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

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ElectrodialysisChapter 11.1. OVERVIEW OF TECHNOLOGYIndustrial 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. ELECTRODIALYZER1.2.1 Structure of an ElectrodialyzerThe basic structure of the vertical sheet-flow type module consists ofstacks 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)12015-5dx.doi.org/10.1016/S0927-5193(07)12015-5.3dkehgekl+lj fdaibcjflFigure 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).AnodechamberPress (fix)FeedingframeStack StackFasteningframeFeedingframeCathode chamberPress (move)Figure 1.2 Filter-press type electrodialyzer (Azechi, 1980).Ion Exchange Membranes: Fundamentals and Applications322Electrodialysis 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). Fastening FrameMaximum 2000 pairs of membranes are arranged between electrodes in anelectrodialyzer, 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. 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. GasketThe shape of the gasket is presented in Fig. 1.3. A solution is suppliedfrom 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 membraneMembraneGasketSlotFigure 1.4 Deformation of an ion exchange membrane (Urabe and Doi, 1978).ManifoldGasketSpacerSlotManifoldFigure 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 inFundamentals), 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 plateFigure 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. Electrode and Electrode ChamberPlatinum plated titanium, graphite or magnetite is used for anode materialand 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. PressAn oil pressure press is usually used adjusting the pressure to be 510 kgcm2.1.2.3 Requirements for Improving the Performance of an ElectrodialyzerIn order to improve the performance of an electrodialyzer, membranecharacteristics 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). 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, and gives rise to thedecrease of the limiting current density of the electrodialyzer (cf. Sections 9.1,11.6 and 11.7 in Fundamentals). In order to operate the elctrodialyzer stably, itbecomes necessary to make the solution velocities between the desalting cellsuniform. Solution Leakage in an ElectrodialyzerThe dimensions of all parts of an electrodialyzer are not always consistentwith the values in the specifications. Small pinholes can open in an elect-rodialyzer because the strength of ion exchange membranes is relatively low.Gaps may occur between the materials composing the electrodialyzer in theassembly works of an electrodialyzer. If a pressure difference between the de-salting cells and concentrating cells exists in these circumstances, solutions leakthrough the membranes and lower the performance of the electrodialyzer(cf. Section 12.2 in Fundamentals). In order to avoid these troubles, we have toremove the pinholes and gaps in the electrodialyzer and control the pressuredifference between desalting cells and concentrating cells.Electrodialysis 3271.2.3.3 Distance between the MembranesDecrease of the distance between the membranes brings about the de-crease of electrical resistance and energy consumption. On the other hand, itbrings about the increase of friction loss of solution flow, blocking of the ma-terials suspended in a feeding solution and the increase of pumping motivepower. Accordingly, it becomes necessary to realize the optimum distance be-tween the membranes. The optimum distance is decided further taking accountof electric resistance of ion exchange membranes and that of electrolyte solutionin desalting and concentrating cells. SpacerMain functions of a spacer in to create space between a cation exchangemembrane and an anion exchange membrane. When solution velocity and theReynolds number are decreased, hydrodynamic pattern exhibits laminar flow,which means that disturbing effect of the spacer is low. In order to increase thelimiting current density, turbulent flow should be induced by increasing theReynolds number (cf. Sections 10.3.4 and 10.5 in Fundamentals). Electric Current LeakageA part of an electric current flows through slots and manifolds causingineffective current leakage. Current leakage is increased by the increases of thenumbers of cell pairs integrated in a stack and the increase of sectional area ofslots and manifolds (cf. Section 12.1 in Fundamentals). These events, however,related with the solution velocity distribution between the cells described inSection Simplicity of Structure of an ElectrodialyzerDisassembling and assembling work is peculiar characteristics in operat-ing an electrodialyzer (cf. Section 1.5.3 in Applications). Excellent durability ofion exchange membranes is owing to careful treatment in this work. So, thesimplicity of the structure is a requirement for performing this work.1.3. ELECTRODIALYSIS PROCESSThe ED process had been explained in detail in several articles (Mintz,1963; Shaffer and Mintz, 1966; Itoi et al., 1978; Yawataya, 1986; Tanaka, 1993).The following is overall description of the process.1.3.1 One-Pass Flow ProcessAn electrolyte solution is fed to an electrodialyzer and desalted solution isdischarged to the outside of the process (Fig. 1.7). When the concentration ofthe feeding solution is invariable, the performance of the system becomes stable.Joining the process in Fig. 1.7 to the succeeding process, a one-pass flowFeeding solutionConcentrated dischargeFeeding solutionDesalted solutionConcentrating cellDesalting cellIon-exchangemembranePumpFigure 1.7 One-pass flow process (Tanaka, 1993).ablxx+dxCoutCinC-dCC0Figure 1.8 Electrolyte concentration change in a desalting cell (Tanaka, 1993).Ion Exchange Membranes: Fundamentals and Applications328multiple continuous system suitable for a large-scale plant is formed. The de-salination in Fig. 1.7 is proceeded as below.In the desalting cell shown in Fig. 1.8, a, b and l are the flow-pass depth(distance between membranes), the flow-pass width and the flow-pass length,respectively. Cin and Cout are the electrolyte concentration at the inlet and theoutlet of the desalting cell. Passing an electric current across the membranes,ions in the desalting cell are transferred toward the concentrating cell. Assumingthe transport of water to be negligible across the membranes under an electriccurrent passing, the material balance between at x and x+dx in Fig. 1.8 isElectrodialysis 329indicated by the following equation, including current density i, linear velocity ina desalting cell u, current efficiency Z and electrolyte concentration at x distantfrom the inlet of a desalting cell C.au 1CdC ZFiCdx (1.1)Voltage applied to a membrane pair (cell voltage) consists of a membrane po-tential and Ohmic loss of a cation exchange membrane, an anion exchangemembrane, a desalting cell and a concentrating cell. In the desalting process,Ohmic loss of a desalting cell iRde (Rde: electric resistance of desalting cell) isdominant in the cell voltage and voltage difference between electrodes is inde-pendent of x (cf. Section 9.1 in Fundamentals). Accordingly, iRde in the desalt-ing cell is estimated to be invariable in the range of x 0l in Fig. 1.8. Further,Rde is inversely proportional to C, so that i/C is assumed to be nearly constantand we can integrate Eq. (1.1) as follows:uaZ CoutCin1Cdx ZFiCZ l0dx (1.2)Solving Eq. (1.2)CoutCin exp lZaFuiC (1.3)Desalting ratio a is defined as follows:a 1 CoutCin 1 exp lZaFuiC (1.4)Here, we can adopt the average value i=C instead of i/C (Yawataya, 1986) in Eq.(1.4) as follows:iC iC 2iCin Cout(1.5)From Eqs. (1.4) and (1.5)iC 2iCin2 a(1.6)Substituting Eq. (1.6) into Eq. (1.4)a 1 exp 2ZiaFCinu=l2 a (1.7)a is calculated using Eq. (1.7), assuming a 0.1 cm, Cin 0.05 eq dm3,Z 0.90, and plotted against u/l taking i as parameter (Fig. 1.9). l vs. u iscomputed setting a 0.90 (Fig. 1.10) indicating u is proportional to l. Ina practical-scale sheet-flow electrodialyzer, flow-pass length l is from less than0.00 0.01 0.02 0.03 0.04 0.05 (s-1) 1.9 Influence of linear velocity, flow-pass length and current density to desaltingratio.0 1 2 3 4 5 6 7 8 9 100510152025303540u (cm s-1)l (m) 1.10 Flow-pass length and linear velocity in a desalting cell.Ion Exchange Membranes: Fundamentals and Applications330Electrodialysis 33112m, and linear velocities in desalting cells u are 310 cm s1. In a tortoise-flowtype electrodialyzer, however, l and u are larger than those in the sheet-flow type(cf. Section 2.2 in Application).1.3.2 Batch ProcessIn Fig. 1.11, the feeding solution is prepared in the circulation tank at first.Next, open valve V1, close valve V2 and circulate the solution between the tankand the electrodialyzer. The solution is electrodialyzed applying constant volt-age until electrolyte concentration of a desalted solution attains a definite value,and then the desalted solution is discharged. The process mentioned above isrepeated periodically. This system is usually adopted in a small-scale plant andthe desalination is proceeded as follows.A definite volume V of electrolyte solution is assumed to be circulatedbetween the circulating tank and the electrodialyzer and it is electrodialyzedapplying a constant voltage between the electrodes. An electric current I andelectrolyte concentration C of the solution decrease with elapsed time t, butI/C does not change with t, because Ohmic loss IRdil is dominant in a cell volt-age and roughly inversely proportional to C. Setting numbers of cell pairs in-tegrated in the electrodialyzer M, the electrolyte concentration in a feedingsolution CF and in a desalted product solution CP, and an operating time inan unit batch cycle t, and assuming the transport of water to be negligible acrossFeeding solutionConcentrated dischargeFeeding solution Desalted solutionCirculation tankV1V2Figure 1.11 Batch process (Tanaka, 1993).Ion Exchange Membranes: Fundamentals and Applications332the membranes under an electric current passing, the material balance in adesalting cell in this batch system (Fig. 1.11) is expressed by the followingequation.VZ CPCFdCC MZF1CZ t0dt (1.8)Integrating Eq. (1.8)CPCF exp MZFVICt (1.9)Expressing an electric current I as I bli and the solution volume V as V QPt,(Qp is product solution volume, water transport across the membranes is as-sumed to be neglected) in Eq. (1.9), the desalting ratio a in the batch system isintroduced as follows:a 1 CFCP 1 exp MblZFQPiC (1.10)Accordingly, numbers of cell pairs M integrated in the electrodialyzer isM FQP lnCF=CPblZi=C(1.11)Here, we estimate M in the following case:QP 1m3 h1 106/3600 cm3 s1, CF/CP 10, a 1CP/CF 0.90,b 50 cm, l 50 cm, Z 0.90, i 0:01 A cm2; CF 5 105 eq cm3,F 96,500C eq1.From Eq. (1.6), i/C is calculated as:iC 2iCin2 a 364Substituting these values into Eq. (1.11), we obtain M 75 pairs.1.3.3 Partially Circulation (Feed and Bleed) ProcessAn electrodialyzer is operated at constant current density supplying adefinite amount of solution and circulating a part of feeding solution (Fig. 1.12).Joining Fig. 1.12 to the succeeding process, a multiple partially circulation sys-tem (Fig. 1.13) suitable for a middle-scale plant is formed. The desalination inFig. 