Ion exchange processing can be accomplished by either a batch method or a column method. In the first method, the resin and solution are mixed in a batch tank, the exchange is allowed to come to equilibrium, then the resin is separated from solution. The degree to which the exchange takes place is limited by the preference the resin exhibits for the ion in solution. Consequently, the use of the resins exchange capacity will be limited unless the selectivity for the ion in solution is far greater than for the exchangeable ion attached to the resin. Because batch regeneration of the resin is chemically inefficient, batch processing by ion exchange has limited potential for application.
Passing a solution through a column containing a bed of exchange resin is analogous to treating the solution in an infinite series of batch tanks. Consider a series of tanks each containing 1 equivalent (eq) of resin in the X ion form . A volume of solution containing 1 eq of Y ions is charged into the fust tank. Assuming the resin to have an equal preference for ions X and Y, when equilibrium is reached the solution phase will contain 0.5 eq of X and Y. Similarly, the resin phase will contain 0.5 eq of X and Y. This separation is the equivalent of that achieved in a batch process.
If the solution were removed from Tank 1 and added to Tank 2, which also contained 1 eq of resin in the X ion form, the solution and resin phase would both contain 0.25 eq of Y ion and 0.75 eq of X ion. Repeating the procedure in a third and fourth tank would reduce the solution content of Y ions to 0.125 and 0.0625 eq. respectively. Despite an unfavorable resin preference. using a sufficient number of stages could reduce the concentration of Y ions in solution to any level desired. This analysis simplifies the column technique, but it does provide insights into the process dynamics. Separations are possible despite poor selectivity for the ion being removed. Most industrial applications of ion exchange use fixed-bed column systems, the basic component of which is the resin column (Figure 5). The column design must:
• Contain and support the ion exchange resin
• Uniformly distribute the service and regeneration flow through the resin bed
• Provide space to fluidize the resin during backwash
• Include the piping, valves, and instruments needed to regulate flow of feed, regenerant. and backwash solutions
After the feed solution is processed to the extent that the resin becomes exhausted and cannot accomplish any further ion exchange, the resin must be regenerated. In normal column operation, for a cation system being converted first to the hydrogen then to the sodium form, regeneration employs the following basic steps:
• The column is backwashed to remove suspended solids collected by the bed during the service cycle and to eliminate channels that may have formed during this cycle. The back- wash flow fluidizes the bed, releases trapped particles, and reorients the resin particles according to size.
Figure 5. Ion exchange unit.
During backwash the larger, denser panicles will accumulate at the base and the particle size will decrease moving up the column. This distribution yields a good hydraulic flow pattern and resistance to fouling by
• The resin bed is brought in con- tact with the regenerant solution. In the case of the cation resin, acid elutes the collected ions and converts the bed to the hydrogen form. A slow water rinse then removes any residual acid.
• The bed is brought in contact with a sodium hydroxide solution to convert the resin to the sodium form. Again, a slow water rinse is used to remove residual caustic. The slow rinse pushes the last of the regenerant through
• The resin bed is subjected to a fast rinse that removes the last traces of the regenerant solution and ensures good flow characteristics.
For resins that experience significant swelling or shrinkage during regeneration, a second backwash should be performed after regeneration to eliminate channeling or resin compression. Regeneration of a fixed-bed column usually requires between 1 and 2 hr. Frequency depends on the volume of resin in the exchange columns and the quantity of heavy metals and other ionized compounds in the wastewater. Resin capacity is usually expressed in terms of equivalents per liter (eq/L) of resin. An equivalent is the molecular weight in grams of the compound divided by its electrical charge, or valence. For example, a resin with an exchange capacity of 1 eq/L could remove 37.5 g of divalent zinc (Zn+2, molecular weight of 65) from solution. Much of the experience with ion exchange has been in the field of water softening: therefore, capacities will frequently be expressed in terms of kilograms of calcium carbonate per cubic foot of resin. This unit can be converted to equivalents per liter by multiplying by 0.0458. Typical capacities for commercially available cation and anion resins are shown in Figure 4. The capacities are strongly influenced by the quantity of acid or base used to regenerate the resin. Weak acid and weak base systems are more efficiently regenerated; their capacity increases almost linearly with regenerant dose. Columns are designed to use either cocurrent or countercurrent regeneration. In cocurrent units, both feed and regenerant solutions make contact with the resin in a downflow mode. These units are the less expensive of the two in terms of initial equipment cost. On the other hand, cocurrent flow uses regenerant chemicals less efficiently than countercurrent flow: it has higher leakage concentrations (the concentration of the feed solution ion being removed in the column effluent), and cannot achieve as high a product concentration in the regenerant. Efficient use of regenerant chemicals is primarily a concern with strong acid or strong base resins. The weakly ionized resins require only slightly greater than stoichiometric chemical doses for complete regeneration regardless of whether cocurrent or countercurrent flow is used. With strong acid or strong base resin systems, improved chemical efficiency can be achieved by reusing a part of the spent regenerants. In strongly ionized resin systems, the degreeof column regeneration is the major factor in determining the chemical efficiency of the regeneration process. To realize 42 percent of the resin's theoretical exchange capacity requires 1.4 times the stoichiometric amount of reagent [2 lb HCl/ft3 (32 g HCI/L)]. To increase the exchange capacity available to 60 percent of theoretical increases consumption to 2.45 times the stoichiometric dose [5 lb HCl/ft3 (80 g HCI/Liter)]. The need for acid doses considerably higher than stoichiometric means that there is a significant concentration of acid in the spent regenerant. Further, as the acid dose is increased incrementally, the concentration of acid in the spent regenerant increases. By discarding only the first part of the spent regenerant and saving and reusing the rest, greater exchange capacity can be realized with equal levels of regenerant consumption. For example, if a regenerant dose of 5 lb HCl/ft3 (80 g HCI/Liter) were used in the resin system, the first 50 percent of spent regenerant would contain only 29 percent of the original acid concentration. The rest of the acid regenerant would contain 78 percent of the original acid concentration. If this second part of the regenerant is reused in the next regeneration cycle before the resin bed makes contact with 5 Ib/ft3 (80 g/Liter) of fresh HCl, the exchange capacity would increase to 67 percent of theoretical capacity. The available capacity would then increase from 60 to 67 percent at equal chemical doses. Figure 5 shows the improved reagent utilization achieved by this manner of reuse over a range of regenerant doses. Regenerant reuse has disadvantages in that it is higher in initial cost for chemical storage and feed systems and regeneration procedure is more complicated. Still, where the chemical savings have provided justification, systems have been designed to reuse parts of the spent regenerant as many as five times before discarding them.
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