'Membrane Separation Systems : Recent Developments and Future Directions

472 Pages·1991·19.711 MB·English

by  R.W. Baker

4. Facilitated Transport
E.L. Cussler
Department of Chemical Engineering and Materials Science,
University of Minnesota
4.1 PROCESS OVERVIEW
4.1 .l The Basic Process
Facilitated transport is a form of extraction carried out in a membrane. As
such, it is different from most of the other membrane processes described in this
report. Many of these, for example ultrafiltration, are alternative forms of
filtration. Others, especially gas separations and pervaporation, depend on
diffusion and solubility in thin polymer films. In contrast, facilitated transport
involves specific chemical reactions like those in extraction.’
Facilitated transport usually has four characteristics that make it different
from other membrane separations:
(1) It is highly selective.
(2) It reaches a maximum flux at high concentration differences.
(3) It can often concentrate as well as separate a given solute.
(4) It is easily poisoned.
Characteristics (2) and (3) are the most powerful evidence that facilitated
transport is occurring.
The way in which facilitated transport works is shown schematically in
Figure 4-l. The U-tube at the top of this figure contains two aqueous solutions
separated by a denser chloroform solution. The aqueous solution on the left
contains mixed alkali metal chlorides; the solution on the right is initially water.
The chloroform solution contains dibenzo- IS-crown-6, a macrocyclic polyether. An
additive like this polyether is called a “mobile carrier”.2 With time, salts in the
left-hand solution in Figure 4-1 dissolve in the chloroform and diffuse to the
right-hand solution. This dissolution is enhanced or “facilitated” by as much as a
million times by the mobile carrier. Moreover, the enhancement is selective: in
this case, the flux of potassium chloride is four thousand times greater than that
of lithium chloride.
242
Facilitated Transport 243
Feed solution stripping solution;
chloroform solution itlally pure water
Same chloroform
solution held In
membrane pores
Figure 4-1. Facilitated Transport as Extraction. The extraction system at the
top of the figure mimics the facilitated diffusion in the membrane at
the bottom of the figure.
244 Membrane Separation Systems
The process at the top of Figure 4-1 is obviously an extraction. The
reduction from extraction to facilitated transport is made possible by reducing the
chloroform phase to a thin sheet, perhaps 30 pm thick, as suggested at the
bottom of Figure 4- 1. This thin sheet is stabilized by capillary forces within the
pores of a microporous hydrophobic polymer membrane. Celgard@ membranes,
made from microporous polypropylene are a common choice. As before, potassium
chloride is selectively extracted from the left-hand ‘feed’ solution into the
membrane; the mobile carrier greatly facilitates the salt’s diffusion, and the right
hand “stripping” solution removes the solutes from the membrane.
The example of facilitated transport illustrated in Figure 4-1 involves the
separation of metal ions with a carrier that selectively reacts with one ion and
not with the others in the solution. These carriers are widely used in solvent
extraction under the name liquid ion-exchange (LIX) reagents. Another type of
facilitated transport process uses carriers that will selectively react with and
transport one component of a gas mixture. One of the most widely studied gas
separation facilitated transport processes is the separation of oxygen from air.
Hemoglobin is a well known natural carrier for oxygen, and many other synthetic
carriers are also known.
In the past, most facilitated transport membranes used selective carriers
dissolved in an organic liquid solvent. In conventional forms of the process, the
carrier-solvent solution is immobilized in the pores of a microporous membrane.
If the solvent is nonvolatile and insoluble in the surrounding media, the pores of
the microporous membrane are made sufficiently small that the liquid is
immobilized by capillarity. Such immobilized liquid membranes (ILMs) are easily
made, and are normally stable for periods of days, weeks or even months, but not
longer.
A second type of facilitated transport membrane is the emulsion liquid
membrane (ELM), first developed by Li at Exxon.s In these membranes, the
solvent-carrier and the aqueous product solution are emulsified together to form
an oil-in-water emulsion. This emulsion is then re-emulsified in the feed solution,
forming a water-in-oil-in-water emulsion. The carrier-solvent phase is the oil
phase, and forms the wails of an emulsion droplet separating the aqueous feed
solution from the aqueous product solution. Permeate ions are concentrated in
the interior of the emulsion droplets. When sufficient permeate has been
extracted, the droplets are separated from the feed solution and the emulsion is
broken, liberating a concentrated product solution and an organic carrier phase.
The organic carrier phase is decanted from the product solution and recycled to
make more emulsion droplets.
4.1.2 Membrane Features
Facilitated membrane diffusion can be much more selective than other forms
of membrane transport.3-6 but the membranes used in the process are usually
unstable. This instability is a tremendous disadvantage, and is the reason why
this method is not commercially practiced.
Facilitated Transport 245
The characteristics of facilitated diffusion are illustrated by the values in
Table 4-1, which compares diffusion coefficient, separation factor and thickness
for polymer membranes and for facilitated transport membranes. The speed of
any membrane separation is directly proportional to the membrane’s diffusion
coefficient and inversely proportional to the membrane’s thickness. The diffusion
coefficient in polymer membranes is much smaller than that in liquid membranes,
but the thickness of the polymer membranes is also much smaller. As a result,
polymer membranes may show separations with speeds comparable to those of
facilitated diffusion.
