Hydrodynamically-Enhanced Light Intensity Distribution in an Externally-Irradiated Novel Aerated Photoreactor: CFD Simulation and Experimental Studies

Abstract

The enhancement of the surface incident radiation on the walls of an externally-irradiated bubble tank photoreactor was studied and modeled by solving the radiation transport equation (RTE) in conjunction with the continuity, momentum and k-e turbulence equations. Computational fluid dynamic (CFD) simulation results were complemented with actinometric runs to determine the effect of the gas flow rate on the radiation loss by reflection at the surface of the gas-liquid mixture due to bursting of the bubbles. The model assumed that the gas-liquid mixture is a semitransparent medium where the light is scattered as a result of specular reflection and refraction when the light rays impinge on the air bubbles. The superficial reflectivity at the top of the gas-liquid mixture was linearly correlated with the superficial gas velocity. In particular, the simultaneous solution of the hydrodynamics and radiation transport equation using CFD allowed us to establish the relationship between the light scattering coefficient and the bubble size and the gas hold-up. The excellent agreement obtained between the experimental data and the CFD model validates the proposed model.

Computer Simulations of Photocatalytic Reactors

“…The specification of the apparent stress gradient (ρUU) is related to the adapted turbulence model [33]. Four hydrodynamic models are most often considered for modeling photoreactors: (a) standard k-ε model (S k-ε) [36,38,45,69,70], (b) the realizable k-ε model (R k-ε) [45,69], (c) the Reynolds stress model (RSM) [45,69], and (d) the low Reynolds number k-ε model (AKN-developed by Abe, Kondoh, and Nagano [21,45,71]). Duran et al performed the CFD for a single-phase flow mass transfer prediction in annular reactors using different hydrodynamic models.…”

A Radiative Transfer Modeling Methodology in Gas-Liquid Multiphase Flow Simulations

“…Modeling radiative transfer in multiphase flows is important in several applications such as solid fuel combustors [1,2], externally irradiated gasifiers [3], photocatalytic reactors [4,5], and photobioreactors (PBRs) [6,7]. While the procedure for coupling radiative transfer with the hydrodynamics has been well established in dilute multiphase flows (local dispersed phase volume fractions less than 10%) such as pulverized fuel combustors [8], the effect of radiative transfer is often neglected or grossly simplified in computational fluid dynamic (CFD) simulations where all the phases are present in significant fractions such as bubbling bed and circulating fluidized bed gasifiers [9,10].…”

Oxidative Photomineralization of Dichloroacetic Acid in an Externally-Irradiated Rectangular Bubble Tank Reactor: Computational Fluid Dynamics Modeling and Experimental Verification Studies

“…Unlike conventional multiphase reactors, the design and analysis of heterogeneous photocatalytic reacting systems requires the solution of the mass, momentum, and radiation transport for the selected geometry. The coupling of these equations unveils important nonlinear interactive phenomena such as hydrodynamically enhanced light intensity distribution within the liquid phase due to the presence of reflecting gas bubbles , and, hence, improved performance of the aerated photoreactor over a single-phase system in which the flowing liquid has been presaturated with the oxidizing gas before entering the reactor.…”

“…The walls of the reactor are made of polished stainless steel but the reflectivity at the walls was simplified as diffuse. The emissivity was estimated to be 0.8 based on previous actinometric experiments by comparing the absorbed radiation with black and silver walls . The UV light enters the reactor from the top after passing a UVA filter; as a result, the light may be deemed diffused after passing through the filter (because the quartz reactor lid-cover has been finely sand-blasted on the inner, light emergent side).…”

“…The emissivity was estimated to be 0.8 based on previous actinometric experiments by comparing the absorbed radiation with black and silver walls. 16 The UV light enters the reactor from the top after passing a UVA filter; as a result, the light may be deemed diffused after passing through the filter (because the quartz reactor lid-cover has been finely sand-blasted on the inner, light emergent side). Strictly speaking, eq 11 should be solved for each one of the nine discrete wavelength intervals displayed in Figure 2; hence, the incident radiation and the local volumetric rate of photon absorption (LVRPA), e λ a , can be calculated for each of the wavelength intervals as 34 The LVRPA for the discrete wavelength spectrum is given by 1 However, a pseudo-mono-chromatic approximation to eq 18 may be obtained 14,34 if weighted optical properties of the medium, such as absorption coefficient (κ λ* * ), scattering coefficient (σ λ* * ), and useful wavelength (λ*), are employed; thus The radiation spectrum distribution after passing the filter (cf.…”