1.13 is achieved as below.Assuming the linear velocity in desalting cells u, current efficiency Z andi/C to be constant in each electrodialyzer, Cout/Cin in each electrodialyzer isexpressed using Eq. (1.3) as follows:Cout1Cin1 Cout2Cin2 CoutnCinn CoutCin b (1.12)Feeding solutionConcentrated dischargeFeeding solutionDesalted solutionFigure 1.12 Partially circulation process (Tanaka, 1993).QR-Q QR-Q QR-Q(Cout)1 (Cout)2 (Cout)nQF=Q QR Q QR QRQR QRQ QP=Q(Cin)1 (Cin)2 (Cin)n CP=(Cout)nQR(Cout)1 (Cout)2 (Cout)n1 2 nFigure 1.13 Multiple partially circulation process (Tanaka, 1993).Electrodialysis 333From the material balance in Fig. 1.13CFQF Cout1QR Q Cin1QRCout1Q Cout2QR Q Cin2QR...Coutn1Q CoutnQR Q CinnQR(1.13)Substituting Eq. (1.12) to Eq. (1.13)CFQF Cin1fQR bQR QgCin1 Cin2bQfQR bQR Qg...Cinn1 CinnbQfQR bQR Qg(1.14)Ion Exchange Membranes: Fundamentals and Applications334Putting Eq. (1.14) togetherCFQF CinnbQn1fQR bQR Qgn (1.15)Substituting QF QP Q, CP (Cout)n (water transport across the membranesis neglected) and Eq. (1.12) into Eq. (1.15), and rearranging the equationCFCP QRQPCinCout 1 1 n(1.16)Desalting ratio a of the process is introduced as follows from Eq. (1.16).a 1 CPCF 1 QRQPCinCout 1 1 n(1.17)In one-path flow system, QR QP holds, so Eq. (1.17) becomesCFCP CinCout n(1.18)A large-scale plant is realized by assembling a multi-stage multiple partiallycirculation process arranging in each stage N units of electrodialyzer incorpo-rated with M cell pairs as indicated in Fig. 1.14. The amount of solution QRcirculating in each stage isQR abuMN (1.19)QR QR(Cin)1 (Cout)1 (Cin)2QR(Cout)2 (Cin)N (Cout)NQF=Q Q Q QCF CP=(Cout)NQR-Q QR-Q QR-QN21N21N21Figure 1.14 Multi-stage multiple partially circulation process (Tanaka, 1993).Table 1.1 Arrangement of electrodialyzers in a multi-stage multiple partially circulationprocessn QR (cm3 s1) N nN1 514,530 25.7 1 262 123,617 6.2 2 63 65,999 3.3 3 34 44,494 2.2 4 25 33,438 1.7 5 26 26,744 1.3 6 1Electrodialysis 335We estimate the numbers and arrangement of electrodialyzer in a multi-stagemultiple partially circulation process (Fig. 1.14) putting the following param-eters: a 0.1 cm, b 100 cm, l 100 cm, u 5 cm s1, i/C 364A cm eq 1,CF/CP 10, QP 200m3 h1 200 106/3600 cm3 s1, Z 0.9, M 400pairs, F 96,500Ceq1. At first, Cin/Cout is computed as follows:CinCout exp lZaFuiC exp 100 0:90 3640:10 96500 5 1:9718Using Eq. (1.16), CF/CP isCFCP QRQPCinCout 1 1 n QR200 106=36001:9718 1 1 n 10Accordingly, QR is expressed as follows:QR 101=n 1200 106=36000:9718cm3 s1 (1)From Eq. (1.19), numbers of cell pairs per unit stage areMN 400N QRabu QR0:1 100 5So, we have N as follows:N QR20000(2)Changing the values of n, QR, N and nM are computed as indicated in Table 1.1using Eqs. (1) and (2).ConcentratedFeeding solutionDesalted solutionC outC q C inq0q0C0q q Figure 1.15 Concentration or separation process.Ion Exchange Membranes: Fundamentals and Applications3361.3.4 Concentration ProcessFig. 1.15 gives a single-stage concentration unit process. The output of amulti-stage multiple process X is expressed by the following equation.X iF blZMnN (1.20)Here, we calculate numbers of electrodialyzers in the process for concentratingseawater by ED and crystallizing NaCl by evaporation. Putting as NaCl outputin the evaporation process: 200,000 t y1, operating time of electrodialyzers:8000 h y1, NaCl yield rate in the evaporation process: 0.97 and NaCl molecularweight: 58.5, we obtain:X 2000008000 3600 0:97 58:5 122:38 eq=sSubstituting X 122.38 eq s1, i 0.03A cm2, b 100 cm, l 100 cm,e 0.92, Z 0.90, M 2000 pairs and F 96,500C eq1 into Eq. (1.20),nN 23.8 is obtained. Accordingly, the numbers of the electrodialyzers in thismulti-stage multiple processes are known to be 24.Electrolyte concentration in a concentrated solution C00 is expressed by thefollowing equation introduced from the overall mass transport equation(cf. Section 6.1 in Fundamentals).C00 12rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiA2 4rBq AA fi m rC0B li mC0(1.21)0 1 2 3 4 5 6 7 8 9 100123456C" (eq dm-3)i (A dm-2) 9 7 5 3 1C' (eq dm-3)Figure 1.16 Dependence of C00 on i and C0.Electrodialysis 337The overall transport number l, the overall solute permeability m and the over-all electro-osmotic permeability f are expressed by the following empiricalequations of the overall hydraulic conductivity r (cf. Section 6.1 in Fundamen-tals).l l1 l2r l1 9:208 106 l2 1:914 103 (1)m mr m 2:005 104 (2)f n1r0:2 n2r n1 3:768 103 n2 1:019 102 (3)Here, we calculate l, m and f by substituting r 1 102 cm4 eq1 s1 intoEqs. (1)(3) as; l 9.399 106 eqC1, m 2.005 106 cm s1,f 1.398 103 cm3C1. Dependence of C00 on i for this membrane pairs iscomputed as shown in Fig. 1.16 by substituting current density i, electrolyteconcentration in a feeding solution C0, l, m, f and r into Eq. (1.21).1.3.5 Separation ProcessAn ion exchange membrane shows the permselectivity between ionshaving the same charged sign, which is defined by the permselectivity coefficientEq. (1.22) for ion A against ion B, TBA (cf. Section 3.2 in Fundamentals).TBA C00B=C00AC0B=C0A(1.22)Ion Exchange Membranes: Fundamentals and Applications338C00i ; is concentration (eq dm3) of ion i in a concentrating cell and it is assumedto be invariable in the cell. Concentration of ion i in a desalting cell C0i is theaverage of the values at the inlet and the outlet as follow:C0i 12C0i;in C0i;out (1.23)C0i;in and C0i;out are concentrations of ion i at the inlet and outlet of a desaltingcell. Ion A is separated from ion B by applying the permselectability of themembrane in the separation process indicated in Fig. 1.15.Here, we define the separation factor of ion B against ion A, in a desaltedsolution S0BA and in a concentrated solution S00BA by the following equations.S0BA C0B;out=C0A;outC0B=C0A(1.24)S00BA C00B=C00AC0B=C0A(1.25)C0A; C0B: Concentration of ion A and ion B in a feeding solution.Desalting ratio of ion i (ai) in Fig. 1.15 is defined as:Ci;out 1 aiC0i (1.26)The material balance of ion i in Fig. 1.15 is shown by the following equationsassuming q0 q00 hold.C0i q0 C0i;outq0 C00i q00 (1.27)C0i;inq0 C0i;outq0 C00i q00 (1.28)q0 is the amount of feeding solution to the process, q0 the amount of a desaltingsolution being supplied to the electrodialyzer and q00 the amount of a concen-trating solution flowing out from the electrodialyzer.Following equations are introduced from Eqs. (1.22)(1.28):S00BA 1 aB1 q0=2q01 aA1 q0=2q0(1.29)S00BA S0BATBA 1 aB1 q0=2q01 aA1 q0=2q0TBA (1.30)When q0 q0 hold, Eqs. (1.29) and (1.30) becomeS0BA 11 aA1 TBA(1.31)Electrodialysis 339S00BA S0BATBA TBA1 aA1 TBA(1.32)In Eqs. (1.31) and (1.32), the following phenomena are found:when TBA41 S0BAo1 S00BA oTBAwhen TBA 1 S0BA 1 S00BA TBAwhen TBAo1 S0BA41 S00BA 4TBA(1.33)Equation (1.33) means that the separatability is inferior to the permselectability.Further, we find the following events in Eqs. (1.31) and (1.32):limaA!0S0BA 1 limaA!1S0BA 1TBA(1.34)limaA!0S00BA TBA limaA!1S00BA 1 (1.35)aA vs. S0BA and S00BA is computed using Eqs. (1.31) and (1.32) and shown inFig. 1.17, in which the relationships in Eqs. (1.33)(1.35) are confirmed.0.0 0.2 0.4 0.6 0.8 1.010-210-111011020S AB S ABA1040.40.0110410.4TAB =0.01Figure 1.17 Relationship between permselectivity coefficient, desalting ratio and sep-aration factor. Open, S0BA ; filled, S00BA :Ion Exchange Membranes: Fundamentals and Applications3401.4. ENERGY CONSUMPTION AND OPTIMUM CURRENT DENSITYIn multi-stage multiple partially circulation system (Fig. 1.14), rectifiersare assumed to be put in each stage, in which an electric current I is supplied toelectrodialyzers through a parallel circuit. I is expressed as follows:IFZMN QRfCinn Coutng (1.36)Voltage V applied to an electrodialyzer is indicated by the following equationputting the voltage in the electrode cell as VP.V MV cell VP (1.37)Vcell is cell voltage as follows (cf. Section 13.2 in Fundamentals):V cell IRK RA Rdil Rconc EM (1.38)RK and RA are electric resistance of a cation and an anion exchange membrane.Rdil and Rconc are electric resistance of a desalting and a concentrating cell. RM ismembrane potential.A reasonable estimate of the optimum current density is introduced byassuming that the major costs are divided into three categories: those directlyproportional to current density, those inversely proportional to current density,and those independent of current density (Eq. (1.39)).Z ai bi c (1.39)where Z is total cost, and a, b, c are the relative proportionality constants. Theoptimum current density iopt is defined by the minimum of Eq. (1.39), so it isintroduced by differentiating Eq. (8.37); dZ/di 0 and expressed as (Leitz,1986):iopt PQR 1=2(1.40)P is depreciation cost ($/m2s), Q the energy cost ($/Ws) and R the cell pairelectric resistance (O m2).1.5. SURROUNDING TECHNOLOGY1.5.1 Filtration of a Feeding SolutionFine materials such as sand, clay, iron components, humus soil and mis-cellaneous inorganic and organic colloid are usually suspended in a raw feedingsolution. In order to avoid invasion of these materials into an electrodialyzer, afeeding solution is filtrated using sand, fibers or cohesive agent and turbidity ofthe solution is decreased less than 0.10.2 ppm. First of all, the following valve-less sand filter is broadly applicable (Tsunoda, 1994). In Fig. 1.18, a raw feedingH31096 H2H1511742318Figure 1.18 Valve-less filter. 1, Feeding solution inlet; 2, filtrating chamber; 3, sandfilter; 4, collecting chamber; 5, filtrate flow out pipe; 6, filtrate outlet; 7, connecting duct;8, washing chamber; 9, siphon pipe; 10, siphon breaker; 11, control valve (Tsunoda,1994).Electrodialysis 341solution is supplied into the filtrating chamber, filtrated through the sand filterand collected in the collecting chamber. The filtrate flows out through the flow-out pipe and supplied to an electrodialyzer at solution level H2. At the sametime, a part of filtrate flows through the connecting pipe into the washingchamber put on the filtrating chamber and is accumulated there at H2. Pro-ceeding with the operation, solution level in the siphon pipe goes up with theincrease of flow resistance in the sand filter. When the level surpasses H3, thesolutions in the filtrating chamber, the collecting chamber and the washing tankare discharged to the outside of the process at one stroke through the siphonpipe due to the siphon function. In this instance, sand is washed and the level inthe washing tank goes down to H1.1.5.2 Scale Trouble Prevention1.5.2.1 Acid DosageTotal carbonic acid dissolved in brine is equilibrated to CO2 gas(3 104 atm.) in air. Total carbonic acid concentration in seawater is25 102mol dm3 at pH 7.0 7.4. 