[membrane science and technology] fundamentals of inorganic membrane science and technology volume 4...

Fundamentals of Inorganic Membrane Science and Technology Edited by A.I. Burggraaf and L. Cot

�9 1996, Elsevier Science B.V. All rights reserved

Chapter 12

Transport and fouling phenomena in liquid phase separation with inorganic an hybrid membranes

d

Christian Guizard and Gilbert Rios

Laboratoire des Mat4riaux et Proc4d4s Membranaires (UMR 5635 CNRS-ENSCM-UMII), Ecole Nationale Sup6rieure de Chimie, 8, rue de I'Ecole Normale, 34053 Montpellier, France

12.1 INTRODUCTION

Regardless of the type of the membrane used for liquid phase separation, three main classes of phenomena must be distinguished concerning solvent and solute transport. They develop:

- near the front face of the filtering element (polarization); - inside the membrane structure (internal mass transport); - at interfaces (fouling). If hydrodynamics and related mass transfer kinetics mainly control the first

two, thermodynamic equilibrium also plays a main part in the third. The occurrence of secondary phenomena, such as solute modifications due to surrounding effects, must also be mentioned due to possible induced interferences.

In this chapter, pressure driven processes involving porous inorganic or hybrid membranes are particularly examined. As recently shown by Bhave [1], differences with traditional organic elements mainly result from the structure and intrinsic properties of materials, either regarding flow (with ceramic membranes, the transport occurs through the intergranular spaces within the top layer, porous sublayers and support, while across polymeric barriers it develops

570 12 m TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION

through the continuous network of openings from one face to the other) or interactions (unlike polymers, metallic oxides present electric charges and thus surface phenomena that strongly depend upon pH and ionic strength of solutions).

Basic phenomena are firstly recalled in a short presentation which is not intended to give a thorough description as this has already been done elsewhere [2], but which aims to provide a comprehensive study of classical routes for modelling. An unified approach of fouling during the separation process itself or even during membrane synthesis is also proposed, which gives a new opening on formed-in-place membranes.

Solvent and solute interactions with membrane material during microfiltration and ultrafiltration are then described and related to process performance, i.e. permeability and rejection. It is worth recalling that molecules in the colloi- dal range, roughly between one and one hundred nanometers, are separated by ultrafiltration, while microfiltration retains larger particles.

As recently shown, small molecules with sizes lower than about one nanometer and ions can be rejected by nanofiltration membranes. In the following, size exclusion, Donnan effect or combined mechanisms operating in such situations are examined.

Finally some more prospective aspects are hinted at, either concerning coupled processes with at least one membrane separation step, or new membranes for facilitated transport or selective separation. Specific advantages of inorganic or hybrid material will be underlined.

12.2 BASIC P H E N O M E N A IN PRESSURE DRIVEN PROCESSES

12.2.1 Modelling of Hydrodynamics and Mass Transport

Following the classical scheme, free transport of solutes and solvent in the boundary layer at the liquid-membrane interface and hindered transport of substances in the porous structure of the membrane material are described successively.

Concentration polarization: Convective transport and retention of solutes by the membrane results in an accumulation of species at wall. Local concentrations, Cw, are higher than in the bulk, CB, and a back-diffusion from near the wall into the bulk liquid phase takes place. This is the so-called "concentration polarization" phenomenon (Fig. 12.1). A simple mass balance leads to the classical equation:

( C w - C p ) / ( C b - Cp) --- exp (Jv/k) (12.1)

with k = D/~), the mass transfer coefficient defined as the ratio of solute diffusivity to boundary layer thickness.

12 - - TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION 571

membrane ~_~

permeate

Cp

--~ gel layer

C w = C g

boundary layer

t t Tt CB

bulk feed

Ap 8 x

Fig. 12.1. Concentrat ion polarizat ion in pressure driven membrane processes. Concentrat ion pro-

file and gel formation at the steady state.

From this relation, it can be easily inferred that the phenomenon will be strongly dependent on:

- membrane morphology and interactions through permeate flux, Jv; - fluid properties (viscosity, diffusivity) and flow conditions through k. Regarding the definition of the observed rejection TRobs between feed and

permeate and the real rejection TR defined from concentration at walls, the following equations are used:

Cp TRobs = 1 = - (12 .2)

Cb Cp

TR = 1 (12.3) Cw

The real rejection can be calculated from the observed rejection using Eqs. (12.1) to (12.3).

1 - Trobs r r = 1 + exp 8 (12.4)

Trobs

As a general rule, concentration polarization will be all the more important as membrane permeability and liquid viscosity will be higher and solute diffusivity will be lower. This is the reason why its effects, ordinarily negligible for nanofiltration, appear to be of major importance during ultrafiltration. Under gel polarization conditions, i.e. in those cases where Cw reaches a maximum gel value Cg (as an example with ultrafiltration of concentrated protein solutions),

572 12 -- TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION

the above analysis even indicates that the driven pressure Ap can no longer influence the permeate flux Jv: operating performance is essentially controlled by mass transfer parameters.

On the assumption that permeate flow does not strongly modify hydrodynamics at walls, k is ordinarily estimated using classical dimensionless correla- tion for non-porous walls, such as the L6v6que equation for laminar flow or the Deissler equation for turbulent flow. More recently, in order to account for specific disturbances that take place at pore entrance when high permeate fluxes prevail, new specific equations have been proposed [3].

Membrane transport: There are two fundamentally different ways of modelling permeable solid materials. In the first the membrane is regarded as a heterogeneous structure through the pores of which solvent and solutes flow in, while in the second the membrane is considered as a homogeneous phase forming a molecular mixture with other species. Heterogeneous models are the natural choice for aggregated inorganic porous materials, while homogeneous models are most usefully considered with dense top polymeric layers of organic-inorganic hybrid membranes.

Starting from the broad point of view of Maxwell and Stefan [4], it may be assumed that in a steady flow the thermodynamic forces acting on solute or solvent are counterbalanced by the frictional forces due to other species. This leads to the following equation

(d ln a i /dz ) + (1 /p~ .dp /dz ) + 1 / @ ~ . d @ / d z = ~..~Xj [(Vj -- Vi ) /Di j ] - ( v i / D i m ) (12.5)

with ai the activity of i and Xj the mole fraction of j at z, p and cI), the local pressure and electric potential, vj and vi the species velocities, Dij and Dim the Maxwell-Stefan diffusivities representative of the friction between species i and j or i and m the membrane, p~ and ~ are two constants in pressure and electric driving forces given by

p~ - R T / V i (12 .6)

~ - RT/F zi (12.7)

with R the gas constant, T the absolute temperature, F the Faraday constant, zi and Vi the charge number and partial molar volume of i. Fluxes with respect to the membrane are equal to

li = Ci" (vi + Jv) (12 .8)

with Ci the local concentration, and Jv the viscous flow velocity in heterogeneous media, a linear function of the driven pressure, Ap, according to Darcy's law.

12 -- TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION 573

Various treatments of Eqs. (12.5) to (12.8) are proposed in the literature for those cases where charge effects are negligible (microfiltration, ultrafiltration or nanofiltration of neutral solutes), and low concentrated solutions are considered, i.e. with negligible non-idealities and activities assimilated to concentrations. Most of the time, membrane morphology is just considered through simple parameters such as effective pore size, tortuosity accounting for the effect of fouling on measurable transport properties. A well-known model thus obtained is due to Kedem and Katchalsky [5]:

J v - ap. ( A F - (~ AI-[) (12.9)

Is = co" (Cw -Cp ) + (1 - o ) . Jv" [ (Cw-Cp) / (ln C w / Cp)] (12.10)

When the rejection, TR, is high, wall and permeate concentrations are better linked through

Z R - (Cw- Cp) / Cw= (~-(1 -F) / (1 - G F)

F = exp [- (1 - o ) . Iv / co]

(12.11)

(12.12)

These equations relate the fluxes for the solvent, Jv, and for the considered solute, Js, to three parameters: Lp the membrane permeability to the solvent, co the membrane permeability to the solute and c~ the reflection coefficient that measures the selectivity of the filtering element. This dimensionless number varies from 0 (pure convective transport, roughly the situation in microfiltration) to I (pure diffusive transport, preferably encountered in reverse osmosis). On the assumption that solute molecules are rigid and spherical, transport parameters may be linked to the membrane morphology, i.e. the mean pore radius or the ratio of the open surface area to the pore length, through hindrance and friction factors respectively accounting for the pore entrance and wall effects [6].

The one-dimensional flux equations (12.9) and (12.10) implicitly assume that local components of flux in the plane of the membrane can be ignored relative to fluxes in the z direction. If we consider z* as a dimensionless axis perpendicular to the faces of the membrane whose values range from zero to unity, a local form of Eqs. (12.9) and (12.10) can be rewritten for solute i:

Jv = - Kvl~ th (dp/dz* - r~RTdCi/dz*) (12.13)

Ji = (1 - (~)CiJv- (P/h)dCi/dz* (12.14)

Alternative definitions for the permeability coefficients have been utilized in Eqs. (12.13) and (12.14) for the purpose of explicitly representing the dependence on membrane thickness h and viscosity ~t. They are related to their coun- terparts in Eqs. (12.9) and (12.10):

ap = K v/~l,h (12.15)

5 7 4 12 - - TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION

m = P / h (12.16)

Solution of Eq. (12.14) yields a concentration profile for the solute wi th in the membrane:

C i ( z * ) -- C i ( 1 )

C i ( 0 ) - Ci(1) 1 - exp [-Pe(1 - z*)]

(12.17) 1 - exp [-Pe]

in which Pe represents the Peclet number:

Pe = (1 - c ~ ) J v h / P (12.18)

The magn i tude of the Peclet number indicates the importance of the convective relative to the diffuse process for solute transport. The solute concentrat ion profiles for representat ive values of Pe are illustrated in Fig. 12.2 according to Bungay [7]. When diffusion is dominan t (Pe --- 0) the concentration varies nearly linearly in z*. For large absolute values of the Peclet number , diffusion is significant only in a thin zone adjacent to the low pressure face of the membrane in which the concentrat ion profile is very steep. For micro- and ultrafil tration membranes , the solute concentration varies little from the value at h igh pressure face. For nanofil trat ion the Peclet number can vary considerably depending on membrane characteristic: almost dense or porous membranes .

C~(z*) - C~(])

Ci(O) - C i ( | )

+10 0.8

+3

0.6 +1

0

0.4 - 3 --1

0.2

0 0 0.2 0.4 0.6 0.8 1.0

Z �9

Fig. 12.2. Steady-state solute concentration profiles hi simultmleous diffusion and convection across a membrmle of uniform properties. Numbers adjacent to profiles hldicate values of the Peclet number whose sign depends upon the direction of the volumetric flux relative to the external solution

concentration difference [7].

