INTRODUCTION TO BAROMEMBRANE TECHNIQUES
André AYRAL
Institut Européen des Membranes UMR n° 5635 CNRS-ENSCM-UM2
CC047, Université Montpellier 2, Place Eugène Bataillon, 34095 Montpellier cedex 5, France
Andre.Ayral@iemm.univ-montp2.fr
Introductory remarks
A membrane can be defined as a thin and selective barrier which enables the transport or
the retention of compounds between two media. Different types of driving forces can be at the
origin of the transport across the membranes. They are associated with different types of
membrane processes. For dialysis of solutes, it is a difference of compound concentration. In the
case of electrodialysis and related electromembrane processes, an electric field between electrodes
located on the two sides of the membrane enables the selective separation of ionic species in
solution. For baromembrane processes, the driving force is a pressure gradient between the feed
and strip compartments (transmembrane pressure or TMP, P) (Figure 1). The treated phases can
be liquids or gas (Table 1). This paper will be focused on baromembrane processes for liquid
treatment.
SUPPORT
PERMEATE
Figure 1: Schematic
representation of tangential
filtration using an asymmetric
tubular membrane.
MEMBRANE
 P
FEED
RETENTATE
PERMEATE
One part of the terminology recommended by the International Union of Pure and
Applied Chemistry (IUPAC) for membranes and membrane processes [1] will be first presented:
–
–
The penetrant (or permeant) is the entity from a phase in contact with one of the membrane
surfaces that passes through the membrane. The permeate is the stream containing penetrants
that leaves a membrane module. The retentate (or raffinate) is the stream that has been
depleted of penetrants that leaves the membrane modules without passing through the
membrane to the downstream.
The permeability coefficient, Pi, [kmol·m·m-2·s-1·kPa-1] is the parameter defined as a transport
flux, Ji, per unit transmembrane driving force per unit membrane thickness. The permeance
15
(pressure normalized flux), [kmol·m-2·s-1·kPa-1] is the transport flux per unit transmembrane
driving force.
The rejection factor, R, is the parameter equal to one minus the ratio the concentrations of a
component (i) on the downstream and upstream sides of a membrane:
–
Table 1: Characteristics of the main baromembrane processes.
Process
Nature of
feed/strip
Microfiltration
MF
Ulltrafiltration
UF
Pore size
[10 - 0.1 µm]
liquid/liquid
[0.1µm – 2 nm]
Origin of selectivity
sieving
effect
sieving +
specific interactions
with the membrane
Pressure
gradient
1 - 3 bars
3 -10 bars
Nanofiltration
NF
< 2nm
Reverse osmosis
RO
dense
retention of solutes and
permeation of solvent
> osmotic
pressure
< 2nm
sieving + additional
specific interactions
1 bar
[100 µm - 10
nm]
sieving effect
0.1 - 5 bars
[few nm - dense]
sieving + additional
specific interactions
0.1 50 bars
ionic conduction of o2by oxides
P(O2)
dense
ionic conduction of H+
by oxides
H transport by metals
P(H2)
Pervaporation
PV
Gas filtration
GF
Gas separation
GS
Gas separation
GS
Gas separation
GS
liquid/gas
gas/gas
dense
R = 1 – [(Ci)downstream/(Ci)upstream])
10 - 40 bars
Elemental
operation
clarification,
debacterisation,
separation
clarification,
purification,
concentration
purification,
water softening,
separation,
concentration
purification,
water
desalination
separation
separation,
dusting
separation,
extraction,
purification
air separation
selective
transport of O2
selective
transport of H2
(1)
– The retention factor, rF, is the parameter defined as one minus the ratio of permeate
concentration to the retentate concentration of a component:
rF = 1 – [(Ci)p/(Ci)r]
(2)
The molecular-weight cutoff (MWCO) is the molecular weight of a solute corresponding
to a 90% rejection coefficient for a given membrane (Figure 2).
16
100
80
R (%)
Figure 2: Rejection versus solute
molecular weight for two low
ultrafiltration membranes. The
MWCO of the two tested
membranes is close to 1500
daltons.
