Selecting The Right Thermodynamic Models For Process Simulation PDF [PDF]

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Engineering Practice Selecting the Right Thermodynamic Models for Process Simulation Software that enables complex processes to be simulated continues to evolve for a wide range of thermodynamic conditions. Selection of the most appropriate models plays a crucial role in adequately representing real-life conditions Atul B. Choudhari Tata Consulting Engineers Ltd.

ne of the first and most important decisions to Select an be made when model­ Are there polar equation of *-©components present? ling a chemical process state (EOS) model system using a computer simulation package is the selection of an ap­ propriate method for simulating ther­ Are the operating Select a modified modynamic properties. Thermody­ conditions near the EOS model critical region? namic-property methods enable the mathematical calculation of pres­ sure, temperature, composition and specific volume for multi-component Are there any light gases or supercritical components present and multi-phase chemical systems. (such as H , CO2, N2)? These calculation methods are also used as a basis for determining the Select an activity coefficient model with Select an activity coefficient model state functions of a system, such as Henry's law (if available) for the liquid for the liquid phase and an EOS for enthalpy and entropy and hence, phase, and an EOS for the vapor phase the vapor phase Gibbs free energy. This article pro­ FIGURE 1. This decision tree can aid in the selection of the most appropriate thermodynamic method vides an overview of the primary thermodynamic-property meth­ ods, and discusses the basis that junction with either the ideal gas the following: underlies each one, the strengths equation or an EOS, to accurately • Steam tables and weaknesses of each, and appli­ incorporate vapor-phase behavior. • API Sour-Water Method cability of each to various chemical This is sometimes referred to as a • Glycol • Amines systems, to assist in the selection dual-model approach. decision. Commercial simulation pack­ 3. Henry’s Law. Henry’s Law is gen­ In general, thermodynamic-prop­ erally employed for environmental ages often use terminology such erty methods can be categorized applications. It is applied along with as ‘‘thermodynamic system” or liquid-activity-coefficient methods, “thermodynamic package" in the according to the following four op­ tions (each of which is discussed in such as non-random two-liquid input data section. Selection of the greater detail below): (NRTL) methods. Henry’s Law deter­ most appropriate thermodynamic 1. Equations of state (EOS). These mines the amount of a supercritical method is not just a decision on the are primarily suited to describing va­ component (non-condensable) gas thermodynamic-property method. por-phase behavior and predicting in the liquid phase. Instead, it involves selection of all the vapor-liquid equilibrium (VLE) be­ 4. Special models. For specific appli­ the relevant equations and methods tween the vapor phase and an ideal, cations, a specially developed ther­ that the simulation will use to predict saturated liquid phase. modynamic package (for instance, vapor-liquid equilibrium (VLE), vaporhaving binary interaction param­ liquid-liquid equilibrium (VLLE), state 2. Liquid-activity-coefficient meth­ ods. These have been developed eters that are specially developed functions (such as enthalpy and specifically to model non-ideal liquid- for the components associated with entropy), physical properties and phase behavior. These methods are these systems) is generally transport properties. more empirical in nature than EOS available in most of the commer­ and therefore require more data cial simulation packages. The types Equations of state (EOS) about the system and its compo­ of systems that are often included Any equation that attempts to re­ nents. They must be used in con­ in these special models include late the pressure, temperature and

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CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM DECEMBER 2019

TABLE 1. EQUATION OF STATE (EOS) APPLICABILITY GUIDE Classification

Low pressure

High pressure

Liquid phase

Polar molecules

Comments

Ideal gas equation

Yes

No

No

No

Strictly applicable to ideal gas systems

Generalized correla­ tions (such as LeeKesler-Plocker)

Yes

Yes

No

Yes

The compressibility factor and accentnc factor attempt to account for non-ideality. These correlations are of a graphical form and are easy to use. The basic premise is that all gas­ eous components with similar accentricity behave similarly at the same reduced tempera­ ture (TJ and reduced pressure (PJ

Virial equations

Yes

No

No

No

Though these equations have reasonable accuracy for non-polar gas mixtures at low to medium pressures, they have limited applicability, as they have not been developed to the extent of the more popular cubic EOS

Cubic EOS (such as Peng Robinson (PR), SRK, RK and others)

Yes