Liquid Membrane Types

Basic Overview of Membrane Design

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

Bulk Liquid Membranes

This setup is useful only for laboratory experiments, and is set up as follows. Following Figure 1, a U-tube cell is used, and some type of carrier, perhaps dissolved in CH2Cl2, 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. With a magnetic stirrer rotating at fairly slow speeds, in the range of 100 to 300 rpm, the transported amounts of materials are determined by the concentrations in the receiving phase. Stability is maintained so long as the stirrer is not spinning too quickly.

Using this setup, transport of the anionic, and neutral species has been demonstrated.


Figure 1. Bulk Liquid Membrane.

Emulsion Liquid Membranes

This setup has a very thin membrane and a large surface area per unit source phase volume, which enhances the transport rate of this membrane. Concentrations in the receiving phase are increased by a large factor, due to the ratio of source phase volume to receiving phase volume, which occurs whenever the organic phase emulsion is added to an even larger quantity of source phase.

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 you still want to maintain that large area per source phase volume ratio mentioned earlier. Figure 2 shows a simple ELM.

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

    1. Anything effecting emulsion stability must be controlled. ie. ionic strengths, pH, etc.
    2. If, for any reason, the membrane does not remain intat 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.
So, why go with the ELM? If you have a system where there is significant proton ionizable carrier activity on the surface of the membrane, than the ILMs (or SLMs) might have large problems with that, depending on exactly what is occuring. For an ELM, that problem is relatively unimportant, since an emulsion is already present.


Figure 2. Emulsion Liquid Membrane.

Thin Sheet Supported Liquid Membranes

The most simplistic in design, the thin sheet supported liquid membrane can be utilized for laboratory scale, but cannot be scaled up for industrial use. Essentially, this is just a porous polymer membrane whose pores are filled with the organic liquid and carrier, set in between your source phase and your receiving phase, which are being gently stirred. See Figure 3.

In this system, you can probably guess that the way to instability is to somehow get rid of the carrier or organic liquid in the pores of that supporting membrane. There are two possible ways for this to occur, I believe. One is through carrier or solvent evaporation, and the other is by creating a large pressure differential across the membrane, effectively pushing the fluid out.


Figure 3. Thin Sheet Supported Liquid Membrane.

Hollow Fiber Supported Liquid Membranes

The design of the HFSLM is akin to a large electrical cable. You have the outer shell, which is a single nonporous material, through which the materials inside cannot be transported. Inside that shell, there are many, many thin fibers running the length of the shell, all in nice, neat rows. What occurs is that the source phase is piped through the system from top to bottom, and the pores in the fibers themselves are filled with the organic phase. The carriers in that phase then transport the source across to the receiving phase, and then the receiving phase is forced out through the sides of the shell. Figure 4 represents this system. Figure 5 is a close up of the cross section of a single hollow fiber.

There are several inherent boons to using the hollow fiber system.

    1. The surface area and membrane thickness provide rapid transportation
    2. The source/receiving phases are more easily recoverable than the emulsion system
    3. The entire source and receiving phase are not in contact with the membrane at any given instant
    4. Leakage and contamination are easily contained
Likewise, there are a few problems associated with this system.
    1. Very hydrophobic membrane solvents are required to maintain integrity
    2. Hollow Fiber System must be cleaned between uses or there will be aqueous and contaminant buildup
    3. Pore Fouling, a cousin to caking in filters, often occurs due to surface effects and particles in the system.
    4. High Capital Costs


Figure 4. Hollow Fiber Supported Liquid Membrane.


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

Two Module Hollow Fiber Supported Liquid Membranes

In an effort to work around one of the problems found in the HFSLM, researchers attempted 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 ...

Why should you use the TMHFSLM system?

    1. Solvents with lower hydrophobicities required
    2. Replacement of solvent and carrier is simple
    3. Relatively High Transport Rate
    4. Leakage and contamination are easily contained
What would keep you from using it?
    1. Transport rates dependant on amount of stirring of membrane phase
    2. Creation of a boundary layer slows system down as compared to either ELM or HFSLM
    3. Fouling a problem
    4. High Capital Costs


Figure 6. Two Module Hollow Fiber Supported Liquid Membrane.

Other Liquid Membranes

This is pretty much a catch-all. All of these other designs lay primarily in the ILM/SLM portion of the membranes, since you can only form emulsions in one or two ways, but you can fill a micropore in just about any configuration.

So, what other kinds are there? In fact, there's probably lots of them, but the only other one that has reached any kind of use is the spiral wound membrane, and that is the one I'm going speak of.

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 you 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.


Figure 7. Spiral Wound Membrane.