12 - - TRANSPORT AND FOULING P H E N O M E N A IN LIQUID PHASE SEPARATION 575

If electric forces influence solute retention, as an example with nanofiltration of electrolyte solutions on membranes presenting a high charge density, references in literature suggest the use of a peculiar form of Eqs. (12.5) to (12.8), the so-called Nernst-Planck equation:

]i = -Pi(dCi/dz + Ci I r " d~/dz ) + Jv ci . (1 -(~i) (12.19)

in which Pi and Ji are the permeability and flux for ion i. Using appropriate boot-strap relations (electroneutrality of solutions or zero electric current conditions) and boundary conditions (at the surface between the membrane and. external solution the assumption is made of Donnan equilibrium) integration can be carried out across the membrane. But ordinarily it is not easy to get the effective charge density and related electrostatic potential of the material [8].

If for very dilute solutions of neutral rigid spherical species, the hydrodynamic theory for the hindered transport previously developed may be considered as at a mature stage, in sharp contrast is the situation for charged and /or flexible solutes in more concentrated solutions where solute-pore and solute- solute interactions remain largely unevaluated. Other important gaps in the present knowledge concerns the prediction of solute adsorption in the pores and its effect on measurable transport properties [9].

12.2.2 Foul ing

There are two different levels where fouling phenomena and related effects may interfere with performance of composite inorganic or hybrid membranes. The first and the more classically reported in literature is the one of the separation process itself, which through various interactions between solution and material (adsorption, surface deposits, pore plugging) generally leads to reduced fluxes and increased retentions. The second, much more less described by authors but of the same nature and with analogous effects, concerns membrane preparation, and the possible interactions between deposited layers. Theses two aspects are linked up with the so-called formed-in-place membranes, obtained by deposition of species onto a ceramic support through cross-flow filtration. In what follows, they will be described in a unified approach.

Membrane synthesis

The formation of ceramic membranes for microfiltration, ultrafiltration or nanofiltration by association of various granular layers is now a common procedure [10]. Each layer is characterized by its thickness, h, its porosity, r and its mean pore diameter, dp. These parameters are controlled by the particle size, d, and the synthesis method. Each layer induces a resistance which may be predicted through the classical Carman-Kozeny model:


I

Fig. 12.3. Simulation of mult i layer deposits with particle diameter ratios of 2.5 (I, non-interpene-

trated layers) and 4 (II, interpenetrated layers) [11].

R = Ap/(Jv" ~t) = 180 h(1 - E)2/(E 3. d 2) = (80 h / E ) . dp 2 (12.20)

It has been shown recently [11] that the experimental hydraulic resistance of such a composite structure can be much larger than the theoretical resistance obtained by simply summing the resistances of the different layers. As an interpretation, the existence of highly resistant transition boundary layers due to infiltration between adjacent media of drastically different particle sizes has been suggested (Fig. 12.3). In order to check this theory, infiltrated and nonin- filtrated TiO2 membranes deposited on o~-alumina support have been prepared. Scanning electron micrograph of their interfaces is shown in Fig. 12.4. From them, complementary resistances have been measured.

TABLE 12.1

Experimental and predicted hydraulic resistances (m -1 unit) for a membrane with 0.2 ~tm mean pore diameter deposi ted on a suppor t with 0.8 ~tm pore diameter [11]

Membrane Model

Suppor t 3.6 x 10 l~ 3.6 x 10 l~

Membrane 7.0 x 10 l~ 7.2 x 10 l~

Noninfi l trated Infiltrated Noninfi l trated Infiltrated

Filtration element 12 x 1010 17 x 1010 12.6 • 1010 15 x 1010

Infiltration zone 1.4 x 10 l~ 6.4 x 10 l~ 1.8 x 10 l~ 4.2 x 10 l~

12 n TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION 577

Fig. 12.4. Cross-section microscopy images showing interpenetrated (a) and non-interpenetrated (b) ceramic porous layers.

A computer model has been also developed to predict the porosity at the layer interface, as a function of the relative sizes of granules. From Table 12.1, it follows that measured and calculated resistances are comparable. This clearly proves the effect of infiltration on performance and shows that to increase the permeabili ty of filtration elements new routes for synthesis limiting the extent of infiltrated zones are needed.

Membrane separation

When a solution is filtered through a membrane which can partially or completely retain one or more of the solutes present, the observed flux strongly decreases with time at first, and then under steady state conditions reaches a level much less than the flux of pure solvent. In addition, the retention of solutes is generally increased. This phenomenon corresponds to on-line membrane fouling.

A clear distinction must be made between membrane fouling and concentration polarization. As already explained, concentration polarization is the devel- opment of a concentration gradient of the retained components near the membrane. It is a function of the hydrodynamic conditions and is independent of the

578 12 m TRANSPORT A N D FOULING P H E N O M E N A IN LIQUID P H A S E S E P A R A T I O N

physical properties of the membrane. The membrane pore size and porosity are not directly affected by concentration polarization. Fouling on the other hand is the deposition of material on the membrane, leading to a change in the membrane behaviour. Fouling is the coupling of deposited material to the membrane through the intermediate step of concentration polarization. From this point of view, fouling includes gel formation. Fouling must also be distinguished from membrane compaction, which is the compression of the membrane structure under the transmembrane pressure, causing a decrease in membrane permeability. If with purely inorganic membrane compaction may be neglected, it is not necessarily the same with organo-inorganic or hybrid membrane under high enough Ap as for nanofiltration. With hybrid filtering elements, swelling of polymeric layers may also induce flux decrease that must not be confused with fouling.

Generally on-line fouling of membranes is extremely complicated and in several aspects is not fully understood. Authors used to consider three succes- sive stages in flux decline. As an example Aimar et al. [12] for the UF of cheese whey with Carbosep M4 membranes (Fig. 12.5) mention a very short initial step (less than 1 mn) corresponding to reversible concentration polarization build up, and then two distinct features: a sharp decrease during the first hour, followed by a slow decrease over several hours. From a comparison of models, they suggest that the sharp decrease could be due to either protein adsorption or particle deposition, and that the longer term decline is related to further

Flux

Stage 1 - Flux loss due to concentration po la r i s a t i on

Stage 2 - Flux loss due to protein deposition

Stage 3 - Flux loss due to par t ic le depos i t i on c consol ida t ion of the foul ing marer ia l

Time Fig. 12.5. Various stages of flux decline durh~g protehl filtration with an ultrafiltration membrane

(Carbosep M4) [12].

12 m TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION 579

convective deposition of species or fouling layer consolidation. Considering solute retention, a continuous increase may be observed as flux decreases. It is likely that both surface and internal fouling, to a greater or lesser extent, occur simultaneously, the predominant mechanism being a function of the experimental conditions, as well as of membrane and feed material properties [13].

In several recent works, the basic mechanisms of protein deposition on microfiltration c~-alumina membranes were thoroughly investigated. Vetier et al. [14] studied the processing of milk using a 0.2 gm Membralox membrane. Starting from scanning electron microscopy and physicochemical analysis,. authors show that at first there is an important surface adsorption phenomenon in the form of a thin film of casein and salts on which other micelles are then deposited. These may be connected by phosphocalcic bonds which form a porous layer that is largely responsible for the retention of serum proteins. It was observed that there was almost no penetration of casein micelles in the top layer, and that the cross-flow velocity also helped to reduce the thickness of the fouling layer. In another study carried out with gelatin solutions and several membranes with pore diameters between 0.2 and 2.0 gm, Freund and Rios [15] gave a further insight into the influence of pore diameter, fluid velocity, concentration and driven pressure as regards the extent and localization (surface deposition or pore plugging) of fouling. A clear distinction is made between irreversible fouling phenomena that are taken into account through a complementary term Rc increasing the hydraulic membrane resistance, and reversible phenomena probably due to highly concentrated layers of mobile macromolecules in the vicinity of interfaces that are associated to an abatement pressure term A~, as indicated in the following equation:

Jv = (Ap- AFI)/~t. (R m + Rc) (12.21)

Rm is the hydraulic resistance of the clean membrane that is deduced from water flux, while An and Rc are estimated through water and solution fluxes of the prefouled element. Another work conducted by Matsumoto et al. [16] also compared the resistances due to internal and surface fouling with ovalbumin. From all these data, it may be concluded that:

- with low velocities (laminar), and with 0.2 gm or to a lesser extent 0.8 gm membrane, surface layer fouling dominates;

- with larger pore sizes or higher velocities, pore plugging prevails. As a whole, the main extent of fouling when protein are treated with alumina membranes is underlined, and it is proved that adsorption turns the initial microfiltration process into a true formed-in-place or dynamic ultrafilter, capa- ble of effectively retaining species with a size ten or one hundred times lower than initial pore size.

Because all inorganic materials (zirconia, titania, glass) do not present the same charge and /or hydrophobic/hydrophilic characteristics, interactions


may strongly vary. Thus surface modification can constitute a route of the highest interest for enhancing or preventing solute interactions with membranes. Another main consequence of previous findings is that, with an asymmetric element composed from different material, performance will not be only dependent on solute-membrane, but also on solute-support interactions.

It is also worth keeping in mind that fouling mechanisms are influenced by modifications in solute configuration due to charge or high concentration effects (as an example, the aggregation of proteins in the concentrated polarization layer which then in turn will induce specific interactions with the membrane material) as well as by specific solute-solute interactions. Recently, Grund et al. [17] suggested that with bovine serum albumin in the presence of a sufficient amount of fatty acid protein-protein interactions may have the greatest effect on the permeability of the protein deposits and hence on the permeate fluxes, while at a sufficient reduced level of fatty acid these interactions appear to play a far lesser role than do protein-membrane interactions. Similar effects may be observed with pH, ionic strength, temperature as shown elsewhere [13]. From a practical view point, this highlights the main role of pretreatment and choice of working conditions as regards cross-flow filtration performance.

Most of the time, fouling can be removed by cleaning solutions or other appropriate means, and membrane performance restored. But in same cases also, foulant behaves into the membrane in such a way that the filtering layer cannot be completely regenerated: this type of fouling, that has received much less attention, determines the lifetime of the membrane.

Formed-in-place membranes

It has been previously shown that dynamically formed or formed-in-place ultrafiltration membranes naturally set down when biological molecules such as proteins tangentially flow along a ceramic microfiltration membrane. This new membranes directly result from infiltration/deposition of macromolecules, i.e. fouling mechanisms, that develop on the original filtering element.

In a similar way, when organic or inorganic polyelectrolytes are added to pressurized salt solutions held in contact with inorganic porous supports (ceramic, stainless steel), salt-filtering layers are formed that present reverse osmosis and/or ion exchange properties with moderate salt rejection as compared to polymeric cellulose acetate but high flux often an order of magnitude higher [1]. Such membranes possess the desirable properties for applications requiring higher temperatures. Studies concluded that hydrous zirconium oxide/polyacrylic acid dual-layers membranes had the best performance characteristics.

Recently, Negrel et al. [18] proposed a new route to get nanofiltration performance starting from a formed-in-place membrane of gelatin deposited on a


2

100 -

v

t -

O

O

...=,,

rr

5

PEG 6OOO

Q PEG 2000

lrll PEG 6o0

10 15 20 4 0

Time( min ) Fig. 12.6. PEG rejection versus time for a formed in place membrane made of tanned gelatin [18].

