Flux recovery in the ultrafiltration of suspended solutions with ultrasound

Journal of Membrane Science 243 (2004) 115–124

Flux recovery in the ultrafiltration of suspendedsolutions with ultrasound

Ruey-Shin Juang∗, Kung-Hsuan Lin

Department of Chemical Engineering and Materials Science, Yuan Ze University, Chung-Li 320, Taiwan

Received 10 January 2004; received in revised form 10 January 2004; accepted 6 June 2004Available online 17 August 2004

Abstract

The use of a horn sonicator (Misonix, 20 kHz) to recover the flux in the ultrafiltration (UF) of suspended solutions was studied. Two typesof solutions, i.e., the Cu2+–polyethylenimine (PEI) solution and the W/O emulsions dispersed in aqueous solution, were selected, whichwere widely found in binding-UF and liquid surfactant membrane (LSM) processes, respectively, for removal of toxic heavy metals fromeffluents. The Amicon YM10 membrane (regenerated cellulose, MWCO 10,000) was used. Experiments were performed as a function of tipp tio of W/Oe ising methodf eep them tion)f l.©

K

1

uietamOnda“tbL

xter-one ofs to aefeedon-atero-cess

de-e inem-

quesverypor-es ofubu-some

0d

osition, ultrasonic power, and solution properties such as target ion concentration, percentage of emulsification, and volume ramulsions to the external aqueous phase. It was shown that ultrasound associated with a hydraulic pressure of 10 psi was a prom

or the recovery of UF flux, particularly for the solution with less fouling potential. Careful control of the ultrasonic intensity could kembranes durable and prevent the organic compounds (PEI in the Cu2+–PEI solution; carrier, surfactant, and solvent in the W/O/W solu

rom degradation. Finally, the fouling of membrane during UF of W/O/W solutions was analyzed by the resistance-in-series mode2004 Elsevier B.V. All rights reserved.

eywords:Flux recovery; Ultrafiltration; Suspended solutions; Ultrasound; Membrane fouling; Resistance-in-series model

. Introduction

In the past 25 years, binding-ultrafiltration (UF) processsing water-soluble polymers has been shown to be promis-

ng for removal of trace metals and radionuclides from wasteffluents, groundwater, and seawater[1,2]. The advantages of

his process are the low-energy requirement involved in UFnd high binding capacity of the polymers[2,3]. However,embrane fouling by such polymers could not be ignored.n the other hand, liquid membrane process is known as aovel way for recovery and separation of various species fromilute aqueous solutions including metals, weak acids/bases,nd biologically important compounds[4–6]. The so-calleduphill transport” has received much attention as an alterna-ive to conventional solvent extraction because liquid mem-rane combines extraction and stripping stages in one unit.iquid surfactant membrane (LSM), in which organic solu-

∗ Corresponding author. Fax: +886 3 4559373.E-mail address:[email protected] (R.-S. Juang).

tion is reformed as small spherical shells to separate enal feed and internal strip aqueous phases, representsthe feasible types of liquid membranes, because it leadmembrane area of 1000–3000 m2/m3 of equipment volum[6]. The separation of W/O emulsions from the externalphase by gravity settling is often not effective and time csuming[6]; thus UF was justified to be a potential altern[7]. Similarly, membrane fouling is serious during UF pcess even with hydrophilic membranes, making this promore critical in industrial applications.

Membrane fouling is characterized by an irreversiblecline in flux [8]. Considerable progress has been madunderstanding the interactions among the foulants, the mbrane, and operating conditions. Although many technihave been developed to overcome fouling, flux reco(membrane cleaning) still appears to be practically imtant in membrane filtration systems. The typical techniqumembrane cleaning are forward flushing (spiral wound, tlar) and back pulsing (hollow fiber)[9,10]. These two waymay be especially useful with colloidal suspensions and s

376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2004.06.013

116 R.-S. Juang, K.-H. Lin / Journal of Membrane Science 243 (2004) 115–124

tubular membranes. Moreover, the chemicals such as deter-gents, acids or bases are often used to clean the fouled mem-branes[11]; however, chemical methods sometimes damagethe membrane materials and causes secondary pollution. Thecharged particles will move away from the membrane sur-face by electrical techniques[12], depending on the electricfield strength, thus reducing the extent of concentration polar-ization and increasing permeate flux. There is the danger ofelectrolysis occurs at the electrodes and gas being generated.Corrosion of electrodes and high power costs has inhibitedthe commercial practice of this technique[13].

