recovery of n-butanol from salt containing solutions by pervaporation

Recovery of n-butanol from salt containing solutionsby pervaporation

Ver�onica García*, Eva Pongr�acz, Esa Muurinen, Riitta L. Keiski

Mass and Heat Transfer Process Laboratory, Department of Process and Environmental Engineering,FIN-90014, University of Oulu, Finland

Tel. +358 8 553 7862; Fax +358 8 553 2369; email: [email protected]

Received 16 September 2007; revised 4 December 2007; accepted 11 December 2007

Abstract

The separation of volatile organic compounds (VOCs) from chemical waste streams containing salts is aninteresting pervaporation (PV) application for recovery purposes. In this study, the separation of n -butanol (n -BuOH) by PV was investigated, and the influence of temperature, membrane choice, feed concentration, and thepresence of sodium chloride (NaCl) was evaluated. Separation factor and partial fluxes were calculated for twohydrophobic commercial membranes (CMX-GF-010-D, CELFA AG, Switzerland and PERTHESE† 500-1,Perouse Plastie, France), when permeating pure water, n -BuOH/water, and n -BuOH/NaCl/water mixtures.Further, permeance was calculated for pure water and n -BuOH/water systems at 408C. Results obtained indicatethat an increase in temperature implies higher VOC fluxes and selectivities. The apparent activation energy for thetransport of n -BuOH and water through the membranes indicate that n -BuOH flux is more affected bytemperature changes than water flux. VOC fluxes through both hydrophobic membranes increase linearly with thedriving force. The separation factor of both membranes towards n -BuOH is unchanged by an enhancement in thedriving force. It was concluded that the Celfa membrane presents higher fluxes and is less selective towards n -BuOH than the P 500-1membrane. The calculated overall permeances indicate a faster transport of compoundsthrough Celfa than P 500-1membrane. Further, the permeance of both membranes towards the transport of waterwas smaller than of n -BuOH.

Celfa and P 500-1 were found not to be permeable to salts. The addition of NaCl caused a minor variation inthe activation energy for the n -BuOH transport through the membranes. Further, the addition of salt has a modesteffect on the PV properties of n -BuOH/water mixtures through P 500-1 and Celfa membrane.

Keywords: Pervaporation; n -Butanol; Salt; Chemical wastewater; Binary and ternary system

Presented at the Third Membrane Science and Technology Conference of Visegrad Countries (PERMEA), Siofok,Hungary, 2–6 September 2007.

*Corresponding author.

Desalination 241 (2009) 201�211

0011-9164/09/$– See front matter # 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.desal. 00 . .02 7 12 51

1. Introduction

Achieving sustainable water management is amajor concern of today. The requirement toreduce the release of polluting substances intothe aquatic environment has increased waste-water disposal costs. Volatile organic compounds(VOCs) such as n-butanol (n-BuOH) are presentin chemical effluents and are considered hazar-dous due to their impact on the environment andhuman health. Article 16 of the Water Frame-work Directive (2000/60/EC) sets out theEuropean Union strategy against the release ofchemical substances such as VOCs [1]. Whilewater directives have evolved from “end of pipe”solutions to preventative and integrated manage-ment approaches, chemical industries havefocused on “end of pipe” water pollution controlstrategies. Traditional treatment methods includedistillation, extraction, and sorption and can belimited by feed conditions, large volume of by-products, and high cost of post treatments [2]. Toovercome environmental and economical draw-backs, industrial effluents should be consideredas a source of valuable compounds, and industryshould invest in more efficient recovery methodsto improve the sustainability of resource man-agement. As a result, the need for water and rawmaterials will be reduced and also the hazard ofthe chemical effluents will be decreased.

Pervaporation (PV) is a separation processthat can be used for recovery purposes. PVinvolves the selective removal of compoundsfrom liquid mixtures as vapor through a nonpor-ous perm selective membrane, by keeping thepressure of the downstream side much lower thanthe saturation pressure of the permeating compo-nents [3]. The enriched permeate is condensedand can be reused within the process, whilethe retentate is concentrated in the nonpreferen-tially permeating component and can either bereused or discharged. The PV process is drivenby a difference in chemical potential and canbe explained by the solution-diffusion model,

i.e., sorption of the feed components on the activesurface layer of the membrane, diffusion of thecomponents through the membrane, and deso-rption at the permeate side [4]. The separationeffect of PV is based on the different permeabil-ities of the compounds to be segregated andcombines the influence of both sorption anddiffusion. The advantages of PV compared totraditional methods include low operating tem-peratures, minimal energy expenditures, avoidingemissions to the environment, and no require-ment of additional component in the feed, whichprevents product contamination [5]. Further, PVcan be integrated into existing processes and withother traditional separation technologies and itsfunctions are independent of vapor�liquid equi-librium [6]. On the other hand, PVefficiency canbe limited by low fluxes, concentration polariza-tion and membrane swelling.

