Masters Degrees (Chemical Engineering)

Permanent URI for this collectionhttps://hdl.handle.net/10413/6658

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Development and testing of a faecal sludge greenhouse solar dryer.
(2023) Naidoo, Pareshin.; Stringel, Santiago Septien.; McGregor, Craig.
Faecal sludge poses a significant global challenge, contributing to pollution and disease transmission if not managed appropriately. Thermal drying presents a viable treatment method, reducing sludge mass and eliminating pathogens. However, the high energy requirements associated with conventionally powered methods of thermal drying raise operating costs, and to address this issue, solar drying was evaluated as an efficient and sustainable alternative for this process. This project aims to develop and test a solar thermal drying system for the treatment of faecal sludge, measure its performance characteristics, and identify areas of improvement and optimization. This project presents an adaptation of a greenhouse-type solar dryer developed by the WASH R&D Centre at the University of KwaZulu-Natal for the treatment of faecal sludge. Greenhouse solar dryers utilise solar thermal energy and ventilation to dry a product. This dryer consists of seven subsystems: an enclosure, absorber wall, ventilation and circulation system, sludge mixing system, drainage system, drying tray, and testing instrumentation. Polymethyl methacrylate and aluminium were used in the construction of the enclosure, which measures 2 × 1.5 × 2 m (length × width × height). A ventilation and circulation system were installed in the greenhouse to remove moisture from the dryer and to improve moisture evaporation by enhancing external mass transfer at high air velocities. A rake system was designed to mix sludge and promote homogenous drying. The sludge was placed in a suspended grid where it could be dried from the top and bottom. A temperature and humidity control system was developed to record both the humidity and temperature at seven different areas in the dryer. The dryer was tested in Durban during the winter season of 2022 using feedstock consisting of water, wet soil, and synthetic sludge as a proxy of real faecal sludge, at UKZN, Howard College campus on the roof of the Chemical Engineering building. Temperatures higher than 40 °C were recorded inside the dryer with ambient outside temperatures lower than 30 °C were recorded, leading to relative humidity lower than 40 %. Functionality tests were conducted to ensure the various subsystems were operating correctly, after which, the system was tested with feedstock. During the water tests, 7 small crucibles filled with water were used in the greenhouse dryer to determine how well water evaporated. Adjustments of both the ventilation and circulation fan speeds were made to determine the best airflow rates for the drying of water. Lower fan speeds proved more effective for water evaporation so the lowest fan speed combinations, ventilation speed 1 (V1) and circulation speed 1 (C1) were used in the wet soil tests and synthetic sludge. Tests with soil and sludge were conducted at these speeds. A large reduction of the moisture content of wet soil was observed during a single day from around 70 % moisture content to less than 20 % in the space of 5 hours. Synthetic sludge was made and tested during the winter season. Synthetic sludge has very similar properties and texture to faecal sludge and therefore is an effective feedstock for measuring the dryer's performance. A 10 kg sample of synthetic sludge was tested in the greenhouse solar dryer between the dates of 05-08-2022 and 08-08-2022. Synthetic sludge with an 80 % moisture content was reduced to 6.5 % over 4 days. Drying times ranged from 5 to 6 hours a day, with an overall drying rate of 0.36 kg/h on average, a specific energy consumption (SEC) of approximately 655 kWh/t and an overall dryer efficiency of 9.6 %. A second test was conducted on the dates of 14-08-2022 and 15-08-2022. Better weather conditions on the second test enabled the dryer to dry 80 % moisture content sludge to 12 % in 2 days. The overall dryer efficiency of approximately 18 % and average SEC of 363 kWh/t were calculated over the two days of testing. The greenhouse solar dryer research and testing indicated that solar thermal dryers can potentially address the drying and disposal of faecal sludge, a matter that is a major problem in developing countries. Further steps to conclude the study include tests with real faecal. sludge, improvement and optimization of the system, and the development of a techno-economic analysis.
Analysis of the products from mixed plastic waste pyrolysis.
(2024) Naidoo, Bradlee Kevin.; Moodley, Kuveneshan.; Naidoo, Prathieka.; Durski, Marcin Hubert.
The utilization of plastic products has grown significantly in the last several decades and consequently expanded the production rate of plastic products, as well as increased pollution rates of waste plastic globally. Pyrolysis processes may be a feasible alternative for the management of non-recyclable plastic waste such as mixed plastic. During this process, plastics (individual or mixed types) are reacted at high temperatures (generally from 300°C to 900°C) to crack and breakdown. Products such as pyrolysis liquid, wax, gas, and char result from the process. South African legislation on pyrolysis emissions has caused several pyrolysis facilities in KwaZulu-Natal to halt operations due to non-compliance. Hence, pyrolysis technologies for the treatment of plastics needs to be analysed and redesigned/optimized to reduce emissions such that it complies with legislation. This work falls under a broader research project with the goal of developing an optimized pyrolysis process with novel aspects to manage mixed waste plastic through catalytic pyrolysis. This dissertation focuses specifically on the analysis of components generated from catalyzed lab-scale plastic pyrolysis. Plastic pyrolysis experiments were conducted using a feedstock of compounded low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP) and mixtures of these plastics at 450°C and vacuum of approximately 42 kPa. Zinc oxide powder was used as the catalyst, and its effect was investigated in comparison to the thermal (uncatalysed) pyrolysis process. The selection of the conditions and catalyst used are described in detail within this dissertation. Product yields of the pyrolysis liquid, wax, char, and gas were determined through weighing and mass balance. Gas chromatography-mass spectrometry (GC-MS) was used to perform chemical analyses on the liquid and wax to determine the types of components constituting these products. Additionally, the emissions were analysed to identify the volatile organic compounds (VOCs) present. The results indicated that the zinc oxide catalyst effectively enhanced the pyrolysis experiments and increased the combined liquid and wax product yields, which are the desirable products. Pure LDPE and HDPE pyrolysis with catalyst to feed ratios of 5:100 produced 78.3 wt % and 78.2 wt % of combined liquid and wax yields respectively. This was relatively high compared to the other pure plastic experiments with and without catalyst. However, uncatalysed HDPE pyrolysis produced a combined liquid and wax yield of 85.4 % but required a longer reaction time. The use of zinc oxide catalyst (5:100 ratio) in LDPE pyrolysis experiments reduced reaction times by approximately 14 hours compared to the uncatalyzed experiment. Reaction times of catalysed (5:100) HDPE and PP pyrolysis experiments were reduced by 1 hour 30 minutes (HDPE) and 15 minutes (LDPE) compared to their respective uncatalyzed experiments. Pure PP pyrolysis produced the highest gas yields (ranging from 17.7 wt % to 19.2 wt %). Mixed plastic feed experiments with higher proportions of compounded LDPE produced the highest combined liquid and wax yields. The usage of catalyst caused a reduction of gas yields in many of the mixed plastic pyrolysis experiments. According to the GC-MS analyses, the use of 5:100 zinc oxide catalyst to feed ratio produced the lowest total VOC peak area percentages for many of the pure plastic feed types, however it seems the catalyst caused greater fractions of VOCs to form in the mixed plastic experiments. Many of the liquid and wax analysis results indicated a large proportion of components within the C12 to C27 range. This indicates that the liquid products potentially had similar properties to diesel fuel and correlates with the results of similar studies in the literature. Furthermore, the 5:100 zinc oxide catalyst to feed specification produced the highest combined liquid and wax yields in many of the experiments. A catalysed (5:100) experiment using a feedstock of 32.3 wt % LDPE, 34.52 wt % HDPE, and 33.18 wt % PP indicated a trade-off between high liquid + wax yields, reasonable VOC content in emissions, lower gas yields, and lower energy usage per gram of product formed relative to experiments with other feed compositions. A preliminary life cycle gate to gate analysis was conducted for the laboratory scale process, and it was determined that the major power consuming units, heating mantle, condensing circulator and vacuum pump contribute approximately equally to the indirect CO2 emissions for the process, assuming they are operating at capacity. Pure LDPE pyrolysis had the highest CO2 emissions per gram of desirable product obtained (3.83 𝑘𝑔 𝐶𝑂2𝑔 ) while PP pyrolysis with a catalyst ratio of 10:100 had the lowest CO2 emissions (0.31 𝑘𝑔 𝐶𝑂2𝑔). In the mixed plastic experiments, the CO2 emissions increased as the amount of LDPE in the feed was increased. The proportion of gas product streams that would be considered volatile organic compounds (VOCs) constituted between 32-83 % for each experiment. The total VOCs was lowest in mixed plastic pyrolysis experiments where approximately equal proportions of compounded LDPE, HDPE and PP was used as the feed, compared to the other sets. It was observed that the use of catalyst (5:100) generally increased the total VOCs and the carbon number of the most abundant VOCs in the gas stream in the mixed plastic experiments.
