Kinetics of Absorption of CO 2 in Concentrated Aqueous Methyldiethanolamine Solutions in the Range 296 K to 343 K Fatos Pani, Alain Gaunand, Renaud Cadours, Chakib Bouallou, and Dominique Richon* Centre de Recherche en Proce ´de ´s de Transformation de la Matie `re, Ecole Nationale Supe ´rieure des Mines de Paris, 60, bd Saint-Michel, 75006 Paris, France The kinetics of CO 2 absorption by aqueous solutions of methyl diethanol amine (MDEA) were measured in the temperature range (296-343) K and MDEA concentration range (830-4380) mol‚m -3 (10-50 mass %). A thermoregulated constant interfacial area Lewis-type cell was operated by recording the pressure drop during batch absorption. The kinetic results are in agreement with a fast regime of absorption according to film theory. MDEA depletion at the interface has a significant effect on the kinetics at the CO 2 pressures (100 to 200 kPa) studied in this work, especially at low temperatures and low MDEA concentrations. Considering only the reaction between CO 2 and MDEA, the CO 2 absorption appears as a first-order reaction with respect to MDEA. The activation energy found for the reaction between CO 2 and MDEA is 45 kJ‚mol -1 , but this value depends significantly (by about 10% in the worst case) on the vapor-liquid equilibrium data used. Introduction Absorption by aqueous solutions of alkanolamines is the dominant industrial process for removing acid gases, mainly CO 2 and H 2 S, from natural gas. Such washing processes are also used in petroleum refining, coal gasifica- tion, and hydrogen production. Instead of ethanolamine (MEA) and diethanolamine (DEA) solutions, industry would prefer to use less corrosive and more advanced solvent systems which could be formulated along with the plant design and operation, according to the feed and exit stream specifications of plants. Methyldiethanolamine (MDEA) and blends with primary or/and secondary amines and sterically hindered amines are new systems that have been studied in the laboratory and sometimes used on industrial scale. In spite of an abundance of literature (Barth et al., 1981, 1984; Blauwhoff et al., 1984; Yu et al., 1985; Critchfield, 1988; Haimour et al., 1987; Versteeg and van Swaaij, 1988a; Littel et al., 1990; Rinker et al., 1995), only a few works (Tomcej and Otto, 1989; Toman and Rochelle, 1989; Xu et al., 1992) deal with absorption kinetics for CO 2 + MDEA + H 2 O with solutions more concentrated than 3 × 10 -3 mol‚m -3 MDEA in water. High MDEA concentrations seem advantageous for absorption kinetics, in spite of the increasing viscosities which lead to decreasing diffusivities and physical transfer. Only Tomcej and Otto (1989) and Xu et al. (1992) investigated absorption of carbon dioxide in aqueous MDEA solution with MDEA concentrations higher than 3400 mol‚m -3 solutions and temperatures higher than 340 K. Rate expressions generally accepted (Yu et al., 1985; Critchfield, 1988; Haimour et al., 1987; Versteeg and van Swaaij, 1988a; Littel et al., 1990; Rinker et al., 1995; Tomcej and Otto, 1989; Toman and Rochelle, 1989) for the forward chemical reaction between CO 2 and MDEA are first order with respect to the concentrations of each of these species, while the estimated energies of activation lie between (33.1 and 71.6) kJ.mol -1 . This paper describes the very efficient apparatus devel- oped to measure absorption kinetics of acid gases into amines solutions. Experiments of CO 2 absorption by aqueous MDEA solutions, starting with CO 2 pressures ranging from (100 to 200) kPa, and in an extended range of MDEA concentrations and temperatures have been made with this apparatus. Experimental Section Chemicals. Twice-distilled water and reagent-grade MDEA are used. MDEA is from different origins: from Aldrich, with a certified minimum purity of 99 mass %, from Merck, with a certified minimum purity of 98 mass %, and from Alfa with a certified minimum purity of 98 mass %. Carbon dioxide is from L’Air Liquide, with a certified purity of 99.995 vol %. Experimental Setup and Mode of Operation. The (6.00 ( 0.02) × 10 -2 m internal diameter thermostated glass reactor (Lewis type, Lewis 1954) shown in Figure 1 is provided with a six-bladed Rushton turbine, (4.25 ( 0.02) × 10 -2 m, in its lower part, a (4.00 ( 0.02) × 10 -2 m dia- meter propeller in its upper part, and four equally spaced vertical PTFE baffles to prevent vortexing. A horizontal PTFE plate and a ring are put midway between the bottom and the top of the cell to set both the level and area of the gas-liquid interface and to make sure of its stability during stirring. The shafts are maintained vertically between two pivots supported by two sapphire bearings fixed in the seats machined on the inside parts of the two stainless steel flanges on the flanges of the cell and the horizontal PTFE plate; they are driven magnetically by adjustable speed motors. This technique avoids leaking, friction, and heat generation due to shafts passing through the envelope of the cell by means of packings. The impeller speed is checked with a stroboscope, it remains constant within 1 rpm during each test. The temperature in the reactor is known within (0.05 K through a 100 Ω platinum probe, calibrated against a 25 Ω STHPB platinum probe from LYON ALEMAND LOUYOT. The temperature is con- trolled by circulating a thermostatic fluid through the glass double jacket. The whole cell is placed inside a ther- moregulated air bath. A tube allows us either to evacuate the cell or to introduce CO 2 into the cell. The total volume available for gas and liquid is (0.3504 ( 0.0005) × 10 -3 m 3 and the gas-liquid interfacial area A is (11.72 ( 0.05) × 10 -4 m 2 . Uncertainties are geometrically estimated. Kinetics of gas absorption are measured by recording the absolute pressure drop through a SEDEME pressure transducer, working in the range (0 to 200) kPa. This transducer is thermostated at a temperature slightly higher than the experiment temperature to avoid liquid condensation in its measuring chamber. For each temper- 353 J. Chem. Eng. Data 1997, 42, 353-359 S0021-9568(96)00251-8 CCC: $14.00 © 1997 American Chemical Society