1530% of this total value is carbonicacid molecules (CO2 and H2CO3), and the remainders are ionic carbonic acidconsisting of HCO3 (more than 95%) and CO32 (less than 5%). In a concen-trating cell in an electrodialyzer HCO3 ions decompose as follows and combineIon Exchange Membranes: Fundamentals and Applications342with Ca2+ ions to form CaCO3 precipitation.2HCO3 3CO23 H2O CO2 " (1)CO23 Ca2 ! CaCO3 (2)In order to avoid CaCO3 precipitation, HCO3 ions are decomposed into CO2gas by dosing with an HCl or a H2SO4 solution into concentrating cells.HCO3 H ! CO2 " H2O (3) Precipitation Controlling Agent DosageSmall amount of precipitation controlling agents such as condensedsodium phosphate Na2[Na4(PO3)6] are dosed into a concentrating cell, resultingwith the absorption of crystalline nuclei to the agents and dissolution of CaSO4or CaCO3 due to the following chelate reaction.Na2Na4PO36 CaX ! Na2Na2CaPO36 Na2XCaX : CaSO4 or CaCO3(4)Carboxyl methyl cellulose (CMC) or poly-acrylic acid is also available instead ofcondensed sodium phosphate.1.5.3 Disassembling and Assembling WorksIn spite of the filtration described in Section 1.5.1, a very small quantity offine particles passes through a filter and invades into an electrodialyzer. Some-times, fine organisms pass through the filter and breed in an electrodialyzer. Thefine particles are adhered on the surface of membranes and spacers in desaltingcells (cf. Section 14.2.2 in Fundamentals), causing the increase of flow resistanceof a solution in the desalting cell with acceleration of concentration polarizationon the membrane surface (cf. Section 14.2.1 in Fundamentals). In order to avoidthese troubles, an electrodialyzer is usually disassembled and washed periodically.The disassembling and washing process is generally as follows (Tanaka, 1987):(1) Electric current interruption(2) Solution feeding interruption(3) Solution discharge(4) Stack extraction(5) Stack disassembling(6) Membrane surface washing(7) Desalting cell, concentrating cell and spacer washing(8) Stack assembling(9) Leak test(10) Integrating stack into an electrodialyzer(11) Solution feeding(12) Electric current passing.Electrodialysis 3431.6. PRACTICE1.6.1 Potable Water Production from Brackish WaterResidents in an isolated island suffer from serious water shortage becauseof their high dependence on rainwater. In order to solve this problem, AsahiChemical Co. constructed the ED plant (2500m3/day) illustrated in Fig. 1.19 in1990 in Ohshima island, Tokyo (Fukuhara et al., 1993). Ohshima Town suppliespotable water to the residents even now. The specifications of this plant areshown in Table 1.2.Raw brackish water pumped up from wells is fed to the suspended solidfilter (SSF), and then it is supplied to the first stage electrodialyzer unit (EDU-A1 and EDU-B1) through the first stage desalination tank (DST-1). A part ofthe filtrated solution is supplied to the concentrated solution tank (CST). Thedesalted solution in DST-1 is further desalted at the second stage electrodialyzerunit (EDU-A2 and EDU-B2) to be about 400 ppm, and then supplied to thewater cleaning tank (WCT).P P P PPPP P P P P PWells ST SSF CST AT EST DST-1 DST-2 WCT DRPWTEDU-A 1 EDU-A 2 EDU-B 1 EDU-B 2DS-A 1ACS-A FS-A CS-AACS-B FS-B CS-BDS-B 1 CCS-ADS-A 2 DS-B 2 CCS-B+ + + + Figure 1.19 Saline water desalting process (Ohshima island). ACS, Anode compartmentsolution; AT, acid tank; CCS, cathode compartment solution; CS, concentrated solution;CST, concentrated solution tank; DR, distributing reservoir; DS, desalinated solu-tion; DST, desalination tank; EDU, electrodialyzer unit; EST, electrode solution tank;FS, frame solution; PWT, product water tank; SSF, suspended solid filter; ST, stock tank;WCT, water cleaning tank (Fukuhara et al., 1993).Table 1.2 Specifications of the electrodialysis plantSite Ohshima, TokyoCapacity, product water 2500m3/day, from 1200 ppm TDS raw water1000m3/day, from 3000 ppm TDS raw waterRaw water source 4 brackish wellsProduct water TDS: 450 ppm or lessChloride ions: 150 ppm or lessElectrodialyzers Asahi Chemical SS-0Conductive area 1780 m3 (318 pairs/stack, 4 stack)Thickness Dilution compartment: 0.5mmConcentration compartment: 0.5mmMode of operation Dual-train, two steps per line automatic, continuousSource: Fukuhara et al. (1993).Table 1.3 Operational resultsProduction rate 2610m3/day, from 1200 ppm TDS raw waterProduct water TDS: 440 ppmChloride ions: 145 ppmpH: 6.8Electric consumption 0.8 kWh m3Source: Fukuhara et al. (1993).Ion Exchange Membranes: Fundamentals and Applications344Organic fouling due to organic acid including in raw brackish water isavoided by integrating anti-fouling anion exchange membranes Aciplex A-201into the electrodialyzer instead of standard type Aciplex A-101 membranes(cf. Section 14.3.2 in Fundamentals).Operational results are shown in Table 1.3. Analysis of raw and productwater are shown in Table 1.4. The values listed in both tables meet the potablewater standard established by the Ministry of Welfare, Japan. Actual operatingcost is determined based on the plant operation in a year as indicated in Table Electrodialysis Desalination System Powered by Photo-Voltaic PowerGenerationBabcock-Hitachi K.K. and Hitachi Ltd. established ED system combinedwith sunlight photo-voltaic power generation in Oshima island and at Saki-yama, Nagasaki (Inoue and Kuroda, 1993). In this system, the electrodialyzerwas operated using direct current power generator using solar batteries andproduced potable water.Fig. 1.20 gives the system in Oshima island, showing a two-stage seawaterdesalting process for producing 103/day of potable water. In the first stage, if it isfine, seawater (35,000mg l1-TDS) is desalted during the daytime to obtain anTable 1.4 Analysis of raw and product waterRaw Watera Product WaterVisual Inspection Colorless, Transparent Colorless, TransparentTurbidity (1) o1.0 o1.0Color (1) o1.0 o1.0pH 7.2 7.3Electrical conductivity (mS cm1) 2500 650Total hardness (CaCO3, ppm) 685 83Calcium hardness (CaCO3, ppm) 406 33Evaporation residue (ppm) 1631 418Silica (SiO2, ppm) 43 42Chloride ion (Cl, ppm) 632 131Sulfuric ion (SO4, ppm) 262 40Hydrocarbonate ion (HCO3, ppm) 210 82Calcium ion (Ca, ppm) 51 14Potassium ion (K, ppm) 17 3Magnesium ion (Mg, ppm) 48 12Sodium ion (Na, ppm) 368 94Source: Fukuhara et al. (1993).aFrom single well.Table 1.5 Operating costUnits Projected ActualVolume of product water m3/day 1950 1853Raw water TDS mg l1 1600 1250Water recovery % 80.0 86.5Electric consumption yen m3 27.82 10.04Stering agents yen m3 0.38 3.73Membrane replacement yen m3 11.80Total yen m3 40.00 13.77Source: Fukuhara et al. (1993).Electrodialysis 345intermediately desalted solution (5000mg l1-TDS) and at the same time, elec-tric power surpluses are stored in batteries (STB). In the second stage, theintermediately desalted solution is further desalted to obtain potable water(400mg l1-TDS) in the rainy or cloudy daytime or in the nighttime using thestored electric power.Fig. 1.21 shows the system in Sakiyama for producing 200m3/day ofpotable water from brackish water. In this system, electric consumption forpumping up raw brackish water is larger. So, in the fine daytime, raw brackishwater (1500mg l1-TDS) is pumped up and is desalted to obtain potable water(400mg l1-TDS). At the same time, brackish water and electricity are stored inSOBCBO STBDATFILDSTISTDSP ED CSPCSTBrineSeaPWTProductwater +FSTSWPSeatFigure 1.20 Seawater desalination process powered by solar generation (Oshima island).CBO, Control board; CSP, concentrated solution pump; CST, concentrated solutiontank; DAT, direct/altering current transducer; DSP, desalted solution pump; DST, de-salted solution tank; ED, electrodialyzer; FIL, filter; FST, filtrated solution tank; IST,intermediately concentrated solution tank; PWT, product water tank; SOB, solar battery;STB, storage battery; SWP, seawater pump (Inoue and Kuroda, 1993).Ion Exchange Membranes: Fundamentals and Applications346the fine daytime for operating the electrodialyzer in the rainy or cloudy daytimeand in the nighttime. The specifications of both systems are shown in Table 1.6.An electric power consumption pattern of the Oshima plant is illustratedin Fig. 1.22. In a batch desalting system, electric power consumption is large atbeginning and it decreases with elapsed time. So, when a large amount of electricpower is generated in the fine daytime, seawater is electrodialyzed to obtainintermediately desalted solution, which is temporarily stored in a tank (high-concentration operation). At the same time, electric power surpluses are storedin STB. In the cloudy or rainy daytime and in the nighttime, the stored solutionis further desalted to obtain potable water (low-concentration operation) pass-ing the stored electric power.Fig. 1.23 shows an electric power consumption pattern of the Sakiyamaplant, which is automatically controlled with a sunlight photo-voltaic powergeneration system, a storage battery system and an ED system. An electricpower generated in the solar system is directly consumed in the electrodialyzer,and the electric power surpluses charge the battery.Operating performance of both plants are indicated in Table 1.7.SOBtCBO STBDCLDATACLAlternating currentFILWELSWPDSTDSPECM fromFSTPWTProductwaterCSTSeaCSPED +to CSTFSTSaline waterDirect currentFigure 1.21 Saline water desalination process powered by solar generation (Sakiyama).ACL, Altering current load; CBO, control board; CSP, concentrated solution pump;CST, concentrated solution tank; DAT, direct/altering current transducer; DCL, directcurrent load; DSP, desalted solution pump; DST, desalted solution tank; ECM, electro-conductivity meter; ED, electrodialyzer; FIL, filter; FST, filtrated solution tank; PWT,product water tank; SOB, solar battery; STB, storage battery; SWP, seawater pump;WEL, well (Inoue and Kuroda, 1993).Electrodialysis 3471.6.3 Electrodialytic Recovery of Wastewater from a Metal Surface TreatmentProcessElectrodialysis is applicable to recovering