PV has been considered a cost effectivetechnique for separating valuable compoundsin different applications. PV is well establishedin the dehydration of alcohols and other organicsolvents [7]. In addition, PV has also beeneffectively applied for the recovery of volatileorganic pollutants from aqueous solutions [2],separation of close boiling components [8], andin the recovery of aroma compounds in the foodindustry [9]. Chemical waste streams generallyare multicomponent systems consisting oforganic compounds and nonvolatile componentssuch as salts, e.g., sodium chloride (NaCl). NaClis commonly present in chemical industry wastestream as a result of acid�base chemistry. Theseparation of n-BuOH from water by PV hasalready been demonstrated by several authors; inapplications of recovery of n-BuOH from ABEfermentation growth [10,11] and from pharma-ceutical waste streams [12,13]. However, data onternary systems, especially mixtures containingsalts, is scarce. In this study, n-BuOH isseparated by PV from synthetic binary andternary systems. Even though the PV process

202 V. García et al. / Desalination 241 (2009) 201�211

depends significantly on the differences insorption and diffusion behavior of the permeantcomponents through the membrane, processparameters can also affect the PV fluxes. Theinfluence of experimental parameters such asmembrane choice, temperature, and feed con-centration was also investigated. In addition, theapplicability of PV for the recovery of n-BuOHfrom salt-containing mixtures was studied.

2. Theory

The driving force of the process can beexpressed as an activity or partial pressuregradient across the membrane. The flux acrossthe membrane in terms of partial vapor pressuredifference can be defined as:

Ji ¼ Qov;iðp0b;i � p00i Þ ð1Þwhere /Qov;i is the pressure-normalized permea-tion flux (permeance) of compound i and /p0b;i and/p00i are the partial vapor pressures of i in the bulkof the feed and in the permeate, respectively. Thepartial pressure of i in the bulk of the feed is afunction of concentration, as expressed by themodified form of Raoult’s law:

p0b;i ¼ �iX0b;ip

0i ð2Þ

where /X 0b;i is molar fraction of specie i at the feed

side, gi is the activity coefficient in the feedmixture and /p0i is the vapor pressure of the purecomponent. The values of gi and/p0i were estimatedby Aspenplus 2004.1. Combining Eqs. (1) and(2), the following equation is obtained:

Ji ¼ Qov;ið�iX 0b;ip

0i � p00i Þ ð3Þ

According to the resistance-in-series model,the two main mass transfer resistances that areconsidered to affect the PV process are (a) theliquid boundary layer resistance, caused by aconcentration polarization phenomenon and (b)the membrane resistance. The fundamentalassumption is that, at steady state, the flux throughall the mass transfer layers is equal,

Ji ¼ Qov;iðp0b;i � p00i Þ¼ Qbl;iðp0b;i � pmb;iÞ ¼ Qm;iðpmb;i � p00i Þ ð4Þ

where /Qbl;i and /Qm;i are the pressure-normalizedpermeation flux across the liquid boundary layerandmembrane, respectively./Qov;i is related to/Qbl;i

and /Qm;i by:

1

Qov;i

¼ 1

Qbl;i

þ 1

Qm;i

ð5Þ

In this study, the effect of the liquid boundarylayer resistance is considered to be negligible, asall experimentswere conducted in a very turbulentflow regime (Reynolds number (Re) ~26,000).

Re was calculated according to the equivalentdiameter dH and feed velocity on the surface ofthe membrane v, as:

Re ¼ dHv�

�ð6Þ

where r and m are the feed density and viscosity,respectively, and dH and v are defined asfollows:

dH ¼ 2ab

aþ bð7Þ

v ¼ Vsðaþ bÞ2�a2b2

ð8Þ

where Vs (m3 s�1) is the circular feed flow rateand a and b are the rectangular slot dimensionsof the membrane module; 4.4�/10�3m and3�/10�3m, respectively.

Combining the resistance-in-series approachwith the solution-diffusion mechanism, as themembrane resistance is assumed to be the majorcontributor to the mass transfer of the permeant,the permeance for each compound i can becalculated as follows:

Ji ¼ Qm;ið�iX 0b;ip

0i � p0iÞ ð9Þ

V. García et al. / Desalination 241 (2009) 201�211 203

3. Materials and methods

PV experiments were conducted using across-flow laboratory scale membrane unit(model P28, CELFA AG, Switzerland) with aneffective membrane area of 2.8�/10�3m2 and afeed capacity of 5�/10�4m3. Two types of flat,commercial hydrophobic membranes were used;CMX-GF-010-D (CELFA AG, Switzerland)and PERTHESE† 500-1 (Perouse Plastie,France). CMX-GF-010-D (Celfa) is a compositemembrane with an active layer thickness of0.01mm, made of polysiloxane polymer andallows applications up to 353.15K (808C).PERTHESE† 500-1 (P 500-1) is a densemembrane of 0.125mm made from a siliconeelastomer that consists of dimethyl and methylvinyl siloxane copolymers. Feed temperatureswere kept constant at 295.15, 303.15, and313.15K (22, 30, and 408C) by means of athermostatic unit. Downstream side pressure wasmaintained below 300 Pa using a rotary-vanevacuum pump (Varian, DS 302). A gear pumpwas used to recirculate the feed from the feedtank through the PV unit. The feed flow ratethrough the membrane module was monitoredusing a flow meter. All the experiments wereperformed with a feed flow rate of 5�/10�5m3

s�1, in order to avoid the influence of concen-tration polarization phenomenon. Temperaturepolarization effects during the experiments werenegligible, because the permeate flux throughthe membrane was relatively small and the cellwas thermally isolated. Prior to each experiment,the membrane was conditioned by permeatingthe feed sample until steady state was attained(1 h). In the case of Celfa membrane, steady stateconditions were assumed to have been reachedwhen partial fluxes varied by less than 5% of themean value over a minimum of 1 h. In the caseof P 500-1, the low flux through the membranedid not allow the measurement of the amount ofpermeate with time. However, the variation infeed composition was found to be negligible

after 1 h. Typically, one experiment lasted for4 h, with permeate and feed samples taken atregular intervals. The permeate was collected bycondensation in liquid nitrogen cold traps, anddissolved in a specific amount of deionizedwater. When quantifying feed and permeatecomposition, the content of n-BuOH (obtainedfrom Kemfine Oy) was determined by gaschromatography with a flame ionized detector(Agilent, 6890N) and NaCl (Merk, pro-analysi)by atomic absorption spectroscopy (PerkinElmer, Aanalyst 4100). PV performance isrepresented in terms of mass flux J and separa-tion factor a, which are defined as J�/m /Dt�/Aand a�/(y1/y2)/(x1/x2), respectively. While m isthe total amount of permeate collected during theexperimental time interval Dt at steady state, Ais the effective membrane area, x1 and x2 theweight fraction of the minor compound andwater in the feed, and y1 and y2 the weightfractions of the minor compound and water inthe permeate. During the experiments, the feedreservoir quantity was much larger than thepermeate quantity transferred across Celfa andP 500-1membranes (500 g B/ 4.2 g and 500 g B/

0.4 g, respectively). As a consequence, variationsin n-BuOH feed concentration were neglected,and the given feed concentration was consideredas an average of the feed concentration valuesduring the experiment. The salt content in themembranes was studied by X-ray excited photo-electron spectroscopy using Scienta SES-200electron energy analyzer with commercial SpecsX-ray source.

4. Results and discussion

Partial fluxes and selectivity exhibited byCelfa and P 500-1 were studied at different feedtemperatures and concentrations. At first, purewater experiments were performed as reference,to assess the membrane’s behavior in contactwith different mixtures under study. Further,


separations of binary (n-BuOH/water) mixtureswere carried out in order to evaluate theinfluence of temperature and the driving forceof the process by means of the feed concentra-tion. Afterwards, NaCl was added to the n-BuOH/water binary system, to study the effect ofthe presence of salt at different temperatures andsalt concentrations.

