Crossflow microfiltration modelling and mechanical means to prevent membrane fouling
The definition, history and applications of Microfiltration (MP) are briefly reviewed in Chapter 1. The physical mechanisms and mathematical models of the filtration process including concentration polarization (CP), gel polarization (GP) and pore blocking are given in Chapter 2. Crossflow microfiltration membrane fouling and the deposition of solids onto the filter surface have been investigated using a process fluid (seawater), latex and a ground mineral. The performance of various membrane materials has also been studied, including: acrylonitrile, polypropylene, PTFE, ceramic and stainless steel. The seawater filtration work showed in Chapter 3 that good filtrate flux rates can be maintained if material fouling or depositing on the membrane can be prevented from entering the membrane structure. A surface deposit may be removed by mechanical means such as backflushing with permeate or compressed air. This aspect of the work indicated that a more comprehensive study of fouling was required. Existing crossflow filtration membrane models did not adequately represent even the simplest filtration when penetration of the membrane structure applied. Such conditions occurred during latex filtration in Chapter 4. Latex of varying sizes and density were manufactured and filtrations using acrylonitrile membranes were performed. Considerable deposition of latex inside the membrane pores occurred despite the nominal rating of the membrane being less than the latex particle diameter. Thus the membranes relied on a depth filtration mechanism rather than a surface straining mechanism for filtration effectiveness. A standard filtration blocking model was modified for use in crossflow microfiltration, coupled with a mass balance on the amount of material filtered. This mathematical model was then used to predict and correlate the rate of filtration flux decay with respect to filtration time during crossflow filtration. The model provided acceptable accuracy and is an improvement on existing empirical models for the flux decay period. Under the circumstances of membrane penetration it is advisable to minimise the amount of material entering the membrane structure. Mechanical means to achieve this were investigated and a novel anti-fouling method using a centrifugal field force and enhanced shear stress at the membrane surface was developed. The filtration of limestone slurries with three different tubular filters are presented in Chapter 5, in which one filter was conventional, the other two novel ones were specially designed for the separation of particles with a density different from that of the liquid, one used a helical channel around the filter, and the other had tangential inlet and outlet endcaps. The centrifugal force produced by the spinning flow around these two filters retarded the approach of particles towards the membrane surface so that the particle deposition was reduced. The results showed such a system was energy efficient, saving 20 % of the energy required to effect a separation of mineral material compared with using the membrane in a more conventional way.
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