Table 4- 1. Representative Membrane Characteristics. Facilitated transport
membranes offer fast diffusion and good selectivity, but are thicker
than polymer membranes.
Diffusion
coefficient Separation Thickness
(cm2/sec) Factor (cm)
Glassy polymer membrane 10-s 4 10-s
Rubbery polymer membrane 10-s 1.3 10-4
Facilitated transport
membrane (liquid) 10-s 50 10-s
The high selectivity of facilitated transport comes from the chemical
reactions between the diffusing solutes and the mobile carrier. These chemical
reactions cover the spectrum of chemistry used in extraction and absorption. In
the system in Figure 4-l. both potassium chloride and lithium chloride are largely
insoluble in the chloroform membrane unless they are complexed. The potassium
ions are strongly complexed by the crown compound, but lithium ions are only
weakly complexed, this is why this membrane is much more selective for potassium
chloride than for lithium chloride.
The coupling between diffusion and chemical reaction gives facilitated
diffusion another unusual feature: the flux is not always proportional to the
solute concentration difference between the feed and permeate side. At low
solute concentration, doubling the concentration difference often doubles the flux,
but at high solute concentration, doubling the concentration difference may have
no effect on the flux at all. This unusual non-linearity occurs because at high
solute concentration, the reactive carrier is fully utilized. In other words,
essentially all the carrier molecules are complexed on the feed side of the
membrane. and essentially none are complexed on the permeate side of the
membrane. Increasing the feed concentration has no effect, therefore, because no
additional complexed solutes can be formed. The resulting data are sometimes
confused with dual mode sorption, a similar, but less well defined effect, which is
described in the section on gas separations.
246 Membrane Separation Systems
Because facilitated transport involves coupled diffusion and chemical
reaction, it can sometimes concentrate as well as separate a specific solute. Such
concentration is sometimes called “coupled transport”. An example, given in
Figure 4-2. shows how a feed solution containing 100 ppm copper can produce a
stripping solution with 1,200 ppm copper. The energy for this separation usually
comes from a second reaction, often implicit and often involving a pH change. In
the copper case, it is the reaction of the carrier with acid. Again, the process is
like extraction, where such concentration is common.
Although facilitated transport membranes are highly selective, they suffer
from a major problem, namely instability, that has precluded industrial adoption.
Four main causes of membrane instability have been identified: solvent loss,
carrier loss, osmotic imbalances, and spontaneous emulsification.697 Each merits
more discussion:
Splvent Los Solvent loss occurs when the liquid membrane solvent dissolves in
the adjacent solutions and eventually ruptures. As a result, facilitated transport
solvents are usually chosen for their relative insolubility in the adjacent solutions.
In practice, rupture usually occurs for some other reason, before the solvent in
the membrane has dissolved significantly, so this mechanism is only infrequently a
significant cause of instability.
Carrier LOST: The mobile carrier makes the diffusing solutes more soluble in the
membrane. Unfortunately, the diffusing solutes also tend to make the mobile
carrier more soluble in the adjacent solutions. Moreover, many carriers are
chemically unstable, including many proposed for separating oxygen from air.
These disadvantages can sometimes be reduced by modifying the carrier
chemistry, for example reducing its water solubility by adding side chains to the
mobile carrier, just as is done in extraction.
Osmotic ImbalanceS: A facilitated transport system often involves concentration
differences between the feed and the permeate solution that are 1.0 M or higher.
These imply osmotic pressure differences of 40 atmospheres or more. Such
pressure differences can force the liquid membrane out of the pores of the
polymer support. This effect can be reduced by using supports with very small
pores or good wetting, but it remains a chief cause of membrane instability.
Soontaneous Emulsification: Most membrane additives are amphoteric. For
example, the oximes used to extract copper ions from aqueous solution into
kerosene can also extract some water. This water is sometimes “stranded” within
the membrane, eventually producing trails of water droplets which coalesce into
channels across the membrane. Such emulsification is less well studied than the
osmotic imbalances, but it may also be a serious cause of membrane rupture.
Other factors that may influence the membrane performance are membrane
viscosity, membrane density and support wetting. These issues are less significant
than those concerned with membrane stability.
Facilitated Transport 247
Copper
Concentration (ppm)
in the acid phase
0 2 4 6
Time (min.)
Figure 4-2. Copper concentration by facilitated transport. While the basic
feed contains only 100 ppm copper, the acidic product has
over 1000 ppm copper.’
248 Membrane Separation Systems
4. I .3 Membrane and Module Design Facton
. .
ed Llauld As described above, liquid membranes for
facilitated transport can be stabilized in the pores of a microporous polymer
membrane. Ideally, the microporous membrane supports should have very small
pores. so that the effects of any applied pressures are minimized. The pores
should also be as homogeneous as possible. In practice, many ultrafiltration and
microfiltration membranes work well. Hydrophobic microporous polypropylene
membranes are commonly used.
