How do I prevent CaSO4 Anhydrite Scale in a CaCl2 vaporization loop? 1

[pjharder]

I have an industrial process with aqueous CaCl2. Water continuously flows in and is flashed out of the system but the CaCl2 inventory does not turn over. Trace sulfur accumulates and winds up as Calcium Sulfate Anhydrite in the 150'c HX. Supersaturation is enevitable and the resulting scale really messes up the Uo.

Does anyone have experience with a similar problem and was able to find a suitable solution, other than mechanical removal of the scale? I have several options I would like to explore and would love to hear ahead of time if some of them will lead me down a dead end path...

- Addition of MgCl2, or other (un)common ions.
- Electronic/magnetic fields. Do these things work? Would they be expected to work when the dissolved solids (as CaCl2) is already ~50%.
- Electrolytic technologies
- Phosphonates or other scale nucleation inhibitors.

Thanks for any help you can give...
PJHarder

 

[kenvlach]

This scaling seems rather inevitable given continuous solids build-up as water evaporates. Also, the solubility of calcium sulfate decreases by more than an order of magnitude when heated to 150 C (see ‘Sulfate removal” thread164-47948).

Re "Magic" water descalers?” thread798-53236. Consenus is a waste of money.

Re common ion. Unless you can alter your process to use NaCl instead of CaCl₂, not much chance. The Ca⁺² has a stronger chemical attraction for the SO₄^-2 anion than other cations (except maybe barium, and possibly not at this T).

Electrolytic descaling should work (same as electrocleaning of metals) for occasional descaling, but long term use in concentrated chloride will lead to HXer dissolution, possibly with pitting. You could use your tank or a graphite electrode to complete the circuit. An earlier thread mentioned some success descaling carbonates from a pump shaft,
‘Removal of Carbonates Depsosition from Pumps by Electrolysis’
thread124-47652

I am not familiar with scale inhibitors for these conditions. Give it a test.

Good luck,
Ken

 

[groags]

We in conjunction with a Chemical company have developed a phosphonate based antiscalent for Anhydrite/Gyspum scale in supersaturated sulphate environment. It works extremely well and we have not had any Ca scale issues since.

I would reccommend an antiscalent of this type for your HX's and piping or vessel walls. We still do have scale build up on pump impellors and we are currently trying to determine why.

However a phosphonate based antiscalent should work well so give it a try.

Ronan

 

[pjharder]

Thanks guys for your pointers. A lot has progressed since I posted the original question. We are about a week away from straping on one of those electronic jobs. I was up-front with the rep and told him that there was a lot of controversy over the 'validity' of the technology. He is letting us do a trial at no cost to us, so I figured what the heck. The worst is that it doesn't work (corrosion was talked about heavily and they assured us that would not be an issue). He is quite confident in not only preventing future scale, but removing buildup that is already there. If it works for us even mildly, that's all we need to know. I'll keep you posted on the results.

Chemical addition is probably our last choice alternative because of the adjoined processes. Phosphorous and other electronic impurities are important to us so unless we strike out completely, it is unlikely we will choose to go that route.

Wide open to other ideas, though.