4.1. Effect of operating temperature on PVproperties

The influence of temperature was studied inthe range of 295.15�313.15K (22�408C). Thetemperature dependence of component fluxes in

the binary system through Celfa and P 500-1membranes can be explained by the Arrheniusequation Ji�/J0,i exp(�/Ea,i /RT), where /Ea;i isthe so called apparent activation energy; J0,i isthe pre-exponential factor in the case of perme-ate total flux, R the universal gas constant, and Tthe temperature in Kelvin. /Ea;i includes theinfluence of temperature on the driving force inPV, as well as the influence of temperature onboth the solubility of given component in themembrane and diffusivity of this componentthrough the membrane [14,15]. The Arrheniusbehavior is illustrated in Fig. 1. As water fluxvalues remain similar when permeating purewater and binary systems, only pure water fluxis plotted in the figure. As shown, partial fluxesincreased with temperature in the PV of purewater and binary systems. Increasing tempera-ture caused an increase in the flux of n-BuOHthrough the hydrophobic membranes. This is aconsequence of an increase in the molecularmobility due to polymer chain segmental motionat higher temperatures as a result of the largeravailable free volume in the polymer matrix[16]. The apparent activation energies for the n-BuOH and water transport through Celfa and P500-1membranes were calculated from theslopes of individual Arrhenius plots for purewater and binary systems and, together with thecoefficient of determination (r2), are shown inTable 1.

Table 1Apparent activation energy (kJ mol�1) and coefficient of determination (r2 ) for the transport of water and n -BuOH, inbinary and ternary systems, and pure water through Celfa and P 500-1membranes

n -BuOH flux Water flux

Celfa P 500-1 Celfa P 500-1

Ea (kJ mol�1) r2 Ea (kJ mol�1) r2 Ea (kJ mol�1) r2 Ea (kJ mol�1) r2

Water 42.77 0.998 45.63 0.995n -BuOH/water 58.53 0.998 40.62 0.956 42.04 0.999 32.30 0.999n -BuOH/NaCl/water 53.23 0.999 45.28 0.990 41.07 0.999 33.37 0.995

–10123456

3.181000/T (K–1)

ln J

i

3.383.333.283.23

n-BuOH Celfa Water Celfan-BuOH P 500-1 Water P 500-1

Fig. 1. Arrhenius type dependence of n -BuOH andwater when permeating pure water and n -BuOH/waterthrough Celfa and P 500-1membrane. Feed composition0.2wt.% n -BuOH.


Further, Celfa and P 500-1membranes fol-lowed the Arrhenius behavior. In both mem-branes Ea,n-BuOH was higher than Ea,water. Asimilar trend is reported for the separation ofethylbutyrate/ethanol/water by polydimethylsi-loxane hollow fiber (Ea values of 49.10, 27.8,and 19.5 kJ mol�1, respectively) by Shepheret al. [17] and for the permeation of apple juiceflavor compounds (Ea,n-BuOH and Ea,water valuesof 106.45 and 92.09 kJ mol�1, respectively) byOlsson and Tr€agårdh using a poly(octyl)methyl-siloxane membrane [18]. A higher value of /Ea;i

implies a more sensitive behavior towards tem-perature changes. Consequently, the partial fluxof n-BuOH was the most affected by temperaturewhen it permeated through the membranes.

Fig. 2 shows the effect of temperature on theselectivity of Celfa and P 500-1membranestowards n-BuOH. The effect of temperature onselectivity depends on the variation of the sorp-tion of the species in the membrane withtemperature, and the relative changes in thediffusion coefficient for the diffusing species.As illustrated, the selectivity of Celfa membranetowards n-BuOH seemed to be positively affectedby temperature. This might be due to highervalues of /Ea;i of the organic solvent compared

with water. Further, P 500-1 showed an increasein selectivity when temperature was raised from303.15 to 313.15K (from 30 to 408C).

It can be concluded that an increase intemperature implies higher n-BuOH fluxes andselectivities. The same trend was observed alsoby Thongsukmak and Sirkar [19]. This is mainlydue to an increase in the driving force for PV bythe raise of vapor pressures with temperature. Inaddition, the diffusivity of the species isenhanced with temperature as the viscosity ofthe liquid decreases. Results obtained in thisstudy seem to suggest that working at highertemperatures would enhance the relative flux ofthe VOC compared to water. Permeating n-BuOH through Celfa membrane would implyworking at higher temperatures than using P500-1membrane in order for the flux to beappreciably increased as higher /Ea;i values areobserved. This can be due to the difference inpermeant membrane affinity.

