Portable, Rapid Analysis of MEA-Triazine via Raman Spectroscopy

Topics in Shale Gas
Oral Presentation

Presented by M. Peterman
Prepared by

Contact Information: merwan@ondavia.com; 510-229-9197


Hydrogen sulfide is a naturally-occurring, poisonous, corrosive, and putrid component of natural gas and crude oil. To avoid corrosion and asset risk, it is beneficial and often legally necessary to remove H2S as early as possible before the fuel products are transported or processed. The sulfur content of crude oil supplies has increased steadily, making H2S treatment increasingly necessary. Many H2S treatment methods have been developed for use at the well-head, in transit, or at specialty scrubbing units at the refinery.
There are regenerative and non-regenerative hydrogen sulfide-capturing chemicals, each with different strengths and weaknesses. Regenerative chemicals, such as monoethanolamine (MEA), have the benefit of sulfur recovery for further use, albeit at the cost of larger capital expenditure. Non-regenerative scavengers are more cost-effective if the H2S concentration is below a few hundred parts-per-million (ppm).
One commonly-used, non-regenerative scavenger, 1,3,5-tri-(2-hydroxyethyl)-hexahydro-s-triazine (MEA-triazine), is typically supplied as an aqueous solution in a range of 30- to 80% by mass (wt-%). It reacts stoichiometrically with hydrogen sulfide, is consumed in the process and forms dithiazine. Triazine can scavenge and incorporate other mercaptans present in the fuel stream; however, the concentration of these contaminants is minimal compared to hydrogen sulfide. Dithiazine is only slightly soluble in water, up to 2-3% by weight. When the dithiazine concentration surpasses the solubility limit in a scavenger solution, it precipitates or polymerizes, producing a difficult to dissolve residue which can lead to production interruptions. These negative outcomes can be avoided by limiting the extent of MEA-triazine consumption.
There are a few monitoring technologies to determine the remaining concentration of triazine-based scavengers. Measuring total amine content via the Kjeldahl method can give an approximation of remaining triazine in solution; however, the process involves multiple steps including digestion, distillation, and titration as well as a priori knowledge of the amine structure. Furthermore, the presence of free or complexed MEA molecules can obscure the true value and complicate the procedure. GC-MS is complicated by the fact that MEA-triazine and its reaction products are not stable throughout the analysis process. These molecules must be derivitized before measurement, lengthening the analysis protocol and introducing a dependence on the intermediate chemical transformation in a variety of background solvents and potential interfering compounds. Proton NMR has been used to study the reaction dynamics of laboratory-scale triazine reactions but this type of study is only useful for developing models for the reaction and its products. Field asymmetric ion mobility spectroscopy has been demonstrated to analyze triazine solutions in the ppm-range but this process can be difficult to accomplish, evidenced by its lack of widespread adoption in the industry.
We describe a Raman-spectroscopy-based method for repeatable triazine concentration monitoring from fresh to fully-spent scavenger. We suggest a technique for optimizing operational efficiency while minimizing both chemical consumption and downtime for hydrogen sulfide abatement in triazine-based scavenger applications. In this research note, we explain and explore the development of an all-optical and simple-to-use Raman spectroscopic method for the analysis of MEA-triazine in less than five minutes.
Raman spectroscopy is analogous to infrared spectroscopy in that it monitors the vibrations of chemical bonds. Each set or combination of bonds has a distinct spectrum, providing a chemical ‘fingerprint’ which uniquely identifies the unknown chemical. The intensity of the observed spectrum is linearly related to the concentration in solution. We use this relationship to produce a fast, laboratory-quality test for MEA-triazine quantification in both fresh and spent scavenger solutions. In recent years, due to the miniaturization of lasers, electronics, and microprocessors, Raman spectrometers have decreased in size, and cost, thereby increasing portability and convenience. This research note details the application of cutting-edge Raman techniques to create a rapid and accurate MEA-triazine monitoring system.