LIQUID MEMBRANE

CONTENTS

1. Abstract……………………………………………05

2. Introduction to Liquid Membrane………………05

3. Types of Liquid Membrane……….………..……..06

4. Liquid Membrane Setup……….………………….08

5. Liquid Membrane Transport Mechanisms………15

6. Liquid Membrane Transport Phenomenon……..17

7. Conclusion………………………………………….22

8. Case Study…………………………………………23

9. Applications……………..…………………………27

10. References…………………………………..…….28

L.I.T. NAGPUR

LIQUID MEMBRANE TECHNOLOGY AND ITS APPLICATIONS

Abstract

Membrane separation process forms an important part in many industries. This process had replaced many traditional separating processes. In this work, we discuss recent progress achieved in this field, focusing on gas and liquid separation using facilitated transport membranes. The main advantages of using a carrier species in a liquid membrane is to enhance the flux, the selectivity and the permeability of the transport. This is more advantages system than many of the separation processes.

Introduction to Liquid Membrane

What is Liquid Membrane?

A liquid membrane (LM) can be defined as a thin liquid film separating two liquid or gaseous phases and controlling the mass transfer between these phases. The main advantage of this kind of membrane is the higher solubility and diffusivity coefficients of compounds in a liquid medium than in a solid one. Addition of a carrier agent increases even more the permeability of the membrane.

The permeation of a compound in a liquid membrane can be divided into the following steps:

1) sorption at the feed interface, 2) complexation reaction with the carrier, 3) diffusion of the species/carrier complex across the membrane, 4) decomplexation reaction at the permeate interface and 5) desorption of the species.

After completing this cycle, the carrier diffuses back to the feed interface to complex more molecules. Using a counter transport it is even possible to permeate one compound against its concentration gradient. This occurs when two ions with the same total charge are transported in opposite directions, because the

Liquid Membrane Technology And Its Applications. 2

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complexation reaction that takes place in the membrane phase reduces the chemical potential of the permeating species.

When the components are in an aqueous phase, liquid membrane separation combines the solventextraction process and solute recovery in a single step. The simplest liquid membranes consist in an organic phase that contains the carrier situated between two aqueous phases. One aqueous phase contains the compounds to be separated (feed phase), while the other aqueous phase will receive the compounds that permeate the membrane (receiving phase).

One of the benefits of using a liquid membrane is that liquid membranes are highly selective and, with the use of carriers for the transport mechanism, specific molecular recognition can be achieved. Liquid membranes are relatively high in efficiency, and as such, are being looked into for industrial applications. It is at this point that we run into the largest of problems.

Stability:

Liquid membranes require stability in order to be effective, and if they are pushed out of the pores or ruptured in some way due to pressure differentials or turbulence, then they just do not work.

Types Of Liquid Membrane:

There are two basic types of liquid membranes

1. Emulsion Liquid Membrane (ELM).2. Immobilized Liquid Membrane (ILM), also called a Supported Liquid

Membrane.

An ELM can be visualized as a bubble inside a bubble inside a bubble type of the system. The inside phase of the bubble is acting as a receiving phase while the


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outside phase of the bubble is acting like the source phase and the bubble itself is acting like a separating skin. In an ELM setup, there are large quantities of bubbles which are distributed uniformly in the system.

Emulsified liquid membranes (ELM) are being used to solve the problem of low interfacial area and reproducibility. In this system, a stable emulsion of the organic and receiving phase is prepared. A second emulsion is then prepared with the first emulsion and the feed phase. This configuration has the advantage of high interfacial area between the phases with the drawback of a typical batch process and the need to use other compounds to stabilize and break the emulsions.

An ILM is much simpler to visualize. Pretty much what is there in some other kind of rigid membrane, with lots of microscopic pores in it. Every one of these pores, then, is filled with this liquid, and in that liquid there is organic liquid and the carrier liquid. This ILM takes things from one side of the rigid membrane and carries it to the other side through this liquid phase. And that is a very brief model of what a LM is.

