[membrane science and technology] membrane contactors: fundamentals, applications and potentialities...

Chapter 9. Supported liquid membranes

1. Introduction

In this Chapter the potentialities, as well as the drawbacks, of supported liquid membranes

are discussed. The different types of facilitated transport that can be established within these

systems are described. Concerning the analysis of the mass transport, the mass transfer

resistances, the concentration profiles and the mass transport equations for both carrier-free

and carried-charged membranes are reported. The above equations are derived for the

supported liquid membrane most investigated in literature: an hydrophobic membrane with an

organic phase immobilized into its micropores for the treatment of aqueous solutions.

However, the analysis made is general and can be extended to the other supported liquid

membrane configurations. A section devoted to the research efforts made worldwide for

improving the supported liquid membrane stability is provided at the end of the Chapter.

2. Facilitated transport

The transport of a species in a supported liquid membrane can occur by simple permeation

through the liquid immobilized into the micropores or by a facilitated transport.

Supported Liquid Membranes 309

In the former case the species is transferred by a solution-diffusion mechanism and the

affinity between the liquid phase and the species determines the selectivity of the process

(see Figure 1).

Figure 1. Transport of i through the immobilized liquid by a solution-diffusion mechanism.

The facilitated transport occurs when the species that diffuses through the liquid is

subjected to a reversible chemical reaction (the so-called complexation). Usually, this type of

transport is obtained thanks to a carrier dissolved in the liquid that reversibly reacts with the

solubilized species and that facilitate its transfers through the membrane as a complex. The

permeation as a complex takes place in parallel with the solution-diffusion of the species into

the liquid phase and is usually higher than the simple permeation (see Figure 2) [ 1,2]. In order

to compare the transport of the flee species with that of the complex, a facilitation factor (F) is

310 Chapter 9

often used which is defined as the ratio of the flux achievable in presence of the carrier to the

flux obtained in a carrier-free membrane [3].

F = Flux in presence o f carrier~ Flux without carrier (1)

Figure 2. Transport of i through the immobilized liquid as a complex and as a free species.

A generic form of the reversible chemical reaction between a species i and a carrier C is:

kc

i + C ~ C-i ka

(2)


where:

kc, complexation rate coefficient;

kd, decomplexation rate coefficient.

The species i complexes with the carrier C at the feed side:

kc i + C ~ C-i (3)

and decomplexes at the strip side:

ka C-i ~ i + C (4)

Depending on the specific application, the transport of the complex can be controlled by

acting on different parameters, such as temperature and pH of the feed and strip streams [4,5].

The facilitated transport can be simple or coupled [1,2]. In the former case, the carrier

reacts with the species at the feed-membrane interface to form the complex, the complex

diffuses through the membrane, the species is released (decomplexation) at the membrane-

strip interface and the carrier re-diffuses to the feed-membrane interface (Figure 3).

Figure 3. Simple facilitated transport.

312 Chapter 9

When the feed stream contains charged species, a coupled facilitated transport can take

place. In particular, if the species have an opposite charge we can have a co-trasport: the

species form a complex with the carrier at the feed-membrane interface, the complex diffuses

through the membrane, the species are released at the membrane-strip interface and the carrier

re-diffuse towards the feed-membrane interface (see Figure 4).

Figure 4. Coupled facilitated transport: co-transport of i and j.

If the species have the same charge, we can have a counter-transport: the carrier complexes

with one of the species at the feed-membrane interface, the complex moves towards the

membrane-strip interface where the carrier releases the species. Once the species is released,

the carrier complexes at the membrane-strip interface with the other species and the complex

re-diffuses towards the membrane-feed interface, where the other species is released. In the


counter-transport it is possible to transfer a species against its concentration gradient (from a

low to a high concentration side), driving the process the concentration gradient of the other

species. Figure 5 shows this type of transport.

Figure 5. Coupled facilitated transport: counter-transport of i andj.

