ScaleChem
... What makes it work

Thermodynamics at their finest
  OLI's aqueous thermodynamic model is the calculation engine for ScaleChem.

Applied to Equilibrium calculations

The OLI Aqueous Model is unique because it can predictively model the speciation of a wide range of chemicals in water. Like other process simulation software packages, the OLI Aqueous Model considers the vapor-liquid equilibrium (VLE) of a given chemistry for the molecular species involved. The OLI Aqueous Model also considers the speciation, if any, of a chemistry, and can accurately predict reactions between chemicals, solids formation, the pH, and the ionic composition of a given chemistry.

The OLI model uses:

the Revised Helgeson Equation of State for predicting the standard state partial molal thermodynamic properties of all species, including organics and complexes, in water;

the Bromley-Zemaitis framework for the prediction of the corresponding excess partial molal thermodynamic properties;

the Pitzer and Setschenow formulation for the prediction of the excess partial molal thermodynamic properties calculation of molecular species in water; and

the Enhanced SRK Equation of State for the prediction of vapor and non-aqueous, liquid phase partial molal thermodynamic properties.

Alkalinity-pH-CO2 Reconciliation

The alkalinity of a water is defined as the base capacity of the water. Alkalinity is sometimes considered equivalent to the carbonate concentration; however, all basic species contribute to alkalinity.

In ScaleChem, the alkalinity calculation is done by calculating the amount of HCl which would be needed to titrate the solution to the alkalinity endpoint (default value: 4.5). There is also a convergence loop around this calculation, since the value of pH is also being adjusted to the calculated value by the addition or subtraction of HCl.

Understanding
Electrolytes

Scaling Tendency

The Scale Tendency is defined as the ratio of the activity product Q for an equilibrium to the solubility product K_sp for the same equilibrium. In other words, ST=Q/K_sp.

When the ratio of Q/K_sp is greater than 1.0, the solid is said to have a thermodynamic driving force to form. When the ratio is less than 1.0, the solid does not have the driving force to form. When the ratio equals 1.0, the solid is considered to be at saturation.

The solubility product, usually represented as K_sp, is the thermodynamic equilibrium constant, a function of temperature and pressure. ScaleChem can accurately calculate the equilibrium constant, K_sp, as a function of temperature and pressure, for all the solids in the ScaleChem chemistry.

This scale tendency is the actual ratio, not the log of the ratio. The log of the ratio is also used in the industry. In other words, Scale IndexSI=log10(ST). This log is calculated by ScaleChem and is included in the report summaries.

CO2 in Gas Reconciliation

ScaleChem also permits a reconciliation of the water analysis based on the fraction of CO2 present in the gas. In this case, CO2 is added to meet a specified vapor fraction, and the pH and alkalinity are calculated and reported.

Density Reconciliation

The density of the water is usually reported on a water analysis as grams/liter or as grams/cc. Occasionally, specific gravity is reported, which is the ratio of the density of the solution to the density of pure water.

If the user selects the Density Reconciliation option in ScaleChem, then ScaleChem calculates the density and uses that value in all other calculations. If this option is not selected, the user measured density is used in all other calculations.

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