Browsing by Author "Ndlovu, Phakamile."

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Carbon dioxide encapsulation in methane hydrates.
(2022) Ndlovu, Phakamile.; Naidoo, Paramespri.; Babaee, Saeideh.; Moodley, Kuveneshan.
Coal mining and petroleum refining processes face extreme pressure under climate change and global warming threats. Hence alternative sustainable and renewable energy sources must be made available for the rising energy demands. Natural gas found in permafrost and seabed areas in the form of gas hydrates possess vast amounts of low-carbon methane gas, which can replace fossil-based energy sources. The capture and storage of carbon dioxide gas in natural gas hydrate beds with the release of methane gas is a sustainable route under intense research. This study investigates the methane-carbon dioxide (CH4-CO2) replacement reaction mechanisms and the improvement of the process using different techniques, namely, additives, secondary gas, and thermal stimulation. Firstly, the gas hydrate dissociation measurements for the former gases utilized in the study were performed. This was followed by kinetic measurements with nanoparticles (aluminum oxide, copper oxide, and graphene nanoplatelets) and chemical additives (zinc oxide powder, graphite powder, and magnesium nitrate hexahydrate crystals) in the presence of sodium dodecyl sulfate (SDS) to affect kinetic or thermodynamic improvement in hydrate formation. The kinetic parameters investigated were induction time, hydrate storage capacity, water consumed in hydrate formation, fugacity of the gaseous phase, and the ratio of gas consumed to moles of water. Graphene nanoplatelets were selected for replacement reaction based on promising results obtained from the kinetic studies. The CH4-CO2 replacement process was performed in a 52 cm3 equilibrium cell using deionized water and nanoparticles. Also, a new experimental setup with a 300 cm3 reaction vessel was designed and assembled for CH4-CO2 replacement in the presence of synthetic silica sand. The results from kinetic studies showed an improvement in the hydrate formation kinetics due to the presence of nanoparticles. The CO2 hydrate formation kinetics obtained a maximum storage capacity of 51 (v/v), with 1.2 wt.% graphene nanoplatelets which also produced a maximum water conversion of 25%. When nanoparticles were added, the induction time for CO2 hydrate in deionized water was reduced from 9 minutes to less than one minute. Graphite powder with a concentration of 1.2 wt.% had the highest rate of gas uptake of 0.0024 (mol of gas/ mol of water. min). In CH4 kinetics, the induction time was reduced from 18 minutes with deionized water to less than one minute due to addition of nanoparticles. A maximum storage capacity of 28.5 (v/v), water-to-hydrate conversion of 13.09%, rate of gas uptake of 0.0089 (mol of gas/ mol of water. min), and gas consumption of 0.0238 moles were obtained with 0.1 wt.% CuO + 0.05 wt.% SDS. Also, CH4-CO2 replacement measurements showed that an 80 mol% N2/20 mol% CO2 gas mixture yielded a CH4 replacement efficiency of 17.04% at a temperature of 274.77 K and pressure of 5.34 MPa. The highest amount of CO2 sequestrated was 57.03%, and 28.77% was the highest CH4 replacement efficiency. These results were obtained using pressurized CO2 with application of thermal stimulation at a temperature of 275.90 K and pressure of 5.66 MPa. In the replacement reaction with silica sand, the maximum amount of CH4 replaced was 37.49% with the pressurized CO2 at a pressure of 7.01 MPa and temperature of 276.43 K. Applying thermal stimulation and adding secondary gas (N2) improved CO2 sequestration from 51.73% to 76.63%. These outcomes are vital in applying hydrates in gas storage and CO2 sequestration.
Commissioning of a refrigerant test unit and assessing the performance of refrigerant blends.
(2017) Ndlovu, Phakamile.; Naidoo, Paramespri.; Raal, Johan David.; Narasigadu, Caleb.; Ramjugernath, Deresh.
This study has two major purposes; to commission and to demonstrate that a new refrigerant test rig can be used for investigating the performance of different refrigerants and refrigerant blends. The motivation for this work is the need for testing new refrigerants or refrigerant blends to replace current refrigerants which are on the verge of being phased out due to environmental concerns (Montreal and Kyoto protocols). These protocols seek to implement refrigerants without any environmental impacts such as global warming potential and ozone depletion. In literature, several refrigerant test rigs that have been assembled and used in the investigation of different refrigerants are outlined, but there is limited coverage of refrigerant blends due to technical difficulties associated with the use of blends. Consequently, this places restrictions on their application, necessitating further research into properties, operating procedures, and equipment development. A refrigerant test rig was designed and assembled at the University of KwaZulu-Natal to operate on the following cycles; simple vapour compression cycle, two-stage vapour – compression cycle, cascade system and vapour –compression cycle with a suction-line heat exchanger. In this study, the simple vapour compression cycle was used, with the refrigerant R134a being employed to validate the reliability and reproducibility of the refrigerant test rig. The main components of the cycle were the evaporator, the condenser, the compressor and the throttle valve. Water was used as the heat load and heat sink medium in the evaporator and the condenser, respectively. The temperature was measured by thermocouples and; pressure transducers were used for the measurement of pressure, and their combined expanded uncertainties were 0.1 ℃ and 0.026 MPa respectively. Commercial blends R507a and R413a, as well as a laboratory synthesised blend R134a/R125 in the ratio (66/34) and (50/50) by wt-%, were used in the investigation. The simulation of the refrigeration cycles was carried out using the Reference Fluid Properties Package (REFPROP) property method, which is a component within Aspen Plus ® V8.6. This software package allowed the prediction of the theoretical performance of the refrigerants, and refrigerant blends studied. One objective of this study was to compare the performance of the test rig against the simulated results to assess the extent of the deviation between the practical and theoretical (ideal) results. Mollier charts were used to analyse experimental data. Refrigerant blend R507 displayed the best performance when compared to the refrigerants investigated in this study, with a coefficient of performance (COP) value of 5.00, while R413a had the lowest COP value of 4.00. Considering environmental aspects, R134a/R125 (66/34 wt %) with COP value of 4.88 has the least negative impact. The deviation between the theoretical and experimental values was within the experimental uncertainty, with a notable difference occurring in the evaporator inlet temperature. The results show that the test rig is fit for use in refrigeration experimental work. Furthermore, refrigerant blends showed good performance on the vapour compression cycles employed in this study proving that it is feasible to use the test rig in the investigation of refrigerant blending.