Ion-Exchange Membranes in the Chemical Process Industry

Review pubs.acs.org/IECR

Ion-Exchange Membranes in the Chemical Process Industry Heiner Strathmann,* Andrej Grabowski, and Gerhart Eigenberger Institute of Chemical Process Engineering, Stuttgart University, Boeblinger Str. 78, D-70199 Stuttgart, Germany ABSTRACT: Ion-exchange membranes play an important role today in deionization of aqueous solutions, in electrochemical synthesis, and in energy conversion and storage. Some of the applications of ion-exchange membranes are mature and well established processes such as the water desalination by electrodialysis or the electrolytic chlorine−alkaline synthesis. Other applications of ion-exchange membranes are still in an early state of their development, such as the redox flow battery. In this publication the principles of state-of-the-art ion-exchange membrane processes and their applications are briefly described. Their advantages and limitations are discussed, and their commercial relevance is indicated. More recently developed products and processes are also addressed. Their basic functions are described, and their present and potential future applications are discussed. Research needs for a further improvement of ion-exchange membranes and their applications are pointed out.

1. INTRODUCTION Ion-exchange membranes are used today on a large industrial scale in processes such as electrodialysis, electrodeionization, diffusion dialysis, membrane electrolysis, and so forth for the production of potable and industrial water, for the treatment of industrial effluents,1 and for the chlorine−alkaline production.2 More recently, interest in other processes such as the electrodialytic water dissociation with bipolar membranes3 or the capacitive deionization4 is rapidly growing. Especially, the application of ion-exchange membranes in energy conversion and energy storage systems such as fuel cells5 or redox flow batteries, reverse electrodialysis, and so forth may play an important role in the future of electrical energy generation without the use of fossil feed stock.

conductors and is increasing with temperature, while the conductivity of electrons in metal conductors is decreasing with temperature. In spite of these basic differences, the electrical current can be described in both cases by the same mathematical relation, that is, by Ohm’s law, which is given by

Here, U and I are the electrical potential difference and the electrical current and R is the electrical resistance. The resistance R is a function of the specific resistance ρ (or specific conductivity κ) of the material, the distance l between the electron sources, and the cross sectional area A of the material through which the electric current is transported: R=ρ

2. FUNDAMENTALS OF ION-EXCHANGE MEMBRANE PROCESSES Membranes used in electromembrane processes are films of ion-exchange polymer swollen in water. In the ideal case they must be permeable for ions of one charge (counterions) and impermeable for the ions of opposite charge (co-ions) and for hydrodynamic water flow. All processes with ion-exchange membranes as key components have several fundamental relations in common which determine their technical feasibility and their economics in different applications. Before entering into a discussion of various processes and their applications, some fundamental electrochemical and thermodynamic equations as well as mass transport phenomena will be considered in relation to the main process parameters. 2.1. Electron and Ion Conductivity and Ohm’s Law. The transport of electric charges, that is, an electric current, can be achieved either by the transport of electrons in solid material such as metals or by transport of ions in electrolyte solutions and ion-exchange membranes. The most important difference between electron and ion conductivity is that ion conductivity is always coupled with a mass transport of ions while, due to the very small mass of an electron, virtually no mass is transported in an electron conductor. Furthermore, the conductivity in electrolytes is several orders of magnitude lower than that of usual electron © 2013 American Chemical Society

(1)

U = RI

l l = A Aκ

(2)

The conductivity of electrolyte solutions depends on the concentration Ci, the equivalent conductivity λeq i , and the charge number zi of the ions i in the solution and is given by κ=

∑ |zi|λieqCi i

(3)

The electrical current I passing through an electrolyte solution under the driving force of an electrical potential gradient is proportional to the flux of electrical charges and is given by

I = FA ∑ zi Ji i

(4)

Here, F is the Faraday constant and Ji is the ion flux density, which is proportional to λeq i , Ci, and the electrical potential gradient Δφ/l: Special Issue: Enrico Drioli Festschrift Received: Revised: Accepted: Published: 10364

January 18, 2013 March 13, 2013 March 13, 2013 March 13, 2013 dx.doi.org/10.1021/ie4002102 | Ind. Eng. Chem. Res. 2013, 52, 10364−10379

Industrial & Engineering Chemistry Research |Ji | =

λieq CiΔφ Fl

Review

(5)

2.2. Electrochemical Potential and the Donnan Equilibrium. Transport of an ion across an ion-exchange membrane or within an electrolyte solution occurs under the driving force of the electrochemical potential gradient of the ion. The electrochemical potential μ̃i is a function of the electrical potential φ and the chemical potential μi of an ion, which at constant temperature is a function of the total pressure p and ion activity ai. It can be described by μĩ = μi + Fziφ = μi◦ + Vmip + RT ln ai + Fziφ

dx

=

dμi dx

+ Fzi

(6)

d ln ai dp dφ dφ = Vmi + RT + Fzi dx dx dx dx (7)

Electrochemical equilibrium between two phases such as an electrolyte solution and an ion-exchange membrane exists when the electrochemical potentials of the ions in the membrane and the solution are equal. Thus, for each ion in equilibrium, it is μĩ m = μĩ s = μi m + Fziφ m = μis + Fziφs

⎞ ⎛ as ⎞ 1 ⎛ ⎜⎜RT ln⎜ ma ⎟ + Vma Δπ ⎟⎟ Fza ⎝ ⎝ aa ⎠ ⎠

m Cco

(10)

Cs = s Cfix

(11)

Here the subscripts co, fix, and s refer to the concentrations of co-ions, fixed, and salt ions. Thus, the co-ion concentration in an ion-exchange membrane decreases with increasing concentration of the fixed ions and increases with increasing electrolyte solution concentration. 2.3. Transport of Ions and Ion-Exchange Membrane Permselectivity. To describe the ion transport in electrolyte solutions and ion-exchange membranes along the coordinate x, the extended Nernst−Planck equation applies.8 This equation is given by Ji = −Di

⎞ as 1 ⎛ = φ m − φs = ⎜RT ln mi + Vmi(ps − p m )⎟ Fzi ⎝ ai ⎠ =

=

2

(8)

The superscripts m and s refer to the membrane and to the solution, respectively. Thus, the electrochemical potential of an ion is composed of two additive terms; the first is the chemical potential and the second is the electrical potential multiplied by the Faraday constant and the charge number of the ion. Introducing the chemical potential μi as defined in eq 6 into eq 8 and rearranging gives the electrical potential difference between the membrane and the adjacent solution when the electrochemical equilibrium is reached. This equilibrium potential is referred to as Donnan potential φDon6 and is given by φDon

⎞ ⎛ as ⎞ 1 ⎛ ⎜⎜RT ln⎜ mc ⎟ + VmcΔπ ⎟⎟ Fzc ⎝ ⎝ ac ⎠ ⎠

The subscripts a and c refer to the anion and cation. As explained subsequently in Section 3, ion-exchange membranes are characterized by the nature of their fixed charges as either cation- or anion-exchange membranes. Mobile ions in the membrane with the same charge as the fixed charge are termed co-ions and those with opposite charge counterions. Equation 10 provides a general relation for the co-ion and counterion distribution at the interface between a solution and an ion-exchange membrane in equilibrium. This equilibrium, which is referred to as Donnan equilibrium, is important in electro-membrane processes. It is the basis for estimating the exclusion of co-ions in a membrane as a function of the ion concentration in the bulk solution. The calculation of the co-ion concentration in the membrane for a multicomponent electrolyte under practical conditions is rather complex. However, for a monovalent electrolyte the coion concentration in an ion-exchange membrane can be estimated under the assumption that the activity coefficient in the solution and in the membrane are approximately 1, that the co-ion concentration in the membrane is much lower than in the solution, and that osmotic effects can be neglected. Under these conditions the co-ion concentration in a homogeneous membrane in contact with an electrolyte solution can be expressed to a first approximation7 by

Here μi° is the chemical potential at standard conditions and Vmi is the partial molar volume of component i, R is the gas constant, and T is the temperature. The driving force for the transport of ions through a membrane, that is, the electrochemical potential gradient in the x-direction perpendicular to the membrane surface, is given by dμĩ

φDon =

Fz C dφ dCi + −Di i i + vCi RT dx dx

(12)

Here, Di is the diffusion coefficient of ion i and v is the linear convective velocity along x. Three terms of eq 12 represent three different modes of mass transport in ion-exchange membranes: • diffusion by a gradient in concentration or chemical potential • migration by an electrical potential gradient • convection by a pressure gradient The transport of ions is governed by the electroneutrality requirement which postulates that on a macroscopic scale negative and positive charges are compensated. By diffusion alone cations and anions move in the same direction and, due to electroneutrality, with the same speed, while in electromigration they are transported in opposite directions and carry different portions of the overall current. In ion-exchange

⎞ as 1 ⎛ ⎜RT ln mi + Vmi(Δπ )⎟ Fzi ⎝ ai ⎠ (9)

