(502i) Solubilities and Mass Transfer Coefficients of Gaseous Mixtures in Physical Solvents for Co2 Capture Applications

Solvent-based processes are of prime interest for acid gas removal from synthesis gases due to the potential minimal impacts of these solvents upon the plant efficiencies. A recent study indicated that 75% of the CO₂ could be captured from an IGCC facility with only a 4% loss in efficiency, without accounting for the cost of CO₂ transportation to a utilization/sequestration site or further processing. These solvent-type processes can be categorized into: (1) chemical solvents; (2) physical solvents; and (3) mixed chemical/physical solvents. Aqueous amines and methyl-diethanolamine (MDEA) are mainly used as chemical solvents; chilled methanol (Rectisol), a mixture of dimethylethers of polyetheleneglycol (Selexol), and a mixture of N-formylmorpholine and N-acetylmorpholine (Morphysorb) as physical solvents; and a mixture of sulfolane and aqueous solution of either diisopropanol amine or MDEA (Sulfinol) is used as a mixed chemical/physical solvent.

For any physical-solvent to be economically feasible, it must have: (1) low vapor pressures in order to prevent its loss; (2) high selectivity for acid gases when compared with those of CH₄, H₂ and CO; (3) low viscosity; (4) thermal stability; and (5) non-corrosive behavior. Unfortunately, only a few commercially employed solvents can meet some of these criteria, in addition, in all the above processes, the post water-shifted fuel gas expected at 505 ? 533 K and over 30 bar, has to be cooled down to ambient temperature (Morphysorb process, and Selexol ~ 312 K) or sub-ambient (Rectisol, 263 K) which is highly energy intensive.

The objective of this study is to investigate the potential use of different physical solvents for selective CO₂ capture from post water-gas-shift reactor streams at elevated pressures and temperatures. In order to achieve this objective an experimental program was devised to obtain the equilibrium gas solubility and volumetric mass transfer parameter for gas mixtures containing not only H₂ and CO₂ but also Ar, CO, and CH₄ in perfluoro-perhydro-benzyltetralin (C₁₇F₃₀), a physical solvent known as PP25.

In order to obtain the equilibrium solubility (C*) and volumetric liquid side mass transfer coefficient (k_La) for each gaseous component in the mixture, an online calibrated Mass Spectrometer was used to continuously monitor the mole fractions of the gaseous components in the mixture during the transient and equilibrium conditions. The partial pressures of each component were then calculated allowing the determination of C* and k_La for each component in the gaseous mixture. The experimental technique was initially tested and validated at low temperature using Selexol as a physical solvent.

Preliminary data indicated that the solubilities of all components in the gas mixture in the Selexol solvent were in a very good agreement with those obtained for each component individually with the same solvent at similar pressures and temperatures. The volumetric liquid-side mass transfer coefficients obtained for the components of in the gas mixture, on the other hand, were slightly lower than those obtained for each single gaseous component in the Selexol solvent. This behavior can be attributed to the competition among the gaseous components in the mixture in order to transfer into the solvent through the same gas-liquid interface (gas bubble surface) which resulted in more resistance to mass transfer and subsequently a decrease of the mass transfer coefficient for each component in the mixture. Experimental data of the solubility and mars transfer coefficients obtained under high pressures and temperatures with the gaseous mixture in PP25 will be also discussed.