Doctoral Degrees (Chemical Engineering)

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Browsing Doctoral Degrees (Chemical Engineering) by Author "Abufalgha, Ayman"

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    Development of a bubble column bioprocess for the application of alkane bioactivation
    (Stellenbosch : Stellenbosch University, 2022-12) Abufalgha, Ayman; Pott, Robert William M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Hydrocarbon upgrading has become an important field worldwide, and particularly in South African industries, due to the significant hydrocarbon resources, especially in the form of coal. An attractive method of upgrading these hydrocarbons to high-value products is through biological activation processes, whereby an oxygen moiety is inserted into the hydrocarbon backbone by microbes. However, this process is affected by several factors such as oxygen availability, bioreactor geometry, and the activity of the organism. Therefore, this work has investigated a bubble column hydrocarbon bioprocess through examining the hydrodynamics and oxygen transfer in multiphase systems under a range of operating conditions, such as hydrocarbon concentration (𝐻𝐶), superficial gas velocity (𝑈𝐺) and solids (deactivated yeast, cornflour, and wildtype hydrocarbon-degrader (Alcanivorax borkumensis SK2)) loading (𝑆𝐿). Furthermore, this work explored the genetic modification of Alcanivorax borkumensis SK2 in order to convert n-alkanes to their alcohol as bioproducts, using the native pathway. Objective 1 examined the hydrodynamics (gas holdup and bubble size) in an air-deionized water-deactivated yeast-hydrocarbons system. It was observed that bubble size and gas holdup increased with increasing UG (1 to 3 cm.s-1), due to the increase in the number of bubbles in the system. whereas an increase in SL (0.5 to 6 g/l) resulted in bubble size increasing, which thereafter caused a decrease of gas holdup in a bubble column reactor (BCR). Yeast addition was found to change the fluid surface tension and viscosity and therefore affected the system hydrodynamics. Objective 2 studied volumetric oxygen transfer coefficient (KLa) in different phases i) air-deionized water, ii) air-deionized water-deactivated yeast, iii) air-deionized water-hydrocarbons, and iv) air- deionized water-deactivated yeast -hydrocarbons in BCR. It was found that KLa was affected differently by each phase system. E.g., KLa increased with increasing UG in all phase systems due to an increase in the number of small bubbles which enhanced gas holdup. Whereas the addition of yeast and hydrocarbons reduced KLa due to increases in the bubble size. In air- deionized water -hydrocarbon-cornflour system (Objective 3), SL and HC affected KLa differently, whereas UG had the most significant effect on KLa and oxygen transfer area. KLa showed an optimal level at SL of 3 g/l, but any further increase resulted in a reduction in KLa. HC had shown an insignificant change in KLa with the range considered (2.5 to 20%v/v). This was a result of changing hydrodynamic conditions, which affected the mass transfer coefficient (KL) behaviour, as there was no corresponding change to the interfacial area. After completion of the first three objectives, it was important for Objective 4 to investigate the effect of SK2 (as a novel biomass) as the solid phase on hydrodynamics of bubble column hydrocarbon bioprocess. It was observed that gas holdup increased linearly with increasing UG from 1 to 3 cm.s-1. Further, SK2 addition resulted in a reduction in fluid surface tension and therefore gas holdup was increased in air-deionized water-SK2 biomass system. Objective 5 detailed the genetic engineering of the SK2 strain by the removal of the alcohol dehydrogenase gene, alkJ1, using a gene knockout technique, with the aim of allowing accumulation of alcohol intermediates. Marked mutants of the alkJ1 gene knockout of SK2 were constructed by insertion of antibiotic resistance cassettes, with alkJ1 flanking regions. It was found that no alcohols were accumulated during the cultivation of modified SK2 in n-octane (10%, 20% and 50%) at 370C and 150 rpm for 30 days of cultivation. This finding suggested that the alkJ1 mutant of SK2 was not suitable for the bioconversion process, and that a second mutation in the alkJ2 gene of SK2 or/and a double mutant of both alkJ1 and alkJ2 may be required in order to remove the alcohol conversion step.