Design, mathematical modelling, and numerical simulation of a novel tubular photoelectrochemical reactor and experimental validation by residence time distribution and mass transfer coefficients

The present work introduces a new design of input/output distributors for tubular photoelectrochemical reactors (T-PECR) with concentric electrodes and external illumination. Expanded meshes of TiO2-coated titanium (outer electrode), and nickel-plated stainless steel (inner electrode) were used as photoanode and cathode, respectively. 3D-simulation studies of the hydrodynamic (laminar and turbulent regimes) in steady state and the mass conservation equation (convection-diffusion model) in the transitory regime were used to design the geometry of the input/output distributors, the T-PECR length, and the inter-electrode gap. Two restrictions were considered for an adequate T-PECR design: to minimize the inlet/outlet effects and thus achieve fully developed velocity profiles. Two experimental approaches were used to validate the theoretical results: (a) residence time distribution (RTD) obtained by an innovative methodology of step-signal input and (b) mass transfer coefficients obtained by the limiting current (I-L) technique. Both regimes showed a concentric core-annular flow and stagnant zone in the reactor shell (between the photoanode and the reactor wall). In the laminar regime, the core flow (inside the cathode) is fully independent of the annular flow (between the cathode and anode). In the turbulent regime, the development of a crisscross flow through the diamond-shaped openings of the electrodes causes the mixing of both core and annular flows. The local mass transfer coefficients (k(m)) on the photoanode surface and the global mass transfer coefficients (ka(ef)) in the diamond-shaped openings of the expanded meshes, estimated with Comsol Multiphysics, are in good agreement with those determined by limiting current experiments and the Plug Dispersion Exchange Model (PDEM). Thus, the use of inlet and outlet distributors improves the homogeneous flow pattern within the T-PECR.