Molecular sieves equilibrium isotherms

For noncoustaut diffusivity, a numerical solution of the conseiwa-tion equations is generally required. In molecular sieve zeohtes, when equilibrium is described by the Langmuir isotherm, the concentration dependence of the intracrystalline diffusivity can often be approximated by Eq. (16-72). The relevant rate equation is ... 

Tphe breakthrough curve for a fixed-bed adsorption column may be pre-dieted theoretically from the solution of the appropriate mass-transfer rate equation subject to the boundary conditions imposed by the differential fluid phase mass balance for an element of the column. For molecular sieve adsorbents this problem is complicated by the nonlinearity of the equilibrium isotherm which leads to nonlinearities both in the differential equations and in the boundary conditions. This paper summarizes the principal conclusions reached from a recent numerical solution of this problem (1). The approximations involved in the analysis are realistic for many practical systems, and the validity of the theory is confirmed by comparison with experiment. 

Figure 5 shows the experimental breakthrough curves obtained by Sheth (14) for saturation and regeneration of a 4A molecular sieve column with a feed stream containing a small concentration of ethylene in helium. The equilibrium isotherm for this system is highly nonlinear, and, as a result of this, the saturation and regeneration curves have quite different shapes. However, the theoretical curves calculated from the nonlinear analysis using the same values of the parameters bqB and D /rz2 for both... 

The n are interaction terms, one for each solute, and are obtained by correlating the mixture equilibrium data. This approach was used by Yon and Turnock (13) to correlate equilibrium data for adsorption on molecular sieves. The analogous expression for the three parameter isotherm is given by ... 

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. 

A comparison of the adsorption isotherm data measured by the batch method with those measured by the HPLC method for the five alcohols in silicalite is given in Figure 6. As shown in Figure 6, a remarkably good agreement is found between the two methods, verifying the validity of the present HPLC technique for the measurement of adsorption equilibrium as well as diffusion in molecular sieve crystals... 

Zeolite could selectively adsorb A -nitrosamines in the solution of methylene chloride or water, and the equilibrium data were fitted to Freundlich-type isotherms. Textural and acid-basic properties of zeolite determined their adsorption capacity. The extraordinary adsorption properties of NaA zeolite for iV-nitrosamines is inferred that the adsorbates inert the channel with the group -N=N-0. Larger amount of A-nitrosamines was adsorbed on ZSM-5 zeolite in water instead in methylene chloride, due to the hydrophobicity of the zeolite. Application of zeolite to remove A-nitrosamines from beer seems successful. Up to 100% of the worst carcinogenic compounds could thus be removed with 1.4 g/L of zeolite which was proven to be better adsorbent than silica or alumina. On Na 3 zeolite and MCM-41 mesoporous material A-nitrosodimethylamine decomposed above 573 K and the liberated NOx could be detected even at 773 K during the TPSR process, indicating the strong adsorption of A-nitrosamines on molecular sieves that makes zeolite become the functional materials for environmental protection. 

These expressions provide a simple and convenient model for the analysis of experimental uptake curves when the equilibrium isotherm is highly favorable and micropore diffusion is rapid. These conditions are amply fulfilled for the adsorption of water at or near ambient temperature in molecular sieve adsorbents. Experimental uptake curves for this system measured by Kyte are shown in Figure 6.9. The experimental conditions and the effective diffusivity calculated according to Eq. (6.27) are giveii in Tables 6.4 and 6.5. Under the experimental conditions the estimated value of the Knudsen diffusivity [from Eq. (5.17)] is much larger than the molecular diffusivity... 

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.]... 

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. 

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

Figure 2.4 Sketch of equilibrium isotherms of oxygen and nitrogen on molecular sieve carbon at2(TC (redrawn from Crittenden 1992, p. 4.17).

The selectivity of a particular molecular sieve for a given ion as a function of exchanger composition is normally measured fi om an ion exchange isotherm, which is an isonormal [45], isothermal and reversible plot of equilibrium distributions of ions between the solution and zeohte phases. It is emphasised that it is only vahd to calculate selectivity coeffidents, and derived thermodynamic data, from isotherms which are reversible (that is, the forward and reverse isotherms coindde within experimental uncertainty). The types of isotherms. 

One can conclude that stepped isotherms and low pressure hysteresis loops may only be observed on monodispersed and large crystals of network types of molecular sieves. The hysteresis loop was examined in detail by scanning the region between the adsorption and the desorption branch of the hysteresis. The points betweoi the two branches were stable for many hours, thus, the low pressure hysteresis is controlled by equilibrium properties and not by kinetical effects. Previous studies provided clear evidence that the stepped isotherms of argon and nitrogen on MFI-crystals can be rationally explained by localised adsorptive molecules at the channel walls and intersections [10]. 

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