INTRODUCTION
Eric M. V. Hoek and MaryTheresa M. Pendergast
University of California, Los Angeles, CA
Volodymyr V. Tarabara
Michigan State University, East Lansing, MI
1 WHAT IS A MEMBRANE?
A membrane is a semipermeable barrier that allows passage of certain compounds, but not
others (Fig. 1). In 1861, Maxwell wrote of “a being whose faculties are so sharpened that
he can follow every molecule in its course” (1). Maxwell’s theorized “sorting demon”
would be able to perfectly recognize and separate individual molecules. While such
an ideal barrier has not yet been fabricated 150 years later, commercially available
membrane technologies reliably perform a wide range of separations and the application
of membrane technology continues to grow.
Separation processes create order from disorder; therefore, there is an inherent energy
input required to enable them. This energy is termed the minimum work of separation
(Wmin ). Mixing of two solutions occurs spontaneously if the free energy of the mixture is
less than the sum of the free energies of the pure solutions. Alternately, the minimum work
required to separate a mixture is equal to or greater than the free energy (G) of mixing,
Wmin ≥ G = H − T S
This free energy can be related to the chemical potential difference between the mixture
and the pure solutions,
∂G = −S∂T + V∂P +
μi ∂ni
The gradient of the chemical potential is the driving force for mixing and determines
the minimum energy that must be input to achieve separation.
Take osmosis and reverse osmosis as a practical example (Fig. 2). A semipermeable
membrane (ideally, infinitely permeable to water and infinitely impermeable to salt)
separates a salt solution from pure water. The chemical potential (μ) of the solution
Encyclopedia of Membrane Science and Technology. Edited by Eric M.V. Hoek and Volodymyr V. Tarabara.
Copyright © 2013 John Wiley & Sons, Inc.
1
2
INTRODUCTION
FIGURE 1 Depiction of two reservoirs separated by a semipermeable membrane. Initially, all
solutes are on one side in a mixed solution. During the separation process, some solutes pass through
the membrane, while others are retained. This is the basic concept of membrane separation.
Level will rise
until osmotic
pressure is
equalized
(a)
Applied
pressure
(b)
FIGURE 2 The phenomenon of osmosis (a) and reverse osmosis processes (b). If a salt solution
and pure water are separated by a semipermeable membrane, natural osmosis dictates that water
will pass through the membrane to dilute the salt solution until equilibrium is reached. In reverse
osmosis, pressure is applied to overcome the osmotic pressure difference causing pure water to
pass through the membrane and concentrate the salty solution.
is reduced in proportion to the concentration of salt, which increases the entropy
(S ) and decreases the free energy. The higher chemical potential of the pure water
drives water molecules through the membrane until the two solutions achieve osmotic
equilibrium—this is osmosis. To reverse the spontaneous process of osmosis and drive
water from the salt solution side to the pure water side (to “reverse osmosis”), energy
equal to the original osmotic pressure difference (i.e., chemical potential difference)
between the two solutions must be input to the salt solution side. Energy in excess of
this amount must also be exerted to account for any irreversible losses.
In order to quantify the performance of a membrane separation (Fig. 3), two primary
metrics are used: selectivity and permeability. Selectivity is the starting point as separation is the primary objective. Selectivity of a membrane can be expressed in terms of
solute rejection (especially for pressure-driven liquid separations),
cp
R =1−
cf
INTRODUCTION
Membrane
module
Pf
3
Pp
Feed
Permeate
Pump
Pr
Retentate
FIGURE 3 Process schematic for a pressure-driven membrane separation, denoting the influent
feed stream and two outflows of permeate and retentate.
where cp is the concentration of the solute in the permeate stream and cf is the concentration of the solute in the feed stream. This equation has the same form as the conversion
in a chemical reactor. Alternatively, selectivity of a membrane can be expressed as a
separation factor,
S
αA/B = A
SB
where S (= cp /cf ) is the sieving coefficient and A and B denote the components
being separated. Separation factor is more commonly used in solute separations (e.g.,
protein fractionation) or phase separations (e.g., O2 /N2 gas separation, ethanol/water
pervaporation).
After it is established that the selectivity of a membrane is appropriate for a given
separation, the permeability of a membrane (to a large extent) dictates the cost of the
ultimate separation. Lower permeability translates into larger membrane area required
(increased capital investment) and/or more energy input required (increased operating
expense). Flux (J ) is the flow of matter through the membrane per unit area of membrane. Permeability of a mixture component is typically derived from measurement of
the component’s flux,
J = −A
dX
dx
where A is the phenomenological permeability coefficient and dX /dx is the gradient
driving permeation. The phenomenological coefficients and gradients are listed for a
variety of transport types in Table 1. For example, in the pressure-driven reverse osmosis
application described above, the driving force is the pressure drop (dP = Pf − Pp ) across
the membrane thickness (dx ).
