Carbon molecular sieves kinetic separation

Air separation G Carbon molecular sieve Kinetic Pressure swing... 

Advanced Materials Experimental membranes have shown remarkable separations between gas pairs such as O9/N9 whose kinetic dian ieters (see Table 22-23) are quite close. Most prominent is the carbon molecular sieve membrane, which operates by ultran iicro-porous molecular sieving (see Fig. 22-48c). Preparation of large-scale permeators based on ultran iicroporous membranes has proven to be a major challenge. 

Selectivity. Selectivity in a physical adsorption system may depend on differences in either equilibrium or kinetics, but the great majority of adsorption separation processes depend on equilibrium-based selectivity. Significant kinetic selectivity is. in general, restricted to molecular sieve adsorbents—carbon molecular sieves, zeolites, or zeolite analogues. 

The primary requirement for an economic adsorption separation process is an adsorbent with sufficient selectivity, capacity, and life. Adsorption selectivity may depend either on a difference in adsorption equilibrium or, less commonly, on a difference in kinetics. Kinetic selectivity is generally possible only with microporous adsorbents such as zeolites or carbon molecular sieves. One can consider processes such as the separation of linear from branched hydrocarbons on a 5A zeolite sieve to be an extreme example of a kinetic separation. The critical molecular diameter of a branched or cyclic hydrocarbon is too large to allow penetration of the 5A zeolite crystal, whereas the linear species are just small enough to enter. The ratio of intracrystalline diffusivities is therefore effectively infinite, and a very clean separation is possible. 

In molecular sieve adsorbents, such as zeolites and carbon molecular sieves, the micropore size distribution is extremely narrow, thus allowing the possibility of kinetic separations based on differences in molecular size. However, this feature is utilized in only a few commercial adsorption separation processes, and in the majority of such processes the separation depends on differences in the adsorption equilibrium rather than on the kinetics, even though a molecular sieve adsorbent may be used. 

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. 

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. 

Carbon molecular sieve (CMS) is useful in air separation processes because of its ability to selectively discriminate on the basis of molecular size and hence adsorb the smaller oxygen molecule over nitrogen. The difference in the adsorption kinetics of various gases allows the separation of gas mixtures into pure components using pressure swing adsorption (PSA). 

Reid, C.R., and Thomas K.M., Adsorption of Gases on a Carbon Molecular Sieve Used for Air Separation Linear Adsorptives as Probes for Kinetic Selectivity, Langmuir, 15, 3206 (1999). 

Kinetic selectivities between O2 and Ar in four adsoibents were compared based on equilibria and kinetics measured at low pressure. The equilibrium and kinetic parameters of Ar on the chosen sample, a Carbon Molecular Sieve, were then measured over a wide pressure range. These equilibrium and kinetic parameters were used to theoretically investigate separation of Ot-Ar mixture by Pressure Swing Adsorption. 

Recently, carbon molecular sieves have been fabricated in the form of planar membranes and hollow tubes by the pyrolysis of polyacrylonitrile in suitable forms (12-16). Very high separation selectivities have been reported with these materials. Their pore sizes are in the range from 3 to 5.2A. Selectivities of greater than 100 1 are observed between molecules which differ by as little as 0.2A in their critical dimensions. Kinetics of adsorption on these materials have been determined (2.,ii,l ) -... 

A particular application of porous carbon as filter is the gas separation [228] by carbon molecular sieves (CMS). This highly microporous (predominant pore size carbon material is able to distinguish a difference of the order of 0.02 nm between the kinetic diameters of O2 and N2 molecules. Thus, the separation of nitrogen from air can be achieved... 

Ahmadpour and coworkers" used carbon molecular sieves prepared naturally occurring substrate (Iranian Walnut shell) and commercial activated carbon (Silcarbon) for air and hydrocarbon separation, using four different methods based on the adjustment of pore opening in the activated carbon structure. They studied the adsorption kinetics of O2, N2, CH4, and C2H4 (Figure 4.19). Comparison of Figures 4.19 (a), (b), (c), and (d) indicates that none of these methods is able to separate O2 from N2 in air. 

In another publication, Mirhabibi and coworkers carried out separation of methane, ethane, and ethylene from nitrogen was carried out by adsorption on a carbon molecular sieve (Figure 4.20) with pore diameter less than 0.4 nm prepared from Iranian natural-nut shell. The kinetic adsorption curves shows that good selectivities... 

Kinetic Separations. As discussed in Chapter 5, carbon molecular sieves have already been used for gas separation that is based on differences in diffusivities of different gas molecules. The same separations should also be possible with carbon nanotubes. To this end, a number of simulation studies have been carried out. Mao and Sinnott (2000 and 2001) have reported molecular dynamics simulation results for diffusion of methane, ethane, n-butane, and isobutene, as well as their binary mixtures, in SWNTs and their bundles. As expected, diffusion of smaller molecules is faster, for example a factor of 25 was obtained for melhane/isobutene in a (8,8) nanotube (Mao and Sinnott, 2001). 

Ultra-micropores, with diameters less than 0.5 nm, present special problems for strongly adsorbed carriers, even for supposedly inert gases such as He [34]. The separation of individual components of gas mixtures in pores with diameters between 0.3 nm and 0.5 nm may strongly depend on the kinetics of adsorption governed by the shape of adsorptive molecules, as well as that of the pores, as shown by M. L. Sykes et al [35], but the differences in adsorbate affinity for the surface may also play a role as shown recently by the author for a carbon molecular sieve [36]. 

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