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Journal of Membrane Science 301 (2007) 221–237

Recovery of vanillin from lignin/vanillin mixture byusing tubular ceramic ultrafiltration membranes

M. Zabkova, E.A. Borges da Silva, A.E. Rodrigues ∗Laboratory of Separation and Reaction Engineering (LSRE), Department of Chemical Engineering, Faculty of

Engineering, University of Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal

Received 24 January 2007; received in revised form 5 June 2007; accepted 13 June 2007Available online 19 June 2007

bstract

The aim of this work is to evaluate the membrane ultrafiltration process for the recovery of vanillin from Kraft lignin oxidation media. Tubulareramic membranes with molecular weight (Mw) cut-off of 1, 5 and 15 kDa are investigated. The influence of lignin and vanillin concentrationnd pH of the mixture on the performance of ultrafiltration is studied in batch mode. Permeate flux, lignin and vanillin content in retentatend permeate streams were measured. A theoretical model is used to calculate the variation of the concentration profile within the polarizationoncentration layer and to estimate the membrane surface concentration during the ultrafiltration operation. The values obtained for the ligninoncentration at membrane surface suggests that there is a growth of a gel layer in the ceramic membranes of higher Mw cut-off for the oper-ting conditions used. By filtrating the lignin/vanillin mixture at the concentration 60/6 g L−1 and pH 12.5 using 1 kDa cut-off membrane, the

bserved rejection of lignin was 0.972. On the other hand, using the membranes with small cut-off leads to significant decline of permeate fluxompared with the membranes with larger cut-off. It was observed that very high cleaning efficiency can be obtained using sodium hydroxideolution.

2007 Elsevier B.V. All rights reserved.

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eywords: Vanillin; Lignin; Separation; Ultrafiltration; Membranes

. Introduction

Since the late sixties membrane filtration processes haveound industrial applications and started to compete with pro-esses like distillation, evaporation or extraction. Membranesake advantage of their physical and chemical stability, repro-ucible performance, selectivity and high surface area per unitolume [1]. They have broad applications in the wastewaterreatment, food processing, biotechnology and pharmaceuticalndustry for the concentration of solutions and size-selective sep-ration of large molecules from low molecular weight species2].

Membrane filtration processes are pressure-driven separationechniques generally classified according to the size of particlesetained by the membrane. The main categories of membranes

∗ Corresponding author. Tel.: +351 22 5081671; fax: +351 22 5081674.E-mail addresses: miriam@fe.up.pt (M. Zabkova), eduardo@fe.up.pt

E.A.B. da Silva), arodrig@fe.up.pt (A.E. Rodrigues).

litlwste

376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2007.06.025

over microfiltration, ultrafiltration and nanofiltration with theange of pore size from 2 �m to 0.5 nm. Ultrafiltration mem-ranes have pore size between 50 and 2 nm, operating pressureifference between 1 × 105 and 1 × 106 Pa and retain higholecular-mass solutes (polymers, proteins, colloids) and smallolecules like water, salts can pass through. Ultrafiltration is

tilized for the concentration of macromolecular species suchroteins, polysaccharides, various polymers and the elimina-ion of macrosolutes from effluents or in water for industrial or

edical applications.The Kraft pulp liquor coming from paper industry is a waste

hich must be treated before disposal. The main purpose ofignin removal from the liquor is to separate the cooking chem-cals, NaOH and Na2S to avoid losses of these chemicals fromhe mill [3]. On the other hand, organic compounds present iniquors, especially lignin, is a very useful raw material from

hich many valuable substances or materials can be produced,

uch as: vanillin, vanillic acid, dispersing agents, syntheticannins, polymer filter sand binding agents, active carbon, ion-xchanger, etc. Thus, the recovery of inorganic chemicals from

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ipfiecoF5wwrwoutside diameter 10 mm. Filtanium membranes for fine ultrafil-tration were made of TiO2 active layer on the ceramic support,with the operating differential pressure of 0 × 105–1 × 106 Paand resistant to pH in the range 2–14. The membranes with 5

22 M. Zabkova et al. / Journal of Me

lack liquor is an important part of pulp and paper industries [4].n a conventional process, the large amount of black liquor gen-rated in pulp and paper industries is handled in a destructive wayo recover inorganic chemicals (is burned as a fuel) and manyaluable organics are vanished [5]. In order to overcome the dis-dvantages found in the conventional treatment process for blackiquor, constant research cover with the search for alternativereatment methods.

Membrane based separation process was the focus of atten-ion of the separation technologists for the past three decades4,6–8]. Numerous works have so far been performed to studyhe treatability of Kraft black liquor using membranes [5,9–17].owadays, the processes used for lignin removal are precipita-

ion and ultrafiltration. Ultrafiltration was generally employedainly for the following purposes: (a) separation of lignin com-

ounds from low molecular weight inorganic, in order to useignin as a fuel or valuable product, (b) fractionation of high

olecular weight lignin compounds and (c) recovery of water4].

One of the major drawbacks with the process of ultrafiltrationor treating Kraft black liquor is the decline of flux with time. Theux decline is attributed to either of osmotic pressure retardation,olute retention at membrane surface (polarization and/or gelormation) or fouling of the membrane, resulting from reversibler irreversible pore plugging [17]. As a result there is a decreasen driving force for the filtration or increase in the resistancegainst transport of the permeating solvent during the filtration.

In the process of Kraft lignin oxidation to produce vanillin18], the reaction media contains degraded molecules of ligninnd sodium salt of vanillin (vanillate) and other species. Thesolation of vanillate from the oxidized solution is an impor-ant stage in the vanillin production. During the ultrafiltrationrocess vanillin goes to the permeate stream because of its lowolecular weight. The lignin as a macromolecule stays in the

etentate and its concentration significantly increases during theatch ultrafiltration process. Therefore, the ultrafiltration tech-ique can be used as a first step in the process of recovery ofanillin from Kraft lignin oxidation.

The objective of this work is to study the membrane ultrafil-ration process particularly focused on the recovery of vanillinrom Kraft lignin oxidation media. For this purpose tubulareramic membranes Kerasep supplied by Orelis (France) witholecular weight cut-off of 15 kDa and Filtanium membranesith molecular weight cut-off of 1 and 5 kDa supplied by Tami

ndustries (France) were tested. The influence of lignin andanillin concentration and pH of the mixture on the ultrafiltrationrocess has been analyzed as well.

. Experimental part

.1. Chemicals and methods

Vanillin (C8H8O3, purity �98%), pellets of sodium

ydroxide (NaOH, purity�98%), and lignin were used in exper-mental work. Lignin used has an average molecular weight

w = 60,000 g mol−1 and the amount of sulfur total impuritiesas 9 wt% according to the supplier Sigma–Aldrich. Hydrochlo-

Ff6pfl

ne Science 301 (2007) 221–237

ic acid at concentration 1 M was used for acidification of theamples. All chemicals were supplied by Sigma–Aldrich. Theixtures of vanillin and lignin and sodium hydroxide were pre-

ared with degassed and deionised water with conductivity valueower than 1 �S cm−1. The pH of the mixture was adjusted bydding NaOH solution.

Vanillin and lignin concentration were measured by UV–vispectrophotometer Jasco (model 7800, Japan) after pretreatmentf the samples. The samples were acidified by concentratedydrochloric acid to precipitate high molecular weight com-onents (lignin). The precipitated solid was separated byentrifugation during 15 min at 83.33 Hz. The supernatant waseparated from the precipitated solid, diluted in deionised waternd analyzed by UV spectrophotometer for vanillin contentt the wavelength of 259 nm [18]. The precipitated solid wasiluted in 2N solution of sodium hydroxide in order to dis-olve the lignin content. The amount of lignin was measuredy UV spectrophotometer at the wavelength of 280 nm [19].ue to inclusion of vanillin inside the lignin precipitate, the

alibration samples were treated by the same technique asbove.

.2. Equipment and experimental set-up

All experiments have been performed by using the exper-mental set-up shown in Fig. 1. It consists of a feed tank,eristaltic pump (Watson-Marlow, England), damper and ultra-ltration unit (Orelis, France). The membranes used inxperiments were Kerasep membranes with molecular weightut-off (molecular weight at which the membrane rejects 90%f solute molecules) of 15 kDa supplied by Orelis (France) andiltanium membranes with molecular weight cut-off of 1 andkDa supplied by Tami Industries (France). Kerasep membranesere made of Al2O3–TiO2 active layer on the ceramic support,ith the service differential pressure of 0 × 105–4 × 105 Pa and

esistant to pH in the range 0–14. The total membrane areaas 0.008 m2; length was 400 mm, inside diameter 6 mm and

ig. 1. Scheme of the experimental set-up for batch ultrafiltration: 1, tank witheed mixture; 2, peristaltic pump; 3, damper; 4, membrane module; 5, membrane;, tank with permeate solution; 7, circulating valve; P1, pressure gauge inlet; P2,ressure gauge outlet; Qfeed, feed flow-rate; Qp, permeate flow-rate; Qr, retentateow-rate.