Here Δπ is the osmotic pressure difference between the membrane and the adjacent solution, which is also referred to as the swelling pressure of the membrane. The Donnan potential φDon cannot be measured directly. It can, however, be calculated from the ion activities in the solution and the membrane and by the swelling pressure Δπ. The numerical value of the Donnan potential φDon can be calculated either from the cation or from the anion activities. For a single salt it is 10365

dx.doi.org/10.1021/ie4002102 | Ind. Eng. Chem. Res. 2013, 52, 10364−10379

Industrial & Engineering Chemistry Research

Review

• high chemical stabilitythe membrane should be stable over the entire pH range from 1 to 14 and in the presence of oxidizing agents The properties of ion-exchange membranes are determined by two parameters, that is, the basic material they are made from and the type and concentration of the fixed ion. The basic material determines to a large extent the mechanical, chemical, and thermal stability of the membrane. Most of today’s ionexchange membranes used in commercially relevant processes are based on hydrocarbon or fluorinated hydrocarbon polymers.9 The type and the concentration of the fixed ions in a membrane determine its permselectivity and the electrical resistance, but they also have a significant effect on the mechanical properties of the membrane and especially on the degree of swelling. The ions often used as fixed charges in cation-exchange membranes are −SO3− and −COO− and in anion-exchange membranes are −N+HR2, −N+R3. It is difficult to optimize the properties of ion-exchange membranes because the parameters determining their properties often act contrary to one another. For instance, a high degree of polymer cross-linking improves the mechanical strength of the membrane but also increases its electrical resistance. A high concentration of fixed ions in the membrane matrix leads to a low electric resistance but causes a high degree of swelling combined with poor mechanical stability. 3.1. Preparation of Ion-Exchange Membranes. The preparation of ion-exchange membranes is described in a large number of detailed recipes in the literature.10−12 For the practical preparation of ion-exchange membranes two rather different procedures are used. The first procedure results in a homogeneous ion-exchange membrane structure and is closely related to the preparation of ion-exchange resins. The difference between membranes and resins arises largely from the mechanical requirements. Generally, ion-exchange resins are mechanically weak and changes in the electrolyte concentration may cause major changes in the water uptake and hence in the volume of the resin. These changes are not acceptable in large sheets of ion-exchange membranes which have been cut to fit in an apparatus. Therefore most ionexchange membranes are reinforced by stable material which gives the necessary dimensional stability. Homogeneous ionexchange membranes are produced by either a polymerization of monomers that carry anionic or cationic moieties or by introducing these moieties into a polymer which may be in an appropriate solution or a solid preformed film. A typical example for the preparation of an ion-exchange membrane by polymerization and sulfonation of monomers is the polymerization of styrene with divinylbenzene. An example for introducing a charged moiety into a polymer is the preparation of anion-exchange membranes by introducing positively charged groups into polystyrene by chloromethylation and amination with triamine. A cation-exchange membrane with exceptional good chemical and thermal stability which is widely used in the electrolytic chlorine alkaline production and as the polymer electrolyte in low temperature fuel cells consists of a polyfluorocarbon material. This membrane is often referred to as the Nafion membrane,2 which is the trade name of DuPont.

membranes the current is carried preferentially by the counterions. The fraction of the current that is carried by a certain ion is expressed by the ion transport number ti, which is given by

ti =

zi Ji ∑i zi Ji

(13)

The sum of the transport number of all ions in a solution is obviously 1. The transport numbers of cations and anions in an aqueous salt solution do not differ very much. In an ion-exchange membrane, however, the concentration of the counterions is much higher than that of the co-ions and the sum of the transport numbers of the counterions is close to 1. The transport number tc of the counterion in a membrane is related to the permselectivity Ψ of a membrane and given by Ψ cm =

tccm − tcs tas

and

Ψam =

taam − tas tcs

(14)

The superscripts cm and am refer to cation- and anionexchange membranes, and the subscripts c and a refer to cation and anion, respectively. An ideally permselective cation-exchange membrane would transmit positively charged ions only and its permselectivity is Ψcm = 1. The permselectivity approaches zero when the transport number within the membrane is identical to that in the electrolyte solution.

3. ION-EXCHANGE MEMBRANES In electro-membrane processes ion-exchange membranes are key components. Their unique properties to transport and separate ionic components selectively make ion-exchange membranes interesting tools in separation and energy conversion processes. Ion-exchange membranes can be classified by their function as a separation medium or according to the material they are made from and their structure. As far as their function is concerned, ion-exchange membranes are classified as • cation-exchange membranes which contain fixed negatively charged ions and which have a selective permeability for cations • anion-exchange membranes which contain fixed positively charged ions and which have a selective permeability for anions • bipolar membranes which consist of a cation- and an anion-exchange membrane laminated together that are used for splitting water in H+ and OH− ions as explained in Section 4.2.1 The most desired properties for ion-exchange membranes are • high permselectivityan ion-exchange membrane should be highly permeable for counterions, but should be impermeable to co-ions • low electrical resistancethe permeability of an ionexchange membrane for the counterions under the driving force of an electrical potential gradient should be as high as possible • good mechanical and form stabilitythe membrane should be mechanically strong and should have a low degree of swelling or shrinking in transition from dilute to concentrated ion solutions 10366



Review

acceptable low electrical resistance and high permselectivity. However, in general the resistance is higher and the permselectivity is lower in heterogeneous than those in homogeneous membranes.

The second widely used ion-exchange membrane preparation technique which leads to a rather heterogeneous structure is based on mixing an ion-exchange resin powder with a binder polymer, such as polyvinylchloride or polyethylene, and extruding the mixture as a film at a temperature close to the melting point of the polymer.13 The structures of a homogeneous and heterogeneous cationexchange membrane are illustrated in Figure 1 a,b. A

4. PROCESSES BASED ON ION-EXCHANGE MEMBRANES Several ion-exchange membrane processes such as electrodialysis or electrolysis can be considered as state-of-the-art industrial processes with a substantial technical and commercial impact.1 Other processes such as continuous electro-deionization or the electrodialysis with bipolar membranes were successfully commercialized in several applications in the last few decades. A number of emerging applications have been identified for these processes14 requiring further technological development. Finally, there are a number of processes with ion-exchange membranes as key components, described in the literature,15 which are based on theoretical considerations or laboratory and pilot tests, such as capacitive deionization or ion flow batteries. A number of the more interesting processes and their present and potential future applications are listed in Table 1. Table 1. Processes Utilizing Ion-Exchange Membranes and Their Most Technically Relevant Applications16 processes

technical relevant applications

(a) State-of-the-Art Processes with Significant Commercial Relevance electrodialysis water desalination, salt preconcentration electrolysis chlorine-alkaline production diffusion dialysis acid and base recovery from waste waters (b) Recently Developed Processes with Increasing Commercial Relevance bipolar membrane electrodialysis production of acids and bases continuous electro-deionization production of pure/ultrapure water chemical to electrical energy fuel cells conversion (c) To Be Developed Processes with Potential Future Relevance reverse electrodialysis electrodialytic energy generation capacitive deionization water desalination and water softening electrodialytic energy storage concentration and redox flow batteries

Figure 1. Schematic drawing illustrating (a) the structure of a homogeneous membrane showing an even distribution of the fixed negative ions in the polymer matrix and (b) the structure of a heterogeneous membrane showing macroscopic ion-exchange particles imbedded in a matrix polymer and the longer pathway of counterions across the membrane.

homogeneous structure is shown schematically in Figure 1a which indicates the polymer matrix with the fixed negative ions and the mobile counterions as well as their pathway through the membrane. The structure may contain a limited number of co-ions which should be as low as possible to achieve a high permselectivity of the membrane. A cation-exchange membrane with a heterogeneous structure is depicted in the schematic drawing of Figure 1 b which shows cation-exchange resin particles imbedded in a generally hydrophobic matrix polymer and pathways of counterions across the membrane. Since the hydrophobic polymer matrix is virtually impermeable for ions, they can only permeate the membrane via the contact points of ion-exchange particles. This leads to a much longer pathway across the membrane and hence an increased resistance. In heterogeneous membranes the number of contact points between the ion-exchange resin particles, which is proportional to the volume ratio of ion-exchange particles to the matrix polymer, has to exceed a percolation threshold in order to achieve a sufficient number of continuous ion pathways through the membrane. If, however, the volume ratio of ionexchange resin to binder polymer is too high, the membrane becomes mechanically weak and water clusters are formed in the matrix polymer, leading to an additional co-ion penetration. For the preparation of heterogeneous ion-exchange membranes the volume ratio of ion-exchange resin to matrix polymer should be between 0.6 and 0.7 to obtain membranes with