4.2. Effect of concentration on PV properties

The influence of the process driving force onpartial fluxes can be evaluated by observing theeffect of feed concentration (keeping constanttemperature and downstream pressure), as anincrease in feed concentration causes a raise inthe activity and partial pressure. n-BuOH/watermixtures containing a n-BuOH concentration inthe range of 0�1.36wt.% and 0�1.00wt.% werepervaporated through Celfa and P 500-1, respec-tively. Figs. 3 and 4 show the effect of n-BuOHconcentration, by means of the driving force, onthe partial flux and selectivity through Celfa andP 500-1membranes, when testing binary systems.

As illustrated in Fig. 3, the n-BuOH fluxincreased linearly with the driving force whenpermeating through both hydrophobic mem-branes. The linearity of the flux/driving forcerelationship indicates that a constant n-BuOHpermeance can be assumed in the studiedconcentration range. Further, the water flux

0

10

20

30

40

50

60

295Temperature (K)

Sep

arat

ion

fact

or

310305300

Celfa membrane P 500-1 membrane

Fig. 2. Effect of temperature on the separation factor ofCelfa membrane and P 500-1membrane towardsn -BuOH in a binary system. Feed composition 0.2wt.%n -BuOH.


remains constant, indicating swelling of bothmembranes. When the n-BuOH feed compositionwas increased, the total fluxes across P 500-1 andCelfa membranes raised from 6.99 to 9.45�/

10�6 kg m�2 s�1 and from 67.91 to 101.71�/

10�6 kgm�2 s�1, respectively. This effect mightbe due to the enhanced diffusion of n-BuOHthrough the membrane as the proportion ofn-BuOH in the feed increased. Fig. 4 showsthat the selectivity of P 500-1 and Celfa towardsn-BuOH remained constant with the increasein the driving force (a�/569/8 and 399/1,respectively). Further, the permeate n-BuOH

concentration when permeating the binarysystem through P 500-1membrane increasedwith the increase of feed n-BuOH concentrationand reached 36.21wt.% at a feed concentrationof 1.00wt.%. Same trend was observed in thepermeation of n-BuOH/water through Celfamembrane, when the permeate contained n-BuOH in the concentration of 34.81wt.% atn-BuOH feed concentration of 1.36wt.%.Finally, it can be concluded that Celfa membranepresents significantly higher partial fluxes thanP 500-1. In contrast, P 500-1 is more selectivetowards n-BuOH than Celfa membrane. This isdue to differences in thickness.

According to Eq. (3), the overall mass trans-fer parameter that characterizes the flux of eachcomponent is the permeance Qov,i and is calcu-lated from the slope of the linear fittings of Ji vs /

�iX0b;ip

0i , in Fig. 3. Values obtained for the

permeation of n-BuOH through P 500-1 andCelfa membranes are Qov,n-BuOH�/2.82�/10�5

kgm�2 s�1 kPa�1 and Qov,n-BuOH�/2.25�/

10�4 kgm�2 s�1 kPa�1, respectively. Waterpermeance through P 500-1 and Celfa mem-branes was calculated from the flux dataobtained in the experimental run when circulat-ing only water, Qov,w�/7.03�/10�7 kgm�2 s�1

kPa�1 and Qov,w�/9.24�/10�6 kgm�2 s�1

kPa�1, respectively.

4.3. Effect of the presence of salt on PVproperties

Impermeable components can influence theperformance of PV significantly. Salts affectboth the mass transfer rate and the driving forcein the transport of organic compounds through anonporous silicone rubber membrane [20]. Thepresence of salt increases the density andviscosity of the feed solution. As a consequence,the transport rate should be slower. Based on thesalting-out effect, activity coefficients of soluteand solvent are also altered, affecting the PVdriving force [21]. The salting-out phenomenon

0102030405060708090

100

0

Sep

arat

ion

fact

or

16014012010080604020

P 500-1 membrane Celfa membrane

Xn-BuOH Yn-BuOH P0 × 103 (kPa)n-BuOH

Fig. 4. Effect of process driving force on the separationfactor of Celfa membrane and P 500-1 towards n -BuOHin n -BuOH/water mixtures at 313.15K (408C).