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M. Zabkova et al. / Journal of Me

nd 1 Da cut-off are identified as fine ultrafiltration membranesTami Industries, France).

.3. Measurement of water flux through membrane

The pure water permeate flow-rate was measured for eachembrane in order to obtain the water flux at different trans-embrane pressure and to determine the membrane resistance

Rm). Water flow-rate in the permeate side was measured at dif-erent pressures which were adjusted by a recirculation valve onhe system starting from the lowest to the highest pressure andice versa. The value of the membrane resistance was also useds an indicator of the membrane cleaning effectiveness.

The measured quantities during the experiment were: Qr,etentate flow-rate (L h−1); Qp, permeate flow-rate (L h−1);P1, relative inlet pressure on the feed side (Pa) �P2, relative

utlet pressure on the retentate side (Pa); T, temperature (◦C).The average water permeate flux is calculated as:

= Qp

Am(1)

here J is permeate flux at time t (L h−1 m−2) and Am is mem-rane surface area (m2). The water permeate flux was measuredt different average transmembrane pressures at 25 ◦C.

The average transmembrane pressure �P (Pa) is obtainedrom,

P = �P1 + �P2

2(2)

here �P1 is the relative pressure at the inlet of the membraneodule (Pa) and �P2 is the relative pressure at the outlet of

he membrane module (retentate side) (Pa). Pure water permeateux data are used to evaluate the membrane hydraulic resistanceRm) according to Darcy’s law:

= �P

η0Rm(3)

here η0 is the solvent viscosity (Pa s).

.4. Membrane cleaning after experiments

The retained molecules accumulate on the membrane surfaceeading to the decrease of the membrane performance and peri-dically cleaning of the membrane must be carried out after eachun.

The lignin solubility increases with increasing pH. Ifeionised water is used (pH 6.1), some lignin can precipitateith lowering the pH and thus block the membrane. Therefore,.1–0.2 M NaOH solution was used to clean the membrane afterach experiment. In the first step of the cleaning procedure theembrane and feeding tubes were rinsed with solution of NaOHithout opening the circulation valve to remove residues inside

he system; later on the circulation valve was opened and theransmembrane pressure �P = 2.0 × 105 Pa adjusted. By “openhe valve” was meant that there is permeate flow out of the

embrane.

2umi

ne Science 301 (2007) 221–237 223

The time of cleaning procedure was 5–6 h. In the next step,he system was rinsed with deionised water in order to obtaineutral pH that was measured in the permeate and the retentateide. After each cleaning procedure the permeate flux of wateras measured and compared with the reference value.

.5. Ultrafiltration of vanillin/lignin mixture

The experiments were performed by using the lab-scalexperimental set-up shown in Fig. 1 in the batch mode. Its a commonly used method where retentate is re-circulatedack to the feed tank and the permeate is withdrawn to aeparated reservoir. The lignin/vanillin mixture was preparedn the feed tank with an initial feed volume depending onhe experiment (7, 5 or 4 L). The studied compositions of theeed mixture were approximately 60 g L−1 of lignin–6 g L−1

f vanillin, 20 g L−1 of lignin–2 g L−1 of vanillin, 5 g L−1 ofignin–0.5 g L−1 of vanillin, and the pH tested was in the range.5 and 12.5.

Before starting the experiment, the feed tank was filled with.1 M solution of sodium hydroxide and the whole systemncluding the tubes was properly washed in order to remove for-ign bodies that membrane can contain. Subsequently, deionisedater was passed through the system and the experimental rigas ready for use. The pH of the mixture was adjusted with 0.1 MaOH. Then the system was filled with the feed solution and

he feed flow-rate adjusted, which was kept constant during allhe experiment duration. The experiment started when the circu-ation valve was open and the average transmembrane pressure�P) adjusted to the value of 1.3 × 105 or 1.55 × 105 Pa.

During the experiment the retentate flow-rate, the permeateow-rate and pressures �P1 and �P2 were measured; the sam-les of permeate and retentate were also collected. After eachxperiment the final volume of permeate withdrawn and the vol-me of concentrated solution in the feed tank was measured andhe volume reduction was determined by:

R = Vp

V0(4)

here Vp is the permeate withdrawn volume and V0 is the initialolume of the feed tank.

.6. Measurement of viscosity

Viscosity measurements were made with routine capillaryiscosimeter Jenna-Glass Schott Mainz (Germany) [20]. Theiscosimeter has been designed in such a way that correction ofinetic energy may be ignored when flow time exceeds 200 s.he solution at the concentration 5, 20, 40, 60 and 80 g L−1 of

ignin at pH 8.5 and 12.5 was prepared. The viscosimeter waslled with the sample of lignin and the flow time between twoarks written on the viscosimeter was measured. The measure-ent was repeated three times for each sample at the temperature

6 ◦C. To evaluate the viscosity (Pa s), the equation η = K × t wassed where the constant K = 0.002865 [20]. The viscosity deter-ined for each concentration of lignin was used to calculate the

ntrinsic viscosity [η] (L g−1).

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24 M. Zabkova et al. / Journal of Me

. Basic principles in ultrafiltration modeling

The membrane ultrafiltration is a size-exclusion, pressure-riven process. The important characteristics of ultrafiltrationrocess are the volumetric flux (permeate flux) and the rejectionf the solute by the membrane. The basic flux equation in aubular membrane may be expressed as:

= �P

ηRm(5)

here J is the volumetric flux, η the viscosity of solution, �P theransmembrane pressure (=P(z) − Pp); where P(z) is the pressureistribution along the flow channel and Pp is the permeate pres-ure of the shell side. For the case of a perfectly mixed system,oth pressures are assumed constant and Rm is the membraneydraulic resistance. The term ηRm can be found as the thicknessf membrane to its permeability ratio and, then, the volumet-ic flux depends inversely upon the thickness of the membrane.ne of the major challenges with the modelling of ultrafiltrationrocess is the estimation of the decline of the flux with time.

Konieczny and Rafa [21] have reported that the basic prob-ems involving the ultrafiltration of pressure membrane processre:

flux decline driving the operation of system;efficiency of the process is becoming independent of trans-membrane pressure (limiting flux);fouling and concentration polarization leading to the reduc-tion of efficiency of membrane.

As it is, Eq. (5) does not include any phenomena accountingor the loss of efficiency in the filtration. These phenomena arehe gel formation, the membrane polarization, the fouling of the

embrane, resulting from reversible or irreversible pore plug-ing, which are very important in UF and need to be includedn any detailed analysis.

Fouling is the accumulation of material on the surface (exter-al) or in the pores (internal) of the membrane. In some UFperations, a membrane fouling will remain until a cleaningtep is performed; after that, the original flux is partial or com-letely restored. Fouling can arise from several phenomena suchs precipitation, adsorption, electrostatic attraction, biologicalrowth, chemical reaction and polymerization. The permeateux decline caused by reversible fouling can be recovered byleaning the membrane with de-ionized water, whereas the flowecline caused by irreversible fouling can be only recovered byeplacing the membrane or cleaning it using chemical reagents22]. The irreversible case is more critical, when membraneores are blocked by the foulants or the foulants are adsorbedn the membrane surface [23].

The rejection coefficient is also an important parameter to

haracterize an UF membrane, it represents a measure of themount of solute retained by the membrane. The most commonefinition of rejection is the observed rejection (Ri

obs) becauseoth permeate and concentrate solute concentration can be mea-

aoom

ne Science 301 (2007) 221–237

ured:

iobs = 1 − Cpi

Cbi

(6)

here Cpi and Cbi are the permeate and the bulk concentrationf the species i, in the feed side of the membrane, respec-ively. However, the observed rejection can be taken as anpproximation of the separation properties of the membrane.or impermeable membrane, the rejection will be equal to 1.

There are other definitions of rejection, named intrinsic rejec-ion (R0), when phenomena leading to decline flux are takinglace in UF processes, such as:

with concentration polarization

R0 = 1 − Cp

Cw(7)

where Cw is the concentration of the rejected solute at themembrane surface.with gel formation,

R0 = 1 − Cp

Cg(8)

here Cg is the gel concentration of the rejected solute.Combining (Eqs. (6) and (7)) or (Eqs. (6) and (8)), one can

btain the relation between the observed and intrinsic rejectionoefficients.

