Sequestrant efficiency

The sequestrant is actually masking the water hardness and the water behaviors like to be softened. For this reason at the beginning of this technology sequestrants were called “internal softeners” and the expected results of the application are exactly the same if we were softening the cooling tower’s water.

• At he heating surfaces

A sequestrant is acting well at the bulk water, but at the heating surface its effectiveness is highly limited. The high skin temperature is providing high kinetic energy to the calcium and anion ions, while the polymeric chain, having a high molecular weight, moves slowly.

The high kinetic energy of calcium and anions permits them to overpass the attraction of the functional groups of the sequestrant or to vibrate in larger radius. So, they can reach its-other, and due to the high scaling potential, to form scales.

So the use of a sequestrant lowers the scaling potential. The sequestrant, acting at the bulk water and not at the place of the scale formation, is not 100% protecting the system. The volume of the scales created is minimized, but not avoided.

• Sequestrant and biofilms.

Sequestrants are biodegradable, but not as easily degraded as threshold inhibitors. Even anionic, is less reactive and deactivated by cationic glycocalyx, like other scale inhibitors.

It is not also easily penetrating the slime masses. This difficulty increases as the calcium hardness increases, means as the number and diameter of surrounding layers increase.

Actually the loss of performances of a sequestrant due to biofilm is mostly related to mechanical reasons and to distance from the heating surface that the sequestrant can approach.

• In Corrosivity

The sequestrant, if overfeed, is by default corrosive, and the corrosion is of the type of electrochemical corrosion.

Blocking the bivalent cations round it, it creates lack of bivalent cations in the water balance.

As to establish again its balance, water tends to dissolve iron , because dissolved iron at its fist stage is bivalent.

• Into a dirty system

The introduction of a sequestrant into a dirty system has the same effects like to start to soften the water.

New scale formation is minimized but not avoided. On the other hand overfeed of sequestrant into the dirty system has the tendency to slightly solubilize existing deposits.

This solubilization is more intensive at the high stress areas of the existing deposits, and can be presented as detachments of the deposits and clogging of the pipes and damages to pumps and accessories.

• When there are side stream softeners.

A sequestrant is competitive to ion exchange resins used for softening. So, softeners cannot function if there is a sequestrant present into the filtered water.

• A sequestrant combined to a Threshold Inhibitor

Such a combination can be very successful, especially into a stable system. It can form a co-operative mixture, in which:

• The sequestrant will:
  • Capture most of the hardness into the bulk water, make it unavailable for reaction. But will not function at the heating surfaces.
  • Redisolve, inactivate and destroys ready formed nuclei of crystallization, while they are traveling into the system.
• The Threshold Inhibitor will function at the scale formation surfaces, increasing the oversaturation limit and has to inhibit less scaling forming ions concentrations.

As to obtain successful inhibition, both have to be fed at their original concentrations. Threshold inhibitor is not effective at all if underfeed, and this is not changed if sequestrant is present. On the other hand the sequestrant efficiency is related to its concentration.

Above combination makes the selection of the cooperative mixture even successful, not cost effective.

In general, the application of a sequestrant “IS NOT A CHEAP APPLICATION”.

select a link to continue or go to the table of contents

[ Up ]