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Journal of Membrane Science 325 (2008) 509–519

Contents lists available at ScienceDirect

Journal of Membrane Science

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m e m s c i

Review article

Liquid membranes for gas/vapor separations

F.F. Krull∗, C. Fritzmann, T. Melin

Aachener Verfahrenstechnik, Chemical Reaction Engineering, RWTH Aachen University, Turmstr. 46, 52056 Aachen, Germany

a r t i c l e i n f o

Article history:

Received 16 July 2008

Received in revised form

10 September 2008

Accepted 11 September 2008

Available online 19 September 2008

Keywords:

Immobilized liquid membrane

Supported liquid membrane

Gas separation

Vapor separation

Preparation

Performance

Materials

a b s t r a c t

A review on developments of liquid membranes (LMs) in the field of gas and vapor separation of the

last 16 years is presented. Liquid membrane configurations employing supports, i.e. immobilized, sup-

portedandcontainedliquidmembranesare focussedand detailedinformationon therespectivematerials,i.e. supports (supplier, type, thickness, pore width, porosity, tortuosity), liquids and carriers, are pre-

sented togetherwith theirspecific separation tasks. Performance of differentLMs in termsof permeability

and selectivity as well as stability (duration of testing, applied differential pressures) are compared and

discussed. Finally, different preparation methods of LMs are illustrated.

© 2008 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509

2. Liquid membrane configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510

3. Mass transfer in liquid membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510

4. Materials for LM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511

4.1. Supports for LM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511

4.2. Liquids and carriers for LM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513

5. Performance of LM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515

6. Preparation of LM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 517

7. Summary of liquid membrane review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518

Abbreviations: BEHA, bis(2-ethylhexyl)amine; CA, carbonic anhydrase; CAN,cel-lulose acetate-nitrate; CLM, contained liquid membrane; CUN, cuprophan; DAE,

diaminoethane; DBC, dibenzo-18-crown-6; DEA, diethanolamine; DETA, diethylen-

etriamine; DEYA, diethylamine; DGA, diglycolamine; DIPA, diisopropanolamine;

EDA, ethylenediamine; ELM, emulsion liquid membrane; FS, flat sheet; HF, hollow

fiber; IL,ionic liquid;ILM, immobilized liquid membrane; LiAlO2, lithium aluminate;

LM,liquidmembrane; MEA,monoethanolamine;MS, molten salt; NA,not available;

PAA, porous anodic alumina; PAMAM, polyamidoamine dendrimer; PAN, poly-

acrylonitrile; PEG, polyethyleneglycol; PES, polyethersulfone; PP, polypropylene;

PS, polysulfone; PTMSP, polytrimethylsilylpropyne; PVDF, polyvinylidinedifluoride;

SLM, supported liquid membrane; TEG, triethyleneglycol.∗ Corresponding author. Tel.: +49 241 8 09 05 83; fax: +49 241 8 09 22 52.

E-mail addresses: [email protected] (F.F. Krull),

[email protected] (C. Fritzmann),

[email protected] (T. Melin).

1. Introduction

For more than 30 years now, liquid membranes have been in

focus of research. Since diffusivities in liquids in comparison to

solids are higher by several orders of magnitude, enhanced per-

meabilities of liquid in comparison to solid membranes can be

expected.

Investigated applications of liquid membranes comprise the

separation/concentration of ions [1,2], the separation of liquid feeds

[3] and the separation of gases or vapors which are subject of the

review at hand.

Several reviews have been written on liquid membrane-based

separations [4–9]. However, to our knowledge only one of these

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510 F.F. Krull et al. / Journal of Membrane Science 325 (2008) 509–519

reviews by Dutta et al. written in 1992 is explicitly and only

concerned with gas and vapor separation by means of liquid mem-

branes [10]. Thus, a review on developments of liquid membranes

forgas andvapor separationscoveringthe last 16 yearsis presented

here.

After presenting possible LM configurations (Section 2), the

mass transfer mechanism in LM will be briefly discussed (Section

3). The core of the review at hand is then given by the literature

review of the last 16 years on LMs employed for gas or vapor sep-

arations. In Section 4, LM configurations and detailed information

on the respective materials, i.e. supports (supplier, type, thickness,

pore width, porosity, tortuosity), liquids and carriers, will be pre-

sented togetherwith theirspecificseparation tasks.In Section 5, the

performance of different LMs in terms of permeability and selec-

tivity as well as stability (duration of testing, applied differential

pressures) will be compared and discussed. Finally, in Section 6,

different preparation methods of LMs will be illustrated.

2. Liquid membrane configurations

Generally, liquid membranes with and without supports can be

differentiated.For thosenot employing supports, the so-called bulkliquid membranes (BLM) and emulsion liquid membranes (ELM)

arefound. The liquid membranesemploying a support can be subdi-

vided into immobilized liquid membranes (ILM), supported liquid

membranes (SLM) and contained liquid membranes (CLM) (Fig. 1).

The simplest form of liquid membrane without support is given

by the BLM consisting of a u-tube and three non-miscible liquids.

BLM are mainly used to study mass transfer from the donor phase

through the membrane phase into the acceptor phase but do not

have any relevancefor large-scale separation processes due to their

large thickness.

In principle, ELM represent a double emulsion consisting of an

acceptor phase being dispersed in a membrane phase and this

emulsion again being dispersed in a donor phase. A species from

the donor phase is absorbed into the membrane phase, diffuses

towards the acceptor phase and finally is desorbed into the latter.

To obtain the permeate, the double emulsion is disintegrated and

the species is extracted from the acceptor phase.

