Doctoral Degrees (Chemical Engineering)

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    A comparative investigation of the technoeconomic feasibility and sustainability of mango waste biorefineries in South Africa: a process modeling approach
    (Stellenbosch : Stellenbosch University,, 2023-03) Manhongo, Tariro Tecla; Chimphango, Annie Fabian Abel; Thornley, Patricia; Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering.
    ENGLISH ABSTRACT: Fruit processing waste (FPW) is a suitable biorefinery feedstock for conversion into bioenergy, biofuels, and chemicals. However, information on which processing routes and product combinations are economically viable and sustainable is limited. Using South Africa as a base developing economy, the availability of FPW as biorefinery feedstocks, economic viability, and sustainability of FPW-based biorefinery systems were evaluated in this study using mango processing waste as a base feedstock. Six biorefinery scenarios were evaluated; (I) combined heat and power (CHP) generation, (II) co-production of pectin and CHP, (III) co-production of pectin, polyphenols and CHP, (IV) co-production of pectin, bioethanol, and CHP, (V) co-production of pectin, polyphenols, bioethanol, and CHP, and (VI) co-production of bioethanol and CHP. In scenarios II to VI, residues from pectin and/or polyphenols recovery and wastewater are anaerobically digested for biogas production and the biogas in all scenarios is co-combusted with mango seed for steam generation (for use within the biorefinery and export to the host dried mango chips facility) and power (for consumption within the biorefinery and export if excess is generated). Aspen Plus process simulation models were developed at a plant capacity of 1500 tonnes per day (1200 tonnes process wastewater + 133.33 tonnes peel + 166.67 tonnes seed), operating for 24 h/day, and 120 days/annum. A discounted cash flow analysis was employed in assessing the economic viability of the six biorefinery models using the mass and energy flows from the models and incorporated in SimaPro-based attributional life cycle analysis models to evaluate the environmental impacts of the biorefineries. Using systems thinking, results from the technoeconomic analysis were employed in estimating the socio-economic benefits of the biorefineries using input-output Jobs and Economic Development Impact assessment models adopted from the National Energy Renewable Laboratory. Indicators for economic, environmental, and socio-economic performances of the biorefineries were normalized, weighted, and aggregated in a multi-criterion decision analysis approach to compare sustainability performances of the biorefineries. Scenarios I and VI are economically unattractive with net present values (NPVs) of -$94.4 and -$120.9 million, respectively. NPVs of the biorefineries increase with the recovery of more products in the order Scenario IV
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    A thermosiphon photobioreactor for photofermentative hydrogen production by Rhodopseudomonas palustris.
    (Stellenbosch : Stellenbosch University, 2023-03) Bosman, Catharine Elizabeth; Pott, Robert William M.; Bradshaw, Steven Martin; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Hydrogen has widely been identified as a commodity chemical. Currently, however, hydrogen is primarily produced through non-renewable methods. Biological hydrogen production through microbial photofermentation offers an environmentally friendly and potentially economically feasible alternative. Although this technology is promising, the costs associated with photofermentation systems need to be reduced and hydrogen productivity increased, to make the technology a competitive alternative to non-renewable hydrogen production methods. This can potentially be realised through cost-reduction strategies in combination with bioremediation – purifying wastewater whilst simultaneously producing a valuable chemical. This work applied a combination of techniques to develop and evaluate a novel thermosiphon photobioreactor (TPBR) for photofermentative hydrogen production, using Rhodopseudomonas palustris (R. palustris). The TPBR implements the thermosiphon effect to passively circulate biomass – the first and currently the only photobioreactor with the potential of operating without any external energy inputs. The TPBR was successfully implemented for photofermentative hydrogen production using R. palustris, achieving maximum hydrogen production rates of up to 0.310 mol·m−3 ·h−1 in the growing state. The effects of light intensity, temperature and biomass concentration on hydrogen production and passive circulation of biomass were investigated. The effects of biomass concentration were found to be most pronounced (0.4 to 1.2 g·L−1 ), while light intensities of 400 to 600 W·m−2 and an internal operating temperature of 31 to 44 °C were found to be suitable for hydrogen production. Exploring the effects of geometry, two novel TPBR designs were proposed – a tubular loop TPBR and a flat-plate TPBR. Using computational fluid dynamics (CFD) simulations, these designs were characterized in terms of fluid flow patterns, temperature profiles and radiation fields. Both TPBR designs showed potential for hydrogen production, achieving temperature gradients sufficient to ensure adequate circulation and velocities to maintain biomass in suspension. CFD simulations indicated light distribution as a possible area for improvement in the existing TPBR. Consequently, a reflector system was developed and implemented for the enhancement of light distribution and hydrogen production in the experimental TPBR – achieving a more uniform light field and an associated 48% increase in hydrogen production. Evaluating the feasibility of outdoor operation, the effects of diurnal light cycles and the emission spectrum of light were investigated. R. palustris was able to produce hydrogen under a sunlight-mimicking light emission spectrum achieving maximum hydrogen production rates of 0.790 mol·m−3 ·h−1 , albeit slightly lower as compared to under near-infrared light where it reached production rates up to 0.891 mol·m−3 ·h−1 . Hydrogen production was found to cease during dark periods in the diurnal light cycles; however, continuing again in the presence of light and achieving maximum Stellenbosch University iii hydrogen production rates of ~0.015 mol·m−3 ·h−1 . This demonstrated promising potential towards outdoor operation of the TPBR, circumventing the requirement for external energy inputs. This dissertation has successfully demonstrated the application of a novel thermosiphon photobioreactor for photofermentative hydrogen production with minimal external energy input. The research comprised determination of suitable operating conditions for hydrogen production, a CFD modelling method for the design of PBRs, two novel TPBR designs and characterization thereof, a light distribution strategy for the enhancement of hydrogen productivity in PBRs, and insight into the passive circulation of biomass in a TPBR and the behaviour of R. palustris under simulated outdoor conditions. Collectively, this research provides knowledge not only improving the TPBR, but which could also be extended to other systems in the biohydrogen field.
