Separation and Purification Technology 94 (2012) 23–33

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Separation and Purification Technology

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Performance characterization and design evaluation of spinning basket membrane(SBM) module using computational fluid dynamics (CFD)

Debasish Sarkar a,⇑, Ankur Sarkar b, Anirban Roy c, Chiranjib Bhattacharjee b

a Department of Chemical Engineering, University of Calcutta, Kolkata 700 009, Indiab Department of Chemical Engineering, Jadavpur University, Kolkata 700 032, Indiac M.N. Dastur and Company (P) Ltd., P-17 Mission Row Extension, Kolkata 700 013, India

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

Article history:Received 21 April 2011Received in revised form 16 March 2012Accepted 28 March 2012Available online 4 April 2012

Keywords:Spinning basketDynamic shear enhanced modulePermeate fluxCleaning runCFD

1383-5866/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.seppur.2012.03.034

⇑ Corresponding author. Tel.: +91 33 2350 8386; faE-mail address: deba_1s@rediffmail.com (D. Sarka

Performance characterization and hydrodynamic simulation of a newly proposed Dynamic shearenhanced (DSE) module, equipped with an inbuilt cleaning facility has been presented in this article.The module, presently in lab-scale has been named as spinning basket membrane (SBM) module becauseof its inherent geometric similarity with the well known spinning basket reactor. The module perfor-mance was evaluated in ultrafiltration of polyethylene glycol (PEG 6000)/water solution, under differentparametric conditions using polyethersulfone membrane of 5 kDa molecular weight cut-off. A detail CFDsimulation of the proposed device was conducted using the standard two equation k � e model becauseof the existing turbulent flow regime under all possible operating conditions. Subsequently the masstransfer characteristic was explained in the light of prevalent hydrodynamic picture. Moreover, in termsof performance rating, it is to be noted that the final regenerated flux after 21 h of continuous run waswell within 85% of the flux obtained at the time of start-up.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Over the last two decades membrane technology has emergedas an alternative to the conventional separation techniques. Thisis expressed in the vast amount of research, which has gone intodeveloping the right membrane type and module for differentkinds of separation process, developing new processes as well assearching for the best possible circumstances for separation. Theresearch effort finally leads to the present day commercializationof different membrane based process ranging from microfiltration(MF) to reverse osmosis (RO). However, with all its potential theuse of membranes are restricted within small scale applicationsbecause of massive flux decline during the operation [1–4]. Rootcause analysis reveals that the reversible concentration polariza-tion and subsequently the irreversible membrane fouling are thetwo major nonidealities, responsible for flux decline. Additionally,the irreversible fouling (solute adsorption, gel layer formation etc.)reduces the effective lifetime of a membrane [5]. Fouling can bereduced by a number of ways, which includes feed pretreatment;membrane surface modification; back flushing; back washing;back pulsing; use of external force fields (most importantly theelectric field) and many others. A comprehensive overview ofmembrane fouling and subsequent remedial techniques has been

ll rights reserved.

x: +91 33 2414 6378.r).

presented by Hilal et al. [6]. They have also pointed out the mono-tonicity of fouling with concentration polarization. So to limit theeffect of fouling, all the possible means to reduce concentrationpolarization must be explored in addition to the other pre and posttreatments of the membrane module. In the late 1960 cross flowmodule had been developed primarily objective to restrict concen-tration polarization. In a standard cross flow module high feedvelocity induced membrane shear is capable of disengaging the re-jected solute, but at the same time it causes a large axial pressuredrop leading to a ‘non-optimal membrane utilization’ [7]. On theother hand, as the membrane shear is entirely dependent on thefeed flow rate the degree of polarization is generally higher for asmall capacity module otherwise high retentate flow rate willdemand higher energy consumption by the pump.

Dynamic shear enhanced (DSE) modules were first developed inthe mid seventies, primarily with an objective to generate a highmembrane shear independent of the feed flow rate. The first DSEmodule, successfully commercialized for plasma collection fromhuman blood was a Couette flow type device. A cylindrical mem-brane was subjected to rotate inside a coaxial cylindrical housingwith a very small clearance [8,9]. Later on, different DSE moduleswere designed and successfully used for a wide range of applica-tion. Rotating disk/single stirred module was one of the earlyversion of DSE devices, which is still in use, particularly for smallscale operations. An upgraded design of rotating disk modulewas commercialized by ABB Flowtek, the optifilter CR in late

Notations

A total membrane surface area (m2)C0 feed concentration (kg m�3)Cp permeate concentration (kg m�3)e specific energy usage rate (W h L�1)_E total power consumption (W)_Emotor power consumption by motor (W)_Epump power consumption by feed pump (W)g acceleration due to gravity (=9.8 m s�2)Gk turbulent kinetic energy generation rate (kg m�1 s�3)J permeate flux (m3 m�2 s�1)k1 constant used in Eq. (3) (W s rad�1)Q feed flow rate (m3 s�1)r radial coordinate (m)r position vector (m)Re Reynolds number (dimensionless)Rm hydraulic resistance of the membrane (m�1)Robs observed rejection (dimensionless)TMP transmembrane pressure (Pa)

