Separation and Purification Technology 90 (2012) 1–9

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Separation and Purification Technology

journal homepage: www.elsevier .com/ locate /seppur

Computational modeling for visualization of flow patterns in a membranetesting device

Iqbal Ahmed a, Ahmad Hussain b, S.M.F. Hasani c, M. Shakaib a,d,⇑, Rosli M. Yunus a

a Faculty of Chemical and Natural Resources Engineering, University Malaysia Pahang, Kuantan, Malaysiab Department of Mechanical Engineering, King Abdulaziz University, Rabigh, Saudi Arabiac Department of Mechanical Engineering, Al-Imam Mohammad Ibn Saud Islamic University, Riyadh, Saudi Arabiad Department of Mechanical Engineering, NED University of Engineering and Technology, Karachi, Pakistan

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 December 2011Received in revised form 31 January 2012Accepted 1 February 2012Available online 9 February 2012

Keywords:Membrane test cellModelingVelocity profileShear stressPressure distribution

1383-5866/$ - see front matter � 2012 Elsevier B.V. Adoi:10.1016/j.seppur.2012.02.004

⇑ Corresponding author at: Faculty of Chemical anding, University Malaysia Pahang, Kuantan, Malaysia. T

E-mail addresses: shakaib@ump.edu.my, mshakaib

Computational fluid dynamics (CFD) modeling is carried out to examine fluid flow behavior in a novelmembrane testing cell. The modeling results in terms of velocity and shear stress distribution show thatfluid movement near the membrane surface in the device is mainly in cross flow direction. The region inthe center is an exception where the fluid hits the membrane in normal direction after leaving the innertube. Stagnant flow zones are observed at the outer wall of the cell were local velocities are very low.Flow recirculation regions are also formed around the central tube before the fluid exits the cell. The com-parison of various heights of tube shows that an inner tube of greater height is preferred due to betterflow patterns and higher magnitudes of wall shear rates.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

The main component of a membrane system is a semi-perme-able membrane which separates impurities from water. The mem-branes available for several processes such as for water treatmentare of different materials and characteristics with various poresizes and pore distributions. This difference in membrane structureis due to considerable variety in the size of impurities or contam-inants that are removed from water. It is known that the mem-brane performance depends on the material and the techniqueused for membrane synthesis. Since the production of a completespiral wound membrane element along with testing on a full sizetreatment system is very expensive, the membranes are initiallyprepared in small sizes of flat shapes. The performance of thesenewly developed membranes is then checked in laboratory cells.The effect of operating conditions on the membranes such as pres-sure, pH and temperature are also determined through these testcells on the basis of cost and time needed for analysis.

An important and crucial feature of the testing cells is that thefluid dynamics within the cell should be similar to one in largercommercial membrane modules in order to obtain reliable results.A different flow pattern in the cell can result in inaccuratepredictions of the product fluxes and solute rejections from the

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Natural Resources Engineerel.: +60 9 5492871.@live.com (M. Shakaib).

membranes. To mitigate the difference in hydrodynamic condi-tions between test cells and membrane modules, suitable test celldesigns are therefore needed [1].

Very few studies exist which analyze flow patterns in such de-vices. For example, Kawachale et al. [2] compared flow in modified(with diffuser disk) and conventional (without disk) cells. The CFDresults in the paper revealed that flow was better distributed in themodified cells. In another work [3], the same authors compared thegas flow through the same testing cells. It was found that poor gasflows (in conventional design) resulted due to the unsuitable loca-tion of inlet port in the cell. Tarabara and Wiesner [4] studied thevelocity and shear stress in the feed channel of Sepa Cf MembraneCell of GE Osmonics [5]. Flow was observed to be parallel and uni-directional in the cell excluding the dead regions at the corners ofthe channel. The study of Cortés-Juan et al. [6] suggested designimprovement of a nanofiltration cell. The results showed thatvelocity and wall shear stress increased sharply and the size ofthe recirculation areas reduced due to design modification. Manyother studies [7–12] exist in literature that includes CFD modelingin membrane channels. These studies however did not discuss thefluid flow in the test cells.

