US 2689826 A
Description (OCR text may contain errors)
Sept. 21, 1954 p, KOLLSMAN 2,689,826
ELECTRODIALYTIC APPARATUS Filed July 2l, 1950 2 Sheets-Sheet 1 n QI 'Ill/1111 INVENTOR. Pa/ Ko smc/'7 BY #rm&.wl
Patented Sept. 21, 1954 UNITED sTATss, ATsNT OFFICE ELEcTRoDIALYTIc APPARATUS Paul nousmaNew York, N. Y.
Application July 21, 1950, Serial No. 175,127 s claims; (o1. 2114-301) This invention relates to the art of modifying the chemical composition of substances by a transfer of ions under the influence of an electric current in a process commonly called electrodialysis.
Basically, this process involves a transfer, or removal, of ions of one volume of fluid through anion impeding and cation impeding dia- `phragms into another volume of iiuid under the influence of` an electrical bias or potential causing the ions to travel in predetermined directions. The volume of the rst iiuid is thus depleted of ions and the volume of the second fluid is enriched.
It is thus possible, for example, to reduce the salt content of saline solutions, for example, sea water, to a point where the desalted product is suitable for industrial and agricultural uses, and even for human consumption.
The present invention provides improvements in, and refinements of, the method of electrodia'lysis as well as of apparatus for practicing the method, making method and apparatus more efficient, resulting in products of higher purity, and greater uniformity, even with dia-lysis diaphragms of moderate quality.
'by drawings, showing, for the purpose of illustration, apparatus for practicing the invention.
The invention also consists of certain new and original features of construction and combination of parts, as well as of steps and combination `of steps, as hereinafter set forth and claimed.
Although the characteristic features of this invention which are believed to be novel will be particularly pointed out in the claims appended hereto, the invention itself, its objects and advantages, and the manner in which it may .be carried out will be better understood by referring to the following description taken in connection with the accompanying drawings forming a part 4of it, in which:
Figure 1 is a diagrammatic representation, in vertical cross section of an improved apparatus embodying the present invention and adatped to `carry out the improved method disclosed herein; and
Figure 2 is an elevational view taken on line 2-2 of Figure 1, and showing, in addition, further details of the apparatus.
In the following description and in the claims various details will be identified by specific names for convenience. Like reference characters refer to like parts in the several figures of the drawings.
In the drawings accompanying and forming a part of this specication, certainspecic disclosure of the invention is made for the purpose of explanation of the broader aspects of the invention, but it is understood that the details may be modified in various respectswithout departure from the principles of the invention and that the invention may be applied to, and practiced by, other structures than the ones shown.
The principles and features of the invention are readily understood by rst considering the basic structure of an apparatus for practicing it. Figure l is a diagrammatic illustration of an apparatus particularly designed for increasing and decreasing the salinity of water by clectrodialysis, but it may be used for the treatment or production of other fluids and compositions.
A tank II is subdivided into a plurality of chambers or cells by separating ion discriminating walls or diaphragms composed of a suitable composition or material imparting to the walls or diaphragme ion discriminating characteristics. Thus, certain diaphragms I2 are anion-permeable V and cation-repellent, While other diaphragms I3 have theopposite characteristic of being cation-permeable and anion-repellent. rThe diaphragms are arranged in alternating sequence with respect to traverse of the tank from one end to the other so that an anion-permeable diaphragm follows a cation-permeable diaphragm and is, in turn, followed by an anion-permeable diaphragm, and so forth.
The chambers of cells may be classified into two terminal cells Il and I5 containing electrodes I6 and il, and a plurality of intermediate cells I8 and I9.
The electrode Iii `is connected to the negative pole of a source of electric energy 2li by a lead 2i thus becoming a cathode, and the electrode Il is connected to the positive pole of a source 2i) by a lead 22 making the electrode Il an anode. The intermediate cells I8 may conveniently be termed concentration cells, and the intermediate cells I9 may `be called dilution cells, according to the character of the electrodialytic action taking place therein.
