Gas diffusion transport characteristics and mathematical description of membrane systems with application for biogas upgrading.

Ogunlude, Priscilla

Abstract

Greenhouse gas emissions (GHGs) and their effects have been a matter of global concern over the past decade. With growing energy demands to support developing economies, there has been a challenge of harnessing and utilizing renewable energy to meet these demands. However, despite the effect of global warming and the problems associated with it, the use of fossil fuels is still increasing. This problem has negatively impacted the climate, because the greenhouse gases produced by burning fossil fuels are increasing the concentration of carbon dioxide in the atmosphere. This study investigates a method that could remedy the situation - channelling biogas for use as a renewable energy source using membrane technology. The study began with observing the behaviour of biogas components as they travel selectively through the membrane support. This was done by conducting gas permeation tests to experimentally show how methane and carbon dioxide gases flow distinctively through ceramic membranes with different physico-chemical properties. Permeation tests were carried out under various operating conditions of temperature (up to 100 degrees Celsius) and pressure (up to 3 bar), using membranes of different pore sizes and characteristics, to ascertain the influence of these parameters on the membrane perm-selectivity. It was identified that the membranes were operating in a parallel flow regime and gas permeability was a function of viscous and Knudsen flow. A mass transfer model was developed to confirm that mass transfer conditions were not limiting in the boundary layer of the membrane surface. The model incorporates the influence of both structural and fluid properties in characterising diffusive and convective flow through the membrane. The analysis showed that under the same pressure drop across the membrane, the mass flux in porous membrane can be over four orders of magnitude higher than in a silica PDMS membrane with the similar thickness. Based on these findings, a dynamic approach was considered to modify the membrane by dip coating technique which allows easy manipulation of the deposition quantity and thickness, and it was possible to achieve a finely modified membrane with reduced pore size that improves the perm-selectivity. A methane selectivity of 1.6 was achieved, which is of the ideal Knudsen selectivity, with a 42% decrease in the CO2 concentration of the gas mixture. Permeation studies of the coated membrane shows that gas permeance is dependent on temperature with little impact due to pressure changes which can be attributed to a reduced impact of viscous flow due to the coating. The reduction in pore size due to coating was to a degree that significantly impacts the viscous flow contribution. The characterization results confirm this pore size reduction with the methane gas permeation rates reduced from 6x10-6 mol/m2sPa to 1x10-6 mol/m2sPa for the support and coated membrane respectively. A numerical method of estimating the pore size of the coated membrane was adopted based on the operating flow mechanism using a series model in conjunction with flow parameters through the layers. It was confirmed that the pore size of the coated membrane was sufficiently reduced to 36nm, which is in the dominant Knudsen range. Hence it was confirmed that, by modifying the membranes with this technique, it is possible to increase the selectivity of the methane by Knudsen diffusion. From this study, it can be established that a membrane's selectivity performance is dependent on an interplay of factors: (i) the structural effect that filters gases based on their interaction with pores; (ii) the thermodynamic equilibrium effect having to do with operating conditions; and (iii) the kinetic effect that considers the different diffusion rates of gas components, making them permeate quicker than others.

Citation

OGUNLUDE, P. 2023. Gas diffusion transport characteristics and mathematical description of membrane systems with application for biogas upgrading. Robert Gordon University, PhD thesis. Hosted on OpenAIR [online]. Available from: https://doi.org/10.48526/rgu-wt-2071771

• Thesis Type: Thesis
• Deposit Date: Sep 6, 2023
• Publicly Available Date: Sep 6, 2023
• DOI: https://doi.org/10.48526/rgu-wt-2071771
• Keywords: Membrane technologies; Ceramic membranes; Membrane permeance; Membrane selectivity; Biogas; Carbon capture; Renewable energy
• Public URL: https://rgu-repository.worktribe.com/output/2071771
• Award Date: Mar 31, 2023