Developing of a Equation of State library with CPA in Rust with application to moisture calculation in natural gas

A library of equations of state - Reos - was developed in the Rust programming language. The package can also be easily used in Python. The main feature of the package is the inclusion of the CPA EoS, whose implementation offers users the flexibility to choose the association schemes of the components in the mixture, the association parameters and combining rules, as well as access to the fraction of non-bonded sites of the molecules. To achieve this, a comprehensive study of the implementation of the associative contribution Helmholtz energy was carried out due to its complexity. The conclusion is that the most effective way to implement it is through the site-oriented paradigm. This paradigm is characterized by an index transformation that considers only the indices of the sites in the mixture, dropping the indices of the components to which the sites belong. To implement this approach, we proposed matrices that characterize the system, which are: the matrix of allowable interactions between sites and the matrix of site owners indicating which component each site belongs to. With these two matrices, all associative properties can be computed through simple matrix operations. To validate the implementation, the calculation of water content in natural gas containing CO₂ was used as an example. In this case, besides the self-association of water, solvation interactions between water and CO₂ are also present. The results are consistent with experimental data, and the Python code that validates the implementation is present in the GitHub of the project, which contains not only the moisture calculation, but phase equilibrium for other systems – water and acid; water and alcohols and mixture of alcohols.