(719f) Understanding Dominating Mass Transfer Resistance Using Frequency Response Method

Knowledge for mass transfer is very important for developing effective adsorbents and catalysts that utilize differences in mass transfer rates for different molecules.  Mass transfer limitations often play an important role in the overall rate of reaction; the rate of conversion and product formation in the catalytic systems.  Various mass transfer steps can be encountered in porous adsorbent or catalyst, such as molecules diffusing through the various systems of pores and surface barrier through pore openings. The slowest mass transfer step (giving longest resistance time) is quite possibly the overall rate limiting step for adsorption and catalytic reaction. It is very important to understand the dominating mass transfer mechanisms and measure accurate mass transfer rate constants in order to develop the suitable adsorbents/catalyst and process design.

Frequency response is a unique method to discriminate among different mass transfer mechanisms by introducing harmonic perturbations at various frequencies of oscillation.  A novel pressure-swing frequency response (PSFR) has been developed to have capability to measure mass transfer at operating conditions close to practical interest.  Examples are given to demonstrate how the dominating resistance can be determined for various systems.  Micropore diffusion is most common for mass transfer in zeolites.  Heat effects can also be very important when heat dissipation is much slower compared to mass transfer (for example for CO2 diffusion).  Particle size difference or/and adsorbent structure can relate to parallel micropore diffusion steps.  Mass transfer study at various pressures and temperatures can also provide a fundamental understanding of loading dependence for diffusion rates.

In this work we will present some examples of application of PSFR technique to study mass transfer in several classes of adsorbents.