Vr velocity relative to the frame of reference (m s�1)Vh tangential velocity of the basket (m s�1)y+ dimensionless distance from the wall (dimensionless)Greek lettersr

reflection coefficient (dimensionless)q density of feed solution (kg m�3)s stress tensor (Pa)sm local membrane shear stress (Pa)p osmotic pressure (Pa)Dp osmotic pressure differential across the

membrane (Pa)l viscosity of the feed (Pa s)g pump efficiency (dimensionless)X rotational speed of the basket (rad s�1)Subscriptm

membranep permeater retentate

24 D. Sarkar et al. / Separation and Purification Technology 94 (2012) 23–33

nineties. The device is presently being used in the treatment of pa-per-pulp and dye effluents [10]. In order to intensify the shear fieldcompared to rotating disk, rotating membrane based multipleshaft disk (MSD) separator, suitable for large scale treatment wasalso introduced commercially by Aaflowsystems (Aalen-Esingen,Germany). In this device, ceramic membrane mounted parallelshafts are arranged over a cycle [11]. The partial overlapping ofthe membranes mounted on counter rotating adjacent shaftsintensifies the velocity gradient and hence the shear stress onthe membrane surface. Performance characteristics of standardMSD pilot in microfiltration of CaCO3 suspension have been re-ported by Ding et al. [12]. The results of the investigation show thatMSD pilot is capable of producing high permeate flux at highestrotational speed and transmembrane pressure. In a separate studyeffect of membrane overlapping on the performance of MSD pilothas been investigated by Jaffrin et al. [13]. It is reported that themembrane overlapping in general increases the permeate flux,but the rate of increase decreases with increasing rotational speedof the pilot. On the other hand, performance studies of a modifiedMSD pilot with overlapping ceramic membrane and non-permeat-ing disks rotating independently has also been reported [14]. Thecorresponding results show that for vane fitted disks permeate fluxincreases significantly in comparison to the standard MSD pilotand at the same time energy consumption per unit volume of thepermeate reduces significantly. Recently, the performance charac-teristics of nylon membrane fitted modified MSD pilot has alsobeen reported though the specific energy consumption in poly-meric membrane MSD was found to be much higher than theirceramic counterpart [15].

However, with all the advantages, the transient decline of per-meate flux cannot be eliminated to an appreciable limit even inthe most advanced DSE module. For example, a standard rotatingstirrer, placed in closed proximity of a counter rotating membrane,the permeate flux is reported to drop by 30% [16]. It is to be notedthat the flux, once reduced, can only be recovered by exhaustivecleaning of the membrane, which may include chemical processor backflushing/backwashing/backpulsing techniques. Because ofthe particular problem, all the membrane based separation devicesoperate in cyclic mode with normal production run intervened byin-place cleaning run [17]. On the other hand, frequent cleaningmay damage the porous configuration of the membrane, which re-sults in reduced membrane life and selectivity [18–20]. Duration ofcleaning run (tc) is also an important parameter to be optimized.

Several researchers have reported that cleaning time has a benefi-cial effect on flux recovery [21–23], though the effect of an increas-ing cleaning time was found to be decreasing gradually [24].

So in general it becomes clear that if a shear enhanced modulecould be designed, which has the ability of self-cleaning withoutusing any external agents it would be highly advantageous notonly from the perspective of minimizing the operating cost but alsoin relation to ensure an uninterrupted production run. The presentwork has been undertaken in an attempt to design and character-ize the performance of a novel shear enhanced device named spin-ning basket membrane (SBM) module considering its structuralsimilarity with the well known spinning basket reactor. In thepresent study, the operational characteristics of the newly de-signed SBM module have been reported as a function of basketrotational speed (X), applied transmembrane pressure (TMP) andfeed concentration (C0) in ultrafiltration of Polyethylene glycol6000 (PEG 6000) solution in water using Polyethersulfone (PES)membrane of 5 kDa molecular weight cut-off. Additionally, in or-der to analyze the performance of the module during normal aswell as cleaning runs it is important to evaluate the distributionof membrane shear stress and transmembrane pressure. However,the analytic solution of the system hydrodynamics is not availablebecause the system operates totally in the turbulent regime(Re � 105). Accordingly it is necessary to use Computational FluidDynamics (CFD) approach for evaluating the hydrodynamiccharacteristics. In the present work, a CFD based hydrodynamicsimulation has been also included in order to understand the oper-ational performance of the module under different parametricconditions.

2. Materials and methods

2.1. Experimental setup

The proposed module has been designed to be like a spinningbasket reactor with flat membranes (each of dimension65 � 145 mm2 with an effective area of 55 � 130 mm2) fitted onalternate sides of adjacent radial arms, while the other sideremains impermeable as shown in Fig. 1. The hollow basket withfour radial arms (which may be increased in a scaled up module)was mounted on a central hollow shaft fitted with a pulley drive.The whole system with suitable sealing arrangement was placed

Fig. 1. Schematic of the spinning basket module (inset showing the photograph of the spinning basket).

D. Sarkar et al. / Separation and Purification Technology 94 (2012) 23–33 25

in a stainless steel cylindrical tank fed by a triplex piston pump. Inorder to enhance the membrane shear rate, the basket wassubjected to high speed rotation in the direction of the membranesurface area vector (outward normal to the membranes). Thepower requirement for the high speed rotation was provided bya three phase induction motor with a belt pulley drive connectedto the central shaft of the SBM module. The clearance betweenthe tip of the radial arms and the cylindrical housing was 6 mm,which causes additional dynamic pressure built up on the mem-brane surface. As a result the effective transmembrane pressurebecomes higher than the applied transmembrane pressure (TMP)developed by the pump. However, the primary advantage of smallclearance can be explained from the perspective of vortex likecirculation flow generated between two adjacent arms. Becauseof the high velocity circulation of the feed solution accumulatedsolutes on the membrane surface were swept away leading to a re-duced degree of polarization. Though we have not estimated theshear rate in the module, but it is expected to be a monotonicallyincreasing function of basket’s rotational speed.

