Dynamic Crossflow Filtration DYNAMIC CROSSFLOW FILTRATION: PRINCIPLE AND APPLICATIONS

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DynamicCrossflowFiltration

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DOI:10.1002/9781118522318.emst074

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EMST emst074.tex V1 - 01/04/2013 6:20 P.M. P. 1

DYNAMIC CROSSFLOW FILTRATION:PRINCIPLE AND APPLICATIONSMichel Y. JaffrinUMR CNRS 7338, Technological University of Compiegne, Compiegne, France

1 INTRODUCTION

1.1 Definition of Dynamic Crossflow Filtration

Crossflow filtration consists in circulating a molecular solution or a particle suspensionusing a pump along a flat or tubular membrane in order to collect by filtration throughthe membrane a permeate containing water and small solutes. At the same time, the feedsolution called retentate is concentrated, as particles or molecules bigger than the mem-brane pore size are rejected by the membrane. This operation necessitates applying onthe feed solution a pressure higher than that of the permeate as well as a tangential veloc-ity sufficient to prevent solute accumulation on the membrane, which induces fouling.It is well known that using high shear rates at the membrane is one of the most effi-cient factors for increasing the permeate flux as it reduces concentration polarization inultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO), and cake build-up inmicrofiltration (MF). But crossflow filtration can only increase the shear rate by raisingfluid velocity or reducing channel thickness, both of which generate a pressure drop alongthe membrane, which decreases the mean transmembrane pressure (TMP) and leads toflux decrease.

This problem can be avoided in dynamic crossflow filtration (DCF), previously calleddynamic or shear-enhanced filtration by creating the shear rate at the membrane with amoving part such as a rotating membrane (1), a disk or a rotor rotating near a fixed circularmembrane (2) or by azimuthal vibrations of circular membranes around a perpendicularaxis (3) or shaking longitudinally a hollow fiber cartridge.

1.2 Advantages and Drawbacks of Dynamic Crossflow Filtration

The first and most obvious advantage of DCF is that it can increase permeate flux by afactor of 3–5 as compared to crossflow filtration (CF) by using high rotation speeds or

1


2 DYNAMIC CROSSFLOW FILTRATION: PRINCIPLE AND APPLICATIONS

vibration amplitudes of about 2.5–3.1 cm. It also increases solute transmission throughthe membrane in MF by reducing cake formation and fouling. In low cut-off UF and inNF and RO, the transfer through the membrane of microsolutes such as ions and chemicaloxygen demand (COD) components is mainly diffusive. This transfer is reduced at highshear rates because concentration polarization is lowered and molecular concentration atthe membrane is smaller. This reduced membrane concentration leads to a higher rejectionof microsolutes by the membrane, which is generally the goal of NF and RO operationsin wastewater treatment (4). So it may be possible to replace an RO membrane by anNF one, which will increase the permeate flux. At the same time, permeate fluxes keepincreasing until high pressures of at least 40 bar in NF and RO, as the pressure-limitedregime is extended by the reduction in concentration polarization, further increasingmaximum fluxes.

A second advantage is that DCF can reduce energy consumption as the feed flowonly needs to be slightly larger than the filtration flow rate because the shear rate isindependent of feed flow and powerful pumps are not necessary. Of course, rotatingmembranes or disks consume energy at a high rotation speed. It is possible to usemoderate rotation speeds giving similar permeate fluxes as CF, but by reducing thespecific energy consumed per unit volume of permeate, which may be minimized byseeking the optimal rotation speed. Manufacturers of rotating disks or membrane modulesclaim at least a 60% reduction in specific energy at optimal speed. The power requiredfor vibrating membranes may also be reduced by operating near the resonant frequencyof the system as is the case with the VSEP described in Section 2.3.

A third advantage is the possibility of reaching dry solid concentrations of 70% ormore in one single pass through the module by using feed flow rates slightly larger thanthe permeate flow rate (5). Rotating disk modules such as the Spintek and the Dyno filterhave their compartments connected in series so that the retentate keeps concentratinguntil it reaches the outlet, avoiding the need for recirculation and for connecting severalmodules in series. They can even be used in dead-end filtration with high permeate fluxes.

The first and most obvious drawback of dynamic filtration modules is their cost and, insome cases, their complexity, which may raise maintenance costs. The second drawbackis the limited membrane area of some models, which does not exceed 25 m2. However,three of them—the Optifilter CR, the Rotostream, and the VSEP—are available with150 m2 of membrane.

Unfortunately, information on investment and maintenance costs is not readily avail-able and cannot be given here. It is clear that cost may be an issue as sales of DCFmodules are still limited and these modules are better suited for end-of-pipe treatmentwhen the initial volume to be treated has been reduced by one or more preliminary CFstages.

2 INDUSTRIAL DYNAMIC FILTRATION SYSTEMS

2.1 Cylindrical Membrane Modules

The first industrial-size systems had cylindrical membranes rotating inside a concentriccylindrical housing, thus generating Taylor vortices in the annular gap between mem-brane and housing. At high rotation speeds, these vortices increased the shear rate at themembrane as well as the turbulence and the permeate flux. The Biodrucker filter builtby Sulzer Ag (Wintherthur, Switzerland) appeared in about 1988 and was described in


DYNAMIC CROSSFLOW FILTRATION: PRINCIPLE AND APPLICATIONS 3

Reference 1. The Benchmark Rotary Biofilter by Membrex, (Garfield, NJ, USA) appearedaround 1990. The maximum membrane area of these units seems to have been 2 m2,which is not sufficient for many applications. The Biodrucker filter is no longer commer-cialized. Membrex Sinthofer presently produces the BHS, a rotary pressure filter with anup to 9 m2 filter area for separating particles in the range of 10–500 μm.

The most successful realization of the rotating cylindrical membrane concept wasin the medical field, with the plasma cell filter developed by Hemascience Co (SantaAna, CA, USA) and introduced in 1985 for collecting plasma from donors (6, 7) andshown in Figure 1. This filter featured a 58 cm2 polycarbonate cylindrical membranewith 0.8 μm pores and 1.34-cm diameter rotating at 3600 rpm in a concentric cylinder.It was later commercialized by Fenwall Inc. (Lake Zurich, IL, USA) and integrated intoan automatic system, the Autopheresis–C, which could collect 600 ml of plasma froma donor in 30 min, faster than with hollow fiber plasma filters of 300 cm2, thanks to itshigher permeate flux. This system is still commercialized in 2012 for transfusion centers.

2.2 Rotating Disks Systems

These systems exist in various configurations, with metal disks mounted on a singleshaft and rotating between fixed circular membranes, or membranes rotating betweenfixed plates and multishaft systems with overlapping rotating ceramic membranes.

2.2.1 Single-Shaft Systems with Rotating Membranes. These systems are simple tobuild and to service, but the counterpressure due to the action of centrifugal forces onthe permeate during its rotation decreases the TMP at high speeds.

