Vapour pressure, prediction

The vapour pressure of BHET is approximately three orders of magnitude lower than that of EG. Nevertheless, evaporation of BHET still occurs in significant amounts under vacuum. In Figure 2.26, the experimentally determined vapour pressure of BHET is compared to the vapour pressure predicted by the Unifac group contribution method [95], The agreement between the measured and calculated values is quite good. In the open literature, no data are available for the vapour pressure of dimer or trimer and so a prediction by the Unifac method is shown in Figure 2.26. The correspondence between measured and predicted data for BHET indicates that the calculated data for dimer and trimer... 

Before beginning the series of runs to determine the relief size, the physical property and kinetic data need to be correlated in the form required, by the code. In some cases, the code may already have the components required on a database. In all other cases, physical property data must be found, estimated or measured and correlated in the appropriate form. Some codes have a front-end program for curve fitting of data. For tempered systems, the vapour/ liquid equilibrium models are of critical importance since errors will cause the code to open the relief system at the wrong temperature and reaction rate. It is therefore worthwhile to spend time to ensure reasonable behaviour of the vapour pressure predictions. Check that all correlations behave sensibly over the entire temperature range of relevance for relief sizing. A good test for the physical property and kinetic data supplied to the code is to first model the (unrelieved) adiabatic calorimetric test which was used to obtain the kinetic data.. . ... 

However, it was Soave s modification [30] of the temperature dependence of the a parameter, which resulted in accurate vapour pressure predictions (especially above 1 bar) for light hydrocarbons, which led to cubic equations of state becoming important tools for the prediction of vapour-liquid equilibria at moderate and high pressures for non-polar fluids. 

Peng and Robinson [31] used a different volume dependency of the attractive term, which results in slightly improved liquid volumes (because for this E.O.S Zc = 0.307, which is closer to the experimental values) and changed slightly the temperature dependence of a to give accurate vapour pressure predictions for hydrocarbons in the 6- to 10-carbon-number range. 

Figure A2.3.6 illustrates the corresponding states principle for the reduced vapour pressure P and the second virial coefficient as fiinctions of the reduced temperature showmg that the law of corresponding states is obeyed approximately by the substances indicated in the figures. The useflilness of the law also lies in its predictive value.

Figure A2.5.11. Typical pressure-temperature phase diagrams for a two-component fluid system. The fiill curves are vapour pressure lines for the pure fluids, ending at critical points. The dotted curves are critical lines, while the dashed curves are tliree-phase lines. The dashed horizontal lines are not part of the phase diagram, but indicate constant-pressure paths for the T, x) diagrams in figure A2.5.12. All but the type VI diagrams are predicted by the van der Waals equation for binary mixtures. Adapted from figures in [3].

Fractional distillation. The aim of distillation is the separation of a volatile liquid from a non-volatile substance or, more usually, the separation of two or more liquids of different boiling point. The latter is usually termed fractional distillation. The theoretical treatment of fractional distillation requires a knowledge of the relation between the boiling points, or vapour pressures, of mixtures of the substances and their composition if these curves are known, it is possible to predict whether the separation is difficult or easy or, indeed, whether it will be possible. 

The micrographs in Fig. 7.88 show clearly how from a knowledge of the AG -concentration diagrams it is possible to select the exact reaction conditions for the production of tailor-made outermost surface phase layers of the most desired composition and thus of the optimum physical and chemical properties for a given system. In addition it shows that according to thermodynamics, there can be predictable differences in the composition of the same outermost phase layer prepared at the same conditions of temperature but under slightly different vapour pressures. 

The equation-of-state method, on the other hand, uses typically three parameters p, T andft/for each pure component and one binary interactioncparameter k,, which can often be taken as constant over a relatively wide temperature range. It represents the pure-component vapour pressure curve over a wider temperature range, includes the critical data p and T, and besides predicting the phase equilibrium also describes volume, enthalpy and entropy, thus enabling the heat of mixing, Joule-Thompson effect, adiabatic compressibility in the two-phase region etc. to be calculated. 

