3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Synthesis and evaluation of SOD-ZMOF-chitosan adsorbent for post-combustion carbon dioxide capture(2017) Singo, Muofhe ComfortSouth Africa emits large amounts of carbon dioxide (CO2) due to its reliance on coal. The emission of CO2 needs to be reduced for clean sustainable energy generation. Research efforts have therefore been devoted to reducing CO2 emissions by developing cost-effective methods for capturing and storing it. Amine-based absorption using monoethanolamine solvent is the most mature technique for CO2 capture despite its huge energy consumption, corrosiveness and difficulty in solvent regeneration. However, CO2 removal by solid adsorbents is a promising alternative because it consumes less energy, and can be operated at moderate temperature and pressure. Metal organic frameworks have received attention as a CO2 adsorbent because they have large surface areas, open metal sites, high porosity and they require less energy for regeneration. This research was aimed at optimizing and scaling-up SOD-ZMOF through structural modification for enhanced CO2 adsorption by impregnating it with chitosan. Scaled-up SOD-ZMOF samples were prepared as described elsewhere and impregnated with Chitosan. Physiochemical properties obtained using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Nitrogen physisorption showed that SOD-ZMOF and SOD-ZMOF-chitosan were successfully synthesized. Qualitatively, the surface area of the SOD-ZMOF synthesized using the scaled up protocol is lower than the one prepared using the non-scaled-up protocol XRD pattern of SOD-ZMOF showed that it was crystalline and was in agreement with literature. The XRD peaks of the SOD-ZMOF decreased after chitosan impregnation showing that chitosan was impregnated on SOD-ZMOF. The FTIR spectrum of SOD-ZMOF showed functional groups present in organic linker used to synthesize SOD-ZMOF, and that of the SOD-ZMOF-chitosan revealed the same functional groups but with disappearance of carboxylic acid functional group. N2 physisorption showed a decrease in BET surface area and pore volume after chitosan impregnation on SOD-ZMOF as well. Performance evaluation of the material was carried out with a demonstration adsorption set-up using a 15%/85% CO2/N2 mixture and as a thermal gravimetric analysis (TGA) using 100% CO2. For both the packed-bed column and the TGA experiments, evaluation was conducted on SOD-ZMOF and SOD-ZMOF with chitosan for comparison. About 50 mg of the adsorbent was used at 25 oC, 1 bar and 25 ml/min for the packed-bed column. For the adsorption with the TGA, 11 mg of adsorbent was used at 25 ℃, 1 bar and 60 ml/min. SOD-ZMOF showed improved adsorption capacity after chitosan impregnation. CO2 adsorption capacity of SOD-ZMOF increased by 16% and 39% using packed-bed column and TGA, respectively, after chitosan impregnation. The increase in adsorption capacity was attributed to the impregnated chitosan that has amine groups that display a high affinity for CO2. A traditional approach was used to investigate the effect of adsorption temperature and inlet gas flowrate on the CO2 adsorption capacity of SOD-ZMOF-chitosan. This was done using both the parked bed column and the TGA. Temperature range of 25-80 ℃ and inlet gas flowrate range of 25-90 ml/min were investigated. Adsorption capacity increased with a decrease in temperature and inlet gas flowrate. For the packed-bed column, maximum of 781 mg CO2/ g adsorbent was obtained at 25℃, 1 bar, 25 ml/min and for the TGA a maximum CO2 adsorption capacity of 23 mg/ g adsorbent at 25 ℃, 1 bar, and 60 ml/min was obtained.Item Numerical simulation of CO2 adsorption behaviour of polyaspartamide adsorbent for post-combustion CO2 capture(2017) Yoro, Kelvin OdafeClimate change due to the ever-increasing emission of anthropogenic greenhouse gases arising from the use of fossil fuels for power generation and most industrial processes is now a global challenge. It is therefore imperative to develop strategies or modern technologies that could mitigate the effect of global warming due to the emission of CO2. Carbon capture and storage (CCS) is a viable option that could ensure the sustainable use of cheap fossil fuels for energy generation with less CO2 emission. Amongst existing CCS technologies, absorption technology using monoethanolamine (MEA) is very mature and widely embraced globally. However, the absorption technology has a lot of challenges such as, low CO2 loading, high energy requirement for solvent regeneration, corrosive nature etc. On this note, the