SOLUBILITY CALCULATIONS FOR ACID GASES IN AMINE BLENDS (PHASE EQUILIBRIA, TREATING)

Treating with alkanolamines is often used to sweeten gases containing only a few parts per million of CO(,2) and H(,2)S. For this reason, the equilibrium solubility of the acid gases plays a key role in design and operation of gas treating plants. Primary amines such as monoethanolamine (MEA) have great affinity for acid gases and are able to produce high purity sweet gas; on the other hand, tertiary amines like methyldiethanolamine (MDEA) have large capacity and are easy to regenerate but, because they do not bind chemically with CO(,2), they are unable to produce a sweetened gas low in this component. Recently, the use of amine blends (two or more amines in aqueous solution) has become a subject of potentially great commercial importance. For example, a solution containing a high concentration of MDEA and a small amount of MEA or DEA (diethanolamine) might be able to produce high purity sweetened gas while maintaining easy regenerability. The range of possible amines and blend formulations is large and it would be quite out of the question to determine the optimum mixture experimentally for each application. A method for predicting equilibrium solubility is needed.;To model blended amine systems, the appropriate reaction equilibrium expressions were written and, using single-amine interaction parameters, the resulting system of equations was solved. A sparse matrix implementation of multi-dimensional Newton-Raphson iteration was used. This solution procedure has been found to allow the VLE model to be used effectively in column design and simulation packages.;MEA-MDEA and DEA-MDEA blends have been studied in detail in this work. It is found that each amine contributes to the overall acid gas solubility in a nonlinear way and that the solubility curves can exhibit maxima and minima as a function of the relative concentrations of the amines. (Abstract shortened with permission of author.);We have developed a rigorous thermodynamic model based on the framework established by Edwards, Newman and Prausnitz (AIChE Journal, 21,248,1975) which uses the extended Debye-Huckel expression proposed by Guggenheim (Phil. Mag., 19,588,1935). Our model is very similar to one developed by Deshmukh and Mather (Chem. Eng. Sci., 36,355,1981) for single-amine solutions and involves the fitting of binary interaction parameters to experimental data. In this work, the interaction parameters found to be important in the activity coefficient expression were fitted to each single-acid-gas single-amine subsystem using all published solubility data. A modified Gauss-Newton method was found quite suitable for this task. The resulting model was then validated by comparing mixed-acid-gas single-amine solubility predictions with published VLE data.