Theses and Dissertations (Chemical Engineering)

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Browsing Theses and Dissertations (Chemical Engineering) by Author "Aoyi, Ochieng, Prof."

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    Hybrid light photocatalysis of aromatic wastes in a fluidized bed reactor
    (Vaal University of Technology, 2022-08) Akach, John Willis Juma Pesa; Tshilenge, John Kabuba, Prof.; Aoyi, Ochieng, Prof.
    The use of solar photocatalysis for the treatment of aromatic chemicals like phenol in wastewater has attracted significant attention due to the low cost of sunlight. However, sunlight is unreliable since its intensity fluctuates during the day. This drawback can be addressed by supplementing sunlight with artificial UV lamps when the solar intensity reduces. In this work, such a hybrid solar/UV lamp reactor, internally illuminated by the UV lamp and externally by sunlight, was designed. Phenol was used as the model pollutant and the nanophase Aeroxide P25 TiO2 was employed as the photocatalyst and fluidized by compressed air. The catalyst and bubble distribution in the reactor was analysed using computational fluid dynamics (CFD) while the Monte Carlo (MC) method was used to model the light distribution and reaction kinetics. Finally, a lamp controller was designed to specify the required UV lamp output as a function of the solar intensity. The CFD simulation using ANSYS CFX 17 showed that a fairly homogeneous distribution of the catalyst was achieved in the reactor. Consequently, accurate simulations of the light distribution could be achieved without considering the hydrodynamics. The MC models revealed that bubbles did not significantly influence light absorption at the optimum catalyst loading. This showed that air was a good medium for fluidization as it could provide good mixing and oxygen electron acceptor without negatively affecting light absorption. The forward scattering behaviour of the P25 TiO2 and the increase in light attenuation with catalyst loading was confirmed in this work. The optimum catalyst loading in the different reactor configurations was 0.15 g/L (tubular solar), 0.2 g/L (annular solar), 0.4 g/L (annular UV lamp), and 0.4 g/L (hybrid light). This resulted in experimental reaction rates of 0.337 mgL-1min-1 (tubular solar), 0.584 mgL-1min-1 (annular UV lamp), and 0.93 mgL-1min-1 (hybrid light). An analysis of the local volumetric rate of energy absorption (LVREA) and reaction rate profiles along the radial coordinate showed a non-uniformity which worsened with an increase in catalyst loading. The reaction order with respect to the volumetric rate of energy absorption (VREA) indicated that solar illumination resulted in a higher electron-hole recombination as compared to UV illumination. This, combined with the higher intensity of the UV lamp, resulted in a higher reaction rate under UV light as compared to sunlight, demonstrating that the UV lamp could be used to supplement sunlight. For a typical sunny day, a lamp controller was designed that could adjust the UV lamp output as a function of the solar intensity to maintain the reaction rate at a reference level while ensuring less energy consumption than an ON/OFF lamp controller. This work demonstrated the feasibility of hybrid solar/UV lamp photocatalysis reactor which could maintain the advantages of solar photocatalysis while mitigating its drawbacks.
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    Integrated anaerobic digestion and UV photocatalytic treatment of industrial wastewater in fluidized bed reactors
    (Vaal University of Technology, 2017-03-28) Apollo, Seth Otieno; Onyango, Maurice S., Prof.; Aoyi, Ochieng, Prof.
    Anaerobic digestion (AD) is usually applied in the treatment of distillery effluent due to the fact that it is effective in chemical oxygen demand (COD) reduction and bioenergy recovery. However, due to the presence of biorecalcitrant melanoidins present in distillery effluent, AD is ineffective in colour reduction. For this reason, ultraviolet (UV) photodegradation, which is effective in melanoidins’ degradation, can be integrated with AD to achieve high efficiency in colour and COD reduction. However, the UV process is energy intensive, majorly due to the electricity requirement of the UV lamp. In contrast, the AD process has high potential of renewable energy production in the form of biomethane, which can be transformed into electrical energy and applied to supplement the energy requirement of the UV process. The aim of this study was to evaluate the efficiency of a combined AD-UV system in colour and COD reduction for the treatment of distillery effluent in fluidised bed reactors. The potential of the application of the bioenergy produced by the AD process to supplement the energy intensive UV process was evaluated and modelled using response surface methodology. In the first place, the optimal hydrodynamic conditions of the fluidised bed reactors were determined using optical attenuation technique. The best homogeneity in the bioreactor, in which zeolite was used as microbial support, was found to be at a superficial liquid velocity of 0.6 cm/s while the best catalyst and gas hold up in the photoreactor were found to be 0.077 and 0.003, respectively. At these conditions, it was found that the initial biological step removed about 90% of COD and only about 50% of the colour while photodegradation post-treatment removed 98% of the remaining colour. Kinetic analysis of the bioreactor showed that ~ 9% of the feed total organic carbon (TOC) was non-biodegradable and this was attributed to the biorecalcitrant melanoidins. Photodegradation post-treatment mineralized the biorecalcitrant melanoidins via a reductive pathway as was indicated by the formation of NH4+ in large quantity compared to NO3-. Kinetic analysis further showed that the rate of substrate utilization in the bioreactor increased with an increase in organic loading rate and it was inversely proportional to the rate of photodegradation post-treatment. Modeling using response surface methodology (RSM) was applied to predict the effects of the operating parameters of the initial AD step on the performance of the photodegradation post-treatment process and the energy efficiency. Energy analysis of the integrated system showed that the AD process could produce 59 kWh/m3 of electricity which could supplement the electricity demand of the UV lamp by 30% leading to operation cost reduction of about USD 4.8/m3. This led to a presumed carbon dioxide emission reduction (CER) of 28.8 kg CO2e/m3.