The membrane support should have a large surface area per volume to allow
a large flux per volume. Plate-and-frame and hollow-fiber supports, such as those
shown in Figures 4-3a and 4-3b. are often used. Capillary hollow-fiber systems
are generally preferred. The support is prepared simply by wetting it with the
membrane liquid. Sometimes, this wetting is enhanced with vacuum or pressure,
but this is rarely essential.
Emulsion Liauid Membranes: A second method of using facilitated transport
membranes is as water-in-oil-in-water emulsions, shown in Figure 4-3~.~~~~~ An
example of an emulsion liquid membrane would be droplets of water emulsified by
rapid stirring in a chloroform solution of surfactant and of mobile carrier, then
dispersed in the mixed salt feed solution by slow stirring. Salts like potassium
chloride diffuse from the mixed feed across the chloroform into the internal
phase. This method is also called “double emulsion membranes,” “liquid surfactant
membranes.” and “detergent liquid membranes.”
The advantage of emulsion liquid membranes is the large area per volume,
which enables separations to take place at great speed. The principal
disadvantage is the complexity. The membrane separation may be fast, but the
emulsion manufacture, the emulsion separation, and the recovery of the internal
phase from the centers of the droplets can be extremely difficult.“*”
Membrane ContactorS: A final method of employing facilitated diffusion is to
make a membrane contactor. A membrane contactor typically uses a hollow-fiber
module containing microporous membranes, as shown in Figure 4-3d. Feed flows
through the lumens of the hollow fibers, and extractant flows countercurrently
on the shell side of this module. Solute entering in the feed is removed by the
extractant. The solute is later stripped from the extractant under different
conditions in a second module. This process is really a hybrid, combining aspects
of both conventional extraction and liquid membranes. Alternatively, one can use
two sets of hollow fibers immersed in a “liquid membrane” solution. One set of
fibers carries the feed, and the other contains the product stream. In either
case, the membrane contactor performs the same function as a packed tower.
Membrane contactors exhibit the advantages of liquid membranes, but avoid
disadvantages of absorption and extraction.“*ls They have a large area per
volume and hence produce fast separations. They are unaffected by loading, by
flooding, or by density differences between feed and extract. In this sense, they
are a poor man’s centrifugal extractor. Membrane stability problems are avoided
by essentially putting one membrane interface in an extraction module and the
other interface in a stripping module. However, although the performance of
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Facilitated Transport 251
membrane contactors in the laboratory is excellent, their performance under
industrial conditions is only currently being explored. The energy requirements of
such a system have yet to be clarified.
4.1.4 Historical Trends
Studies of facilitated transport originated in biochemistry.10 Living
membranes often show fluxes that are faster and more selective than would be
expected to be achieved by the membrane components. Living membranes often
concentrate specific solutes, but are easily poisoned by particular drugs. These
observations led to the postulates of mobile carriers. Theories of mobile carriers
were later tested by studies on blood, where hemoglobin facilitates diffusion of
oxygen and of carbon dioxide.
The high fluxes, high selectivity and specific concentration achieved in
biological membranes spurred research into synthetic membranes and carriers. The
simultaneous development of microporous polymer films abetted this development
by providing membrane supports. By 1975, facilitated transport processes were
being studied for acid gas treatment, metal ion recovery, and drug purification.
None of these processes became commercial. Ironically, biochemical interest also
waned, deflected into studies of chemically selective pores.
Since 1980, the most important research studies in the field of facilitated
transport have focussed on membrane stability, an issue which remains unresolved.
4.2 CURRENT APPLICATIONS
Many fundamental studies of facilitated transport systems have been
performed. Table 4-2 summarizes a recent review by Noble et a1.s. and shows the
broad scope of facilitated diffusion research to date.
The solutes shown can be roughly divided into three groups. The largest
group includes metals, especially in their ionic forms. This group underscores the
similarity between facilitated transport and extraction. A second group, gases,
builds on parallels between facilitated transport and absorption. A third group
consists of organics of moderate molecular weight, with an emerging bias towards
pharmaceuticals. These fundamental studies have elucidated the detailed chemistry
responsible for many types of facilitated diffusion.
In contrast to these fundamental efforts, commercial successes are sparse.
The most serious, publicly detailed effort has been by Rolf Marr and his
associates at the University of Graz, in Austria.g These workers developed a
large-scale emulsion liquid membrane process for the selective extraction of zinc
from textile waste. The process is shown schematically in Figure 4-4. They
used liquid membranes of diamines dissolved in kerosene. The product solution is
first emulsified in the carrier medium. This emulsion is then re-emulsified with
the feed solution. In the permeation step the zinc migrates into the interior
product solution. The emulsion is then passed to a settler and the feed solution,
now stripped of zinc, is removed as a raffinate. In a final step, the carrier-
product emulsion phase is passed to a de-emulsifier where the emulsion is broken
to produce a carrier solution, which is recycled, and a product solution containing