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2.05 a0005 Nanofiltration Operations in Nonaqueous Systems
L G Peeva, S Malladi, and A G Livingston, Imperial College London, London, UK
2010 Elsevier B.V. All rights reserved.
2.05.1 Introduction 1
2.05.2 Membranes for Separations in OSs 2
2.05.2.1 Polymeric Membranes 2
2.05.2.1.1 Integrally skinned asymmetric polymeric membranes 2
2.05.2.1.2 TFC membranes 3
2.05.2.1.3 Postformation treatment 3
2.05.2.1.4 Commercially available polymeric membranes 3
2.05.2.2 Ceramic OSN Membranes 5
2.05.2.2.1 Commercial ceramic membranes 7
2.05.3 Membrane Characterization 8
2.05.3.1 MWCO and Flux 8
2.05.3.2 Swelling 8
2.05.3.3 SEM and Atomic Force Microscopy 9
2.05.3.4 Pore-Size Measurement 12
2.05.3.5 Positron Annihilation Lifetime Spectroscopy, X-Ray Photoelectron Spectroscopy,
Contact Angle, Surface Charge, and Surface Tension 12
2.05.4 Applications of Separation in OSs 13
2.05.4.1 Fine Chemical and Pharmaceutical Synthesis 13
2.05.4.2 Food and Beverage 15
2.05.4.3 Refining 17
2.05.5 Conclusions 19
References 19
s0005 2.05.1 Introduction
p0005 Membrane-based separation processes, such as gas
separation, reverse osmosis (RO), nanofiltration
(NF), ultrafiltration (UF), microfiltration (MF), elec-
trodialysis (ED), and pervaporation (PV), have been
developed for various applications [1]. NF, which is
intermediate between RO and UF, is a pressure-
driven process used for removing solutes, such as
divalent ions, sugars, dyes, and organic matter,
which have molecular weight (MW) in the range
of 2001000 g mol
1
, from aqueous feed streams [1].
A recent innovation is the extension of pressure-
driven membrane NS processes to organic solvents
(OSs). This emerging technology is referred to as
organic solvent nanofiltration (OSN), or alterna-
tively as solvent-resistant nanofiltration (SRNF)
[2]. Aqueous NF, in many cases, involves separation
between charged solutes and other compounds in an
aqueous phase, whereas, by contrast, OSN is used for
separations between molecules in organicorganic
systems. Another membrane-based process widely
used with OSs is PV where separation occurs by
differential permeation of liquids through a
membrane, with transport of liquids through the
membrane effected by maintaining a vapor pressure
gradient across the membrane [3]. Membrane-based
separations, in general, use significantly less energy
than thermal processes, such as distillation, and this is
of particular interest given the current high energy
prices. This chapter focuses on describing the state of
the art in OSN.
p0010 Sourirajan [4] reported the first application of
membranes to nonaqueous systems in 1964 for the
separation of hydrocarbon solvents using a cellulose
acetate membrane. Later, Sourirajan and co-workers
[57] used membranes to separate OS mixtures and
organic and inorganic solutes using cellulose acetate
membranes. From 1980 onward, major oil companies,
such as Exxon [811] and Shell [12, 13], and chemical
companies, such as Imperial Chemical Industries (ICI) AU3
