liquid membrane technology review

2Emerging TechnologiesReceived: 6 March 2009 Revised: 24 June 2009 Accepted: 3 July 2009 Published online in Wiley Interscience: 14 August 2009

(www.interscience.wiley.com) DOI 10.1002/jctb.2252

Liquid membrane technology: fundamentalsand review of its applicationsM. F. San Roman, E. Bringas, R. Ibanez and I. Ortiz

Abstract

OVERVIEW: During the past two decades, liquid membrane technology has grown into an accepted unit operation for a widevariety of separations. The increase in the use of this technology owing to strict environmental regulations and legislationtogether with the wider acceptance of this technology in preference to conventional separation processes has led to aspectacular advance in membrane development, module configurations, applications, etc.

IMPACT: Liquid membrane technology makes it possible to attain high selectivity as well as efficient use of energy and materialrelative to many other separation systems. However, in spite of the known advantages of liquid membranes, there are very fewexamples of industrial applications because of the problems associated with the stability of the liquid membrane.

APPLICATIONS: Liquid membrane technology has found applications in the fields of chemical and pharmaceutical technology,biotechnology, food processing and environmental engineering. On the other hand, its use in other fields, such as in thecase of hydrogen separation, the recovery of aroma compounds from fruits, the application of ionic liquids in the membraneformulation, etc., is increasing rapidly.c 2009 Society of Chemical Industry

Keywords: liquid membrane; transport mechanisms; membrane contactors; hollow fibre

INTRODUCTIONMembranes are barriers that separate two fluid phases andallow the selective permeation of solutes from one side of thebarrier to the other.1 Solids are not the only materials that havebeen used as membranes and it is possible to use a liquid as amembrane. Liquid membranes are present in various forms indaily life; an oil layer on a water surface is a typical organic liquidmembrane of an immiscible liquid phase, beer froth, foam onsoap, detergent or surfactant solutions, oil films coated on ametalsurface popularly used in rust protection and lubrication arefamiliar liquid films separating two phases.2

A new age in the development of liquid membrane technologybegan in the 1960s when Li et al. patented the use of emulsionliquid membrane systems (EMLs) for industrial scale desalinationand the separation of hydrocarbons.2 In ELMs, the oil dropletscontaining the receiver aqueous phase are dispersed in the feedaqueous phase. The total volume of the receiving phase insidethe oil droplets is at least ten times smaller than that of the sourcephase (Fig. 1). The resulting system is thus a water/oil/waterdouble emulsion. Oil/water/oil systems are also possible. Afurther advantage of ELM processes is the creation of very largesurface area to volume ratios.3 The thickness of the membrane(organic film) is very small, while the surface area is enormous,resulting in very fast separations.4 Large interfacial areas can beattained in units that occupy significantly less floor space andcost significantly less than traditional solvent extraction columns.However, issues of emulsion stability and membrane leakagehave limited their commercial potential. Thin membranes areprone to leakage or rupture and the inner phase can sufferfrom swelling instability. Current research has been directed at

increasing emulsion stability without significantly reducing theextraction rate with, for example, the development of chemicalcompounds that act as both emulsifiers and extractants.3

The potential for industrial applications of supported liquidmembrane (SLM) was first reported by Ward and Robb in 1967.They showed that an aqueous bicarbonatecarbonate solutionmembrane supported in porous cellulose acetate film was morepermeable by CO2 than by O2.2 The SLM technology resolvedthe problems of ELM stability. The immobilization of the organicliquid containing an active complexing agent (carrier) in suitablediluent within a porous structure or solid membrane (polymericor inorganic membranes) promises physical stability for practicaluse.3 In this technology the membrane is clamped between twocompartmentswhich are filledwith an aqueous source and receiv-ing phase (Fig. 2).5 A number of studies on facilitated transport ofgases1,6,7 such as CO2, O2, H2S, NO, SO2 and CO and in particular,the separation of the components of a liquid solution by a liquidmembrane conducted by Cussler8 for sodium transport acrossmembranes containing a variety of carriers, including stearic acid,lecithin, andmonensin throughSLMsare available in the literature.

Common configurations of SLMs are flat sheet supported liquidmembrane (FSSLM) and hollow fibre supported liquid membrane(HFSLM). Planar or flat geometry is very useful for laboratory

Correspondence to: I. Ortiz, Dept. Ingenier a Qumica y Qumica Inorganica,ETSIIyT, Universidad de Cantabria, Avda Los Castros s/n, 39005 San-tander, Spain. E-mail: [email protected]

Dept. Ingenier a Qumica y Qumica Inorganica, ETSIIyT, Universidad deCantabria, Avda Los Castros s/n, 39005 Santander, Spain

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3Liquid membrane technology: a review www.soci.org

Continuous outer phase(feed)

Liquid membrane phase

Inner phase(stripping phase)

Figure 1. Emulsion liquid membrane (ELM).

Feedphase

Strippingphase

Liquid membrane

Support

Figure 2. Supported liquid membrane (SLM).

purposes; however, for industrial purposes, a planar geometryis not very effective since the ratio of surface area to volume istoo low. Recently, the separation technology based on the useof membrane contactors such as hollow fibre and spiral woundmodules that provide high surface area to volume ratio,5 hasdeveloped beyond academic curiosity5,9,10 and found commercialapplications. Membrane contactors have led to the integration ofdifferent configurations of liquid membranes creating new formsof separation processes such as emulsion pertraction (EPT)1113 orpseudo-emulsion-basedhollow-fibre stripdispersion (PEHFSD)5,14

and hollow fibre renewal liquid membrane (HFRLM)1517, whichmaximise the efficiency of the process with long-term stability. Aschematic classification of the all liquid membrane technologiesis shown in Fig. 3.