4.1. State-of-the-Art Ion-Exchange Membrane Processes and Their Application. Electrodialysis is today by far the most important of the state-of-the-art ion-exchange membrane processes. It has found large scale application in water desalination and deionization processes in the food and chemical industry. The electrolytic chlorine−alkaline production with a chemically stable, fluorinated cation-exchange membrane is an important commercial process for the efficient and environmentally friendly production of chlorine and sodium hydroxide. Diffusion dialysis is also a well-established process in the steel industry to recover acids from spent pickling solutions. But its commercial relevance is still rather low. 4.1.1. Electrodialysis. The principle of conventional electrodialysis is illustrated in Figure 2 which shows a series of alternating anion- and cation-exchange membranes arranged between two electrodes. The cation- and anion-exchange membranes are separated by a spacer gasket and form individual cells, through which an electrolyte solution is pumped. When an electrical potential difference between the electrodes is established the cations migrate toward the 10367



Review

exchange membrane are shown in Figure 4a. At the boundary layer on the diluate (anode) side, the concentration of ions is

Figure 2. Schematic diagram illustrating the principle of electrodialysis.

cathode. They pass through the cation-exchange membrane but they are retained by the anion-exchange membrane. Likewise the anions migrate toward the anode and pass through the anion-exchange membrane but are retained by the cationexchange membrane. The overall result is that an electrolyte is concentrated in alternate compartments while its ion content is depleted in the other compartments. A cation-exchange membrane, a concentrate containing cell, an anion-exchange membrane, and a diluate containing cell is referred to as a cell pair or a repeating unit. In an industrial size electrodialysis stack 100 to 400 cell pairs are arranged between the electrodes.17 Various spacer and stack constructions such as the so-called sheet flow or the tortuous path flow stack design are used in practical applications. The concept of a sheet flow stack is illustrated in Figure 3 which shows the arrangement of the spacers and membranes in a stack. Not only do the spacers separate the membranes and provide the proper mixing of the solutions in the cells, but in their frames, they also contain the manifolds for the two different flow streams in the stack. Concentration Polarization, Limiting Current Density, and Membrane Fouling. Concentration polarization in electrodialysis is the result of differences in the transport numbers of ions in the solution and in the membrane and of the electroneutrality requirement. The transport number of a counterion in an ion-exchange membrane is generally close to 1 and that of the co-ion close to 0 while in the solution the transport numbers of anions and cations are not very different. The resulting concentration profiles at both sides of a cation-

Figure 4. Schematic drawing illustrating (a) the concentration profiles of a salt in the laminar boundary layers on both sides of a cationexchange membrane and the flux of ions in the solutions and the membrane and (b) the current as a function of the applied voltage.

reduced since the cations are pulled through the membrane toward the cathode with a transport number which is higher than in the solution, while the anions are pulled to the anode with few anions passing the cation-exchange membrane. Because of the electroneutrality requirement cation and anion concentrations in the solution are always equal. The net result is a reduction of the electrolyte concentration in the diluate solution at the surface of the membrane. On the other side of cation-exchange membrane (the concentrate side) the electrolyte concentration at the membrane surface is increased for similar reasons. Figure 4a shows the direction of fluxes Ji and

Figure 3. Schematic drawing illustrating the construction of a sheet flow stack design. 10368



Review

polarity and switches the flow streams so that the diluate cell become the brine and vice versa. During a relatively short time the “new” diluate cell is rinsed from concentrated solution and then the diluate stream can be used as product. Thus, in electrodialysis reversal there is always a certain amount of the product lost to the waste stream. In brackish water desalination this loss is generally less than 2%. Electrodialysis reversal has been very effective not only for the removal of precipitated colloidal materials but also for removing precipitated salts and is used today in most electrodialysis water desalination systems. Electrodialysis Process Costs. The total costs in electrodialysis are the sum of fixed costs associated with the amortization of the plant capital costs and the plant operating costs. The capital investment related costs are proportional to the required membrane area for a given capacity plant. The operating costs are proportional to the required desalination energy per unit product. There are more operation related costs such as pumping energy, monitoring and control devices, and general maintenance. But all cost items are also strongly affected by the plant capacity, its location, and the composition of the feedwater.22 The capital costs in electrodialysis are mainly determined by the required membrane area for a given feed and product concentration. Other items such as pumps and process control equipment are considered as a fraction of the required membrane area. This fraction depends on the plant capacity. The required total area of each membrane type for a given capacity plant can be calculated from the current density, the feed, and the product solution concentrations by

the concentration profiles on both sides of a cation-exchange membrane. The superscripts mig and diff refer to migration and diffusion, the superscripts d and c refer to diluate and concentrate solution, the superscripts b and m refer to bulk phase and membrane surface, respectively, and the subscripts a and c refer to anion and cation. The accumulation of ions on the concentrate side of the membrane can result in a precipitation of salts when the concentration exceeds the solubility limit of the electrolyte. In the diluate cell the concentration is decreased as shown in Figure 4a. At a certain voltage a complete depletion of ions at the membrane surface occurs. The respective current density is referred to as limiting current density. Figure 4b shows the current density as a function of the applied voltage in an electrodialysis stack. The curve exhibits three regions. In the first region, indicated by (I), the current density is increasing nearly linear with the applied voltage. When the abovementioned limiting current density is reached, a further increase of the applied voltage results only in a small increase of current due to secondary effects, such as electroconvection18 as indicated in region (II). At a certain applied voltage the current density increases again in region (III) due to water dissociation and possible electroconvection at the membrane/ diluate surface.18,19,49 The current density in this region is referred to as overlimiting current density.20 In practical applications the limiting current density should not be exceeded because water dissociation does not increase ion separation but results in a pH-value decrease in the diluate and increase in the concentrate solution. Since the limiting current density is proportional to the diluate concentration, large membrane areas are required for a given capacity plant when low salt concentration in a desalination process is required. A second problem which affects the efficiency of electrodialysis is membrane fouling by suspended solids which carry positive or negative electrical charges such as polyelectrolytes, humic acids, surfactants, and biological materials. These components are deposited on the membrane surface, increasing the membrane resistance, and must be removed. In industrial electrodialysis the problem has been eliminated to a large extent by reversing in certain time intervals the polarity of the applied electrical potential. This results in a release of charged particles that have been precipitated on the membranes. This technique is referred to as “clean in place” or electrodialysis reversal,21 and its principle is illustrated in Figure 5. In certain time periods, the flow control system reverses the electrode

A=

FQ std(C f − C d) iξ

(15)

Here i is the electric current density passing through a cell pair, A is the active area of the cell pair, Q is the volume flow, C is the ion concentration expressed in equivalents per volume, and ξ is the current efficiency. The subscript st refers to the stack, and the superscripts d and f refer to diluate and feed solutions. The operating current density i should be below the limiting current density which is determined experimentally for the given stack design, feed, diluate, and brine concentration. The total investment related costs for a given plant capacity depend not only on the required membrane area and the price of the membranes but also on their useful life under operating conditions which is in water application 5−8 years. The operating costs are composed of labor, maintenance, and energy costs. The labor and maintenance costs are directly proportional to the size of the plant and usually calculated as a certain percentage of the investment related costs. The energy required in an electrodialysis process is an additive of two terms: the electrical energy to transfer the ionic components from one solution through the membranes into another solution and the energy required to pump the solutions through the electrodialysis unit. The energy EV required to desalinate 1 m3 of water by electrodialysis is given by EV =

A FrCP i f (C − C d ) ξ

(16)

Here rACP is the area resistance of the cell pair, which can be estimated from measurements of membrane resistance and conductivities of solutions.

Figure 5. Schematic drawing illustrating the removal of deposited negatively charged components from the surface of an anion-exchange membrane by reversing the electric field. 10369



Review

of the low limiting current density which requires a large membrane area. Thus, electrodialysis can only be cost effectively applied in water desalination in a certain range of feedwater salt concentrations and required product water qualities. 4.1.2. Diffusion Dialysis. Diffusion dialysis with ionexchange membranes is mainly used today to recover acids or bases from a mixture with salts.23 The process is based on the fact that ion-exchange membranes generally show a high permeability for counterions while being more or less impermeable for co-ions. Exceptions are protons and hydroxide ions which can easily permeate both cation- and anionexchange membranes. The transport mechanism in diffusion dialysis is more complex than in conventional dialysis due to the electrostatic interaction between positive and negative charges and the electroneutrality requirement. The process and equipment design is very similar to that used in conventional dialysis. In diffusion dialysis the driving forces for the transport of ions through the ion-exchange membrane are gradients in their chemical potentials only. There is no external electrical potential applied. The principle of the process is illustrated in Figure 7 showing part of a diffusion dialysis stack for the

The desalination energy is proportional to the difference between the feed and the diluate concentration and the resistance of the solutions and membranes in the stack. Thus, for a given feed solution and required diluate concentration the desalination energy is directly proportional to the current density, while according to eq 15 the required membrane area for a given plant capacity is decreasing with increasing current density. The total desalination costs are the sum of energy costs and investment costs. Therefore, the total desalination costs will reach a minimum for a certain applied current density. The relation between energy costs, membrane costs, total costs, and current density is illustrated in Figure 6 which shows the

