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Artificial Photosynthetic Systems: Artificial photosynthesis involves capturing energy from the sun and storing it in the form of chemical fuels. The focus of this research is to create engineered solar fuel generators for the photocatalytic production of hydrogen, a leading candidate for the fuel of the future. Our team develops photoelectrochemical cells and multifunctional photocatalysts activated by solar and ultraviolet radiation for hydrogen generation by water splitting. We design and build original photoreactors for the scalable production of hydrogen and other chemical fuels. Our target is to create sustainable ways of producing solar fuels by utilizing earth-abundant materials and cost-effective processes. Photoreactors for Water Purification: The ultraviolet (UV) reactor is today’s fastest growing water treatment technology. The primary emphasis of this research is to formulate the next generation of UV photolytic and photocatalytic reactors, by studying their fundamentals including their hydrodynamics, kinetics, and optics. We develop models of UV reactor performance and evaluate them through extensive experimental studies. Our current strategic project focuses on the development of a new generation of UV reactors operating with UV-LED and UV Microplasma. The research program we have been leading in this field has and will continue to have a significant impact on product development in the UV reactor industry. Computational Modeling of Chemical and Biological Systems: Computational fluid dynamics (CFD) plays a significant role in the study of chemical and biological systems and various phenomena happening within these systems. This research program focuses on combining fundamental physical models with CFD to develop chemical and biological reactor performance models for virtual prototyping and design optimization. We also perform experimental analysis, including laser-based imaging techniques, for model evaluation. The optical diagnostic methods utilized include particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF). The results of this research are applied to advance the design of new reactors. Ongoing projects include developing models and performing validating experimental studies for: the reaction kinetics and hydrodynamics of fluidized bed reactors, microfluidic devices for stem cell research, and micro fuel cells for portable electronic devices.
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Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - April 2022)
In this research, UV-LED-activated metal-oxide gas sensors were studied, where the electrochemical properties of the sensing material were enhanced by manipulation of sensor operating parameters as well as material structure and composition.In the first phase, the sensing characteristics of several favorable metal oxide semiconductors were investigated under different wavelengths, irradiances, and pulsation frequencies. The results showed that UVA-LEDs (i.e. 365 nm) could result in better responses due to resonant absorption. At a constant photon energy (i.e. 3.4 eV), increasing irradiance enhanced the photo-desorption of adsorbed components and reduced the response. Pulsed UV irradiation could significantly enhance the response as a result of increased residence time of adsorbates.Also, a set of strategies were applied to investigate and modify the sensing layer structure and composition. Firstly, Ag was incorporated into ZnO nanoparticle structure which resulted in an increase in sensor response toward 5 ppm NO₂ (ΔR/R = 0.98), compared to pristine ZnO (0.5) potentially due to layer charge carrier enhancement. Secondly, a sensing material structure was designed based on ZnO nanowires decorated with Pt nanoparticles, as metallic co-catalytic sites. The ZnO nanowires showed an increased response (1.6) compared to nanoparticles due to their higher surface area. Furthermore, decorating the surface of ZnO nanowires with Pt nanoparticles remarkably enhanced the sensing performance, whereas 0.1 wt% Pt decorated ZnO nanowires sensor exhibited nearly 4-times higher and 50s faster response compared to ZnO nanoparticles, in identical photo-activation settings. Thirdly, ZnO nanowires were used as a core for a thin layer of a secondary semiconductor to develop ZnO-In₂O₃ and ZnO-SnO₂ core-shell gas sensors. The relative responses of the ZnO-In₂O₃ (2.49) and ZnO-SnO₂ (2.21) sensors were higher than that of pristine ZnO NWs (1.6), possibly due to the improved photon absorption, and increased active sites on the surface of the nanowires.Lastly, carbon mesoporous materials (CMMs) with various ZnO loading concentrations were tested against NO₂ and NH₃, where the sensors responded 1.91 (NO₂) and 1.35 (NH₃) in optimal loading concentrations. This improvement in response could be attributed to the high surface area, extended separation, and more oxygen vacancies (OVac) induced by C-dopant in ZnOx/CMM sensors.