TABLE 1 Phenomenological Equations Relating Fluxes to Driving Forces (Gradients)
Governing Relation
Fick’s law
Darcy’s law
Fourier’s law
Newton’s law
Ohm’s law
Type of Transport
Flux
Coefficient
mass flux
volume flux
heat flux
momentum flux
electrical flux
Jm (kg/(m ·s))
Jv m3 /(m2 ·s)
Jh J/(m2 ·s)
Jn (kg/(m·s2 ))
Ji C/(m2 ·s)
2
=
=
=
=
=
−D (m /s)
−K /μ (m2 /(Pa·s))
−λ (W/(m·K))
−μ (kg/(m·s))
−G (S)
2
Gradient
×
×
×
×
×
dc/dx
dP /dx
dT /dx
dv /dx
dE /dx
4
2
INTRODUCTION
SOME BASIC HISTORY OF MEMBRANE SCIENCE AND TECHNOLOGY
Membrane science has benefited from the fundamental contributions of multiple fields,
including chemistry, biology, physics, and engineering. As early as in 1748, traces of
membrane science can be found; one example being Nollet’s studies of water permeation
through a diaphragm that led him to coin the term osmosis (2). Work through the early
FIGURE 4 Timeline indicating the scientific developments and technological milestones in membrane science (2–30).
INTRODUCTION
5
1900s was focused on understanding the phenomena involved in barrier and interfacial
systems as well as ideal liquid and gas systems (1, 3–10). Membranes were employed as
model systems to study transport processes, such as diffusion, osmosis, and dialysis. Later
work focused more on understanding membrane transport and on developing membranes
as a technology (11–17). A few important scientific and technological milestones are
laid out in Fig. 4.
Membrane technology has evolved dramatically over the last century. Collodion (nitrocellulose) ultrafiltration and microfiltration membranes—described by Fick as early as
in 1855 (5)—were produced commercially for laboratory separations in the early 1900s
(18, 19). The 1940s saw the rise of commercial ion exchange membranes, which were
previously employed exclusively in fundamental studies (20–22). This was followed by
the opening of the first successful electrodialysis facility in 1952 and the first sea salt
production facility in 1961 (23). Around this time, cellulose acetate membranes emerged.
Cellulose acetate films were first shown to be capable of desalinating water in 1959 (24).
In 1963, Loeb and Sourirajan (25) demonstrated the phase separation technique, used
widely to this day, for the formation of integrally skinned asymmetric cellulose acetate
membranes for seawater desalination. About a decade later, Cadotte (26) developed interfacial (thin-film) composite membranes. By 1979, the first reverse osmosis seawater
desalination facility had opened (27) and in 1982, the first pervaporation facility opened.
These early developments—both scientific and technological—laid the foundations for
modern membrane science and technology.
3 THE IMPORTANCE OF MEMBRANE SCIENCE AND TECHNOLOGY
Essentially, all life forms require membranes for critical biological functions. Biological membranes serve as a protective layer for genetic and metabolic materials. They
selectively transmit water, ions, and organic compounds in and out of cells and organs.
Passive diffusion of carbon dioxide, oxygen, and water across cellular membranes ensures
biological equilibrium is maintained. Transmembrane protein channels, such as aquaporins, actively regulate the passage of specific species, such as sugars and amino acids
to aid in metabolism. Much work has gone into understanding the structure and function of biological membranes, leading to the creation of biomimetic membranes and
processes.
Membrane separations have traditionally played an important role in biotechnology
and medicine (31, 32). For almost 70 years now, since Kolff (33) formed his model
of an artificial kidney—not an exact replica, but a functional dialyzer for medical
treatment—scientists have tried to replicate the intricate structures and functions of the
body. Artificial cells, organs, and liposomes all rely on the ability to mimic biological
membranes by controlling the permeation of certain chemical species. Today, membranes
are used in controlled drug delivery in order to allow for slow release of a pharmaceutical from a reservoir into the bloodstream of a patient. Microfiltration and ultrafiltration
have long been used for sterile filtrations of fermentation media, protein concentration,
and buffer exchange, where high throughput is needed, but extreme selectivity is not.
More precise size separations required for protein purification and protein–virus separations are currently being aided by advances in membrane materials and high performance
tangential flow filtration systems.