Ultrasound is the wave at a frequency above 16 kHz andhas been widely used as a method for cleaning materials be-cause of cavitation phenomenon[14]. When ultrasound isirradiated through a liquid medium, an alternating adiabaticcompression and rarefaction cycle of the medium occurs.Simply put, cavitation is the formation, growth, and suddencollapse of micro bubbles in liquids, which are formed in therarefaction cycle of the ultrasonic wave when a large negativepressure is applied to a liquid medium[15,16]. The compres-sion cycle can cause bubbles to collapse, with a release ofenergy, which causes cleaning of the membrane surface. Thecollapse of the bubbles has sufficient energy to overcome theinteraction between the foulant and membrane and to removethe foulant from the surface of polymer membrane[8].

fluxoCt rme-a thec ocia-t otoe -s d ont g oft tiallya

fluxe onicms ul-t auset thisw radi-a am-i lu-t sedh it hasb tentos orica mu-ln ns ast resenL ling

of membrane during UF process using the resistance-in-seriesmodel.

2. Materials and methods

2.1. Apparatus and membranes

All UF experiments were performed in a batch cell, inwhich the diameter was 150 mm and the volume was 3.5 L.The applied pressure (�P) was controlled by N2 gas. Thehydrophilic membrane YM10 (Amicon, regenerated cellu-lose) was selected. It had a MWCO of 10,000, and a purewater permeate fluxJw of 55–70 dm3/(m2 h) after 5 min ofoperation at 207 kPa.

The flux experiments were conducted with ultrasound(Misonix Sonicator 3000) equipped with a Sonabox acousticenclosure to reduce noise. This sonicator offers a powerfulgenerator with a 600 W output, a 20 kHz convertor, temper-ature control, auto-tuning, and external cooling device in-terface. The tapped titanium horn type 200 (tip diameter12.7 mm) was mostly used in this work, and type 305 (tipdiameter 19.1 mm) was tested in some cases for comparison.The double (peak-to-peak) amplitude of the radiating faceof both tips is 120�m on the sonicator operating at outputcontrol setting 10.

2

da ch asC icalr ddingva ei ointo utionw

sol-v dus-t an,r .5%)a Co.a que-o eousp fE asew .1 MN ofd ec ink

2

sselw 0

Many studies have been carried out to enhance thef membrane filtration using ultrasound treatment[17–22].hai et al.[18] used an ultrasonic bath (45 kHz, 2.73 W/cm2)

o clean UF and MF membranes fouled by peptone petion in a cross-flow filtration cell. They suggested thatleaning of the fouled membranes by ultrasound in assion with water cleaning is effective. In addition, Matsumt al. [19] and Duriyabunleng et al.[20] found that ultraound is effective for removing the cake layer depositehe surface of MF membrane and preventing the plugginhe membrane pores when ultrasonic energy is tangenpplied to the membrane in a bath.

