Titel
"Chemical and Phase Equilibrium Calculation for Geochemical Systems"
Abstract
Most studies of chemical and phase equilibrium (CPE) calculations consider mixtures of non-dissociating compounds. However, many processes entail weak electrolyte equilibrium, sour water scrubbing, gas sweetening and salt precipitation. Calculation of CPE with Gibbs energy minimization in such systems is very limited. Different minimization methods can be classified as stoichiometric and non-stoichiometric. Stoichiometric methods perform the minimization with respect to reaction extents. They are intuitive but inefficient for multiple reactions and multiphase mixtures. Non-stoichiometric methods minimize the Gibbs energy under material balance constraints which are incorporated in the Gibbs energy Lagrangian.
Two non-stoichiometric methods, the Lagrange multipliers method and the modified RAND method are tested on electrolyte systems. The Lagrange multipliers method converges linearly to the solution due to its nested-loop implementation. The modified RAND method utilizes composition derivatives of fugacity/activity coefficients to attain quadratic convergence. Its major advantages are quadratic converge for non-ideal systems, the same treatment of all components in all the phases and monitoring of the Gibbs energy to guide converge. The electrolyte systems involve a vapor phase of volatile solutes, an aqueous phase where speciation reactions take place and a pure solid phase. Activity coefficients for the aqueous phase were obtained using Pitzer’s model. Both algorithms can converge to the equilibrium solution of the highly non-aqueous phase and the solid phase does not affect convergence of either the initialization or the CPE calculations.