Solubility of light reservoir gasses in water by the modified Peng-Robinson plus association equation of state using experimental critical properties for pure water

Cubic-Plus-Association (CPA) equations of state (EOS) have found special interest in thermodynamic modeling of processes in which the solubility of light gasses in water is important. In this class of EoSs the association contribution of Helmholtz free energy proposed by Wertheim has been added to cubic EoSs such as Soave-Redlich-Kwong (SRK) and Peng-Robinson (PR) to extend their application to associating substances. In the development of CPA EoSs it is a common practice to adjust the pure component properties like the critical temperature, the critical pressure and the acentric factor in addition to the association parameters.This work focuses on the extension of a modified version of PR (mPR) EoS to water by addition of the compressibility factor contribution based on Wertheim theory. However, in contrast to other CPA EoSs the critical properties and the acentric factor are not considered as adjustable parameters and their experimental values are used. The temperature dependent parameters of the mPR EoS are modified by introduction of two correction factors which are correlated as functions of reduced temperature. Different association site schemes were examined for water. The results show that the 2-site association scheme produces the best predictions. The average absolute percent deviations of 0.43% and 0.03% for liquid density and vapor pressure in the range of 0.422 < Tr < 0.980 were obtained. Then the solubilities of light hydrocarbon gasses (CH4, C2H6, C3H8 and n-C4H10), and some common industrial non-hydrocarbon gasses (H2S and CO2) were calculated over wide ranges of pressure and temperature. Binary interaction coefficients (BIP) were regressed as a function of temperature. Accurate results show that the proposed model is able to produce excellent description of pure water properties as well as the solubilities of light gasses in water.

Research highlights
► The modified Peng-Robinson equation of state together with the Wertheim association contribution is used to present a new cubic plus association equation of state.
► The model uses experimental critical properties and acentric factor for water.
► Solubility of light gasses (C1 to n-C4 hydrocarbons, CO2 and H2S) are calculated by the model.
► Accurate results are obtained for the vapor-liquid coexistence curve of water and solubility of light gasses in water.