Packed Bed Systems

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For plants with external backwash vessels, resin should be handled manually or with a suitable hydraulic ejector system.

In packed bed systems it is important that the freeboard level (free space between top of resin bed and inert resin or nozzle plate) is minimized to ensure optimal plant performance. This is especially important for strong acid and strong base resins. The freeboard needs to be calculated accurately taking the resin bed height with resin in the most swollen form. Strong acid cation resins are most swollen in the hydrogen form, while strong base anions have maximum volume in the hydroxide form. Weak acid and weak base resins have peak volume in the exhausted form. Determine the required resin volume accordingly.

Fill 1/2 of the backwash vessel with water. Load 1/2 of the resin volume into the vessel. Backwash the resin bed for 15-20 minutes at an expansion of 80-100%  to remove fines and classify the bed. Let the bed settle and drain to 10 cm (4 inches) above top of resin bed. If possible, determine the resin volume. Make sure the service vessel is filled halfway with water, and transfer the resin. Repeat the procedure with the second half of the resin.

Commissioning

Proceed through a normal acid and caustic regeneration procedure. If supplied in the exhausted form, a double or triple regeneration is recommended. This does not mean going through the whole regeneration cycle two or three times, but instead ensuring that double or triple the amount of chemical is applied. This may involve refilling the chemical measure.

New resins should be rinsed to drain separately when regenerated the first time. Only apply recycle mode in subsequent regenerations to prevent contamination of both the resin bed and the treated water with residual organic matter that may be present on newly manufactured resin.

After proceeding through the displacement rinse and rinsing to specificity, check the freeboard levels to ensure the fill levels meet plant design specifications.

An alternative approach for this commissioning is to fill 90-95% of the resin first, carry out a double regeneration and top-up the resin volume to the desired freeboard level. A single regeneration is carried out after the top-up. This procedure enables very low freeboards to be accurately adjusted.

Inert resin can be either added manually to the service vessel before adding the resin, or transferred via the backwash vessel – where possible – after completion of the resin transfer.