(346d) Mass Transfer Studies of Carbondioxide Absorption in Sodium Hydroxide in Millichannels

During the past several decades, CO₂ levels in the atmosphere have been dramatically increasing due to the man-made emissions such as fossil fuel-fired power plants. This results in global warming thus leading an increase in the need to study different ways to capture the emitted CO₂ directly from the exhausts of power plants or atmosphere. There are several ways to remove CO₂, such as absorption into a liquid solvent, adsorption into a solid, cryogenic separation, permeation through membranes and photochemical conversion. In most industries, the absorption of CO₂ in chemical solvents (in absorption towers) is used for CO₂ capture. In these towers, the mass transfer along with chemical reactions take place between the gas and liquid phase.It is important to understand these processes in detail. These flow patterns are difficult to maintain in large scale industrial absorbers. So to get accurate information controlled gas-liquid absorption experiments are carried out in milli-channels in this work under controlled atmosphere. The absorption experiments of CO₂ in varying concentrations of sodium hydroxide solution are carried out in T-junction glass milli-channels with a circular cross section (inner diameter of 2mm). The gas and liquid flow rates are controlled by a mass flow controller (MFC) and a Harvard syringe pump respectively. The slug flow in the channel is recorded using a camera and the videos are analysed. The gas slug of pure CO₂ is found to decrease in size along the length of the channel due to absorption of gas in the liquid. This is also captured with the model developed and the mass transfer characteristics are studied. The pressure drop across the channel is determined by sum of the pressure drops from the gas slugs and the liquid plugs. A dimensionless correlation for the mass transfer coefficient is developed in terms of Sherwood number and compared with the existing correlations in the literature. They are found to be in close agreement with each other. In this case, due to the presence of chemical reaction, the enhancement of mass transfer is obtained. This is quantified with the help of an enhancement factor.

Keywords— Absorption, Enhancement factor, Mass transfer coefficient, Sherwood number