TRANSPORT MECHANISMS IN LIQUID MEM-BRANE TECHNOLOGYIn a liquid membrane the barrier is formed by a thin gasor liquid film which separates two miscible liquids or gasesand which controls the mass transfer between both phases.A component or solute from the fluid phase is transportedacross the membrane to another phase (stripping phase) bydiffusion (passive diffusion or single transport), or by a com-bination of diffusion and chemical reaction in the organic

phase (facilitated transport and coupled transport) and/or inthe stripping solution (passive diffusion transport with chemi-cal reaction).18,19 The addition of a carrier to a liquid membranesystem, which complexes rapidly and reversibly with the de-sired species, can improve the membrane permeability andselectivity, which are significantly dependent on carrierguestinteraction.

In general, the driving force for mass transport is a potentialgradient through the different diffusional layers (external layerand liquid membrane) described by Ficks first law:

Ji = Di dCidx

(1)

where Ji is the diffusive mass transport flux of solute i, Di is thediffusion coefficient of solute i, Ci is the solute concentration andx is the direction of diffusion. Assuming a linear concentrationgradient and integrating Equation (1), the following expression isobtained:

Ji = DiCi = kiCi (2)

where and ki represent, respectively, the thickness of the dif-fusional layer and the corresponding mass transport coefficient.1

Some representative examples of mass transport mechanism inliquid membranes will now be described.

Passive diffusion or single transportPhenol and phenolic derivatives are chemical products commonlyencountered in aqueous effluents frommanufacturing processes.Phenol recovery using the liquidmembrane technique is based onthe solubility difference in the phenol between the aqueous andthe organic phases.20 Fig. 4 shows that phenol initially present inthe aqueous phase is dissolved in the liquid organic membrane(methyl isobutyl ketone (MIBK)). Then, phenol diffuses acrossthe membrane phase into the NaOH-containing internal phase(stripping phase), where it reacts with NaOH to form sodiumphenolate. Since this component is not soluble in the organicphase, it is trapped in the internal phase. The reaction maintainsthe phenol concentration effectively at zero in the strippingphase, giving a high driving force and thus a high extractionrate.1

Facilitated transportScholander21 in 1960 reported oxygen mass transport throughaqueous hemoglobin solutions bymeans of a facilitated transportmechanismwith thehemoglobin acting as a carrier that transportsoxygen. Figure 5 describes this situation; the hemoglobin reactswith oxygen at the interface i1 (phase Iphase II) to form

Liquid Membranes

FSSLMFlat Sheet Supported Liquid

Configuration

BLMsBulk Liquid Membrane

ELMsEmulsion Liquid Membrane

SLMsSupported Liquid Membrane

HFSLMHollow Fibre Supported Liquid

NDSXNon-dispersive Solvent Extraction

EPT (PEHFSD)* Pseudo-emulsionbased strip dispersionEmulsion Pertraction TechnologyHFRLMHollow Fibre Renewal Liquid

Figure 3. Classification of liquid membrane technologies.

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4www.soci.org MF San Roman et al.

Figure 4. Extraction of phenol: passive transport with chemical reaction.

O2

** Hemoglobin

HEM-O22

O2

O2 + HEM** ---> [HEM** O2] [HEM** O2] ----> O2 + HEM**Interface i1 Interface i2

Phase I Phase II

Liquid membrane

O21

HEM-O21

O22

Phase III

Figure 5. Permeation of oxygen across a membrane using hemoglobin asthe carrier agent.

oxyhemoglobin, which then diffuses across the membraneas far as the interface i2 (phase IIphase III). There, thereaction is reversed: oxygen is liberated into the permeategas, and the hemoglobin is regenerated. The hemoglobin thendiffuses back to the feed side of the membrane (interfacei1) to pick up more oxygen. In this way, the hemoglobinacts as a shuttle to selectively transport oxygen through themembrane.18

Coupled transportAs a result of the growing world population and rapid industrialdevelopment, an increasing amount of waste-water is producedworldwide. Waste-water contaminants may eventually enter thefood chain and cause severe health and environmental problems.Current restrictive legislation on waste-water effluent dischargehas led to the need for enhanced treatment processes that arecapable of removing toxic metal ions, among other contaminants.Many examples have been published in which metal ions areremoved from water with protons as counter-transport ions.22

The most typical example of this process is metal cationexchange with H+ ions, facilitated by acidic carriers; the metalions (i.e. Mn+), contained in aqueous solutions, are selectivelyextracted by the carrier (HX) at the organic membrane/feedinterface. The carrier picks up the metal, moves across themembrane as a complex (MXn) and finally exchanges Mn+

with the charged species (i.e. H+) on the other side of themembrane. To preserve electrical neutrality the carrier actsas a shuttle carrying the flux of Mn+ and H+ in oppositedirections.23,24 The removal of zinc (M2+) from waste-water usingbis(2-ethylhexyl)phosphoric acid (D2EHPA) as the carrier in n-heptane,25 with sulfuric acid (H+) as the stripping phase is a wellknown system that can illustrate the fundamentals of counter

Feedphase

Liquid membrane

Strippingphase

Zn+2

H+

H+

Zn+2

ZnX2(i1)

Flux Zn+2Flux H+

Phase I Phase II

Phase III

ZnX2(i2)

H+(i2)

H+(i1)

HX(i2)

HX(i1)

Zn+2(i2)

Zn+2(i1)

Figure 6. Mechanism of the facilitated coupled counter-transport.

transport (Fig. 6).

