What is the principle behind feeding the rich liquid from the sulfur unit into the regeneration tower to remove the absorbed hydrogen sulfide?
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RT, could some expert explain this? Thank you
The main reactions involving MDEA in the absorption of H2S and CO2 are as follows: 2R3N + H2S ⇌ (R3HN)2S; (R3HN)2S + H2S ⇌ 2R3HNHS; R3N + H2O + CO2 ⇌ (R3HN)2CO3; (R3HN)2CO3 + H2O + CO2 ⇌ 2R3HNHCO3. H2S can react directly and rapidly with amines to form amine sulfides and hydrogen sulfides, while CO2 also participates in these reactions, but at a relatively slower rate. The aforementioned reactions are both reversible; H2S is absorbed at lower temperatures, while it is released at higher temperatures. Alcohol amines are weak organic bases that can absorb hydrogen sulfide and carbon dioxide from gases at temperatures between 20 and 50°C. When the temperature rises to 105°C or higher, they decompose and release the previously absorbed hydrogen sulfide and carbon dioxide, thereby regenerating the amine solution. Commonly used desulfurization solvents in China include monoethanolamine (MEA), diethanolamine (DEA), diethylenepropylamine (DIPA), N-methyldiethanolamine (MDEA), and composite MDEA.
In that case, the amine liquid regeneration tower should not be considered a distillation tower; then why does the regeneration tower still require reflux?
Recycling is generally done to extend the time or increase the concentration, etc
If there is no liquid-phase reflux, then at high temperatures a large amount of water vapor and H2S will rise together; an excessive gas-phase load may also carry amine liquid. Liquid-phase reflux is provided to, first, wash the amine out of the gas phase and prevent the loss of amine liquid ； Secondly, it reduces the water vapor content in the gas phase, increases the concentration of H2S, and reduces the cooling load on the overhead cooler.
An aqueous solution of ethanolamine is alkaline at low temperatures and can undergo an acid-base neutralization reaction with hydrogen sulfide; when heated to over 100 degrees, it becomes neutral and hydrogen sulfide is released (desorbed). The reflux of acidic water from the top of the tower is intended to obtain acid gas with a higher purity; in practice, this increases the concentration of hydrogen sulfide in the acidic water after cooling, thereby yielding acid gas with a higher purity.