Novoflow Co (Oberndorf, Germany) produces a CRD pilot available with 1–3 ceramicmembrane disks of 152-mm diameter for MF and UF. Its pressure is limited to 2.5 barand the rotation speed to 1000 rpm. A larger model, the SSDF-312, uses ceramic disksof 312-mm diameter permitting up to 15 m2 membrane area, while the SSDF-500, with500-mm-diameter disks, can reach 25 m2 of membrane area. Pore sizes available rangefrom 7 nm to 2 μm and the recommended TMP to maintain a steady permeate flow rateis between 0.2 and 2 bar. Novoflow can manufacture filter disks with almost any flatmembrane, using its New Seal technology (5), up to a maximum diameter of 850 mm.They report operating costs of ¤ 7.4 m−3 of permeate with their rotating modules againsta cost of ¤ 15.3 m−3 with ceramic tubular modules for the same fluid. Units of up to240 m2 can be ordered by assembling several modules.

Another multidisk system is the one produced by Spintek (Huntington, CA, USA) with27.5-cm-diameter membranes for a maximum area of 2.3 m2 and is shown in Figure 2.Initially available with polymeric membranes, it is now offered with 1, 5, or 10 ceramicmembranes, permitting feed volumes up to 11 m3/day. Its maximum speed is 1800 rpmand has been used by Dal Cin et al. (8) for separating water from cutting oil emulsionsby UF. They observed the existence of a maximum flux due to permeate counterpressureand depending on TMP.

2.2.2 Single-Shaft Systems with Metal Disks Rotating Between Fixed CircularMembranes. The Dyno filter, based on a Czech patent, came on the market in 1970.The present module, manufactured by Bokela GmbH (Karlsruhe, Germany) and shownin Figure 3, has several compartments in series, with steel rotors rotating on a horizontalshaft between fixed circular membranes mounted on support plates. Membrane diameters



Rotating cylinder

Fixed cylinder

Plasma flow

Outlet blood flow

Plasma collectingchannel

Membrane

Inlet blood flow

Membranesupport

and plasmachannel

FIGURE 1 Schematic diagram of rotating cylindrical filter for plasma collection from donors.

range from 13.7 to 85 cm and the maximum membrane area is 12 m2 with a maximumpressure of 600 kPa and a maximum rotation speed of 1350 rpm (9). These devices areavailable with polymeric or ceramic membranes.

The permeate is collected at the bottom of each support plate so that the retentateis continuously concentrated as it circulates from the inlet to the outlet. A solid matterconcentration of 65% can easily be reached in a single pass. Another interesting feature



FIGURE 2 Sketch of Spintek rotating membrane module.

is that a wash liquor (such as water) can be introduced at the top of each supportplate in order to dilute the retentate, which corresponds to a diafiltration. Industrialapplications include the preparation of Arabic gum, emulsions, cocoa butter, recovery ofwhite pigments, kaolin, TiO2, and CaCO3 molecules, as well as thickening of beverageresidues. Ehrfeld and Bott (10) reported a power consumption of 25–30 kW for a Dynomodule of 8 m2 membrane area during concentration of a white pigment suspension from13.6% to 41% in weight with a final viscosity of 25 Pa s. They also used the same moduleto remove salt from a microfine pharmaceutical suspension by diafiltration with an MFmembrane at 6% solid concentration. The permeate flux decreased gradually from 350to 250 l/h m2 with a mean value for the batch of 320 l/hm2 at a pressure of 2 bar anda temperature of 30 ◦C. A selection of industrial applications of the Dyno filter is givenin Table 1. Another similar early system was the DMF of Pall Corporation (MA, USA)patented in 1992 with a 2 m2 membrane area intended for biotechnological applications,but it is no longer commercialized.

The Optifilter CR, developed by ABB Flootek (Finland) in 1995 and presently com-mercialized by Metso Paper (Raisio, Finland), features blades instead of disks, rotatingbetween stationary flat circular membranes with a tip azimuthal speed of 10–15 m/s



FIGURE 3 Dyno rotating disk module (Bokela, Germany) (Please refer to the online version forthe color representation of the figure.)

TABLE 1 Examples of Industrial Applications of Dyno Filtera

Product Type Product or ProcessCharacteristics

FeedConcentration,%W

RetentateConcentration,%w

PermeateFlux, L/hm2

Industrialwastewater

Variable properties 0.3 11 900

Red mud 30 65 300TiO2 Abrasive, high

porosity34–40 50–59 400–1000

Ultramarine 17 55 300Yellow pigment 4.5 20 400Molybdenum

orangeHigh viscosity 5 50 750

Silica acid, SiO2 13 40 800Bentonite High water

absorption2.4 6.7 100,150

Borine carbide Abrasive 21 52 150Glaze for ceramic Removing slimy

contents33 72 150

Silicon carbide Extremely abrasive 26 66 100Nanoscale

chemical productNanoparticles

5–50 nm5 30 170

a From Reference 10 with permission.

(Fig 4). The membrane diameter can reach 1 m and the total membrane area can exceed140 m2 with a 132-kW motor (11). These devices can deliver permeate flow rates of upto 230 m3/h. Smaller units of 84 m2 (75 kW) and 15 m2 (30 kW), with 0.55-m diametermembranes are also available. They are used by more than 30 plants, mostly for treatmentof pulp and paper effluents or pigment recovery.



PeripheralStrip Rotor

Support layer

Clamp ring

Support plate

Membrane

Feedwater in

Filtratewater out

Concentratewater out

FIGURE 4 OptiFilter CR, Metso paper. (Please refer to the online version for the colorrepresentation of the figure.)

2.2.3 Multishaft Systems with Rotating Membranes. These systems appeared around1999 with the introduction of large ceramic membrane disks. Andritz KMPT GmbH(Vierkirchen, Germany) has commercialized a two-shaft unit with 152-mm-diameterceramic membranes rotating at 1000 rpm in a pressurized tank with membrane arearanging from 0.14 to 16.4 m2. Available pore sizes range from 7 nm to 2 μm and max-imum pressure is 2.5 bar. These systems can handle fluids with viscosity up to 3 Pa s.The retentate can be recirculated in the tank by feed and outlet pipes, while the permeateis collected in the hollow shafts. It applications include cell harvesting; proteins andenzymes recovery; biomass treatment; processing and sterilization of juices, vegetableoil, and gelatin; water recovery from starch processing; biodiesel preparation; and recov-ery of solvents and valuable liquids. The membrane shear rate is the maximum in theoverlapping region between two membranes. As the disks are counterrotating, the highpermeate flow rates can create local zones of high concentration that limit the perfor-mance. Grimm (12) observed that the differential rim speed between disks was 7.3 m/sand the entrance and exit of overlapping regions generated a very efficient cleaning effectdue to turbulence. Goldnik and Ripperger (13) reported the washing of concentrated TiO2and Al2O3 suspensions by diafiltration with a 6-ceramic disk module of 0.14 m2 mem-brane area rotating at 1400 rpm. Pore size was 0.2 μm and temperature 20 ◦C. The Al2O3



suspension, with a mean particle size of 3.5 μm, gave a mean permeate flux of 320 l/h m2

in comparison to 113 l/h m2 for the TiO2 one with a mean particle size of 0.4 μm. Thelatter produced a thicker layer on the membrane. These fluxes rose at the beginning ofthe test and fluctuated around their mean values during the entire 76-min test.