Impact of degree of sulphonation of lignosulphonic acid on molecular weight distribution of spent liquours.
(2024) Mokoena, Bonisile Ignacia.; Sithole, Bishop Bruce.; Potthast, Antje.
Lignosulphonates are produced during the chemical pulping of wood species through acid sulphite or neutral sulphite semi-chemical processes. Lignosulphonates are applied in several commercial products, including but not limited to dispersants, resins, binders, concrete admixtures, water treatment aids (coagulants), etc. These complex macromolecular structures exhibit different chemical properties, depending on the pulping process and tree species used. How lignosulphonates are valorised will, therefore, depend on their inherent chemical properties. The molecular weight and the sulphonic acid content in particular are two properties that will affect how lignosulphonates are used. In this study, the molecular weight and sulphonic acid of magnesium lignosulphonate (MgLS) and sodium lignosulphonate (NaLS) were studied. Due to the processing contaminants that interfere with molecular weight and sulphonic acid measurements of spent liquors purification of the MgLS and NaLS streams was carried out using a combination of Dowex 50wx8 and XAD-7 resins. The Dowex resin was used for de-ashing, and the XAD-7 resin to adsorb the lignosulphonate molecule. Ultrafiltration was also included as a comparison for the Dowex-XAD-7 purification. Approximately; 5 % ash was retained on samples purified by ultrafiltration and less than 3% ash was retained by samples purified by Dowex-XAD-7. The sample yield after Dowex-XAD-7 and ultrafiltration was similar for MgLS samples, between 32 and 36% sample was extracted . There was, however, a difference in sample yield when NaLS liquor was purified. A 24 and 32 % sample recovery was achieved using Dowex-XAD-7 and ultrafiltration, respectively. The yield was higher for the ultrafiltrated product than for the XAD-7 product. Samples purified by ultrafiltration generally showed high molecular weight and sulphonic acid content. The molecular weight for NaLS and MgLS samples purified by ultrafiltration measured approximately 6 and 20 kDa, respectively. Where as NaLS and MgLS samples purified by Dowex-XAD-7 reported a molecular weight of 3 and 12 kDa, respectively. The sulphonic acid content ranged between 3 and 5% for ultrafiltrated samples where as the Dowex-XAD-7 extracted samples ranged between 2 and 3%. The samples purified by XAD-7 resins were then sequentially fractionated using ultrafiltration with molecular weight cut-off (MWCO) sizes between 100 kDa to 1kDa. The molecular weight of the fractionated products decreased with a decrease in MWCO size ranging between 100 and 1kDa. This decrease was from 17.6 to 0.6 kDa and 14.8 to 0.8 for MgLS and NaLS samples, respectively. For the MgLS, the sulphonic acid content increased from 0.93 to 1.25 mmol/g with a decrease in molecular weight, whereas the fractions from the NaLS feed seemed to decrease with a decrease in molecular weight.
A generalized novel group contribution kinetic modeling approach to linear alkene metathesis.
(2023) Bansi, Nikhiel Isaiah.; Lokhat, David.
Traditional modeling approaches for linear metathesis systems involve the use of complicated approaches – This paper proposes a generalized novel kinetic modeling technique, in which the substituent (alkyl) groups on the transition states are postulated to have a unique effect on the rate at which the controlling step (transition state dissociation) occurs. Metathesis involves the redistribution of the carbon-carbon double bond across constituents, to create longer chain hydrocarbons, which significantly enhances product value in an industrial context. This work proceeds by unpacking the mechanics of the 1-Hexene (linear) metathesis pathway and utilizes this as a basis for developing a group contribution kinetic modeling approach. The substituent (alkyl) groups present in the forming transition state complexes were used to define (identify) rate constants that would potentially control the rate of formation of the various transition state complexes. A key assumption in this work was that the largest externally attaching substituent, and the substituent existing in the metallacyclobutane complex, be selected as the groups that define the formation of the respective transition state complexes. These observations resulted in a system of 25 uniquely defined (identified) rate parameters that were derived based on the defining groups observed within the mechanisms and are sufficient to describe the metathesis system under investigation. Literature experimental data, at a range of temperatures (420℃ – 460℃) was readily available and was used for the purpose of fitting the identified rate parameters, by simultaneously solving the system of differential equations that result from this system. Given the system size (25 parameters), and complexity of the interactions in the system, evolutionary and swarm optimization techniques were found to be fit for this purpose. It was found that combination between a genetic algorithm (GA) and particle swarm optimization (PSO) approach yielded identified parameters that minimized the overall error of the prediction. Kinetic parameters were identified, and Arrhenius plots were developed – These allowed for the activation energies (𝐸𝑎) and pre-exponential factors (𝐴𝑜) to be determined for each parameter. This was tested using literature experimental datasets, and predictions were found to present with a modified fitness value between 1.968 and 50.55. This illustrates that the novel group contribution kinetic modeling approach was suitable to define the interactions in the system. Further research is required to generalize this result over other alkene metathesis systems; however, this work proves that the approach is viable. Extending this work to ring closing metathesis (RCM), is a future area of research that is of particular importance, as pharmaceutical intermediates result from these processes, and represents the unique opportunity to create a synergistic industrial landscape.
High temperature production and desulphurisation of syngas.
(2015) Ali, Johra Said.; Kiambi, Sammy Lewis.; Carsky, Milan.
Synthetic gas (syngas) produced by the gasification of coal provides a cheap fuel alternative for the production of electricity. However advanced utilization of syngas is limited due to the contaminants which can seriously deactivate the catalysts used in the downstream reactions as well as downstream equipment such as gas turbines. Among the contaminants, sulphur compounds produced in the gasification process, which are mainly H2S with small amounts of COS must be removed. The method presented here is downstream sulphur capture by a metal oxide at high temperatures. In this study, a laboratory scale unit was used to produce and clean synthetic gas (syngas) containing 1.0-1.15 mole % H2S from a liquid hydrocarbon fuel consisting of 86 % methanol, 14 % propanethiol by mass, and 18 mole % oxygen gas as the oxidant. The gasifier operates at 830 0C at atmospheric pressure. Desulphurisation occurs in a fixed bed reactor packed with zinc oxide spherical pellets as the sorbent. Experiments were performed to determine whether the use of the liquid hydrocarbon mixture as fuel in the laboratory could actually produce H2S containing syngas, and eventually compare the composition of the gas produced experimentally to the one predicted by a model. Desulphurisation experiments were performed by varying reaction temperatures (350 0C and 550 0C) and sorbent particle sizes (1.63-2.03 mm) at atmospheric pressure using sorbent with varying surface areas (average of 5 m2/g and 25 m2/g) at high and low gas velocities (average of 3430 h- 1and 610 h-1). These variations were performed in a 2*2 factorial design to determine the effect of these factors on the desulphurisation process and observe whether there is any interaction between them. Statistical analysis was used to determine the significance of each factor on the sorbent sulphur sorption capacity. A packed bed model using shrinking core model was used to describe the desulphurisation process. GC analysis specific to sulphur compound detection showed consistent production of H2S during gasification. In addition to this the composition of the predicted syngas was validated through a GC equipped with a TCD detector and there was a good agreement giving the expected ratio of 2:1 of H2 and CO respectively. Small particle size sorbent with 5.3 m2/g surface area and an average space velocity of 610 h-1 at 350 0C reaction temperature had the highest sulphur sorption capacity of 3.71 Wt. % which was a 20% conversion of zinc oxide to zinc sulphide. There was no effect of increase in temperature on the process. The low surface area sorbents were more effective than the increased surface area sorbents. These results were verified by the statistical analysis performed on the sulphur sorption capacity obtained during experimentation. The packed bed model results were not in agreement with the experimental results.
