AMINE TREATMENT
Description
Alkanolamines are the most commonly used chemical solvents for H2S and CO2 removal. They may be divided as primary, secondary and tertiary according to the hydroxyl groups bound to the amine nitrogen. The best known amines are Mono-Ethanol-Amine (MEA), Di-Ethanol-Amine (DEA) and Methyl-Di-Ethanol-Amine (MDEA). In addition to simple aqueous solutions of amine, proprietary amine formulations with additives are widely used. Formulated solvents give additional selectivity to H2S removal in presence of CO2 and, in some cases, enhanced absorption capacity too. Amine treatment is a regenerative process. The absorption of acid contaminants takes place approximately at ambient temperature and the regeneration of amine takes place at the boiling point in the stripper column. The basic amine process flow sheet mainly consists of the absorption and stripper columns, lean and rich solution heat exchanger, steam reboiler, acid gas cooler, lean solution cooler and solution circulation pumps.
Application
Remove acid contaminants, like hydrogen sulphide (H2S) and carbon dioxide (CO2), from natural gas, refinery gas and synthesis gas to meet pipeline, LNG and petrochemical plant specifications or to comply with environmental requirements.
The extent of residual acid compounds is, usually, 4 ppm of H2S and 2% CO2 in natural gas to pipelines and 50-100 ppm of H2S in refinery gas used as fuel gas within the process units.
Operating conditions
A very wide range of treating pressures and contaminant concentrations can be accommodated. Operating conditions can vary according to the amine used to treat the gas. MEA is not selective but offers high reaction speed with acid gases. MEA aqueous solution concentration is limited to 20% wt. due to corrosion problems that develop at higher concentrations. DEA reacts with both CO2 and H2S and is not selective. DEA solution concentration as high as 35% wt. can be used with very high acid gas loading. MDEA is highly selective to H2S absorption. MDEA concentration can be up to 50% wt. and the acid gas loading is restricted only by the relative partial pressure of acid compounds and by the required selectivity. Normally the use of MDEA is limited to gas treating in which both H2S and CO2 are present and in which the H2S has to be recovered as elemental sulphur in a Claus plant. Typically, in refineries, it is preferable to move from MEA or DEA to MDEA because the utilities consumption is reduced and plant capacity is increased in terms of acid gas loads.