With all its shear enhancement it is obvious that still the pro-posed module will suffer from the drop of transient permeate fluxfrom the start up to the respective steady state as is the case of anymembrane module irrespective of whether it is shear enhanced ornot. Here comes the uniqueness of the proposed module. Once theflux reaches its steady state the basket is to be rotated in the re-verse direction (in the direction normal to the impermeable sideof the radial arms) after releasing the applied TMP to atmosphericpressure by operating the back pressure regulator (BPR) fitted inthe retentate line. As a result a local vacuum in the order of 1

2 qV2h

will be created on the membrane surface, so that the effectiveabsolute pressure on the retentate side becomes close to Patm � 1

2qV2

h , whereas on the permeate side it will remain atmospheric. Be-cause of the counter rotation induced pressure difference the accu-mulated solute is expected to be disengaged from the membranethereby reducing the hydraulic resistance of the same. This willresult in a recovered permeate flux in the next normal run.

For the present work, the SBM pilot made of SS316 was manu-factured by Gurpreet Engineering Works, Kanpur, UP (India) as per

the specified design. The schematic of the complete filtrationbench is depicted in Fig. 2. The induction motor was fitted with avariable speed drive and a reversing switch for efficient speed con-trol and reversal of the rotational direction, respectively.

2.2. Material

Polyethylene glycol (PEG-6000, AR grade) of molecular weightrange of 5000–7000 dissolved in water was used as feed solutionand was obtained from Merck, India. Moist PES membrane (asym-metric, molecular weight cut-off: 5000 Da) was obtained fromKoch Membrane Systems (USA). The rectangular flat sheet mem-brane was operable in the pH range of 2.5–10.5. Typical operatingpressure for the membrane was 210–830 kPa (maximum operatingpressure- 970 kPa) with operating temperature range of 5–54 �C(maximum operating temperature: 65.5 �C).

2.3. Analysis

Solution concentrations were measured with a refractometer(Brix 0–30%, Erma Inc., Tokyo, Japan). The density and viscosityof the solution were measured by using pycnometer and Ostwaldviscometer, respectively at 303 K.

2.4. Design of experiment

In order to explore the operational characteristics of theproposed module experiments were designed in such a way so thatthe effect of three process parameters, namely the applied trans-membrane pressure (98.0, 196.1, 392.3 and 588.4 kPa), feed con-centration (10, 20, 30 and 40 kg m�3) and rotational speed(10.47, 20.95, 31.4 and 41.9 rad s�1) could be investigated. Anytwo of the parameter were kept constant while the third one is var-ied in order to get the actual nature of the dependence. A constantretentate flow rate of 10�4 m3 s�1 was maintained for all the exper-imental runs. In cleaning run, the module was operated with thesame rotational speed (in opposite direction) as that of the corre-sponding normal run with released transmembrane pressure

Fig. 2. Schematic of the complete filtration bench.

26 D. Sarkar et al. / Separation and Purification Technology 94 (2012) 23–33

(TMP = 0) for a duration of 300 s. It is to be noted that the speed aswell as the duration of the cleaning run was totally arbitrary. Aftereach cleaning run the module was again subjected to normal run.In the present study, the duration of each normal run was fixedto be of 2 h (the duration was determined by the time requiredfor the permeate flux to reach its steady value). It is also to benoted that each continuous experiment marked by a definiteTMP, X and C0 consists of ten normal runs intervened with ninecleaning runs, i.e., with a total duration of 21 h.

In order to compare the performance of SBM with other stan-dard shear enhanced modules experiments with the same mem-brane–solute combination (PES/PEG-6000) have been performedin Single Stirred (SS) and Rotating Disk Membrane (RDM) modules.The detailed specifications and the geometry of these two wellknown shear enhanced modules are described elsewhere [16].For the RDM module, the membrane speed was fixed at its highestpermissible limit i.e., at 62.5 rad s�1. It is also to be noted that theexperiments of SS and RDM modules have been conducted at thesame TMP, X (for both of the modules the stirrer speed was chosento be the parameter equivalent to the basket rotation speed, X forthe sake of comparison) and C0 as that of the proposed SBMmodule.

2.5. Experimental procedure

For the present system four rectangular membranes were fixedon four rectangular porous supports on the alternate faces of fourradial arms. In order to overcome compaction effect of the mem-branes, the module was pressurized with distilled water for at least3 h at a transmembrane pressure of 600 kPa, which was higherthan the highest operational pressure. This was followed by actualexperiments. In order to determine the transient permeate flux10 cm3 of permeate was collected in a measuring cylinder andthe time for collection was recorder. Each normal run was contin-ued until at least two successive flux reading was nearly equal. Asstated earlier each normal run was followed by cleaning run withduration of 300 s. Once an experiment with a tentative duration of21 h was over, all the four rectangular membranes were thor-oughly cleaned with distilled water at least for 2 h to remove any

deposit. The water flux was checked again to detect any variationin the hydraulic resistance of the membrane. The same procedurewas repeated for each experiment with fixed TMP, X and C0. It isalso to be noted that for the purpose of comparison similar exper-imental procedure was followed for the other two mentioned shearenhanced modules [16].

3. Model and methods used in CFD analysis

3.1. Model equations

The equations solved in hydrodynamic simulation are the stan-dard conservation equation of mass and momentum using thecommercial CFD package of GAMBIT and FLUENT. Since the basketrotates through the fluid, the simulation was conducted in rotatingreference frame (RRF) of the basket. The absence of baffles or anyother similar stator arrangement indicates that the RRF, insteadof multiple reference frame (MRF) is the preferred option for treat-ing the flow within the FLUENT environment. As a consequence ofusing RRF, the acceleration of the fluid is augmented by additionalterms that appear in the momentum equation. Assuming the fluidto be incompressible, the steady state, modified conservationequations becomes:

r:V r ¼ 0 ð1Þ

r:ðqV rV rÞ þ 2qX� V r þ qX�X� rþ q@X@t� r ¼ �rP þrs ð2Þ

Where, Vr is the velocity relative to the frame of reference, srepresents the stress tensor, X is the angular velocity of RRF, P isthe pressure and r is the position vector with respect to the RRF.Since the reference frame is non-inertial (i.e., @X

@t ¼ 0), the terms2qX � Vr and qX �X � r are included in Eq. (2) as coriolis andcentrifugal forces, respectively.