Extraction Reaction

Zn+ 2(A) + 2HX (M) Zn X2 (M) + 2H+(A)

Stripping Reaction

Zn X2 (M) + 2H+(S) 2HX (M) + Zn+ 2(S)

A : Aqueous Phase; M : Membrane; S : Stripping Solution

This is called coupled transport and it resembles facilitatedtransport in that a carrier agent is incorporated into themembrane. However, in coupled transport the carrier agentcouples the flow of two species. Because of this cou-pling, one of the species can be moved against its con-centration gradient, provided the concentration gradient ofthe second coupled species is sufficiently large.18 When thefeed solution contains ionic species, coupled transport oc-curs in order to maintain the electroneutrality of the solu-tion.

Considering that extraction and back-extraction reactions arelocated, respectively, at interfaces i1 and i2, the mass transportfluxes (J), at steady state, through the phase I (PI) stagnant layer,the liquid membrane (LM) and the phase II (PII) stagnant layer canbe described according to Ficks law by means of the followingequations:

JPIZn+2 = k

PIL,Zn+2

(CPIZn+2 C

PI,i1Zn+2

)(3)

JPIH+ = kPIL,H+ (CPI,i1H+ CPIH+

)(4)

JLMZnX2

= kLMm,ZnX2

(Ci1ZnX2

Ci2ZnX2

)(5)

JLMHX

= kLMm,HX

(Ci2HX

Ci1HX

)(6)

JPIIZn+2 = k

PIIL,Zn+2

(CPII,i2Zn+2 C

PIIZn+2

)(7)

JPIIH+ = kPIIL,H+ (CPIIH+ C

PII,i2H+

)(8)

where kL and km are, respectively, the mass transport coefficientin the aqueous phase stagnant layer (phase I and II) and themembrane mass transport coefficient. The solute concentrationsat interfaces i1 and i2 areunknownandthus,a relationshipbetweenthem should be established to make the flux calculation possible.For instantaneous reactions, concentrations can be related by

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means of equilibrium parameters as follows:

KExtraction =Ci1ZnX2

(CPI,i1H+

)2

CPI,i1Zn+2

(Ci1HX

)2 (9)

KStripping =(Ci2HX

)2 CPII,i2Zn+2

Ci2ZnX2

(CPII,i2H+

)2 (10)

The methodology detailed above can easily be extended to thedifferent mass transport mechanisms described in this section.Furthermore, an extended guideline to characterize the masstransport phenomena in liquid membrane systems can be foundin a recently published paper by Bringas et al.26

Another typical example is the extraction of hexavalentchromium using secondary or tertiary amines. In acidic solution(pH = 1) the prevalent chromium species is HCrO4 and theextractionprocess isdescribedbythe followingreversible reaction:

HCrO4 + H+ Feed phase

+ R3N (carrier)

(R3NH)HCrO4 (Liquid membrane)

This process is called facilitated coupled co-transport. In thiscase, the pH or the concentration of the H+ is used as thedriving force. The (chromium) species can be transported acrossthe membrane, against its concentration gradient, until it hasbeen transferred from the feed to the strip side. This situationis very common in practice when the feed phase contains verydilute solutions of the target species.23,24 Two components aretransported simultaneously; once again, a reaction is necessary tomaintain the driving force across the membrane.22 If the carrier isa neutral or a basic extractant (i.e. amines, solvating extractants, E),the metal (i.e. Mn+) can be transported together with negativelycharged ions (counterion), X in the same direction.

MEMBRANE CONTACTORSSeparation processes based on solvent extraction are traditionallycarried out using conventional equipment, such as towers,columns, mixer-settlers, etc.27 The main target in the designof the equipment is to maximize the mass transfer rate byproducing as much interfacial area as possible and reduce thedisadvantages of the two fluid phases to be contacted. The use

of hollow fibres (HF) has led to a development in applications inthe separation of gas mixtures as well as liquid mixtures.1,4,6,10 Inthe latter, dispersion-free solvent extraction using micro-porousmembranes has been developed to overcome the disadvantagesof conventional equipment. In addition the technology also offersadditional advantages: (i) no flooding because the two fluid flowsare independent; (ii)nomixing;emulsionformationdoesnotoccur,because there is no fluid/fluid dispersion (iii) no density differenceis required; (iv) it provides a large and known interfacial areaper volume (v) scale-up is more straightforward using membranecontactors with a modular design which means a wide range ofcapacities10 can be processed.

The best known commercialised module is the Liqui-Cel

ExtraFlow module offered by Liqui-Cel MEMBRANE CONTAC-TORS. This module uses microporous polypropylene fibres thatare woven into a fabric wrapped around a central tube feeder thatsupplies the shell side fluid. The inner diameter and wall thicknessof the fibres are usually 200220 m and 30 m, respectively. Thefibres are potted into a solvent-resistant epoxy or polyethylenetube, and the shell casing is polypropylene, PVDF or PVC. TheExtra-Flow module contains a central shell side baffle, which im-proves efficiency byminimizing shell side by-passing. The smallestLiqui-Cel commercialmodule is 2.62 inches diameter and contains1.4 m2 of contactarea,while the largestone is14.0 inchesdiameterand offers 220 m2 of contact area.10

