Measurement of effective diffusivity from effluent concentration of a flow through diffusion cell

Abstract

The overall effective diffusion coefficient for 3.3 mm extrudates of 5. 4 Zeolite and CO,/Nz a t 292°K and 142 kN/m2 was measured in a continually purged, well mixed, constant volume diffusion cell. The change of COs concentration in the effluent stream resulting from a step change in the influent stream was used with appropriate parameters in the analytical solution to calculate diffusion coefficients for the 0 to 0.1 vol. % and 0.1 to 1.0 vol. % COs concentration ranges. A. C. FROSTUnion Carbide Corp.Tarrytown, NY SCOPEOverall effective diffusion coefficients, or their subsidiary components, have been found from the dynamic response of a packed bed to either pulse or step inputs, as typified by Hashimoto and Smith (1973) While almost all of these methods were capable of isolating the individual internal rate constants that made up the overall effective diffusion coefficient from the external rate constants, they either were mathematically complicated, required special sample preparation, o r w e r e not conducive to high temperature-pressure operation.Ideally, a system should be able to operate over a wide range of conditions, be free of complicating external mass transfer resistances, and be described in relatively simple mathematical terms. These criteria might be met with a simple flow through diffusion cell if the transient weight pick-up were determined from the history of the effluent concentration, and if the mixing of the gas passing through the cell were sufficiently severe to insure a uniform concentration throughout the cell and a negligible bulk film resistance around each pellet.This work covers the development of a cell designed so that the passing gas was well mixed and could flow freely around each pellet. The transient change in the effluent concentration was monitored as the gas feed was switched from one composition to another. This transient response was used with the appropriate mathematical model describing this system to calculate the overall effective diffusion coefficient for 3.3 mm extrudates of SA Zeolite and COs/Nz at 292°K and 142 kN/m2 over the 0 to 0.1 vol. % COz and 0.1 to 1.0 vol. 8 COO concentration ranges. The ability of the mathematical model to take into account the effect of changes in sample size and flow rate was tested with runs having different values for these parameters. Furthermore, the bulk film mass transfer resistance was examined as a function of the gas throughput rate. CONCLUSIONS AND SIGNIFICANCEThe diffusion coefficient measured over the 0.1 to 1.0 vol. % COz concentration range was uninfluenced by changes in the purge gas flow rate o r the number of extrudates in the cell. Consequently, the analytical solution, which did not account for a bulk film mass transfer resistance, was satisfactory under these conditions. Furthermore, the measured overall effective diffusion coefficient for the 3.3 mm extrudates of SA Zeolite was found to be influenced by macropore as well as micropore diffusion.The diffusion coefficient measured over the 0 to 0.1 vol. % COP conc...

Use of stirred batch reactors for the assessment of adsorption constants in porous solid catalysts with simultaneous diffusion and reaction. Theoretical analysis

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Transient effectiveness factors in the dynamic analysis of heterogeneous reactors with porous catalyst particles

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Importance of finite adsorption rate in the evaluation of adsorption and diffusion parameters in porous catalysts

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