US3354068A - Method of deionization utilizing a low electrostatic potential - Google Patents

Description

21. 1967 R. A. RIGHTMIRE ETAL 3,354,068

METHOD OF DEIONIZATION UTILIZING A LOW ELECTROSTATIC POTENTIAL Filed June 7, 1962 5 Sheets-Sheet 3 65A WATER SEA WATEQ Dwcmeewe mzocess CHAIZC'HNG DEUCE-55 nesmeo WATEQ 22 CONCENTRATED F15 E. 5A LT 5OLUT/ON #55 J cia STOEAEE' INVENTORS WASTE v 1 5 5 W575 POBfiWAfP/QHTM/RE BYGLENA/ R 590 w/v P/c/m RD 4 Po wLAND AT'TOQNE 5.,

United States Patent 3,354,068 METHOD OF DEIUNIZATION UTILIZING A LOW ELECTROSTATEC POTENTIAL Robert A. Rightmire, Twinsburg, Glenn R. Brown, Solon,

and Richard L. Rowland, Cleveland, Ohio, assignors to The Standard Oil Company, a corporation of Ohio Filed June 7, 1962, Ser. No. 200,722 1 Claim. (Cl. 204180) This invention relates generally to the application of an electrostatic field across the interphase boundary between an electron conductor and an ion conductor to permit the separation of ions. Such a method and apparatus is particularly adapted to the removal of a polar solute from a polar solvent in a polar solution. The principles of this invention, for exemplary purposes, will be described in reference to the removal of solute ions from aqueous media, it being understood that these principles are applicable to other types of polar solutions as well.

Generally, conductors in which electrical charges are stationary, as opposed to those in which the charges flow, are said to be in an electrostatic condition. The charges in an electrically neutral conductor may be separated in several ways known to those skilled in the art including separation by induction from an adjacent charged body. For the purpose of this description, an ion conductor will be deemed the equivalent of an electron conductor with respect to its charged condition. Thus, the imposition of a charge is tantamount to the separation of electrons and protons in an electron conductor and to the separation of cations and anions in an ion conductor. The separation of cations and anions in an ion conductor may also include some polarization of ion-dipoles which are not completely dissociated in the polar solvent of the ion conductor. Compounds of polar molecules which are readily dissociated into ions in a polar solvent will be identified throughout as polar solutes. The interphase boundary between the electron and ion condutcors will be referred to hereinafter as the interface. The ion conductor which is composed of the polar solvent and polar solute will also be referred to as the polar solution.

When an electrical potential is applied across the interface between an electron conductor and an ion conductor, it is believed that there normally exists at such interface an array of charged particles of ions and oriented or polarized dipoles. The resulting disposition of charges on the surface of the electron conductor at the interface and the adjacent array of ions and dipoles in the ion conductor has been termed the double layer. While this concept is theoretical and perhaps an over-simplification of a more complex arrangement of charged ions and dipoles, it nevertheless will diagrammatically serve as an exemplary basis for describing the method and apparatus of the present invention.

In the environment set forth above, the electron conductor may take the form of an electrode, for example a porous carbon electrode, and the ion conductor may take the form of a polar solution, for example salt water or brine to form the internal circuit. Under the application of a suitable external electical potential, the electrode may be made to possess either a net positive or a net negative charge, or no net charge at all. The maximum potential at which no net charge exists may be referred to as the potential of the electrocapillary maximum which varies with the electrode material and with the nature and concentration of the polar solution. It is readily apparent that the electrode potential relative to that of the eletrocapillary maximum determines whether the charge is positive or negative.

In the environment of an internal circuit embodying an ion conductor in combination with electron conductors, for example an electrochemical apparatus, there is an tion.

applied electrical potential below which no electrical current will flow in the internal circuit. This potential corresponds to the potential at which the polar solute will decompose or otherwise promote discharge of ihe solute ions at the respective interfaces. Hence, this potential will be hereinafter referred to as the decomposition potential for the particular polar solute. The thermodynamics of such circuit are such that the decomposition potential necessarily exceeds the potential of electrocapillary maximum, and in the electrostatic range of potential in between, no current will flow in the internal circuit. It is believed that in this range of potential, the double layer at the interface behaves like a capacitor of relatively high specific capacity, which approximates the condition of a single parallel plate capacitor.