and Union Carbide [14], began to file patents on the
use of polymeric membranes to separate molecules
present in organic solutions. The applications include
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oil recovery [810], enrichment of aromatics [1518],
and homogeneous catalyst recycle [14]. Major mem-
brane producers, including Grace Davison [1922]
and Koch [23], began research and acquisition
programs, and products started to be commercially
available from the mid-1990s onward. The largest
success so far industrially has been the
MAX-DEWAX
TM
process installed at ExxonMobil
Beaumont refinery for the recovery of dewaxing sol-
vents from lube oil filtrates [20], while the most recent
addition to commercial offerings is the launch by
Membrane Extraction Technology (MET) in 2008 of
the DuraMem
TM
series of highly solvent-stable OSN
membranes for the separation of organic solutes from
various OSs [24]. These efforts have prompted a rapid
rise in the number of academic publications and pro-
cess development projects in industry. By way of
illustrating the surge in interest in OSN, Figure 1
shows a rough estimate of the number of patents and
papers published on the application of membranes for
nonaqueous operations before the 1990s, during
the1990s, and from 2000 onward.
s0010 2.05.2 Membranes for Separations
in OSs
p0015 Both polymeric and inorganic materials have been
used for the preparation of OSN membranes. In what
follows, we present a brief summary of the currently
commonly available OSN membranes.
s0015 2.05.2.1 Polymeric Membranes
p0020 Compatibility of polymeric membranes with a wide
range of OSs is a very challenging issue in the OSN
membrane production. Polymeric membranes gener-
ally fail to maintain their physical integrity in OSs
because of their tendency to swell or dissolve.
Nevertheless, several polymeric materials exhibit
satisfactory solvent resistance (e.g., polyimides
(PIs)) or can be made more stable, for example, by
increasing the degree of crosslinking (e.g., silicone
and polyacrylonitrile (PAN)). An overview of sol-
vent-resistant polymeric materials used for
membrane preparation can be found elsewhere [1,
25]. Most polymeric OSN membranes have an asym-
metric structure, and are porous with a dense top
layer. This asymmetry can be divided into two
major types: the integrally skinned asymmetric
type, wherein the whole membrane is composed of
the same material; and the thin-film-composite
(TFC) type, wherein the membrane-separating
layer is made of a different material from the
supporting porous matrix.
s0020 2.05.2.1.1 Integrally skinned asymmetric
polymeric membranes
p0025 Integrally skinned asymmetric membranes are most
commonly prepared by the phase-inversion immer-
sion precipitation process. A solution of the polymer
is cast as a thin film onto a nonwoven fabric, dried for
a few seconds to create a dense top layer, and then
immersed in a coagulation bath, which contains a
0
Before 1990 199099
Years
20
40
60
80
100
120
140
N
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r