A variety of models for volumetric flux calculation has beeneported in literature. Cheng and Wu [24] have mentioned somesed models for analysing the mean volumetric flux of ultrafil-ration: the gel-polarization model [25], the resistance-in-series

odel [26], the osmotic pressure model [27], the boundary layerodel [28]. With regard to the ultrafiltration of black liquor,hattacharya and co-workers have extensively studied the fluxecline prediction in (rectangular and radial) cross-flow ultra-ltration and in stirred batch cells [4,5,13–16,29,30]. In theirodels, decline in permeate flux over the period of operation

s attributed mainly to the concentration polarization leading tohe build up of osmotic pressure of solution and the develop-

ent deposited layer on the membrane surface. The osmoticressure controls the initial decline and the deposited layer issually responsible by the progressive flux decline [4]. How-ver, gel polarization model has already been used to correlatexperimental UF data of Kraft and spent sulphite liquor [31].ovasin [32] has commented that the use of gel polarizationodel is criticized since the actual gelation is not usual in UF;

e also discussed mass transfer phenomenon in the UF of spentooking liquor. In this work, the unsteady-state model of theurrent ultrafiltration system is presented and the permeate fluxs represented to take into account the osmotic pressure and therowth of deposited layer at the membrane wall (formation of

s a tool to help on the flux decline analysis, i.e., the presencef resistances to the permeate flux, and to allow the estimationf the rejected solute concentration at the surface of the UFembrane.

M. Zabkova et al. / Journal of Membrane Science 301 (2007) 221–237 225

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.1. Description of the modeling for flux decline analysis

The concentration polarization is a phenomenon in whichhe solute that is rejected by the membrane accumulates on the

embrane surface and then its concentration at surface is higherhan that in the bulk. Fig. 2 shows a scheme of the polariza-ion on tubular membrane surface. Since the flux of the specieshrough the membrane is low, its concentration will build up athe membrane wall and such build up will result in back diffu-ion of the solute into the bulk fluid. If the concentration of theejected solute on the membrane exceeds a certain value, namedel concentration (Cg), the rejected solutes can form gels, cakesr slimes at the membrane wall. At the beginning of ultrafiltra-ion, only a polarization layer is taking place. After some time,elation point can be reached and then the net solute flux doesot lead to a further increase of the concentration at the mem-rane surface; however, the thickness of the gel layer can change33].

Considering the scheme of Fig. 2, the governing soluteonservation equation, in cylindrical coordinates, within polar-zation concentration layer is expressed as

∂C

∂t+ J

∂C

∂r= D

r

∂

∂r

(r∂C

∂r

)(9)

here C is the concentration of species and D is the diffusivityf the species within polarization layer. The assumptions are: (i)onstant diffusivity in the range of variation of the solute concen-ration in the layer [34,35]; (ii) negligible gradient concentrationn the axial direction in comparison to the radial direction; (iii)ymmetry condition to the θ-direction. Eq. (9) can be solvedsing the following initial and boundary conditions:

t t = 0, C = Cb(t = 0) (10)

t t > 0, C = Cb(t) for r = R − δpol (11)

t t > 0, JC|r=R − DdC

dz

∣∣∣∣r=R

= JCp for r = R (12)

here δpol is the thickness of the concentration polarization layer

=D/k; k is the mass transfer coefficient for the solute transporthrough the membrane). Eqs. (11) and (12) are valid before thencipient gel formation, when after that the thickness of gel layerδg) needs to be considered and C = Cg for r = R − δg.

taid

ion in tubular ultrafiltration membrane.

The permeate flux can be calculated on the frame of phe-omenological equation as:

= UF driving force

resistance(13)

The driving force in UF operations is the transmembraneressure applied on the system that can be decreased due ton increase in the osmotic pressure difference across the mem-rane. The flux decline in the membrane process also comesrom the presence of several kinds of the resistances. Tu et al.36,37] have reported that the flux decline in UF can be attributedo: (i) solute accumulation at the membrane surface (concentra-ion polarization phenomena and gel formation); (ii) irreversiblehanges in the gel layer; (iii) solute sorption on the membraneurface; (iv) internal diffusion of solute into the membrane (porelockage). Satyanarayana et al. [4] have studied the reductionf the flux for the Kraft black liquor in some UF systems –tirred, radial and flat cell – and observed the influence of thesmotic pressure and the deposited layer resistances, which ariseue to solute retention at the membrane surface. In our work,he flux equation is expressed such a way to take into accountoth short and long term flux decline discussed in details inlsewhere [38]. The short term flux decline is osmotic pressureontrolled and occurs in start-up of the UF operation; and theong term flux decline emerges from the growth of the mass ofejected solute deposited on the membrane surface (polarizednd, eventually, gel layer) and, therefore, a concentration polar-zation (and/or gel formation) controlling takes place. Assuminghese considerations, the permeate flux can be calculatedrom

= �P − �π

η(Rm + RT)(14)

here �π is a difference of osmotic pressure between the wallembrane and the permeate side (=πw − πp), η the viscosity of

he solution, Rm the hydraulic membrane resistance and RT ishe total resistance accounting the polarization resistance (Rp)nd the gel layer resistance (Rg) [RT = Rp + Rg]. The osmoticressure can be measured directly or estimated from other ther-odynamics quantities. For the case of diluted ideal solutions,

he van’t Hoff equation could be applied (i.e., π(C) = a′C, where′ is an empirically determined constant). As known, the polar-zation resistance (Rp) is a time dependent variable and increasesuring membrane filter operation until the system reach steady

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26 M. Zabkova et al. / Journal of Me

tate. In this way, the time evolution of Rp can be given as39]:

dRp

dt= kp(R∞

p − Rp) (15)

here kp is a parameter related to the velocity of formation of theolarization layer and R∞

p represents a steady state polarizationayer, i.e., a UF system working under a set of operation vari-ble will have a R∞

p resistance, limited by the bulk turbulence,t steady state condition. This resistance have been estimatedhrough the theories based on the power laws and consideredo be a function of the transmembrane pressure drop, tangen-ial velocity as well as feed, bulk and permeate concentrations36,37,40]. We have assumed that the formation of this polar-zation layer will depend on the bulk concentration, �P and theydraulic condition and it can be given by:

∞p = αp �PnCm

b (16)

here αp is a parameter that is a function of flow axial velocitypplied in the UF module. The gel layer resistance is also time-ependent caused by the increase the gel layer thickness. Theate of Rg can be obtained through mass balance in the polarizedayer resulting the equation [41],

dRg

dt= φCb

(J − k ln

(Cg

Cb

))(17)

here φ is the thickness to density of the gel layer ratio.onsidering the osmotic pressure phenomenon and mentioned

esistances, the volumetric flux can be written as:

= �P − a′R0C|r=R

η(Rm + RT(t))(18)

It is worth mentioning that in membrane processes, mainlyn ultrafiltration and microfiltration, internal pore fouling mighte a important source of resistance to the flux decline becauseither larger molecular over cut-off membrane can go throughore diffusion causing pore blockage or small molecules mightccumulate and obstruct the membrane pores. In ultrafiltrationf the mixture lignin and vanillin using 1, 5 and 15 kDa cut-offembranes, lignin is mostly rejected, since it consists of largeacromolecules with molecular weight in range of 60 kDa; and

he smaller molecules of vanillin (molecular weight of 152 Da)hall diffuse completely through the pore. Lignin is expectedo be mostly responsible for surface fouling and the vanillinransport through the membrane would be accused of some inter-al pore fouling. In this work, the effect of internal membraneouling on the UF process has been neglected based on our exper-mental observations. Pore internal fouling is usually classifieds irreversible fouling when pores may be completely or par-ially blocked by solute particles or solute adsorption can takelace leading to pore clogging; however, some authors also con-ider that pore internal fouling can have a reversible nature. Ast will be shown, the cleaning step of the membrane using alka-

ine solution has restored the original membrane permeabilitynd we can assume that irreversible fouling is not taking placen significant extent. Moreover, as commented in [36,37], fre-uently fouling influences membrane rejection characteristics

T

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ne Science 301 (2007) 221–237

nd our experimental results have shown that vanillin rejectionemains constant (RV

obs ≈ 0) for all analyzed UF membranes.ence, the modelling approach considers that flux decline isainly governed by progressive growth of the deposited layer

t the surface membrane during the ultrafiltration operation.Since vanillin transport across the membrane is facilitated

nd no vanillin rejection is observed, one might assume thatne species is not affected by the presence of the other species.owever, this fact cannot be valid if one high molecular weight

pecies causes gel deposit which might change the retentionharacteristics of the membrane and then such gel-layer mightetain the small molecules. De and Bhattacharya [42] havenvestigated such situation where a mixture of sucrose and poly-inyl alcohol (PVA) ‘ultrafiltrated’ in 10 kDa cut-off, whichoes not retain sucrose at all in the absence of PVA (a highel-forming molecular weight solute), provides a substantialejection of sucrose molecules in presence of PVA gel. Accord-ng to our experimental results no changes in vanillin rejectionre observed over the analyzed period of the filtration; so thisodeling approach has considered that the flux decline occurs

ue to the lignin solute retention causing osmotic pressure andoncentration polarization phenomena and the contribution ofanillin for the variation of flux becomes insignificant in thisF operation.

