Production of value added products from biodiesel-derived crude glycerine
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The increasing growth of the biodiesel industry has resulted in the increase of crude glycerine generated as a by-product. Typically, for every 100 kg of biodiesel produced, 5-10 kg of crude glycerine is produced. The disposal of crude glycerine, which contains impurities such an unreacted methanol, triglycerides and catalysts, creates a number of challenges to the environment and, added to this, glycerine purification processes are not economically feasible. Thus, the process of the conversion of crude glycerine into valuable products is promising for the safe disposal of biodiesel production wastes. Crude glycerine can be converted into a variety of products through catalytic or biological conversion pathways. Research indicated that biological pathways showed advantages over catalytic pathways in terms of product selectivity and because glycerine can be fermented to a number of fuels and products. A number of microorganisms are able to utilise glycerol as a sole carbon source despite the presence of impurities in the feed mixture. The exploitation of pretreatment methods would improve the conversion of and product yields from crude glycerine fermentations. The objective of the study was to evaluate the possible routes/pathways for producing value-added products from the fermentation of crude glycerine and washed crude glycerine with Clostridium diolis. Batch fermentations were performed at 37oC, a pH of 6.8 and an agitation at 150 rpm under N2 sparging and anaerobic conditions. The concentration of glycerol used ranged from 50 g.L-1 to150 g.L-1. Fermentations were conducted for 48 hours with samples taken every 3 hours for analyses. The crude glycerine used in this study had a relatively high pH (10.84) and contained methanol (309.9 g/L). Moreover, the crude glycerine was then washed with petroleum ether which reduced the methanol content (0.23 g/L) and the pH (8.94). An analysis of the fermentation products resulted in the production of 1,3-propanediol, lactic acid, butyric acid and acetic acid after analysis by high-performance liquid chromatography (HPLC). The most prominent product was 1,3-propanediol with a concentration of 17.83 g/L after 24 hours of fermentation with 50 g/L of pure glycerol as carbon source and 18.89 g/L of 1,3-propanediol when fermented with 50 g/L of washed crude glycerine. Impurities in the crude glycerine inhibited the growth of C. diolis with a maximum inhibition rate of 92% in crude glycerine and a maximum inhibition rate of 73 % in washed crude glycerine. Subsequently, the effect of ultrasound on the growth of Clostridium diolis and the production of 1,3-propanediol from pure glycerol at a concentration of 50 g/L was investigated. The use of ultrasound irradiation showed that the fermentation time could be reduced from 24 hours to 20 hours when fermented on 50 g/L pure glycerol. An activation time of 2 minutes prior to fermentation resulted in high concentrations of 1,3-propanediol (16.89 g/L), decreased fermentation time and increased bacterial growth. The exposure of Clostridium diolis to ultrasound for 10 minutes did not significantly produce 1,3-propanediol formation and slow bacteria growth was observed. The study indicates that crude glycerine inhibits the growth of C. diolis and that washed glycerol has the potential of being used as a sole carbon source for anaerobic fermentation to produce value-added products.
- Engineering