Among several carriers available, tertiary amines and quatemary ammonium salts can be

used to form complexes with anions [1] and crown ethers and oximes (LIX-series) to complex

cations [ 1, 6-10]. In Table 1 are reported some of the carriers used. Although the huge number

of carriers already known, the synthesis of new carriers with better properties is in progress

worldwide, as will be more discussed in Chapter 11.

314 Chapter 9

Table 1. Some of the carriers used

Carrier Species transferred Reference

18-crown-6 ether

Tetraoctyl ammonium bromide,Trioctylmethyl ammonium chloride

Potassium, copper(II), silver (I), gold (III), silver(I), zinc(II) ions

Nitrate ions

LIX84, LIX864, LIX64N, LIX860, LIX984N

Bis(2-ethylhexyl) hydrogen phosphate (D2EHPA)

Silver(I) ions

Copper(II), cadmiun (II), zinc(II) ions

Copper (II), zinc(II), cobalt(II), nickel(II)

Benzene, Ethylene

Cobalt (II) salt, haemoglobin, Oxygen porphyrins

[1], [6], [11-13]

[5]

[7-10], [14], [15]

[41, [161, [171

[18], [19]

[20-22]

3. Mass transfer equations

When a facilitated transport takes place, the total transport of a species through the

membrane consists of two contributes: the facilitate and the simple transport. The former

refers to the transport performed by the carrier, whereas the simple transport is related to the

permeation of the species through the liquid and is regulated by a solution-diffusion

mechanism. This dual mechanism leads to a total flux that is not necessarily proportional to

the driving force. Therefore, appreciable fluxes can be obtained even at very low

concentrations of the species to be transferred.

The facilitated transport is characterized by five steps that occur in series (solubilization of

the species, complexation, diffusion of the complex, decomplexation and desorption of the


species) and, depending on the specific case, each of them can represent the limiting step and

control the rate of transfer. Generally, the solubilization and desorption steps can be supposed

instantaneous and the controlling step can be established by comparing the relative rates of

complexation, decomplexation and the diffusive transport of the complex. Figure 6 reports the

mass transfer resistances involved in presence of a facilitated transport.

Figure 6. Resistances involved in facilitated transport.

The most widely studied type of supported liquid membrane consists of a hydrophobic

support, with micropores filled by an organic phase, that is located between two aqueous

phases (feed and strip) [11, 23]. The organic phase is immiscible with the aqueous streams

and is retained in the pores thanks to the surfacial tensions and the capillarity.

316 Chapter 9

For the development of the mass transfer equations we will refer to this specific system,

although the same iter can be followed for the case of hydrophilic membranes with w a t e r -

filled pores and organic feed and strip phases as well as for gaseous feed and strip streams.

A species i contained in the feed stream during its movement towards the strip phase finds

three mass transfer resistances offered by the feed, the membrane and the strip (Figure 7)

and its total flux can be expressed as:

J, - K ( c f - c , 9 (5)

Figure 7. Resistances encountered by the species i during its permeation from the feed to the strip phase.

The concentration profiles that are established because of the mass transfer resistances are

depicted in Figure 8 for a flat membrane.


Figure 8. Concentration profiles of the species i during its permeation from the feed to the strip phase. Flat membrane.

The overall mass transfer coefficient can be related to the individual mass transfer

coefficients by considering, as already made in Chapters 4 and 5, that at steady state the flux

through the feed side equals the flux through the membrane as well as the flux through the

strip side'

Ji -- k i J (Ci f- Cfm) -- kim slm (C fme- CSm) -- kiw s (CSme- Cff) (6)

where:

k f mass transfer coefficient in the aqueous feed for the species i; kin ~tm, mass transfer coefficient in the supported liquid membrane for the species i; kJ , mass transfer coefficient in the aqueous strip for the species i; Ci f , concentration of the species i at the f e e d - membrane interface; Cfme, concentration of the species i at the feed-organic interface, organic side," CiSm, concentration of the species i at the membrane - strip interface," C~me, concentration of the species i at the organic - strip interface, strip side.

