Mark F. Mathias

Effect of solids loading method on bed porosity and gas flux distribution in a fixed-bed reactor

Abstract We examine the effect of the method of loading particulate solids into a commercial fixed-bed reactor on porosity distribution through the resulting bed. Cylindrical extrudates were released into a vertical vessel through a loading chute at the top center with a single hollow cone as a distributor device. Bed porosity and gas flow variations through the solids were deduced using a temperature response analysis recognizing axial convection as the dominant heat transfer mode. Porosity distribution was extremely sensitive to the position of a circular orifice in the chute above the distributor cone. With the orifice just above the distributor, particles slid off the cone in a well-defined sheet landing at about one-third the vessel radius, from which solids spread into the vessel by sliding down the rising-bed surface at the angle of repose. This produced a radially non-uniform porosity resulting in a gas flux at the bed center, roughly half that at the reactor wall. Much more uniform porosity was obtained by raising the orifice in the loading chute such that falling particles approached terminal velocity before striking the cone, dispersing more broadly over the vessel cross-section.

Modelling probe beam deflection experiments in binary bathing electrolytes

Optical probe beam deflection allows monitoring of concentration gradients within a mass-transfer boundary layer next to an electrode, and the data are used to infer chemical fluxes. A binary electrolyte is often used since electroneutrality requires the charge density due to anion and cation to be equal throughout the system, resulting in a single concentration gradient and a direct proportionality between the beam deflection signal and the diffusive flux. As migration contributes significantly to the flux of each ion in a binary electrolyte, this flux contribution must be considered to determine the ionic fluxes using the beam deflection signal. In this paper, we develop a model that accounts for flux by diffusion and migration in a binary electrolyte, uses measured time-dependent current and beam deflection responses as the model input, and computes the cationic and anionic fluxes through the system. The model assumes one-dimensional semi-infinite geometry and allows both cations and anions to transport through an interface into or out of an adjacent phase. The model is useful for monitoring exchange of ions between a liquid bathing electrolyte and a film containing redox sites, an exchange required to maintain film electroneutrality as the redox sites are oxidized or reduced. The use and value of the model are illustrated with previously published data from a conducting polymer film, poly(1-hydroxyphenazine).