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Title: Mechanisms of membrane fouling by macromolecules at multiple scales during ultrafiltration
Author: Shi, Xiafu
ISNI:       0000 0004 6060 8907
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2014
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The thesis aims to gain a better understanding on the mechanisms of the complicated macromolecular fouling in ultrafiltration (UF). The work is divided into three main parts. Firstly, comprehensive literature reviews on both membrane fouling and cleaning were carried out for a better overview on this problem. The findings such as the identification of the main foulants and the current knowledge on fundamental fouling mechanisms, directly contributed to further parts of the thesis. Secondly, a multiscale approach was developed to form generalised framework for modelling complex fouling scenarios. Two complex fouling models combining multi- ple fouling mechanisms were derived accordingly. The models were then applied to the filtration data collected from UF experiments (constant-pressure and dead-end) on three individual macromolecular solutes, i.e., dextran blue (DB), polyethylene oxide (PEO), and humic acid (HA), respectively. During the experiments, the effect of macromolecular concentration and transmembrane pressure was investigated. Using the appropriate combined model, the overall and initial fouling behaviours and the predominant fouling mechanisms at different stages of filtration were identified. The fouling parameters in the combined models were determined and found to be consistent with the existing theories. The switch points between the dominant fouling mechanisms were assessed using two methods (integral and differential), respectively. Comparing all the information together gave a comprehensive understanding of the physics involved in the macromolecular fouling. Finally, the effect of the deformability of a macromolecule on membrane fouling was studied. The sieving results from the experiments indicated a flux-dependent permeation during UF of the DB solution, hypothetically due to the elongational deformation of the large DB molecule (a linear polymer at 2000 kDa MWCO) under high velocity gradient at the pore entrance, allowing the molecule to adapt a smaller transversal size to enter pores at nominally 100 kDa cutoff. This not only increased the chances of permeation but also the probability of severe irreversible fouling. Subsequently, a mesoscopic model using dissipative particle dynamics (DPD) was developed to investigate the blocking event at the pore entrance in the presence of a deformable linear macromolecule. The simulation results shed lights on the threshold permeating flux at which the molecular chains start to deform.
Supervisor: Field, Robert ; Hankins, Nicholas Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Chemical engineering ; ultrafiltration ; fouling ; membrane