C H A P T E R 1

M E M B R A N E S AND M E M B R A N E P R O C E S S E S

In its earlier stages of development, membrane science and technology focused mostly on naturally occurring membranes despite evidences of studies of synthetic membranes in the mid-nineteenth century. This is not surprising in light of the fact that all life forms use natural membranes for separation of nutrients, selective protection from toxins, photosynthesis, etc. In the early to middle twentieth century, it began to evolve from a fairly narrow scientific discipline with limited practical applications to a broader field with diverging applications that support a unique industry which started in the late 1960s and early 1970s. Higher energy costs in recent decades have made the membrane processes even more economically competitive with conventional separation technologies such as distillation, crystallization, absorption, adsorption, solvent extraction, or cryogenics. An excellent overview on the historic growth of membrane technology has been published previously by one of the membrane pioneers, H. K. Lonsdale (1982).

1.1 MEMBRANE PROCESSES

A simplified working definition of a membrane can be conveniently stated as a semipermeable active or passive barrier which, under a certain driving force, permits preferential passage of one or more selected species or components (molecules, particles or polymers) of a gaseous and/or liquid mixture or solution (Figure 1.1).

The primary species rejected by the membrane is called retentate(s) or sometimes just "solute" while those species passing through the membranes is usually termed permeate(s) or sometimes "solvent." The driving force can exist in the form of pressure, concentration, or voltage difference across the membrane. Depending on the driving force and the physical sizes of the separated species, membrane processes are classified accordingly: microfiltration (M~, ultrafiltration (UF), reverse osmosis (RO), dialysis, electrodialysis (ED) and gas separation (Table 1.1). Their current status of the technology is summarized in Table 1.2 [Baker et al., 1991]. Any major and minor technical problems associated with each membrane process that need to be solved for further process improvement and applications development are listed along with the technical hurdles that are essentially overcome.

Membrane processes can be operated in two major modes according to the direction of the feed stream relative to the orientation of the membrane surface: dead-end filtration and crossflow filtration (Figure 1.1). The majority of the membrane separation applications use the concept of crossflow where the feed flows parallel to and past the membrane surface while the permeate penetrates through the membrane overall in a

2 Chapter I

direction normal to the membrane. The shear force exerted by the flowing feed stream on the membrane surface help remove any stagnant and accumulated rejected species that may reduce permeation rate and increase the retentate concentration in the permeate. Predominant in the conventional filtration processes, dead-end filtration is used in membrane separation only in a few cases such as laboratory batch separation. In this mode, the flows of the feed stream and the permeate are both perpendicular to the membrane surface.

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Figure 1.1 Schematic diagram of a membrane process: (a) crossflow filtration mode and (b) dead-end filtration mode

Two of the most important parameters that describe the separation performance of a membrane are its permselectivity and permeability. Permeability is typically used to provide an indication of the capacity of a membrane for processing the permeate; a high permeability means a high throughput. A high throughput is useless, however, unless

Membrane and Membrane Processes 3

TABLE 1.1 Membrane separation processes

Process ' Microf'dtrauon

UltmFfltration

Gas separation

Driving force Typical permeate Pressure difference, Water and 10 psi dissolved species Pressure difference, Water and salts 10-100 psi Pressure difference, Gases and vapors 1-100 atm

Reverse osmosis Pressure difference, Water 100-800 psi

Dialysis Concentration difference

Ions and low molecular-weight organics

Electrodialysis Voltage Ions

(Adapted from Lonsdale [1982])

Typical retentate Suspended materials

Biologicals, colloids and macromolecules Membrane- impermeable gases and vapors Virtually all suspended and dissolved materials Dissolved and suspended materials with molecular weight > 1,000 All non-ionic and macromolecular species

another important membrane property, permselectivity, also exceeds an economically acceptable level. On the other hand, a membrane with a high permselectivity but a low flux or permeability may require such a large membrane surface area that it becomes economically unattractive. Simply put, permselectivity is the ability of the membrane to separate the permeate from the retentate. For MF, UF and RO, it is usually conveniently expressed in terms of a rejection or retention coefficient:

R= 1- Cp/Cr (1-1)

where Cp and Cr represent the concentrations of the rejected species in the permeate and retentate, r e s t i v e l y . The rejection coefficient essentially gives a percentage of the rejected species that "leaks" through the membrane. Since the coefficient refers to retention by the membrane of a given species in certain defined experimental conditions, caution should be exercised in its extensive use especially beyond the test conditions.

