h other solids or high-viscosity liquids forms a “black shoe polish”-type mixture as Figure 4:
of HSAS become unstable, releasing a portion of the acid, which then attacks bare carbon steel. The more salts and the hotter the reboiler temperature, the more likely this is to occur. This puts the hottest areas of the system at the most risk of corrosion: the reboiler shell, vapour return line, and bottom of the regenerator. HSAS should be controlled to less than 2.5 wt% in methyldiethanolamine (MDEA) systems, and 3 wt% in all other amine systems. Since Fe+2 ions are not visible in the amine, it is not immediately obvious that the system is corroding simply by looking at the lean amine. A quick corrosion test refiners can do is contact a lean amine sample with H 2 S and observe if FeS forms. This can be done in two ways: bubble H 2 S gas through the amine or add high-H 2S reflux water. If the amine turns green or dark (as shown in Figure 5 ), this is an indication of FeS formation, which in turn means the amine contained Fe+2 ions. These ions will be converted to FeS in the absorbers, precipitating and settling in low points (vessels and piping). This can become fouling material in the absorber or may be filtered out of the rich amine. FeS should not be present in a properly regenerated lean amine. Preventing fouling To prevent absorber fouling, do not allow solids to enter or form inside the column. Stopping them from entering requires effective feed gas separation and lean amine filtration. Preventing solids formation is more difficult. Most solids that foul absorbers are composed
are a result of regenerator corrosion. When the regenerator (or reboiler) corrodes, free iron (Fe+2) ions are released into the amine. When amine enters the absorber and encounters the sour gas, a reaction between Fe and H 2 S occurs, and FeS is formed. FeS is not soluble and therefore becomes a potential fouling material as soon as it develops. The process by which iron circulates around amine systems is called known as ‘the iron pump’, a term coined by Shell (see Figure 3 ). Fouling due to FeS is made worse in two situations: the frequent addition or overdosing of antifoam, and in packed absorbers. The mixture of antifoam and FeS, along with other solids or high-viscosity liquids, forms a ‘black shoe polish’-type mixture, as shown in Figure 4 . Fe+2 ions are released into the amine from carbon steel corrosion by heat stable amine salt (HSAS). At higher concentrations, certain types Figure 4: Black Shoe Polish Fouling of a Packed Absorber Figure 4 Black shoe polish fouling of a packed absorber
ns are released into amine from carbon steel corrosion by heat stable amine salt At higher concentrations, certain types of“heat stable” amine salts become e , releasing aportion ofthe acid which then attackbare carbon steel. The more salts hotter the reboiler temperature, the more likely this is to occur. This puts the
amine had Fe+2 ions in it. These ions will be converted to FeS in the absorbers, precipitating and settling in low points (vessels and piping), becomes fouling material in the absorber, or is filtered out of the rich amine; iron sulphides should not be present in a properly-regenerated lean amine.
of FeS. The goal then is to minimise the release of Fe+2 into the amine, which involves holding the concentration of HSAS such that they are not corrosive. In refineries, HSAS are usually formed because of various reactions with hydrogen cyanide (HCN). It is typically found in gases from coker and FCC units, but may also be in other refinery gas streams. It is possible to remove HCN from
Figure 5: Example of amine color change with the addition of reflux water. Before (left) and after (right) Figure 5 Example of amine colour change with the addition of reflux water. Before (left) and after (right)
Preventing Fouling To prevent absorber fouling, don’t allow solids to enter nor form inside the column. Stopping them from entering requires effective feed gas separation and lean amine filtration. 52
Refining India
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