o
f

l
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a
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e

r
e
p
o
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t
s
Patents
Papers
200008
Figure 1
f0005
AU4 Number of patents and papers published before 1990s, during 1990s, and 2000s on membranes for nonaqueous
operations.
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nonsolvent for the polymer. The solvent starts to
diffuse out of the homogeneous liquid polymer film,
while the nonsolvent simultaneously diffuses into the
film. Due to the presence of a nonsolvent, phase
separation takes place in the polymer film and the
polymer precipitates as a solid phase, forming a
porous asymmetric membrane structure. The ther-
modynamic properties of the casting system and the
kinetics involved in the exchange of solvent and
nonsolvent affect the morphology of the membrane,
and, consequently, its permeability and solute rejec-
tion [26]. The phase separation can also be induced
by other methods, such as lowering the temperature
(thermal precipitation), by evaporating the volatile
solvent from the polymer film (controlled evapora-
tion), or by placing the cast polymer film in a
nonsolvent vapor phase (precipitation from the
vapor phase) [2]. More detailed information about
membrane preparation techniques can be found
elsewhere [1].
s0025 2.05.2.1.2 TFC membranes
p0030 Composite membranes consist of at least two differ-
ent materials. Usually, a selective membrane material
is deposited as a thin layer upon a porous sublayer,
which serves as support. The advantage of this type
of membrane over the integrally skinned ones is that
each layer can be optimized independently in order
to achieve the desired membrane performance.
There are several well-established techniques for
applying a thin top layer upon a support: dip coating,
spray coating, spin coating, interfacial polymeriza-
tion, in situ polymerization, plasma polymerization,
and grafting. Details of these techniques can be found
elsewhere [1]. Due to the large variety of preparation
techniques, almost all polymeric materials can be
used to produce these types of membranes. The top
layer and the support both contribute to the overall
membrane performance.
s0030 2.05.2.1.3 Postformation treatment
p0035 In order to increase the separation performance of
asymmetric membranes and to increase their long-
term stability, several postformation treatments or
conditioning procedures can be used, such as anneal-
ing (wet or dry), crosslinking, drying by solvent
exchange, and treatment with conditioning agents
[2]. Posttreatment procedures could be applied to
both types of polymeric membranes mentioned
above.
s0035 2.05.2.1.4 Commercially available
polymeric membranes
p0040 Despite the fast development of research in the area of
separation in OSs, there are still a limited number of
membranes that have been commercialized.
According to our knowledge of the membrane market,
there are currently five companies producing SRNF
membranes. The commercially available solvent-
stable membranes include the Koch and Starmem
TM
membrane series, the SolSep membranes, the newly
launched DuraMem
TM
membrane series, and the
Inopor series of ceramic membranes.
p0045 Koch SelRO membranes. Koch Membrane Systems
(USA) [27] was the first company to enter the OSN
market with three different membranes designed for
solvent applications. However, the hydrophobic
membranes, such as SelRO MPF-60 (molecular
weight cutoff (MWCO) 400 g mol
1
, based on rejec-
tion of Sudan IV (384 g mol
1
) in acetone) and
SelRO MPF-50 (MWCO 700 g mol
1
, based on
rejection of Sudan IV in ethyl acetate), have already
been removed from the market. Only the hydrophilic
MPF-44 membrane (MWCO 250 g mol
1
, based on
rejection of glucose (180 g mol
1
) in water) is still
available, in flat sheet as well as in spiral-wound
(MPS-44) module configuration [2].
p0050 It is believed that the MPF series membranes are
TFC-type membranes, comprising a dense silicone-
based top layer of submicron thickness on a porous
crosslinked PAN-based support. Membrane produc-
tion may be associated with a patent from Membrane
Products Kyriat Weitzman (Israel) [28], in which a
crosslinked PAN support was first treated with sila-
nol-terminated polysiloxane as a pore protector, and
then immersed in a solution of polydimethylsiloxane
(PDMS), tetraethyl silicate, and a tin-based catalyst
for final coating and crosslinking. A scanning electron
microscopy (SEM) picture [29] of the MPF-50 mem-
brane is presented in Figure 2. Koch also distributes
a UF membrane (nominal MWCO 20 000 g mol
1
),
based on crosslinked PAN, available in both flat
sheets (MPF-U20S) and spiral-wound (MPS-U20S)
elements [2, 27].
p0055 According to the manufacturers information, both
membranes are claimed to be stable in methanol,
acetone, 2-propanol, cyclohexane, ethanol, methyl
ethyl ketone (MEK), butanol, methyl isobutyl ketone
(MIBK), pentane, formaldehyde, hexane, ethylene
glycol, dichloroethane, propylene oxide, trichlor-
oethane, nitrobenzene, methylene chloride,
tetrahydrofuran (THF), carbon tetrachloride, aceto-
nitrile, diethylether, ethyl acetate, dioxane, xylene,
MESE 00036
Nanofiltration Operations in Nonaqueous Systems 3
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and toluene, and claimed to have limited stability in
dimethylformamide (DMF), N-methyl pyrrolidone
(NMP), and dimethylacetamide [27].
p0060 The MPF series of OSN membranes were the first
freely available membranes on the market, and,
therefore, they have been subjected to extensive stu-
dies and have been tested in many applications, for
example, the recovery of organometallic complexes
from dichloromethane (DCM), THF, and ethyl acet-
ate, and of phase-transfer catalysts (PTCs) from
toluene, the separation of triglycerides from hexane,
and for solvent exchange in pharmaceutical manu-
facturing. Extensive fundamental studies on solvent/
solute transport mechanisms in OSN membranes
have also been performed on MPF membranes [2].