Membrane 2
60
40
Membrane 1
20
0
0
500
1000
M
W
1500
(g.mol
-1
2000
2500
)
The separation coefficient, SC(AB) is the ratio of the compositions of component A and
B in the downstream relative to the ratio of compositions of these components in the upstream. For
example, if compositions are expressed in mole fractions (X A and XB):
SC(AB) = [XA/XB]downstream / [XA/XB]upstream
(3)
The separation factor, SF(AB) is the ratio of the compositions of components A and B in
the permeate relative to the composition ratio of these components in the retentate. For example:
SF(AB) = [XA/XB]Permeate / [XA/XB]Retentate
(4)
As mentioned in Table 1, the four main baromembrane processes for liquids are
microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and the reverse osmosis (RO). Their
IUPAC definitions [1] are as follows:
–
Microfiltration is a pressure-driven membrane-based separation process in which particles and
dissolved macromolecules larger than 0.1 µm are rejected.
– Ultrafiltration is a pressure-driven membrane-based separation process in which particles and
dissolved macromolecules smaller than 0.1 µm and larger than about 2 nm are rejected.
– Nanofiltration is a pressure-driven membrane-based separation process in which particles and
dissolved molecules smaller than about 2 nm are rejected.
– Reverse osmosis is a liquid-phase pressure-driven separation process in which applied
transmembrane pressure causes selective movement of solvent against its osmotic pressure
difference.
As a consequence, MF membranes are able to reject suspended solids like clays and
paint pigments or bacteria. UF membranes are able to reject colloids, macromolecules, viruses,
proteins. NF membranes are able to reject one important part of inorganic salts or diluted organic
molecules. Reverse osmosis membranes are mainly used for water desalination and the production
17
of ultrapure water. The working domains of these four processes in term of transmembrane
pressure and fluxes are shown in Figure 3.
Figure 3: Working domains for
MF, UF, NF and RO (from [2]).
Flux [L/m2/h]
1000
MF / UF
100
NF
RO
10
1
0,1
1
10
100
Transmembrane pressure [bar]
Transport mechanisms and separation
The overall performance of membranes is related to two main characteristics of such
separative layers, their permeance (and the associated flux for a fixed transmembrane pressure)
and their permselectivity (separation efficiency).
MF, UF, NF membranes are porous membranes whereas RO membranes are dense
membranes. Taking into account the IUPAC classification of the pores (Table 2), MF membranes
and high UF membranes are macroporous membranes, low UF membranes are mesoporous
membranes and NF membranes are microporous membranes.
Table 2: IUPAC classification of the pores as a function of their size
Micropores
<2nm
Ultra-micropores
Super-micropores
<0.7 nm
> 0.7 nm
Mesopores
Macropores
2 50 nm
> 50nm
For porous membranes, the pore size mainly manages the cut-off of the membrane.
However for the retention of the smallest entities by the smallest pores, the transport mechanisms
are more complex than simple sieving. Specific physical or chemical interactions become
preponderant and settle the membrane selectivity.
For macroporous membranes, the membrane intrinsic permeability, D, and the
membrane thickness, x, fix the viscous flux for a given transmembrane pressure P. The viscous
flux of liquid, J, across a porous medium is given by the Darcy’s law:
J  
D

PL
(5)
where J is the flux (volume per unit of porous material area and per unit of time; m.s -1), PL, the
pressure gradient in the liquid (Pa.s-1), , the liquid viscosity (Pa.s), and D, the intrinsic
18
permeability of the porous material (m2). Assuming a linear gradient of pressure across the
membrane, the relation can be expressed as follows:
J 
D

(P / x)  L p P
(6)
where Lp is the permeance of the membrane for the used liquid.
