CONSTRAINED GIBBS ENERGY METHODS IN PROCESS MODELING
Systematic methods and tools for managing the complexity
Advances in Computational & Numerical Methods (T4-4)
Keywords: Multi-phase Gibbs energy minimisation, constraint potentials, reaction kinetics, surface energy, ion exchange
The multi-phase analysis of complex systems by Gibbs free energy minimisation has gained increasing popularity with the development of efficient computers and advanced data-bases. The advantages of the multi-phase methods have been widely recognised and they are becoming accepted in different applications, ranging from chemical engineering and metallurgy to materials processing as well as pulp and paper production and energy and environment technologies.
The Gibbs energy minimisation requires the use of the chemical potentials (partial molar Gibbs energies) of the constituents of the system. Usually, these appear at their equilibrium values as a result of the minimisation calculation, the mass balance constraints being the necessary subsidiary conditions. Yet, there are several such physical circumstances, where chemical potentials appear constrained also by other factors. In this paper a method will be presented by which constrained chemical potentials can be applied with multi-phase Gibbs energy minimisation software. The constrained potentials arise typically due to an additional work or energy effect or they are due to a pre-defined affinity. In the Gibbs energy minimisation performed by the Lagrange method the new constraints are set as additional Lagrangian multipliers.
Examples of the constrained Gibbs energy method will be presented in terms of the electrochemical Donnan equilibria in aqueous systems containing semi-permeable interfaces, the phase composition in surface-energy controlled systems and in systems with affinities controlled by chemical reaction kinetics. For each of these applications advanced computation methods are being developed which can be utilized with the existing multi-phase thermodynamic (Gibbs energy) data. The new method has been successfully applied in calculating distribution coefficients for metal cations together with pH-values in pulp suspensions, in calculation of surface tension of various mixtures and in thermochemical process modelling involving chemical reaction rates. It can be used in assessment of thermodynamic properties, service and design applications, process scale-up, study of new chemical concepts and search for new, more economic process alternatives.
Presented Tuesday 18, 16:20 to 16:40, in session Advances in Computational & Numerical Methods (T4-4).