Analysis of potential small satellite launch operations at the Denel Overberg test range.
(2022) Arunakirinathar, Aravind.; Pitot de la Beaujardiere, Jean-Francois Philippe.; Brooks, Michael John.
One of the primary objectives of the South African First Integrated Rocket Engine (SAFFIRE) programme of UKZN’s Aerospace System Research Group (ASReG) is to develop the capacity for orbital injection missions to Low Earth Orbits (LEOs) from South Africa. The most likely launch site for these missions is the Denel Overberg Test Range (OTR) near Cape Agulhas in the Western Cape. In order to determine the suitability of OTR as a launch site, it is imperative to gain an understanding of the performance, mechanics and structural loads of a vehicle entering orbit. The goal of this dissertation is to analyse the performance of a variety of modern two-stage launch vehicles as they travel along orbital injection trajectories into LEOs from OTR. This study considers solutions for the ascent-to-orbit trajectory for various launch vehicles. The primary method was to utilise trajectory optimisation methods and this was achieved by developing an optimal control solver, which makes use of direct Hermite-Simpson collocation methods, and a sequential quadratic programming solver. In order to improve the robustness and speed of the solver, formulae for the first order analytical derivative information of direct Hermite-Simpson collocation were developed. The optimal control solver was then validated using various linear and nonlinear examples from literature. The optimal control solver was used to analyse the performance of various hypothetical missions conducted by the following established launch vehicles: Rocket Lab’s Electron, SpaceX’s Falcon 1, SpaceX’s Falcon 9, and ASReG’s proposed small satellite launch vehicle, CLV. As a baseline comparison, all vehicles were launched from OTR into various LEOs. The payloads, trajectories, control histories and structural loads of these vehicles for injection were investigated. Finally, the effect of perigee altitude, inclination, and eccentricity of orbits on the extracted results was studied. The payload performance of the launch vehicles considered were relatively similar to that provided by each vehicle’s corresponding payload user guide. On all missions, the altitude of the Electron, Falcon 9 and CLV would constantly increase with range, however the Falcon 1 would tend to rise, dip, and then rise once more on missions to orbits with a perigee altitude of 200 km. Such trajectories are referred to as lofted trajectories and are common among vehicles with a low upper stage thrust to weight ratio (Patton and Hopkins, 2006), such as the Falcon 1. The tangent yaw and pitch of the thrust direction was highly linear for all analysed missions. This result allows for a reasonable control law which can be used to determine trajectory solutions using indirect optimal control methods. This study demonstrates the viability of the Denel Overberg Test Range as a competitive base of operation for space launch missions to LEO.
Development of catalysts for possible hydrogenation of xylose to xylitol.
(2022) Rajkumar, Shivek.; Sithole, Bishop Bruce.
This project aimed to synthesize and characterize a novel catalyst for the transfer hydrogenation of xylose to xylitol. A literature review revealed that palladium supported on an aluminium-tin mixed metal oxide support was the best catalyst to facilitate the reaction due to palladiums electron configuration, superior hydrogen bonding capacity, strong metal-support interactions and amphoteric nature of the support material. Catalysts of nominal palladium loading 5 wt.% supported on tin-alumina mixed metal oxides of molar composition 10 %, 20 % ,30 %, 40 % and 50 % Sn were successfully synthesised using the microwave assisted sol-gel method and characterised using BET, EDX, SEM and XRD techniques. Catalyst surface area decreased with tin composition from 200 m2/g at 10% Sn to 60 m2/g at 50% tin. SEM imaging confirmed nanoparticle formation with varying morphology depending on tin composition. EDX confirmed even dispersion of 5 wt. % Pd with one anomaly after impregnation and calcination. EDX maps indicate high density of Sn as it is more dense than Al. XRD confirmed the crystalline structure with the formation of both alpha and gamma-alumina phases. A sustainability analysis indicated that the biological process is more environmentally friendly than the chemical route. Future work would entail testing the catalysts for their intended application.
Biogasoline production by catalytic cracking of waste cooking oil.
(2023) Nzengu, Ditunga Patient.; Mohammadi, Amir Hossein.
Government, Heads of State, and international organizations gather from time to time to investigate and implement strategies required to eradicate global warming and address environmental issues. The principal causes of global warming and ecological issues are industrializations, excavation, cutting down of trees, and production of fossil fuels, to mention only a few. Besides global warming and climate change, it is essential to know that the other issue related to fossil fuels is that they generate from a finite source. This source is dwindling as time passes. The potential disappearance of fossil fuels is also a cause of the high prices of crude oil, primarily in the regions with less or without crude oil. The reasons mentioned prove that there is a need to alter to a renewable source of energy-generating from an infinite source. Several investigations are taking place at international and national (South Africa) levels to develop the production of biofuels considered clean fuels. In an attempt to improve the quality of biofuels, many works are taking place behind the scenes regarding feedstock quality, feedstock source, feedstock transportation, and technologies required to produce biofuels. Regarding biofuels, products like biodiesel and bioethanol, including biogas, are at an advanced stage of development in terms of technologies and commercialization in specific locations such as America (USA), Brazil, and Canada. However, biogasoline production is not yet advanced, even though many vehicles use gasoline. Biogasoline has the quality to be blended or to be used as a replacement for conventional gasoline (fossil-based gasoline) in vehicles’ engines. This project investigates the feasibility of manufacturing biogasoline from waste cooking oil, one of the available feedstocks. Waste cooking oil is converted to biogasoline via catalytic cracking, thermal cracking, and a two-step (hybrid) method. All the methods mentioned earlier see their applications in this research study. The methods requiring a catalyst were conducted using nanocatalysts which carry out the reactions effectively in an optimized way. The nanocatalysts' structure causes the enzyme to be motionless. The dormant state of enzymes affects the biocatalytic efficiency through the increased load of enzymes and surface area. The nanocatalyst was composed of Ammonium molybdate tetrahydrate [(NH4)6Mo7O24.4H2O] and cobalt nitrate hexahydrate [Co(NO3)2.6H2O], including ɣ-aluminium oxide. After mixing the catalyst samples, they were dried and calcined. Then the catalyst samples were analysed using the following techniques: x-ray diffraction, inductively coupled plasma optical emission spectroscopy (ICP-OES), scanning and transmission electron microscopy. The thermogravimetry analyser (TGA) method was used to determine the regeneration temperature of the nanocatalyst. The TGA results showed that the regeneration temperature for three nanocatalyst samples is 600oC. However, the nanocatalyst calcined at 600oC is selected for this study since it has a completed regeneration cycle. The regeneration cycle at this temperature starts from the evaporation of water. Then there is an increase of inorganic compounds caused by partial oxidation, and finally, coke combustion. These three processes show that the regeneration cycle of the nanocatalyst at the selected temperature is complete. After preparing the nanocatalyst, the waste cooking oil underwent pre-treatment to remove salt and food particles. The remaining sulphur after cleaning is 4%, roughly 71% of which is removed from the oil. Pre-treatment of waste cooking oil concluded, and the resulting product was used to conduct the thermal cracking at varying reaction temperatures (400, 450, and 500oC) and reaction times. This method's highest biogasoline is 24.96%, obtained at 500oC and 60 minutes, respectively, reaction temperature and reaction time. The optimum reaction time is 60 minutes. The catalytic cracking and two-step methods were conducted at a constant reaction time of 60 minutes (optimum time) while varying the catalyst load (1, 3, and 5 grams) and the reaction temperature. For catalytic cracking, the percentage yields increase with the reaction temperature at a constant catalyst load of one gram. The highest percentage yield is 12.7% at a reaction temperature of 550oC. The optimum waste cooking ratio of oil is one gram of nanocatalyst to 44 grams of waste cooking oil. The biogasoline percentage increases with the reaction temperature for the two-step process (hybrid method). The highest biogasoline percentage yield is 29.63% at reaction temperature and catalyst load of 550oC and 3 grams, respectively. One of the aims of this research was to investigate the effects of the calcination temperature of the nanocatalyst at a constant reaction temperature of 475oC. The experimental procedure yields 42.36% of biogasoline for 5 grams catalyst load. The calcination temperature of the catalyst used is 700oC. The catalytic processes (catalytic cracking and two-step process) were optimized. The optimized results for cracking are 11.51%, 3.35g, 482oC, and 0.28h-1, respectively; biogasoline percentage yields, catalyst weight, reaction temperature, and weight hourly space velocity. The optimized results for the two-step process are 41.78%, 4.32g, 567.2oC, and 0.22h-1, respectively, biogasoline yields, catalyst weight, reaction temperature, and hourly space velocity. It is vital to conduct further studies in biogasoline production using thermal cracking methods (with and without catalysts) and the two-step process method. The study must include the addition of metals such as copper and nickel to nanocatalysts and consider changing the operating conditions such as temperature and carried gas pressure. These techniques provide a great platform to step up biogasoline production at an industrial scale and conduct a techno-economic assessment. It is also vital to conduct a techno-economic assessment of biogasoline production to establish its production cost and selling price. Documenting the production method, the suitable catalyst, and the feedstock is essential. The studies have proved that lignocellulosic biomass is affordable and environmentally sane to produce biogasoline.
