Operating methods
There are three basic operating methods for ion exchangers. They consist of batch, column and
moving bed operations. Following is an introduction to each type of system.
- Batch operation consists of dumping resin and solution into a tank, mixing and
allowing for equilibrium. Solution is filtered off. Resin is then prepared for the next
cycle. This process is limited by the selctivity of the resin under equlibrium. The
total capacity used is due to selectivity.
- Column operation is like several batch reactors in series. It is carried out in a
row of containers. The purpose of column operation is to work around the limitation of selectivity. H
It is realized that the
improvement in each stage is less than that obtained in preceding one. It can be seen with an
unfavorable selectivity there is a need for more stages (law of diminishing returns).
Column operation is analogous to a distallation column and the same techniques can be applied to
analize the process.
- Three types of column operations are down flow, up flow and counter flow. Most beds
operate with down flow operation. This is where feed and resin pass down through the resin bed.
On the contrary, upflow operation is when the feed and resin are raised throught a bed.
The final flow is counter flow and it consists of the feed flowing down from the top
and the regenerate flows up from the bottom.
- Moving beds are the most economical beds of the three. The principle is resin and solution flow
through the system. The resin is contacted counter-currently with the exhausting stream and
regenerated stream. The physical chemistry is similar to fixed bed and can also be thought similarly as a
distillation column. The advantages of operation is there is a continuous product of
uniform quality at less space, capital and labor. The problem is a complexity of
the design problemf for a operating system.
Mixbed Ion Exchange
An anionic and a cationic resin together can remove all ions from water. If one is charged
with H+ and the other with OH-, they drive the equilibria. Deionized
water costs less than distilled water and is often a fully satisfactory substitute.
A mixed bed exchanger cannot be regenerated
easily because the reagents for one resin would put the other into an undesirable
form. For example, contact with sodium hydroxide would convert one resin
to the desired hydroxyl form while putting undesired sodium ions on the
other. The resins must be separated so that each may be regenerated. This
is accomplished by designing a process that has resins with different sedimentation
characteristics. If the densities or particle sizes are sufficiently different,
upflow through the bed will classify the resins with the one that settles
more rapidly underneath the other. Careful hydraulic balancing allows one
reagent to suspend and regenerate the lower resin while another reagent
suspends and regenerates the upper resin.
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