wastewater and nickel in a metalsurface treatment process. The process was developed by Asahi Glass Co. asfollows (Itoi et al., 1986).A nickel plating process includes several rinsing processes. In this process,Ni concentration in the effluent from the first rinsing stage is high. So that theeffluent is usually returned to the electro-plating bath and the effluent from thefinal rinsing stage is discharged. The ED process is designed to collect Ni ions inthe first rinsing stage and return them to the electro-plating bath for the purposeof increasing recovery ratio of Ni and decreasing Ni content in the waste fromthe final rinsing stage.Fig. 1.24 is a continuous process operating in the car components man-ufacturing factory. The process includes two stages of rinsing baths and one unitTable 1.6 Specifications of the desalting processOshima SakiyamaSolar generatorType Silicon single crystal Silicon single crystalCapacity 25 kW 65 kWModule number 64W 390 module 47W 1380 moduleStorage batteryType Lead storage battery Lead storage batteryCapacity 115 kWh 230 kWhVoltage 96V 192VElectrodialyzerCapacity 10m3/day 200m3/dayRaw water Seawater (35,000mg l1-TDS) Saline water (1500mg l1-TDS)Type Filter-press Filter-pressGasket dimension 185 2000mm 370 2000mmEffectivemembrane area0.238m2 0.476m2Distance betweenmembranes0.8mm 0.8mmNumber of cell pair 250 600System control 3 modes operation 4 modes operation1. High-concentrationoperation1. Pump up+desaltingoperation2. Low-concentration operation 2. Pump up operation3. Stand by operation 3. Desalting operation4. Stand by operationSource: Inoue and Kuroda (1993).Potable water productionIntermediately concentratedsolution productionSolar battery outputElectrodialysisElectric current (A)6 8 10 12 14 16 18 20 22 24TimeAuxiliary machineFigure 1.22 Electric power consumption pattern of the seawater desalination plant(Oshima island) (Inoue and Kuroda, 1993).Ion Exchange Membranes: Fundamentals and Applications348Auxiliary machinecurrentElectrodialysis currentPump up currentSolar battery outputStand byTimeDesalinationDesalination + Pump up20181614121086Electric current (A)Figure 1.23 Electric power consumption pattern of saline water desalination plant(Sakiyama) (Inoue and Kuroda, 1993).Table 1.7 Operating performance of the electrodialyzerOshima Sakiyama1986 1987 1988 1900 1901Sunlight quantity (kWh m2) 3.60 3.73 3.86 4.08 3.60Generation quantity (kWh/day) 49.3 50.6 57.0 162 145Water production (m3/day) 3.45 3.44 4.04 229 194Electric power consumption (kWh m3) 14.3 14.7 14.1 0.71 0.75Source: Inoue and Kuroda (1993).Electrodialysis 349of electrodialyzer, which is designed to maintain the Ni concentration in the firststage rinsing bath to about 5 g l1 when the concentration in the Ni plating bathis about 84 g l1. The specifications of the electrodialyzer are presented inTable 1.8. The limiting current density for NiSO4 or NiCl2 is extremely lowcomparing to that for Na2SO4 or NaCl. So, it is important to control the currentdensity to prevent the precipitation of Ni(OH)2 caused by water dissociation.Further, the organic substances in the solution added into the electro-platingbath possibly give rise to generate the organic fouling of anion exchange2nd RIT1st RITEPBChemicaldosing*PRT+ *EDCXC AXCFILDIL CON ELR 2nd RIW DEW1st RIWFigure 1.24 Electrodialysis Ni2+ electro-plating wastewater recovery process. AXC,Anion exchange column; CXC, cation exchange column; CON, concentrate; DEW, de-mineralyzed water; DIL, diluate; ED, electrodialyzer; ELR, electrode rinse; EPB, electroplating bath; FIL, filter; PRT, pretreatment; RIT, rinse tank; RIW, rinse waste (Itoi et al.,1986).Table 1.8 Specifications of the electrodialysis plantElectrodialyzer Model DU-111Ion exchange membrane Selemion CMV/AMVSize of membrane 0.49 0.98mEffective membrane area 0.336m2Number of membrane pairs 40 pairsDistance between membranes 2.0mmFlow velocity 3.0 cm s1Current density 1.0A dm2Maximum voltage 50VSource: Itoi et al. (1986).Ion Exchange Membranes: Fundamentals and Applications350membranes. So, it is necessary to remove the organic substances by means of aspecial pretreatment.Material balance of Ni2+ ions and water in the process is indicated inFig. 1.25 showing that the recovery ratio is larger than 90% and a dilutedsolution is perfectly recycled to the first stage rinsing bath. Current efficiencyand electric power of ED were, respectively, greater than 90% and 2 kWh kg1Ni ion.Cost estimation is presented in Table 1.9.Take out from plating bath 13 l/h (Ni2+ 84.2 g/l)1 st stage Rinsing tankTake out to 2nd stage 13 l/h (Ni2+ 5g/l)Feed to ED 3 m3/h (Ni2+ 5g/l)Concentrated stream To plating bath 12.4 l/h (Ni2+ 83 g/l)Electrodialyzer Diluate stream 3 m3/h Ni2+ 4.65 g/lFigure 1.25 Ni2+ and water balance in the electrodialysis process (Itoi et al., 1986).Table 1.9 Cost estimation of electrodialytic recovery of rinsing waste in the nickelelectroplating processNickel recovery rate (as NiSO4 6H2O) 3460 kgmonth1Electricity consumption (as NiSO4 6H2O) 0.7 kWhkg1Equipment installation cost 15million yen1Electricity unit cost 10 yen kWh1Purchasing price of nickel salt (as NiSO4 6H2O) 360 yen kg1Profit of Nickel salt recovery360 yen kg1 3460 kgmonth1 1,245,600 yenmonth1Runnig costElectricity 10 yen kWh1 0.7 kWhkg1 3460 kgmonth1 24,220 yenmonth1Maintenance and consumable item (annually 3%)15,000,000 0.03/12 month 37,500 yenmonth1Amortization (7 years)15,000,000 yen (7 year 12 month)1 178,570 yenmonth1Interest (annually 9%)15,000,000 yen 0.09/12 month 112,500 yenmonth1Running cost total 352,790 yenmonth1Profit 892,810 yenmonth1Source: Itoi et al. (1986).Electrodialysis 3511.6.4 Reuse of Wastewater by Electrodialytic TreatmentIon exchange membrane ED technology is applicable to wastewater treat-ment for realizing resource saving and protection of environment. TokuyamaInc. developed the technology to establish a closed system by means ofHCl, H2SO4NaOHFlocculantDesalted solutionDesigned process is surrounded by the dotted line.ProcessAcid wasteNeutralizationClarifierTraider receivingFilterElectrodialysisConcentrated solutionFigure 1.26 Electrodialytic treatment process of industrial waste (Matsunaga, 1986).Solution Na Ca Cl SO4Circulating solution 0.255N 13 ppm 0.168 N 0.087 NConcentrated solution 3.870 220 3.329 0.541to processfrom clarifierMITSAFDSTCHFED12CST ConcentratedsolutionHCl65.4 l /hFigure 1.27 Electrodialytic treatment process of industrial waste. CHF, Check filter;CST, concentrated solution tank; DST, desalted solution tank; ED, electrodialyzer; MIT,middle tank; SAF, sand filter (Matsunaga, 1986).Ion Exchange Membranes: Fundamentals and Applications352Electrodialysis 353electrodialytic reuse of wastewater in a plating process as shown in Figs. 1.26and 1.27 (Matsunaga, 1986). Typical components of a raw feeding solution in this system.pH: 9.5, NaCl: 17.6 g l1, Na2SO4: 7.1 g l1, Ca: 48mg l1, SS: 20mg l1.Main component: Fe(OH)3. The others: Fe, Cu, Zn, P etc. Conditions of electrodialytic treatment.Quantity of a desalted solution: 10 ton/month. Degree of desalination:maximum. Degree of concentration: maximum. Temperature: normal. Op-erating time: 24 h/day, 30 days/month, 12months/year. Operating system:partially circulation. Ion exchange membrane: Neocepta C5S-8T (monova-lent cation selectively permeable cation exchange membrane) and NeoceptaACH-45T (anion exchange membrane). Operating results.Current density: 3.0A dm2. Cell voltage: 0.5V/pair (1820 1C). Electro-lyte concentration in a desalted solution: Cde,Na 0.250.32 eq dm3,Cde, Ca 1315mg l1.Electrolyte concentration in a concentrated solution Ccon,Na 3.804.02 eq dm3, Ccon,Ca 220240mg l1. Quantity of a concentrated solu-tion: 6066 l h1 (1.441.58m3/day). Quantity of a desalted solution 10.912.1 ton/month. Running cost.Electric power: 262 kWh/day 15 yen/kWh 3930 yen/dayLabor: 0.2 people/day 15,000 yen/people 3,000 yen/dayThe others (Ion exchange membrane, HCl, electrode, filter): 1330 yen/dayTotal: 8260 yen/day.1.6.5 Simultaneous Treatment of Wastewater by Electrodialysis and ReverseOsmosisYunichica Ltd. developed a treatment process of wastewater includingtoxic metallic ions discharged from a semi-conducting material manufacturingprocess (Ishibashi, 1986).The process is illustrated in Fig. 1.28 consisting from the following system. High-Concentration SystemHS gas dissolving in the wastewater is deaerated and neutralized addingNaOH. Fine particles suspending in the wastewater are removed using a micro-osmosis filter. Then, a clear salt solution is supplied to an ion exchange mem-brane ED unit and concentrated to 3.54.0M. The concentrated solution isevaporated using a vacuum evaporator (EV) to obtain Na2SO4 and NaCl crys-tals, which are reused after re-purification. The desalted solution obtained fromthe ED unit is further desalted using a reverse osmosis (RO) unit. ConcentratedConcentratedsolutionEV Separated salt 570 kg/day TDSTDS 240g/l TDSHigh concentration system 36g/l 1.5g/lED RO TDS0.1g/lLowconcentration systemIXPure water Heigh pure waterProcessED: Electrodialysis RO:Reverse osmosis IX: Ion exchange EV:EvaporationPolishingFigure 1.28 Simultaneous treatment process of wastewater by electrodialysis and re-verse osmosis (Ishibashi, 1986).Ion Exchange Membranes: Fundamentals and Applications354solution from the RO unit is returned to the ED unit. The specifications of bothunits are as follows: Electrodialysis unitTokuyama TS-25-160 type integrated with Neocepta C66-5T/AFSBatch wise operating systemDesalting performance 18m3/dayConcentration of a desalted solution 1.5 g l1 Reverse osmosis unitMiddle pressure hollow fiber B-9Desalting performance 15m3/dayConcentration of a desalted solution 100 ms1.6.5.2 Low-Concentration SystemAfter deaeration, the solution is filtered passing through a carbon filterand neutralized using weak basic ion exchange resins. Then the solution is sup-plied to a cation exchange and an anion exchange column to obtain pure water.The pure water is fed to the semi-conducting material manufacturing processAnion exchange layer Cation exchange membraneMn+H+ H+Anode Cathode+ ++++++Figure 1.29 H+ ion permeable cation exchange membrane. Mn+, n valent metalic ion(Katayama, 2004).Electrodialysis 355directly or via a polishing column filled up with cation and anion exchangeresins and nonionic resins.1.6.6 Electrodialytic Recovery of AcidAcid is recovered from a waste acid applying a H+ ion permselectivecation exchange membrane placed an anion exchange layer on a cation exchangemembrane as illustrated in Fig. 1.29. Here, multivalent cations Mn+ do not passthrough the membrane due to repulsive