The above studies provided valuable information onnhancement with ultrasound, but use of the ultrasethod and its effectiveness is limited[8,22]. Online ultra-

onic irradiation has low efficiency and high cost. Anrasonic bath is only useful in laboratory studies bechere is a high waste of acoustic energy in the bath. Inork, an ultrasound horn was used and a flat cell was irted with the sonicator. The aim of this work was to ex

ne the flux recovery during the UF of Cu(II)/polymer soion and W/O/W solution with ultrasound. The polymer uere was weakly basic polyethylenimine (PEI) becauseinding ability for many heavy metals and a high conf amino groups (primary, secondary, tertiary)[2]. Besidesurfactant, a promising carrier di(2-ethylhexyl)phosphcid (D2EHPA) was added in the organic phase to for

ate W/O/W solution in actual LSM operation[5]. The inter-al aqueous phase contained Cu(II)-EDTA chelated anio

racers because they were not transported through the pSM [7]. Finally, an attempt was made to analyze the fou

t

.2. Reagents and solutions

PEI with a MW of 25,000 (Aldrich Co.) was provides a 50 wt.% aqueous solution. Other inorganic salts suuSO4 and NaOH were supplied by Merck Co. as analyt

eagent grade. The Cu–PEI solution was prepared by aarious molar concentration ratios of CuSO4 (fixed at 1 mM)nd PEI in deionized water (Millipore Mill-Q), in which th

nitial pH was 9.5 without any adjustment because the pf zero charge of the PEI is near 8.2 (not shown). The solas agitated for 2 h before UF experiments.The surfactant Span 80 (sorbitan monooleate) and

ent kerosene, obtained from Wako Pure Chemical Inries, Ltd., Japan and Union Chemical Works Ltd., Taiwespectively, were used as received. D2EHPA (purity, 98nd disodium salt of EDTA were the products of Mercknd were used without further purification. The external aus phase was deionized water, and the internal aquhase consisted of equimolar CuSO4 and disodium salt oDTA (Cint,0 = 7.8 mM). The pH of internal aqueous phas adjusted to be 4.0 by adding a small amount of 0aOH to ensure that all Cu(II) tracers exist in the formivalent chelated anions with EDTA[23]. The organic phasontained 6 vol.% of D2EHPA and 3 vol.% of Span 80erosene.

.3. Preparation of W/O emulsions

The W/O emulsions were prepared in a cylindrical veith the aforementioned sonicator[24]. The horn type 20

R.-S. Juang, K.-H. Lin / Journal of Membrane Science 243 (2004) 115–124 117

was used. The volumes of the internal aqueous phase (Vint,0)and organic phase (Vorg,0) were fixed at 0.1 and 0.05 L, re-spectively. After careful pouring of internal aqueous phaseand organic phase to the vessel in sequence, the tip of thehorn was placed at the aqueous–organic interface and thesonicator was started at various preset powers (15–93 W) for5 min. The temperature was controlled at 25◦C.

To determine the percentage of emulsification, the W/Oemulsion was poured into the external aqueous phase (Vext,0= 0.15 L), and was gently agitated to form the so-called theW/O/W solution. After settling for 5 min, the external aque-ous sample was taken for Cu(II) analysis (Cext) when it wasseparated from the W/O emulsions by passing W/O/W solu-tion through YM10 in a stirred cell with a diameter of 62 mmand a volume of 0.2 L (Advantec UHP 62, Japan) at�P =69 kPa. From the mass balance of the tracer in external andinternal aqueous phases, we have

Cext = (Vint,0 − Vint)Cint,0

Vext,0 − Vint,0 − Vint(1)

where the subscript ‘0’ refers to the initial state. BecauseVint,0, Vext,0, Cint,0, andCext are known, we can obtainVint.The percentage of emulsification (E) was thus calculated by

E(%) = Vint × 100 (2)

2

ndera dri-c rnalv thec ur-i theC ortd . Thee (0 dis-t was

F withu

termed as tip height (H), which was varied mostly from 20to 65 mm. The temperature was maintained at 25◦C by cir-culating constant-temperature water in the jacket around thecell. The permeate fluxes (J) were measured by an electricalbalance (Mettler AG204), connected to a PC.