Figure 6. Schematic diagram illustrating the various cost items in electrodialysis as a function of the applied current density.

various cost items as a function of the applied current density for a given plant capacity and feed and product water composition. The current density which leads to a minimum in overall desalination costs depends to a large extent on the membrane costs and their lifetime under operating conditions. However, the current density must not exceed experimentally determined limiting current density. Advantages and Limitations of Electrodialysis in Water Desalination. Electrodialysis is well established in water desalination as a reliable process for more than half a century. A main advantage of electrodialysis compared to reverse osmosis is that very little feed pretreatment is required since membrane fouling and scaling is reduced to a minimum due to reverse polarity operation. Also much higher brine concentrations can be achieved in electrodialysis than in reverse osmosis, since there are no osmotic pressure limitations. The chemical and mechanical stability of ion-exchange membranes guarantees a long useful life even in feed waters with aggressive and oxidizing components. Compared to distillation processes electrodialysis has the advantages of lower energy costs as well as investment costs for certain feed and product water compositions. Compared to conventional ion-exchange processes, electrodialysis has the advantage that no regeneration chemicals are required which makes the conventional ionexchange process rather costly for deionization of feed solutions with high salt concentrations. But electrodialysis has several severe technical and economic limitations. A major disadvantage especially for the production of potable water is the fact that only ions are removed while uncharged components such as microorganisms or organic contaminants will not be eliminated. Another disadvantage of electrodialysis is the relatively high energy consumption when solutions with high salt concentrations have to be processed. Likewise, the investment costs are prohibitively high when very low salt concentrations must be achieved in the diluate because

Figure 7. Schematic drawing illustrating the principle of diffusion dialysis utilizing anion-exchange membranes to recover an acid from a mixture with salts.

recovery of HCl from a salt mixture. It is composed of anionexchange membranes forming cells in which in alternating series a feed solution of an HCl/FeCl2 mixture and a stripping solution of water is introduced, since anion exchange membranes are permeable for H+, Cl−, and H+ permeate the membranes while the other cations (Fe2+) are retained. The mass transport in diffusion dialysis is determined by the transport of ions and by the osmotic water flux through the membranes. It can be determined by the respective balance equations. Various models describing the mass transport in diffusion dialysis are suggested in the literature. However, most of these models do not consider electrochemical interactions of the different ions such as osmotic and electroosmotic effects. In practical applications of diffusion dialysis the osmotic flux of water from the stripping solution to the feed solution is generally quite significant because of the concentration difference between the two solutions. In a typical industrial size dialyzer used for instance to recover acids from a mixture with metal salts, the volume of the stripping solution decreases as much as 20%. 10370



Review

commercially available such as the electrodialysis with bipolar membranes and the continuous deionization. Other applications with ion-exchange membranes such as in electrochemical synthesis have so far only little technical relevance. 4.2.1. Electrodialysis with Bipolar Membranes. The conventional electrodialysis can be combined with bipolar membranes and utilized to produce acids and bases from the corresponding salts.25,26 As mentioned earlier, a bipolar membrane is a laminate of an anion on a cation-exchange layer. In this process monopolar cation- and anion-exchange membranes are installed together with bipolar membranes in alternating series in an electrodialysis stack as illustrated in Figure 9.

A typical application of diffusion dialysis is the recovery of acids from spent pickling solutions used in the metal industry as shown in Figure 7. Contrary to Figure 7, a stack with 100 to 200 cell pairs is operated with counter current flow of the feed and the stripping solutions, which allows high acid recovery rates. So far, however, diffusion dialysis has not been used on a large commercial scale. Apparently high costs and poor chemical stability of the membranes affect the profitability of the process. 4.1.3. Electrochemical Synthesis Utilizing Ion-Exchange Membranes. In electrochemical synthesis such as the electrolytic production of chlorine and caustic soda or hydrogen and oxygen, ion-exchange membranes play a dominant role. Today, the chlorine/alkaline electrolysis is by far the most important production process for sodium hydroxide.24 The process is energy efficient and has no toxic byproducts, which are harmful for the environment and cause a waste disposal problem. The principle of the process is illustrated in the simplified schematic drawing of Figure 8 which shows an electrolytic cell

Figure 9. Schematic drawing illustrating the principle of the electrodialytic production of an acid (HCl) and a base (NaOH) from the corresponding salt (NaCl) with bipolar membranes. Repeating cell unit consisting of a cation-exchange membrane (cm), a bipolar membrane (bpm), and an anion-exchange membrane (am).

A typical repeating unit of an electrodialysis stack with bipolar membranes is composed of three cells, two monopolar membranes, and a bipolar membrane. The outer cells of the repeating unit in Figure 9 are fed with a salt solution and the inner cells with water or a diluted acid and base. When an electrical potential gradient is applied across a repeating unit, protons and hydroxide ions are generated in the bipolar membrane by splitting of water. With the cations and anions removed from the salts solution, an acid and a base are produced on either side of the bipolar membrane. The process design is closely related to that of the conventional electrodialysis using the sheet flow stack concept. However, because of the significantly higher voltage drop across a cell unit, only 50 to 100 repeating cell units are placed between two electrodes in a stack. The utilization of electrodialysis with bipolar membranes to produce acids and bases from the corresponding salts is economically very attractive and has a multitude of interesting potential applications in the chemical industry as well as in biotechnology and water treatment processes.27 Its key component is the bipolar membrane. The bipolar membrane (bpm) schematically illustrated in Figure 9 consists of a laminate of an anion- and a cation-exchange membrane with a 4 to 5 nm thick catalytic transition layer in between. In Figure 9 this transition layer has been artificially magnified. Water is diffusing through both membrane layers into the transition layer where it gets electro-catalytically dissociated into H+ and OH−ions, which migrate toward the cathode and anode into the outer solutions.

Figure 8. Schematic drawing illustrating the principle of the electrolytic production process of chlorine and sodium hydroxide.

separated by a cation-exchange membrane, forming two compartments. The compartment with the anode contains the anolyte, that is, a 25 wt % NaCl solution, and the other compartment with the cathode contains the catholyte, that is, dilute sodium hydroxide. When an electrical potential between the electrodes is applied, the chloride ions in the anode compartment migrate toward the anode where they are oxidized and form chlorine which is released as gas. The sodium ions from the salt solution migrate through the cation-exchange membrane toward the cathode where they are reduced to sodium metal, which immediately reacts with the water to sodium hydroxide and hydrogen which is then released as gas. A multitude of the cell units shown in Figure 8 are usually integrated in a stack, using bipolar electrodes. High performance perfluorinated membranes with excellent chemical stability are used in the process. The chlorine−alkaline and the hydrogen−oxygen productions are extensively described in the literature and will not further be discussed. 4.2. Recently Developed Processes with Increasing Commercial Relevance. In recent years the application of ion-exchange membranes was extended beyond its original use in water desalination. Several of these processes are today 10371



Review

Figure 10. Schematic drawing illustrating different stack concepts used in continuous electrodeionization: (a) conventional stack with diluate cells filled with a mixed bed ion-exchange resin, (b) stack with cation-exchange and anion-exchange resins in separate diluate cells and continuous regeneration of the ion-exchange resins by H+ and OH− ions produced in a bipolar membrane.

for the reluctant use of bipolar membrane electrodialysis are poor membrane stability at very high or low pH-values and insufficient permselectivity at high ion concentrations, which results in a substantial product salt contamination, low current efficiency, and short membrane life.28 Nevertheless, there are a number of smaller scale applications in the chemical process industry, in biotechnology, in food processing, and in wastewater treatment. 4.2.2. Continuous Electrodeionization. Continuous electrodeionization is very similar to conventional electrodialysis. However, the cells which contain the diluate stream are now filled with ion-exchange resins. The conductivity in these cells is substantially increased, and highly deionized water can be obtained as a product. The process design and the different hardware components needed in continuous electrodeionization are very similar to those used in conventional electrodialysis. The main difference is the stack construction. In a continuous electrodeionization stack the diluate cells and sometimes also the concentrate cells are filled with an ion-exchange resin. The different concepts used for the distribution of the cation- and anion-exchange resins in the cells are illustrated in Figure 10. In the conventional process the diluate cell is filled with a mixed bed ion-exchange resin with a ratio of cation- to anion-exchange resin being close to 1 as shown in Figure 10a. The mixed bed ion-exchange resin in the diluate cells removes both the cations and the anions of a feed solution. Due to an applied electrical field the ions migrate through the ion-exchange bed toward the adjacent concentrate cells, and highly deionized water is obtained as a product. The ion-exchange resin increases the conductivity in the diluate cells, and the stack resistance is significantly lowered. But continuous electrodeionization using a stack with mixed bed ion-exchange resins in the diluate cell has also disadvantages. The most important one is the poor removal of weak acids and bases such as boric or silicic acid.29 Much better removal of weakly dissociated electrolytes can be obtained in a system in which the cation- and anion-exchange resins are placed in separate beds with a bipolar membrane placed in between as illustrated in Figure 10b.30 It shows a diluate cell filled with a cation-exchange resin facing toward the cathode, separated by a bipolar membrane from a diluate cell facing the anode. A cation-exchange membrane, a cation-

