This invention relates generally to electrodialysis and, more particularly to electrodialysis employing high brine concentrations.
Electrodialysis is a membrane separation technology in which stacked pairs of selective cationic and anion selective membranes are typically used to segregate increasingly dilute salt streams from concentrated salt streams. Stacks of membrane pairs can be very large and can include 10 to 100 or more pairs of alternating membranes. At one end of the stack, electrochemical reactions are produced by a cathode in electrolyte solution. At the other end of the stack, another reaction is created by an anode in electrolyte solution. In the usual process, the electrolyte stream is separated from the dilute salt and the concentrated salt flows. The electrolyte solution is continuously applied to the electrodes. Furthermore, care is taken to collect the electrolyte streams from the anode and cathode reaction zones and remix the electrolyte prior to reapplication to the electrodes.
As will be appreciated by those skilled in the art, the general purpose of keeping solution flowing past the electrodes is to minimize the build-up of hydroxide ion at the cathode by virtue of acid destruction, and the build-up of excess acid at the anode by virtue of hydroxide destruction. Remixing the two electrolyte streams has the purpose of blend neutralizing the acid and base conditions that naturally occur.
The reactions at the anode and the cathode are integral to the process. It is the transfer of the electrons in the half-cell reactions that cause continued polarization that in-turn, causes the flux of salt ions across the selective membranes. Without initiation of the electrolysis reaction, electrodialysis cannot proceed.
In practice, there are several junctions within conventional electrodialysis processing that will or can impede efficient ion transfer. These include:
1) Boundary diffusion resistance at all membrane surfaces;
2) Resistance to current transmittal in all solutions;
3) Relative difference in dilute and concentrate streams concentrations;
4) Potential drop at each membrane; and
5) Reaction initiation and transfer of ions at the electrodes.
When electrodialysis is used in a conventional manner to treat light brine (e.g., brine that in general contains less than 1% salt and in some cases salt in a relative amount of as few as few hundred parts per million, i.e., high voltage and low amperage), one must be careful to avoid what is referred to as “limiting current” conditions. Limiting current is caused by applying too high a voltage such that instead of salt ion transfer, water at or near the membrane is caused to “split” into hydrogen ion and hydroxide ion. This can cause transfer of hydrogen ions and hydroxide ions across the respective membrane instead of the beneficial transfer of salt ions. Thus, amperage is generated by transfer of non-beneficial ions caused by the water splitting. This condition can be destructive to the membranes for a variety of reasons.
However, when electrodialysis is used or applied at conditions to treat heavy brine or high salt concentrations (e.g., brine containing 1-8% salt down to 0.5% salt; i.e., low voltage and high amperage) such operation is very far from the “limiting current” conditions that pertain to conventional operation. Thus, such operation presents challenges and issues not addressed by conventional practice or operation.
An electrodialysis unit is typically composed of two fundamental components: electrodes cells consisting of an anode and a cathode; and a membrane stack of hydraulically isolated concentrate and diluate streams (the treatment cells) disposed therebetween. The electrode cells and the membrane stack are hydraulically isolated, but intimately associated electrochemically. Simplistically, the process can be viewed as an electric potential; pitted against a series of resistances; that generates an electric current at the electrodes. The electric current is translated into ion flux across the membrane stack. A partial list of the process dynamic and equilibrium resistances that can cause loss of potential (voltage drop) are summarized in the following Table 1, below.