.1.1. Governing mass balances equationsConsidering the system shown in Fig. 1 the overall mass

alance is

d(V )

dt= AmJ = Qp (19)

here V(t) is the volume of the solution in the entire systemfeed tank, pipes and UF module).

The mass balance of the species (i = lignin, vanillin) over theystem can be written as,

d(VCfi)

dt= QrCri − QfCfi = −QpCpi (20)

here Q is the flow-rate and Ci is concentration of the species. The subscripts f, r and p mean feed, retentate and permeatetream, respectively, in the module of membrane. Eq. (20) cane rewritten as:

d(Cfi)

dt= Qp

V(Cfi − Cpi) (21)

he global and solute mass balances for the membrane modulere,

f = Qr + Qp (22)

fiQf = CriQr + CpiQp (23)

he system is assumed as perfectly mixed that means the reten-ante concentration is the same as the bulk concentration (Cbi).

he concentration of species i in the retentate or bulk is:

ri = Cbi = Cfi

1 − θRiobs

(24)

M. Zabkova et al. / Journal of Membrane Science 301 (2007) 221–237 227

Fig. 3. Water permeate flux (m3 s−1 m−2) through the membrane as a functionoO

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4

st

Fig. 4. Flux of water with the new membrane and after each experiment fol-lowing cleaning procedure. Membranes: 15, 5 and 1 kDa cut-off. Cleanings�

1

z

vTitcstzemi

(

(

a

w

afcmc1

f the transmembrane pressure (kPa). Membranes: 1, 5 and 15 kDa cut-offs.pen system.

here θ is the permeate to feed flow-rate ratio (=(Qp/Qf)). Its worth noting that for batch operation, the parameter θ is notonstant during the UF process because the permeate flow-rates decreasing.

Concentration polarization leads to an increase in solute con-entration at the membrane wall as shown in Fig. 2. Underigh-pressure operation, the concentration at the membrane canncrease to the point of incipient gel precipitation, and a dynamicecondary membrane is formed on the top of the primary struc-ure. In a batch operation, because of the increased concentrationf lignin in the feed tank, the lignin solute might form a gel layern the feed surface of the membrane.

. Results and discussion

.1. Measurement of the water flux trough the membrane

The viscosity of water at 25 ◦C is 0.891 × 10−3 kg m−1 s−1

nd the value of the membrane resistance (Rm) obtained fromxperimental results was 2.806 × 1013 m−1 for 1 kDa mem-rane, 1.222 × 1013 m−1 for 5 kDa and 2.806 × 1012 m−1 for5 kDa membrane. The permeate fluxes of water as a function ofransmembrane pressure (�P) for membranes of 1, 5 and 15 kDare shown in Fig. 3. Of course, with increasing the cut-off of theembrane the water flux is higher.The cleaning efficiency is shown in Fig. 4 where the water

ux in the new membranes and after each experiment/cleanings reported. The use of 0.1–0.2 M NaOH as a cleaning agenteems to be acceptable. In the case of membrane 1, 5 and 15 kDafter each experiment and subsequent cleaning the water fluxbtained was almost the same as the original flux with the freshembranes.

.2. Viscosity measurements

The intrinsic viscosity [η] characterizes the capacity of theolute to increase the viscosity of the solution and is defined ashe limiting value of the specific viscosity/concentration ratio at

dc

l

tep is carried out by using 0.1 M NaOH solution. Transmembrane pressureP in which the flux was measured: 1.3 × 105 Pa for membrane 15 kDa and

.5 × 105 Pa for membrane 1 and 5 kDa.

ero concentration and given by: [η] = limc→0

(ηsp/c). The intrinsic

iscosity is expressed in unit of reciprocal concentration (L g−1).he specific viscosity is defined as: ηsp = (η − η0)/η0, where η0

s the viscosity of the pure solvent and η is the viscosity ofhe solution. The variation of the viscosity number (ηsp/c) withoncentration depends on the type of molecule as well as theolvent. To obtain the intrinsic viscosity it is necessary to findhe viscosity at different concentrations and then extrapolate toero concentration (infinite dilution). The intrinsic viscosity [η]xpressed in L g−1 was obtained from extrapolation of experi-ental data. The data were extrapolated in two ways to yield the

ntrinsic viscosity:

1) From viscosity number ((η − η0)/η0c) versus concentrationplot.

2) From a plot of ([ln(η/η0)/c]) versus concentration.

The equations, which are extrapolated to zero concentration,re:

η − η0

η0c= [η] + K1[η]2c (25)

[ln(η/η0)]

c= [η] + K2[η]2c (26)

here K1 and K2 are the constants [43].The values of intrinsic viscosities obtained by extrapolation

re slightly different. The value of the intrinsic viscosity obtainedrom the extrapolation of data plotted as viscosity number versusoncentration was 0.00386 L g−1 and from the plot of logarith-ic viscosity number versus concentration 0.00395 L g−1. The

onstants K1 and K2 are dimensionless with values of 2.345 and.167 at pH 8.5, respectively. The values of intrinsic viscosity

etermined from the viscosity number plot were employed inalculations of hydrodynamic radius of molecule.

The hydrodynamic radius of the molecule of solubilisedignin was calculated according to the Einstein viscosity relation

2 mbrane Science 301 (2007) 221–237

[

[

w

R

wg

twnp

e

D

wicntr

S

wsη

tat

4

4

lipocpl

mipbfaa

6

Fig. 5. Variation of the permeate flux in ultrafiltration of mixtures with differentcompositions of lignin/vanillin, at two values of pH (12.5 and 8.5), by usingmembranes of (a) 1 kDa cut-off, (b) 5 kDa cut-off and (c) 15 kDa cut-off. Oper-at

2nstfwbp

28 M. Zabkova et al. / Journal of Me

44] if the porous sphere particle model is used, i.e.,

η] = 0.025NA

M

(4

3πR3

e

)(27)

hich leads to:

e =(

30[η]M

πNA

)1/3

(28)

here Re is the hydrodynamic radius of molecule, NA the Avo-adro’s number and M is the molecular weight.

The value of molecular weight used in the calculation washe average molecular weight of lignin Mw = 60,000 g mol−1

hich is given by the supplier Sigma–Aldrich. The hydrody-amic radius obtained for lignin molecule in water solution atH 8.5 was 155 A.

The diffusion coefficient (D) is given by the Stokes-Einsteinquation [17]:

= kBT

6πη0Re(29)

here kB is the Boltzmann constant, T the temperature and η0s the solvent viscosity. The calculated value of the diffusionoefficient at pH 8.5 was D = 1.623 × 10−11 m2 s−1 (in infi-ite dilution). Numerous correlations have been published forhe calculation of the mass transfer coefficient. In the turbulentegime, one of the most used equations is [45]:

h = 0.023Re0.8 Sc0.33 (30)

ith Reynolds number (Re) equal to (dhvρ/η0) where ρ the den-ity (kg m−3), dh the hydraulic diameter of the flow channel (m),0 the viscosity of solvent (kg m−1 s−1) and v is the velocity inhe flow channel (m s−1); Schmidt number (Sc) equal to (η/ρD)nd Sherwood number (Sh) equal to (kdh/D), where k is the massransfer coefficient (m s−1).

.3. Ultrafiltration of vanillin/lignin mixture

.3.1. Permeate fluxThe optimal membrane cut-off used to recover vanillin from

ignin vanillin mixture is not so obvious since there are twomportant parameters involved that should be considered. Thesearameters are the purity of vanillin in the permeate and theptimal permeate flux. Therefore, when membrane with a lowut-off is used the vanillin in permeate is obtained with highurity. In contrast, the permeate flux on such membrane is veryow and that implies the use of high membrane area.

The permeate flux is the permeate flow per unit area of theembrane (L h−1 m−2). It is an important parameter because it

nfluences the full scale operation cost. The factors affecting theermeate flux are the feed composition, the cut-off of the mem-rane and the temperature. The variation of the permeate fluxesor the mixture of lignin and vanillin at different concentrations

nd at different values of pH on the ceramic membranes of 1, 5nd 15 kDa cut-off are shown in Fig. 5.

From Fig. 5a–c, where the lignin/vanillin mixture of0/6 g L−1 in 1 kDa cut-off membrane and of 60/6 and

a

ia

tional conditions are in Table 1; (dots) experimental points, (lines) adjust fromhe mathematical model.

