Evaluation of the Mass Transfer Rate in Columns of Small Diameter Zeolite Particles

The mass transfer rate in a zeolite adsorbent column is generally assumed to be controlled by intraparticle diffusion, which is a reasonable assumption for large diameter particles (> 1 mm). Under this assumption, breakthrough curves can be used to determine a mass transfer coefficient (MTC) with a linear driving force (LDF), often assumed constant in column dynamics models. A similar assumption is often applied to the mass transfer rate of smaller diameter zeolite particles (< 1 mm). However, since the diffusion path inside the particle is significantly reduced, the assumption of intraparticle diffusion controlling the mass transfer rate is no longer necessarily valid. By reducing the internal diffusion resistance of the particle, mass transfer external to the particle becomes more prominent in determining the overall adsorption rate.

In this presentation, we will evaluate if the assumption of intraparticle diffusional resistance as rate limiting is appropriate for small diameter X-zeolite particles. Breakthrough curves were measured over a range of gas velocities and pressures for a commercial Li-X zeolite (~0.5 mm in diameter). The mass transfer zone (MTZ) lengths calculated from the breakthrough curves provide insight into the mechanism controlling the mass transfer rate. Additionally, the breakthrough curves were matched to simulated breakthrough curves to determine if a constant MTC is suitable for the zeolite particle.