TABLE 1CathodeElectrode potential (Eo) for H+ reduction to H2CathodeDiffusion of H+ to the electrodeCathodeOvervoltage (unknown excess above Eo)/watersplitting at electrodeCathodeBack diffusion of anion from the electrode cellCathode/MembraneDiffusion of cation into electrode cellStackMembrane StackDiffusion of cation across the cation selectivediluate/concentratemembrane/Back diffusion of counter ionMembrane StackDiffusion of anion across the anion selectivediluate/concentratemembrane/Back diffusion of counter ionMembrane StackResistance of membraneDiluate/ConcentrateOsmotic pressure as a Nernst PotentialAnodeElectrode potential (Eo) for H2O/OH−oxidation to O2AnodeDiffusion of OH− to the electrodeAnodeOvervoltage (unknown excess above Eo)/watersplitting at electrodeAnodeBack diffusion of cation from the electrode cellAnode/Membrane StackDiffusion of cation into membrane stack
Electrodialysis processing is conventionally operated in a salt concentration range of a few hundred ppm up to about 1% (10,000 ppm) total dissolved solids (TDS). Furthermore, conventional operation typically avoids cases where the divalent cation content of the water exceeds more than a few hundred milligrams per liter. Under such conditions, ion flux is usually limited by the “limiting current” at the membrane interfaces within the membrane stack. The voltage across the electrodes can be adjusted in order to avoid the limiting current condition. The limiting current is usually defined as a severe depletion of ions at the membrane surfaces such that the water boundary at the membrane “polarizes”. Effectively, the driving force to move ions (the stack voltage) exceeds the diffusion rate of ions to (and from) the membrane. Under polarized conditions, water “splits” into hydrogen ion and hydroxide ion. The ionized water is transported in lieu of the target ions. Thus, at this limiting current condition, power is wasted by transferring hydrogen ion instead of the target cation (example: sodium ion) and/or hydroxide ion instead of the target anion (example: chloride). The polarized water also can cause rapid degradation of the membrane materials. Therefore, under these conventional salt ranges, the majority of development in the science of ED technology is driven by the prediction and avoidance of the polarization of water at the membrane surfaces.
In such conventional processing, the chemistry of the electrolyte is relatively simple. For example, a typical electrolyte solution contains about 30 g/L Na2SO4 and has a conductivity of about 28-30 mS/cm. This is usually sufficient since the water to be treated, 1% TDS as NaCl or less, has a conductivity of less than 16 mS/cm.
A steady flux of ions across the membrane stack balances the electrochemical reaction at the electrodes. Hydrogen ions are reduced to hydrogen gas at the cathode. The loss of H+ in the catholyte causes an ion imbalance that requires the transport of a cation (e.g., sodium ion) from the membrane stack into the cathode cell. Oxygen is produced at the anode at the expense of hydroxide ion. The resultant loss of hydroxide by such oxidation necessitates the transport of a cation (e.g. sodium ion) away from the anolyte into the membrane stack in order to appease the loss of hydroxide at the cathode. The result is that the catholyte becomes basic and sodium rich while the anolyte becomes acidic and sodium poor. In order to maintain pH and sodium balance between the catholyte and the anolyte, it is common (after suitable degassing) to re-mix catholyte and anolyte to make a common electrolyte for re-application to the electrodes.
However, the application of electrodialysis to high brine conditions, i.e., 0.5% to 8% TDS, presents many and various challenges. For example, with the application of electrodialysis to high brine conditions the opportunities to create a “limiting current” condition at the membrane surfaces are typically severely limited as the ionic strength of the solution is so high.
Moreover, in highly concentrated brines containing significant concentrations of multivalent cations, such as magnesium, calcium, barium, and iron, the cations can cross from the water (usually from the diluate cell located adjacent to the cathode cell) into the cathode cell. Precipitation of calcium and barium as hydroxides or sulfates, or the precipitation of iron as a hydroxide at the electrodes is problematic and can result in a rapid increase in resistance to ion flux at the surface of the electrode. In practice this is observed as a decrease in current (amps) for a given voltage. In conventional electrodialysis, the presence of such multivalent cation ions cause a need for frequent cleaning of the electrodes. Conventional cleaning usually involves alternating flushes of both the electrode cells and the treatment cells with a strong acid (e.g. 1% HCl) followed by strong base (e.g., 1% NaOH). Conventional washing typically requires significant down time. An additional disadvantage of conventional cleaning is the need for handling and storage of strong chemical cleaning agents.
Further, while pole reversal has previously been applied during conventional electrodialysis, such conventional practice has typically involved switching the polarity of the electrode for extended periods of time, up to several hours per switch. In order to accommodate such pole reversal, there must also be a hydraulic shift that switches the concentrate stream to become the diluate stream, and vice-versa.
In view of the above, there is a need and a demand for improved electrodialysis processing and, more particularly to improved electrodialysis processing when using or in conjunction with the employment of high brine concentrations.