Physical Solvents for Selective Co2 Capture at Elevated Pressures and Temperatures

Yannick J. Heintz1, Laurent Sehabiague1, Badie I. Morsi1, Kenneth L. Jones2, and Henry W. Pennline2. (1) Chemical and Petroleum Engineering, University of Pittsburgh, 1249 Benedum Hall, 3700 O'Hara Street, Pittsburgh, PA 15261, (2) U.S. Department of Energy, National Energy Technology Laboratory, 626 Cochrans Mill Road, P.O. Box 10940, Pittsburgh, PA 15236

Acid gas removal, including CO2, from Integrated Gasification Combined Cycle (IGCC) power generation facilities has been conventionally carried out using: (1) a chemical process employing methyl-diethanolamine (MDEA) or (2) a physical process utilizing either chilled methanol (Rectisol) or a mixture of dimethylethers of polyetheleneglycol (Selexol). The MDEA process requires high thermal energy (heat) for solvent regeneration. The Rectisol process is complex, and refrigeration makes it the most expensive acid gas removal process. The Selexol process is more expensive than the MDEA process, and the chilling option could increase the process costs. In an IGCC application, these physical and chemical processes, however, require cooling and subsequent reheating of the stream before the gas turbine which undeniably decreases the plant thermal efficiency and increases the overall cost of the process. Thus, there is a need for developing an alternative process which should be economical and absorb CO2 without significant cooling of the gas streams.

Extensive literature review revealed that perfluorinated compounds (PFCs) have low reactivity and high chemical stability due to the high energy of their C-F bonds. They have high boiling points and low vapor pressures because of the strength of the C-F bond and the high molecular weight. They also have no dipole and very low molecular interactions due to the repulsive tendency of fluorine atoms. These unique properties lead to high gas solubility, low vapor losses, and low forces required for expelling the gas molecules upon decreasing pressure or increasing temperature. Thus, PFCs show a great potential for CO2 capture from post-shift fuel gas streams at elevated pressures and temperatures.

The main objective of this study is to investigate the potential use of perfluorinated compounds as physical solvents for CO2 capture from post water-gas-shift reactor streams under elevated pressures and temperatures. After obtaining the physical properties and measuring the gas solubility and the hydrodynamic and mass transfer parameters (gas holdup, Sauter mean bubble diameter, and volumetric mass transfer coefficient) for CO2 in different PFCs, the data were used in Aspen Plus software to simulate the absorption/regeneration process in order to improve the CO2 capture. The Peng-Robinson equation of state was used in the simulation to calculate the vapor-liquid or vapor-liquid-liquid equilibria. Also, some other parameters were adjusted in the Aspen plus in order to fit the experimental data obtained.

The simulation was conducted at 48 bar with Selexol and PP25 solvents (perfluoro-perhydro-benzyltetralin, C17F30). The temperature was 312 K and 511 K for Selexol and PP25, respectively. The solvent and gas feed flowrates in both processes were 17427.36 kg/s and 102.52 kg/s, respectively. The shifted gas composition is given in Table 1.

After absorption, the Selexol process, considered in this study as a “benchmark process,” was regenerated at the same absorber temperature (312 K) where the pressure was decreased gradually till 1 bar. The regeneration of the PP25 process, however, was carried out with a series of reactors in which the pressure and the temperature were lowered at different steps till almost complete solvent regeneration. The CO2 removal efficiency and the solvent lost for PP25 process were compared with those of Selexol in order to evaluate the feasibility of the PP25 process.  

Table 1 Shifted gas composition used in Aspen plus simulation

Components Mole Percent
Ar 0.480
CH4 0.240
H2 37.498
N2 0.330
CO 6.270
CO2 23.869
H2O 30.678
NH3 0.160
H2S 0.475
Total 100.000