Speaker
Description
The presence of shell-core structures in some extruded pellets is a well-known phenomenon [1], but it is very difficult to quantify the thickness of the external layer and its impact on mass transport kinetics. We present a methodology to characterize mass transport in extruded pellets that shows for the first time the ability to distinguish uniform structures from shell-core structures. The measurements are carried out with the zero length column (ZLC) technique [2] including both full equilibration and partial loading experiments [3]. Two commercial extruded materials from UOP, a Honeywell company, and UTSA-16 extrudates from SINTEF were used in this study. The two zeolite materials are shown to behave significantly differently when normal paraffins are used as the probe molecules. HISIV 3000 (silicalite) pellets show ZLC responses that are perfectly consistent with macropore diffusion control, while 5A pellets show a distinct “surface resistance” in addition to the internal diffusion process. The MOF pellets also show a distinct surface resistance, which can be detected using the ZLC and a suitable probe molecule.
The slower mass transport in the external shell is shown to be a macropore diffusion process by performing tests with different carrier gases. A detailed shell-core model is developed to analyze the responses in greater detail and determine the thickness of the external skin by the simultaneous regression of multiple experiments (different flowrates and partial loading runs) in the whole pellet and a fragment. This allows to determine quantitatively the thickness of the external layer, which is only a few microns thick.
The use of the ZLC provides a very efficient method to detect skin effects on pellets which are an important feature for quality control for manufactures. It also provides the ability to determine the effect of the skin resistance, allowing to estimate physical parameters that can be used to describe more accurately mass transport in adsorption and catalytic processes.
References:
1. Roberts CW. Molecular sieves for industrial separation and adsorption applications. In: Townsend RP, editor. The properties and applications of zeolites, London; The Chemical Society 1980 p. 103–120.
2. Eic M. and Ruthven DM, A new experimental technique for measurement of intracrystalline difusivity. Zeolites 8 (1988) 40–45. https://doi.org/10.1016/S0144-2449(88)80028-9
3. Brandani S. and Mangano E. The Zero Length Column Technique to Measure Adsorption Equilibrium and Kinetics: Lessons Learnt from 30 Years of Experience. Adsorption, 27 (2021) 319–351. https://doi.org/10.1007/s10450-020-00273-w