0/2 g L−1 in 15 kDa cut-off membrane are shown, one canotice that through the ultrafiltration of these solutions at theame composition but different pH the flux differs slightly. Inhe case of membrane with 15 kDa cut-off the permeate fluxor the solution of lignin/vanillin at concentration of 60/6 g L−1

as approximately 1.5 times higher at lower pH. The differenceetween permeate fluxes at the lignin concentration 20 g L−1 atH 8.5 and at pH 12.5 was also present but not as noticeable ast higher concentration.

The flux decline with time is a result of membrane foul-ng which is usually assigned to the polarization phenomenand gel layer formation. Nevertheless, the “fouling potential”

mbra

osmMasppmbss

mTpmdt(cnapm[nflahsea

mttusot[pacttnbtsortiSc

stw

lp[twdaTbghghdbC

fWcmFmflcwFcv6v3Atomtos(auoracrwo

M. Zabkova et al. / Journal of Me

f the membrane is also affected by the membrane surface andolute characteristics such as sorption of solute molecules onembrane surface, their chemical and electrostatic interactions.any membranes have a positive zeta-potential at low pH andnegative zeta-potential at high pH. The pH at which the tran-

ition from positive to negative occurs is called the isoelectricoint. The isoelectric point is in general obtained from zeta-otential measurements at different pH [46]. An equal charge ofembrane and filtered molecule reduces the fouling properties

ecause of the repulsion of the membrane whereas an oppo-ite charge leads to immediate formation of a fouling layer andtrong flux decline [47].

The active layer of the ultrafiltration and fine ultrafiltrationembranes used in our study consists from mixed Al2O3 andiO2 or TiO2 active layer. There are several works alreadyublished related to determining electric charge of such asembranes using streaming potential measurements or recently

eveloped electroviscous technique. Almecija et al. [48] reportshat when using UF ceramic membranes at the extreme pH values3 and 10), the whey proteins and the membrane have the sameharge sign (positive and negative, respectively) and fouling isot favored because of the repulsion. On the other hand, at pH 4nd 5 (around the isoelectric points of the most abundant wheyroteins) fouling dominates due to the aggregates of unchargedolecules deposition on the membrane [48]. de la Rubia et al.

49] found out that the ceramic membrane (TiO2/ZrO2) has aegative apparent zeta potential at pH range 2.4–9.2. Permeateux and solute rejection of humic acids decreased significantlyt low pH which is partially attributed to the accumulation ofumic macromolecules at the membrane surface in a layer. Theurface charge of membrane becomes less negative and thus thelectrostatic repulsion between the membrane surface and humiccid macromolecules is reduced [49].

Benfer et al. [47] reported that the fine ultrafiltration TiO2embrane and ultrafiltration Al2O3/TiO2 membranes exhibit

he isoelectric point in the weak acid range (3.8–4.6). Belowhis value the membrane surface is positively charged and at val-es higher than isoelectric point the membrane has a negativeurface charge. Dong et al. [50] reported monotonic decreasef zeta potential of Kraft lignin as the pH decreased from 10o 1, which the isoelectric point for Kraft lignins is at pH 1.050,51]. The negative charge of lignin in alkaline solutions isredominantly due to dissociation of phenolic hydroxyls (pKabout 9.5) [52]. As a result, the lignin particle in alkali is moreharged and more mobile than in neutral solution. According tohese facts, one can suggest that at the pH 12.5, the lignin andhe membrane surface are charged negatively, at the pH 8.5 alsoegatively but with less value. The expected flux decline causedy fouling due to electrostatic interactions should be higher athe lower pH (8.5). From Fig. 5a–c, one can verify that the oppo-ite situation is taking place. The measurements of zeta potentialf the membranes were not available; nevertheless, based on theesults already published, it can be assumed that zeta potential of

he membrane and of the studied solution in the pH range stud-ed (8.5–12.5) have no influence on the permeate flux decline.imilar result was observed by Huisman et al. [53], where theritical flux for the crossflow microfiltration of silica particle

oslo

ne Science 301 (2007) 221–237 229

uspensions was independent of the particle zeta potential andhe membrane zeta potential, even if the membrane zeta potentialas changed from positive to negative [53].It is well known that Kraft lignin has a broad molecu-

ar mass distribution [54]. During the cooking, the lignin isartly degraded and hydrophilic groups are formed in the lignin55,56]. Since the lignin is naturally hydrophobic, the ioniza-ion of these groups is essential in order to make the ligninater-soluble [57,58]. Based on this, the observed high fluxecline with higher pH of the filtrated solution could be mostlyttributed to the hydrophobicity membrane surface and solute.he hydrophilic membranes surfaces facilitate water transporty forming hydrogen bonds with the water molecules and thusenerate a higher permeate flux [59]. Lignin is a naturallyydrophobic compound, but with increasing pH the hydrophilicroups became more ionised. Thus, the groups can form strongydrogen bonds with the water molecule, organic moleculesisplace water molecules from the hydrophilic sites of the mem-rane, and prevent water transport though the membrane layer.onsequently, the generated permeate flux is lower.

In Fig. 5, one can also verify that the flux decreases slightlyrom the beginning of experiment and no steady state is reached.

e have noticed that the permeate fluxes are quite low inomparison to the measurements of pure water fluxes thoughembranes immediately after permeate started to be collected.or the case of 1 kDa cut-off, the flux of pure water at the trans-embrane pressure of 1.55 × 105 Pa was 21 L h−1 m2 while theux of lignin/vanillin mixture was only 4–5 L h−1 m2. For thease of 5 kDa, the flux decreased by a factor of 5 when the feedas a mixture of lignin/vanillin instead of pure solvent. Fromig. 5c, one can verify that this reduction is dependent on the con-entration of the components of the mixture in addition to the pHalue. For the 15 kDa cut-off membrane and feed concentration0/6 g L−1 the reduction was 6.4 and 9.6 times (dependent on thealue of pH); for feed 20/2 g L−1 the flux reduction was around.3 times and for 5/0.5 g L−1 feed the flux was 1.7 times lower.s expected, the initial permeate flux is lower as the concentra-

ion of lignin in the initial feed solution is larger. A time-analysisf the mass transfer phenomenon taking place in UF operationay suggest that the permeate flux decline might be divided in

wo intervals: (i) in first seconds the osmotic pressure mechanismccurs and (ii) at longer times a progressive growth of rejectedolute at the membrane wall becomes the governing mechanisma quasi steady-state concentration polarization layer is set-upnd, eventually, gel layer can emerge). In general, during theltrafiltration process the gel layer is not formed at the beginningf the process; actually, the polarized layer and the membraneesistance, combined with the increase of the osmotic pressure,re the governing resistances. From our experimental data, wean notice that total resistance is always higher than membraneesistance from the beginning of sample collection. On the otherords, if flux is calculated considering only the influence ofsmotic pressure, its value will be much more higher than that

btained from the first flux measurement (around to 1 min); thisituation suggests that, at least, the polarization concentrationayer is already formed when the flux measurements are startingn. Considering the experimental routine, in which the solution

230 M. Zabkova et al. / Journal of Membrane Science 301 (2007) 221–237

Table 1Experimental conditions used in ultrafiltration of the lignin/vanillin mixture using tubular ceramic membranes of 1, 5 and 15 kDa cut-off

Run C0feed (g L−1) pH Qfeed (L h−1) V0 (L) �P (×105 Pa)

Lignin Vanillin

1 kDa cut-off1 60 6 8.5 130 5

1.552 60 6 12.5 120 4

5 kDa cut-off1 60 5 12.5 120 4 1.55

15 kDa cut-off1 5 0.5 8.5 109

5 1.302 20 2 8.5 1023 20 2 12.5 102

ivEtim

tadtttatcpsm

eavuriticpttaptodtb(

tsiccvtttcTtvel1a

flctibtamtngaiiesue

4 60 6 8.55 60 6 12.5

s circulated through the system without opening the permeatealve, one can assume a very high value for the parameter kp inq. (15) once the polarized layer has been already grown-up at

he membrane surface. In Table 1, experimental conditions usedn ultrafiltration of lignin/vanillin mixture using tubular ceramic

embranes 1, 5 and 15 kDa cut-off are shown.The mathematical model used to describe the experimen-

al permeate fluxes for different composition of feed solutiont different pH assumes an increase of total resistance (RT)uring the ultrafiltration. The model is solved in such a wayo fit the measured flux data within a range of optimizationolerance (minimum 10−8) and, then, to estimate the total resis-ance acting against the transport through the membrane. By thedvection–diffusive equation, the concentration profile withinhe polarized layer is known as well as the variation of walloncentration over the period of UF process. The computationalackage gPROMs is used applying the MAXLKHD solver in theolution of this parameter estimation problem. The discretizationethod is the orthogonal collocation on finite elements method.From this numerical procedure, the total resistance can be

