Electrodialysis with bipolar membranes

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Electrodialysis with bipolar membranes utilizes water dissociation of bipolar membranes in an electrodialysis stack for the production of acids and bases from the corresponding salts. The process and the stack design are in many aspects similar to conventional electrodialysis. The key element in electrodialysis with bipolar membranes is the bipolar membrane. Its performance determines to a very large extent the technical feasibility and economics of the process [25].

4.2.1 The bipolar membrane, and its structure and function

The function of the bipolar membrane is illustrated in Fig. 16a, which shows a bipolar membrane consisting of an anion- and a cation-exchange

Electrodialysis

Figure 16 Schematic diagram illustrating the function of a bipolar membrane showing (a) a bipolar membrane and (b) the 4-5 nm thick transition region at the interphase of the two cation- and anion-exchange layers.

Figure 16 Schematic diagram illustrating the function of a bipolar membrane showing (a) a bipolar membrane and (b) the 4-5 nm thick transition region at the interphase of the two cation- and anion-exchange layers.

layer arranged in parallel between two electrodes. If a potential difference is established between the electrodes, all charged components will be removed from the interphase between the two ion-exchange layers. If only water is left in the solution between the membranes, further transport of electrical charges can be accomplished only by protons and hydroxyl ions, which are regenerated due to the water dissociation in a very thin, that is, 4—5 nm thick, transition region between the cation- and anion-exchange layers of the bipolar membrane as shown in Fig. 16b. The water dissociation equilibrium is given by

The energy required for water dissociation can be calculated from the Nernst equation for a concentration chain between solutions of different pH values [28]. It is given by

where AG is the Gibbs free energy, F is the Faraday constant, R is the gas constant, T is the absolute temperature, and ApH and A j are the pH and the voltage differences between the two solutions separated by the bipolar membrane. For 1 molL-1 acid and base solutions in the two phases separated by the bipolar membrane, AG is 0.022 kWhmol-1 and Aj is 0.828 V at 25 °C.

The transport rate of H+ and OH ions from the transition region into the outer phases cannot exceed the rate of their generation. However, the generation rate of H+ and OH ions in a bipolar membrane is drastically increased compared to the rate obtained in water due to a catalytic reaction [26-28]. Therefore, very high production rates of acids and bases can be achieved in bipolar membranes.

4.2.2 System and process design of electrodialysis with bipolar membranes

The design of an electrodialysis process with bipolar membranes is closely related to that of a conventional electrodialysis desalination process. The main difference is in the stack construction and additional energy requirements for water dissociation. Furthermore, the mono- and bipolar membranes as well as other hardware components must have excellent chemical stability in strong acids and bases.

Stack design in bipolar membrane electrodialysis. A typical arrangement of an electrodialysis stack with bipolar membranes is illustrated in Fig. 3, which shows the production of an acid and a base from the corresponding salt in a repeating cell unit, which consists of three individual cells containing the salt solution, the acid and the base, and three membranes, that is, a cation-exchange, an anion-exchange, and a bipolar membrane. In industrial-size stacks, 50-100 repeating cell units may be placed between two electrodes.

The main difference between an electrodialysis desalination stack and a stack with bipolar membranes used for the production of acids and bases is the manifold for the distribution of the different flow streams. Since in most practical applications, high acid and base concentrations are required, the stack is usually operated in a feed and bleed concept as shown Fig. 17.

4.2.3 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 costs analysis in electrodialysis desalination. The contributions to the overall costs are the sum of the investment-related costs and the operating costs.

Investment costs in electrodialysis with bipolar membranes. The investment costs are directly related to the required membrane area for a certain plant capacity and can be expressed as a certain percentage of the total required membrane area for a given capacity plant, which can be calculated from the current density by

Water

Salt solution

Base

Salt solution

Water

Figure 17 Schematic diagram indicating the production of acids and bases from the corresponding salt in a stack with feed and bleed operation.

where Aunit is the required cell unit area containing a bipolar membrane, and a cation- and an anion-exchange membrane; i the current density; Qpro the product volume flow; F the Faraday constant; X the current utilization; and Cpro the concentration of the product.

Operating costs in electrodialysis with bipolar membranes. 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 total energy for the production of an acid and a base from the corresponding salt is as in electrodialysis desalination given by the total current passing through the stack and the voltage drop across the stack. The voltage drop across the stack is the result of the electrical resistance of the membranes, that is, that of the cation- and anion-exchange membranes and the bipolar membranes and the resistances of the acid, the base and the salt containing flow streams in the stack. In addition to the voltage drop required to overcome the various electrical resistances of the stack, additional voltage drop is required to provide the energy for the water dissociation which is given by Eq. (30). Assuming that the three cells of a cell unit in the stack have the same geometry and flow conditions, the total energy consumption in an electrodialysis stack is given analog to the energy in conventional electrodialysis expressed by [29]

N unit AunitX

where Espc pro is the energy needed for the production of a certain amount of acid and base; i is the current density passing through the stack; Nunit is the number of cell units in a stack; Aunit is the cell unit area; C and C are the concentration and the average concentration in a cell; D is the thickness of the individual cells; A is the equivalent conductivity; ; is the area resistance; X is the current utilization; R is the gas constant; T is the absolute temperature; F is the Faraday constant; ApH is the difference in the pH value between the acid and base; the subscript pro refers to product and the subscript i refers to salt, acid, and base; the superscripts am, cm, and bm refer to the cation-exchange, the anion-exchange, and the bipolar membrane; the superscripts out and in refer to cell outlet and inlet; Q is the flow of the acid or base through the stack; and t is the time.

The average concentrations of the acid, the base, and the salt in the bulk solutions are the integral average of the solutions given by ln(C°u7qn ) Ci = ( t ' ' ) (33)

Operation of an electrodialysis unit with bipolar membranes requires three pumps to circulate the salt solution, the acid, and the base through the stack.

The energy required for pumping flow streams through the stack is given by

Qprot Qpro where Ep,,spec is the total energy for pumping the solutions through the stack per unit product, keff is an efficiency term for the pumps, Q is the volume flow rate, the superscript fs refers to the different flow streams, and the subscripts p and pro refer to pumping and product, respectively.

The total costs of the electrodialytic water dissociation with bipolar membranes are the sum of fixed charges associated with the amortization of the plant investment costs, the energy costs, and of the operating costs, which include maintenance costs and all pre- and posttreatment procedures.

Problems in practical application of bipolar membrane electrodialysis. In addition to the precipitation of multivalent ions in the base containing flow stream and the stability of the ion-exchange membranes in strong acids and bases, a serious problem is the contamination of the products by salt ions, which permeate the bipolar membrane, which in general is not completely perm-selective. Furthermore, the current utilization is affected by the leakage of H+ and OH~ ions through the monopolar anion- and cation-exchange membranes. Especially, when high concentrations of acids and bases are required, the salt contamination is generally quite high and the current utilization low. This is a major limitation for the practical application of bipolar membranes for the effective production of acids and bases from the corresponding salt solutions.

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