Investigating the effects of wood chemistry and cooking conditions on pulp properties at a dissolving wood pulp mill.
(2021) Khatshane, Zininzi.; Sithole, Bishop Bruce.; Braunstein, Ron.
Pulp viscosity is the main criteria used to determine the endpoint of the pulping process in acid bisulphite pulping for production of dissolving pulp (DP). However, pulping performance is highly dependent on the relative abundance, structure, and reactivity of wood chemical components, mainly cellulose, hemicellulose and lignin. Thus, the chemistry of wood components has become an important factor when selecting wood species for use in the production of DP. It is believed that the chemical composition of wood components in tree species can be used to predict the pulp properties after pulping the wood. Although this correlation has been reported in Kraft pulping studies, there is very limited knowledge of such correlations in the acid sulphite pulping processes. This study entailed an investigation to ascertain if there are correlations between wood chemistry components and the resulting pulp properties from acid sulphite pulping process. The study was conducted on wood samples from Eucalyptus species, coded in this study as (E.001, E.002, E.003, E.004), Acacia mearnsii (Wattle) and additional clones and hybrids of eucalyptus species (W962-71, G438-105, A189-97, 100 GN). The samples were pulped using the magnesium acid sulphite cooking process, at wood specific base charge of 2.5 and 3.0. The wood chemistry of the trees and properties of their resultant brown pulp samples were determined. The results showed that there were variations in wood chemical composition among the Eucalyptus species that were evaluated. A lower base ratio of 2.5 produced pulps with lower viscosity and lower kappa than those produced at base ratio 3.0. These pulping conditions resulted in about 1 to 2% lower pulp yields than those reported for standard mill pulping conditions. Comparison of data on wood chemistry composition and pulp properties seemed to indicate good correlations on samples cooked at base ratio of 3.0 with a correlation coefficient (r) maximum of 0.71: the correlation was between alpha cellulose and total pulp yield. Correlation coefficients above 0.5 are considered significant. The results suggested that for some eucalyptus species pulped at base ratio of 3.0, a higher syringyl to guaiacyl S/G ratio in wood resulted in a higher delignification rate, resulting in lower residual lignin (K number) and higher pulp brightness. Eucalyptus wood samples with higher total lignin produced lower total pulp yields and brightness values whereas those with higher alpha cellulose content produced higher pulp yields with higher brightness values. Analysis of a combination of various wood components, namely, S/G ratio, hemicellulose content, and alpha cellulose content led to derivation of a formula that showed these parameters could be used to predict the viscosity of resultant pulps: the correlation coefficient was approximately 0.7 was obtained at base ratio 3.0, whereas 0.67 was obtained at base ratio 2.5.
Establishment of a micro-biorefinery in a rural community: beneficiation of food waste into high value materials.
(2023) Mchunu, Thandiwe.; Sithole, Bishop Bruce.; Okole, Blessed.
Farmers are faced with great challenges such as ever rising costs of fertilizers, pesticides, natural disasters, climate change and many more factors that affect the quality and quantities of produce per season. Regardless of these challenges, food is desired with the ever-growing demand. The population growth also means the exponential increase of organic waste which is estimated to ten million tons annually in South Africa. This organic waste often ends up at landfills, whereas it has valuable nutrients needed to enhance production of fresh food. The spent grain and Spanish reed, food waste and plant waste were used to produce organic compost using the Bokashi process. This research aims to demonstrate the yield obtained from the application of various kinds of organic compost mixes made using the Bokashi process. The compost was further utilized in growing vegetables such as Chillies, Bell peppers, Swiss Chard, and Tomatoes It was discovered that compost from spent grain and Spanish reed and 90% Food waste, showed a better yield across the vegetables grown and had highest values of Nitrogen, Phosphorus and Potassium. The use of organic waste and transforming it into usable compost was found to be a feasible business solution and the compost provided the essential elements needed to produce fresh vegetables. The products were also well acceptable at the market because of high quality.
Establishing a bioconversion process for the production of succinic acid using industrial feedstocks.
(2022) O’Brien, Frances Faith.; Sithole, Bishop Bruce.; Chunilall, Viren.; Ramchuran, Santosh Omrajah.
One of the leading challenges of the current global situation is the decline of non-renewable, fossil fuels. Due to this rapid depletion, there is a shift towards replacing petrochemical products with equivalent, ideally superior bio-based substitutes. The bio-chemical of interest that was studied in this work is bio-succinic acid which is considered a platform chemical. Bio-based procedures have the attractive advantage of potentially obtaining a high-value product from an underutilised product/waste stream. In this dissertation, the industry that was focused on was the sugar sector, this vital industry is under pressure it is therefore crucial that alternative revenue avenues are identified. A literature study highlighted the importance of succinic acid, detailed both the upstream and downstream literature methods and addressed the impact that biochemical processes could have within South Africa. Small scale flask studies were conducted using succinate-producing microorganisms, on synthetic C5 and C6 sugar medias, namely xylose and glucose. The results from these studies showed that L. paracasei and C. glutamicum were the top performing strains on the C6 sugar (glucose) media and as a result these strains were then grown on C6 industrial material, namely sugarcane juice and molasses. These flask studies concluded that C. glutamicum grown on molasses was the superior combination, with a succinic acid concentration of 18.81 ± 0.75 g.L-1 and a productivity of 0.67 ± 0.07 g.L-1.hr-1 being achieved. The process was then successfully scaled up to 30L reactors where a succinic acid concentration of 28.89 ± 3.57 g.L-1 was reached, which was higher than the ‘ideal’ glucose reactor run. Downstream processing of the harvested broth was conducted using the precipitation method. Process development was performed, and the final method resulted in a final succinic acid recovery of 54.47 ± 14.02 % and 58.20 ± 2.24 % for the glucose and molasses-based medias respectively. In conclusion, molasses has the potential as an alternative carbon source in the production of succinic acid. The biochemicals sector is still a novel concept within South Africa, and as this platform gains more traction such studies show the ‘value’ of industry’s waste/by-product streams, especially for the sugar industry.
Process simulation, optimization and economic valuation of waste tyre pyrolysis for fuel production.
(2023) Mchunu, Emmanuel Nkosinathi.; Mohammadi, Amir Hossein.
