Selective adsorption of nitrogen

Methane/Nitrogen Gas Separation over the Zeolite Clinoptilolite by the Selective Adsorption of Nitrogen... 

Early work of Barrer (3), McKee (4) and Domine and Hay (5) showed that calcium A, calcium X, mordenite and several types of natural zeolites could be used to enrich air by a selective adsorption of nitrogen. Several pressure-swing-adsorption processes utilizing zeolite adsorbents have been developed which yield a product containing up to 95% oxygen at rates of 20 tons per day (6,7). 

Aluminium dissolves with H2 evolution, and this hydrogen remains chemisorbed on nickel, presumably in a dissociated form. Raney nickel catalysts are often doped with other metals in order to improve the catalytic activity the selectivity decreases in the order. Mo > Cr > Fe > Cu > Co. These metals are fused with the Ni-Al alloy and remain on the final catalyst, probably as oxides. It is believed that the role of the doping metals is to strengthen the selective adsorption of nitrogenous substrates. 

J. Dewar Found selective adsorption of oxygen from a mixture with nitrogen, during the uptake of air by charcoal 1904... 

In addition, the large pore volume, pore size flexibility, and structural variety of MCM-41 can be extensively used for the selective adsorption of a diversity of gases and liquids [39,40], An extremely high sorption capacity for benzene has been demonstrated [40], Widespread work has been carried out on the sorption properties of some adsorbates, such as nitrogen, argon, oxygen, water, benzene, cyclopentane, toluene, and carbon tetrachloride, as well as certain lower hydrocarbons and alcohols on MCM-41 [122],... 

As a result of their systematic study of the adsorption of nitrogen and other gases on a number of different adsorbents, Emmett and Brunauer (1937) came to the conclusion that Point B marked the boundary between monolayer and multilayer adsorption. It will be recalled that Point B was defined as the beginning of the middle, nearly linear, region of an adsorption isotherm. On an empirical basis a number of other characteristic features (designated A, C, D, and E) were rejected in favour of Point B. The main reasons for the selection of Point B were (1) because it gave fairly consistent values of surface area, and (2) because at this point the isosteric heat of adsorption appeared to undergo an appreciable decrease. 

The aim of this study is to eompare pore structure characteristics of two industrial catalysts determined by standard methods of textural analysis (physical adsorption of nitrogen and mercury porosimetry) and selected methods for obtaining parameters relevant to transport processes (multicomponent diffusion and permeation of gases). 

Another process likely to occur is calcite precipitation, when pH and calcium concentration are both high. The NOM then adsorbs on the calcite surface. Suess (1973a, 1973b) measured the selective adsorption of organics onto calcite and mentioned the possibility of h] drated organo-calcium complexes on calcite surfaces. He described the thick layer of organics on the calcite surface as nitrogen-rich, possibly protein-like substances. 

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. 

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. 

The question of whether adsorption should be done ia the gas or Hquid phase is an interesting one. Often the choice is clear. Eor example, ia the separation of nitrogen from oxygen, Hquid-phase separation is not practical because of low temperature requirements. In C q—olefin separation, a gas-phase operation is not feasible because of reactivity of feed components at high temperatures. Also, ia the case of substituted aromatics separation, such as xylene from other Cg aromatics, the inherent selectivities of iadividual components are so close to one another that a simulated moving-bed operation ia hquid phase is the only practical choice. 

Final purification of argon is readily accompHshed by several methods. Purification by passage over heated active metals or by selective adsorption (76) is practiced. More commonly argon is purified by the addition of a small excess of hydrogen, catalytic combustion to water, and finally redistiHation to remove both the excess hydrogen and any traces of nitrogen (see Fig. 5) (see Exhaust control, industrial). With careful control, argon purities exceed 99.999%. 

Additives. Because of their versatility, imparted via chemical modification, the appHcations of ethyleneimine encompass the entire additive sector. The addition of PEI to PVC plastisols increases the adhesion of the coatings by selective adsorption at the substrate surface (410). PEI derivatives are also used as adhesion promoters in paper coating (411). The adducts formed from fatty alcohol epoxides and PEI are used as dispersants and emulsifiers (412). They are able to control the viscosity of dispersions, and thus faciHtate transport in pipe systems (413). Eatty acid derivatives of PEI are even able to control the viscosity of pigment dispersions (414). The high nitrogen content of PEIs has a flame-retardant effect. This property is used, in combination with phosphoms compounds, for providing wood panels (415), ceUulose (416), or polymer blends (417,418) with a flame-retardant finish. 

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