Graduate Thesis Or Dissertation
 

A computer program to solve aqueous inorganic equilibrium systems using Pitzer's method to determine activity coefficients

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  • Chemical equilibrium is a major factor in many natural and industrial systems. The ability to predict ionic equilibrium is extremely valuable in industry in solving problems without the costs involved in full scale experiments. An example of an application in the pulp and paper industry is the prediction of non-process element solubility in process streams. A computer program, ISIS, was developed to estimate the solubility of inorganic salts in aqueous, inorganic solutions. The model incorporates a two step Gibbs free energy minimization algorithm and Pitzer's method for ionic activity coefficient prediction. The program, ISIS, was tested on three cases. The test cases were: KCl solubility in a NaCl and water solution, NaCl solubility in a MgCl₂ and water solution, and CaSO₄ solubility in a NaCl and water solution. The equilibrium predictions in the test cases were very good, with the mean of the absolute value of the relative error ranging from 4.3% for the first case, 17% for the second case, to 3.8% in the third case. ISIS accurately predicted the solid phase amount and chemical composition in each of the test cases. A parametric study was conducted on the three test cases to examine the effects of the activity coefficient predictor, and of uncertainty in chemical potentials, and third virial coefficients and mixing terms in the Pitzer activity coefficient prediction model. The effects of the activity coefficient predictor were determined by comparing predictions made with the Pitzer model versus an extended Debye-Huckel equation and an ideal solution assumption. The differences were great, with the other activity coefficient predictors resulting in errors greater than five times the error with the Pitzer activity coefficient model. The effect of the chemical potentials was large, especially in the trace species case, CaSO₄ in a NaCl and water solution. A relative change of less than a tenth of a percent in the solid species chemical potential resulted in an increased error of ten times the original error. The effect of assuming the third virial coefficients and the mixing terms to be zero in the Pitzer activity coefficient predictor can be large. Errors up to 90% in the mean activity coefficient were found. It was concluded that the computer program ISIS could predict accurately the solubility of inorganic salts in aqueous, inorganic solutions. The accuracy of the prediction would be greatly affected by the accuracy of the chemical potentials and the availability of the third virial coefficient and mixing terms in the Pitzer activity coefficient predictor. Future work is recommended in collecting accurate chemical potentials and Pitzer interaction terms to increase the database for ISIS and similar programs. The inclusion of ISIS into a steady state chemical process simulator is also recommended.
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