Corrosion of PVC in Hypo/Cl2 duty

[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? Also believe free radicals may be released in the above reactions - do these try to combine with the PVC? Can anyone help please?<br>

 

[BarryMartin]

I would need more information to help. Typically PVC lined FRP is used for a combination of strength and corrosion resistance. The PVC does not have good strength at elevated temperatures. At 140F it needs to be continuously supported.

Typically PVC has reasonably good performance. There are some better materials available however. If you are simply trying to get the most life out of the existing equipment, then controlling the process is key. When you suggest overchlorination, it sounds like control is not good. However, you suggest that you do have excess caustic. As long as the circulation rate is high and the cooling is sufficient, I'd be surprised at a problem unless there was a problem with the materials (i.e fabrication or intital material quality).

 

[suzuki]

WE design,engineer, and fabricate chemical process equipment worldwide including Dual-Lamiante constructions and can tell you that there are several types of PVC that are out in the market place and each one will have different corrosion resistancy, several of our clients are very specific when we fabricate such a tower in PVC/FRP Dual-Laminate constructions.

http://www.viatec.com

 

[ghopkins]

What is your process temperature? Do you know the concentration of free chlorine. There are several papers published by the Swedish Corrosion Institute comparing various thermoplastics - including vinyls, polyolefins, and flouropolymers. Each type of thermoplastic has its place depending on the service temperature, composition of the process constituents, and permeability.

With more information, we can help provide an answer. Normally, PVC is an extremely compatible material at ambient conditions. What is the material of construction of the packing? Has it seen the same degradation of the liner? If not and its a thermoplastic, perhaps you should consider that for the lining material.

Let me know if you need more help or want to handle it off-line.
ghopkins@rl-industries.com