Determination of mass transfer coefficients in high-pressure CO2-H2O flows in microcapillaries using a colorimetric method

Abstract : This study presents a method for the experimental determination of the local volumetric mass transfercoefficientkLaLin a high-pressure two-phase flow of water (H2O) and carbon dioxide (CO2) in a micro-capillary using a colorimetric method. H2O and CO2 are fed continuously and co-axially injected athigh-pressure (10 MPa) and moderate temperature (303 K) into a microcapillary. Under the flow condi-tions studied, a segmented flow of CO2 in H2O is formed. The CO2 dissolves into the H2O-rich phase,thereby reducing the pH to about 3.3, depending on the pressure and temperature. The pH of the H2O-rich phase is determined over the entire length of the capillary using a pH sensitive indicator coupledwith high-speed imaging and analysis. The concentration of CO2 in the water-rich phase is deduced fromthis pH value using literature experimental data. The CO2 concentration data and the unit-cell model,which has been modified to account for high pressure conditions, have then been used to determinethe volumetric mass transfer coefficient, kLaL, of CO2 into the liquid phase along the entire length ofthe microreactor. The experimentally derived kLaL ranges between 1 and 13 s-1.