염기자리를 첨가한 카본 흡착제의 이산화탄소 흡착 특성

송현곤, 이건홍

포항공과대학교 화학공학과

Adsorption of Carbon Dioxide on the Base-site-containing Carbon Adsorbents

Hyun-Kon Song and Kun-Hong Lee

Department of Chemical Engineering

Pohang University of Science and Technology

Introduction

The adsorption processes for separation of carbon dioxide have been developed due to its low energy requirement and clean operation, even though they have the limitation of application to large-scale separation[1]. Adsorbent is one of the most important parts consisting of the adsorption separation process. The practical adsorbents, activated carbons or zeolites, have the carbon dioxide adsorptivity at room temperature due to their large surface area. However, they loose their adsorptivity as temperature increases. Since the effluent gas from industrial processes has temperature higher than room temperature, the adsorptivity at high temperature have to be enhanced for simplification of the separation process.

The effluent gas streams mainly consist of nitrogen and carbon dioxide. Since carbon dioxide has quadrupole and polarizability greater than those of nitrogen[2], modification of the electrostatic field on the surface of adsorbents results in the enhancement of the physical affinity for carbon dioxide, and thus the adsorption capacity of given adsorbents with a greater extent than for nitrogen. Introduction of alkali metals or alkaline earth metals into the surface of an adsorbent gives base sites which have strong affinity for the molecules of acidic gases like carbon dioxide[3,4].

In the present study, surfaces of the three carbon materials were chemically modified by impregnating with calcium acetate solution. Calcination of these adsorbents results in a homogeneous distribution of calcium oxide on the surfaces. The effect of introduction of base sites were investigated by measuring the amount of adsorbed carbon dioxide at various temperatures.

Experimental

Interaction between calcium oxide and carbon dioxide was studied through differential scanning calorimeter(TA instruments, DSC 3000), X-ray diffraction technique (Rigaku, DMAS-B (CuKa)) and carbon dioxide adsorption on calcium oxide in the gas adsorption apparatus(Micromeritics, Accusorb 2100E). Calcium oxide was prepared by calcining calcium acetate monohydrate(Sanyo Pure Chemicals CO., LTD., min. 98.0%) at 700℃ for 2 hours.

Three kinds of carbon materials were used as support materials ; two kinds of  activated carbons (each designated AC1 (The Kansai Coke and Chemicals Co., Ltd, Maxsorb) and AC2 (Aldrich, 27,809-2)) and carbon black (designated CB (Cabot corporation, Vulcan XC 72 GP-3211)). 10g of calcium acetate monohydrate was dissolved in 20ml of deionized water for 5 hours with agitation, followed by vacuum filtration. 1g of each support material was impregnated with this solution for 5 hours at 25℃ by agitation, followed by vacuum drying for c.a. 10 hours at 120℃. Finally, the impregnated support materials were calcined at 700℃ for 2 hours in the flowing nitrogen of 1 l/min. Each adsorbent was designated AC1(Ca), AC2(Ca) and CB(Ca).

Nitrogen adsorption isotherms at 77K were obtained to reveal the microstructure of adsorbents. The surface area was calculated by BET equation and the pore size distribution by Kelvin equation.

Results and Discussion

1. Bulk calcium oxide

Interaction between calcium oxide and carbon dioxide was investigated using DSC, X-ray diffractometer and carbon dioxide adsorption. DSC thermograms of calcium oxide (Fig. 1.) showed that the first exothermic peak at c.a. 300℃ was irreversible with respect to pressure swing and reversible with respect to temperature swing at 600℃. The X-ray diffraction technique revealed that the pattern of calcium oxide reacted with carbon dioxide at ca. 300℃ was similar to that of calcium oxide as a starting material. However, calcium oxide reacted with carbon dioxide at 600℃ showed calcium carbonate peaks due to bulk carbonation. The irreversibility of the adsorption of carbon dioxide on calcium oxide was confirmed by the successive adsorption of carbon dioxide on calcium oxide with evacuation between each adsorption.

It can be concluded from above observations that the main interaction between carbon dioxide and calcium oxide can be classified in two temperature domains ; irreversible chemisorption up to c.a. 300℃ and bulk carbonation near 400℃ and above. Similar results has been reported previously[5].

2. Adsorbents

Microstructures of the support carbon materials and the base-site- containing carbon adsorbents were examined through nitrogen adsorption experiments. The morphological parameters were collected in Table 1.

