Piperazine-Based Mixed Solvents for CO2 Capture in Bubble-Column Scrubbers and Regeneration Heat

This work used piperazine (PZ) as a base solvent, blended individually with five amines, which were monoethanolamine (MEA), secondary amines (DIPAs), tertiary amines (TEAs), stereo amines (AMPs), and diethylenetriamine (DETA), to prepare mixed solvents at the desired concentrations as the test solvents. A continuous bubble-column scrubber with one stage (1 s) was first used for the test. Six parameters were selected, including the type of mixed solvent (A), the ratio of mixed solvents (B), the solvent feed rate (C), the gas flow rate (D), the concentration of the mixed solvents (E), and the liquid temperature (F), each one having five levels. Using the Taguchi experimental design, only 25 runs were required. The outcome data, such as the absorption efficiency (EF), the absorption rate (RA), the overall mass-transfer coefficient (KGa), and the absorption factor (φ), could be determined under steady-state conditions. The optimal mixed solvents were found to be A1 (PZ + MEA) and A2 (PZ + DIPA). The parameter importance and optimal conditions for EF, RA, KGa, and ϕ were determined separately; the verification of all optimal conditions was successful. This analysis found that the importance of the parameters was D > C > A > E > B > F, and the gas flow rate (D) was the most important factor. Subsequently, multiple-stage scrubbers were used to capture CO2. Comparing 1 s and 3 s (three-stage scrubber), EF, RA, KGa, and φ increased by 33%, 29%, 22%, and 38%, respectively. The desorption tests for the four optimal scrubbed solutions, including multiple stages, showed that the heat of regeneration for the three scrubbers was 3.57–8.93 GJ/t, in the temperature range of 110–130 °C, while A2 was the best solvent. Finally, the heat regeneration mechanism was also discussed in this work.