0.2 ~tm alumina support from SCT-US Filter, and then applying various chemical (tanning) and /or physical (thermal) treatments to the macromolecular deposit. Separation performance were characterized using PEG (polyethylene glycol) as tracer molecules (Fig. 12.6). With the new membrane, high fluxes were obtained probably due to the fact that protein deposit are thinner than top layers of commercial nanofilters.

It is worth noting that the idea, if not the means, of this new generation of organic-inorganic composite membrane is not very far from the one proposed by Guizard et al. [19] of impregnating organic polymers such as polyphosphazenes or heteropolysiloxanes in mesoporous inorganic supports of SiO2, A120 3, ZrO 2 or T iO 2 oxides. This also bears witness of the interest of a global view of fouling, integrating at the same time synthesis and on-line separation problems.

12.2.3 Specific Aspects Attached to Ceramic Membranes

Major developments of membrane processes using ceramic membranes have been aimed at microfiltration or ultrafiltration applications. Up to now the most important applications for these membranes are found in aqueous media for the separation of particles, bacteria, colloids, macromolecules. Recently, ceramic nanofilters based on sol-gel derived microporous materials have been described [20]. They extend separation capability of ceramic membranes to ions and organics.


Behind the general parameters (viscosity, transmembrane pressure, temperature, flow velocity) which can influence cross-flow filtration with ceramic membranes two aspects must be considered to be more specific of this sort of membrane. One is related to the geometry (tubular multichannel or honeycomb) found for the major part of commercially available membranes, the other is the amphoteric behaviour of metal oxides used in the preparation of these ceramic membranes.

Membrane geometry

The tubular or monolith geometry of ceramic membranes proceeds from inherent constraints in ceramic material processing. Due to the mechanical properties of sintered ceramic materials which can be summed up in a high compressive resistance and a brittle character, cylindrical shapes offer the best compromise between pressure resistance and cross-flow filtration adaptability. Typical geometries for ceramic membranes are shown in Fig. 12.7.

While microfiltration is operated at low transmembrane pressure (-~ I bar), higher pressures are required for ultrafiltration (up to 10 bar) and nanofiltration (10-20 bar). Here also pressure vessels with pressure resistant sealing are easier to design for ceramic tubes than for ceramic plates. In micro- and ultrafiltration, high cross-flow velocity (2-8 m/s ) is generally recommended to minimize concentration polarization and induced fouling. Thanks to their abrasive resistance, tubular or multichannel ceramic membranes allow to attain a high wall shear rate with particles charged liquids without prefiltration. Figure 12.8 shows the details of a membrane module commercialized by Tech-Sep (Groupe Rhone-Poulenc) in which a number of tubular or multichannel ceramic membranes are arranged in a parallel way. In this case tubular membranes are 1.2 m long, 10 mm in diameter and up to 252 tubes can be inserted in one module.

The problem encottntered with the first generation of tubular ceramic membranes was that they required a high pumping energy and a high volume/surface

honeycomb

multichannel

Fig. 12.7. Different types of geometries found in commercial ceramic membranes.

12 m TRANSPORT A N D FOULING P H E N O M E N A IN LIQUID PHASE SEPARATION 583

Fig. 12.8. Example of a module obtained by assembling a number of multichannel membranes in a stainless steel pressure vessel (from Tech-Sep, Groupe Rhone Poulenc).

ratio. This p rob lem has been in major par t overcome by the second genera t ion of ceramic m e m b r a n e s exhibit ing a monol i th geometry. The characteristics of

the main monol i th ceramic membranes commercia l ly available are repor ted in

Table 12.2. A better v o l u m e / s u r f a c e ratio results f rom these ne w geometr ies

whi le intrinsic proper t ies of ceramic micro- and ultrafi l trat ion m e m b r a n e s are

preserved. C o m p a r e d to organic po lymer membranes , mul t ichannel ceramic

m e m b r a n e s exhibit in termedia te h y d r o d y n a m i c per formances be tween those

of tubular and hol low fibres po lymer membranes . Liquid flow in a p ipe can be

TABLE 12.2

Characteristics of commercially available monolith ceramic membranes

Mmlufacturer Trade name Membrane Support material material

Membrane Geometry of Chalmel pore membrane inside diameter element diameter

CERAMEM

CERASIV

metal oxides

metal oxides

TECH-SEP KERASEP TM TiO2 ZrO2

US FILTER MEMBRALOX | metal oxides

cordierite 0.05- 0.5 ~tm

ocA1203 5 nm- 1.2 ~m

czA1203- 2 nm- TiO2 1/.tm

czA1203 4 nm- 1.4 ~m

honeycomb monolith

monolith

monolith

monolith

1.8 mm

3 m m

4.2 mm 6 m m

8 m m

4.5 mm 2.5 mm

6 m m

4 m m

5 8 4 12 m TRANSPORT A N D FOULING P H E N O M E N A IN LIQUID PHASE SEPARATION

described according to the following parameters: the kinematic viscosity ~t = rl/p, the hydraulic diameter d h and the feed flow velocity v. Laminar or turbulent flow in the pipe can be achieved for specific values of v and d h according to Reynolds number Re:

dhV 9Vdh R e - ~ - ~ (12.22)

For undisturbed flow through a straight pipe, the change from laminar to turbulent flow occurs at a Reynolds number of about 2000. The behaviour of fluid flow in a porous channel with suction is different than that in a non-porous-walled channel. Mellis et al. [21] showed that in a porous stainless steel microfiltration tube, axial, ReF, and wall, Rew, Reynolds numbers influence axial pressure drop and related friction factor. Wall suction has its maximum effect on the axial pressure drop at intermediate flow rates (1,000 < ReF < 15,000). Moreover the value of ReF for transition to turbulent flow increases with R e w .

The conclusion is that wall flux has a feedback effect on itself through the axial pressure drop and, hence, the transmembrane pressure drop. More recently, Nakao et al. [3] proposed to refer to axial and radial Reynolds numbers to improve mass transfer prediction at wall for high flux operations. Regarding the influence of membrane geometry on Reynolds number one can see that with monolith channels of a few mm in diameter, a turbulent flow can be attained with reasonable pumping energy. This is an important point in the economics of processes based on the utilization of ceramic multichannel membranes compared to tubular geometry.

M e m b r a n e m a t e r i a l

Several characteristics of ceramic membranes, resulting from metal oxide grains sintering, are worth underlining:

- a composite structure made of a porous ceramic multilayer structure, - a pore geometry resulting from sintering of packed particles, - an amphoteric behaviour of the ceramic surface in contact with water. The volume flux through the different porous layers cannot correctly be

described by the basic Hagen-Poiseuille equation more specifically suited for transport through membranes consisting of a number of parallel pores. As above-mentioned volume flux in inorganic membranes can best be described by the Carman-Kozeny relationship, Eq. (12.20), taking into account the possible interpenetrating of the different layers.

The amphoteric behaviour of metal oxides in contact with water has thoroughly been described by many authors [22-24]. This basic property results in charged surfaces depending on pH condition. In a first approximation, connected porosity in ceramic membranes can be represented by an array of


a) aquo complex formation H O - H

H I , ~ H

7 , , o / o \ o I I

�9 ... :... M M M M M M �9 " " " " " : ' : . . . . . " ' : " " " : " ] " : . . . . . . . . . : . i " . . " . . . . . . . . . . . 7 . : : : " : i ' : . i : ' : : . : : % . " . . . . " �9 . . . . . . . �9 . . .

: solid surface

b) amphoteric behavior

i i~i:i~i:ili:ili i:i~!il . : : . . . : : : : : : : . : : : . : : : -: ~ .

. . : : : : : : : : : : : . : : : : . : . . : / / :::::::::::::::::::::::::::::::

iiiiiiiii!iiiiii:: M - - OH + H20

?:~ii::ii!i::::ii!i!::ii;::::i . . . . . . . . . . . .

M - OH2+ + O H

M - O + H30 +

Fig. 12.9. M e c h a n i s m of charged surface fo rmat ion due to ampho te r i c behav iou r of meta l oxides.

microcapillaries. This approximation allows to apply the so-called microcapil- lary model for transport of electrolytes [25]. The consequences of applying pressure and potential gradients across capillaries (or porous media) filled with electrolytes are based on phenomena such as electro-osmosis, streaming potential and electroviscous retardation effects accompanying flow of electrolyte solutions. This has a direct influence on permeability and selectivity of micro- and ultrafiltration ceramic membranes and in a more pronounced manner in the case of ceramic nanofilters for which the effective thickness of diffuse electrical layer formed on pore walls is of the same order of magnitude than pore radius. A short description of the formation of this diffuse electrical layer on metal oxide surface is given hereafter.

The occurrence of neutral and charged (+) surfaces at the oxide-solution interface has been attributed to the formation of metal aquo complexes as shown schematically in Fig. 12.9.

One can see from Fig. 12.9 that the oxide surface can be negatively or positively charged as a function of pH. There exists a pH value defined as the zero point of charge (zpc) for which there is no charge on the surface. The negative surface charge originates from acidic dissociation of the surface hydroxyl groups (at pH > zpc) and increases with increasing pH. Cations present in the solution adsorb at the surface according to the double layer theory. At pH < zpc the positive surface charge is explained on the basis of proton addition to the neutral aquo complex, together with or without the replacement of the surface hydroxyl groups by anions present. It results from the adsorption of ions on charged oxide surfaces the formation of a double layer as shown in Fig. 12.10.

It is generally accepted that the electrical double layer around charged oxide surfaces consists of two parts [22]:


t3"0 ~ ~d

iiiiiiiiii!!iiiiiiiiiiiii ii:!!i'~ii~i2!!!?!:!i!'~!~ i:iii~gii;iiiiii!iii!iii!ii !:iiiiiii!iii-%ii ::::::::::::::::::::::::: ::::::::::::::::::::::::::: ::::::::::::::::::::::::::: iiiii~ili i:!!i:ii::ii!iiiii ~ii!il;!!ii!ii!ii!iiiii ili!!;i~ii!iiiiii~i;ili~i~i . .....-.......,..... .... :::::::::::::::::::::::::::

|

@

(3"0+(~13+(3'd-'0 [

Q, structured water

bulk liquid

N~--q S t e r n l a y e r ~ G o u y l a y e r

Fig. 12.10. Schematic representation of double layer formation in presence of electrolytes.

- an extended diffuse outer part or Gouy layer, the potential distribution of which is well described by the Poisson-Boltzman equation,

- an inner part or Stern layer of a few molecular layers thickness, reflecting the specific properties of the counterions and the nature of the surface.

The potential profile through the double layer of an oxide surface in presence of electrolytes, Fig. 12.11, can be described by successively accounting for the resulting charges at the surface of the oxide, in the plane of specifically adsorbed ions and at the interface between Stern and Gouy layers. The above description of double layer formation on oxide surfaces in presence of electrolyte solution can be applied to pore wall in ceramic membranes. When the porous volume of a ceramic membrane is filled with an electrolyte solution pore walls are uniformly charged and surrounded by a diffuse electrical layer with an effective thickness which is given by the Debye length ~,D = 1/~c. ~c is the reciprocal of the Debye length and is given as:

1/2

f 2 2CiO / _ _F Z i

~c ~eRT (12.23)

with e the permittivity of the solvent, zi the ion valence and C ~ the ion concentration at the pore axis r = 0.

