Optimal design of engineered gas adsorbents: Pore-scale level

Abstract This study presents an optimization methodology and results for the structure of gas adsorbents at a pore level by evaluating the effect of pore geometry, size and overall adsorbent porosity on ultimate working capacity of adsorbents used in pressure swing adsorption (PSA) processes. Three model pore network topologies are studied: parallel, grid-like and branched structures. These are “ near ” optimal structures for an adsorbent particle and their relative performance is compared in a two-step PSA cycle. The macropore network of such structured adsorbents is optimized through maximization of an objective function i.e . working capacity WC , defined as the number of moles adsorbed per unit volume of slab. Molecular and Knudsen diffusion are considered as the sole transport mechanisms in the macropore channels. An unexpected finding of this optimization technique is that the branched structure with a porosity of less than 50% represents an optimum structure with highest working capacity for the system considered. Furthermore, for faster cycles the advantage of branched structures is more obvious, reflecting the advantages of the pore network in facilitating diffusion more efficiently than other structures. A macropore channel density (defined as the density of macropores per metre of external surface) of below 10 is suggested for optimum performance for both “fast” and “slow” PSA cycles. The theoretical results of this study will be used as a priori results for the design of adsorbents at the macro-scale (bed) level.

[1]  Asterios Gavriilidis,et al.  Catalyst Design: Optimal Distribution of Catalyst in Pellets, Reactors, and Membranes , 2005 .

[2]  A. Liapis,et al.  Network model investigation of gas transport in bidisperse porous adsorbents , 1991 .

[3]  L. Hegedus.  Catalyst Pore Structures by Constrained Nonlinear Optimization , 1980 .

[4]  A. Wheeler.  Reaction Rates and Selectivity in Catalyst Pores , 1951 .

[5]  Muhammad Sahimi,et al.  Statistical and continuum models of fluid-solid reactions in porous media , 1990 .

[6]  J J Meyers,et al.  Network modeling of the convective flow and diffusion of molecules adsorbing in monoliths and in porous particles packed in a chromatographic column. , 1999, Journal of chromatography. A.

[7]  Y. Yortsos,et al.  A pore-network study of bubble growth in porous media driven by heat transfer , 1996 .

[8]  Gang Wang,et al.  Optimizing transport in nanostructured catalysts: A computational study , 2007 .

[9]  L. Hegedus,et al.  Design of monolith catalysts for power plant nitrogen oxide (No.+-.) emission control , 1991 .

[10]  Stefan Gheorghiu,et al.  Optimal bimodal pore networks for heterogeneous catalysis , 2004 .

[11]  Athanasios I. Liapis,et al.  Network modeling of the intraparticle convection and diffusion of molecules in porous particles packed in a chromatographic column , 1998 .

[12]  Frerich J. Keil,et al.  Optimization of Three-Dimensional Catalyst Pore Structures , 1994 .

[13]  F. Keil.  Diffusion and reaction in porous networks , 1999 .

[14]  Gang Wang,et al.  Optimal Distributor Networks in Porous Catalyst Pellets. I. Molecular Diffusion , 2007 .

[15]  F. Keil,et al.  Simulation and experiment of multicomponent diffusion and reaction in three-dimensional networks , 1999 .

[16]  J. Kärger,et al.  Flow and Transport in Porous Media and Fractured Rock , 1996 .

[17]  J J Meyers,et al.  Determination of the pore connectivity and pore size distribution and pore spatial distribution of porous chromatographic particles from nitrogen sorption measurements and pore network modelling theory. , 2001, Journal of chromatography. A.

[18]  G. Froment,et al.  The effectiveness of mass fractal catalysts , 1997 .

[19]  S. Hyun,et al.  Equilibrium adsorption of ethane, ethylene, isobutane, carbon dioxide, and their binary mixtures on 13X molecular sieves , 1982 .

[20]  Marc-Olivier Coppens,et al.  Calculation of the Optimal Macropore Size in Nanoporous Catalysts and Its Application to DeNOx Catalysis , 2008 .

[21]  M. Sahimi.  Flow phenomena in rocks : from continuum models to fractals, percolation, cellular automata, and simulated annealing , 1993 .