The supported liquid membrane (SLM) is the most attractive for industrial separations involving gaseous or liquid solutes. Figure 2 shows a supported liquid membrane (SLM), which consists of a micro porous support containing a liquid phase impregnated with the carrier. Liquid is held inside the support pores by capillary forces, as described by the Laplace-Young equation, so caution must be taken so as not to exceed the maximum operation pressure.

Supported liquid membranes may be prepared using microporous membranes of various geometries. The hollow fiber geometry is particularly advantageous because it allows much higher module packing densities than the flat sheet plate and frame modules or the tubular membrane modules. Another advantage is that hollow fiber modules require low investment and operating costs due to the reduced equipment requirements.

Fig. Supported Liquid Membrane


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Liquid Membrane Setup

Main aspects of setup

While there are two basic types of liquid membrane, different setups have been studied in an effort to increase the efficiency of the entire liquid membrane operation in industrial use.

1. Bulk Liquid Membrane.2. Emulsion Liquid Membrane.3. Thin Sheet Supported Liquid Membrane.4. Hollow Fiber Supported Liquid Membrane.5. Two Hollow Fiber Supported Liquid Membrane.6. Spiral Wound Membrane.

1. Bulk Liquid Membrane

This kind of membrane is used in universities, laboratories and in pilot scale. This membrane can be setup in coaxial cylinders and in U tube. In Figure 1, a U-tube cell and coaxial cylinder is used, and some type of carrier which is dissolved in organic liquid is placed in the bottom of the tube. That is the organic membrane phase. Two aqueous phases are placed in the arms of the U-tube, floating on top of the organic membrane. A stirrer is provided in the form of magnetic stirrer with very low speed. The speed is kept nearly about 100 to 300 RPM. The reason behind the slow speed is to maintain the stability. The transported amounts of materials which are transfer from source phase to receiving phase determined by the concentrations in the receiving phase.


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Figure 1. Bulk Liquid Membrane

2. Emulsion Liquid Membrane.

An ELM can be visualized as a bubble inside a bubble inside a bubble type of the system. The inside phase of the bubble is acting as a receiving phase while the outside phase of the bubble is acting like the source phase and the bubble itself is acting like a separating skin. In an ELM setup, there are large quantities of bubbles which are distributed uniformly in the system.

The membrane formed in this type of setup is very thin. As there are large numbers of bubble and each bubble has its own identity the surface area per unit volume of the bubble is very large as compare to any other system. Because of all these factors the transport rate through the membrane are increased. Generally in


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this kind of system the volume ratio of source phase to that of receiving phase is very high.

For stability, all that is required is for both the membrane solvent and the carrier molecule to be mildly hydrophobic. Compared to the hollow fiber system, the volume ratio is not large, since the organic and receiving phase volumes are equal, and large source volumes cannot be used if we still want to maintain that large area per source phase volume ratio mentioned earlier. Figure 2 shows a simple ELM.

This system has several disadvantages, all having to do with the formation of the emulsion.

1. Anything effecting emulsion stability must be controlled. i.e. Ionic strengths, pH, etc.

2. If, for any reason, the membrane does not remain intact during operation, the separation achieved to that point is destroyed.

3. In order to recover the receiving phase, and in order to replenish the carrier phase, you have to break down the emulsion. This is a difficult task, since in order to make the emulsion stable; you have to work against the ease of breaking it back down.

Figure 2. Emulsion Liquid Membrane.

3. Thin Sheet Supported Liquid Membrane.

This type of membrane is supposed to be the most simple in design. At the same time this are used in various aspects and in large number. This setup consists of a polymer film having number of pores. The carrier is added in the organic fluid and this fluid forms the membrane on the pores. The source phase and receiving


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phase are separated by this kind of setup. A gentle stirring is provided for the proper mixing. In this type the system can be collapsed because of the evaporation of organic fluid or by creating large pressure drop across the system.

Figure 3. Thin Sheet Supported Liquid Membrane.

