In scavenging applications, MEA-triazine is typically supplied as an aqueous solution in a range of 20% to 80% by mass. It is consumed during its reaction with hydrogen sulfide. One mole of MEA-triazine reacts with two moles of hydrogen sulfide, creating 2-(1,3,5-dithiazinan-5-yl)ethanol (MEA-dithiazine) and two moles of free monoethanolamine (MEA) as primary reaction products. This reaction is illustrated below. The MEA-triazine (TRZ) molecule reacts with one sulfide, creating MEA-thiadiazine (TDZ). The TDZ then reacts with another sulfide, producing MEA-dithiazine (DTZ). The optimization of your chemical usage requires monitoring these chemicals for scavenging process control.
MEA-triazine and MEA-dithiazine provide stable, clear, reproducible Quantitative Raman Spectra, as presented in the figure to the right. The spectra are markedly different for molecules which possess one-third structural similarity. This difference is manifested in the triplet of wide and strong peaks between 800 and 1000 relative wavenumbers (rel. cm-1) for the MEA-triazine spectrum. These peaks are related to the ring breathing and ring-incorporated methylene hydrogen rocking vibrations for the intact triazine molecule.
Substitution by sulfur atoms in the course of hydrogen sulfide scrubbing removes these triazine marker peaks and creates a sharp and strong peak at 670 rel. cm-1 concomitant with a wider feature at 580 rel. cm-1. The peak at 580 rel. cm-1 is in a largely silent region of the MEA-triazine spectrum. The strong peak at 670 rel. cm-1 is assigned to a C-S stretch10—a bond which is only formed after sulfur exposure. The combination of these two peaks is a clear indicator of the presence of dithiazine in the sample solution.
Dithiazine
The Raman spectrum provides a direct measure the dithiazine concentration in an active triazine-based scavenging solution, information that is essential for scavenging process control. Traditionally, an operator might run their scavenger tower until breakthrough. This approach is risky: at high concentrations, dithiazine polymerizes, forming insoluble and difficult to remove deposits. Alternatively, the scavenging system can be operated for a fixed period of time or for a calculated period of time. This approach is full of guesswork and estimates, errors that either lead to solids or waste money through overly frequent chemical changes.
Thiadiazine
The Raman spectrum includes additional information about the scavenging process. The figure above highlights the TRZ and DTZ signals. But it also highlights TDZ, the intermediate reaction component. If you only measure the TRZ concentation, you will miss half the scavenging capacity of TRZ.
The ability to operate a system even 10% longer without fear of solids formation can save tens to hundreds of thousands of dollars per year. Not only is there a chemical cost, but also the time spent by your team handling change-outs. With proper process control using OndaVia technology, you can keep your headcount reasonable and performing value-added tasks. Meanwhile, you reduce chemical usage for a chemical known to fluctuate in cost and availability.