Dianne E. Wiley

Spacer characterization and pressure drop modelling in spacer-filled channels for ultrafiltration*

A correlation has been developed that allows the characterization and design of net-like feed channel spacers in any combination of geometric characteristics: angle, mesh size, thickness, strand size and voidage. Flow visualization was used to determine the flow path in a spacer-filled channel and to assess the effect of spacer characteristics on fluid mixing. Pressure losses in the spacer-filled channel have been modelled by inclusion of viscous drag on the channel walls and spacer, form drag of the spacer and kinetic losses due to directional flow change. This semi-empirical model permits evaluation of spacer performance. Grober’s mass transfer laminar flow correlation for developing concentration and velocity profiles was modified to account for spacer geometry. The equation obtained predicts mass transfer in spacerfilled channels with a deviation of less than 10% for the majority of spacers tested. The penalty for improving flux is increased pressure loss along the channel. An illustrative economic analysis which includes operating costs (which are pressure drop related) and capital costs (which are flux related) is used to identify optimal spacer designs for ultrafiltration.

Techniques for computational fluid dynamics modelling of flow in membrane channels

Abstract Accurate modelling of the flow and concentration polarisation in pressure driven membrane processes is inhibited by the complex couplings in the flow equations along with any added effects of variable solution properties. A generic computational fluid dynamics (CFD) model has been developed which incorporates these effects and describes the flow across the membrane wall. The results have been validated against classical solutions available in the literature. Extended work indicates that overly simplified expressions for the dependence of viscosity and diffusivity on concentration produce velocity and concentration profiles that may grossly misrepresent reality.

CFD simulations of net-type turbulence promoters in a narrow channel

The most common spacers or turbulence promoters for membrane processes are net-like materials which enhance mass transfer as well as provide passage for feed solutions. The enhanced membrane performance of spacer-filled channels is determined by the fluid flow patterns induced by the spacer filaments. Insight into the effect of spacer characteristics can be obtained by computational fluid dynamics. In this research, the commercial finite volume package FLUENT was used to visualise the flow pattern in a rectangular membrane channel. Three transverse filament arrangements were simulated. The results show that both high shear stress regions and eddies are present in the channel due to the spacer cylinders. The mass transfer enhancement on the wall/membrane surface is directly related to the high shear stress value, velocity fluctuation, and eddy formation. The peak shear stress and velocity fluctuation are repeated after each spacer cylinder, while the eddies are generally found before and after each cylinder. The CFD simulation also suggests that reducing the transverse filament distance will reduce the distance between shear stress peaks and consequently introduce larger shear stress regions near the wall region and increase the number of eddies, which will benefit membrane mass transfer. However, the penalty for this is that energy losses will also be significantly increased. The selection of optimum spacer geometry design involves a trade-off between these competing effects.

Enzymatic and detergent cleaning of a polysulfone ultrafiltration membrane fouled with BSA and whey

Abstract Optimisation of membrane cleaning protocols requires in depth understanding of the complex interactions between the foulant and the membrane. In addition it is important to consider the economic impact of cleaning procedures including the costs of the cleaning process itself together with the effect of the procedures on membrane lifetime and efficiency. For foulants containing proteinaceous components enzymatic cleaners play a vital role in scissioning specific points in the protein strands while detergent cleaners also interact with the protein strands at specific points but in addition rapidly solubilize any small loose protein fragments. Our results show that it is most effective to clean first with an enzyme and then with a detergent, or, if both are present in the same cleaner it must be formulated in such a way that the action of each component does not interfere with any others. Enzymatic cleaners are most effective when operated at a concentration that optimises the cutting of the proteins. Use of higher concentrations does not increase the enzymatic cleaning efficiency. The efficiency of detergent cleaners usually increases with concentration up to a point where the cleaner attacks the membrane itself. The use of a water rinse during the cleaning procedure can be an effective method of removing loose foulant pieces at little extra cost, but is only effective if the rinsing is carried out at the same temperature as the chemical cleaning, otherwise the rinsing causes compaction of the fouling layer. Pretreatment of a polysulfone membrane with enzymatic or detergent cleaners has no effect on subsequent fouling and cleaning. For multiple usage cycles the optimum operating conditions would be those in which the “active” sites on the membrane that are responsible for strong irreversible fouling remain occupied after the first cycle so that subsequent cycles achieve 100% flux recovery and therefore no further decline in membrane performance.

A CFD study of unsteady flow in narrow spacer-filled channels for spiral-wound membrane modules

In spiral-wound membrane modules, spacers are used to enhance wall shear stress and to promote eddy mixing, thereby reducing wall concentration and fouling. Insights into the effect of spacer filaments on flow patterns in narrow channels were obtained using a computational fluid dynamics (CFD) code. The flow patterns were visualized for different filament configurations incorporating variations in mesh length, filament diameter and for channel Reynolds numbers up to 1000. The simulated flow patterns revealed the dependence of the formation of recirculation regions on the filament configuration, mesh length, filament diameter and the Reynolds number. When the channel Reynolds number is increased above 300, the flow becomes super-critical showing time-dependent movements for a filament located in the center of a narrow channel; and when the channel Reynolds number is increased above 500, the flow becomes super-critical for a filament adjacent to the membrane wall. For multiple filament configurations, flow transition can occur at channel Reynolds numbers as low as 80 for the submerged spacer at a very small mesh length (lm/hch = 1) and at a slightly larger Reynolds number at a larger mesh length (lm/hch = 4). The transition occurs above Rech of 300 for the cavity spacer (lm/hch = 4) and above Rech of 400 for the zigzag spacer (lm/hch = 4).