For liquid membranes employing supports, the most compact

form of a LM is given by an ILM where a liquid is held inside the

pores of a porous support (e.g. a porous solid membrane) by means

of capillary forces. The support has to be wettable by the liquid

for this configuration. If the support is not wetted by the liquid, a

SLM can be prepared where a liquid is located on top of the porous

support. The CLM represents a SLM with two porous supports on

both sides. This configuration offers the possibility of replenish-

ing or regenerating the membrane phase during operation. Thus

a breakdown of the membrane function caused by evaporation of

the membrane liquid can be avoided by means of continuous liq-

uid replenishment. As to stability of LM configurations employing

supports, further requirements will be discussed in more detail in

the following sections. In terms of mass transfer, the LM employ-

ing supports work according to the same principles as the ELM,

i.e. a solution-diffusion mechanism, which will be explained in the

following Section 3.

In the literature the terms SLM, ILM and CLM are often mixed

up or assigned to different liquid membraneconfigurations. Mostly,

the term supported liquid membrane is used for the liquid mem-

brane configuration where the membrane liquid is situated within

the support corresponding to an immobilized liquid membrane

according to the classification given in Fig.1. However, regardless of

what an author in theliterature names a configuration andwhether

that naming fits the classification given above, all naming of liq-

uid membrane configurations in the work at hand is done to the

classification given in Fig. 1.

The review at hand exclusively concerns liquid membranes

employing supports used for gas and vapor separation applications.

In the following, the term liquid membrane is used as synonym for

liquid membranes employing supports.

3. Mass transfer in liquid membranes

Liquid membranes work according to a solution-diffusion mass

transfer mechanism as do dense solid membranes. Including the

mass transfer steps in the respective feed and permeate phases, a

gas molecule is transported across the membrane in seven steps:

(1) Convective transport of the molecule towards the membrane.

(2) Diffusion of the molecule through the boundary layer at thefeed–membrane interface.

(3) Absorption into the membrane phase.

(4) Diffusion through the liquid membrane.

(5) Desorption into the permeate phase.

(6) Diffusion of the molecule through the boundary layer at the

permeate–membrane interface.

Fig. 1. Liquid membrane configurations.



F.F. Krull et al. / Journal of Membrane Science 325 (2008) 509–519 511

Fig. 2. Facilitated transport ILM.

(7) Convective transport of the molecule into the permeate

phase.

The actual solution-diffusion mechanism is given by the steps

3–5 only.

Given the assumption of similar diffusivities of two gases in a

liquid, the selectivity of a liquidmembrane is based on the sorption

selectivity between the two gases in the feed phase. In case this

sorption selectivity is very low or lacking, carrier species may be

employed (Fig. 2).

A molecule of the preferred gas is reversibly bound by a car-

rier and transported across the membrane either via diffusion

of the carrier–molecule complex or via a hopping mechanism

of the molecule from one carrier to another [8,11–14]. At the

permeate–membrane interface, the molecule dissociates from the

carrier and is desorbed into the permeate phase. This transport

mechanism is often called facilitated transport.

In the case of facilitated transport, the selectivity of a sep-

aration is mainly influenced by the availability of free carriermolecules. On the one hand, the solubility of carrier molecules

in the membrane phase is limited. On the other hand, the mobil-

ity of the carrier molecules and the formed complexes determine

the number of free carrier molecules at the feed–membrane inter-

face. As long as free carrier molecules are available, the flux

across the membrane increases non-linearly with increasing driv-

ing force across the membrane. From the point where diffusion

of unsaturated carriers towards the feed–membrane interphase

and diffusion of saturated carriers from the feed–membrane inter-

phase become the limiting steps in the solution-diffusion process,

i.e. the chemisorption and diffusion process, the flux increases

due to the seizure of facilitated transport. However, superim-

posed physical diffusion might lead to a linear increase with

increasing driving force. At given transport of undesired speciesby means of physical diffusion, the selectivity of a membrane

also increases non-linearly up to the point of full carrier sat-

uration at the feed–membrane interface. Due to the very low

diffusive flux of undesired species at low concentration differ-

ence across the membrane, selectivity shows maximum values

within the range from zero to full carrier saturation concen-

tration difference across the membrane. From the point of full

carrier saturation at the feed–membrane interphase, selectivity

might either show constant values (ratio of physical diffusion

between desired and undesired species stays constant) or even

decrease independent of the applied gas-concentration differ-

ence across the membrane (enhanced diffusive flux of undesired

species in contrast to slightly or non-enhanced flux of the desired

species).

A different approach to increase or obtain a sorption selectiv-

ity of the membrane liquid is given by the use of homogeneous

catalysts (Fig. 3).

In contrast to a carrier, the catalyst increases the solubility of a

gas inside the membrane liquid due to conversion of the gas to aproduct, which diffuses towards the permeate phase. However, in

this case a back-diffusion of reaction products cannot be excluded

since the partial pressure for the product in the gas phases is very

low at both sides of the membrane.

The combination of the unit operations homogeneous catalysis

and gas separation into one clearly leads to process integration.

To present, homogeneous catalysis inside an ILM has only been

investigated by Carlin et al. [15,16].

4. Materials for LM

As in every membrane process, separation characteristics of the

process are determined by the properties of the membrane mate-

rial. Hence, the materials of LM, i.e. liquids and carriers as well asthe supports are subject of the Sections 4.2 and 4.1, respectively.

4.1. Supports for LM

As shown in Section 3, the liquid and carriers are responsible

for LM properties in terms of permeability and selectivity, while

the choice of the support merely affects the permeability by its

porosity. However, the right choice of support ensures sufficient

stability of the LM configuration.

In Tables 1 and 2 the separation tasks, configurations and

employed supports reported in the literature of the last 16 years

are given chronologically, starting with the most recently reported

configuration.

As can be seen from Tables 1 and 2, the vast majority of sup-ports are flat sheet polymeric supports. Only some authors employ

polymeric hollow fiber supports [22,24,33,35] andHuanget al. and

Baltus et al. employ a ceramic support [17,25].

