Molecular sieves, separation

Despite the difference ia the nature of the surface, the adsorptive behavior of the molecular sieve carbons resembles that of the small pore zeoHtes. As their name implies, molecular sieve separations are possible on these adsorbents based on the differences ia adsorption rate, which, ia the extreme limit, may iavolve complete exclusion of the larger molecules from the micropores. 

Ellis, L., Aromatic hydrocarbons in crude oil and sediments Molecular sieve separations and biomarkers, RhD thesis, Curtin University of Technology, 1994. 

Molecular sieving Separation on n- and isoparafins using 5A zeolite Separation of xylenes using zeolite... 

Different ways have been proposed to prepare zeolite membranes. A layer of a zeolite structure can be synthesized on a porous alumina or Vycor glass support [27, 28]. Another way is to allow zeolite crystals to grow on a support and then to plug the intercrystalline pores with a dense matrix [29], However, these two ways often lead to defects which strongly decrease the performance of the resulting membrane. A different approach consists in the direct synthesis of a thin (but fragile) unsupported monolithic zeolite membrane [30]. Recent papers have reported on the preparation of zeolite composite membranes by hydrothermal synthesis of a zeolite structure in (or on) a porous substrate [31-34]. These membranes can act as molecular sieve separators (Fig. 2), suggesting that dcfcct-frcc materials can be prepared in this way. The control of the thickness of the separative layer seems to be the key for the future of zeolite membranes. 

Existing literature and data will be used to avoid personal or institutional bias. One system involves relatively inert (nitrogen) physisorption on chars of interest as sorbents, catalysts, molecular sieves, separation substrates, storage media, etc. All of the sorption data presented here exist in tabular form, such that verification of this work and development of alternate theories, verification, and methodology is facilitated. 

Zeolite materials are used commercially as shape/ size selective catalysts in the petrochemical and petroleum refining industry, and as molecular sieving separation media for gases and hydrocarbons. For both applications, zeolites are used in powder composite form such as pellets and granules. In this entry, we focus on zeolite membranes. We define zeolite membranes as a continuous phase of zeolite-based materials (pure zeolite or composite) that separate two spaces. Zeolite membranes are generally uniform thin films attached to a porous or a nonporous substrate. They can also be self-standing without a substrate. Note that we have included zeolite films and layers on nonporous substrate in this entry because we believe many of the synthesis strategies and applications reported for those nonporous substrates are easily transferred to a porous substrate to prepare a zeolite membrane. 

AEs, the difference in transport activation energy and adsorption energy may be positive or negative. When AEs < 0, transport due to selective surface flow will increase with decreasing temperature with AEs > 0 it will decrease. Plainly stated, adsorption and hence selectivity increase with decreasing temperature. This is the opposite of temperature influence for molecular sieving separation (see below). 

The reduction in the free diameter of the windows by blocking cations causes a dramatic reduction in the diffusivity of the guest molecules. The extent to which the windows are obstructed depends on the number and nature of the cations since different cations show differing affinities for the window sites. By appropriate choice of cationic form it is sometimes possible to develop kinetic selectivity and even, in certain cases, to obtain a molecular sieve separation between species which can both diffuse easily in an unobstructed sieve. A schematic representation showing the effective apertures for some cationic forms of A and X zeolites, as well as in some other sieves, is shown in Figure 1.5. 

The main applications of PSA are to be found in the production of oxygen from air, dehumidification of gases and purification of hydrogen. Other applications include removal of carbon dioxide, recovery of radioactive waste gas, enrichment recovery of rare gases, purification of helium, purification of natural gases, separation of isomers and separation of carbon monoxide. Separation of iso-parafllns from normal paraffins is accomplished by using a shape-selective adsorbent such as a molecular sieve. Separation of carbon monoxide involves chemical adsorption on complex adsorbents. 

FIGURE 9.12 Schematic representation of membrane-based gas separations, (a) Knudsen-flow separation, (b) surface-difftision, (c) capillary condensation, (d) molecular-sieving separation, and (e) solution-difftision mechanism. 

Carboxen 1010 PLOT Carbon molecular sieve separation of hydrogen, nitrogen, CO, methane, carbon dioxide, and C2 and C3 hydrocarbons. 

Deng, Z., Pera-Titus, M., Guo, Y., and Giroir-Fendler, A. (2010) Molecular sieving separation of hexane isomers within nanocomposite (B)-MFI-alumina hollow fiber membranes a modeling study. Ind. Eng. Chem. Res., 49,... 

The possibility of producing thin coherent defect free zeolite membranes that will allow industrially important molecular sieving separations to be carried out as a continuous flow process has attracted much attention over the past decade... 

In nanoporous materials diffusion is sterically hindered so that the diffusional activation energy (and hence the permeance) are strongly dependent on molecular size (see Fig. 2), thus giving rise to the possibility of size selective molecular sieve separations. In extreme cases where one of the components is sterically excluded from the pore system a highly efficient molecular sieve separation may be achieved (provided that the membrane is coherent). However,... 

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