Experimental and numerical study of carbon dioxide mass transfer and kinetics in amine solutions

Abstract

In CO2 absorption processes large mass and heat transfer driving forces occur in several places, such as the bottom of absorber, water wash section and desorber. Mass transfer is a science that describes the relative motion of species in a mixture of chemical species as a result of mass concentration gradient or difference. Good fundamental models for the combined heat and mass transfer including chemical reaction and convective mechanisms are still lacking. A proper description of the transfer processes is vital for improved absorption process simulation and optimization.

The objective of this work has mainly been focused on kinetic and mass transfer modeling in CO2 absorption by aqueous amine solutions. In addition, part of the work has been devoted to a comparison of pilot plant data and simulation of the pilot plants with different process simulators.

The thesis firstly presents the characterization work of a wetted wall column in order to obtain the absorption measurements from the whole experimental setup as accurately as possible. A characterization of both gas and liquid phase resistance were done in the wetted wall column so that the gas and liquid mass transfer coefficients could be obtained under a variety of operation conditions.

The mass transfer rates for the reaction of CO2 into aqueous amine systems were measured for MEA and EDA with both unloaded and CO2 loaded solutions. Good agreement between the experimental data in this work and published data from available literature is found. Hence, kinetic models are developed, based on a soft model concept, on a concentration basis and finally, for MEA, on an activity basis. Predictions based on these models are compared and it was found that there is no significant difference between the models with regard to predictive capability of mass transfer flux within the experimental concentration and temperatures ranges.

A heat and mass transfer model for the absorption process was developed based on the penetration theory, and validated by the absorption rate experiments from the wetted wall column. Also available literature data were included. The prediction shows less than 13% average absolute deviation to experimental data. Regarding mass transfer in the gas phase, this work shows results from tests of the gas phase mass transfer models tested. The Fick’s law and Maxwell-Stefan equation were used in an assumed case with significant convective transport and multi-component mixtures. The results show significant differences with respect to gas phase resistance.

The numerical solution methods for the PDEs, and their implementation, were found to be a key factor in obtaining stable, robust and fast computations. The penetration model for combined heat and mass transfer was implemented and tested using a variety of numerical methods. It was found that the orthogonal collocation method is by far the fastest. Based on this numerical scheme, the combined heat and mass transfer model was developed in this work.

The simulation work contains the results and conclusions from the testing of 6 different simulators on sixteen data sets from four different pilot plant studies based on 30 wt% MEA solutions as solvent. Of the simulators four were commercial simulators and two in-house codes. The simulations were performed on an as equal basis as possible given the constraints of the various simulators. Basically all the simulators are capable of giving reasonable predictions on overall performance, i.e. CO2 absorption rate. The reboiler duties are less well predicted with some scatter, but still reasonable. Concentration profiles in gas and liquid could also be reasonably well predicted. However, temperature profiles were quite scattered. This gives confidence in simulation results obtained from such tools, but at the same time gives ample room for improvement.