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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.
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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.
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    Extending SAFT-VR Mie to the global phase behaviour of CO2 and its mixtures
    (Stellenbosch : Stellenbosch University, 2022-04) Smith, Sonja Almi Milé; Schwarz, Cara Elsbeth; Cripwell, Jamie Theo; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH SUMMARY: Understanding the phase behaviour of CO2-containing mixtures is important for many industrial processes, amongst others supercritical fluid fractionation and enhanced oil recovery. These mixtures are complicated by the CO2 quadrupole moment, and, because these processes are often conducted near CO2’s critical point, critical phenomena. These characteristics make thermodynamic modelling of CO2-containing systems challenging. Many equations of state (EoSs) with firm theoretical foundations have been developed. The Statistical Associating Fluid Theory, or SAFT EoS, is rooted in statistical mechanics where macroscopic properties are calculated by considering the energy contributions of molecular interactions. The SAFT with Variable Range Mie-potential (SAFT-VR Mie) model was the focus of this project, because it is arguably the most advanced of the SAFT-variants and shows promise as a holistic predictive tool. The industrially relevant Cubic Plus Association (CPA) model was included for comparative purposes. The overarching aim of this project was to improve the predictive modelling of CO2- containing mixtures, thereby developing a single model that describes the global phase behaviour of these mixtures. To achieve this, the models’ descriptions of quadrupolar interactions and of the critical region needed improvement. To account for quadrupolar interactions, SAFT-VR Mie (VRM) and CPA were extended with the quadrupolar terms of Gross (G) and Larsen & coworkers (L), and three new models were proposed: VRM-G, VRM-L, and CPA-G. CPA extended with the Larsen quadrupolar term was developed in previous work (Bjørner & Kontogeorgis, Fluid Phase Equilibria 2016;408:151􀀀69), and is called qCPA. The quadrupolar models were evaluated by modelling the phase equilibria of binary mixtures containing CO2 or benzene + n-alkanes, 1-alkanols, water, or esters. The quadrupolar models’ improvements are most pronounced in the CO2 + n-alkane systems. The quadrupolar models predict these systems’ phase behaviour accurately at subcritical conditions, and offer improved qualitative descriptions at supercritical conditions. In the CO2 + 1-alkanol systems, good predictions are obtained when accounting for both quadrupolar and cross-association interactions. A single set of CO2 association parameters, determined from a sensitivity analysis, were used to predict the VLE behaviour of CO2 + 1-alkanol mixtures ranging from ethanol to 1-decanol. There is still room for improvement, specifically regarding the water- and ester mixtures. In the water mixtures, the additional quadrupolar terms do not improve the descriptions of the nonpolar models. To obtain good qualitative descriptions of the phase boundaries, the cross-association description is the most important. In the ester mixtures, the polar models do not capture the balance between dipolar, quadrupolar, and dipole-quadrupole interactions adequately. Based on the results for the CO2 + n-alkane and CO2 + 1-alkanol mixtures, VRM-G and qCPA were identified as the best quadrupolar model options in SAFT-VR Mie and CPA, respectively. These models are based in mean-field theory, and therefore cannot describe the critical region. To this end, VRM-G and qCPA were treated with renormalisation corrections, yielding VRM-G + RG and qCPA + RG. Both models improve the description of pure component properties in and around the critical region, without losing accuracy outside the critical region. The RG-models were extended to mixtures using the isomorphism approach and applied to binary n-alkane and CO2 + n-alkane systems. qCPA + RG only offers significant improvement for the more symmetric systems; this improvement, however, does not worsen prediction of binary VLE outside the critical region. In VRM-G + RG, remarkable predictions of the critical loci are obtained without binary interaction parameters. VRM-G + RG also describes the phase behaviour of these systems outside the critical region accurately, thus achieving the overarching aim of developing a global model for CO2-containing mixtures. The following contributions stem from this research: 1. The development of VRM-G and VRM-L, published in Journal of Chemical & Engineering Data 2020;65(12):5778 􀀀 5800; 2. The development of CPA-G, published in Fluid Phase Equilibria 2021;528:112848; 3. The development of VRM-G + RG.