Investigation of hybrid spring-membrane system for fouling control

Desalination 276 (2011) 117–127

Contents lists available at ScienceDirect

Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Investigation of hybrid spring-membrane system for fouling control Nina Zhou a,⁎, A.G. Agwu Nnanna b a b

Department of Mechanical Engineering, Water Institute, Purdue University Calumet, 2200 169th Street, Hammond, IN, 46323, USA Department of Mechanical Engineering, Water Institute, Purdue University Calumet, 2200 169th Street, Hammond, IN, 46323, USA

a r t i c l e

i n f o

Article history: Received 2 December 2010 Received in revised form 9 March 2011 Accepted 11 March 2011 Available online 19 April 2011 Keywords: CFD Experiments Flow pattern Membrane fouling Flux enhancement

a b s t r a c t This paper presents numerical and experimental investigation of flux augmentation in hybrid springultrafiltration membrane system. The numerical simulation is based on the Eulerian multiphase model. Momentum exchange coefficient was used to account for interaction between the liquid–solid phases. The helical spring insert was modeled as filaments with defined pitch. The experimental facility consists of a two-pass spring-membrane system subjected to iso-transmembrane pressure and inlet velocity. The wall velocity, shear stress and turbulence kinetic energy (TKE) were greatly enhanced and varied intensely by the spring insert. These fluctuations and resulting scouring forces deterred particle deposition on the membrane surface, while the generation of eddy currents after each filament effectively increased local mixing and suppressed development of concentration polarization layer. The simulation results revealed that the volume fraction (VF) distribution was non-uniform between two filaments. Flux was improved by 25%; and the spring diameter can be up to 30% of membrane internal diameter without incurring penalty in pressure drop. Furthermore, observation from a membrane autopsy revealed non-uniformity in buildup of fouling; the fouling pattern mimicked the helical spring contour. Analysis of the membrane surface showed that approximately 60% of the surface was un-fouled. This experimental evidence is consistent with numerical results. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Tubular ultrafiltration (UF) membrane systems are commonly used for water treatment. They can sufficiently remove suspended solids in wastewater, and it is considered as an economical and attractive process. However, the main challenge for the wide application is fouling which is due to the concentration polarization and fouling development caused by particle accumulation on the membrane surface, which reduces productivity and membrane performance. Regular methods [1–6] such as pretreatment, periodic back washing, forward washing, chemical cleaning, membrane surface modification and physical cleaning using ultrasonication are considered to be either time- or capital-consuming. There is a need for improved technologies to control particulate fouling and for improved understanding of fouling characteristics associated with various membrane treatment systems and source water [7]. By far, a lot of works were reported to mitigate fouling by different hydrodynamic approaches. The hydrodynamic methods such as flow pulsations for creating unsteady flow were applied [7,8]. Wayne [7] used the frequent and periodic reversal of the transmembrane pressure (TPP) to reduce flux resistances, the results reveals that the crossflow shear rates did not significantly affect the non-pulsed permeate flux, TPP can significantly reduce membrane fouling. The ⁎ Corresponding author. Tel.: + 1 219 989 4128; fax: + 1 219 989 2898. E-mail addresses: [email protected] (N. Zhou), [email protected] (A.G.A. Nnanna). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.03.034

other reported technique is based on the use of inserts [8–10]. Gupta et al. [8] used helical baffles in mineral membrane by filtrating bakers’ yeast and dodecane-water emulsion. Their results showed more than 50% increase in permeate flux, and that flux is enhanced with increase in helices number. Furthermore, reduction in diameter of baffle by 40% resulted in a small variation in permeate flux. Ahmad et al. [10] used a helical baffle for cross flow microfiltration to reduce fouling. They reported that helical inserts resulted in increase fluid velocity and wall shear rates and produced secondary flows or instabilities, and flux enhancement more than 100%. Their study suggests the flux augmentation is a function of the number of turns, and decrease in flux was observed when the number is more than 4 turns per 50 mm baffle length. Liu et al. [11] evaluated the effect of axial baffles in noncircular channeled membranes on the critical flux. Cylindrical rod, helical baffles and alternating direction helical baffles were used, the results indicated that the introduction of alternating direction helical baffles led to the greatest improvements due to the turbulence generated and mixing in the flow channel. In recent years, Computational Fluid Dynamics (CFD) has emerged as a powerful tool in membrane science field, it provides insight into the phenomena taking place inside membrane modules, to assist the design processes and improve the performance of membrane modules [12]. Berman [13,14], and Yuan et al. [15] were the first to solve the Navier–Stokes equations for a laminar flow in a porous slit and in a porous tube, respectively. They assumed that the axial flow was fully developed and that the shape of the non-dimensional velocity profile was invariant with the axial distance. Ghidossi et al.