For preparation of an ILM, the support must be wettable by the

membrane liquid whereas it should be non-wettable for prepa-

ration of an SLM. Further, the support should be chemically and

thermallyinert to avoida mechanical breakdown of the membrane

function. Wettability is ensured if hydrophobic supports are com-

bined with organic liquids or hydrophilic liquids with hydrophilic

supports. The latter represents the majority in the reported config-

urations.

The support mainly influences the mechanical and long-term

stability of a LM configuration. According to the so-called bubble

point equation, which is derived from a thermodynamical point




Fig. 3. ILM with simultaneous gas or vapor separation and catalytic reaction.

Table 1

Separation tasks, configurations and supports of liquid membranes.

Separation task Configuration Support Ref.

C6H12/C6 H14, C5H10/C5H12, C5H8/C5H12 ILM PAA [17]

CO2/CH4 ILM (SLM) PTFE support for ILM which is sandwiched between two

hydrophobic PTFE supports

[18]

CO2/He ILM PES, PS [19]SO2/N2, SO2/CH4, SO2/CO2 ILM Hydrophilic PES [20]

H2O vapor/air/VOC ILM (SLM) Hydrophilic CA, hydrophobic PVDF [21]

CO2/air CLM PP [22]

H2, CO, O2 , N2 ILM NA [23]

CO2/air CLM PP [24]

CO2, N2 ILM PAA [25]

CO2/CH4 ILM Hydrophilic PVDF [26]

CO2/N2 and CO2/CH4 ILM PES [27]

CO2/CH4/H2, water vapor/air, benzene vapor/cyclohexane vapor ILM Hydrophilic PTFE (ILM) supported on hydrophobic PVDF

support

[28]

C3H6/C3H8 ILM Hydrophilic PTFE (ILM) supported on hydrophobic PVDF

support

[29]

C4H8/C4H10 ILM Hydrophilized PVDF [30]

CO2/N2 ILM Hydrophilized PVDF, PP [31]

H2S/CO2, H2S/CH4 ILM PP [32]

CO2/N2 ILM Hydrophilic Supports: PAN, CUN, hydrophilized PS fibers [33]

CO2/CH4 ILM Hydrophilic PTFE support (ILM) supported on PVDF support [34]

Table 2

Separation tasks, configurations and supports of liquid membranes continued.

Separation task Configuration Support Ref.

CO2/N2, CO2/O2 ILM Hydrophilized PVDF,

hydrophilized PP, PAN

fibers, hydrophilized PS

fibers

[35]

Water vapor/air ILM Hydrophili c PTFE support

(ILM) supported on PVDFsupport

[36]

SO2/air ILM Porous LiAlO2 [37]

CO2/N2 ILM Hydrophilic PVDF,

hydrophilized PP

[38]

CO2/air ILM Hydrophilic CAN [39]

CO2/N2 ILM Hydrophilic PVDF [40]

SO2/N2 ILM Hydrophilic PVDF [41]

CO2/N2 ILM Hydrophilic CAN [42]

CO2/CH4 ILM Hydrophilic PVDF [43]

CO2/CH4/H2 I LM/S LM P P (I LM)/ PTMS P (S LM) [44]

CO2/C2H6 ILM PP [45]

SO2/N2 ILM NA [46]

CO2/CH4 ILM PP [47]

O2/N2 (air) ILM Stainless steel woven wire

mesh

[48]

NH3/N2, NH3/H2 ILM Stainless steel woven wire

mesh, woven zirconia cloth

[49]

of view of expulsion of liquid from a capillary (Young-Laplace-

Equation)

p =4kp cos( )

dp, (1)

a small contact angle between support and liquid (ILM configu-

ration) and a small pore diameter dp of the support ensure higher

achievable differential pressures p across the membrane [50,51].

With increasing interfacial tension between liquid and gas phase

higher differential pressures are possible. The factor kp denotes

deviation of the experimental pressure from the theoretical bubblepoint, i.e. 0 < kp ≤ 1.

In Tables 3–6 , details on the supports of the reported configu-

ration in the literature are presented.

The thickness of the liquid membrane most often corresponds

to the thickness of the support and lies between 25 and 380 m.

Exceptions to this are given by the configurations of Gan et al. [23].

Anadditional layer of ionic liquidis placedon top of thefully wetted

support to avoid gas permeance through the solid polymeric sup-

port. Thisexampleshows thatthe gas permeability of the polymeric

support itself must be negligible if liquid membrane permeability

is to be determined and highly selective membrane configurations

are to be achieved.

Chen et al. investigated the effect of liquid membrane thickness

on permeability [38]. Details on this will be given in Section 6.




Table 3

Supports for liquid membranes.

Support material Supplier/type Shape Thickness [m] Ref.

PAA Whatman FS 60 [17]

PTFE support for ILM which is

sandwiched between two

hydrophobic PTFE supports

NA FS 35 [18]

PS, PES Pall, HT Tuffryn (PS), Supor (PES) FS 152 (PS), 145 (PES) [19]

Hydrophilic PES NA FS 150 [20]Hydrophilic CA, Hydrophobic PVDF NA FS 52 (CA), 45 (PVDF) [21]

PP Membrana/Hollow fiber HF NA [22]

membrane mat, Celgard/X40 - 200

NA Sterlitech Corp., YMHLSP1905 Polymer FS 167+ 118 [23]

PP Membrana/Hollow fiber FS 250 [24]

membrane mat, Celgard/X30 - 240

PAA Whatman FS 60 [25]

Hydrophilic PVDF Millipore/Durapore FS 100 [26]

PES Pall FS 152 [27]

Hydrophilic PTFE (ILM) supported

on hydrophobic PVDF support

PTFE: NA, PVDF: Millipore/Durapel

hydrophobic

FS 35–55 [28]

Hydrophilic PTFE (ILM) supported

on hydrophobic PVDF support

PTFE: Toyo Roshi Kaisha Ltd. Japan, PVDF:

Millipore/Durapel hydrophobic

FS 45–70 [29]

Hydrophilized PVDF NA FS 100 [30]

Hydrophilized PVDF, PP PVDF: Millipore, PP: Celgard/2500 FS PVDF: 100, PP: 25 [31]

PP Celgard/3401 FS 25 [32]

Yamanouchi et al. [28] and Duan et al. [29] report about dif-

ferent membrane configurations employing the same support for

different separation tasks. Hence a range of thicknesses is given in

Table 3.