4. Hollow Sheet Supported Liquid Membrane

The HSSLM can be visualized as a solid outer shell. Inside that shell there are many thin fibers running the length of the shell in neat rows. In this type of system the organic fluid with carrier are passed through the system and the pores of each fiber in the system is filled with that phase. When the source phase is passed through the hollow fibers and receiving phase is passed through the system, the carrier transport the source phase to receiving phase and receiving phase is removed from the side of the shell. Figure 4 represents this system. Figure 5 is a close up of the cross section of a single hollow fiber.

Advantages.

1. The surface area and membrane thickness provide rapid transportation2. The source/receiving phases are more easily recoverable than the

emulsion system as both are transported through different system.3. The entire source and receiving phase are not in contact with the

membrane at any given time4. Leakage and contamination problem can be easilysolved.

Disadvantages.


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1. Very hydrophobic membrane solvents are required to maintain integrity2. Hollow Fiber System must be cleaned between uses or there will be

aqueous and contaminant buildup3. Fouling are easily appeared in pores due to film effect.4. High Capital Costs

Figure 4. Hollow Fiber Supported Liquid Membrane.

Figure 5. Close-up Cross Section of a Hollow Fiber.


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5. Two Module Hollow Fiber Supported Liquid Membrane.

In an effort to work around one of the problems found in the HFSLM, there is another setup, one that looks something like the sketch in Figure 6. The way this works is that the source phase is piped in through one channel of hollow fibers, and the receiving phase in and out through another, with a stirred membrane phase in contact with both. So the question remains .. .

Advantages.

1. Solvents with lower hydrophobic ties required2. Replacement of solvent and carrier is simple3. Relatively High Transport Rate4. Leakage and contamination are easily contained

Disadvantages.

1. Transport rates dependant on amount of stirring of membrane phase2. Creation of a boundary layer slows system down as compared to either

ELM or HFSLM3. Fouling a problem4. High Capital Costs


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Figure 6. Two Module Hollow Fiber Supported Liquid Membrane.

6. Spiral Wound Membrane

The spiral wound membrane is essentially a flat membrane sandwich, wrapped around a perforated tube, through which the effluent streams out of the membrane. As one can see in Figure 7, that sandwich is actually four layers; a membrane, a feed channel, another membrane, and a permeate channel, which forces all the separated material towards that perforated tube in the center.

This type of membrane is a sort of intermediate step between the generic flat, laboratory membrane and the hollow fiber membrane, at least in terms of surface area per unit volume and stability.


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Figure 7. Spiral Wound Membrane.

Liquid Membrane Transport Mechanisms.

From the above discussion it is clear that there are number of ways by which liquid membrane can be set up. But the main important thing that has to be discussed is how this mechanism takes place.

If we actually followed the stages of the cation transport, we will notice that there are two stages involving diffusion. That's probably the best place to start because there are in fact two major categories of transport, active and passive transport, but in understanding those, one should also understand the rules governing diffusion.

1. Basic Diffusion2. Active Transport3. Passive Transport


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Rules Governing Diffusion

Simple Diffusion

The flux of a gas through a membrane under a concentration gradient in one dimention system is dictated by Fick's First Law,

Active Transport

When talking of active transport, we are most often referring to carrier kinetics which allow for transport against a concentration gradient. If we rewrite Fick's First law and begin to define the flux in terms of (a) the conjugate forces, (b) the coupled flux, and (c) the coupled chemical reactions, then we end up with a function which look as

In work done by Kedem and Caplan, it was shown that if the sum of Ji×Xi + Jr×Xr is positive, the flux may be negative and uphill transport is possible.

Passive TransportPassive transport is basically transport with the concentration gradient,

driven by a difference in chemical potential. The flux for a species is therefore


http://www.rpi.edu/dept/chem-eng/Biotech-Environ/patillo/membrane.biochem/mem.mechanism.html#fickslaw

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following Fick's Law, and can be expressed in terms of the diffusion coefficient, the concentrations, and the thickness of the membrane.

There is also about fourteen different models for relating the flux of a metal ion through a membrane, but there is no way I will cover these; the math involved would take up too much room on my account, so I'll break down a few into a sentence or so. If you couldn't tell by that wonderful lead-in, these are really general break-downs. One of these is the case where

M + L ML

which is diffused, and the same equilibrium constant is applied to both interfaces. Essentially what we get is that the transport ceases when the concentration in both phases becomes the same, and, more importantly, flux is directly proportional to the total carrier concentration in the membrane.