A larger system, the MSD system, designed in 2001 by Aaflow (Aalen, Germany)features 31.2-cm-diameter ceramic membranes stacked on eight parallel shafts to give amaximum membrane area of 80 m2 (Fig. 5). All disks rotate at the same speed and areenclosed in a cylindrical housing. The membrane shear rate is unsteady and maximumin the overlapping regions (14–16). This system seems to have been abandoned afterthe purchase of Aaflow by Westfalia Separator GbmH. Other systems, the Rotostream(Canzler, Dueren, Germany) (17, 18) and Hitachi (Japan) (19), available up to 150 and100 m2 membrane areas, respectively, have their parallel axes in the same plane. Thesesystems are simpler to build than single-shaft systems with metal disks rotating betweenfixed membranes as they avoid the construction of separate compartments, which mustbe connected either in parallel or in series to receive the feed flow.

FIGURE 5 Industrial MSD module with eight parallel shafts and 31-cm ceramic disks. Source:Courtesy of Westfalia Separator. (Please refer to the online version for the color representation ofthe figure.)



2.3 Vibrating Systems

2.3.1 VSEP Module. The best-known vibrating system is the vibrating shear-enhancedprocess (VSEP) proposed in 1992 (20) by New Logic Research (Emeryville, CA, USA).It is also the most widely used dynamic filtration system, as 376 industrial VSEP mod-ules have been installed worldwide since its introduction. It consists of a stack of circularmembrane compartments mounted on a vertical shaft (Fig. 6). Each compartment con-tains two membranes separated by a spacer, drain cloths, and an O-ring. The membranediameter is 30 cm for pilots and 60 cm for large modules. The shaft is spun in azimuthaloscillations of 3-cm maximum amplitude at the rim by a vibrating base at a resonantfrequency slightly higher than 60 Hz. The shear rate at the membrane is produced by theinertia of the fluid, which moves 180 ◦ out of phase with the membrane. It thus variessinusoidally with time and is maximum at the membrane periphery (3). The use of res-onance minimizes the power necessary for the vibrations, which is only 9 kW for thelargest units of 150 m2 membrane area (Fig. 7). These modules can withstand pressuresof 40 bar and are suitable for NF and RO, in addition to MF and UF. They have beenused in a large number of applications such as in bioethanol production from yeast toremove solids after the fermentation process and before distillation, with a mean per-meate flux of 66 l/h m2 while the solid concentration rose from 3% to 18%. In landfillleachate treatment, a VSEP equipped with NF membranes permitted sludge concentrationby a factor of 10 while producing a clear permeate that could be recycled in the plant.After drying the retentate, the volume of the final solids was reduced to 2% of the initialleachate volume.

2.3.2 Case Studies.2.3.2.1 Black Liquor Treatment. It is necessary for economic reasons to concentrate theblack liquor generated in pulp mills in order to reduce the energy costs of evaporation.Using successive UF and NF steps, the VSEP can concentrate fibers, organic lignins,

EP flow diagram

Clear permeate Feed

Concentrate

Membrane

Membrane

Permeate channel

Orain cloth

O-ringO-ring retainer

Stainless steel trayDrain cloth

FIGURE 6 Schematic diagram of circulation in a VSEP membrane stack. Source: Courtesy ofNew Logic Research. (Please refer to the online version for the color representation of the figure.)



FIGURE 7 Industrial VSEP vibrating modules. Source: Courtesy of New Logic Research.(Please refer to the online version for the color representation of the figure.)

and total dissolved solids (TDS) up to 35% in weight of the black liquor without themembrane fouling encountered in CF. Nearly 80–90% of the black liquor was recoveredas reusable water or for discharge in rivers.

2.3.2.2 Waste Oil Treatment. Two VSEP modules with 100 m2 of membrane area wereinstalled in Portland (Oregon, USA) to recycle 76 m3/day of used engine oil. The initialflux at 90 ◦C was 24 l/h m2 and fell to 6.5 l/h m2 when the permeate volume reached80% of the initial volume, for a mean flux of 21 l/h m2. Raising the temperature to120 ◦C doubled the permeate flux. The purified waste oil can be refined to serve as greenlubrifiant.

2.3.2.3 Clarification of Phosphate Feedstock for Fertilizer Production. VSEP moduleswere used to separate calcium sulfate and suspended solids from phosphoric acid byMF and to remove contaminants in single-pass mode. Tests were conducted at 70 ◦C todecrease fluid viscosity and increase the flux.

2.3.2.4 Titanium Dioxide Dewatering. Titanium dioxide (TiO2) is an important pigmentused in the production of quality papers and in the production of paints. During the cal-cination process, TiO2 is suspended in an aqueous slurry at a 30% weight concentration,which must be dewatered to reach 70% concentration. This step is usually done by anevaporator-dryer, but UF using a VSEP significantly reduced energy costs.

2.3.2.5 PVC Latex Concentration. UF with a VSEP raised the initial 38% concentrationof suspended solids to 68%, corresponding to the gel point of this solution. A spray dryer



was used subsequently to convert the concentrated solution to a dry powder. The reductionby UF of the volume treated by the spray dryer decreased the energy consumption andmultiplied the plant capacity with the same dryer by a factor of 2–3.

2.3.2.6 Treatment of Pulp and Paper Mills’ Effluents. The VSEP, when equipped withUF or NF membranes, can significantly reduce biological oxygen demand (BOD), COD.and TDS, thus minimizing treatment cost. VSEP can be used as a secondary treatmentimmediately after the primary clarification, and the permeate can be recycled in theplant. It can also be used as a tertiary treatment after a classical biological treatment andsecondary clarification.

To remove ink from recycled papers, VSEP is used as a first NF stage to recycleor discharge in rivers more than 90% of waste water as clean permeate with a BODreduction of over 80%. The retentate containing the ink is further concentrated by asecond VSEP to raise its concentration to 30% of total solids.

New Logic Research estimates that the three best applications of VSEP technol-ogy worldwide are the treatment of biogas, landfill leachates, and cooling tower blow-downs. Treatment of used oil, gas waste waters, and ethanol stillage is also promising“(G. Johnson, personal communication)”.

Pall Corporation has obtained a license to commercialize its own version of the VSEP,the PallSep Biotech, but only for biotechnological and pharmaceutical applications. It isavailable with a microencapsulated hydrophilic polyethersulfone (PES) membrane of0.2-μm pores with areas of 0.2, 1, and 5 m2. According to Pall, this membrane, togetherwith the high shear rate due to vibrations, allows an increased transmission of microso-lutes.

3 ESTIMATIONS OF MEMBRANE SHEAR RATES IN VARIOUS SYSTEMS

The estimation of the membrane shear rate as a function of rotation speed or vibrationfrequency is important as it is the main parameter governing the permeate flux andmembrane selectivity such as rejection. Equations for these rates are presented for varioussystems.