Management of waste tyres is increasingly becoming a global challenge; the problem is only projected to get worse in future as worldwide tyre production is continually on the rise in response to the growing global population. Managing waste tyres through traditional landfilling approach has proven to be an unsustainable approach due to the associated environmental concerns. The said concerns include posing toxic gas emitting fire risks, contaminating ground water and housing disease carrying insets. Despite these challenges, a huge percentage of tyres are still managed through legally and illegally landfilling. Waste tyre recycling and retreating has proven to be a useful and sustainable management alternative, these technologies are however limited to only handle up to 13 % of global waste tyres. The detrimental environmental problems associated with traditional waste tyre management has resulted in a significant rise in a number of studies focusing on investigating waste tyre management alternatives. Of the means investigated, waste tyre pyrolysis has emerged as the most manageable approach as it yields easy to handle products and has the ability to handle huge waste tyre volumes. In addition to the said benefits, tyre pyrolysis technology harnesses energy from tyres that would have been lost to the landfills. Tyre pyrolysis products are energy rich oil, char and gases. The process of tyre pyrolysis entails volatilization of tyres at temperatures above 400 oC in the absence of oxygen. There are a number of governing factors associated with tyre pyrolysis process, viz. type of pyrolysis rector used, operating temperature, heating rate, particle size, residence time, operating pressure, and a presence of the catalyst. The governing factors outlined are mainly focused on maximizing the oil yield and minimizing char and non-condensable gaseous products. Of the governing factors influencing tyre pyrolysis process, operating temperature has the biggest impact. According to the literature studies, the typical optimum oil yield is between 38 % and 60 % by weight of waste tyre, achievable between 425 oC and 720 oC. On this study, a ASPEN Plus simulation computer software was used to develop a tyre pyrolysis process model. From the developed simulations a number of investigations were undertaken to understand the impact of the key process governing factors. Investigations conducted were on the operating temperature, reactor type, dimensions of the reactor, residence time, reactor operating pressure, heating rate, presence of the catalyst and tyre particle size The reactor type investigations showed that the reactors with some level of mixing favoured higher oil yield. This premise was evident in the investigation between the CSTR and PFR. The CSTR presented oil yield increase from 50.93 % to 51.13 % while the PFR showed an increase from 44.02 to 44.13 % for the temperature range between 400 and 700 oC. To incorporate a kiln reactor comparison, the temperature was kept constant at 550 oC on all the reactors investigated. The CSTR and kiln reactor showed higher oil yields than PFR, with oil yields of 51.01 %, 50.93 % and 44.09 % respectively. The results obtained from the kiln reactor and CSTR showed to be relatively similar since the kiln reactor was modelled using CSTRs in series. In the reactor size investigation which is linked to the residence time. For the PFR, no further improvements on the yield were noticed at the reactor diameters above 1m. The CSTR showed the yield to remain constant above a 1 hour residence time. The investigation on the operating pressure showed that the higher oil yields are achieved at lower operating pressure. When the pressure was increased from 0.1 atm to 1 atm, the CSTR oil yield decreases from 50.98 % to 50.97 %. Increasing the reactor heating rate showed positive impact on the oil yield, an improvement from 50.75 to 50.89 % was achieved when the heating rate was increased from 5 to 15 K/min. However, at 20 K/min the oil yield decreased to 50.87 %. The use of the catalyst showed positive impact on the oil yield, an increase from 49.92 % to 51.7 % was noted from no catalyst basecase. Two size classes were considered in the particle size investigation, viz. 0.1 mm – 0.8 mm and 0.8 mm – 4 mm. The investigation showed that the lower particle size results in higher oil yield. The CSTR showed an increase with the increase in the residence time, an increase of 0.04 % points was noted when the residence time was increased from 0.1 to 1 hr. A different tool was used to check the validity of the simulation findings, for this investigation a numerical model from literature was employed. The model was based on a laboratory study incorporating the particle size, temperature and feed rate. The un-optimised conditions of the model showed oil yield of 26.3 % while the optimum conditions showed oil yield of 47.9 %. The optimum conditions were identified be at a temperature of 400 oC, particle size of 1.0 mm and a feed rate of 0.78 kg/hr. Based on the results of this study encompassing both numerical and simulation findings, conclusion were drawn that the optimum oil yield is obtainable from the CSTR operated at a temperature range between 400 oC and 550 oC, tyre particle size less than 1 mm, operating pressure below 1 atm, heating rate between 10-15 K/min and residence of time of less than 1 hour. The economic evaluations showed that the tyre pyrolysis plant only starts yielding positive ROI at least after 3.7 years of operating. This is relatively an acceptable ROI period pertaining to the investment decision making.
Beneficiation of pulp and paper mill sludge: production of cellulose nanofibrils (CNFs).
(2022) Jele, Thabisile Brightwell.; Sithole, Bishop Bruce.
Depletion of landfill space, the cost of waste disposal, and environmental concerns are pushing Pulp and Paper Mills (PPMs) to make changes and improvements in the way waste is handled. Pulp and paper mill sludge (PPMS) is a solid by-product of the wastewater treatment of the PPM industry; thus, it is considered a waste. This study was conducted to investigate the feasibility of producing cellulose nanofibrils (CNFs) from waste material such as PPMS instead of using high purity cellulose products such as chemical pulps as the starting material. Three PPMS samples collected from different South African (SA) mills were chemically and physically characterised to investigate their suitability for various beneficiation pathways. The overall objective was to analyse and allocate the most suitable beneficiation process to each PPMS sample based on the characterisation results and literature. The possible beneficiation pathways of the pulp and paper mill sludge (PPMS) samples were discussed in line with their characteristics. The characteristics of PPMS were influenced by the pulping technique employed at each mill, the raw material and the type of effluent treatment employed. Thermal analysis revealed that the calorific values of all PPMS samples studied were too low for energy harvesting (thermal processing). The high ash content of PPMS C (de-inking) and PPMS A (recycle) was suitable for biocomposites whose strength could be enhanced by fillers present in PPMS samples. The higher glucose content in PPMS B (virgin) compared to PPMS A (recycle) and PPMS C was favourable for bioethanol and bio-oil production. The high cellulose and low ash content of PPMS B were found suitable for the production of nanocellulose. Highly fibrillated CNFs were produced from three different PPMS samples by grinding using the automated Super mass colloider (SMC) after washing and bleaching to remove impurities. Chemical composition analysis showed a considerable reduction in ash content due to screen washing, successful cellulose concentration, and lignin removal by bleaching. The crystallinity index (CrI) was calculated to be 51.1%, 58.1% and 59.4% for CNF A, CNF B and CNF C, respectively. TGA analysis showed a progressive decrease in thermal stability from untreated PPMS to CNFs.The overall yield for the production of CNFs from PPMS A, PPMS B and PPMS C was 27.2%, 32% and 42.8%, respectively. Finally, a description of the transition from manual grinding using the SMC to an automated operation was done. The automated system was designed to pump the feedstock through the SMC via sample holding tanks for a predetermined number of passes. The automated operation alleviated challenges associated with the manual operation, which included high labour demands, loss of material due to spillages, thus affecting yield and posing a safety risk in the working environment, inconsistent product due to human error in counting the number of cycles, low productivity due to long working hours. The introduction of the automated SMC system was a worthwhile investment justifiable by improving efficiency and operator safety.
Techno-economic assessment of algae conversion to biofuels.
(2023) Duma, Ndumiso Sweet-man.; Mohammadi, Amir Hossein.; Chetty, Manimagalay.
