Carbon molecular sieves isotherms

Many simple systems that could be expected to form ideal Hquid mixtures are reasonably predicted by extending pure-species adsorption equiUbrium data to a multicomponent equation. The potential theory has been extended to binary mixtures of several hydrocarbons on activated carbon by assuming an ideal mixture (99) and to hydrocarbons on activated carbon and carbon molecular sieves, and to O2 and N2 on 5A and lOX zeoHtes (100). Mixture isotherms predicted by lAST agree with experimental data for methane + ethane and for ethylene + CO2 on activated carbon, and for CO + O2 and for propane + propylene on siUca gel (36). A statistical thermodynamic model has been successfully appHed to equiUbrium isotherms of several nonpolar species on 5A zeoHte, to predict multicomponent sorption equiUbria from the Henry constants for the pure components (26). A set of equations that incorporate surface heterogeneity into the lAST model provides a means for predicting multicomponent equiUbria, but the agreement is only good up to 50% surface saturation (9). 

Type I isotherms are characteristic of microporous solids having relatively small external surface area (activated carbons, molecular sieve zeolites, metal organic frameworks, etc.). They are usually obtained by most gases and vapors on activated carbons. 

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. 

The reversible Type I isotherm (Type I isotherms are sometimes referred to as Langmuir isotherms, but this nomenclature is not recommended) is concave to the p/pa axis and na approaches a limiting value as p/p° — 1. Type I isotherms are given by microporous solids having relatively small external surfaces (e.g. activated carbons, molecular sieve zeolites and certain porous oxides), the limiting uptake being governed by the accessible microporc volume rather than by the internal surface area. 

Many new adsorbents have been developed over the past 20 years including carbon molecular sieves, new zeolites and aluminophosphates, pillared clays and model mesoporous solids. In addition, various spectroscopic, microscopic and scattering techniques can now be employed for studying the state of the adsorbate and microstructure of the adsorbent. Major advances have been made in the experimental measurement of isotherms and heats of adsorption and in the computer simulation of physisorption. 

Two kinetic (CMS-Kl, CMS-K2) and one equilibrium (CMS-R) carbon molecular sieves, used originally for separation of gaseous mixtures, were investigated. The adsorption Nj isotherms at 77 K, in static conditions where obtained. In the case of the two first sieves the adsorption was so low that the calculation of parameters characterizing the texture was impossible. The volume of nitrogen adsorbed on the sieve CMS-R is remarkable From obtained results parameters characterizing micropore structure according to Dubinin -Radushkevich equation and Horvath - Kawazoe method were determined. 

For description of textural properties of carbonaceous adsorbents, adsorption/desorption isotherms of vapours and gases in static conditions as well as mercury porosimetry are used. The latter method often leads to destruction of porous structure of investigated materials while the usage of the former one is affected by the specific properties of molecular sieves described above. Taking into account these limitations, in this work the authors have made an attempt of determination of porous structure of carbon molecular sieves with the used of the pycnometric technique. 

Figure 14.23 Adsorption isotherm for ethylene on a carbon molecular sieve at 323.15 K (50°C)...

For commercial applications, an adsorbent must be chosen carefully to give the required selectivity, capacity, stability, strength, and regenerability. The most commonly used adsorbents are activated carbon, molecular-sieve carbon, molecular-sieve zeolites, silica gel, and activated alumina. Of particular importance in the selection process is the adsorption isotherm for competing solutes when using a particular adsorbent. Most adsorption operations are conducted in a semicontinuous cyclic mode that includes a regeneration step. Batch slurry systems are favored for small-scale separations, whereas fixed-bed operations are preferred for large-scale separations. Quite elaborate cycles have been developed for the latter. 

This equation reduces to the famous Langmuir isotherm when n = 1. The adsorption equation (2.4-19) without adsorbate - adsorbate interactions works well with adsorption of hydrocarbons and carbon dioxide on activated carbon and carbon molecular sieve with n ranging from 2 to 6. 