3.1 Rotating Cylindrical Membranes

The rotation of the fluid in the annular gap between two concentric cylinders gener-ates toroidal instabilities (Taylor vortices) above a certain rotation speed. This speedcorresponds to a value of Taylor number Ta > 42 with

Ta = ωR1/2e3/2

ν(1)

where ω denotes the angular speed, R the inner cylinder radius, e the annular gap, and ν

the kinematic viscosity. These vorticies increase mixing and the shear rate on the innerand outer cylinders, which can both receive a membrane. When Ta > 400, the flowbecomes turbulent. Taylor (22) measured the torque on the inner cylinder at differentspeeds and geometries and proposed the following equation for the shear rate

γ = 0.23Ta1/2 ωR

e= 0.23ν3/2R5/4e−1/4ν−1/2 (2)



The shear rate decreases if the annular gap and fluid viscosity increase and increaseswith increasing speed and radius as expected. This equation is valid in the range60 < Ta < 3000, which includes both laminar and turbulent regimes. By conductingexperiments with skim milk and bovine serum albumin on a small module at variousrotation speeds and annular gaps, Lopez-Leiva (23) obtained an empirical equation forthe Sherwood number Sh in the turbulent regime for 400 < Ta < 4000 as

Sh = kme

D= 0.070Ta0.64Sc1/3 (3)

where km is the mass transfer coefficient, D is the diffusivity, and Sc = ν/D is theSchmidt number. This equation estimates the permeate flux J in the mass-transfer-limitedregime using the classical equation of Blatt et al. (24):

J = km ln

(Cw

Cb

)(4)

where Cw is the solute concentration at the membrane and Cb the bulk concentration. Bycombining Equations 2–4, it can be seen that the permeate flux varies as γ 0.96, almostproportional to the shear rate, as observed also by Belfort et al. (25).

3.2 Systems With Rotors or Disks Rotating Near a Fixed Membrane

The internal fluid mechanics of these systems was investigated by Bouzerar et al. (26)using a 15-cm-diameter metal disk rotating near a single membrane. For laminar flow,at low rotation speed, the local shear rate γl at radius r was calculated from the NavierStokes equation as

γl = 0.77 (kω)1.5rν−0.5 (5)

where ω is the disk angular velocity and kω is the angular velocity of the inviscidfluid core between the disk and membrane. This coefficient, which is less than 1, wascalculated from pressure measurements at various speeds and found to be equal to 0.42for a flat (or smooth) disk. It is higher if radial vanes are added to the disk and reached0.86 with eight vanes that were 6-mm high (27).

In turbulent flow at higher rotation speeds, the local shear rate at membrane γt wasestimated from the Blasius friction coefficient for a flat plate, to be

γt = 0.0296 (kω)1.8 r1.6ν−0.8 (6)

This shear rate increases faster when r or ω increases than does the laminar shear rateand it is of course maximum at the disk rim (r = Rd).

The mean shear rate over the membrane area is given by

γtm = 0.0164 (kω)1.8 R1.6d ν−0.8 = 0.55 γmax (7)

where γmax is the maximum shear rate at the disk rim.



The pressure distribution in the invisicid fluid between the disk and the membrane isgiven by the Bernouilli equation as

p(r) =(

1

2

)ρ(kωr)2 + p0 (8)

where p0 is the pressure at the membrane center. As it is more practical to measure thepressure at the disk rim pc with a pressure tap, Equation 8 can be used to find

pc =(

1

2

)ρ(kωRd)

2 + p0 (9)

and from Equations 8 and 9

p(r) =(

1

2

)ρ(kω2)(R2

d − r2) + pc (10)

Thus, the mean TMP ptm over the membrane, assuming that permeate pressure isatmospheric, is calculated as

ptm = pc −(

1

4

)ρ(kωRd)

2 (11)

3.3 Systems with Rotating Membrane Disks on Single and Multiple Shafts

In the case of rotating membranes mounted on a single shaft as in the Spintek or theNovoflow CRD and SSDF filters, membrane shear rates have been given by Murkes (28)for laminar (Eq. 12) and turbulent (Eq. 12a) flows as

γdl = 1.81(kω)1.5 rν−0.5 (12a)

γdt = 0.057(kω)1.8 r1.6ν−0.8 (12b)

It can be noted that these shear rates on rotating membranes are larger than those inthe case of a disk rotating near a stationary membrane (Eqs. 5 and 6). But they increaseless rapidly with r and ω than in Equations 5 and 6. The permeate collected by themembranes rotates and is submitted to centrifugal forces, causing a counterpressure thatreduces the effective TMP. This pressure can become negative near the rim at highrotation speeds, causing a decrease in mean permeate flux.

The estimation of the membrane shear rate is more complex in the case of multipleshafts systems with overlapping membranes such as the MSD, KMPT devices, or theRotostream of Canzler, as Equation 12 would only hold during short intervals in nonover-lapping membrane parts. Thus, the membrane shear rate is unsteady and maximum whentwo membranes overlap. Ding et al. (15) showed, from geometrical arguments, that thedifferential velocity between two overlapping membranes rotating at the same speed ina two-shaft pilot was uniform and equal to diω where di is the intershaft distance and ω

the angular velocity. A useful parameter may also be the disk outer rim velocity, whichcan easily reach 20 m/s, about three to four times larger than in tubular modules.



3.4 Vibrating Systems

The flow field induced by torsional oscillations of two parallel disks as in the VSEP hasbeen investigated by Rosenblatt (29). This flow is periodic and the membrane shear rate,which depends on time as well as on radius r , has been calculated by Al Akoum et al. [3].The instantaneous local shear rate on the membrane is given by

γ (r , t) =(

rd

R2

)d(πF )1.5 ν−0.5 (cos ω1t − sin ω1t) (13)

where d is the membrane displacement at radius r , itself proportional to r , R2 is the outerradius, F the oscillations frequency = ω1/2π . The maximum with time of this shear rateat the disk periphery is given by

γmax1 = 20.5d1(πF )1.5ν−0.5 (14)

where d1 is membrane displacement at the periphery. The mean shear rate over themembrane area is

γm = 23/2(R32 − R3

1)

3πR2(R22 − R2

1)γmax1 = 0.330γmax1

(15)

Although the shear rate can be increased by raising the vibration amplitude d1, NewLogic Research has limited the maximum displacement at the rim to 30 mm both in30-cm-diameter pilots and in the 60-cm-diameter industrial modules. In fact, the recom-mended amplitude for continuous operation is about 25 mm, probably for maintenancereasons. At d1 = 30 mm, the maximum shear rate for ν = 10−2 m2/s, the kinematicviscosity of water at 20 ◦C, is equal to 112 103/s against 92 103/s for d1 = 2.5 cm.

Beier et al. (30) have used longitudinal vibrations in hollow fiber modules in order toincrease the shear rate and permeate flux (Fig. 8).

γ = V0

[ω2

2ν

]0.5[sin ω2t − cos ω2t] (16)

where ω2 is the angular frequency, d2 their displacement amplitude, and V0 = d2 ω2 isthe velocity amplitude of the fibers. Its maximum with time, when ω2t = −π /4 or 3π /4is

γmax2 = 20.5V0d2

(ω2

2ν

)0.5

(17)

As π F = ω1/2, Equation 14, may be rewritten as

γmax1 = 20.5d1

(ω1

2

)1.5ν−0.5 (18)

and the ratio of maximum shear rates for VSEP and vibrating hollow fibers will be

γmax1

γmax2=

(d1

d2

) (ω1

ω2

)1.5

(19)

If the membrane displacements and angular frequencies were the same, the hollowfiber module would have a larger mean shear rate on the membrane as it is uniform



PC

Hollow fiber

Steel plate -closedends of the

hollow fibers

Feed pump

Scale

Permeate gap

Hollow-fibermembranemodule

Hollow rod

Feed tank

Frequencycontroller

P

Rotation head

Permeate

Permeatepump

Pressuretransducer

FIGURE 8 Oscillating hollow fiber cartridge. Source: From Reference 30 with permission.

inside the fibers, while it decreases toward the center in the VSEP. But for practicalreasons, the vibration amplitude of the fibers is limited to about 3 mm and the fibervibration frequency to 30 Hz (30) against 3 cm and 60 Hz for the VSEP, giving a ratioγmax1/γmax2 equal to 28. A similar system tested by Genkin et al. (31) had a highervibration amplitude (4 cm) but a lower frequency of 2 Hz. Thus, in both systems, theeffective shear rate was much lower than that in the VSEP.