The energy required for the water dissociation can be calculated from the Nernst equation for a concentration chain between solutions of different pH-values. It is given by ΔG = F Δφ = 2.3RT ΔpH

(17)

Here ΔG is the Gibbs free energy and ΔpH and Δφ are the pH-value and the potential difference between the two solutions separated by the bipolar membrane. For 1 mol/L acid and base in the two phases separated by the bipolar membrane, ΔG is 0.022 kW·h/mol and Δφ is ca. 0.83 V at 25 °C. Compared to the ohmic potential drop over the membranes, the required potential drop for water splitting in the transition layer is much more pronounced. Electrodialysis with Bipolar Membrane Process Costs. The determination of the costs for the production of acids and bases from the corresponding salts follows the same general procedure as applied for the cost calculation in electrodialysis desalination. The overall costs are the investment related costs and the operating costs. The investment related costs are dominated by the membrane costs and are proportional to the required membrane area for a given capacity plant. They are a function of the current density applied in a given stack operation. A unit cell contains a bipolar membrane and a cation- and an anion-exchange membrane. The bipolar membrane is rather expensive, and its useful lifetime as well as that of the anion-exchange membrane is rather limited in strong bases. The operating costs in electrodialysis with bipolar membranes are strongly determined by the energy requirements which are composed of the energy required for the water dissociation in the bipolar membrane and the energy necessary to transfer the salt ions from the feed solution and protons and hydroxide ions from the transition region of the bipolar membrane into the acid and base solutions. The energy consumption due to the pumping of the solutions through the stack can generally be neglected. Applications of Bipolar Membrane Electrodialysis. Since bipolar membranes became available as commercial products, a large number of applications have been identified and studied on a laboratory or pilot plant scale. However, in spite of the obvious technical and economical advantages of the technology, large scale industrial plants are still quite rare. The main reasons 10372



Review

However, there is some contamination of the diluate by cations permeating the anion-exchange membrane from the concentrate into the diluate if the membrane is not completely semipermeable. Since this is generally not the case a complete removal of ions cannot be achieved in practical applications. Electrodeionization is now longer than 25 years on the market, but for better competitiveness against traditional ionexchange deionization, further achievements in product water quality and in operating and manufacturing costs are desirable. 4.2.3. Ion-Exchange Membranes in Energy Conversion. Ion-exchange membranes play an increasingly important role in energy storage and conversion systems such as battery separators and fuel cells.32 They are also considered in generating electrical energy by mixing salt solutions and surface water. The most prominent application of ion exchange in energy conversion systems is in low temperature fuel cells.33 The principle of a fuel cell based on the electrochemical oxidation of hydrogen by oxygen is illustrated in Figure 12. It shows a single

exchange resin, a bipolar membrane, an anion-exchange resin, and a concentrate cell form a repeating unit between two electrodes. The main difference between the system with the mixed bed ion-exchange resins and the system with separated beds is that in the mixed bed systems anions and cations are simultaneously removed from the feed and the solution leaving the diluate cell is neutral. In the system with separated ion-exchange beds and bipolar membranes the cations of the feed will first be exchanged in the cell filled with a cation-exchange resin by the protons generated in the bipolar membrane with the result that the solution leaving the cation-exchange bed is acidic. This solution is then passed through the cell with the anionexchange resin where the anions are exchanged by the OH− ions generated in the bipolar membrane while at the exit of the anion-exchange filled cell the solution is again neutral. Both the mixed and the separated bed ion-exchange continuous electrodeionization systems are widely used today on a large industrial scale.31 The main application of continuous electrodeionization is the production of so-called ultrapure water, which is used in many industrial processes and in chemical and analytical laboratories. Generally, well or surface water is purified in a series of processes that include water softening, microfiltration, reverse osmosis, ultrafiltration, UV sterilization, and mixed bed ion exchange, which is used as a final polishing process producing water with a conductivity of less than 0.06 μS cm−1. While processes such as reverse osmosis, micro- and ultrafiltration, or UV sterilization can be operated in a continuous mode, the mixed bed ion exchanger, which is necessary to reach the required low conductivity, must be regenerated in certain time intervals. This regeneration is not only labor intensive and costly but also requires extremely long rinse down times to remove traces of regeneration chemicals. By replacing the mixed bed ion-exchanger by a continuous electrodeionization, using separate ion-exchange beds and bipolar membranes, the ultrapure water production can substantially be simplified, yielding consistently high quality water in a completely continuous electrodeionization process as illustrated in Figure 11.

Figure 12. Schematic drawing illustrating the principle of the fuel cell based on the electrochemical oxidation of hydrogen by oxygen.

unit cell consisting of two porous electrodes and two catalyst layers, separated by a cation-exchange membrane. Hydrogen gas is passed through the porous anode. It reacts in the catalyst layer forming protons and releasing electrons at the anode to an electric circuit. The protons diffuse through the ion-exchange membrane. They react in the catalyst layer at the surface of the porous cathode with oxygen gas to form water and take up electrons from the electric circuit. The overall reaction in a fuel cell is the oxidation of hydrogen by oxygen to water. Instead of hydrogen also hydrocarbons such as methanol or ethanol are used as fuel and usually air is used as the oxidation medium instead of oxygen. Since an extensive literature on proton exchange membrane (PEM)-fuel cells is available, this process is not further detailed here. 4.3. Developing Processes with Ion-Exchange Membranes as Key Components. Processes with ion-exchange membranes as key components that are presently developed on a laboratory scale are the capacitive deionization, energy generation from ion concentration gradients, and redox or concentration gradient flow batteries. None of these recent developments has reached large scale applications so far. 4.3.1. Capacitive Deionization. Capacitive deionization is an electrosorption process that can be used to remove ions

Figure 11. Simplified flow diagram of an ultrapure water production line.

The advantages of the ultrapure water production with an integrated continuous electrodeionization unit, compared to the use of a mixed bed ion exchanger, are a simpler process, no need of regeneration chemicals, less raw water consumption, and a substantial reduction in costs. But both systems, that is, the mixed bed ion-exchange resin as well as that with bipolar membranes and separated beds, have certain limitations. In the system with the mixed bed ion-exchange resin the removal of weakly dissociated acid is very poor. In the system with separated beds of ion-exchange resins and bipolar membranes the removal of weak acids is very efficient. 10373



Review

from an aqueous solution.4,34,35 A capacitive deionization cell unit consists of two electrodes, usually made of activated carbon, separated by a spacer that acts as a flow channel for an ion containing solution. In capacitive deionization electrons are not transmuted by oxidation and reduction reactions but by electrostatic adsorption. The system resembles a “flow-through capacitor”. If an electrical potential is applied between the electrodes, ions are removed from the solution and adsorbed at the surface of the charged electrodes. When the carbon electrodes are saturated with the charges, the ions are released from the electrodes by reversing the potential; that is, the cathode becomes the anode and vice versa the anode becomes the cathode. Thus, capacitive deionization is a two step process. In a first step ions are removed from a feed solution by migration from the feed solution toward the electrodes under an electrical potential driving force and electrosorption at the electrodes. The result is a deionized product water. In a second step the adsorbed ions are released from the carbon electrodes and transported back into the feed solution by reversing the electrical potential, and concentrated brine is obtained. The process is illustrated in Figure 13, which shows schematically a

This potential difference for close positioned electrodes is limited by a maximum 1.2 V. The advantage of the process is the relatively low energy requirement. The disadvantage is the large surface area of the electrode which is necessary when feed solutions with high salt concentration are deionized. The most reasonable practical applications of capacitive deionization are expected in partial deionization of brackish or well waters to the salt content required for drinking or household use water. The main advantage of capacitive deionization compared to other membrane deionization processes or thermal desalination is the relatively low energy consumption and, due to the polarity reversal in every operating cycle, a better resistance to scaling and fouling. 4.3.2. Reverse Electrodialysis. The production of energy by mixing seawater with river water through ion-exchange membranes is a process referred to as reverse electrodialysis.38 The process which is illustrated in Figure 14 has found some interest as a clean and sustainable energy source.39

Figure 14. Schematic drawing illustrating the concept of reverse electrodialysis used to generate electrical energy by mixing river and seawater. Figure 13. Schematic diagrams illustrating the capacitive deionization process with ion-exchange membranes between the feed solution and the porous carbon electrodes: (a) deionization step with sorption of ions and (b) regeneration step due to a change of polarity, producing concentrated brine.

