MDEA desulfurization selectivity
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This post was last edited by liaifeng on 2018-9-6 19:46. The literature states that \"regardless of the gas-liquid ratio, the selectivity always deteriorates as the number of trays increases.\" How should this sentence be understood? It is known that as the number of plates increases, the hydrogen sulfide level in the purified gas becomes acceptable, and carbon dioxide levels also decrease; in other words, the carbon dioxide content in the purified gas drops when the number of plates increases. And it is said here that selectivity worsens as the number of plates increases; does he mean that with more plates, less carbon dioxide and hydrogen sulfide are absorbed per unit mole of amine solution? There is no conflict between the two.
R2R’N + H2S → R2R’NH+ + HS— (fast reaction). Since the nitrogen atom in MDEA does not possess lone pairs, it cannot react directly with CO2; such a reaction can only occur in an aqueous solution: R2R’N + CO2 + H2O → R2R’NH+ + HCO3— (slow reaction). R2R’N + CO2 → no reaction. Clearly, there is a difference in reaction rates between MDEA and H2S/CO2, whereas there is no such difference forMEA and DIPA when reacting with H2S/CO2. Therefore, MDEA has a better selective absorption capacity for H2S in the presence of CO2. A slow reaction does not mean no reaction at all; due to the slow rate of reaction, hydrogen sulfide is absorbed first. As long as the number of trays is sufficient for adequate absorption of hydrogen sulfide, additional trays can be added. The MDEA that has already absorbed hydrogen sulfide will then absorb carbon dioxide on those extra trays, resulting in a reduced selectivity in the end.
The deterioration refers to the selective absorption by MDEA; the reactions of carbon dioxide and hydrogen sulfide with MDEA are slow and fast reactions, respectively. Increasing the number of tower plates means increasing the reaction time, and as time goes on, the absorption of carbon dioxide increases, leading to a decrease in selectivity.
Relatively speaking, MDEA is more capable of absorbing H2S, but it does not absorb CO2. When H2S absorption is nearing saturation, increasing the number of tower plates will further facilitate CO2 absorption, resulting in a decrease in selectivity.