3.2. Computational grid and the boundary conditions

The default interior of the SBM module, as shown in Fig. 1 wasmeshed with the help of GAMBIT (version 2.3). The interior as a

D. Sarkar et al. / Separation and Purification Technology 94 (2012) 23–33 27

whole was considered to be the computational domain of thestudy, and it was discretized into arbitrary unstructured polyhedra.The grids were compressed near the solid surfaces in order toensure adequate resolution. The near wall grid resolution tech-nique finally resulted into 676, 864 unstructured polyhedra gridspanning over the default interior of the module. As the flowregime was turbulent, the grids were adapted in order to furtherresolve the flow in the near wall regions. The boundary conditionsas used in the simulation are as under,

� Because of the employed RRF scheme, the basket and the shaftwere set to have velocity. On the other hand, the cylindrical cas-ing of the device was assigned a rotational velocity equal inmagnitude but opposite in sign to the rotation of RRF.� All the solid surfaces were assigned to the well known no-slip

velocity condition.� The feed inlet was specified as pressure inlet type. As the feed

and retentate lines of the module are fitted with pressure regu-lators, the pressure change from inlet to exit was well known.

3.3. Turbulence modeling and solution scheme

The imparted computational domain was solved using FLUENT(version 6.3) using pressure based solver with an implicit formula-tion. The gradient option was Green-Gauss cell based, in which theaverage face value of any flow variable is computed using the val-ues at its straddling cells. The main turbulence model used herewas the standard two equations k � e model [25]. The k � e modelis a semi-empirical model that has been proven to provide engi-neering accuracy in a wide verity of turbulent flows. However,the model may not be well suited for flows in which anisotropyof turbulence significantly affects the mean flow [26]. But the pres-ent computational domain was symmetric and simple in its struc-ture, which restricts the possibility of large anisotropic effect.Accordingly, the choice of k � e model could be justified.

As mentioned earlier, RRF scheme was employed for the pur-pose of simulation, which means a single reference frame attachedto the basket, was considered in solving the conservation equa-tions. Additionally, segregated solution algorithm with implicitsolver formulation was employed. In this formulation each discretegoverning equation is linearized implicitly with respect to thatequation’s dependant variable. This results in a system of linearequations with one equation for each cell in the domain. The pres-sure equation was discretized using PRESTO (PREssure STaggeringOption) scheme, which is most suitable for high-speed rotatingflows. PRESTO scheme uses the discrete continuity balance for a‘‘staggered’’ control volume about the face to compute the ‘‘stag-gered’’ (i.e., face) pressure. For turbulent flows with high Reynoldsnumber, SIMPLEC (SIMPLE-Consistent) discretization scheme isgenerally recommended for pressure–velocity coupling, accord-ingly for the present study the corresponding algorithm has beenused. On the other hand, for momentum, turbulent kinetic energy(k) as well as for dissipation rate (e) second order upwind schemewas selected for the sake of second order accuracy. For quad/hex/polyhedral grids, generally better results are obtained using thesecond order discretization, especially for complex flows [26].

The under relaxation factors were kept at their default values.With the adopted special discretization, and using a 3.07 GHz pro-cessor [Intel� Core TM i3 540 at 3.07 GHz, 4 GB RAM, 500 GB HDD],the simulation time was estimated to be about 11 min.

3.4. Grid adaptation

The grids generated by using GAMBIT, as mentioned in the Sec-tion 3.2 were sufficiently dense; however for highly turbulentregime grid refinements were required to better resolve the flow

in the near wall region. By using solution adaptive refinement, cellswere added where they are required without affecting the otherregions of the flow field. When adaptation is used properly, theresulting mesh is optimal for the solution because the solution it-self is used to determine where more cells are added. For the pres-ent study, Boundary Adaptation approach based on normaldistance has been used in order to ensure that the centroid ofthe first cell near the moving basket and the casing was such thaty+ � 1, which is the optimum for enhanced wall treatment.

4. Results and discussion

4.1. Hydrodynamic simulation

The primary objective of the CFD analysis was to investigate thehydrodynamic condition inside the module and subsequently toexplain its performance under different parametric conditions.Fig. 3 shows the variation of velocity field for the maximum andthe minimum rotational speed condition of the basket (X = 10.47and 41.9 rad s�1) under fixed TMP (=98 kPa). In the plot differentshades represent different velocity magnitudes. Because of theno-slip boundary condition, the velocity was found to be the max-imum at the tip of each radial arm, wherefrom it has progressivelydecreased towards the basket axis. As expected, the local velocitymagnitude was observed to increase with X. For any radial dis-tance, the velocity magnitude was much higher on the membraneside than the corresponding solid side of the same radial arm. Thetrend was similar for all the different rotational speeds. The veloc-ity field was found to remain unchanged with the applied TMP.