The ultraviolet light emitting diode (UV-LED) has recently emerged as a new UV source. It offers design flexibility due to its small size and ability to alter its radiation profile. In view of the variety of design possibilities for a UV-LED reactor, a computational model could be of great value for simulating the reactor and providing insight into its performance. Given the UV-LED’s ability to emit various radiation wavelengths and because it is a directional UV source, the challenges of simulation for UV-LEDs are greater than those for UV lamps, which typically have a single wavelength and an almost radial radiation profile. This study proposes a method of simulating UV-LED reactors for water treatment in the Eulerian framework through the integration of the kinetic, hydrodynamic, and radiation models, representing UV-LED systems. Additionally, the concept of an ideal UV-LED system is proposed, which can provide insight into the efficiency of any UV-LED reactor design concept. In this study, UV-LED is modeled as a polychromatic point source with a specific radiation profile. The radiant energy field of a UV-LED reactor is developed by considering the absorption of media, refraction and reflection at air-quartz-water interfaces, and the reflection from the internal wall surfaces. The developed radiation model was applied to different UV-LEDs, and the numerical predictions were successfully evaluated using actinometry and radiometry. The radiation model was employed to obtain the kinetic rate constants of the challenge microorganisms (e.g., Escherichia coli and MS2) suspended in water, and biodosimetry results showed that modifying microorganisms’ 254 nm fluence-response data based on the germicidal efficiency of UV-LED could yield acceptable kinetics information. Therefore, during the development of the integrated model, the germicidal factors of UV-LEDs were used to homogenize the germicidal radiant power outputs of UV-LEDs. The integrated model of reactor performance was evaluated through experimental studies of the challenge organisms in two UV-LED reactors under different operating conditions, including flow rates, flow regimes, radiant powers, and UV-LED configurations. The integrated model could predict the overall reactor performance and provide information that enabled to improve the efficiency 10 times.
Safe drinking water is essential to humans, thus natural raw water requires necessary treatment, especially removing pathogenic microorganisms for disinfection. In addition to conventional chemical disinfectant, ultraviolet (UV) radiation has been increasingly used for water disinfection. Recently a new UV source - UV light-emitting diodes (UV-LEDs) - has emerged with many special features, which is believed to be a promising alternative to conventional UV lamps for water disinfection. This research focused on two special features of UV-LEDs, multiple wavelengths and pulsed irradiation, to explore the effect and potential on water disinfection.UV-LEDs in different UV wavelength ranges were combined in various manners to investigate the effect of multiple wavelengths on microorganisms inactivation in water. The results showed the effect of UV-LEDs multiple wavelengths depends on the wavelength combinations among UVA (315 – 400 nm), UVB (280 – 315 nm) and UVC (200 – 280 nm), the manner to apply different wavelengths (e.g. simultaneous, sequential), as well as different types of microorganisms (e.g. bacteria, virus). Combinations of UVC/UVB always achieved additive effect on microorganisms inactivation due to the same photochemical reactivation induced by UVC/UVB on DNA that follows the Second Law of Photochemistry. However, combining UVA with UVC/UVB simultaneously or applying UVA after UVC/UVB reduced the inactivation of bacterium E. coli due to DNA repair and photoreactivation effect of UVA. A special wavelength combination was developed by applying UVA as pretreatment followed by UVC inactivation, which achieved dramatic inactivation improvement and significant reactivation reduction on E. coli. The effects and mechanisms of this special combination were thoroughly investigated and revealed in this research.The effect of UV-LEDs pulsed irradiation was examined by applying pulsed irradiation with various pulsed patterns (frequency and duty rate) on different microorganisms in pure water and wastewater. Comparable inactivation were obtained by UV-LEDs continuous irradiation and various pulsed irradiation on all the four microorganisms examined, which clarified the role of pulsation on UV disinfection.The findings in this research promote a better understanding on UV disinfection and are of considerable significance to take full advantage of UV-LEDs for water disinfection.
