Numerical study of flow maldistribution and depressurization strategies in a small-scale axial adsorber

Abstract

Flow maldistribution and local high velocity in an axial adsorber is numerically studied to investigate the potential occurrence of sorbent pulverization and uneven utility. A considerable maldistribution induced by the entrance effect and local high velocity caused by rapid gas discharge during depressurization is observed. Three types of gas distributors and different depressurization strategies are then proposed and studied to determine their capabilities to create uniform velocity profiles. Results show that locating the predistributor in the dead zone is critical to flow distribution. The maldistribution factor (Mf) can decrease to a minimum of 0.055 when a perforated inlet plenum is used with a conventional distributor. In addition, the internal ring can effectively reduce wall effects. Moreover, both gas expansion and desorption have a significant influence on the evolution of local velocity during depressurization. In this step, local high velocity can possibly exceed incipient fluidization velocity and cause attrition and pulverization of the sorbent. To a certain extent, employing methods to control the depressurization rate is necessary. Applying linear depressurization (p ¼ 101can reduce local high velocity, and thus, improve flow conditions. List of symbols a Thermal diffusion coefficient, m 2 s -1 d p Diameter of adsorbent particle, m D im Mass dispersion rate, m 2 s -1 e f Total fluid energy, kJ kg -1 e p Total solid medium energy, kJ kg -1 F Momentum source term, kg m -2 s -2 DH Heat of adsorption, J mol -1 K i Langmuir parameter, mol kg -1 kPa -1 k 1 Langmuir temperature dependence constant, mol kg -1 kPa -1 k 2 , k 4 Langmuir temperature dependence constant, K k 3 Langmuir temperature dependence constant, kPa -1 k Mass transfer rate coefficient, s -1 k t Turbulent kinetic energy, m 2 s -2 k eff Effective bed thermal conductivity, W m -2 K k p Solid medium thermal conductivity, W m -2 K k f Fluid phase thermal conductivity, W m -2 K Mf Maldistribution factor M i Molar weight of component i, kg mol -1 M w Molar weight of fluid, kg mol -1 q Solid-phase adsorbate concentration, mol kg -1 q * Adsorbate concentration in equilibrium with gas phase, mol kg -1 r Radial coordinate, m R Gas constant, J mol -1 K -1 /radius of bed, m S i Mass source term of the ith component, kg m -3 s -1 S m Total mass source term, kg m -3 s -1 T Temperature, K u i Velocity in the i direction, m s -1 u Velocity vector, m s -1 y Distance from the adsorber side wall, m y i Mass fraction of component i z Axial coordinate, mGreek symbols e Porosity of the fixed bed e t Dissipation rate of turbulence kinetic energy, m 2 s -3 e b Bulk porosity q p Density of adsorbent particle, kg m -3 q f Fluid density, kg m -3

Two-Dimensional Modeling of Pressure Swing Adsorption (PSA) Oxygen Generation with Radial-Flow Adsorber

“…The governing equations are consistent with previous research [27,28], and the simulation results were verified experimentally by use of the product of the O 2 concentration on the axial flow adsorber [28].…”

“…It can be seen that the four radial-flow adsorbers give similar patterns of pressure changes with the order of the value of CF-π>CF-Z>CP-Z>CP-π, indicating that the effect of the flow type on the pressure change in the bed during the radial-flow PSA process is small. In order to compare the gas distribution of the adsorber in the PSA oxygen generator, a maldistribution factor integrating the surface and corresponding velocity, Mf, was introduced as in [27,36].…”

“…In this regard, the centripetal flow mode is much more suitable for PSA oxygen generation. In order to compare the gas distribution of the adsorber in the PSA oxygen generator, a maldistribution factor integrating the surface and corresponding velocity, M f , was introduced as in [27,36].…”

“…Numerical simulation is a very effective method to study the PSA process of oxygen generation [22][23][24][25][26]. However, most of the studies are based on the 1-D assumption, while in actual PSA processes there are often inconsistencies perpendicular to the flow direction [27,28]. Few studies have been reported in the research of PSA oxygen generation based on 2-D or 3-D radial-flow adsorbers.…”

3-D Modeling of Gas–Solid Two-Phase Flow in a π-Shaped Centripetal Radial Flow Adsorber

“…It is very effective to study gas separation by numerical simulation [22][23][24][25][26]. However, most of the studies are based on the 1-D or 2-D RFA [27], and few studies have been reported in research of PSA oxygen production based on 3-D RFA. In actual PSA processes, the gas separation in the adsorption bed is a coupling process of multiple complicated processes such as gas flow, heat transfer and mass transfer.…”