318 Chapter 9

The equilibrium conditions at the aqueous-organic interfaces are described by the

distribution coefficients:

C f me = mi Ci f (7)

CiSm: mi CiSme (8)

and the overall mass transfer coefficient can be expressed in terms of single mass transfer

coefficients by:

1/K = I/kwf + 1/(k,m stm mO + 1/k~w ~ (9)

Concerning the distribution coefficients determination, Coelhoso et al. [24] pointed out

that, when the extraction and stripping are carried out simultaneously, their values can be

different with respect to those obtained for the single experiment of extraction or stripping. In

particular, the osmotic pressure difference between the two aqueous streams that might occur

during the process under certain conditions has to be taken into account for a correct

evaluation.

In order to calculate the flux, the individual mass transfer coefficients have to be

determined. Referring to the resistances offered by the phases, the same correlations reported

in Chapter 4 can be used. For what concerns the membrane mass transfer resistance, we can

distinguish two different situations: carrier-free membranes and carrier-charged membranes.


3.1.Carrier-free membranes

In carrier-free membranes, the mass transfer through the micropores is function of the

diffusion coefficient of the species in the organic phase and can be derived by [25]:

kim slm = Dio c/r6 (1 O)

where:

Dio, diffusion coefficient o f the species i in the organic phase.

3.2.Carrier-charged membranes

When the organic phase contains a carrier, the flux of the species i is given by the sum of

two contributes:

J~ = D,o c/r6 (cUe - C[m) + D c-,o e/r6 (c~_fme - C~-7~ ( l l )

where:

D c-io, diffusion coefficient o f the complex in the organic phase; Cc-{me, concentration of the complex at the feed-organic interface," Cc_iSm, concentration of the complex at the membrane-strip interface.

The first term of the sum represents the diffusion of the free species through the organic

phase, whereas the second refers to the diffusion of the species as a complex and depends on

the diffusion coefficient of the complex into the organic liquid as well as the concentration

difference of the complex across the immobilized liquid.

By introducing the binding constant of the complexation reaction between the species and

the carrier and assuming that the concentrations at the membrane-strip interface of both the

320 Chapter 9

free species and the complex are negligible, it is possible to re-write the second term as

function of the concentration of the species i at the feed - organic interface, Cife [ 1 ].

The equation (1 l) becomes:

J~ = D,o c C L e / r 6 + kmr C[me (12)

with the membrane mass transfer coefficient for the complex (kmcomplex) function of

different parameters such as its diffusion coefficient, the carrier concentration, the binding

constant and the concentration of the species at the feed-organic interface [ 1 ].

The membrane mass transfer coefficient of the carrier-charged membrane can be, then,

written as"

kim str" = D~o e / r6 + kmcomptex (13)

In carrier-charged membranes the selectivity of the process is mainly depending on the

affinity between the species to be transferred and the carrier. If the carrier complexes with

more than one species, the facilitated transport becomes competitive. A typical example of

this phenomenon is the transport of acid gases, as reported by Way and Noble [26] who found

that the presence of CO2 strongly reduced the transport of H2S: a rapid decrease in the H2S

flux was observed when C02 was added to the feed stream.

Table 2 summarizes the main properties of supported liquid membranes.


Table 2. Main properties of supported liquid membranes

Membranes

Phase in the micropores

Feed and strip phases

Carrier

Microporous hydrophobic/hydrophilic

Organic/aqueous; immiscible with the feed and strip phases

Aqueous/organic/gaseous; immiscible with the immobilized phase

Highly soluble in the immobilized phase; poorly soluble in the feed and strip phases; high selective for the species of interest

4. Main potentialities and drawbacks

Supported liquid membranes allow to efficiently treat solutions leading to high levels of

purification. With respect to liquid-liquid systems they operate with lower amounts of

extractant (the quantity employed is just that charged into the micropores). This means that

expensive extractants can be used. Figure 9 shows a comparison between a liquid-liquid

system and a supported liquid membrane for the purification of a water stream by means of an

organic extractant. The difference in the extractant amount used in the two systems is clearly

evident.

322 Chapter 9

Figure 9. Liquid-liquid system (a) and supported liquid membrane (b) for water purification by an organic extractant.