Starmem
TM
membranes. The Starmem
TM
mem-
branes series (Starmem is a trademark of W.R.
Grace and Company) are distributed by MET
(UK) [24]. The series consist of hydrophobic inte-
grally skinned asymmetric OSN membranes with
active surfaces manufactured from PIs. An active
skin layer less than 0.2 mm in thickness with a pore
size of <5 nm covers the PI membrane body [19, 21].
An SEM picture of a typical Starmem
TM
membrane
is presented in Figure 3. Starmem
TM
122 has an
MWCO of 220 g mol
1
, Starmem
TM
120 has an
MWCO of 200 g mol
1
, and Starmem
TM
240 an
MWCO of 400 g mol
1
. These quoted MWCOs
are manufacturer values obtained using toluene as
a solvent and are quoted as the MW at 90% solute
rejection, estimated by interpolation from a plot of
rejection versus MW for a series of n-alkanes. The
membranes are claimed to be stable in alcohols (e.g.,
butanol, ethanol, and iso-propanol); alkanes (e.g.,
hexane and heptane); aromatics (e.g., toluene and
xylene); ethers (e.g., methyl-tert-butyl-ether);
ketones (e.g., methyl-ethyl-ketone and methyl-iso-
butyl-ketone); and others (e.g., butyl acetate and
ethyl acetate). All membranes are available as flat
sheets or spiral-wound elements [24].
p0065 Starmem
TM
membranes have also been widely
used to study solute and solvent transport through
OSN membranes, and tested in different applica-
tions, such as product separation and catalyst
recycle; chiral separations; solvent exchange in phar-
maceutical manufacturing; ionic liquid-mediated
reactions; and microfluidic purification and
(a)
MPF 50
50 m
5 m
SolSep 3360
(b)
Figure 2
f0010
AU5 Scanning electron microscopy (SEM) images of the skin layer (top) and the entire cross section of the: (a) Koch
membrane MPF-50, (b) SolSep 3360. Adapted from Van der Bruggen, B., Jansen, J. C., Figoli, A., Geens, J., Boussu, K.,
Drioli, E. J. Phys. Chem. B 2006, 110, 1379913803.
MESE 00036
4 Nanofiltration Operations in Nonaqueous Systems
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membrane bioreactors (MBRs) for biotransforma-
tions [2]. Starmem
TM
membranes are the only OSN
membranes applied at a large scale, in the refining
industry for solvent recovery from lube oil dewaxing
(MAX-DEWAX
TM
) [21]. Further details on this
large-scale application are provided in the subse-
quent section.
p0070 SolSep membranes. The Dutch company SolSep [30]
offers five NF membranes with different stabilities
and nominal MWCO values between 300 and 750 g
mol
1
, and one UF membrane with an MWCO
around 10 000 g mol
1
. According to the manufac-
turer, the membranes are stable in alcohols, esters,
and ketones, and some of them are also stable in
aromatics and chlorinated solvents. Typical charac-
teristics of the SolSep membranes, as presented by
the manufacturer, are summarized in AU6 Table 1 and
are claimed as being produced as spiral-wound-type
modules [31]. While there is not much information
available on the type of membrane material used for
their preparation, it is believed that the SolSep mem-
branes are of TFC type and some of them were
proven to have a silicone top layer [29], as illustrated
in Figure 2. The top layer of SolSep 3360 is clearly
thicker than the barrier layer of MPF-50; conse-
quently, lower solvent permeability is reported for
similar MWCO [2, 29] AU7 . There is relatively limited
information for the performance of these membranes
in the literature [2, 29, 3133]. Filtration data for
SolSep NF030306 were recently reported in ethanol,
i-propanol, toluene, xylene, hexane, heptane, cyclo-
hexane, and butyl acetate [33].
p0075 DuraMem
TM
. DuraMem
TM
range of highly stable
OSN membranes is manufactured by MET [24].
Membranes are of integral asymmetric type and are
based on crosslinked PI [34, 35]. These membranes
are available with different MWCO curves (180
1200 g mol
1
) and possess excellent stability in a
range of solvents, including polar aprotic solvents
such as DMF and NMP. The membranes have a
sponge-like structure and are stable in most OSs,
including toluene, methanol, methylene chloride,
THF, DMF, and NMP. The membranes have been
operated continuously for 120 h in DMF and THF
and showed stable fluxes and good separation perfor-
mances, with DMF permeability in the range
of (18) 10
5
l m
2
h
1
Pa
1
(18 l m
2
h
1
bar
1
)
[35]. Possible re-imidization and loss of crosslinking
at elevated temperatures limit their range of applica-
tion to temperatures <100

C.
s0040 2.05.2.2 Ceramic OSN Membranes
p0080 Ceramic materials (silicium carbide, zirconium oxide,
and titanium oxide) endure harsh temperature condi-
tions and show stable performance in solvent
medium, and, therefore, are excellent materials for
membrane preparation. Ceramic membranes gener-
ally have an asymmetric structure, in which a thin
membrane layer with one or more intermediate layers
is applied to a porous ceramic support. The support
defines the external shape and mechanical stability of
the membrane element. Common configurations are
disks, which are produced by film casting or pressing
of dry powder, and tubes, which are commonly pro-
duced via extrusion of ceramic powders with the
addition of binders and plasticizers. These supports
are subsequently sintered at 12001700

