Separation of organic and inorganic compounds in solid waste obtained from biodiesel production process

Abstract

In biodiesel production, typical homogeneous catalyst for transesterification of vegetable oil are basic solutions, such as KOH, NaOH or NaOCH3. After the reaction, the remaining catalyst in the product must be neutralized by acid, for example H2SO4. As a result, the solid salt is then obtained. During separation, this solid waste also carries some free fatty acid, unreacted triglycerides and possibly glycerol with it. Therefore, the solid waste can have both inorganic and organic compounds. To minimize amount of this solid waste, several methods are proposed. The first approach is the elimination of organic part by combustion, resulting in higher purity of inorganic salt. The second approach is the extraction and precipitation. Both approaches are focused in this study. The organic part is combusted in electrical furnace at different temperature and varying time. The result shows that organic part is successfully removed by thermal combustion at temperature above 700°C with time longer than 20 minutes, with 91-93% of sample recovery. For extraction and precipitation, 6.85 g of inorganic salt can dissolved in 50 ml water and then alcohol (e.g. methanol and ethanol) is used to precipitate the inorganic salt from extractant. It is found that, methanol can precipitate salt higher than ethanol at same ratio and can extract maximum amount of salt at water: alcohol ratio 5:4. For EtOH and recycled EtOH were 5:5 and 5:7 respectively. For economic evaluation, it is found that the product price for combustion which is 5.55 bath/kg-K2SO4 is cheaper than extraction and precipitation which is 6.30 bath/kg-K2SO4, both product price are estimated in minimum conditions. Thus the payback period of combustion is shorter than extraction and precipitation.