Furthermore, if a carrier is added to the organic phase, the transport of the species through a

supported liquid membrane can be higher than in a liquid-liquid system due to the diffusion of

the species-carrier complex besides the diffusion of the species through the organic liquid.

The transport rate is, thus, enhanced and, if the carrier is high specific for the species of

interest, very high selectivities can be reached. Finally, the fact that appreciable fluxes can be

obtained even at very low concentrations of the species to be transferred, represents another

interesting advantage of this class of membranes.

Although these potentialities, supported liquid membranes are not yet developed at

industrial level because of a series of constraints that limit their effective application. First of

all, the loss of organic phase from the membrane micropores. We already stated that, in order

to keep the membrane pores organic-filled, it is essential that the organic phase/carrier is


immiscible with the aqueous streams. This is not, however, a sufficient condition for avoiding

the organic loss, that can occur as formation of emulsion droplets or by evaporation [23, 27].

The loss can be caused also by the application of a differential pressure across the membrane

higher than capillary forces or by the creation, during the process, of an osmotic pressure

gradient across the membrane that favours large flows of water [28]. The support structure

plays also an important role. For example, lower pore sizes lead, in general, to higher stability

[28]. However, lower pore sizes implies also lower mass transfers and then higher porosities

are required in order to work with reasonable fluxes. Furthermore, the substrate structure

affects the minimum thickness needed to maintain the supported liquid membrane integrity.

The stability of the membrane is also related to the carrier behaviour with time. For efficient

operations, the carrier has to keep its properties as longer as possible. However, carriers can

be poisoned by impurities present in the streams and can be subjected to deactivation (e.g., in

the case of oxygen transfer, irreversible oxidation of the carrier can occur [29]).

Moreover, in order to be implemented at commercial scale, supported liquid membranes

have to offer fluxes of industrial interest. This last point is related to the properties of the

immobilized solution and of the carrier (e.g., viscosity, solubility of the carrier in the liquid

medium) as well as the support structure and thickness (a too large thickness leads to low

fluxes). In Table 3 the main advantages and drawbacks of supported liquid membrane are

reported. Table 4 summarizes the requirements needed to improve the supported liquid

membrane performance in order to propose their application at large scale.

324 Chapter 9

Table 3. Main advantages and drawbacks of supported liquid membranes

Advantages Drawbacks

High selective

Low amount of extractant needed to perform the separation

High transport rates

Appreciable fluxes achievable also at low concentrations of the species to be transferred

Loss of the immobilized liquid

Carrier deactivation

Still low fluxes for industrial application

Table 4. Requirements needed for improving the performance of supported liquid membranes

Membrane

Immobilized phase

Low thickness; low pore size; high porosity; improved immobilization techniques; new materials for high temperatures operations

Low volatility; low viscosity

Carrier High selectivity; long life-time; no deactivation under the working conditions; optimum value for the binding constant in order to avoid carrier saturation; low solubility in the feed and strip phases; optimum value of concentration in order to avoid a high increase of the viscosity


5. Research efforts for improving the supported liquid membrane stability

The development of techniques devoted to the stabilization of supported liquid membranes

had interested many research groups all over the world. Some of the research efforts made are

reported and discussed in the following.

5.1. Fixed carrier membranes

Fixed carrier membranes consist of solid polymeric structures that incorporate the carrier

thanks to physical or chemical bounds (Figure 10). The membrane does not contain any liquid

phase, so the liquid loss from the membrane support is avoided.

Figure 10. Scheme of a fixed carrier membrane.

The mechanism of transport that occurs in this system is still not well understood and two

different theories have been proposed. One theory [30] supposes that the species jumps from

one carrier site to another (Figure 11). Referring to this hypothesis, the distance between two

326 Chapter 9

carrier sites plays an important role in the mass transfer, because, when below a limit value

(very low carrier concentration), the jumps of the species become more difficult and the flux

is strongly reduced.

Figure 12. Migration of the species from one carrier site to another.


Several works on fixed carrier membranes are reported in literature [32-34]. Gherrou et al.