As shown in Fig. 12.11, the charge density G0 of the native surface of the pore walls is partially compensated (r~0 > r~) or overcompensated ( I c~01 > I c~l ) by a charge density r~ of specifically adsorbed counter ions in the Stern-layer. The resulting charge density (~d at the shear surface between Stern-layer and diffuse Gouy-layer results in an electrical potential ~)(r) in a perpendicular direction r


(3"O

4)o

~d

(~B G'd

DifFuse layer

,, ~ r

1 Stern-layer

specific adsorption =:> cyo > CYl~

(3"O

4)o

i

~d I "

super soeciJ

O'B (Td

Diffuse

i layer

. . . ~ v

,er-equivalent :=> I~01> I ,1 )ecific adsorption

Fig. 12.11. Representation of potential evolution in a perpendicular direction to the oxide surface when counter ions are specifically adsorbed on the surface.

to the pore wall. This potential, called zeta potential, (~d, c a n be related both to the characteristics of metal oxide membrane and to the feed ionic strength.

According to the space charge model (SC), when a solution is flowing in the porous structure under a pressure gradient, pore wall is reduced to the Stern- surface between the static and the mobile portions of solution. The pore radius equivalent to the Stern-surface is called hydrodynamic radius rh with

r h = r p - l (12.24)

where rp is the original pore radius and I usually taken as a counter-ion diameter. So that when a pressure gradient, Ap, acts through the membrane, the solution close to the pore wall stays immobile while the rest of it moves along the pore. This movement leads to the appearance of an electrical potential drop from one side to the other side of the membrane, A~g. This electrical potential results from an electrical field which develops because the flux of the counter-ions is greater than that of the coions into the membrane pores. This electrical field generates an electrokinematic flow of the counter-ions that is opposed to the previous one thereby satisfying the constraint that there is no net current flow through the membrane. The combination of ~)(r) and A~g effects corresponds to a dynamical contribution to the total electric potential profile, according to the space charge model which was originally proposed by Osterle et al. [26-28].

5 8 8 12 m TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION

O(r,z) = (ziF/RT) ~(r) + ~ ( z ) (12.25)

The z-dependent potential ~(z), in zero current conditions, is related to streaming potential Vp. In the case of ceramic membranes ~)d and Vp are the two quantifies which influence both retention of charged solutes and volume flux.

- - ~)d is related to ionic strength and pH of the feed solution; - Vp is related to ionic strength but also to the transmembrane pressure-

driven flow resulting from the pressure gradient Ap applied to the membrane.

As a whole some general rules can be pointed out concerning the effect of ~)d and Vp on membrane behaviour. The spatial extent of the double layer in the radial direction of the pores is characterized by the Debye-length so as a high ionic strength leads to a short Debye-length and a weak electric effect on transport. The ionic strength is related to ions concentration in the feed solution but also to the valence of co- and counter-ions. Multivalent counter-ions which adsorb in the double layer have a more marked effect than monovalent counter- ions in diminishing the spatial extent of the double layer and the resulting zeta potential. On the contrary, due to their higher electric charge, multivalent coions are more rejected than monovalent coions. Taking into account the distribution of charges in the radial direction of pores, volume flux in the axial direction can be described by the addition of two opposite contributions: the convective-diffusive flow and the back electrokinematic flow due to streaming potential. The electrokinematic flow for a porous ceramic membrane can be related to the following parameters:

4?s - (ziF/R T)(~d (12.26)

a dimensionless zeta potential dependent upon ceramic surface characteristics and pH of feed solution;

r E = r p / ~ D (12.27)

a dimensionless length also called electrokinetic radius accounting for the ionic strength of ionic feed solution;

Ls = Lp/Lo (12.28)

a dimensionless hydraulic permeability resulting from the variation of rE and ~)s. L0 is the pure water permeability for the membrane with the same pore radius and a zero surface charge density while Lp is the membrane permeability in presence of a counter-electrokinematic flow.

Due to Donnan exclusion principle [29] charged membranes can reject inorganic salts even though they have pores much larger than the salts and this ion rejection is known to decrease with increasing feed ionic strength. The example of 1.1 electrolyte filtration through different pore sizes at a pH far from the

121 TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION 589

Dolman b--x~ Streaming effect

b o u n d ~ ~ membrane layer

boundary layer

,-.. ..... n+@

1 @ C ) ~ . . . . _ ~ , ' - @ . - @ : [ : " . . @ ~ @ ~@ @ o

Stem-layer

......... | .......... ~ . ~ / / / / / / / / ~ ~

............. .| ........ | . i _ _ ~ | 1 7 4 @.@.@. o.:..,~-...,.,,-- @.@__| @

!] .......... @ G | =.=..-:::,.,7.=.i-.-. | |

e -~ ~ ~ ~ ~ 1

rp~l nanofiltration

rp>l ultrafiltration

| | ....... G . . . . . e . . . . . |

|174 g4:::~---"-:::G-~ | |174

G _ N ~ @ ~ @ | | ..... | G ...... | e o

rp>> 1 microfiltration

convective flow ~ -.-.:r,~--- electrokinematic flow

Fig. 12.12. Influence of zeta-potential (Stem-layer thiclaless l) and Streaming-potential (electrokinematic flow) on ion rejection and volume flux for porous ceramic membranes exhibiting negatively charged pore walls. Cases of micropores (nanofiltration), mesopores (ultrafiltration) and macropores

(microfiltratio11).

isoelectric point (high zeta-potential for ceramic membrane materials) is given in Fig. 12.12. According to the above described dimensionless parameters the occurrence of electrokinematic flow (Ls < 1) is expected for rE < 1. Usual ly electrokinematic effect is likely to occur for nanofil tration and ultrafil tration

590 12 - - TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION

membranes when deci- to centimolar electrolyte solution are used. Neverthe- less this effect can be expected for microfiltration membranes in the case of very low ionic strength resulting in a Debye-length which were calculated to reach several tens of nm [30-32].

12.3 R E C E N T D E V E L O P M E N T S I N M I C R O F I L T R A T I O N A N D

U L T R A F I L T R A T I O N W I T H C E R A M I C M E M B R A N E S

Ceramic membranes were first applied to microfiltration processes. Several authors published a comprehensive description of basic transport phenomena involved in ceramic macroporous structures [1,33]. Lately improvements in ceramic membrane processing led to commercial ultrafiltration membranes exhibiting a mesoporous structure with transport phenomena close to those encountered in microfiltration. As described in Section 12.2, the major limitation in membrane performances for micro- and ultrafiltration processes is caused by concentration polarization and fouling. Methods that help to reduce concentration polarization and fouling can be classified into three categories: (i) chemical cleaning methods including strong acid and basic solutions or oxidiz- ing agents due to high chemical resistance of ceramic membranes; (ii) physical methods such as backflushing and the use of turbulence promoters; (iii) hydrodynamic methods related to module design. In fact two aspects have been more specifically investigated in recent years concerning cross-flow filtration systems based on ceramic membranes:

- the hydrodynamic of microfiltration and ultrafiltration systems and its influence on membrane performance in terms of fouling reduction and permeability enhancement;

- the influence of membrane material (metal oxides in most cases) on selectivity and permeability.

12.3.1 Hydrodynamic of Micro- and Ultrafiltration systems

In a review on cross-flow microfiltration Belfort et al. [34] outlined the importance of module design and hydrodynamic operating conditions in order to improve performances of cross-flow filtration using macroporous membranes. The authors suggest that unsteady flow conditions can be even more effective in disturbing the flux-limiting effects of concentration polarization and fouling [35]. Various approaches to inducing instabilities in bulk flow across a membrane surface include designing membrane surfaces with organ- ized roughness, pulsation of axial and lateral flow, and the use of curvilinear flow under conditions that promote instabilities or vortices. A number of these devices shown in Figs. 12.14 and 12.15 can be adapted to ceramic membranes.

12 ~ TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION 591

mlalltltJm~mmltl~,.-..~

r

porous wall

parabolic flow

with pulsation

Fig. 12.13. Effect of pulsat ions on flow profile in a smooth-wal led duct.

Permeate flux enhancement by pressure and flow pulsations has been investigated by many authors [36-38]. The effect of flow oscillations in a smooth- walled duct is shown in Fig. 12.13.

Pulsa te flows were applied to mineral microfiltrations membranes during apple juice filtration [36] illustrating the advantage of this method to enhance permeability compared to steady flow regime. With carefully chosen pulsations permeate flux increased up to 45% at I Hz pulsation frequency. More- over well defined pulsations decreased the hydraulic power dissipated in the retentate per unit volume by up to 30%. In an other work on cross-flow filtration of plasma from blood [37] permeate flux increase was also observed when pressure and flow pulsations at I Hz are superimposed on the retentate.

Pulsate flow can also be used to good advantage in rough walled ducts and those with inserts. Simulation of cross-flow filtration for baffled tubular channels and pulsate flow were reported by Wang et al. [38]. Wall and central baffles, in a similar way as in Fig. 12.14, with and without pulsations have been considered. Reynolds numbers up to 200 have been used in simulation that is notably lower than values used to obtain turbulent flow in smooth porous- walled channels. Concentration polarization effects have been included in calculation. The addition of pulsations improved the fluxes, the relative improvements being greater for the wall baffles. However the absolute values of the predicted fluxes were found greater for the central baffles.

It has been suggested in the literature [34] that filtration devices producing Taylor or Dean vortices can help depolarization of the solute build up on membranes. This seems to be an attractive way because of excellent bulk fluid mixing, high wall shear rates and weakly decoupled cross-flow with transmembrane flux. Unfortunately there are some severe limitations on a technical and economical point of view with such devices. Build up and scale up of these modules are expensive with difficulties in repairing and changing membranes. A good compromise between economic and technical constraints has been described by Charpin et al. [39]. It consists in the preparation of mineral (metal

592 12 ~ TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION

( a ) ~ " K , ~ protuberance ~.x~-:-_--_--_--_--: _ _ _ _ - ~ , ~ , ~

. . . . . . . . . . . . . . . . . . . . .

~ - ,.:...~,,<..,.,.:.- :.: ,. :.:: ;.~. !~, !; ~.: ..;.. : ..;?r..:.. :.':...-. '..-:....>:.?! :.-.:.2.;; ::~;~; ~"~ ~:7 :~!:': :".:;:~:?.: "'.i~!~":.~>; ":r I porous wail i

..... ~-.~. . . . . . . . . . . . . . . . . . . . . . . . . . ] porous wall [, . . . . . . ~ .