For separation of the components of a liquid mixture, hollowfibre contactors (HFC) or hollow fibre membrane contactors(HFMC) can be used in different configurations. The simplestoperation relies on the performance of a single fluidfluidseparation process. The aqueous phase containing the soluteflows through the inner side of the hollow fibres. In the case ofhydrophobic fibres the organic phase containing the solvent witha carrier is pumped through the shell side and fills the membranepores. When the fibre is hydrophilic the pore is filled with theaqueous phase. To prevent entrainment of the organic phase inthe aqueous phase a higher pressure is applied to the non-wettingliquid than to thewetting liquid.28 With this configurationonlyonestep is possible; extraction (membrane-based solvent extraction(MBSE) or back-extraction (membrane-based solvent stripping(MBSS).29 Two modules are necessary to perform simultaneousextraction and stripping steps. This technology is referred to theliterature in severalways: non-dispersive solventextraction (NDSX)(or dispersion-free solvent extraction (DFSX),30 and hollow fibrenon-dispersive solvent extraction (HFNDSX).31 Fig. 7 shows a flowdiagram of the separation process.

Pump

Pump

PumpFeed tank

Organic tankStrippingtank

Flowrate

Hollow fibremodule

Hollow fibremodule

Figure 7. Experimental setup of non-dispersive solvent extraction (NDSX).



In recent years process intensification has been studied bythe simultaneous performance of both extraction and strippingin the same HF module and a number of different alternativeconfigurations have been reported, namely:

i) Hollow fibre supported liquid membrane, HFSLM. In thiscase, the pore membrane is impregnated with an organicsolvent and the feed and stripping phases flow outside thefibre. At the interface between the feed solution and themembrane, the solute is extracted into the organic phase;then, it diffuses by itself and/or in a complex form to theother side of the membrane where it contacts the flowingstripping phase. At this stripping phase-organic phase (liquidmembrane) interface, the solute is back-extracted into thestrippingphase. By varying theflow ratesof the feedand stripphases the solute canundergoconcentration. TheHFSLMhasmany advantages such as low capital, operating and energycosts, low volume of organic phase, high mass transfer areaper unit volume, etc. Despite the numerous advantages,this technology has not been adopted to industrial scale.The most important reason for this is liquid membraneinstability32 that can lead to: (i) loss of organic phase (orextractant); (ii) progressive wetting of the support pores asa consequence of the aqueous phase filling the pores; andfinally; (iii) the displacement of the fluidfluid interface dueto high differential pressure created by fluid flow.1,33

Authors suchasNeplenbroeket al.,34 studied the instabilitybehaviour of several supported liquid membranes for asystem in which nitrate ions were removed from an aqueousfeed phase and concentrated in a stripping phase. Thecomposition of both the aqueous phases and the liquidmembrane was determined after the aqueous phases hadflowedparallel to themembranes for a periodof 6 days. Fromthe experimental data it could be observed that membranestability depended largely on the type of solvent and on themolecular structure of the carrier. Furthermore, membranestability improved when the salt content in the strippingphase increased. Subsequent studies performed with thesame system advanced a new hypothesis of formation ofan emulsion induced by shear forces for the degradationmechanism of supported liquid membranes. A decreasein the salt concentration in the aqueous phases and anincrease in flow velocity of these phases parallel to themembrane surface both lead to an increase in instability ofthe liquidmembranewhile emulsion formation is stimulatedby these circumstances.32,35 Finally, to enhance membranestability a careful choice of membrane conditions andmaterials is important. Once the phases in the system arefixed, several suggestions can be found in the literature toimprovemembrane lifetime.Gelationof the liquidmembranephase to prevent loss by emulsification is, in particular, veryencouraging but difficult to apply on an industrial scale.32

ii) Hollow fibre contained liquid membrane, HFCLM. In the HF-CLM, the extraction and enrichment of valuable componentsare carried out simultaneously in a single hollow fibre con-tactor containing one bundle of fibres for extraction and adifferent one for the back-extraction process. The organicphase circulates between the fibres transporting the solutefrom the feed phase to the stripping phase.33,36 The sep-aration process can also be carried out in two membranecontactors (non-dispersive solvent extraction technology,NDSX).The amount of the organic phase will be somewhat

Feedphase

Liquid membrane

Stripping phase

Figure 8. Enlarged view of a hollow fibre with the feed phase in the lumenside and emulsion phase in the shell side.

higher when two modules are used compared with onemodule, but the HFCLM module size would be larger andmodule fabrication is more difficult and expensive.37 The HF-CLMprovides an alternative to reduce organic loss and offersimprovements to this configuration based on (a) automaticand easy replacement of the liquid membrane, (b) the useof hollow fibre leads to a high membrane surface area perunit permeator volume and low effective liquid membranethickness, and finally (c) the HFCLM may be used in a fargreater variety of modes than a HFSLM.

iii) More recently, several applications have been reported usingemulsionpertraction(EPT)orpseudo-emulsion-basedhollowfibre strip dispersion (PEHFSD) technology.5,14 Emulsionliquidmembranes (ELM) are attractive separation techniquesallowing removal and recovery of many compounds due tothe large interfacial area in the stripping processes. However,ELM techniques have not been adopted on a large industrialscale due to the lack of long-term stability and difficultoperations, such as the necessary emulsification and de-emulsification steps. With this background and to overcomethese problems, a new alternative technology (EPT) has beendeveloped combining the advantages of ELMs and NDSX,with the permeation fluxes comparable with a traditionalstirred contactor system, and where extraction and strippingof the metal ions take place simultaneously. In EPT an ELM iscoupled to a hollow fibre contactor with the aqueous phasecontaining the target species separated from the emulsionphase by a hydrophobic microporous membrane (Fig. 8).The emulsion phase consists of an organic phase with adissolved extractant in which an aqueous strip solution isdispersed as aqueous droplets. There is no need to makethe dispersion very stable because the droplets of the stripsolutiondonotpenetrate into themembraneporesasa resultof its hydrophobic character.1113 The interface ismaintainedinside the mouth of the pore of the hollow fibre membrane,where a feed solution containing the solute comes intocontact with the organic solution. The solute is extractedinto the pseudo-emulsion and subsequently stripped in thestripping solution contained in the pseudo-emulsion. Oncethe separation process is stopped, the organic and stripsolutions are separated.14