Portable electronic devices for the next generation demand a quick charging and long-lasting energy power system. Micro direct methanol fuel cells (µDMFCs) are considered as one of the appropriate alternatives to rechargeable battery technology for portable power devices. Although a significant amount of work has been done with µDMFCs, it is still a design challenge to miniaturize the fuel cell and to provide adequate power. The conventional bipolar fuel cell architecture contains a membrane electrode assembly sandwiched between two flow field plates. In this research, we present an approach to enhance the maximum power density of µDMFCs without affecting the total fuel cell volume by depositing extra anode catalyst on the fuel flow channel walls. An air-breathing µDMFC with extra anode catalyst deposited on the channel walls was developed, and the effects of key design parameters and operating conditions on the fuel cell performance were examined by measuring the overall cell and individual electrode polarization curves. The fuel cell with extra anode catalyst on the channel walls improved the maximum power density by 20% compared to the conventional design with only a catalyst coated membrane. The fuel cell design approach with catalyzed flow field channel walls was also demonstrated in an air-breathing micro Fe(II)/Fe(III) redox anode fuel cell (μRAFC). The μRAFC with graphite channel walls as an anode improved the maximum power density by 281% compared to the μRAFC with inactive channel walls. The impacts of key operating conditions on the cell performance were also evaluated.A 3D simplified model for the µDMFC design with catalyzed channel walls was developed and applied to evaluate the key parameters. It was found that the fuel cell performance was mainly limited by the kinetics of the methanol oxidation reaction. For the fuel cell with anode catalyst both on the membrane and the channel walls, increasing the anode catalyst loading on the channel walls improved the contribution of the anode on the wall to the total anodic current, and reducing the channel dimensions only slightly improved the cell performance.
Application of ultraviolet (UV) radiation for water treatment has been increasing steadily in the past two decades. Further, significant improvements in semiconductor technology have made ultraviolet light emitting diodes (UV-LEDs) a viable alternative to conventional UV sources for water treatment. However, utilizing UV-LEDs for water disinfection comes with challenges related to their radiation measurements due to their specific structure, operation, and radiation pattern. Without a standardized measurement method, the efficacy of this new radiation source on the inactivation of waterborne microorganisms could not be determined accurately. In this study, in order to determine the fluence delivered to a microorganism’s solution, first, a method was developed to properly operate, control, and measure the output of the UV-LEDs. It was found that, not only the operational conditions affect the UV-LEDs output, but also the measurement techniques were critical in obtaining accurate results. Then, the radiation distribution was simulated. The radiation model was validated by two common measurement techniques, chemical actinometry and radiometry. Using the validated model, common radiation modeling presumptions such as the point source assumption and symmetry assumption for radiation profile of UV-LEDs were evaluated. Subsequently the radiation model and the operational method were implemented to develop a protocol for fluence determination of UV-LED systems. In this protocol, the average fluence was estimated by measuring the irradiance at a few points for a collimated and uniform radiation on a petri dish surface containing microorganism solution. Finally, the developed fluence determination protocol was tested in different setups to evaluate the radiation distribution and its effect on microbial inactivation kinetics measurements. A novel setup was presented for UV-LED kinetics studies; further, the inactivation kinetics of a common waterborne microorganism, E. coli, was measured. This study includes a fundamental holistic insight for fluence determination of UV-LED systems. The developed protocols for UV-LED operation and fluence determination studies help researchers to perform reliable UV-LED inactivation studies and obtain precise kinetics data.
In this study, novel approaches for the development of solar-responsive photocatalysts for water splitting are investigated, with a focus on the gallium-zinc oxynitride solid solution (GaN:ZnO).A facile synthesis technique was developed for the fabrication of nanoporous GaN:ZnO photocatalyst. The synthesis time was reduced substantially to 12 min (from original 10+ h) as the result of effective solid–solid and gas–solid reactant interactions at the nanoscale. The synthesized photocatalyst samples were characterized for their optical, structural, and photochemical properties. Despite the short synthesis time, the prepared nanoporous GaN:ZnO photocatalyst maintained the overall visible-light water splitting activities at reasonable rates, reaching to the maximum apparent quantum efficiency of 2.71% at 420–440 nm.Decoration of the photocatalyst surface with the optimal amount of various hydrogen and oxygen evolution co-catalyst materials through photo-deposition and impregnation was investigated. Our experimental and characterization data suggest a mechanism for minimizing the effect of the undesired charge recombination and reverse reaction through the utilization of structural nanopores as the active water splitting regions.To reduce the recombination of photo-excited charges, the hybridization of GaN:ZnO photocatalyst on highly conductive graphene support was studied. Effective electrochemical interaction between composite components was confirmed through material characterization, photo-induced conversion of graphene oxide to reduced graphene oxide (rGO), and visual observation of co-catalyst nanoparticles on the surface of the conductive nanosheets. The GaN:ZnO-rGO composite photocatalyst exhibited ~70% improvement in photocatalytic hydrogen evolution. Finally, a number of approaches for the synthesis of one-dimensional (1-D) GaN:ZnO photocatalysts were studied. A novel direct fabrication route for 1-D GaN:ZnO through gold-catalyzed atmospheric pressure chemical vapour deposition was proposed. The material characterization data indicated that the proposed method is capable of preparing 1-D GaN:ZnO nanostructures with a wide range of morphologies, including nanofibers and nanowires, via vapour–liquid–solid epitaxy. In addition, via the proposed method, the dimensions of the obtained nanomaterials can be tailored. The synthesized GaN:ZnO nanowires demonstrated promising sacrificial hydrogen evolution compared to the powder and nanofiber photocatalysts.The work presented in this research provides an in-depth understanding of the nanoscale fabrication and optimization of GaN:ZnO photocatalysts for visible-light hydrogen generation.