~ x ~ ~ J ~ ' ' ' - - ' - - ' ' ~ I inserts

( b ) - - - - - - ~

| , s ,,. /.~,;:. i ;.,;~ ;,:,,4.s. ;~,.~:,f.~.~,,:..,,,:.~. : L ' , , : , : i : ~::., ' , ' , ' . : : ' , . : ;~ ' , . : ..~,;i'.4.is.~i.:s:~.::Ss:.~:./:;:'.,'z.: ~ . . - . 5 ; ~ . : �9

t!,.!~.-.'~.-;_.:.;C::'_,:!.:_-_.,;..,7.~.:.:_:~.,::,."._~ :-_".~i ..................... ;; ~: :"F.":,'.'~: ~ :'":~"~:"Y~ "~)~/ ..... .?;';"~ :1 }porous wa . ! . . . .

Fig. 12.14. M e t h o d s for haduc ing f low instabil i t ies: (a) plachag objects (p ro tube rances , baffles) on to the m e m b r a n e sur face to fo rm a r o u g h surface, (b) plachag objects hato the f low charmel a w a y f r o m

the m e m b r a n e surface.

f l o w

x , ' . " . ~ ' ~ ' . ' / : ' / ' "~ ~. " " ' / .,~. z , : " ",'..:. .~ ,~, ' , ' . ' , :,,..;.~,.~ ~ ~ . . . . , .

/ - - - tubular helicoidal shape

,j

~ . ~ , ,~?,~..~'

Fig. 12.15. Schematic represen ta t ion of a tube wi th ma il~ler surface exhibi t ing an hel icoidal profile.

or ceramic) membranes exhibiting an inner helicoidal structure as s h o w n in Fig. 12.15. These helicoidal shaped tubes can be sealed in modules in the same w a y than with classical tubes or monoliths.

12 m TRANSPORT A N D FOULING P H E N O M E N A IN LIQUID PHASE SEPARATION 593

Rotating disc systems have also been described as efficient devices to overcome flux limitation due to matter deposit on membrane surface during cross- flow filtration [40]. The problem of erosion of a macroscopic particle solid deposit on a rotating disc membrane has been quantified by Aubert et al. [41]. The influence of the transmembrane pressure, the thickness of the initial deposit and the pore size on the critical shear stress have been investigated and described by empirical fits. It results from this study that fouling is more effi- ciently eliminated at high Ap and large pore size.

12.3.2 Influence of Membrane Material on Permeability and Solute Rejection

The influence of metal oxide derived membrane material with regard to permeability and solute rejection was first reported by Vernon Ballou et al. [42,43] in the early 70s concerning mesoporous glass membranes. Filtration of sodium chloride and urea was studied with porous glass membranes in close- end capillary form, to determine the effect of pressure, temperature and concentration variations on lifetime rejection and flux characteristics. In this work experiments were considered as hyperfiltration (reverse osmosis) due to the high pressure applied to the membranes, 40 to 120 atm. In fact, results repro- duced in Table 12.3 show that these membranes do not behave as hyperfiltration membranes but as membranes with intermediate performances between ultra- and nanofiltration in which surface charge effect of metal oxide material plays an important role in solute rejection.

Rejection data for NaC1 were explained according to a low-capacity ion exchange mechanism. The ion exchange mechanism put forward in this work is not consistent with the porous structure of the membranes and the high t ransmembrane pressure used in the filtration experiments . Ion exchange

TABLE 12.3

Rejection of NaC1 (58.5 g mole -1) and Urea (56 g mole -1) ushlg mesoporous glass membranes over a range of solute concentration, from Ref. [42]

Solute Feed concentration R

g/1 tool/1

NaC1 0.47 0.008 0.86 NaC1 1.30 0.022 0.68

NaC1 9.11 0.156 0.42 Urea 1.74 0.029 0.41 Urea 3.80 0.063 0.38 Urea 11.32 0.189 0.37


mechanism is better related to the working conditions described by Singh and Singh [44] for zirconium phosphate membranes. Regarding the work of Vernon Ballou [43] pore sizes were calculated from nitrogen isotherm data. Narrow pore volume distributions of unused glass membranes were found between 1.9 and 2.2 nm pore radius. One can see that NaC1 rejection cannot be explained by an hyperfiltration mechanism based on selective diffusion of water through the membrane. Moreover the decrease of NaC1 rejection when ionic strength of the feed solution increases is in favour of mass transport through an array of metal oxide microcapillaries filled with electrolytes. Unfortunately these glass membranes were not stable over a long period of time due to solubility of silica. Interpretation of the results were altered by a loss of rejection and an increase of permeability mainly caused by broadening of pore size distribution with time.

Since then, the evidence of pH effect on cross-flow micro- or ultrafiltration using ceramic membranes has been given in the literature [45-48]. Hoogland et al. [45] showed that permeability of a Ceraflo (Norton) (x-alumina membrane towards pure water and mineral slurries was dependent of pH. For pure water the maximum of permeability was found in a pH range near the isoelectric point of the membrane (low pH) while the membrane resistance was highest far from the isoelectric point (high pH) when the charge of the membrane is strongly negative. At high pH, this effect can be explained by the flow through a charged porous barrier which leads to electro-osmosis phenomenon and an effective loss of permeability. Fluxes measured with mineral slurries (silica particles) were also dependent on pH. Higher permeabilities for the membrane were found at low and high pH while flux decline was maximum at intermedi- ates pH. In this case two phenomena due to pH act simultaneously. One is related to the alumina membrane, the other to silica particles. At low pH near the isoelectric point of the particles, there is formation of large-size flocs gener- ated by aggregation of weakly charged particles. These flocs prevent penetration of the individual particles inside the porosity and are easily removed from the membrane surface by the effect of cross-flow. At high pH both the membrane and the particles exhibits negative charges which lead to repulsive forces at the membrane-solution interface and depolarization of the membrane. At intermediate pH polarization and membrane resistance are maximum. One important parameter, the ionic strength of filtered solutions, was not investigated in this work.

The effects of pH and ionic strength on the performance of an (x-alumina microfiltration membrane from U.S. Filter was evaluated by Nazzal and Wies- ner [46]. Concerning pH effect on flux, results obtained in this work perceptibly differ from the previous one. Here the membrane operated at a significantly higher permeation rate at a pH well below the isoelectric point of the membrane. This variance can be explained considering the isoelectric point of the membrane was found at pH = 8.3 in this case while it was at pH = 3.5 in the

12 - - TRANSPORT A N D FOULING P H E N O M E N A IN LIQUID PHASE SEPARATION 5 9 5

previous work. It can be observed that in both cases maximum of flux for pure water was measured at pH 3 to 4.

Results concerning filtration studies with 0.2 ~tm titanium dioxide membranes supported on stainless steel or ceramic porous tubes were recently reported by Porter et al. [47,48]. Solutions containing sodium nitrate alone and in the presence of anionic, direct and acid dyes were filtered with adjusted solution pH. Electrolyte rejections and colour rejections were measured at pH values from 4 to 10. They showed that the charged membrane was responsible for ion rejection at low ionic concentration while rejection decreased to near 0% as the salt concentration was raised to 5000 ppm..These results are consistent with long range forces associated to Debye-length which can reach several hundred Angstroms in the solution for very low ionic concentrations.

12.4 N A N O F I L T R A T I O N W I T H C E R A M I C M E M B R A N E S

In the early 1970s, several authors described separation membrane processes with intermediate performances between reverse osmosis (RO) and ultrafiltration (UF). Typically retention for these membranes was in the range of 50-70% for sodium chloride while it was in the 90% for organics. In the 1980s a suggested definition for these membranes was based on a molecular weight cutoff of 1000. Then "nanofiltration" was considered a suitable name for such a process which rejects molecules in the nanometer range. Presently basic properties of nanofiltration membranes can clearly be defined compared with ultrafiltration or reverse osmosis membranes:

- a molecular weight cutoff of less than 1000 (membranes with MW cutoff of 1000 and above are considered UF membranes),

- a lower transmembrane pressure and a higher flux than for RO, - a mixed mass transport mechanism involving convective and diffusive

fluxes for both solutes and solvent, - in most cases membrane charged either positively or negatively due to

their materials, - a marked influence of Donnan mechanism in the case of an aqueous feed

solution containing mixed electrolytes. It results from these basic properties that nanofiltration offers unique per-

formances for the separation of salts and organics. A negative salt rejection has been evidenced in these membranes which can be explained with reference to the above-mentioned capillary model in which the structure of nanofiltration membranes is represented by a bundle of charged capillaries with a pore radius in the nanoscale. In practice, this negative salt rejection effect can be usefully exploited in industrial desalting-concentration processes of molecules exhibiting molecular weight of less than 1000. In fact nanofiltration membranes are finding increased applicability in various fields but their transport mechanism

5 9 6 12 E TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION

is not yet well understood. Up to now a number of published papers deal with the description of transport properties of organic nanofiltration membranes [49-51]. On the other hand, few data are available in the literature concerning ceramic nanofilters. In the following, recent results concerning separation properties of ceramic nanofilters are presented showing that some of these basic properties are relevant to describe mass transport and solute rejection observed with microporous ceramic membranes.

Ceramic nanofilters are a new class of ceramic membranes which obey the basic properties of nanofiltration membranes. Some similarities can be noted between organic and inorganic NF membranes behaviour; however specificities exist with ceramic membranes due to the amphoteric properties of metal oxides in water media. Basically the structure of ceramic nanofilters can be described according to concepts developed for nanophase materials. The active layer is made of a supported microporous layer with a thickness in the micron range. This microporous structure which results from sintering of ceramic grains of less than 10 nm in size leads to membrane materials with a high surface area. Metal oxides already used for the preparation of micro- and ultrafiltration membranes can also be used for nanofilters. Microporous y-alumina, titania, zirconia and silica supported layers have been described by Julbe et al. [20] with suitable characteristics for nanofiltration. However, regarding industrial applications of these membranes for aqueous filtration, zirconia and titania are preferred to silica or y-alumina because of their stability in large pH and temperature ranges.

The main characteristics of nanofiltration membranes made of oxide ceramics is that they exhibit a microporous structure with charged pore walls depending on pH and ionic strength of feed solutions. Three main cases are distinguished in the discussion of mechanisms involved in permeation and separation processes using microporous ceramic nanofilters:

- separation of neutral solutes in absence of electrolyte; - separation of pure electrolyte mixtures, - separation of solutions containing both organics (ionisable or not) and

electrolytes;

12.4.1 Separation of Neutral Solutes in Absence of Electrolytes

When Donnan contribution can be neglected (case of neutral solutes), membrane cut-off can be determined based on respective sizes of model solutes and membrane pores. Mass transport can be described using both basic concepts of ultrafiltration and specific aspects of transport in micropores.

Pure solvent flow can be described as a convective flow with a linear dependence to transmembrane pressure as shown in Fig. 12.16. With nanofiltration membranes a minimum value of pressure gradient has to be applied before to observe

12 - - TRANSPORT AND FOULING P H E N O M E N A IN LIQUID PHASE SEPARATION 597

Jv

l increasing pore diameter

Ap Fig. 12.16. Schematic representation of hydraulic permeability versus transmembrane pressure for

a microporous membrane.

a solvent flux through the membrane. This is due to the occurrence of important capillary forces in the case of micropores of less than 2 nm in diameter.