Liquid Membrane Transport Phenomenon

Types of TransportThere are four basic types of transport systems, each of which has its own

mechanisms and carrier types. In each of these systems, the one big item to notice is that regardless of mechanism, the complexes formed are that charge-neutrality must be maintained. Now then, the four systems are

1. Cation Transport2. Anion Transport3. Neutral Guest Transport4. Switchable Transport


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Cation Transport

Cation Transport can occur in either of two ways, either symport or antiport, as shown in Figures 8 and 9. In the symport configuration, a neutral carrier moves the guest and co-transported anion together across the membrane. This occurs in four stages. If the outer side of the membrane is in contact with Aqueous Phase I, and the inner side of the membrane is in contact with Aqueous Phase II, then

1. At the Phase I interface of the membrane, the guest salt forms complex with the carrier.

2. That complex diffuses across the membrane.3. The release of that guest salt occurs at the Phase II interface of the

membrane.4. The carrier diffuses back across the membrane, which makes itself ready

for another transport.

Figure 8. Cationic Symport.


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Antiport transport is similar to symport transport but instead of neutral carrier, anionic carrier has been used. So the four stages are a slight bit different.

1. At the Phase I interphase, the carrier tries to form a neutral complex with the guest cation.

2. The ion-pair diffuses across the membrane.3. Cation-exchange reaction releases the guest cation to Phase II.4. The carrier complex with the counter-transported ion diffuses back across

the membrane.

Figure 9. Cationic Antiport.


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Anion Transport

As summarized in Figure 10, anionic transport is similar in mechanism to the cationic transport, the only difference is that here cationic carrier is used instead of anionic carried in antiport configuration.

Figure 10. Anionic Antiport and Symport Transport.


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Neutral Guest Transport

Neutral species are transported symport, using various carriers. Oxygen and CO have been transported as gases through the liquid membrane, but they use a mechanism different that that shown in Figure 11.

Figure 11. Transport of Neutral Guest.

Switchable Transport

The use of photochemistry and electrochemistry has recently been investigated in increasing the rates at which the carrier complexes dissociate, which would therefore increase the transport rate. A sample of this is shown in Figure 12, but one can imagine coming up with exactly what is being down. Essentially, the switchable transport system works in addition to the regular transport system, and only the second step of the

AC – e- AC+ A + C+

reaction is accelerated.


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Figure 12. Switchable Transport.

Conclusion

Membrane processes are an alternative to the traditional energy-intensive processes for some difficult separations. Existing membranes have low selectivity for use in economically feasible process.

Thus, facilitated transport membranes have been used in many applications to increase permeability and selectivity simultaneously. Liquid membranes have been investigated in many studies, given that solubility and diffusivity coefficients of compounds in a liquid medium are higher than those in a solid one. The disadvantage of this type of membrane is the lack of stability due to the loss of solvent and carrier. On the other hand, a comparatively larger number of studies dealing with carrier fixed membranes can be found in the literature. The main reason for this is the greater stability of the immobilized carrier. Despite this feature, the matrix must have sufficient segmental motion of the chains, and there is a minimum carrier concentration, below which no facilitated transport occurs.

Case Study


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Immobilized Liquid Membrane for Propylene-Propane Separation

A. Feed GasesIndustrial grade propylene and industrial grade propane were used as feed

gases and pure nitrogen was used as sweep gas.

B. Carrier SolutionSilver Nitrate (AgNO3, GR Pro Analysis) was used as the carrier of

propylene. An aqueous solution of silver nitrate was prepared by dissolving silver nitrate in deionized water.

C. Flat Sheet Membrane ModulePolyvinilydene diflouride (PVDF) flat sheet membranes (Durapore from

Millipore, filter diameter 142mm) were used as the support of the liquid membrane. After being immersed in the carrier solution, the membrane filter was sandwiched between two compartments of the module. Once prepared, the membrane filter could be used for 3-4 weeks with no change in separation and permeation properties.

