Friday, September 09, 2011

Common Anion Resin Foulants

In a previous blog, we discussed common cation resin foulants. Here are some typical issues that we see with anion resin.

Surface waters, such as rivers, streams, reservoirs, lakes, etc, can contain hundreds of PPM of natural and man-made organic matter. Natural organics are commonly formed from decaying vegetation. This decaying vegetation includes tannins, tannic acid, humic acid, and fulvic acid. Organics block strong base sites, which reduces the resin’s Salt Splitting Capacity (SSC). SSC is a major component of the resin capacity. It is a measure of the number of sites acting as a strong base on the anion resin, which acts together with strong acid sites to perform mix-bed polishing. As these sites become blocked, this reduces the anion resin’s capacity. Organic fouling of anion resin is noticed by the tea-colored to dark brown color of the water flowing from the outlet of an anion unit during regeneration.

Precipitated iron, also known as ferric iron, or clear water iron (ferrous iron) that becomes oxidized into small solid iron particles can coat ion exchange resin, both Anion and Cation resin, reducing the resin’s ion exchange capacity. Not only does iron coat the surface of the resin, but it can also penetrate within the resin bead. Clear water iron (ferrous iron) can be removed from strong acid cation resin through a softener; however, if clear water iron comes in contact with oxidizing agents, such as air/oxygen or chlorine, ferrous iron turns to ferric iron and this will clog resin beads, decreasing their capacity and prevent ion exchange.

Aluminum Sulfate, also known as alum, is used in water treatment to remove turbidity and suspended solids, causing these smaller particles to stick together to form larger particles, so they can be removed by filtration. Typically, Aluminum can be found in water due to the carryover of using Alum. Aluminum can precipitate into a jelly-like substance and coat resin, which will make the exchange sites inaccessible and reduce the capacity.

Silica Fouling is commonly caused by improper regeneration in which the regenerate temperature is too low, or if the caustic used to regenerate contains too much silica. At low pH levels, silica can turn into colloidal silicic acid, which causes shorter runs and poor quality on anion resin beds.

These substances can coat the resin causing short service cycles and poor product water quality. Dirt particles and broken resin beds will stick to oil and grease which also causes channeling of the resin bed. Oil fouling typically occurs from leaks in oil-lubricated pumps.

If you notice a loss in capacity of your anion vessel, it would be best to take a sample of the resin and have it tested to investigate any potential problems. To learn the best way to sample your resin, please check out this previous blog post.

No comments: