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2.05a0005 Nanofiltration Operations in Nonaqueous SystemsL 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 gasseparation, reverse osmosis (RO), nanofiltration

(NF), ultrafiltration (UF), microfiltration (MF), elec-trodialysis (ED), and pervaporation (PV), have beendeveloped for various applications [1]. NF, which isintermediate 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 rangeof 2001000 g mol1, 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 asorganic 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 anaqueous phase, whereas, by contrast, OSN is used forseparations between molecules in organicorganicsystems. Another membrane-based process widely

used with OSs is PV where separation occurs by

differential permeation of liquids through amembrane, with transport of liquids through themembrane effected by maintaining a vapor pressuregradient across the membrane [3]. Membrane-based

separations, in general, use significantly less energythan thermal processes, such as distillation, and this isof particular interest given the current high energyprices. This chapter focuses on describing the state ofthe art in OSN.

p0010Sourirajan [4] reported the first application ofmembranes to nonaqueous systems in 1964 for theseparation of hydrocarbon solvents using a cellulose

acetate membrane. Later, Sourirajan and co-workers[57] used membranes to separate OS mixtures andorganic and inorganic solutes using cellulose acetatemembranes. From 1980 onward, major oil companies,such as Exxon [811] and Shell [12, 13], and chemical

companies, such as Imperial Chemical Industries (ICI) AU3and Union Carbide [14], began to file patents on theuse of polymeric membranes to separate moleculespresent 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 acquisitionprograms, and products started to be commerciallyavailable from the mid-1990s onward. The largestsuccess so far industrially has been theMAX-DEWAXTM process installed at ExxonMobilBeaumont refinery for the recovery of dewaxing sol-vents from lube oil filtrates [20], while the most recentaddition to commercial offerings is the launch byMembrane Extraction Technology (MET) in 2008 ofthe DuraMemTM series of highly solvent-stable OSNmembranes for the separation of organic solutes fromvarious OSs [24]. These efforts have prompted a rapidrise in the number of academic publications and pro-cess development projects in industry. By way ofillustrating the surge in interest in OSN, Figure 1shows a rough estimate of the number of patents andpapers published on the application of membranes fornonaqueous operations before the 1990s, duringthe1990s, and from 2000 onward.

s0010 2.05.2 Membranes for Separationsin OSs

p0015 Both polymeric and inorganic materials have beenused for the preparation of OSN membranes. In whatfollows, we present a brief summary of the currentlycommonly available OSN membranes.

s00152.05.2.1 Polymeric Membranes

p0020Compatibility of polymeric membranes with a widerange of OSs is a very challenging issue in the OSNmembrane production. Polymeric membranes gener-ally fail to maintain their physical integrity in OSsbecause of their tendency to swell or dissolve.Nevertheless, several polymeric materials exhibitsatisfactory solvent resistance (e.g., polyimides(PIs)) or can be made more stable, for example, byincreasing the degree of crosslinking (e.g., siliconeand polyacrylonitrile (PAN)). An overview of sol-vent-resistant polymeric materials used formembrane preparation can be found elsewhere [1,25]. Most polymeric OSN membranes have an asym-metric structure, and are porous with a dense toplayer. This asymmetry can be divided into twomajor types: the integrally skinned asymmetrictype, wherein the whole membrane is composed ofthe same material; and the thin-film-composite(TFC) type, wherein the membrane-separatinglayer is made of a different material from thesupporting porous matrix.

s00202.05.2.1.1 Integrally skinned asymmetric

polymeric membranesp0025Integrally skinned asymmetric membranes are most

commonly prepared by the phase-inversion immer-sion precipitation process. A solution of the polymeris cast as a thin film onto a nonwoven fabric, dried fora few seconds to create a dense top layer, and thenimmersed in a coagulation bath, which contains a

0Before 1990 199099

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Figure 1f0005AU4 Number of patents and papers published before 1990s, during 1990s, and 2000s on membranes for nonaqueousoperations.