(iv) Hollow fibre renewal liquid membrane (HFRLM). Recently, anew liquid membrane technique called hollow fibre renewalliquid membrane (HFRLM) has been reported. This is basedon the surface renewal theory and integrates the advantagesof fibre membrane extraction and liquid film permeation.In the HFRLM process, hydrophobic hollow fibres are usedwhich are prewettedwith theorganic phase so that theporesbecome completely filled with the organic phase. The feedphase is pumped through the shell side of the module. The



stirredmixtureof theorganic andstrippingphase, containinguniformlydispersedorganic droplets, is pumped through thelumen side of the fibres. Due to the wetting affinity betweenthe organic phase and the walls of hydrophobic fibres, a thinorganic film, i.e. a liquidmembrane layer, is developedwithinthe lumen side of the fibres. The shear force resulting in theflowing fluids causes the film liquid to form microdropletson the surface of the liquid membrane layer, which will peeloff from the surface of the liquid membrane layer and enterthe lumen side flowing fluid. At the same time, the organicdroplets, dispersed fluid flowing in the lumen side, will fillthe surface of the liquid film; thus renewal of the liquidmembrane proceeds. This effect and the huge mass transferarea due to the direct contact of organic droplets and thevigorously mixed aqueous phase can accelerate the masstransfer rate and greatly reduce the mass transfer resistancewithin the lumen side. In addition, the HFRLM technique hassome potential advantages of long-term stability, high masstransfer rate, easy operation, etc.1517

LIQUID MEMBRANE SEPARATIONS USINGHF CONTACTORS. APPLICATIONS ANDPERSPECTIVESMembrane contactors have been applied to a wide range ofgas/gas, gas/liquid and liquidliquid separations in differentfields such as chemical (including nuclear) and pharmaceuticalindustries, biotechnology, food processing and environmentalengineering,anditsuse inotherpotentialapplications is increasingrapidly, as in the case of the hydrogen separation or the recoveryof aroma compounds from fruits.

Some representative examples will now be described.

Separation of CO2 from a gas streamCO2 is one of the major greenhouse gases.38 Conventionaltechniques used for the separation of CO2 from gas streams,such as chemical and physical absorption, solid adsorption,cryogenic distillation, etc. are energy-consuming and not easy tooperate because of the frequent problems that include flooding,channelling and entrainment. Among these methods, the mostwell-established is the separation of CO2 from gas streamsby absorption into alkanolamine solutions using conventionalcontactor equipment such as packed or tray columns. In packed

towers or columns, CO2 contacts the absorbent to form a weakcomplex and the aqueous solution is then transferred to aregeneratingunit to releaseCO2 byheating. After this, the solutionis cooled and recirculated to the absorption equipment. Krullet al,39 reviewed the potential of liquidmembranes for gas/vapourseparations. They observed that the most common separation isthe removal of CO2 from gas streams containing CH4 or air, sinceabout two-thirds of the reviewed articles report CO2 separations,stressing the industrial relevance of this separation task. Otherapplications include the removal of SO2, H2S and NH3.39 Forgas/gas or gas/liquid separation, the term liquid membrane isused as a synonym for liquid membrane employing supports. Inthese applications, the most compact form of a liquid membraneis given by SLM, where a liquid (carrier) is held inside the pores of aporous support by means of capillary forces. In this configurationthesupporthas tobewettableby the liquid. The long-termstabilityof a liquid membrane configuration is mainly dependent on thevolatility of the membrane liquid. Ionic liquids (RTISL) or liquidmolten salts offer improved membrane stability, although theyare still in the minority.5,39 RTILs are used in supported ionic liquidmembranes (SILMs) as a highly permeable and selective transportmedium. The use of SILMs as media for separation CO2 from N2andCH4 seems to be especially promisingwith imidazoliumbasedsalts.40

The vast majority of liquid membrane configurations are flatsheet configurations, employing polymeric supports; however,liquid membranes employing hollow fibre membrane contactorshave increased since the 1980s.38 A promising alternativethat combines membrane separation technology and chemicalabsorption technology for gas separation is called membrane gasabsorption technology.41 These processes have demonstrated anumber of advantageswhen comparedwith conventional packedcolumns, including a larger interfacial area, independent control ofgas and liquid flow rates anda knowngasliquid interfacial area.42

Membrane gas absorption is based on a gasliquid contact acrossa hydrophobic microporous membrane (Fig. 9). This membraneforms a permeable barrier between the liquid and the gas phase,which permits mass transfer between the two phases without onephase dispersing into the other. The gas fills the hydrophobicmembrane pores andmeets the liquid on the opposite side of themembrane. The liquid phase pressuremust be slightly higher thanthat of the gas phase to prevent dispersion of gas bubbles into theliquid. In this situation the solution does not penetrate the poresand the gas/liquid interface is immobilized in the mouth of themembrane pores on the solution side.38,41,43

CO2 from agas stream

- Polypropylene membranes- Polyolefin membranes- Polytetrafluoroethylene

MembraneContactor

gas streamoutlet without CO2

- NaOH aqueous solution- Distillated water- Mixtures of salts and amino-acids

Type of membranes

Absorbent

Liquid streamwith CO2

Figure 9. Recovery of CO2 from gas stream using hollow fibre contactors.



