Mass transfer characteristics of liquid films flowing down a vertical wire in a counter current gas flow

Modeling of carbon dioxide and hydrogen sulfide pollutants absorption in wetted-wire columns with alkanolamines

In this work, the modeling of flow equations and associated transported phenomena in wetted-wire columns (WWC) has been carried out by using the CFD method. The studied processes in this column included the absorption of H2S and CO2 gases from the gas stream by absorbent solution. In this regard, laboratory results were available only for CO2 absorption in a column with a single wire or 109 wires. Moreover, the fact that modeling of a wetted-wire column needs robust hardware. As a result, firstly, the process of CO2 absorption with monoethanolamine (MEA) solution in a column with a wire was modeled by COMSOL Multiphysics version 5.6. Then, the results of various parameters were compared with laboratory results (the error percentage was calculated to be 2.4%). It was observed that by increasing the liquid flow rate, the distance between the beads decreased and beads with larger diameters and higher velocities formed. Meanwhile, for the first time, the temperature profile inside the column was determined along the column, the temperature of the liquid phase increased. The gas stream after a slight increase in temperature, left the column with a temperature close to the incoming liquid. After model validation, other processes were investigated, resulting from changing desired gas for separation or liquid solution. Finally, different absorbents’ abilities were predicted to absorb gaseous pollutants and obtained that in terms of absorption efficiency, second-type alkanolamines perform better than other types in the simultaneous absorption of CO2 and H2S.

Electrochemical regeneration of hexavalent chromium from aqueous solutions in a gas sparged parallel plate reactor

In this study anodic oxidation of Cr₂(SO₄)₃ was carried out in an air-sparged divided parallel plate cell. Variables studied were current density, Cr₂(SO₄)₃ concentration, and superficial air velocity. The rate constant of Cr₂(SO₄)₃ oxidation was found to increase with increasing current density and Cr₂(SO₄)₃ concentration. The effect of air sparging was found to depend on Cr₂(SO₄)₃ concentrations, at high Cr₂(SO₄)₃ concentration (> 0.1 M) air sparging does not affect the rate constant of the reaction denoting that the reaction is charge transfer controlled. As Cr₂(SO₄)₃ concentration decreases below 0.1 M the reaction becomes under mixed diffusion and chemical control and the rate constant increases with increasing air superficial velocity, the lower Cr₂(SO₄)₃ concentration the higher the contribution of diffusion to the reaction rate. The current efficiency of the process ranged from 20 to 85% depending on current density and Cr₂(SO₄)₃ concentration. Electrical energy consumption which ranged from 1.8 to 14.4 kW h/kg of Cr⁶⁺ was found to increase with increasing current density and decreases with increasing Cr₂(SO₄)₃ concentration. Air sparging was found to decrease electrical energy consumption in the case of dilute solutions << 0.1 M Cr₂(SO₄)₃.

Effects of Nozzle Geometry on the Fluid Dynamics of Thin Liquid Films Flowing down Vertical Strings in the Rayleigh-Plateau Regime

Thin-liquid films flowing down vertical strings undergo instability, creating wavy film profiles and traveling beads. Previous studies assumed that the liquid film thickness and velocity profiles within the healing length from a nozzle were specified by the Nusselt solution, independent of the nozzle geometry. As a result, the influence of the nozzle diameter on the flow characteristics, such as the liquid bead size, spacing, and traveling speed, was largely overlooked. We report an experimental and numerical simulation study on liquid-film flows in the Rayleigh-Plateau regime while systematically varying the nozzle diameter from 0.5 to 3.2 mm at different mass flow rates (0.02, 0.04, 0.06, and 0.08 g/s). We find that the nozzle diameter does have a strong influence on the flow regime and the flow characteristics. We identify the thickness of a nearly flat portion of a liquid film that precedes the onset of instability, which we term the preinstability thickness, as a critical flow parameter that governs the size, spacing, and frequency of liquid beads that develop downstream. By defining the liquid film aspect ratio α in terms of the preinstability thickness, we capture a flow transition from the Rayleigh-Plateau (RP) instability regime to the isolated droplet regime. Improved understanding of the flow regimes and characteristics assists in the systematic design and optimization of a wide variety of processes and devices, including fiber coating and direct contact heat and mass exchangers.