In situ extraction of volatile fatty acids from anaerobic digestion systems

James, Gerard (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

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.

AFRIKAANSE OPSOMMING: In onlangse dekades het anaerobiese vertering (AD) -tegnologie beduidende belangstelling verkry as gevolg van beleidmakers se voorneme om nie-herwinbare bronne te verminder, en vir die prosessering van organiese afval. AD staar egter bedryfsuitdagings soos suur-ineenstorting, en optimeringsprobleme in die gesig, omdat voermateriaal veranderlik is met gereeld onderbroke voorraad. Hierdie tesis het beoog om addisionele produkte uit AD te ontwikkel deur die koproduksie van vlugtige vetsure (VFAs) en biogas, deur die kontinue verwydering van VFAs deur in situekstrahering. Gasstroping en vloeistof-vloeistof ekstrahering (LLE) is geïdentifiseer as potensiële ekstraheringsmetodes. Gasstroping is deur ’n Aspen™-ekwilibriummodel ondersoek. Die model het aangedui dat 100% herwinning van VFAs bereik kan word met ’n massa ratio (gas na VFA) van 230 vir suiwer koolstofdioksied en 150 vir ’n ekwimolêre mengsel van koolstofdioksied en metaan. Gasekwilibriumeksperimente vir beide mengsels is met die model vergelyk. Die hoogste persentasie VFAs geëkstraheer was 0.91 ±1.042% deur koolstofdioksied te gebruik by pH 6.0 en 0.55% vir die ekwimolêre mengsel by pH 3.5. ’n Kontinue gasstropingseksperiment het aangedui dat 4.48% VFAs (0.013 g) geëkstraheer is uit ’n 20 mL sintetiese VFA-oplossing (14.65 g/L) deur 40.2 L van ekwimolêre gas. Die resultate het aangedui dat die model die lewensvatbaarheid van gasstroping as ’n in situherwinningmetode vir VFAs aansienlik oorskat het. Dis tot die gevolgtrekking gekom dat gasstroping oneffektief was, en ’n alternatiewe in situmetode was voorgestel deur LLE te gebruik. Uit literatuur is trioktielamien (TOA) en tributielfosfaat (TBP) en drie verdunners (kanola-olie, lampolie, en oleïelalkohol) geïdentifiseer as gepaste oplosmiddels. Hierdie oplosmiddels is ondersoek in vloeistof-vloeistof ekwilibriumeksperimente. Hierdie eksperimente het aangedui dat daar ’n sterk afhanklikheid op pH vir die ekstrahering van VFAs was. Die hoogste grade van ekstrahering by pH 5 was waargeneem vir TOA/oleïelalkohol (50.48 ±0.13%) en die laagste vir TOA/kanola-olie (25.64 ±8.42%). Biochemiese metaanpotensiaal (BMP)-toetse is uitgevoer, deur die drie beste oplosmiddels te gebruik, om die bio-verdraagbaarheid van die oplosmiddels met AD-bakterieë te toets. Uit hierdie eksperimente het die steekproewe wat TOA/kanola-olie en TBP/lampolie bevat beter gedoen as die kontrole in totale gasproduksie (168.00 mL ± 26.15 mL en 145.67 ± 5.03 mL) en metaanpersentasie (12.62 ± 2.82% en 14.68 ± 6.73%). Die kontrole het 114.50 ± 39.42 mL gas (9.73 ± 1.33% metaan) geproduseer. Oor ’n 28-dae verteringsperiode, was 2.40 ± 0.30 g/L en 5.84 ± 0.36 g/L VFAs in 10 mL oplosmiddel suksesvol herwin uit TBP/lampolie en TOA/oleïelalkohol, onderskeidelik. ’n 17 L AD-bioreaktoer is aangepas deur ’n in situekstraheringspyp binne die reaktor te plaas, wat aan ’n sirkuleerder en lotekstraheringseenheid gekonnekteer was. TOA/oleïelalkohol is gekies vir die 17 L opskaal, gebaseer op die toetse vir ekwilibrium en bio-verdraagbaarheid. 0.078 g VFAs is geëkstraheer en 6.71 L biogas is geproduseer met ’n metaanpersentasie van 43% in die opskaal. Om af te sluit,in situ LLE-ekstrahering kan industrieel toepaslik wees as ’n potensiële ko-vervaardigingsproses vir biogas en VFA

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