effects between the cations and theanion exchange layer. The concept mentioned above is applicable to recover anacid from an aqueous solution dissolving for instance Fe(NO3)3 with HNO3 asshown in Fig. 1.30. In this system, cation exchange membranes correspond tothe membrane illustrated in Fig. 1.29, and they permeate H+ ions selectively anddo not permeate Fe3+ ions. On the other hand, anion exchange membranepermeate NO ions selectively rather than H+ ions. Consequently, HNO3 isrecovered in the concentrating chamber and Fe(NO3)3 is remained in the de-salting chamber.Table 1.10 shows the material balance in an electrodialyzer (effectivemembrane area: 90m2) developed by Tokuyama Inc., which treated a wastedacid solution (0.54M acid solution including H2SO4 and HCl with 0.30M ofAl3+ ions) generated in an etching process of aluminum products (Katayama,2004). Acid concentration in a recovered acid is 1.90M including 0.01M of Al3+ions. Acid recovering ratio and leak ratio are, respectively, 88% and 0.6%.Energy consumption and current efficiency are, respectively, 12 kW and 55%.1.6.7 Seawater Concentration for Salt ProductionElectrodialysis is applied to concentrating seawater for producing salt inJapan. The seawater concentrating process is illustrated in Fig. 1.31 (Tomita,1995). Seawater pumped up from sea is filtered and supplied to the elect-rodialyzer via the filtered solution tank. Concentrated seawater is circulatedTable 1.10 Material balance in an electrodialyzer for acid recoveringSolution SolutionQuantity (l h1)CompositionH+ (M) Al3+ (M) SO42 (M) Cl (M)Waste acid (beforeelectrodialysis)742 0.54 0.30 0.65 0.16Deacid solution(afterelectrodialysis)671 0.07 0.33 0.50 0.08Water 115 0 0 0 0Recovered acid 186 1.90 0.01 0.79 0.35Source: Katayama (2004).+AnodeWaterWaste acid HNO3 Fe(NO3)3H+ H+ H+Fe3+C A C A CDe-acidified solution Fe(NO3)3Recovered acid HNO3NO3 NO3H+NO3H+Fe3+CathodeNO3Figure 1.30 Acid recovery by means of ion exchange membrane electrodialysis. C, H+ion permselective cation exchange membrane; A, H+ ion low-permselective anion ex-change membrane (Katayama, 2004).Ion Exchange Membranes: Fundamentals and Applications356between the concentrated seawater tank and the electrodialyzer, and its gain issupplied to an evaporating process to obtain salt crystals. CaCO3 scale precip-itation in concentrating cells is prevented by adding hydrochloric acid to theconcentrated seawater to decompose HCO3 and CO32 ions (cf. Section 1.5.2 inApplications). A part of filtrated seawater is supplied to anode chambers.Titanium is adopted as the anode material. In order to avoid membrane de-struction due to Cl2 and HClO generated by an anode reaction, a perfluorinatedion exchange membrane is integrated between the cathode chamber and the2169103 4 587C+ Figure 1.31 Electrodialytic seawater concentration process. 1, Diluted seawater tank; 2,cathode solution tank; 3, concentrated seawater tank; 4, washing solution tank; 5, HCltank; 6, filtrated seawater; 7, concentrated seawater output; 8, HCl; 9, electrodialyzer; 10,anode solution (Tomita, 1995).Electrodialysis 357adjacent stack. The wasted solution from the anode chamber is mixed with thefiltrated seawater to suppress the growth of microorganisms in seawater. Cath-ode material is plated with Pt. An HCl solution is supplied to the cathodechamber to neutralize OH ions generated by the cathode reaction. A washingsystem is provided for washing the inside of desalting cells by acid or chemicalreagents and dissolving adhered substances (cf. Section 14.2.3 in Fundamentals).When the turbidity of raw seawater is 2 ppm, it is decreased to about0.05 ppm by filtering through two-stage sand filters (cf. Section 1.5.1 in Appli-cations). In spite of such filtration, fine particles pass through the filter andinvade into the electrodialyzer and precipitate on the membrane surfaces.Fe(OH)3 components precipitated on the membrane possibly give rise to waterdissociation (cf. Sections 8.8.4 and 8.9 in Fundamentals). Sometimes, fine or-ganisms pass through the filter and breed in the electrodialyzer. These troublesare avoided by disassembling and washing the electrodialyzer at the interval of46 months (cf. Section 1.5.3 in Applications). In order to increase currentefficiency and avoid CaSO4 scale precipitation, membrane surfaces are treated togive monovalent ion permselectivity (cf. Section 3.7 in Fundamentals).Figure 1.32 Composition of an electrodialyzer (Tokuyama). 1, Fastening bolt; 2, fas-tening and feeding frame; 3, concentrated seawater inlet; 4, diluted seawater inlet; 5,diluted seawater outlet; 6, concentrating cell; 7, desalting cell; 8, desalting cell manifold; 9,concentrated seawater outlet; 10, cation exchange membrane; 11, anion exchange mem-brane; 12, concentrating cell manifold (Tomita, 1995).Ion Exchange Membranes: Fundamentals and Applications358Compositions of electrodialyzers developed by Tokuyama Inc., AsahiGlass Co. and Asahi Chemical Co. are illustrated in Figs. 1.321.34 (Tomita,1995). Typical performance of an electrodialyzer is exemplified in Table 1.11(Tanaka, 1991).1.6.8 Salt Production Using Brine Discharged from a Reverse Osmosis SeawaterDesalination PlantConcentrated brine is discharged from a RO seawater desalination proc-ess. It seems advantageous to use this brine as raw material for salt production.Figure 1.33 Composition of an electrodialyzer (Asahi Glass Co.). 1, Cathode chamber;2, anode chamber; 3, cathode plate; 4, anode plate; 5, intermediate frame; 6, desalting cell;7, concentrating cell; 8, cation exchange membrane; 9, anion exchange membrane; 10,packing cell frame; 11, intermediate packing; 12, blind cell frame (Tomita, 1995).Figure 1.34 Composition of an electrodialyzer (Asahi Chemical Co.). 1, Diluted sea-water; 2, special gasket; 3, concentrating cell; 4, turn-Buckle; 5, desalting cell; 6, con-centrated seawater; 7, fastening frame; 8, cation exchange membrane; 9, anion exchangemembrane (Tomita, 1995).Electrodialysis 359Table 1.11 Performance of an electrodialyzer for concentrating seawater (plant A, 1987)Current density (A dm2) 2.66Temperature (1C) 25.3Cl current efficiency (%) 88.6Na current efficiency (%) 80.8Energy consumption (kWh t1 NaCl) 178.6Concentrated solution puritya (%) 90.90Constitution of concentrated solutionNaCl (g dm3) 190.6Cl (eq dm3) 3.568SO4 (eq dm3) 0.015Ca (eq dm3) 0.064Mg (eq dm3) 0.169K (eq dm3) 0.095Na (eq dm3) 3.256Source: Tanaka (1991).aNaCl(g)/Total electrolyte (g).Ion Exchange Membranes: Fundamentals and Applications360The operating performance and energy consumption in a salt manufacturingplant were investigated for an ion exchange membrane ED system to whichdischarged brine from a RO plant is supplied as follows (Tanaka et al., 2003).The salt manufacturing process (NaCl production capacity: 200,000ton/year) is illustrated in the dotted frame in Fig. 1.35. Discharged brine (elec-trolyte concentration: 1.5 eq dm3) from a RO plant is assumed to be supplied toan ion exchange membrane electrodialyzer. The concentrated solution obtainedfrom the electrodialyzer is supplied to a multiple-effect EV, in which salt iscrystallized. The salt obtained from the evaporator is supplied to an ionexchange membrane electrolytic bath, in which sodium hydroxide and chlorineare produced. Energy consumption in the salt manufacturing process is assumedto be supplied by a simultaneous heat-generating electric power unit consistingof a boiler and a back-pressure turbine.Fig. 1.36 shows the flows of electricity and steam in a salt manufacturingplant. Boiler steam is introduced to a turbine and generates electricity, which isdistributed to electrodialyzers, etc. The back-pressure of a back-pressure turbineis supplied to a heater in a No. 1 evaporator in multiple-effect evaporators.Evaporated steam in a No. 1 evaporator is supplied in turn to the followingevaporators. Pressure and temperature of boiler steam are set as 6Mpa and4801C in this study. The temperature difference between heating steam andevaporated steam is fixed to 201C at each evaporator. The number of evaporatoris kept to a minimum, but the quantity of electricity does not exceed the electricpower consumption in this salt manufacturing plant. An electric power shortfallis assumed to be made up by purchased electric power, which is generated by acondensing turbine.Seawater 0.6 eq/dm3Desalted solution Discharged brineCin=1.5 eq/dm3ROCCoutEDCl2 T BEVNaOHNaClcrystalsIMSalt manufacturing processH2OFigure 1.35 RO-ED combined salt manufacturing process. RO, Reverse osmosis; ED,electrodialyzer; EV, evaporation; IM, ion exchange membrane electrolysis; B, boiler; T,turbine. Diagonals in RO, ED and IM unit box represent the membranes (Tanaka et al.,2003).Electrodialysis 361The energy required for producing salt F is plotted against current densityI/S in both cases of RO discharged brine ED and seawater ED. The plots areshown in Fig. 1.37, which indicates that the energy consumption in a salt man-ufacturing process using RO discharged brine is 80% of the energy consumptionin the process using seawater. The optimum current density at which the energyconsumption required in the salt manufacturing process being minimized is 3Adm2 for both RO discharged brine ED and seawater ED.1.6.9 Desalination of Amino Acid and Amino Acidic SeasoningsAmino acid is an amphoteric electrolyte, and its behavior is different fromthat of usual electrolytes. Itoi and Utsunomiya (1965) electrodialyzed aqueoussolutions of methionine and glysine containing sodium formate as follows; (a)methionine 25 g l1, HCOONa 20 g l1. (b) glysine 70 g l1, HCOONa 20 g l1.The solution was supplied to the electrodialyzer (membrane pairs: 9,effective membrane area: 209 cm2) incorporated with Selemion CSG and AST.Changing pH and maintaining cell voltage at 15V (average current density:nearly 1A dm2), amino acid permeation ratio (the ratio of amino acidPurchsed electric powerGGgen+G=Etotal PNaClROH*S*Discharged brine1.5 eq/dm3solutionGenerated electric powerGgenB BPT EDW = w PNacl50C 30C70CStQcond EV NO1 EV NO2 EV NO3Tcond=90CHcondScondFigure 1.36 Energy flow in a salt manufacturing process. B, Boiler; BPT, back-pressureturbine; EV, evaporator; ED, electrodialyzer; RO, reverse osmosis unit (Tanaka et al.,2003).Ion Exchange Membranes: Fundamentals and Applications362transported across a membrane pair against that dissolving in a feeding solu-tion) was measured and shown in Fig. 1.38, indicating that the amino acidpermeation ratio becomes minimum near the isoelectric point of amino acid.Changing current density and keeping pH near the isoelectric point of the aminoacid, the amino acid permeability (quantity ratio of amino acid transportedacross a membrane pair against electricity) was measured and shown inFig. 