Theses and dissertations (Engineering and Built Environment)
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Item Multiscale modelling of biogas purification using montmorillonite adsorbent(2024-05) Khuzwayo, Thandeka Ntombifuthi; Ngema, Peterson Thokozani; Ramsuroop, Suresh; Lasich, Madison M.Biogas, a renewable energy source derived from organic materials, offers significant potential for creating sustainable power sources and minimize environmental pollution. However, the presence of contaminants like carbon dioxide (CO2) and hydrogen sulfide (H2S) in biogas can reduce its usefulness and efficiency in a number of applications. To address this issue, this research focuses on the purification of biogas using clay adsorbent. This study investigates the adsorption capacity of clay minerals, such as montmorillonite, in removing CO2 and H2S from biogas. In this study, Grand Canonical Monte Carlo (GCMC) simulations were performed using a self-consistent forcefield to predict adsorption isotherms for methane, carbon dioxide, ethane, and hydrogen sulfide in montmorillonite lattice. The experimental setup involved a Pressure Swing Adsorption (PSA) column, where biogas passes through the adsorbent, leading to the adsorption of impurities while maintaining the methane content, thus enhancing the overall biogas quality. The model was fitted with Langmuir adsorption isotherms for all species at different pressures and ambient temperature, coupled with batch equilibrium approach to model the PSA system. The equilibrium modelling of a pressure swing adsorption system to purify CH4/CO2 feedstock was demonstrated in such that a system can be incorporated into a solar biogas reforming process, targeting purity of 93-96 mol-% methane, which was readily achievable. The modelling of PSA indicate that the system could produce over 96% of methane and a recovery of around 82% at low pressure. The findings suggest that the choice of clay adsorbent and optimization of process parameters can significantly enhance the purification efficiency of biogas via pressure-swing adsorption. The strong selectivity of the montmorillonite adsorbent has affinity to adsorb carbon dioxide and other species at low pressures, even though nitrogen require more pressure to be adsorbed onto the montmorillonite bed.Item Non-oxidative conversion of methane into carbon and petrochemicals over Fe, W,& Mo catalyst systems supported on activated carbon and HZSM-5(2021-04) Musamali, Ronald Wafula; Isa, Yusuf MakarfiNon-oxidative conversion of methane (NOCM) is an environmentally benign route for producing carbon and valuable petrochemicals from methane. Unlike other methane conversion processes like Fischer-Tropsch and methanol synthesis which have been scaled up to commercial level, NOCM process development remains at laboratory scale due to various challenges such as catalyst deactivation due to coking, process thermodynamics, low conversion, and limited selectivity towards useful products. In this present work, a study of non-oxidative conversion of methane into carbon and petrochemicals was done over Fe, W, & Mo catalyst systems supported on activated carbon (AC) and HZSM-5. The catalyst systems were prepared by various techniques at different metal loadings. The prepared catalysts were characterized for phase identification, structural properties, surface area, presence of functional groups, and tested for non-oxidative methane conversion at different operating conditions in a packed bed reactor. Products from non- oxidative conversion of methane were analysed using gas chromatography. To accomplish the research objectives, synthesized binary catalyst systems were developed step by step. Phase one of the study involved synthesis of 24 single metal catalyst systems supported on activated carbon and HZSM-5 between 1.8-7.2% metal loading and tested for non-oxidative methane conversion. Prepared catalysts were screened based on methane conversion. Phase two of the study involved synthesis of 5.4% bimetallic catalyst systems supported on AC/ HZSM-5 and applied for non-oxidative methane conversion. Catalytic activity of Fe-Mo, W-Mo and Fe- W on AC and HZSM-5 supports were evaluated based on methane conversion and product distribution. In the final phase of the study, trimetallic binary catalyst systems (Fe-W-Mo) on AC and HZSM-5 supports were synthesized, characterized, and their catalytic activity evaluated at different metal loading, different metal composition, and different process conditions. The effect of support and catalyst preparation method on catalyst activity was also evaluated. Based on the results obtained, catalyst Fe-Mo/HZSM-5 showed little activity in terms of methane conversion with low C2 and high coke formation whereas catalyst W-Mo/HZSM-5 was very active in methane conversion but less selective towards C2 and aromatic hydrocarbons. On the other hand, catalyst Fe-W showed low methane conversion and low coke formation but exhibited high selectivity toward aromatics. A 5.4% binary catalyst system (Fe-W-Mo/HZSM-5) with equal metal loading did not show much improvement on methane conversion, selectivity towards C2 hydrocarbons, aromatics, and coke. However, when Fe and W metal loading were higher than Mo in this 5.4% binary catalyst system, there was notable increase in methane conversion and coke but C2 formation decreased. On the contrary, when Mo loading was increased and Fe and