In initial feed and permeate, the concentration of Cu(II)was analyzed by an atomic absorption spectrophotometer(Varian FS220). In the UF of Cu–PEI solution, the total or-ganic carbon content (TOC) was also measured to determinethe rejection of PEI by a TOC analyzer (O.I. Analytical 1010).The reproducibility of the concentration measurements waswithin 5% (mostly, 2%). The rejection of Cu(II) or PEI (R)was calculated as follows:

R = 1 −ion concentration or TOC

content in the permeate

ion concentration or TOC

content in the initial feed

(3)

The droplet sizes of the W/O emulsions were mea-sured using laser Doppler electrophoresis (Malvern Zetasizer3000HS). The used membranes were immediately flushedwith water after filtration, and then regenerated in sequenceby rinsing with 0.1 M NaOH, 1.4 mM NaOCl, and 10 mMHCl in an ultrasonic cleaner (Brandson B20, USA) for10–30 min each. TheJw value of the cleaned membrane wasm fs d. Int fore

3

3

dba asonw seo sw sed( nici onlyf

witht rtheri di-c abaticc adi-a i-c ures.T e form andv rt oft s the

Vint,0

.4. Experimental procedures

Fig. 1shows the experimental setup of flat-sheet UF un ultrasonic field. The diameter and volume of the cylinal cell used was 150 mm and 2 L, respectively. An exteessel with a solution volume of 1.3 L was connected toell; so the working liquid level was fixed at 110 mm dng the experiments. The cell was assembled, to whichu–PEI solutions with different ratios of [Cu(II)] to [PEI]

he W/O/W solutions (Vint,0/Vorg,0 = 2,φw/o = 1 or 0.1) withifferent percentages of emulsification were then pouredxperiment was started at different ultrasonic powersP =–150 W) upon complete addition of the solutions. The

ance from the membrane to the tip of ultrasound horn

ig. 1. Experimental setup for ultrafiltration of suspended solutionsltrasound.

easured, and only the membrane with a deviation oJwmaller than 5% from its fresh one was repeatedly usehis work, more than 100 cycles were mostly permittedach YM membrane such as YM10.

. Results and discussion

.1. UF fluxes of pure water with ultrasound

Fig. 2 shows the permeate fluxes of pure water (Jw) atifferent ultrasonic powers (P) and tip heights (H). It shoulde noted that the applied pressure (�P) is still maintainedt 69 kPa in these experiments and hereinafter. The reill be described in the following section. Unlike the uf horn type 305 (Fig. 2b), the flux Jw linearly increaseith increasing ultrasonic power as horn type 200 is u

Fig. 2a). This is probably due to much weaker ultrasontensity (power/area) of the horn type 305, which areour-ninths of the horn type 200.

With the horn type 200, the pure water flux increasesip height up to 65 mm, but starts to decrease with a funcrease in tip height for a given ultrasonic power. As inated above, the ultrasound produces an alternating adiompression and rarefaction of the liquid media being irrted[14–16]. In the rarefaction part of ultrasonic wave, mrobubbles form due to sufficiently large negative presshe bubbles can be either stable about their mean sizany cycles or transient when they grow to certain size

iolently collapse or implode during the compression pahe wave. That is, the threshold tip height (65 mm) cause


Fig. 2. Effect of ultrasonic power and tip height on UF fluxes of pure water.

compression cycle to be well developed and the collapse ofthe bubbles releases sufficient energy to remove the foulantfrom the membrane surface. Here, the ratio of threshold tipheight to cell diameter (65/150) is about 0.45.

It is also found from experiments that too high ultrasonicpowers (>80 W) slightly destroy the structure of the mem-brane when the smallest tip height is used (10 mm). Underother conditions, the YM membrane can be repeatedly usedby checking theJw values after each experiment. In fact,Masselin et al.[25] studied the effect of 47 kHz ultrasonicwaves on the properties of polymeric membranes immersedin an aqueous bath including water permeability, porosity,and mean pore size. They observed that the polyethersulfonemembrane is affected by ultrasonic treatment over its entiresurface but the polyvinylidenefluoride and hydrophilic poly-acrylonitrile membranes present no significant change.