0

5

10

15

20

25

30

35

0

J n-B

uOH

thro

ugh

P 5

00–1

× 1

07

(kg

m–2

s–1

)

J n-B

uOH

thro

ugh

Cel

fa ×

10–7

(kg

m–7

s–1

)

050100150200250300350400

16014012010080604020

P 500-1 membrane Celfa membrane

Xn-BuOH Yn-BuOH P

0 × 103 (kPa)n-BuOH

Fig. 3. Effect of process driving force on n -BuOH fluxwhen permeating n -BuOH/water system through Celfaand P 500-1membranes at 313.15K (408C).


is based on a reduction in the solubility of thecompounds (increase of the activity coefficient)in the presence of salts and interactions betweensolvent molecules and ions based on solvationand hydration. When the solubility of thecompound is enhanced (decrease of the activitycoefficient), the effect is known as salting-in[22]. Thus, the PV driving force for thecompounds being salted out and in, are enhancedand decreased, respectively. The effect of saltson the PV properties will be a balance betweenthe salting out effect and the retarding effect ofthe increased viscosity [20]. The salting outeffect will be characteristic of the given organic-salt combination [21]. Possible fouling of themembrane by salt penetration and physicalblocking of the pores on the permeate side bymicro-salt crystallites can also affect the PVproperties [13,23].

Cocchini et al. studied the effect of NaCl onthe extraction of 2-nitrotoluene through siliconerubber membranes [24]. It was found that themass transfer and driving force for the processwere strongly dependent on the presence of salts.Lipnizki et al. studied the effect of NaCl, MgCl2,and glucose on the permeation of 1-propanol andwater through the commercial hydrophobicPDMS membrane PERVAP 1060 and 1070[23]. They observed a significant positive effecton the selectivity towards the organic compoundwith an increasing salt concentration in thefollowing order MgCl2�/NaCl�/glucose. Inaddition, photoelectron spectroscopy XPS stu-dies showed that NaCl penetrated into thestructure of the membrane and was present onthe membrane surface. The same occurrence wassuggested by Shah et al [13]. Further, Kujawskiet al. have recently investigated the influence ofthe presence of NaCl on the PV performance ofPERVAP 1060membrane in contact with water�methyl acetate solutions [25]. The presence ofNaCl caused the increase of the selectivity of themembrane towards the organic compound due to

a decrease in water flux. In all these cases themembranes were found to be impermeable tosalts. On the contrary, Ravindra et al. reportedNaCl in the permeate side when processingreaction liquors with a salt concentration higherthan 10wt.% using a dense chitosan membrane[26]. In addition, Zwijnenberg et al. described asolar driven PV process for the production ofdesalinated water from highly contaminatedwaters using a polyetheramide-based polymerfilm with polyamide/polyethylene nonwovensupport shell. They also reported the presenceof salts in the condensate at mg L�1 level(99.998% salt retention) [27]. An explanationfor these observations might be the hydration ofions in aqueous solutions forming water clusters.As a consequence, the affinity of the ionstowards the membrane increases [23]. Once theions penetrate into the membrane structure,diffusion of salts along with other compoundsuntil the other side of the membrane occurs [26],and ions are being transported as aerosol byadvection [28].

Preliminary tests were conducted to check thepermeability of the membranes towards NaCl bypermeating NaCl/water and n-BuOH/NaCl/water mixtures. When analyzing the permeate,the electrolyte could be detected by AAS at ppblevel. In order to exclude the possibility ofsample contamination, the PV of CdCl2/watermixtures was conducted. Results indicate that nocadmium was detected in the permeate; there-fore, we can conclude that the Celfa and P 500-1membranes were not permeable to salts. Further,XPS experiments indicated no sodium above theestimated detection limit of 0.2% of the amountof silicon in the membrane.

Solutions containing NaCl were pervaporatedin the temperature and chloride concentrationrange of 295.15�313.15K (22�408C) and0�0.9wt.%, respectively. Results indicate thatthe addition of NaCl to n-BuOH/water mixturescaused a minor influence in the temperature


effect on n-BuOH fluxes through Celfa andP 500-1membranes (Table 1) as the values ofapparent activation energy for n-BuOH andwater transport were about constant.

The effect of salt on the PV properties ofCelfa and P 500-1 in the permeation of n-BuOH/water system is shown in Figs. 5 and 6. Asobserved in Fig. 5, the presence of electrolytein the feed solution has no influence onn-BuOH flux, when it permeated with waterthrough the hydrophobic membranes. Water

flux, 66.41 kgm�2 s�19/1.12 through Celfaand 21.60 kgm�2 s�19/1.27 through P 500-1,were independent of the presence of salt. NaCl isdescribed by Cocchini et al. to have a weakinfluence on water activity [21]. As Fig. 6demonstrates, the selectivity of Celfa andP 500-1membranes towards n-BuOH was notaffected by the presence of salt.