As a general rule, the membrane technology is a competitive separation method for small to medium volumetric flowrate applications and for either primary separation or when the purity level required is in the 95 to 99% range.

4 Chapter 1

TABLE 1.2 Current status of membrane technology

Problems

Process Major Minor Microf'dtration Reliability Cost

(fouling)

Ultrafiltration Reliability Cost (fouling)

Reverse osmosis Reliability Selectivity

Electrodialysis Fouling Cost Temperatur e stability

Gas separation Selectivity Cost Flux

Pervaporation Selectivity Cost Reliability

Coupled and Reliability facilitated (membrane transport stability)

(Adapted from Baker et al. [ 1991 ])

Mostly solved Selectivity

Selectivity

Cost

Selectivity Reliability

Reliability

Comments Better fouling control could improve membrane lifetime significantly Fouling remains the principal operational problem of ultrafiltration. Current fouling control techniques are a substantial portion of process costs Incremental improvements in membrane and process design will gradually reduce costs Process reliability and selectivity are adequate for current uses. Improvements could lead to cost reduction, especially in newer applications Membrane selectivity is the principal problem in many gas separation systems. Higher permeation rates would help to reduce costs Membrane selectivities must be improved and systems developed that can reliably operate with organic solvent feeds before major new applications are commercialized Membrane stability is an unsolved problem. It must be solved before this process can be considered for commercial applications

Membrane and Membrane Processes 5

1.2 POLYMERIC MEMBRANES

While the market size of the membrane industry worldwide varies from one estimate to another, it is generally agreed that the industry grew to be approximately $1 billion annually in 1986 and was expected to exceed $4 billion annually in the mid-1990s. The breakdown of four major market components according to various sources can be summarized as given in Table 1.3. The industry so far is dominated by polymeric membranes which after many years of research and development and marketing by several giant chemical companies particularly in the U.S. start to enjoy niche applications ranging from desalination of sea and brackish waters, food and beverage processing, gas separations, hemodialysis to controlled release.

TABLE 1.3 Worldwide membrane market size

Process Major Applications 1986 Sales 1996 Sales (Forecast) Microfiltration Biotechnology, $550 million $1.5 billion

chemicals, electronics, environmental control, food & beverage, pharmaceutical

Ultrafiltration Biotechnology, $350 million $1.1 billion chemicals, environmental control, food & beverage

Reverse osmosis Desalinating sea water, $120 million $530 million electronics, food & beverage Environmental control Gas separation $20 million $1.5 billion

Total $1 billion $4.6 billion

1.2.1 Membrane Materials and Preparation

The first widespread use of polymeric membranes for separation applications dates back to the 1960-70s when cellulose acetate was cast for desalination of sea and brackish waters. Since then many new polymeric membranes came to the market for applications extended to ultrafiltration, microfiltration, dialysis, electrodialysis and gas separations. So far ultrafiltration has been used in more diverse applications than any other membrane processes. The choice of membrane materials is dictated by the application environments, the separation mechanisms by which they operate and economic considerations. Table 1.4 lists some of the common organic polymeric materials for various membrane processes. They include, in addition to cellulose acetate, polyamides,

6 Chapter 1

polyimides, polysulfones, nylons, polyvinyl chloride, polycarbonate and fluorocarbon polymers.

