Mitigating corrosion in sweet gas units : a comparison between laboratory.
Résumé
An extensive review of acid gas removal units covering 5 decades and more than 120 amine units showed that units treating sweet gases (i.e. with CO2 only) presented the worst risks of corrosion. In some extreme cases, a quite uniform corrosion in an active mode was observed on stainless steel grades such as AISI 410 and 304L, despite apparently mild corrosive conditions. Strong scaling of the heat exchangers by corrosion products was also observed. A detailed analytical survey of those corrosive DEA units was then performed over more than two years. Immediately after fresh solvent swap, dissolved iron concentration increased until stabilization at several hundreds of mg/L after a few months. Good correlation was found between dissolved iron, amine degradation, and the amount of scaling products that was recovered in the heat exchangers. Comparisons with similar units using activated MDEA revealed dissolved iron levels several orders of magnitude below those in DEA. In order to understand more precisely the driving forces for steel corrosion and for the precipitation of corrosion products, a laboratory program was launched. Comparisons between different amines in rich and lean conditions were performed in autoclave. A specific protocol was developed which aimed at degrading the amine solutions and at providing dissolved iron before corrosion tests to image the degradation of industrial solvent in actual life of plant. The laboratory degraded DEA was successfully compared to an industrial DEA solution sampled in a gas sweetening unit experiencing corrosion problems. Active corrosion of carbon steel (CS), AISI 410and even AISI 304L was reproduced in laboratory conditions with hot rich DEA and with industrial DEA solution as well. Industrial trends of strong precipitation of iron carbonate in the amine - amine exchangers could be explained. Iron solubility was found to depend on amine loading, with a lower solubility in rich conditions, thus a strong tendency to precipitate. Based on a the same protocol to obtain degraded MDEA and energizedMDEA, the study further looked at the behavior of those commonly used amines made of on the shelf and generic amine molecules. The experience and know-how gathered from the feed-back, inspection and laboratory experiments is applied to understand the performance of existing units, for which carbon steel has been used extensively. When improvements are welcome, the key corrosion mitigating issues are exposed in term of appropriate amine selection for sweet or sour gases, flow velocities and procedures for solvent preservation. The article also discusses about material selection and replacement of carbon steel by appropriate corrosion resistant alloy (CRA) when needed and only on selected areas, as normal maintenance operations following periodic inspections. This considerably extends the service life, while also enabling operation above the initial specifications. For newly built compact units, the design criteria focus on application of AISI 316L, now preferred instead of lower grades like 304L or AISI 410, to areas potentially prone to corrosion. With appropriate use of CRA's, acid gas loadings of 0.85 or even higher can be considered in design, without velocity limitations on the rich amine lines or stringent material selection. Furthermore, the amine units based on the AdvamineTM technologies remain fully versatile and flexible to encompass the widest range of amines, including MDEA and energizedMDEA.