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    Modelling of Petroleum Wastewater Photodegradation in a Fluidized Bed Reactor
    (Vaal University of Technology, 2018-04) Nyembe, N.; Lerotholi, M.; Aoyi, Ochieng, Prof.
    Petroleum wastewater is highly contaminated with toxic organic pollutants that are harmful to the environment. The heterogeneous photocatalytic oxidation (HPO) process has shown the ability to remove these pollutants through the application of a fluidized bed reactor (FBR). The purpose of the study was to apply response surface modelling (RSM) and computational fluid dynamics (CFD) to optimize the operating conditions for the photodegradation process in an FBR. This was done by investigating the hydrodynamics, photodegradation efficiency and reaction kinetics; that gave a holistic view on the performance of the FBR. The hydrodynamic study focused on modelling the axial liquid velocity, gas hold-up and turbulence quantities due to their substantial impact on the design and performance of the FBR. This was done by implementing the Eulerian-Eulerian approach which solves the continuity and momentum equations for each phase. In addition, the standard k-ε turbulence model was used to capture the turbulent characteristics in the liquid phase. A numerical optimization technique (desirability) was used to determine the optimal simulation setting methods; that were found to be a fine grid size (500 000 cells), 2nd Order Upwind discretization scheme and a small time step size (0.001) and gave the best desirability (0.985). The axial liquid velocity was maximal towards the centre of the reactor and decreased towards the wall. The same trend was seen with the local gas hold-up, where it was high towards the centre and low near the wall region. This was an indication that the bubbles tended to gather towards the central region as they move up. Furthermore, the bubbles had a spherical–like shape due to the low superficial gas velocity and operating within the homogeneous regime. The turbulent kinetic energy increased at distances away from the distributor region, due to the bubbles accelerating, and it balanced well with the energy introduced by the bubbles. Central composite design (CCD), which is a type of response surface modelling technique, was used to investigate and optimize the photodegradation operating parameters. The maximal degradation efficiency in the current study was found to be 65.9%, which was relatively low when compared to literature (80.84%). This was attributed to the increase in the catalyst particle size from nanometer to micrometer. Furthermore, the second-order empirical model that was developed, using the analysis of variance (ANOVA), presented a sufficient correlation to the photodegradation experimental data. The optimal photodegradation operating conditions were found to be: superficial gas velocity of 17.32 mm/s, composite catalyst loading of 1.0 g/L, initial pH level of 3.5 and reaction time being 210 min. Using the Langmuir-Hinshelwood model, it was found that the photocatalytic degradation of petroleum wastewater follows pseudo first-order reaction kinetics. Since the photocatalytic degradation mechanism of phenol follows three stages whereby the second stage is the photocatalytic degradation on the surface of the catalyst to form by-products. This is the rate dominant stage and follows the pseudo firstorder reaction kinetics.
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    Solar photocatalytic degradation and adsorption of emerging pharmaceutical contaminants in wastewater
    (Vaal University of Technology, 2014-09-15) Akach, John Willis Juma Pesa; Onyango, Maurice S., Prof.; Aoyi, Ochieng, Prof.
    Pharmaceutical pollutants in wastewater have become an increasing concern in recent years. Adsorption and photocatalytic degradation of pharmaceutical pollutants have proved to be very efficient in the removal of pharmaceutical contaminants. In this study, a composite catalyst of powdered activated carbon (PAC) and TiO2 bound by silica xerogel (CTS composite) was synthesized and characterised using SEM, XRD and XRF. The composite catalyst was then used to adsorb and photodegrade the pharmaceuticals sulfamethoxazole (SMX), diclofenac (DCF) and carbamazepine (CBZ) in a three phase fluidised bed photocatalytic reactor using sunlight to activate the TiO2. The solar radiation intensity at the Vaal University of Technology and the hydrodynamic behaviour of the reactor were also investigated. Additionally, the effect of catalyst composition and loading, hydrodynamics and solution characteristics on the adsorption and photodegradation of the substrates was investigated. It was found that the solar radiation intensity varied with the hour of day, weather and seasons of the year. SEM showed that the porosity of the composite catalyst increased with increase in the PAC loading and a decrease in the silica xerogel loading. The XRD results showed that the silica xerogel and the PAC did not alter the composition of the P25 TiO2. XRF showed that the method used in the preparation of the substrates resulted in the desired composition of the catalyst. The optimum CTS composition was 60% silica xerogel loading and 10% PAC/TiO2 ratio. The best mass of the composite catalyst was 1.5 g/l. Using the optimal composite composition resulted in over 90% removal of the substrates with low residual solution turbidity of less than 3.5 formazin attenuation units (FAU). The optimum hydrodynamic condition was obtained when the reactor inclination angle and superficial air velocity were 75° and 0.014 m/s, respectively. However, a reactor inclination angle of 75° and a superficial velocity of 0.007 m/s gave the best adsorption and photodegradation of the substrates. Reducing the initial concentration of the substrates resulted in an increase in the efficiency of removal of the substrates. The adsorption and photodegradation of SMX was observed to increase with a decrease in pH and was maximum at pH 4. The adsorption of SMX and DCF was found to follow the Langmuir isotherm model. These results show that the use of the synthesised composite catalyst in the fluidised bed reactor provided a stable and efficient system capable of long term use. The results from this work also show that this system can be used for the removal of pharmaceutical substrates at low concentrations.