Carbon dioxide adsorption isotherms

Lozano-Castello D, Cazorla-Amoros D, Linares-Solano A, and Quinn DF. Micropore size distributions of activated carbons and carbon molecular sieves assessed by high-pressure methane and carbon dioxide adsorption isotherms. J. Phys. Chem. B, 2002 106(36) 9372-9379. 

Figure 1 shows CO2 adsorption isotherms and Table 1 contains the micropore volume calculated from both nitrogen and carbon dioxide adsorption isotherms. 

Physical sorbents for carbon dioxide separation and removal were extensively studied by industrial gas companies. Zeolite 13X, activated alumina, and their improved versions are typically used for removing carbon dioxide and moisture from air in either a TSA or a PSA process. The sorption temperatures for these applications are usually close to ambient temperature. There are a few studies on adsorption of carbon dioxide at high temperatures. The carbon dioxide adsorption isotherms on two commercial sorbents hydrotalcite-like compounds, EXM911 and activated alumina made by LaRoche Industries, are displayed in Fig. 8.F23,i24] shown in Fig. 8, LaRoche activated alumina has a higher carbon dioxide capacity than the EXM911 at 300° C. However, the adsorption capacities on both sorbents are too low for any practical applications in carbon dioxide sorption at high temperature. Conventional physical sorbents are basically not effective for carbon dioxide capture at flue gas temperature (> 400°C). There is a need to develop effective sorbents that can adsorb carbon dioxide at flue gas temperature to significantly reduce the gas volume to be treated for carbon sequestration. 

Walton KS, MiUward AR, Dubbeldam D, et al. Understanding inflections and steps in carbon dioxide adsorption isotherms in metal-organic firameworks, J Am Chem Soc 130(2) 406-407, 2008. 

The observed results of carbon dioxide adsorption were presented in the form of CO2 isotherms in Fig. 1 and 2. Active carbon is a typical example for hydrophobic adsorbents whereas zeolite 4A is an example for hydrophilic adsorbents. For the former, specific pore volume accessible for benzene is 0.227 cm /g, whereas for the later, these pore volume was an order of magnitude lower, i.e. 0.021 cm7g. The volume of other pores, inaccessible for benzene is 0.017 cm /g for carbon and 0.172 cm /g for zeolite. It can be noticed, that active zeolite-carbon adsorbents have to be characterized by maximally developed structure of both kinds of pores. 

Adsorption microcalorimetry, finally, allows to check the differential enthalpies of adsorption derived from various adsorption isotherms through the isosteric method. What is more, it is much safer and meaningful in the first, raising, part of the isotherm, specially when it is close to the ordinate, such as for instance for carbon dioxide adsorption. This part is probably the most interesting from the viewpoint of specific interactions and gas separation or storage. 

A problem with active carbon is that the usual /-plot is not obtained for adsorption isotherms measured on carbon. At 77 K adsorption is often limited because migration of adsorbed molecules over the surface is required to enter narrow pores. At 77 K the mobility of adsorbed species is often not sufficient. Carbon dioxide adsorption is therefore employed to assess the surface area of activated carbon supports. 

Figure 12 24. Equilibrium isotherms for carbon dioxide adsorption on molecular sieves, types 4A and 5A. Data ofU0P(1993/ ...

Adsorption isotherms for several of the hydrocarbons typically removed from gas streams are given in the previous section of this chapter entitled Properties of Gas Adsorption Carbons. Additional data can be obtained from the carbon manufacturers. A set of equilibrium data for the hydrocarbons and natural gas sulfur compounds on activated carbon (Pittsburgh BPL) has been presented by Grant et al. (1962,1964). These data and some additional experimental data for isobutane and carbon dioxide adsorption on Columbia NXC 8 x 10 carbon have been used by Hasz and Bairere (1964) as a basis for predicting isotherms for all of the common constituents of natural gas using the Polyani potential theory of adsorption (1914). 

The Oij-method (1,2) provides a useful means to detect the presence of mesoporosity in a porous solid it can also be successfully applied to evaluate the volume of micropores, especially in solids devoid of mesopores. However, the capillary structure of activated carbons is frequently composed of both kinds of porosity and so the evaluation might become more difficult. Attempts were made by various authors to transform experimental isotherms, with the purpose to adapt them to a single kind of porosity. This can be done by elimination of adsorption on the surface of mesopores, as is the case when the Dubinin theory of volume filling of micropores is applied (3). Another interesting approach -the carbon dioxide subtraction method (CDS)- has recently been proposed (4), with the aim to obtain an isotherm for porous systems devoid of micropores. In the present paper some related problems are considered, with particular attention to the evaluation of the carbon dioxide adsorption data. The research was carried out on a suite of progressively activated chars from humic acids. 