FIGURE 11.27. Equilibrium isotherms (a) and experimental uptake curves (b) for sorption of O2 and N2 on Bergbau-Forschung carbon molecular sieve. (From ref. 37 excerpted by special permission from Chemical Engineering, Vol. 8., No. 25, 87 (1978). Copyright 1978, by McGraw-Hill, Inc., New York, N.Y. 1(X)20.]... 

The influence of sulfur surface compounds on the adsorption of polar and nonpolar vapors of varying molecular dimensions was examined by Puri and Hazra. The adsorption of water vapors increased appreciably at relative pressures lower than 0.4 and decreased at higher relative pressures. The effect increased with increase in the amount of sulfur fixed and was attributed to the variation of the pore-size distribution caused by the fixation of sulfur along the pore walls. The adsorption isotherms of methanol and benzene vapors indicated that these larger molecules found smaller and smaller areas as more and more sulfur was being incorporated into the pores. Bansal et prepared carbon molecular sieves by blocking pores of PVDC charcoals by... 

The pyrolysis of benzene over the active carbon surface results in the deposition of the carbon on the surface of the substrate carbon as weU as in the microporous system and at some preferred sites. The adsorption isotherms of organic molecules of varying sizes and shapes indicated that the carbon gets deposited preferentially in the pore entrances reducing entrance diameter resulting in the formation of carbon molecular sieves. Pore-size distribution curves indicated that the treatment with benzene between 3 and 6 hrs reduces the mean pore dimensions to 0.6 nm, and a larger time of treatment reduces pore entrances to less than 0.6 nm. 

Carbon Molecular Sieves. Excellent carbon molecular sieve materials have been developed in industry. On these sorbents, the O2, N2, and Ar isotherms are approximately equal because they all adsorb by van der Waals interactions, and their polarizabilities are approximately the same. But the diffusivily ratio for 02(Ar)/N2 is approximately 30 due to differences in molecular size (Chen et al., 1994). 

Figure 2. Adsorption isotherms on carbon molecular sieve at 25 C. ---------oxygen------------nitrogen.

The purpose of this paper is to present such a model which can be regarded as a synthesis of fundamental considerations resulting from the activities in porous sorbents characterization and classical thermodynamics developments. This model is able to represent in a correct way adsorption data of oxygen, argon, nitrogen and methane on a given carbonaceous adsorbent using a unique pore size distribution function whatever the adsorbate. The procedure was applied on four different activated carbons and on a carbon molecular sieve. The adsorption isotherms were measured at 283 K, 303 K and 323 K and for pressures up to 2200 kPa. 

Carbon molecular sieves (CMS) adsorbents are produced by pyrolysis of carbonaceous materials followed by carefully controlled deposition of carbon within the pores [43]. In contrast to activated carbons which have a broad distribution of micropore size (generally in the 10 - 100 A range) the pores of a carbon molecular sieve are very small (< 10 A) and the pore size distribution in narrow. As a result the adsorption behavior is similar to that of a zeolite. Carbon molecular sieves are widely used for production of nitrogen from air (by selective adsorption of oxygen). There is little difference between the equilibrium isotherms of O2 and N2 on CMS but as a result of its slightly smaller molecular size oxygen is adsorbed very much faster (diffiisivity ratio 10 - 100). The sorption kinetics show some interesting features. 

In order to determine the PSD of the micropores, Horvath-Kawazoe (H-K) method has been generally used. In 1983, Horvath and Kawazoe" developed a model for calculating the effective PSD of slit-shaped pores in molecular-sieve carbon from the adsorption isotherms. It is assumed that the micropores are either full or empty according to whether the adsorption pressure of the gas is greater or less than the characteristic value for particular micropore size. In H-K model, it is also assumed that the adsorbed phase thermodynamically behaves as a two-dimensional ideal gas. 

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