Wall Effects (wall + effects)

Distribution by Scientific Domains


Selected Abstracts


Axial Dispersion and Wall Effects in Narrow Fixed Bed Reactors: A Comparative Study Based on RTD and NMR Measurements

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 8 2004
D. Tang
Abstract Axial dispersion and wall effects in narrow fixed beds with aspect ratios < 10 were investigated, both by classical methods and by NMR imaging. The residence time distribution (RTD) in the center and at the wall was measured, system water/NaCl-solution as tracer, and subsequently compared with radial velocity profiles based on NMR imaging. The influence of the aspect ratio and Rep on dispersion and on the degree of non-uniformity of the velocity profile was studied. The NMR results are consistent with the RTD and also with literature data of numerical simulations. For low aspect ratios, dispersion/wall effects have a strong influence on the reactor behavior, above all, in cases where a low effluent concentration is essential, as proven by breakthrough experiments with the reaction of H2S with ZnO. [source]


The effect of column diameter and bed height on minimum fluidization velocity

AICHE JOURNAL, Issue 9 2010
Akhil Rao
Abstract Experiments show that the minimum fluidization velocity of particles increases as the diameter of the fluidization column is reduced, or if the height of the bed is increased. These trends are shown to be due to the influence of the wall. A new, semicorrelated model is proposed, which incorporates Janssen's wall effects in the calculation of the minimum fluidization velocity. The wall friction opposes not only the bed weight but also the drag force acting on the particles during fluidization. The enhanced wall friction leads to an increase in the minimum fluidization velocity. The model predictions compare favorably to existing correlations and experimental data. 2010 American Institute of Chemical Engineers AIChE J, 2010 [source]


The effect of mixer properties and fill level on granular flow in a bladed mixer

AICHE JOURNAL, Issue 2 2010
Brenda Remy
Abstract The discrete element method was used to study the effect of mixer properties and fill level on the granular flow of monodisperse, cohesionless spheres in a bladed mixer. For fill levels just covering the span of the blades, a three-dimensional (3-D) recirculation zone develops in front of the blades, which promotes vertical and radial mixing. Increasing fill level reduces the size of the recirculation zone, decreases bed dilation and hinders particle diffusivities. However, above a critical fill level, the behavior of the particles within the span of the blade is found to be invariant of fill level. At low-fill levels, the pressure within the particle bed varies linearly with bed height and can be approximated by hydrostatics. At higher fill levels, a constant pressure region develops within the span of the blades due to the angled pitch of the blades. Cylinder wall friction is shown to significantly influence granular behavior in bladed mixers. At low-wall friction, the 3-D recirculation zone observed for high-wall friction conditions does not develop. High-wall friction leads to an increase in convective and diffusive particle mixing. Shear stresses are shown to be a function of wall friction. Blade position along the vertical axis is shown to influence flow patterns, granular temperature and stress. The effect of increasing the mixer diameter at a constant particle diameter was also studied. When the mixer diameter is larger than a critical size such that wall effects are minimized, the observed granular behavior follows simple scaling relations. Particle velocities and diffusivities scale linearly with mixer size and blade speed. Normal and shear stress profiles are found to scale linearly with the total weight of the particle bed. 2009 American Institute of Chemical Engineers AIChE J, 2010 [source]


Water gas shift reaction via Pd-based membranes

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 3 2009
Silvano Tosti
Abstract The water gas shift reaction has been studied in tubular Pd-based membranes: a thin walled dense tube and a composite Pd-ceramic tube have been considered. A computer code based on a finite element model has been developed for modelling the membrane reactor. The model accounts for the reaction kinetic, the hydrogen diffusion through the porous ceramic support and permeation through the PdAg membrane and for the partial pressure gradients of hydrogen generated at the permeate side of the membrane when a flow of purge gas is introduced. The code has been used to assess the influence of temperature, lumen pressure, presence of wall effects and sweep gas mode on the reaction conversion and hydrogen yield of the membrane reactors. At 200 kPa of lumen pressure and counter-current sweep mode, it was found that both reaction conversion and hydrogen yield increase with temperature: the dense and the composite membranes exhibit very close values of conversion (more than 99% at 400 C) and hydrogen yield (96,97% at 400 C). In co-current mode, the highest values of both reaction conversion and hydrogen yield have been assessed at 350 C, while it was demonstrated that the beneficial effects of increasing the lumen pressure up to 400 kPa are maximum at 300 C. Copyright 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source]


Axial Dispersion and Wall Effects in Narrow Fixed Bed Reactors: A Comparative Study Based on RTD and NMR Measurements

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 8 2004
D. Tang
Abstract Axial dispersion and wall effects in narrow fixed beds with aspect ratios < 10 were investigated, both by classical methods and by NMR imaging. The residence time distribution (RTD) in the center and at the wall was measured, system water/NaCl-solution as tracer, and subsequently compared with radial velocity profiles based on NMR imaging. The influence of the aspect ratio and Rep on dispersion and on the degree of non-uniformity of the velocity profile was studied. The NMR results are consistent with the RTD and also with literature data of numerical simulations. For low aspect ratios, dispersion/wall effects have a strong influence on the reactor behavior, above all, in cases where a low effluent concentration is essential, as proven by breakthrough experiments with the reaction of H2S with ZnO. [source]