- Title
- Transport phenomena in foam fractionation
- Creator
- Li, Xueliang
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2012
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- This dissertation consists of experimental and theoretical approaches towards a better understanding of some principal aspects of the foam fractionation process: (i) interfacial adsorption, (ii) foam stability, (iii) foam hydrodynamics and (iv) process intensification and device design. Studies on interfacial adsorption include a flexible and robust numerical solution to the Ward-Tordai equation that can be used with any adsorption isotherm. The scheme makes use of the trapezium rule of numerical integration, coupled with the bisection method of root-finding to guarantee local consistency. The scheme is found to be robust and efficient. However, it is made clear that Ward-Tordai equation is only applicable for the adsorption of non-ionic surfactants onto quiescent planar or convex interfaces. For the adsorption of ionic surfactants onto the surface of a rising bubble in a foam fractionation column, experiments are carried out to study the rate and extent of adsorption. It is found that adsorption equilibrium of SDS (sodium dodecyl sulphate) in both the liquid pool and the foam layer can be achieved within 0.2 m of column depth. However, the equilibrium surface excess on a rising bubble is lower than that on a quiescent interface. Studies of foam stability include direct observation of inter-bubble gas diffusion in foams with different liquid fractions and the effect of environmental humidity on foam stability. Inter-bubble gas diffusion is found to be slow and only becomes significant in long term foam stability tests. However, inter-bubble gas diffusion itself does not lead to the collapse of a foam layer. Instead, it is shown conclusively that the bursting of bubbles at the free surface of a foam and the collapse of a foam layer are affected by the humidity gradient in the freeboard of a foam column. A mechanism based on the Marangoni instability is proposed to explain the dependency of foam stability on humidity. The mechanism is supported by experimental observations of the bursting of an isolated bubble under conditions of non-uniform evaporation via high speed video recording. The behaviour of pneumatic foams flowing vertically through an expansion or a contraction is theoretically and experimentally studied. It is demonstrated that although a sudden contraction of flow area decreases the in situ liquid fraction, it does not affect the volumetric liquid over-flow rate. Conversely, a sudden expansion of flow area decreases both the liquid fraction and the volumetric liquid over-flow rate. Implications of this analysis for foam fractionation device design, optimisation and process intensification are discussed. Finally, based on the theoretical analysis, a mechanistic explanation is proposed for a process intensification device. The device consists of a plate with a tube (foam riser) mounted in the centre. The plate has the same diameter of the foam column whilst the tube is narrower. When a foam riser-plate assembly is inserted into a conventional foam column, the foam is forced through a contraction of flow area followed by an expansion. A significant reduction in liquid flux is observed, without diminishing the surface area. However, due to the adsorption isotherm of SDS, the enrichment enhancement measured in the experiments herein is modest. It is believed that if the device is applied to other systems such as proteins where the adsorption isotherm favours high enrichment, the enhancement of enrichment will be more significant. The device is not optimised for any specific system in the current study. Several major aspects of foam fractionation technology that have only been addressed separately by different researchers previously are investigated on a systematic basis here. It is hoped that the work has not only provided enhanced mechanistic understanding of the foam fractionation process, but that it will also promote industrial adoption of the technology.
- Subject
- foam fractionation; interfacial adsorption; foam stability; Marangoni effect; drift-flux; process intensification
- Identifier
- http://hdl.handle.net/1959.13/932169
- Identifier
- uon:11273
- Rights
- Copyright 2012 Xueliang Li
- Language
- eng
- Full Text
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| View Details Download | ATTACHMENT02 | Thesis | 4 MB | Adobe Acrobat PDF | View Details Download |