The design of a stack to be used in reverse electrodialysis is very similar to the stack used in electrodialysis. The main difference is that the cells, arranged in parallel between the electrodes, are rinsed in alternating series by sea and by river water. The ions in the seawater indicated as Na+ and Cl− ions permeate from the seawater into the river water through the corresponding ion-exchange membrane and produce brackish water due to their electrochemical potential gradient. This leads to an electrical current between the cathode and anode. The maximum amount of energy that can theoretically be recovered is the Gibbs free energy of mixing fresh water and seawater which is given by

capacitive deionization unit and the ion transport during the sorption and desorption step of the deionization process. During the deionization step anions are prevented to diffuse into the product water by a cation-exchange membrane on the cathode and an anion-exchange membrane on the anode as shown in Figure 13a.36 During the regeneration step under reverse polarity conditions the cation-exchange membrane prevents the transport of anions toward the anode and the anion-exchange membrane the transport of cations toward the cathode as shown in Figure 13b and, thus, avoids an ion adsorption at the electrodes during the regeneration step. A key component in this process is the carbon electrode. The number of ions adsorbed at the electrodes is directly proportional to the available surface area. Therefore, the specific surface area, that is, the surface area per unit weight of the electrodes, should be as high as possible.37 Carbon nanotubes on activated carbon are the most promising materials for the preparation of electrodes. Their specific surface area is up to 1100 m2/g. Another parameter is the applied potential difference which should not exceed a certain maximum value to avoid water splitting in electrode reactions.

ΔGm = (Gc − Gd) − G b

(18)

Here ΔGm is the Gibbs free energy of mixing, Gb is the Gibbs free energy of the mixture, that is, the brackish water, Gc is the Gibbs free energy of the concentrate, that is, the seawater, and Gd is the Gibbs free energy of the diluate, that is, the river water. If 1 m3 of fresh water is mixed with 1 m3 of seawater having a total salt content of ca. 35 kg/m−3 the theoretically obtained maximum energy is ca. 0.4 kW·h. The power that could be obtained by mixing the water of a river such as the Rhine which discharges ca. 2000 m3/s into the North Sea with seawater corresponds to a power plant of ca. 4700 MW. 10374



Review

Different from the previous example of mixing river and seawater, higher ion concentrations can be obtained during the charging cycle, leading to higher energy output during discharging. But the limitations discussed in Section 4.3.2 are similar and will be quantified below. The reversed electrodialysis with bipolar membranes is indeed a complete reversal of the process shown in Figure 9. In the case of HCl and NaOH neutralization, H+ ions diffuse from the acid solution across the cation-exchange layer of the bipolar membrane into the reaction layer of the membrane, where they are neutralized by OH− ions, permeating the anion-exchange layer of the bipolar membrane from the base solution. If the bipolar membrane is strictly semipermeable, the reaction layer contains only water and its ion concentration is that of pure water and will not change. Then the ion concentration in the reaction layer is very low and due to the water dissociation equilibrium it is constant, that is, independent of the diffusion rate of H+ and OH− ions into the reaction layer. The drop of the H+ and OH− ions concentration in the reaction layer generates an electrical potential difference Δφ, which in the ideal case represents the “open voltage” of the battery. It can be calculated for a given acid and base concentration by eq 19. The main difference between a concentration flow battery and the neutralization flow battery (with bipolar membranes) is this open cell voltage. In the concentration flow battery the open voltage is proportional to the concentration difference of the diluate and concentrate solution. Assuming that the brine concentration of a monovalent salt is 1 M in the concentrate and 0.01 M in the diluate, under these conditions the open cell voltage is ca. 0.23 V. During the discharge the concentrations in diluate and concentrate change very rapidly and become equal when only 50% of ions are transfered from the concentrate to the diluate. Then the open cell voltage has dropped to zero. In the neutralization flow battery, however, the open cell voltage is proportional to the concentration difference between the acid, respectively the base, and the ion concentration in the reaction layer of the bipolar membrane, which is always constant and extremely low, since it is determined by the water dissociation equilibrium. That means that a change in the acid and base concentrations has only little effect on the open cell voltage. Assuming an ideal neutralization flow battery with a 1 M acid solution and pure water in the reaction layer of the bipolar membrane, the voltage calculated by eq 19 between the acid and the water is ca. Δφ1 = 0.42 V. Since the same voltage applies for Δφ2 between the water and the base, the total open cell voltage amounts to ca. 0.84 V, which corresponds to the necessary voltage for water splitting in bipolar membranes, mentioned previously. If the acid and base concentrations change by a factor of 10, the open voltage will change by less than 5%, since the H+ and OH− ion concentration in the reaction layer is still orders of magnitude lower. These calculations apply for the case with ideally permselective membranes and no ohmic losses. But the general conclusion, that the neutralization flow battery with bipolar membranes allows for considerably higher cell voltages and that it is much less sensitive to concentration changes, will also apply in reality. A detailed study of the potential of neutralization flow batteries including the influence of co-ion leakage and ohmic losses is a matter of ongoing research. One major problem in the use of electrodialysis and electrodialysis with bipolar membranes as flow batteries is the poor permselectivity of today’s ionexchange monopolar and bipolar membranes which leads to a significant self-discharge.

Unfortunately, the maximum energy that could be recovered in practice is significantly lower. It can be calculated from the “open voltage” multiplied with the current carried by the ions diffusing from the seawater into the river water and the electrical resistance of the stack. The open voltage represents an equilibrium state between two solutions of different ion concentrations separated by an ion-exchange membrane. The open voltage can be expressed by U0 =

RT ln ∏ ai F i

(19)

Here U0 is the so-called “open voltage”, R the gas constant, T the absolute temperature, and F the Faraday constant, and ai are the equivalent ion activities. The maximum power output Wmax that can be achieved from an electrodialysis stack is given by Wmax = IUst0 = I 2R st =

(Ust0 )2 R st

(20)

Here Rst is the stack resistance. For the maximum power output the stack resistance is a critical parameter. It is determined by the resistances of the membranes and the solutions between the membranes. The resistances should be minimized by making the membranes as well as the individual cells in the stack as thin as possible and by reducing concentration polarization effects. A critical cost factor is also the required membrane area for a given capacity of a reverse electrodialysis power plant, which is determined by the power density wmax and given by wmax =

Wmax A

(21)

The maximum power density that can be achieved today in practical applications is on the order of 1 W/m2. Therefore, it seems that membrane costs must also be drastically decreased to make reverse electrodialysis a competitive energy production process.56 4.3.3. Ion-Exchange Membranes in Electrical Energy Storage Systems. Ion-exchange membranes in energy storage systems have been a subject of research for many years. Especially the redox flow batteries have been studied extensively.40 More recently, interest in electrodialysis as a flow battery has increased and will be briefly discussed in the following. Electrodialysis Battery. There are two types of electrodialysis ion flow batteries. One is based on a reversal of regular electrodialysis and is referred to as “concentration flow battery”,40 and the other is based on reversal of electrodialysis with bipolar membranes and is referred to as “neutralization flow battery”.41 Both batteries have a charging and a discharging cycle. The charging cycle is in both processes, as far as the hardware components and the operation is concerned, identical as described earlier for electrodialysis (Figure 2) and electrodialysis with bipolar membranes (Figure 9). During discharge both processes are reversed. In reversed electrodialysis the concentrate and diluate solutions are mixed by diffusion of the cations through the cation-exchange membrane and of the anions through the anion-exchange membrane as already explained in Figure 14. The obtained open voltage is the electromotive force generated by the concentration difference of the diluted and concentrated solutions as described by eq 19. 10375



Review

Figure 15. Membranes produced by press-molding and swollen in water: (a) cross-section of an anion-exchange membrane; (b) top-view of a cation-exchange membrane (right side edge of the active membrane area); (c) photograph of the cation-exchange membrane prepared for assembly; and (d) illustration of the flow channel between two profiled membranes.