[6] recently have prepared and characterized a new fixed carrier membrane containing crown

ethers to facilitate the diffusion of silver(I), copper(II) and gold(Ill) ions. Depending on the

amount of carrier immobilized, the mass fluxes varied, with a maximum around 1.13 10-3

g/cm 2. For higher values, multilayers of carrier or aggregates on the membrane matrix were

formed, with a consequent drastic reduction of the three mass fluxes.

From a comparison between the fixed carrier membrane developed and a supported liquid

membrane containing the same carrier, it resulted that higher fluxes than supported liquid

membrane can be achieved at a certain amount of carrier loaded. The prepared membranes

stayed stable over 15 days.

5.2. Composite membranes

The loss of the liquid phase immobilized into the micropores can be reduced by adding a

layer to the support structure, leading to a composite membrane. Wijers et al. [35] used this

method to enhance the performance of a supported liquid membrane for copper selective

transport. The polymer chosen for the layer was a sulphonated poly(ether ether ketone),

because of its high permeability for copper ions. Figure 13a shows the stabilization layer

applied at both sides of the support. Authors claimed out that the lifetime of the membrane

was substantially extended and that also the copper flux was higher. This last result was

attributed to the reduction of the liquid membrane thickness, due to the partial penetration of

the sulfonated PEEK into the pores of the support (Figure 13b). However, a deeper

328 Chapter 9

penetration of the stabilization layer could lead to a reduction of the selectivity because of the

non specifically diffusion of ions through it (Figure 13c).

Figure 13. Supported liquid membrane stabilization by a surface layer (a). Partial (b) and deeper (c) penetration of the layer in the pores.

5.3. Gelation of the liquid membranes

The gelation technique was proposed by Neplenbroek et al. [36]. The method consists in

the application of a homogenenous gel in the pores of the support or of a thin dense gel layer

on the support side(s). Authors found it very effective for increasing the stability for nitrate

transport with TeOA as carrier. However, other authors who tested the technique, did not

achieve the same positive results, probably due to instability phenomena [ 15].


5.4. Interfacial polymerization

The interfacial polymerisation allows to achieve thin film composite membranes by

performing a polymerisation between two slightly miscible phases each containing one

monomer. The process occurs by the following steps (see Figure 14):

- immersion of the support in the solution containing the monomer 1;

- immersion of the support in the solution containing the monomer 2;

-reaction at the support surface (phases interface) and formation of a dense polymeric

layer.

sup mpregnated LS~176176 I LS~176176 I

monomer 2 , >

composite membrane

Figure 14. Scheme of the interfacial polymerisation technique.

Several authors investigated the potentialities of this technique for different processes.

Kemperman et al. [37] obtained good results for the selective nitrate transport by using

piperazine and trimesoyl chloride as monomers to form the top layer on a polypropylene

support. In particular, the top layer did not reduce the flux of ions and was impermeable to the

liquid membrane, leading to stable operations and no flux decrease after 350 h of operation,

330 Chapter 9

whereas the flux through the uncoated membrane reduced to zero after only one day of test.

An attempt to apply the technique to a hollow-fiber geometry has been made by Kemperman

et al. [38]. The removal of nitrate ions from water was the process also considered here. The

support used was asymmetric with the smallest pores at the lumen side, where the top layer

has been formed. With respect to the un-coated fibers, the system was more stable, however,

the application of a uniform layer was not achieved, with consequent difficulties in the

reproducibility of results.

The interfacial polymerisation did not always led to improvements of the liquid supported

membrane performance. Lower fluxes than the uncoated membranes [39], un-uniform coating

and poor adhesion of the layer to the substrate [40] are some of the problems still unsolved.

5.5. Plasma polymerization

Yang et al. [15] proposed the use of coatings obtained by plasma polymerisation as a

mean for stabilizing supported liquid membranes containing LIX 984N for copper transport.

Monomers they used were hexamethyldisiloxane and heptylamine, whereas the support was

hydrophobic and microporous. The technique acted only on the support surface and allowed

to improve the stability of the supported liquid membrane thanks to the reduction of the

surface pore size of the support by the coating formation. Authors found that flux of copper

depended on the degree of coverage and on the reduction in the contact angle of the surface.


