Analysis of fluid-dynamical and physico-chemical aspects of capillary membrane backwashing
Final Report Abstract
The economic operation of inside-out dead-end driven capillary membranes for Ultrafiltration (UF) crucially depends on the efficiency of the temporally realized reversal of the permeate flow direction (backwash). In order to optimize the backwash (BW) process a fundamental understanding of the complex mechanisms taking place inside the capillary during BW is required. These mechanisms, however, are still widely unexplored and respective experimental studies are scarce. The intention of the present study was therefore to identify and describe the mechanisms that determine BW performance by a combined numerical and experimental approach. For the experimental studies specialized filtration and BW devices were designed and constructed, which were operated with four representative particle suspensions each possessing characteristic physicochemical features. The particle size and distribution in the BW water and even the influence of adhesion time were investigated in experiments with only single UF capillaries. For this purpose flushed out BW water was collected time-resolved and analyzed individually for the including particle masses and its particle size distribution. Three different BW concepts were investigated and compared and optimal BW concepts identified. Finally, these concepts were validated and compared with conventional concepts by conducting long term experiments with handmade UF modules comprised of a bundle of UF capillaries. In single capillary experiments pronounced hydraulic classification effects were observed during the filtration and the BW process. These effects obviously led to an entirely inhomogeneous distribution of particle sizes and concentrations within the capillary in radial and axial direction. The associated formation of “hot spots” with high local particle densities were identified to most probably act as seeding points for the formation of plugs that can expand through the whole radial capillary lumen. Prolonged idle time before BW further revealed the high significance of (time dependent) adhesion effects for the layer structure and BW efficiency. The consequences of this influence, however, seem to be depending on the relation between the particle-particle and particle-membrane interactions, which, in turn, strongly depends on the physicochemical characteristics of the respective particles. Since it was found that particles in the medium sized range of 5 to 50 µm are preferably removed by backwashing, larger and smaller particles remain for longer times in the capillary. Since these particles are more prone to time dependent adhesion, they are more likely to be involved in the development of irreversible fouling. Comparing experimental and theoretical results both agreements and deviations can be identified. Like the experimental research the theoretical investigation found that the accumulation tendency and formation of enrichment areas of particles strongly depend on the particle size and operational parameters. Numerically it was calculated that it is possible that in case of large axial velocity gradients during backwash process particles are driven not just into the bulk of the solution and out of the capillary but also towards the capillary wall. Under acting adhesive forces (not considered in the numerical studies) they will probably adhere to the wall or to still sticking particles which will of course increase the likelihood of strong inhomogeneity of the structure of the remaining particle layer. This inhomogeneity of the layers structure actually was a major result found in the experimental part.
Publications
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Backwash of dead-end capillary membrane: Numerical simulation of multiphase flow with initial homogeneous particle distribution, IWA World Water Congress & Exhibition, Busan, Korea, 16-21.09.2012
Mansour H., Keller A., Panglisch S., Gimbel R., Kowalczyk W.
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Experimental and theoretical investigation of basic capillary membrane backwash mechanisms, International Water Association Specialist Group, International Conference on Particle Separation, Berlin, Germany, 18-20.06.2012
Keller A., Mansour H., Kowalczyk W., Gimbel R.
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Influence of initial particle distribution on multiphase flow in dead-end capillary membrane during backwash, 9th European Fluid Mechanics Conference (EFMC9), Rome, Italy, 9-13.09.2012
Mansour H., Keller A., Gimbel R., Kowalczyk W.
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Numerical simulation of multiphase flow with initial homogeneous particle distribution during backwash of dead-end capillary membrane, 10th World Congress on Computational Mechanics (WCCM2012), Sao Paulo, Brazil, 8-13.07.2012
Mansour H., Keller A., Panglisch S., Gimbel R., Kowalczyk W.
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Numerical simulation of particle distribution in capillary membrane during backwash, Membranes, 2013, 3, 249-265
Mansour H., Keller A., Gimbel R., Kowalczyk W.
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Studies on the removal of fouling layers in UF capillary membranes during backwash, 10th Aachen Conference Water and Membranes, Aachen, Germany, 29-30.10.2013
Keller A., Mansour H., Kowalczyk W., Gimbel R.
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Effects of operating conditions on the flow through a porous wall in dead-end capillary membrane during backwash, 11th WCCM, 5th ECCM, 6th ECFD, Barcelona, Spain, 20-25.07.2014
Mansour H., Keller A., Gimbel R., Kowalczyk W.