118

N. Zhou, A.G.A. Nnanna / Desalination 276 (2011) 117–127

Nomenclature D df dp CD C1ε C2ε Cμ f Gk,m Kpq k P Pp Re TMP → Vp → V q

→ Vm αp αq ε μ μq μ t,m ρm ρp ρq σk σε τp

tube diameter, m filament diameter, m particle diameter, m drag coefficient constant in the k–ε model constant in the k–ε model constant in the k–ε model drag function production of turbulence kinetic energy, m2/s2 momentum exchange coefficient, kg/m3·s turbulent kinetic energy, m2/s2 pressure, kPa pressure of particle, kPa Reynolds number transmembrane pressure, kPa velocity of particle phase, m/s velocity of liquid phase, m/s mixture velocity, m/s volume fraction for particle phase volume fraction for water phase turbulent dissipation rate, m2/s3 viscosity, Pa·s particle viscosity, Pa·s turbulent viscosity, Pa·s mixture density, kg/m3 particle density, kg/m3 liquid density, kg/m3 constant in the k–ε equations constant in the k–ε equations particulate relaxation time, s

[16] simulated a hollow fiber ultrafiltration membrane. They showed the flexibility of the relation developed for a wide range of conditions. Wardeh et al. [17] developed FORTRAN code and simulated the concentration polarization in spacer-filled channels. Liu et al. [11] simulated the turbulent flow in baffle-filled membrane tubes they inserted central baffle and wall baffle periodically in membrane tubes. Their results show that the turbulence as well as wall shear stress fluctuated due to the baffles. Ahmad et al. [18] investigated different spacer filament geometries on concentration polarization control in narrow membrane channel by CFD method, the results suggested that triangular filament has the highest degree of concentration minimi-

zation ability and pressure drop, the criterion of optimum spacer's geometries is found to be dependent on feed Reynolds number. Based on three-dimensional CFD simulations, Shakaib et al. [19] revealed significant influence of spacer geometric parameters such as filament spacing, thickness and flow attack angle on mass transfer in spacerobstructed membrane feed channels. Results revealed the maximum mass transfer coefficient and minimum shear stress occur near reattachment point, the diamond-shaped spacers with large bottom filament spacing was considered to be the better one. Shakaib et al. [20] carried out three-dimensional CFD simulation to study impact of different geometric parameters of parallel type spacers in spacerobstructed feed channels of membrane elements on fluid flow behavior. They found that the velocity profiles and average shear stress values significantly depended on parameters such as transverse filament spacing and filament thickness. Santos [21] investigated flow pattern and mass transfer in membrane module channels filled with flow-aligned spacer using CFD, the results showed that the transition flow regime could be determined for the spacers, besides, it was found that the presence of longitudinal filaments was shown to not significantly affect the flow structure; the modified friction factor could be used for selecting the best spacer in terms of mass transfer efficiency. Cao [22] used CFD to obtain fluid flow pattern in spacerfilled narrow membrane channel with three different traverse filament arrangements. The results indicated the regions where high shear stress occurred and eddies appeared resulted from presence of spacer cylinders. The increase in mass transfer on membrane surface directly related to high shear stress value, velocity fluctuation and eddy formation. Li [23,24] carried out both simulation and experiments to optimize and design spacers. They reported the effect of concentration pattern and mass transfer coefficient distribution on the surface of membrane feed channels. A review of literature revealed that most of the numerical works [17–20,22] were not experimentally validated, and some works were reported and mainly studied the qualitative and quantitative properties of fluid dynamics in membrane system, however, the multiphase model was not applied to take into account the mass transfer and the contaminants concentration distribution [12,18,20,22]. For practical application of baffle-membrane system, experimental data is needed to confirm the numerical results. Reported studies of baffle-membrane systems have also shown that it presents manufacturing challenges and leads to pressure drop due to surface modifications [8–12]. To the author's knowledge little or no information has been reported in open literature on flux enhancement and fouling control using spring inserts. The aim of this numerical and experimental study is to investigate fouling mechanism in the spring-membrane system. The presence of spring induces turbulence; alter flow pattern and rate of particle deposition on membrane surface. The numerical model takes into

Fig. 1. Representation of the geometry considered.

N. Zhou, A.G.A. Nnanna / Desalination 276 (2011) 117–127

account the solid–liquid–membrane interplay and provides insight on the effects of wall velocity, shear stress, and turbulence kinetic energy on fouling phenomena. Numerical parametric studies were performed to correlate spring characteristics with membrane performance.

