Patent Publication Number: US-2012031763-A1

Title: Electrodialyzer

Description:
TECHNICAL FIELD 
     This invention relates to an electrodialyzer and, in particular, relates to an electrodialyzer of a low power consumption type. 
     BACKGROUND ART 
     As an example of using an electrodialyzer, a seawater desalination apparatus is known. For example, Patent Document 1 describes a seawater treatment apparatus comprising a reverse osmosis separation apparatus for desalinating seawater to obtain fresh water and an electrodialyzer for further concentrating concentrated water discharged from the reverse osmosis separation apparatus. 
     Patent Document 2 discloses an electrodialysis method with a plurality of pairs of electrodialysis chambers having a group of ion exchange membranes with selective ion permeability, which supplies conductivity waters with different electrolyte concentrations in series with respect to the plurality of pairs of electrodialysis chambers and energizes them to cause a large amount of current to flow in the low electrolyte concentration water, thereby improving the electrolyte removal ratio. 
     Further, Patent Document 3 describes using a multistage electrodialyzer for obtaining high concentration bittern. 
     However, all of Patent Documents 1 to 3 omit a detailed explanation of the electrodialyzer itself. Therefore, it is not possible to infer a problem in the electrodialyzer from Cited Documents 1 to 3. 
     Herein, referring to  FIG. 1 , a problem in an electrodialyzer will be clarified. As shown in  FIG. 1 , the electrodialyzer has a structure comprising an anode  101 , a cathode  102 , negative ion (anion) exchange membranes  103 , and positive ion (cation) exchange membranes  104 , wherein a plurality of pairs of the cation exchange membranes and the anion exchange membranes that are alternately disposed are sandwiched between the two (pair of) electrodes and water to be treated flows between the ion exchange membranes. 
     When a voltage is applied to the pair of electrodes, cations in the water move toward the cathode  102  side while anions in the water move toward the anode  101  side. In this event, the cations can pass through the cation exchange membrane, but cannot pass through the anion exchange membrane. On the other hand, the anions cannot pass through the cation exchange membrane, but can pass through the anion exchange membrane. As a result, desalination chambers  106  and concentration chambers  105  are formed. For example, when seawater is introduced as feed water to be treated, the water in the desalination chambers  106  is obtained as fresh water. In a normal electrodialyzer for seawater desalination, a plurality of pairs of ion exchange membranes are disposed between a pair of electrodes for making the electrodialyzer compact and inexpensive, and the electrodialyzer is operated at an interelectrode voltage of several 100V and at an electrode current density of several 10 mA/cm 2 . For example, if the electrode area is 1 m 2  (10000 cm 2 ) and the current density is 10 mA/cm 2 , the total current becomes 100A and, if the interelectrode voltage is 100V, the required power becomes 10 kW and thus very large power is consumed. 
     Further, the power increase becomes a more serious problem in Patent Documents 2 and 3 in which the electrodialyzer has the multistage structure. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: JP-A-H9-276864 
         Patent Document 2: JP-A-2003-94063 
         Patent Document 3: JP-A-2005-287311 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     As described above, the conventional electrodialyzer has the problem that very large power is required. 
     Therefore, this invention aims to provide an electrodialyzer with less power consumption. 
     Means for Solving the Problem 
     With respect to the above-mentioned problem, the present inventors have studied whether or not it is possible to reduce a voltage to be applied between electrodes, as a means for reducing the power consumption. 
     Specifically, first, the present inventors have paid attention to the fact that when a voltage is applied to the electrodes, the following phenomenon occurs between ions and water molecules in water and the electrodes. Specifically, when the voltage is applied to the electrodes, current does not flow between the electrodes immediately after the application of the voltage while the cations in the water start to move toward the cathode and the anions in the water start to move toward the anode (first phase). Further, when the voltage continues to be applied and exceeds a certain threshold voltage, electrons are transferred between the electrodes and the ions and water molecules in the water, i.e. the electrode reaction occurs, as a second phase so that the current starts to flow between the electrodes (second phase). This threshold voltage depends on the ion species and concentration in the water and the temperature thereof and further depends on an electrode material and so on. 
     Herein, the present inventors have found that, in order to move the ions, the conventional electrodialyzer is operated at a voltage where the electrode reaction proceeds in the above-mentioned second phase, and that this causes the power consumption to be large. Specifically, since the conventional electrodialyzer has the structure in which the plurality of pairs of ion exchange membranes are sandwiched between the pair of electrodes, the amount of ions to be moved by the pair of electrodes is large. Therefore, it is necessary to provide a large potential difference between the electrodes and, as a result, the interelectrode voltage exceeds an electrode reaction threshold voltage. 