D. Experimental Apparatus and ProcedureThe schematic diagram of the experimental setup is shown in Fig. 2. All

tubing used to connect all parts of the setup was stainless steel (AISI 316). The experimental procedure is asfollows. Propylene and propane, after passing through mass flow controllers (Brooks Instruments, model 5850S), were mixed and entered the humidifier. The humidified feed passes through a temperature control system and enters the membrane cell. A combination of a heater and a cooler were used as the temperature control system. The feed gas was introduced to the upper compartment of the cell and the sweep gas, nitrogen, was supplied to the lower compartment. The main product, permeate, was collected from the lower compartment and the secondary product, retentate, was collected from the upper compartment. A back pressure regulator (BPR, Tescom, Germany) was used on the retentate line to control the pressure of the system. During all experiments, sweep gas was at atmospheric pressure. The experiments were conducted at room temperature


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(298±5K). All the experimental data were obtained after an initial permeation period of 4-6 hr.

Fig. Schematic diagram of the supported liquid membrane system

E. AnalysisThe gas composition was determined by a Gas Chromatograph (Agilent

6890N) equipped with a Flame Ionization Detector (FID, Agilent Technologies Inc. column, HP Al/S, 0.53 mm in diameter, and 50 m in length).

F. Result And DiscussionThe effect of trans-membrane pressure and carrier concentration on

membrane performance is shown in Figure. Different mixtures of propylene-propane were used as feed gas. Facilitated transport is a combination of two processes: absorption (on the feed side) and stripping (on the permeate side). Increasing the pressure is in favor of absorption and decreasing the pressure is in favor of stripping. Thus, the more the pressure on the feed sides, the more the absorbed propylene on the feed side. Due to the pressure difference between feed side and permeate side, the complexed propylene is decomposed on the permeate side. Therefore, the more the trans-membrane pressure, the more the driving force for separation. Hence, more propylene was transported across the membrane and separation factor was increased. In facilitated transport membranes, propylene permeation occurs via two mechanisms:


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1. Fickian diffusion and 2. Facilitation transport.

In the absence of carrier (Ag+) in the membrane, propylene was permeated only via Fickian diffusion. When carrier was added to the membrane system, propylene was permeated via Fickian diffusion and facilitated transport. Based upon facilitated transport mechanism, when more carriers are available in the membrane, more propylene can be transported along the membrane thickness and this will cause membrane to have a better separation performance. In an immobilized liquid membrane with a constant carrier concentration and at constant trans-membrane pressure, when a few concentration of propylene was in the feed stream, the separation factor was higher in comparison with the case when a large concentration of propylene was in the feed stream. The reason is that, in the former case, most of the propylene molecules can react with carrier molecules and can transport across the membrane via facilitated mechanism, but in the latter case, as more propylene molecules were in the membrane, all of them cannot react with the carrier molecules and cannot transport across the membrane via facilitated mechanism. Hence, separation factor in the former case is greater than in the latter case.

Performance of membrane system for the separation of 30:70 vol.% propylene-propane mixture


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Applications of Liquid Membrane.

These types of Liquid Membrane are used in

1. The most promising avenue for new uses of liquid membranes resides mainly in the biochemistry and biological fields.

2. The use of carriers utilizing proteins, antibiotics, or other molecules naturally found in cell membranes can provide fast, efficient, and almost continuous service for the researcher.

3. Some possible uses for liquid membranes would be in the treatment of wastewater.

4. In the recovery of metals from dilute solutions and for controlling certain problems in the oil well control industry.

5. Separation of oxygen from the air to obtain highly oxygen enriched air.6. It is used in various separation processes such as Propane/Propylene

Separation, Glucose/Fructose Separation etc.


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References

1. Indian Journal of Chemical Technology Vol. 12 July 2005.R Anupama and K Palanivelu

2. www.rpi.edu Article written by Chris Pattillo

3. World Academy of Science, Engineering and Technology 47 2008.4. Journal of Membrane Science 280 (2006) 330–334.5. Chemical Separations With Liquid Membranes

Richard A. Bartsch and J. Douglas Way


http://www.rpi.edu/

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