"Introduction" In: Encyclopedia of Membrane Science and Technology

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)ofmixing,

≥ = − 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

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 Applied pressure

(a) (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), c R = 1 − p cf INTRODUCTION 3

Membrane Pf module 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, = SA αA/B SB = where S ( cp/cf)isthesieving 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, dX J =−A 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 Type of Transport Flux Coefficient Gradient 2· =− 2 × Fick’s law mass flux Jm (kg/(m s)) D (m /s) dc/dx 3 2· =− 2 · × Darcy’s law volume flux Jv m /(m s) K /μ (m /(Pa s)) dP/dx 2· =− · × Fourier’s law heat flux Jh J/(m s) λ (W/(m K)) dT /dx · 2 =− · × Newton’s law momentum flux Jn (kg/(m s )) μ (kg/(m s)) dv/dx 2· =− × Ohm’s law electrical flux Ji C/(m s) G (S) dE/dx 4 INTRODUCTION

2 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 (nitro- cellulose) 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. Biolog- ical 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 aquapor- ins, 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 func- tion 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 pharmaceuti- cal 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 hydrodesulfur- ization 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 nitro- gen 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 com- mon 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 clas- siﬁcation 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 col- lection of reference materials in an attempt to offer a one-stop reference on all aspects of membrane science and technology.

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