3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Improving polysulfone membrane resistance to carbon dioxide induced plasticization during natural gas separation using functionalized carbon nanotubes(2019) Aberefa, Oluseyi AdebisiCommercial natural gas, generally methane (CH4), is described as the most efficient energy source with a calorific value per mass of approximate 50.1 MJ/kg. However, raw natural gas is made up of mostly methane and a considerable quantity of other components like C2+, CO2, water and hydrogen sulphide (H2S) in varying composition from well to well depending on the geographical location and condition. Membrane technology for gas separation technology has become an acceptable method for CO2 removal from natural gas. Majority of commercial natural gas separation membrane systems are polymeric due to processing feasibility and low cost. However, conventional polymeric membranes have reached a trade-off threshold between permeability and selectivity, and of great concern is CO2 induced plasticization due to harsh industrial conditions such as high feed pressure. The addition of carbon nanotubes (CNTs) into polymers appears to offer a unique solution to the deficiencies of conventional polymeric membrane systems. In this study, carbon nanotubes (CNTs) were produced using a catalyst-assisted chemical vapour deposition method. Ferrocene was used as the catalyst. Effects of carbon sources (acetylene and methane) and production conditions (temperature and type of carrier gas) on the physicochemical property of as-produced CNTs were investigated using field emission scanning electron microscopy (FESEM), energydispersive X-ray spectroscopy (EDS), electronic precision balance (EPB) and Raman spectroscopy. The best of as-produced CNTs was purified and functionalized using carboxylation protocol. The surface chemistry, thermal stability, textural property and crystallinity of the functionalized CNTs (FCNTs) were obtained using Fourier transform infrared spectroscopy (FTIR), thermo-gravimetric analysis (TGA), N2 physisorption at 77 K and X-ray diffraction (XRD), respectively. CNT produced from methane and argon at 900 oC displayed the best quality with ID/IG ratio of 0.17 while the CNT produced from acetylene and mixture of argon and hydrogen at 1000 oC has the highest yield of 1.78 mg/s. FTIR confirms successful functionalization of CNTs. The degree of functionalization obtained from TGA is consistent with that of EPB. N2 physisorption at 77 K indicates an increase in pore volume and average pore size of the FCNTs indicating more adsorption sites for the adsorbate. Thereby suggesting that FCNT could be a good filler in membrane synthesis for gas separation. Dense mixed matrix membranes (MMMs) were synthesized by incorporating functionalized CNTs (FCNTs) at different wt. % (0.2, 0.5, 1.0, and 1.5) into polysulfone (PSF) using evaporation method. The MMMs were characterized using field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), atomic force microscopy (AFM) and texture analysis (TA). Single gas permeation experiment was conducted using a custom-built permeation set-up via constant volume variable pressure technique while mixed gas permeability was measured on a constant pressure variable volume gas permeation apparatus. Surface and cross-section morphology of the MMMs as depicted by FESEM show that the membranes are dense, indicating that the membranes consist of dense structure whose porosity is not observable under an electron microscope. Depth-at-max analysis of AFM images of the synthesized membranes further supports that the pores of the MMMs possess dense structure. TGA indicates improved thermal stability of the MMMs with increasing wt. % loading of FCNTs. DSC results show that the glass transition temperature (Tg) of the MMMs was also improved with the addition of FCNTs. However, the increase in wt. % loading of FCNTs does not significantly increase the Tg of the MMMs. Tensile strength and Young’s modulus of the MMM obtained from TA show an increase in these mechanical properties with the addition of FCNTs. Single gas permeation results indicate an increase in CO2 permeability as the wt. % loading of FCNTs increases up to 1.0 wt. % loading. Mixed gas separation shows similar trend without sacrificing CO2/CH4 selectivity. CO2 induced plasticization of the membranes was investigated experimentally and by phenomenological modelling. The sorption isotherms showed a typical dual-mode sorption model behaviour for all the types of synthesized membranes with the CO2 concentration increasing rapidly at low pressures, which indicated a hole-filling of the Langmuir sites. At higher pressure, the rate of increase in the concentration declines due to the saturation of the Langmuir sites, the further increase in the concentration at higher pressure is because of the penetrant sorption