Briefly, then, in accordance with this invention, we have found that operation of such a circuit in the electrostatic range of potential will effectively charge the polar solution sufficiently to enable withdrawal of the polar solvent while leaving the ions of the polar solute electrostatically coupled in the'double layer at the corresponding interface, thereby effectively isolating and removing the polar solute from the polar solution. By sequentially applying a plurality of different charges to the polar solution, a plurality of polar solutes may be sequentially removed. By reversing the charge in the circuit, the isolated polar solute ions may subsequently be discharged into another portion of polar solution to increase the concentration of solute in such portion and the concentrated solution removed to regenerate the electrode surfaces for further operation in the removal of polar solute. This invention is particularly useful in connection with aqueous media and more especially for desalination of sea water and brines to render them potable.

A preferred embodiment of a method and apparatus for carrying out this invention will be hereinafter more particularly described in conjunction with the accom panying drawings of which:

FIG. 1 is a diagrammatic representation of the double layer effect.

FIG. 2 is a diagrammatic and schematic illustration of an apparatus in accordance with the present invention which is especially adapted for the treatmentof sea water to remove salt.

FIG. 3 is a diagrammatic illustration of a multistage apparatus embodying the principles of the present inven- FIG. 4 is a diagrammatic and schematic illustration of single stage apparatus useful in accordance herewith.

With more particular reference to FIG. 1, there is here shown in diagrammatic form a representation of the double layers which are believed to exist at the interfaces of positively and negatively charged electrodes separated by an ion transfer medium. At every interface there is thought to exist an array of charged particles and oriented dipoles which is known as the double layer. This layer rises from the fact that in general the two phases which form the interface are at different electrical potentials. In the present case one phase is the electrode shown as metal in FIG. 1 and the other is the electrolyte. At the negative electrode, the structure is believed to consist of a layer of electrons, a layer of adsorbed ions and a diffuse layer consisting of an ionic atmosphere in which the ions of one side are in excess of their bulk concentration and those of the other side are less than their bulk concentration. This atmosphere falls off rapidly, being generally much less than angstroms in depth. Also-at the interface, there may be a neutral molecule which may or may not be oriented. As illustrated in FIG. 1, the electrodes have an excess of positive and negative charges, respectively. When the electrodes have equal amounts of both positive and negative charges, they have no'net charge. The maximum electrode potential at which theelectrode is charge-free is known as the potential of the electrocapillary maximum. This potential varies for different electrode materials and also depends on the nature and concentration of the electrolyte. This potential is of importance because of the electrode potential relative to that of the electrocapillary maximum determines whether positive or negative species are adsorbed, and whether ions or dipoles predominate at the electrode.

As hereinbefore noted, there is a range of electrode potentials. for which a current does not flow across the interface of the double layer. For example, when a platinum electrode is immersed in concentrated hydrochloric acid, this range would vary from the positive electrode potential at which chlorine is evolved and the negative potential at which hydrogen is evolved. At intermediate potentials, no charging or discharging of ions is thermodynamically possible. In this range ionic adsorption occurs as the. double layer is charged. For every electron which flows onto the electrode, a positive ion must be adsorbed from the solution side. The interface is electrically similar to a capacitor of high specific capacity.

Referring now more particularly to. FIG. 2, there is here shown in diagrammatic and schematic form one form of apparatus embodying the principles of the present invention. Since, as indicated above, elements of the apparatus must undergo a charge cycle and a discharge cycle, there is shown in FIG. 2 a pair of; enclosed columns 10 and 11 of spaced electrodes. Columns 10 and 11 may be made, of any convenient material of construction such as hard rubber, plastic, glass or metal. Where an electrical conductor, e.g. a metal is employed, suitable means for insulating the electrodes from each other must be provided. The electrodes are high surface, area carbon blocks which are porous and which serve as, the electron conductors in the apparatus. First alternate electrodes 12 of column 10 are connected by suitable leads 15 to the positive side of a DC. power source 16., Second alternate elec-. trodes 13 of column 10 are connected in series by suitable leads 17 with the second alternate electrodes 13A of colima 1 he first alte nate e ec ro s 12 of column 11 are in turn connected to the negative side of the DC. powe so rce 1 iolu an is P ovided with a suitable inl t 19 f n roducing aqueou ion containin ed m. r ample sea water, into column 10. Inlet 19 is in turn contr ated. to he e which by means, f, a s pa inlet lso eeds col mn. ll. i h an aque us. o con i g dium, e.g. sea water. Column is also provided with a suitab e outle 2 whi h, thr ug su a le p p n leads to either storage means or waste disposal means not forming part-of this invention and therefore not shown. In like manner, column 11 is provided with an outlet 23 which is also adapted to be conducted to waste or storage.