Growing global energy demands and an increased environmental awareness have resulted in a demand for renewable energy sources. Photocatalytic water splitting has long been explored as a direct solar-to-chemical energy conversion method in the hopes of creating a sustainable, emissions-free hydrogen production process. In this thesis we present the first focused effort on hydrogen production via photocatalytic water splitting in a UV-irradiated fluidized bed reactor. This novel approach was taken to address the mass-transfer effects, poor radiation distribution, parasitic back-reaction and photocatalyst handling difficulties that limit the efficiency and scalability of existing water splitting systems.By fluidizing platinum-deposited TiO₂ spheres in a 2.2M Na₂CO₃ solution, steady hydrogen production rates of 211 μmol/hr with an apparent quantum efficiency of 1.33% were achieved upon UV-irradiation. This represents a marked 44% increase in efficiency when compared to results obtained by suspended slurry TiO2 photocatalysts in the same reactor. A mathematical model describing the performance of the fluidized bed water splitting system was derived and then employed to estimate several key parameters. From the model, it was found that high rates of mass transfer in the separator unit could minimize the negative effects of the parasitic back reaction and greatly improve the overall rate of hydrogen evolution. Indeed, it was demonstrated experimentally that slight modifications to the liquid-gas separator to improve mass transfer resulted in a 350% increase in the rate of hydrogen evolution. The application of the model to the design of fluidized bed water splitting systems is described.Advanced, fluidizable nanowire and nanorod photocatalysts that can withstand the rigors of fluidization are described here for the first time. We present two novel, scalable methods that allow for the growth of anatase nanowires or rutile nanorods on porous glass particles, whose deep surface features protect the nanostructured films from mechanical attrition. It was found that the photocatalytic activity of anatase nanowires grown via a chemical bath deposition process was over three times greater than that of hydrothermally grown rutile nanorods when employed for photocatalytic hydrogen production and degradation of a model contaminant (Rhodamine B). The factors controlling nanowire growth and performance are discussed.
A computational fluid dynamics (CFD) model for the simulation of immobilized photocatalytic reactors for water treatment was developed and evaluated experimentally. The model integrated hydrodynamics, species mass transport, chemical reaction kinetics, and irradiance distribution within the reactor. For the development of this integrated CFD model, each of the above phenomena was individually evaluated against experimental data and proper models were identified. The experimental evaluation was performed using various configurations of annular reactors and UV lamp sizes, over a wide range of hydrodynamic conditions (350
Effective mixing of pulp fibre suspensions is essential for many pulp and paper manufacturing processes. Pulp suspensions display non-Newtonian rheology and possess a yield stress, which complicates mixing. In order to improve our understanding of pulp mixing in agitated vessels, a series of studies was undertaken to assess the suitability of using computational fluid dynamics (CFD) to model these systems.CFD simulations for laboratory-scale and industrial-scale mixing chests were developed with the pulp suspensions treated as Bingham fluids. The computed flow fields were used to determine the dynamic response of the virtual mixers, which was then compared with experimental measurements providing very good agreement under conditions of moderate agitation. Comparison between calculations and measurements of torque and axial force was also good (relative average error of 12% to 24%). The simulation results provided insight into the mixing flow occurring within the systems, showing the formation of caverns around the impeller(s), the location of stagnation regions and the presence of channeling. However, the accuracy of these predictions was limited by the Bingham model used to describe the suspension's rheology and the uncertainty to which the suspension's yield stress could be measured. To assess the degree to which the approximated rheology contributed to the CFD results, the mixing of a model fluid having a well-defined rheology (Newtonian glycerin solution) was extensively investigated in a laboratory-scale vessel using a typical industrial geometry (rectangular chest, side-entering axial flow impeller). The flow fields were measured using particle image velocimetry (PIV) and compared with CFD computations for identical operating conditions. Good agreement was found (avg. 13.1% RMS deviation of local axial velocities) confirming that the approach used in the CFD model was adequate to calculate the complex mixing flow fields for a Newtonian fluid. These results encouraged further research on extending the combined CFD and PIV application on a system more closely representative of pulp suspension agitation. Calculations were then performed to select a suitable non-Newtonian model fluid and appropriate operating conditions to model pulp suspension mixing in the laboratory-scale vessel, with the dimensionless cavern diameter as the mixing criterion for conservation between the two systems.
