Optimization of the production of biodiesel from recycled grease trap waste

Abstract

This thesis investigated the technical and economic feasibilities of biodiesel produced from recycled grease trap waste (GTW) which accumulates in the wastewater system coming from food outlets and food processing sectors. The study was carried out by performing four principal tasks, these being: (1) extraction of fats, oils and grease (FOG) from GTW and investigation of its characterisation as a potential feedstock for biodiesel production; (2) development of an analytical methodology for the quantitative analysis of free fatty acid ethyl ester profiles; (3) optimisation of the process parameters for the synthesis of biodiesel from the extracted FOG via esterification and transesterification reactions; (4) scaling-up the laboratory-based process using a process simulator, and evaluating the economic feasibility of producing biodiesel from GTW. The GTW investigated in this study was provided by Peats Soil and Garden Supplies Ltd. Pty., (Adelaide, South Australia). The provided GTW was collected from the floating grease layer in a storage tank in which the lipid content was concentrated due to its lower density (approximately 51 wt%). Lipid extraction was then performed at ambient temperature (25ºC), employing hexane (HEX) and diethyl ether (DEE) as the solvents. A 97% lipid yield was obtained under the optimum operating conditions, these being: 1:1 DEE to GTW ratio (v/w); 300 rpm stirring speed, and 5 hr reaction time. Due to the high level of free fatty acids (>80 wt%) in the extracted oil, the esterification reaction was then conducted to reduce the FFA level in the feedstock. Two approaches, an esterification reaction with the use of the co-solvent acetone, and an esterification reaction without acetone were investigated and optimized. The optimal conditions for both reactions were determined using response surface methodology (RSM) based on central composite design (CCD). Optimum conditions for the esterification reaction without acetone were found to be 6:1 ethanol to oil molar ratio, 3 hr reaction time, 75ºC reaction temperature, and 3 wt% H₂SO₄ loading per FOG weight. In the presence of acetone, the optimal conditions were 35% v/v acetone to FOG, 6:1 ethanol to oil molar ratio, 3 hr reaction time, and 3 wt% H₂SO₄ loading. The ester yields obtained were 81 wt% and 80 wt%, respectively. The crude oil obtained from the previous pre-treatment was then subjected to a transesterification reaction to convert the remaining glycerides to ethyl esters. Two approaches, which were specified by the priority of the added components, were investigated to determine the optimal operating parameters. The results showed that a 96.7 wt% ester yield could be obtained under the optimum conditions, which were; 4:1 ethanol to oil molar ratio, 1 hr reaction time, 65ºC reaction temperature, and 1 wt% KOH loading. Importantly, approximately 30% of the excess ethanol could be reduced when ethanol and the base catalyst were heated first, while the extracted oil was added later at a specific rate. It was also found that the characterization of the GTW-derived biodiesel obtained satisfied most of the performance and compositional parameters required by the Australian Biodiesel Standard. After obtaining the optimal process parameters, two simulation models were constructed using Aspen Plus® V8.8 to generate the equipment and utility data required for the economic evaluation. Those simulation models were classified by the use of the co-solvent acetone in the esterification reaction. The result showed that the minimum production price of GTW-derived biodiesel was US$1,337.5/t, obtained through the process without using acetone. It can be concluded that the production of biodiesel from recycled greases is technically feasible and the economic aspect is also promising, although further studies need to be conducted to produce a biodiesel fuel which all satisfies the current mandatory standards.