Molecular sieve carbons

A detailed study of diffusion in 5A carbon molecular sieve has been published by Chihara, Suzuki, and Kawazoe who used both the chromatographic and gravimetric methods. The concentration dependence of the 

FIGURE 5 1. Concenlralion dependence of diffusional time constant for Nj in molecular sieve carbon. (From ref. 82. with permission.) 

FIGURE 5.23. Arrhenius plot showing temperature dependence of limiting diffusivity for small molecules in molecular sieve carbon MSC-5A. (From ref. 81, reprinted with permission.) 

Essentially the same model as applied by Karger et al. to diffusion in 

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Fig. 2. Pore size distribution of typical samples of activated carbon (small pore gas carbon and large pore decolorizing carbon) and carbon molecular sieve (CMS). A / Arrepresents the increment of specific micropore volume for an increment of pore radius.

Fig. 8. Variation of activation energy with kinetic molecular diameter for diffusion in 4A 2eohte (A), 5A 2eohte (0)> carbon molecular sieve (MSC-5A) (A). Kinetic diameters are estimated from the van der Waals co-volumes. From ref. 7. To convert kj to kcal divide by 4.184.

In certain adsorbents, notably partially coked 2eohtes and some carbon molecular sieves, the resistance to mass transfer may be concentrated at the surface of the particle, lea ding to an uptake expression of the form... 

Fig. 9. Uptake curves for N2 in two samples of carbon molecular sieve showing conformity with diffusion model (eq. 24) for sample 1 (A), and with surface resistance model (eq. 26) for example 2 (0)j LDF = linear driving force. Data from ref. 18.

Fig. 16. Plots showing (a) variation of (c F/2)J. / ) with 1 for O2 (left plot, X, 0.84- 0.72 mm = 20-25 mesh Q 0.42-0.29 mm = 40-50 mesh) and N2 (right plot, on 3.2-mm pellets) in Bergbau-Forschung carbon molecular sieve and (b) variation of HETP with Hquid velocity (interstitial) for fmctose (soHd symbols), and glucose (open symbols) in a column packed with KX 2eoHte crystals. From refs. 22 and 23.

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). 

New Adsorbent Materials. SihcaUte and other hydrophobic molecular sieves, the new family of AlPO molecular sieves, and steadily increasing families of other new molecular sieves (including stmctures with much larger pores than those now commercially available), as well as new carbon molecular sieves and pillared interlayer clays (PILCS), will become more available for commercial appHcations, including adsorption. Adsorbents with enhanced performance, both highly selective physical adsorbents and easily regenerated, weak chemisorbents will be developed, as will new rate-selective adsorbents. 

Fig. 3. Pressure swing adsorption nitrogen generation system. CMS = carbon molecular sieve.

The extent of purification depends on the use requirements. Generally, either intense aqueous extractive distillation, or post-treatment by fixed-bed absorption (qv) using activated carbon, molecular sieves (qv), and certain metals on carriers, is employed to improve odor and to remove minor impurities. Essence grade is produced by final distillation in nonferrous, eg, copper, equipment (66). 

Adsorbents are natural or synthetic materials of amorphous or microcrystalhne structure. Those used on a large scale, in order of sales volume, are activated carbon, molecular sieves, silica gel, and activated alumina [Keller et al., gen. refs.]. 

Hydrophobic Activated carbon Carbon molecular sieves... 

The classifications in Table 16-3 are intended only as a rough guide. For example, a carbon molecular sieve is truly amorphous but has been manufactured to have certain structural, rate-selective properties. Similarly, the extent of hydrophobicity of an activated carbon will depend on its ash content and its level of surface oxidation. 

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. 

Nandi, S.P and Walker, P.L. Jr., Carbon molecular sieves from the concentration of oxygen from air. Fuel, 1975, 54, 169 178. 

As an example of the selective removal of products, Foley et al. [36] anticipated a selective formation of dimethylamine over a catalyst coated with a carbon molecular sieve layer. Nishiyama et al. [37] demonstrated the concept of the selective removal of products. A silica-alumina catalyst coated with a silicalite membrane was used for disproportionation and alkylation of toluene to produce p-xylene. The product fraction of p-xylene in xylene isomers (para-selectivity) for the silicalite-coated catalyst largely exceeded the equilibrium value of about 22%. 

Carbon molecular sieve membranes Resistant to contaminants Intermediate hydrogen flux and selectivity Intermediate hydrogen flux and selectivity High water permeability Pilot-scale testing in low temperature WGS membrane reactor application Need demonstration of long-term stability and durability in practical applications... 

Liu, P.K.T., Carbon Molecular Sieve Membrane as Reactor for Water Gas Shift Reaction, Proceedings of 2006 U.S. DOE Hydrogen Annual Merit Review Meeting, Arlington, VA, May 2006. 

Suda, H. and Haraya, Alkene/Alkane permselectivities of a carbon molecular sieve membrane, /. Chem. Soc. Chem. Commun., 93, 1997. 

Rate Nitrogen from air by PSA using carbon molecular sieve Nitrogen and methane using titanosilicate ETS-4... 

KURASEP [Kuraray Separation] A process for separating nitrogen from air by a variant of the PSA process, using carbon molecular sieve as the adsorbent. Developed by Kuraray Chemical Company. 

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