The pore width of the supports lies between 0.005 and 13m.

Porosity of the supports ranges from 0.4 and 0.83 while tortuosity

– if determined – ranges from 1.0 to 3.05.

4.2. Liquids and carriers for LM

Tables 7 and 8 show the different liquids and carriers employed

in the LM configurations of the last 16 years.

In gas separations, the long-term stability of a LM configu-

ration is mainly dependent on the volatility of the membrane

liquid. Hence, low to non-volatile liquids such as glycerol or tri-

ethylene glycol (TEG) are most suitable to avoid a breakdown of

the membrane function due to evaporation. Ionic liquids (ILs) or

liquid molten salts (MS) do show even lower to non-measurable

vapor pressures. While molten salts have already been used about

16 years ago [48], ILs have gained importance in the last 6 years

[23,25,27].

In case a volatile liquid like water is used as membrane phase,

feed and sweep gas humidification is inevitable to avoid the loss of

the membrane function although according to

pv

p0v

= e−M/RTdp (2)

the vapor pressure pv inside a porous support is lower in compari-

son to the normal vapor pressure p0v for a smaller pore diameter dp

of an employed support ([52], p. 58). The terms M and represent

Table 4

Supports for liquid membranes continued.

Support material Supplier/type Shape Thickness [m] Ref.

Hydrophilic supports: PAN, CUN, NA HF PAN: 50, CUN: 10, [33]

Hydrophilized PS fibers PS: 40 and 275

Hydrophilic PTFE support PTFE: Toyo Roshi Kaisha Ltd.

Japan

FS 25 [34]

(ILM) supported on PVDF support PVDF Millipore/Durapel

hydrophobic

Hydrophilized PVDF PVDF Millipore FS, HF PVDF: 100, Celgard: 25, [35]

Hydrophilized PP, PAN fibers PP: Celgard/2500 PAN: 50 MWCO, PS: 40

Hydrophilized PS fibers PAN: Sepracor, PS: Minntech

Hydrophilic PTFE support (ILM) supported on PVDF support PTFE: Toyo Roshi Kaisha Ltd. Japan, PVDF Millipore/Durapel

hydrophobic

FS 35 [36]

Porous LiAlO2 NA FS NA [37]

Hydrophilic PVDF, hydrophilized PP PVDF: Millipore, PP:

Celgard/2500

FS PVDF: 100, PP: 25 [38]

Hydrophilic CAN Millipore/Type AA-WP FS 150 [39]

Hydrophilic PVDF Millipore FS 100 [40]

Hydrophilic PVDF Millipore/Durapore VVLP FS 100 [41]

Hydrophilic CAN Millipore/AA WP Type FS 150 [42]

Hydrophilic PVDF Millipore/Durapore VVLP FS 100 [43]

PP (ILM)/PTMSP (SLM) PP: Celgard/3401, PTMSP: NA FS PP: 25, PTMSP: NA [44]


NA NA FS NA [46]


Stainless steel woven wire mesh Pall FS 200 [48]

Stainless steel woven wire mesh Steel mesh: Pall, Zirconia FS Wire mesh: 200, [49]

Woven zirconia cloth cloth: Zircar Ceramics Ceramic cloth: 380




Table 5


Support material Pore width [m] Porosity Tortuosity Ref.

PAA 0.1 NA 1 [17]

PTFE support for ILM which is sandwiched

between two hydrophobic PTFE supports

0.1 0.71 NA [18]

PES, PS 0.2 0.75–0.85 NA [19]

Hydrophilic PES 0.22 0.8 NA [20]

Hydrophilic CA, hydrophobic PVDF 0.22 (CA), 0.15 (PVDF) NA NA [21]PP 0.04 0.4 NA [22]

NA NA NA NA [23]

PP 0.04 0.4 NA [24]

PAA 0.02 NA 1.0 [25]

Hydrophilic PVDF 0.10 NA NA [26]

PES 0.20 0.8 NA [27]

Hydrophilic PTFE (ILM) supported PVDF

support on hydrophobic

1.00 0.83 NA [28]

Hydrophilic PTFE (ILM) supported on

hydrophobic PVDF support

1.00 0.83 NA [29]

Hydrophilized PVDF 0.10 0.7 2.58 [30]

Hydrophilized PVDF, PP PVDF: 0.10 PVDF: 0.7, PP: 0.45 PVDF: 2.58, PP: 2.54 [31]

PP NA 0.5 1.25 [32]

Hydrophilic supports: PAN, PAN: NA, PAN: NA, NA [33]

CUN, hydrophilized PS fibers CUN: 0.005, CUN: 0.6–0.7,

PS: 0.10 and 0.20 PS: 0.3–0.4 and 0.7–0.8

Table 6


Support material Pore width [m] Porosity Tortuosity Ref.

