Establishment of an efficient tray-drying process for qualitative biodiesel production from a locally isolated microalga Tetradesmus obliquus cultivated in polyhouse raceway ponds

Abstract

In this present investigation, a locally isolated green alga, Tetradesmus obliquus (Turp.) Kütz., grown in 16,000 L optimized culture volume in raceway ponds, was explored for the development of a rapid algal drying protocol using a self-designed tray-dryer. The biphasic nitrogen starved cultures were subjected to drying with 5, 7.5, and 10 mm algal slurry thickness at three different temperatures viz. 60, 80, and 100 °C. The wet algal slurry with ~90% initial moisture content (MC) required 12, and 15 h of drying period at 80 °C to achieve maximum lipid yield (~425 mg/g dry algal weight) for 5, and 7.5 mm initial slurry thickness, respectively. Newton's model constant (k) was calculated from the available datasets which showed that the increase in slurry thickness resulted in lower k values indicating a slower drying process with higher thickness. Partial drying up to 10% residual MC at 80 °C showed 93% lipid recovery in comparison to complete drying at the same temperature. The drying period was significantly reduced to only 6–8.5 h in comparison to complete drying. The energy consumption of this tray-drying technique at 80 °C with 10% residual MC was calculated to be 10.81, and 9.93 kWh/ kg dry biomass, for 5, and 7.5 mm initial algal slurry thickness, respectively. The lipid produced from the tray-dried biomass after conversion to biodiesel, was further analyzed in GC–MS. It was found that the saturated and mono-unsaturated fatty acids constituted 90% of the total fatty acid methyl ester (FAME) yield in the obtained algal biodiesel. Furthermore, the characterization of various fuel properties of the produced biodiesel revealed the values were within the definite limits of Indian and international biodiesel standards, thus, demonstrating the bright possibility of producing qualitative biodiesel from T. obliquus using this rapid tray-drying technique.