Browsing by Author "James, Gerard"

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    In situ extraction of volatile fatty acids from anaerobic digestion systems
    (Stellenbosch : Stellenbosch University., 2020-03) James, Gerard; Pott, Robert William M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: In recent decades anaerobic digestion (AD) technology has gained significant interest due to policymakers’ intent to reduce non-renewable resources, and for the processing of organic wastes. AD is, however, faced with operational difficulties such as acid crash, and optimisation problems since feedstocks are variable and often intermittent. This thesis aimed at developing additional products from AD by investigating the co-production of volatile fatty acids (VFAs) and biogas, by the continuous removal of VFAs by in situ extraction. Gas stripping and liquid-liquid extraction (LLE) were identified as potential extraction methods. Gas stripping was investigated by an Aspen model. The model indicated that 100% recovery of VFAs could be achieved with a mass ratio of 230 for pure carbon dioxide and 150 for anequimolar mixture of carbon dioxide and methane. Gas-equilibrium experiments for both mixtures were compared to the model. The highest percentage of VFAs extracted was 0.91 ±1.42% using carbon dioxide at pH6.0 and 0.55% for the equimolar mixture at pH 3.5. A continuous gas stripping experiment showed that 4.48% ofVFAs (0.013g) were extracted out from a 20mL of synthetic VFA solution (14.65g/L) using 40.2L of equimolar gas. The results indicated that the model significantly overestimated the viability of gas stripping as an in situ recovery method for VFAs. Gas stripping was concluded to be inefficient, and an alternative in situ method was proposed using LLE. From literature, trioctylamine (TOA) and tributyl phosphate (TBP) and three diluents (canola oil, lamp oil, and oleyl alcohol) were identified as suitable solvents. These solvents were investigated in liquid-liquid equilibrium experiments. These experiments showed that there was a strong dependence on pH for the extraction of VFAs. The highest degree of extraction at pH 5.0 was observed for TOA/oleyl alcohol (50.48 ± 0.13%) and the lowest for TOA/canola oil(25.64 ± 8.42%). Biochemical methane potential (BMP) tests were conducted, using the three best solvents, to test the biocompatibility of the solvents with AD bacteria. From these experiments, the samples containing TOA/canola oiland TBP/lamp oil performed better than the control in total gas production (168.00mL ± 26.15mLand 145.67± 5.03mL) and methane percentage (12.62± 2.82% and14.68± 6.73%). The control produced 114.50± 39.42 mLof gas (9.73 ± 1.33% of methane). Over a 28 day digestion period, 2.40 ±0.30g/L and 5.84 ± 0.36g/L of VFAs in 10mL of solvent were successfully recovered from TBP/lamp oil and TOA/oleyl alcohol, respectively.A 17LAD-bioreactor was modified by placing an in situ extraction tube inside the reactor, connected to a circulator and batch extraction unit. TOA/oleyl alcohol was selected for the 17L scale-up, based on the equilibrium and biocompatibility tests. 0.078g of VFAs were extracted out and 6.71 L of biogas was produced with a methane percentage of 43% in the scale-up. To conclude, in situ LLE extraction may be industrially applicable as a potential co-production process for biogas and VFAs.