Abstract
Cubic equations of state (EOS), such as the Peng-Robinson EOS, are routinely used by the oil and gas industry for the design of recovery and processing operations of gases at high pressures. Nonetheless, many non-cubic equations of state, derived from statistical thermodynamics and with solid theoretical basis, have been proposed in recent years. Among them, EOS of the SAFT-family are possibly those with widest acceptance. In this work, we compare the performance of the Peng-Robinson and PC-SAFT EOS in the calculation of dew points, bubble points, and critical points of natural gases. Binary interaction parameters in both EOS were set equal to zero in all calculations to test the predictive capability of the models. Calculations were performed for 19 synthetic natural gases for which experimental data are available in literature. For most mixtures, predictions of the PC-SAFT EOS are in better agreement with the experimental data.
Highlights
Pressure-temperature (P-T ) phase diagrams can provide important information for the design of recovery and processing operations of natural gases at high pressures
The fugacities of each component i in the vapor and liquid phases should be equal in the dew and bubble points, i.e.: ln fiV ) and liquid ( (fiL)(T, P, x) − ln fiV (T, P, y) = 0 where x and y are the mole fractions in the liquid and vapor phases, respectively
We selected 19 synthetic natural gases (SNG) in which all components are well characterized substances, i.e., without lumped heavy fractions such as C6+. This eliminates the effect the lumping procedure might have in the equations of state (EOS) comparison
Summary
Pressure-temperature (P-T ) phase diagrams can provide important information for the design of recovery and processing operations of natural gases at high pressures. Three points are of particular interest in this type of diagram: the vapor-liquid critical point, which is the endpoint of the bubble and dew point curves, the cricondenbar and the cricondentherm, which are the saturation points with largest pressure and temperature in the diagram, respectively. Their experimental determination can be difficult and expensive and, there is motivation to develop and use equations of state (EOS) capable of accurately predicting the phase behavior of natural gases. Compared to cubic EOS, they generally have a sounder theoretical basis even if at the cost of greater mathematical complexity and larger computational effort. It remains to be evaluated whether this extra computational effort translates, or not, into significantly better phase behavior predictions
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