Low shear crossflow

Particulate deposition, fouling and adsorption of molecular species at the membrane surfaces often lead to lower than expected permeate fluxes. While chemical cleaning and periodic backflushing/backpulsing with permeate or compressed gas can temporarily increase fluxes, large installed membrane areas may be required to achieve the desired separation rates. This, in conjunction with relatively high pumping duties, means that both capital and 

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Multi-element candle Multi-element leaf Plate frame press Precoat Nutsche and multielement leaf Precoat plate frame press Recessed plate filter press Sheet filter Single leaf Nutsche Bag Cartridge Dead-end membrane Fibre bed Low shear crossflow Sand bed Simplex strainer Belt press Duplex strainer High shear crossflow Rotary disc Rotary drum Sand bed Tower press Diaphragm filter press Expression (screw) press Horizontal element tube press Vertical diaphragm filter press Vertical element tube press... 

Figure 1.51 Examples of the membrane arrangements used in low shear crossflow membrane filters, (a) spiral wound (b) hollow fibre (c) plate and frame (d) tubular monolith (atech innovations).

Rather than using a continuous rotation of the filter elements or interstitial discs, another variant employs vibration of the elements to enhance filtration. In this case up to 100 double-sided, flat filter elements separated by thin spacers are clamped together within a cylindrical vessel and caused to vibrate by a motor drive assembly close to the resonant/natural frequency of the unit. The oscillatory motion in the plane of the horizontal elements produces a shear rate up to 150000 s which is many times greater than that observed in typical low shear crossflow filters. As the feed flow rate and... 

Eor a given crossflow filtration, the dominant particle back transport mechanism may depend on the shear rate and the particles size [4]. Brownian diffusion is only important for particles smaller than only a few tenths of a micron in diameter with relative low shear, whereas inertial lift is important for particles larger than several tens of microns with higher shear rates. Shear-induced back transport appears to be important for intermediate particle sizes and shear rates. Li et al. [7] reported that the shear-induced mechanism was able to predict fluxes comparable with the critical fluxes identified by the DOTM. 

High shear crossflow filters offer many of the advantages of the low shear filters described in Section 1.6.2, but with the potential benefit of higher... 

Instead of using a furrowed or dimpled membrane support plate, Sobey [41] observed that a single flow deflector in a flat membrane channel could produce many vortices under oscillatory flow conditions, an effect named the vortex wave. An important feature of the vortex wave is that it could occur under low crossflow velocity conditions or with laminar flow so that it can be used for shear-sensitive fluids. Millward et al. [42] tested the effect of vortex waves on plasma filtration with waves produced by flow deflectors with cross-sectional area of 1 x 1 mm in a 2.25 mm high channel as shown in Figure 8.22. The aim was to improve membrane applications for the separation of plasma from cellular blood components for both donor... 

Hollow fiber High wall shear rates (4000-14,000 s- ) High surface area-to-volume ratio Low liquid holdup Low energy consumption Narrow channels more prone to blockage Crossflow velocity between 0.5 and 2.5 m/s Reynolds numbers 500-3000 Max. operating pressure limited to approx 2 bar High membrane replacement cost... 

Fouling could be related to concentration polarisation, which has been calculated for the stirred cell mass transfer coefficient and crossflow systems. Solubility tests showed that in both systems the solubility of calcium and organics is exceeded at the fluxes typical for NF. Operation at low flux, low transmembrane pressure and high shear reduces the deposition of insoluble matter at the membrane surface and thus minimises fouling. 

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