Antoine’s 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). 

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). 

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

There are two disadvantages to the existing vapour pressure tables. Rrst of all, like any experimental data, there is no agreement between sources. This is worsened by the decision to take only one value into account for each chemical substance. This fact may encourage the user to take on trust the figure proposed, which is sometimes unjustified. Secondly, these values are given for a temperature that does not always correspond to the thermal conditions in which the chemical substance will be handled. Some references, to overcome this difficulty, offer several values. For instance, Weka most often gives three values, 20, 30 and 50°C, and the coefficients A, B, C in Antoine s equation can thus be calculated ... 

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)... 

For the semi-batch stirred tank reactor, the model was based on the following assumptions the reactor is well agitated, so no concentration differences appear in the bulk of the liquid gas-liquid and liquid-solid mass transfer resistances can prevail and finally, the liquid phase is in batch, while hydrogen is continuously fed into the reactor. The hydrogen pressure is maintained constant. The liquid and gas volumes inside the reactor vessel can be regarded as constant, since the changes of the fluid properties due to reaction are minor. The total pressure of the gas phase (P) as well as the reactor temperature were continuously monitored and stored on a PC. The partial pressure of hydrogen (pnz) was calculated from the vapour pressure of the solvent (pvp) obtained from Antoine s equation (pvpo) and Raoult s law ... 

The C value in Antoine s equation appears to be the least critical parameter, and some workers use a fixed value of 230 for all compounds. However, Thomson has found that there is a correlation between C and the boiling point of a substance. For elements which yield monatomic vapors, and for subtances which boil below — I50°C the correction leads to... 

For pure components, use Antoine s equation with constants given in Table 4.1. 

Use the Matcad genfit function to fit the data to Antoine s equation. 

Mujtaba (1989) simulated the same example for the first product cut using a reflux ratio profile very close to that used by Nad and Spiegel in their own simulation and a nonideal phase equilibrium model (SRK). The purpose of this was to show that a better model (model type IV) and better integration method achieves even a better fit to their experimental data. Also the problem was simulated using an ideal phase equilibrium model (Antoine s equation) and the computational details were presented. The input data to the problem are given in Table 4.7. 

Temperature known. For this case, the problem is straightforward. The vapor pressure of each species can be found readily from Antoine s equation and then inserted into Equation (27) to find the total pressure. 

Q=t+c. When t=—c, p=0, implying (like the equations of Groshans,10 Buchwald, de Heen, and Gerber, see below) an evaporation limit ( 7.VIIIK). It agrees with Cailletet and Colardeau s values ( 6.VIIB) for the vapour pressure of water up to the critical temperature. Antoine s equation may be... 

Define saturation pressure as a function of temperature, in this case, Antoine s equation is used. (T must be in K)... 

We assume (1) the validity of Antoine s equation and Raoult s law, (ii) that pressure head and surface tension effects on the boiling point are negligible. 

Solution 5.1b. In the case of placing an organic fluid in the cylindrical body of the thermometer immersed in the water, two factors affect the pressure gauge reading the expansion of the air as well as the increased vapor pressure of the organic fluid. The vapor pressure of the fluid is calculated according to Antoine s equation, while the overall pressure is determined from the ideal gas law. Amontons law is inapplicable because the total number of moles of gas in the vapor phase of the cylinder will increase. (We shall also assume that the change in volume of the liquid is inconsequential.)... 

From Antoine s equation, we first calculate the vapor pressure of ethanol from which we can then derive its initial mole fraction in the vapor phase (at 25 C) ... 

Use the Antoine s equation in Appendix D to estimate the latent heat of vaporization for toluene at its normal boiling point, 110.63°C, and its normal melting point, -95°C. Compare your estimates with the experimental values, which are near 364 J/gm and 453 J/gm, respectively [17]. 

D14. We are feeding 100 kmol/h of a mixture that is 30 mol% n-butane and 70 mol% n-hexane to a flash drum We operate with V/F = 0.4 and = 100°C. Use Raoult s law to estimate K values from vapor pressures. Use Antoine s equation to calculate vapor pressure,... 

D2. What is the dew point of a vapor that is 30 mol% n-butane, 50 mol% n-pentane, and 20 mol% n-hexane at p = 760 mm Hg Use Raoult s law to predict K values. Find vapor pressures from Antoine s equation. 

The vapor pressure of ethanol can be accurately estimated by Antoine s equation,... 

The vapor pressure of either benzene or toluene is given by Antoine s equation as a function of temperature as in the previous equations Eq. riO.1021 and (10.103). 

Antoine s equations The Antoine equation is a simple three-parameter fit to experimental vapor pressures measured over a restricted temperature range ... 

For species in which tabulated data is not available, the most common technique for estimating the vapor (or saturation) pressure is to use Antoine s equation, which is written as... 

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