Conventional Process

Residual water(CrO42-)

Mixed

Liquid membrane(HF contactors)

Organic phase (Aliquat 336)

Stripping phase (NaOH)

ConcentratedCr(VI)

H2SO4

Advanced Separation Process

Reduction Cr(VI) to Cr(III)Ca(OH)2

MixedTreated Water

Sludge: Cr(OH)3Ca(SO)4

Figure 10. Treatment of effluents containing Cr(VI).

Hexavalent chromium removal from waste-waterDuring the past century the widespread use of chromiumin industrial applications such as leather tanning, metallurgy,electroplating processes, etc., caused chromium contamination ofsurface and ground waters. Public concern arose as a result of theevidenceofCr(VI)carcinogenicity inhumansandbio-accumulationin flora and fauna.11 Various technologies were developed for theremoval of Cr(VI), including chemical precipitation, ion exchange,solvent extraction, reverse osmosis, diffusive dialysis, adsorption,etc. The conventional and the most commonly used method forCr(VI) removal is chemicalprecipitationwhereCr(VI) is first reducedtoCr(III) because the latter is less toxic, less soluble, and lessmobilethanCr(VI), but theprocess is very tedious requiringa largeamountof chemicals. Cr(III) is then converted to the hydroxide form andremoved after precipitation (Fig. 10).11,44

In recent decades, the use of liquid membranes for the separa-tion and concentration of Cr(VI) has received considerable atten-tion due to several advantageous characteristics such as commer-cial availability of selective extractants, high separation rates, pos-sibility of metal recovery, etc.9 Different systems (membrane tech-nologies/extractant agent/back-extractant agent) have beenstudied for the removal of Cr(VI). Several works have reportedthe use of primary and secondary amines such as Adogen 382 andAdogen 283, tertiary amines such as trioctylamine and Alamine336, and quaternary ammonium salts such as Aliquat 221 andAliquat 336 for the extraction of Cr(VI).45 The stripping agentmost widely used has been NaOH. Hollow fibre contactors con-taining between 2100 and 32 500 hollow fibres and 0.2319.3 m2

of effective mass-transfer area have been used in different con-figurations; e.g. single membrane module (only removing thesolute), dual membrane modules (both removal and recovery ofthe solute) and emulsion pertraction (EPT) for the separation andconcentration of Cr(VI).11,4648

Concentration factors higher than 18 000 have been reportedwhen using tertiary amines or quaternary ammonium salts as theextractant and NaOH as the stripping agent, permitting the reuseof chromium as chromate in electroplating.11

Extraction and recovery of penicillin GPenicillin G is an important intermediate for many othermedical substances in the antibiotic industry. It is obtained in atypical biotechnological fermentation process. After incubationand fermentation periods of 40 and 190 h, respectively, thefermentation broth contains a maximum of 2% of Penicillin G.In the traditional process Penicillin G is extracted periodicallywith butyl acetate at pH 22.5, which causes a loss of about20% and makes the fermentation broth unusable for further

processing.49 The process requires a reduced temperature of05 C to carry out the total separation and expensive centrifugalextractors are needed. The reason for the low temperature and theexpensive centrifugal extractors is that, at the pH used, PenicillinG is an extremely unstable weak acid. Thus, at pH 2.0 and ambienttemperature, 1% of aqueous Penicillin G will decompose in 10 s.50

Otherdisadvantagesof thisphysical extraction systemare thehighconsumption of butyl acetate and high power consumption forrecovery of the butyl acetate from the raffinate. On the other hand,anextractionprocess that combines reactionwith separation, suchas liquidliquid extraction has much greater selectivity, i.e. whenthePenicillinG is extractedbya selective carrier suchasAliquat336at a higher pH,where it ismuchmore stable, decomposition lossesare greatly reduced.51 An efficient alternative to conventionalliquidliquid extraction is the use of the removal and strippingof Penicillin G by membrane contactor technologies, using theconfiguration of hollow fibre non-dispersive solvent extraction(HFNDSX) (Fig. 11).52 The authors checked the viability of thesimultaneous extraction/stripping of Penicillin G using two large-scale hollow fibre modules. The recovery process of Penicillin Ghas been studied at pilot scale using HFmodules with an effectivesurface area 7.7 m2.

CONCLUSIONSLiquidmembrane technology is one of themost efficientmethodsof separation. This technology does not use pressure or voltagebecause the separation is based on a concentration difference.Passive diffusion (single transport), facilitated transport or coupledtransport (counter or co-transport) make the active transport ofthe components from feed phase to receptor or stripping phasepassing through the liquidmembrane possible. Liquidmembranetechnology presents high selectivity and high enrichment factors,although, due to the stability problems of liquidliquid interfaces,there is a growing tendency to immobilize in porous structures orsolidmembraneswhichpromise physical stability for practical use.This form is called supported liquid membrane (SLM). Dependingon their configuration, SLM can be a flat sheet supported liquidmembrane (FSSLM) or a hollow fibre supported liquid membrane(HFSLM).