stimated for all of studied membranes. Fig. 6 describes the vari-tion of RT during the ultrafiltration of the mixture of lignin andanillin at different concentrations and pH. For all cases, the val-es of the total resistance are always higher than the membraneesistance measured using pure water (shown in Table 1) and isncreasing with time. Fig. 6 also shows the calculated values ofhe wall concentration of the rejected solute (lignin) versus timen this UF process. For the 1 kDa cut-off membrane, the wall con-entration profile for the solution 60/6 g L−1 (lignin/vanillin) atH 8.5 describes a parabolic behaviour (with concave down) inhe UF period, while at pH 12.5 the value of Cw is decreasing inhe interval. One can state that the variation of the concentrationt the surface of membrane is being essentially dependent on theermeate flux and the bulk concentration (or permeate concen-ration) of the rejected solute. An increase either to the value of Jr of Cb leads to the increase in Cw. However, as permeate flux is

ecreasing and bulk concentration is increasing, both contribu-ions are competitive and it is possible to achieve this presentedehaviour. For the case of pH 12.5, the effect of the flux declineand consequently larger effect of diffusive term in Eq. (9)) on

asoi

101101

he build-up of retained lignin at the membrane wall is moreignificant than that caused by the increase of the concentrationn bulk side; at pH 8.5 there is a period where the effects areounterbalanced. Since it is observed in these two cases the walloncentration does not reach to the gel concentration (here, thealue of Cg is taken to be 265 g L−1 [31,32]), one may assumehat the total resistance is basically the polarization resistance inhis UF operation. Hence, the Eq. (16) can be applied to predicthe resistance due the polarization and α, m and n parametersan be estimated to be used in the range of the variation of Cb.his correlation might be useful to give an idea of its contribu-

ion in flux decline of further UF experiments. For example, thealues of the α, m and n parameters which could describe thestimated polarization resistance behaviour in ultrafiltration ofignin/vanillin using 1 kDa cut-off membrane are, respectively,4.92, 4.2 and 1.0 for the solution at pH 8.5 and 4.8 × 102, 2.9nd 1.0 for the solution at pH 12.5.

For the membranes of larger Mw cut-off (5 and 15 kDa), theux is too high and wall concentration approaches the gel con-entration very quickly. De and Batacharaya [15] have reportedhat black liquor does not exhibit distinct gel layer formationn the range of moderate operating conditions of UF in stirredatch cells. Dafinov et al. [10] and Wallberg et al. [3] have usedubular ceramic membranes (5 and 15 kDa) to treat black liquornd, at applied conditions, the formation of a gel layer on theembrane surface has taken place. In this work, no experimen-

al measurement is made to know the wall concentration, butumerical estimations for this concentration indicate that theel layer has been formed (when Cw = Cg). This layer acts asnother barrier and, for such situations, the total resistance isnvolving both polarization and gel resistances. From our exper-mental work, it would be hard to differentiate the contribution ofach resistance to the current flux decline for the different casestudied. Although, there are some ways to evaluate the individ-al contribution of these resistances from the set of independentxperiments and after they can be introduced to the modelling

pproaches to predict the permeate flux and the rejection ofpecies. Further study should be considered to compute eachne of these resistances in current UF process. Furthermore, its worth mentioning that no considerable vanillin retention is

M. Zabkova et al. / Journal of Membra

Fig. 6. Variation of the total resistance during ultrafiltration of lignin/vanillinmixture in composition: (a) 60/6 g L−1 using 1 kDa cut-off membrane, (b)60/5 g L−1 using 5 kDa cut-off membrane and (c) 60/6 g L−1, 20/2 g L−1 and5ec

oabtittritcct

rs1afltvo

oiptvlf

pltTlv

cfmhier

4

aa6uesw

icrmucifcw

/0.5 g L−1 using 15 kDa cut-off membrane at pH 8.5 and at pH 12.5; (dots)xperimental points, (lines) from the mathematical model. Small graphics: con-entration at the membrane surface (Cw) as function of time.

bserved from those experiments where the total resistance isssumed the sum of polarization and gel resistance. As statedefore, gel formation can affect the rejection characteristics ofhe membrane, but as vanillin is a small molecule in compar-son with the lignin molecule and pores of the membranes,his phenomenon seems to have no influence on vanillin rejec-ion (that remains around to zero—Figs. 7–9). The gel layeresistance depends mostly on gel layer thickness but it is alsonfluenced by the change of solution properties with time since

he batch method is applied. The solution is becoming more con-entrated, more viscous and the molecular weight distributionhanges since certain molecular weight fraction is withdrawn inhe permeate.

a1ic

ne Science 301 (2007) 221–237 231

Experimental values of volume reduction, lignin and vanillinetention measured at the end of experiment performed on thetudied membranes are summarized in Table 2. For the case ofkDa cut-off membrane, the final volume of permeate collectedt the end of the experiment (≈21 h) due to very low permeateux was approximately 10% of initial volume in the tank. Using

his small cut-off membrane, although the permeate flux wasery low comparing to higher cut-off membranes, the highestbserved rejection of lignin was obtained (Robs = 0.972).

The experiment performed on the membrane with 5 kDa cut-ff presents an observed rejection of lignin equal to 0.968 thats slightly lower than for the membrane 1 kDa cut-off but theermeate flux was one order of magnitude higher. In this case,he collected volume of permeate was 27% from initial solutionolume in the tank after 17 h of operation. The concentration ofignin measured in the tank at the end of experiment increasedrom 60 to 90 g L−1.

In Table 2, one can also see the experimental resultserformed with the membrane of 15 kDa cut-off. For theignin/vanillin feed solution of 20/2 g L−1, the observed rejec-ion of lignin was 0.958 at pH 8.5 and 0.873 for pH 12.5.he rejection of lignin significantly decreased also at the

ignin/vanillin concentration of 60/6 g L−1 when increasing thealue of the pH.

The observed rejection of vanillin as low molecular weightompound is approximately zero at both tested pH of solutionsor all experiments. For the ultrafiltration using the 1 kDa cut-offembrane, high-purity vanillin can be recovered. On the other

and, in this case, the effectiveness of the ultrafiltration processs low mainly due to low permeate flux from the beginning of thexperiment and with a tendency to decrease more with volumeeduction.

.3.2. Products in retentate and permeate sidesEach stream (retentate and permeate) was analyzed for lignin

nd vanillin content. The concentration histories of vanillinnd lignin in retentate and permeate at concentration of lignin0 g L−1 and vanillin 6 g L−1 and at pH equal to 8.5 and 12.5sing membrane of 1 kDa cut-off are shown in Fig. 7. Thexperiments were performed at constant transmembrane pres-ure (�P = 1.55 × 105 Pa). The feed flow rate was 130 L h−1

hen pH 8.5 and 120 L h−1 when pH 12.5.The numerical profiles for both species vanillin and lignin

n retentate and permeate sides are also shown in Fig. 7. Ofourse, the concentration of lignin and vanillin in permeate oretentate side depends on the observed rejection. Since the batchethod was applied, the feed concentration increases during the

ltrafiltration as the retentate is recycled to the tank. Thus, theoncentration of lignin in the tank increased from 60 to 63 g L−1

n the feed solution with pH 8.5 and from 60 to 67 g L−1 for theeed solution with pH 12.5. If the observed lignin rejection isonstant, the concentration of lignin in each stream must increaseith increasing concentration in the feed. Because the perme-

te flux using membrane of 1 kDa cut-off is very low and only0% volume reduction was reached, the concentration of ligninn the feed did not increase significantly as well as the ligninoncentration in the retentate and permeate. Similarly, vanillin

232 M. Zabkova et al. / Journal of Membrane Science 301 (2007) 221–237

Fig. 7. Variation of the vanillin and lignin concentration in retentate and permeate side for an ultrafiltration using membrane of 1 kDa cut-off. Initial feed solution:60 g L−1 lignin and 6 g L−1 vanillin. (a) and (b) pH 12.5, (c) and (d) pH 8.5. Operational conditions are in Table 1; (dots) experimental points, (lines) from themathematical model.

F ) pers s are

cai

v

t

TE

R

112

51

112345

ig. 8. Variation of the vanillin and lignin concentration in (a) retentate and (bolution: 60 g L−1 lignin and 5 g L−1 vanillin at pH 12.5. Operational condition

oncentration measured in the samples of retentate and perme-

te remained constant during the experiments, i.e., no rejections observed for vanillin molecules.

