Browsing by Author "Teke, George Mbella"

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    Design and Investigation of a Semi-Partitioned Bioreactor for Extractive Fermentation using Computational Fluid Dynamics simulations and experimental studies
    (Stellenbosch : Stellenbosch University, 2022-12) Teke, George Mbella; Pott, Robert William M.; Gakingo, Godfrey K.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Fermentation technology is employed to convert substrates to products with the help of micro-organisms in various industries (food, pharmaceutical, cosmetic and chemical industries) with some successful examples including pharmaceutical production, bioethanol production for fuel, and beer production. Although this technology is used in industry, micro-organism mediated conversion still faces challenges. For instance, product or by-product inhibition can lead to low product yields and productivity. One route to circumvent product inhibition is via extractive fermentation. This combines a fermentation unit and an extraction unit with the aim of continuous in-situ product recovery. To facilitate extractive fermentation, different separation principles (gas-liquid, solid-liquid and liquid-liquid), modes of operation, and physical bioreactor configurations have been explored. For the most part, many research studies have focused on separation principles (dominated by liquid-liquid separation) and mode of operations, with less attention on the design of novel physical bioreactor configurations. Hence, advancing extractive fermentation technology requires new or modified bioreactor configurations or systems that will aid future optimization studies. This dissertation proposes an adapted fermentation system with the design and investigation of a Semi-Partition Bioreactor (SPB) for in-situ liquid-liquid extractive fermentation based on both Computational Fluid Dynamics (CFD) simulations and experimental studies. The first objective towards achieving the aim was to design, develop and demonstrate the operation of the SPB in the abiotic production of lactic acid (LA). This was done through adapting a standard bioreactor (the mixer) with the addition of an inserted tube (the settler). By investigating three physical configurations, results showed that mixing will be affected with the settler’s inclusion as seen with a decrease in mixing time. In addition, increasing the settler diameter was found to be better for continuous settling and removal of the organic liquid in the SPB. During abiotic production of LA, a stable concentration of 1 g/L of the latter was recorded in the SPB, illustrating the workability in in-situ extractive fermentation. Building from objective 1, objective 2 was focussed on understanding the hydrodynamics of the SPB based on a single-phase CFD model and experimental investigations. The hydrodynamic results showed that the presence of the settler led to a destruction of macro-flow patterns which could have had an influence on SPB mixing. Also, it was shown that when modelling an SPB, a transient approach should be preferred over one that assumes a constant mass flux (exchange) between the mixer and settler as seen with a 14.8% and 57% accuracy between experiment and CFD modelling of the mixing time by these two approaches respectively. With several insights on the SPB hydrodynamics obtained from conducting experiments in line with objective 2, the single-phase model was extended to a multiphase two-fluid model in objective 3. This was done in order to understand the SPB's hydrodynamics and mass transfer behaviour in a more realistic set-up that accounts for the different phases that would be in an extractive fermentation process. The results showed that a minimum agitation speed was necessary for sufficient liquid-liquid mixing. Also, the effectiveness of the SPB in lowering the concentration of the target product was shown to be due to liquid-liquid partitioning or mass transfer as opposed to a dilution effect arising from recycling the extractant phase. In the final objective (objective 4), key learnings from experiments conducted in the previous objectives were employed to come up with an SPB that was used to produce lactic acid as the bioproduct. Lactic acid production is usually prone to product inhibition and so the principles of extractive fermentation were put to the test. The results showed an increased concentration of the overall LA produced, with better yield and productivity of lactic acid (25.10 g/L, 0.75 g/g and 0.35 g/g, respectively) in an SPB as opposed to a standard bioreactor (14.94 g/L, 0.60 g/g and 0.20 g/g, respectively). From these results, the SPB design is recommended to be used to produce bioproducts susceptible to product inhibition.