If the slopes of the straight lines shown inFig. 2are calcu-lated and plotted vs. tip height (Fig. 3a). Two symmetric lineswith a comparable slope of 1.77× 10−3 and−1.74× 10−3

are obtained, respectively. This probably implies that the flowprofiles are progressively built up to be well established whenthe tip height is increased up to 65 mm.

On the other hand, ultrasound has little effect on pure waterflux Jw in the absence of applied pressure as shown inFig. 3b.Ultrasound can be considered as a pseudo stirrer[22] becauseiB(e essed

Fig. 3. Determination of threshold tip height and the increased applied pres-sure due to ultrasound.

in terms of “increased”�P [22]. Under the conditions stud-ied, an applied pressure of 69 kPa gives the best effect forthis purpose and thus is selected in the work. Actually, Li etal. [8] have used a horn sonicator (20 kHz, 82.9 W/cm2) toclean flat sheet nylon MF membranes fouled by Kraft papermill effluent. They found that ultrasound associated with for-ward flushing is an effective method for the recovery of theflux.

3.2. UF of Cu–PEI solutions with ultrasound

Fig. 4 shows the UF fluxes of Cu–PEI solutions with ul-trasound at different tip heights and ultrasonic powers. Inthis work, the ultrasound is started when the flux decreasesless than about 3 dm3/(m2 h) and is then stopped when theflux recovered reaches the steady state. It is found that 30%,50%, and 70% of the fluxJ is recovered (relative toJw) usingan ultrasonic power of 30, 57, and 93 W, respectively, atH= 20 mm. Moreover, the efficiency of flux recovered atH =40 mm is higher than that atH = 20 mm whenP < 60 W. Asexpected from the pure water flux experiments (Fig. 2), thebest efficiency is obtained atH = 65 mm, in which 70–80%of the flux is recovered as long asP > 30 W. On the otherhand, the ultrasonic power required to achieve a given fluxcan be clearly seen inFig. 5.

de-g et nd

t can enhanceJw in the presence of applied pressure (Fig. 2).ased on the relationship ofJw = 0.39�Pwithout ultrasound

not shown), whereJw is in dm3/(m2 h) and�P is in kPa, thenhancement of the flux due to ultrasound may be expr

It is known that ultrasound is an effective method torade the organic compounds[14,15]. An attempt was mad

o justify the possibility of PEI degradation with ultrasou


Fig. 4. Flux recovery in the UF of Cu–PEI solutions with ultrasound atdifferent ultrasonic powers and tip heights.

Fig. 5. UF fluxes of the Cu–PEI solutions with ultrasound at different ultra-sonic powers and tip heights.

Fig. 6. Test of PEI damage during the UF of the Cu–PEI solutions withultrasound.

by monitoring the PEI and Cu2+ concentrations in the perme-ate.Fig. 6 shows the results, indicating that the degradationis negligible at a concentration ratio of [Cu2+]/[PEI] of 1/5.This is not the case for a ratio of 1/2, meaning that the bindingreaction between Cu2+ and PEI in aqueous phase is possiblyenhanced with ultrasound under the conditions studied[15].This argument may be supported from the rejection results asshown inFig. 7. It is found that the rejection of Cu2+ seemsto increase with increasing ultrasonic power up to 93 W, al-though the effect of PEI rejection is less insignificant.

3.3. UF of W/O/W solutions with ultrasound

The effects of operating parameters on the flux of W/O/Wsolutions with ultrasound are shown inFigs. 8–10. In gen-eral, ultrasound recovers the flux more efficiently for UF ofW/O/W solution with a lower percentage of emulsification(E). Moreover, a low ultrasonic power (e.g., <30 W) cannotrecover the flux when the solution has at a highE value (e.g.,30%) and a volume ratio of W/O emulsions to the external

F withu

ig. 7. Rejections of Cu(II) and PEI in the UF of the Cu–PEI solutionsltrasound.


Fig. 8. Flux recovery in the UF of W/O/W solutions with ultrasound atdifferent ultrasonic powers and percentages of emulsification (H = 65 mmandφw/o = 1.0).

aqueous phase (φw/o) of 1.0. This is probably because the“stirring” velocity induced by low-power ultrasound cannotovercome the settling velocity of W/O emulsions. However,this problem can be solved if the volume ratioφw/o is reducedto 0.1 (Fig. 10).