119

2. Computational fluid dynamics model The system under investigation is equipped with an inside-out ultrafiltration membrane. The feed water is a solid/liquid mixture with particle fraction of 2% driven by externally applied pressure.

Fig. 2. Comparison of wall and porous media boundaries. (a) Velocity contour for wall and porous media boundaries. (b) Pressure distribution along centerlines of two channels. (c) Velocity profile along the vertical lines at the middle of two channels.

120

N. Zhou, A.G.A. Nnanna / Desalination 276 (2011) 117–127

The Eulerian multiphase model is used to describe the feed water as continua with phase volume fractions, αp and αq for the particle and water phase respectively; where, αp + αq = 1. Assuming incompressible Newtonian flow, the continuity of mass and momentum transport relations for the particle phase is shown in Eqs. (1) and (2),
 ∂ αp ρp ∂t


 vp = 0 + ∇⋅ αp ρp →


 ∂ αp ρp → vp ∂t

ð1Þ



 2→ vp → vp = −αp ∇P−∇Pp + μ ∇ vp + ∇⋅ αp ρp →
 v −→ v +K → pq

q

ð2Þ

p

The water phase is described by equations similar to Eqs. (1) and (2). The inter-phase momentum interaction is given in Eq. (3), The Schiller and Neumann model [25] is employed to describe the drag force resulting from the presence of particles in the feed. This is expressed as Momentum exchange coefficient αp ρp f τp

Kpq =

ð3Þ

where τp =

ρp d2p 18μ q

ð4Þ

Drag function f =

CD Re 24

ð5Þ

Drag coefficient

CD =

 8
0:687 > < 24 1 + 0:15Re > :

Re 0:44

Re ≤ 1000

ð6Þ

Re N 1000

Eqs. (3)–(6) are combined and substituted into Eq. (2), to produce the velocity profile. The Reynolds number for the flow system is 1.31 × 1010 hence the standard k–ε mixture turbulence model was employed to account for the turbulence effects. The turbulent kinetic energy, k, and turbulent dissipation rate, ε, equations describing this model are  

 μ t;m ∂ðρm kÞ → ∇k + Gk;m −ρm ε + ∇⋅ ρm vm k = ∇⋅ σk ∂t

ð7Þ

 
  μ t;m ∂ðρm εÞ ε
C1ε Gk;m −C2ε ρm ε vm ε = ∇⋅ ∇ε + + ∇⋅ ρm → k σε ∂t

ð8Þ

Eqs. (9)–(11) are substituted into Eqs. (7) and (8). The resulting equations yield turbulence kinetic energy distribution and dissipation rate for the system. The Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) first order accuracy numerical algorithm is used in the simulation. A refined mesh was applied near the boundary of the membrane to improve computational accuracy. 2.1. Model geometry and boundary conditions The membrane system was modeled as 2-D (two-dimensional) geometry in the R–Z plane, and is axi-symmetric about its centerline. As shown in Fig. 1, the simulated flow domain is two concentric tubular channels with the length of 320 mm and inner diameter of 11.93 mm. The extended tubes having a length of 160 mm are attached to both the inlet and outlet of membrane tubes. Four membrane surfaces labeled “mem1, mem2, mem3 and mem4,” are considered as the upper and lower surface of the membrane channels in the 2-D geometry. The empty and the spring-membrane channels are represented by the cross sections of the membrane tubes as depicted in Fig. 1. The helical-membrane tubes are represented by rectangular channels filled with circular shaped spacer filaments, these filaments were located on upper and lower surface in staggered arrangement. The filament or wire diameter d is 0.2 mm, the pitch, l between two filaments on the same membrane surface is 50 mm, and the spring diameter is approximately 11.9 mm. The permeate flux is typically much less than the feed flow rate. For this reason, the membrane wall was modeled as an impermeable boundary [12,20,22]. This assumption was validated in this work through preliminary numerical experiments of non-porous and porous membrane surfaces. The permeability is 1.0 × 10−6 m2 which is of the same order of magnitude as commercially available UF membranes. Data in Fig. 2a reveals that the velocity profile for the permeable boundary mimics that of impermeable surface. The permeate is less than 2% of the feed flow rate. This suggests that permeate velocity has negligible effect on velocity distribution inside the membrane channels. The pressure profiles in Fig. 2b show a difference of 1.3%, and the velocity profiles along the vertical lines located in the middle of the channels show the perfect match for the different boundary conditions (shown in Fig. 2c). Based on the negligible difference in pressure and velocity, a non-permeate membrane wall is assumed in this study. The no-slip boundary condition is assigned on the filament wall. A constant inlet velocity and pressure of 2 m/s and 138 kPa, respectively are maintained in the membrane channel. 3. Experimental facility Fig. 3 presents the schematic of the experimental setup. The membrane is a cross flow tubular PVDF ultrafiltration module

