(476as) Physical Solvents for Selective Co2 Capture at Elevated Pressures and Temperatures

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