C and an
open-pore ceramic body is obtained with pore size
between 1 and 10 mm, depending on the initial
18117128D
WRC 1.0 kV x500 60.0 m WRC 1.0 kV x10.0K 3.00 m
18117128D
Figure 3
f0015
Grace Davison Membranes STARMEM
TM
polyimide membrane at 500 and 10 000 magnification of active
separation layer. Adapted from White, L. S. J. Membr. Sci. 2002, 205, 191202.
MESE 00036
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MESE 00036
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particle size and shape. A thin layer is applied to this
support, typically by suspension coating using nar-
rowly classified ceramic powders dispersed in an
appropriate solvent. The pore size again is controlled
by the size of the powder. The finest available powders
have a particle size of about 60100 nm, from which
membranes with pore size of about 30 nm can be pro-
duced (the upper range of UF) [36]. To reduce the
pore size even further, an additional thin defect-free
layer is added, usually, by the so-called solgel process.
The process starts with a precursor, which is often an
alkoxide. The alkoxide is hydrolyzed in water or OS,
which yields a hydroxide able to polymerize and form
polyoxometalate. At this stage, the viscosity of the
solution increases, which is an indication that polymer-
ization has started. Viscosity modifiers or binders are
frequently added to the sol prior to layered deposition
on the porous support via dip or spin coating, where
the final gelation occurs. Finally, the gel is dried and,
via controlled calcination and/or sintering, the actual
ceramic membrane is produced. A typical multilayered
structure of a ceramic membrane is presented in
Figure 4. Further details on the process of membrane
preparation can be found elsewhere [1, 2, 36].
p0085 The major challenge in opening up the range of
molecular separations in solvents that is possible with
ceramic membranes was the evolution toward a lower
pore size 1 nm. For a long time, the MWCO of the
membranes was retained 1000 g mol
1
. However,
by the end of the last century, NF membranes were
developed based on silica membranes doped with
zirconia and titania. A TiO
2
-based NF membrane,
with a pore size of 0.9 nm and a cutoff of 450 g mol
1
,
has been commercialized under the name Inopor

by
a spin-off company of HITK (Germany) [37], and has
been successfully applied since 2002 in a treatment
plant for harsh colored textile wastewaters [38, 39].
p0090 The intrinsic hydrophilicity of the oxide pore sur-
faces of the existing ceramic NF membranes lowers
the permeability of apolar solvents through these
membranes. Approaches to cope with this by prepar-
ing mixed oxides were not successful. The
modification of the pore surface, by coupling of silane
compounds to the hydroxyl groups, has been found to
be a better solution. The silylation of ceramic mem-
branes has been patented and is semi-commercially
available from HITK (Germany) [36]. The mem-
branes exemplified in the patent show cutoff values
of about 600, 800, and 1200 g mol
1
in toluene using
polystyrene standards [36], and have been used to
retain transition-metal catalysts in apolar solvents [2].
s0045 2.05.2.2.1 Commercial ceramic
membranes
p0095 Inopor series membranes. The Inopor

company [40]
currently offers a range of ceramic UF and NF
membranes in the form of monochannel and multi-
channel tubes with lengths up to 1200 mm, as
summarized in Table 2. The membranes are offered
as hydrophilic version; however, on customer
request, they can be prepared to be hydrophobic.
There is no specific information on the companys
website regarding the hydrophobic membranes, but it
is believed that the literature-cited HITK-T1
(HITK, Germany) is a silylated TiO
2
-based version
of the above-mentioned membranes. With a nominal
MWCO of 220 g mol
1
, this membrane showed
methanol and acetone permeabilities (0.4 l m
2
14 kV X370 50 m 17 26 SEI 14 kU X6, 800 2 m 16 26 SEI
Figure 4
f0020
Typical multilayered structure of a ceramic membrane from the Inopor

