Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Modelling of a recirculating photocatalytic microreactor implementing mesoporous N-TiO 2 modifed with graphene Ahmed Yusuf a,b , Habeebllah Oladipo a,c , Lütfye Yildiz Ozer a,b , Corrado Garlisi a , Vittorio Loddo d , Mohammad R.M. Abu-Zahra a,c , Giovanni Palmisano a,b,c, ⁎ a Department of Chemical Engineering, Khalifa University of Science and Technology, P.O. Box 127788, Abu Dhabi, United Arab Emirates b Center for Membrane and Advanced Water Technology, Khalifa University of Science and Technology, P.O. Box 127788, Abu Dhabi, United Arab Emirates c Research and Innovation Center on CO 2 and H 2 , Khalifa University of Science and Technology, P.O. Box 127788, Abu Dhabi, United Arab Emirates d Department of Engineering (DI), “Schiavello-Grillone” Photocatalysis Group – Università degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy HIGHLIGHTS • Photocatalytic degradation of 4-nitrophenol in microreactor under solar irradiation. • The used CFD model incorporates total recirculation mode of operation. • The CFD model accounts for adsorption in the dark. • The CFD model predicts experimental data in a wide range of reactant concentrations. ARTICLEINFO Keywords: Microreactor Photocatalysis Total recirculation CFD modelling ABSTRACT The use of microreactors in (photo)catalytic processes ofers new possibilities for studying and optimizing many mass and photon transfer limited reactions. In this study, we propose a scalable computational fuid dynamics (CFD) model for the prediction of photocatalytic degradation of a model pollutant (4-nitrophenol) using im- mobilized N-doped TiO 2 grown over reduced graphene oxide (N-TiO 2 /rGO) in a photocatalytic microreactor working in continuous fow-recirculation mode. The mode of operation used in this study allows the reduction of mass transfer limitations inherent to heterogeneous photocatalytic reactions taking place on immobilized cat- alysts. A CFD model was developed for efective prediction of experimental results using COMSOL multi-physics. The experiment and the model results clearly showed a good agreement. The model parameters were determined through ftting the model with the experimental data, adsorption rate constants were estimated to be 1.76 × 10 4 m 3 mol −1 h −1 and 0.0252 h −1 for monolayer (k ads,m and k des,m ), 1.76 × 10 4 m 3 mol −1 h −1 and 0.0126 h −1 for multilayer (k ads,n and k des,n ); and the intrinsic rate constant (k s ) was 2.02 h −1 . This proposed model herein could serve as a practical tool to improve and optimize an extensive number of photocatalytic reactions for (waste)water applications in microreactors operating in recirculation mode. 1. Introduction Heterogeneous photocatalysis (HPC) continues to gain major at- tention as an elective advanced oxidation technique in a number of environmental protection applications, including degradation of re- calcitrant and emerging organic molecules or micropollutants [1,2]. One of the major setbacks limiting large-scale deployment of photo- catalytic processes is the reaction environment. Slurry reactors are the conventional photo-reactors used for studying photocatalytic reactions at laboratory and pilot scales. These photocatalytic reactors are nor- mally stymied by low photonic efciency due to non-uniform dis- tribution of light in suspension, mass transfer limitation resulting from slow difusion, and extra cost associated with photocatalyst separation from the reaction medium. Notably, immobilization of photocatalysts on substrates, such as glass [3,4], represents an efective strategy to avoid additional cost related to the catalyst recovery. Despite the un- doubted advantages of this approach, reduced exposed photocatalyst surface area and difcult scale-up are severe setbacks of this https://doi.org/10.1016/j.cej.2019.123574 Received 1 May 2019; Received in revised form 17 September 2019; Accepted 19 November 2019 ⁎ Corresponding author at: Department of Chemical Engineering, Khalifa University of Science and Technology, P.O. Box 127788, Abu Dhabi, United Arab Emirates. E-mail address: giovanni.palmisano@ku.ac.ae (G. Palmisano). Chemical Engineering Journal 391 (2020) 123574 Available online 21 November 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved. T