Modelling removal of sulphur dioxide from flue gas in purification devices

Thesis

Many chemical filters contain reactive components where harmful substances are removed or transformed. In this thesis, we consider the problem of removal of sulphur dioxide from flue gas using filters made from a porous catalytic medium. This is inspired by a filter, designed by W. L. Gore and Associates, Inc., which converts gaseous sulphur dioxide into liquid sulphuric acid via a chemical reaction that occurs on the surface of microscopic catalytic pellets. During the device operation, the liquid sulphuric acid accumulates within the filter and reduces its efficiency. We derive a series of mathematical models that explore various aspects of this problem and ultimately serve to predict the performance of the device. We begin by considering a fundamental fluid dynamics problem of spreading of a thin film under surface tension with liquid injection due to the chemical reaction. We then formulate a microscale model for the gas and liquid transport within the porous filter material and, separately, present a radially symmetric model for a single catalytic pellet, which we use to estimate the unknown reaction rate constant based on real observations. Although we do not explicitly account for the porous scaffold of the filter, the model set-up is appropriate for the case of hydrophobic material. We move on to upscale the microscale equations to a set of device-scale equations using homogenisation techniques. We obtain numerical solutions and asymptotic predictions for various limits that compare well, and also explore the effect of changing the system parameters, such as the gas speed, on the effective cleansing of flue gas. In addition, we develop a model for two neighbouring pellets, one of which is completely submerged by sulphuric acid, which is useful to understand the long-term behaviour of the system once a continuous layer of liquid forms near the surface of the filter sheets. We also study a simplified problem where the filter is made from hydrophilic material and identify the key differences in the resulting liquid transport. Finally, we present a model for the hygroscopy of sulphuric acid, which helps evaluate the effect of water absorption on the liquid growth within the filter. All the models we develop retain generality and can be applied to other physical and industrial processes.

Authors: Kiradjiev, K

• DOI: 10.5287/ora-j0a4odeng
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: fluid dynamics; homogenisation; porous media; filtration; sulphur dioxide; asymptotic analysis
• Subjects: Asymptotic expansions; Homogenization (Differential equations); Chemical reactions; Gases--Filtration; Fluid dynamics