Master's Student Supervision (2010 - 2021)
Solar fuels are an attractive medium for long term storage of solar energy, but the low efficiency of photocatalysts in visible light prevents large scale adoption. Graphitic carbon nitride (g-CN) is a promising photocatalyst for solar fuel generation with a medium bandgap and demonstrated ability for photocatalyst engineering to improve charge recombination and charge mobility.In this work the improvement of g-CN material is investigated via phosphorous doping and post synthesis techniques to stabilize the g-CN surface chemistry and prepare the material for heterojunction integration. Further, the improved visible light absorption and activity of a self- assembled 2D-2D precious metal free g-CN type II heterojunction z-scheme is presented, targeting photocatalytic CO2 reduction and hydrogen evolution for solar fuel applications. The synergistic combination of highly reducing electrons in the conduction band of g-CN and very oxidizing holes in the valence band of BiVO4 enables tunable, small bandgap, visible light active materials to perform artificial photosynthesis. Strong interfacial ohmic contact induced by the electrostatic attraction of z-scheme constituent materials to promote charge separation is investigated with the addition of reduced graphene oxide (rGO) acting as an electron transfer medium.A realistic flat plate circulating batch reactor operating in the vapor phase, illuminated by a Xe arc solar simulator and coupled with online gas chromatography is employed for the evaluation of photocatalyst materials. This z-scheme employs facile synthesis techniques to achieve a visible light active, tunable band edge constituent material with similar reaction overpotentials toiiisingle, UV active materials such as TiO₂. Photoreduction of CO₂ improves over 100% and water splitting improves nearly 40% for the g-CN/BiVO₄ composite and is attributed to strong interfacial z-scheme charge transfer. Protonation and phosphorous doping are shown to influence the amount and oxidation state of the amine functional groups present on the g-CN surface, with protonation demonstrating nearly 4X increase in CO₂ reduction compared to washed g-CN and attributed to increased CO₂ adsorption. The work presented provides a pathway to electrostatic self-assembly of 2D-2D z-scheme heterojunctions for improved photocatalytic solar fuel generation.
Pathogenic microbes are quickly becoming one of the biggest human health threats of our time. Ultraviolet (UV) radiation enables the optogenetic control of microorganismal replication without requiring chemical addition. However, microbes with UV-resistant and repairable nucleic acids have challenged the germicidal efficiency (GE) of present UV sources. In this regard, targeting intercellular proteins responsible for nucleic acid excitation, repair, and infection can be the solution.The newly-emerged microplasma UV technology is capable of irradiating far-UVC (200 – 240 nm) with unique spectral power distributions in a flat form, which opens new pathways for the development of novel UV disinfection systems. This study is the first to identify the mechanism-analyzed and kinetic-modelled GE of microplasma UV, radiating around proteins UV-absorption and decomposition peak. The microplasma UV lamp is initially characterized in terms of radiation profile and the impact of operating parameters on the power output. It is shown to be an instant-on and fast stabilized source. The radiant power output is a linear function of the electrical current and is not influenced by the lamp operating temperature and intermittent on/off cycles.Afterwards, a protocol is also developed for obtaining reliable kinetic data for microplasma UV-induced reactions. An experimental setup is proposed for the kinetic studies, where the characteristics of the incident irradiance of the lamp, including uniformity, collimation, anddivergence, are quantitatively evaluated. Two studied cases of microbial inactivation and the chemical photo-initiated oxidation in individual protocol-based setups confirm the reproducibilityof the fluence-based kinetic data independent of the reactor size.Eventually, the GE of microplasma UV is studied against two surrogates for challenging microorganisms, Escherichia coli (E. coli) and bacteriophage MS2, and compared with literature values for current UV sources: about 2-fold GE for MS2 inactivation and one-third repair for E. coli is achieved. Emitted microplasma UV photons induce significant nucleic acid repair deficiency disorder and dramatic infection proteins excitation to enhance the genome inactivation. The reactive oxygen species are found to not play a role in this enhancement. Present results nominate promising inactivation sources for severely resistant microorganisms, thereby paving the way toward sustainable disinfection systems.