The mixture density and velocity, ρm and vm are expressed in Eqs. (9) and (10) ρm = αp ρp + αq ρq → → αp ρp vp + αq ρq vq → vm = αp ρp + αq ρq

ð9Þ ð10Þ

The turbulent viscosity μ t, m is given in Eq. (11), μ t;m = ρm Cμ

k2 ε

ð11Þ

Fig. 3. Schematic of the filtration system. (1) Feed tank with clay water; (2) Diaphragm pump; (3) Pressure gauge; (4) Stainless steel membrane test case with membrane tubes; (5) Flow control valve; (6) Graduate cylinder.

N. Zhou, A.G.A. Nnanna / Desalination 276 (2011) 117–127

fabricated by PCI Membranes Ltd. It has a Molecular Weight Cut-off of 100 kDa, inner diameter of 11.93 mm, length of 300 mm, and active filtration area of 240 cm2. A stainless steel compression spring fab-

121

ricated by Central Spring Corporation is inserted into the membrane. The spring and wire diameters are 10.67 mm and 1 mm, respectively; and it has a total of 95 coils, a pitch of 2 mm, stiffness of 55.2 N/m,

Fig. 4. (a) Transient flow domain. (b) Velocity and particle VF profiles for different locations in empty channel. (c) Velocity and particle VF profiles for different locations in springmembrane channel. (d) Vectors of the fluid, colored by particle VF.

122

N. Zhou, A.G.A. Nnanna / Desalination 276 (2011) 117–127

and 270 mm long. The spring-membrane unit is installed into a stainless steel module, MIC-RO 240, manufactured by PCI Membranes Ltd. The MIC-RO 240 is a one shell pass/two tube pass module. This configuration provides an opportunity for installation of two tubular membranes in series hence increasing the permeate flux. The feed water flows in the tube-side into the spring-membrane unit while the permeate exits the shell-side via a port with internal diameter of 9.5 mm. The feed water is a synthetic wastewater prepared under standard laboratory conditions. It is a mixture of clay soil with natural organic matter, NOM, and municipal tap water. The mixture is agitated to facilitate homogeneity and uniform dispersion of NOM. Through sedimentation pretreatment process, supernatant with particle volume fraction of 2% is collected as feed to the membrane system. Supernatant in the feed tank is propelled into the springmembrane channel by ALL-FLO 3/8″ pump with maximum volumetric flow rate of 34 l/min. Periodic agitation of the supernatant was performed to diminish potential for sedimentation. Transmembrane pressure was monitored using diaphragm pressure gauge, GP200E, with a measurement range of 0–1379 kPa ± 2.5%. The permeate volumetric flow rate was measured and recorded for further data reduction while the retentate is re-circulated to the feed tank. The test matrix is designed to address the influence of a spring on the development of fouling and flux augmentation. A comparative analysis of fouled membrane surfaces was performed for test conditions of empty- and spring-membranes. The experiments began by setting the transmembrane pressure at 345 kPa and the cross flow velocity at 1.03 m/s. The pump is activated and baseline data is collected using municipal tap water. This is followed by sets of experiments using supernatant. A set consisted of three tests conducted under the same conditions to ensure reproducibility of measured data. The duration of each run and permeate sampling rate is 2 h and 10 min, respectively. After each run, the membrane was regenerated using clean water, an immersible ultrasonic transducer, and sodium hydroxide solution. The ultrasonic transducer produces micro bubbles and waves near membrane surface to dislodge particulates in the pore. The main source of uncertainties is error due to the measurement of pressure, flow rate, and physical dimensions. The error associated

Fig. 6. Wall velocity profiles for empty channel and spring-membrane channels with different filament diameters.

with pressure is ±2.5%, the caliper for measurement of physical dimensions has ±0.5% error. 4. Results and discussions 4.1. Numerical simulation The transient simulation presented in the following section is to obtain the detailed understanding of the particle deposition mechanism as well as the relation between the changes of flow pattern and the particle migration. The transient simulation reaches steady state after sufficient time. The steady state simulation provides the overall information for the empty channel and spring-membrane channel as well as the effect of filament diameter and pitch. 4.1.1. Transient simulation The transient simulation was conducted in order to investigate the mechanism of particle deposition process on membrane surface. The simulation domain shown in Fig. 4a has a total length of 110 mm, and it is selected as a portion of entire system depicted in Fig. 1, the

Fig. 5. Contours of velocity magnitude for empty and spring-membrane channels at 2 m/s.

