Page 375 -

ION EXCHANGE APPLICATIONS IN WATER TREATMENT '! 2.33

The service cycle times can normally vary from a minimum of approximately 8 h to sev-
eral days, depending on the process, the volume of resin used, and the mix and total con-
centration of ions in the feed water. Systems that cycle more frequently are generally less
reliable. On the other hand, systems that regenerate infrequently can encounter difficul-
ties due to ions migrating deeply onto the resin beads, foulants that harden into the resin
over time, and bacterial growth that occurs in stagnant resin beds.
For the purpose of roughing out a system, a reasonable starting point is to use a flow
rate of 10 gpm/ft 3 and a resin bed depth of 3 ft. The final resin volume should be fairly
close to the first estimate. Most ion exchange processes have very fast kinetics, and the
space velocity (or gallons per minute per cubic foot) is not normally an important factor
in determining the capacity or efficiency of the ion exchange bed. However, flow rates
in excess of 5 gprrdft 3 may lead to linear flow rates greater than 15 gpm/ft 3.
Resin bed depths of less than 24 in. are generally not recommended, primarily to avoid
inefficient operation. Even though the theoretical height of the ion exchange zone is usu-
ally less than 6 in., the "actual" height of the exchange zone is distorted by imperfections
in the distribution and collection systems within the vessel. A considerable fraction of the
total resin bed may be lost to these imperfections. Also, between 2 and 4 in. of the total
bed is usually lost simply because it lies beneath the bottom distributions. This is a sig-
nificant fraction of the total resin volume when the bed depth is less than 24 in. Bed depths
greater than 6 ft are generally avoided because of concerns of exceeding the previously
mentioned pressure drop limitations of the resin.

BEGENEBA TION METHODS

There are several different methods for regenerating resins. The method chosen will largely
determine how efficiently the ion exchange resins will operate and how complex the re-
generation will be. The most commonly employed regeneration method is called coflow
or cocurrent regeneration. The resin is regenerated in the same direction as the service
flow. This method is used in almost all salt cycle exchangers, i.e., softening, dealkaliza-
tion, etc. The usual cycle consists of a backwash to purge the resin bed of suspended
solids and resin fines and fragments, followed by chemical injection of a solution of the
regenerant salt, acid, or base through the resin bed, followed by a rinse cycle to flush the
regenerant from the resin bed. Since the regenerant flow is in the same direction as the
service flow, the ions at the top of the bed have to be pushed downward all the way
through the resin bed before they can be purged. This makes the coflow method some-
what inefficient. It also leaves a portion of the exchanged ions remaining in the resin at
the bottom of the bed, where they can cause leakage in subsequent service cycles. The
leakage is highest at the beginning, and as the service cycle progresses, the leakage be-
comes less. The effect is more noticeable at low regenerant dose levels. This leakage phe-
nomenon is very slight in softeners but very noticeable in nitrate removal salt exchange
units. The nitrate leakage at the beginning of the cycle is significantly higher than the
leakage in mid-cycle. The overall leakage can vary by over 2 to 1 from beginning to end
of the service cycle.
Various means of improving the efficiency of cocurrent regeneration can be employed.
One of the least complicated is called thoroughfare regeneration. This is commonly used
in systems that have two or more exchangers in series that are regenerated with the same
regenerant chemical. The dilute regenerant first is passed through the last polishing col-
unto and then is passed through the preceding columns before it flows to waste. The re-
generant chemical is more efficiently used and gives lower and less variable leakage in
the final effluent. Another method of improving efficiency in coflow exchangers is to re-