The test units are used commonly in centers and institutes in-volved in membrane related research. However due to variouslimitations of the existing cells such as short circuiting of the fluidflow, non-uniform pressure on membrane surface, permanentfixing of membrane within cassette, requirement of the bulky com-ponents like cell body/holder and external pressure source for

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clamping the cell components, a membrane testing device was de-signed [13]. This device as shown in exploded form in Fig. 1 illus-trates that unit contains feed and permeate chamber, coupler/connector and an interior (inner) tube. The membrane along withthe support net and seals are placed in the middle while the cou-pler connects the two portions of the cell using a threaded joint.The designed apparatus makes cleaning, assembly/disassemblyeasy, is compact and light weight. It neither requires multiple nutsand bolts nor need any hydraulic source to clamp and fix the cell.Further the device can be used for pressure driven processes suchas reverse osmosis (RO), ultrafiltration (UF), thermal driven processsuch as membrane distillation (MD) and concentration driven pro-cesses like dialysis and forward osmosis as shown in Fig. 2.

In this paper we perform a hydrodynamic study of this newmembrane testing device. The objective is to determine the flowdistribution in the cell and thus confirm its reliability.

2. Computational modeling

The differential equations are the continuity and the momen-tum equations for three dimensional modeling. The equations are

Fig. 1. Schematic of the membrane testing device.

solved using a CFD code FLUENT [14] which is based on finite vol-ume technique [15]. The equations are discretized using the QUICKscheme and pressure–velocity coupling is obtained through SIM-PLE algorithm [16,17]. The fluid used is water which is assumedto be incompressible and isothermal with constant density andviscosity.

The membrane modules with large surface areas are known tohave low permeation velocities that are typically 3–4 orders ofmagnitude lower than feed velocities. Therefore, CFD simulationsare carried out only for the feed side of the cell and no-permeationand no-slip conditions are assumed at the membrane walls. Thecomputational domain is of cylindrical shape of diameter 4 cmand height 12 cm while the inlet tube has a diameter of 4 mm.The domain is subdivided into a number of finite hexahedral vol-umes by generating a grid as shown in Fig. 3. To ensure grid inde-pendence, the chosen grid size is small enough and contains about135,000 cells. Velocity is specified at the inlet while gage pressureis set to 0 Pa at the outlet. Convergence criterion is set to 1 � 10�5

for the scaled residuals of continuity and velocity components. Fur-thermore, the convergence is confirmed by the stabilized values ofwall average shear rates and velocities at the monitoring pointswithin the computational domain.

3. Experiment

Experiment is performed to obtain fouling patterns on themembrane surface and then to compare with the shear stress con-tours found from simulations. The membrane used in the cell is ofpolyamide material for ultrafiltration application. The inner diam-eter of the cell is 4 cm while height is 12 cm which is same as usedin CFD simulations. Similarly, the diameter of the inlet tube is4 mm and height is 11 mm (1 mm above membrane surface). Theoverall size of the membrane is equal to diameter of cell (4 cm).The seal of flat shape used to prevent leakage is placed on themembrane surface and its thickness is 4 mm. This reduces theeffective area of the membrane in the cell to approximately8 cm2. The operation of the cell is carried out for 30 min in orderto create a fouling layer on the membrane surface.

4. Results and discussion

The flow field in the membrane testing cell is shown with thehelp of velocity contours and vectors in Fig. 4. The inner tube is1 cm from bottom (or 11 cm from top i.e. h = 11 cm) in this Fig. 4while Reynolds number based on diameter of inner tube is 800.The contour and vector plot in Fig. 4 illustrates that fluid after flow-ing within the inner tube in the centre, reaches the bottom surface(membrane) with higher velocity. It then expands, distributes andcontinues to flow with higher velocity on the membrane surface. Astrong fluid recirculation is also observed on both sides of the innertube. The water finally exits the cell through the outlet port. Thevelocity profiles at the bottom wall are shown in Fig. 5 to gain bet-ter understanding of the hydrodynamics near the membrane.Fig. 5a shows that the fluid has maximum velocity in the centerwhere the fluid from inner tube hits the membrane. The vectorsin the center appear to be inward showing that fluid direction is in-clined or normal at this location. Excluding this region the flowdirection seems to be tangential to the membrane surface. At theouter wall of the cell the fluid velocity is lower which shows astagnation zone at this location. The contours of z-component ofvelocity (normalized by inlet velocity) in Fig. 5b are shown nearthe bottom surface that provides more information related to theflow direction. In the centre, the z-velocity has a higher negativevalue which indicates that the fluid jet approaches the surfaceperpendicularly and hits it directly. For the remaining portion

Fig. 2. Various types of processes in the laboratory cell.

Fig. 3. Domain and boundary conditions for CFD analysis.

