[Membrane Science and Technology] Ion Exchange Membranes - Fundamentals and Applications Volume 12 || Chapter 1 Electrodialysis

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  • ElectrodialysisChapter 11.1. OVERVIEW OF TECHNOLOGY

    Industrial application of ion exchange membranes started at first in thefield of electrodialysis (ED) (cf. Preface) and it induced the development of thefundamental theory. This fact is easily understandable from those phenomenaexplained in Fundamentals, which are described by taking the ED into account.The development of the fundamental theory led to further development of theED technology. After that ion exchange membrane technology developed in thesucceeding technology such as electrodialysis reversal (EDR), bipolar membraneelectrodialysis (BP), electrodeionization (EDI), electrolysis (EL), fuel cell (FC)etc. describing in the succeeding chapters. Looking over these historical details,we notice that the ED becomes the fundamental technology and it is applied tothe succeeding technologies based on the ion exchange membranes. In thischapter, we discuss the main subjects such as the structure of electrodialyzer, EDprocess, practical application of ED etc.1.2. ELECTRODIALYZER

    1.2.1 Structure of an ElectrodialyzerThe basic structure of the vertical sheet-flow type module consists of

    stacks in which cation exchange membranes, anion exchange membranes, gas-kets (desalting cells and concentrating cells) are arranged alternately (Fig. 1.1).Fastening frames are put on both outsides of the stack which is fastened uptogether through cross bars setting in the frames. The deformation of the mem-branes is prevented by regulating hydrostatic pressure in the fastening frames.Inlet manifold slots and outlet manifold slots are prepared at the bottoms andheads of the gaskets, respectively. Spacers are incorporated with the gaskets toprevent the contact of cation exchange membranes with anion exchange mem-branes. Many stacks are arranged through the fastening frames. Electrode cellsare put on both ends of the electrodialyzer, which are fastened by a press puttingon the outsides of electrode cells (Fig. 1.2).

    An electrolyte solution to be desalinated is supplied from solution feedingframes to entrance manifolds, flows through entrance slots, current passingportions and exit slots, and discharged from exit manifolds to the outside of thestack (Figs. 1.1 and 1.2). A concentrated solution is usually supplied to con-centrating cells in a circulating flow system, and discharged to the outside of thestack through an overflow extracting system.DOI: 10.1016/S0927-5193(07)12015-5

    dx.doi.org/10.1016/S0927-5193(07)12015-5.3d

  • kehge

    kl

    +

    l

    j fd

    ai

    bc

    jf

    l

    Figure 1.1 Structure of a stack (filter-press type). a, Desalting cell; b, concentrating cell;c, manifold; d, slot; e, fastening frame; f, feeding frame; g, cation exchange membrane; h,anion exchange membrane; I, spacer; j, feeding solution; k, desalted solution; l, concen-trated solution (Azechi, 1980).

    Anodechamber

    Press (fix)

    Feedingframe

    Stack Stack

    Fasteningframe

    Feedingframe

    Cathode chamber

    Press (move)

    Figure 1.2 Filter-press type electrodialyzer (Azechi, 1980).

    Ion Exchange Membranes: Fundamentals and Applications322

  • Electrodialysis 323Effective membrane area is in the range from less than 0.5m2 to aboutmaximum 2m2. In order to reduce energy consumption, it is desirable to de-crease the electric resistance of the membrane and gasket thickness. Gasketmaterial is selected from synthesized rubber, polyethylene, polypropylene,polyvinyl chloride and ethylenevinyl acetate copolymer etc. The spacer is usu-ally incorporated with the gasket and a solution flows dispersing along thespacer net.

    1.2.2 Parts of an ElectrodialyzerThe electrodialyzer is composed of the parts as follows (Urabe and Doi,

    1978).

    1.2.2.1 Fastening FrameMaximum 2000 pairs of membranes are arranged between electrodes in an

    electrodialyzer, so as to let disassembling and assembling works be easy. Themembrane array is divided further into several stacks consisting of 50400 pairs.Fastening frames are fixed by bolts on both ends of the stack. The fasteningframe is usually served as a solution feeding frame, so that a desalting and aconcentrating solution are supplied to each gasket cell from the feeding frameincorporated in every stack. Material of the fastening frame is selected frompolyvinyl chloride, polypropylene and rubber-lining iron etc.

    1.2.2.2 Solution Feeding FrameA solution feeding frame is integrated for feeding solutions to each de-

    salting and concentrating cell. Manifold holes are prepared at correspondingpositions of the holes fitted in the gasket. Solutions are usually supplied throughthe manifolds to each stack, but as the case may be supplied to each plural stack.