Hydrophilic PTFE support (ILM) supported on PVDF support 1.00 0.83 NA [34]

Hydrophilized PVDF PVDF: 0.10, PVDF: 0.7, PP: 0.45, PVDF: 2.58, PP: 2.54, [35]

Hydrophilized PP, PAN fibers Celgard: 0.08, PAN: NA, PS: 0.3–0.4 PAN: NA, PS: NA

Hydrophilized PS fibers PAN: 70000 MWCO, PS: 0.70

Hydrophilic PTFE support (ILM) supported on PVDF support 1.00 0.83 NA [36]

Porous LiAlO2 NA NA NA [37]

Hydrophilic PVDF PVDF: 0.10, PVDF: 0.7, PVDF: 2.58, [38]

Hydrophilized PP PP: 0.04 PP: 0.45 PP: 2.62

Hydrophilic CAN 0.80 0.82 3.05 [39]

Hydrophilic PVDF 0.10 0.7 2.58 [40]

Hydrophilic PVDF NA 0.63 2.61 [41]

Hydrophilic CAN 0.80 NA 3.05 [42]

Hydrophilic PVDF 0.10 0.7 3.23–3.35 [43]PP (ILM)/PTMSP (SLM) Celgard: 0.05× 0.125, PTMSP: NA PP: 0.5, PTMSP: NA PP: 1.25, PTMSP: NA [44]

PP 0.075× 0.25 0.45 1.75 [45]

NA NA NA NA [46]

PP 0.04 0.45 2.1 [47]

Stainless steel woven wire mesh 4.0–13.0 NA NA [48]

Stainless steel woven wire mesh, woven zirconia cloth 4.0–13.0 NA NA [49]

Table 7

Separation tasks, liquids and carriersof liquid membranes; brackets in carrier column indicate thatauthorsreporton experimental results obtained with pure liquids as well

as with liquid–carrier mixtures.

Separation task Liquid Carrier Ref.

C6H12/C6 H14, C5H10/C5H12, [Ag][(1-hexene)Tf2N], [Ag][(1-pentene)Tf2N], [Ag][(1-isoprene)Tf2N], Functionalized liquid [17]

C5H8/C5H12 [Ag][(DMBA)2Tf 2N], AgNO3 in [BMI][Tf2N], [Ag][PrNH2)2Tf2N]

CO2/CH4 [C3NH2mim][CF3SO3], [C3NH2mim][Tf2N], [C4mim][Tf2N] Functionalized liquid,

except [C4mim][Tf2N]

- no carrier

[18]

CO2/He [hmim][Tf2N] None [19]

SO2/N2, SO2/CH4, SO2/CO2 [emim][BF4], [bmim][BF4], [hmim][BF4], [bmim][PF6], [bmim][Tf2N] None [20]

H2O vapor/air/VOC LiCl ·H2O None [21]

CO2/air H2O CA, DEA, NaHCO3 [22]

H2, CO, O2 , N2 ILs: [C4-mim][Tf2N], [C10-mim][Tf2N],[N881][Tf2N], [C8Py][Tf2N] None [23]

CO2/air H2O DEA [24]

CO2, N2 ILs: [C4-mim][Tf2N], [C8F13-mim][Tf2N] None [25]

CO2/CH4 H2O DETA,DAE, DEYA, BEHA [26]

CO2/N2 and CO2/CH4 ILs: [C2-mim][Tf2N], [C2-mim][CF3SO3],[C2-mim][dca], [thtdp][Cl] None [27]

CO2/CH4/H2, water vapor/air, benzene

vapor/cyclohexane vapor

IL [pmim][I] (K2CO3) [28]

C3H6/C3H8 TEG AgBF4 or AgNO3 [29]

C4H8/C4H10 Glycerol AgNO3 [30]

CO2/N2 Glycerol carbonate PAMAM, sodium

glycinate

[31]

H2S/CO2, H2S/CH4 Salt hydrate tetramethyl-ammonium fluoride tetrahydrate [(CH3 )4N]F ·4H2 O Functionalized liquid [32]




Table 8

Separation tasks, liquids and carriers of liquid membranes continued; brackets in carrier column indicate that authors report on experimental results obtained with pure

liquids as well as with liquid–carrier mixtures.

Separation task Liquid Carrier Ref.

CO2/N2 Glycerol Sodium carbonate, sodium glycinate [33]

CO2/CH4 TEG, DGA (KHCO3), (DEA) [34]

CO2/N2 , CO2/O2 PAMAM (Glycerol) [35]

Water vapor/air TEG, PEG None [36]

SO2/air MS: 0.78 Li2 SO4, 0.135 K2SO4 and 0.085 Na2SO4 None [37]CO2/N2 Glycerol Glycine-Na, Glycine-Na2CO3, EDA [38]

CO2/air H2O PEG, DBC, K2CO3 [39]

CO2/N2 Glycerol Na2 CO3 [40]

SO2/N2 H2O None [41]

CO2/N2 H2O K2CO3/KHCO3 [42]

CO2/CH4 H2O DEA, MEA [43]

CO2/CH4 /H2 MS hydrates: tetramethylammmoniu m

fluoride tetrahydride, tetraethylammonium

acetate tetrahydrate

Functionalized liquid [44]

CO2/C2H6 PEG DEA [45]

SO2/N2 PEG DEA [46]

CO2/CH4 PEG-400 DEA, DIPA [47]

O2/N2 (air) MS: anhydrous LiNO3, dry NaNO3 Functionalized liquid [48]

NH3/N2, NH3 /H2 MS: LiNO3 and ZnCl2 Functionalized liquid [49]

the molar mass and the density of the liquid, while the terms R andT denote the universal gas constant and the absolute temperature

in Kelvin, respectively.

In combination with glycerol, feed gas humidification also

decreases the viscosity of the membrane phase since the viscos-

ity of glycerol is highly dependent on the water content of a

glycerol–water mixture [53,54]. Thus, the diffusivity of dissolved

species in glycerol is enhanced resulting in higher permeabili-

ties.

Due to the difficulty in finding liquids displaying a sorption-

selectivity for different gases, carrier species are often employed,

which should be dissolvable in the liquid to a high extent (cf. e.g.

[24,29]). Thehigherthe solubility of a carrier species, thehigher the

allowable feed partial pressure of a desired permeant before satu-

ration in carrier species takes place. In the past 30 years, several

review articles on facilitated transport concepts in liquid mem-branes by the use of carriers have been written, e.g. [7,9].

