Reaction Mechanism and Interphase Mass Transfer of CO2 from Biogas by Hot Potassium Carbonate Solvent

Through chemical equilibrium thermodynamics analysis of CO2 gas-liquid absorption process, the number of independent reactions in the absorption reaction system was calculated and a four-independent reaction hypothesis was presented. According to the calculation of the independent reaction equilibrium constants, ionization of H2CO3 into H+ and HCO3- was postulated. Although the equilibrium constant was 6.018 9×10-7, the concentration of H+ ions was still very low due to the large presence of CO32- ions in the solution, such that the ionization degree of H2CO3 can not be ignored. Moreover, it was proposed that the reaction mechanism of CO2 gas-liquid absorption was a serial-parallel complex. The absorption rate of CO2 was believed to be determined by its parallel reactions with H2O and OH-. Meanwhile, the critical roles of CO32- ions were analyzed in great detail. Firstly, the interaction between H+ and CO32- directly promoted the absorption of CO2; secondly, the concentration of OH- ions in solution was increased as the decrease of concentration of H+ ions. The latter indirectly accelerated the reaction rate between CO2 and OH- ions. A dual-drive reactor was used to study the gas-liquid mass transfer process in H2O-CO2 system and the main conclusions were as following: when the gas phase stirring rate reached above 140 r/min, the mass transfer resistance of the gas film can be negligible; based on the linear regression analysis, the relationship between the liquid mass transfer coefficient and the speed of liquid phase stirring paddle was verified and the regression correlation was obtained. By calculating the residual, the maximum value was 11.312%, and the correlation was considered to be robust. © 2019, Chinese Society of Agricultural Machinery. All right reserved.