F atda

Fig. 10. Flux recovery in the UF of W/O/W solutions with ultrasound atdifferent ultrasonic powers (H = 65 mm andφw/o = 0.1).

Unlike the UF of Cu–PEI solution, tip height has lit-tle effect on the efficiency of flux recovery as shown inFigs. 8 and 9. For example, for UF of the solution withE= 15% andφw/o = 1.0 using an ultrasonic power of 145 W,about 30% of the flux is recovered whenH = 40 and 65 mm;but 50–60% of the flux is recovered for UF of the solutionwith φw/o = 0.1, even asE = 30% (Fig. 10). That is, besidesultrasonic power,φw/o is the most andE is the second-mostcrucial factors affecting the efficiency of flux recovery withultrasound. It is noted thatφw/o mainly affects droplet sizeof the W/O emulsion, rather than the viscosity of W/O/Wsolution[24,26].

Similarly, the destruction of the suspended W/O emulsionsis checked here by monitoring the concentrations of Cu(II)tracers (or exactly, the Cu-EDTA chelated anions) duringthe process. In the power range of 24–150 W, no consider-able changes are found in the permeate. Although the higherthe ultrasonic power, the higher the efficiency of flux recov-ery, a medium power is always suggested (e.g., < 100 W or80 W/cm2). The main aim is to prevent organic-phase com-positions (carriers, surfactants, solvents, etc.) from possibledegradation under ultrasonic irradiation, making the actualLSM operations efficient and practical.

3

usem ions.H O/Ws allya olar-i ce-i uxd or so-l odeli shipso

ig. 9. Flux recovery in the UF of W/O/W solutions with ultrasoundifferent ultrasonic powers and percentages of emulsification (H = 40 mmndφw/o = 1.0).

.4. Fouling analysis of the UF of W/O/W solutions

As clearly indicated above, the W/O/W solutions caore serious membrane fouling than the Cu–PEI solutence, an attempt was made to analyze the UF flux of W/olutions with ultrasound. The decline of UF flux is usunalyzed by the following three mechanisms: (i) the gel p

zation model, (ii) the osmotic model, and (iii) the resistann-series model[27,28]. In the third model, the permeate flecreases due to the resistances caused by fouling

ute adsorption and concentration polarization. This ms adopted here because it easily describes the relationf permeate flux with operating parameters[29–32].


In the resistance-in-series model, the permeate fluxJ canbe expressed as[29,30]

J = �P

Rm + Rf + Rp(4)

whereRm is the intrinsic resistance of the membrane,Rpthe resistances due to the concentration polarization and gellayer, andRf denotes the resistances due to other foulingphenomena such as solute adsorption and pore blocking. Ac-cording the expression ofEq. (4), these resistances involvethe viscosity term[31,32], soRm,Rf , andRp have the unit of(Pa s)/m.

In general,Rp is proportional to the amount and specifichydraulic resistance of the deposited layer. Because the de-posited layer is compressible,Rp will be a function of appliedpressure. In this regard, we may assume[29–32]

Rp = ϕ �P (5)

Thus,Eq. (4)becomes

J = �P

Rm + Rf + ϕ �P(6)

The droplet size of the W/O emulsions, expressed as theSauter diameterd32, was in the range 0.2–1.4�m[24], whichis far larger than the pore size of YM10 membrane. It wase usedm ningf d.

T antϕ

fi Ft

Oo s of(

aW

ϕ

w efi ev=b

l

T

Fig. 11. Determination ofRm, Rf , andϕ defined inEq. (7).

Fig. 12. Typical plot for determination of the constantsbi’s defined inEq.(9).