in Henry’s law sites. The permeation isotherm of pure PSF indicated plasticization onset at CO2 pressure of 12 bar. The MMMs showed no sign of plasticization over the pressure range tested (0-20 bar). This indicated an improvement in the CO2 plasticization resistance of PSF by the addition of FCNTs. The developed model assumed three possible interface interactions in the MMMs; polymer matrix – polymer matrix, CNT – polymer matrix, and aligned CNT – CNT interface interactions. The model also accounted for the plasticization of the membrane. The model equation was solved by using the Levenberg-Marquardt algorithm in the Matlab environment to obtain the plasticization potential and the fraction of the gas held in Langmuir sites that have mobility within the membrane. The model CO2 permeability is in good agreement with CO2 permeability obtained experimentally. The PSF plasticization potential obtained is similar to ones reported in the literature. The fraction of the gas held in Langmuir sites that have mobility within the membrane increases with an increase in FCNT wt. % loading which shows high diffusivity through the microvoids with respect to diffusivity through the polymer matrix. This is an indication of an increase in Langmuir sorption sites for the penetrant thereby improving gas permeabilitItem Synthesis and performance evaluation of Nanocomposite SAPO-34/ceramic membranes for CO₂/N₂ mixture separation(2017) Kgaphola, Kedibone LawrenceGlobal warming, resulting from emission of greenhouse gases (GHGs), is the cause of drastic climate changes that threatens the economy and living conditions on the planet. Currently, recovery and mitigation of these greenhouse gases remains a technological and scientific challenge. Various recovery processes for the mitigation of GHGs have been reported including among others carbon capture and storage (CCS). The most mature and applied technology in CCS process involves the absorption of carbon dioxide on amine based solvents. However, studies have shown that this process has several drawbacks that include low stability and high energy required to strip off the absorbed CO2 and regenerate the solvent. This presents an opportunity for the development of new materials for CO2 capture such as zeolite membranes. Previous studies have shown that the separation of CO2 can be achieved with high selectivity at low temperatures using thin-film SAPO-34 membranes (thin layers on supports). This is because CO2 adsorbs strongly on the membranes compared to other gases found in flue gas. In the thin-film membranes supported on ceramic or sintered stainless steel, thermal expansion mismatch may occur at higher operating temperatures resulting in loss of membrane selectivity due to the formation of cracks. A new method is required to overcome the aforementioned problems, thereby enhancing the separation application of the membranes at higher temperatures. The effective separation and capture of CO2 from the coal-fired power plant flue gas is an essential part in the CCS process (Figueroa et al., 2016; Yang et al., 2008). Currently, the capture stage is a huge contributor to the overall cost of CCS (Yang et al., 2008). This is due to the high-energy intensity and inefficient thermal processes applied in the separation and capture in various industrial applications (Yang et al., 2008). This work presents the use of nanocomposite SAPO-34 zeolite membranes synthesized via the pore-plugging hydrothermal method for the separation of CO2 during post-combustion CO2 capture. The SAPO-34 membranes used were supported on asymmetric α-alumina as membrane supports. The membranes were characterized with a combination of dynamic and static physicochemical techniques such as Basic Desorption Quality Test (BDQT), X-ray diffraction (XRD) spectroscopy, Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). The characteristic peaks at 2θ = 21°, 26°, and 32° on the XRD pattern confirmed the presence of SAPO-34 with a rhombohedral crystalline structure. The SEM images showed the formation of the cubic crystalline which were consistent with the reported morphology of SAPO-34. FTIR spectra showed the presence of the essential double-6 membered rings (D6R) and TO4 structural groups in surface chemistry of crystalline materials further confirming the presence SAPO-34. The TGA confirmed that the membranes possessed high thermal stability. To assess the feasibility of the synthesis process, the nanocomposite zeolites were grown within the tubular supports. The SEM images of the cross-section of the membrane confirmed the presence of the zeolites within the pores of the support confirming the fabrication of nanocomposite membranes by the pore-plugging synthesis method. The permeation tests used a dead-end filtration mode to measure the single gas permeance and the ideal selectivity of CO2 and N2 were calculated. The