TABLE 1.4 Commonly used membrane materials and their properties

Material Application(s) Approximate pH maximum range working temperature (oc)

Cellulose acetates RO, UF, MF 50 3-7 Aromatic RO, UF 60-80 3-11 polyamides Fluorocarbon RO, UF, MF 130-150 1-14 polymers Polyimides RO, UF 40 2-8 Polysulfone UF, MF 80-100 1-13 Nylons UF, MF 150-180 Polycarbonate UF, MF 60-70 Polyvinyl chloride 120-140 PVDF UF 130-150 1-13 Polyphosphazene 175-200 Alumina (gamma) UF 300 5-8 Alumina (alpha) MF >900 0-14 Glass RO, UF 700 1-9 Zirconia UF, MF 400 1-14 Zirconia (hydrous) DM (RO, UF) 80-90 4-11 Silver MF 370 1-14 Stainless steel (316) MF >400 4-11

The preparation methods of organic polymeric membranes depend on the structural characteristics of the membranes suitable for specific applications. For example, to prepare dense symmetrical membranes where the structures are more or less homogeneous throughout, solution casting and melt forming have been used. To produce microporous symmetrical membranes, common techniques such as irradiation (will be discussed in Chapter 3), stretching or template leaching can be employed. For asymmetric membranes, a large array of methods can be applied: phase inversion (or called solution-precipitation), interfacial polymerization, solution casting, plasma polymerization and reactive surface treatment. The phase inversion process, used in making the earliest commercial microporous membranes (cellulosic polymers), remains one of the most widely used techniques for fabricating a wide variety of commercial membranes today. The phase inversion process is applicable to any polymers that can be

Membrane and Membrane Processes 7

dissolved in a solvent and precipitated in a continuous phase by a miscible nonsolvent. It is capable of making polymeric membranes with a wide range of the pore size by varying polymer content, temperature, composition of the precipitation medium and the type of solvent.

It is not the intent of this book to get into any details of organic polymeric membranes. The readers, therefore, are referred to some recently published books in this field for the synthesis, characteristics and applications of various organic membranes [Belfort, 1984; Lloyd, 1985; Sourirajan and Matsuura, 1985; Baker, 1991].

1.2.2 Membrane Elements and Modules

Polymeric membrane elements and modules which consist of elements come in different shapes. The shape strongly determines the "packing density" of the element or module which is indicative of the available membrane filtration area per unit volume of the element or module; the packing density, in turn, can affect the capital and operating costs of the membranes. The packing density is often balanced by other factors such as ease and cost of maintenance and replacement, energy requirements, etc. Most of the polymeric membranes are fabricated into the following forms: tube, tubes-in-shell, plate- and-frame, hollow-fiber, and spiral-wound.

As a technology borrowed from the filtration industry, the plate-and-frame design is comprised of a series of planar composite membranes sandwiched between spacers that act as rigid porous supports and flow channels. The most common usage of this configuration is the electrodialysis cells. The tube-in-shell configuration resembles that in a shell-and-tube heat exchanger. Each tube can be cleaned, plugged off or replaced independently of other tubes. This can be an advantage but also a disadvantage particularly when hundreds or thousands of tubes are packed in a shell. The spiral-wound configuration can be viewed as a plate-and-frame system on flexible porous supports that has been wrapped around a central porous tube several times. The porous supports also provide the flow path for the permeate. The feed flows axially into the feed channels. Retentate continues through these channels to the exit end. The permeate flows spirally to the central tube where it is collected. The available membrane filtration area per unit volume of the module is about one and a half times that for a plate-and-frame design. This very compact configuration, however, makes it difficult for mechanical cleaning and is not suitable for particulate feed streams. Hollow fiber has the maximum packing density, even higher than the spiral-wound design. A hollow fiber module may consist of thousands of hair-like hollow membrane fibers, usually 50 to 100 gm in diameter, assembled into a bundle and contained in a vessel. Hollow fiber membranes have been used extensively in reverse osmosis. This configuration is also susceptible to fouling of particles or macromolecules.

