An efficient chemical absorption method capable of
cyclic fixed-bed operations under moist
conditions for the recovery of carbon dioxide from flue gases has been
proposed employing K2CO3-on-carbon. Carbon dioxide was chemically absorbed by the reaction
K2CO3 + CO2 +
H2O ⇄
2KHCO3 to form potassium hydrogencarbonate. Moisture,
usually contained as high as 8−17%
in flue gases, badly affects the capacity of conventional adsorbents
such as zeolites, but the
present technology has no concern with moisture; water is rather
necessary in principle as shown
in the equation above. Deliquescent potassium carbonate should be
supported on an appropriate
porous material to adapt for fixed-bed operations. After
breakthrough of carbon dioxide, the
entrapped carbon dioxide was released by the decomposition of
hydrogencarbonate to shift the
reaction in Eq.1 in reverse on flushing with steam, which could be
condensed by cooling to afford
carbon dioxide in high purity. Among various preparations of
alkaline-earth carbonates (X2CO3,
X = Li, Na, K) on porous materials,
K2CO3-on-activated carbon revealed
excellent properties
for the present purpose. Preparation and characterization of
K2CO3-on-carbon and
illustrative
fixed-bed operations under flue gas conditions in laboratory columns
and a bench-scale plant
are described.
A Cl'-intercalated hydrotalcite-like compound (HTAL) was prepared by neutralizing magnesium and aluminum chlorides with sodium hydroxide. The HTALs were characterized by chemical analyses, X-ray diffraction (XRD), differential thermal (DTA)/thermogravimetry (TG) analyses, and scanning electron microscopic (SEM) observation. The HTAL crystal with a single unit cell, aa = b0 = 0.31 and c0 = 2.33 nm as the lattice constant, was formed by aging at 353 K. The crystal growth proceeded for all three axes a, b, and c up to 5 h of aging, while the stacking along the c axis was preferentially observed by aging beyond 5 h. Phosphate ion-exchange properties of HTAL were investigated by a batch method. The HTAL obtained by longer aging time showed a slightly larger phosphate uptake, owing to better crystallization of the hydrotalcite structure. The pH dependence of the phosphate ion exchange showed that phosphate uptake has a maximum around pH 7. Chemical analyses of HTALs before and after the phosphate loading showed that phosphate ions are mostly ion-exchanged with interlayer Cl' ions, and the composition of phosphate ions in the solid phase is similar to that in the solution phase. The isotherm for phosphate uptake followed the Langmuir equation at pH 7.53 and 310 K; it gave an ion-exchange capacity of 2.37 mmol of P/g for phosphate.
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