Fig. 4 represents the radial variation of the membrane shearstress at X = 10.47 and 41.9 rad s�1 once again. The simulatedstress values were recorded at a definite axial height from the bot-tom of the cylindrical housing of the device (=0.09 m). ForX = 10.47 rad s�1, the local membrane shear (sm) has smoothly in-creased with radial distance, though the rate of increase was highernear the arm tip. On the other hand, for X = 41.9 rad s�1, sm haspassed through a local maxima, close to the central axis of the bas-ket (at r = 0.013 m), thereafter it has again decreased forming aminima (at r = 0.032 m). For r > 0.032 m, a monotonically increas-ing trend was maintained towards the arm tip. This may be attrib-uted to the formation of an intense circulation flow between twoadjacent radial arms due to the high speed basket rotation. Thecorresponding pathlines may have undergone a stretch deforma-tion with the increase of rotational speed. As a result of this defor-mation, it became probable for a set of pathlines, characterized byhigh velocity magnitude to touch the membrane surface at somepoint. At the point of contact, the relative velocity must vanish inorder to satisfy the no-slip boundary condition with respect tothe rotating reference frame. However, in the neighborhood of thatpoint, the local velocity was expected to pretty high as the chosenpathlines were characterized by high velocity. Accordingly, a highvelocity gradient was expected to exist leading to an intermediatemaxima in the shear stress profile at high X. Subsequently with thecontact point, a small wake region was supposed to form in thevicinity of the maximum intermediate shear stress point. It is wellknown that in any wake region, the vorticity tensor must be highenough to locally minimize the associated stretch tensor, whichmust lead to a point of minimum intermediate shear, as observedin the present study.

In addition to the membrane shear stress profile, the variationsof turbulent kinetic energy generation rate (Gk) on the membranesurface with the radial distance for two extreme X values areshown as an inset of the same figure. Gk actually represents a lossin mean kinetic energy and a gain of turbulent kinetic energy. Thekinetic energy is cascaded down from large to small eddies in every

Fig. 3. Velocity profile for (a) X = 10.47 and (b) X = 41.9 rad s�1 under fixed condition of TMP (=98 kPa).

Fig. 4. Variation of the membrane shear stress with the radial distance at X = 10.47and 41.9 rad s�1 (inset showing the variations of turbulent kinetic energy gener-ation rate (Gk) on the membrane surface).

28 D. Sarkar et al. / Separation and Purification Technology 94 (2012) 23–33

turbulent flow. High Gk is therefore very much desirable in a well-designed DSE module as small scale eddies are primarily responsi-ble for the rejected solute scoop-up from the membrane surfaceThis in turn reduces the effect of concentration polarization. Forthe proposed module, Gk was observed to be appreciably high(mean Gk value was around 60 kg m�1 s�3 for X = 10.47 rad s�1,whereas the same was as high as 400 kg m�1 s�3 for X = 41.9rad s�1); which indicates an efficient energy cascading trend ofthe present design.

Fig. 5 represents the contours of dynamic pressure on the bas-ket surface during cleaning runs conducted at two extreme rota-tional speeds (X = 10.47 and 41.9 rad s�1). The figure clearlyindicates that the dynamic pressure on the solid surface was higherthan that on membrane for a specific X. In order to obtain the totalpressure on the membrane surface the dynamic pressure is tosubtracted from the static/thermodynamic pressure as the mem-brane surface becomes the trailing face during a cleaning run. On

the contrary, the same is to be added to the static pressure for eval-uating the total pressure on the solid surface. Accordingly, the totalgauge pressure on the membrane surface must be negative,whereas on the solid surface it becomes positive. As mentionedpreviously and also indicated in Fig. 3 that local velocity magni-tudes were different on the parallel faces of a radial arm. So thecorresponding dynamic pressure distributions must be differentleading to an asymmetric distribution of total gauge pressureabout the zero gauge pressure line. The radial variations of totalgauge pressure at a definite axial height (once again chosen to be0.09 m from the bottom of the module housing) over the mem-brane as well as the solid surface for X = 10.47 and 41.9 rad s�1

are shown as an inset of the same figure. The figure clearly repre-sents the existence of local vacuum over the membrane surfaceduring a cleaning run. The vacuum was found to the more intensenear the central axis and towards the arm tip for both of the rota-tional speeds, though the relative magnitudes were different. So itbecomes evident that the entire membrane surface was notcleansed uniformly. The maximum cleaning efficient region re-mained localized over the central portion of the membrane. How-ever, with all its nonuniformity a moderate degree of cleaning wasachieved in the present study, which was reflected in the highervalue of regenerated flux after each cleaning run compared tothe steady flux at the end of the previous normal run. This indi-cates that the combined effect of local vacuum, membrane shearand turbulent kinetic energy production rate was sufficient enoughto disengage the accumulated solute over the membrane surface,at least for the present solute/membrane combination.

4.2. Variation of the permeate flux with transmembrane pressure(TMP)

At a constant TMP, the unsteady permeate flux presented an ini-tial decay from high initial value over the first 1.5 h, after which ithas leveled off to a steady value. Because of this each normal runwas continued for duration of 2 h. The variation of permeate fluxwith time for different TMP (but at fixed feed concentration androtational speed) for the first three normal runs is shown inFig. 6. The figure indicates that for the first three normal runs

Fig. 5. Dynamic pressure contours on the basket surface during cleaning runs conducted at (a) X = 10.47 and (b) 41.9 rad s�1 (inset showing the variation of total pressure(gauge) on the membrane as well as the solid surface of the basket).

Fig. 6. Variation of the unsteady permeate flux with time under different TMP forthe first three normal runs.

Fig. 7. Variation of steady permeate flux (as obtained at the end of first normal run)with TMP for different rotational speed (X).