8 Chapter I

1.3. GENERAL COMPARISONS OF MEMBRANES

ORGANIC AND INORGANIC

The advantages of inorganic membranes have been recognized for a long time. In fact, studies of the use of some inorganic membranes such as platinum and porous glass were evident even in the last century. Thermal and pH resistance characteristics of inorganic and organic membranes are compared in Table 1.4 where approximate ranges of operable temperature and pH are given. Although inorganic polymers have not been used commercially yet, polyphosphazene which is under development is included in the table. It can be seen that thermal stabilities of organic polymers, inorganic polymers and inorganic materials as membrane materials can be conveniently classified as <100- 150~ 100-350~ and >350~ respectively. It should be stressed that the use of pH stability is only a crude indication of the chemical stability of the membrane material. An example is silver which can withstand strong bases and certain strong acids. Although silver is resistant to strong hydrochloric or hydrofluoric acid, it is subject to the attack by nitric and sulfuric acids and cyanide solutions. Therefore, the wide operable pH range of silver can not be construed that it is resistant to all acids.

The operable temperature limits of inorganic membranes are obviously much higher than those of organic polymeric membranes. The majority of organic membranes begin to deteriorate structurally around 100~ Thermal stability of membranes is becoming not only a technical problem but also an economic issue. In gas separation applications, for example, if the membrane can withstand the required high process temperatures, the need to ramp down the temperature to maintain the physical integrity of an organic membrane and to ramp up the temperature again after separation can be eliminated.

Inorganic membranes generally can withstand organic solvents, chlorine and other chemicals better than organic membranes. This also permits the use of more effective and yet corrosive cleaning procedures and chemicals. Many organic membranes are susceptible to microbial attack during applications. This is not the case with inorganic types, particularly ceramic membranes. In addition, inorganic membranes in general do not suffer from the mechanical instability of many organic membranes where the porous support structure can undergo compaction under high pressures and cause decrease in permeability.

It is obvious that in a high temperature or harsh chemical environment, inorganic membranes could become the only recourse to many challenging separation applications.

Another very important operating characteristics of inorganic membranes that is not shown in Table 1.4 has to do with the phenomena of fouling and concentration polarization. Concentration polarization is the accumulation of the solutes, molecules or particles retained or rejected by the membrane near its surface. It is deleterious to the purity of the product and the decline of the permeate flux. Fouling is generally believed to occur when the adsorption of the rejected component(s) on the membrane surface is strong enough to cause deposition. How to maintain a clean membrane surface so that

Membrane and Membrane Processes 9

the membrane can be continuously used without much interruption has been a key operational issue with many membranes. First of all, some inorganic membranes such as microporous alumina membranes and surface treated porous glass membranes are more fouling resistant due to their low protein adsorption. Secondly, many inorganic membranes are less susceptible to biological and microbial degradation. And finally with some inorganic membranes, for example porous ceramic and metallic membranes, it is possible to apply short bursts of permeate streams in the reverse direction through the membrane to dislodge some clogged pores of the membrane. This is referred to as backflush. This way the maintenance cycle of the membrane system can be prolonged. More details of this aspect of operation will be discussed in Chapter 5.

Because of the unique characteristics of inorganic membranes mentioned above, the search for inorganic membranes of practical significance has been continuing for several decades. With the advent of ceramic membranes with superior stabilities coming to the separation markets, the potentials for inorganic membranes as separators and/or reactors are being explored at an accelerated rate never witnessed before.

1.4 TYPES OF INORGANIC MEMBRANE STRUCTURES

To facilitate discussions on the preparation methods, characteristics and applications of inorganic membranes in the following chapters, some terminologies related to the types of membranes according to the combined structures of the separating and support layers, if applicable, will be introduced.

