Sulfide scavenging by-products in NGLs

A midstream natural gas company faced a challenge in their collection system: solids were building up at their gas processing plant. Initially, the customer suspected their triazine-based sulfide treatment system was generating dithiazine in the natural gas liquids. After attempting to adjust their treatment system to reduce by-product formation, they started to look upstream at the stations feeding into their plant.

Quantitative Raman Spectroscopy is a set of proprietary tools that enable fast, easy chemical measurement using an all-optical technique. OndaVia has a large suite of analysis methods applicable to the oil & gas industry. Based on customer requests, we developed a test method for monoethanolamine (MEA) and dithiazine in natural gas liquids and slop water samples. For hydrocarbon analysis, this method requires an extraction step into aqueous solution. The water analysis proceeds via our standard amine part-per-million methods. The result is a few-minute test with portable instrumentation that provides quantitative, ppm-level measurements of amine or dithiazine in natural gas liquids.

Quantitative Raman Spectroscopy

Raman spectroscopy is a powerful tool for chemical analysis—an optical technique that measures the vibrational and rotational modes within a molecular system. The sample is illuminated with monochromatic light (a 785-nm diode laser, in our case). The light interacts with the molecular bonds, causing some scattered photons to shift in energy. The resulting scattered light provides structural information that may be used as a “chemical fingerprint”. The intensity of the Raman response is weak; however, it is directly proportional to the number of molecules—in other words, it is a direct measure of concentration.

OndaVia Portable Analysis Laboratory

At OndaVia, we apply Raman spectroscopy for quantitative analytical chemistry. We combine proprietary methods, solvents, software, and algorithms to perform fast, accurate chemical analyses in a range of matrices. The OPAL-104 instrument consists of a compact Raman spectrometer, proprietary reagents, and consumable, analyte-specific analysis tubes.

Presence of H₂S scavengers and dithiazine in natural gas liquids

MEA-triazine (hexahydro-1,3,5-tris(hydroxyethyl)-s-triazine, CAS# 4719-04-4) is an important and popular hydrogen sulfide scavenger in natural gas treatment. Triazine-based scavengers are often applied shortly before transportation, resulting in residual scavenger within the gas. This residual scavenger becomes a tramp amine, which in turn leads to corrosive heat stable salt deposits. More importantly for gas processing, the triazine converts to dithiazine during the scavenging process, a material that can polymerize, causing fouling problems.

Dithiazine has a low vapor pressure, allowing it to be transported alongside the gas stream as a soluble compound in vapor phase. While challenges related to dithiazine in processing equipment and facilities are generally recognized, the phenomenon of dithiazine precipitation in transmission and downstream distribution pipelines and facilities is of growing concern.

The reasons for dithiazine carryover in sales gas remain unknown. Anecdotal suggestions propose that dithiazine might be carried through with a sales gas stream. This effect is particularly true when a scavenger is introduced downstream of, or in the absence of, a conditioning/glycol process. This transported dithiazine then precipitates elsewhere when solubility conditions become unfavorable. This precipitation may occur, for example, within regulating valves on a fuel gas skid at a gas turbine-driven compressor station. The result is a crystalline or amorphous build-up occurring in the lower-pressure/lower-temperature piping downstream of a pressure regulator, in filters, or in other equipment.

To prevent dithiazine in natural gas liquids and sales gas, we recommended that users of MEA-triazine take measures to avoid carryover in sales gas. Operators of pipelines and downstream facilities must conduct assessments and implement necessary measures in the event of dithiazine precipitation. Furthermore, in upstream processing (prior to conditioning), issues such as improper formulation, overspending, or overdosing of the triazine solution may lead to solid formation in processing equipment.

Analysis of amines and dithiazine in natural gas liquids

OndaVia offers ppm-level analysis kits for MEA-triazine (OV-OP-B010-PPM), dithiazine (OV-OP-B011-PPM) and monoethanolamine (OV-OP-B003). These kits offer 10% accuracy over 5- to 100-ppm with a test that requires under two minute to perform. The user mixes the sample with our proprietary reagent, adds this mixture to a vial of nanoparticles, and then analyzes the nanoparticle-sample mixture in a portable Raman spectrometer.

Our analysis kit requires samples to be in an aqueous solution, as an amine signal in crude would be obscured by the organic background. To measure amine- and triazine-compounds in hydrocarbons, we developed an extraction method leveraging commonly used methods for amine analysis. For sample extraction, we weighed approximately 5-10-g of NGL sample from a cylinder, shown in Figure 2, into a 50-ml centrifuge vial. We then added an equal mass of 10-mM HCl.

The vial was vortexed for two minutes, and then centrifuged to separate the water and hydrocarbon phases. Although some liquid is lost during this process, the non-volatility of MEA and dithiazine guarantees they remain in aqueous solution. We collect 100-uL of the aqueous phase, add reagent and 1M NaOH to adjust the pH above 12.7. We added this prepared sample to the nanoparticle vial, and subsequently analyzed the sample using our calibration curves.

The results are presented in Table 1. The amine values are in the 20-40-ppm range with a clear, strong signal. These results are clear proof that triazine-based scavenging is occurring upstream of the gas collection site. Meanwhile, the dithiazine signal ranges from near no-detect to close to 40-ppm.

Amine and dithiazine analysis in slop water

Slop water analysis provides insights into system operation and performance over time. Water analysis is even simpler than NGL analysis, as the extraction step is not needed. Amines will concentrate in the aqueous phase. Dithiazine will build in concentration to its relatively low solubility limit.

The analysis results are presented in Table 2. In this case, we see a higher concentration of amine at all sites. The amine concentrates in the aqueous phase up to a few hundred parts-per-million. At two sites, however, the concentration is over 2% monoethanolamine in the water. This level amine could be an indicator of extreme over-treatment, high sulfide levels that require high triazine dosing, or slop water that has not been flushed for some time.

Looking at the dithiazine numbers, we see a similar trend to the amine concentration. Generally, there is a few hundred ppm. But at the sites with high amine there is also high dithiazine, over 0.6% by weight. These results point to these two sites as locations that should be investigated for problems.

Conclusion

OndaVia has developed analysis methods for ppm-level amines and triazines in aqueous solutions. Analysis of these compounds in NGLs requires an extraction step into dilute aqueous acid. This process uses generally available equipment to transfer the compounds of interest into an aqueous solution in under five minutes. This simple approach expands the capabilities of OndaVia’s analysis approach into matrices heretofore unreachable with Raman spectroscopy.