4 REVIEW OF RECENT RESEARCH ARTICLES USING DCF

As there are few articles describing industrial applications written by companies, we willreview articles written by academic researchers using commercial laboratory pilots or, insome instances, their own prototypes as they provide more detailed information on thepotential of these techniques.

4.1 Rotating Disks Modules

4.1.1 Applications in MF. A single membrane module denoted as a UTC module,with a 15-cm-diameter disk rotating near a fixed circular polymer membrane (26, 27,32) was designed in our laboratory and built using stainless steel to withstand pressuresup to 40 bar in order to permit NF and RO tests. In addition to smooth disks, severaldisks with four to eight radial vanes of various heights were also tested (Fig. 9). Thepresence of vanes increased the fluid entrainment coefficient k from 0.45 for a smooth



RotatingDisk

Fluid inlet Pc

membrane

Gap

e

Permeate

Pressure tap

Retentate

W6 mm

S2–6 mm

ω

FIGURE 9 Schematic diagram of a UTC rotating disk module with the disk equipped withvanes. Source: From Reference 32 with permission. (Please refer to the online version for thecolor representation of the figure.)

disk to 0.65 for a disk with eight 2-mm-high vanes and 0.86 for 6-mm vanes (27).The effect of disk geometry on the variation of permeate flux with TMP is shown inFigure 10 in MF of yeast suspensions. This effect gets bigger with increasing TMP ashigh shear rates extend the pressure-limited regime. The flux increased by 171% whena smooth disk was replaced by one with 6-mm vanes, which increased not only theshear rate but also the level of turbulence. Of course, the presence of vanes increases thefriction forces on the disk and the power required for its rotation. But the gain in fluxis such that, in many cases, the specific energy consumed per cubic meter of permeateis lower with vanes, as observed by Brou et al. (33). As most of the power in a smallpilot is consumed by the friction of the shaft and in the motor, in order to make theirresults representative of larger systems for which most of the power is consumed bythe friction on the disks and membranes, they measured the net electrical power PN asthe difference between the powers consumed by the motor during filtration and when

0

200

400

600

800

1000

1200

1400

1600

1800

0 20 40 60 80 100 120 140 160

Mean transmembrane pressure (kpa)

Per

mea

te fl

ux (

L/h

m2 )

Baker yeast C = 3 g/ I, T = 25 °C, Qp = 30 I /h, N = 2000 rpm

Membrane PALL dp = 0.2 μm

Disk with vanes s = 6 mm, n = 8

Disk with vanes s = 4 mm, n = 4Disk with vanes s = 4, n = 8

Smooth diskWater

FIGURE 10 Variation of permeate flux with TMP for a 3 g/l yeast suspension at 2000 rpm forvarious types of disks. Source: From Reference (27) with permission.



the module was empty. The specific energy consumed per cubic meter of permeate isPN divided par the filtration flow rate QF. The variation of this specific energy is plottedin Figure 11 as a function of the rotation speed for different disk geometries during MFof a polysaccharide suspension with a viscosity of 4 10−3 Pa s. It can be seen that thisspecific energy increases with increasing rotation speed but is minimum for the disk with6-mm-high vanes and maximum for the smooth disk. If the module had been equippedwith two membranes, one on each side of the disk, the permeate flow rate would havebeen doubled and the specific energy halved.

4.1.2 Applications in UF. Ding et al. (34) used the UTC module in UF of skim bovinemilk together with a larger 26-cm-diameter unit in stainless steel equipped with a 460-cm2

50-kDa membrane on each side of the disk. The permeate flux kept increasing with TMPuntil at least 900 kPa at high speed (1500 rpm), which produces the same peripheralvelocity (20.4 m/s) as in the small module at 2600 rpm. Vanes only 2-mm high had alarge effect on the flux, which was doubled at all speeds, probably because the spacebetween the disk and the membrane was small. The permeate flux for both modules andboth types of disks, smooth or with vanes, plotted versus maximum membrane shear ratein log–log coordinates in Figure 12, was found to be given at VRR = 1 by

J = 0.301 γm0.552 R2 = 0.909 (20a)

and, at VRR = 1.8, by

J = 0.199 γm0.522 R2 = 0.986 (20b)

This example confirms that the permeate flux from modules of different sizes anddesigns follows the same relation with the membrane shear rate.

Nuortila-Jokinen and Nystrom (35) compared a cross-rotational CR 500 filter (RaisioFlootek, Finland) pilot with 1.75 m2 of membrane area in UF of paper mill process waters

0

10

20

30

40

50

60

0 500 1000 1500 2000 2500 3000

Rotation speed (rpm)

PN

/Qf (

kWh/

m3 )

MF dp = 0.2 μm, T = 30 °C

Disk with vanesDisk with vanesSmooth disk

k s(mm) μ(mPas)

0.84 6 4.52

0.65 2 4.050.45 0 4.05

FIGURE 11 Specific energy consumed per cubic meter of permeate as a function of rotationspeeds for various disks. Source: From Reference 33 with permission. (Please refer to the onlineversion for the color representation of the figure.)



10

100

1000

1000 10,000 100,000 1,000,000γm (1/s)

J (L

/hm

2 )UHT skim milk, T = 45 °C, 50 kDa PES membrane

J = 0.301 (γm)0.552 R2 = 0.909

J = 0.199 (γm)0.572 R2 = 0.986

Disk with vanes, large module, s = 2 mm, VRR =1

Smooth disk, large module, VRR =1

Disk with vanes, small module, s = 6 mm, VRR =1

Smooth disk, small module, VRR =1

Disk with vanes, small module, s = 6mm, VRR =1.8

FIGURE 12 Variation of maximum permeate flux with mean membrane shear rate for the twomodules and various types of disks at VRR = 1 and for a 6-mm-vane disk at VRR = 1.8. Source:From Reference 34 with permission.

at 50-kDa cut-off with PCI tubular polymeric and mineral membranes of same cut-off.They reported that with the CR filter rotating at 470 rpm with a peripheral rotor speed of12 m/s gave significantly higher fluxes, by a factor of four to five, than those of tubularmembranes with axial velocities of 2.1–2.5 m/s. After an initial flux decline during thefirst 8 h, the CR 500 filter could sustain a quasi-steady flux around 250–350 l/h m2 forclose to 100 h.

The reduction of ionic surfactant (sodium dodecyl benzene sulfonate) in aqueoussolutions was investigated by Moulai Mostefa et al. (36), using the UTC module andPES membranes of 10, 20, and 50 kDa. The maximum surfactant rejection (92%) wasobtained with the 10-kDa membrane, against 58% at 50 kDa and permeate fluxes at1400 kPa, were very high, 400 l/h m2 at 10 kDa and 600 l/h m2 at 50 kDa for a solutionat critical micellar concentration (Fig. 13). At a TMP of 900 kPa, no increase of flux wasobserved above 500 rpm, indicating that the flux was pressure limited, probably due tostrong interactions between surfactant molecules.

