The application of benchtop NMR spectroscopy for investigating the performance of H2S scavengers

Brown, B. A. Magn Reson Chem 2020, 58, 1249-1255

Hydrogen sulfide is commonly found in natural gas and can be fatal at very low concentrations. ^2,3 Commonly, this product is sequestered using H₂S scavengers, such as triazines.⁴ A common reaction pathway is shown in Figure 1, wherein triazine 1 sequentially reacts with two equivalents of H₂S to ultimately form 3 via intermediate 2.⁵ Under the right conditions, the two equivalents of monoethanolamine (MEA, 4) formed during these transformations can further react with H₂S.⁶

Figure 1. Proposed triazine (1) reaction pathway with H₂S to sequentially form thiadiazine (2) and dithiazine (3), along with two equivalents of MEA (4).

As the scavenger (1) gets overspent, the formation of solid deposits of dithiazine (3) can cause a myriad of issues, leading to the need for characterization of the reaction products to determine the remaining scavenging capacity of the triazine (1).⁷ While the products of these reactions are well-characterized by NMR,⁸ the high upfront and recurring costs of traditional high-field instruments (¹H operating frequencies ≥300 MHz), coupled with their size and requirements for expert staff to maintain and operate, means these instruments are challenging to incorporate in many industrial sectors. The rise in popularity and performance of high-resolution benchtop NMR spectrometers has made this technique accessible to more laboratories and industries, making it ideal to monitor these types of transformations.

In this study, one unspent scavenger and four spent scavengers (obtained from the field) were studied using ¹H and ¹³C NMR spectroscopy. Controlled exposure to H₂S also allowed to characterize its reactivity with the scavenger over time. Studying the reactivity of the triazine (1) with varying amounts of H₂S was performed by passing a mixture of gas through a pH-controlled solution of the scavenger and measuring the “break-through” time, at which point the scavenger was deemed to be 100% spent. From these results, the experiments were repeated to prepare scavenger solutions which were 80% and 50% spent. The results indicated that the spent scavenger samples looked almost identical to the samples obtained from the field, including the residual presence of 1, indicating that it is not entirely consumed. Interestingly, the presence of 3 was minimal in the 50% spent solution, with only a small amount starting to appear in the 80% spent solution. Indeed, 3 only became a major reaction product once break-through had occurred. These studies demonstrate that benchtop NMR instruments can be used to monitor the reactivity of 1 towards H₂S. They also show that amounts of both 1 and 2 are still present at the break-through point, indicating that some other component is inhibiting further reactivity.

This blog post is a summary of work published by Brown in Magnetic Resonance in Chemistry.¹ For more information on the work discussed herein, including figures of NMR spectra and an in-depth discussion, the reader is encouraged to read the full publication.

References

(1) Brown, B. A. Magn. Reson. Chem. 2020, 58, 1249–1255.
(2) Sour Gas: https://www.aer.ca/providing-information/by-topic/sour-gas (accessed April 10, 2024).
(3) Guidotti, T. L. Int. Arch. Occup. Environ. Health 1994, 66, 153–160.
(4) Salma, T.; Briggs, M. L.; Herrmann, D. T.; Yelverton, E. K. Hydrogen Sulfide Removal from Sour Condensate Using Non-Regenerable Liquid Sulfide Scavengers: A Case Study. SPE Rocky Mountain Petroleum Technology Conference. Society of Petroleum Engineers: Keystone, Colorado 2001, p. 5. https://doi.org/10.2118/71078-MS.
(5) Bakke, J. M.; Buhaug, J.; Riha, J. Ind. Eng. Chem. Res. 2001, 40, 6051–6054.
(6) Taylor, G. N.; Prince, P.; Matherly, R.; Ponnapati, R.; Tompkins, R.; Vaithilingam, P. Ind. Eng. Chem. Res. 2012, 51, 11613–11617.
(7) Taylor, G. N.; Matherly, R. Ind. Eng. Chem. Res. 2011, 50, 735–740.
(8) Buhaug, J. Investigation of the Chemistry of Liquid H₂S Scavengers , Norwegian University of Science and Technology, Trondheim, Norway 2002