Spatial Scale Effects on Rayleigh Convection and Interfacial Mass Transfer Characteristics in CO₂ Absorption

Abstract

Concentration gradient-induced Rayleigh convections in the CO 2 absorption process were investigated by the hybrid Lattice-Boltzmann/finite-difference method (LBM-FDM). The spatial scale effects on Rayleigh convection were studied by simulating Rayleigh convections with different liquid layers. The scale of convection increased with the liquid layer height but the average mass transfer coefficient showed the adverse tendency. The Rayleigh convection had a pronounced effect on the surface renewal. For better assessment of the renewal intensity, two statistical quantities were proposed. The transient variations of these quantities provided a good following feature with the mass transfer coefficient which confirms their accuracy and precision in characterizing the mass transfer process.

Modeling with statistical hydrodynamic quantities of mass transfer across gas–liquid interface with Rayleigh convection

“…(16), the mass transfer rate in the mixing vessel was found to be a function of the power dissipation in particular applications, such as mixing vessels, packed beds and pipe flow. This positive dependence of mass transfer upon energy dissipation is expected to be valid in a Rayleigh-unstable system because the kinetic energy determines the vortex pattern in the liquid layer, which is helpful to the surface renewal and a key factor for mass transfer [23]. In the concerned absorption process, the vortices-contained kinetic energy will be dissipated by the molecular viscosity.…”

“…In these correlations, the dimensionless Sh number is typically assumed to be correlated with the Ra number. However, recently, Guo et al [23] investigated the interfacial mass transfer of CO 2 absorption into an initially quiescent liquid layer with variations in layer height and found an obvious discrepancy between the mass transfer coefficient and the predicted value based on such correlations. Guo's work suggested that due to the complexity of the system, the model using the Ra number to predict the mass transfer rate is flawed.…”

“…The ratio of the convection term and diffusion term of Eq. From the viewpoint of field synergy [62][63], the convection term can be rewritten as the coupling between the velocity vector and concentration gradient [64][65]: (23) where ș is the angle between the velocity vector and the concentration gradient. The convection depends not only upon the production of the velocity vector and concentration gradient but also upon their included angle.…”

Validation of Simulation and Mass Transfer Coefficient Prediction with Interfacial Convection

“…Early studies of the mass transfer coefficient relied mainly on the experimental method, i.e., macroscopic parameters measured through experiments were correlated with the measured mass transfer coefficient through some dimensionless number, such as the Hugmark equation with the Reynolds number Re , Sherwood number Sh , or Schmidt number Sc . Guo pointed out that correlations of the dimensionless number (such as Ra ) cannot accurately predict the mass transfer coefficient through analyzing various cases of Rayleigh convection with different liquid layer heights. To circumvent this problem, some hydrodynamic parameters, such as the pseudo‐dissipation rate ε *, viscous dissipation rate Φ , and surface divergence, were adopted to predict the mass transfer coefficient k in Rayleigh convection.…”