A robust multiphase isenthalpic flash model for water/solvent/hydrocarbon systems at high pressures and elevated temperatures

Abstract

Based on the assumption that only the presence of water and solvent is considered in the aqueous phase, a robust and pragmatic water-associated isenthalpic flash (WAIF) model has been developed to perform multiphase isenthalpic flash calculations for water/solvent/hydrocarbon mixtures at high pressures and elevated temperatures. More specifically, the phase boundary diagram in enthalpy-temperature (H-T) space is constructed first for a given water/solvent/hydrocarbon system at a specified pressure. Then, the phase transition enthalpies and temperatures are determined from the H-T diagram, and thus one-, two-, or three-phase isenthalpic flash calculations can be conducted directly without resorting to stability test for a given system at a specified enthalpy and pressure. In addition, a recursive method has been proposed to initialize the temperature so that the energy conservation equation and material balance equation can be solved simultaneously to overcome the convergence difficulties. The newly proposed isenthalpic flash model has been compared with the water-free isenthalpic flash (WFIF) model and conventional three-phase isenthalpic flash (CTIF) model through three case studies. The WAIF model is found to handle the narrow-boiling constraints effectively and accurately. Furthermore, as for Water/C3H8/C16H34 mixture and Water/CO2/C16H34 mixture, a good agreement is achieved between the WFIF model and the WAIF model as the solubility of solvents (i.e., C3H8 and CO2) in the aqueous phase is quite small. With respect to Water/dimethyl ether (DME)/C16H34 mixture, however, the WAIF model is found to more accurately predict the phase behaviour of water/solvent/hydrocarbon mixtures than the WFIF model since the solubility of DME in the aqueous phase cannot be neglected. The case studies demonstrate that the newly developed water-associated isenthalpic flash model for water/solvent/hydrocarbon systems is robust, efficient, and accurate.