D. Sarkar et al. / Separation and Purification Technology 94 (2012) 23–33 29

average drop of permeate flux from the initial condition to thesteady state was nearly 18% for different TMP. The result is obviousbecause over the same duration the effective thickness of thepolarized layer increases from zero to some steady value. Thegrowth of the polarized layer increases the osmotic pressure differ-ential between the retentate and the permeate side of themembrane resulting a reduced permeate flux according to the wellknown osmotic pressure model [27]. On the other hand, Fig. 6 alsoreveals that during any normal run both the transient and the stea-dy permeate flux increases significantly with TMP. In order to bemore informative variation of the steady flux as obtained at theend of first normal run with respect to the applied TMP at differentrotational speeds (X) is shown in Fig. 7. The steady flux wasobserved to increase almost linearly with TMP at different X. How-

ever, the rate of change was more intense on the higher side valuesof rotational speed. It may be attributed to the fact that high rota-tional speed of the basket largely limits the concentration built upon the membrane surface because of the vigorous circulation flowbetween two successive radial arms of the module as indicated inthe Fig. 7. As a result the favoring effect of TMP on the steady fluxbecame more pronounced at high X.

4.3. Cleaning run performance for normal runs conducted at differentTMP

All the cleaning runs were performed at the same rotationalspeed as that of the corresponding normal run with zerotransmembrane pressure. The Fig. 8 shows the variation of the

Fig. 8. Variation of regenerated permeate flux (as obtained after each cleaningruns) with the cleaning run number for normal runs conducted at different TMP.

Fig. 9. Variation of the unsteady permeate flux with time under different feedconcentration (C0) for the first three normal runs.

30 D. Sarkar et al. / Separation and Purification Technology 94 (2012) 23–33

regenerated permeate flux after every cleaning run for normal runsconducted at different TMP. After the first cleaning run the averagerecovery of permeate flux (described as Jrecovered�Jsteady

Jsteady� 100) is nearly

19%, which means the regenerated flux was nearly 96% of the ini-tial flux, whereas the corresponding steady flux at the end of thefirst normal run was on an average 82% of its initial counterpart.The fact clearly indicates that the combined effect of generatedvacuum, membrane shear and the turbulence production rate(Gk) on the membrane surface could act even at the length scaleof solute adsorption, which is definitely a molecular scale. And thatis why energy cascading becomes so important in the presentmodule because more be the available energy at the minimumlength scale more be the degree of cleaning leading to a higherregenerated flux in the subsequent normal run. If the variation ofthe regenerated permeate flux from one cleaning run to the nextis investigated, as shown in Fig. 8 it becomes clear that after 9thcleaning run, the average value of the regenerated flux was morethan 90% of the average initial flux (i.e., the average flux at thebeginning of the first normal run) for different TMP. This marks aunique feature of the proposed module and probably no existingmodule can restrict the drop of permeate flux within 10% of the ini-tial value for a total effective run time duration (total normal runtime in an experimental run) of 19 h, which corresponds to ninenormal runs with nine intermediate cleaning runs.

Regarding the cleaning mechanism, it is to be noted that if thesolute adsorption on the membrane surface is irreversible (whichis definitely not the case for PEG 6000/PES combination) and theheat of adsorption is high like the case of protein, the created vac-uum as well as membrane shear and turbulent generation rate maynot be sufficient to reduce the adsorbed layer thickness. In thatcase, the cleaning run would not remain as efficient as in the pres-ent case; still the module is expected to function well from theviewpoint of shear enhancement.

4.4. Variation of permeate flux with feed concentration

Variation of the permeate flux with time for four different feedconcentrations over the first three successive normal runs are asshown in the Fig. 9. The initial flux values were same for four con-centrations indicating no initial polarization effect. The flux behav-ior was exactly consistent with the trends predicted by osmoticpressure model. As the initial osmotic pressure differentialDp = pr � pp was very small compared to the applied transmem-brane pressure the initial flux, J(0) became independent of the feedconcentration as Jð0Þ ¼ TMP

lRm, where Rm is the hydraulic resistance of

the membrane. With time osmotic pressure differential has in-

creased due to accumulation of the rejected solute on the retentateside leading to a reduced driving force (=TMP � rDp) and hence areduced permeate flux. So the initial sameness of the permeate fluxwas lost and the relative difference was increased unless and untilthe steady state was reached, where osmotic differential has lev-eled off to definite but different values marked by different feedconcentration. In the first normal run, the initial flux has decreasedby 14% for a feed concentration of 10 kg m�3, where as a 25% de-crease was observed for the highest feed concentration of40 kg m�3. This was evident as Dp is a monotonically increasingfunction of feed concentration.

Regarding the cleaning run performance it may be noted that abetter recovery was observed for the cases of high feed concentra-tion. A closer investigation reveals that 21% flux recovery wasachieved after the first cleaning run for normal run conducted witha feed concentration of 40 kg m�3; whereas the same was only 12%for 10 kg m�3. This establishes that the disengaging capacity of thecleaning run was practically independent of the amount of accu-mulated solutes at least for the specific feed concentration range,which also indicates a complete reversibility of solute adsorptionfor the present solute-membrane combination during normalrun. The variation of the regenerated flux with different cleaningruns has not been shown separately because the trend is expectedto be similar as shown in Fig. 8.

4.5. Variation of permeate flux with rotational speed and thecharacteristics of the subsequent cleaning runs

The variation of permeate flux with time for different rotationalspeed of the basket (X), but under constant TMP and feed concen-tration for the first three normal runs is shown in Fig. 10. Contraryto Fig. 9 the initial fluxes were different for different rotationalspeed. This can be explained from the perspective of increasing dy-namic pressure and turbulence generation rate with rotationalspeed as shown in Fig. 4. So it becomes obvious that in general ahigher permeate flux is expected for the case of a high rotationalspeed. In the first normal run 115% steady flux enhancement wasobserved for a rotational speed change from 10.47 to 41.9 rad s�1.Whereas for the second and the third normal runs the changeswere 117% and 113%, respectively for the same change of rota-tional speed. This indicates that the effect of rotational speedwas practically independent of the number of normal runs at leastfor first three runs. On the other hand, Fig. 10 once again revealsthe efficient characteristics of the cleaning runs. As it can be clearlyobserved that on an average the flux recovery was nearly 18%,

Fig. 10. Variation of the unsteady permeate flux with time different rotationalspeed (X) for the first three normal runs.

