Today the thermo-physical properties data is represented in the form of a set of combined theoretical and empirical predictive algorithms that rest on evaluated data. These techniques have been tested and incorporated into interactive computer programs that generate a large variety of properties based upon the specified composition and the appropriate state variables. Equations of state, correlations, or empirical models are used to calculate thermos-physical properties of fluids or mixtures. Examples of this include Helmholtz energy based equations, cubic equation of state, BWR pressure explicit equations, corresponding states models, transport models, vapor pressure correlations, spline interpolations, estimation models or calculation methods for vapor-liquid equilibrium or solubility, and surface tension correlations. Further fitting techniques, and group contribution methods are incorporated. The following broad level properties are often used in simulation tools:
- Thermodynamic properties including equation of state, phase equilibria, p-V-T behavior, heat capacity, enthalpy, thermal expansion, sound speed, and critical phenomena.
- Transport properties including thermal and electrical conductivity, viscosity, mass diffusion, thermal diffusion, non-Newtonian behavior, and thermal, thermoacoustic, and other diffusion waves.
- Optical and thermal radiative properties including dielectric constant, refractive index, emissivity, reflectivity, and absorptivity.
- Interfacial properties including solid-solid interfaces, surface tension, interfacial profiles, interfacial transport, and wetting.
Databases are now available for hydrocarbon mixtures, including natural gas, as well as a number of pure and mixed fluids of industrial importance. IAPWS, NIST and Coolprop are a few examples of such resources that provide valuable tools for turbomachinery and refrigeration engineers, and chemical and equipment manufacturers. One example is the IAPWS-IF97 that divides water and steam properties into five distinct regions.
Another example is properties of R134a expressed as 32 term, modified Benedict-Webb-Rubin (MBWR) equation of state, the accuracy of equation of state is estimated to be ± 0.2 % in density, ± 1 % in constant volume heat capacity and ± 0.6 % in sound velocity. The thermos-physical property databases provide core information for process modeling and development. The completeness, correctness, currency and reliability of the data as well as the integrity and management of the database itself are important factors in the ultimate reliability of the modeled process.[:]