Adsorption by Carbon Molecular Sieves

The development of ultrafine porous structure in active carbons (i.e., the preparation of molecular sieve carbons) has been the subject matter of a large number of investigations because these materials find applications in industrial separation processes. These materials have some distinct advantages over the zeoUte sieves.  

Furthermore, CMS obtained from certain carbonaceous materials such as coal can be obtained at very low cost and do not polymerize olefin material in petroleum industry, whereas zeolite sieves can cause such polymerization. CMS also have sufficient strength to be used in a fluidized process for which zeolites are unsuitable due to their inherent weakness. 

FIGURE 4.18 Adsorption kinetics curves of Nj/Oj and CO2/CH4 on CMS. (After de Salazar, C.G., Sepulveda-Estribano, A., and Rodiiguez-Reinoso, R, 24th Bienn. Conf. on Carbon 1999, Ext. Abstr., p. 36. With permission.) 

Pedrero et al. - prepared CMS by chemical vapor deposition on a lignin-based microporous carbon. The textural characterization of the CMS was carried out by adsorption of N2 at 77 K and CO2 at 273 K. The sieving properties of the CMS were determined by the kinetics of adsorption of O2/N4 and CO2/CH4 mixtures. The adsorption capacities of the carbon for O2 and CO2 decreased slightly with deposition. When the pyrolytic carbon deposited was 0.3%, the adsorption of nitrogen was reduced drastically, and the adsorption of CH4 was impeded. The oxygen selectivity (ratio of O2 to N2 adsorbed in 2 min.) was increased rapidly to a value of 6, while the decrease in the adsorption of N2 was only 20%. Similar behavior was observed in the case of CO2/CH4 mixtures. 

FIGURE 4.21 High resolution electron transmission electron microscopy (HRTEM) microstructure of original Persian nutshell charcoal. (After Ahmadpour, A., Abedinzadegan, M., Mahadiarfar, M., Rashidi, A.M., Jalilian, A., and Mirhabibi, A.R., Carbon 02 Intern. Conf. on Carbon, Beijung, Sept. 15-19, 2002. With permission.) 

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Gawrys, M., Fastyn, P., Gawlowski, J., Gierczak, T. and Niedzielski, J. (2001) Prevention of water vapour adsorption by carbon molecular sieves in sampling humid gases. Journal of Chromatography A, 933, 107-16. 

This book has been written in eight chapters, which cover activated carbons their surface structure the adsorption on solid surfaces and the models of adsorption adsorption from solution phase the preparation, characterization of, and adsorption by carbon molecular sieves important applications of activated carbons with special emphasis on medicinal and health applications and the use of activated carbons in environmental clean up. 

EPA. 1988f Method T02. Method for the determination of volatile organic compounds in ambient air by carbon molecular sieve adsorption and GC/MS. Compendium of methods for the determination of toxic organic compounds in ambient air. Atmospheric Research and Exposure Assessment Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, RTP, NC. EPA/600/4-89/017. 

Figure 22.4 Air separation by carbon molecular sieve (CMS) (a) uptakes of O2 and Nj from air on a CMS sample, (b) schematic drawing of a Nj pressure swing adsorption (PSA) system, (c) performance of a Nj PSA process.

Schrbter, H.J. and Jiintgen, H. (1989). Gas separation by pressure swing adsorption using carbon molecular sieves. In Adsorption Science and Technology, NATO ASI Series, Vol. 158 (A.E. Rodrigues, M.D. Levan, and D. Tondeur, eds). Kluwer Academic, pp. 269-83. 

Characterization of IOM-CMS Materials by Molecular Probe Adsorption. Since carbon molecular sieves, and the IOM-CMS materials prepared herein, are both polycrystalline and amorphous, we have characterized their sieving properties phenomenologically by molecular probe adsorption. The probe... 

H. J. Schroter and H. Jungten, Gas Separation by Pressure Swing Adsorption Using Carbon Molecular Sieves, in "Adsorption Science and Technology, NATO ASI... 

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). 

Mercury is emitted from the mercury cell process from ventilation systems and by-product streams. Control techniques include (1) condensation, (2) mist elimination, (3) chemical scrubbing, (4) activated carbon adsorption, and (5) molecular sieve absorption. Several mercury cell (chloralkali) plants in Japan have been converted to diaphragm cells to eliminate the poisonous levels of methyl mercury found in fish (9). 

CO2 adsorption capacities with dry sorbents before and after attrition were shown in Fig.3. We found variation of CO2 adsorption capacity during operation by examining effect of attrition on adsorption capacity. So, adsorption experiments for each sorbent fluidized for 30hours were carried out. As a result, percentage losses of adsorption capacity of molecular sieve 5A and molecular 13X were 14.5% and 13.5%, but those of activated carbon and activated alumina were 8.3% and 8.1% respectively. This is because retention time of molecular sieve 5A and molecular 13X decreased due to elutriation of particle generated from attrition. 

Purification of Air Prior to Liquefaction. Separation of air by cryogenic fractionation processes requires removal of water vapor and carbon dioxide to avoid heat exchanger freeze-up. Many plants today are using a 13X (Na-X) molecular sieve adsorbent to remove both water vapor and carbon dioxide from air in one adsorption step. Since there is no necessity for size selective adsorption, 13X molecular sieves are generally preferred over type A molecular sieves. The 13X molecular sieves have not only higher adsorptive capacities but also faster rates of C02 adsorption than type A molecular sieves. The rate of C02 adsorption in a commercial 13X molecular sieve seems to be controlled by macropore diffusion 37). The optimum operating temperature for C02 removal by 13X molecular sieve is reported as 160-190°K 38). 

To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. 

Carbon molecular sieves are produced by controlled pyrolysis and subsequent oxidation of coal, anthracite, or organic polymer materials. They differ from zeolites in that the micropores are not determined by the crystal structure and there is therefore always some distribution of micropore size. However, by careful control of the manufacturing process the micropore size distribution can be kept surprisingly narrow, so that efficient size-selective adsorption separations are possible with such adsorbents. Carbon molecular sieves also have a well-defined bi-modal (macropore-micropore) size distribution, so there are many similarities between the adsorption kinetic behavior of zeolitic and carbon molecular sieve systems. 

Physical adsorption at a surface is extremely rapid, and the kinetics of physical adsorption are invariably controlled by mass or heat transfer rather than by the intrinsic rate of the surface process. Biporous adsorbents such as pelleted zeolites or carbon molecular sieves offer three distinct resistances to mass transfer the external resistance of the... 

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 most striking feature of Figure 8.2 is the effect of the additional degree of freedom provided by a parallel-sided slit. Indeed, this difference in the packing density in slits and cylinders will be seen to be of great importance when we consider the adsorptive properties of molecular sieve carbons and certain zeolites. 

Assessment of uitramicroporosity on carbon molecular sieves by water adsorption... 

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. 

By using potassium as a carbon gasification catalyst, it is possible to obtain activated carbons of large adsorption capacity (large micropore volume), but with micropores of small dimensions. Nevertheless, these materials could not be converted into carbon molecular sieves by carbon deposition from benzene pyrolysis. Success was achieved with chars which were activated only to a limited extent [16]. 

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