5. ON-GOING RESEARCH IN ELECTROMEMBRANE PROCESSES For many applications such as brackish water desalination the properties of today’s ion-exchange membranes are quite satisfactory. For other applications such as the production of boiler feedwater or the treatment of industrial effluents and in connection with the use of membranes in electrochemical synthesis and energy conversion, higher conductance and permselectivity as well as better chemical stability of membranes are desirable. A substantial amount of research is presently focused on amelioration of the cost/performance ratio of ion-exchange membranes. Research directions in the development of deionization with electromembrane processes are targeted to improve product water quality, to decrease energy and water consumption, to obtain a more energy efficient deionization, and to reduce manufacturing costs. An improvement of heterogeneous membrane properties has been studied in refs 42 and 43 applying an alternating electric field during membrane manufacturing, which aligns ionexchange particles into chains. As schematically illustrated in Figure 1b, this shall create more straight and frequent paths for ion conduction and thus increase the membrane conductivity. Improvement of membrane properties, for example, chemical and thermal stability, is specially required for use in batteries, fuel cells, and electrochemical synthesis.44−48 On-going developments in electrodialysis stack design are concentrated on a better control of concentration polarization effects to increase the limiting current density and reduce overall process costs.49,50 Use of ion-conductive spacers in electrodialysis51 and related processes remains one of the important research topics. Conductive spacers in flow channels of reverse electrodialysis39 show very promising results with increased stack conductance and power density. In electrodialysis with bipolar membranes52 the replacements of a conventional spacer by a conductive one

decreases stack resistance and energy consumption. Optimization of the turbulence promotion and of the contact area of conductive spacers with adjacent membranes, as well as the design of flow channels, are critical issues in further developments with conductive spacers. Another promising direction in the development of electromembrane processes is the use of ion-exchange membranes with a macro-structured surface, that is, membranes with integrated spacers53 or profiled membranes,31,54 like the ones illustrated in Figure 15. Instead of the traditional flow channel, formed by a nonconductive spacer between two flat membranes, a channel between two profiled membranes stacked together exhibits good turbulence promotion and an increased active membrane surface, larger than those that traditional flat membranes of the same size would have. This intensifies mass transport in the channel and increases channel conductivity with subsequent reduction of energy consumption. Use of profiled heterogeneous ion-exchange membranes in electrodialysis has been studied.31 67.5 vol % of dry ionexchange powder was mixed with polyethylene binder and pressed in a mold into flat membranes and profiled membranes with trapezoid notches on both sides, as illustrated in Figure 15. Notches on one side of the membrane are perpendicular to the notches on the opposite side and to that of the adjacent membrane. When counter-polar membranes are stacked together, a flow channel between them is formed (Figure 15d), where all notches are oriented 45° to the main flow direction and 90° to the adjacent membrane. A laboratory size electrodialysis stack of profiled membranes was compared with a similar stack of flat membranes and spacers (Sefar Nitex 06 750/47) for deionization of softened tap water and of reverse osmosis permeate at equivalent operating conditions. Figure 16 shows that, at the same potential difference of the cell pair, the electrodialysis with profiled membranes reaches a 10376



Review

dissociated electrolytes does not seem to be of importance for the studied heterogeneous membranes. It therefore appears reasonable to reduce the contact area between the membranes, for example, by using a V-shaped profile instead of trapezoidal one. Figure 18 shows the profile of such notches produced by calendering of heterogeneous membranes.

Figure 16. Diluate conductivity as a function of the potential drop over an electrodialysis cell pair at 4.5 cm/s flow velocity: (a) softened tap water (κF = 330 μS/cm); (b) reverse osmosis permeate (κF = 5.5 μS/cm).

Figure 18. Cross-section of a profiled membrane with V-shaped notches produced by calendaring of the heterogeneous membrane FTCM (FumaTech, Germany).

higher desalination degree than that with flat membranes and spacers. The advantages in the performance of the stack with profiled membranes increase with decreasing feedwater conductivity. Using reverse osmosis permeate as feedwater, only marginal deionization could be achieved with flat membranes and spacer, while the conductivity had been further reduced down to ca. 2 μS/cm with profiled membranes. The main reason is the higher conductance of the electrodialysis stack, resulting in a higher current and more intensive ions transport. For the same degree of deionization the energy consumption with profiled membranes is lower than with flat membranes as shown in Figure 17 for a test with softened tap water as feed. This can be explained by the higher conductance of the channels with profiled membranes compared to one where membranes are separated by the nonconductive spacer. Water splitting at the contact point and the effect of subsequent ion-exchange on the removal of ions and weakly

Making profiled membranes from homogeneous ionexchange material53 could further improve their desalination performance. Summarizing, electrodialysis with appropriately profiled ionexchange membranes is superior to conventional electrodialysis with flat membranes and spacers, in particular with respect to energy consumption, which is the major part of the operating costs. Absence of spacers can reduce materials and assembly costs. Therefore reduced operating and investment costs give profiled membranes a high chance to replace flat membranes in existing electrodialytic applications. Further, the higher degree of deionization achieved with profiled membranes extends the application of electrodialysis to the treatment of more diluted solutions and the production of diluates with lower conductivities. In general, profiled ion-exchange membranes can be used advantageously in a wide spectrum of electrochemical processes. They can be applied in electrochemical ionexchange55 and in electrodeionization. In reverse electrodialysis higher power density was achieved with profiled heterogeneous membranes compared to the flat membranes with spacer.56 Also in capacitive deionization a use of profiled electrodes coated with an ion-exchange polymer layer could reduce resistance and energy consumption, as well as improve accessibility of electrode material for electrosorbed ions, thus increasing the effective capacity of the electrodes and intensifying the deionization process. Extensive research on technological improvement continues in synthesis and testing of new materials in electromembrane processes: new kinds of membranes for electrochemical synthesis, fuel cells, and other applications;57 differences from resin beads forms of ion-exchange materials, like fibers and monoliths for electrodeionization; electrode materials with larger specific surface and facilitated surface diffusion for capacitive deionization; and so forth.

Figure 17. Energy consumption EV as a function of the deionization degree. 10377



Review

(6) Donnan, F. G. Theorie der Membrangleichgewichte und Membranpotentiale bei Vorhandensein von nicht dialysierenden Elektrolyten. Z. Elektrochem. Angew. Phys. Chem. 1911, 17, 572−581. (7) Strathmann, H. Introduction to Membrane Science and Technology; Wiley-VCH: Weinheim, Germany, 2011. (8) Spiegler, K. S. Transport Process in Ionic Membranes. Trans. Faraday Soc. 1958, 54, 1408−1428. (9) Xu, T. W. Ion-Exchange Membranes: State of their Development and Perspective. J. Membr. Sci. 2005, 263, 1−29. (10) Helfferich, F. Ion-Exchange; McGraw−Hill: London, 1962. (11) Bergsma, F.; Kruissink, C. H. Ion-Exchange Membranes. Fortschr. Hochpolym-Forsch. 1961, 21, 307−362. (12) Simons, R. Preparation of High Performance Bipolar Membranes. J. Membr. Sci. 1993, 78, 13−23. (13) Molau, G. E. Heterogeneous Ion-Exchange Membranes. J. Membr. Sci. 1981, 8, 309−330. (14) Kemperman, A. J. B. Handbook Bipolar Membrane Technology; Twente University Press: Endschede, The Netherlands, 2000 (ISBN 9036515203). (15) Sata, T. Recent Trends in Ion-Exchange Research. Pure Appl. Chem. 1986, 58, 1613−1626. (16) Strathmann, H. Electrodialysis, a mature technology with a multitude of new applications. Desalination 2010, 264, 268−288. (17) Huffmann, E. L.; Lacey, R. E. Engineering and Economic Considerations in Electromembrane Processing. In Industrial Processing with Membranes; Lacey, R. E., Loeb, S., Eds.; Wiley: New York, 1972; pp 39−55. (18) Rubinstein, I.; Zaltzman, B.; Futerman, A.; Gitis, V.; Nikonenko, V. V. Reexamination of electrodiffusion time scales. Phys. Rev. E 2009, 79, 021506. (19) Nikonenko, V. V.; Pis’menskaya, N. D.; Volodina, E. I. Rate of generation of ions H+ and OH− at the ion-exchange membrane/dilute solution interface as a function of the current density. Russ. J. Electrochem. 2005, 41 (11), 1205−1210. (20) Ibanez, R.; Stamatialis, D. F.; Wessling, M. Role of Membrane Surface in Concentration Polarization at Cation-Exchange Membranes. J. Membr. Sci. 2004, 239, 119−128. (21) Katz, W. E. The Electrodialysis Reversal Process. Desalination 1979, 28, 31−40. (22) Lee, H. J.; Sarfert, F.; Strathmann, H.; Moon, S. H. Designing of an Electrodialysis Desalination Plant. Desalination 2002, 142, 267− 285. (23) Kobuchi, Y.; Motomura, H.; Noma, Y.; Hanada, F. Application of Ion-Exchange Membranes to Recover Acids by Diffusion Dialysis. J. Membr. Sci. 1987, 27, 173−179. (24) Coulter, M. O., Ed. Modern Chlor-Alkali Technology; Ellis Horwood, Ltd.: Chichester, U.K., 1980; Vol. 1 (published for the Society of Chemical Industry). (25) Xu, T. Electrodialysis Processes with Bipolar Membranes in Environmental ProtectionA Review. Resour., Conserv. Recycling 2002, 37, 1−22. (26) Wilhelm, F. G.; Pünt, I.; Van der Vegt, N. F. A.; Strathmann, H.; Wessling, M. Asymmetric Bipolar Membranes in Acid-Base Electrodialysis. Ind. Eng. Chem. Res. 2002, 41 (3), 579−586. (27) Liu, K. J.; Chlanda, F. P.; Nagasubramanian, K. J. Use of Bipolar Membranes for Generation of Acid and Base: An Engineering and Economic Analysis. J. Membr. Sci. 1977, 2, 109−124. (28) Gineste, J. L.; Pourecelly, G.; Lorrain, Y.; Presin, F.; Gavach, C. Analysis of Factors Limiting the Use of BPM: A Simplifird Model to Determine Trends. J. Membr. Sci. 1996, 112, 199−208. (29) Ganzi, G. C. Electrodeionization for High Purity Water Production. In New Membrane Materials and Processes for Separation; AIChE Symposium Series; Sirkar, K. K., Lloyd, D. R., Eds.; AIChE: New York, 1988; Vol. 84, pp 73−83. (30) Grabowski, A.; Guiquing, Z.; Strathmann, H.; Eigenberger, G. The Production of High Purity Water by Continuous Electrodeionization with Bipolar Membranes. J. Membr. Sci. 2006, 281, 297− 306.