Fig. 11. Variation of regenerated permeate flux (as obtained after each cleaningruns) with the cleaning run number for normal runs conducted at differentrotational speed (X).

Fig. 12. Variation of steady state observed rejection (Robs) with TMP at differentrotational speed (X) (insert showing the same variation with X at different TMP).

D. Sarkar et al. / Separation and Purification Technology 94 (2012) 23–33 31

which means that for all the three normal runs the average regen-erated flux was roughly 98% of the initial flux values of the corre-sponding runs. The details of the cleaning run characteristics fornormal runs performed at different rotational speeds has beenshown in Fig. 11. The regenerated permeate flux after 9th cleaningrun was observed to have nearly the same kind of dependence onthe rotational speed as the initial flux. A closer investigationreveals that for an increase of rotational speed from 10.47 to41.9 rad s�1 the regenerated permeate flux after 9th cleaning runwas increased by 120%. In order to analyze the cleaning run perfor-mances it is to be noted that regenerated permeate flux after the9th cleaning run was found to be 86% of the initial flux forX = 10.47 rad s�1, whereas the same was 92% for X = 41.9 rad s�1.This is obvious as for the first case all the cleaning runs were con-ducted at 10.47 rad s�1, whereas for the second it was operated at41.9 rad s�1. As a result the local vacuum created on the membranesurface during the second case was nearly 10 times higher thanthat in the first (clearly indicated in Fig. 5) leading to a higher dis-engagement rate.

4.6. Variation of the ‘observed rejection’

Variation of the observed rejection (Robs), defined asRobs � 1� Cp

C0under steady state, i.e., at the end of first normal run

with TMP at different rotational speed is shown in Fig. 12. Anincreasing trend of Robs was observed with TMP. This can be ex-plained in terms of increased permeate flux, because of increasedpressure of course through the membrane leading to a higher sol-ute rejection at higher TMP. Further, the rejected solute forms anadditional mass transfer resistance, which acts like a secondarymembrane in series with the actual membrane. This phenomenonpromotes higher rejection at increased TMP because high TMP notonly increases the permeate flux but also induces higher compac-tion of the deposited layer and thereby increases the rejection. Acloser investigation reveals that the rate of change of observedrejection with respect to the applied TMP was much higher onthe higher side values of rotational speed (Robs was observed to in-crease by 19% for the change of TMP from 98.0 to 588.4 kPa atX = 41.9 rad s�1, where as for the same change of TMP, the changeof Robs was only 9.5% at X = 10.47 rad s�1). The trend can be ex-plained in terms of the nature of variation of Robs with respect toX, as depicted in the insert of the same figure (Fig. 12). High speedbasket rotation was found to reduce Robs, which could be attributedto a reduced polarized layer thickness at higher X. This phenome-non establishes the shear enhancing character of the SBM moduleas indicated by the membrane shear stress profile shown in Fig. 4.Rotation effect was observed to be more pronounced in differenti-ating Robs at different X on the lower side values of the appliedTMP. However, with the increase of TMP, the relative differenceof Robs was observed to decrease because the TMP has a moredirect influence on the polarized layer thickness and on the sec-ondary mass transfer resistance, thus on Robs than X. Though notseparately studied but the observed rejection is supposed toincrease with the feed concentration as the deposited solute layerwill be more compact and dense at higher feed concentrations.

4.7. Energetic considerations

The total electrical power consumed was measured with a watt-meter. The total power is the sum of power supplied to the feedpump and to the induction motor. The variation of specific energyusage (e), defined as the total electrical power supplied divided bythe permeate flow rate with rotational speed at different TMP is asshown in Fig. 13. Variation of the same with TMP but at different Xhas been also depicted as an inset of the same figure. The figuresclearly indicate that the specific energy usage of the proposedmodule was a monotonic function of TMP and X as well. In partic-

Fig. 13. Variation of specific energy usage (e) with rotational speed (X) at differentTMP (insert showing the same variation with TMP at different X).

32 D. Sarkar et al. / Separation and Purification Technology 94 (2012) 23–33

ular, the specific energy usage was observed to decrease with TMP,while with X, it was increasing. The figure also indicates that bothof the trends are nearly linear (the average correlation coefficientwith TMP was 0.94, whereas with X the same was 0.96). It is wellknown that for an induction motor the power consumption isproportional to the rotational speed. So for the present case_Emotor ¼ k1X, where k1 is a constant. On the other hand, for a triplexpiston pump the power requirement ð _EpumpÞ can be expressed as_Epump ¼ gQ�TMP

g , where g is the pump efficiency and Q is the feedflow rate. Accordingly, the total power requirement becomes:

_E ¼ _Emotor þ _Epump ¼ k1XþgQ � TMP

g

) e ¼_E

Q Per¼

_EJA¼

_Emotor þ _Epump

JA

¼ k1XJðTMP;C0;XÞA

þ gQ � TMPgJðTMP;C0;XÞA

ð3Þ

QPer is the total permeate flow rate, accordingly QPer = JA, where J isthe permeate flux and A is the total membrane surface area. FromEq. (3) one can obtain:

@e@ðTMPÞ ¼

gQgJA� e

J@J

@ðTMPÞ ð4aÞ

and

Table 1Comparison of the proposed SBM module with the reported shear enhanced membrane m

Module Membrane/feed combination Steady peflux � 105

Vibratory shear enhancedprocess (VSEP)

Ceramic membrane (plane, MWCO150 kDa)/low heat skim milk

1.2

Rotating disk membrane(RDM)