One of the most promising biomasses for the production of biofuels is microalgae. This is because microalgae have a high growth rate and a highiCO2icapture ability when compared to other biomasses. Furthermore, biofuels produced from microalgae are deemed eco-friendly due to their low sulphur content, superior lubricating efficiency, and non-toxicity nature. As opposed to carbon-based fuels, biofuels are viable alternatives with the potential to meet the increasing demand for energy (Jafari & Zilouei, 2016). Because of its potential of being inexhaustible and a low-cost renewable energy carrier, biofuels research has increased (Akobi et al., 2016). This research investigated the thermochemical and biochemical conversions for producing algal biofuels on a technical, economic, and environmental basis. The primary feed considered was wet algal biomass with a 20 wt%. Each investigated process was simulated on Aspen Plus ® v12. The processing units considered for the thermochemical conversion on Aspen Plus were hydrothermal liquefaction (HTL) for depolymerization, hydrotreating for removing contaminants by using H2, and hydrocracking for removing contaminants by using a high-activity catalyst and H2. The primary processing units considered for the biochemical conversion simulation included pre-treatment where dilute sulphuric acid is fed, conditioning with the assistance of dilute ammonia, fermentation with the aid of S. cerevisiae, purification, and finally, anaerobicidigestion of the production of biogas. The process properties for the investigated conversion methods wereiobtainedithroughimass and energy balance calculations. The thermochemical conversion had a mass ratio of 0,39 and an energy efficiency of 47,45%. The biochemical conversion had a mass ratio of 0,98 and an energy efficiency of 73,11%. The processes were both optimized using the Aspen Energy Analyzer (AEA). The thermochemical simulation had a 23,56% energy savings and a 17,3% carbon emissions reduction. The base case simulation for the biochemical conversion had no design alternatives to improve the heat exchanger network (HEN). The fixed capital investment (FCI) for the thermochemical conversion was 18,3% lower than for the biochemical conversion. The internalirateiofireturn (IRR) for the thermochemical method was 27,36% and 29,61% for the biochemical conversion. The economic evaluation was completed using the discounted cash flow analysis. Both the thermochemical and biochemical processes were profitable. The thermochemical method had a discounted payback period of 7,2 years (break-even point at seven years five months) and seven years (break-even point at six years ten months) for the biochemical method. The environmental impacts of both processes were evaluated using OpenLCA. Typically, OpenLCA employs the cradle-to-gate approach. The assessment used the Agribalyse v3.0.1 database, and the LCIA method used was the ReCiPe 2016 midpoint (H) method and the CML-IA baseline method. The thermochemical method was the more sustainable method. The global warming impact was 42,25% less, the human toxicity was 41,46% less, and the freshwater aquatic ecotoxicity was 38,3% lower than the biochemical method. The investigation is summarized in the Table 0-1 below: Table 0-1 : Summary of the processes studied.
Co₂ solubility measurements and modelling in amine-NMP solvent blends.
(2022) Dijkman, Rebecca-Lynn.; Naidoo, Paramespri.; Nelson, Wayne Michael.; Osman, Khalid.
In this study, solvent blends of monoethanolamine (MEA) or 2-(2-aminoethoxy)ethanol (DGA) with N-methyl-2-pyrrolidone (NMP) or water (H2O) were selected for investigations of carbon dioxide(CO2) solubility due to the high CO2 absorption capacities of the individual solvents. A static synthetic apparatus, consisting of a stirred equilibrium vessel and a gas reservoir, each submerged in their own temperature-controlled environment, was used to measure the CO2 solubility for the systems and conditions stated above. Isothermal solubility measurements were performed at 40 °C over a pressure range of 0.1 – 1.5 MPa for the systems of CO2 in various solvent blends. These included 20% MEA–80% NMP, 30% MEA–70% NMP, 51% DGA–49% H2O, 40% DGA–60% H2O, 30% DGA–70% H2O, 51% DGA–49% NMP, 40% DGA–60% NMP and 30% DGA–70% NMP (by mass). The recorded temperature-pressure-overall composition (T-P-z) data were converted to T-P-mole fraction (T-P-x) data. Results were displayed on pressure vs. CO2 loading (P-𝛼𝐶𝑂2) graphs andcompared to literature data. Further comparisons were made between the various solvent blends. Thermodynamic modelling of the experimental data for the DGA systems was performed using MATLAB®. Due to a solvent blend of a water-lean amine and a physical solvent, two models were fitted to the experimental data by regression of the model parameters, and the results combined and displayed on P-𝛼𝐶𝑂2 graphs with the respective experimental data. The Posey-Tapperson-Rochellemodel was used for DGA, and the Peng-Robinson equation of state (PR-EOS) with modified van der Waals-Berthelot mixing rules was used for NMP in water-lean cases. Only the Posey-Tapperson-Rochelle model was used for amine-water systems. The results indicated that the water-lean blends, MEA-NMP and DGA-NMP, have a higher CO2 loading at the same pressure when compared to the corresponding MEA-H2O and DGA-H2O blends. An increase from 30% DGA–70% H2O to 40% DGA–60% H2O (by mass) resulted in a viscosity increase of 0.65 Pa.s at 40 °C and an increase in CO2 loading of 0.079 molCO2/molamine at 0.63 MPaand 40 °C. Comparing the 30% MEA–70% NMP and 30% DGA–70% NMP (by mass) blends, it was observed that the DGA blend had an increase in CO2 loading of 0.12 molCO2/molamine at 0.24 MPa and40 °C. Thermodynamic modelling for the CO2-51DGA-49H2O system gave a root mean square error of 3.75%, an absolute average deviation (AAD) of 98.24 and an average absolute relative deviation (AARD) of 22.69%, while modelling for the CO2–51 wt% DGA–49 wt% NMP system gave a root mean square error of 0.61%, an AAD of 13.13 and an AARD of 2.94%. Based on the calculated error and AARD, regression for the Posey-Tapperson-Rochelle and PR-EOS model parameters gave the closest results to the CO2–30 wt% DGA–70 wt% NMP measured data. From this work, it was concluded that in terms of the viscosity and CO2 loading at 40 °C, the DGA-NMP blends show promise compared to the DGA-H2O and MEA-NMP blends. The 40 wt% DGA–60 wt% NMP solvent blend was the best-performing DGA-NMP blend. Further experiments to determine the changes in viscosity and CO2 loading of regenerated solvents for a range of DGA-NMP blends are recommended, and further modelling analyses for data prediction are recommended for continuation of this work.
An analysis and control of volatile organic Compound (VOC) emissions from petroleum storage tanks.
(2022) Naidoo, Theasha.; Moodley, Kuveneshan.; Naidoo, Prathieka.
Climate change is a growing phenomenon with its effects becoming more prominent to life on earth. According to the latest report by the Intergovernmental Panel on Climate Change (IPCC), some of the effects of climate change are irreversible. However, the implementation of large-scale reduction strategies on emissions may limit climate change over the long-term and provide short term air quality benefits. Petrochemical industries are a major contributor of Volatile Organic Compound (VOC) emissions as the need for storage facilities are expanding to accommodate for the increase in demand of organic liquids storage capacity. The Durban South Basin is a major industrial hub consisting of South Africa’s largest capacity oil refinery (SAPREF) and Engen refinery, soon-to-be tank terminal, located near a residential area. Therefore, the implementation of emission monitoring and reduction strategies are critical in ensuring climate resilience and the health and well-being of residents living within close proximity to the refineries. While there has been some progress in addressing climate change, emission data indicates that storage tanks contribute 42% of VOC emissions to total emissions from oil refineries. Due to limited studies conducted, there is a gap in the knowledge and understanding of proper monitoring and control practices of VOC emissions from petroleum storage tanks in Durban, South Africa. Therefore, the aim of this study is to provide strategies for implementation, such as simulation modelling using Aspen Plus ® and recommended process conditions, to achieve safe control and handling of emissions and to perform an Environmental Impact Assessment (EIA) for analyses of its potential effect on the environment and health of communities. Estimation of VOC emissions for crude oil and petroleum products (Ultra-Low Sulphur Diesel (ULSD), Unleaded Petrol 95 (ULP 95), Jet Fuel (JET A1) and Marine Gas Oil (MGO) were based on the AP-42 method, Aspen Plus ® simulations, manual flash calculations according to the Rachford-Rice iterative method and empirical correlations (such as the Vasquez-Beggs and Valko-McCain empirical correlation methods). The effects of atmospheric conditions, tank roof type, type of stored organic liquid and varying parameters (such as temperature, pressure and feed flowrate) on the VOC emissions from petroleum storage tanks were assessed to determine the most suitable monitoring method. The potential effect of Nitrogen blanketing (using the API 2000 7th ed. Standards) and Vapour Treatment on the reduction of VOC emissions from petroleum storage tanks were studied to determine its effectiveness as a control method. This study found that the Aspen Plus ® simulation method is an effective tool in monitoring VOC flashing emissions due to its reliability from its repeatability with the estimation crude oil test system in which the Aspen Plus ® and literature VOC measurement was consistent. Its ability to account for variations using the thermodynamic property models (Soave-Redlich-Kwong (SRK) for crude oil and Peng-Robinson (PENG-ROB)) for the product mixtures) further justifies the use of Aspen Plus ® as an effective monitoring method. Manual flash calculations under-estimated emissions across the organic mixture systems due to its less rigorous approach as it uses simplified equations which includes estimates of process conditions whereas Aspen Plus ® is able to account for variations is process conditions. The estimates determined using empirical correlations were mostly invalid due to the limited appliable range. All mixtures indicated a significant reduction in working and breathing losses when stored in an Internal Floating Roof Tank (IFRT) compared to a Floating Roof Tank (FRT). However, MGO was the exception. It was observed that these tanks should operate at 90 % capacity, with turnovers of 0 – 10 per year and a white painted shell, to ensure minimum emissions. Optimal operating tank temperatures should be maintained at 293.15 – 303.15 K and at pressures below 91 kPa. The installation of vapour recovery units is recommended for FRTs, and these measures are 90 % efficient. Due to the high API gravity of the ULP 95 mixture, the ULP 95 mixture should be targeted as a key mixture for control of VOC emissions as it has the potential to emit greater VOC emissions.