5.6. Microencapsulated liquid membranes

The idea to introduce in a polymeric matrix small droplets of the liquid phase containing

the carrier was developed by Bauer et al. [41 ]. Authors prepared an asymmetric structure with

a top layer made of thin open cell that were filled with the liquid phase and a carrier for

oxygen molecules (Figure 15). Although this system allowed to avoid the carrier loss, it

showed a short lifetime both due to a loss of solvent and oxidative degradation of the carrier

complexes. Figoli et al. [29] tried to overcome the above limitations by developing a

membrane system where the solvent and the carrier were confined in capsules (Figure 16).

The process investigated was the transport of oxygen and the preparation technique was

optimised for this specific case. First of all, the capsules had to be permeable to the oxygen

and impermeable to the solvent and, in order to obtain high fluxes, their thickness had to be

lower than 1 ~m. An uniform distribution of the capsules in the polymeric matrix had to be

also ensured for a good performance of the system. Authors also proposed several

modifications of the standard procedures for the encapsulation step. From SEM analysis of

the prepared membranes it resulted the presence of aggregate droplets into the polymeric

structure, thus, more efforts are needed to obtain a homogeneous distribution of the capsules.

Liquid membrane in the thin layer

332 Chapter 9

Figure 15. Asymmetric membrane with the thin layer containing the liquid membrane.

Figure 16. Microencapsulated liquid membrane.

However, in order to show the potentialities of the idea, authors calculated the O2/N2

selectivity for different polymeric matrixes and carrier phase permeabilities as a function of

the capsule content. The highest value of selectivity (50) was obtained for polymethylpentene

as polymeric material.


5.7. Bicontinuous microemuision membranes

Polymeric matrixes containing interconnected water channels (pore size, 4-60 nm) have

been prepared by polymerising bicontinuous microemulsions in situ by Figoli [42] for the

facilitated transport of oxygen. The bicontinuous microemulsion consisted in an

interconnected network of water and oil channels stabilized by an interfacial surfactant.

During its polymerisation the oil channels solidifies leading to the polymeric support, while

the water phase does not change and forms the liquid membrane phase. By acting on the

surfactant concentration and on the amount of water and/or oil, the final membrane structure

(e.g., width of the water channels) can be easily controlled. Applied for the carrier-facilitated

oxygen transport, these membranes have as advantages the stability of the liquid membrane

against transmembrane pressure gradients, due to the nanometer pore size, and the possibility

to reimpregnate them, according to their percolating porous network. Furthermore, if they are

used as coating layer of a composite membrane, the support is not wetted by the liquid

membrane and all the resistance is concentrated in the coating thickness. New water-soluble

carriers prepared by Fiammengo et al. [43] containing porphyrine have been incorporated in

the prepared membrane and experimental tests on the facilitated transport of oxygen have

been performed [42]. Higher facilitation factors have been achieved at low oxygen partial

pressures of the feed, indicating the potentialities of the system substantially for gas streams

with low oxygen content.

334 Chapter 9

5.8. Convective flow of the carrier solution

Teramoto et al. [44] proposed a new type of configuration for gas separation. They used a

dead-end type filtration cell equipped with an ultrafiltration membrane. The system is

characterized by a continuous supply to the feed side of a carrier solution that permeates the

membrane with the gas stream. In this way the membrane surface is always covered by a thin

layer of the liquid membrane and the gas is transferred both by the molecular diffusion and by

the convection of the liquid membrane. From tests made on CO2 transport the system resulted

stable for more than two months.

5.9. Support reimpregnation

In order to reduce the loss of the liquid membrane charged into the micropores, different

techniques of reimpregnation have been developed. For example, Takahashi and Takeuchi

[45] added a small amount of the liquid membrane to the strip solution. More recently, Ho

and Poddar [46] proposed a "supported liquid membrane with strip dispersion", consisting in

the strip phase dispersed into an organic membrane solution that wets tha pores of a

hydrophobic microporous support. In both cases the feed stream was kept at higher pressure

than the strip one, in order to avoid its contamination by the liquid membrane. The higher

amount of the extractant employed and the need to include a recovery step after the removal

stage represent the main drawbacks of the methods.