Nowadays, different configurations have been developed usingmembrane contactors to overcoming those problems related tothe instability of the liquid membrane, these range from theuse of single function modules to carry out a single fluidfluidseparationprocess (extractionMBSEorback-extraction stepMBSS)to the dual function modules where the simultaneous extraction



Penicillin V (Na+, K+ salt)Incubation

FermentationFiltration

Extraction(solvents)

Liquid membrane(HF contactors)

Carbontreatment Filtration

Crystallization(Na+, K+)

-Washed, dried-

Penicillin G (Na+, K+ salt)

Organic phase (Aliquat 336)

Stripping phase (Salts Na+, K+)

Penicillin G (Na+, K+ salt)40-90 hours

Butyl acetate

pH=2-2.5, 0-5C

Separation Conventional Process

Advanced Separation Process

Figure 11. Process of extraction of Penicillin G.

and stripping steps are carried out in the same contactor (HFSLM,HFCLM, EPT or PEHFSD, HFRLM).

Finally, it should be mentioned that liquid membrane technol-ogy has been studied in multiple separation applications such asseparationof a gas fromgaseous component streams,wastewatertreatment, pharmaceuticals, protein extraction, etc, showing thepotential of the technology.

ACKNOWLEDGEMENTFinancial support from projects CTQ2008-5545/PPQ, CTQ2008-00690/PPQ and CTQ2008-3225/PPQ from the Spanish Ministry ofInnovation and Science (MICINN) is gratefully acknowledged.

REFERENCES1 HoWSW and Sirkar KK, Membrane Handbook. Chapman & Hall, New

York (1992).2 Araki T and Tsukube H, Liquid Membranes: Chemical Applications. CRC

Press, FL (1990).3 Kentish SE and Stevens GW, Innovations in separations technology

for the recycling and re-use of liquid waste streams. Chem Eng J84:149159 (2001).

4 Pabby A,Rizhi SHSandSastre AM,Handbook of Membrane Separations.Chemical, Pharmaceutical, Food and Biotechnological Applications.CRC Press, New York (2009).

5 Sastre AM, Kumar A, Shukla JP and Singh RK, Improved techniquesin liquid membrane separations: an overview. Sep Purif Methods27:213298 (1998).

6 Teramoto M, Takeuchi N, Maki T and Matsuyama H, Gas separationby liquid membrane accompanied by permeation of membraneliquid through membrane physical transport. Sep Purif Technol24:101112 (2001).

7 Figoli A, Sager WFC and Mulder MHV, Facilitated oxygen transportin liquid membranes: review and new concepts. J Membr Sci181:97110 (2001).

8 Cussler EL, Membranes which pump. AIChE J 17:13001303 (1971).9 De Gyves J and Rodriguez San Miguel E, Metal ion separations by

supported liquid membranes. Ind Eng Chem Res 38:21822202(1999).

10 Gabelman A and Hwang S-T, Hollow fiber membrane contactors.J Membr Sci 159:61106 (1999).

11 Bringas E, San Roman MF and Ortiz I, Separation and recovery ofanionic by the emulsion pertraction technology. Remediation ofpolluted groundwaterswith Cr(VI). Ind Eng Chem Res 45:42954303(2006).

12 Ho WSW and Poddar TK, Newmembrane technology for removal andrecovery of chromium from waste waters. Environ Prog 20:4452(2001).

13 Ortiz I, San Roman MF, Corvalan SM and Eliceche AM, Modeling andoptimization of an emulsion pertraction process for removal andconcentration of Cr(VI). Ind Eng Chem Res 42:58915899 (2003).

14 Sonawane JV, Pabby AK and Sastre AM, Au(I) extraction by LIX-79/n-heptane using the pseudo-emulsion-based hollow-fiber stripdispersion (PEHFSD) technique. J Membr Sci 300:147155 (2007).

15 Ren Z, Zhang W, Liu Y, Dai Y and Cui C, New liquid membranetechnology for simultaneous extraction and stripping of copper(II)from wastewater. Chem Eng Sci 62:60906101 (2007).

16 Ren Z, Zhang W, Dai Y, Yang Y and Hao Z, Modeling of Effect of pH onmass transfer of copper(II) extraction by hollow fiber renewal liquidmembrane. Ind Eng Chem Res 47:42564262 (2008).

17 Ren Z, Zhang W, Li H and Wei L, Mass transfer characteristics of citricacid extraction by hollow fiber renewal liquidmembrane. Chem EngJ 146:220226 (2009).

18 Baker RW, Membrane Technology and Applications. McGraw-Hill, NewYork (2000).

19 Mulder M, Basic Principles of Membrane Technology. Kluwer AcademicPublishers, The Netherlands (1996).

20 Urtiaga AM, Ortiz I, Salazar E and Irabien JA, Supported liquidmembranes for the separation-concentration of phenol. 1. Viabilityandmass-transfer evaluation. Ind Eng Chem Res 31:877886 (1992).

21 Scholander PF, Oxygen transport through hemoglobin solutions.Science 131:585590 (1960).

22 Franken T, Liquid Membranes academic exercise or industrialseparation process? Membr Technol 85:610 (1997).

23 Kocherginsky NM, Yang Q and Seelam L, Recent advances insupported liquid membrane technology. Sep Purif Technol53:171177 (2007).

24 Danesi PR, Separation of metal species by supported liquidmembranes. Sep Sci Technol 19:857894 (198485).

25 Ortiz I, Wongswan S and de Ortiz ESP, A systematic method for thestudy of the rate-controlling mechanism in liquid membranepermeation processes. Extraction of zinc by Bis(2-Ethylhexyl)phosphoric acids. Ind Eng Chem Res 27:16961701 (1988b).