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the z-velocity is nearly zero showing cross flow movement adja-cent to the membrane surface. The velocity vectors and contoursin Fig. 5 show that flow structure in the present cell approximatesreasonably well the flow patterns encountered in commercialmembrane modules over a major portion of the membrane surface.In the central region, the flow is however not tangential but thissmall region probably would not cause much error since the flowin cross flow modules is also not perfectly uniform and tangentialthroughout the channel length. The membrane modules mayinclude flow non-uniformities due to developing/fully developed

regions, high velocity and recirculation zones of various sizes with-in the channel as reported in earlier studies.

The distance of inner tube in Figs. 4 and 5 is 1 cm from the bot-tom (h = 11 cm). The distance is now varied to study the effect ofthis parameter and determine suitable values. The comparison isshown in Fig. 6. In Fig. 6a, when the tube is 3 cm above the mem-brane surface, the velocity contours are similar to ones in Fig. 4.Fluid leaves the tube, strikes the bottom and causes a major recir-culation zone in the bottom portion of the cell. Cross flow exists ona significant region close to the membrane surface as observed ear-lier when distance was 1 cm (from bottom). In Fig. 6b, the tubelength is further shortened and the fluid exits the tube 6 cm abovethe bottom. A comparison of Fig. 6b with 6a shows that velocitybecomes slower when the fluid drops from a greater height(6 cm). Recirculation zone is also less significant in this case. Thevelocity vectors near the membrane bottom show that size of stag-nant zone near the membrane is greater when the height h is 6 cm.When h = 9 cm, it appears from vector plot that fluid remains at-tached and stuck to the bottom whereas when h = 6 cm fluid is rel-atively less attached to the membrane surface. In Fig. 6c no tube isused in the cell. The fluid from the inlet port falls directly towardsthe bottom. The contours for various heights show that in the cen-tral portion the velocity is relatively higher when the tube is pres-ent. In Fig. 6c, since it does not include the inner tube, the fluidstream diverges as it moves towards the bottom thus decreasingthe velocity. The velocity profiles without the tube however doshow some similarity to the cases when the inner tube is present.For example the flow recirculation regions in Fig. 6c are found to beapproximately of the same size as in Fig. 6b. The pressure distribu-tion along with flow patterns within the cell are shown in Fig. 7.The local pressures are relatively higher in the center of the bottomsurface which is more obvious in the cell with tube height h = 9 cm.The pressure values at other locations are almost uniform in themembrane testing device. At the outlet the pressure rapidly dropsto 0 Pa (gage pressure). The shear stress at the membrane surface isalso determined and is shown as a line plot on the bottom surfacein Fig. 8a and b for two different values of Reynolds numbers. The

Fig. 4. Flow patterns in the testing cell (h = 11 cm).

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trends in Fig. 8a indicate that shear stress for all heights has a verysmall value in the center due to the fact that flow direction is nor-mal to the bottom surface. The low shear stress region in the centerwith values less than 0.3 N/m2 is less than 5% of the total mem-brane area. After encountering a low shear stress region in the cen-tre, the shear rates increase rapidly. The shear stress values thendecrease gradually and finally become zero at the corners. The neg-ligible shear stress at the ends would not be a problem since theportion of the membrane at this location is covered with seal toprevent leakage of the solution and is therefore not included inthe membrane effective area.

The comparison of different tube heights in Fig. 8a shows thatwhen the distance between the tube and bottom surface is lesssuch as 1 or 3 cm (h = 11 or 9 cm), the shear stress is high on a ma-jor portion of the membrane. When the distance is higher such as6 cm or when the tube is not used, the shear stress is low. The sizeof stagnant zone is also large as can be seen from very low values ofshear stress at the ends. The effect of inlet velocity/Reynolds num-ber is shown in Fig. 8b. As expected, at low Reynolds number, thewall shear stress is less when compared with higher Reynoldsnumber flows and it is least when the cell does not contain anytube. The local minimum value of shear stress exists in the centerwhen cell contains the tube. Without the use of tube, the fluidstream is affected by the outlet port on right side. It thereforestrikes the bottom at a location shifted from the center and mini-mum shear stress does not lie at x = 0.

The results of shear stress show that membrane testing cellwith different tube heights or even without the presence of tubecan yield flow situations in the cell that are comparable with thefluid dynamics in large modules and hence are able to produce

sufficiently accurate results. To use the inner tube however ispreferred as its absence results in very low shear stress at themembrane surface. The simulations show that the test device canyield cross flow in the cell without the need of any complicatedconstruction such as fixing of a diffuser ring as used in study ofKawachale et al. [2,3]. The device, in addition, does not involveany complexity of multiple inlet ports and thus double pistonpumps to equally distribute the fluid onto the membrane as sug-gested by Cortés-Juan et al. [6].