The dilution cells I 9 are preferably narrower than the concentration Acells I8, width being measured between the bordering diaphragms.
Speaking first of the dilution cells I9, the cells have inlet ports 23 at, or near, the bottom admitting fluid into the dilution cells from an inlet duct 24 which is suitably manifolded with respect to all the dilution cells.
The endmost diaphragms extend above the tops of the intermediate diaphragms to corinne between them a common pool 25 with which the open ends 26 of the cells I9 communicate freely. A large outlet port 2l determines the height to which the liquid level 28 may rise. An outlet duct 29 (Figure 2) leads from the port 2l to discharge processed uid from the processing tank II.
The terminal diaphragms have ports 30 and 3| controlled by adjustable gate members 32 and 33 for admission of a controlled volume of iiuid from the pool 25 into the terminal chambers I4 and I5 which the fluid leaves through ducts 34 and 35'.
Fluid in the pool may freely enter the concentration chambers I8 which are open at the top at 36. The fluid iiows through the concentration chambers in a downward direction, opposed to the direction of flow through the chambers I8, and leaves the concentration chambers through restricted metering passages 3l which form discharge ports for these chambers. The restricted passages 31 are dimensioned to permit only a fraction of the volumetric iiow admitted through the inlet ports 23 to pass into a common discharge duct 43 as will also be explained later in greater detail.
The `fluid of the terminal chambers is preferably handled separately because of certain electro-chemical reactions which may be induced by the physical presence of the electrodes in the chambers, making it generally undesirable to mix the product of the terminal cells with the products of other cells.
From the arrangements of the ports, ducts and cells or chambers it is evident that the direction of flow through the dilution cells is upwards, or opposed to gravitation while the direction of iiow through the concentration cells is downward following gravity.
Fluid is supplied through the inlet duct 24 at a predetermined controlled slow rate which is maintained sufliciently slow to insure a pre- `determined degree of dilution, by reason of iondepletion, to take place, during the ow of the fluid from the bottom of the cells to the top.
The flow through the concentration cells is preferably maintained at a fraction of the total volumetric flow passing through the dilution cells, a preferred range of ratios being that in which the ow through the concentration cells is restricted to between one-half and one-twelfth the volumetric flow passing through the dilution cells. This is preferably accomplished by installation of flow restricting passages 31 of the proper dimension.
Since most electrodiallytic processes involve a transfer of a certain amount of uid through the diaphragms it is convenient to compare the volumetric flows through the dilution and the concentration cells by reference to the volume entering the dilution cells through the inlet duct 24 and to the volume leaving the concentration cells through the discharge duct 43 into which the restricted passage 31 lead. Thus the volume of fluid entering the dilution cells includes that portion of fluid which permeates the diaphragms of the dilution cells, and the volume withdrawn from the concentration cells includes 4 the fluid gain due to passage of fluid through the diaphragms into the concentration cells.
The supply of fluid through the inlet duct 24 may conveniently be maintained at a predetermined volumetric rate by use of supply tank 38 (Figure 2) connected to the duct 24. The liquid level 39 in the supply tank is maintained constant by a supply valve 4I) controlled by a iioat 4I admitting sufcient fluid from a supply pipe 42 to maintain the liquid level slightly above the liquid level 28, the difference in the two levels being so selected as to provide for a predetermined static pressure which, in turn, results in a predetermined rate of flow through the chambers I9. It is thus possible to control the rate of ilow through the dilution cells by adjustment of the float 4I.
The operation of the apparatus may be conveniently explained by a specic example. It may be assumed that the apparatus is being used for the production of fresh water of a high degree of purity and the simultaneous production of concentrated sea water or brine. When the operation of the device as applied to Water purification is understood, it will easily be seen how other compounds in solution may be treated in the apparatus.
It may be assumed that an electrical potential is applied at the electrodes at the time salt-containing raw water enters through the inlet duct 24 into the apparatus whose concentration cells are also filled with water, preferably containing a slight amount of salt in order to cause a current to ow through the cells from one electrode to the other. The raw water fed into the apparatus is preferably rst filtered to free it from mechanical impurities. The raw water ows slowly through the dilution cells from the bottom towards the top and is gradually deionized by reason of the action of the electric current causing the ions of the raw water to permeate the diaphragms.
