Gas 2025 Issue

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Fe (ppmw) Rich loading Lean loading

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Jan 2020 New pump

June 2020 New L/R exchanger

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Jan 2018 New contractor

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Figure 5 Iron content vs lean and rich loadings

Regenerator and reboiler corrosion To mitigate corrosion in the regenerator, regeneration of 95% of the rich amine must happen before entering the reboiler. A second rule is to reduce reboiler and vapour return line corrosion to maintain the CO 2 content of the reboiler return below 1 mol%. According to the simulation, only 89% of the rich amine is regenerated prior to the reboiler. The vapour return line contains 1.2-2.5 mol% CO 2 . The following operating param- eters need to change to regenerate more in the column, thus reducing the corrosion potential:  The L/R heat exchanger needs to be upgraded to achieve a rich outlet temperature of at least 190°F.  Have sufficient hot oil flow/temperature to the reboiler to achieve a regenerator overhead temperature of at least 215°F (102ºC) (preferred >220°F, 104ºC). The simulated water reflux flow rates are near 0.5 GPM or less. The expected flow rate should be closer to 3-5 GPM for a properly regenerated MEA solvent. By raising the regener - ator overhead temperature close to the recommended val- ues, the water reflux rates will increase accordingly. Corrosion in amine units can be initially detected via:  A change in solution colour, clarity, and visible solids.  Levels of dissolved iron, manganese, chromium, and Pure MEA solvent is clear; however, in a process with H2 S, corrosion of steel surfaces forms iron sulphide, which tends to turn the solvent pale yellow, dark yellow, green, brown, and finally to a black colour as corrosion continues unabated. This takes place as the iron content builds from virtually zero ppmw to greater than 50 ppmw. If there is not enough H 2 S to form an appreciable passivation iron sulphide (FeS) layer on the unit piping/equipment, the majority of the iron will be in the form of iron carbonate (Fe 2 CO 3 ). Colours build from clear to yellow, to gold, to brown to black as Fe 2 CO 3 levels increase. Iron and manganese are typically representative of car- bon steel corrosion. Iron levels represent current corrosion nickel in the amine solvent and analysis.  Levels of certain suspended solids.

rates, while manganese levels represent historic corrosion. Chromium and nickel are typically present in solution when stainless steel corrodes. Usually, chromium dissolves in the solution and can be detected in a laboratory analysis, while nickel precipitates as a solid that is filterable. Iron content below 5 ppmw is acceptable, whereas chromium content greater than 1 ppmw is problematic. Figure 5 provides a plot of the iron in solution compared to the lean and rich amine loadings. Corrosion at high temperatures If new equipment was installed, an increase in soluble iron would be consistent with the high initial corrosion rates of fresh steel. Usually, after the first few weeks, corrosion rates tend to decline as a protective passivation film forms on the inner metal surfaces. An event in December 2019 was the initial onset of iron in the solution; although relatively small, it was still noticeable against the historic values below the detectable limits of the test. Given the anomaly in the rich amine loading at the same period, another change occurred in either the operating mode or equipment. Our assumption is a reduction in rates to allow for the equipment changes. Overall, the soluble iron levels were acceptably low. However, in early January, there was a massive increase in the soluble iron content. This coincided with the installation of a new pump off the regenerator column. This replacement caused corrosion because of the elevated temperatures and insufficient H2 S or CO 2 present to form protective films. Since corrosion is a chemical reaction and all chemical reactions proceed faster and more aggressively as the solution heats up (corrosion rates can often double for every 16ºF increase in temperature), reboiler and lower regenerator column corrosion will always be high. Figure 6 provides a plot of chromium levels. As with iron, the chromium content made a similar increase around the same time frame. With chromium levels this high, it gener- ally indicates a significant failure of stainless steel compo - nents. Generally, the locations of interest would be the trays

Gas 2025