Determination of macropore diffusion in molecular sieve particles by pulse gas chromatography

Abstract

The experimental conditions necessary for reliable determination of the intraparticle diffusivity of gases in porous solids by pulse gas chromatography (PGC) are investigated. Diffusivity measurements carried out with non-adsorbable tracer gases in packings of molecular sieve particles of different sizes indicate that the experimentally observed contributions of external mass transfer and intraparticle diffusion resistance may not depend on the particle size as theory predicts. With decreasing particle diameter the external mass transfer resistance becomes increasingly dominant as compared to intraparticle diffusion resistances. This finding follows from comparative experiments with packings of non-porous glass beads and porous molecular sieve particles of equal size. The sensitivity of the mass transfer rate parameter determination is markedly improved by using an experimental arrangement with a column to particle diameter ratio which corresponds to a “single pellet string reactor” (S.P.S.R.). The realibility of the determination of the intraparticle diffusivity with an S.P.S.R. has been checked by independent diffusivity measurements carried out on pressed single pellets of the molecular sieve. Steady-state counter-diffusion and dynamic pulse response measurements yielded an effective diffusivity comaparable to that determined with the PGC method using an S.P.S.R. arrangement.