Separation of gases based on adsorption requires the reversibility in adsorption and desorption. The base-site-containing carbon adsorbents prepared in this work possess highly irreversible surfaces due to the strong chemisorption. This irreversibility, however, can be circumvented by retreating the adsorbents with carbon dioxide. Reversible adsorption-desorption were observed after one dose of carbon dioxide adsorption for all experiments.

Adsorption experiments of carbon dioxide were carried out at 25, 100 and 250℃ with the base-site-containing carbon adsorbents. Similar experiments were also done with unmodified carbon adsorbents to investigate the effect of chemical modification on the adsorption of carbon dioxide.

The isotherms for the reversible adsorption were fitted into the Henry's equation since the adsorption pressure was in the range of very low relative pressure. The amounts of adsorbed carbon dioxide on the carbon adsorbents decrease with increasing temperature, and the same is true for the chemically modified carbon adsorbents. However, since the interaction between the base site and carbon dioxide is stronger than that between carbon and carbon dioxide, considerable amount of carbon dioxide can be adsorbed on the base-site-containing carbon adsorbent at temperatures as high as 250℃. The infrared spectra confirms that chemisorbed carbon dioxide on calcium oxide is the unidentate species[7]. Unidentate species is considered to induce stronger affinity for carbon dioxide than carbon surface. Adsorption of carbon dioxide on the carbon adsorbents are unlikely at this temperature without the chemical modification.

The amount of the irreversible adsorption and Henry's constant for the reversible adsorption were given in Table 2. Henry's constant represents the amount of carbon dioxide adsorption. These values shows the enhancement of adsorptivity due to the addition of base sites at high temperature virtually. As a prominent result, there was no irreversibility of carbon dioxide adsorption on carbon support materials. AC1(Ca) and AC2(Ca) showed the chemisorption at the first carbon dioxide dose only for the carbon dioxide adsorption at 250℃ while CB(Ca) at 25, 100 and 250℃. This chemisorption is caused by calcium oxide. The zero chemisorbed amount at 25℃ and 100℃ on AC1(Ca) and AC2(Ca) seems to be due to small surface coverage of CaO relative to large surface area of their support materials. It can be concluded that the CO2 affinity of the base sites plays a major role in adsorption of CO2 at high temperature.

Fig.1. Interactions of carbon dioxide with calcium oxide. (Heating rate 20℃/min.) :

(a)	calcium oxide obtained from	calcium acetate mono hydrate,

(b)	fresh sample,

(c)	sample in (b) after evacuation	at 330℃ for 2 hours,

(d)	sample in (c) after evacuation	at 600℃ for 2 hours.

Table 1. Microstructures of the carbon adsorbents

	AC1	AC1(Ca)	AC2	AC2(Ca)	CB	CB(Ca)
CaO load, %		27.65		27.35		26.72
SBET, m2/g	3,000	1,500	1,371	630	225	141
Vtot, cm3/g	1.60	0.97	1.26	0.62	0.31	0.19
r, Å	11	13	18.4	19.7	28	27

SBET, total surface area calculated by BET equation; Vtot, total pore volume ; r, average pore radius.

Table 2. Adsorption of carbon dioxide on the carbon adsorbents

adsorbent		Henry's constant for reversible CO2 adsorption (□mol/m2/atm)				irreversible adsorption quantity (□mol/m2)
		25℃	100℃	250℃		25℃	100℃	250℃
CaO						13	23	64
AC1		0.919	0.274	0.048		0	0	0
AC1(Ca)		0.950	0.458	0.140		0	0	1.1
AC2		1.65	0.368	0.140		0	0	0
AC2(Ca)		2.84	0.700	0.698		0	0	0.77
CB		3.41	0.882	0.204		0	0	0
CB(Ca)		1.21	1.16	0.657		0.57	0.40	4.61

References

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2.	Weast, R. C.: "CRC Handbook of Chemistry and Physics", 66th ed., p E-68, CRC press, Inc., Boca Raton, Florida(1985-1986)

3.	Uhlhorn, R. J. R., Keizer, K. and Burggraaf, A. J.: "J. Membr. Sci.", 46, 225(1989)

4.	Cho, Y., Han, K., and Lee, K.: "J. Membr. Sci.", 104, 219(1995)

5.	Cazorla-Amoros, D., Linares-Solano, A., Joly, J. P. and Salinas Martinez de Lecea, C.: "Catalysis Today", 9, 219(1991)