Phase envelopes of multicomponent mixtures: From the simple to the complex

BENELLI, FEDERICO E.; PISONI, GERARDO O.; CISMONDI, MARTÍN

Congreso; 11th WORLD CONGRESS OF CHEMICAL ENGINEERING; 2023

Phase behavior can be of interest in a lot of processes in the chemical engineering field. The petroleum andseparation industries can be an example where this is of importance since an indispensable number of unit operationsare related to phase equilibria, like a separator in a reservoir extraction field. In this kind of operation, it is relevant tobe able to predict how a fluid could behave under certain conditions or to find the conditions where a desired statehappens.The P-T flash is probably the most useful and frequently used phase equilibrium calculation in industry. It providesquantitative and qualitative information for a phase separation at a single specification of pressure and temperature. Itis also possible to explore a wide region of specifications by performing a series of flash calculations for varying T-Pconditions, which in fact has become a widely used approach for determining regions of complex phase diagrams withmultiple-phase regions. However, this can be computationally very expensive. A better way of predicting the overallpicture for the phase behavior of a mixture can be naturally based on the explicit calculation of the whole phaseenvelope, including not only two-phase but also three-phase boundaries when necessary.An efficient algorithm to trace two-phase envelopes was developed by Michelsen is very much used to this day. It isalso discussed in detail in the book by Michelsen and Mollerup [1]. This method firstly finds an easy to converge pointand uses the information given in that point to calculate the next one, this being repeated until the whole phaseenvelope is traced.Complex multi component mixtures, like reservoir fluids, can be prone to present more complex behaviors than simpletwo-phase equilibria, mostly as a cause of the high asymmetry of the systems due to the presence of not onlyhydrocarbons but also polar components like water, methanol and other additives, heavy compounds like asphaltenesor, sometimes, high concentrations of CO2. These types of asymmetry in a mixture can cause the existence of three-phase equilibrium besides the usually expected two-phase one.A method that extends the previously named algorithm by Michelsen to calculate three-phase envelopes where anincipient phase could be either a vapor or a liquid has been proposed by Cismondi [2].In this work we explore how compositional variations can affect the phase behavior of complex mixtures, starting fromsimple hydrocarbon systems and then going to more complex ones, increasing the asymmetry of the system by theaddition of other components, like water and carbon dioxide. We also show the influence of these different compoundsin the appearance of a third phase and both similarities and differences in the topology of the complete envelopes.