These relations are analogous to the Poiseuille’s relation giving the expression of the
laminar flow of a viscous fluid in a cylindrical tube of radius r and length L:
Flow  
4
r P
8L
(7)
The intrinsic permeability D of the porous medium can be estimated using models taking
into account the irregularity of the porous medium (tortuosity, non-circular sections, etc). The
Carman-Kozeny’s model is a simple and usually precise model which leads to the following
expression of D:
D 

2
5[(1   ) S
 S ]2
(8)
where  is the porosity, S, the specific surface area and S, the skeleton density.
In the case of the smallest pores (mesopores and micropores) the developed area is very
large and as a consequence, the intrinsic permeability is very low. Moreover the osmotic pressure
cannot be neglected and the general transport equation is:
J  Lp (P  )
(9)
where  is the osmotic pressure difference across the membrane.
The osmosis phenomenon is summarized in Figures 4a and 4b. Figure 4c schematically
represents the principle of the reverse osmosis. The osmotic pressure difference can be calculated
by a virial expansion from the concentrations of the solute in the feed and in the permeate.
However, in the case of forced flux, a concentration polarization layer is observed in the front side
of the membrane. The wall concentration of solute (concentration of solute at the surface of the
membrane) is higher than in the bulk of the feed and as a consequence, the osmotic pressure
difference is increased.
The mechanisms of separation and transport in nanofilters will be now more specifically
detailed. In the case of amphoteric organic NF membrane, the dissociated electrolytes go easily
across the membranes whereas neutral organic molecules are efficiently rejected (Figure 5). Due to
the absence of high salt concentration factor in the retentate, the osmotic pressure is not high
enough to prevent the effect of the transmembrane pressure [4]. An second important type of
nanofilters corresponds to electrically charged organic or inorganic membranes. In this case, the
rejection rate of the electrolytes will depend on their chemical nature, on their charge, on their
dissociation level and on the composition of the treated mixtures (Figure 6) [4].
19
Organic molecules
300<M<1000
water
Figure 5. Schematic representation of
transport across an amphoteric organic NF
membrane.
Figure 6: Schematic representation of
transport across a negatively charged NF
membrane in the case of a mixture of
monovalent and multivalent ions.
The pH value of the aqueous solutions is a very important parameter because it defines
the surface charge of the membrane as illustrated by Figure 7 and Table 3 in the case of oxide
ceramic materials.
20
pH<ZPC
Table 3: Zero Point of Charge (ZPC) of different oxides.
Oxide ceramic
Zero point of charge
(pH unit)
-Al2O3
-Al2O3
Anatase TiO2
ZrO2
Mixed silica-alumina
8.5-9.1
9.1
5.9-6.6
6.4
<4.1 – 7.2
Oxide
pH>ZPC
Figure 7: Schematic representation of the
surface for a solid oxide and pH values
lower or higher than its zero point of
charge.
The effect of pH on the selectivity of a titania-based nanofilter is schematically
illustrated in Figure 8.
Figure 8: Effect of pH on the
selectivity of a titania-based
nanofilter towards negatively
charged ions.
water
Water
Complex electrokinetic phenomena occur during the forced flow of the ionic solutions
through the confined volume of the micropores because the thickness of the double layer formed
on the charged pore surface and the pore size are in the same range.
In a first approach, a NF membrane can be assimilated to a non-ideal RO membrane [4].
From the following parameters:
– Jv and Js, the volume flux and the solute flux, respectively;
– the driving forces : P and 
– the hydraulic permeance, Lp; the solute permeance,  and, the reflection coefficient, ;
it can be written:
Jv = Lp.( P -  )
(10)
Js =  + (1 - ).Jv.c
(11)
21
with c, the average concentration of solute.
 = (P/) Jv = 0
(12)
Js = cs".Jv
(13)
with cs", the concentration of solute at the permeate side of the membrane.
Jv [(1-) x / P] = ln[cs" / (cs" - cs’ (1-))]
(14)
with x, the thickness of the membrane, cs’, the concentration of solute at the feed side of the
membrane and P, the permeability coefficient of the solute.