W metal loading reduced, there was a subsequent decrease in methane conversion and coke formation but C2 and aromatics formation increased by a big margin. From X-ray diffraction (XRD) results, M2C on HZSM-5 produced by transformation of highly dispersed MoO3, was the most active site for the activation of the C-H bond in methane molecules, but these sites were less active for further decomposition of CH∗ radicals. Based on methane conversion, catalytic activity of Fe-W-Mo 3 catalyst systems showed the same trend both on AC and HZSM-5 although methane conversion values were higher on AC than on HZSM-5 support. A wider range of product distribution was realized on catalysts supported on HZSM-5 than on AC support. This was attributed to the HZSM-5 zeolite channel structure and its inherent acidity which promoted shape selectivity towards benzene and its derivatives. Further, methane reacted with Mo6+ on HZSM-5 zeolite to produce CH3+ (a methoxy species on the Bronsted acid sites of the zeolite) and [Mo-H]5+ which were further transformed into a molybdenum-carbene species (Mo=CH2). These species further reacted with CH4 to produce C2 intermediates. The Bronsted acid sites located inside the zeolite channels and shape selectivity of HZSM-5 zeolite were responsible for activation of C- H bond and conversion of the C2 intermediates into benzene and other higher carbon hydrocarbons. Despite intensive research in this area, and to the best of the author’s knowledge, no work on the development of a catalyst system for quantitative control of methane conversion and product distribution using Fe, W, and Mo catalyst systems loaded on AC/HZSM-5 has been reported. Therefore, the novelty in this work lies in the development of a tuneable binary catalyst system for quantitative control of product distribution in methane conversion to carbon and petrochemicals.Item Anaerobic co-digestion with industrial wastewater for biomethane production(2020-10-20) Adedeji, Jeremiah; Chetty, MaggieThe increasing demand for energy has led to the utilization of fossil fuels more abundantly as a quick alternative for generation of energy. The use of these sources of energy however as led to the generation of greenhouse gases which tend to cause climate change, thus affecting the ecosystem at large. Thus, there have been the search for alternative sources which cannot be depleted but do generate minimal greenhouse gases. One of such alternate sources is industrial wastewater which have shown to have high concentration of nutrients in the form of organic contents which can be converted by micro-organisms into energy, usually known as biogas, comprising majorly of CH4, CO2 and H2. Another important factor is that industrial wastewaters are a renewable energy source which are continuously generated due to increasing urbanisation and population growth. In this study, the characteristics of three agro-industrial based wastewaters used shows their potential for application in anaerobic co-digestion”. Anaerobic co-digestion method was utilized to harness the synergetic effect of both sewage sludge and agro-industrial wastewater as co-substrate for the generation of biomethane. The result of the effect of varying mix-ratio of the substrates on biomethane production of sugar wastewater and dairy wastewater indicated that mix-ratio of 1:1 for sewage sludge to sugar wastewater operated at 35oC was suitable for optimum generation of biomethane of 1400.99 mL CH4/g COD added and COD reduction of 54%. The model generated using design expert was found to navigate the design space and could perfectly predict the yield of biomethane effectively for the sugar wastewater mix. The biomethane potential tests (BMP) experiment using varying inoculum-substrate ratio (ISR) showed that operating at mesophilic temperature of 25oC with ISR of 1:2 and 2:1 for sugar wastewater and dairy wastewater respectively does increase the methane production within the first three (3) weeks. The kinetic models that best fit the anaerobic co-digestion for sugar wastewater was the first order model while the simplified Gompertz model favoured the dairy wastewater perfectly. The biomethane potential tests indicate significant increase the biomethane production and as well reduction in the volatile solid and chemical oxygen demand (COD) content. In conclusion, both sugar and dairy wastewater can be recommended as co-substrates for anaerobic digestion of sewage sludge for increased and improved biomethane production while simultaneously reducing their COD content at the same time.Item Decomposition of methane into carbon and hydrogen over Ni-Li/CaO catalysts(2018) Musamali, Ronald Wafula; Isa, Yusuf MakarfiOverdependence on fossil-based fuels and their effect on environment is a global concern by energy stake holders. Bulk of present day hydrogen comes from gasification of coal, steam reforming and partial oxidation of hydrocarbons. Steam reforming accounts for over 50% of world hydrogen production despite producing carbonaceous gases which are harmful to the environment and poisonous to both; proton exchange fuel cells and alkaline fuel cells. Natural gas is a preferred feed for hydrogen production, because it is abundantly available on earth. Catalytic decomposition of ammonia can produce clean hydrogen but ammonia itself is an air pollutant. Catalytic decomposition of methane into carbon and hydrogen is an attractive option to