Hypo towers, corrosion of PVC liners. Corrosion mechanism? 1

[pita]

We operate a continuous sodium hypo chlorite manufacturing plant. Caustic is circulated over a packed tower which is mounted directly to top of the recirc tank . Both vessels are PVC lined FRP. The PVC liner has suffered extensive corrosion after only 13 mnths operation. Corrosion on this duty is usual but 5 to 8 yr life is the norm. <br>Corrosion is generally in form of close spaced vertical grooving around the inside of the tank – above the main liquid level (there is no corrosion below liquid), similar but deeper than hydrogen grooving of carbon steel acid tanks. Preferential attack of welds is evident.<br>We believe the problem is associated with overchlorination, followed by decomposition, of the recirculating hypo/excess caustic solution. <br>We are trying to identify the corrosion mechanism: is it from heat of decomp and/or corrosive&nbsp;&nbsp;products formed after decomp? Can anyone help please?<br>

 

We are having very good experience with our Hypo scrubbers and tanks, which have had no problems for past 20 years. We ensure that the operating temp. never exceeds 30 deg C, the Chlorine content is limited to 120 gms/litre by a continuous on line REDOX analyser which gives alarms if values go high, a minimum of 0.8 % Caustic lye is ensured in the hypo product. The material used is FRP-VINYL ESTER in the reaction tanks (with 18% caustic lye) and PVC-FRP in the scrubber and Chlorine drying tower ( with 98% Sulphuric acid) . All materials have been reliable and never needed replacement. I am willing to share more experiences

nattu

 

[processeng]

Pita:

I can't specifically identify the corrosion mechanism for your problem but maybe this is of interest.

When caustic is available the following reaction takes place:

Cl2 + 2 NaOH = NaOCl + NaCl + H2O

once all the caustic is consumed or because of localized chlorine excesses relative to caustic the following takes place:

Cl2 + H2O = HOCl + HCl
2HOCl + NaOCl = NaClO3 + 2HCl
2HCl + NaOCl = H2O + Cl2 + NaCl

so somewhere in those products and reactants is the culprit.

Given that it is probably very difficult to identify which component is actually causing the grooves it is probably better to prevent overchlorination in the first place. You can do this in several ways (also hypo overchlorination contributes to product decomposition so by preventing it you'll improve product quality).

A higher caustic excess in the hypo solution will help prevent it but this is often dictated by the hypo product spec so the only way to increase it is by increasing the tower circulation rate (same concentration but higher flow so effectively providing more caustic relative to the chlorine flow). Check for flooding!

Alternately, keep the chlorine from contacting the liquid running down the tank walls by use of an insert in the gas inlet nozzle.

And better yet, keep all liquid off the tank walls altogether and then you'll give your PVC its best chance at a long and happy life. If the tank and tower are the same diameter consider some wall wipers installed below the packing support. If not, an extension piece to the tower that goes into the tank.

One thing to be aware of is that there are different grades or qualities of PVC available. Some of it is crap (filled with recycled plastic and low quality binders etc) and it is probably this quality material that got installed in your tower. I have successfully used PVC/FRP for 8 to 10 years and as you say it can be done but probably takes the right quality PVC.