Assuming, for reasons of simplicity, that the only salt present in the raw water is sodium chloride, the positively charged sodium cations tend to travel towards the cathode I5. The sodiu'm cations pass through the cations permeable diaphragms I3 and accumulate in the concentration cells I8 which they are unable to leave because of the cation-blocking properties of the diaphragms I2 which bar their path.
Similarly chlorine anions pass through the anion permeable diaphragm I2 and accumulate in the concentration cells I3 from which their exit is barred by the anion-blocking properties of the diaphragms I3.
The sodium and chlorine ions in the concentration cells recombine as sodium chloride and cause the salt concentration in the cells I3 to increase, while simultaneously the salt concentration in the dilution cells decreases.
Since purified water is present in the pool 25 during normal operation of the apparatus, the purication of water flowing through the dilution cells may be carried to a very high degree, and Water leaving through the outlet port 2l has a particularly high degree of purity.
The flow through the concentration cells takes place at a volumetric rate which is only a fraction of the volumetric rate of flow through the dilution cells. For this reason the salt enrichment per volumetric unit of fluid in the concentration cells reaches a higher degree than the salt depletion in the dilution cells. Assuming, .for example, that the volumetric ilow through the 5, concentration cells is one sixth of the volumetric :dow through the dilution cells,.itlis evident Vthat the concentration taking place` in the concentration compartment `is six times as great per volumetric unit of fluid as the loss of salt in the dilution cells so that the water leaving the concentration `compartment through the discharge ports contains six times the amount of salt as the sea water entering the dilution cells.
The aforementioned iiowand concentration ratios involve several economic advantages. Firstly, it seems that `the transfer of fluid, or. in other words, the loss of water by passage from the dilution compartments into the concentration compartments at any particular point of the diaphragm is, in approximation inversely proportional to the concentration on the other side of the diaphragm at the :point Where the loss occurs.
Since, furthermore, the loss of fluid appears to be proportional to the transfer of ions, the presence of a higher ion concentration near the bottom of the concentration cells lessens the loss of fluid from the dilution cells in which the greatest loss also tends to occur near the bottom. Thus the high ion concentration in the concentration cells tends to reduce the loss of fluid from the dilution cells.
The high concentration of the fluid leaving the concentration cells makes the fluid suitable for further commercial use, which it might not have, if the concentration were less. Thus the resultant brine may be used for manufacture of dry salt and other uses.
In addition, greater economy is achieved due to the `fact that the iiuid in the cells I 8 offers little resistance to electric current because of the high concentration by reason of he reduced volumetric rate of flow.
It is easily seen that the ion depletion in the dilution cells per inch of advance from the inlet ports 23 to the pool 25 proceeds at a slower linear rate than the ion enrichment per inch of advance from the pool 25 to the discharge ports 31.
The volumetric rate of flow through the dilution cells I9 may be controlled either by control of the fluid pressure or by the dimensions of the ports 2l, or both, in such a way that the fluid leaving the device through the outlet lduct 29 has the desired degree of dilution, and the volumetric flow through the concentration compartment is so controlled, as to maintain the ion enrichment at a predetermined ratio with respect to the ion depletion in the adjoining cells. For example, the ratio may be one to six or one to ten, or any other gure, as conditions `may require. This is conveniently effected by control of the out-flow, for example by installation of suitably restricted discharge ports 2B.
A particular feature of the counter flow arrangement of the illustrated apparatus is its favorable effect on the current density near the bottom of the cells in order to remove the greatest possible number of ions per unit of time from the ilow entering the dilution cells. A high current density near the bottom of the cells is promoted by the concentration cells in which the greatest concentration and hence the :greatest conductivity is likewise near the bottom, and not near the top, as it would be in an `installation which does notemploy the principle of opposite ilow on opposite sides of the diaphragms.