N. Zhou, A.G.A. Nnanna / Desalination 276 (2011) 117–127

123

Fig. 7. Filament pitch effect. (a) Contour of velocity distribution. (b) Contour of particle VF distribution.

distance between the inlet and the first filament is 50 mm, and the pitch between two filaments is 30 mm. A sequence of points labeled 1–5 are along the line which is 0.5 mm away from the lower surface of the tube, these points are 3 mm, 10 mm, 15 mm, 25 mm and 28 mm away from the first filament and shown in Fig. 4a. Two cases: empty channel and the spring-membrane are compared to investigate the effects of a spring. Initially, at t = 0, the feed fluid has 0.2% particles starts to flow into the channel. The inlet velocity is 2 m/s and the outlet pressure is 138 kPa. The historical data for the velocity and particle volume fraction (VF) at the five different locations for the empty channel are plotted in Fig. 4b. The velocity field reaches steady state at 0.04 s, while the particle VFs at five locations increase at the same rate and reach the same constant value after 0.05 s. For the spring-membrane channel, by examining the velocity profile, it is observed that the velocities for the five locations reach the iso-value after 0.04 s, the maximum iso-velocity (1.6 m/s) is found at

point 3, which is approximately 1.6 m/s higher than the values for point 5. The iso-velocity magnitude is 5 b 1 b 2 b 4 b 3. From the VF profile, the particles initially accumulate at points 2 and 3 and followed by 4, 5 and 1. It is found that the particles deposit very fast at locations 3, 4, but at point 1, the particle VF is very low. It is much faster to reach the steady state at points 2–4, which means in real time, the particles are carried toward the membrane surface by the fluid flow is balanced out by diffusion away from the membrane and their convection by tangential flow. For the two low velocity locations: 1 and 5, the VF at 5 exponentially increases and is difficult to reach the steady state even the VF is as high as 0.28% after 0.1 s; for point 1, the VF slowly reaches constant (0.01%) and is found to be the lowest value among the five points. When the fluid carries particles into location 5, the particles are more likely entrapped at point 5. Besides, the particles deposited at other locations have the chance to migrate along the flow direction to location 5, once the particles are accumulated at this location, they are difficult to move

124

N. Zhou, A.G.A. Nnanna / Desalination 276 (2011) 117–127

Fig. 8. Wall velocity profiles for empty channel and spring-membrane channels with different filament diameters.

and are not easily carried away by the fluid. Examining the velocity distribution as depicted in Fig. 4d, shows that when the fluid flow passes over one filament, most of the fluid flows towards points 2, 3 and 4. At these locations, the mass transfer is significant, and the VFs are relatively higher than that at point 1, because at point 1, there are less fluid flows into this zone, the presence of particles at this point is mainly resulted from diffusion due to the reverse flow. Overall, the maximum velocity does not result in the lowest particle VF, which dues to the significant mass transfer in the regions with high velocity. The particles are readily carried to these regions and then a portion of the particles migrate to different locations; on the other hand, the significant mass transfer also leads to high permeate flux. At location 1, the particles are mainly transported by diffusion due to the small velocity and reverse flow; the point 5 is considered as the detrimental site because the velocity at this region is very low but with the maximum particle VF, which will result in significant fouling and low permeate flux. The obtained results indicate that the particle deposition mechanism and VFs at different locations have significant difference in the spring-membrane system. 4.1.2. Steady state simulation In Fig. 5, the distinctions between the empty and springmembrane system are noted. At the stagnation
 point  upstream du behind each filament, the fluid N 0 because of dz
 accelerates  favorable pressure gradient, dp dz b 0
, reaches a maximum velocity,  4 dp = 0 , and decelerates 3 and dz
m/s  Re = 2.39 × 10 , when
   du b 0 because of the undesirable pressure gradient, dp dz N 0 . dz The fluid reaches a separation point near the membrane surface where the velocity gradient is zero. This results in development of boundary layer separation, a condition for which the boundary layer detaches

Table 1 Average value at distance 0.5 mm from channel walls for different diameter filaments inserted in membrane tubes. Diameter

Average wall velocity (m/s)

Average turbulence kinetic energy (m2/s2)

Average wall shear stress (kPa)

Static pressure drop (kPa)

2 mm 3 mm 4 mm

1.0701 1.1378 1.1965

0.1027 0.1219 0.1268

49.0204 57.0441 62.8432

12.5226 19.2788 26.2577

Fig. 9. Turbulence kinetic energy distributions for empty channel and spring-membrane channel.

