This application claims priority from international application number PCT/IB2016/000590, filed Apr. 13, 2016, which claimed benefit from provisional application No. 62/148,059, filed Apr. 15, 2015.
The invention relates to water treatment using mixed bed separators. In particular the invention concerns regeneration of the cation and anion resins in a mixed bed separator with minimal cross-contamination.
Ion-exchange resins are widely used in different separation, purification and decontamination processes. Common examples are water softening and water purification. The resins are often used in a mixed bed containing both anion resins and cation resins in a working vessel through which water is passed for ion exchange to remove contaminants. The use of ion-exchange resins is often an alternative to use of natural or artificial zeolites.
The resins form an insoluble matrix or support structure usually in the form of small beads (approximately 0.5 to 1 mm diameter), fabricated from an appropriate organic polymer such as polystyrene sulfonate. Usually the resin beads are porous so as to provide a high surface area. Trapping of ions on the bead surfaces occurs with the simultaneous release of other ions in the ion exchange process carried out by the ion resin beds.
Anion resins used in ion exchange are strongly or weakly basic, so as to attract anions, which are negatively charged. Cation resins are strongly or weakly acidic, so as to attract cations, which are positively charged.
In an ion exchange process the resin beads must be regenerated periodically, since they become saturated. Regeneration of anion resins typically involves treatment of the resin beads with a strongly basic solution, such as aqueous sodium hydroxide. Regeneration of cation resins is typically done with a strongly acidic solution, such as hydrochloric acid or sulfuric acid. During regeneration the regenerant solution is passed through the resin bead and trapped ions are released, by combining with the regenerant chemical, thereby renewing the resin's exchange capacity.
Mixed bed resins comprise both cation and anion resins intermixed in a working vessel. When regeneration is needed, the cation resin beads must be separated from the anion resin beads, then each separately regenerated with the respective regenerant acidic or alkaline chemical. Known processes for mixed bed regeneration have included transporting the mixed bed resin from the working vessel to a specially dedicated separation column, e.g. a tall column of relatively small diameter which might have a cylinder height of about four meters to contain a mixed resin bed of about two meters (100% free room). In this column, a backflush of water, upward through the bed of mixed media, will cause the less dense anion resin beads to rise above the cation resin beads, thus forming an anion layer separate from and above a cation layer.
Regeneration of the cation and anion resins has then been accomplished by several different processes prior to the current invention. In one process the anion and cation layers have been regenerated simultaneously, with an alkaline solution passed downwardly through the anion layer while at the same time an acid solution was directed upwardly through the cation layer. Both regenerant chemicals, spent from regenerating the respective ion resin layers, were directed out of the column through one or more exit ports at the level of the interface between the two layers. The anion and cation resins were then rinsed and mixed again as the resins were returned to a working vessel. Use of this method has resulted in a significant cross-contamination region of resin above and below the interface or separation plane between the cation and anion resin layers. The cross-contamination made the mixed bed resin less effective in removing contaminants, in the working vessel.
Another prior method has been to separate the anion and cation resins in one column, using a water backflush as discussed above, but then the two layers were separately transferred into two different columns, where the cation and anion resins were regenerated separately, with little cross-contamination (normally no more than about 0.2%). With the withdrawal of the cation and anion layers a mixing zone at and adjacent to the interface was deliberately left in the separation column. The mixing zone typically comprised a region about 20 to 30 cm in height. Once the cation and anion resins were separately regenerated, they were mixed and/or transported back to the working vessel.
In a variation of the three-column system described above, a two-column system has also been used, wherein the anion resin layer was removed prior to regeneration into a second column, then regenerated in the second column, while the cation layer remained in the separation column and was regenerated in that column. After the separate regenerations the two resins were then mixed in the second column and then transported back to the working vessel. Again, a cross-contaminated layer was left in the separation column.
U.S. Pat. No. 4,191,644 describes processes similar to the above.
Single-column regeneration of anion and cation resins is inefficient, with approximately 10% cross-contamination which remains in the resin bed when returned to the working vessel. Regeneration involving two or three separate columns is far more effective in producing cleanly regenerated resins but is considerably more costly. It would be desirable to employ a process that achieves efficient regeneration using only a single column.