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2 Nanofiltration Operations in Nonaqueous Systems

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nonsolvent for the polymer. The solvent starts todiffuse out of the homogeneous liquid polymer film,while the nonsolvent simultaneously diffuses into thefilm. Due to the presence of a nonsolvent, phaseseparation takes place in the polymer film and thepolymer precipitates as a solid phase, forming aporous asymmetric membrane structure. The ther-modynamic properties of the casting system and thekinetics involved in the exchange of solvent andnonsolvent affect the morphology of the membrane,and, consequently, its permeability and solute rejec-tion [26]. The phase separation can also be inducedby other methods, such as lowering the temperature(thermal precipitation), by evaporating the volatilesolvent from the polymer film (controlled evapora-tion), or by placing the cast polymer film in anonsolvent vapor phase (precipitation from thevapor phase) [2]. More detailed information aboutmembrane preparation techniques can be foundelsewhere [1].

s0025 2.05.2.1.2 TFC membranesp0030 Composite membranes consist of at least two differ-

ent materials. Usually, a selective membrane materialis deposited as a thin layer upon a porous sublayer,which serves as support. The advantage of this typeof membrane over the integrally skinned ones is thateach layer can be optimized independently in orderto achieve the desired membrane performance.There are several well-established techniques forapplying 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 foundelsewhere [1]. Due to the large variety of preparationtechniques, almost all polymeric materials can beused to produce these types of membranes. The toplayer and the support both contribute to the overallmembrane performance.

s0030 2.05.2.1.3 Postformation treatment

p0035 In order to increase the separation performance ofasymmetric membranes and to increase their long-term stability, several postformation treatments orconditioning procedures can be used, such as anneal-ing (wet or dry), crosslinking, drying by solventexchange, and treatment with conditioning agents[2]. Posttreatment procedures could be applied toboth types of polymeric membranes mentionedabove.

s00352.05.2.1.4 Commercially available

polymeric membranes

p0040Despite the fast development of research in the area ofseparation in OSs, there are still a limited number ofmembranes that have been commercialized.According to our knowledge of the membrane market,there are currently five companies producing SRNFmembranes. The commercially available solvent-stable membranes include the Koch and StarmemTM

membrane series, the SolSep membranes, the newlylaunched DuraMemTM membrane series, and theInopor series of ceramic membranes.

p0045Koch SelRO membranes. Koch Membrane Systems(USA) [27] was the first company to enter the OSNmarket with three different membranes designed forsolvent applications. However, the hydrophobicmembranes, such as SelRO MPF-60 (molecularweight cutoff (MWCO) 400 g mol1, based on rejec-tion of Sudan IV (384 g mol1) in acetone) andSelRO MPF-50 (MWCO 700 g mol1, based onrejection of Sudan IV in ethyl acetate), have alreadybeen removed from the market. Only the hydrophilicMPF-44 membrane (MWCO 250 g mol1, based onrejection of glucose (180 g mol1) in water) is stillavailable, in flat sheet as well as in spiral-wound(MPS-44) module configuration [2].

p0050It is believed that the MPF series membranes areTFC-type membranes, comprising a dense silicone-based top layer of submicron thickness on a porouscrosslinked PAN-based support. Membrane produc-tion may be associated with a patent from MembraneProducts Kyriat Weitzman (Israel) [28], in which acrosslinked PAN support was first treated with sila-nol-terminated polysiloxane as a pore protector, andthen immersed in a solution of polydimethylsiloxane(PDMS), tetraethyl silicate, and a tin-based catalystfor final coating and crosslinking. A scanning electronmicroscopy (SEM) picture [29] of the MPF-50 mem-brane is presented in Figure 2. Koch also distributesa UF membrane (nominal MWCO 20 000 g mol1),based on crosslinked PAN, available in both flatsheets (MPF-U20S) and spiral-wound (MPS-U20S)elements [2, 27].