The rejection factor of the solute is equal to:
R = 1 – (cs" / cs’)
(15)
R can be expressed as a function of the parameter F related to the flux of solvant J v:
F = (-R)/(1-R)
with: F = exp (-Jv A)
(17)
and
(16)
A = [(1-) P] x = (1-) / P
(18)
Thus, the rejection factor of the solute which is a function of the solvent flux can be
finally expressed as :
R = (1-F) / (1- F)
(20)
A second approach takes into account the separation mechanism of Donnan induced by
the steric ion retention (cation ou anion) [4]. Let us consider the case of the complete retention of
an organic anion organique, X-, as its counter-ion, Na+, and another salt, NaCl, can go across the
membrane.
The concentrations in the feed are equal to:
[Cl-] = cs’
(21a)
[X-] = cX’
(21b)
[Na+] = cs’ +  cX’
(21c)
The rejection factor is given by :
R = 1 – (1-) / (1 – F)
with  = (1+  cX’ / cs’)0.5
(22)
(23)
As a consequence, the salt retention factor becomes negative when the flux tends to zero
and decreases as the organic solute concentration increases. This model takes into account the
surface charged membranes with a pore diameter larger than the size of the ions in the feed. Two
parameters can be defined:
22
–
–
–
the effective charge density, X;
a structural parameter, (v/k2.x) with k, a tortuosity factor; x, the thickness of the
membrane and v, the volume fraction of water in the membrane.
Two normalized parameters are experimentally used:
the normalized flux of solvent as a function of the surface porosity of the membrane, A k:
Jv,N = Jv. x/Ak
–
(24)
the ratio of the effective charge density by the counter-ion concentration in the feed.
 = X/ca
(25)
From this approach, the behavior of a charged nanofilter can be more precisely predicted:
– for a unique elctrolyte in solution, the evolution of the rejection factor R as a function of the
volume flux Jv is the same as for a neutral solute. The increase of the charge density of the
membrane induces an increase of R. R is usually higher for divalent co-ions than for
monovalent co-ions whereas the opposite behavior is observed for the counter-ions;
– for mixtures of electrolytes, the rejection factors for both the co-ions and the counter-ions
strongly depend on the volume flux, the molar fraction of electrolytes and of . In that case a
negative rejection factor can be observed for monovalent co-ions The divalent co-ions can be
separated (Figure 9a) whereas it is not the case for the counter-ions (Figur 9b).
Figure 9: (a) Rejection rate for a mixture of chloride and sulfate ions versus the flux for a
negatively- charged membrane and  = 10; (b) Rejection rate for a mixture of Mg 2+ et Na+
ions versus the flux for a positively-charged membrane and  = 10.
It must noted that the transport properties of ceramic nanofilters for aqueous solutions
have been extensively studied and modeling tools are now available like Nanoflux®, a computer
software for modeling ion and molecular transport across nanofiltration membranes and industrial
plants [5,6].
23
Configuration and geometry
Different types of ideal continuous flows used in membrane-based separation devices are
distinguished by IUPAC (Figure 10). In this case of liquid treatment, the most usual type
corresponds to cross-flow (Figure 10d). Tangential filtration limits fouling phenomena.
Figure 10: Types of ideal continuous flows used in membrane-based separation devices
(from [1]).
In addition with considerations on
permeability and on permselectivity, another
important parameter is the ratio of filtering
surface to membrane volume because it defines
the final size of the membrane units. The organic
membranes can be easily shaped as hollow fibers
which are then assembled as bundles. Membrane
sheets can be rolled as spiral wounds. These two
configurations are commonly applied for organic
polymer modules (Figure 11).
Figure 11: Types of modules used
membrane-based separation (from [1]).
24
in
The stiffness of the ceramic materials hinders the preparation of compact spiral modules
accessible with flexible organic membranes. The ceramic membranes are usually tubular
membranes, well-adapted for tangential filtration. Multichannel membranes (Figure 12),
honeycomb structures and recently, ceramic hollow fibers were adopted to improve the
compactness of the filtration units using ceramic membranes.
Porous ceramic tube
Figure 12: Tubular ceramic
membranes (from [7]).