Membranes can be divided into two categories according to their structural characteristics which can have significant impacts on their performance as separators and/or reactors (membrane reactors or membrane catalysts): dense and porous membranes. Dense membranes are free of discrete, well-defined pores or voids. The difference between the two types can be conveniently detected by the presence of any pore structure under electron microscopy. The effectiveness of a dense membrane strongly depends on its material, the species to be separated and their interactions with the membrane.

The microstructure of a porous membrane can vary according to the schematic in Figure 1.2. The shape of the pores is strongly dictated by the method of preparation which will be reviewed in Chapter 3. Those membranes that show essentially straight pores across the membrane thickness are referred to as straight pore or nearly straight pore membranes. The majority of porous membranes, however, have interconnected pores with tortuous paths and are called tortuous pore membranes.

When the separating layer and the bulk support designed for mechanical strength are indistinguishable and show an integral, homogeneous structure and composition in the direction of the membrane thickness, it is called a symmetric or isotropic membrane. Since the flow rate through a membrane is inversely proportional to the membrane

10 Chapter I

thickness, it is very desirable to make the homogeneous membrane layer as thin as possible. However, very thin stand-alone membranes typically do not exhibit mechanical integrity to withstand the usual handling procedures and processing pressure gradients found in many separation applications. A practical solution to the dilemma has been the concept of an asymmetric or composite membrane where the thin, separating membrane layer and the open-cell mechanical support structure are distinctly different. In this "anisotropic" arrangement, separation of the species in the feed stream and ideally the majority of the flow resistance (or pressure drop) also takes place primarily in the thin membrane layer. The underlying support should be mechanically strong and porous enough that it does not contribute to the flow resistance of the membrane element to any significant extent.

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If a membrane has a graded pore structure but is made in one processing step, frequently from the same material across its thickness, it is called an asymmetric membrane. If, on the other hand, the membrane has two or more distinctively different layers made at different steps, the resulting structure is called a composite membrane. Almost invariably in the case of a composite membrane, a predominantly thick layer provides the necessary mechanical strength to other layers and the flow paths for the permeate and is called the support layer or bulk support. Composite membranes have the advantage that the separating layer and the support layer(s) can be tailored made with different materials. Permselective and permeation properties of the membrane material are critically important while the material for the support layer(s) is chosen for mechanical strength and other consideration such as chemical inermess. The composite membranes can have

Membrane and Membrane Processes 11

more than two layers in which the separating layer is superimposed on more than one support layer. In this case the intermediate layer serves a major purpose of regulating the pressure drops across the membrane-support composite by preventing any appreciable penetration of the very fine constituent panicles into the pores of the underlying support layer(s). Typically the intermediate support layer(s) is also thin.

The aforementioned membranes are made prior to end use. There is a different type of inorganic membranes that are formed in-situ while in the application environment. They are called dynamic membranes which have been studied a great deal especially in the 1960s and 1970s. The general concept is to filter a dispersion containing suspended inorganic or polymeric colloids through a microporous support to form a layer of the colloids on the surface of the support. This layer becomes the active separating layer (membrane). Over time this permselective layer is eroded or dissolves and must be replenished as they are washed away in the retentate. Commonly used materials for the support are porous stainless steel, carbon or ceramics. A frequently used dynamic membrane material is hydrous metal oxides such as zirconium hydroxide although some organic colloids such as polyvinyl methyl ether or acrylic acid copolymers have been used as well. Dynamic membranes have been investigated for reverse osmosis applications such as desalination of brackish water, but found to be difficult to provide consistent performance and the added cost of the consumables makes the process unattractive economically. Today only limited applications for recovering polyvinyl alcohol in the field of textile dyeing is commercially practiced.

