Kinetics, modeling, and simulation of carbon dioxide absorption into mixed aqueous loaded solution blends of methyldiethanolamine with monoethanolamine and with piperazine

Chemical solvents such as alkanolamines are commonly used to aid the removal of greenhouse gases, primarily carbon dioxide (CO 2), from low pressure and low concentration of CO2 of flue gas streams of power plant through the process of absorption. Data available on the kinetics of absorption of newly formulated and energy efficient mixed amines such as blends of methyldiethanolamine (MDEA) with monoethanolamine (MEA) and piperazine (PZ) are scarce at various CO2 loadings and for a wide range of temperatures (298K to 333K). This research involves the development, for the first time, of new, comprehensive, numerically solved one dimensional (1-D) and two dimensional (2-D) absorption rate/kinetics models to interpret the experimental kinetic data obtained with a laminar jet apparatus for the absorption of carbon dioxide (CO2) in CO2 loaded mixed solutions of these two mixed amine systems. The first kinetics system consisted of three MDEA/MEA weight ratios ranging from 27/03 to 23/07, over concentration ranges of 2.316-1.996 kmol/m3 for MDEA and 0.490-1.147 kmol/m3 for MEA. The second experimental kinetics system data consisted of MDEA/PZ wt% concentration ratio of 27/3, 24/6 and 21/9, and CO 2 loading from 0.0095 to 0.33 mole of CO2 per mole of amine obtained under no interfacial turbulence and over a temperature range of 313 to 333 K. The models take into account the coupling between chemical equilibrium, mass transfer, and the chemical kinetics of all possible chemical reactions involved in the CO2 reaction with MDEA/MEA and MDEA/PZ solvents. The partial differential equations of the 1-D model of the MDEA/MEA kinetics system were solved by two numerical techniques: the finite difference method (FDM) based on in-house coded Barakat-Clark scheme and the finite element method (FEM) based on COMSOL software. The Barakat and Clark (1966) scheme is being applied, for the first time, to solve the blended solvent MDEA/MEA kinetics model system represented in this research work. The FEM comprehensive model was then used to solve the set of partial differential equations in the 2-D cylindrical coordinate system setting. Both FDM and FEM produced very accurate results for both the 1-D and 2-D models, which were much better than the previously published simplified model. The limitations to develop the kinetics of reaction for CO2 absorption into mixed amines by using a simplified model were obviously confirmed. Also, this allows us to confirm that FEM could be used for the numerical solution in order to eliminate the extra effort of coding the program of the numerical scheme, proposed to solve the PDEs representing the comprehensive kinetic model. The reaction rate constant obtained for MEA blended into MDEA at 298-333 K was kMEA =5.127 X 108 exp (-3373.8/T).;In addition, the 2-D kinetic model, used for the first time, for both blends of amines MEA/MDEA and MDEA/PZ, provided the concentration profiles of all the species in both the radial and axial directions of the laminar jet, thus enabling an understanding of the correct sequence in which the reaction steps are involved in the reactive absorption of CO2 in aqueous mixed amines under study.;Keywords: Absorption, Carbon dioxide, Kinetics, Numerical modeling, Monoethanolamine, Methyldiethanolamine, Piperazine, Comprehensive models.;The obtained experimental kinetics data for the mixed MDEA/PZ were interpreted with a newly developed absorption-rate/kinetics model in order to obtain the kinetics parameters of the CO2 absorption into MDEA/PZ solutions. The new, comprehensive, numerically solved 1-D and 2-D absorption rate/kinetics model has been developed, for the first time; by solving the partial differential equations of this model with a numerical technique: the finite element method (FEM), based on COMSOL software. The reaction rate constant for PZ blended into MDEA at 313-333 K was obtained.