Heat of reaction and VLE of post combustion CO2 absorbents

Abstract

The energy required to operate CO2 capture systems at present significantly reduces the overall efficiency of fossil fuel based power generation and other industrial processes and leads to increased fuel requirements, solid wastes generation and environmental impacts relative to the same type of base plant without capture. Minimization of energy requirements for capture, together with improvements in the efficiency of the energy conversion processes will continue to be of highest priority for future improved technologies. Energy requirements for CO2 capture are directly related to the solvents used; being required in the solvent regeneration stage for heating up the solvent, for providing the required heat of desorption for the chemically bound CO2, and for producing sufficient steam for stripping in the upper part of the desorber.

Little experimental data of the heats of absorption/desorption have been published in the open literature. An even smaller number of publications may be found on the modelling of the heat of absorption. Heats of absorption have very often been calculated, using the Gibbs-Helmholtz equation, from data on partial pressure of CO2 as function of temperature and loading.

One objective of this work has been the measurement and prediction of heats of absorption of CO2 with aqueous solutions of a range of amines and their mixtures.

Heats of absorption were in this work measured in a reaction calorimeter at low system pressure, (max 3 and 10 bar) and in an industrially important range of temperatures (40 to 120 ºC). A method was developed and validated in this work for the first time for measuring heats of absorption that were differential both in temperature and loading.

It was shown in this work that it is possible to predict overall heats of absorption of CO2 using temperature dependent correlations for the equilibrium constants of each of the key reactions taking place in the systems CO2 + amine + H2O. For the first time, heat contributions from the individual reactions for basic amines (MEA and MDEA) were separated out and discussed. It was shown that experimental heats of absorption values may be used together with partial pressures of CO2 for fitting some of the equilibrium constants. Two equilibrium models – the Deshmukh-Mather model (Deshmukh and Mather, 1981) and the electrolyte-Non-Random-Two-Liquid model (Chen and Evans, 1986), were used in this work for calculating the activity coefficients and concentrations of species distribution in the solutions. Based on these data, the extents of reactions were calculated and used for the estimation of the heat contributions of the individual reactions. Predictions of the overall heats of absorption using both models were compared.

The other main objective of this work was to measure vapour-liquid equilibria (VLE) on the unloaded amine + water solution. These data are necessary for proper thermodynamic modelling. For engineering purposes, it is often necessary to make estimates of activity coefficients for mixtures, where only fragmentary data, or no data at all, are available.

Accurate data for vapour pressures of pure components are also very important in the calculation of activity coefficients. They are highly sensitive to the experimental conditions and to the purity of the materials used. For proper experimental determination of vapour-liquid equilibria (VLE) it is essential that the vapour pressures of the pure components be measured with the same apparatus and for the same lots of material as are used for the other measurements so that they are an integral part of the data set.

Vapour pressure of pure components and equilibrium vapour pressure and compositions of both vapour and liquid phases for binary and ternary amine + water solutions were measured in a modified Swietoslawski ebulliometer under pressures of maximum 1 bar at temperatures from 40 to 100 ºC, The data were used in calculations of experimental activity coefficients for the components in the liquid and vapour phases.

Binary molecule-molecule interaction parameters were fitted in this work to the experimental activity coefficients of amines and water. It was shown that these parameters may be used in activity coefficient models. Also, experimental activity coefficients may be used for testing VLE models on their ability to predict the system behaviour at zero loading.