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 oil & gas fields has increased steadily, making H2S treatment increasingly necessary. Many H2S treatment methods have been developed for use at the well-head, in transit, or within specialty scrubbing units at the refinery. These treatments include triazine-based sulfide scavengers and amine-based treatment approaches.
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, triazine-based sulfide scavengers are more cost-effective if the H2S concentration is below a few hundred parts-per-million (ppm).
MEA-triazine production
One common, relatively-inexpensive, non-regenerative scavenger is MEA-triazine [1,3,5-tri-(2-hydroxyethyl)-hexahydro-s-triazine]. MEA-triazine is produced by mixing equal molar concentrations of MEA with formaldehyde, illustrated below. Three moles of each compound are consumed, producing one mole of MEA-triazine and three moles of water via an exothermic reaction. If produced using pure MEA and paraformaldehyde, this reaction can theoretically produce a solution that is 80.2% by mass MEA-triazine. More commonly, a concentrated solution of 50% formaldehyde is mixed with MEA to produce a 60% by mass solution.
Although the reaction is conceptually simple, there are many variables in the production process. The quality of reactants, especially
formaldehyde, is one challenge. The heat produced in the reaction must be controlled to prevent material degradation and to protect equipment. Moreover these solutions are further diluted for in-field applications. Additional solvents such as methanol or glycol may be added to adjust performance or operating temperature range. Dilution by water or solvents can be sources of error, especially if performed in the field.
Quantitative Raman Spectroscopy for triazine-based sulfide scavengers analysis
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 coupled with an internal standard to produce a fast, laboratory-quality test for MEA-triazine quantification in fresh scavenger solutions, an approach that has been used successfully for analyte quantitation in other complex sample matrices. 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.
MEA-triazine spectrum
The MEA-triazine spectrum in aqueous solution is presented to the right. This signature is manifested in the triplet of wide and strong peaks between 800 and 1000 relative wavenumbers (rel. cm-1). These peaks are related to the ring breathing and ring-incorporated methylene hydrogen rocking vibrations for the intact triazine molecule.
Focusing on the region between 800 and 1100 rel. cm-1 in the MEA-triazine spectrum, the figure to the right also illustrates the Raman response for a variety of concentrations from 20 to 90% by mass with three sample spectra at each concentration. Using this data with a multivariate data analysis technique, we extract a relationship between the normalized values at a few specific points in the spectrum and the concentration of MEA-triazine in the sample. The optimal variance occurs at the 870 rel. cm-1 peak attributed to the MEA C-C bond; however, because MEA is a reactant and a by-product during scavenging reactions, we chose the 921 rel. cm-1 peak (asymmetric N−C+C−C stretching + CH2 rocking) as it is specific to the triazine species. The resulting predictive model is univariate on the 921 rel. cm-1 peak
MEA-triazine reaction products
The reaction that produces triazine-based sulfide scavengers starts with formaldehyde and an amine. For MEA-triazine, this reaction starts with equimolar concentrations of formaldehyde and monoethanolamine. This reaction is simple, in theory. Monoethanolamine is 60 g/mol; formaldehyde is 30 g/mol. Mix 2 parts formaldehyde with 1 part monothanolamine, by weight and you’ll get MEA-triazine. The reaction is exothermic and highly-favored., and will quickly run to completion.
(blue) 46% MEA-triazine, 13% excess MEA, 0% excess formaldehyde; (green) 53% MEA-triazine, 0% excess MEA, 0% excess formaldehyde; (black) 44% MEA-triazine, 0% excess MEA, 8% excess formaldehyde.
But, nothing is ever this simple. 1) There are many concentrations of formaldehyde available: formalin is 37% formaldehyde, often with a few percent methanol as a stabilizer; heated trucks can deliver 50% formaldehyde; and paraformaldehyde is polymerized, 100% formaldehyde. They vary in cost and complexity of handling. Additionally, the reaction produces water, three moles per mole of triazine; and a lot of heat, too much of which causes the MEA-triazine to degrade.
The combination makes it challenging to produce a high-quality product if you won’t have the tools to measure the components. Not only is it essential to measure your incoming products, but also to monitor the reaction products to verify their presence or non-presence.
Quantitative Raman Spectroscopy provides a tool for measuring the presence of excess reactants. In the spectrum presented on the left are spectra for three triazine samples with and without excess reactants. All spectra are scaled to the internal standard peak at 1001-cm-1. Formaldehyde appears in the spectra most strongly at 908-cm-1. Excess MEA appears as an increase in signal at 870-cm-1 relative to the MEA-triazine ring breathing peak at 921-cm-1.
Using this information, OndaVia equipment measures the concentration of excess reactants in an MEA-triazine solution, which provides insight into your manufacturing process or confidence in your triazine quality.
