Hydrodynamics and mass transfer in a tubular airlift photobioreactor

In photobioreactors, which are usually operated under light limitation, sufficient dissolved inorganic carbon must be provided to avoid carbon limitation. Efficient mass transfer of CO2 into the culture medium is desirable since undissolved CO2 is lost by outgassing. Mass transfer of O-2 out of the system is also an important consideration, due to the need to remove photosynthetically-derived O-2 before it reaches inhibitory concentrations. Hydrodynamics (mixing characteristics) are a function of reactor geometry and operating conditions (e.g. gas and liquid flow rates), and are a principal determinant of the light regime experienced by the culture. This in turn affects photosynthetic efficiency, productivity, and cell composition. This paper describes the mass transfer and hydrodynamics within a near-horizontal tubular photobioreactor. The volume, shape and velocity of bubbles, gas hold-up, liquid velocity, slip velocity, axial dispersion, Reynolds number, mixing time, and mass transfer coefficients were determined in tapwater, seawater, and algal culture medium. Gas hold-up values resembled those of vertical bubble columns, and the hydraulic regime could be characterized as plug-flow with medium dispersion. The maximum oxygen mass transfer coefficient is approximately 7 h(-1). A regime analysis indicated that there are mass transfer limitations in this type of photobioreactor. A methodology is described to determine the mass transfer coefficients for O-2 stripping and CO2 dissolution which would be required to achieve a desired biomass productivity. This procedure can assist in determining design modifications to achieve the desired mass transfer coefficient.