UF membrane/diluted skim milk (1:2) 5.9

Vibratory shear enhancedprocess (VSEP)

Desal AG RO membrane/skim milk 5

Rotating disk membrane(RDM)

NF membrane/diluted skim milk (1:2) 11.1

Vibratory shear enhancedprocess (VSEP)

UF membrane/ UHT skim milk 1.54

Spinning basketmembrane (SBM)

Polyethersulfone (PES) membrane(MWCO5 kDa)/PEG 6000/water

11.4

@e@X¼ k1

JA� e

J@J@X

ð4bÞ

Subsequently from Eq. (4a) and b the pure second order deriv-atives become:

@2e

@ðTMPÞ2¼ � e

J@2J

@ðTMPÞ2ð5aÞ

and

@2e

@X2 ¼ �eJ@2J

@X2 ð5bÞ

From the data represented in Figs. 7 and 9, it becomes evidentthat for the present module, the permeate flux was a nearly a linearand monotonically increasing function of TMP and X, at least with-in the framework of the present experiment (for example, the aver-age correlation coefficient for the variation of steady permeate fluxas obtained at the end of the first normal run with TMP was 0.93

and the same with X was as high as 0.98). So @2e@ðTMPÞ2

� @2e@X2 � 0 as

@2 J@ðTMPÞ2

� @2 J@X2 � 0, which is exactly consistent with Eq. (5a) and b.

Accordingly, it may be concluded that the energy usage rate (e)must vary linearly with both TMP and X, provided Q remains un-changed. For the present study as mentioned earlier the retentateflow rate was fixed at 1.6 � 10�4 m3 s�1, where as the maximumsteady permeate flow rate was 1.3 � 10�5 m3 s�1, which is negligi-ble compared to the retentate flow. Hence the feed flow rate (Q),which is sum of permeate and retentate flow rates may be consid-ered to remain unchanged for all the experimental runs.

DSE modules in general consume higher energy compared totheir cross flow counterpart. Primarily for this particular limitationDSE modules are still not widely accepted as it was expected to be.So for any newly proposed shear enhanced module energy costshould be considered seriously. A legion of studies on DSE moduleswith different types of solute/membrane combination was re-ported. The performances of some of them in terms of permeateflux and power usage rate are reviewed here in comparison tothe proposed SBM module and are presented in Table 1 [28–32].Amongst the other studies, the highest permeate flux as reportedby Luo et al. [31] was 11.1 � 10�5 m3 m�2 s�1, at the cost of1.3 kW power usage rate, in the nanofiltration of diluted skim milkusing a standard Rotating Disk Membrane (RDM) module. Com-pared to the reported RDM module, the permeate flux, as obtainedfrom the proposed module was nearly 10% higher, whereas theenergy usage rate was found to be 28% lower. Additionally, forthe flux recovery, a standard RDM unit would require either chem-ical cleaning or periodic back washing. On the contrary, in theproposed module, the flux recovery was achieved by an inbuiltmechanism and membrane cleaning was accomplished withoutthe frequent use of cleaning agents or washing liquid. From the

odules in terms of power consumption and permeate flux.

rmeate(m3 m�2 s�1)

Motor speed(rpm)

Maximum powerconsumption (kW)

Reference

2000 36.9 Grangeon andLescoche [28]

2500 1.4 Ding et al. [29]

2000 18.15 Frappart et al. [30]

2500 1.3 Luo et al. [31]

2000 15.41 Akoum et al. [32]

400 0.94 Present study

D. Sarkar et al. / Separation and Purification Technology 94 (2012) 23–33 33

foregoing discussion, it is clear that the proposed SBM module canperform efficiently than the other existing DSE units.

5. Conclusion

The design and performance characterization of a novelDynamic shear enhanced (DSE) module with inbuilt cleaning facil-ity has been proposed in this article. The module, which principallycontains a hollow shaft mounted spinning basket with four radialarms, has been named as spinning basket membrane (SBM)module. Experiments under different parametric conditions oftransmembrane pressure, rotational speed of the basket and feedconcentration were conducted in ultrafiltration of polyethyleneglycol 6000 (PEG 6000) with polyethersulfone (PES) membraneof 5 kDa molecular weight cut-off. The results showed that the fluxdecline could be restricted within 12% of its initial value after 21 hof continuous operation. The module was periodically operated incleaning mode, simply by reverse rotation of the basket withreleased transmembrane pressure avoiding frequent chemicalcleaning or backwashing technique adopted in conventional mem-brane separation units. The performance comparison revealed thatin the proposed device higher permeate flux could be obtained atthe cost of lower energy usage rate compared to the other existingDSE modules. This represents the novelty of the present design.Moreover, in order to investigate the hydrodynamic characteris-tics, a detail CFD analysis of the SBM module was conducted usingthe commercial CFD software package FLUENT, with the computa-tional grids generated from the pre-processor package GMBIT. Thesimulation clearly establishes that the proposed module is able togenerate high shear and turbulence generation rate at the mem-brane surface. This actually restricts the growth of polarized layerduring normal operation and disengages the accumulated solutefrom the membrane surface during the subsequent cleaning oper-ation. Considering the experimental results and CFD simulation itmay be concluded that the proposed SBM module, after properscale-up may serve the membrane based industries as a uniquecost effective device.

Acknowledgments

This work was carried out utilizing the infrastructures devel-oped under the Project, entitled ‘‘Development of Novel High ShearMembrane Module’’, funded by Department of Science and Tech-nology (DST), Govt. of India, under SERC scheme (vide sanctionletter no. SR/S3/CE/058/2009 dated December, 2009). The contri-bution of DST is gratefully acknowledged.

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