     Accordingly, the present inventors have considered that if electrodialysis is carried out at an interelectrode voltage that does not exceed an electrode reaction threshold voltage, ions can be moved with almost no current flowing and thus a low power consumption type electrodialyzer is realized. In order for this, a high voltage cannot be applied, and therefore, it is necessary to construct a system that moves as small an amount of ions as possible with a pair of electrodes. As a consequence, the present inventors have found that, using as a basic unit a structure in which a pair of a cation exchange membrane and an anion exchange membrane are disposed between a pair of electrodes, electrodialysis can be carried out at an interelectrode voltage that does not exceed an electrode reaction threshold voltage. 
     Specifically, according to one aspect of this invention, there is obtained a low power consumption type electrodialyzer which has a structure in which, between opposed electrodes, a cation exchange membrane is provided on the anode side while an anion exchange membrane is provided on the cathode side, and which is adapted to move ions by supplying seawater between the electrodes and the ion exchange membranes and supplying seawater or fresh water between the cation exchange membrane and the anion exchange membrane and by applying a voltage, substantially not causing current to flow, between the electrodes. 
     On the other hand, the present inventors have made further studies on a means that reduces a voltage to be applied between electrodes as compared with conventional. 
     As a result, the present inventors have also found that, using as an electrode material a material with higher electron emission characteristics than that of a conventional electrode material, the electrode reaction can proceed at a lower voltage than conventional, i.e. the voltage to be applied between the electrodes can be reduced as compared with conventional. 
     According to another aspect of this invention, there is provided an electrodialyzer having a structure in which a plurality of pairs of anion exchange membranes and cation exchange membranes are disposed in parallel and are sandwiched on both sides thereof by an anode and a cathode, the electrodialyzer characterized in that Pt or Se is used as at least a part of a surface of the anode and LaB 6  is used as at least a part of a surface of the cathode. 
     Effect of the Invention 
     According to this invention, it is possible to reduce the power consumption amount of an electrodialyzer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a schematic structure of an electrodialyzer. 
         FIG. 2  is a diagram showing a potential-current curve of an aqueous solution. 
         FIG. 3  is a diagram of a two-stage electrodialyzer according to a first embodiment of this invention. 
         FIG. 4  is a diagram showing the operation of a first stage of the electrodialyzer according to the first embodiment of this invention. 
         FIG. 5  is a diagram showing the operation results of the first stage of the electrodialyzer according to the first embodiment of this invention. 
         FIG. 6  is a diagram showing the operation of a second stage of the electrodialyzer according to the first embodiment of this invention. 
         FIG. 7  is a diagram showing the operation results of the second stage of the electrodialyzer according to the first embodiment of this invention. 
         FIG. 8  is a diagram showing a cathode reaction potential-current curve when Pt is used as an anode and LaB 6  or Pt is used as a cathode in the electrodialyzer of  FIG. 1 . 
         FIG. 9  is a diagram for explaining a reduction in power consumption which is realized by a reduction in total voltage using LaB 6  as an electrode material in the electrodialyzer of  FIG. 1 . 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinbelow, preferred embodiments of this invention will be described in detail with reference to the drawings. 
     First, the first embodiment will be described. 
     The first embodiment is such that electrodialysis is carried out at an interelectrode voltage that does not exceed an electrode reaction threshold voltage. 
     First, referring to  FIG. 2 , the threshold voltage measurement results will be described. 
     In an acrylic vessel containing a 3 wt % sodium chloride aqueous solution, an anode and a cathode were disposed so as to face each other at a distance of 3 cm, and a voltage was applied between the electrodes using an external power supply. A platinum plate with a thickness of 0.1 mm was used as a material of each of the anode and the cathode.  FIG. 2  shows current amounts plotted versus interelectrode voltages. As shown in  FIG. 2 , it is seen that when a positive voltage is applied as an electrode voltage difference, current hardly flows up to 2V, while, when a negative voltage is applied as an electrode voltage difference, current hardly flows up to −2V. This means that it is possible to generate a potential gradient in water present between the electrodes with almost no current flowing up to an interelectrode voltage of 4V. 
     Next, referring to  FIG. 3 , the basic structure of an electrodialyzer according to the first embodiment of this invention will be described. Although the electrodialyzer is composed of a plurality of stages, a description will be given herein using a two-stage structure as an example. That is, the illustrated electrodialyzer comprises a first-stage electrodialysis portion and a second-stage electrodialysis portion. 
     The first-stage electrodialysis portion shown in  FIG. 3  has a negative ion (anion) exchange membrane  304 , a positive ion (cation) exchange membrane  305 , and an intermediate electrode  303  between an anode  301  and a cathode  302 . The intermediate electrode  303  is an electrode of a structure having a large number of small holes that allow a liquid to pass therethrough in both left and right directions in the figure, and is grounded. Therefore, chambers adjacent to each other through the intermediate electrode  303  can be regarded as the same chamber. 