5.10. Hollow-fiber contained liquid membranes

A different configuration of liquid membranes, aimed at the improvement of the stability,

has been proposed by Sirkar et al. [47]. The system consists in the use of couples of

microporous hollow fiber membranes containing the feed and the strip phase in the fiber

bores, the liquid membrane being at the shell side. The membranes can be either hydrophobic

and hydrophilic. If the feed and strip phases are aqueous and the membranes are hydrophilic,

the interfaces between the aqueous streams and the organic liquid membrane are established

at the outer diameters of the fibers (Figure 17). The hollow fibers are located into the module

in such a way that the fibers containing the feed phase are close to those containing the strip

phase.

Figure 17. Organic liquid membrane at the shell side of hydrophilic hollow fibers for water treatment.

336 Chapter 9

With respect to conventional supported liquid membranes, this configuration offers several

advantages such as the improved stability of the liquid membrane, that can be easily

replenished during the process. Moreover, by packing a high number of hollow fibers into the

module, it is possible to operate with low liquid membrane thickness. However, some

possible limitations or difficulties related to this system have to be taken into account.

For example, the membrane thickness is not known a priori. At this purpose, Sirkar et al.

[47] defined an effective membrane thickness as the thickness of a hypothetical liquid film

that offers the same mass transfer resistance of the contained liquid membrane and developed

a procedure for its calculation.

Authors suggest some guidelines to follow during the process for optimizing the efficiency

of the proposed configuration.

In order to promote the mass transfer from the feed phase to the strip phase without

exceeding the breakthrough conditions, the operating pressures of the feed, liquid membrane

and strip have to be carefully chosen and controlled. For example, in gas treatments, the

higher is the pressure at the feed side, the higher is the driving force available for the

transport. However, higher feed pressures mean higher aqueous liquid membrane pressures

and, then, a possible breakthrough into the sweep fibers can occur. This phenomenon can be

controlled by working with small membrane pores. At high operating pressures the fiber

strength has also to be sufficient to prevent the fiber deformation. When gases are involved,


the pressure drops along the fiber can affect the driving force and have to be considered in the

design step.

Another important point is that a minimum distance between the fibers has to be ensured in

order to avoid that the feed and the strip phases could mix.

Authors performed a deep and detailed analysis of the mass transfer resistances involved.

They also made a comparison between the performance achievable by the hollow fiber

contained membrane and that of two separate hollow fiber contactors, such as that of a

conventional membrane permeator for gas separation. As a general remark, the production of

large scale modules containing two sets of fibers is more complex than that of modules where

only a single set of fiber is packed, due to the difficulty in obtaining the desired distribution of

fibers and a low liquid membrane thickness.

Among the several studies made, the application of the proposed configuration to isomer

separation and lipase-facilitated separation of organic acids [48-49] will be reported in

Chapter 11.

A variation of the hollow-fiber contained liquid membrane above described is the three-

phase contactor with parallel or cross flow or pulsation of the liquid membrane. These types

of contactor have been used in several works [50-52]. More recently, a new three-phase

contactor with distributed U-shaped bundles of hydrophobic hollow-fibers has been tested for

the pertraction of dimethylcyclopropanecarboxylic acid and phenol [53]. The advantage of

this configuration is that bundles of fibers can elongate without deformation or

338 Chapter 9

maldistribution of fibers. Authors found that the pulsation of the membrane phase increases

the transport rate by 35-61% and that a plateau is reached at a pulsation velocity of 1.1 mms 1.


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Handbook, Chapman and Hall, New York (1992) 833-866

[4] P. Argurio. Membrane processes coupled with metal binding reactions in water treatment. PhD

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4. University of Twente, Enschede, The Netherlans

[6] A. Gherrou, H. Kerdjoudj, R. Molinari, P. Seta and E. Drioli. Fixed sites plasticized cellulose

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