The idea of functionalizing chemical gas sensors at room temperature as well as making them smaller and more efficient has initiated important progresses in the last few years among scientists worldwide. Ultraviolet Light Emitting Diode (UV-LED) technology has shown its capability to fulfill the gap between laboratorial and industrial production of room temperature gas sensors.In this research, a review on the performances, preparation techniques, and most influential factors of several photo-activated metal oxide semiconductor gas sensors under UV-LED irradiation was conducted. Further, a comparative study on the development of sensitive gas sensors using ZnO and In₂O₃ semiconductors for NO₂ gas detection was performed. The results indicated that the sensitivity of In₂O₃ to NO₂ is approximately two times greater than that of ZnO for all the experimented irradiances. The highest sensitivities with complete recovery for the ZnO and In₂O₃ based sensors were obtained at 1.2 mW/cm² and 2.8 mW/cm² irradiances, respectively. In general, the In₂O₃ sensors required a higher UV irradiance compared to ZnO sensors, to prevent permanent adsorption of target gas molecules on the surface.To further increase the sensitivity and reduce the response time, n-type semiconductor oxides of ZnO and In₂O₃ were coupled using co-precipitation method, to obtain nano-crystalline composite sensing materials. The composition, structure and optical properties of the prepared samples were characterized by EDS, XRD, SEM, XPS and UV-Vis analyses. The composite materials showed higher sensitivity towards NO₂ with a 200s decrease in response time compared to pristine samples. A favorable composition ratio of [In]:[Zn] was determined to be 1:2 for the nano-composite particles, with 2.21 sensitivity as the highest sensing performance to 5 ppm NO₂. The high sensitivity of this combination is attributed to the morphology and composite porous structure, as well as lower band-gap of the target composite. The irradiance of 1.7 mW/cm² provided the highest sensitivity, short response time and a complete recovery for the ZnO/In₂O₃ composite structures, within the experimented range. It’s believed that, ZnO favors the flow of charge carriers and increases the surface area, while In₂O₃ acts as active light absorption centers and enhances chemisorption ability in the composite.
The wide applicability of mechanically stirred tanks in industry demands a comprehensive understanding of the physical and chemical phenomena controlling the performance of these fundamental units. The rheological complexity of some industrial fluids can create unfavorable mixing environments like dead zones that limit the contact area among the components being mixed. Also, the complex three dimensional nature of the flow generated by the impellers makes difficult the prediction of the flow properties, especially when the fluid viscosity is a function of the shear rate. Some research groups have investigated mixing flow of these kinds of fluids in conventional stirred tanks with top-entry impellers. But, little has been done to characterize the flow behavior in tanks with side-entry impellers.In order to improve our understanding and provide insight into the flow mixing occurring in stirred tanks with side entry impellers, the flow field generated by different impellers in scale-down vessels filled with glycerine and carbopol solutions, was studied using the flow visualization technique, particle image velocimetry (PIV). Moreover, a computational model was built to predict flow variables and mixing characteristics unattainable with the experimental technique. The capabilities of the model were evaluated based on the velocity fields obtained experimentally. Good agreement was found between the predicted and measured macroscale flow structures and global mixing parameters. However, the models were unable to predict the symmetric flow observed during the experiments at high rotational speeds, likely due to the approach taken to simulate the flow, which provides a steady state velocity profile for one specific impeller locationOverall the results showed the formation of dead zones and segregated regions when mixing the non-Newtonian solutions. The size of the dynamic regions and the average velocity near the impeller were improved by increasing the suction area. Likewise, large pitch ratios were found to enhance the active mixing zone and the axial discharge. While, radial discharge and a strong tangential flow arose when the viscous forces dominate the flow. In conclusion, the flow features were defined by the Reynolds number in the vicinity of the impeller and the restrictions imposed by the walls of the vessel.
