Process grease : a possible feedstock for biodiesel production
Venter, Roelof Jacobus
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The utilisation of waste process grease (WPG) as feedstock for biodiesel production was investigated in this study. WPG is a lubrication oil used in the metalworking industry and is considered a hazardous waste material. WPG contains vegetable oil and animal fat which are used as base oils in the lubricant formulation. Three different production routes were followed to produce biodiesel using WPG as feedstock. The first production route involved the conventional two-step production process comprising the acid esterification of the free fatty acids, followed by alkaline transesterification. The second production route involved the extraction of free fatty acids in the WPG by means of liquid-liquid extraction and the production of biodiesel from the extracted free fatty acids through acid esterification. The produced biodiesel was purified by means of chromatography. A third process route was the saponification of the WPG using aqueous sodium hydroxide followed by acidulation with hydrochloric acid. The resulting acid oil was purified by means of column chromatography, using a hydrophobic resin as the stationary phase prior to esterification through acid catalysis to produce biodiesel. The crude biodiesel was purified using column chromatography with silica gel as stationary phase. The optimum reaction conditions for the reduction of the free fatty acid content of WPG in route 1 to 0.5% were a methanol to oil ratio of 8:1 and a reaction temperature of 65 °C with a catalyst loading of 4 wt%. Acetonitrile was found to be the most effective extraction solvent for the reduction of sulphur compounds in the free fatty acid feedstock in route 2. A reverse phase chromatographic system with a hydrophobic stationary phase and methanol as the mobile phase was found to be an effective system to reduce the sulphur to below 10 ppm as specified by the SANS 1935 biodiesel standard in route 3. Both the conventional two-step process (route 1) and the liquid-liquid extraction process (route 2) were found not to be suitable for the production of biodiesel from WPG as the sulphur content of the produced biodiesel for routes 1 and 2 was 8 141 ppm and 4 888 ppm, respectively. The sulphur content of the produced biodiesel following route 3 was 9 ppm. The latter approach reduced the sulphur content of the biodiesel to acceptable levels that conform to the SANS 1935 standard to be used in a B10 biodiesel blend. A biodiesel yield of 45%, calculated as the mass of biodiesel produced as a percentage of the total mass of dried WPG used, was achieved with route 3. The biodiesel conformed to most of the specifications in the SANS1935 standard for biodiesel. The presence of a relatively high concentration of saturated fatty acids reflected in the higher cetane number of 74.7, the high cold filter plugging point of +10 and the oxidative stability of > 6 hours. A comparative cost analysis for route 3 indicated that the production cost of biodiesel, compared to the cost of petroleum diesel is marginally higher at the current Brent crude oil price of $102.41 per barrel. The production of biodiesel from WPG will be economically viable once the crude oil price has risen to about $113 per barrel.
- Engineering