26 Bringas I, San Roman MF, Irabien JA and Ortiz I, An overview on themathematical modeling of liquid membrane separation processesin hollow fiber contactors. J Chem Technol Biotechnol (Accepted 8May 2009). DOI 10.1002/jctb.2231.

27 Kumar A, Haddad R and Sastre AM, Integrated membrane processfor gold recovery from hydrometallurgical solutions. AIChE J47:328340 (2001).

28 Alonso AI and Pantelides CC, Modeling and simulation of integratedmembrane processes for recovery of Cr(VI) with Aliquat 336.J Membr Sci 110:151167 (1996).

29 Kertesz R, Simo M and Schlosser S, Membrane-based solventextraction and strippingof Phenylalanine in HF contactors. J MembrSci 257:3747 (2005).

30 Gupta SK, Rathore NS, Sonawane JV, Pabby AK, Janardan P,Changrani RD, et al, Dispersion-free solvent extraction of U(VI)in macro amount from nitric acid solutions using hollow fibercontactactor. J Membr Sci 300:131136 (2007).

31 Kumar A, Haddad R, Alguacil FJ and Sastre AM, Comparativeperformance of non-dispersive solvent extraction using a singlemodule and the integrated membrane process with two hollowfiber contactors. J Membr Sci 248:114 (2005).

32 Kemperman AJB, Bargeman D, Van Den Boomgaard Th andStrathmann H, Stability of supported liquid membranes: state ofthe art. Sep Sci Technol 31:27332762 (1996).

33 Yang XJ, Fane AG and Soldenhoff K, Comparison of liquid membraneprocesses for metal separations: permeability, stability, andselectivity. Ind Eng Chem Res 42:392403 (2003).

34 Neplenbroek AM, Bargeman D and Smolders CA, Supported liquidmembranes: instability effects. J Membr Sci 67:121132 (1992).


10

www.soci.org MF San Roman et al.

35 Neplenbroek AM, Bargeman D and Smolders CA, Mechanism ofsupported liquid membrane degradation: emulsion formation.J Membr Sci 67:133148 (1992).

36 Sengupta A., Basu R and Sirkar KK, Separation of solutes fromaqueoussolutions by contained liquid membranes. AIChE J 34:16981708(1988).

37 Bringas E, On the design of selectivemembrane separation processes.Remediation of polluted groundwaters with Cr(VI). Doctor ofChemistry Engineering Dissertation, University of Cantabria, Spain(2008).

38 Li J-L. andChen B-H,ReviewofCO2 absorptionusingchemical solventsin hollow fibermembrane contactors. Sep Purif Technol 41:109122(2005).

39 Krull FF, Fritzmann C and Melin T, Liquid membranes for gas/vapourseparations. J Membr Sci 325:509519 (2008).

40 Bara JE, Carlisle TK, Gabriel CJ, Camper D, Finotello A, Gin DL, et al,Guide to CO2 separations in imidazolium-based room-temperatureionic liquids. Ind Eng Chem Res 48:27392751 (2009).

41 Yan S, Fang M, Zhang W-F, Wang S-Y, Xu Z-K, Luo Z-Y, et al,Experimental study on the separation of CO2 from flue gas usinghollow fiber membrane contactors without wetting. Fuel ProcessTechnol 88:501551 (2007).

42 Li K, Kong J and Tan X, Design of hollow fibre membrane modules forsoluble gas removal. Chem Eng Sci 55:55795588 (2000).

43 Mavroudi M, Kaldis SP and Sakellaropoulos , Reduction of CO2emissions by a membrane contacting process. Fuel 82:21532159(2003).

44 Venkateswaran P and Palanivelu K, Studies on recovery of hexavalentchromium from wastewater by supported liquid membrane

using tri-n-butyl phosphate as carrier. Hydrometal 78:107115(2005).

45 Alonso AI, Irabien A and Ortiz I, Nondispersive extraction of Cr(VI)with Aliquat 336: influence of carrier concentration. Sep Sci Technol31:271282 (1996).

46 Ortiz I, Galan B and Irabien A, Kinetic analysis of the simultaneousnondisepersive extraction and back-extraction of chromium(VI).Ind Eng Chem Res 35:13691377 (1996).

47 Alonso AI, Galan B, Gonzalez M and Ortiz I, Experimental andtheoretical analysis of a nondispersive solvent extraction pilotplant for the removal of Cr(VI) from a galvanic process wastewaters.Ind Eng Chem Res 38:16661675 (1999).

48 HoWSW and Annandale NJ, Supported liquid membrane processfor chromium removal and recovery. United States Patent, #US/6,171,563 B1/ (2001).

49 Miesiac I and Szymanowski J, Pertraction of penicillin G in hollow fibercontained liquid membranes. J Radioanal Nucl Chem 228:7781(1998).

50 Yang C and Cussler EL, Reactive extraction of penicillin g in hollow-fiber and hollow-fiber fabric modules. Biotechnol Bioeng 69:6673(2000).

51 Smith E and Hossain Md.M, Extraction and recovery of penicillin Gin a hollow-fiber membrane contactor. Asia-Pacific J Chem Eng2:455459 (2007).

52 Lazarova Z, Syska B and Schugerl K, Application of large-scale hollowfiber contactors for simultaneous extractive removal and strippingof penicillin G. J Membr Sci 202:151164 (2002).


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