producing clean hydrogen because its products are carbon and hydrogen. In this work, five different catalysts comprising of varying quantities of nickel and lithium, supported on calcium oxide were synthesized by incipient wetness impregnation method and designated according to weight % as; 30%Ni/CaO, 37.5%Ni-12.5%Li/CaO, 25.0%Ni- 25.0%Li/CaO, 12.5%Ni-37.5%Li/CaO and 50%Li/CaO. The synthesized catalysts were characterized by (XRD, SEM, BET and TEM) and tested for methane decomposition. From the XRD patterns of the synthesized catalysts, distinct crystalline phases of CaO and NiO were positively identified in 50%Ni/CaO according to their reference JCPDS files. Introduction of Lithium hydroxides improved the crystalline structure of the Ni/CaO catalyst. SEM analyses of the catalyst material using Image-J software confirmed that all catalyst materials were nanoparticles ranging from 3.09-6.56nm. BET results confirmed that, all the catalysts are mesoporous with pore sizes ranging from 20.1nm to 45.3nm. Introduction of LiOH to Ni/CaO generates mesoporous structures by destructing the lattices of the CaO structure during the formation of Ni-Li/CaO species. Particle size distribution in TEM analyses revealed that, a higher nickel loading in the catalyst favours the formation of carbon nanotubes while higher lithium hydroxide loading favours the formation of carbon fibres (CF). Low yield of carbon fibres from methane decomposition on unsupported Ni catalyst in 50%Ni/CaO was attributed to the presence of large Ni particles with low index planes which were incapable of dissociating the unreactive methane molecule. The aim of this work was to synthesize a catalyst for use in decomposition of methane into carbon and hydrogen, that addresses drawbacks of traditional solid metal catalysts such as sintering and coking. From the experimental results, 37.5%Ni-12.5%Li/CaO catalyst recorded 65.7% methane conversion and 38.3%hydrogen yield while 50%Ni/CaO recorded the lowest methane conversion of 60.2% and a hydrogen yield of 35.7% at 650℃. Outstanding performance of the 37.5%Ni-12.5%Li/CaO catalyst is attributed to the incorporation of lithium hydroxide which provided more catalyst active sites and a molten environment for proper dispersion of the nickel metal. The solid 50%Ni/CaO catalyst readily deactivated due to coking unlike the supported molten 37.5%Ni-12.5%Li/CaO catalyst in which methane decomposition reaction took place by both surface reaction and chemisorption.Item Optimization of anaerobic co-digestion of sewage sludge using bio-chemical substrates(2018) Madondo, Nhlanganiso Ivan; Chetty, ManimagalayThe anaerobic process is increasingly becoming a subject for many as it reduces greenhouse gas emissions and recovers carbon dioxide energy as methane. Even though these benefits are attainable, proper control and design of the process variables has to be done in order to optimize the system productivity and improve stability. The aim of this research was to optimize methane and biogas yields on the anaerobic co-digestion of sewage sludge using bio-chemical substrates as co-substrates. The first objective was to find the bio-chemical substrate that will generate the highest biogas and methane yields. The anaerobic digestion of these substrates was operated using 6 L digesters at 37.5℃. The substrate which generated the highest biogas and methane yield in the first batch experiment was then used for the second batch test. The objective was to optimize the anaerobic conditions (substrate to inoculum ratio, co-substrate concentration and temperature) in-order to optimize the biogas and methane yields. The second batch test was achieved using the conventional One-Factor-At-A-Time (OFAT) and the Design of Experiment (DOE) methods. Final analysis showed that the bio-chemical substrates could be substrates of interest to biogas generators. Amongst the substrates tested in the first batch experiment glycerol (Oleo-Chemical Product waste) generated the highest methane and biogas yields of 0.71 and 0.93 L. (g volatile solids added)-1, respectively. It was believed that glycerol contains significant amount of other organic substances such as lipids that have higher energy content than the other bio-chemical substrates, thus generating larger biogas and methane yields. Moreover, digestion of sewage sludge alone produced biogas yields of 0.19 L /g VS and 0.33 L/g COD, and methane yields of 0.16 L/g VS and 0.28 L/g COD. Generally, co-digestion yields were higher than digestion yields of sewage alone. Using the OFAT method the results of the second batch test on glycerol demonstrated highest amounts of volatile solids (VS) reduction, chemical oxygen demand (COD) reduction, biogas yield and methane yield of 99.7%, 100%, 0.94 L (g VS added)-1 and 0.75 L (g VS added)-1 at a temperature, substrate to inoculum ratio and glycerol volume of 50℃, 1 (on VS basis) and 10 mL, respectively. Above 22 mL and substrate to inoculum ratio of 1, the process was inhibited. The DOE results suggested that the highest methane and biogas yields were 0.75 and 0.94 L (g VS added)-1, respectively. These results were similar to the OFAT results, thus the DOE software may be used to define the biogas and methane yields equations for glycerol. In conclusion, anaerobic co-digestion of bio-chemical substrates as co-substrates on sewage sludge was successfully applied to optimize methane and biogas yields.