1 Effects of viscosity on CO 2 absorption in aqueous piperazine/2methylpiperazine Ye Yuan, Brent Sherman, Gary T. Rochelle* The University of Texas at Austin, Department of Chemical Engineering. 200 E Dean Keeton St. Stop C0400, Austin, TX 78712-1589 Abstract: Piperazine (PZ) has high absorption rate, good stability, low viscosity and high capacity, while a narrow solid solubility window limits its application. Aqueous 2-methylpiperazine (2MPZ) and 2MPZ/PZ blend are attractive as they preserve most of the benefits of PZ and overcome its solubility issue. 8 m 2-methylpiperazine (2MPZ) and 4 m/4 m 2MPZ/PZ were previously studied by Chen. At lower amine concentration, viscosity decreased and the liquid film mass transfer coefficient (k g ’) also increased. Based on k g ’ avg and viscosity normalized capacity, defined as capacity/(/ #$%& ) 0.175 , the optimal amine concentration is 4 m for 2MPZ and 2.5 m/2.5 m for PZ/2MPZ. The k g ’ avg is 8.3*10-7 mol/s*Pa*m 2 and 8.0*10-7 mol/s*Pa*m 2 , respectively. Normalized capacity is 0.87 and 0.89 mol/kg solvent, respectively. In short, 4 m 2MPZ and 2.5 m/2.5 m 2MPZ/PZ are competitive solvents for amine scrubbing. Compared to 8 m PZ, the solid solubility of 8 m 2MPZ and 4 m/4 m 2MPZ/PZ is improved; however, the k g ’ was 70% and 85% less than that of 8 m PZ, respectively. Equation 1 is the pseudo-first-order rate expression of k g ’. The diffusivity of 2MPZ at higher concentration is lower because of the higher viscosity. Assuming the chemical reaction rate (k 2 ) and physical solubility H CO2 do not vary much as concentration changes, an optimal concentration for k g ’ should exist. * ’= - ./0 ∗2 0 ∗[45678] : ./0 (1) The absorption rate (k g ’) and CO 2 solubility of 2 m, 4 m, 6 m, and 8 m 2MPZ were measured at four CO 2 loadings across the lean and rich operating range at 40 ºC using the wetted wall column (WWC). The results show increasing P* as loading increases. Given the CO 2 solubility, capacity was calculated using Equation 2 ∆C = $=>?>@AB ?>C∗(E=?F@GH IJKL ME=?F@GH N.IJKL ) PCHQ$=>?>@AB RS%&∗ST (2) The average liquid-film mass transfer coefficient (k g ’ avg ) with P* CO2 between 0.5kPa and 5kPa was calculated by dividing the log-mean average of the flux over the log-mean average of the driving force. Viscosity, k g ’ avg , capacity, and normalized capacity are listed in Table 1 and plotted in Figure 1. In the operating range of P* from 0.5kPa to 5kPa, 4 m 2MPZ shows the highest k g ’ avg , followed by 6 m and 2 m 2MPZ, with 8 m 2MPZ the lowest. As shown in Figure 1, when concentration goes up, viscosity and capacity increase, and normalized capacity and