# High Temperature

## Using the OLI High Temperature Code

### ABSTRACT

An equation of state has been developed for the representation of the phase behavior of high-temperature and supercritical aqueous systems containing salts and nonelectrolytes. The equation includes a reference part that is based on a model for hard-sphere ion pairs and dipolar solvent molecules. In addition to the reference part, the equation contains a perturbation part, which is expressed by a truncated virial-type expansion. To enhance the predictive capability of the EOS for normal fluids such as hydrocarbons, the equation has been reformulated using the three-parameter corresponding-states principle. For salt-water systems for which little experimental information is available, a predictive procedure has been developed that relies on similarities in the fluid phase behavior of various salt-water systems. This procedure utilizes the equation of state for NaCl+H2O as a prototype system and introduces a transformation of parameters for the salt of interest. The equation accurately represents vapor-liquid equilibria, solid-liquid equilibria and densities for systems containing water, salts and hydrocarbons.

Here are links to the manual and example files.

ESP 9.3 High Temperature Manual(PDF, 375 kB)

**Which thermodynamic approach is necessary that can accurately represent the chemical equilibria in both the water-rich and the CO2-rich phases over broad ranges of temperature, pressure, and electrolyte composition?**

In view of the importance of the CO2–water–salt systems in geochemistry and chemical and petroleum engineering, various computational models have been developed to represent the properties of such systems. In general, these models fall into two distinct categories:

(1) γ–φ models, in which an activity coefficient formulation is used to reproduce the behavior of aqueous solutions whereas an equation of state provides the fugacity coefficients of components in the gas phase and

(2) φ–φ models, in which a homogeneous equation of state is used to reproduce the properties of both the liquid and gas phases.

Thus, it can be said that there are two streams of research – in one stream activity coefficient models are extended to high T and P and in the other stream EOS models are developed.

The main reason why EOS models have not been widely embraced at “normal” temperatures (i.e., below 300 C) is twofold. First, they do not seem to be working well up to high concentrations. Second, combining them with speciation calculations requires sophisticated numerical algorithms and even if such an algorithm is developed, the computations are bound to be very time-consuming (at least an order of magnitude longer than gamma-phi computations because of the inner loops of solving the EOS).

At high temperatures (above 300 C), the situation is completely different because EOS models are the only possibility. There has been some research in this area by |Duan et al. who has continued the development of EOS.