Triazine-based H₂S scavengers are simple in principle: one triazine molecule scavenges two hydrogen sulfide molecules, passing from MEA-triazine to MEA-thiadiazine to MEA-dithiazine. (There are some who claim it scavenges three sulfides, but this reaction does not occur in real-world situations.) The operating question of “how much scavenging capacity remains?” is often translated into “how much triazine remains?”. But this question does not consider the deeper details about the scavenging reaction. The real question should be: how much dithiazine is present and when will it form solids?
The complete scavenging reaction is shown above. At the first step, MEA-triazine takes one H₂S, releasing one MEA in the process while creating MEA- thiadiazine. In the second step, a second H₂S molecule interacts with the MEA-thiadiazine to produce MEA-dithiazine, also releasing an MEA molecule.
The literature says that the MEA-thiadiazine molecule is short-lived; others claim it has never been observed via chromatographic methods. These claims lead to a belief that there is no thiadiazine in a scavenging solution—that there is only triazine and dithiazine.
But the experimental conditions in the literature do not mirror what happens in the real world. In a 1000-gallon tower, the odds that two H₂S molecules will quickly find one triazine so that the reaction proceeds immediately through both steps is highly unlikely. In fact, if a tower is well-mixed, the tower will at some point have a few-%-level concentration of thiadiazine—itself an active H₂S scavenger.
This process is exactly what we observe with Raman spectroscopy. During a scavenging reaction, we observe the triazine peak disappear relatively quickly, while the dithiazine peak does not immediately grow in a proportional manner. The difference between the two substances is thiadiazine. Even more powerful, we measure the MEA concentration directly. Over the course of this reaction, the MEA signal changes little—the MEA is conserved as would be expected.
What does this mean for your scavenging process? Well, it means that a measurement of triazine does not tell you residual scavenging capacity. Thiadiazine is an excellent sulfide scavenger! We suggest that monitoring the increasing dithiazine concentration is the right approach. Not only is the dithiazine a strong and increasing signal, but also it is the “problem” compound that causes scale.
Want to monitor your scavenging reaction properly? Drop us a note. We have the tools to measure triazine, dithiazine, and thiadiazine with one quick, simple test.