adsorption technology using solid sorbents is being considered for CO2 capture due to its competitive advantages such as flexibility, low energy requirement for sorbent regeneration, non-corrosive nature etc. On the other hand, adsorbents have a very vital role to play in adsorption technology and there is need to understand the behaviour of adsorbents for CO2 capture under different operating conditions in order to adapt them for wider applications. On this note, the study contained in this dissertation investigated the adsorption behaviour of a novel polymer-based adsorbent (polyaspartamide) during post-combustion CO2 capture using experimental study and mathematical modelling approach. Polyaspartamide is an amine-rich polymer widely used in drug delivery. In addition, its rich amine content increases its affinity for CO2. Its porosity, thermal stability and large surface area make it a promising material for CO2 capture. In view of this, polyaspartamide was used as the adsorbent for post-combustion CO2 capture in this study. This dissertation investigated the kinetic behaviour, the diffusion mechanism and rate limiting steps (mass transfer limitation) controlling the CO2 adsorption behaviour of this adsorbent. Furthermore, effect of impurities such as moisture and other operating variables such as temperature, pressure, inlet gas flow rate etc. on the CO2 adsorption behaviour of polyaspartamide was also investigated. Existing mathematical models were used to understand the kinetics and diffusion limitation of this adsorbent during CO2 capture. Popularly used gas-solid adsorption models namely; Bohart- Adams and Thomas model were applied in describing the breakthrough curves in order to ascertain the equilibrium concentration and breakthrough time for CO2 to be adsorbed onto polyaspartamide. Lagergren’s pseudo 1st and 2nd order models as well as the Avrami kinetic models were used to describe the kinetic behaviour of polyaspartamide during post-combustion CO2 capture. Parameter estimations needed for the design and optimization of a CO2 adsorption system using polyaspartamide were obtained and presented in this study. The Boyd’s film diffusion model comprising of the interparticle and intra-particle diffusion models were used to investigate the effect of mass transfer limitations during the adsorption of CO2 onto polyaspartamide. Data obtained from continuous CO2 adsorption experiments were used to validate the models in this study. The experiments were conducted using a laboratory-sized packed-bed adsorption column at isothermal conditions. The packed bed was attached to an ABB CO2 analyser (model: ABB-AO2020) where concentrations of CO2 at various operating conditions were obtained. The results obtained in this study show that temperature, pressure and gas flow rate had an effect on the adsorption behaviour of polyaspartamide (PAA) during CO2 capture. Polyaspartamide exhibited a CO2 capture efficiency of 97.62 % at the lowest temperature of 303 K and pressure of 2 bar. The amount of CO2 adsorbed on polyaspartamide increased as the operating pressure increased and a decrease in the adsorption temperature resulted in increased amount of CO2 adsorbed by polyaspartamide. The amounts of CO2 adsorbed on polyaspartamide were 5.9, 4.8 and 4.1 mol CO2/kg adsorbent for adsorption temperatures of 303, 318 and 333 K, respectively. The maximum amount of CO2 adsorbed by polyaspartamide at different flow rates of 1.0, 1.5 and 2.5 ml/s of the feed gas were 7.84, 6.5 and 5.9 mmol CO2/g of adsorbent. This shows that higher flow rates resulted in decreased amount of CO2 adsorbed by polyaspartamide because of low residence time which eventually resulted in poor mass transfer between the adsorbent and adsorbate. Under dry conditions, the adsorption capacity of polyaspartamide was 365.4 mg CO2/g adsorbent and 354.1 mgCO2/g adsorbent under wet conditions. Therefore, the presence of moisture had a negligible effect on the adsorption behaviour of polyaspartamide. This is very common with most amine-rich polymer-based adsorbents. This could be attributed to the fact that CO2 reacts with moisture to form carbonic acid, thereby enhancing the CO2 adsorption capacity of the material. In conclusion, this study confirmed that the adsorption of CO2 onto polyaspartamide is favoured at low temperatures and high operating pressures. The adsorption of CO2 onto polyaspartamide was governed by film diffusion according to the outcome of the Boyd’s film diffusion model. It was also confirmed that intra-particle diffusion was the rate-limiting step controlling