Kinetic mechanism study and waste tyre characterisation for valuable chemicals production.
(2022) Mwelwa, Maxwell Katambwa.; Mkhize, Ntandoyenkosi Malusi.; Iwarere, Samuel Ayodele.
Researchers have extensively investigated waste tyre pyrolysis kinetics and mechanisms. However, available literature shows limitations as the studies were primarily aimed at tracking the devolatilisation of the main components of the feedstock than the formation of chemicals. In the present study, truck tyre crumbs of less than 1 mm were pyrolysed in a multi-shot pyrolyser connected to a mass spectrometer. The aim is to investigate the kinetics and reaction pathways that favour the formation of specific valuable chemicals (such as limonene and indene) in tyre-derived oil to maximise their production and content. The multi-shot pyrolyser (EGA/PY-3030D) equipment and accessories have been used to track the temperature and heating rate influence on the formation of chemicals. For temperature influence, experiments were conducted in single-shot mode and at different temperatures (400, 500, 600, and 700 °C), based on thermogravimetric analysis data. Chromatograms analysed regarding the area % of the considered chemicals proved that the selectivity to limonene decreases as temperature increases, reaching a minimum of 28.5% around 700 °C. In contrast, there was no discernible effect of temperature on indene in the considered range of temperature. The selectivity ratios of limonene to isoprene and indene to styrene presented a plateau between 400 and 600 °C. This trend evidenced that several equilibrated reactions steps could have taken place between limonene and isoprene as well as indene and styrene. Above 600 °C, the selectivity ratio of limonene to isoprene decreases, characteristic of cyclisation, hydrogenation and aromatisation reactions. The influence of heating rate on the production of the chemicals was conducted through Direct Evolved Gas Analysis (EGA) experiments at heating rates of 20, 30, 40, 50 and 100 °C /min and optimum temperature of 500 °C, obtained from temperature influence on the production of the chemical. The selectivity to limonene presented a continuous increase with the increase of heating rate from 30.8 % at 20 °C /min to 37.8 % at 50 °C /min and 42.7% at 100 °C /min. Hence, limonene formation is maximised at higher heating rates. The kinetic study conducted through combined model fitting and model-free approaches presented values in the range of 122 and 145 kJ.mol-1. The highest value has been obtained using the differential method (144.4 kJ.mol-1) with a difference of about five units with the two model-free methods of Kissinger (138.5 kJ.mol-1) and Starink's (139.7 kJ.mol-1). The Friedman's (122.0 kJ.mol-1) and Coats-Redfern (130.6 kJ.mol-1) methods present lower activation energy values with a difference of about 22 and 16 with the differential method. Therefore, a simplified reaction map has been presented and includes equilibrated reactions that contribute to the production of limonene-isoprene and styrene-indene.
Characterisation of faecal sludge from urine diversion toilets: impact on black soldier fly larvae growth.
(2022) Mutsakatira, Ellen Tafadzwa.; Mercer, Susan Jessica.
The area managed by eThekwini Municipality in KwaZulu-Natal, South Africa, was extended in 2002 to include approximately 80 000 households without basic sanitation. The municipality adopted urine diversion dehydrating toilets (UDDT) as a dry on-site sanitation option. The adoption of the UDDTs over other sanitation methods was due to UDDTs being cost effective, the geographical location of these households and the impracticality of providing a sewer system. The UDDTs offered waterless sanitation in a water-constrained environment and the pedestal separated faeces and urine at source, making it possible to handle the treatment of the two excreta streams separately. These UDDTs overcame the problem faced during the desludging of ventilated improved pit (VIP) toilets as UDDT vaults are smaller and allow for easier manual emptying because of the lower moisture content of the faecal sludge (FS). It was the responsibility of the household to empty the vaults once they were full, bury the FS on the property and plant a tree to mark the burial site. The urine was directed to a soakaway located near the unit. In 2017, the eThekwini Water and Sanitation (EWS) unit implemented an UDDT emptying campaign due to the householders’ dissatisfaction over emptying UDDT sludge and the possibility of exposure to FS sludge after burial on the household property. In addition, there was growing concerns over health risks due to potentially pathogenic sludge. A solution was required to treat the waste from the UDDTs, and through funding from the Bill & Melinda Gates Foundation, a project was initiated to investigate the feasibility of using the black soldier fly (BSF) larvae technology. Under this project, a full-scale BSF larvae processing plant was established in Durban, South Africa, with the aim of treating 20 tons of UDDT FS per day. The BSF larvae have been shown to be consumers of a wide range of decomposing organic matter, including kitchen waste, human waste, animal waste, restaurant waste, and vegetable waste. In 2017, there was very little data available on the characteristics of UDDT FS and the impact of UDDT FS on the bioconversion process using BSF larvae. A study was therefore initiated to determine the bioconversion and waste reduction capabilities using the BSF larvae. This study was undertaken by monitoring and determining different material flows in batch reactors fed on two types of substrates: (i) UDDT sludge, and (ii) a mix of UDDT sludge and primary sludge (PS) by performing mass balances. Due to the unavailability of data from the full-scale plant and problems experienced with commissioning components of the plant, small-scale studies were conducted under the same conditions as at the full-scale plant, and the data from these studies was then used to predict the overall material flows of the treatment plant. The small-scale studies were performed in sheds at the BSF larvae plant under uncontrolled environmental conditions, and data was generated from either on-site measurements, or through laboratory analysis of the substrates before and after treatment. This data was then extrapolated to generate material flows for the full-scale BSF larvae waste management plant. The mass balances were carried out on a dry matter basis, wet matter basis, volatile solids, and ash. The growth of larvae was monitored every three days over a period of 13 days. Laboratory analyses were carried out on the UDDT sludge, and a mix of UDDT sludge and PS to analyse the difference in the composition before and after treatment with the BSF larvae. Laboratory analyses included total solids, volatile solids, ash, chemical oxygen demand, calorific value, carbon to nitrogen ratio, and protein content. The change in depth of the substrates during the small-scale trial was measured to determine the reduction in volume due to treatment with the BSF larvae. Humidity and temperature fluctuations were also monitored in the sheds to investigate the effect of changes in these conditions on the process. Due to poor waste management services in the areas where UDDTs are installed, users of UDDTs generally dispose of other wastes (termed trash) in the toilets, including metal objects. The presence and concentration of heavy metals such as Pb, Cd, As, Cr, Cu and Ni was determined through laboratory analyses of the BSF larvae and remaining substrate (residue) after 13 days of treatment, as the presence of these metals can affect the market potential of the BSF larvae and residue. The outcomes from this project showed that the use of BSF larvae has potential as a process for FS management with resource recovery. Treatment by BSF larvae reduced the mass of UDDT sludge by 28% wet basis in the small-scale trial operating in an uncontrolled and low maintained system and achieved a bioconversion of 6% wet basis. The study also showed that bioconversion could be increased by co-digestion of UDDT sludge with PS as it increased from 6% to 9% with the addition of 15% mass of PS. It was also found that the characteristics of the UDDT sludge directly affected the characteristics of the BSF larvae, which can potentially influence the market value of the BSF larvae. For example, UDDT sludge had a high ash content which resulted in an increase in the ash content of the BSF larvae fed on UDDT sludge. Furthermore, the bioconversion process was shown to be sensitive to temperature and humidity as changes in these conditions affected the relative waste reduction and bioconversion rates. Outcomes from this study indicate that the bioconversion and waste reduction of the UDDT sludge could be improved by co-digestion with an organic substrate and using an environment with controlled temperature and humidity.