     Likewise, the second-stage electrodialysis portion has a negative ion (anion) exchange membrane  1304 , a positive ion (cation) exchange membrane  1305 , and an intermediate electrode  1303  between an anode  1301  and a cathode  1302 . However, the layout of the anode, the cathode, and the ion exchange membranes is reversed in phase with respect to that of the first-stage electrodialysis portion. Specifically, in the first-stage electrodialysis portion, with respect to the center position of a flow direction, indicated by arrows, of an aqueous solution, the anode  301  is disposed on the left side (i.e. one side) in the figure while the cathode  302  is disposed on the right side (i.e. the other side) in the figure, and the cation exchange membrane  305  is disposed on the anode  301  side while the anion exchange membrane  304  is disposed on the cathode  302  side. On the other hand, in the second-stage electrodialysis portion, with respect to the center position of a flow direction of the aqueous solution, the cathode  1302  and the anion exchange membrane  1304  are disposed on the left side (i.e. one side) while the anode  1301  and the cation exchange membrane  1305  are disposed on the right side (the other side). 
     In the case of an electrodialyzer having a larger number of stages, an anode and a cation exchange membrane, and a cathode and an anion exchange membrane are alternately disposed on the left and right sides with respect to the center position of a flow direction. 
     The flow of the aqueous solution in the illustrated electrodialyzer is as follows. Specifically, in the case of seawater desalination, in the first stage, seawater is supplied, as feed water to be treated, into a chamber  306  between the anode  301  and the cation exchange membrane  305  and into a chamber  308  between the cathode  302  and the anion exchange membrane  304 , while, seawater or fresh water is supplied into a chamber  307  sandwiched between the cation exchange membrane  305  and the anion exchange membrane  304 . 
     The treated water having passed through the chambers  306 ,  307 , and  308  in the first-stage electrodialysis portion flows into and passes through chambers  1306 ,  1307 , and  1308 , which are similar to those of the first stage, likewise in the second-stage electrodialysis portion. As described above, however, the layout of the electrodes and the ion exchange membranes is reversed in phase in the second-stage electrodialysis portion. 
     Next, referring to  FIGS. 4 to 7 , the operation principle of the electrodialyzer according to the first embodiment of this invention will be described. 
     First-Stage Electrodialysis Portion: 
     In the first-stage electrodialysis portion, as shown in  FIG. 4 , cations are reduced in the chamber  306  sandwiched between the anode  301  and the cation exchange membrane  305  while anions are reduced in the chamber  308  sandwiched between the cathode  302  and the anion exchange membrane  304 . The results thereof are shown in  FIG. 5 . 
     In this manner, water from the chamber  306  where the cations are reduced and water from the chamber  308  where the anions are reduced (conversely speaking, water in which the anions and the cations are respectively concentrated) in the first-stage electrodialysis portion are sent to the second-stage electrodialysis portion. 
     In the case of seawater, as a voltage to be applied between the anode  301  and the cathode  302  in the first-stage electrodialysis portion, a voltage of +2V or less is applied to the anode  301  with respect to the intermediate electrode  303  while a voltage of −2V or less is applied to the cathode  302  with respect to the intermediate electrode  303 . That is, since the voltage not more than the threshold voltage shown in  FIG. 2  is applied between the anode  301  and the cathode  302 , it is possible to generate a potential gradient in the water with almost no current flowing between both electrodes. 
     Second-Stage Electrodialysis Portion: 
     Referring to  FIG. 6 , as described above, the structure of the second-stage electrodialysis portion is reversed in phase with respect to that of the first-stage electrodialysis portion. The water in the chamber  306  where the cations are reduced in the first-stage electrodialysis portion as shown in  FIG. 5  is supplied into the chamber  1308  sandwiched between the cathode  1302  and the anion exchange membrane  1304  in the second-stage electrodialysis portion as shown in  FIG. 6 , while, the water in the chamber  308  where the anions are reduced in the first-stage electrodialysis portion as shown in  FIG. 5  is supplied into the chamber  1306  sandwiched between the anode  1301  and the cation exchange membrane  1305  in the second-stage electrodialysis portion as shown in  FIG. 6 . 
     In the second-stage electrodialysis portion, a voltage of +3 to 4V is applied to the anode  1301  with respect to the intermediate electrode  1303  while a voltage of −3 to 4V is applied to the cathode  1302  with respect to the intermediate electrode  1303 . That is, the voltage larger in absolute value than that of the first-stage electrodialysis portion is applied to the second-stage electrodialysis portion. 
     Since the water having passed through the chamber  306  in the first-stage electrodialysis portion and thus containing anions at a concentration unchanged passes through the chamber  1308  sandwiched between the cathode  1302  and the anion exchange membrane  1304  in the second-stage electrodialysis portion, the anions flow into the chamber (concentration chamber)  1307 , provided in the center, through the anion exchange membrane  1304  so that the anion concentration in the chamber  1308  is significantly reduced. 