PERMEATE
Porous
multichannel
tube
Porous layers
Before closing this section devoted to configuration and geometry, the immersed
membrane technology must be mentioned. It that case, large structures consisting in organic
hollow fibers grouped and fixed on frames are immersed in big tanks filled with water. In that case
the direction of the water flow is from the external side to the internal side of the fibers. Usually
MF hollow fibers are used and this type of filtration unit can efficiently replace sand beds.
Membrane materials, membrane preparation
and characterization
The membrane materials can be classification following different criteria:
based on microstructure: porous or non-porous (dense);
based on isotropy: symmetric (isotropic) or asymmetric (anisotropic);
based on material nature: organic polymers or inorganic solids.
The main part of the used membranes are organic membranes but for specific conditions
of utilization inorganic (or organic-inorganic) membranes have been also developed.
A lot of different organic polymers are used to prepare membrane like cellulose acetate,
regenerated cellulose, polysulfone, polyethersulfone, polyamide, polyvinylidedefluoride,
polyacrylonitrile,…
The main inorganic materials used for membranes are oxide ceramic, porous glasses,
carbon-like solids, porous metals (like sintered stainless steel) or dense metals (like Pd or Pd-based
alloys for H2 separation).
The interest of the ceramic membranes is related to the intrinsic characteristics of the
used materials :
– mechanical strength allowing large pressure gradients without significant strain;
–
–
–
25
–
chemical resistance which permits applications in corrosive aqueous media or in organic
solvents;
– refractarity for using at high temperatures.
Other specific properties are the ability to counter-pressure cleaning, to sterilization and
their insensibility to bacterial attacks.
As discussed before, in the case of small mesoporous, microporous (or dense)
membranes, the intrinsic permeability is very low inducing important pressure drops. In that case
asymmetrical membrane structures are preferred in order to minimize the thickness of the
separative layer.
Dense organic membranes are prepared by conventional plasturgy methods like melt
extrusion, compression moulding, solution casting or coating. For porous organic membranes, the
most used class of techniques is called phase inversion techniques [8]. Phase separation
mechanisms can generally be subdivided in three main categories depending on the parameters
that induce demixing [8]. By posing a change in one of these parameters at one particular side of
the film, asymmetric boundaries are posed on the polymer film which can be expressed in the
resulting structure. By changing the temperature at the interface of the polymer solution, heat will
be exchanged and demixing can be induced (temperature induced phase separation or TIPS). The
original polymer solution can also be subjected to a reaction which causes phase separation
(reaction induced phase separation) (RIPS). The most used technique is based on diffusion induced
phase separation (DIPS). By contacting a polymer solution to a vapour or liquid, diffusional mass
exchange will lead to a change in the local composition of the polymer film and demixing can be
induced (Figure 13) [8].
Other methods can be used to prepare porous organic membranes like leaching (for
selective removal of a porogen phase), sintering of polymer powders or track etching (irradiation
followed by caustic etching).
Porous ceramic membranes for liquid treatments are usaually based on a macroporous
support and successive layers with decreasing thickness and pore size (Table 4 and Figure 14). The
macroporous support and the macroporous intermediate layers are produced by conventional
ceramic methods. The sol-gel route is well-adapted to prepare the mesoporous and microporous
separative top layers required for low UF and NF membranes, respectively. Macroporous and
mesoporous alumina discs obtained by anodizing aluminum foils are also commercially available.
Table 4: Characteristics of the intermediate and of the separative top layers (from
26
[7]).
Process
Average number
of layers
Thickness of the
separative layer
Pore size in the
separative layer
nature of the
porosity
Microfiltration
1-3
few ten µm
5 – 0.1 µm
macroporosity
ultrafiltration
3-4
few µm
5nm
mesoporosity
nanofiltration/
gas separation
4-5
< 1 µm
1nm
microporosity
(a)
(b)
Figure 13: Diffusion induced phase separation processes and exemples of membrane
morphologies (from [8]). (a) Schematic representation of three DIPS processes: (A)
precipitation with nonsolvent vapor, (B) evaporation of solvent, (C) immersion
precipitation. Main direction of diffusion of the different species is indicated by arrows.
