Mass transfer characteristics of CO2 absorption into liquid droplets
Doctoral thesis, Peer reviewed
MetadataShow full item record
Original versionHan, J. Mass transfer characteristics of CO2 absorption into liquid droplets. Doctoral dissertation, Telemark University College, 2014
CO2 capture from exhaust gases has been paid more and more attention in order to avoid global warming. One of the methods for removing CO2 from the flue gas streams is the use of absorption and aqueous alkanolamine solutions as absorbents. Alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA) and N-methyldiethanolamine (MDEA) are widely used in CO2 capture because of their high CO2 absorbing capacity and lower energy consumption. Physical properties such as density and surface tension of the pure compounds of amines, the mixtures with water and CO2 loaded aqueous amine solutions are important for optimal designing of absorption-desorption processes and the related engineering calculations. The absorption of CO2 into aqueous amine solutions by the spray method is a possible process for bulk removal of CO2 from a gaseous stream. A deep understanding of the mass transfer characteristics in the spray column is very important for the optimization design of the column and the selection of absorbent. The study of mass transfer between CO2 and the unit part of a spray - individual droplets is crucial for better understanding the mass transfer characteristics in the spray column. In this work, densities in liquid solutions of water + monoethanolamine (MEA), water + diethanolamine (DEA) and water + N-methyldiethanolamine (MDEA) have been measured at temperatures from (298.15 to 423.15) K by Anton Paar density meters DMA 4500 and DMA HP. The mass fraction of amine ranged from 0.3 to 1.0. Excess molar volumes of the binary system were derived and correlated by a Redlich-Kister equation. The model uses a third order Redlich-Kister equation and a linear relationship with the temperature for unloaded aqueous MEA solutions, while a fourth order Redlich-Kister equation and a second polynomial function with respect to the temperature for unloaded aqueous DEA and MDEA solutions. Densities of CO2 loaded aqueous MEA solutions (water + MEA + CO2) were measured at temperatures from (298.15 to 413.15) K by Anton Paar density meters DMA 4500 and DMA HP. The mass fraction of MEA in water was 0.3, 0.4, 0.5 and 0.6. Densities in liquid solutions of water + DEA + CO2 and water + MDEA + CO2 were measured at temperatures from (298.15 to 423.15) K by Anton Paar density meters DMA 4500 and DMA HP. The mass fraction of DEA and MDEA in water was 0.3 and 0.4. The CO2 loading ranged from 0.1 to 0.5. Molar volumes of the ternary system were derived and correlated by the equation from Weiland et al. at each temperature. The parameters were in turn fitted by a polynomial function of the temperature. The agreement between the measured density results and the correlated data is good. The uncertainties of density measurements were analyzed. Surface tensions of aqueous MEA solutions were measured at temperatures from (303.15 to 333.15) K by the sessile drop method. A Rame-Hart Model 500 Advanced Goniometer with DROPimage Advanced v2.4 was employed. The mass fraction of MEA ranged from 0 to 1.0. Measured surface tensions of aqueous MEA solutions in this work were compared with Vázquez et al.?? data. The experimental surface tensions were correlated with temperature by a linear relationship. The correlated surface tensions by the linear equation and the experimental data have very good agreement. The surface tensions of aqueous MEA solutions were correlated with mole fraction of MEA by both an empirical model and the chemical model. The chemical model shows better agreement with the experimental surface tension data than the empirical model. The uncertainties of surface tension measurements were analyzed. In order to study the mass transfer characteristics between CO2 and liquid droplets, a novel experimental set-up was constructed. This system produces individual droplets by pushing the liquid through a needle with the help of pressurized nitrogen. The droplets fall through a gas chamber one by one and finally deposit under kerosene. Pure CO2 is filled in the gas chamber to eliminate the gas side mass transfer resistance. A temperature control box was built outside the chamber in order to perform the absorption experiments under controlled temperatures. The pressure inside the chamber keeps constant and the same as the atmosphere by an overflow section. The experiments can be performed at different droplet falling heights by adjusting the length of the overflow tube. The volume flow rate of CO2 was measured by a soap film flow meter to calculate the absorption rate. A high speed camera system was used to determine the size of droplet, droplet formation time and droplet formation rate. The absorption of CO2 into the kerosene can be measured before the droplets start dripping. The results from this blank experiment will be subtracted to determine the concentration of CO2 that is absorbed by liquid droplets. Because the density of kerosene is much smaller than the solvent, the droplets deposit under kerosene very fast. Hence, the coalescence effect can be eliminated. The liquid phase mass transfer coefficients of CO2 absorption by liquid droplets were measured at different temperatures, droplet formation times and droplet falling heights. Physical absorption (CO2 + water droplets) and chemical absorption (CO2 + droplets of 30% MEA solutions) were both investigated. The liquid phase mass transfer coefficients of CO2 absorption into water droplets during droplet life-time (formation and fall together) were measured at temperatures T = 303.65 K and 323.15 K, droplet falling heights h = 0.41 m and 0.59 m, and droplet formation times t1 = (0.352 to 2.315) s. It was found that there exists convection inside the water droplets which significantly enhances the mass transfer between CO2 and water droplets. The convection increases as the droplet formation time decreases. The absorption rates of CO2 into water droplets during droplet formation were measured at different droplet formation times and temperatures T = 297.15 K and 323.15 K. The measured absorption rates of CO2 absorption into water droplets during formation at 297.15 K agree well with Dixon and Russell?? data. The correlation of the absorption rate of CO2 into water droplets during formation with droplet formation time at 323.15 K was determined. The correlation between the Sherwood number and the Reynolds number of CO2 absorption by water droplets during droplet fall at 323.15 K was obtained. The absorption rates of CO2 into droplets of 30% MEA solutions during droplet formation were measured at 323.15 K and different droplet formation times. It was found that the mass transfer between CO2 and droplets of 30% MEA solutions was not affected by the droplet formation time, which is probably because the convection inside droplets of 30% MEA solutions is small and the intensity of convection does not change very much over the range that these measurements covered. The liquid phase mass transfer coefficients of CO2 absorption into droplets of 30% MEA solutions during droplet life-time (formation and fall together) were measured at temperatures T = 303.65 K and 323.15 K, and droplet falling heights h = (0.07 to 0.53) m. The correlation between the Sherwood number and the Reynolds number of CO2 absorption by droplets of 30% MEA solutions during droplet fall at 323.15 K was obtained. The enhancement factors of CO2 absorption by droplets of 30% MEA solutions are estimated. The liquid phase mass transfer coefficients without chemical reaction for CO2 into water droplets and that for CO2 into droplets of 30% MEA solution are compared.