Investigation of the Phase‐Separation Point During CO ₂ Absorption in the DETA–Sulfolane–Water System

ABSTRACT The phase‐separation behavior of liquid–liquid biphasic amine absorbents is typically identified using bubbling reactors, where visual observation is employed to determine the onset of phase separation. However, this approach lacks accuracy in detecting the true phase‐separation point and provides no means of predicting its occurrence. In this study, an optical diagnostic method based on Mie scattering was developed to achieve real‐time and precise detection of phase separation in biphasic absorbents. By employing a linear‐array CMOS sensor to capture forward‐scattered light from the sample, the intensity and variation of the scattered light were analyzed to identify microdroplets and gas bubbles formed during CO 2 absorption. This technique enables accurate determination of the phase‐separation point for the DETA–sulfolane–water absorbent system. To quantitatively describe the phase‐separation boundary, a generalized solubility product ( K sp ) was proposed to correlate the concentrations of CO 2 , DETA, sulfolane, and water at the phase‐separation point. An empirical expression for the DETA–sulfolane–water system was obtained as K sp = [CO 2 ] 0.45 [DETA] 0.051 ([Sulfone]/[H 2 O]) 0.6 with an average fitting error of 8.078%. Furthermore, Arrhenius‐type regression of K sp across 40°C–80°C yielded an apparent thermal effect parameter of 4.053 kJ mol −1 CO 2 , indicating that phase separation is governed primarily by solubility–desolvation equilibria rather than chemical reaction energetics. This work integrates optical scattering diagnostics with thermodynamic modeling to establish a predictive framework for analyzing and forecasting phase separation in biphasic CO 2 absorbents, providing theoretical guidance for the design and optimization of next‐generation low‐energy solvent systems.