p0055According to the manufacturers information, bothmembranes are claimed to be stable in methanol,acetone, 2-propanol, cyclohexane, ethanol, methylethyl ketone (MEK), butanol, methyl isobutyl ketone(MIBK), pentane, formaldehyde, hexane, ethyleneglycol, dichloroethane, propylene oxide, trichlor-oethane, nitrobenzene, methylene chloride,tetrahydrofuran (THF), carbon tetrachloride, aceto-nitrile, diethylether, ethyl acetate, dioxane, xylene,

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Nanofiltration Operations in Nonaqueous Systems 3

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and toluene, and claimed to have limited stability indimethylformamide (DMF), N-methyl pyrrolidone(NMP), and dimethylacetamide [27].

p0060 The MPF series of OSN membranes were the firstfreely available membranes on the market, and,therefore, they have been subjected to extensive stu-dies and have been tested in many applications, forexample, the recovery of organometallic complexesfrom dichloromethane (DCM), THF, and ethyl acet-ate, and of phase-transfer catalysts (PTCs) fromtoluene, the separation of triglycerides from hexane,and for solvent exchange in pharmaceutical manu-facturing. Extensive fundamental studies on solvent/solute transport mechanisms in OSN membraneshave also been performed on MPF membranes [2].

StarmemTM membranes. The StarmemTM 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 withactive surfaces manufactured from PIs. An activeskin layer less than 0.2 mm in thickness with a poresize of

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membrane bioreactors (MBRs) for biotransforma-

tions [2]. StarmemTM membranes are the only OSN

membranes applied at a large scale, in the refining

industry for solvent recovery from lube oil dewaxing

(MAX-DEWAXTM) [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

mol1, and one UF membrane with an MWCOaround 10 000 g mol1. 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 inAU6 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 DuraMemTM. DuraMemTM 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 membranesare available with different MWCO curves (1801200 g mol1) and possess excellent stability in arange of solvents, including polar aprotic solventssuch as DMF and NMP. The membranes have asponge-like structure and are stable in most OSs,including toluene, methanol, methylene chloride,THF, DMF, and NMP. The membranes have beenoperated continuously for 120 h in DMF and THFand showed stable fluxes and good separation perfor-mances, with DMF permeability in the rangeof (18) 105 l m2 h1 Pa1 (18 l m2 h1 bar1)[35]. Possible re-imidization and loss of crosslinkingat elevated temperatures limit their range of applica-tion to temperatures

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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 themembranes was retained 1000 g mol1. However,by the end of the last century, NF membranes were

developed based on silica membranes doped with

zirconia and titania. A TiO2-based NF membrane,

with a pore size of 0.9 nm and a cutoff of 450 g mol1,has been commercialized under the name Inopor by

a spin-off company of HITK (Germany) [37], and hasbeen successfully applied since 2002 in a treatmentplant for harsh colored textile wastewaters [38, 39].

p0090The intrinsic hydrophilicity of the oxide pore sur-faces of the existing ceramic NF membranes lowersthe permeability of apolar solvents through thesemembranes. Approaches to cope with this by prepar-ing mixed oxides were not successful. Themodification of the pore surface, by coupling of silanecompounds to the hydroxyl groups, has been found tobe a better solution. The silylation of ceramic mem-branes has been patented and is semi-commerciallyavailable from HITK (Germany) [36]. The mem-branes exemplified in the patent show cutoff valuesof about 600, 800, and 1200 g mol1 in toluene usingpolystyrene standards [36], and have been used toretain transition-metal catalysts in apolar solvents [2].

s00452.05.2.2.1 Commercial ceramic

membranesp0095Inopor series membranes. The Inopor company [40]

currently offers a range of ceramic UF and NFmembranes in the form of monochannel and multi-channel tubes with lengths up to 1200 mm, assummarized in Table 2. The membranes are offeredas hydrophilic version; however, on customerrequest, they can be prepared to be hydrophobic.There is no specific information on the companyswebsite regarding the hydrophobic membranes, but itis believed that the literature-cited HITK-T1(HITK, Germany) is a silylated TiO2-based versionof the above-mentioned membranes. With a nominalMWCO of 220 g mol1, this membrane showedmethanol and acetone permeabilities (0.4 l m2

14 kV X370 50 m 17 26 SEI 14 kU X6, 800 2 m 16 26 SEI

Figure 4f0020 Typical multilayered structure of a ceramic membrane from the Inopor series [40] at magnification 370 (edge

view) and 6000 (top-layer edge view).

