(769d) Bicine Production in Promoted-Mdea Solvents
The correlation between equipment corrosion and the presence of the amino acid bicine in acid gas scrubbing systems utilizing promoted methyldiethanolamine (MDEA) solvents is fairly well established. Preventing the formation of bicine in operating MDEA solvents is accepted as the best way to control corrosion. But the conditions conducive to bicine production are still being debated, with little consensus existing in the literature regarding the role of degradation product intermediates, dissolved oxygen, and H2S scavengers (aldehydes) in bicine formation. Without a clear understanding of those conditions, bicine production will continue to be a problem for natural gas conditioning and refinery based amine solvent systems. This paper explores the conditions which lead to bicine production in MDEA-based solvents based on new laboratory results, and presents pathways which explain its formation.
Bicine production has been linked to the degradation intermediate product diethanolamine (DEA) in aqueous MDEA solvents (Closmann PhD dissertation, 2011) in the laboratory, and to triethanolamine (TEA) (Howard and Sargent, 2001) in field systems. MDEA-based solvents typically accumulate significant amounts of DEA, but little or no TEA. However TEA has been identified in MDEA degradation experiments, with its presence documented and explained mechanistically (Bedell, 2010). Bicine has been identified in separate oxidation experiments with DEA and TEA, leading to different explanations for its formation. Further, the role of two-carbon aldehydes in bicine production in degraded MDEA solvents is likely.
One explanation for the production of bicine in MDEA-based solvents comes from Closmann (2011), and involves the formation of DEA and other secondary amines through nucleophilic substitution reactions at the higher temperatures associated with the stripper reboiler (120+ °C). Once formed, the strong nucleophilic behavior of the secondary amine will lead to a reaction with carbonyl carbons associated with one and two-carbon aldehydes to form a substituted alcohol intermediate which can undergo rearrangement to form another aldehyde. This aldehyde will also behave as an intermediate, and will be readily oxidized to its respective amino acid. These two-carbon aldehyde interactions will lead to the formation of bicine. One such pathway for the formation of bicine through interaction between two reactive intermediates, DEA and hydroxyacetaldehyde, was proposed by Closmann (2011). The presence of both those intermediates can be explained in degraded MDEA solvents. The presence of other aldehydes (glyoxal) intentionally marketed as H2S scavengers in incoming feed gas streams has been documented in field applications, and provides further support to this pathway for bicine production. In addition to the formation of secondary amines as intermediate products, our paper also explores these secondary amine-aldehyde interactions.
Another possible explanation for the presence of bicine in working solvents is the formation of TEA as an MDEA degradation intermediate. TEA, like other tertiary amines, could be formed through the same SN2 substitution reactions serving to explain the formation of DEA and methylmonoethanolamine (MMEA) in degraded MDEA solvents. With the oxidation of a single hydroxyethyl group, TEA would be oxidized to bicine. We performed isolated oxidation experiments which readily demonstrated the formation of bicine from TEA. Those experiments were conducted in a jacketed glass reactor maintained at 55 °C, and fed a constant headspace gas of 98% oxygen at 100 cc/min. Dissolved transition metal sulfate salts of iron, chromium and nickel were added to catalyze oxidation activity in the aqueous solvent. The rate of bicine production in 40% (wt) TEA was 0.27 mmols/kg-hr, which was ~20X the rate observed in a 7 molal MDEA/2 molal PZ solvent oxidized at similar conditions in the same reactor. The formation of bicine in TEA was detected without a lag period, whereas a lag time of greater than 25 hours was observed in the promoted MDEA oxidation experiment, demonstrating the need to form and accumulate a reactive intermediate such as DEA in the promoted MDEA solvent. We will discuss the implications of a TEA-based pathway for bicine production in MDEA solvents.
We will also demonstrate that analogous pathways lead to the formation of other important amino acids in MDEA-based solvents. Those amino acids include hydroxyethyl sarcosine (HES) and N-(2-hydroxyethyl)glycine (HEG). We determined that the total carbon and nitrogen associated with these two amino acids is at least as much as that associated with bicine, and that the total nitrogen of all three of these amino acids can exceed 2% of all nitrogen in a working natural gas scrubbing solvent.