Analysis of Vapor-Liquid Equilibrium Parameters of Multicomponent Hydrocarbon Mixtures Using Cubic Equations of State
Authors: Isaeva Anna , Grushnikov Ilya , Dobrozhanskiy Vladimir 
Published: October 2018

Computer simulation of interphase mass transfer processes in natural hydrocarbon mixtures, as well as their qualitative laboratory studies, allow the specialist to study the processes in the oil reservoir more deeply. Today, the industry uses a number of software solutions (PVTSim, WinProp CMG, Aspen HYSYS, etc.), simulating interphase mass transfer in oils, gas condensate mixtures, etc. In most of these industrial software products, a method for calculating the phase equilibrium based on the equation of state of a multicomponent hydrocarbon system is implemented (flash calculations). For this purpose, as a rule, various versions of the Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK) cubic equations of state are used. At the same time, in the last decades the search for a reliable and simultaneously simple equation of state is one of the most important directions of research (Amao 2014; Yan 2011; Ahmed 2010; Brusilovsky 2002; Gross 2001; Brusilovsky 1992).

One of the modifications of cubic equations of state is the Brusilovsky equation (Brusilovsky 2002; Brusilovsky 1992). When developing this equation, the task implied the formulation of a cubic equation of state describing the PVT properties of natural hydrocarbon mixtures at pressures up to 100 MPa and temperatures up to 200° with sufficient accuracy for engineering purposes. The existing equations of state were reduced to a single form and a new generalized equation of state was proposed (Brusilovsky 2002; Brusilovsky 1992).

The aim of the present paper is to identify the advantages of using a particular equation of state (PR, SRK, Brusilovsky) to describe the vapor-liquid equilibrium of hydrocarbon mixtures under various thermobaric conditions. To achieve this goal, a software in Python was designed that implements an iterative algorithm for calculating the equilibrium ratios (K-values) of components for the multicomponent hydrocarbon mixtures (flash calculations) using the Brusilovsky equation of state. The designed software is available for public access at the Internet under the link:

https://gist.github.com/GrushnikovIU/982ba4dff03c0fa1fa5753b590a477d2.

Since the PR and SRK equations of state can be considered as a special case of the Brusilovsky equation of state, the designed software made it possible to compare the K-values calculated using different equations of state. For some mixtures, the calculated K-values were compared with the known experimental K-values (Kogan 1966). It has been revealed in which cases the use of the generalized Brusilovsky equation of state can give advantages in comparison with the application of the PR and SRK equations of state. The results obtained in the present paper made it possible to formulate recommendations on the use of the considered equations of state (PR, SRK, Brusilovsky) in modeling the vapor-liquid equilibrium of hydrocarbon mixtures of various compositions.

With the help of the software designed, additional studies were carried out (calculations of the mixtures’ composition of the vapor and liquid phases, the dew-point curve and the boiling curve) in order to demonstrate its capabilities.