In comparison to conventional liquids (organic or aqueous mix-

tures) liquid molten salts offer the possibility of an intrinsic carrier

function, i.e. one of the salt ions acts like a carrier [32,44,49]. In the

following, these liquids are referred to as functionalized liquids.

5. Performance of LM

In Tables 9–12 the different reported gas and vapor separation

tasks and the performance of the employed liquid membrane con-

figurations in terms of permeability, selectivity, pressure stability

and long-term stability are given.

The most encountered separation task is the removal of CO2

from gas streams containing CH4

or air. About 2/3 of the reviewed

Table 9

Performance of liquid membranes.

Separation task Selectivity Flux/permeability Flux/permeability unit Ref.

C6 H12/C6H14, C5H10/C5H12,

C5H8 /C5 H12

C6 H12/C6H14: 1.16–531,

C5 H10/C5H12: 0.97–546,

C5 H8/C5H12: 18.9–795

C6H12: 1.64–123.27, C5H10:

6.03–137.14, C5 H8: 15.66–209.58

GPU

(1E−6 cm3 (STP)cm/cm2 s cm

HG)

[17]

CO2/CH4 CO2/CH4: 10–120 CO2 : 500–2500 Barrer [18]

CO2/He CO2/He: 8.7–3.1 CO2 : 744–1200 Barrer [19]

SO2/N2 , SO2/CH4, SO2/CO2 CO2/N2 : 126–223, SO2 /CH4:

87–128, SO2/CO2: 9–19

SO2 : 7280–9350 Barrer [20]

H2O vapor/air/VOC NA H2O vapor: 1.14E−4 kg/m2 s [21]

CO2/air CO2/air: 152–234 CO2 : 1 × 10−8to5× 10−8 mol/(m2 s Pa) [22]

H2, CO, O2, N2 H2/CO max 4.3 H2: 15–1250, CO: 0–1450 Barrer [23]

CO2/air CO2/N2 : 90–442, CO2/O2: 68–270 CO2 : 1.25× 10−8to5.01× 10−8 mol/(m2 s Pa) [24]

CO2, N2 CO2/N2 : 72–127 N2: 2.1 × 10−11

to3.2 × 10−11

, CO2:1.5 × 10−9to4.0 × 10−9 mol/(barcm

2

s) [25]

CO2/CH4 CO2/CH4: 20–1000 CO2 : 2 × 10−7to1.7 × 10−6, CH4:

2 × 10−9to9× 10−9

mol/(m2) s kPa) [26]

CO2/N2 and CO2 /CH4 CO2/N2 : 15–61 CO2/CH4: 4–20 N2: 10–48, CO2: 350–1000, CH4 :

31–94

Barrer [27]

CO2/CH4 /H2, water vapor/air,

benzene vapor/cyclohexane

vapor

CO2/CH4: 20, CO2/H2: 13, water/air:

1000, benzene/cyclohexane: 20

CO2 : 3 × 10−8 cm3(STP)cm/(cm2 scmHG) [28]

C3 H6/C3H8 C3 H6/C3H8: ca. 25–70 C3H6: 8 × 10−9to1× 10−7 cm3 cm/(cm2 scmHg) [29]

C4 H8/C4H10 C4 H8/C4H10: as high as 850 C4H8: 260 Barrer [30]

CO2/N2 CO2/N2 : 90–130 without carrier,

4.5–1000 with carrier

CO2 : 80–380 without carrier,

34–6600 with carrier

Barrer [31]

H2S/CO2, H2S/CH4 H2S/CO2: 6–8, H2S/CH4: 34–140 H2S: 192–813, CO2: 30–109, CH4:

4.3–7.4

Barrer [32]

CO2/N2 CO2/N2 : 7.8–5830 CO2 : 2.3 × 10−9to3.13× 10−5 cm3/(cm2 scmHg) [33]

CO2/CH4 DGA CO2 /CH4: 100, TEG CO2/CH4:

30

CO2 : DGA 1 × 10−8, TEG 1.5 × 10−8 cm3(STP)cm/(cm2 scmHg) [34]




Table 10

Performance of liquid membranes continued.

Separation task Selectivity Flux/permeability Flux/permeability unit Ref.

CO2/N2, CO2/O2 CO2/N2: 760–16,300, CO2 /O2:

920–2300

CO2 (N2 mix): 300–3200, CO2

(O2 -mix): 2000–4400

Barrer [35]

Water vapor/air Water vapor/air: 2000 1× 10−6 cm3(STP)cm/(cm2 scmHg) [36]

SO2/air NA SO2: 1 × 10−7to3.5 × 10−7 kmol/m2 s [37]

CO2/N2 CO2/N2: 29–6990 CO2: 0.64× 10−5to3.94× 10−5 scm3/(cm2 scmHg) [38]

CO2/air NA CO2: 2 × 10−6to12× 10−6 cm3/(cm2 scmHg) [39]CO2/N2 CO2/N2: 100–3440 CO2: 2.5× 10−6to30.0 × 10−6 cm3/(cm2 scmHg) [40]

SO2/N2 NA SO2: 1 × 10−4to6× 10−4 mol/(m2 skPa) [41]

CO2/N2 NA CO2: 2.8× 10−9to6.5 × 10−9 mol/(cm2 s) [42]

CO2/CH4 MEA - CO2 /CH4: 100–2000, DEA -

CO2/CH4: 100–2000

MEA - CO2 : 5 × 10−6 to6× 10−5,

MEA - CH4: 2 × 10−8to9× 10−8,

DEA - CO2: 6 × 10−6to7× 10−5,

DEA - CH4: 3 × 10−8to1× 10−7

cm3/(cm2 scmHg) [43]

CO2/CH4/ ILM CO2 /H2: 9.1–120, ILM - CO2: 100–1720 ILM: barrer [44]

H2 CO2/CH4: 2–10, SLM CO2/H2 :

