Investigation of liquid-liquid extraction process for separation of glycerol and biodiesel
Creators: name: Tizvar, Roza
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2007
Subjects: subject: Engineering, Chemical.

description: Biodiesel is a sustainable and environmentally friendly source of energy which is now being used as an alternative for fossil fuels worldwide. Glycerol is the main by-product of biodiesel produced via transesterification and must be removed from biodiesel according to the ASTM Standard D-6751-02. This purification has usually been carried out with liquid-liquid extraction with water as a solvent for glycerol. In commonly-used alkali-catalyzed transesterification of waste cooking oil, the use of large quantities of water alone, as extraction solvent, results in formation of soaps, then emulsions and further difficulties in process downstream. This problem does not exist in acid-catalyzed transesterification of waste cooking oil. The main objective of the current study was to evaluate the efficiency of other potential solvents, such as hexane and methanol, as well as water in a liquid-liquid extraction unit for separation of glycerol and biodiesel. In order to accomplish this task, first of all, the reliability of the UNIFAC activity coefficient model to predict the phase equilibria of such systems was evaluated. The technical feasibility of the use of hexane, methanol and water as solvents in a single-stage mixer-settler was then investigated. The biodiesel was produced via acid-catalyzed transesterification of waste cooking oil with methanol and the stream entering the mixer was assumed to be free of unconverted oil and acid catalyst, containing only biodiesel, glycerol and methanol. Furthermore, the biodiesel was assumed to have the properties of methyl oleate, which is the major component of biodiesel made from canola oil and methanol. The ASTM limit for glycerol content of biodiesel (&lt;0.02 wt%), the residence time in the settler, the ratio of the liquid phase volumes in the settler and the biodiesel loss were the major performance measures considered. Four different solvent systems were found suitable: (1) water combined with residual methanol (with optimal mass ratio of biodiesel:methanol:water of 1:0.10:0.97), (2) a mixture of hexane and methanol (with optimal mass ratio of biodiesel:hexane:methanol of 1:1.15:1.62), (3) a mixture of hexane and water combined with residual methanol (with optimal mass ratio of biodiesel:hexane:methanol:water of 1:0.79:0.10:0.69) and (4) a mixture of hexane, methanol and water (with optimal mass ratio of biodiesel:hexane:methanol:water of 1:2.27:0.87:0.30). In acid-catalyzed production of biodiesel from waste cooking oil and methanol, the units located following the mixer-settler were designed and used in the economic evaluation of the entire biodiesel plant. The technically feasible solvent systems were then optimized based on maximizing the after-tax return on investment as a measure of the annual profitability. Although all the processes showed poor economic potentials with negative annual after-tax return on investment, the relative values were important to compare these processes. The biodiesel plants using hexane and water solvents combined with residual methanol or additional methanol with optimal biodiesel:hexane:methanol: water mass ratio of 1:0.43:0.10:0.24 or 1:2.19:0.90:0.34, respectively, yielded the highest annual after-tax return on investment of -35%. The process using water combined with residual methanol, with optimal mass ratio of biodiesel:methanol:water of 1:0.10:0.86 showed to be the least economically beneficial system, mainly because of its relatively low annual revenue and low total capital investment. In contrast, the process using water combined with residual methanol and the process using a mixture of hexane and water combined with additional methanol yielded the lowest and the highest biodiesel break-even prices of 1.70 and 1.91 $/L, respectively.