The rate of flow through the individual dilution cells is controlled inA such a way that the product at the tops 2.6 of "all the cells `Il) is of 6. uniform purity. It is rather di'icult to control the `rate of flow through the individual dilution cells E9 by proper dimensioning of their individual inlet ports 23 in such a way that the volumetric flows through all the dilution cells I9 is `precisely the same. Nonmuniformity of flow has certain disadvantages, as will be seen from the followin specific examples:
`It may be assumed, that the volumetric ilow through all the cells is equal with the exception of one cell through which the ow passes at a slower rate. Under these circumstances all the cells, except the one, yield a product of insufficient purity, since the uid in the one cell becomes deionized sooner than the fluid in the other cells. As soon as the fluid in the one cell becomes deionized its conductivity' approaches zero and the current ceases to flow. Deionization in the other cells ceases, and the fluid discharged from them has a lower degree of purity, than desired. I
A similar result is obtained if the rate of flow through all the cells is the same, but the fluid passing through one oi the cells happens to have a lower initial ion content than the fluid in the others. In this case deionization is completed too soon in the one cell and the fluid in the other cells is insufliciently deionized because the current ceases to ow.
In these two examples, it should be understood that reference is made to the flow of current at a certain level above the bottom of the cells, since obviously current will still continue to flow near the bottom where the ion content of the flows is sufficiently great. However, near the top of the dilution cells the current density becomes so low upon complete deicnization of the fluid in one or a few cells near the top that the electrodialytic action ceases in all the other cells at approximately the same level, at which, due to the higher initial iiow rate or the higher initial ion concentration the fluid has not yet reached the state of complete deionization.
These undesirable conditions may be eliminated or reduced by an automatic control of the flow through the individual cells in dependence on the gravity of the fluid in the respective cell.
Due to the relatively slow rate of flow in the illustrated form of apparatus and due to the fact that the pressures at all the inlet ports 23 and at the tops 2@ of the dilution chambers are equal, the gravity control may be carried out in an extremely simple manner by using the weight of the liquid columns as the controlling means. In other words, the specic gravity of each fluid column proper in the individual dilution cells retards or accelerates the flow through the cells. It has been found that extremely small differences in gravity are suificient to provide a very effective control of the rate of flow. The control operates as follows:
It may be assumed that the fluid ows at a more rapid rate through one cell than through the other. In this case the rapid flow in the one cell causes the iluid to be exposed to the dialyzing current for a shorter time than in the other cells, resulting in less deionization and a higher specic gravity of the fluid in the one cell than in the others. The greater weight of the fluid column in the one cell causes less liuid to enter the cell, thus retarding the flow. The retarded iicw is subject to the dialyzing current for a longer period of time whereby a proportionately greater number of ions is removed therefrom, causing the weight of the fluid column to become less and the ow rate to increase. In this manner a proper mean rate of flow is automatically maintained in each cell resulting in a yield of fluid from all the dilution cells which is of substantially uniform purity,
If a fluid of a particularly high degree of purity is desired, for example, if drinking water is to be produced from sea water, it is preferable to subject the iluid to the action of a dialyzing current more than once. I am aware that it has formerly been proposed to treat fluids in successive stages to produce a product of great purity. However, the present modification of the basically known idea of stage-by-stage operation, involves several novel and advantageous aspects, which result in an improvement over conventional and known operations.
In the apparatus disclosed herein the same dialyzing current flows through all the dilution cells and an equal number of ions are driven out of all the dilution cells within a predetermined period of time. Unequalities in the degree of purity of the output is largely counteracted by the effective flow control, hereinbefore described. However for very exacting demands the effect of the variations in the purity of the output of the individual dilution cells is eliminated, and a higher total degree of purity is attained in a rather economical manner by the arrangement illustrated more particularly in Figure 2.