30–360, CO2/CH4: 140–800

SLM - CO2 : no permeability given

due to unknown thickness of active

membrane

CO2/C2H6 CO2/C2 H6: 5–145 CO2: 2.0× 10−12to7.8× 10−12 m3 m/(m2 s kPa) [45]

SO2/N2 Pure PEG: SO2/N2: 113–143,

DEA-PEG SO2/N2: 25–70

SO2: 1.9× 10−11to− 3.1 × 10−11 m3 m/(m2 s kPa) [46]

CO2/CH4 DEA-PEG membrane CO2/CH4:

1.5–43, DIPA-PEG membrane

CO2/CH4: 1.5–30

DEA-PEG membrane CO2: 360

-11,000, DIPA-PEG membrane CO2:

350–8700

Barrer [47]

O2/ NaNO3 membrane: 4–80 NaNO3 membrane - O2: 58–1110 Barrer [48]

N2 (air) LiNO3 membrane: 11–320 LiNO3 membrane: 240–7700

NH3/N2 LiNO3 membrane NH3/N2: 80–245 LiNO3 membrane: 6500–9900 NH3 Barrer [49]

NH3/H2 ZnCl2 membrane (wire mesh)

NH3 /N2: at least 1000 NH3/H2 at

least 3000,

ZnCl2 membrane (wire mesh) NH3:

10× 105,

ZnCl2 membrane (zirconia cloth) ZNCl2 membrane (zirconia cloth)

NH3 /N2: 1400 NH3: 2.2 × 105

articles report on CO2 separations, stressing the industrial rele-

vance of thisseparation task.Other separation tasksare given by the

removal of SO2, H2SandNH3. Water vapor transportis investigated

byIto [36] and Yamanouchi et al. [28] who also reporton separation

of benzene vapor from cyclohexane vapor. Finally, alkene/alkane

separations are examined by different authors [29,30]. This sepa-ration task as well as SO2removal are of major industrial relevance.

Since generally only very few reports and no review article on

membrane-based SO2removal are available, the number of five

reports cited in the review at hand stands out.

Permeability is reported in different units. The conversion of

these units proves to be problematic due to the fact that not all

membrane thicknesses and partial pressure differentials are avail-

able. Hence, the original units found in the respective literature are

reported.

In his review on LM for gas separations Dutta claimed that LMperformance is still not comparable to polymeric membrane per-

formance [10]. Also nowadays, this issue is worth to be discussed:

Many authors claim their configurations as comparable to poly-

meric membranes in terms of selectivity and permeability. Given

Table 11


Separation task Differential pressure Long-term stability Ref.

C6H12/C6 H14, C5H10/C5H12, C5H8/C5H12 112 kPa NA [17]

CO2/CH4 0 More than 260 days [18]

CO2/He 15.7 psi Up to 125 ◦C [19]

SO2/N2, SO2/CH4, Atmospheric NA [20]

SO2/CO2

H2O vapor/air/VOC Atmospheric NA [21]CO2/air Atmospheric 50 days [22]

H2, CO, O2 , N2 7 bar Stable up to 10 bar during experiments [23]

CO2/air Atmospheric 5 days [24]

CO2, N2 17 psia upstream, 12 psia

downstream

NA [25]

CO2/CH4 160 kPa feed, 135 kPa Sweep -

0,25bar ptrans

NA [26]

CO2/N2 and CO2/CH4 0–20 kPa NA [27]

CO2/CH4/H2, water vapor/air, benzene vapor/cyclohexane vapor Atmospheric feed, 0.1–1.3kPa

vacuum permeate

NA [28]

C3H6/C3H8 Atmospheric feed, 1.3kPa vacuum

permeate

2–3 weeks [29]

C4H8/C4H10 2 atm 3 weeks [30]

CO2/N2 6 psi More than a week [31]

H2S/CO2, H2S/CH4 None NA [32]

CO2/N2 35.8–52.4 kPa Up to 300 h at 1.27 atm 40m

glycine-Na-glycerol ILM

[33]




Table 12


Separation task Differential pressure Long-term stability Ref.

CO2/CH4 186 kPa feed, 1.3 kPa vacuum At least 160 h up to 250 kPa [34]

CO2/N2 , CO2/O2 PVDF: 0.68atm, PP: 1.7atm, hollow fibers: 1.8atm NA [35]

Water vapor/air Atmosperic feed, vacuum <0.2 kPa permeate NA [36]

SO2/air Atmospheric feed and sweep NA [37]

CO2/N2 2.36 atm feed to 1 atm sweep Up to 25 days, humidified feed, dry sweep [38]

CO2/air 1 atm feed to 664 Pa sweep NA [39]CO2/N2 2.58 atm feed to 1 atm sweep Stable up to 10 days, humidified feed, dry

sweep, 1.5atm retentate pressure

[40]

SO2/N2 Atmospheric Stable during experiments [41]

CO2/N2 Atmospheric feed /560 m mHg sweep Stable during experiments [42]

CO2/CH4 Atmospheric NA [43]

CO2/CH4 /H2 400 cmHg ILM, 545 cmHg SLM NA [44]

CO2/C2H6 Atmospheric NA [45]

SO2/N2 Atmospheric NA [46]

CO2/CH4 1 atm NA [47]

O2/N2 (air) 76 cmHg LiNO3 membranes: 10–42h, NaNO3

membranes: at least 18 h

[48]

NH3/N2, NH3 /H2 80 cmHg ZnCl2: 21 days [49]

the fact that state of the art polymeric membrane selectivity for

CO2ranges between 5 and 45 with a permeability of up to 600

barrer [55], various of the reported LM configurations prove to be

comparable. For example Hanioka et al. report permeabilities of

500–2500 barrer with selectivities of 10–120 for the separation of

CO2andCH4even exceeding the standardsof polymericmembranes

[18].