In the presence of solutes with small molecular weights, concentration polarization is likely to occur but with much less effect than in the case of ultrafiltration as explained in Section 12.2.1. A theoretical model concerning separation of sucrose and raffinose by ultrafiltration membranes has been proposed by Baker et al. [53] which assumes transport of solvent and solute exclu- sively through pores. This model can apply to ceramic nanofilters as they exhibit a porous structure with a pore size distribution. The retention characteristics of a given membrane for a given solute is basically determined by its pore-size distribution. The partial volume flux jv through the pores which show no rejection to the solute can be expressed as a fraction of the total volume flux Jv.

jv= f . Jv (12.29)

The solute rejection is then given as a function of the total water flux, of the solute diffusion coefficient Ds and of the pore fraction el permeable to the solute:

fexp Z R - 100 1 -- ~al"/ds ) h (12.30)

f - 1+ exp / l iDs )

It follows from Eq. (12.30) that as Jv goes to zero, the exponential term goes to unity and the rejection coefficient reduce to zero. On the contrary as Jv tends to become very large, the exponential term tends towards infinity, and the rejection coefficient approaches a specific limiting value for a given solute.

The same evolution of the rejection coefficient with volume flow and indi- rectly with transmembrane pressure was predicted by Tremblay [54] using the finely porous model proposed by Merten [55] and modified by Mehdizadeh

598 12 u TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION

Separation factor (%)

100 r ~," " ~ ' ~

I . ; / / / -

80 , /~ / �9 .

60 I/.1'1 "/ 0 ; : e

0 40 r't " 0

20

0 i

Increasing Pressure Gradient

I

10000 100 1000 100000

Solute molecular weight

Fig. 12.17. Evolution of separation factor versus solute molecular weight for different pressure gradients. Results obtained from mass transfer across micropores using radially averaged friction

factors [54].

and Dickson [56] in which a radially friction factor b -1 has been included. This friction factor is directly related to the ratio ;~ of the solute radius to the pore diameter and represents the friction between a solute molecule translating along the centre line of a cylindrical pore and the pore wall.

1 P dCi 1 Ji- - ~ ~ d---~ + b Ci Jv (12.31)

= (1 - ~ ) 2 b

with b given by the Faxen equation

b = 1 - 2.1044;~ + 2.089~ 3- 0.948~ 5

(12.32)

(12.33)

It results from this approach that separation factors will depend on the ratio ;~ and on the operating pressure. As shown in Fig. 12.17 the influence of friction factor on separation factor is predominant at high pressure gradient.

12.4.2 Salt Rejection of Electrolyte Solutions

Salt rejection of a single electrolyte by a nanofiltration membrane in the absence of Donnan contribution can be described by Eqs. (12.9) and (12.10) according to the work of Spiegler and Kedem [57]. With ceramic nanofilters the Donnan contribution has to be taken into account due to the amphoteric

12 - - TRANSPORT AND FOULING PHENOMENA IN LIQUID PHASE SEPARATION 5 9 9

behaviour of metal oxide surface resulting in membrane materials with charged pore wall. The extended Nernst-Planck equation (12.19) has been applied by Tsuru et al. [58-60] to predict ion rejection in the case of charged membranes for single and mixed electrolytes. This approach showed good agreement with mass transport description obtained from irreversible thermodynamics. The general tendencies for ion rejection are as follow"

- for a single electrolyte solution, rejection dependency on volume flux is the same as that of neutral solutes. Increasing the charge density in the membrane make rejection higher. Rejection of divalent coion electrolyte is expected to be higher than that of monovalent coion electrolyte, while divalent counter-ion rejection seems lower than that of monovalent counter-ion electrolyte;

- for a mixed electrolyte solution, rejections are shown as strongly dependent upon the volume flux, mole fraction, and the ratio of the feed concentration to the membrane charge density. Mono- and divalent coions are suggested to be separated effectively, and the monovalent coion to show negative rejection under a certain condition. However, mono- and divalent counter-ions are not so effectively separated as coions under ordinary conditions.

Recently Wang et al. [61] proposed the comparison of different models from the literature to describe electrolyte transport through nanofiltration membranes. The space charge (SC) model described in Section 12.2.3 was compared with the Toerell-Meyer-Sievers (TMS) model. The SC model assumes that ion concentration and electric potential have a distribution in the radial direction in the membrane, while the TMS model supposes that both of them held uniform. The evolution of ion rejections versus Peclet number (Pe) for a 1-1 electrolyte (KC1) were compared for the two models respectively with increasing charge density at constant pore radius (5 nm) and for decreasing pore radius at a constant charge density (3.336 C.m-2). In agreement with general expressions derived from linear, non-equilibrium thermodynamic theory [7] the rejection was found to increase with Pe. This is consistent with the fact that at small Pe number there is a dominant contribution of diffusion to electrolyte transport while contribution of convection is dominant at large Pe number. With decreasing pore radius, the rejections calculated from the two models tend to coincide and shows almost the same value for pore radius of I nm. This can be explained because an overlap of double layers into the pores due to a Debye-length equal or larger than the pore size. This overlap of double layer renders the distribution of concentration and electric potential uniform in agreement with the TMS model. According to definitions of electrokinetic radius rE and dimensionless hydraulic permeability Ls given by Eqs. (12.27) and (12.28) the authors calculated evolution of Ls versus rE. In Fig. 12.18, curves Ls =firE) drawn at different potential gradients show that a maximum effect of the electrical force is expected for rE --- 1 and high potential gradient.


Ls 0.95

0.9

0.85

0.8

0.75 . . . . . . . . . . 0.1 1

" 1%(;) I �9 I .047 [ b 1.094 [ c 1.234 I ~ 1.46e l e 1.936

f | 2.34 g 14 .68

J ! �9

10

i" E

Fig. 12.18. Dimensionless water permeabil i ty as functions of electrokinetic radius at different potential gradient (q0) [61].

In the above-considered works the behaviour of electrically charged nanofilters towards electrolyte solution has been mainly regarded with respect to Donnan analysis to explain the coion rejection. Bardot et al. [62] looked at the effect of transmembrane transport kinetics on counter ion rejection through an alumina/polysulfone composite membrane in the case of electrolyte mixtures. The rejection phenomenon is based on a "decompensation" of the convective and electric flows of a given counter-ion as a consequence of the addition of counter-ions with a different mobility. It has been shown both theoretically and experimentally (for negative charged membranes) that the same physics accounts for not only the improvement of the retention of more mobile counterions upon addition of less mobile but also for a significant deterioration of the retention (down to the negative one) of less mobile counter ions upon addition of more mobile. Experimental correlations of the phenomenon with the ratio of mobilities of counterions, the concentration of starting electrolyte and transmembrane pressure difference (Pe number) have been in complete agreement with theoretical predictions. However the influence of the ceramic support versus pH of the feed solution, which can be of great influence on ion rejection, is not discussed.

The evidence of electrokinetic salt rejection by a microporous inorganic material was given by Jacazio et al. [63] based on the model of Osterle [26-28]. Experiments were carried out on the salt rejecting properties of compacted clay through which saline solutions were forced under high pressures. In accordance with the model the performance of the porous material was shown to depend on three main parameters: the ratio of the Debye length to effective pore


radius; a dimensionless wall potential related to the ~ potential; and a Peclet number based on the filtration velocity through the pore. Comparison between the experimentally determined and theoretically predicted rejections of potassium chloride in the case of effective pore radius in the range 1-2 nm were shown to be excellent.

Regarding the rejection of salt mixtures with inorganic membranes Alami- Younssi et al. [64] investigated the performance of a y-alumina membrane. In aqueous media containing indifferent electrolytes such as NaC1, the point of zero charge (zpc) of the y-alumina is near 8.5; in the presence of divalent anions or cations which are able to form surface complexes respectively with the surface groups A1OH ~ or A10, the zpc of the material can be shifted, respectively, towards higher or lower pH values, pH values for feed electrolyte solutions were measured to be in the range 5-5.9, which means that the membrane is positively charged. Results are discussed only in terms of effective charge of the membrane and valence of the co- and counter-ions present in the feed solution. In this case the membrane is positively charged and the rejection obeys to the prediction of Tsuru concerning mixed electrolyte solutions. Meas- ured rejections are reported in Table 12.4. Rejections were shown to depend on the charge of the ions and decrease in the order: (divalent cation, monoanion) > (monocation, monoanion) or (dication, dianion) > (monocation, dianion).

Another work from Rios et al. [65] also deals with performance of a positively charged y-alumina membrane fed with single NaC1, MgC12, Na2SO4 and MgSO4 solutions at various concentrations (10 -4 to 10 -1 mole-l-I), or even with electro-

TABLE 12.4

Rejection with a y-alumina membrane of mixed electrolyte water solutions [64]

Sodium and calcium nitrates

[Ca 2+] feed (M) 0 10 -3 10 -2 10 -2 10 -2

[Na+] feed (M) 10 -2 10 -2 10 -2 10 -3 0

Rejection NO 3-- (%) 68 75 75 93

Rejection Ca 2+ (%) 90 95 95

Rejection Na + (%) 68 63 38 47

96

96

Potassium and sodium nitrates

[Ca 2+] feed (M)

[K +] feed (M)

0 10 -3 10 -2 10 -2

10 -2 10 -2 10 -2 10 -3

10 -2

0

Rejection NO 3- (%)

Rejection Ca 2+ (%)

Rejection Na + (%)

55 18 45 50

32 60 56

55 15 18 25

68

68


lyte mixtures (NaC1 + MgC12; Na2SO 4 + MgSO4) of constant counter ion concentration (10 -3 mole.l-I). Results that confirm the previous trends are explained using a new simplified model based on Eq. (12.19) that makes the assumption of Donnan effect at pore entrance. This model accounts for electrokinetic phenomena inside the pores and also considers differences in ion mobility.

Zirconia nanofilters (partially stabilized or not) have been investigated by Guizard et al. [66,67] with respect to rejection performance towards model solutes. These membranes were synthesized by the sol-gel process using zirconium and magnesium alkoxide precursors, the later being used as stabilizer agent for the cubic zirconia phase. Pore diameter for these membranes is in the range I to 2 nm depending on preparation conditions. In agreement with data published in the literature a zpc near 7 has been evidenced. In this work special attention has been paid to the influence at one hand of pore size and specific surface area of the membranes, on the other hand of transmembrane pressure and ionic force of feed solutions. It has been shown that these parameters clearly relate to the dimensionless zeta-potential %, the electrokinetic radius rE and the dimensionless hydraulic permeability Ls resulting from the variation rE and %. The rejection versus pH of chloride and sulfate ions using a Na2SO4/NaC1 mixture (200 ppm) is shown in Fig. 12.19. At a pH < zpc chloride and sulfate must be regarded as counterions for the membrane while at pH > zpc they behave as coions. One can see that results are in good agreement with the prediction; sulfate are better rejected than chloride when they are coions of the

Rejection (%)

I 0 0 ' , ,

8 0

60

40

2 0

' ! ' ' ' I

-- Su,fate j - - ~ Chl~ I

'--'~. iP."C~ ............ i

. . . . . . . . . . . . . . . . . . . . . . . . . . -

, , , ! , , , ! , , ,

i i

i i 2 4 6 8 10 12

pH Fig. 12.19. Rejection of an electrolyte mixture Na2SO4/NaC1 (200 ppm) by a zirconia nanofilter.