the adsorption of CO2 onto polyaspartamide. According to the results from the kinetic study, it can be inferred that lower temperatures had an incremental effect on the kinetic behaviour of polyaspartamide, external mass transfer governed the CO2 adsorption process and the adsorption of CO2 onto polyaspartamide was confirmed to be a physicochemical process (both physisorption and chemisorption).Item Synthesis and performance evaluation of nanocomposite ceramic-sodalite membranes for pre-combustion CO2 capture(2017) Oloye, OlawaleGlobal climate change and other environmental disasters have been attributed to continuous anthropogenic carbon dioxide (CO2) emission into the atmosphere. Today, researchers are constantly seeking measures to reduce anthropogenic CO2 emission. Traditionally, absorption technology with use of monoethanolamine (MEA) is used for separating / capturing of anthropogenic CO2. However, the use of MEA is associated with numerous shortcomings, including inefficient energy usage, high operating and capital cost, amine degradation, solvent loss and excessive equipment corrosion. Alternatively, zeolite based membrane systems are promising technique that prove handy and useful than the traditional processes (absorption with monoethanolamine). However, zeolitic membranes with zeolite coating on the supports (i.e. thin-film supported zeolite membranes) are susceptible to abrasion and thermal shock at elevated temperatures due to temperature mismatch between the supports and the membranes, making them to lose selectivity at early stages. On the contrary, nanocomposite architecture membranes, synthesized via pore-plugging hydrothermal route, are more thermally stable and membrane defects are controlled. Nanocomposite zeolite (sodalite) membranes have been proposed for gas separations, most importantly in the separation of H2/CO2, a major component in pre-combustion carbon capture. In addition, sodalite, a porous crystalline zeolite made up of cubic array of β-cages as primary building block having cage aperture in the range of 0.26 and 0.29 nm, is a potential candidate for the separation/purification of light molecules such as hydrogen which has a cage aperture of 0.27 nm under certain process conditions. In this work, nanocomposite architecture hydroxy sodalite membrane with sodalite crystals embedded within α-alumina tubes were successfully synthesized using the pore-plugging hydrothermal synthesis technique and characterized using techniques such as scanning electron microscopy (SEM) and X-ray diffraction (XRD). The morphology of the synthesized membranes shows that sodalite crystals were indeed grown within the porous structures of the support. Furthermore, Basic Desorption Quality Test (BDQT) and gas separation measurement were conducted to evaluate the quality of the as-synthesized membrane in industrial gas separation applications. The effects of operating variables such as pressure at 1.1 bar, 2.0 bar and 3.0 bar. Also, the effects of temperature were conducted on the nanocomposite membrane at 373 K, 423 K and 473 K. Finally, the gases permeation results were fitted with the well-known Maxwell-Stefan model. Results indicated that, the nanocomposite sodalite / ceramic membrane is a potential candidate for removal of H2 from H2/CO2 mixture. The gas permeation measurement from the one-stage nanocomposite membrane shows that the membrane displayed H2 and CO2 permeance of 3.9 x 10-7 mols-1m-2Pa-1 and 8.4 x 10-8 mols-1m-2Pa-1, respectively. However, the morphology of two-stage nanocomposite membrane shows that the support was more plugged with sodalite crystals and the permeance of H2 and CO2 were 7.4 x 10-8 mol.s-1.m-2.Pa-1 and 1.1 x 10-8 mol.s-1.m-2.Pa-1, respectively. Consequently, the H2/CO2 ideal selectivity for the one-stage nanocomposite membrane improved from 4.6 to 6.5 in the two-stage nanocomposite membrane. In conclusion, the two-stage synthesized membrane shows better improvement. The porous support was well plugged and separation performance was evaluated. However, occluded organic matters present in the cages of hydroxy sodalite could have adverse effect on the gas permeation performance of the membrane. It is expected that an organic-free sodalite supported membrane (such as silica sodalite supported membrane) could out-perform the hydroxy sodalite supported membrane reported in this work in term of membrane flux because there will be enough pore space for gas permeation.Item Optimization of the synthesis and performance of Polyaspartamide (PAA) material for carbon dioxide capture in South African