from the surface and vortex is consequently formed. Particles tend to accumulate on membrane surface and cause concentration polarization and cake layer, resulting in decline of permeate flux [12]. The presence of eddies enhances diffusivity and produces scouring action, which interrupts the formation of concentration boundary layer at membrane surface. This effect results in retardation of fouling formation and consequent improvement in mass transfer and permeate flux across the membrane. To further elucidate the phenomena presented in Fig. 5 and to study the effect of filament diameter, spatial velocity profiles near (0.5 mm offset from the bottom membrane wall in first channel) the membrane wall are plotted in Fig. 6 for various filament diameters at constant pitch. The filament diameters investigated are 2, 3, and 4 mm for internal membrane diameter of 11.93 mm. The maximum fluid velocity, Vmax occurs periodically between filaments. It is observed that there exists sub-fluctuation velocity between filaments prior to the emergence of the peak value due to reverse flow behind each filament. The reverse flow contributes to the formation of vortex. After the first filament, negligible increase in Vmax is noted. The Vmax is related to the membrane and filament diameters according to Vmax = DV/(D − df), where V is the velocity in empty membrane, D is the tube diameter and df is the diameter of filament. An increase in filament diameter results in a decrease in membrane free-flow area and consequently an increase in fluid velocity. This is consistent with the mass conservation requirement for an incompressible fluid in which for constant volumetric flow rate, the velocity increases with decreasing cross-sectional area. Fig. 7a shows the velocity contour as a function of pitch between filaments. Three cases with 30 mm, 40 mm and 50 mm pitches were studied for filament diameter of 4 mm. A continuous bulk stream high velocity is observed for the 30 mm pitch while the velocity magnitude periodically changes for the 50 mm pitch. Data from Fig. 7b shows that the volume fraction of the particle depends on filament pitch. It is seen that the particles concentrated in the middle of the bulk stream for 30 mm spacing hence a decrease in filament spacing result in the particles flowing along the central area of the membrane. The presents of the U-bend connection does not affect the water velocity filed between the filaments significantly as shown in Fig. 7(a). However, in Fig. 7(b), the particles move toward the upper surface of the tube due to the effects of the U-bend connection. Such disturbance is damped and disappeared after the particle passing through about four filaments. Aggregation of foulant depends on the force and number of bonds holding the particles together. Aggregate breakage and erosion of cake

N. Zhou, A.G.A. Nnanna / Desalination 276 (2011) 117–127

125

Fig. 10. Contours of particle volume fraction for empty and spring-membrane channels at 2 m/s.

layer will occur if the wall shear stress, τ, exceeds the binding strength. The erosion rate E (ratio of deposited mass to total mass), is proportional to τ. The wall stress, τ due to the filaments in the membrane module is related to velocity gradient in Fig. 6 according to the relation, τ = -μ(du/dz), and is presented in Fig. 8 for filament diameters 2, 3, and 4 mm, and constant pitch of 50 mm. The maximum stress and sub-fluctuations in wall stress periodically occur in the same locations as the velocity distribution in Fig. 6. This is attributed to the fact that the shear stress is a function of velocity. The shear stress increases with the diameter of filaments. Data from Fig. 8 shows that τ is a function of filament diameter. Table 1 lists the average stress values for various filament diameters. It indicates that the larger diameter results in higher wall shear stress. The distribution of turbulence kinetic energy (TKE) due to local velocity fluctuations is presented in Fig. 9 for the empty and springmembrane system. Maximum TKE occurs between two filaments at locations different from where the maximum wall velocity and shear stress occurs. However, the maximum TKE location coincides with the region of vortex formation as depicted in Fig. 5. TKE results in flow instability and dissipates due to flow stabilization. This phenomenon replicates when flow encounters each filament. Data in Fig. 9 shows that TKE increases with filament diameter, with the least occurring in the empty membrane system. The difference between the TKE in spring- and empty membrane is approximately 0.1 m2/s2. This implies that the spring induces turbulent flow in the membrane channel. Turbulent flow impacts the concentration boundary layer, and decrease the aggregation and deposition of particles on the membrane surface. An examination of particle distribution in the empty and springmembrane channels as depicted in Fig. 10, shows that most of the particles are deposited near the upper region of the second membrane tube. This indicates that accumulation of foulants is most pronounced downstream along the membrane than upstream near the entry length. Data from Fig. 10 reveals that the average particle fractions on the bottom wall are 27.0% and 0.3% of the feed concentration in the first membrane for empty and spring-membrane, respectively. Additional study is needed to gain insight if this behavior is attributable to entrance/exit effect. The presence of U-bend connecting the first and second membranes results in pressure and kinetic energy losses and consequent deceleration of the feed water. Decrease in fluid acceleration could results in increased particle deposition on the upper membrane channel.