    1.2.2.3 GasketThe shape of the gasket is presented in Fig. 1.3. A solution is supplied

    from the inlet manifold put at the bottom, flows through the slot and is fed intothe current passing portion. Then the solution is discharged through the outletslot to the manifold fitted to the head. The gasket has the following functions:(1) prevents solution leakage from the inside to the outside of the electrodialyzer,(2) adjusts the distance between a cation exchange membrane and an anionexchange membrane, (3) prevents solution leakage between a desalting cell and aconcentrating cell occurring at slot sections. In order to prevent the solutionleakage, it is desirable to adopt a soft material for the gasket. On the other hand,it is desirable to adopt a hard and stable material to avoid dimension changesduring long-term operation. The material of the gasket is selected from rubber,ethylenevinyl acetate copolymer, polyvinyl chloride, polyethylene etc. Thethickness of the gasket is in the range of 0.52.0mm.

  • Deformation of a membrane

    Membrane

    Gasket

    Slot

    Figure 1.4 Deformation of an ion exchange membrane (Urabe and Doi, 1978).

    Manifold

    Gasket

    Spacer

    Slot

    Manifold

    Figure 1.3 Gasket (Urabe and Doi, 1978).

    (a) (b) (c)

    Figure 1.5 Structure of slots (Urabe and Doi, 1978).

    Ion Exchange Membranes: Fundamentals and Applications3241.2.2.4 SlotIt is important to reduce the inside solution leakage (cf. Section 12.2 in

    Fundamentals), which arises through pinholes and cracks in the membranes orthrough gaps due to the membrane deformation at the slot as shown in Fig. 1.4.In order to prevent these troubles, a lot of devices are proposed as exemplified inFig. 1.5 in which (a) decrease the width of the slot, (b) bend the slot, (c) insert thesupport in the slot.

  • (a) Expanded PVC

    (c) Diagonal net (d) Mikoshiro texture (e) Honeycomb net

    (b) Wave porous plate

    Figure 1.6 Structure of spacers (Urabe and Doi, 1978).

    Electrodialysis 3251.2.2.5 SpacerThe function of a spacer is to keep the distance between the membranes.

    In addition, the spacer increases the limiting current density due to solutiondisturbance (cf. Sections 10.2 and 10.3 in Fundamentals). The spacer is selectedtaking account of the requirement such as; (1) low friction head loss, (2) lowelectric current screening effect, (3) easy air discharge, (4) less blocking of flow-pass caused by the precipitation of fine particles suspended in a feeding solution.The structures of a spacer are classified in Fig. 1.6 as (a) expanded polyvinylchloride, (b) wave porous plate, (c) diagonal net, (d) mikosiro texture and (e)honeycomb net.1.2.2.6 Electrode and Electrode ChamberPlatinum plated titanium, graphite or magnetite is used for anode material

    and stainless or iron is used for cathode material. The shape of electrodes isclassified into net, bar and flat. A partition is inserted between an electrodechamber and a stack for preventing the mixing of solutions. In an anode cham-ber, oxidizing substances such as chlorine gas evolve. An ion exchange mem-brane is easily deteriorated by contact with the oxidizing substances, so it isnecessary to use two sheets of partitions and put a buffer chamber between thetwo partitions. Material of the partition is an ion exchange membrane, an as-bestos sheet or a battery partition.

  • Ion Exchange Membranes: Fundamentals and Applications326An acid solution is added into a cathode solution and the electrodialyzer isoperated under controlling pH of the cathode solution for preventing the pre-cipitation of magnesium hydroxides in the cathode chamber. A feeding solutionor a concentrated solution is supplied into the electrode chamber. The concen-tration of oxidizing substances in the anode solution is reduced by adding so-dium sulfite or sodium thiosulfate into the solution being discharged.Sometimes, a sodium sulfate solution is supplied to an anode and a cathodechamber, achieving the neutralization by mixing the effluent of both chambers.

    1.2.2.7 PressAn oil pressure press is usually used adjusting the pressure to be 510 kg

    cm2.

    1.2.3 Requirements for Improving the Performance of an ElectrodialyzerIn order to improve the performance of an electrodialyzer, membrane

    characteristics should be naturally improved. At the same time, the circum-stances in an electrodialyzer in which the membranes work should be better.Here, we describe the definite problems lowering the circumstances in an elect-rodialyzer and requirements for improving the circumstances and performanceof an electrodialyzer (Urabe et al., 1978).

    1.2.3.1 Solution Velocity Distribution between Desalting CellsIn an electrodialyzer, ion exchange membranes and desalting and con-

    centrating cells are arranged alternately and a solution is supplied into desaltingcells. In this flow system, the solution velocity distribution in desalting cells doesnot become uniform. This phenomenon causes the concentration distributionand current density distribution in the electrodialyzer, an

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