Polymer, solvent and nonsolvent are represented with P, S and NS respectively.
Components which are not necessary to be present in the original polymer solution and
coagulation bath are put between brackets. (b) Membranes prepared from PSf-solutions
(resp. 15 w% (left) and 25w% (right) PSf in NMP) by coagulation into mixtures of water
and IPA (ratio indicated).
Figure
14:
Scanning
electron
microscope image of the cross-section
of a commercial UF alumina
membrane (Pall Exekia). The average
pore size of the support, of the two
intermediate layers and of the
separative top layer are 10 µm, 0.8 µm,
0.2 µm et 5 nm, respectively.
27
Concerning the membrane characterization, the main parameters are the hydraulic
permeance determined by filtering pre-filtered deoinised water at different transmembrane
pressures and the sieving properties measured by the MWCO from the rejection curve (Figure 2).
Other important characteristics are the mechanical strength, the chemical resistance and the
chemical compatibility, the porosity and the pore size distribution.
Membrane applications
The main domains of application of baromembrane techniques for liquids are reported in
Figure 15. Among the major applications, it can be mentioned the water desalination
(electrodialysis or reverse osmosis), the production of tap water, the treatment of waste waters, the
preparation of food, beverage, dairy or pharmaceutical products, the treatment and the recycling of
industrial effluents. The mainly used membrane processes are microfiltration and ultrafiltration.
However the number of applications of NF membranes is increasing more and more. For example,
the plant of Méry sur Oise (France) produces tap water from the river Oise, using nanofiltration
technology for a production capacity of 140000 m3 per day [2]. NF applications to non-aqueous
liquids appear also as very promising for the recovery of organic solvents or of catalysts in fine
chemistry syntheses.
Chemistry 5.6%
Food 12.9%
Desalination 20.4%
Metallurgy 4.9%
Mining 1.4%
Oils and gas 1.5%
Tap water 15.4%
Other industries 9.6%
Waste waters 4.3%
Pharmaceutical 10.4%
Microelectronics 5.1%
Refineries 1.4%
Energy 3.8%
Papermills 3.1%
Figure 15: Domains of application of baromembrane techniques; Data source: McIlvaine
Company (2004) (from [2])
28
Conclusion
In baromembrane processes, the driving force for the selective transport across the
membrane is a pressure gradient between the feed and strip compartments (transmembrane
pressure).
Differents types of membranes processes exist for the treatment of liquids like
microfiltration MF, ultrafiltration UF, nanofiltration NF, reverse osmosis RO, applied for
elemental operations like clarification, debacterisation, separation, purification, concentration,
water softening.
A large number of membrane materials and of membrane configurations are now
commercially available and can be selected as a function of the application requirements.
References
[1]
Terminology for membranes and membrane processes,W.J. Koros, Y.H. Ma, and T.
Shimidzu, Pure & Applied Chemistry, 68 (7) (1996) 1479-1489.
[2] Procédés membranaires pour le traitement d’eau potable, Revue des technologies et
estimation des coûts, Y. Poussade, and J.C. Schrotter, Veolia environnement, 1 ière Réunion
Ouest Africaine sur l’Intégration des « Sciences et Technologies à Membranes, 6-8 juin
2007, Dakar, Sénégal.
[3] Membrane technologies, Peter S. Cartwright, Cartwright Consulting Co., CWQA
Professional Development Seminar, May 4, 2007.
http://cwqa.com/attachments/File/Membrane%20Technologies%20-%20CWQA%205-0407.ppt
[4] La nanofiltration, C. Guizard, Fiche de synthèse du Club Français des Membranes, Paris,
2001.
[5] J. Palmeri, J. Sandeaux, R. Sandeaux, X. Lefebvre, P. David, C. Guizard, P. Amblard, J.F.
Diaz and B. Lamaze, Desalination. 147 (2002) 231.
[6] http://www.nanomempro.com/Front/offers.php?id=26
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L.Cot.) (Elsevier, Amsterdam, 1996).
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29