4.2. Experimental results Fig. 11a shows the temporal evolution of flux for constant transmembrane pressure and inlet velocity of 345 kPa and 1.03 m/s, respectively. It is observed that the initial fluxes for all the springfilled and empty membrane systems are approximately 260 l/m2h (lmh) and 180 lmh, respectively, a 37% difference. The fluxes declines after 2 h of filtration process, however steady state flux achieved in the spring-filled membrane is 30 lmh higher than the one in the empty membrane. Data presented in Fig. 11b and c shows flux enhancement of 25%, 20% and 10% for the respective two-springs, one-spring in first tube and one-spring in second tube systems. These flux augmentation results in no pressure drop since the spring occupied only 1.75% of the membrane volume. Previous studies [8,11,12] on flux enhancement using rod, and axial baffles reported an increase in pressure drop. It is shown from numerical studies in this paper that the presence of a spring in membrane tube increases wall velocities and shear stress, and alters the flow pattern. The rotational flows generated in the vicinity of the spring induce turbulence which scours the membrane surface. This turbulence can greatly disturb the concentration polarization mechanism by reducing the buildup [11] and further increases mass transfer. A membrane autopsy was performed to examine the accumulation and distribution of fouling on membrane surface. Fig. 12a shows uniformly distributed thick layer of fouling on the empty membrane surface. Fig. 12b reveals non-uniformity in buildup of fouling, and alternating fouled and un-fouled profiles throughout springmembrane surface. This fouling pattern mimics the helical spring contour. The width of the un-fouled strip is approximately 2 mm and equivalent to the spring pitch, about 60% of the surface is un-fouled. This experimental evidence supports the numerical results presented in this study that spring-membrane system increases local high velocity field, shear stress, turbulence and consequently the scouring force which suppresses formation of concentration boundary layer and fouling. 5. Conclusion The performance of the hybrid spring-membrane system for fouling control was evaluated through numerical simulation and experiments. The results obtained from the transient simulation presented the detailed process of particle deposition mechanism. The

126

N. Zhou, A.G.A. Nnanna / Desalination 276 (2011) 117–127

Fig. 12. Fouled membrane inner surfaces after two-hour filtration. (a) Without spring (b) with spring.

Fig. 11. Flux enhancement by spring insertion TMP = 345 kPa, velocity = 1.03 m/s. (a) Flux evolution over 2 h filtration. (b) Flux enhancement over time. (c) Average flux and flux enhancement after 2 h filtration for different experimental runs.

particle VF increased over time and reached steady state for those sites before the flow encountered each filament; but the accumulation of particles at the region right before the filament increased dramatically and resulted in the maximum VF. At low velocity regions, which are before and behind the filament, the different particle deposition mechanism was significant. The particles transported to the region behind the filament due to diffusion by the reserve flow and led to the lowest VF; but once the particles were carried or

migrated into the region before filament, they were easily entrapped and resulted in high VF. These two regions are considered as low mass transfer regions and will result in low permeate flux. The steady state simulation for the entire spring-membrane system demonstrated that the presence of spring led to the change of flow pattern, the generation of eddy behind each filament, and the significant fluctuation of wall velocity, wall shear stress and TKE. The eddies and turbulence flow enhance diffusivity and produce scouring action, which interrupts the formation of concentration boundary layer at membrane surface. The shear stress is a function of velocity, and aggregation of foulant will break if the wall shear stress exceeds the binding strength. Particles concentrated in the middle of the bulk stream for 30 mm spacing hence a decrease in filament pitch resulted in the particles flowing along the central area. Accumulation of foulants was most pronounced in downstream. Experimental results showed that permeate flux enhanced 25% for the spring-membrane system and 60% of the surface was un-fouled after membrane autopsy. This experimental evidence supports the numerical results presented in this study that spring-membrane system increases local high velocity field, shear stress, turbulence and consequently the scouring force which suppresses formation of concentration boundary layer and fouling. References [1] G. Crozes, C. Anselme, J. Mallevialle, Effect of adsorption of organic matter on fouling of ultrafiltration membrane, Journal of Membrane Science 84 (1993) 61. [2] R. Ben Amar, B.B. Gupta, M.Y. Jaffrin, Apple juice clarification using mineral membranes: fouling control by backwashing and pulsatile flow, Journal of Food Science 55 (1990) 1620.