     Since the water having passed through the chamber  308  in the first-stage electrodialysis portion and thus containing cations at a concentration unchanged passes through the chamber  1306  sandwiched between the anode  1301  and the cation exchange membrane  1305  in the second-stage electrodialysis portion, the cations flow into the central chamber  1307  through the cation exchange membrane  1305  so that the cation concentration in the chamber  1306  is significantly reduced. The results thereof are shown in  FIG. 7 . 
     In this manner, the water to be treated passes through the first stage and the second stage. When the number of stages is increased, by passing the water alternately through the anode side→the cathode side→the anode side→the cathode side, while, passing the water, on the other hand, alternately through the cathode side→the anode side→the cathode side→the anode side in the third and subsequent stages in the same manner as in the first and second stages, the cations and the anions in the water are concentrated into chambers  307 ,  1307  each provided in a central portion. As the number of stages increases so that the ion concentrations in chambers  306  ( 1306 ) and  308  ( 1308 ) decrease, it is necessary to increase a voltage to be applied. 
     However, since the operation is carried out under the condition where almost no current flows, i.e. the interelectrode voltage is set so that the current density becomes 1 mA/cm 2  or less and preferably 0.1 mA/cm 2  or less, to thereby remove cations and anions, it is possible to reduce the NaCI concentration with a significantly smaller power consumption amount as compared with the conventional electrodialysis method (current density: several 10 mA/cm 2 ). 
     Next, the second embodiment of this invention will be described with reference to  FIGS. 1 ,  8 , and  9 . 
     The second embodiment is such that LaB 6  is used as at least a part of a surface of a cathode in an electrodialyzer. 
     Referring to  FIG. 1 , one example of an electrodialyzer according to the second embodiment will be described. 
     The basic structure of the electrodialyzer according to the second embodiment is the same as that shown in  FIG. 1 , but Pt is used as the anode  101  and LaB 6  or Pt is used as the cathode  102 . 
     As described before, in  FIG. 1 , the electrodialyzer has the structure in which the plurality of pairs of anion exchange membranes  103  and cation exchange membranes  104  are disposed in parallel and are sandwiched on both sides thereof by the anode  101  and the cathode  102 . Hitherto, a Pt-plated Ti electrode or the like is used as a material of each of the electrodes. 
     The operation of the electrodialyzer intended for seawater is as described above, but will be briefly described again. Cations and anions are always present in an aqueous solution containing salt. 
     (1) When seawater is supplied into the vessel provided with the anode  101  and the cathode  102  and a voltage is applied between the two electrodes, ions are attracted to the opposite-polarity electrodes due to electrophoresis. Herein, if the cation exchange membrane  104  is present between the two electrodes, the migrating anions cannot pass through this cation exchange membrane  104 . On the other hand, the cations can pass through this cation exchange membrane  104  to move to the electrode side. If the anion exchange membrane  103  is present between the two electrodes, this shall be reversed. 
     (2) Since, as described above, the cation exchange membranes and the anion exchange membranes  103  are alternately inserted between the anode  101  and the cathode  102  in the electrodialyzer, two water flow paths or compartments are formed. Then, when salt water as a liquid to be treated is supplied and a current is caused to flow between the two electrodes, the above-mentioned ion movements alternately occur so that there can be formed the flow path in which both anions and cations are concentrated and the flow path in which both anions and cations are removed, i.e. diluted. As a consequence, a salt-concentrated liquid (Condense) and a desalinated liquid (Dilute) are obtained at outlets of the two flow paths. 
     Only several ion exchange membranes are shown in  FIG. 1 , but in an actual electrodialyzer, for example, 300 pairs of anion exchange membranes  103  and cation exchange membranes  104  are disposed in parallel for efficiently using the current. Hitherto, when desalination is carried out to reduce the seawater salt concentration from 3.5% to 2.7% in an electrodialysis cell having a structure in which the anion exchange membranes  103  and the cation exchange membranes  104  are sandwiched on both sides thereof by Pt-plated Ti electrodes, a voltage of about 250V is required. A rough breakdown of the voltage is a 300-pair intermembrane voltage of 240V and a voltage of 10V at the electrode portions. 
     The second embodiment pays attention to LaB 6  as an electrode material in such an electrodialyzer, thereby realizing a reduction in power consumption. 
     In terms of features such as high melting point, low work function, and high electron emission rate, LaB 6  is widely used as a thermion emission material for electron microscopes and so on. The low work function corresponds to high electron emission capability. Work functions of several materials are shown below as examples, wherein the superiority of LaB 6  is apparent. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Material 
                 Work Function (eV) 
               
