Kinetics of Water Vapor Adsorption on Single-Layer Molecular Sieve 3A: Experiments and Modeling

Abstract

The objective of the current work was to shorten the gap for fundamental adsorption kinetic data required for the development of advanced adsorption unit-operation models to be incorporated into an overall plant-level model for spent nuclear fuel reprocessing. The kinetics of water-vapor adsorption on molecular sieve 3A was investigated at 25–80 °C and water dew points from −69 to 17 °C. Water uptake curves were fitted with three kinetic models including the linear-driving-force model, the shrinking-core model, and the Langmuir kinetic model. The results suggest that the water-vapor adsorption on molecular sieve 3A under the investigated experimental conditions was controlled by both external film resistance and internal macropore resistance. The contribution of the external film resistance varied from 25% to 50% of the total mass-transfer resistance depending on the adsorption temperature. It was also found that the Langmuir kinetic model fitted individual sets of kinetic data very well, but the Langmuir adsorption constant obtained from curve fitting decreased with increasing adsorption temperature and with increasing water vapor pressure. This result indicates a significant surface heterogeneity of molecular sieve 3A and also implicitly verifies that the Langmuir isotherm model is unable to represent isotherms of water adsorption on molecular sieve 3A.

A mechanistic modeling framework for gas‐phase adsorption kinetics and fixed‐bed transport

“…Previous studies have shown that silver ions inside the mordenite crystals were reduced to metallic silver and formed silver nanoparticles on the surface of the crystals during the reduction by hydrogen . Physical properties and chemical characteristics of the MS3A and Ag 0 Z were described previously . In addition to those reported properties, the average macropore radii of MS3A and Ag 0 Z were measured in this work using mercury porosimetry, which were 35 nm and 26.5 nm, respectively.…”

“…Both solid adsorbents were microporous crystalline alumina silicates that have micropores and channels in the crystals. The commercial MS3A that were used in prior studies were spherical beads with a radius of 1.18 mm, and the Ag 0 Z used in prior studies were extruded cylinders with a radius of 0.8 mm.…”

“…Both the H 2 O and I 2 uptake experiments were performed with continuous‐flow gas adsorption systems, which have been reported previously . Each of the systems had a H 2 O/I 2 vapor generating unit, a microbalance unit, a heating unit, and a data acquisition system.…”

“…There were fairly large spaces between the particles so the adsorption data essentially represented the adsorption processes on each single particle. Details of the procedures were reported in previous papers …”

“…This process is governed primarily by advection and molecular diffusion . For the microscale, adsorption is governed by film mass transfer from bulk solution to the outside of the adsorbent, pore, and surface diffusion inside the adsorbent domain, and surface reactions or adsorption equilibria . To model these mechanisms requires a mass balance on each adsorbate as it travels between, around, and inside the particles.…”