5. Conclusions

Separation of organic compounds from aqu-eous solutions holds importance for its potentialof preventing water pollution and recoveringvaluable materials. PVof n-BuOH through Celfaand P 500-1membranes was conducted atdifferent n-BuOH concentrations and tempera-tures in binary systems. In addition, the effect ofsalt content was analyzed. Results indicate thatthe membranes used are effective for the recov-ery of n-BuOH, even when an electrolyte ispresent. Fluxes through the membranes and theseparation factor increased with temperature,with n-BuOH flux more influenced by changesin temperature than water flux. An increase inPV driving force implies increase in n-BuOHflux through the membranes. Selectivity valuesthat P 500-1 and Celfa exhibit toward n-BuOHremained constant. Celfa and P 500-1mem-branes are found not to be permeable towardsNaCl. The presence of electrolyte caused a minorchange in the temperature effect on n-BuOHfluxes through the membranes. Permeate flux andseparation factor were independent of the exis-tence of NaCl. Comparing the performance ofboth membranes, it can be concluded that Waterflux, 66.41 kg m�2 s�19/1.12 through Celfaand 21.60 kg m�2 s�19/1.27 through P 500-1,were independent of the presence of salt. Celfamembrane presents higher fluxes but is lessselective towards n-BuOH than P 500-1. Thecalculated overall permeances indicate a fastertransport of compounds through Celfa than

0

10

20

30

40

50

60

70

80

90

100

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

J n-B

uoH

x 10

7 (k

g m

–2 s

–1)

[CI–] (Wt%)

Celfa membraneP 500-1 membrane

Fig. 5. Effect of the presence of salt on the permeationflux of n -BuOH when permeating n -BuOH/NaCl/watermixtures through Celfa and P 500-1membrane at313.15K (408C) and a feed composition 0.2wt.%n -BuOH.

0

10203040

506070

0(Cl–) (wt%)

Sep

arat

ion

fact

or

Celfa membrane P 500-1 membrane

10.90.80.70.60.50.40.30.20.1

Fig. 6. Effect of the presence of salt on the separationfactor that Celfa and P 500-1membranes exhibit towardsn -BuOH when permeating n -BuOH/NaCl/water, mix-tures at 313.15K (408C) and a feed composition 0.2wt.%n -BuOH.


P 500-1membrane. Further, the permeance ofboth membranes towards the transport of waterwas smaller than of n-BuOH.

Acknowledgements

The authors thank Ms. Audrey Bouchery andMr. Ismo Koskenkorva for their assistance inconducting the experiments. Mr. Marko Huttulafrom the Electron Spectroscopy Group at theDepartment of Physical Sciences, University ofOulu is gratefully acknowledged for the X-rayexcited photoelectron spectroscopy analysis ofmembranes. KemFine Oy and Dr. Pekka Oinasare thanked for their collaboration. The valuablecomments of Ms. Junkal Landaburu-Aguirre andthe two anonymous reviewers were also greatlyappreciated. Finally, the Academy of Finland(project no. 111416), the Maj and Tor NesslingFoundation, and the Thule Institute are acknowl-edged for their financial support.

Nomenclature

A effective membrane area (m2)dH equivalent diameter (m)Ea apparent activation energy (kJ mol�1)J permeation flux (kg m�2 s�1)J0 pre-exponential factor (kg m�2 s�1)m mass of permeate (kg)p partial pressure (kPa)p0 vapor pressure of the pure component

(kPa)Q permeance (kg m�2 s�1 kPa�1)R universal gas constant (kJ mol�1 K�1)Re Reynolds numberr2 coefficient of determinationT temperature (8C, K)Dt experimental time interval (s)Vs circular feed flow rate (m3 s�1)v velocity on the surface of the mem-

brane (m s�1)x liquid weight fraction (kg kg�1)

X liquid molar fraction (mol mol�1)y permeate weight fraction (kg kg�1)

Greeksa separation factorg activity coefficientr feed density (kg m3)m viscosity (kg m�1 s�1)

Superscriptsƒ permeate? feedm membrane

Subscriptsi componentov overallb bulkbl liquid boundary layer

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recovery of n-butanol from salt containing solutions by pervaporation

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