The product of all the dilution cells I9 is collected in the pool 25 and withdrawn through the port 21 and the duct 29. The now of the treated fluid through the duct 29 involves intimate mixing of the products of the several dilution cells so that the fluid is of uniform concentration or dilution as it enters a second dialyzer tank Ill through an inlet duct H24. The dialyzer III corresponds in all details to the dialyzer shown in Figure 1, but the potential applied to its electrodes may be higher because of the low ion concentration of the iiuids to be treated. A lead l2| leading to the anode of the dialyzer lll is visible. After ow of the fluid through the dilution cells of the second dialyzer the fluid is again collected in a pool and is then withdrawn through a duct |29. The fluid obtained at the duct |29 is of a high degree of purity and the second step of dialyzation is efflcient and economical, since the products of the cells of the rst dialyzer are not individually subjected to the second treatment, but only after thorough mixing. This is important since no unduly high voltages need be employed to overcome the effects of the presence of a more highly purified product in some of the cells, resulting in a greater ohmic resistance, than in the others.
It will be noted that, aside from the -transfer of ions through the diaphragms, no electrochemical electrode rea-ction takes place in any of the intermediate cells, since the cells do not contain electrodes.
The electrodes IB and I1 are lmade of a material resisting decomposition. Carbon and graphite are suitable materials for the anodein an apparatus for purifying wa-ter, and iron or nickel-chromium may serve as material for the cathode.
Since the dilution cells represent a greater ohmi'c resistance per unit of width than the concentration cells, -the dilution cells may be made narrower than the concentration cells.
In actual practice the thickness of the fluid lms in the ycells is considerably less than shown in the drawings in which many dimensions are exaggerated or reduced for the sake of clearness. It has been found particularly advantageous toA make the spaces between the diaphragms narrower than the thickness of the diaphragms and to employ diaphragms of high electric conductivity. For example, a spacing of one or two millimeters has 'been found advantageous `for diaphragms of a thickness of three millimeters.
Similarly, the height of the cells is not shown in its correct -proportion with respect to the other dimensions. The height of the ycells may natu` rally be considerably greater than shown so that the fluid columns in the cells are of substantial length.
In the practice of the improved method of electrodialysis flows of fluid to be deionized are confined between flows of fluid into which ions are to be transferred through ion-discriminating diaphragms. The fluids are maintained in ythe state of flux in opposite directions `past the diaphragms and the volumetric flow of the iiuid into which ions are to be transferred' is maintained smaller than -the volumetric flow of the iiuid to be deionized. By this arrangement the concentration on both sides of the diaphragm is greatest near the bottom of the cells and the fluid transfer -through the diaphragms is minimized as hereinbefore set forth.
The volume of fluid withdrawn from the `concentration cells may be supplied in part from the output of the dilution cells, but may be replenished entirely by fluid transfer through the diaphragms. It is evident that in the treatment of fluids in steps or stages by passage, in succession, through several ion exchange units as represented by Figures 1 and 3, the fluid supplied to the concentration cells in the rst stage or unit need not be as highly purified as in the succeeding stages, since the purity of the fluid at the top of the concentration cells need not be greater than the desired purity of the iiuid leaving the dilution cells.
Referring -to the illustrated forms of apparatus, it is seen that the flow of iiuid to be deionized is split into a plurality of substantially equal branches all of which are subjected to the same current. It follows that the rate of deionization per inch of flow is the same in all -the branches assuming that the flows are equal. This is conveniently -controlled by proper adjustment of the individual ports lthrough which the fluid enters or leaves the cells.
Thus numerous changes, additions, omissions, substitutions and modifications in the apparatus and `method steps, as well as other applications of the method and apparatus may be made withou-t departing from the spirit, the teaching iand the principles of the invention.
For the sake of brevity the term ion-discriminating diaphragm is used in the claims to identify membranes which have the inherent property of being permeable to ions of one sign and passage resistant to ions of the opposite sign.
