Lipid extraction from microalgae for biodiesel production
Creators: name: Halim, Ronald
Publisher: name: Monash University
Year: 2017
Subjects: Uncategorized

description: Microalgae appear to be a promising source of biodiesel as they have high photosynthetic rates and can accumulate substantial amount of lipids in their biomass (up to 77% of dry cell mass in some species). Unlike oilseed crops (such as rapeseed, soybean, and corn) which require freshwater and arable land for their cultivation, many of the marine microalgal strains can be cultivated in saltwater on non-arable lands. As such, large-scale microalgal cultivation should pose minimal interfere with valuable agricultural resources. The development of an effective and sustainable process to isolate intracellular lipids from the microalgal biomass is critical for an economically viable up-scaling of microalgal technologies. The primary objective of this thesis was to examine the mechanism underlying the lipid extraction process. We investigated the use of two types of laboratory-scale lipid extraction technologies: supercritical carbon dioxide (SCCO2) extraction and organic solvent extraction. In addition to the optimization of the lipid extraction process, the study examined the auxiliary cell disruption step which assisted lipid extraction by liberating lipids from the encapsulation of microalgal cells. SCCO2 extraction was found to produce a higher lipid yield than traditional organic solvent extraction using hexane (0.058 g lipid / g dried microalgae after 80 min vs 0.032 g lipid / g dried microalgae after 330 min). Lipid yield of the SCCO2 extraction process increased with decreasing temperature and increasing pressure. By increasing the ratio of water adsorbent (diatomaceous earth) to microalgal biomass in the extraction column, the SCCO2 process was able to extract lipids from wet microalgal paste (water content = 70 wt% of biomass) as effectively as it did from dried biomass. This modification, however, reduced the capacity of the extraction vessel by 88%. Commercial-scale use of the SCCO2 extraction must consider the high cost associated with its infrastructure and operation. Despite the routine use of classical Folch (chloroform/methanol) protocol for the determination of microalgal lipid content, the variables that affect organic solvent extraction of microalgal lipids are not well understood. A first-order kinetic model that described the microalgal organic solvent lipid extraction process as an equilibrium-driven phenomenon was developed and fitted to experimental results (0.68 ≤ r2 ≤ 1.00). The model was the first of its kind ever reported and provided a novel perspective in understanding the mechanism underlying the lipid extraction process. According to this model, the amount of intracellular lipid that can be transferred to the extraction solvent was governed by the equilibrium constant, K (unitless), while the rate of lipid mass transfer across the cell membrane was jointly controlled by the overall mass transfer coefficient, k (1/min), and the equilibrium constant. The efficiency of organic solvent extraction of microalgal lipid with either a methanol-based system or a hexane-based system was shown to increase with a rise in the cell concentration of the extraction mixture, a rise in the speed of agitation, a rise in the extraction temperature, and a rise in the degree of cellular disruption. The presence of water up to 20 vol% of the extraction solvent did not appear to adversely affect the efficiency of the extraction process. The tolerance of the extraction process to relatively large amounts of water demonstrated that the extraction process can be applied to wet microalgal paste or even concentrate solution with minimal loss of efficiency. This finding was crucial for the economics of the extraction process as it removed the necessity for an energy-intensive drying process prior to the lipid extraction step. An increased presence of water in excess of the aforementioned threshold level, however, rapidly decreased extraction efficiency. Cell disruption assists lipid extraction by releasing intracellular lipids from microalgal cellular structures. The use of high-pressure homogenization (HPH), ultrasonication, bead beating, and sulfuric acid treatment as laboratory-scale disruption methods were investigated for microalgal cells. The disruption efficiency of each method was evaluated through percentage reduction of intact cell concentration. With an average disruption efficiency of 73.8 % of initial intact cells, HPH was found to be the most efficient cell disruption method. Sulfuric acid treatment, bead beating, and ultrasonication followed HPH with an average disruption efficiency of 33.2, 17.5 and 4.5 % of initial intact cells, respectively. The use of HPH and ultrasonication as microalgal cell disruption methods were examined in further detail. The kinetics of cell disruption under both methods appeared to follow a first-order decay model whereby disruption rate was shown to decrease with time. The disruption rate constant for either method was found to increase linearly with parameter that affected the level of energy input (operating pressure for HPH and power for ultrasonication). On the other hand, initial cell concentration appeared to affect the disruption rate constant of both methods in a non-linear fashion. Disrupting microalgal cells with either HPH or ultrasonication prior to lipid extraction increased both lipid and triglyceride yields by respectively 590 – 786 % and 358 – 484 %.