Supercritical carbon dioxide extraction of oil from microalgae for biodiesel production

Nowadays, there is a growing interest in biomass as a renewable energy resource due to pollution caused by the fossil fuels, which is supplying the majority of world's energy demand, and the increasing energy need. Microalgae, amongst the biomass types, comes into prominence in the recent years due to its high oil content and high production rate. In oil extraction from microalgae to be used as biofuel, usage of supercritical fluid extraction, due to its adjustable selectivity and the ability to perform high purity extraction, is an important alternative to organic solvent extraction. In this thesis, oil has been extracted using supercritical carbon dioxide technique from 16 g pellets of "Desmodesmus sp." type of microalgae. Optimum extraction conditions were searched in the extraction experiments by investigating the effects of pressure, temperature, carbon dioxide flow rate, water content of microalgae and particle size of the sample on the oil yield. Experiments were conducted in the temperature range of 40-80 °C, at pressures 200, 250 and 350 bar, and CO2 flow rates of 1.4, 2.9 and 5.7 g/min. Based on the experiments performed at constant temperature (50 °C), constant pressure (250 bar) and varying flow rates, the experiments were continued to be performed at 2.9 g/min considering the total oil yield and reasonable extraction times. Influence of pressure on both the oil yield and fatty acid methyl ester (FAME) fraction has been investigated at pressures of 200, 250 and 350 bar, at constant temperature (50 °C) and CO2 flow (2,9 g/min). Highest oil yield (9.45%) was obtained at 350 bar and highest FAME fraction (90.5%) was obtained at 250 bar. Although the FAME fraction was around 70% in experiments at 350 bar, the total FAME amount (6.1% of the total algae mass) is higher than FAME amounts at 250 bar. In order to investigate the influence of temperature on oil yield, experiments have been conducted at 5 different temperatures between 40 °C and 80 °C at 200 bar pressure and 2.9 g/min flow rate. Highest oil yields have been obtained at 40 °C and 50 °C, a fast decrease in yield has been observed as the temperature increased. This behavior can explained to be due to decrease in the solubility of triacylgliserol, which is forming major part of the obtained lipids, while increasing the temperature at low pressures. To investigate the influence of sample type pellets, crushed pellets and freeze-dried microalgae cells have been used. In comparison to pellets that has been used in most part of the experiments, higher oil yield has been obtained from crushed pellets and freeze-dried microalgae cells. A clear decrease in oil yield has been observed when samples with 25% and 80% water content has been used in order to investigate the influence of water content to oil yield instead of the samples with 4.13% water content that has been used in most of the experiments. Empirical kinetic mass transfer model has been fitted to the experimental results and it has been observed that a vast majority of the obtained results stay in the ±5% error margin.