Static Turbulence Promoters in Membrane Filtration

Journal of Membrane and Separation Technology, 2015

Dynamic filtration (DF) consists in creating a high membrane shear rate by disks rotating near a fixed membrane or by rotating or vibrating membranes. The shear rate can exceed 3 105 s-1 in some modules and significantly increases permeate flux and membrane selectivity as compared to cross flow (CF) devices. This paper describes several DF industrial modules and gives equations for calculating shear rates at rotating and vibrating membranes. It reviews 23 recent articles from 2008 to 2014, dealing with diverse applications: separation of microalgae from sea water by UF, clarification of rough beer, concentration of CaCO3 suspensions, treatment of dairy effluents and shipboard wastewaters, inulin extraction from chicory juice, treatment of oil field water, and separation of bovine albumin from yeast. In several applications, the maximum permeate flux at initial concentration ranged from 270 to 760 Lh-1m-2. Modules with ceramic membranes rotating around several shafts inside a housing seem to be preferable to the concept of multi-compartments modules with metal disks rotating between fixed membranes. Since the cost of DF modules is higher than that of spiral wound ones, it is better to apply DF to ”end of pipe treatment” after an initial concentration by CF.

Journal of Membrane Science, 1997

Dominant mechanisms of particle transport in cross-flow membrane filtration are unified to obtain a generalized model for time-dependent permeate flux. The unified model extends an earlier model based on shear-induced diffusion and a concentrated flowing layer to include Brownian diffusion and inertial lift. It is applicable over a broad range of contaminant sizes encompassing macromolecules, colloidal and fine particles, and large particles. The combined theory predicts an unfavorable particle size, of the order of 10 1 ~tm, where the net back-transport away from the membrane attains a minimum, leading to maximum cake growth. For the system simulated in this work, this implies minimum permeate fluxes in the size range of 0.01~0.1 ~tm, depending on the operating time. Inside-out hollow-fiber geometry is predicted to be favorable for feed suspensions with small particles and/or low concentrations which form thin resistive cakes. However, larger particles, which form thick cakes, may result in reduced surface area available for filtration due to curvature effects in inside-out membranes, making the slit or outside-in geometry more favorable for these particles. Fine particles (< 0.1 ~tm) are predicted to demonstrate mass-transport limited behavior. For larger particles, different combinations of fiber radius and cross-flow velocity, resulting in the same shear rate, demonstrate different permeate fluxes.

Desalination and Water Treatment, 2014

This study presents a closed-loop process for forming pre-coated dynamic membrane used in membrane bioreactor. Powder-activated carbon (PAC) particles are suspended by using axial-flow agitator whilst simultaneously deposited upon the filter medium by cross-flow filtration. The revolution speed of agitator is varied in the range of 150-450 rpm. Computational fluid dynamic is used to calculate velocity and shear rate adjacent to membrane surface. The force balance of the particles considers the driving force of dynamic membrane formation, drag force due to negative pressure-driven of suction pump, lift force due to mechanical mixing, and adhesive force due to particle-particle interactions. The effect of two major terms including drag and lift forces on the porosity of dynamic membrane is systematically investigated. The lower drag force results in the higher porosity of dynamic membrane. The results also show that the denser dynamic membrane is formed when the lift force becomes lower. The porosity of dynamic membrane is ranged from 0.38 to 0.46 while the thickness of dynamic membrane is altered between 100 and 500 μm. A dimensionless immobilized parameter is derived to predict the formation of pre-coating dynamic membrane. The results obtained from modeling show that dynamic membranes formed at higher values of immobilization parameter have a more cohesive structure. The results obtained from the experiments and the model reveal that the thickness of 400 μm of PAC layer is considered as a stability threshold thereafter the structure of dynamic membrane becomes loose and unstable. The rejection capacity of dynamic membrane is also evaluated at different thicknesses by using Formazin solution. The results reveal that the turbidity of filtrate decreases with increasing the thickness of dynamic membrane. The results display that rejection capacity of pre-coating dynamic membrane is comparable to that of microfiltration membrane.

Chemical Engineering Research and Design, 2016

This paper describes a dynamic membrane filtration system that combines crossflow filtration and centrifugal separation in a rotating tubular membrane. Because of the noslip boundary condition, membrane rotation leads to higher centripetal force near the lumen wall than introduction of a rotating flow. In this fundamental exploratory study, hollow glass microspheres (5 to 35 µm diameters) serve as model low-density, separate-phase foulants during filtration of aqueous suspensions through a tubular ceramic membrane (nominal 0.14 µm pore size). At low crossflow rates, membrane rotation at 1725 rpm decreases fouling and shifts the microsphere size distribution in the membrane cake towards smaller diameters. Force balance calculations suggest that centripetal force should move particles with diameters >~17 μm away from the lumen surface. Moreover, azimuthal and longitudinal shear stresses will also selectively remove larger particles from the membrane cake. Computational fluid dynamics (CFD) simulations show that the rotational flow does not fully develop in the membrane lumen and the fluid radial velocity peaks before the membrane wall. Both of these factors will decrease movement of low-density particles away from the lumen wall. Nevertheless, consistent with experimental data, CFD simulations show greatly decreasing encounters of particles with the membrane wall as particle size increases.

Water Research, 2018

Oily water production is one of the many drawbacks of petroleum and several other industries. Finding effective ways for the treatment of produced water remain one of the main areas of interest in membrane sciences. Albeit the many advantages of membrane technology, they suffer from the unavoidable problem of fouling, which results from the accumulation of dispersed materials at the surface of membranes. Membrane modification and operational optimization have been approached as a potential cure of the problem of fouling. In this work we introduce a new and novel method that minimizes the development of fouling and in the same time utilizes no chemicals (i.e., environmentally friendly). The core of this method is based on alternating the pressure in the feed channel in a periodic manner and is therefore named the periodic feed pressure technique, PFPT. The idea is to make pinned droplets at the surface of the membrane lose essential forces that keep them sticking to the surface. The drag force due to permeation flux and the capillary force due to interfacial tension represents the two forces that largely contribute to the pinning of oil droplets at the surface of the membrane. Other forces including buoyancy and lift forces are generally small to be of significant influence. The idea of the PFPT is, therefore, to eliminate the force due to permeation drag. This is done by setting the transmembrane pressure (TMP) to zero at fixed intervals allowing pinned oil droplets to dislodge the surface. When the TMP is set to zero, permeation flux stops and the force due to permeation drag vanishes. This significantly reduces the overall residence time of pinned oil droplets, minimizing the chance for other oil droplets to cluster and coalesce with pinned ones. The PFPT does not cause any damage to the support layer of the polymeric membrane, which is a drawback of back-flushing methodology. The novel PFPT displays minimal 2 membrane fouling and very similar permeation recovery despite only half the cycle time is in filtration mode. In this work, we show how the permeation flux is recovered and provide comparisons between the PFPT and regular filtration methodology. Furthermore, we compare the overall amount of filtrate at the end of the experiments using both methods. It is interesting to note that, the amount of filtrate using the PFPT is very much comparable to that obtained using regular filtration methodology and even higher. By optimizing the frequency of the cycle and the amplitude of the pressure change, it is possible to customize the PFPT to various membrane technologies and to achieve the highest recovery of the flux. Visual inspections of the membranes post operation and post rinsing indicate that membranes undergoing filtration using the PFPT achieves a very clean surface compared with those undergoing regular filtration processes. This method is a promising solution to membrane fouling that is easy to implement without any additional use of chemicals or equipment. Computational fluid dynamics (CFD) investigation is also conducted on microfiltration processes to show why this technique works.