Trithiane, the impossible triazine by-product

I have previously shared some thoughts on a forgotten by-product of the triazine-based scavenger reaction, thiadiazine. But when scale forms, there’s another compound that often gets implicated – a compound that cannot be generated by sulfide scavenging – trithiane.

In principle, the scavenging reaction is simple: one triazine molecule scavenges two hydrogen sulfide molecules. The final component of this reaction is a dithiazine species with two monoethanolamine molecules. Here’s the diagram:

Diagram illustrating the triazine reaction as it scavenges hydrogen sulfide in two steps to yield MEA-dithiazine.

Trithiane production is also simple, in theory. Mix three formaldehyde molecules with three hydrogen sulfide molecules. You get:

trithiane
1,3,5-trithiane

But there are those that believe it scavenges three sulfides, creating 1,3,5-trithiane (CAS 291-21-4). Trithiane is not an appreciable by-product of H2S scavenging with triazine. Think you have trithiane deposits? You don’t. This fact has been demonstrated in the academic literature time and time and time again, including recently by Monica Roman et al. in their nice ACS Omega paper.

This reaction is not going to occur to any appreciable extent in a real-world sulfide scavenging application. Hypothetically, if your triazine scavenger was at a low pH (<8), you could have free formaldehyde to react with sulfide. But any %-level triazine will have a much higher pH. It would be far-fetched to find a situation where you are dosing triazine, your pH is low, and your sulfide is high.

Instead, the reaction terminates at dithiazine. As the dithiazine concentration increases, the presence of other species in the scavenging solution causes the dithiazine to polymerize. The resulting solids are dithiazine polymer.

Why blame trithiane?

So, why do I get asked about trithiane, even through all the literature points otherwise? My guess is folks find hard-to-remove deposits in their system that appear as a hard, white precipitate on valve stems and other hardware. These deposits do not dissolve in simple solvents and it is easy to see how one might think this is trithiane.

In reality, these deposits are simply the polymerized form of dithiazine, described as amorphous dithiazine in the literature. Once formed, they are rock hard and impervious to most solvents. You can etch them with concentrated acid. Or you can physically remove them with a sandblaster or jack-hammer.

Solids formation? Use Quantitative Raman Spectroscopy

Years ago, we received the valve stem shown up top in the mail. It was removed from a natural gas pipeline in the US Pacific Northwest. That white material? Analyzed directly via Raman spectroscopy: it’s dithiazine. Dissolved to ppm-levels and measured with Quantitative Raman Spectroscopy: it’s definitely dithiazine!

Your best bet is to avoid these deposits all together. That’s where OndaVia comes in. Our Quantitative Raman Spectroscopy tools not only identify these deposits, but more importantly, allow you to monitor the scavenging reaction to avoid forming them.

Want to take your scavenging operations to a new level? Drop us a note. Our equipment gives you the unique insights that save money and protect your assets.

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