2 nd Mercosur Congress on Chemical Engineering 4 th Mercosur Congress on Process Systems Engineering 1 AN EQUILIBRIUM DESCRIPTION FOR THE ABSORPTION OF CO 2 /H 2 S WITH AQUEOUS SOLUTIONS OF ETHANOLAMINES Leandro C. Barbosa, José L. de Medeiros * , Ofelia Q. F. Araújo Escola de Química - Universidade Federal do Rio de Janeiro Abstract. In the petroleum industry an important operation is the sweetening of gas streams by removing CO 2 and H 2 S via absorption with aqueous solutions of ethanolamines. Among the amines used, monoethanolamine (MEA) and methyldiethanolamine (MDEA) are of relevance, exhibiting complementarity when used together dissolved in water. Depending on application, the combination of these amines may lead to an efficient, cost effective, more durable and less corrosive solvent for acid gas removal. Up to date models for CO 2 /H 2 S absorption with aqueous ethanolamines are described in the Literature by non-equilibrium frameworks that, invariably, propose an impressive set of ions participating on several kinetic-controlled chemical reactions that are also coupled with interfacial transfer of mass and heat. These models are so numerically involved that a mere separation calculation may be too heavy in terms of the necessary machine power. Besides, they usually require lots of physical parameters that are inexistent in the Literature or are, frankly, difficult to determine with accuracy. Taking advantage of the fact that all relevant ions are weak and produced by species with low tendency for dissociation, this work circumvents the need of electrolyte thermodynamics by specifying only molecular species in both phases. Following the premises of the Chemical Theory, non volatile, molecular complex species are proposed as products of reactions involving an amine (MEA or MDEA), water and an acid gas (CO 2 or H 2 S). Complex species remain in the liquid phase. The chemical reactions which generate them are in chemical equilibrium, enabling a good representation of gas absorption (positive reaction coordinate) and gas stripping (negative reaction coordinate). Equilibrium properties of liquid and vapor phases, for real and complex species, were predicted by conventional equations of state. Model parameters – chemical equilibrium constants – were estimated via Maximum Likelihood procedures over isothermal extracts of experimental equilibrium data on the system H 2 O/MEA/MDEA/ CO 2 /H 2 S. Keywords: CO 2 absorption, aqueous ethanolamines, chemical theory 1. Introduction With increasingly stringent environmental regulations, processing of low quality oils and intensification of natural gas use, analysis and selection of technologies for removal of acid gases (CO 2 , H 2 S) become important issues in the design of hydrocarbon processing plants. The removal of CO 2 and H 2 S from gaseous streams can be achieved by absorption with liquid solvents, adsorption on solids, cryogenic separation, permeation through membranes and chemical conversion. Among these alternatives, absorption with liquid solvents is at present the most economical technology for treating large volumes of gas. Absorption of CO 2 and H 2 S is a well established technology in the chemical industry. Besides the selection of an appropriate solvent, it demands continuous operation of two interlinked (packing or tray) columns. The first column – the absorber – contacts the gas stream with lean solvent at moderately high pressures and moderately low temperatures. The rich solvent (in terms of CO 2 and H 2 S) goes to a second column – the stripper – where at expenses of lower pressures, higher temperatures and contact with a stripping agent (e.g. steam), acid gases are liberated in the top vapor product, while the lean solvent is recovered as bottom product and returned to the absorber after some cooling and making-up. * To whom all correspondence should be addressed. E-mail jlm@eq.ufrj.br Address: Escola de Quimica, E, Centro de Tecnologia, Universidade Federal do Rio de Janeiro, Ilha do Fundao, 21949-900, Rio de Janeiro, RJ, Brazil