Sarkar and Bhattacharjee et al. (37) described an original system consisting of a rotorrotating near a contrarotating membrane disk, used in UF of a polyglycol solution inwater with a 5-kDa membrane. They varied separately the angular speeds of membrane(ω1) and rotor (ω2). They presented an analytic solution for inner fluid mechanics of thissystem, which they validated against their experimental data. Differences between modeland experimental were less than 7% for permeate flux and rejection. Unfortunately, thedata presented do not permit determining if it was more efficient to increase ω1 or ω2. Inanother paper, the same group (38) applied the same module to the recovery of proteinsfrom casein whey, using successively a 30-kDa membrane to concentrate casein and a5-kDa membrane to recover lactose in permeate. With the rotor at rest, they obtained at50 kDa and a speed of 400 rpm stabilized fluxes of 230 l/h m2, at a pH of 2.8. The initialflux at 5 kDa and 392 kPa was also 230 l/h m2, but it decayed rapidly and stabilized after15 min to 100 l/h m2.



0

200

400

600

800

1000

0

200

400

600

800

1000

1200

1400

1600

0 500 1000 1500 2000 2500 3000

Ptm

(kP

a)

Rotation speed (rpm)

Flu

x, J

(L

/hm

2 ) 50 kDa20 kDa10 kDaPtm

FIGURE 13 Variation of permeate flux and TMP versus disk rotation speed for various mem-brane cut-offs T = 24 ◦C, VRR = 1, disk with 6-mm vanes SDBS at critical micellar concentration,Pc = 900 kPa. Source: From Reference 36 with permission.

Espina et al. (39) proposed a two-stage MF–UF DCF process in order to fractionatemilk proteins using an MSD pilot with ceramic membranes of 0.2 μm pores and a UTCRDM for the UF stage. Casein micelles were extracted in the MF retentate at a permeateflux ranging from 95 l/h m2 at VRR = 1–20 l/h m2 at VRR = 5, with a mean yield inpermeate of 81% for α-La and 76% for β-Lg whey proteins. The separation between α-Laand β-Lg was carried out with a PES 50-kDa membrane. Permeate fluxes in UF werevery high, ranging from 600 l/h m2 at VRR = 1 to 340 l/h m2 at VRR = 5.5. The meanα-La recovery in permeate was about 32%, while that of β-Lg in retentate was 90%.This work confirmed the high potential of rotating ceramic membranes for milk proteinfractionation, probably because whey proteins transmission in MF and α-La transmissionin UF were not affected by increasing concentration.

4.1.3 Applications in NF and RO. Frappart et al. (40) investigated the treatment ofdairy process waters simulated by diluted skim milk at one-third concentration using theUTC RDM module equipped with a Desal 5-DK NF membrane. The goal was to recoverand concentrate in retentate lactose and milk proteins, while obtaining a permeate with aCOD below the allowed limit of 125 mg O2/l for discharge in rivers and a conductivitybelow the limit of 1000 μ S/cm. They compared data collected with a smooth diskand a disk with 6-mm vanes at 2000 rpm, 45 ◦C and 20, 30, and 40 bar. Variations ofpermeate flux with VRR in concentration tests are shown in Figure 14. As expected,the permeate flux was maximum at 40 bar and for a disk with vanes that produced thehighest membrane shear rate (γm) of 4.4105/s at VRR = 1. The drop in flux when VRRincreases is due to the increase in retentate concentration Cb according to Equation 4and the decrease in shear rate due to the rise of kinematic viscosity with concentration,according to Equation 7. With a smooth disk, γm was equal to 1.1 × 105/s at VRR = 1,and the flux was smaller and dropped faster with increasing VRR. The regime becamemass-transfer-limited faster and was smaller than the flux at 30 bar with vanes. Whilethe contribution of vanes to permeate flux is large at high TMP, it is small at 20 barbecause, at this low pressure, the flux is still pressure limited.



0

50

100

150

200

250

1 10 100

VRR

J (L

/hm

2 )

4000 kPa; 6 mm vanes4000 kPa; Smooth3000 kPa; 6 mm vanes3000 kPa; Smooth2000 kPa; 6 mm vanes2000 kPa; Smooth4000 kpa; 6 mm vanes; undiluted

N = 2000 rpm ; T = 45 °C

VRR max = 13.6

VRR max = 12.2

VRR max = 8.5VRR max = 8.9

VRR max = 14.8

FIGURE 14 Variation of permeate flux with VRR in semi-log coordinates at 2000 rpm andvarious pressures for an RDM with two types of disks. Source: From Reference 40 with permission.

Corresponding variations of permeate COD with VRR are displayed in Figure 15.This COD is minimum at the highest pressure as COD rejection is highest becauseof the packing of the solute layer at the membrane and increases exponentially withVRR. The COD limit is exceeded at VRR = 4, meaning that a further RO step may benecessary at higher concentration. It must be noted that the feed COD is much higher at36,000 mg O2/l than in real dairy effluents. Similar results were obtained for permeateconductivity proportional to the ionic concentration shown in Figure 16, except that its

0

200

400

600

800

1000

1200

0 1 2 3 4 5 6 7 8 9 10VRR

Per

mea

te C

OD

(m

gO2/L

)

4000 kPa; 6 mm vanes4000 kPa; Smooth3000 kPa; 6 mm vanes3000 kPa; Smooth2000 kPa; 6 mm vanes2000 kPa; SmoothCOD rejection standard

N = 2000 rpm; T = 45 °C

FIGURE 15 Effect of TMP on variation of permeate COD with VRR at 2000 rpm with bothtypes of disks. Source: From Reference 40 with permission.



00 1 2 3 4 5

VRR

500

1000

1500

2000

2500

3000

3500

4000

4500

6 7 8 9

Per

mea

te c

ondu

ctiv

ity (

μS/c

m)

4000 kPa; 6 mm vanes4000 kPa; Smooth3000 kPa; 6 mm vanes3000 kPa; Smooth2000 kPa; 6 mm vanes2000 kPa; SmoothConductivity rejection standard

N = 2000 rpm; T = 45 °C

FIGURE 16 Effect of TMP on variation of permeate conductivity with VRR at 2000 rpm withboth types of disks. Source: From Reference 40 with permission.

variation with VRR was linear. The conductivity standard was exceeded at VRR ofgreater than 2.5 at 40 bar and at VRR = 2 at 20 bar.

4.2 Modules with Rotating Disk Membranes

We summarize results of the tests performed in our laboratory on CaCO3 suspensionswith a two-shaft MSD pilot (Westfalia separator) (15) equipped with 12 ceramic disksof 9-cm diameter, shown in Figure 17. Their nominal pore size was 0.2 μm and the totalmembrane area was 0.121 m2. Disks from each shaft overlap for 15.5% of their surface,and the membrane shear rate is the maximum during overlapping. Figure 18 comparesthe permeate fluxes obtained with the normal configuration of 12 disks (denoted DF)and when only 6 disks are present on a single shaft (denoted s). These fluxes are nearlyequal at the two lower speeds, while at 1930 rpm, the s mode gives the highest flux.The reason is that at 1930 rpm, the permeate flow rate is high with 12 disks (144 l/h) incomparison with the inlet flow of 180 l/h and the retentate concentration reaches 500 g/lfor a feed of 200 g/l, while this concentration was only 312 g/l with 6 disks.

