[curiouscat]

Would 5% HNO3 (aq.) react with Na2SO4 (s) to liberate H2SO4?

Context: One typical CIP (Cleaning in place) procedure for Stainless Steel process equipment involves successive treatments with distilled water, 10% NaOH and 5% HNO3, all reagents heated to approx. 90 C and 2 hour exposure to circulating reagent in each case.

Normally none of these reagents damages Stainless Steel under the given conditions.

But in one specific process there is likely to be Sodium Sulfate present. dil. H2SO4 can be corrosive to Stainless Steel especially at high temperature.

So I am wondering if there is any risk in using the standard CIP procedure in this case.

Any comments?

[Arkcon]

Generally, CIP protocols are written to avoid such occurrences.  Briefly, there should be enough of a water rinse to be sure the sodium sulfate is mostly gone.  As an example, you might be asked to measure the conductivity of the output di water, and not continue unless it is indistinguishable from the input di water.  I would suspect there is a water rinse between the NaOH and the nitric, although there doesn't have to be.  Again, you monitor the output of the rinsate (say in this case, by pH)  to be sure you're ready for the next step.  Or indeed, after the nitric, to be sure its rinsed away from the equipment and be sure its ready to use.

[curiouscat]

Thanks @Arkon.

Couple of points:

The CIP procedure I quoted was vendor provided. It is generic & hence I was wondering what might be the required modifications for our specific process. Hence the question.

Yes indeed there's a water rinse in between the NaOH & the nitric.

The problem is that this equipment being an evaporator does get extensive solid deposition during normal operations. So getting it clean to the point where the rinse-ate is identical in quality (say conductivity) to the input DI water is  practically impossible. Even if I flushed it for 24 hours all the deposits rarely come off.

To clarify, unlike a food or pharma CIP the goal here is not to get the equipment scrupulously clean to avoid contamination, bacterial growth or a safety hazard. The goal is to get as much deposit of the solids off to enhance heat transfer and to prevent physical choking, corrosion etc.

It is in that context that I am wondering if accidental H2SO4 formation would be something to worry about.

[DrCMS]

If you just need to clean off the solid then use a good solvent for the solid and pump it round for a fixed time before switching to fresh solvent and repeating until the amount dissolved in the solvent is below an acceptable level.

[curiouscat]

If you just need to clean off the solid then use a good solvent for the solid and pump it round for a fixed time before switching to fresh solvent and repeating until the amount dissolved in the solvent is below an acceptable level.

Water is a good solvent for Na2SO4. If there's a better option I'd love to hear.

[DrCMS]

If the solids you want to remove are only sodium sulphate then your earlier comment that you could wash it with water for 24hrs and still not remove all the deposits seems odd. 

Have you tried using something like a polyacrylate dispersant or similar antiscalant?

[curiouscat]

If the solids you want to remove are only sodium sulphate then your earlier comment that you could wash it with water for 24hrs and still not remove all the deposits seems odd. 

Have you tried using something like a polyacrylate dispersant or similar antiscalant?

Agreed. I think you are right. There must be other solids. Basically even traces or undetectable quantities of other solids would tend to concentrate especially if they are liable to deposit out.

In general, I'd expect the same scaling solids that would occur in a typical water application or an older boiler using unsoftened water. Reason being that the upstream process that produces this Na2SO4 solution is being fed normal water.

That must be the reason the CIP recommends sequential washes with strong base and acid. Which is totally fine except I am just being cautious about the side product H2SO4 generation. It could be innocuous if the build up in conc. would be small. But maybe not.

Excellent point about the dispersant. I will try those.