Robust calculations show that the incidence of solar energy on the earth’s surface by far exceeds all human energy needs. Undoubtedly, the most trusted way of utilizing solar energy is to convert and store it in the form of an energy carrier such as hydrogen. Semiconductors capable of absorbing light energy so-called photocatalysts can potentially drive water splitting reaction for hydrogen generation. In this research, fundamental studies on a new class of solar-activated supported photocatalysts for water splitting application are presented. This resulted in significantly higher rates of H₂ production in comparison to the existing supported TiO₂ series under visible light. The composition comprises silico-aluminates (zeolite) as the support, titanium dioxides (TiO₂) as the semiconductor, cobalt compounds as hydrogen evolution sites and heteropolyacids (HPAs) as multifunctional solid acids with excitability under visible light. Using this composition, I ended up with at least 2.6 times higher hydrogen evolution rates under visible light in comparison to Degussa P25, the best commercially available titania product. The chemical point of view of this successful combination was investigated, attributing the higher photocatalytic activity of the synthesized chemical compositions to the basicity of the matrix. The more basicity properties besides HPA presence can overcome the negative impacts of titania interactions with the zeolite which are band gap widening and anodic shift of the TiO₂ band edges. Furthermore, the effect of cobalt precursors (nitrates and chlorides) on the photocatalytic activity of the prepared photocatalysts was also investigated. Although nitrate-based photocatalysts exhibited an improvement in the UV-VIS absorbance spectra toward visible light, they caused an almost 30% lower H₂ production rate in comparison to the chloride salts. Overshadowing the poisoning and parasitic effects of Cl⁻ anions on the photooxidation sites in the zeolite-supported composition was another notable outcome of this study. This suggests emulation of the core-shell photocatalysis concept insofar with providing a reasonable distance between redox sites. The results indicate the importance of zeolite’s structural and chemical properties as the photocatalyst support. This can be addressed through the selection of suitable zeolite types, taking an important step in the development of visible-light-activated photocatalysts based on earth-abundant materials.
The oxygen delignification stage is implemented in modern kraft pulp mills to cut the cost of producing bleached bright pulp and reduce emissions from the bleaching process. While several oxygen delignification kinetic models are presented in the literature, most models are derived from and limited to, specific pulp blends. In this work, an oxygen delignification kinetic model was developed based on lignin model compound chemistry found in the literature to create a universal kinetic model applicable to a range of pulp blends. The kinetic model splits the pulp into three reactive lignin groups (fast, slow and non-reacting) each with their own kinetic constants. Along with the starting kappa number, the proposed testing protocol measures kappa number from oxygen delignification experiments at 90°C for three hours (to determine non-reacting lignin fraction) and at 50°C for five minutes (to determine the fast lignin fraction). From the three kappa measurements, the fraction of each lignin group is determined and combined with their respective kinetic constants to create the overall kinetic model. Coupling separately developed mass transfer governing equations for the pulp suspension with the kinetic model, the overall oxygen delignification stage model was developed. The oxygen delignification model was compared with experiments performed both in the laboratory and in an industrial pulp mill. Laboratory oxygen delignification experiments on four pulp blends (three softwood and one hardwood) showed good agreement between experimental data and simulation values indicating the proposed testing protocol and kinetic constants are able to model the oxygen delignification reaction. In particular, oxygen delignification reaction result at 50°C for five minutes in conjunction with delignification result at 90°C for three hours was able to determine the split between fast, slow, and non-reacting lignin groups. Model simulations using the determined lignin fraction split was able to model experimental data especially well for 90°C experiments that mimics the conditions experienced in an industrial setting. Experiments at a pulp mill showed agreement within one kappa number between measured values and model simulations signifying the applicability of the proposed overall oxygen delignification model to simulate an industrial process.