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Chemical Identification with Raman Spectroscopy: Part 2

Raman spectroscopy can be a powerful addition to your research suite. In my last post, I demonstrated how a single spectrum can be used to identify five different compounds—the five TEX compounds in BTEX. Even in a xylene sample containing up to 25% ethylbenzene, it is possible to identify a 1% toluene contaminant.

One of the measurements about which I am frequently asked is the analysis of triazine-based hydrogen sulfide scavengers. Why? Triazine compounds are widely used for H2S scavenging, yet they are not easy to measure, either during production or during their application in the field. The use of these materials is increasing, as the world’s oil supplies are ever-trending towards higher sulfur content. Hydrogen sulfide is a corrosive, hazardous gas. It can build-up in the headspace of a tank, creating a very hazardous situation for workers. It can affect “asset integrity”—in other words, cause corrosion and equipment failures. Simple, effective methods for H2S removal are a requirement for the production, transport, and processing of hydrocarbons.

The compound MEA-triazine (hexahydro-1,3,5-tris(hydroxyethyl)-s-triazine, CAS# 4719-04-4) is a relatively inexpensive, easy-to-use, and effective H2S scavenger. MEA-triazine is produced by mixing monoethanolamine (MEA) with formaldehyde, three molecules to three molecules. The ring forms and water is released. We’ve done this in our lab. Take a drop of MEA, add a drop of formaldehyde, and the solution turns viscous and slightly yellow. Simple, right? Not so fast. The reaction is exothermic. Left uncontrolled, it will generate enough heat to damage the triazine. Plus, formaldehyde is a tricky material. It comes as an aqueous solution and likes to polymerize. Careful thought and control must be put into the reaction and quality chemical supplies.

In the field, MEA-triazine will scavenge two H2S molecules, in the process releasing two MEA molecules and forming an MEA-dithiazine (2-(1,3,5-Dithizinan-5-yl)ethanol, CAS # 88891-55-8) molecule. The MEA can continue scavenging H2S, while the dithiazine will polymerize, form solids, and deposit within the treatment system. The challenge is using the triazine to maximum capacity while not generating enough dithiazine that the system scales.

And this is where Raman spectroscopy comes into play. Not only can we analyze the material as produced for QA/QC applications, but we can also monitor the scavenging process in the field. In the attached figure, I show the spectra for MEA-triazine (black), MMA-triazine (green, CAS# 108-74-7), and MEA-dithiazine (blue). There are a few interesting features to note. We attribute the peak at approximately 920-cm-1 to the triazine ring, present in both triazine molecules. The peak at 870-cm-1 is due to the MEA group on the triazine ring, present in MEA-triazine and in MEA-dithiazine. The peak at 670-cm-1 is due to the sulfur in the dithiazine molecule. Raman enables rapid identification of these compound, while providing structural information that can help identify poor quality materials or the potential for solids formation.

But we take Raman a step further, making it a quantitative method. In an upcoming post, I’ll demonstrate how we can use these spectra to determine the concentration of each compound.