Process intensification of biodiesel production from algae using foam flotation column

Abstract

To achieve cost effective biodiesel production from microalgae, the elimination of drying steps (responsible for about 84.1 % of process energy cost) is the single most important strategy to adopt. This project looks at combining three major components of algal biodiesel production, namely: harvesting, oil extraction, and transesterification into a one-unit operation. The intensified process hereby proposed, combines the advantage of three main ingredients to achieve its aim. Cell lysing, frothing and algae cell capture by surfactants; hydrophobicity inducement, cell lysing, as well as catalyst properties of acid; and the oil extraction and reactant properties of methanol, were combined in the process. The effect of operating condition (airflow), surfactant type (CTAB, MTAB, DAH, & DPC); and concentration, and media chemistry (pH and ionic strength) on the foam flotation harvesting of marine algae (N. oculata) was investigated. The impacts of cell properties (size, morphology, hydrophobicity, and surface charge) were also studied. Using 20 mg L-1 CTAB, 1.2 L min-1 air flow, at pH 6, the highest enrichment ratio of 14 was achieved in 77 % of cells recovered. When airflow and CTAB were 3.6 L min-1 and 60 mg L-1 , respectively, 83 % of cells were recovered, albeit a low enrichment of 4. Cell properties (morphology, hydrophobicity, and surface charge) and media chemistry (pH and ionic strength) are strong determinants of flotation performance. The results suggest that foam flotation harvesting of N. oculata is more collision dependent than attachment controlled and the frequency of collision and attachment can be enhanced by reducing ionic strength as well as pH whilst increasing airflow. As a prerequisite for biodiesel production from algae in a flotation column, foam stability as well as efficient methanol injection must be guaranteed. The impact of methanol and its injection pattern on flotation process has been investigated. For foam stability to occur, the percentage of methanol in the liquid pool must not exceed 50 % wt./wt. Using 50 % methanol

in a C. vulgaris culture, 98 % of cells (CF =18.3) were recovered via cocurrent methanol injection foam floatation, while moisture content (water and methanol) was 156 wt. %. Countercurrent methanol injection into the column top was achieved with the aid of methanol distributors. This countercurrent methanol delivery system guarantees more effective and economic methanol usage than cocurrent methanol injection, delivering methanol at 76 % concentration. Using the countercurrent methanol feed system in a riser-enabled column, it was possible to convert D. salina lipids to biodiesel with yields of 9.3 ± 0.2 and 11.2 ± 0.3% after 1 and 24 h respectively without any additional dewatering processes. Besides the novelty of this process, it has potential for huge reduction in cost of production, due to cost savings by elimination of drying. However, these levels of yield have to be improved on in order to ensure profitability. To this end, it is expected that the concept demonstrated by this work will rekindle hope that was once associated with algal biodiesel as an alternative to liquid fossil fuels