Computation of Complete Fluid State Phase Diagrams at Set Initial Global Composition for Multicomponent Chemically Reactive Systems

M.J. MOLINA; S. B. RODRIGUEZ-REARTES; M. S. ZABALOY

Virtual

Conferencia; PetroPhase2021 (Virtual): The 21st International Conference on Petroleum Phase Behavior and Fouling; 2021

ExxonMobil

The constant global composition (z) phase envelope (PE) of a multicomponent non-reactive system is a quite familiar type of phase diagram line which shows, in the pressure (P) versus temperature (T) plane, a number of curves, such as the bubble point line and the dew point line (z is a vector of global component mole fractions). In a given point of the PE, a phase of finite size (major phase) of composition z is at equilibrium with a phase of differential size (incipient phase) of composition generally different from z. In strict terms, and limiting these statements just to the fluid state, other lines might contribute to the PE, e.g., a cloud point line where a major liquid phase of composition z is at equilibrium with an incipient liquid phase. The PE is the boundary between the homogeneous region and the two-phase (heterogeneous) region. The information on the phase behavior of the multicomponent system of interest becomes more complete if some additional types of lines are plotted within the heterogeneous region. For instance, the constant phase fraction lines are useful, e.g., to identify the retrograde behavior region. The system may also present the so called three-phase envelopes (3PE). This type of line is the boundary between a two-phase region and a three-phase region. In a point of a 3PE, two phases of finite size are at equilibrium with an incipient phase, (still being the global composition equal to z). Another useful type of additional line is the constant overall volume line (isochore) which may have homogeneous segments, two-phase segments, etc. The set of all PE segments plus all the auxiliary lines (e.g., 3PEs) constitute a complete constant z diagram which is named ?isopleth? (IP). This kind of diagram is very useful since it can always be visualized in the P vs. T plane, regardless the number of components in the multicomponent system. For instance, the classification of reservoir fluids in categories such as gas condensate, volatile oil, etc., is based partly on the isopleth corresponding to the composition of the reservoir fluid [1]. For reactive systems, the global composition z changes throughout the reaction course. Sometimes, for reactive systems, the phase state evolution in time is somehow studied by computing a number of IPs, each for a different z, where each z vector becomes defined by a set conversion value. This was done, e.g., in ref. [2]. Such analysis is valid only if the rate at which the phase equilibrium is achieved is much higher than the rate at which chemical equilibrium is reached. Since the truthfulness of such large difference in rates should be hard to verify, we are left with no other option than computing the simultaneous phase and chemical equilibria, as a step previous to the rigorous study of the complex transport phenomena taking place in the system. The proper (reactive) phase diagram will now be (in part) made of bubble, dew and cloud point lines where, for each point, a couple of reactive phases are at equilibrium, one of them being incipient. The ?reactive? nature will be found also in all lines within the heterogeneous region such as (reactive) 3PEs (R-3PEs). Clearly, z loses its constancy when going from non-reactive IPs to reactive IPs (R-IPs). Both, in IPs and in R-IPs the global mole fractions of the atoms remain constant, while the global mole fractions of the components are constant only for the non-reactive IPs. Thus, we distinguish between substance-wise isopleths (IPs) and atom-wise isopleths (RIPs). In the present work, calculation algorithms have been developed for all types of lines present in atom-wise (reactive) isopleths (RIPs). In particular, lines of constant conversion, with, both, homogeneous and heterogenous segments have been considered. The corresponding non-linear systems of equations account for the equality of chemical potential, and for the chemical equilibrium and mass conservation conditions. Some applications are presented, in particular the case of production of biodiesel, whose presence in blends with diesel from non-renewable origin is often required by regulations. References. [1] K. S. Pedersen, P. L. Christensen, J. A. Shaikh, Phase Behavior of Petroleum Reservoir Fluids, 2nd Ed. CRC Press. (2015) [2] L. Gharnati, N. E. Musko, A. D. Jensen, G. M. Kontogeorgis, J. D. Grunwaldt, J. Supercrit. Fluids, 82 (2013) 106.