BDQT was essential in the study of the quality of the as-synthesized nanocomposite membranes. The quality of the membranes increased with an increase in the synthesis layers of the membranes. However, with an increase in synthesis layers, the membrane thickness also increases. The membrane thickness affected the gas permeance for CO2 and N2 significantly. The permeance of the N2 gas decreased from 10.73 x10-7 mol.s-1.m2Pa-1 after the first synthesis to 0.31 x10-7 mol.s-1.m2Pa-1 after seven synthesis layers. Alternatively, the more adsorbing gas CO2 decreased from 12.85 x10-7 mol.s-1.m2Pa-1 to 2.44 x10-7 mol.s-1.m2Pa-1. The performance of these zeolite membranes depends significantly on the operating conditions. Hence, we studied extensively the influence of the various operating conditions such as temperature, feed pressure and feed flowrate in this work. Results indicated that the membrane separation performance in this study is largely dependent on the temperature. In addition, the ideal selectivity decreased significantly with an increase in temperature. High temperatures results in less adsorption of the highly adsorbing CO2 gas, the permeance reduces significantly, while the permeance of the less adsorbing N2 increased slightly. The feed flow rate has less effect on the adsorbing gas while the non-absorbing gas increased resulting in a decrease in the ideal selectivity as well. The nanocomposite membranes in this study have a low flux compared to their thin film counterparts. An increase in feed pressure significantly increased the flux significantly as well as the ideal selectivity. Maxwell-Stefan model simulation was done in this study to describe the permeance of pure CO2 single gas permeance as a function of temperature. This model considered explicitly the adsorption-diffusion mechanism, which is the transport phenomenon, involved in the transport of CO2 through the zeolite membrane. The description of the support material was included in the model as well. However, the model was only applied to the CO2 gas permeation well within the experimental data. We then compared the model was with the experimental results and a good correlation was observed. In conclusion, SAPO-34 nanocomposite zeolite membranes were obtained at low temperatures (150 °C) with a short synthesis time (6 h). In addition, the high thermal stability of the as-synthesized SAPO-34 membranes makes them ideal for high temperature CO2 separation such as the intended post-combustion carbon capture. The BDQT revealed that the quality of the membranes was related to the thickness of the membranes. Therefore, better membrane quality was obtained with relatively thicker membranes. The separation performance evaluation was conducted on the membrane with the greatest quality. Our findings demonstrate that the performance of the membranes depends extensively on the operating conditions.Item Alternatives to distillation: multi-membrane permeation and petrol pre-blending for bio-ethanol recovery(2016) Stacey, Neil ThomasSeparation of materials is crucial to the operation of the majority of chemical processes, not only for the purification of final products but also for the processing of feed-stocks prior to chemical reaction. The most commonplace method of materials separation is distillation which, unfortunately, is often an energy-intensive process and contributes significantly to mankind’s energy consumption and carbon dioxide emissions. Alternative approaches to separation are therefore a crucial element of the ongoing pursuit for sustainability in chemical industries. There are two principal ways of going about this. The first is to replace distillation units with alternative unit operations that can achieve the same separation with less energy expenditure. The second approach is overall flowsheet revision, fundamentally changing a separation cycle to minimize its energy requirements. The greatest improvements to energy efficiency will be achieved by applying both approaches in tandem. However, each must be developed separately to make that possible. This thesis lays the groundwork for radical revision of major separation operations by showcasing a new overall flowsheet for bioethanol separation that promises tremendous improvements in separation efficiency, reducing the energy usage involved in ethanol purification by as much as 40% in some scenarios. It also develops a novel method for the design of multi-membrane permeation units, showing how area ratio can be manipulated to fundamentally alter separation performance from such units, resulting in superior separation performance to conventional units, achieving higher recoveries than conventional setups. With membranes being an increasingly popular separation method, the potential for superior performance from multi-membrane units promises improvements in separation efficiency.