Carbon dioxide (CO2) concentrations in the atmosphere have increased significantly over the past century. Many methods have been devised to reduce CO2 industrial emissions, e.g., CO2 postcombustion absorption by amine-based solvents. Solvent degradation losses are very critical in this process, due to economic and environmental issues. The two main degradation pathways of amine-based aqueous solutions in the presence of CO2 are oxidative and thermal degradation. In this work, a lab-scale pilot plant has been set up to carry out degradation experiments during continuous and dynamic cycles of absorption and stripping with three different amine solvents: MEA (monoethanolamine) used as benchmark solvent for CO2 capture, a blend of 1MPZ (1-methylpiperazine) and PZ (piperazine), and a blend of MDEA (methyldiethanolamine) and MEA. The experimental data have been used to assess the performance of CO2 absorption over time and experimental conditions. The variation of CO2 fraction at the gas outlet of the reactor has been used as an indicator of solvent degradation. To simulate the behavior of the plant at different experimental conditions and with each solvent, a dynamic model has been developed, on the basis of the validation of a fast reaction regime. It reproduces accurately the pilot plant’s behavior during the absorption and stripping phases. Among the solvents’ physical properties, the effect of viscosity appears to be the most critical for the CO2 absorption efficiency. Kinetics of solvent degradation has finally been optimized to match experimental observations. Of the three solvents studied, 1MPZ/PZ is the most stable, whereas MEA and MDEA/MEA have quite similar degradation rates.
This paper reports new experimental data and calibrated semi-empirical correlations concerning density and viscosity of aqueous solutions of Potassium Lysinate (LysK), a promising bio-based solvent for postcombustion CO2 capture from flue gases, an application of relevance for industrial decarbonization. The analysis covers a LysK molality range in the solution from 2.16 m to 4.44 m, temperature intervals between 288.15 K and 358.15 K, and a CO2 loading range of 0.3 molCO2/molLysK -0.98 molCO2/molLysK. Newly collected experimental results supported the development and calibration of semiempirical correlations for density and viscosity of LysK solutions suitable to predict these thermophysical properties in the range of conditions tested. These models can predict the behaviour of the thermophysical properties investigated with high accuracy (for loaded solutions, average absolute deviation for density equal to 1.48 kg/m 3 and for viscosity of 0.04 mPas) within the whole range of LysK concentration, temperature and CO2 loading explored. Validation against literature data lying outside the calibration range shows an accuracy for density correlation ≤3%, while for viscosity model deviations are larger and vary between 1% and 15% of the measured value.
The elimination of impurities from natural gas (water, CO2, H2S, mercaptans) is an obligation: in Europe, a max. of 20 mg/Sm3 in total sulfur (at 15°C and 1 atm) is allowed, as well as 2.5 vol% in CO2. It maximizes the calorific value of delivered gas and limits chemical risks (corrosion, toxicity) associated with this transition fuel to cleaner energy sources. Acid gas separation is commonly performed by reactive gas-liquid absorption. Raw gas and absorptive solution are brought into counter current contact in an absorber at 40-80°C and 30-80 bar. Then the acid-gas loaded solvent is sent to a stripper where acid gas desorption from the solvent takes place at T between 105 and 140°C and P ≤ 2.5 bar. The solvent is an aqueous base (e.g., amine) solution. A good solvent has a high reactivity with the dissolved gas during absorption and a low regeneration energy, hence the advantage of mixtures of two amines. Here we study an aqueous solution of a small amount of piperazine (PZ) added to methyldiethanolamine (MDEA). PZ reacts faster with CO2 than MDEA, and the major reaction product with CO2 is the hydrogen carbonate ion HCO3-, which lowers the solvent regeneration duty. This work aims at characterising the kinetics of the absorption of CO2 in an aqueous solution of MDEA-PZ. Indeed, no consensus has been found on the reaction mechanism of CO2 in this amine mixture; the synergies between MDEA and PZ observed in the literature remain poorly explained. However, its knowledge is essential to optimize solvent formulation and industrial unit design.
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