6
INTRODUCTION
Growth of the field of membrane science and technology took off soon after World War
II, following in the steps of rapid developments in polymer science (34). A large demand
for advanced technologies in water quality analysis and treatment was also spurred at
that time, as drinking water sources in Europe had become severely polluted during the
war and rapid population growth in arid regions, such as California, prompted interest
in “saline water conversion” in the United States (34). Since Hassler, McCutcheon,
and others theorized on the ability to desalinate water with synthetic membranes at the
UCLA (University of California, Los Angeles), as early as in 1950, researchers have
sought to use membranes to produce potable water from alternative sources (28). Today,
research continues on membrane-based methods of purifying alternate water sources such
as seawater, brackish water, and wastewater to drinking water standards to relieve water
stress. Currently, membrane technology is widely applied for pretreatment (microfiltration
and ultrafiltration), for softening and removal of dissolved metals and organic molecules
(nanofiltration), and for desalination (reverse osmosis). Reverse osmosis is also used
in the high technology sector for ultrapure water production. Membrane separations are
favored over many other treatment options because, in principle, they require no chemical
additives, thermal inputs, or spent media disposal; however, this is rarely achieved in
practice owing to fouling.
Gas separations are widely applied in industry. The earliest large-scale application of
gas separations was for hydrogen recovery from ammonia purge gases starting in the
1970s (35). The use of this technology has since increased as the need for pure hydrogen
gas increased in applications such as hydrotreating, hydrocracking, and hydrodesulfurization due to new environmental regulations. Today, gas separations are broadly used
to treat flue gas, syngas, and other off-gases for emissions control and carbon dioxide
capture. Other industrial applications of gas separations include air separation for nitrogen production, carbon dioxide separation for power production, recovery of organics in
mixed gas streams, and natural gas dehydration.
While membrane technologies are most widely applied in the realm of medicine and
water treatment, other applications exist and continue to grow. Membrane processes are
incorporated in various energy applications, including batteries, fuel cells, and osmotic
energy recovery (e.g., pressure-retarded osmosis). The food and beverage industry relies
on membranes for the production of pure water, filtration of fluid streams, meat packaging
materials, and production of carbon dioxide. Membranes are also applied in textiles, for
example, with high performance clothing such as Gore-Tex. Environmental applications
constitute a large demand for membrane technology. Pervaporation, nanofiltration, reverse
osmosis, and gas separations are applied in environmental remediation applications to
treat and remove regulated chemical and microbial contaminants.
4
SCOPE AND OBJECTIVES OF THIS ENCYCLOPEDIA
The field of synthetic membranes is quite rich and is continually offering challenges in
basic scientific research. At the same time, technology and applications of membranes are
growing in such areas as gas separations, energy generation, water purification, biotechnology, and many others to hold exciting prospects in coming years. While there are
books and journals covering many aspects of membrane science and technology, the
information in research journals is generally not accessible in a consolidated form to
reveal the history and current state of the science and state of the art in this field. This
INTRODUCTION
7
encyclopedia serves as a comprehensive reference on membrane science and technology,
covering all aspects at both fundamental and practical levels. To keep the content of this
encyclopedia up-to-date, new articles will be added periodically to the online edition and,
as necessary, existing articles will be revised to reflect current advances in the field. In
this way, this work should be viewed as a dynamic, ongoing project, which offers the
opportunity to critically assess, on a regular basis, the status of advancements in materials science of membranes and their engineering applications. This encyclopedia includes
five parts.
Part I: Membrane Separation and Transport focuses on basic principles and properties
of membranes and the primary separation mechanisms. Other topics include common metrics for membrane performance (e.g., permeability and rejection/retention),
and inherent issues of concentration polarization and membrane fouling, and currently practiced and emerging methods of addressing these challenges.
Part II: Membrane Materials, Characterization and Module Design focuses on classification of membrane materials, morphologies, and properties of commercial and
developmental membranes; methods of their synthesis, manufacture, and characterization; and on the design of membrane modules.
Part III: Membrane Processes focuses on membrane processes including: filtration
and osmotic processes, gas separations, electrochemical processes, and membrane
reactors.
Part IV: Membrane Applications focuses on membrane applications including water
treatment, energy generation, gas separations, processing of foods, biotechnology,
and many others.
Part V: Membrane Terminology, Societies, Conferences, and Symposia includes a collection of reference materials in an attempt to offer a one-stop reference on all
aspects of membrane science and technology.
REFERENCES
1. Mulder M. Basic Principles of Membrane Technology. London: Kluwer Academic Publishers;
1996.