1.39. Inspecting Fig. 1.39 and the limiting current density measured for anHCOONa solution, it is concluded that the amino acid permeability becomesminimum by applying the limiting current density. Concentration changes ofNaCl and essence in the batch system ED of soy sauce are indicated in Tables1.12 and 1.13.0 1 8 9 3 4 5 6 70102030405060(RO discharged brine electrodialysis)/ (seawater electrodialysis)(103M cal/h)I/S (A/dm2): RO discharged brine electrodialysis : Seawater electrodialysis: RO discharged brine electrodialysis / Seawater electrodialysis Figure 1.37 Energy consumption in a salt manufacturing plant (Tanaka et al., 2003).Electrodialysis 3631.6.10 Desalination of Natural EssencesIn an extraction process of natural essences, several kinds of salt, acid andalkali are added. Accordingly salt is obtained as a by-product in a final stage.Salt content in the natural essences is expected to be controlled for maintaining ataste and human health. Tokuyama Inc. developed the following desalinationprocesses of natural essences (Ideue, 1986; Yamamoto, 1993).An ED system was set up using ion exchange membranes met the foodhygiene standard established by the Ministry of Welfare, Japan, and incorpo-rated with polyvinyl chloride or polypropylene materials suitable for food pro-duction. It was further necessary to pay attention to prevent solution stagnationand cultural contamination. Further cleaning in place (CIP) was necessary tooperate the apparatus stably. The electrodialyzer was operated in a batch systemusing ED system indicated in Fig. 1.40, in which a conductivity control indicatorwas set at an exit of the electrodialyzer for detecting the concentration of adesalted solution. The solution feed and solution discharge were automaticallycontrolled by switch valves operating with the conductivity control indicator.Natural essences include meat essences, seafood essences, flesh essencesetc. They were extracted with the aid of a NaCl solution. Salt added in theextraction process was desalinated by means of ED mentioned above. Constit-uent changes in desalination of extracted meat essences, fish essences and fruitMethionineGlycineCell voltage : 15 V0 2 4 6 8 10 12 14pH80706050403020100Amino acid permentation ratio (%)Glicine 70 g/l, HCOONa 20 g/lMethionine 25 g/l, HCOONa 20 g/lFigure 1.38 Relationship between solution pH and amino acid permeation ratiothrough ion exchange membranes (Itoi and Utsunomiya, 1965).Ion Exchange Membranes: Fundamentals and Applications364flesh essences are shown in Table 1.14. Specifications of an electrodialyzer forseafood essences are shown in Table Electrodialysis of Milk and Whey1.6.11.1 Composition of Milk and Whey (Ideue, 1986; Tomita et al., 1986)Whey is obtained as a by-product of a cheese production process. Theoutput of whey amounts to nine times of that of cheese, so it is an importantsubject in the dairy industry to utilize the whey effectively. The effective utilizationof permeates derived from an ultra-filtration process of milk and whey is also abig problem in the dairy industry. The composition of milk, whey and ultra-filtration permeates is indicated in Table 1.16. Ash content in dry matter of thewhey and permeate is so high that it is expected to reduce their ash content by ED.Powdered milk for baby rising is prepared using cow milk as main rawmaterials. The constituents of breast milk and cow milk are not same as shownin Table 1.17. Total ash and casein compositions of cow milk are, respectively,3.4 and 4.4 times of those of the breast milk. Accordingly, baby rising powderedGlycineMethioninepH: 6.3- 0.5 1.0 1.5 2.0Average current density (A/dm2)Amino acid permeability (g/Ah)Glycine 70 g/l, HCOONa 20 g/lMethionine 25 g/l, HCOONa 20 g/lFigure 1.39 Relationship between current density and transport rate of amino acid (Itoiand Utsunomiya, 1965).Table 1.12 Electrodialytic demineralization of soy sauceStart EndSolution quantity (l) 11.6NaCl concentration (g l1) 191 31Essence concentration (g l1) 163 191pH 4.7 4.7Density 1.181 1.103Current efficiency (%) 90Note: Effective membrane area: 209 cm2; Numbers of membranes: 10 pairs; Currentdensity: 3.5A dm2; Applied voltage: 2558V.Source: Itoi (1983).Electrodialysis 365Table 1.13 Concentration changes of components in electrodialysis of soy sauceComponent Ratio (Before ED/After ED)NaCl 0.800Total amino acid 0.94Glutamic acid 0.93Aspartic acid 0.95Lysine 0.98Leucine 0.98Isoleucine 0.98Alanine 0.96Phenylalanine 0.87Valine 0.95Total nitrogen 0.96Essence part 1.009Source: Itoi (1983).Ion Exchange Membranes: Fundamentals and Applications366milk is prepared by adding whey to cow milk. Before this mixing operation,however, it is necessary to extract excessive ash from the milk or whey by ED. Limiting Current Density in Electrodialysis of Milk and Whey(Nagasawa et al., 1973; Nagasawa et al., 1974)In ED of a dairy product solution, protein is condensed and attached onthe desalting surface of an anion exchange membrane at above limiting currentdensity due to occurrence of water dissociation. At the same time, insoluble saltsuch as calcium phosphate etc. precipitates on the concentrating surface of theanion exchange membrane. Accordingly, it is important to increase the limitingcurrent density to operate a practical electrodialyzer stably. Nagasawa et al.evaluated the limiting current density as follows.In ED of a skim milk solution, limiting current density was evaluated basedon the relationship between current density and voltage. The relationship betweenconductivity of the skim milk solution k and the limiting current density ilim wasexpressed by ilim 1.08 103k. (i/k)lim was known to be proportional to the0.6th power of solution velocity of skim milk V. (i/k)lim/V0.6 vs. temperature Tand dry matter content of skim milk S is given in Fig. 1.41 which indicates that(i/k)lim/V0.6 decreases with the increase of T and S. Flow length and distancebetween the membranes did not influence to ilim. From the investigation men-tioned above, the following limiting current density equation was introduced asilim QS1=61:01665S0:1TV0:6k (1)Q is a constant. S, T and V can be voluntarily fixed, so the above equation isexpressed by the following equation.ilim Rk (2)1 21083453739LC6P1LAF1F1AI VIP1P1P1F1+ _LAF1CCIFigure 1.40 Electrodialysis process of natural essences. 1, Raw solution tank; 2, desalted solution tank; 3, drain; 4, concentratedsolution tank; 5, concentrated waste solution; 6, electrode solution tank; 7, water supply; 8, ammeter; 9, electrodialyzer; 10, desaltedsolution; Vi, voltage indicator; Ai, ampere indicator; Fi, flow indicator; Pi, pressure indicator; CCi, conductivity control indicator, LC,level control; LA, level alarm (Yamamoto, 1993).Electrodialysis367Table 1.14 Desalination of fish essencesRaw Essence Desalted Essence Desalting RatioConductivity 15.4mS cm1 2.74mS cm1 82.2%Viscosity 4 cp 6.4 cpAshNa 2975 ppm 560 ppm 81.2%K 241 ppm 19 ppm 92.1%Ca 38 ppm 2.4 ppm 93.7%Mg 45 ppm 1.9 ppm 95.8%Cl 3299 ppm 583 ppm 82.3%Source: Yamamoto (1993).Table 1.15 Specifications of an electrodialyzer for desalinating seafood essencesRequirementRaw solutionNaCl 14 g l1Protein 100 g l1Specific gravity 1.05Viscosity 4.0 cppH 5.5Treating amount 0.5 t h1SpecificationsEffective membrane area 256m2Number of membrane pair 200 pairsIon exchange membrane Neocepta CM-1, AM-1Model TS-25-200Operating system Automatic batch systemRunning cost 1517 yen t1Source: Yamamoto (1993).Table 1.16 Typical composition of milk and UF-permeateFat (%) Protein(%)Lactose(%)Ash (%) Water(%)Ash/DryMatter(%)Cow milk 3.3 2.9 4.5 0.7 88.6 6.1Cheese whey 0.3 0.7 4.5 0.6 93.7 9.5Permeate derivedfrom UF of milk0 0.2 4.4 0.5 94.8 9.6Source: Tomita et al. (1986).Ion Exchange Membranes: Fundamentals and Applications368Table 1.17 Comparison of composition between breast milk and cow milkWater(%)Ash (%) WheyProtein (%)Casein(%)Fat (%) Lactose(%)Breast milk 88.0 0.2 0.68 0.42 3.5 7.2Cow milk 88.6 0.7 0.69 2.21 3.3 4.5Source: Tomita et al. (1986).10520 10 20 30 40Temperature (C)(i/)lim/V0.6 x 102S % : 0.3, : 8.3, : 16, : 25Figure 1.41 Effects of temperature and total solid content on (i/k)lim/V0.6 (Nagasawaet al., 1974).Electrodialysis 369R is a constant. We know ilim from k, and it was confirmed that the elect-rodialyzer is operated stably at under ilim estimated from Eq. (2). Ion Exchange Membrane (Okada et al., 1975) for the Demineralizationof Milk or WheyIn the demineralization of skim milk or whey including rich proteinaceousmaterials or other organic matters, conventional membranes applied to thetreatment of saline water are not feasible because the life span of the membranesis shortened and the performance is deteriorated. In order to make membranesapplicable to the demineralization process of milk or whey, the investigation wasperformed to give the following characteristics to the membranes.Anti-Organic FoulingThe specific conductance of various membranes k was determined in ademineralization experiment of Gouda cheese whey. The membranes are,Aciplex K-101 (conventional cation exchange membrane, Asahi Chemical Co.),A-101 (conventional anion exchange membrane) and A-201 and A-211 (both areA-1011008060402000 2000 4000 6000 8000A-211A-201Operating time (hr)Remaining specificconductance(%) K-101Figure 1.42 Membrane conductivity changes with operating time (Okada et al., 1975).Ion Exchange Membranes: Fundamentals and Applications370newly developed anion exchange membranes). k of K-101 is not changed largelywith running time as indicated in Fig. 1.42. k of A-101 shows very rapid de-crease. However, k of A-201 and A-211 membranes does not change signifi-cantly for 6000 h. A-201 and A-211 membranes show excellent anti-fouling(cf. Section 14.3.2 in Fundamentals).Anti-Alkaline CircumstanceOrganic fouling due to deposition of proteinaceous materials or fattysubstances from the dairy products is cleaned by washing in an alkaline deter-gent by means of CIP method. Exposure of the membrane to OH ions gen-erated from water dissociation vitiates its performance. Therefore, life span ofthe membrane is shortened without the durability against a basic solution incontact. Fig. 1.43 shows the durability evaluated by measuring specific con-ductance k of the membranes immersed in a 1% NaOH solution. k of conven-tional cation exchange membrane K-101 does not change, however, that ofanion exchange membrane A-101 decreases remarkably with immersing dura-tion. On the other hand, decrease of k is suppressed for anti-organic foulinganion exchange membrane A-201 and A-211. Particularly, A-211 membraneexhibits excellent anti-alkaline performance.Permeability to Larger Organic Ions of an Anion Exchange MembraneAn anion exchange membrane which does not permeate larger organicanions causes pH lowering in demineralization of dairy products. This is becauseOH ions caused by water dissociation generated on a desalting surface of ananion exchange membrane permeate the membrane instead of the larger organicanions toward a concentrating cell. This phenomenon induces pH increase in theconcentrating cell and gives rise to the precipitation of inorganic salts such asCa3(PO4)2, CaCO3, CaSO4 etc. On the other hand, pH in the desalting cell is1009080706050403020100Remaining specific conductance(%)0 100 200 300 400 500A-101Immersing duration (hr)A-201A-211K-101Figure 1.43 Membrane conductivity changes with time in alkaline circumstances. 