The air separation properties of zeolite types A, X, and Y have been studied using grand canonical Monte Carlo simulations of nitrogen, oxygen, and argon adsorbed in these zeolite lattices. Nitrogen is adsorbed preferentially due to the quadrupole-ion electrostatic interactions with the extra framework cations. The localization of adsorption sites for nitrogen near cations and the more diffuse distributions of oxygen and argon within zeolite cavities are clearly illustrated. Predicted nitrogen/oxygen selectivity for 5A from simulations is in good agreement with that determined experimentally. The effect of the calcium-sodium ion exchange on the predicted nitrogen/oxygen selectivity is examined, and is shown to be sensitive to the magnitude of the charges assigned to the extra framework cations.

Investigation of the Air Separation Properties of Zeolites Types A, X and Y by Monte Carlo Simulations

The recent status of pressure swing adsorption (PSA) as a process for separating multicomponent gas mixtures is reviewed. The applications of a new generation of adsorbents, such as zeolites, carbon molecular sieves, and, more recently, pore engineered molecular sieves, are described in detail. The more important theories of adsorption from gas mixtures as well as those of the PSA process are described briefly. The commercial applications of PSA the process-present and potential-are discussed at length.

Separation of gases by pressure swing adsorption

Due to their crystallochemical properties, clay minerals feature different types of structural surfaces which have their own adsorption energy distribution. To study that type of surface heterogeneity, the DIS (derivative isotherm summation) method has been developed by us. Now, a modified version of the DIS method is derived by using the Jagiello-Rudzinski approach and assuming that the local energy distributions are represented by the Dubinin-Asthakov distributions. Two different types of isotherms equations are used, one to describe adsorption in micropores and another one for describing adsorption on external surfaces. The derivatives of experimental adsorption isotherms with regard to ln(p/p s ) are simulated by combinations of the derivatives of corresponding local adsorption isotherms. This best fit provides information on adsorption capacity of the local existing domains, on the symmetry oftheir energy distribution function, and on the parameters characterizing the lateral interactions in each adsorption domain. Using the new equations for the local isotherm derivatives allows now to simulate very accurately derivatives of experimental adsorption isotherms, obtained by using our high-resolution quasi-equilibrium volumetric technique. This was proven in the case for three different well characterized clay minerals: a structural microporous one (palygorskite) and two nonporous lamellar ones (kaolinites). The obtained parameters allow a description ofthe adsorption energy distribution of their different surfaces, their textural parameters, as well as the energy distrubution in micropores for the microporous samples. In addition to the experimental adsorption isotherms, the related experimental heats ofadsorption were employed as a second independent source of information about the energetic heterogeneity of the studied clay minerals. Using the parameters determined from adsorption isotherms, the corresponding isosteric heats of adsorption were calculated and compared with experimental values. The simultaneous good fit of the experimental isotherm derivatives and of the experimental heats of adsorption was a solid check for the correctness ofthe determined parameters characterizing the adsorption energy distributions and the lateral interactions between the adsorbed molecules.

An Improved Derivative Isotherm Summation Method To Study Surface Heterogeneity of Clay Minerals

Investigation of competitive adsorption is carried out using the Xe–Ar mixture in zeolite NaA as a model system. The Xen clusters are trapped in the alpha cages of this zeolite for times sufficiently long that it is possible to observe individual peaks in the NMR spectrum for each cluster while the Ar atoms are in fast exchange between the cages and also with the gas outside. The 129Xe nuclear magnetic resonance spectra of 12 samples of varying Xe and Ar loadings have been observed and analyzed to obtain the 129Xe chemical shifts and the intensities of the peaks which are dependent on the average argon and xenon occupancies. The detailed distributions, f(XenArm), the fractions of cages containing n Xe atoms and m Ar atoms cannot be observed directly in this system, that is, individual peaks for XenArm mixed clusters are not observed in the NMR spectrum. This information is, however, convoluted into the observed 129Xe chemical shifts for the Xen peaks and the distributions Pn, the fraction of cages contain...

http://www.stemwomen.org/topics/pubs/V150.pdf

Competitive adsorption of xenon and argon in zeolite NaA. 129Xe nuclear magnetic resonance studies and grand canonical Monte Carlo simulations

Numerical simulation of a pressure swing adsorption oxygen unit

Pure-component equilibrium data for nitrogen, oxygen, and argon, and multicomponent equilibrium data for air in 20 × 40 mesh 5A molecular sieve were measured at 297.15, 233.15, and 203.15 K and up to pressures of 4 bar. The multicomponent data were analyzed by a statistical thermodynamic model, ideal adsorbed solution theory, an extended Langmuir-Sips equation, and an extended Langmuir equation. The multicomponent system exhibited nonideal behavior at 233.15 and 203.15 K. Isotherm parameters determined in this work may be useful in the design and evaluation of molecular sieve air separation systems.

K. G. Ikels

Papers

Equilibria of nitrogen, oxygen, argon, and air in molecular sieve 5A