coal-fired power plants(2016) Chitsiga, Tafara LeonardGlobal climate change is among the major challenges the world is facing today, and can be attributed to enhanced concentrations of Greenhouse Gases (GHG), such as carbon dioxide (CO2), in the atmosphere. Therefore, there is an urgent need to mitigate CO2 emissions, and carbon capture and storage (CCS) is amongst the possible options to reduce CO2 emissions. Against this background, this work investigated the synthesis and performance evaluation of Polyaspartamide (PAA) adsorbent for CO2 capture. In particular, the effect of the presence of water-soluble amines in the amine-grafted poly-succinimide (PSI) (referred to as Polyaspartamide (PAA) adsorbent), was investigated. Methyl Amine (MA) and Mono-Ethanol Amine (MEA) were employed as water-soluble amines and the effect of changes in their concentration on CO2 adsorption capacity was investigated as well. Water-soluble amines were incorporated to allow water solubility of the adsorbent paving the way for freeze-drying to improve the geometric structure (surface area, pore volume and pore size) of the adsorbent. Initially, the PSI was loaded with Ethylenediamine (EDA), forming PSI-EDA. The water-soluble amines were grafted to PSI-EDA, with the EDA added to improve the chemical surface of the adsorbent for CO2 capture. NMR and FTIR analyses were performed and confirmed the presence of MA and MEA amine groups in the PAA, thereby indicating the presence of the grafted amines on the backbone polymer. BET analysis was performed and reported the pore volume, pore size and surface area of the freeze-dried material. It was observed that the physical properties did not change significantly after the freeze-drying compared to literature where freeze-drying was not employed. An increase in adsorption capacity with an increase in MA and MEA concentrations in MA-PAA and MEA-PAA samples was observed. At low amine concentrations (20% amine and 80% EDA grafted), MEA-PAA was observed to exhibit higher adsorption capacity compared to the MA-PAA samples. At high amine (100% amine grafted) concentrations, MA-PAA samples displayed higher adsorption capacity. Three runs were performed on each sample and the results obtained were reproducible. The best adsorption capacity obtained was 44.5 g CO2/kg Ads. Further work was then performed to understand the effects of operating variables on CO2 adsorption as well as the interactive effect using the Response Surface Methodology approach. The experiments were done by use of CO2 adsorption equipment attached to an ABB gas analyzer. A central composite design of experiment method with a total of 20 experiments was employed to investigate three factors, namely, temperature, pressure and gas flow rate. Six regression models were drawn up and mean error values computed by use of Matlab, followed by response surfaces as well as contours, showing the influence of the operating variables on the adsorption capacity as well as interaction of the factors were then drawn up. The results obtained displayed that each of the factors investigated, temperature, pressure and gas flowrate had an incremental effect on the adsorption capacity of PAA, that is, as each factor was increased, the adsorption capacity increased up to a point where no more increase occurred. Adsorption was seen to increase for both an increase in gas flowrate and adsorption pressure to a maximum, thereafter it starts to decrease. A similar trend was observed for the interaction between temperature and pressure. However, the interaction between gas flowrate and temperature was such that, initially as the temperature and the gas flowrate increase, the adsorption capacity increases to a maximum, thereafter, the temperature seizes to have an effect on the adsorption capacity with a combined effect of decreasing temperature and increasing gas flowrate resulting in a further increase in adsorption capacity. It was confirmed that the operating variables as well as the flow regime have an effect on the CO2 adsorption capacity of the novel material. The highest adsorption capacity was obtained in the pressure range 0.5 bar to 1.7 bar coinciding with the temperature range of 10 oC to 45 oC. The interaction of gas flowrate and adsorption pressure was such that the highest adsorption capacity is in the range 0.8 bar to 1.5 bar which coincides with the gas flowrate range from 35 ml / min to 60 ml / min. In conclusion, the best adsorption capacity of 44.5 g / kg via the TGA and 70.4 g / kg via the CO2 adsorption equipment was obtained from 100 % MA grafted PSI.