SCALE2000: reaction-transport software dedicated to thermokinetic prediction and quantification of scalesApplicability to desalination problems

Mineral scaling prediction in industrial processes involving high salinity solutions at high temperatures remains notoriously intricate, due particularly to difficulties incurred by modelling the thermodynamic and kinetic behaviour of the mineral deposits in such systems. In addition to these complex thermokinetic processes, the quantification of the nature and extent of scale deposit requires the simultaneous incorporation of the mass transport effects resulting from fluid flow through different units of the considered flow sheet. The latest version (V3.1) of SCALE2000, a thermokinetic geochemical calculation program based on Pitzer's ion interaction model, can now take into account the fluid flow effect using a specific approach based on a scheme of serially connected homogeneous reactors. On the other hand, the SCALE2000 thermodynamic database allows the study of the Na-K-Ca-Mg-Sr-Ba-Fe-C1-SO4-HS-OH-H-AcCO3-HCO3-H2S-CO2-AcH-SiO2-H20 system from 25 to 250°C, 1 to 1000 bar and for salinities as high as 300 g/l. The brine densities are also calculated using the Pitzer's formalism and the gas fugacity is calculated using equations of state recently published in the literature. Moreover, SCALE2000 simulates the mixing of waters with contrasting physicochemical properties, as well as fluid heating/cooling consequences. Thus SCALE2000 is well adapted to perform scaling risk prediction applied to industrial desalination facilities. The reliability of the SCALE2000 results was checked against measurements of individual mineral solubility and kinetics data derived from the literature. Furthermore, recent applications of SCALE2000 to geothermal energy production and oilfield exploitation cases [ 1,2] demonstrated the ability of the software to predict and quantify the scaling phenomenon in complex hydrogeochemical systems. The main features of the software as well as a specific example of scaling risk calculation likely to be encountered in desalination processes are presented.

SCALE2000 is able to perform thermodynamic and kinetic calculations in the system Na-K-Ca-Mg-Sr-Ba-Fe-C1-SO 4-HS-OH-H-AcCO3-HCO3-H2S-CO2-AcH-SiO2-H20 according to Pitzer's formalism in the range of 0-250°C and 0-1000 bar, and for solutions of ionic strength as high as 6 M. It takes into account 19 minerals (calcite, dolomite, strontianite, magnesite, siderite, witherite, aragonite, vaterite, halite, sylvite, anhydrite, gypsum, celestite, barite, quartz, chalcedony, amorphous silica and pyrrhotite) and two gas species (CO2 and H2S), as well as all relevant chemical reactions among aqueous species entering the composition of these minerals and gases. This software can perform: . many kinds of calculations including solubility of the main known minerals responsible for scale problems (carbonates, sulphates, sulphides, silicates, halide minerals, etc.) encountered in desalination techniques and industrial processes based on electrolytic aqueous systems; • simulation of the effect of temperature and pressure on the scale formation risks, of the effect of binary mixing of strongly physicochemical differing waters, and quantitative prediction of scale formation based on detailed kinetic formalism; o calculation of density (specific volume) for electrolytic systems (i.e., natural and industrial brines) taking into account the nature of the dissolved salts; • an accurate evaluation of the osmotic coefficient (i.e., osmotic pressure), a crucial parameter in the reverse osmosis method used in desalination. Hence, SCALE2000 can be used with confidence to achieve realistic scaling risk diagnoses in various industrial contexts. Its present form constitutes a scientifically validated research tool which is also oriented to engineering applications. Relying on its powerful thermodynamic, kinetic, and coupled chemistry-transport capabilities, SCALE2000 could be easily adapted to integrate other practical aspects (evaporation, number o f reactors, etc.) o f the desalination industry.