Another important direction of ongoing development concerns new concepts and designs of the stacks58,59 and the combination of electromembrane technologies with other processes.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

LIST OF SYMBOLS

A ai C EV F G i J l p Q rACP R R t T U Vm w W x z

area, m2 activity of i component, mol/m3 molar or normal concentration, mol/m3 or eq/m3 energy consumption per volume of diluate, W·h/m3 Faraday constant, 96486 A·s/equiv electric conductance, S current density, A/m2 molar flux vector, mol/(m2·s) length, m pressure, Pa flow rate, m3/s area resistance of cell pair, Ohm·m2 universal gas constant 8.31 J/(mol·K) electric resistance Ohm transport number of ions, − absolute temperature, K voltage, V partial molar volume, m3/mol power density, W/m2 power, W linear coordinate, m ion charge number, eq/mol

Latin Symbols

Greek Symbols

φ κ λeq i μ μ̃ ν π ρ ξ Ψ

■

electric potential, V specific electrical conductivity, S/m equivalent conductivity of the ion i, S·m2/eq chemical potential, J/mol electrochemical potential, J/mol linear flow velocity, m/s osmotic pressure, Pa specific electrical resistivity, Ohm·m current efficiency, − membrane permselectivity, −

REFERENCES

(1) Schaffer, L. H.; Mintz, M. S. Electrodialysis. In Principles of Desalination; Spiegler, K. S.; Academic Press: New York, 1966; pp 3− 20. (2) Eisenberg, A.; Yeager, H. L. Pefluorinated Ionomer Membranes; ACS Symposium Series 180; American Chemical Society: Washington, DC, 1982. (3) Pourcelly, G.; Bazinet, L. Developments of Bipolar Membrane Technology in Food and Bio-Industry. In Handbook of Membrane Separations; Pabby, A. K., Rizvi, S. S. H., Sastre, A. M., Eds.; CRC Press: Boca Raton, FL, 2009; pp 581−633. (4) Welgemoed, T. J.; Schutte, C. F. Capacitive De-Ionization Technology: an Alternative Desalination Solution. Desalination 2005, 183, 327−340. (5) Williams, K. R. An Introduction to Fuel Cells; Elsevier: London, 1966. 10378



Review

(31) Grabowski, A. Electromembrane Desalination Processes for Production of Low Conductivity Water; Logos: Berlin, 2010. (32) Joerissen, L.; Garche, J.; Fablan, Ch.; Tomazic, G. Possible use of vanadium redox-flow batteries for energy storage in small grids and stand-alone photovoltaic systems. J. Power Sources 2005, 127, 98−104. (33) Kordesch, K.; Simader, G. Fuel Cells and Their Applications; VCH: Weinheim, Germany, 1996. (34) Biesheuvel, P. M.; van der Wal, A. Membrane Capacitive Deionization. J. Membr. Sci. 2010, 346, 256−262. (35) Porada, S.; Bryjak, M.; van der Wal, A.; Biesheuvel, P. M. Effect of electrode thickness variation on operation of capacitive deionization. Electrochim. Acta 2012, 75, 148−156. (36) Lee, J. B.; Park, K. K.; Eum, H. M.; Lee, C. W. Desalination of a Thermal Power Plant Waste Water by Membrane Capacitive Deionization. Desalination 2006, 196, 125−134. (37) Biesheuvel, P. M.; Zhao, R.; Porada, S.; van der Wal, A. Theory of Membrane Capacitive Deionization Including the Effect of the Electrode Pore Space. J. Colloid Interface Sci. 2011, 360, 239−248. (38) Dlugolecki, P.; Nymeiyer, K.; Metz, S. J.; Wessling, M. Current Status of Ion-Exchange Membranes for Power Generation From Salinity Gradients. J. Membr. Sci. 2008, 319, 214−222. (39) Długołęcki, P.; Dąbrowska, J.; Nijmeijer, D. C.; Wessling, M. Ion Conductive Spacers for Increased Power Generation in Reverse Electrodialysis. J. Membr. Sci. 2010, 347, 101−107. (40) Ponce de León, C.; Frías-Ferrer, A.; González-García, J.; Szánto, D. A.; Walsh, F. C. Redox flow cells for energy conversion. J. Power Sources 2006, 160, 716−732. (41) Zholkovskij, E. K.; Müller, C.; Staude, E. The Storage Battery with Bipolar Membranes. J. Membr. Sci. 1998, 141, 231−243. (42) Oren, Y.; Freger, V.; Linder, C. Highly conductive ordered heterogeneous ion-exchange membranes. J. Membr. Sci. 2004, 239, 17−26. (43) Liu, D.; Yates, M. Z. Tailoring the structure of S-PEEK/PDMS proton conductive membranes through applied electric fields. J. Membr. Sci. 2004, 322, 256−264. (44) Kariduraganavar, M. Y.; Nagarale, R. K.; Kittur, A. A.; Kulkarni, S. S. Ion-exchange membranes: preparative methods for electrodialysis and fuel cell applications. Desalination 2006, 197, 225−246. (45) Kerres, J.; Ullrich, A.; Hein, M. Preparation and characterization of novel basic polysulfone polymers. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2874−2888. (46) Atanasov, V.; Kerres, J. Highly Phosphonated Polypentafluorostyrene. Macromolecules 2011, 44 (16), 6416−6423. (47) Peinemann, K.-V., Nunes, S. P., Eds. Membranes for Energy Conversion; Membrane Technology, Vol. 2; Wiley-VCH: Weinheim, Germany, 2007. (48) Komkova, E. N.; Stamatialis, D. F.; Strathmann, H.; Wessling, M. Anion-exchange membranes containing diamines: preparation and stability in alkaline solution. J. Membr. Sci. 2004, 244, 25−34. (49) Volodina, E.; Pismenskaya, N.; Nikonenko, V.; Larchet, C.; Pourcelly, G. Ion transfer across ion-exchange membranes with homogeneous and heterogeneous surfaces. J. Colloid Interface Sci. 2005, 285, 247−258. (50) Larchet, C.; Zabolotsky, V. I.; Pismenskaya, N.; Nikonenko, V. V.; Tskhay, A.; Tastanov, K.; Pourcelly, G. Comparison of different ED stack conceptions when applied for drinking water production from brackish water. Desalination 2008, 222, 489−496. (51) Kedem, O.; Maoz, Y. Ion conducting spacer for improved electrodialysis. Desalination 1976, 19, 465−470. (52) Zhang, K.; Wang, M.; Gao, C. Ion conductive spacers for the energy-saving production of the tartaric acid in bipolar membrane electrodialysis. J. Membr. Sci. 2012, 387−388, 48−53. (53) Balster, J.; Stamatialis, D. F.; Wessling, M. Membrane with integrated spacer. J. Membr. Sci. 2010, 360, 185−189. (54) Zabolotskii, V. I.; Loza, S. A.; Sharafan, M. V. Physicochemical properties of profiled heterogeneous ion-exchange membranes. Russ. J. Electrochem. 2005, 41, 1053−1060. (55) Hawkins, J.; Nyberg, E.; Kayser, G. Textured ion exchange membranes. U.S. Patent 2006016685, 2006.

(56) Vermaas, D. A.; Saakes, M.; Nijmeijer, K. Power generation using profiled membranes in reverse electrodialysis. J. Membr. Sci. 2011, 385−386, 234−242. (57) Balster, J.; Srinkantharajah, S.; Sumbharaju, R.; Pünt, I.; Lammertink, R. G. H.; Stamatialis, D. F.; Wessling, M. Tailoring the interface layer of the bipolar membrane. J. Membr. Sci. 2010, 365, 389−398. (58) Dermentis, K. Continuous electrodeionization through electrostatic shielding. Electrochem. Acta 2008, 53, 2953−2962. (59) Grabowski, A. Electrodeionization device with hydrodynamic flow splitting. U.S. Patent US8313630 (B2), 2012.

10379


Ion-Exchange Membranes in the Chemical Process Industry

Recommend Documents