DC Po e s ce 6 isp ov de ith a suita le sw tching means 25 to reverse the polarity; also, provision is made to reverse the effluent from outlets 22 and 23 to orag nd to ast respect y- The apparatus of FIG. 2, or a much simpler version having only two electrodes, may be used in the deionization, or partial deionization, of a wide variety of other simple and complex solutions. Thus, where ion concentration control is important in chemical processes, electrochemical appara-tus of the herein described type may be utilized to provide electrochemical means for controlling ion concentration, e.g. electrochemical buffering. Proper.

balance between ion-saturated and ion-unsaturated electrode pairs in response to a concentration sensing device can remove or supply ions to a polar solution as the situation demands. 4

In FIG. 3 there is shown successive pairs of units such .5, shown in FIG. .2 assembled in series for the sequential selective removal of different polar solutes. Tanks 31 and 32 forming one pair, and tanks 33 and 34 forming a sec- 0nd pair, contain electrodes as shown in FIG. 2. Any number of such pairs may be connected in series, as by a connecting line 35. Direct current sources 36 and 37 individually control tanks 31 and 32, 33 and 34, respectively, and are each set at a potential which will impose a charge on the respective pairs 31 and 32, and 33 and 34, which is preferably slightly below the decomposition charge for the particular polar solute being removed. Source 36 0perates at a potential lower than source 37 since the polar solute having the lowest decomposition potential must be removed first. Suitable piping enables simultaneous charging and discharging of the tanks in each pair.

The advantages of this type of deionization process derive primarily from the low energy requirement for the process due to the fact that energy used during the adsorption stage is mostly recoverable during the desorption stage.

To exemplify the utilization of the apparatus of the present invention, the removal of sodium chloride from sea water is illustrative. Although the theoretical decomposition potential for sodium chloride as determined from the standard oxidation potentials for sodium and chlorine is approximately 4 volts, optimum charge can be attained by applying incrementally increasing potentials to a maximum of about 1.6 volts. A potential of 0.4 volt is above the electrocapillary maximum potential for sodium chloride in sea water adjacent porous carbon electrodes, i.e. at this potential a charge can be built up at the electrode surfaces without a flow of current in the internal circuit. As the charge builds, it requires a greater driving force to effect an increase in charge. The relationship between charge and potential is exponential, and above about 1.6 volts, the increase in charge for increasing voltage is relatively so small as not to warrant the. application of higher voltages.

FIG. 4 schematically and diagrammatically illustrates an exemplary power supply for use in connection with charging the columns of the present invention. Since it is uneconomical to apply a voltage during the entire charging cycle which is substantially greater than the charge built up in the double layer, it has been found expedient to charge to a lower voltage, than increase the voltage by steps until maximum charge has been secured. The switching arrangement enables stepped application of the batteries which may conveniently be rated at approximately (1.4 volt each. To step the voltage up to a maximum of 1.6 volts in 0.4 volt increments, starting with 0.4 volt potential across leads 15 and l7, switches 1, 3, 4, 6, 7 and 9. are closed. When no more current flows in the circuit, additional voltage can be supplied by re-. arranging the switches so that switches 1,3, 4, 6 and 8 are closed. Again when current ceases to flow in the circuit, the level of 1.2 volts may be attained by closing switches 2, 4, 6 and 8. The final step of 1.6 volts is secured by closing switches 2, 5 and 8.

In column 10 (FIG. 4), the double layers are completed by adsorbing anions, e.g. chlorine, at the positive electrodes and cations, e.g. sodium, at the negative elec-. trodes. Salt depleted water exits from the bottom of column 10. After the column is saturated, i.e. when the current approaches zero, the double layer is discharged in series with the external direct current source 16 which simultaneously provides for adsorption and charging of the double layer in the adjacent column (FIG. 2).. Alternatively, instead of discharging into an adjacent column, the charge may be directed to an energy storage device, or for the performance of useful Work, e.g. operation of a pump, by connecting the motor of such pump across the leads 15 and 17 (FIG. 4).

During the discharging cycle, sea water is continuously passed through the column 10 sweeping out of the sodium and chlorine ions which are now no longer held in the double-layer relationship above explained. Thus, the concentration of the polar solute increases in the polar solu- 7 tion during the discharge cycle. After desorption is complete, the aqueous medium in the void spaces in the porous electrodes is washed out with fresh sea water, and the charging cycle is repeated.