series [40] at magnification 370 (edge

view) and 6000 (top-layer edge view).
MESE 00036
Nanofiltration Operations in Nonaqueous Systems 7
E
L
S
E
V
I
E
R
A
U
T
H
O
R
P
R
O
O
F
h
1
bar
1
), while rejecting Victoria blue (506 g
mol
1
) for 99% from methanol, and erythrosine B
(880 g mol
1
) for 97% from acetone, and demon-
strated efficient catalyst recovery for Pd2,29-
bis(diphenylphosphino)-1,19-binaphthyl (BINAP)
(849 g mol
1
) with rejections around 94.5% [2].
s0050 2.05.3 Membrane Characterization
p0100 Membrane characterization methods can be divided
into two categories: (1) functional characterization
and (2) physicalchemical characterization [41].
Functional parameters, such as flux and rejection,
determine the selection of a membrane for a specific
application [42]. Physicalchemical parameters
include porosity, pore size, pore-size distribution,
hydrophobicity, hydrophilicity, skin layer thickness,
and charge [41]. One of the current challenges in
OSN research is to establish the physicalchemical
structure of the membranes, and then to use that to
predict the functional performance.
s0055 2.05.3.1 MWCO and Flux
p0105 Flux or permeation rate is the volume of liquid flow-
ing through the membrane per unit area and per unit
time and is generally expressed in terms of l m
2
h
1
and the permeability by l m
2
h
1
bar
1
. Rejection of
a solute i (R
i
%) is calculated by R
i
(%) (1 C
pi
/
C
ri
) 100% where, C
pi
and C
ri
are the concentration
of solute i in the permeate and retentate, respectively.
The separation performance of OSN membranes can
also be expressed in terms of MWCO obtained by
plotting the % rejection of solutes versus their MW
(typically 2001000 g mol
1
) and interpolating the
data to find the MW corresponding to 90% rejection.
Oligomeric forms of polyisobutylene [4346], poly-
ethylene glycol (PEG) [47, 48], polystyrene [46, 49],
linear and branched alkanes, and dyes have been used
as solutes to estimate MWCO of OSN membranes
[49]. The properties of solutes and solvents, such as
structure, size, charge, and concentration, are found to
affect the performance of OSN membranes [4351].
Figure 5 shows MWCO curves for Starmem
TM
122
in different solvents using polystyrene oligomers.
MWCO of some of the commercially available mem-
branes are summarized in Table 3. The selection of
membranes for OSN applications depends upon the
MWCO specified by the manufacturer. However,
different methods used for evaluating MWCO of
membranes lead to inconsistencies, making the selec-
tion of a suitable membrane for a desired application
difficult. A simple and reliable method was developed
by See Toh et al. [49] to determine MWCO of OSN
membranes using a homologous series of polystyrene
oligomers spanning the NF range (2001000 g mol
1
)
and which are soluble in a wide range of solvents.
p0110 OSN membranes, with high solvent fluxes and
high retention of organic solutes, are required for
various applications. Fluxes of OSs through commer-
cial membranes are reported in the literature [29, 52
62]. Initial flux decrease was found to be a common
phenomenon, usually attributed to membrane com-
paction, with the variation between initial and
steady-state fluxes depending upon membrane and
solvent [56]. Solvent flux through the membrane also
increases with rise in temperature driven by reduc-
tions in viscosity of solvents, increases in solvent
diffusion coefficients [62], or by increases in polymer
chain mobility [63, 64]. The nature of the membrane
(hydrophilic or hydrophobic), physical properties of
solvents, such as dipole moment, dielectric constant,
and solubility parameter, affect membranesolvent
interaction, which in turn affects solvent flux [57, 65].
s0060 2.05.3.2 Swelling
p0115 Polymer swelling plays an important role in flux and
rejection of some OSN membranes [6567]. Ho and
t0010 Table 2 Ceramic membranes supplied by the Inopor company
Membrane Top layer material
Mean pore size
(nm) Cutoff (gmol
1
) Open porosity (%)
Inopor

ultra TiO
2
30 - 3055
TiO
2
5 8500
ZrO
2
3 2000
Inopor

nano SiO
2
1 600 3040
TiO
2
1 750
TiO
2
0.9 450
MESE 00036
8 Nanofiltration Operations in Nonaqueous Systems