Effect of p H [66,67].

12 - - TRANSPORT A N D FOULING P H E N O M E N A IN LIQUID PHASE SEPARATION 603

Permeabi l i ty

( l /h.mZ.bar)

3 0 l . . ' ' ' ' " I . . . . I ' " ' I ' " ' " ' " -,!

E , i " L . i "~ ' :! z o F - t .... ...................... - .............................. . ~ - ' - .................... [ ............................ q

, i i " " " - q m : I . . :, .

. . . . I . . . . . . . . . . & . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- 1 0 �9

, i iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii 0 ,

0 $ 1 0 1 5 2 0

Pressure (bar)

Fig. 12.20. Permeability versus transmembrane pressure for two zirconia nanofilters with different microporous volumes: (a) 8x10 -2 cm3/g, (b) 6.6-10 -2 cm3/g [66,67].

membrane. On the contrary both sulfates and chlorides are not rejected when they are counterions of the membrane.

Charge density at the pore wall is a key parameter for the description of electrolyte rejection by charged nanofiltration membranes. In ceramic nanofilters charge density can be related to the specific surface area measured on ceramic membrane material. In the case of a negative charged membrane material (pH > zpc), Figs. 12.20 and 12.21 show respectively the influence of t ransmembrane pressure on membrane permeabili ty and the sulfate rejection versus flux for two membranes exhibiting different specific surface area. Calcu- lated hydraulic radius was almost the same for the two membranes (rh ---- 0.43 nm) so that permeabili ty can be discussed in term of the Donnan effect and related streaming potential for electrically charged porous membranes assum- ing that membrane thickness is the same in both cases. The effect of electrokinetic flow on membrane permeability is shown in Fig. 12.20. When t ransmembrane pressure increases permeability increases at first and then decreases due to the opposite contribution of electrokinematic flow to convective flow. This can be explained by the occurrence of a non-negligible streaming potential for a t ransmembrane pressure higher than 3 bar. Moreover the electrokinematic flow effect was more marked for the membrane with the higher surface area which is consistent with a higher charge density. If we consider now sulfate rejection by the two membranes, Fig. 12.21, a better rejection was obtained with the membrane exhibiting the higher surface area and consistently the higher


Rejection (%)

1 0 0

..... o ........ ..................... i ..................... i . . . . . . . . . . . . . . .

6O

40

2O

0

0 $0 100 150 200 250 300

Permeate flux (l/h.m z)

Fig. 12.21. Sulfate rejection versus volume flux through two zirconia nanofilters with different microporous volumes: (a) 8-10 -2 cm3/g, (b) 6.6.10-2 cm3/g [66,67].

charge density. However the evolution of sulfate rejection with flux is not totally explained by Tsuru calculation [59] on sulfate rejection through negatively charged polymer membranes. With these zirconia nanofilters a maximum in rejection versus flux was evidenced while an increase of rejection followed by a plateau has been described with polymer nanofiltration membranes.

12.4.3 Separation of Aqueous Ionized Molecule-Salt Solutions

The case which consists of a mixture of a mono-monovalent salt like NaC1 and a multifunctional organic anion A n- containing n negatively charged groups per molecule in a sodium salt form has been described by Perry and Linder [50]. It has been assumed that the monovalent anion C1- is permeable through the membrane and the organic anion A ~- is fully rejected. Accordingly a new expression for salt rejection was proposed:

TR = 1 - [(1 - r~)~/(1 - r~F)] (12.34)

F is defined as in Eq. (12.12) and [3 as

[3- (1 + n CAb/Csb) 1/2 (12.35)

with CAB and Csb respectively the concentration of the organic anion and the concentration of monovalent anion in the feed solution. When only pure salt is p r e s e n t CAB-- 0, [~ = 1, Eq. (12.34) becomes identical to Eq. (12.11).


Schirg and Widmer [52] published mathematical models for the calculation of retention and selectivity for nanofiltration of aqueous dye-salt solutions. A modification of Eqs. (12.11) and (12.12) has been proposed in which the integral salt permeability r could be described by the introduction of an exponential function

c0 = (x C~ (12.36)

with (x a constant and 7 a coefficient for salt permeability dependence on concentration.

Both calculations by Perry and Schirg have been performed to describe and to predict the rejection characteristics of organic nanofiltration membranes when ionic and chargedmolecular solute mixtures are used in the feed solution. Recently experiments were carried out with ceramic nanofilters [67] which showed that similar properties can be obtained. As an example, results concerning the rejection of a dye/electrolyte mixture at pH = 9 through a zirconia nanofilter are reported in Table 12.5. --

As a general conclusion to this part dedicated to nanofiltration with ceramic membranes one can assume that the general behaviour of these membranes can be assimilated to the behaviour of electrically charged organic nanofiltration membranes. However some specificities exist with ceramic nanofilters due to a sintered metal oxide grains derived porous structure and an amphoteric character

TABLE 12.5

Rejection of a mixture of an organic anion (bromocresol green) and salt anions (SO4, C1-) through a negatively charged zirconia nanofilter [67]

Anion Concentration Mw Pressure (Ap) Rejection (ppm) (bar) (%)

Bromocresol green

A- 200 698 10 63 20 71

NaC1/Na2SO 4 mixture

C1-

SO~

2000 58 10 3 20 6

2000 142 10 39 20 40

Bromocresol green/NaC1/Na2SO4 mixture

A- 200 698

C1- 2000 58

SO4 2000 142

10 70

10 0

10 48

606 12 - - TRANSPORT A N D FOULING P H E N O M E N A IN LIQUID P H A S E S E P A R A T I O N

in water media. At this time few experimental data are available in view of an assessment of existing or new mathematical models well adapted to ceramic nanofilters. Further experiments with different categories of ceramic membrane material are needed to establish general principles of nanofiltration with ceramic membranes.

12.5 PROSPECTIVE ASPECTS

12.5.1 Organic-Inorganic Hybrid Membranes and Related Processes

At the present time, organic-inorganic hybrid membranes do not exist at the commercial stage. However, recent results have shown the interest of these membranes in a non-limited list of applications such as gas separation, perva- poration, chemical and biological sensors, facilitated transport, ultra- and nanofiltration. The main interest of organic-inorganic membranes is that they can combine basic properties of both organic and inorganic membrane materials. Accordingly improved properties are expected from this new category of membrane. A short overview of recent works dedicated to these membranes is given hereafter which illustrates their potentiality in liquid phase separation.

A first way to obtain an organic-inorganic hybrid membrane is to have a polymer material either deposited or grafted at the surface or embedded in the top-layer porosity of a ceramic support. For example Castro et al. [68] investigated the permeability behaviour of polyvinylpyrrolidone-modified porous silica membranes. The surface of 0.4 ~tm-pore-size silica membranes was modified with a covalently bonded polyvinylpyrrolidone brush layer. Hydraulic permeability measurements performed with six different solvents and both unmodified and modified membranes suggest that the permeability of the modified membrane is determined by the configuration of the terminally an- chored polymer chains. In the modified ceramic-supported polymer membrane, the swelling of the polymer brush layer increased as the solvent power increased, resulting in a decrease in the pore radius and subsequently the permeability. In a previously mentioned study Bardot et al. [62] used nanofiltration membranes made by internal coating of porous tubular supports of R-alumina with sulfonated collodion followed by coagulation in an appropriate bath. More recently Sarrade et al. [69] have developed a hybrid nanofiltration membrane highly effective for separating non-charged solutes of molecular weight as low as 500-1000 Dalton in supercritical carbon dioxide medium.

This is a combined organic-inorganic membrane that comprises a macroporous ~-alumina substrate (tubular or multichannel), an intermediate mesoporous inorganic titanium oxide layer (thickness: 1 ~tm) and a microporous Nation polymer top-layer (thickness: less than 0.1 ~tm). The overall performance and


TABLE 12.6

Transport parameter value of a Nation/titania hybrid membrane [69,70]

EG PEG 200 PEG 400 PEG 600 PEG 1500

(~ 6.010 -2 4.410 -1 5.710 -] 8.1 10 -1 9.610 -1 co (m.s -1) 4.410 -5 2.1 10 -6 8.810 -7 1.710 -7 3.1 10 -8

t ranspor t mechanism th rough this membrane have been s tudied using ethylene glycol (EG) and various polyethylene glycols (PEG) as model solutes [70]. Starting from Eqs. (12.9) to (12.12), the membrane permeabili t ies to water, Lp, and solutes, co, as well as the reflection coefficients, r~, were de termined at first. These values are reported in Table 12.6 and in Fig. 12.22. Using the theory p roposed by Verniory et al. [6] to account for h indered t ransport in pores, the mean pore radius was est imated from these parameters. It is wor th not ing that the mean value of 0.6 n m calculated for the membrane is consistent wi th the pore d imension (0.8 nm) directly measured using the biliquid pe rmporome t ry [71]. It has been shown that, regardless of the size of the solute molecule, convective t ransport is always more impor tant than diffusive transport . In

1,0

0,8

0,6

0,4

0,2

x Membrane TN

m Membrane A

0,0 0,0 0,2 0,4 0,6 0,8 1,0

rs (nm)

Fig. 12.22. Variation of reflection coefficient (~ versus equivalent radius of model solutes rs for an alumina nanofilter (A) [65] and a Nation/titania hybrid membrane (TN) [70].


accordance with nanofiltration behaviour this membrane also exhibited effective separation of ionic species.

Organic-inorganic polymer at the molecular level are also of interest as shown in the following examples. A new concept in nanofiltration has been proposed by Guizard et al. [72] based on a hybrid polymer (cyclic polyphos- phazene) supported on a zirconia ultrafiltration membrane. Excellent chemical and temperature resistances were obtained for these membranes due to intrinsic properties of polyphosphazenes as well as a high rejection of small organics and a good selectivity concerning multivalent versus monovalent ions. The reflection coefficient r~ was markedly related to the transmembrane pressure leading to adjustable working conditions. Another example is in an alternative way to selective transport of metal ions through liquid membranes, such as transport mediated by crown-ether and other macrocyclic ligands which has been extensively investigated during the last twenty years. No practical separation processes arose during this period mainly because liquid membranes suffer poor stability and thus short lifetimes: the membrane degradation is essentially due to the loss of carrier by dissolution in the aqueous phase and by emulsion formation at the membrane interfaces. Consequently the recent developments in facilitated transport membrane processes are focused on new membrane systems with improved lifetimes. One of these systems is based on the carrier grafting onto a solid membrane matrix. Grafting of benzo-15-crown-5 in a heteropolysiloxane membrane was investigated by Lacan et al. [73] in view of facilitated transport of alkaline ions. Very stable membranes over several months were obtained without loss of carrier during transport experiments. It has been demonstrated that covalently bound carriers allow facilitated transport of K § ions versus Li + ions to take place with high diffusion rates, high facilitation factor and good selectivity. These membranes open a new way in the application of facilitated transport to practical separation processes.

