Research On CO₂Absorption And Desorption Based On Hollow Fiber Membrane Technology

At present, solvent based post combustion CO2capture (PCC) technology was widely expected to be the most effective approach to control CO2emission from coal-fired power stations in a short period. Of which, CO2absorption&desorption based on hollow fiber membrane contactors are reported to be a promising technology with good mass transfer performance of CO2absorption and low energy intensity for CO2desorption. However, how to choose suitable absorbents and maintain operating stability are the main issues for CO2membrane absorption/stripping. In this thesis, three main aspects around CO2membrane absorption/stripping were studied, including the absorbent selection&development, process simulation&optimization and stability studies of membrane.In terms of single absorbents, we studied the structural-performance relationship for CO2membrane absorption/stripping from amine based absorbents. The effects of amino group number, hydrogen atom number in amino group, carbon chain length, functional group size and position on CO2membrane absorption/stripping were investigated. The study of structural-performance relationship will facilitate to guide the prediction of absorbent structure for CO2capture. We also established a comprehensive evaluation for amine based absorbents in CO2membrane absorption/stripping. It has been found that TEPA and EDA were the most recommended absorbents for CO2membrane absorption. For CO2membrane stripping, MDEA was first recommended; other absorbents such as TETA, DEA and EDA were prospective candidates. In addition, we have proposed the concept of high-temperature CO? membrane absorption to reduce CO2capture cost. Potassium sarcosinate (KS) was selected to be the most suitable absorbent for high-temperature COt membrane absorption by screening24amino acid salts solution.For blended amines, solvent formulation optimization and blending mechanisam were studied intensively by developing a binary amine system based CO2membrane absorption model. The predicted results, including overall mass transfer coefficients and CO2removal ratio, agreed very well with those determined experimentally. The recommended optimal blended amine compositions are:for MEA/MDEA,30wt%MEA with a MDEA proportion of0.1-0.3; for DEA/AMP,15wt%DEA with an AMP proportion of0.5-0.8; and for MDEA/PZ,20wt%MDEA with a PZ proportion of0.3.Based on the theories of CO2reaction kinetics, MEA-H2O-CO2vapor liquid equilibrium (VLE) and fluid flow&mass transfer, a two-dimensional (2-D) mathematical model was developed to simulate the CO2stripping from the CO2-ich monoethanolamine (MEA) solvent in a hollow fiber membrane contactor. The modeling results agreed well with literature experimental results. To improve the understanding and facilitate the optimization of the membrane based CO2stripping process, we investigated the effects of different operating variables including liquid flow velocity, sweeping gas flow rate, regeneration pressure, and temperature on CO2desorption performance. The sensitive analysis of membrane contactor’s dimensions and membrane welting ratio on CO2desorption were also studied. Finally, the operating condition of CO2membrane stripping was optimized,20kPa was repoeted as the optimal the operating pressure.In terms of membrane stability studies, three representative absorbents MEA,2-Amino-2-hydroxymethyl-1,3-propanediol (THAM) and KS were chosen to investigate the mechanism of membrane wetting and change of membrane resistances in the membrane absorption process at60℃. It has been found that transmembrane pressure has significantly increased the membrane resistance; higher transmembrane pressure will lead to high ratio of membrane wetting. KS with a high surface tension has less deformation of membrane and tendency of increasing membrane resistance than other absorbents.Finally, we evaluated and optimized the energy demand in CO2membrane stripping process. For MEA absorbent, the operating pressure of20kPa has the potential to achieve the lowest energy consumption with28%of decreasing compared with conventional thermal induced regeneration. Among all the amine based absorbents screened, MEA presented the lowest regeneration energy consumption in CO2membrane stripping process, with the following of EDA and DEA absorbents. If sweeping steam could be abandoned or abundant low-grade waste heat was available, CO2membrane stripping technology will have more significant advantage on reducing energy consumption, which will promote the commercial application of CO2membrane strippin.