Techno-economic assessment of decentralised waste beneficiation in South Africa.
(2022) Kushveena, Gokul.; Stark, Annegret.
The introduction of the waste hierarchy promotes the diversion of waste away from landfills towards value-adding opportunities. However, there is no clear methodology for the comparison of waste beneficiation pathways for decentralised waste treatment options in South Africa. As a result, most waste is landfilled in the country even though it contains large amounts of recyclable and recoverable materials. This study investigates the use of techno-economic assessments to determine the viability of decentralised waste treatment options and compare them to larger scales in three selected South African locations (Calvinia, Garden Route District, and eThekwini). These preliminary results may estimate economics before intense, expensive, and time-consuming design and scoping are performed. A critical investigation of currently available literature from various data sources is performed to determine the national and local municipal waste landscape. This scoping exercise is essential for determining the variations of waste volumes and composition dependent on municipality size, which affect the capacity of the various treatment options. Hence, feedstock quantities are established for use in the techno-economic assessment. The three technologies assessed are engineered composting, anaerobic digestion, and pyrolysis of plastics, as examples of technologies that can be applied in various municipalities. Based on a simple cost-benefit analysis, techno-economic parameters are used to investigate the project feasibility based on historical Capital Expenditure data and certain assumptions for Operational Expenditure. Furthermore, a Discounted Cash Flow analysis assesses the variables of Net Present Value (NPV), Pay Back Period, and Internal Rate of Return. These assessments are considered preliminary, with an accuracy ranging between 30% and 50%. Sensitivity analysis determines the different breakeven points of the different economic parameters, indicating feasibility in the event of market fluctuations. Six different scenarios are investigated: the separation and recycling of waste streams (scenario 1), engineered composting (scenario 2a), anaerobic digestion (scenario 2b), pyrolysis of plastics (scenario 2c), and combination scenarios (composting and pyrolysis (scenario 4a), as well as anaerobic digestion and pyrolysis (scenario 4b)). The separation and recycling of waste from each location found a credit of R2.4 to R17.8 / t waste based on specific assumptions. The increase in municipality size positively influences the feasibility of this scenario. The economic parameter of the Net Present Value is used to analyse the feasibility of the remaining scenarios. None of the scenarios is economically feasible in Calvinia, with Net Present Values less than zero. Composting is feasible in the Garden Route District and eThekwini with a Net Present Value of R44 million and 89 million, respectively. Based on electricity sales, anaerobic digestion is feasible in Garden Route District (R14.5 million) and eThekwini (R208.6 million). Plastic waste pyrolysis is feasible in the Garden Route District and eThekwini. The combined pyrolysis scenario with either composting or anaerobic digestion also produces feasibility in the Garden Route District and eThekwini, with higher Net Present Values for the latter scenario. The inclusion of Extended Producer Responsibility targets decreases the Net Present Value of pyrolysis by reducing the amount of product and thus the revenue from fuel. Suppose Extended Producer Responsibility targets were fulfilled, and the recyclate stream was diverted to the Buy Back Centres for sales, the overall income could not overcome the capital and operational costs in any location. The different parameters investigated through sensitivity analysis determine the leverage points for each process and inform the influence of market fluctuations due to legislative instruments such as the Extended Producer Responsibility. The landfill credit can be considered a key parameter since the true cost of landfilling is not widely recognised and may be underestimated. This is exhibited by the minimum landfill cost of R577, R711 and R365/ t waste for the feasibility of the separation and recycling scenario. Separation and recycling is the only economically viable scenario in Calvinia, indicating that decentralised separation and recycling of waste is possible and should be considered in these locations. However, scenarios with higher capital and operational costs (scenarios 2(a)-(c) and 3) are not feasible. In such cases, incentives and government funding schemes must be investigated to support waste treatment opportunities in decentralised locations. As scale increases, all investigated scenarios became feasible in Garden Route District Municipality and eThekwini, indicating that the possible combinations of waste from different decentralised locations (i.e. multiple smaller municipalities like in the Garden Route District Municipality) could result in feasible treatment options in these parts of the country. Furthermore, between 35 and 72% of landfilled waste can be diverted through the different scenarios (depending on Extended Producer Responsibility targets and local compositions), indicating that such process implementation can aid South Africa in reaching diversion targets set in the National Waste Management Strategy and Polokwane Declaration.
Assessment and feasibility of converting municipal solid waste to biogas: a South African case study.
(2021) Gaogane, Gaogane Jephtah.; Sithole, Bishop Bruce.; Trois, Cristina.
The present energy crisis in South Africa warrants the need for an alternative and sustainable energy supply. As a sustainable clean energy carrier, biogas has been demonstrated to be a promising renewable energy source for the generation of heat and electricity. The organic fraction of Municipal Solid Waste (MSW) has been reported as a promising feedstock for biogas production and characterisation of MSW is the basis towards successful waste to energy programs. The use of inappropriate equipment and technology choices based on insufficient data on waste volumes and composition have resulted in the failure of many interventions previously introduced in South African municipalities. Assessing the composition and quantity of available biomass for anaerobic digestion (AD), suitable pre-treatment technologies to enhance biogas production as well as optimization of AD parameters such as pH, temperature and substrate ratio were the core components of this research project. The digestate was evaluated and potential use as fertilizer and feedstock for pyrolysis assessed. A mesophilic bench-scale AD of Cow Dung (CD) and Fruit and Vegetable waste (F&V) obtained from a F&V market was conducted. F&V forms the greatest waste in the country, and this facility generates on average 2560 tonnes of waste per annum. This study concludes that utilisation of MSW for AD relies heavily on characterisation data, which is only possible through separation at source. The study recommends the development of a municipal organic waste facility and equally important, diversion of high-end food chains from entering the landfills. Other technologies such as Mechanical Biological Treatment (MBT) can be revisited for possible waste to energy programs at landfill leading to landfill space savings and reduced pollution. Waste pickers at landfills can be employed for this purpose as separation specialists. Bench-scale AD revealed that the benefits of substrate pre-treatment outweighed the effects of co-digestion ratio and pH. Reducing particle size from 1-2mm to <1mm, doubled the methane gas generation in a much shorter time and removed pH induced microbial inhibition in unbuffered reactors. Optimal pH was observed at 7.5-8.5. A co-digestion ratio of 80:20 (CD:F&V) produced higher methane yield for all pH variations in comparison to 60:20. The Digestate measured an average volatile solids loss of 46.4% with a C:N ratio of 12 and a heating value (HV) of 4.30 MJ/kg. Metal analysis of digestate showed presence of Nb, Sr, Si, N, P and K as constituents returned to the soil. The investigation of the digestate as a potential feedstock for pyrolysis yielded a carbon rich biochar with an HV value of 11.5 MJ/kg at 500 oC and a bio-oil rich in phenols, ketones and carboxylic acids which are important industry products.

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