Compact rapid pressure swing adsorption processes - impact of novel adsorbent monoliths

Regarding the development of energetically efficient units for gas separation and gas purification, Pressure Swing Adsorption (PSA) processes offer a profitable approach. Particularly in small scale applications the compact design of the adsorber is crucial. To achieve the required high productivity, short cycle times must be applied and hence small adsorbent particles must be used, providing sufficiently fast adsorption kinetics. This has been characterized by the term Rapid Pressure Swing Adsorption (RPSA). However, high pressure drop and low mechanical stability of the adsorbent are the limiting factors for particle size and thus process optimization. The approach considered in this work is based upon the usage of novel monolithic adsorbent-polymer materials, featuring low pressure drop and high mechanical stability. The monoliths are manufactured by extrusion of highly filled zeolitic polymer matrices using thermoplastic materials as plasticizing aid and binder. After the forming process, the added wax is removed by thermal after-treatment, creating a secondary pore structure in the polymer matrix and hence specifying its resulting mass transport properties. The potential of this development is evaluated by comparing both adsorption equilibria and adsorption kinetics of the new-type adsorbents with commercial adsorbent pellets used in randomly packed beds. The thermodynamic equilibrium has been quantified in an automated, isothermal high vacuum apparatus based on a static-volumetric principle. The mass-transfer is determined by a common dynamic-column breakthrough method. The adsorption of water vapor on a Zeolite 4A-Polyamide compound is considered as example case. In a next step, proper models for adsorption equilibria and kinetics have been developed, fit to the measured data and implemented into a detailed RPSA process model. This model has been validated with an experimental setup of a single column RPSA unit. Since the approach of the Cyclic Steady State (CSS) by dynamic simulation is extremely time consuming, various efficient numerical methods have been studied. Among these, the usage of a Continuous Countercurrent Model (CCM) has been found to be the most promising approach. The scope of the CCM has been determined by comparing it to both the detailed model and the experimental data. In order to analyze the behavior of the RPSA process, extensive simulations have been carried out. In comparison to a conventional design based on adsorbent pellets it is shown that the novel adsorbent monoliths largely eliminate the kinetic and pressure drop limitations faced in compact design, resulting in a substantial enhancement of productivity. Finally, recommendations for the optimal process design procedure are presented. The success of the manufacture and application of the monolithic adsorbents in the air drying example encourages for an extension of the strategy to other PSA-systems like hydrogen purification and oxygen enrichment, whose compact design in mobile fuel cell systems is the principle challenge.