Modeling and Simulation of Pressure Equalization Step Between a Packed Bed and an Empty Tank in Pressure Swing Adsorption Cycles

Background:

It is of paramount importance to pay a great attention when modeling pressure equalization step of pressure swing adsorption cycles for its notable effect on the accurate prediction of the whole cycle performances. Studies devoted to pressure equalization between an adsorption bed and a tank have been lacking in the literature. Many factors could affect the accuracy of the dynamic simulation of pressure equalization between a bed and an empty tank.

Methods:

The method used for the equilibrium pressure evaluation has a significant impact on simulation results. The exact equilibrium pressure (Peq) obtained when connecting an adsorption column and an empty tank could only be obtained by numerical simulation given the complexity of the set of partial differential equations.

Results:

It has been shown that, with some simplifying assumptions, one can analytically determine Peq with satisfactory precision. The analytical solution proposed permits to assess rapidly the equilibrium pressure and the equilibrium mole fraction of the adsorbable species in the tank (Yeq) and without the need to resort to a cubersome modeling.

Conclusion:

Based on the comparisons presented, one can conclude that the agreement between the experimental and numerical results relative to Peq and Yeq is very satisfactory.

Modeling spreading-pressure-dependent binary gas coadsorption equilibria using gravimetric data

The present work proposes an approach to building nonideal coadsorption models in a thermodynamically consistent fashion, including the effects of pressure and spreading pressure, from simple gravimetric measurements. This is an "inverse problem" of parameter determination from appropriate and limited experimental data. The approach relies on the nonideal adsorbed solution theory, which includes activity coefficients and their dependence on spreading pressure, and on an original form of the excess Gibbs energy of mixing. A fully analytical development leads to explicit relations between the infinite dilution activity coefficients and three sets of independent information: the parameters of this excess Gibbs function, the limiting slopes of measured binary gravimetric curves at two different total pressures, and the properties of the single-component isotherms. From there, the four parameters of the model may be determined quasi-analytically and uniquely. The method is exemplified with the coadsorption of CO(2) and CH(4) on activated carbon, and a heterogeneous set of data. On one hand, the total adsorbed mass of the two components is measured at 1 bar by "incremental gravimetry." On the other hand, data obtained from independent batch-type equilibration measurements at 2 bar allow a comparison of calculated and measured data for the individual component concentrations. It is emphasized, however, that only total adsorbed mass data are needed for application of the method.

Description of multicomponent adsorption and absorption phenomena from a single viewpoint

On the basis of phenomenological thermodynamics, an equation has been derived that represents one of the interphase equilibrium equations and provides a description of both multicomponent adsorption and absorption phenomena from a single point of view. The approach is based on a free-volume state equation for the adsorbed phase. It is shown that the well-known Henry, Langmuir, Fowler-Guggenheim, Temkin, and BET adsorption equations (generalized for gas mixtures) follow directly from the equation proposed with constants having a clear physical meaning.