Remarks on adsorbent surface barrier to adsorbate mass transport
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Remarks on adsorbent surface barrier to adsorbate mass transport As the author passed away while this paper was undergoing peer review, the revisions suggested have been implemented by Timothy C. Golden, Air Products and Chemicals, Inc., Allentown, USA Shivaji Sircar1 Received: 12 December 2019 / Revised: 6 February 2020 / Accepted: 7 April 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract The existence of a skin resistance for adsorbate mass transport at the surface of a pelletized adsorbent particle or at the surface of adsorbent crystals within a particle has been proven by many macroscopic and microscopic experiments. An isothermal and isobaric column dynamic test method may be used to approximately estimate the relative magnitude of the skin resistance. Keywords Surface barrier · Zeolites · Mass transfer · LiLSX
1 Introduction The potential of existence of a surface barrier (skin resistance) for adsorbate mass transport at the surface of an adsorbent particle is well documented in the literature. The barrier may exist at the surface of (a) a pelletized adsorbent (amorphous or crystalline) particle (with or without a binder), and (b) the individual crystals within an adsorbent particle. Several instances of visible surface barrier of type (a) in pelletized extruded and beaded, abrasion-resistant zeolites have been reported by using scanning electron microscopes (Ogawa et al. 1983; Kumar and Sircar 1986; Moran et al. 2018). Formation of microscopically invisible surface barriers of type (b) have been reported on various types of zeolite crystals by PFG NMR study (Vasenkov et al. 2001); interference microscopy (Kortunov et al. 2004); oscillating microbalance and IR micro imaging (Zhang et al. 2009); thermal frequency response method (Bourdin et al. 1996); zero length chromatography (Teixeira et al. 2013): etc. micro imaging technique has also revealed that the surface barriers on the individual crystals of a zeolite can be different (micro diversity), even though the shape and size of the crystals are similar (Saint Remi et al. 2016). Such a distribution is
* Shivaji Sircar [email protected] 1
Department of Chemical and Biomedical Engineering, Lehigh University, Bethlehem, PA 18105, USA
known to lead to problems on discriminating the existence of surface barriers by “bulk” techniques, which are applied to assemblages of crystals/particles—rather than to a particular crystal/particle (as a unique property of micro-imaging). For circumventing this problem, Brandani et al. have introduced the so-called “partial loading technique” (Brandani et al. 1995). This technique makes use of the characteristic differences in the molecular distribution inside the crystal/ particle for diffusion and barrier limitation, which are predominantly affected by the mean value (rather than by the distribution) of the surface resistances. Such differences become, as a matter of course, immediately visible in microimaging experiments with the individual crystals/particles (Chmelik et al. 2009). Analy
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