Factors influencing critical flux in membrane filtration of activated sludge

Membrane filtration of biomass is usually accompanied by significant flux decline due to cake-layer formation and fouling. Crossflow filtration with flux controlled by pumping the permeate can produce stable fluxes if a ‘critical flux’ is not exceeded. Below critical flux the transmembrane pressure is typically very low and increases linearly with imposed flux. Above the critical flux the transmembrane pressure rises rapidly signifying cake-layer formation which is usually accompanied by a continued rise in transmembrane pressure and/or a drop in delivered flux. A range of microfiltration and ultrafiltration membranes with pore sizes from 0.22 to 0.65 µm and molecular weight cut-off of 100 kDa was used. The feed was an activated sludge mixed liquor with concentration in the range of 3–10 g dm−3. The results show that the critical flux depends on feed concentration and crossflow velocity, being higher for higher crossflow velocity or lower feed concentration. Critical flux was also dependent on membrane type, being lower for hydrophobic membranes. Although the transmembrane pressure was higher for the larger pore size membrane, no significant difference in critical flux was observed among different pore size membranes. © 1999 Society of Chemical Industry

Computational fluid dynamics modelling of flow and permeation for pressure-driven membrane processes☆

While models of membrane systems with varying degrees of complexity have been developed since the early 1960s, reports of the use of computational fluid dynamics to provide in-depth insights into the separation phenomenon have only recently appeared in the literature. This paper describes the validation and application of a computational fluid dynamics model of pressure-driven membrane processes involving selective removal of components in the feed channel and their transfer to the permeate channel. The effects of changes in rejection, wall permeation rates and solution properties on velocity and concentration profiles are presented for empty channels and channels with eddy promoters. For high polarisation applications typically encountered in conventional ultrafiltration applications, the results demonstrate the need for very fine meshes near the membrane wall, together with high order numerical schemes and accurate modelling of rejection and physical property variations in order to obtain accurate and reliable predictions of the polarisation and flow phenomenon.

The cleaning of ultrafiltration membranes fouled by protein

The relationship between membrane fouling and cleaning was investigated in terms of flow conditions, transmembrane pressures, pH, membrane properties, and cleaning agents using a stirred batch cell and aqueous albumin solution. Fouling was less at the pH extremes than at the isoelectric point for both retentive and partially permeable membranes. Membranes with partial permeability showed a greater tendency for fouling and were less responsive to cleaning than retentive membranes. The results in the stirred cell were shown to be similar to those for a crossflow module under similar operating conditions.

Fouling Control in a Submerged Flat Sheet Membrane System: Part II—Two‐Phase Flow Characterization and CFD Simulations

Abstract Gas‐liquid two‐phase flow has been shown to be very effective in reducing fouling for different membrane modules with different feeds, including submerged flat sheet membranes used in membrane bioreactors for treatment of wastewater. Although gas‐liquid two‐phase flow occurring on the lumen side of tubular or hollow‐fiber membranes has been very well characterized the two‐phase flow regime in submerged membrane processes is different to that inside external membranes. Characterization of two‐phase flow in submerged flat sheet membrane modules has not been previously reported and hence the use of two‐phase flow in these modules has not yet been optimized. This paper reports on characterization of two‐phase flow for a submerged flat sheet membrane module with the aim of identifying the most effective flow profiles for fouling minimization. In order to better understand the fouling control process by two‐phase flow, CFD simulations were also conducted. It was found experimentally that an increase in the bubble size leads to an increase in the cleaning effect, however, for bubbles larger than the channel gap between the submerged flat sheet membranes, any further increase in the bubble diameter had only a minor effect on the cleaning process. CFD simulations revealed that flux enhancement was primarily due to an increase in the overall shear stress on the membrane and to more turbulence generated by introduction of the gas phase.

Fouling Control in a Submerged Flat Sheet Membrane System: Part I – Bubbling and Hydrodynamic Effects

Abstract Submerged flat sheet membranes are mostly used in membrane bioreactors for wastewater treatment. The major problems for these modules are concentration polarization and subsequent fouling. By using gas‐liquid two‐phase flow, these problems can be ameliorated. This paper describes a study of the use of gas‐liquid two‐phase flow as a fouling control mechanism for submerged flat sheet membrane bioreactors. The effect of various hydrodynamic factors such as airflow rate, nozzle size, intermittent filtration, channel gap width, feed concentration, imposed flux, and the use of membrane baffles were investigated. Experiments conducted on model feeds showed that fouling reduction increased with air flow rate up to a given value and beyond this flowrate no further enhancement was achieved. The effect of bubbling was also found to increase with nozzle size at constant airflow. Using intermittent filtration as an operating strategy was found to be more effective than continuous filtration and it also reduced energy requirements. The study showed the importance of the size of the gap between the submerged flat sheet membranes. As the gap was increased from 7 mm to 14 mm, the fouling became worse and the degree of fouling reduction by two‐phase flow decreased by at least 40% based on suction pressure rise (dTMP/dt). This is the first study which has reported the effects of baffles in improving air distribution across a flat sheet submerged membrane. It was found that baffles could decrease the rate of fouling by at least a factor of 2.0 based on the dTMP/dt data, and significantly increase critical flux.