The micropore volumes for D-8 and D-19 carbons obtained from the adsorption isotherms of n-butane and iso-butane are smaller than that evaluated by nitrogen and carbon dioxide adsorption (cf.. Table 2) this means that the smallest micropores, which are... 

Fig. 2.20 Adsorption isotherms of carbon dioxide at — 78 5°C in TK 800 outgassed at 25 C and lOOO C (triangles). The BET monolayer is indicated as n on each isotherm.

A factor militating against the use of other adsorptives for pore size determination at the present time is the lack of reliable r-curves. The number of published isotherms of vapours such as benzene, carbon tetrachloride or the lower alkanes, or even such simple inorganic substances as carbon dioxide, on a reasonable number of well-defined non-porous adsorbents, is very small. 

Whereas determination of chemisorption isotherms, e.g., of hydrogen on metals, is a means for calculating the size of the metallic surface area, our results clearly demonstrate that IR studies on the adsorption of nitrogen and carbon monoxide can give valuable information about the structure of the metal surface. The adsorption of nitrogen enables us to determine the number of B5 sites per unit of metal surface area, not only on nickel, but also on palladium, platinum, and iridium. Once the number of B5 sites is known, it is possible to look for other phenomena that require the presence of these sites. One has already been found, viz, the dissociative chemisorption of carbon dioxide on nickel. 

This paper presents isotherms for the adsorption of methane, nitrogen, carbon dioxide, and water vapor on a Pocahontas No. 3 (lvb) and a Pittsburgh (hvab) coal. Rate data for the adsorption of methane on these coals will be described in a subsequent paper. 

Carbon dioxide isotherms at —78°C. are also shown in Figure 3. The adsorption of CO2 was relatively slow however, adsorption and desorption points taken at 24-hour intervals seem to represent equilibrium conditions. The pressure scale for CO2 isotherms is given as observed pressure divided by an extrapolated liquid vapor pressure at —78°C. (A plot of the logarithm of vapor pressure of liquid CO2 as a function of reciprocal absolute temperature is linear, and this straight line was the basis of extrapolation. At —78°C. the... 

With binary and ternary supercritical mixtures as chromatographic mobile phases, solute retention mechanisms are unclear. Polar modifiers produce a nonlinear relationship between the log of solute partition ratios (k ) and the percentage of modifier in the mobile phase. The only form of liquid chromatography (LC) that produces non-linear retention is liquid-solid adsorption chromatography (LSC) where the retention of solutes follows the adsorption isotherm of the polar modifier (6). Recent measurements confirm that extensive adsorption of both carbon dioxide (7,8) and methanol (8,9) occurs from supercritical methanol/carbon dioxide mixtures. Although extensive adsorption of mobile phase components clearly occurs, a classic adsorption mechanism does not appear to describe chromatographic behavior of polar solutes in packed column SFC. 

Figure A. Adsorption isotherms for the carbon dioxide/activated carbon system. Reproduced with permission from Ref. 33. Copyright 1972, Chemical Society of Japan.

Figure 7.6 Comparison of the adsorption isotherms of carbon dioxide and methane at 304 K on (a) dehydrated and (b) hydrated forms of MIL-53. Grey- and black-filled circles represent two different experiments. na = amount adsorbed.122 (Reprinted with permission from P. L. Llewellyn et al., Angew. Chem., Int. Ed. 2006, 45, 7751-7754. Copyright Wiley-VCH Verlag GmbH Co. KGaA.)...

Brunauer and Emmett,2 however, take the view that, on porous iron catalysts, the first effect of van der Waals adsorption is to cover the surface with a layer one molecule thick. In the case of several permanent gases, and also of carbon dioxide and butane, if the adsorption isotherms are measured not too far above the boiling-point of the gases, the first layer is complete at 50 mm. pressure or less. If the pressure is raised up to atmospheric, further quantities are adsorbed, and there appears a nearly linear relation between the pressure, and the amount adsorbed in excess of the first monomolecular layer but the increase of adsorption, as the pressure is raised above that at which the first layer is complete, is much more gradual than the increase with pressure, at low pressures, before the surface is completely covered.3... 

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