For separation of propylene/propane mixtures a similar result

is achieved comparing LM configurations to data given in a review

article by Burns and Koros [56]. Selectivity of polymer membranes

for this task ranges from 1.4 to 27 with permeabilities of up to

6600barrer. Duan et al. report on permeabilities of 80–1000 barrer

with selectivities of 25–70 for this separation task [29].

However, two aspects need to be considered when comparing

theconfigurations cited beforehand.On theone hand, in most caseshigh selectivity values of a membranemost often go along with low

permeability values. Thus,both values have to be taken intoaccount

when comparing two specific membrane configurations. On the

other hand, the unit Barrer represents permeabilityof a membrane

with respect toits thickness. While fora thickmembraneits perme-

ability given in Barrer might be high, its absolute flux might be low.

Thus, a comparison of absolute flux values across two membranes

should be the comparison of choice. A detailed comparison of flux

values of different liquid as well as solid membrane configurations

lies beyond the scope of the work at hand. A comparison of abso-

lute fluxes between polymeric and liquid membranes even proves

to be impossible since also for polymeric membranes information

on membrane thicknesses is lacking in the review article of Burns

et al. and Powell et al. However, since in most configurations, LMthickness still is limited to the thickness of the employed supports,

a minimization of LM thickness going along with a maximization

of flux remains a desirable aim.

Most of the reported liquid membrane configurations work at

atmospheric conditions or slightly elevated pressures. The highest

reported differential pressure is 7 bar [23].

Data concerning the long-term stability of the membrane con-

figurations are only given by some authors. Here the most stable

configuration shows long-term performance for about 260 days

[18].

Although, regarding permeability and selectivity, LM perfor-

mance seems to have improved very much in the last 16 years,

their long-term stability still constitutes the major challenge when

aiming at industrial application.

6. Preparation of LM

After the materials for preparation of a liquid membrane con-

figuration have been chosen, the preparation itself is carried out. In

general, an ex situ and in situ preparation of LM can be differenti-

ated.

In the ex situ preparation, a support is wetted with the mem-

brane liquid by soaking or impregnation. Depending on the pore

size and the wettability of the support, the time for prepara-

tion differs between hours or even days. After successful wetting

of the support, excess liquid is normally wiped off to obtain a

thin membrane. Generally the membrane thickness corresponds

to the support thickness, since the support is totally wetted. This

method represents by far the most applied preparation method

for LM.Chen et al. also apply the ex situ preparation method, but report

on a wicking method to partially wet a flat support with a mem-

brane liquid and thus adjust the liquid layer thickness [38]. A flat

support is put into contact with glycerol as a membrane liquid,

which is intruding into the pores of the support due to capillary

forces. Depending on the wetting time,a differentliquid layer thick-

ness is achieved.

In general, the use of feed gas humidification somehow impairs

the effect of a lower membrane thickness, since water vapor

might condense inside open pore spaces of the partially wetted

support representing a diffusion resistance for permeating gas

species.

In the in situ membrane preparation, the porous support is

mounted to a membrane module and contacted with membraneliquid inside the module. Again, the liquid wets the support due

to capillary forces. After some time of impregnation, the liquid

is expelled out of the lumen of the membrane module via a gas

stream. Such membrane preparation is reported by Chen et al. [33]

for preparation of hollow fiberliquidmembraneconfigurations. Pez

et al. also report about an in situ preparation of LM [48] employing

liquidmolten salts as membrane phase,which areliquid at temper-

atures well above room temperature. To prepare these membranes,

salt granules are placed on top of a wire mesh or a zirconia cloth

inside a membrane module. The closed module is then heated up

above the melting temperature of the salt granules, which become

liquidand wetthe pores of thesupport forming a liquidmembrane.

Since some of the employed salts are oxygen and water sensitive,

the whole equipment is inertised with argon gas.




Besides the preparation of single layer liquid membranes, Ito et

al. report on double layer liquid membranes [28,29,34]. One of the

two supports is wetted with the membrane liquid. This ILM is then

placed on a non-wettable support with smaller pore size to obtain

a more stable configuration.

7. Summary of liquid membrane review

The following summary and conclusions can be drawn from the

literature review:

• The vastmajorityof liquid membraneconfigurationsare flatsheet

configurations, employing polymeric supports, which are manu-

ally prepared via ex situ methods. Exceptions are given by liquid

membranes employing hollow fiber supports being prepared in

situ [33].• In general, the liquid membrane thickness corresponds to the

support thickness. Application of inorganic supports is lacking

due to their thickness greater than 200m, being too high for

liquid membrane thickness.• Although the use of non-volatile liquids such as liquid molten

salts or ionic liquids promises improved membrane stability,these liquids are still in the minority. In case of ionic liquids this

might be related to the water-sensitivity of some ILs and in case

of liquidmolten salts to their oxygen sensitivity as well as to their

elevated melting temperature.• The main gas separation task is the separation of CO 2from gas

mixtures. Other tasks reported are the separation of alkenes and

alkanes, ammonia,hydrogen sulfide, oxygen andsulfur oxide. Gan

et al. also report on the separation of hydrogen and problems on

choosing a proper polymeric support showing no permeability

for hydrogen [23].• The performance of LM in terms of permeability and selectiv-

ity of some reported configurations seems to be comparable to

polymeric membranes. However, the pressure and long-termsta-

bility of LM is still low and represents the major challenge forfuture developments when aiming at industrial application. Rea-

sonsfor LM instability couldbe a thermal or chemicaldegradation

or swelling (increased pore size) of the membrane support. Fur-

ther,a changein interfacial tension of themembrane liquiddue to

traces of surfactants within thecontacting gases could be respon-

sible.

Acknowledgements

Financial funding from the German Research Foundation

(Deutsche Forschungsgemeinschaft), grant ME 1714/9-1 is grate-

fully acknowledged. Further, we would like to thank Mr. Sachin

Uphadyay and Mr. Heiner Giese for their contributions to prepa-

ration of the review at hand.

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