Numerical analysis of the effects of air on light distribution in a bubble column photobioreactor

Abstract

Light distribution inside photobioreactors (PBR) is a crucial parameter for the determination of growth of phototropic microorganisms and reactor productivity. In order to compute the light propagation inside PBR, scattering due to the presence of microorganisms is often neglected, since it is difficult to measure experimentally and it is not trivial to handle numerically. Moreover, absorption is usually assumed constant, but it is affected by the concentration of microorganisms and the presence of gas bubbles. In the present contribution we study how the flow hydrodynamics and local gas fractions inside a bubble column PBR affect the light distribution. First, we perform numerical simulations of a bubble column flow at different gas superficial velocities. Afterwards, we use instantaneous air volume fractions to calculate the effective scattering and absorption coefficient of the mixture, as well as the effective scattering phase function. Finally, we compute the polychromatic light distribution inside the PBR by means of a Lattice-Boltzmann solver. On the one hand, we find that gas bubbles affect both spatial distribution and magnitude of the light intensity field and their impact increases at higher gas superficial velocity. On the other hand, we also observe that the biomass counteracts these effects already at concentrations less than 1 kg/m3 so that the role of the gas phase on light fields seems to be of minor importance in PBR.