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h1 bar1), while rejecting Victoria blue (506 gmol1) for 99% from methanol, and erythrosine B(880 g mol1) for 97% from acetone, and demon-strated efficient catalyst recovery for Pd2,29-bis(diphenylphosphino)-1,19-binaphthyl (BINAP)(849 g mol1) with rejections around 94.5% [2].

s0050 2.05.3 Membrane Characterization

p0100 Membrane characterization methods can be dividedinto two categories: (1) functional characterizationand (2) physicalchemical characterization [41].Functional parameters, such as flux and rejection,determine the selection of a membrane for a specificapplication [42]. Physicalchemical parametersinclude porosity, pore size, pore-size distribution,hydrophobicity, hydrophilicity, skin layer thickness,and charge [41]. One of the current challenges inOSN research is to establish the physicalchemicalstructure of the membranes, and then to use that topredict 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 unittime and is generally expressed in terms of l m2 h1

and the permeability by l m2 h1 bar1. Rejection ofa solute i (Ri%) is calculated by Ri (%) (1 Cpi/Cri) 100% where, Cpi and Cri are the concentrationof solute i in the permeate and retentate, respectively.The separation performance of OSN membranes canalso be expressed in terms of MWCO obtained byplotting the % rejection of solutes versus their MW(typically 2001000 g mol1) and interpolating thedata 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 asstructure, size, charge, and concentration, are found toaffect the performance of OSN membranes [4351].Figure 5 shows MWCO curves for StarmemTM 122in different solvents using polystyrene oligomers.MWCO of some of the commercially available mem-branes are summarized in Table 3. The selection ofmembranes for OSN applications depends upon theMWCO specified by the manufacturer. However,different methods used for evaluating MWCO ofmembranes lead to inconsistencies, making the selec-tion of a suitable membrane for a desired applicationdifficult. A simple and reliable method was developedby See Toh et al. [49] to determine MWCO of OSNmembranes using a homologous series of polystyreneoligomers spanning the NF range (2001000 g mol1)and which are soluble in a wide range of solvents.

p0110OSN membranes, with high solvent fluxes andhigh retention of organic solutes, are required forvarious applications. Fluxes of OSs through commer-cial membranes are reported in the literature [29, 5262]. Initial flux decrease was found to be a commonphenomenon, usually attributed to membrane com-paction, with the variation between initial andsteady-state fluxes depending upon membrane andsolvent [56]. Solvent flux through the membrane alsoincreases with rise in temperature driven by reduc-tions in viscosity of solvents, increases in solventdiffusion coefficients [62], or by increases in polymerchain mobility [63, 64]. The nature of the membrane(hydrophilic or hydrophobic), physical properties ofsolvents, such as dipole moment, dielectric constant,and solubility parameter, affect membranesolventinteraction, which in turn affects solvent flux [57, 65].

s00602.05.3.2 Swelling

p0115Polymer swelling plays an important role in flux andrejection of some OSN membranes [6567]. Ho and

t0010 Table 2 Ceramic membranes supplied by the Inopor company

Membrane Top layer materialMean pore size(nm) Cutoff (g mol1) Open porosity (%)

Inoporultra TiO2 30 - 3055

TiO2 5 8500ZrO2 3 2000

Inopornano SiO2 1 600 3040

TiO2 1 750

TiO2 0.9 450

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8 Nanofiltration Operations in Nonaqueous Systems