The apparatus is useful in combination with other deionization or purification apparatus, for example in the treatment of brackish waters, as a pre-treatment or posttreatment deionizer. In like manner, the apparatus may be used for partial removal of ionized solute followed by other means, e.g. chemical means, to complete the deionization.

The apparatus and principles of this invention are also applicable to the removal of a plurality of different polar solutes from a given polar solution. While natural saline solutions such as sea water contain sodium and chloride ions, they also contain numerous other ions, e.g. calcium, iron, magnesium, aluminum, sulphate, bromide, carbonate, etc. By passing a polar soltuion containing a plurality of ionized solutes serially through a series of apparatus units of the type shown in FIG. 2, each operated at a dif ferent charge level, specific to and below the decomposition charge of the particular solute in question, and in order of increasing decomposition charge for the particular solute in question, and in order of increasing decomposition charge for the particular solute, a plurality of solutes may be separated from the solvent medium. A form of apparatus for accomplishing this is diagrammatically shown in FIG. 2.

The term sal and its counterpart desalination is used herein in its broadest and chemical sense and is not limited to sodium chloride. Thus, any salt which is composed of a metal radical and an acid radical which may be dissolved in water to the maximum of its solubility therein at ordinary temperatures is contemplated hereby.

Other modes of applying the principle of this invention may be employed instead of those specifically set forth above, changes being made as regards the details herein disclosed provided the elements set forth in any of the following claims, or the equivalent of such be employed.

It is, therefore, particularly pointed out and distinctly claimed as the invention:

In a method of removing a polar solute from a solution comprising a polar solvent in which the solute is dissolved and ionized,

the steps of providing at least one first pair of spaced electrodes of porous electrically-conductive material,

moving a first stream of said solution past said electrodes to form a first pair of spaced interfaces at said electrodes,

applying a direct current electrical potential to said first pair of electrodes to electrostatically charge said first interfaces and isolate in the region of one of said first interfaces, ions of positive charge, and to isolate in the region of the other of said first interfaces, ions of negative charge, without current flow between said first interfaces through said first stream of solution,

maintaining said electrostatic potential across said first interfaces in the range between the electrocapillary maximum of said solute at said interfaces and the decomposition potential of said solute at said interfaces,

removing solute depleted solution away from the vicinity of said first interfaces as a recovered 65 product, providing at least one second pair of spaced electrodes of porous, electrically-conductive material, moving a second stream of said solution past said second electrodes to form second spaced interfaces at said second electrodes,

subsequently applying a direct current electrical potential, including the acquired potential from said first interfaces to said second interfaces to electrostatically charge said second inter- 5 faces and isolate in the region of one of said second interfaces, ions of positive charge, and to isolate in the region of the other of said second interfaces, ions of negative charge, without current flow between said second interfaces 10 through said second stream of solution,

regulating said potential across said second interfaces in the range between the electrocapillary maximum of said solute at said second interfaces and the decomposition potential of said solute at said second interfaces,

removing solute depleted solution away from the vicinity of said second interfaces, and repeating the foregoing cycle by discharging said second interfaces to said first interfaces as a portion of a potential applied thereto to again remove ions from a first stream, whereby ions are first attracted to said first interfaces for separation from a portion of said first stream, then the charge accumulated at said first interfaces is moved to said second interfaces to thereafter attract ions to said second interfaces for separation from a portion of said second stream, at the same time discharging said first interfaces to release accumulated ions from said first interfaces back into another portion of said first stream, thus recharging said first interfaces for recycle, and subsequently reversing the process to again accumulate ions at the first interfaces and discharge the second interfaces for regeneration of the second interfaces.

References Cited UNITED STATES PATENTS 1,051,182 1/ 1913 Ackerman 204-27 5 1,831,075 11/1931 Neely 204-302 2,864,750 12/ 1958 Hughes et a1. 204-149 495,242 4/ 1 893 Cabell 204-150 1,326,105 12/1919 Schwerin 204-180 FOREIGN PATENTS 29,049 1912 Great Britain. 232,551 1/ 1961 Australia.

OTHER REFERENCES ROBERT K. MIHALEK, Primary Examiner.

MURRAY TILLMAN, WINSTON A. DOUGLAS,

JOHN H. MACK, Examiners.

J. BATTIST, L. G. WISE, E. ZAGARELLA,

Assistant Examiners.

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