2. Nollet JA. Investigations on the causes for the ebullition of liquids. J Membr Sci 1995;100:1.
3. Reuss F. Charge-induced flow. Proc Imp Soc Nat Moscow 1809;2:327.
4. Graham T. On the law of the diffusion of gases. J Membr Sci 1995;100:17.
5. Fick A. On liquid diffusion. J Membr Sci 1995;100:33.
6. Graham T. On the absorption and dialytic separation of gases by colloid septa. Part I—Action
of a septum of caoutchouc. J Membr Sci 1995;100:27.
7. van’t Hoff JH. The role of osmotic pressure in the analogy between solutions and gases. J
Membr Sci 1995;100:39.
8. Nernst W. Die elektromotorische Wirksamkeit der Jonen. Z Phys Chem 1889;4:129.
9. Planck M. Über die erregung von electricitat und warme in electrolyten. Ann Phys Chem
1890;39:161.
10. Donnan FG. Theory of membrane equilibria and membrane potentials in the presence of
non-dialysing electrolytes. A contribution to physical-chemical physiology. J Membr Sci
1995;100:45.
8
INTRODUCTION
11. Meyer KH, Sievers IF. The permeability of membranes I—the theory of ionic permeability
I. Helv Chim Acta 1936;19:649.
12. Teorell T. General discussion: artificial membranes. Trans Faraday Soc 1937;33:1035, 1086.
13. Schmid G. Zur Elektrochemie Feinporiger Kapillarsysteme. Zeitschrift Fur Elektrochemie
1950;54:424.
14. Meares P. The conductivity of a cation-exchange resin. J Polym Sci 1956;20:507.
15. Kedem O, Katchalsky A. Thermodynamic analysis of the permeability of biological membranes to non-electrolytes. Biochim Biophy Acta 1958;27:229.
16. Lonsdale HK, Merten U, Riley RL. Transport properties of cellulose acetate osmotic membranes. J Appl Polym Sci 1965;9:1341.
17. Michaels AS. New separation technique for CPI. Chem Eng Progr 1968;64:31.
18. Bechhold H. Colloid studies with the filtration method. Z Phys Chem-Stoch Ve 1907;
60:257.
19. Zsigmondy R, Bachmann W. On new filters. Z Anorg Allg Chem 1918;103:119.
20. Kressman TRE. Ion exchange resin membranes and resin-impregnated filter paper. Nature
1950;165:568.
21. Murphy EA. Apparatus for the electrical treat. US patent 2,331,494. 1943.
22. Juda W, McRae WA. Coherent ion-exchange gels and membranes. J Am Chem Soc 1950;
72:1043.
23. Baker RW. Membrane Technology and Applications. 2nd ed. New York: John Wiley & Sons,
Ltd; 2004.
24. Reid CE, Breton EJ. Water and ion flow across cellulosic membranes. J Appl Polym Sci
1959;1:133.
25. Loeb S, Sourirajan S. Sea water demineralization by means of an osmotic membrane.
In: Robert F Gould, editor. Saline Water Conversion-II . American Chemical Society;
1963.
26. Cadotte JE. Reverse osmosis membrane. US patent 4,039,440. 1975.
27. Muirhead A, Beardsley S, Aboudiwan J. Performance of the 12,000 m3 /d seawater reverse
osmosis desalination plant at Jeddah, Saudi Arabia January 1979 through January 1981.
Desalination 1982;42:115.
28. Hoek EMV. RO technology: UCLA’s contributions to research and education. Australian
Water J 2006;42.
29. Abel JJ, Rowntree LG, Turner BB. On the removal of diffusible substances from the
circulating blood of living animals by dialysis. J Pharmacol Exper Ther 1913–1914;
5:275.
30. Judd S, Judd C. The MBR Book: Principles and Applications of Membrane Bioreactors for
Water and Wastewater Treatment. Butterworth-Heinemann; 2011.
31. van Reis R, Zydney A. Membrane separations in biotechnology. Curr Opin Biotech 2001;
12:208.
32. Chang TMS. Volume I, Artificial Cells: Biotechnology, Nanomedicine, Regenerative Medicine,
Blood Substitutes, Bioencapsulation, and Cell/Stem Cell Therapy. Singapore: World Scientific
Publishing Company Incorporated; 2007.
33. Kolff WJ, Berk HThJ, Welle M ter, van der Ley AJW, van Dijk EC, van Noordwijk J, The
artificial kidney: a dialyser with a great area. Acta Med Scand 1944;117:121.
34. Glater J. The early history of reverse osmosis membrane development. Desalination 1998;
117:297.
35. Bernardo P, Drioli E, Golemme G. Membrane gas separation: a review/state of the art. Ind
Eng Chem Res 2009;48:4638.