(235d) CO2 Capture under Elevated Pressures and Temperatures Using Fluorinated Solvents

The 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 for acid gas removal,
however, require cooling and subsequent reheating of the stream before the gas
turbine which undeniably decreases the plant thermal efficiency and thusly
increases the overall cost of the process. Thus, there is a need for the
development of an alternative process which should be economical and absorb
carbon dioxide 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, minimum
vapor losses, and low forces required for expelling the gas molecules upon
decreasing pressure or increasing temperature. Thus, PFCs show an immense
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. In order to achieve this objective an
experimental program was devised to obtain the equilibrium gas solubility, and
the hydrodynamic and mass transfer parameters (gas holdup, Sauter mean bubble
diameter, and volumetric mass transfer coefficient) for CO₂ in three
different PFCs, namely
Perfluoro-perhydrofluorene (C₁₃F₂₂),
Perfluoro-perhydrophenanthrene (C₁₄F₂₄), and
Perfluoro-cyclohexylmethyldecalin (C₁₇F₃₀), known as PP10,
PP11, and PP25, respectively.

The transient physical gas absorption
technique was employed to measure the volumetric mass transfer coefficient; and
the gas solubility was determined when the thermodynamic equilibrium was reached
in the reactor. The expanded liquid height method and a photographic method were
used to obtain the gas holdup and the Sauter mean bubble diameter, respectively.
The data were statistically designed and obtained in a gas-inducing, one-gallon
Zipper-Clave agitated reactor, equipped with sight-windows in wide ranges of
operating conditions: pressures (6 - 30 bar), temperatures (300 - 500 K), mixing
speeds (10 - 20 Hz), and liquid heights (0.14 - 0.22 m).

The equilibrium solubilities of CO₂ in PP10,
PP11, and PP25 were found to increase with pressure at constant temperatures.
The CO₂ solubilities in PP25 appeared to be greater than those in the
other two PFCs due to the fact that its molecular weight is greater than those
of the other two PFCs. The results showed that CO₂ is more soluble in
the Selexol solvent than in the PFCs only at low temperatures (≤ 333 K). The
Selexol process, however, is customarily operating at temperatures of about 312
K, indicating that the Selexol solvent would not be effective at high
temperatures typifying those at the exit of the gasifier system. Thus, this
study proved the thermal and chemical stability and the ability of the PFCs to
absorb CO₂ at temperatures up to 500 K and pressures as high as 30
bar.
The volumetric mass transfer coefficients (k_La)
of CO₂ in PP10, PP11, and PP25, increased with increasing mixing
speed, pressure, and temperature due to the increase of the gas-liquid
interfacial area (a) and the liquid-side mass transfer coefficient (k_L).
The increase of the gas-liquid interfacial area with these operating variables
was attributed to the increase of the gas holdup (ε_G) and the
decrease of the Sauter mean bubble diameters (d_S). The
volumetric mass transfer coefficients of CO₂ in the three PFCs,
however, decreased with increasing liquid height above the impeller due to the
decrease of the gas holdup and increase of the Sauter mean bubble diameter,
which led to the decrease of the gas-liquid interfacial area. The volumetric
mass transfer coefficients for CO₂ in PP25 were smaller than those in
PP11, and both were smaller than those in PP10, indicating that the volumetric
mass transfer coefficients decrease with increasing the PFCs viscosity. Also,
under the operating conditions investigated, the gas-liquid interfacial areas of
CO₂ in the three PFCs appeared to control the behavior of the
volumetric mass transfer coefficients in the gas-inducing reactor used.