sufficient to reach the kinetic regime and to obtain a system free of mass-transfer limitations.