Taamneh and Ripperger (41) compared the performance of an MSD laboratory pilotin single- and double-shaft configurations to quantify the gain in flux due to overlappingmembranes. From the electrical power measurements, they concluded that the membraneshear stress in the double-shaft configuration was about twice that in the single shaft one.This was confirmed by the absence of cake formation with two shafts. At a speed of750 rpm, the flux remained steady at 1900 l/h m2, while it dropped rapidly to 400 l/h m2

with one shaft.As ceramic membranes for the MSD were only available in limited pore sizes or cut-

offs, Tu and Ding (42) replaced them with disks equipped with two nylon membranesof the same pore diameter (0.2 μm) to concentrate CaCO3 suspensions. The maximumpermeate fluxes were higher at 300 kPa and 1930 rpm for nylon membranes, reaching



FIGURE 17 Photograph of an MSD 12 ceramic disks pilot from Westfalia Separator. (Pleaserefer to the online version for the color representation of the figure.)

0

200

400

600

800

1000

1200

1400

1600

0 50 100 150 200 250 300TMP (kPa)

Per

mea

te fl

ux (

L/h

m2 )

N = 1930 DF N = 1037 DF N = 738 DF

N = 1930 s N = 1037 s N = 738 s

MSD: CaCO3, C = 200 g/L, T = 24 °C, Qi = 240 L/h, dp = 0.2 µm

FIGURE 18 Comparison of permeate fluxes obtained at 200 g/l in single-shaft(s) configuration(6 disks) with those in double-shaft (DF) configuration (12 disks) as a function of TMP and threerotation speeds. Source: From Reference 15 with permission.



850 l/h m2 versus 760 l/h m2 for ceramic membranes because of their higher permeabilityand hydrophilicity.

4.3 Vibrating Modules

4.3.1 Applications in MF and UF. Zouboulis and Petala (43) treated landfill leachatewaste waters with a VSEP laboratory pilot using, successively, MF at 0.1 μm, UF (100and 10 kDa), and NF membranes. Their data confirmed that VSEP high shear ratesincreased COD and small solutes removal as compared to CF while maintaining largeand stable permeate fluxes of 100 l/h m2 at 10 kDa and 150 l/h m2 at 100 kDa withvibrations of 25-mm amplitude corresponding to a maximum shear rate of 80,000/s. Thisflux fell to 45 l/h m2 in NF. Final rejections in NF permeates were 90% for COD and92% for ions, much higher than for CF.

Permeate fluxes given by a VSEP laboratory pilot and an RDM module equippedwith the same nylon membrane (Ultipor, Pall Corp) with 0.2 μm pores were comparedas a function of TMP by Jaffrin et al. (32) in MF of yeast suspensions at 3 g/l. Thiscomparison is displayed in Figure 19 for the VSEP at 60.75 Hz and disks rotating at2000 rpm, with and without vanes. The values of maximum shear rates at the membranerim are also indicated in this figure. It is interesting to note that the VSEP had a 10%larger shear rate than the smooth disk module and gave a slightly higher flux. The diskwith vanes produced the largest flux, but had a shear rate three times larger. When shearrates were varied by changing the yeast concentration in the retentate without permeaterecycling, the corresponding permeate fluxes were plotted as a function of the shear ratein Figure 20. They all fall on the same line, according to the equation as shown inFigure 20.

J = 4.310−6 γ 1.459max (21)

Baker yeast C = 3 g/L, Qi = 30 L/h, Mem brane PALL dp = 0.2 μm

0

200

400

600

800

1000

1200

1400

1600

1800

0 20 40 60 80 100 120 140 160

TMP (kPa)

J (L

/h m

2 )

Disk with 6 mm vanes

Smooth disk

VSEP

N = 2000 rpm, T = 25 °C

N = 2000 rpm, T = 25 °C

F = 60.75 Hz, T = 20 °C

γm = 3.05 105/s

γm = 0.99 105/sγm = 1.12 105/s

FIGURE 19 Variation of permeate fluxes with TMP in MF of 3 g/l yeast suspensions for thesmall rotating disk module with two types of disks and the VSEP. Source: From Reference 32with permission.



Baker yeast, Qi = 30 L/h, Membrane PALL dp = 0.2 µm, concentration tests

1

10

100

1000

10 000

10 000 100 000 1 000 000

γm (1/s)

J (L

/h m

2 )

Small module, s = 6 mm, N = 2000 rpm, T = 25 °C

Small module, s = 0 mm, N = 2000 rpm, T = 25 °C

VSEP, F = 60.75 Hz, T = 20 °C

J = 4.3 10–6 γm1.459

R2 = 0.930

FIGURE 20 Variation in permeate fluxes with maximum membrane shear rate during concen-tration tests of yeast suspensions by MF using RDM with smooth disks and a disk with vanes andthe VSEP. Source: From Reference 32 with permission.

The exponent of the shear rate in Equation 21 is larger than those in Equation 20,reflecting a larger influence of the shear rate when changing the frequency or the rotationspeed. This is due to the double action of increasing the yeast concentration, which bothdecreases the shear rate by raising fluid viscosity and increases yeast concentration onthe membrane, which also decreased the flux, causing a higher rate of flux decreasewith γmax. This results confirms that in dynamic filtration, the permeate flux is mainlygoverned by shear rate and not by module geometry.

4.3.2 Applications in NF and RO. Frappart et al. (44) investigated the reduction ofion concentration and COD in dairy process water simulated by skim milk diluted toone-third concentration using a VSEP pilot equipped with a Desal Ag membrane and aUTC RDM module with the same membrane and a disk with 6-mm vanes. The variationin permeate fluxes with VRR is plotted in Figure 21 in semi-log coordinates for threefrequencies for the VSEP and at 1000 and 2000 rpm for the RDM at a TMP of 40 barand a temperature of 45 ◦C. Fluxes at VRR = 1 were very large, reaching 180 l/h m2 forthe VSEP and 185 l/h m2 for the RDM. At the lowest frequency of 60 Hz, the VSEP fluxwas lowest and the mass transfer limited at VRR of greater than 2. The maximum fluxwas obtained with an RDM at 2000 rpm due to a maximum shear rate of 4.55 × 105/s.But the VSEP flux at maximum frequency of 60.75 Hz was only slightly lower, althoughits maximum shear rate was only 1.35 × 105/s. In fact, the shear rate of the RDM at1000 rpm was 1.31 × 105/s, very close to that of the VSEP vibrating at 60.75 Hz, but itsflux was significantly lower, implying that the periodic variation of the VSEP shear rateis more efficient in raising the flux than the steady shear rate of the RDM.

Variations in retentate and permeate conductivities with VRR are represented inFigure 22 The permeate conductivity increases exponentially with VRR for the VSEP at



0

20

40

60

80

100

120

140

160

180

200

1 10 100

VRR

J (L

/h m

2 )

60.75 Hz60.2 Hz60 Hz2000 rpm; 6 mm vanes1000 rpm; 6 mm vanes

TMP = 4000 kPa; T = 45 °C

J = –65.8 Ln(VRR) + 156

R2 = 0.999VRR max = 12

FIGURE 21 Variation in permeate fluxes for an RDM and a VSEP pilot at various shear rateswith VRR (in semi-log coordinates). Source: From Reference 44 with permission.