What is claimed is:
l. An apparatus for increasing and decreasing the ion ycontent of liquids, comprising means forming a plurality of chambers, not less than ve, the -chambers being arranged in line; aniondiscriminating and cation-discriminating `diaphragms between said chambers for establishing a selective path for ions from one chamber into an adjoining ichamber under the inuence of an electrical potential, said diaphragms being arranged in substantially vertical position and in alternating sequence with respect to traverse from one terminal chamber through the intermediate chambers to the other terminal cham-.
ber; an electrode in each of the terminal cham bers, one electrode to serve as an anode, the other electrode to serve 'as a cathode; means including inlet ports near the bottom of certain alternate `chambers for supplying raw liquid to be deionized into said alternate chambers; means forming a common space above, and in communication With, all of said intermediate chambers, said common space lying above the upper ends of said diaphragms for collecting liquid from said alternate chambers rising into said common space by reason of flow and lreduced specic gravity, said common space having an overflow outlet port above the tops of the diaphragms through which deionized liquid may be withdrawn; means including discharge ports near the rbottom of other -chambers lying between said alternate lchambers for discharging concentrate from said other chambers, liquid from said common space entering said other chambers at points above the level of said discharge ports, whereby the flow through said other chambers is substantially opposed in direction to the ow through said alternate chambers.
2. An apparatus for increasing and decreasing the ion content of liquids, comprising means forming a plurality of chambers, not less than ve, the chambers being arranged in line; aniondiscriminating and cation-discriminating diaphragms between said chambers for establishing a selective path for ions from one chamber into an adjoining chamber under the inuence of an electrical potential, said diaphragms being arranged in substantially vertical position and in alternating sequence with respect to traverse from one terminal chamber through the intermediate chambers to the other terminal chamber; an electrode in each of the terminal chambers, one electrode to serve as an anode, the other electrode to serve as a cathode; means including inlet ports near the bottom of certain alternate chambers for supplying raw liquid to be deionized into said alternate chambers; means forming a common space above, and in communication with, all of said intermediate chambers, said common space lying above the upper ends of said diaphragms for collecting liquid from said alternate chambers rising into said common space by reason of flow and reduced specific gravity, said common space having an overow port above the tops of the diaphragms through which deionized liquid may be withdrawn; means including discharge ports near the bottom of other chambers lying between said alternate chambers for discharging Iii concentrate from said other chambers; and means including supply ports for admitting liquid from said common space into said terminal chambers.
3. An apparatus for increasing and decreasing the ion content of liquids, comprising means forming a plurality of chambers, not less than five, the chambers being arranged-in line; aniondiscriminating and cation-discriminating diaphragms between said chambers for establishing a selective path for ions from one chamber into an adjoining chamber under the iniiuence of an electrical potential, said diaphragms being arranged in substantially vertical position and in alternating sequence with respect to traverse from one terminal chamber through the intermediate chambers to the other terminal chamber; an electrode in each of the terminal chambers, one elecl trode to serve as an anode, the other electrode to serve as a cathode; means including inlet `ports near the bottom of certain alternate chambers for supplying raw liquid to be deionized into said alternate chambers; means forming a common space above, and in communication with, all of said intermediate chambers, said common space lying above the upper ends of said diaphragms for collecting liquid from said alternate chambers rising into said collection chamberby reason of flow and reduced specific gravity, said common space having an overflow outlet port above the tops of the diaphragms through which deionized liquid may be withdrawn; means including discharge ports near the bottom of otherl chambers lying between said alternate chambers for discharging concentrate from said other chambers; and means for admitting liquid from said common space into the terminal electrode-containing chambers; and means separate and distinct from said outlet port and said discharge ports for discharging liquid from said terminal chambers.
References Cited in the iile of this patent UNITED STATES PATENTS Number Name Date 1,326,106 Schwerin Dec. 23, 1919 1,546,908 Lapenta July 21, 1925 1,986,920 Cross Jan. 8, 1935 2,411,238 Zender Nov. 19, 1946 2,636,852 Juda et al Apr. 28, 1953 OTHER REFERENCES Helvetica Chimica Acta, vol. 23 (1940), pages 795 thru 800.
Journal of The Electrochemical Society, vol.
` 97, No. 7, July 1950, pages 139C thru 151C.
"Journal of Physical and Colloid Chemistry,` vol. 54 (1950), pages 204 thru 226.