               
                   
                   
               
             
            
               
                   
                 Se 
                 5.9 
               
               
                   
                 Pt 
                 5.7 
               
               
                   
                 Pd 
                 5.1 
               
               
                   
                 Ti 
                 4.3 
               
               
                   
                 LaB 6   
                 2.4 
               
               
                   
                   
               
            
           
         
       
     
     An electrode reaction in an electrodialyzer is an electron exchange reaction between an aqueous solution and an electrode and it is expected that as the electron donating ability increases, a cathode reaction that donates an electron to a molecule or an ion in the solution is promoted.  FIG. 8  shows a cathode reaction potential-current curve when Pt is used as the anode  101  while LaB 6  is used as the cathode  102 . 
     As is clear from  FIG. 8 , when a constant current is caused to flow using LaB 6  as the cathode  102 , the electrode reaction proceeds at a lower voltage due to its high electron emission characteristics as compared with the case where Pt is used as the cathode  102 . As a consequence, using LaB 6  as the electrode of the electrodialyzer, it is possible to reduce the power consumption amount in the electrode reaction. 
     In order to maximize the effect of the LaB 6  electrode, the number of ion exchange membranes in the electrodialyzer is preferably as small as possible. As shown in  FIG. 9 , in the above-mentioned electrodialysis cell in which the 300 pairs of anion exchange membranes  103  and cation exchange membranes  104  are disposed in parallel, a voltage of 240V between the 300 pairs of membranes and a voltage of 10V at the electrode portions are required for carrying out desalination to reduce the seawater salt concentration from 3.5% to 2.7%. In this case, even if the voltage at the electrode portions can be reduced from 10V to 5V using LaB 6  as the cathode  102 , since the intermembrane voltage of the 300 pairs of ion exchange membranes is 240V, a voltage reduction is only 2% (=5/250) relative to the total voltage being the sum of the electrode portion voltage and the intermembrane voltage and thus the influence upon a reduction in the power consumption of the entire electrodialyzer is not great. 
     Accordingly, in order to maximize the effect of using LaB 6  as the cathode  102 , it is preferable to reduce the number of ion exchange membranes in the electrodialyzer. For example, if the number of pairs of ion exchange membranes is reduced from 300 pairs to 50 pairs, the intermembrane voltage becomes 40V while the electrode portion voltage can be reduced from 10V to 5V and, therefore, the reduction effect relative to a total voltage of 50V due to the use of the electrode of LaB 6  is improved from 2% described above to 10% (=5/50) so that a large reduction in power consumption is enabled. 
     The cathode  102  may be made of LaB 6  alone as described above, but may alternatively be such that at least a part of a surface of a material (e.g. W, Mg, Ti, or the like) different from LaB 6  is coated with a LaB 6  film. On the other hand, as the anode  101 , Se may be used instead of Pt. The anode  101  may be made of Pt or Se alone and may alternatively be such that at least a part of an electrode surface of a different material is coated with a Pt or Se film. 
     INDUSTRIAL APPLICABILITY 
     This invention is applicable not only to a seawater desalination apparatus, but also to a salt or bittern manufacturing apparatus. 
     Description of Symbols 
     
         
         
           
               301 ,  1301  anode 
               302 ,  1302  cathode 
               303 ,  1303  intermediate electrode