12.5.2 Coupled Membrane Processes

Inorganic membranes, and to a less extent hybrid membranes, possess a high degree of resistance to chemical and abrasion degradation as well as tolerate a wide range of pH and temperature values. All these properties make them very useful for coupling with other processes and open up new fields of applications. In what follows, some examples of such integrated processes involving at least one membrane separation stage are presented.

Membrane bioreactor

The idea of coupling membrane separation with bioreaction is not a new one. A lot of works published in the literature bear witness to this fact. But most of


the time organic membranes and aqueous phase reactions are considered [74]. For areas of biotechnology or food engineering, a main advantage of inorganic materials is that they may be repeatedly autoclavable and are very stable against microbiological attack [32]. With them, reactions in pure organic solvent may be also successfully faced. As an example, the enantiomeric resolution of menthol (+) into methyl (-) laurate, through a biological catalysis method involving a lipase from Candida rugosa and lauric acid as substrate, was recently investigated using n-heptane as the solvent medium [75]. A zirconia membrane with a pore diameter of about 4 nm was chosen to'retain the biocatalyst. This lets the substrate and product molecules pass. The transmembrane pressure was selected so as to get a space time leading to an optimum reaction yield. At the reactor outside, menthol (+) and methyl (-) laurate were separated from permeate, and then menthol (-) was regenerated from ester.

Nanofiltration plus supercritical fluid extraction

Supercritical fluid extraction is used to recover small organic solutes with molecular weight below 1500 daltons. In a state of continuity between vapour and liquid, supercritical fluids exhibit intermediate transport properties with lower viscosities than liquid and higher diffusivities than gases. Because of its

(~ A P > 74 bars I T > 31 "C Valve

CO 2 I I

I I P < 60 bars I I

exc6ange r ' ~ ' " ~ I I , l - " " " ~

I ! I _I I T .4 - "

I Extract I I /

Mixt extracts

Fig. 12.23. Nanofiltration/supercritical fluid extraction coupled processes.


attractive critical temperature/pressure conditions, stability, low cost and non- toxicity, CO2 is today the most widely used SCF. As previously indicated, nanofiltration now provides new ceramic and hybrid membranes with cut-off in the range of 300 to 1000 daltons. On principle, nanofiltration plus supercritical extraction aims to both enhance the selectivity of extraction and lower energy consumption. A schematic view of the process is given in Fig. 12.23.

Regarding selectivity, it may be thought that membrane sieving effect will induce a separation of supercritical fluid mixture into fractions respectively containing high (retentate) and low (permeate) molecular weight solutes. From an economic viewpoint, a substantial energy saving may be expected due to the fact that only permeate flow that just represents a small part of total CO2 will be submitted to a strong pressure reduction from extract recovery, while low soluble heavy compounds will be continuously deposited from retentate by means of a small pressure and/or temperature effect. Experiments have proved that silica [76], or titane-nafion membranes [77] were able to endure supercritical fluid conditions with no alteration. With y-alumina membrane, fouling strongly develops probably due to chemisorption. Working with model molecules such as ethylene glycol and polyethylene glycols (PEG 200-400-600), the process capability to extract and separate various size solutes has also been checked [78].

Ultrafiltration plus electrophoresis

It is worth recalling that the flux and the selectivity of ultrafiltration may be improved when treating electrically charged solutes ~ as an example, alkaline gelatin molecules (pI = 4.7) processed at pH = 6.0 present a negative charge by superposing upon the driven pressure an electric field which acts on the retained solute to control concentration polarization. This is the so-called "electro-ultrafiltration process". In the past various works have underlined the influence on performance of such parameters as pressure, fluid velocity, electric field strength or starting conditions particularly with ceramic membranes [79]. With membranes cylindrically shaped, and for instance when processing a negatively charged solutes, a classical setting diagram consists both in install- ing a stainless steel wire as anode through the centre of the membrane and in closely surrounding the outside of the supporting tube by a cathode made of a large mesh stainless wire lattice (Fig. 12.24). Because the supporting tube is placed along with the membrane itself in the electric field acting area, disadvan- tages may result from the use of this traditional set-up: excessive energy consumption, parasite and uncontrolled effects (such as electro-osmosis fluxes). A new concept has been recently proposed with inorganic membranes to overcome some of these difficulties. It consists in designing electronic conductive membranes in which the original feature is the possibility of using the


Membrane support

, / /

t' b Electric, potential I

Central electrode

~ ctive membrane

External electrode

for non-conductive membrane

Retentate

Fig. 12.24. Schematic view of electro-ultrafiltration using conductive inorganic membranes.

active layer both as a filter and as an electrode. RuO2-TiO2 membranes coated on an alumina support belong to this class. With them, performance may be enhanced by applying the electric field only inside the filtration module, spe- cially when the membrane is used as the anode, a classical way to work RuO2-TiO 2 electrodes [80,81].

Cross-flow filtration with mobile turbulence promoters

It is well known that pumping of the fluid has a major effect on flux in the mass transfer controlled region for UF/MF process. Indeed agitation and mixing of the fluid near the membrane surface sweep away the accumulated solutes, thus reducing the thickness of boundary layers. This is the simplest and most effective method of controlling the effect of concentration polarization.


J (I/h.m2)

5 0 -

40

30

20

Fluidized bed (Stainless steel beads 3mm in diameter and 7.9 in density)

Empty robe ,_..._._....p..---

a i i I , I ~ I , I

- - 10 2O 3 0 4O 5 0

/k ~ ,'k u(~) s = 0,42 E = 0,6 ~ = 0,7 s = 0,8

Minimum fluidization

Fig. 12.25. Permeate flux versus tangential fluid velocity with a gelatine solution (10 g 1-1, Ap = 1.5

bar) ushlg a tubular a lumina membrane filled with a f luidized bed.

The magnitude of the effect of flow rate on the mass transfer coefficient will depend on whether the flow is turbulent or laminar, as well as on rheological properties of the fluid, the key factor being the shear stress at wall. Another less common method to effect permeate flux increases is through the introduction of turbulence promoters in the flow conduit. Up to now more attention has been given to fixed promoters due to damage that ordinarily results from the move- ments of free agents at the very fragile surface of traditional organic membranes [82]. On the contrary, ceramic membranes (alumina as an example) are resistant enough to endure the continuous bombardment of fltfidized particles [83] or even the friction of transported solids [84]. With such devices, high permeate fluxes may be obtained (Fig. 12.25) with no sharp decrease in solute rejection (Fig. 12.26), even at tangential fluid velocity as low as a few ten centimetres per second.

The analysis of mass transfer coefficients and hydraulic resistances showed that moving particles insure a significant reduction of the mass transfer boundary layer, as well as a continuous mechanical erosion of the deposit at wall. Polarization is strongly modified. From a practical viewpoint, low retentate velocities may offer some interesting developments in those cases where fragile molecules are to be treated, or long enough residence times are needed. Solid particles could also be used as catalyst (enzymatic supports as an example) for heterogeneous reactions, adsorbent for coupled MF/adsorpt ion processes. As shown in Figs. 12.25 and 12.26 even the existence of opt imum working conditions for fluidized bed devices at an intermediate bed porosity could be turned to advantage to elaborate new permeate flux control strategies.


Ti

100

80

F l /dJze , d bed

Empty tube

70 , I , t , , , t , t , , , J U (cm/s )

0 10 20 30 40 50

Fig. 12.26. Gelatin rejection versus tangential fluid velocity usin~ a tubular alumina membrane filled with a fluidized bed (feed solution 10 g 1-, Ap = 1.5 bar).

12.6 CONCLUSION

Different aspects of liquid phase separation using inorganic membranes should be emphasized compared with organic membrane behaviour. The first characteristic of inorganic membranes designed for liquid filtration is that they exhibit a non-deformable porous structure with pore size adapted to three main processes: macropores for microfiltration, mesopores for ultrafiltration and micropores for nanofiltration. Modelling of mass transfer across these membranes is related to basic phenomena involved in liquid flow through porous media. The Darcy law applies to convective volume flux: through macro- and mesoporous membranes while a convection-diffusion mechanism better ex- plains solvent flux in the case of microporous membranes. Due to pore shape resulting from packing and sintering of mineral particles the Carman-Kozeny model which includes specific surface area and tortuosity provides a better description of the permeability coefficient than the Hagen-Poiseuille law.

The second characteristic of inorganic membranes used in liquid phase separation is that most are made of ceramic oxides. If solute rejection basically originates in size effects related to pore dimension, specific properties are attached to ceramic membrane material. The amphoteric behaviour of metal oxide surfaces is certainly the most important one as membranes can exhibit negative or positive charge density depending on the pH of feeding solutions. Two parameters, zeta-potential and streaming potential, greatly influence rejection and permeability of electrolyte solutions all the more as membranes exhibit small pore size and large specific surface area.


Fouling, responsible for flux decline, is also an impor tan t parameter to deal wi th in the descript ion of t ransport mechanisms wi th inorganic membranes . Three main causes have been identified as impor tan t contributions to fouling of inorganic membranes . It has been suggested that the format ion of ceramic membranes can be a first cause of flux decline as far as association of adjacent granular layers results in highly resistant bounda ry layers. A second phenomenon responsible for flux decline is the on-line membrane fouling which is a function of the hyd rodynamic conditions and is independent of the physical propert ies of the membrane. Finally interaction be tween membrane material and molecules or macromolecules can result in the formation of a dynamic layer on the original filtering element. This layer can be regarded as a formed-in- place membrane wi th specific separat ion propert ies and it is responsible for an addi t ional resistance to the vo lume flux.

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3. S. Nakao, T. Anazawa, T. Tsuru and S. Kimura, Influence of high permeation on concentration polarization in ultrafiltration. Presented at ISMMP'94, 5-10 April 94, Hangzhou, P.R. China.

4. J.A. Wesselingh and R. Krishna, Mass Transfer. Ellis Horwood, New York and London, 1990.

5. O. Kedem and A. Katchalsky, Thermodynamic analysis of the permeability of biological membranes to non-electrolytes. Biochim. Biophys. Acta, 27 (1958) 229.

6. A. Vemiory, R. Dubois, P. Decoodi and J.P. Gassee, Measurement of the permeability of biological membranes. J. Gen. Physiol., 62 (1973) 489.

7. P.M. Bungay, Transport principles m Porous membranes, in: P.M. Bungay, H.K. Lonsdale, M.N. de Pinho (Eds), Synthetic Membranes: Science, Engineering and Applica- tions. NATO ASI Series, Series C: Mathematical and Physical Sciences, Vol. 181, 1986. p. 109.

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