ED Stacks

Yet another stack design utilizes unit cells. Unit-cell stacks were specifically developed for concentrating solutions. Each concentrating cell consists of one cation-exchange membrane and one anion-exchange membrane sealed at the edges to form an envelope with a spacer screen inside. The envelopes also have 

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The bipolar membranes are used in a more or less conventional ED stack together with conventional unipolar membranes. Such a stack has many acid—alkah producing membranes between a single pair of end electrodes. The advantages of the process compared to direct electrolysis seem to be that because only end electrodes are required, the cost of the electrodes used in direct electrolysis is avoided, and the energy consumption at such electrodes is also avoided. 

Performance. The performance of an ED stack may be estimated by considering the material balance around the stack ... 

FIG. 22-60 Expl oded view of a sheet-feed ED stack. Manifolds are built into the membranes and spacers as the practical way to maintain a narrow cell gap. Coutiesy Elsevier.)... 

A schematic of the production of acid and base by electrodialytic water dissociation is shown in Fig. 22-61. The bipolar membrane is inserted in the ED stack as shown. Salt is fed into the center compartment, and base and acid are produced in the adjacent compartments. The bipolar membrane is placed so that the cations are paired with OH" ions and the anions are paired with H. Neither salt ion penetrates the bipolar membrane. As is true with conventional elec trodialysis, many cehs may be stacked between the anode and the cathode. 

FIG. 22-61 Electrodialysis water dissociation (water splitting) membrane inserted into an ED stack. Starting with a salt, the device generates the corresponding acid and base by supplying and OH" from the dissociation of water in a bipolar membrane. CouHesy Elsevier.)... 

A rule of thumb for a modern ED stack is that the pumping energy is roughly 0.5 kWh/m, about the same as is reqmred to remove 1700 mg/f dissolved salts. 

The ED stack is the unit holding together anionic and cationic membranes assembled in parallel as in a filter press between two electrode-end blocks in such a manner that the stream undergoing ion depletion (i.e., the diluate or diluting stream) is kept separated from the other solution (concentrate or concentrating stream) undergoing ion enrichment. Figure 4 shows an exploded view of it. 

FIG. 4 Exploded view of an ED stack with indications of its main items 1, anode 2, cathode 3, steel frame 4, plastic end plate 5, inlet anode compartment 6, anode chamber 7, inlet cathode compartment 8, cathode chamber 9, inlet-concentrating compartment 10, inlet-diluting compartment 11, cation-exchange membrane 12, spacersealing frame 13, spacer net 14, anion-exchange membrane 15, screws. 

In a great number of papers dealing with the design of ED stacks, and especially in the recent and comprehensive paper by Lee et al. (2002d), the solute mass transfer coefficient (km) is expressed as a nonlinear function of the superficial flow velocity (vs) ... 

By applying the second Kirchhoff s law to the equivalent electrical circuit of the ED stack (Figure 9), the overall potential drop across an ED stack can be written as ... 

The overall electric resistance of the ED stack can be expressed as follows ... 

Both the first (ohmic) and second (polarization controlled) regions can be noted in Figure 10, reporting the results of typical limiting current measurements performed on an ED stack composed of only 19 cationic membranes (CMV type, see Table II) and model solutions containing 9 and 28 mol of NaCl per m3 for superficial velocities (vs) ranging from 3.4 to 5.9 cm/s (Fidaleo and Moresi, 2005a). 

The overall energy consumption (L) in an ED stack, as that sketched in Figure 9, can be calculated as ... 

FIG. 12 Schematic layout of a transport depletion ED stack used to demineralize whey c, cationic membrane ERS, electrode-rinsing solution n, neutral membrane. 

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