C. vulgaris

This paper outlines the experimental work conducted for the optimization of the growth condition of micro algae Chlorella vulgaris. A series of three flat plate external loop air lift reactors with different surface area to volume ratios were specially constructed for the experimental investigations.

The gas flow rate is the main variable that is manipulated in order to change the mass transfer coefficient of CO2 into the reactors. Increasing the gas flow rate increases in mass transfer coefficient resulting in an increase in the Chlorella vulgaris growth rate. This increase in growth rate comes at a higher energetic cost associated with the pneumatic air input for mass transfer and mixing. The ratio between the energy contained within the biomass and the energy used in its cultivation is defined as the net energy ratio.

Experiments conducted in this study record the optical density, dry weight, temperature and pH of the three reactors under different rates of aeration. From this data, the growth rate, biomass concentration and energy contained within the algae biomass is calculated for each set of experiments conducted.

This data is compared across the different surface area to volume ratios to derive the trend exhibited. It was found that the reactor with the surface area to volume ratio of 36 achieved the highest maximum growth rate of 0.36 hr-1 and 0.28 hr-1 at a 3% CO2/air mixture flow rate of 1 l/min and 0.5 l/min respectively. The next highest growth rate of 0.18 hr-1 is achieved in the reactor with a surface area to volume ratio of 27 at a gas flow rate of 0.5 l/min.

The highest net energy ratio of 0.065 is achieved by the reactor with the largest surface area to volume ratio of 48 at a gas flow rate of 0.5 l/min. The next highest NER of 0.047 is recorded for the reactor with a surface area to volume ratio of 27 at a gas flow rate of 0.25 l/min.

Mass transfer and light measurement tests are conducted to understand the growth rate data recorded. It was found that the reactor with the highest surface area to volume ratio achieves the best mass transfer at all gas flow rates investigated. This is primarily due to a lower circulation speed identified in hydrodynamic studies. This lower liquid circulation speed results in a longer gas hold up which increases the efficiency of mass transfer. The reactor with a surface area to volume ratio of 27  results in the fastest rate of circulation which in turn results in a 6.78 fold increase in the mass transfer coefficient for a doubling of the volume air to volume liquid ratio from 0.11 to 0.22. The next highest jump in kLa of 1.57 is achieved by the reactor with a surface area to volume ratio of 36 for an increase in the volume air to volume liquid ratio from 0.07 to 0.14.