Mass transfer rate in semi-aqueous amines for CO₂ capture




Yuan, Ye, Ph. D.

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Amine scrubbing is the most promising solution to address CO₂ emission from power plants. Solvent development can significantly reduce the capital and energy cost of the process. This work rigorously studies the CO₂ mass transfer and solubility at flue gas treating process condition for aqueous and semi-aqueous amines. A second-generation aqueous amine solvent: 2methylpiperazine (2MPZ) blended with piperazine (PZ) that has been developed with good overall performance. The effect of viscosity on absorption rate and heat exchanger has been identified. Optimal concentration for 2MPZ/PZ is found to be 5 m (5 mole/kg water). Thermodynamic and kinetic model has been developed for 2MPZ/PZ in Aspen Plus to allow economic assessments, and process modeling. Semi-aqueous MEA/PZ composes of physical solvent, water, and amine has been characterized. Ultra-fast absorption rate at lean loading has been achieved. The effect of viscosity, diffusivity, CO₂ activity (physical solubility), and amine activity on mass transfer rate (kg') has been studied. kg' increases because of reduced operating CO₂ loading (higher MEA concentration at the same P [superscript *] [subscript CO2]), greater CO₂ physical solubility, and greater MEA activity. The increase in kg' becomes less significant at higher loading due to low diffusivity by high viscosity. The mass transfer model of CO₂ diffusion and reaction with semi-aqueous MEA was built in MATLAB [superscript ®]. Sensitive analysis shows the relationship between rate and solvent physical/thermal properties. The pseudo first order approximation is not applicable to semi-aqueous MEA because of surface depletion of MEA. The energy use of CO₂ capture by amine scrubbing can be estimated by adding minimum work and lost work. Semi-aqueous amines reduces the lost work in the condenser due to less water evaporation in the stripper, which. However; second generation amine processes use advanced stripper configurations can accomplish the same effect with little additional capital cost. Besides viscosity, thermal conductivity and heat capacity also effect the heat exchanger cost. Comprehensive normalized capacity has been developed. An advanced solvent with high normalized capacity can reduce the CAPEX/OPEX of the heat exchanger no matter the solvent is water lean or not. [Mathematical equation].


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