Multicomponent equilibrium calculations in process design: study of some acid digester reactors

Abstract

The calculation of multicomponent thermodynamic equilibrium by Gibbs energy minimization has become an efficient tool in modern engineering practice. When calculating hydrometallurgical reactors a parametrized model for activities in concentrated aqueous solution is required. A comparative literature study was performed for several electrolyte models using standard deviations of the mean activity and osmotic coefficients with a number of electrolytes. The Pitzer model results with the smallest variation. Moreover its practical application area is wide due to the large number of published parameters.

With the activity coefficient model chosen the equilibrium calculation of a multicomponent aqueous reactor is based on known thermodynamic and activity data of the heterogeneous system. Omission of sime reactions from the calculation system may further be necessary because of slow reaction kinetics. In that case the minimization algorithm results with a close to equilibrium approximation.

The advantage of the thermodynamic algorithm is the possibility to follow both the chemical composition of all reactor phases and the energy balance of the reactor. Thus the calculation results can be verified by either composition or heat balance measurements. Simultaneously, quantified estimates of effluents and side-products can be made.

The well-known equilibrium routines Solgasmix and ChemSage were used for sulphate — sulphuric acid equilibria in concentrated solutions. A reactor calculation was performed for sulphuric acid digestion of ilmenite ore in TiO₂-pigment manufacturing and for series of reactors in the NPK-fertilizer process. The calculated maximum concentrations of such effluents as SO₂, H₂S and respectively NO_x NH₃ and fluorine were compared with Kemira Oy’s process data.