1 %NaOH at 371C (Okada et al., 1975).Electrodialysis 371estimated to be lowered because H+ ions are remained in the desalting cell.Fig. 1.44 shows pH changes upon 90% ash reduction of Gouda cheese wheyusing three types of anion exchange membranes. A-211 membrane scarcelybrings about the decrease in pH. This is because pore radius in A-211 membraneis large enough (cf. Section in Fundamentals), so that organic acids suchas citric acid, lactic acid, amino acid as well as phosphoric acid easily permeatethe membrane and that water dissociation does not easily occur.From the experiment described here, it is concluded that A-211 is the mostsuitable anion exchange membrane. CIP operations of an electrodialyzer incor-porated with A-211 membranes realize 90% ash reduction and life span exten-sion. Electrodialysis System (Okada et al., 1975) for the Demineralization ofMilk or WheyFig. 1.45 illustrates the four stack ED process MED SV1/2 4-4 developedby Morinaga Milk Industry Co. The specifications and operating conditions ofthis system are shown, respectively, in Tables 1.18 and 1.19. In Fig. 1.45, thepHA-211A-201 (and A-101)76540 10 20 30 40 50 60 70 80 90Relative deashing rate [%]Figure 1.44 pH changes of whey in demineralization (Okada et al., 1975).Ion Exchange Membranes: Fundamentals and Applications372pump P1 feeds the whey to the balance tank WB-1. Succeedingly, the whey issupplied to the 1st stack of the ED system, demineralized to some extent andoverflows from WB-1 to WB-2. The whey in WB-2 circulates through pump P3and second stack, then part of which overflows into WB-3. In the similar way,the whey flows via P4, third stack, WB-4, fourth stack, WB-5, and finally de-mineralized whey flows out via P6 and supplied to the succeeding process such aspasteurization, evaporation and drying. At each balance tank, the conductivityof the whey is monitored to control the desalting rate. The feed rate of the wheyto this system is automatically controlled by means of conductivity measurementin the effluent of demineralizing stream.1.6.12 Desalination of Sugar LiquorThe sugar manufacturing system is classified into the cane sugarfinesugar manufacturing system and the beet sugar manufacturing system. The rawmaterial for the former is sugar canes and that for the latter is beet sugar. In thesugar manufacturing process, the greater part of organic nonsugar componentsin raw sugar is removed by means of defecation, carbonation, adsorption (bonechar, active carbon, ion exchanger etc.). In these treatments, inorganic compo-nents are not removed and transferred to an evaporation process, in whichresidual organic nonsugar components are separated from sugar crystals andremained in molasses. In the course of repeating evaporation and separation, theinorganic and organic nonsugar components are gradually accumulated in theCB-5 CB-4 CB-3 CB-2 CB-1P7P8P9P10P11Con ConI4V4V3V22nd1stV1DilDilP2P1WheyP3 P4 P5P12H2SO4Va1 WaterDemineralizedwheyRinse solutionforfastening frame&cathode frameDC : RectifierV1-4 : Voltage meterI1-4 : Amperage meterCon. : Concentrating compartmentsDil. : Diluting compartmentsP6PB-51st : Stack - 12nd : '' - 23rd : '' - 34th : '' - 4WB-4WB-3WB-2WB-1DCforanode frame(one pass & waste)ConCon4thDCDilDilDCDC3rdI3I2I1Figure 1.45 Flow diagram of Morinaga continuous electrodialysis process (Okada et al., 1975).Electrodialysis373Table 1.18 Specifications of MED SV1/2 4-41. Center-press 12. Stack 43. Electrode 4 pairs4. Cell pair 150 cell pairs/stack5. Distance between membranes 0.75mm6. Effective membrane area 50 dm2/cell7. Spacer Sheet-flow type8. No. of channel 19. Width of the channel 500mm10. Length of the channel 1000mmSource: Okada et al. (1975).Table 1.19 Operating conditions of demineralization of whey (Morinaga ED system)1. Quality of whey to be treated1.1 Total solids 20%1.2 Ash content 1.60%1.3 Specific conductance 0.013 S cm11.4 Sediment test (200ml, 1000 G) less than 0.1ml2. Operating conditions2.1 Level of applied d-current density 3000 ka (mA cm2)2.2 Linear velocity in the compartment 12 cm s12.3 Operating temperature 201CSource: Okada et al. (1975).ak: Specific conductance.Ion Exchange Membranes: Fundamentals and Applications374molasses and finally they are discharged to the outside of the system as wastemolasses. The waste molasses includes considerable amount of sugar compo-nents, so it is utilized as raw materials for fermentation or animal food, however,its economical value is extremely low comparing to that of sugar itself. Becauseof the background described above, the technology development was expectedfor preventing sugar component transfer to waste molasses and increasing sugarrecovering ratio. Further desalting technology by means of ion exchange mem-brane ED came to be attracted because the sugar recovering ratio is influencedby residual inorganic components in syrup.Application of ED in sugar manufacturing industry was investigated fromthe latter half of 1950s. However, it was difficult to put this program intopractice. This is because water dissociation and organic fouling are apt to occuron anion exchange membranes (cf. Section 14.3 in Fundamentals). In order toavoid these troubles, Taito Co. and Asahi Chemical Co. developed the tech-nology using neutral membranes consist of polyvinyl alcohol instead of anionexchange membranes (Sugiyama et al., 1982; Kokubu et al., 1983). TheElectrodialysis 375advantages and disadvantages of this method (Transport depletion method) areas follows.TaDeCuCudMoMo(MopHAsCMKCSSPCSNoCaSo1.ble 1saltinrrentrrentm2lasselasseoBx)lasseh (%aOgO2OliO2O32O5O2ulfate: I:Cl2 aurce:Advantages(a) Neutral membranes do not deteriorate due to organic fouling.(b) Sugar does not decompose, because pH decrease caused by waterdissociation does not occur on the neutral membrane.(c) Current density can be increased, because water dissociation doesnot occur..20g reffide)s vs cs ponte aDuddKEatiociennsityolumonceuritysolishple-ing.okub2. Disadvantages(a) Removing efficiency of anions is low.(b) Current efficiency Z is low, because transport number of anions ofa neutral membrane tAA0:5: If we assume the transport number ofa cation exchange membrane tK 1:0; Z tK tA 1 0:5:In the first stage in a sugar manufacturing process, raw molasses (originalsyrup) is treated to remove organic materials and evaporated to obtain A sugarffect of electrodialysis on pretreatment for molassesI II III(%) 66.18 63.60 2.58cy (%) 43.55 34.01 9.54(A 3.04 3.07 0.03Start End Difference Start End Differencee (l) 10.00 9.76 D 0.24 10.00 9.76 D 0.24 0.00ntration 51.35 46.85 D 4.50 50.45 46.45 D 4.00 D 0.50(%) 51.70 59.01 7.31 52.66 58.45 5.79 1.526.35 6.35 0.00 6.40 6.40 0.00 0.00Start End DesaltingratioStart End Desaltingratiod)0.19 0.06 68.42 0.22 0.13 40.91 27.510.74 0.33 55.41 0.72 0.42 41.67 13.745.47 1.59 70.93 5.98 2.01 66.39 4.543.99 0.15 96.24 3.65 0.19 94.79 1.450.41 0.33 19.51 0.44 0.41 6.82 12.690.69 0.41 40.58 1.22 1.00 18.03 22.550.18 0.19 D 5.56 0.21 0.18 14.29 D 19.840.19 0.14 26.3212.42 4.73 61.92 12.78 5.19 59.39 2.53stage centrifuging after CaCl2 adding; II: Duple-stage centrifuging withoutu et al. (1983).B sugar B molasses Ca(OH)2 CaCl2 1st sludge 1st centrifuged molasses Water2nd sludge 2nd centrifuged molassesSeparated molassesFinal sludgeSeawaterDischarged water WaterDesalted molasses (C molasses)C sugarHeatingWater2nd boiling3rd boiling1st centrifugingRefrigeratorElectrodialyzerSteamSteam2nd centrifuging3rd centrifugingMixingFigure 1.46 Desalination of B molasses (Kokubu et al., 1983).Ion Exchange Membranes: Fundamentals and Applications376and A molasses. In the next stage, B sugar and B molasses are obtained fromA molasses through the similar process. Finally, C sugar and C molasses areobtained from B molasses. Table 1.20 shows ED experiment of B molasses,which is centrifuged two times after adding CaCl2 (Case I) and without addingCaCl2 (Case II). The experiment indicates that desalting ratio and current effi-ciency in Case I are increased comparing those in Case II. This is because theminerals such as CaO, SiO2, SO3, P2O5 etc. are removed by the CaCl2 treatmentin Case I.Based on the experiment described above, Taito Co. designed the EDtreatment process of B molasses as shown in Fig. 1.46. In this process, BTable 1.21 Bacterial strains and media for cell countStrains Media (Agar)Staphylococcus aureus Mannitol saltSalmonella heidelberg DeoxycholateEscherichia coli K-12 DeoxycholatePseudomonas aeruginosa DeoxycholateProteus vulgaris DeoxycholateKlebsiella pneumoniae DeoxycholateBacillus subtilis NutrientSource: Sato (1989).C+ A A VIVIIIIIIFSPAS BSS CSP PAnode CathodeFSCFigure 1.47 Schematic diagram of an electrodialytic disinfection diagram. C, Cationexchange membrane (Selemion CMV); A, anion exchange membrane (Selemion AMV);CS, cathode solution; AS, anode solution; BSS, bacteria cell suspending solution; dis-tance between the membranes, 1 cm; membrane area, 18.4 cm2 (Sato, 1989).Electrodialysis 377molasses is treated at first by defecation mixing with Ca(OH)2 and CaCl2. Afterheating and two stages of centrifugation, the second centrifuging molasses issupplied to the electrodialyzer via the refrigerator to obtain desalted molasses(C molasses). C sugar is crystallized in the evaporation of C molasses. Byintegrating the ED step mentioned above in the sugar manufacturing process, itbecame possible to obtain D sugar from C molasses.Cell viability (%)1009080706050403020100Current density (A/dm2)0.54 0.81 1.08 1.35 1.65: S. aur.,: K. pneu., : B. sub (spore).: Sal. heid, : E. coli, :Ps. aerug., :Pr. vulg.,Figure 1.48 Relation between bacteria cell viability and current density. Flow rate,3 cm3/min; duration time, 60min (Sato, 1989).Ion Exchange Membranes: Fundamentals and Applications3781.6.13 Electrodialytic DisinfectionSato et al. (1984) investigated water disinfection by means of ion exchangemembrane ED. The merits of this method are as follows.