Table 1Correlation between tip height (H) and the constants (correlation coefficient> 0.990)a

bi’s in Eq. (9) ci’s in Eq. (11)

b1 = (−1.0 + 0.077H) × 10−8 c1 = (1.05− 0.12H + 0.004H2) × 1012

b2 = 0.14− 0.049H c2 = 0.39− 0.055Hb3 = 1.53− 0.031H c3 = 8.62− 0.23H + 0.002H2

a Unit of H is mm.

xperimentally observed that the pure water flux of theembrane could be fully recovered after ultrasonic clea

or 30 min. Hence, the termRf in Eq. (6)cannot be neglecteRewritingEq. (6), we have

1

J= ϕ + Rm + Rf

�P(7)

he values ofRm andRf , as well as the proportional const, are determined from experimental data. TheRm value isrst obtained to be 9.1× 109 (Pa s)/m from the separated Uests for pure water (Fig. 11) according to

1

Jw= Rm

�P(8)

n the basis ofEq. (7), therefore, the values ofRf andϕ arebtained from the slope and intercept in the linear plot1/J) versus (1/�P), as typically shown inFig. 11.

Basically, theϕ value is a function of ultrasonic power (P)nd percentage of emulsification (E) at a given tip height (H).e may assume the following form[29,30]:

= b1Pb2Eb3 (9)

herebi’s are the empirical constants. Theb3 value can brst determined at a fixedPas shown inFig. 12a. An averagalue ofb3 of 0.7 is taken in theP range of 30–150 W atH40 mm. Then, we can determine the values ofb1 andb2 toe 1.44× 108 and−1.19, respectively, according to

n(ϕE−0.7) = ln b1 + b2 ln P (10)

The correlations between tip height andbi’s are listed inable 1.


TheRf value is also a function ofP andE at a given tipheight (H); thus we have

Rf = c1Pc2 exp(c3E) (11)

In a similar manner, the averagec2 value is first deter-mined to be−1.28 in theE range of 3%–30% atH =40 mm (Fig. 13a). Moreover, the values ofc1 andc3 are ob-tained to be 2.25× 1012 and 2.39, respectively, accordingto

ln(RfP1.28) = ln c1 + c3E (12)

Table 1also lists the correlations between tip height andci’s. Once the parameters are known under various condi-tions, we can compare the experimental and calculated fluxes.The standard deviation (S.D.) defined below, whereN is thenumber of data points, is found to be 16%, indicating that thepresent model is acceptable.

S.D.(%)= 100×√∑

[(Jcalc/Jexpt) − 1]2

N − 1(13)

In the MF of Kraft paper mill effluent using nylon mem-branes with 0.2�m mean pore size, Li et al.[8] found thatthe membrane resistance, the reversible resistance due toparticle polarization and cake layer, and the irreversible re-s 9.1%o e to-t nica-t thef tancec stem,RH× s

TT

E H = 40 mm H = 65 mm

Rf × 10−10 ϕ × 10−5 Rf × 10−10 ϕ × 10−5

3 1.81 1.85 1.66 2.131.21 1.61 0.81 1.800.88 1.01 0.75 1.100.41 0.40 0.38 0.480.35 0.15 0.29 0.18

1 3.63 5.02 2.70 5.522.12 3.32 2.10 3.361.15 1.79 0.92 2.090.64 1.58 0.59 1.660.45 0.84 0.38 1.28

3 13.3 13.4 11.3 14.13.22 5.48 2.56 6.652.11 2.49 1.46 2.541.24 1.57 1.18 2.200.