To test the grid considered in this work, the average shear stressis determined at bottom surface at different number of cells. Theplot in Fig. 9 shows that shear stress has a very lower value whenthe number of cells in the computational domain are less. Theshear stress value then increases and is almost same for 135,000and 200,000 cells (difference less than 0.5%). This shows that thegrid used for simulations is fine enough to produce accurateresults.

The grids are also compared on qualitative basis in terms of shearstress distribution. Fig. 10 indicates that contours are almost similarand regions of high/low shear stress are same at 135,000 and200,000 cells. This further verifies that grid is adequate for the pres-ent modeling work. The shear stress profiles (in Fig. 10) show a sim-ilar behavior as was noticed in Fig. 8. The shear stress from a verylow value in the center rises rapidly and then gradually decreasesand becomes almost zero at the ends (near the circumference). Pic-ture of the membrane tested in the new membrane device is also in-cluded in Fig. 10 to experimentally evaluate the CFD findings. In anultrafiltration process, it is very well known that the water perme-ates through the membrane while the solutes (impurities) do notpass and mostly carried away by the concentrate stream. However,

Fig. 5. (a) Velocity vectors and (b) contours of z-velocity (normalized by inlet velocity) at the bottom surface (h = 11 cm).

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many of the impurities/particles remain at the membrane surfacethereby resulting in fouling to reduce permeate flux and quality.The image of the membrane shows that retained particles are al-most equally dispersed on the membrane surface. This means thatdespite the fact that tube is oriented perpendicular to the bottomsurface, the flow spreads on the entire membrane surface and per-meation of solution takes place through a major portion of mem-brane area. The region near the circumference/outer portion of thefouled image is the seal which is used to prevent fluid leakage. Thisregion is unfouled and lighter as the permeation does not take placethrough this portion.

The membrane image provides support to the CFD numericalresults which indicates that flow is distributed on major portionof the membrane surface in the cell. The fouling pattern in the im-age shows reasonable agreement with the profiles of shear stressas more fouling is seen on the portion where local shear stress val-ues are significant and vary between 0.4 and 1.25 N/m2. Highershear stress on the membrane surface is generally useful to reduceconcentration polarization and remove foulants as reported in var-ious studies. Lower fouling is usually seen in regions of highershear stress, but the fouling pattern can also depend on the natureof the particles and it is possible to have a different trend as

Fig. 6. Velocity contours in membrane cell: (a) h = 9 cm, (b) h = 6 cm and (c) h = 0 cm.

Fig. 7. Pressure distribution in membrane cell: (a) h = 9 cm, (b) h = 6 cm and (c) h = 0 cm.

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pointed out in some of the previous literature. For example,Fimbres-Weihs and Wiley [9] compared the contours of Sherwoodnumber in their CFD work with particulate fouling patternsobtained experimentally by Neal et al. [18] and found that the loca-tions of higher mass transfer (or Sherwood number) were same aslocations of higher particulate fouling. The most likely reason for

such patterns is the higher weight/density of the particles whencompared to the surrounding fluid [9]. The particles did not followflow streamlines and were retained at the membrane surface dueto inertial effects. The region of higher shear stress can hence besame as the region of higher fouling; that is the portion of mem-brane through which the permeation takes place. The similarity

Fig. 8. Shear stress variation on bottom (a) Re = 800 and (b) Re = 200.

Fig. 9. Effect of number of cells on average shear stress.

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Fig. 10. Contours of shear stress at different grids and image of a fouled membrane.

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of the shear stress contours and fouling patterns in Fig. 10 corrob-orates the accuracy of results in this CFD work and the reliability ofthe testing device.

5. Conclusions

Flow features such as velocity and shear stress are obtained in anew cell developed for membrane testing. The results show thatalthough the orientation of the inner inlet tube is normal to themembrane surface but the direction of fluid flow is mostly tangen-tial. The velocity vectors are seen aligned with the membrane sur-face showing cross flow similar to one present in the large sizedmodules except for a few locations like the center and corners ofthe membrane cell. The comparison of various heights of innertube shows that velocity is lower when the tube outlet is at a largerdistance from the membrane (bottom) and liquid solution dropsfrom a greater height. The shear stress is also lower in this case.Larger tube height is suitable as it increases the wall shear stressand reduces the size of the stagnant zone. The pattern of the depos-ited impurities on an ultrafiltration membrane used in the cell wasfound similar to the shear stress contours obtained on the mem-brane surface using CFD analysis.

Acknowledgement

The authors acknowledge the support provided by UniversityMalaysia Pahang for this work.

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