0

100

200

300

400

500

600

700

800

0 1 2 3 4 5 6 7 8 9 10

VRR

Per

mea

te C

ondu

ctiv

ity (

µS/c

m)

0

2000

4000

6000

8000

10,000

12,000

Ret

enta

te C

ondu

ctiv

ity (

µS/c

m)

60.75 Hz60.2 Hz60 Hz2000 rpm; 6 mm vanes1000 rpm; 6 mm vanes

TMP = 4000 kPa; T = 45 °C

FIGURE 22 Variation in permeate and retentate conductivities for both systems at various shearrates with increasing VRR. Source: From Reference 44 with permission.

60 Hz, while its increase is slower for other tests at a higher shear rate. However, thesefluxes remain much below the limit for rejection in rivers of 1000 μ S/cm and belowthe reuse limit of 40 μ S/cm up to VRR = 3 for the RDM at 2000 rpm and the VSEP at60.75 Hz. The retentate conductivity also increases with VRR and reaches a plateau at10,000 μ S/cm at VRR = 8.



5 DISCUSSION

Although many articles have reported the high performances of various DCF modulesin many applications in MF, UF, NF, and RO concerning permeate fluxes, membraneselectivity, or energy consumption, their diffusion in industry is still slow, especiallyin Europe. It is true that in laboratory tests, which are generally of short duration, it ispossible to use maximum shear rates, while in industrial production it is necessary to limittheir performance in order to keep maintenance and energy costs moderate. For instance,New Logic recommend limiting the vibration amplitude to 2.5 cm against a maximumof 3.0 cm, even in 60-cm-diameter VSEP modules, which reduces the shear rate to about1.1 × 105/s at low viscosity and permeate flux by about 10% of the maximum (3).Novoflow recommends limiting the tangential velocity at the disk rim to about 7–8 m/s,which is only 2 m/s higher than the axial velocity achieved in tubular membranes, whilea 15-cm-diameter RDM is capable of tangential velocities exceeding 20 m/s.

On the contrary, in RDM and rotating membranes, the maximum membrane shearrate is theoretically unlimited. It increases with the disk radius Rd at power 1.6 and withangular velocity at power 1.8, according to Equation 7 and can easily exceed 5 × 105/s.However, such high shear rates will consume a great deal of energy and they may damagesome solutes. It seems that most manufacturers of DCF modules stress on energy savingsand retentate concentrations more than on permeate flux increase, although large fluxincreases will reduce membrane area and investment cost for the same output.

6 CONCLUSION

The high potential offered by DCF in terms of permeate flux and microsolute transmis-sion in MF and UF and the high rejection in NF and RO have been well confirmed inthe scientific literature by many investigators using small pilots. Among DCF modulemanufacturers, Bokela GmbH, for instance, emphasizes high permeate flux aspects.Others such as KMPT and Novoflow stress energy savings aspects of up to 80% becauseof the reduction in feed flow rates, the high retentate solid concentrations that can beachieved with a single large module in a single pass, and the relatively uniform TMPthat optimizes membrane efficiency. But these manufacturers, generally, only publishshort articles of case studies without a detailed quantitative analysis to back their claims.

Constraints imposed by continuous industrial production also limit the performanceof DCF modules in order to avoid costly maintenance.

It is clear that DCF modules will cost much more per square meter of membranethan do spiral wound modules, and they are not intended for initial treatment of hugevolumes of fluid. But they can be used in end-of-pipe treatment when the goal is to reachhigh solid concentrations in order to minimize waste volumes. In wastewater treatment,it may be possible to replace an RO step by an NF one using DCF because of increasedrejection at high shear rates with a large gain in permeate flux. If sales of DCF modulescontinue to increase, their cost will decrease and their total operating cost per cubic meterof permeate may become competitive with conventional modules, as the gain in energycost will offset a slightly higher investment cost per cubic meter of the product.

The number of DCF modules permitting NF and RO is presently limited, but it shouldincrease in the near future as benefits of high shear rates seem to be higher in NF and ROthan in UF and MF in terms of flux permeate and selectivity, which are augmented by the



reduction of concentration polarization. The availability of large ceramic NF membranedisks should permit this development as it is simpler and cheaper to build modules withceramic membranes rotating in a housing similar to the MSD or the KMPT than inmulticompartment systems with rotors between fixed membranes.

ACKNOWLEDGMENTS

This article could not have been written without the scientific contributions of mycoworker Prof L. Ding and my former PhD students O. Akoum, G. Beaudoin, A. Brou, V.Espina, M. Frappart, and N. Moulai Mostefa. We also thank Veolia Waters, France,and Aaflow, Germany, for their gift of a VSEP pilot, and an MSD pilot, and Alting,Novasep, and Osmonics Co. for supplying membranes.

LIST OF SYMBOLS

d vibration amplitude (m)J permeate flux (J/h m2)

P pressure (Pa)Sh Sherwood numberTa Taylor number

Greek symbols

γ shear rate (1/s)ν kinematic viscosity (m2/s)ρ density (kg/m3)

ω angular velocity (rad/s)

ABBREVIATIONS

CF cross flowDCF dynamic cross flowMF microfiltrationNF nanofiltrationRDM rotating disk moduleRO reverse osmosisTMP transmembrane pressureUF ultrafiltration

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FURTHER READING

Membranes food applications . Peinemann KV, Pereira Nunes S, Giorno L,

Membrane Technology. Volume 3, WILEY-VCH Verlag GmbH.Q2

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Please note that the abstract and keywords will not be included in the printed book,but are required for the online presentation of this book which will be published onWiley’s own online publishing platform.

If the abstract and keywords are not present below, please take this opportunity toadd them now.The abstract should be a short paragraph upto 100 words in length and keywordsbetween 5 to 10 words.

Abstract:

Dynamic crossflow filtration increases membrane performance through the use of mem-brane shear rates that are much higher than in conventional crossflow filtration. Theproduction of these high shear rates requires specific modules with metal disks or rotorsrotating at high speed near fixed membranes or rotating or vibrating membranes. Theadvantages and drawbacks of dynamic crossflow filtration will be discussed and currentlyavailable pilots and industrial dynamic filtration modules will be described. Specificmethods for calculating the shear rate at membranes, which is the key factor govern-ing permeate flux, will be presented for various types of modules. Recent applicationsof dynamic filtration published in the scientific literature, together with industrial casestudies and comparisons with conventional crossflow will be reviewed.These systems can be advantageous for end-of-pipe treatment when the goal is to max-imize retentate concentration in order to reach at least 65% of solid concentration. Inpharmaceutical and biotechnological applications, where a high purity is required, thehigher membrane selectivity of dynamic filtration (better solute transmission in micro-filtration and higher microsolute rejection in nanofiltration and reverse osmosis) is animportant asset.Industrial strategies of manufacturers of modules, New Logic Research (CA, USA), whichhas sold over 376 industrial vibrating modules; Bokela (Germany), which produces therotating disk Dyno filter; and Metso Paper (Finland), which sells the Optifilter with rotorsbetween, will be summarized.

Keywords:

rotating and vibrating membranes; rotating disk modules; shear-enhanced filtrationQ3

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Queries

Q1. Please provide publisher location for Ref. 28.

Q2. Please provide the year of publication and publisher location for Reference ”Peine-mann et al.”

Q3. Please provide a minimum of 5 rotating and vibrating membranes

Documents

Dynamic Crossflow Filtration DYNAMIC CROSSFLOW FILTRATION: PRINCIPLE AND APPLICATIONS