Fig. 13. Typical plot for determination of the constantsci’s defined inEq.(11).

with increasingE or decreasingP. Further information isneeded to improve the efficiency of flux recovery such asthe velocity profiles in the cell produced by ultrasound andtheir relationships with other operating parameters includ-ing tip position, applied pressure, and “stirred” cross-flowvelocity.

istance due to adsorbed layer are 6%, 34.9%, and 5f the total resistance, respectively. They found that th

al resistance of the membrane cleaned with a horn soor (20 kHz, 82.9 W/cm2) reduces by 84% compared toouled membrane. This means that the irreversible resisan be decreased with ultrasound. In the present UF syf is less thanRm only under the conditions of smallE, largeand/or highPas shown inTable 2(remember thatRm = 9.110−9 (Pa s)/m). Theϕ value (i.e., theRp value) increase

able 2he values ofRf andϕ calculated under various conditionsa

(%) P (W) H = 20 mm

Rf × 10−10 ϕ x 10−5

30 2.59 0.8148 2.55 0.6981 1.48 0.61

118 1.19 0.34144 0.69 0.11

5 30 4.47 4.2448 3.61 3.1681 2.58 1.60

118 1.28 1.48144 0.63 0.64

0 30 15.5 27.148 4.77 9.6081 3.43 7.72

118 1.51 3.83144 1.42 2.10

a Units ofRf andϕ are (Pa s)/m and s/m, respectively.

76 1.42 0.13 1.87


4. Conclusions

The recovery of permeate flux during flat-sheet UF ofCu–PEI solutions and W/O/W solutions using YM10 mem-brane was investigated with a low-frequency horn ultrasound(20 kHz). The following results were obtained:

(1) With horn type 200 (diameter 12.7 mm), a threshold tipheight of 65 mm was obtained when cell diameter was150 mm. Pure water experiments showed that ultrasoundassociated with a hydraulic pressure of 10 psi would en-hance the flux to the largest extent.

(2) Ultrasound was a promising method for flux recovery,particularly for the Cu–PEI solution with less foulingpotential. For example, 70–80% of the flux was recoveredas long asP > 30 W atH = 65 mm in the UF of Cu–PEIsolutions ([Cu2+]/[PEI] = 1/5).

(3) The volume fraction of W/O emulsions in the W/O/Wsolution (φw/o) appeared the most crucial factor in de-termining the efficiency of flux recovery. For example,atP = 145 W andH = 65 mm about 30% of the flux wasrecovered for a solution withE = 15% andφw/o = 1.0;however, 50–60% of the flux was recovered for a solutionwith φw/o = 0.1, even whenE = 30%.

(4) Under the conditions studied, careful control of ultra-and

/W

( a-e-in-anal-

A

ci-e rate-f

c

e

J permeate flux of suspended solutions(dm3/(m2 h))

Jw permeate flux of pure water (dm3/(m2 h))P ultrasonic power (W)�P transmembrane pressure (Pa)R rejection defined inEq. (3)Rf resistance due to fouling phenomena defined

in Eq. (4)(Pa s/m)Rm intrinsic resistance of membrane inEq. (4)

(Pa s/m)Rp resistance due to concentration polarization de-

fined inEq. (4)(Pa s/m)V volume of the solution (mL)

Greek lettersφw/o volume ratio of W/O emulsion to the external

aqueous phaseϕ proportional constant defined inEq. (5)(s/m)

Subscriptsext external aqueous phase in the W/O/W solutionint internal aqueous phase in the W/O/W solutionorg organic phase in the W/O/W solution0 initial (total)

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[ m-n 104

sonic power could keep the YM membrane durableprevent the organic compounds (PEI in the Cu2+–PEIsolution; carrier, surfactant, and solvent in the W/Osolution) from degradation.

5) The UF flux of the W/O/W solutions with horn ultrsound could be reasonably analyzed using resistancseries model (standard deviation, 16%). Resistanceysis indicated thatRf was less thanRm only under theconditions of smallE, largeH and/or highP. In addi-tion, the value ofRp increased with an increase inE or adecrease inP.

cknowledgements

Financial support for this work by the ROC National Snce Council, under Grant NSC 91-2214-E-155-008, is g

ully acknowledged.

Nomenclature

CE extractant carrier concentration in the organiphase (vol.%)

CS surfactant concentration in the organic phas(vol.%)

d diameter of emulsion droplet (nm)E percentage of emulsification (%)H tip height (mm)

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Documents

Flux recovery in the ultrafiltration of suspended solutions with ultrasound