In Fig. 8, one can observe concentration profiles of lignin andanillin in retentate and permeate side for the ultrafiltration using

vtat

able 2xperimental measured values in ultrafiltration of the lignin/vanillin mixture using tu

un C0feed (g L−1) pH VR Robs (lignin)

Lignin Vanillin

kDa cut-off60 6 8.5 0.102 0.95160 6 12.5 0.103 0.972

kDa cut-off60 5 12.5 0.275 0.968

5 kDa cut-off5 0.5 8.5 0.500 0.916

20 2 8.5 0.440 0.95820 2 12.5 0.440 0.87360 6 8.5 0.313 0.94360 6 12.5 0.340 0.865

meate side for an ultrafiltration using membrane of 5 kDa cut-off. Initial feedin Table 1; (dots) experimental points, (lines) from the mathematical model.

he 5 kDa cut-off membrane. The concentration of lignin and

anillin in the feed is 60 and 5 g L−1, respectively, and pH equalo 12.5. This experiment was performed at �P = 1.55 × 105 Pand Qf = 120 L h−1. It is worth noting that the lignin concentra-ion in the retentate for the 5 kDa cut-off membrane becomes

bular ceramic membranes of 1, 5 and 15 kDa cut-off

Robs (vanillin) η (×10−3 kg m−1 s−1) Rm (×1013 m−1)

≈0 1.192.806≈0 1.19

≈0 1.19 1.222

≈0 0.887

0.2806≈0 0.954≈0 0.954≈0 1.190≈0 1.190

M. Zabkova et al. / Journal of Membra

Fig. 9. Variation of the vanillin and lignin concentration in retentate and perme-ate side for an ultrafiltration using membrane of 15 kDa cut-off. Composition ofthe initial lignin/vanillin mixture: (a) and (b) 60/6 g L−1 at pH 12.5; (c) and (d)60/6 g L−1 at pH 8.5; (e) and (f) 20/2 g L−1 at pH 12.5; (g) and (h) 20/2 g L−1 atpe

mtmiTttosdtan

io5t

oi

ldtirmta1tt2

udcsctpaalbmsbmpfic1

tddbmKTbo1uTfatl

H 8.5; (i) and (j) 5/0.5 g L−1 at pH 8.5. Operational conditions: Table 1; (dots)xperimental points, (lines) from the mathematical model.

ore concentrated than that presented in retentate from sys-em with 1 kDa cut-off membrane. In fact, with 5 kDa cut-off

embrane the permeate flux is higher and the reduction volumes larger, and the retentate product is concentrated more rapidly.he 5 kDa cut-off membrane shows almost the same lignin rejec-

ion and a much larger permeate flux than those achieved withhe 1 kDa cut-off membrane; such membrane could be the betterption if one has to choose the more adequate membrane con-idering the aspects of the operation time of ultrafiltration andegree of recovery. As expected, for a larger membrane cut-off,he concentration of vanillin in the permeate side was constantnd no obstruction for the penetration of vanillin molecule wasoted.

Fig. 9 shows the lignin and vanillin concentration profiles

n retentate and permeate sides obtained from the ultrafiltrationf the lignin/vanillin mixture at concentrations 60/6, 20/2 and/0.5 g L−1 at two values of the pH (12.5 and 8.5; except to 5/0.5hat is only at pH 8.5) using the 15 kDa cut-off membrane. The

wtet

ne Science 301 (2007) 221–237 233

perational conditions for each experimental run are describedn Table 1.

Depending on the permeate flux and the rejection of theignin, the concentration of lignin in the retentate increasesuring the experiment. It is also evident as the feed concen-ration is increasing with time, lignin concentration increasesn permeate side as well, but the observed rejection of ligninemains constant. For the case of mixture 60/6 g L−1, as per-eate flux is higher when the pH of the feed solution is 8.5,

he lignin content in retentate side becomes more concentratedt lower time in comparison to that shown for the case of pH2.5. Using this cut-off membrane, the lignin concentration inhe permeate side is higher for higher pH. This observation onhe permeate product is also verified for the case of mixture0/2 g L−1.

Under different pH conditions, the behaviour of both the vol-metric flux and the rejection of the solute by the membraneuring the UF operation appear to be linked to: (1) the surfacehemistry between the transition metal oxides and the organicspecies (lignin and vanillin) and (2) the chemical and physi-al properties of the filtrated solution. For the former, eitherhe effect of molecules and membrane charges or hydrophilicroperties of the solute and membrane surface are important,s discussed above. The phenomena of lignin molecule associ-tion and aggregation to different degrees in various solventsead to different molecular weight distributions. This fact haseen already investigated in the literature [60–63] and the mainechanisms of molecular association are hydrogen bonding,

tereoregular bonding, lyophobic bonding and charge transferonding [64]. Generally, it can be stated that association ofolecules is strongly dependent on the molecular weight and

H of filtrated macromolecular solution [60]. It was found thator small molecular weight compounds the major association isn pH between 10 and 13; in contrast for high molecular weightompounds the considerable association occur at pH between2 and 13.5 [65].

The association of the molecules at high pH leads to par-icle size increase mostly due to polyelectrolyte swelling andissociation of ionisable functional groups [62] while when pHecreases the association is due to the formation of hydrogenonds. From literature, it was noticed from the experiments usingembranes of 10- and 20-kDa cut-off that the rejection of theraft lignin decreased with increasing pH of the solution [66].he same results were found in our experiments using the mem-rane of 15 kDa cut-off but they are in contradiction with thosebserved on the membrane of 1 kDa cut-off (although, for thekDa cut-off membrane, different feed flow-rates have beensed in ultrafiltration of the mixture at pH 8.5 and pH 12.5).his disagreement between the rejection of lignin at two dif-

erent pH of solution and using different membrane cut-off (1nd 15 kDa) could be related to a change of governing associa-ion mechanism. One can speculate that at pH 12.5, the biggerignin molecules provide more sites for association compared

ith smaller lignin molecules and therefore it is feasible that

he larger ones join mainly the small ones (instead of agglom-ration between small molecules themselves). This fact reduceshe smaller lignin molecules fraction in solution and the rest

2 mbrane Science 301 (2007) 221–237

o1masam

gttofco(

4

rtmatwbscmvaom

Fig. 10. Permeate flux in ultrafiltration of the initial feed mixture of 2 g L−1

lignin and 6 g L−1 vanillin at pH 12.5 by using the 1 kDa cut-off membrane.Operational conditions: �P = 1.55 × 105 Pa and Qf = 120 L h−1. Model assum-ipa

tohcflaff(m

F6p

34 M. Zabkova et al. / Journal of Me

f agglomerated complexes can be more effectively rejected bykDa membrane while using 15 kDa membrane even associatedolecules can pass through the membrane to the permeate. The

ssociation of molecules at lower pH can be predominant onmaller lignin molecules leading to the formation of large stablessociated complexes, which are not allowed to pass through theembrane of 15 kDa cut-off.Another comparison can be performed among Fig. 9c, d;

, h; and i, j, which show cases of different concentrations athe same pH of solution. One would expect that lignin rejec-ion was increasing for an increasing value of concentrationf this species in the mixture; however, the value of RL

obsor the case of 20/2 g L−1 did not follow this trend. As onean see for each feed, the same vanillin concentration wasbserved in permeate and in retentate during the experimentsi.e., RV

obs ≈ 0).

.3.3. Staging membranesIt was observed that for the lower cut-off membrane, higher

ejection of lignin could be obtained. On the other hand, ultrafil-ration of solution at high lignin concentration using low cut-off

embrane leads to significant decline of the permeate flux. Onepproach to solve the difficulty is to introduce more ultrafiltra-ion stages. For this purpose, one experiment was carried outith lignin and vanillin solution at the similar concentration ofoth components as obtained by ultrafiltration of lignin/vanillinolution at a concentration 60/6 g L−1 using membrane of 1 kDaut-off. The permeate flux during ultrafiltration of lignin/vanillinixture at the concentration 2 g L−1 of lignin and 6 g L−1 of

anillin at pH 12.5 is shown in Fig. 10. The results for the perme-te flux obtained from the mathematical model which includesnly osmotic pressure are also compared in this figure. It is worthentioning that the solution was prepared from fresh lignin and

((

m

ig. 11. Variation of lignin and vanillin concentration in retentate and permeate sig L−1 vanillin at pH 12.5 by using the 1 kDa cut-off membrane. Operational conditiohenomenon without (full line) and with (dotted line) polarization resistance; (dots) e

ng osmotic pressure phenomenon without (full line) and with (dotted line)olarization resistance; (dots) experimental points. Small graphic: concentrationt the membrane surface (Cw) as function of time.

his model solution did not reflect the real permeate compositionf the mentioned experiment essentially due to the content ofigh molecular weight fraction. The experiment is performed atonstant transmembrane pressure �P = 1.55 × 105 Pa and feedow rate 120 L h−1 (initial feed volume: 4 L). The membranerea is Am = 0.008 m2 and a′ empirically determined constantrom van’t Hoff equation is 42.9 Pa m3 kg−1. The mass trans-er coefficient is calculated using the diffusion coefficient DEq. (29)), which includes the hydrodynamic radius of the ligninolecule (Re) estimated only for the average molecular weight

Mw = 60,000 g mol−1). The estimated value of k from the Eq.30) is 2.97 × 10−6 s−1.

