Exploring the tension between energy consumption, light provision and CO2 mass transfer through varying gas velocity in the airlift bioreactor

Abstract

The airlift principle is a key operating principle of many aerated vertical tube photobioreactors and the aeration arm of horizontal tube reactors. It can also be incorporated into low cost hanging bag reactors. In each of these reactor systems, it is well recognised that algal productivity is typically limited by the supply of light and the rate of CO2 supply. It is desirable to maximise algal productivity by optimising these factors while minimising energy consumption in the reactor. In this paper, the relationship between gas velocity and light supply is investigated. Experimental algal growth in 3.2L internal airlift reactors showed that when CO2 supply was sufficient, light was the limiting factor. Productivities of 0.186gL−1d−1 at 148μmolm−2s−1 light, and 0.254gL−1d−1 at 300μmolm−2s−1 were obtained at a superficial gas velocity of 0.02ms−1. Reductions in algal concentration and productivity at lowered superficial gas velocity, even when the critical CO2 supply rate was met, suggested poor light distribution when liquid circulation and mixing were reduced. This was supported by simulated algal growth, modelled using a realistic light history obtained from Positron Emission Particle Tracking (PEPT) and a light attenuation model. This is the first reported use of PEPT in algal reactors, used here to determine realistic trajectories of algal cells through the airlift system, thereby allowing the cell's light history to be mapped as a function of gas velocity. Critical minimum superficial gas velocities were determined for light distribution and CO2 supply, dependent on the light intensity provided. Algal productivity in the airlift system can thus be maximised on the basis of energy efficiency.