3 replies to this topic

[jounetsu]

I am trying to find data on materials that are compatible with Piranha / Caro's Acid / Peroxymonosulfuric acid. And also those that are not.

It is simply a mixture of concentrated sulphuric acid, and hydrogen peroxide in various ratios. It is typically formed in situ i.e. a batch reactor, as the material is unstable and breaks down quickly.

So far I have gleaned that at low temperatures PVC may be a suitable lining material for a non-resistant metallic tank at lowers temperatures, and one of the hastelloy alloys could be intrinsically resistant. However at the very exothermic nature of the reaction complicated matters, and I cannot be certain at what temperature materials would lose their resistance. Reaction temperatures of over 150°C are not unknown, which I guess would put PVC out of the question.

The only thing I am certain of is that a borosilicate lined glass reactor would probably be okay at all temperatures.

Another aspect I am curious about is the potential for explosion of the material, some sources suggest that peroxide concentrations of 50% or greater will lead to a dangerously unstable mixture.

Any input gratefully received,

Thanks,

Paul

[fseipel]

Piranha is extremely hazardous.   I have used it to clean glassware and glass-lined reactors after incidents that baked material on the surface. Piranha is compatible with teflon and borosilicate glass.  If you make up the solution in the typical 3:1 ratio conc. (98%) sulfuric to H2O2 sol'n, you'll be close to the 75% conc. H2SO4, at which PVC is not acceptable for sulfuric acid alone.  Face shield and long gloves and apron are minimum PPE.  You can try chromic acid as an alternative to piranha but it is carcinogenic.  I'm assuming you're trying to clean a very stubborn deposit with this solution.  Piranha can cause an explosion when it contacts organic residues so doing this on an industrial scale may be extremely hazardous; I've never heard of it being prepared in large tanks.  Is powerwashing an option?  Hot caustic?  Citric acid?  There are other alternative to chromium that aren't carcinogenic; e.g. nochromix for cleaning & that will avoid the exotherm issue.  You are correct, mixing the peroxide and sulfuric is quite exothermic.

[jounetsu]

The scenario I'm considering where pirhana is formed is a potential mis charge of a batch reactor. What I'm really trying to figure out is just how bad it could potentially be!

Since they are proposed to be rubber lined mild steel, I suspect the consequences could be pretty horrendous.

Obviously some level of safety must be built in to avoid the mis charge scenario, but the possible consequences are difficult to determine based on the data I can find.. An accurate understanding of the potential consequences will be a good starting point for determining the requirements for safety integrity levels etc

Thanks for the reply, fseipel.

Regards,

P

[fseipel]

I worked at a plant that had a large organic peroxide explosion.  Having seen the 1/2" thick carbon steel dished heads of a 2000 gallon reactor deformed to a hemispherical shape, I definitely respect peroxides.  The 18" oval manway blew off (large rupture disc there).  Those were organic peroxides as opposed to H2O2.

 

As the tank heats up, hydrogen offgassing may create sufficient backpressure to self-regulate the addition -- i.e. not enough pressure in addition line to overcome internal tank pressure -- depending upon pumping rate, tank PRV set pressure and vent scheme.  Inclusion of an addition safety valve interlocked on temperature may merit consideration.  If tank is not mixed, and addition is slow, the very dense conc. H2SO4 may settle to bottom of tank with H2O2 solution above -- then on mixing, a potentially violent exotherm may occur -- depends on addition arrangement, rate, etc.  Also if tank is/can be equipped with a headspace pressure transmitter, interlocking addition valve on pressure may be a good idea.

 

You may want to take a very small sample of the rubber and a steel washer in a beaker, and slowly add H2O2 to conc H2SO4 to form the piranha.  It is a very strong oxidizer.  If it doesn't outright dissolve the rubber quickly, it will likely begin to do so, with the solution turning black as the rubber is bleached and dissolved.  The metal will be attacked and hydrogen gas will be generated.  Often any residues that are left are white from the bleaching.  Since the piranha will offgas Hydrogen if it attacks metals, you may want to evaluate the possibility of a hydrogen gas explosion, or do the test in a closed container with a flowmeter in vent or bubbler so you can monitor offgas rate.

Edited by fseipel, 19 October 2013 - 08:44 AM.