3. Electronic Theses and Dissertations (ETDs) - All submissions
Permanent URI for this communityhttps://wiredspace.wits.ac.za/handle/10539/45
Browse
Search Results
Item Synthesis and performance evaluation of chitosan based adsorbent for CO2 capture(2017) Osler, KerenCarbon dioxide capture is essential to reducing CO2 emissions in an attempt to mitigate climate change. Absorption via amine based solvents is currently the mature technology that is applied for the capture of CO2. However, amines can pose health and environmental risks when emitted into the air from CO2 capture plants. Furthermore, the efficiency penalty caused by CO2 capture via absorption and the huge costs associated with the regeneration of the spent amine based solvent poses a threat to the economic viability of CO2 capture by the absorption process. Adsorption technology is an alternative to absorption technology. Adsorption technology seems promising due to its moderate energy consumption (which stems from the ability to operate at moderate temperatures and pressures) as well as being health and environmentally benign. Recently, extensive research has been conducted on designing adsorbents that have the ability to adsorb large quantities of CO2 with a low energy consumption. The challenge in CO2 adsorption technology is to design an adsorbent that is not only non-toxic, biodegradable and cost effective but also has the ability to selectively and efficiently remove CO2 gas from a mixed gas stream. This study proposes chitosan, a biodegradable, non-toxic polymer, as one such adsorbent. Chitosan has the potential to be a suitable adsorbent for CO2 capture because it contains the desired amine groups which act as CO2 adsorption sites. In this study, chitosan was studied as an adsorbent in order to confirm that it is suitable for CO2 capture. Chitosan and chitosan impregnated carbon nanotubes (CNTs) (chitosan/MWCNTs) composite adsorbents were synthesized. Chitosan was impregnated onto MWCNTs in order to enhance the physical properties (surface area, pore size and pore volume), CO2 adsorption capacity and CO2 affinity of the composite adsorbent. The synthesized materials (chitosan and chitosan/MWCNTs) were characterized and evaluated for CO2 adsorption. Chitosan was successfully synthesised from chitin. This was confirmed using FTIR spectroscopy. The synthesised chitosan had desirable properties for CO2 capture. This was confirmed using TGA and custom built CO2 adsorption equipment. The synthesised chitosan samples were inexpensive, had the desired amine groups and were thermally suitable at industrial CO2 capture operational temperatures. The CO2 adsorption capacity of the synthesised chitosan was generally low when compared with literature. The highest CO2 adsorption capacity achieved by the synthesised chitosan in this study was 11 gCO2/kg adsorbent. However, it is important to consider that the polymer is derived from a waste material and as such it is possible to cost effectively utilize a large amount. The amount of CO2 adsorbed by the synthesised chitosan is dependent on the number of amine groups present. Against this background this study aimed to increase the number of amine groups present. This was done using response surface methodology (RSM) to develop a polynomial regression model. The developed polynomial regression model is able to predict the DDA of the synthesised chitosan based on the synthesis variables used. The polynomial model was validated using chitosan from literature and found to be statistically significant. The polynomial model showed the optimum synthesis conditions to yield the highest DDA. Chitosan was successfully impregnated onto MWCNTs. This was confirmed using FTIR spectroscopy. The synthesised chitosan/MWCNT adsorbent was not suitable for CO2 capture. This was confirmed using Raman spectroscopy, N2 physisorption, SEM TGA and custom built CO2 adsorption equipment. The CO2 adsorption capacity of the synthesised chitosan/MWCNTs was low when compared to literature. This is attributed to the fact that the MWCNTs used in this study are not suiTable as adsorbents for CO2 capture as they showed a low CO2 adsorption capacity before chitosan impregnation. However, the CO2 adsorption capacity of the MWCNTs was improved by 650 % after chitosan impregnation. Reports from literature, where CNTs were impregnated with other amines did not show such a significant increase in CO2 adsorption capacity. It is hypothesized that if chitosan were impregnated onto more suiTable CNTs for CO2 capture is would improve the CO2 adsorption capacity of that CNT by 650 %. Thus, yielding a suitable non-toxic, biodegradable adsorbent for CO2 capture. It is concluded that chitosan possesses properties that make the polymer suitable for use as an adsorbent for CO2 capture.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.