There is another type of membrane called liquid membrane where a liquid complexing or carrier agent supported or immobilized in a rigid solid porous structure function as the separating transport medium (membrane). The liquid carrier agent completely occupies the pores of the support matrix and reacts with the permeating component on the feed side. The complex formed diffuses across the membrane/support structure and then releases the permeant on the product side and at the same time recovers the carrier agent which diffuses back to the feed side. Thus permselectivity is accomplished through the combination of complexing reactions and diffusion. This is often referred to as facilitated transport which can be used for gas separation or coupled transport which can separate metal compounds through ion transport. An example of the former is some molten salts supported in porous ceramic substrate that are selective toward oxygen and of the latter is some liquid ion exchange reagent for selectively transporting copper ions. In this configuration, the composite of the liquid membrane and its support can be considered to be a special case of dense membranes. Despite their potentials for very high selectivities, liquid membranes suffer from physical instability of the membrane in the support and chemical instability of the carrier agent. Consequently, liquid membranes have not seen any significant commercial applications and is not likely to be a major commercial force in the separation industry in the next decade [Baker et al., 1991 ]. The liquid membrane process will not be treated in this book, but the use of inorganic membranes as the carriers for liquid membranes will be briefly discussed in Chapter 7.

12 Chapter 1

1.5 INORGANIC MEMBRANES ACTIVELY PARTICIPATING IN CHEMICAL REACTIONS

Among the frontier developments in the field of inorganic membranes, a general area is particularly promising. It involves the use of a membrane as an active participant in a chemical transformation. This process integration stems from the concept of a membrane reactor where the membrane not only plays the role as a separator but also as part of a reactor. With their better thermal stability than organic membranes, inorganic membranes logically become potential attractive candidates for the use in many industrially important chemical reactions that are frequently operated at high temperatures and, in some cases, under harsh chemical environments.

For the above reason, a significant portion of this book will be devoted to the various issues centering around the utilization of inorganic membranes as both a separator and a reactor. Chapters 8 through 11 will deal with those subjects.

1.6 SUMMARY

While organic membranes have enjoyed over two decades of commercial success, inorganic membranes are just making their inroads to a growing number of commercial applications. This new market development is fueled by the availability of consistent quality ceramic membranes introduced in recent years. Compared to their organic counterparts, inorganic membranes made of metals, ceramics and inorganic polymers typically exhibit stabilities at high temperatures and extreme pH conditions.

Their historical developments and various membrane preparation methods will be discussed in Chapters 2 and 3, respectively. Chapter 4 reviews the general separation and non-separation properties of the membranes and the methods by which they are measured. Chapter 5 presents commercial membrane elements and modules and their application features which are followed by discussions of liquid-phase separation applications in Chapter 6. Many of those applications are commercially practiced. Potential gas separation and other applications (such as sensors and supports for liquid membranes) will be discussed in Chapter 7.

Those higher thermal and chemical stabilities not only make inorganic membranes very suitable for separation applications, but also for reaction enhancement. The duel use of an inorganic membrane as a separator as well as reactor offers great promises which along with potential hurdles to overcome will be treated in Chapters 8 through 11. The concept and related material, catalytic and engineering issues are addressed.

Market and economic aspects as well as major technical hurdles to be resolved prior to widespread usage of inorganic membranes are finally summarized in Chapter 12.

Membrane and Membrane Processes 13

REFERENCES

Baker, R.W., E.L. Cussler, W. Eykamp, W.J. Koros, R.L. Riley and H. Strathmann, 1991, Membrane Separation Systems - Recent Developments and Future Directions (Noyes Data Corp., New Jersey, USA).

Belfort, G. (Ed.), 1984, Synthetic Membrane Processes (Academic Press, Orlando, USA).

Lonsdale, H.K., 1982, J. Membrane Sci. 10, 81. Lloyd, D.R. (Ed.), 1985, Materials Science of Synthetic Membranes (ACS Symposium

Ser. 269, American Chemical Society, Washington, D.C., USA). Sourirajan, S., and T. Matsuura (Eds.), 1985, Reverse Osmosis and Ultrafiltration (ACS

Symposium Ser. 281, American Chemical Society, Washington, D.C., USA).