|CO2 is the most dominating anthropogenic greenhouse gas. Combustion of fossil fuels, to meet energy demands is one of the major sources of CO2 emission. Release of CO2 from burning fossil fuel and cement industry has increased significantly during last few decades. In order to cope with environmental consequences of Greenhouse gas (GHG) emissions, a transition towards renewable energies is required, i.e. biogas is one such source of renewable energy. Biogas can be upgraded by separating CO2 from methane (CH4). This upgraded biogas contain <95% of methane which can be used as an alternative fuel source for vehicles, power grid stations and domestic use.
It is essential to have an optimized upgrading process in terms of low energy consumption and high efficiency. Membrane technology is an emerging green technology that offers several advantages over other commercially available technologies. Especially, when determining the performance of membrane systems; both permeability and selectivity are major parameters. In order to make membranes commercially viable and compete with conventional amine absorption, a commercial scale membrane module with high CO2 permeance and high selectivity is required. Different types of materials like ceramics, carbon, polymers and ionic liquids have been investigated for their CO2 separation performance. Polymeric membranes seem more suitable to be used at commercial scale due to their easy fabrication, low cost, and reasonably high membrane performance, respectively. One approach is the use of innovative membrane materials such as polymers embedded with functionalized nano-particles, fixed site carrier membranes (FSC) or facilitated transport membranes (FTM). FTM have attracted considerable attention in CO2 separation due to the potential of achieving both high selectivity and permeance.
The content of this thesis was designed keeping in view the transport mechanism through facilitated transport membranes. Performance of FTM can be enhanced by increasing the degree of swelling of the membrane material. The emphasis of this thesis is to develop a novel FTM nanocomposite membranes composed of mimic enzyme, nanocellulose and poly vinyl alcohol (PVA). Different types of nanocellulose have been used and the concentration in PVA has been optimized for their concentration. Furthermore, the Zn mimic enzyme Zn-cyclene has been added to the highest performance type of nanocellulose/PVA and optimized for enzyme concentration. In addition, the effect of pH has been investigated. The membrane morphology was observed by scanning electron microscope (SEM). It was found that by increasing the nanocellulose concentration in PVA, the thickness of the selective layer also increased accordingly. Hence, a methodology was developed to cast equal thickness membranes and later those membranes were tested for their CO2 permeation. All casted membranes were also tested for CO2 separation performance using a high pressure membrane rig at 5, 10 and 15 bar of pressure, respectively.
As experimental work was designed to test the membranes for high pressure applications therefore, mechanical, and thermal behaviour of all the formulated membranes has been investigated in detail as part of this research work. XRD, DMA, TGA, DCS, FTIR and Mechanical testing are different techniques that have been used during membrane analysis. In order to maximize facilitated transport through the membrane the effect of pH, relative humidity and concentration of nanocellulose on the degree of swelling for all formulated membranes has been investigated in details.
Membrane characterisation shows that addition of nanocellulose increased the elastic modulus, crystalinity and swelling properties of PVA films. 1% crystalline nanocellulose (CNC) showed optimized results with respect to CO2 separation application at high pressure. Zn mimic enzyme concentration was optimized for 2 wt% PVA and 1% CNC wt/wt PVA. It has been found that only very small amount of enzyme is required as the optimal loading of enzyme was 5µmol/gram PVA.
In short, this research work consists of mechanical characterisation of films and CO2 separation performance of nanocomposite membranes from methane. Resultant membranes can be used as a potential technology for future biogas upgrading system.