Kinetics of competitive adsorption and reaction systems

Narasimhan Sundaram, Purdue University

Abstract

The description of sorption processes forms the core of this thesis. Pressure drop, hitherto ignored, was successfully incorporated into pressure swing adsorption (PSA) processes. These effects were shown to be important in rapid cycling and important scaling parameters were identified. Such rapid cycling is believed to be one of the modes of operation of fixed bed sorption processes that can be utilised for intensification and scaling. Industrial cycles were classified by a simple scheme based on whether they exhibited pressure drop effects. Most conventional cycles were verified to be under negligible pressure drop. The existence of shock waves due to crossing of characteristics was observed and modelled using material balances. The importance of pressure drop in the desorption step was discovered. This work has motivated other research groups to conduct experiments on pressure drop in PSA. After the above study on operating variables, the fixed bed exchange equations were identified as Abel equations and solved for a system of two interfering solutes which occurs in chromatography or in air separations in PSA. Such a description is crucial to laying the framework for intensification and scale-up. The solution was compared to numerical predictions of breakthrough behavior. The geometry of contacting, chemistry of resin or manipulation of operating variable did not play a role but the asymptotic condition of constant pattern was invoked. It was found that a suitable window of feed and initial bed composition pairs existed where the Thomas kinetics for the two-solute problem decoupled to give accurate profiles. Kinetic parameters which are more reliable and independent of resin macro-structure could be extracted. Conventional mass transfer rate expressions were shown to be of the same form. Thomas solutes compete for adsorption sites much like product and substrate compete for enzyme in the classical Michaelis-Menten batch reaction. An interesting insight into the dynamical behavior of systems involving species competing for a single resource was obtained. The solution technique involving the Abel type equation was applied to this problem and complete concentration profiles for this important reaction were obtained. All the existing limits of behavior such as pseudo-steady state and quasi-equilibrium were satisfied. The transient portion which is best suited for parameter estimation is well described by the technique for enzyme kinetics. Considerable refinement over existing initial rate methods is possible. In comparison to singular perturbation the relations developed are easy to use and valid over a much wider range of parameters.

Degree

Ph.D.

Advisors

Wankat, Purdue University.

Subject Area

Chemical engineering

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