The predicted permeate flux using the osmotic pressureodel is higher than the flux experimentally obtained which

de during the ultrafiltration of the initial feed mixture of 2 g L−1 lignin andns: �P = 1.55 × 105 Pa and Qf = 120 L h−1. Model assuming osmotic pressurexperimental points.

mbra

matostifwcutawvldr

1htrFecpocrcefi

ffgfrcbwimiugf

5

bmwc1

cuemw

mconwm1iom

daaeptmiuvlfem

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A

M. Zabkova et al. / Journal of Me

eans that some additional resistance should be taken intoccount to describe the permeate flux properly. If the experimen-al behaviour of flux is used to estimate the operating resistance,ne can find that it is increasing with time and then the rejectedolute is gradually being deposited on the membrane wall. Sincehe flux predicted by the osmotic pressure model is higher, its expected that the calculated lignin concentration on the sur-ace of membrane is also higher than the wall concentrationhen the resistance to the transport through the membrane is

onsidered in addition to the osmotic pressure. The found val-es of Cw suggest that polarization concentration resistance isaking place during the measurement period (its value does notpproach the assumed gel lignin concentration). Therefore, evenith this low initial feed solution – 2 g L−1 lignin and 6 g L−1

anillin – , it seems that there is already a formation of polarizedayer and in order to predict the experimental behaviour of fluxecline, the modelling approach should include the polarizedesistance.

The observed retention of lignin on ceramic membrane withkDa cut-off was 0.981. The permeate flux was significantlyigher in comparison to that when high concentration was fil-rated. The concentration histories of vanillin and lignin inetentate and permeate side in this analysis are presented inig. 11. The concentration of lignin in the tank analyzed at thend of experiment increased from 2 to 3.2 g L−1. The vanillinontent in permeate and retentate side remained constant. Therocess was carried out up to 41% volume reduction and 9.7 gf vanillin was recovered in permeate. Since the low con-entration of lignin in the solution, the flux decline does notepresent significant limitation in the process as it is at highoncentrations. This could be a way to optimize the recov-ry of vanillin from Kraft lignin oxidation media using ultra-ltration.

Besides the limitation of permeate flux caused by the sur-ace fouling attributed to concentration polarization and gelormation, it is possible that the growth and adhesion of microor-anisms on surface or internal porous of membrane (biologicalouling) can occur [36]. In fact, the reaction mixture leaves theeactor at high temperature (170–200 ◦C) and at strong alkalionditions (pH 12–13). In the industry, the ultrafiltration woulde carried on as a second step (after reactor) in a continuousay. Kraft lignin and vanillin are ideal organic substrates, but

t is also needed to consider the diversity of microbial com-unity that can form the biofilm at such alkaline conditions

s limited. Hence, some experimental tests under long timeltrafiltration would help to clarify the presence of microor-anisms in the deposited layer and the question of biologicalouling.

. Conclusion

The recovery of vanillin from Kraft lignin oxidation mediay ultrafiltration has been investigated. The appropriate size of

embrane cut-off can significantly reduce the high moleculareight components from the lignin/vanillin mixture. Tubular

eramic membranes with different molecular weight cut-offs:, 5 and 15 kDa were tested. Due to high physical and

lni

ne Science 301 (2007) 221–237 235

hemical resistance the ceramic membranes can be appliednder strong pH conditions and high temperature. The influ-nce of the lignin and vanillin concentration and pH of theixture on the ultrafiltration process has been studied asell.The highest value of lignin rejection was obtained with the

embrane 1 kDa cut-off, for the lignin/vanillin mixture at theoncentration 60/6 g L−1 and pH 12.5 (RL

obs = 0.972). On thether hand, using the membranes with small cut-off leads to sig-ificant decline of permeate flux comparing with the membranesith bigger cut-off. For example the permeate flux using 1 kDaembrane was only 4 L h−1 m2, while for membrane 5 kDa was

4 L h−1 m2. One possibility to solve this problem can be tontroduce more ultrafiltration stages starting from larger cut-ff membrane. No vanillin rejection is found for the studiedembranes.The influence of pH of the lignin/vanillin mixture on the flux

ecline and observed rejection was also analyzed. Since ligninnd these ceramic membranes present negative charges at thenalyzed values of pH, potential fouling could not be consid-red to justify the difference found in flux decline at differentH. The observed high flux decline at higher pH of the fil-rated solution could be mostly attributed to the hydrophobicity

embrane surface and solute. It was also found that increas-ng the pH of the solution leads to decrease of lignin rejectionsing membranes with 15 kDa cut-off whereas the opposite waserified using membrane 1 kDa. The different mechanisms ofignin molecules association that depend on molecular weightraction, concentration, pH and ionic strength seem to be rel-vant to justify the rejection of lignin behaviour shown by theembranes.By using the presented modeling approach, it is verified

hat there is a growth of the rejected solute on the mem-rane surface resulting in a progressive raise on total resistanceuring the batch UF operation. It is shown that wall concen-ration is directly linked to permeate flux and lignin rejectionor bulk concentration). Under the applied operating condi-ions, the estimated wall lignin concentration suggests thator the ultrafiltration of the mixture of lignin/vanillin usinghe 5 and 15 kDa cut-off membranes, there is a gel layeresistance. However, it was observed that very high clean-ng efficiency can be obtained using alkali cleaning (NaOH).here was no evidence of irreversible fouling or plugging theembrane pores. All tested cut-off membranes recovered after

xperiments and subsequent cleaning procedure the same per-eability as was found with “new” membranes. Therefore,

he ultrafiltration process can be used as an efficient wayor the recovery of vanillin from Kraft lignin oxidation prod-ct.

cknowledgments

Miriam Zabkova (SFRH/BD/8007/2002) gratefully acknow-edges the Ph.D. grant from Fundacao para a Ciencia e Tec-ologia. Financial support for project POCI/EQU/61738/2004s acknowledged.

236 M. Zabkova et al. / Journal of Membrane Science 301 (2007) 221–237

Nomenclature

a′ coefficient in van’t Hoff equation (Pa m3 kg−1)Am membrane surface (m2)Cb solute concentration in the bulk (g L−1)Cg gel concentration of the rejected solute (g L−1)Cp solute concentration in the permeate (g L−1)Cw concentration of the rejected solute at the mem-

brane surface (g L−1)dh hydraulic diameter of the flow channel (m)dp particle diameter (m)D diffusion coefficient (m2 s−1)J permeate flux (L h−1 m−2)k mass transfer coefficient (m s−1)kB Boltzmann constant

(1.381 × 10−23 m2 kg s−2 K−1)kp kinetic coefficient for formation of polarized layer

(m s−1)K1, K2 constants in Eqs. (20) and (21)Kg permeability of the gel layer (m2 Pa−1 s−1)L length of the membrane (m)m parameter in Eq. (16)Mw average molecular weight of the solute (g mol−1)n parameter in Eq. (16)NA Avogadro’s number (6.022 × 1023 mol−1)�P average transmembrane pressure (Pa)�P1 relative inlet pressure on the feed side (Pa)�P2 relative outlet pressure (Pa)Pp permeate pressure of the shell side (Pa)Qfeed feed flow-rate (L h−1)Qp permeate flow-rate (L h−1)Qr retentate flow-rate (L h−1)Re Reynolds number, Re = dhvρ/η0R0 intrinsic rejectionRobs observed rejectionRe hydrodynamic radius (m)Rg resistance of the gel layer (m−1)Rm membrane hydraulic resistance (m−1)Rp polarization resistance (m−1)RT total resistance (m−1)Sc Schmidt number, Sc = η/ρDSh Sherwood numbert time (s)T temperature (K)VR volume reductionv velocity in the flow channel (m s−1)V0 initial volume of the feed tank (m3)Vp permeate withdrawn volume (m3)z axial variable (m)

Greek lettersα parameter in Eq. (16)δg thickness of the gel layer (m)δpol thickness of the polarization layer (m)φ parameter in Eq. (17)η0 solvent viscosity (Pa s)

η viscosity of the solution (Pa s)[η] intrinsic viscosity (L g−1)�π osmotic pressure (Pa)θ flow-rate ratio of permeate and feedρ density of the solute (kg m−3)

Subscripts and superscriptsf feedi species (vanillin or lignin)j stream in the membrane modulusL ligninp permeater retentate

R

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