(1) The operation is proceeded at normal temperature.(2) Comparing to chlorine disinfection, the electrodialytic disinfection ismore powerful and proceeded during shorter time.(3) The process is not harmful to human body.Bacterial strains in Table 1.21 were cultivated at 371C for 18 h. The cul-tivated solution suspending 108 cells cm3 of bacteria cells was supplied into thedesalting chamber (chamber III) in an ED system in Fig. 1.47 and electrodi-alyzed for 60min. Viability of the cells is plotted against current densities andshown in Fig. 1.48. Here, limiting current density is 0.81A dm2. Bacteriaviability is seen to be decreased with the increase of current density in a range ofover limiting current densities and becomes zero at 1.63A dm2. Electron E. coli cellWater dissociationOH- H+ H+Anode CathodeAnion exchange membraneDesalting chamber Cation exchange membraneFigure 1.49 Mechanism of disinfection in electrodialysis (Sato, 1989).Electrodialysis 379microscope observation revealed that bacteria cell conformation is shrank underapplying over limiting current density. The mechanism of electrodialytic disin-fection in this study is estimated as follows.In Fig. 1.49, Escherichia coli cells are suspended in a solution in the de-salting chamber (Chamber III). Passing over limiting current in this system, theelectrolyte concentration in the desalting chamber is decreased and electric re-sistance of the solution is increased. In this situation, water dissociation is gen-erated on the anion exchange membrane (cf. Section 8.8.1 in Fundamentals) andH+ ion concentration in the desalting chamber is increased. An E. coli cell is anelectron conducting substance, so H+ ions pass through the E. coli cells. Thisphenomenon is similar to that in electrodeionization (cf. Chapter 4, Fig. 4.8 inApplication). E. coli cells are estimated to be destroyed by H+ ions passingthrough the cells.The investigation described here should be analyzed from the biologicaleffects of alkaline electrodialyzed water at the cellular level (Takahashi, 2006;Kikuno, 2006).REFERENCESAzechi, S., 1980, Electrodialyzer, Bull. Soc. Sea Water Sci., Jpn., 34(2), 7783.Fukuhara, K., Hamada, M., Azuma, I., 1993, Efficient desalination of brackish water byelectrodialysis, Industrial Application of Ion Exchange Membranes, vol. 2. Researchgroup of electrodialysis and membrane separation technology, Soc. Sea Water Sci.,Japan, pp. 159167.Ideue, K., 1986, Desalination with ion exchange membranes in food industry, Food Dev.,21(7), 5459.Ion Exchange Membranes: Fundamentals and Applications380Inoue, S., Kuroda, O., 1993, Electrodialysis desalination system powered by photovoltaicpower generation, Industrial Application of Ion Exchange Membranes, vol. 2. Re-search group of electrodialysis and membrane separation technology, Soc. Sea WaterSci., Japan, pp. 151157.Ishibashi, T., 1986, Simultaneous treatment of waste water by electrodialysis and reverseosmosis, Industrial Application of Ion Exchange Membranes, vol. 1. Research groupof electrodialysis and membrane separation technology, Soc. Sea Water Sci., Japan,pp. 177180.Itoi, S., 1983, Electrodialytic demineralization of soy sauce and amino acid seasonings,Food Industry and Membrane Utilization, Saiwai Shobo Inc., Tokyo, pp. 157162.Itoi, S., Komori, R., Terada, Y., Hazama, Y., 1978, Basis of electrodialyzer design andcost estimation, Ind. Water, No. 239, pp. 2940.Itoi, S., Nakamura, K., Kawahara, T., 1986, Electrodialytic recovery of waste water frommetal surface treatment process, Industrial Application of Ion Exchange Membranes,vol. 1. Research group of electrodialysis and membrane separation technology, Soc.Sea Water Sci., Japan, pp. 206211.Itoi, S., Utsunomiya, T., 1965, Electrodialysis of aqueous solution of amino acid con-taining electrolyte by ion exchange membrane, Asahi Glass Res. Report, 15, 171178.Katayama, S., 2004, Waste water treatment and recovering of useful substances, In: Seno,M., Tanioka, A., Itoi, S., Yamauchi, A., Yoshida, S. (Eds.), Functions and Appli-cations of Ion Exchange Membranes, Industrial Publishing & Consulting Inc., Tokyo,Japan, pp. 151169.Kikuno, R., 2006, Microbicidal effect of strong alkaline electrolyzed water, 2005 AlkalineElectrodialyzed Water Symposium, Tokyo, Kitasato University, September 3, 2006.Kokubu, T., Yamauchi, T., Miyagi, S., 1983, Application of electrodialysis in sugarindustry, Food Ind., 9, 2631.Leitz, F. B., 1986, Measurements and control in electrodialysics, Desalination, 381401,presented at the International Congress on Desalination and Water Re-use, Tokyo(1977).Matsunaga, Y., 1986, Reuse of waste water by electrodialytic treatment, Industrial Ap-plication of Ion Exchange Membranes, vol. 1. Research group of electrodialysis andmembrane separation technology, Soc. Sea Water Sci., Japan, pp. 188196.Mintz, M. S., 1963, Criteria for economic optimization are presented in the form ofcomparative performance equations for various methods of operation, Ind. Eng.Chem., 55, 1928.Nagasawa, T., Okonogi, S., Tomita, M., Tamura, Y., Mizota, T., 1973, Demineralizationof skim milk by means of electrodialysis with ion permselective membrane, I. Rela-tionship between limiting current density and specific conductivity of demineralizingsolution, Jap. J. Zootech. Sci., 44(8), 426431.Nagasawa, T., Okonogi, S., Tomita, M., Tamura, Y., Mizota, T., 1974, Demineralizationof skim milk by means of electrodialysis with ion selective membrane, II. The effects oflinear velocity, temperature and total solid content etc. on the limiting current density,Jap. J. Zootech. Sci., 45(11), 578584.Okada, K., Tomita, M., Tamura, Y., 1975, Electrodialysis in the treatment of dairyproducts, Symposium Separation processes by membranes, ion-exchange and freeze-concentration in food industry, Paris, March 1314.Sato, T., 1989, Investigation on water disinfection applied ion exchange membraneelectrodialysis and production of pyrogen-free water, Thesis, Yokohama NationalUniversity, Yokohama, Japan.Electrodialysis 381Sato, T., Tanaka, T., Suzuki, T., 1984, Disinfection of a colon bacillus by means of ionexchange membrane electrodialysis, J. Electrochem. Jpn., 52, 239243.Shaffer, L. H., Mintz, M. S., 1966, Electrodialysis, In: Spiegler, K. S. (Ed.), Principles ofdesalination, Academic Press, New York, London, pp. 200289.Sugiyama, M., Takatori, Y., Touyama, R., Nakamura, A., Yamauchi, T., Kawate, H.,Yamaguchi, A., 1982, On the desalination process of sugar solutions by electrodialysisusing neutral membranes, J. Sugar Refining Technology, 30, 2632.Takahashi, R., 2006, Basic analysis of biological effects of potable alkaline electrodialy-zed water at the cellular level, 2005 Alkaline Electrodialyzed Water Symposium, To-kyo, Kitasato University, September 3, 2006.Tanaka, Y., 1987, Concentration of seawater using electrodialysis, In: Kawasaki, J.,Kunime, T., Sakai, K., Hakuta, T. (Eds.), Membrane Separation Technology HandBook, Science Forum, Tokyo, pp. 211215.Tanaka, Y., 1991, Membrane separation, In: Seno, M., Abe, M., Suzuki, T. (Eds.), IonExchange, Kodansya Scientific Co., Tokyo, pp. 211227.Tanaka, Y., 1993, Electrodialysis, In: Sakai, K. (Ed.), Theory and Design of MembraneSeparation Process, Industrial Publishing & Consulting Inc., Tokyo, pp. 69107.Tanaka, Y., Ehara, R., Itoi, S., Goto, T., 2003, Ion exchange membrane electrodialyticsalt production using brine discharged from a reverse osmosis seawater desalinationplant, J. Membr. Sci., 222, 7186.Tomita, A., 1995, Electrodialyzer, In: Ogata, N. (Ed.), Engineering in Salt Manufactur-ing, vol. 2, Electrodialysis, Japan Salt Industry Foundation, Tokyo, pp. 85101.Tomita, M., Tamura, Y., Mizota, T., 1986, Electrodialysis of milk and whey, IndustrialApplication of Ion Exchange Membranes, vol. 1. Research group of electrodialysisand membrane separation technology, Soc. Sea Water Sci., Japan, pp. 147158.Tsunoda, S., 1994, Present status and latest trends of deep bed filtration, Bull. Soc. SeaWater Sci., Jpn., 48, 2737.Urabe, S., Doi, K., 1978, Electrodialyzer, Ind. Water, 239, 2428.Yamamoto, Y., 1993, Desalination of natural essences by electrodialysis, Industrial ap-plication of ion exchange membranes, vol. 2. Research group of electrodialysis andmembrane separation technology, Soc. Sea Water Sci., Japan, pp. 181188.Yawataya, T., 1986, Ion Exchange Membranes for Engineers, Kyoritsu Shuppan Co.Ltd., Tokyo, pp. 9498.ElectrodialysisOverview of TechnologyElectrodialyzerStructure of an ElectrodialyzerParts of an ElectrodialyzerFastening FrameSolution Feeding FrameGasketSlotSpacerElectrode and Electrode ChamberPressRequirements for Improving the Performance of an ElectrodialyzerSolution Velocity Distribution between Desalting CellsSolution Leakage in an ElectrodialyzerDistance between the MembranesSpacerElectric Current LeakageSimplicity of Structure of an ElectrodialyzerElectrodialysis ProcessOne-Pass Flow ProcessBatch ProcessPartially Circulation (Feed and Bleed) ProcessConcentration ProcessSeparation ProcessEnergy Consumption and Optimum Current DensitySurrounding TechnologyFiltration of a Feeding SolutionScale Trouble PreventionAcid DosagePrecipitation Controlling Agent DosageDisassembling and Assembling WorksPracticePotable Water Production from Brackish WaterElectrodialysis Desalination System Powered by Photo-Voltaic Power GenerationElectrodialytic Recovery of Wastewater from a Metal Surface Treatment ProcessReuse of Wastewater by Electrodialytic TreatmentSimultaneous Treatment of Wastewater by Electrodialysis and Reverse OsmosisHigh-Concentration SystemLow-Concentration SystemElectrodialytic Recovery of AcidSeawater Concentration for Salt ProductionSalt Production Using Brine Discharged from a Reverse Osmosis Seawater Desalination PlantDesalination of Amino Acid and Amino Acidic SeasoningsDesalination of Natural EssencesElectrodialysis of Milk and WheyComposition of Milk and Whey (Ideue, 1986; Tomita et al., 1986)Limiting Current Density in Electrodialysis of Milk and Whey (Nagasawa et al., 1973; Nagasawa et al., 1974)Ion Exchange Membrane (Okada et al., 1975) for the Demineralization of Milk or WheyAnti-Organic FoulingAnti-Alkaline CircumstancePermeability to Larger Organic Ions of an Anion Exchange MembraneElectrodialysis System (Okada et al., 1975) for the Demineralization of Milk or WheyDesalination of Sugar LiquorElectrodialytic DisinfectionReferences


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