Antoine equation, vapor pressure

DECHEMA m-model UNIFAC activity coefficients Equation of state —ideal gas law Vapor pressure—Antoine equation Excess enthalpy from UNIFAC model Specifications ... 

Compilation of vapor-pressure data for organic compounds data are correlated with the Antoine equation and graphs are presented. 

Vapor-pressure data correlated with the Antoine equation. Results displayed graphically. 

Numerous mathematical formulas relating the temperature and pressure of the gas phase in equilibrium with the condensed phase have been proposed. The Antoine equation (Eq. 1) gives good correlation with experimental values. Equation 2 is simpler and is often suitable over restricted temperature ranges. In these equations, and the derived differential coefficients for use in the Hag-genmacher and Clausius-Clapeyron equations, the p term is the vapor pressure of the compound in pounds per square inch (psi), the t term is the temperature in degrees Celsius, and the T term is the absolute temperature in kelvins (r°C -I- 273.15). 

Vapor Density. Substitution of the Antoine vapor-pressure equation for its equivaient iog P in the ideai gas equation gives... 

Many empidcal expressions have been proposed to represent the temperature dependence of the vapor—hquid saturation pressure, as shown by the vaporization curve of Figure 2. The most popular is the Antoine equation ... 

Vapor pressure as a function of the temperature is correlated by the Antoine equation ... 

Theoretically based correlations (or semitheoretical extensions of them), rooted in thermodynamics or other fundamentals are ordinarily preferred. However, rigorous theoretical understanding of real systems is far from complete, and purely empirical correlations typically have strict limits on apphcabihty. Many correlations result from curve-fitting the desired parameter to an appropriate independent variable. Some fitting exercises are rooted in theory, eg, Antoine s equation for vapor pressure others can be described as being semitheoretical. These distinctions usually do not refer to adherence to the observations of natural systems, but rather to the agreement in form to mathematical models of idealized systems. The advent of readily available computers has revolutionized the development and use of correlation techniques (see Chemometrics Computer technology Dimensional analysis). 

Extensive tabulations of Antoine parameters are available for many chemicals of importance to engineers, chemists, and environmental scientists (9,19,20). Caution is in order when using tabulated Antoine constants because several forms of the correlating equation are found in the Hterature. In particular, there are variations in the sign before the second term, the units of temperature, and the use of natural or decimal logarithms of the vapor pressure. 

The equation developed by the Design Institute for Physical Property Data (DIPPR) is another successful correlating tool for vapor pressure (4). It is an empirical extension of the Antoine equation and has two additional constants, D and E ... 

Mathematical Consistency Requirements. Theoretical equations provide a method by which a data set s internal consistency can be tested or missing data can be derived from known values of related properties. The abiUty of data to fit a proven model may also provide insight into whether that data behaves correctiy and follows expected trends. For example, poor fit of vapor pressure versus temperature data to a generally accepted correlating equation could indicate systematic data error or bias. A simple sermlogarithmic form, (eg, the Antoine equation, eq. 8), has been shown to apply to most organic Hquids, so substantial deviation from this model might indicate a problem. Many other simple thermodynamics relations can provide useful data tests (1—5,18,21). 

Correlation Methods Vapor pressure is correlated as a function of temperature by numerous methods mainly derived from the Clapeyron equation discussed in the section on enthalpy of vaporization. The classic simple equation used for correlation of low to moderate vapor pressures is the Antoine S equation (2-27). 

Table 13-1, based on the binary-system activity-coefficient-eqnation forms given in Table 13-3. Consistent Antoine vapor-pressure constants and liquid molar volumes are listed in Table 13-4. The Wilson equation is particularly useful for systems that are highly nonideal but do not undergo phase splitting, as exemplified by the ethanol-hexane system, whose activity coefficients are snown in Fig. 13-20. For systems such as this, in which activity coefficients in dilute regions may...

Antoine s equation An equation used for calculating saturation vapor pressure. 

At/Ap)j = slope of vapor pressure curve. This may be calculated from Antoine type vapor pressure equation or obtain from a plot. 

For the monomers in the polymerization under consideration the fugacity coefficients were estimated by Redlich-Kwong equation of state and were found to be close to unity. The activity coefficients (8) for the monomers were estimated by Scatchard-Hildebrand s method (5) for the most volatile monomer there was a temperature dependence but none for the other monomer. These were later confirmed by applying the UNIFAC method (6). The saturation vapor pressures were calculated by Antoine coefficients (5). 

Blanco et al. (1994) reported measurements of the vapor pressure (P,at) for p-xylene, y-picoline, piperidine, pyridine and tetralin. The data for piperidine and pyridine are given in Table 3.4. A suitable equation to correlate these data is Antoine s relationship given next... 

Silva and Weber (1993) reported vapor pressure measurements for the 1-ch oro-l,l-Difluoroethane (RI42b) and 1,1 -Difluoroethane (R152a) refrigerants. The data are given in Tables 3.5 and 3.6 respectively. Use Antoine s equation to correlate the data for R142b and the following equation for R152a (Silva and Weber, 1993)... 

Vapor pressure in bar can be predicted for temperature in Kelvin from the Antoine equation using coefficients in Table 4.73. Data... 

Example 4.5 2-Propanol (isopropanol) and water form an azeotropic mixture at a particular liquid composition that results in the vapor and liquid compositions being equal. Vapor-liquid equilibrium for 2-propanol-water mixtures can be predicted by the Wilson equation. Vapor pressure coefficients in bar with temperature in Kelvin for the Antoine equation are given in Table 4.113. Data for the Wilson equation are given in Table 4.126. Assume the gas constant R = 8.3145 kJ-kmol 1-K 1. Determine the azeotropic composition at 1 atm. 

The system methanol-cyclohexane can be modeled using the NRTL equation. Vapor pressure coefficients for the Antoine equation for pressure in bar and temperature in Kelvin are given in Table 4.176. Data for the NRTL equation at 1 atm are given in Table 4.186. Assume the gas constant R = 8.3145 kIkmol 1-K 1. Set up a spreadsheet to calculate the bubble point of liquid mixtures and plot the x-y diagram. 

Find the vapor pressure of the refrigerant in the condenser P ond at Tcond and the vapor pressure in the evaporator P vap at Tevap using a correlation for the saturated liquid - vapor pressure, such as the Antoine equation (see Chapter 4) ... 

Although Eq. (6.37) is commonly used to represent the vapor pressure as a function of temperature, it is by no means the best equation for the purpose. For many compounds, a more accurate representation of the vapor pressure is given by the Antoine equation,... 

This can be empirically modified by introducing additional parameters to give the three-parameter Antoine equation by replacing T with (T + C), where C is a constant, which is the most common vapor pressure correlation used to represent experimental data (Zwolinski and Wilhoit 1971, Boublik et al. 1984, Stephenson and Malanowski 1987, and other handbooks). 

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