Steady-State and Dynamic Systems for Diffusion Parameters Determination: Advantages and Disadvantages
Advancing the chemical engineering fundamentals
Transport Phenomena in Porous/Granular Media - I (T2-7a)
Keywords: Transport parameters, Single-Pellet String Column, Wicke-Kallenbach diffusion cell, Graham’s diffusion cell
Two types of methods (dynamic/steady-state) commonly used for structure characteristics (effective diffusion coefficients and transport parameters) determination were compared. Chromatographic technique as a dynamic method and counter-current gas diffusion measurement as a steady-state method were used. The chromatographic column was in a configuration generally known as single-pellet string column (SPSC). In SPSC, particles are packed one by one into a column with diameter that exceeds only slightly (10–20 %) the particle dimension. A pulse of tracer gas is injected into the carrier-gas stream, which flows at constant flow rate through the column. The parameters (effective diffusivities and other transport characteristics) were determined by time-domain matching through application of the convolution theorem.
Classic Wicke-Kallenbach and Graham’s diffusion cell for studying of steady-state counter-current diffusion in porous solids were used for evaluation of transport characteristics. Diffusion cell was same for both experimental arrangements. It consists of two flow-through compartments separated by impermeable disc with fixed samples. For the classic Wicke-Kallenbach cell outlet gas streams from both compartments were analyzed for the content of the gas. Graham’s diffusion cell is based on validity of Graham’s law. The cell arrangement enables to stop the gas inlet and outlet of the one cell compartment and to determine the net volumetric diffusion flux through the tested porous materials.
The main advantage of the SPSC chromatographic method is that enables determination of transport characteristics for various shaped porous materials (asterisk, asterisk with hole, lens, extrudate, washcoat, etc.), while diffusion cell methods required only cylindrical shape pellets. Another advantage of SPSC is the averaging of obtained transport characteristics over many pellets. On the other hand, during measurements carried out in diffusion cell only diffusion transport process takes place, while in chromatographic column except diffusion transport other processes, such as convection and axial dispersion, transport of the tracer through a laminar film around solid particles and adsorption could proceeded.
Acknowledgement
The financial support of the Grant Agency of the Czech Republic (203/03/H140) and Grant Agency of Academy of Sciences of the Czech Republic (A4072404) is gratefully acknowledged.
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Presented Tuesday 18, 11:00 to 11:20, in session Transport Phenomena in Porous/Granular Media - I (T2-7a).