N. Zhou, A.G.A. Nnanna / Desalination 276 (2011) 117–127 [3] G. Gesan, G. Daufin, U. Merin, J.P. Labbe, A. Quemerais, Fouling during constant flux cross-flow microfiltration of pretreated whey. Influence of transmembrane pressure gradient, Journal of Membrane Science 80 (1993) 131. [4] A. Nabe, E. Staude, G. Belfort, Surface modification of polysulfone ultrafiltration membranes and fouling by BSA solutions, Journal of Membrane Science 133 (1) (1997) 57–72. [5] A. Hamza, V.A. Pham, T. Matsuura, J.P. Santerre, Development of membranes with low surface energy to reduce the fouling in ultrafiltration applications, Journal of Membrane Science 131 (1–2) (1997) 217–227. [6] H. Ma, C.N. Bowman, R.H. Davis, Membrane fouling reduction by backpulsing and surface modification, Journal of Membrane Science 173 (2) (2000) 191–200. [7] Wayne F. Jones, Richard L. Valentine, V.G.J. Rodgers, Removal of suspended clay from water using transmembrane pressure pulsed microfiltration, Journal of Membrane Science 157 (1999) 199–210. [8] B.B. Gupta, J.A. Howell, D. Wu, R.W. Field, A helical baffle for cross-flow microfiltration, Journal of Membrane Science 99 (1995) 31–42. [9] B.J. Behouse, G. Costigan, K. Abhinava, A. Merry, The performance of helical screwthread inserts in tubular membranes, Separation and Purification Technology 22–23 (2001) 89–113. [10] A.L. Ahmad, A. Mariadas, M.M.D. Zulkali, Reduction of membrane fouling using a helical baffle for cross flow microfiltration, In: Regional Symposium on Membrane Science and Technology 2004, 21–25 April, Puteri Pan Pacific Hotel, Johor Bahru, Johor, Malaysia, 2004. [11] T.Y. Chiu, A.E. James, Effects of axial baffles in non-circular multi-channel ceramic membrane using organic feed, Separation and Purification Technology 51 (2006) 233–239. [12] Yuanfa Liu, Gaohong He, Xudong Liu, Gongkui Xiao, Baojun Li, CFD simulations of turbulent flow in baffle-filled membrane tubes, Separation and Purification Technology 67 (2009) 14–20. [13] A.S. Berman, Laminar flow in channels with porous walls, Journal of Applied Physics 24 (1953) 12. [14] A.S. Berman, Effects of porous boundaries on the flow of fluids in systems with various geometries, Proceedings of the Second United Nations International Congress on the Peaceful Use of Atomic Energy, vol. 4, 1958, p. 358.

127

[15] S.W. Yuan, A.B. Finkelstein, Laminar pipe flow with injection and suction through a porous wall, Transactions of the ASME 78 (1956) 719–724. [16] R. Ghidossi, J.V. Daurelle, D. Veyret, P. Moulin, Simplified CFD approach of a hollow fiber ultrafiltration system, Chemical Engineering Journal 123 (2006) 117–125. [17] S. Wardeh, H.P. Morvan, CFD simulations of flow and concentration polarization in spacer-filler channels for application to water desalination, Chemical Engineering 86 (2008) 1107–1116. [18] A.L. Ahmad, K.K. Lau, M.Z. Abu Bakar, Impact of different spacer filament geometries on concentration polarization control in narrow membrane channel, Journal of Membrane Science 262 (2005) 138–152. [19] M. Shakaib, S.M.F. Hasani, M. Mahmood, CFD modeling for flow and mass transfer in spacer-obstructed membrane feed channels, Journal of Membrane Science 326 (2009) 270–284. [20] M. Shakaib, S.M.F. Hasani, M. Mahmood, Study on the effects of spacer geometry in membrane feed channels using three-dimensional computational flow modeling, Journal of Membrane Science 297 (2007) 74–89. [21] J.L.C. Santos, V.M. Geraldes, S. Velizarov, J.G. Crespo, Investigation of flow patterns and mass transfer in membrane module channels filled with flow-aligned spacers using computational fluid dynamics (CFD), Journal of Membrane Science 305 (2007) 103–117. [22] Z. Cao, D.E. Wiley, A.G. Fane, CFD simulation net-type turbulence promoters in a narrow channel, Journal of Membrane Science 185 (2001) 157–176. [23] F. Li, W. Meindersma, A.B. de Haan, T. Reith, Optimization of non-woven spacers by CFD and validation by experiments, Desalination 146 (2002) 209–212. [24] F. Li, W. Meindersma, A.B. de Haan, T. Reith, Optimization of commercial net spacers in spiral wound membrane modules, Journal of Membrane Science 208 (2002) 289–302. [25] L. Schiller, Z. Naumann, A Drag Coefficient Correlation, Zeitschrift des Vereins Deutscher Ingenieure 77 (1935) 318–320.