There are very few automatic procedures in the literature to calculate three-phase envelopes (VLLE boundaries). The best existing procedures require selecting each step manually to complete the three-phase envelope. In this article, a density marching procedure and a saturation point calculation at specified density of one of the incipient phases developed recently for drawing two-phase envelopes is extended to three-phase envelopes. Procedures have been presented to determine three-phase bubble and dew points reliably. Two examples are presented to demonstrate the method. The method has direct application in process simulation and thermodynamic property programs. © 2014 American Chemical Society.

G. Venkatarathnam

Department of Mechanical Engineering

chemistry

Engineering research

AUTOMATIC PROCEDURES

DEW POINTS

In-process

MARCHING METHOD

PHASE ENVELOPE

SATURATION POINT

Application programs

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IIT Madras

Industrial and Engineering Chemistry Research

Some of the most widely used commercial process simulators and property programs cannot calculate P-xy diagrams of even simple binary mixtures satisfactorily when the specified temperature is greater than the critical temperature of one of the two components because of the limitations of the methods used by them currently. In this paper, we present a new method for drawing P-xy diagrams of binary mixtures that overcomes the limitations of the present methods. In the proposed method, the density of the vapor/liquid phase is varied systematically and the saturation point calculated at each of the specified density (instead of the conventional specified concentration) to generate P-xy diagrams. Two different approaches have been presented for the calculation of saturation points at specified density. The methods presented are an extension of the density marching method presented earlier for drawing two-phase and three-phase isopleths. A number of zeotropic and azeotropic mixture examples are presented to demonstrate the methods. © 2017 American Chemical Society.

Density Marching Method for Drawing Phase Envelopes. 3. P-xy Diagrams of Binary Mixtures

Different methods have been proposed in the literature for the calculation of phase envelopes of mixtures. The phase envelopes of even simple mixtures have at least two stationary points on the P-T plane [∂P/∂T = 0 at the point of maximum pressure (cricondenbar) and ∂T/∂P = 0 at the point of maximum temperature (cricondentherm)]. Even more stationary points exist for complex mixtures. Therefore, a monotonic variation of either pressure or temperature cannot be used to trace the full phase envelope. Different methods have been proposed in the literature to help choose the right variable at the right location of the phase envelope to automatically trace the entire phase envelope. Although most methods work well for simple mixtures, many of them do not work well with wide-boiling mixtures or with mixtures that exhibit open-ended dew lines. In this article, we compare phase envelopes with space curves and show that the use of an independent variable helps trace complex phase envelopes automatically. It is shown that density is the ideal independent variable. A density-based method is developed to determine saturation points and automatically construct complex phase envelopes. The proposed method has immediate application in property programs and process simulators and was implemented in version 9.1 of NIST REFPROP property program released in May 2013. © 2014 American Chemical Society.

Density marching method for calculating phase envelopes

The phase of a fluid (liquid, vapor) at a given state is normally identified by comparing the properties of the fluid at the given state with saturation properties. In this paper we present a thermodynamic method for determining the phase of a fluid from the partial derivatives of pressure, volume and temperature without reference to saturated properties. The method has immediate application in a variety of phase equilibria calculations, particularly for liquid-liquid or vapor-liquid-liquid equilibria calculations in process simulators. © 2010 Elsevier B.V.

Fluid Phase Equilibria

Identification of the phase of a fluid using partial derivatives of pressure, volume, and temperature without reference to saturation properties: Applications in phase equilibria calculations

Chemical Process Design and Simulation

Index

International Cryogenics Monograph Series,Cryogenic Mixed Refrigerant Processes

Simulation of cryogenic processes

The Journal of Supercritical Fluids

Global phase equilibrium calculations: Critical lines, critical end points and liquid–liquid–vapour equilibrium in binary mixtures

Density marching method for calculating phase envelopes. 2. Three-phase envelopes

Journal	Data powered by TypesetIndustrial and Engineering Chemistry Research
Publisher	Data powered by TypesetAmerican Chemical Society
ISSN	08885885
Open Access	No