An alternative method of raising the vapour pressure of a solvent is to increase the experimental temperature. The consequence should be both a decrease in the rate of degradation and an increase in the limiting degree of polymerisation (i. e. higher final R.M.M. value) as a result of the lower intensities of cavitational collapse at the higher temperatures (see Section 2.6.2). Tab. 5.5 and Fig. 5.16 [41] show these predictions are borne out in practice. 

The Redlich-Kwong equation gives a somewhat better critical compressibility (Zc = 0.333 instead of 0.375 as results from the Van der Waals equation), but is still not very accurate for the prediction of vapour pressures and liquid densities. 

This expression, known as the Kelvin equation, has been verified experimentally. It can also be applied to a concave capillary meniscus in this case the curvature is negative and a vapour pressure lowering is predicted (see page 125). 

The process of pesticide volatilization from a leaf surface is considered first in terms of the component physical processes of sublimation and molecular diffusion through a saturated boundry layer. Predicted volatilization rates based solely on pesticide vapour pressures often bear little relation to field observations due to myriad interactions of the pesticide with the leaf and the surrounding microenvironment. Observed pesticide fluxes above sprayed agricultural fields together with microclimatological characteristics of coniferous forests are then used to predict general patterns of pesticide volatilization from a treated coniferous stand. 

Although saturated vapours can be reproduced precisely in the laboratory the application of equilibrium vapour pressures to the prediction of field volatilization rates are fraught with difficulties. The pesticide may interact with other spray components to change the physical characteristics of the deposits. As pointed out by Hartley (4) a pesticide which can exist in a supercooled state (eg. impure DDT in thin films) will be more volatile and more soluble than if it is crystalline. As a rough approximation a crystalline substance becomes one-third to one-fourth as volatile as the supercooled liquid for each... 

As was already discussed in the previous sections, dimeric molecules contribute significantly (up to 10%) to the total vapour pressure of the lanthanide trihalides. However, experimental information is only available for a few systems, which is often highly uncertain. As a result it is difficult to predict trends in the lanthanide series or estimate unknown values. 

Determination of pure component parameters. In order to use the EOS to model real substances one needs to obtain pure component below its critical point, a technique suggested by Joffe et al. (18) was used. This involves the matching of chemical potentials of each component in the liquid and the vapour phases at the vapour pressure of the substance. Also, the actual and predicted saturated liquid densities were matched. The set of equations so obtained was solved by the use of a standard Newton s method to yield the pure component parameters. Values of exl and v for ethanol and water at several temperatures are shown in Table 1. In this calculation vH and z were set to 9.75 x 10"6 m3 mole"1 and 10, respectively (1 ). The capability of the lattice EOS to fit pure component VLE was found to be quite insensitive to variations in z (6[Pg.90]

Substance Name CAS Number Vapour Pressure at 20°C (mPa) Measured Water Solubility (mg/L) Measured Henry s Constant (Pa m3/mol) Calculated Predicted from as VP/Saa HENRYWIN aq ... 

Liang, C. and Gallagher, D.A., QSPR prediction of vapour pressure from solely theoretically-derived descriptors, J. Chem. Inf. Comput. Sci., 38, 321-324, 1998. 

Mujtaba (1989) simulated the example considered by Boston et al. (1980) presented in section 4.2.4.1.1 using CMH model. The volume holdups used by Boston et al. were converted to molar holdups at the initial conditions. These were 0.00493 lbmol for each internal plates and 0.0493 lbmol for the condenser. Equilibrium k values were calculated using Antoine s vapour pressure correlation and enthalpies by the same procedure outlined in section 4.2.4.2.I. The simulation results are presented in Table 4.6. Note the slight differences in predictions (Table 4.4 and 4.6) are due to the use of different types of models (CVH and CMH) and thermodynamic property calculations. 

Yuan, W., Hansen, A.C., Zhang, Q., Vapour pressure and normal boiling point predictions for pure methyl esters and biodiesel fuels, Fuel, 84, 943-950, 2005... 

In all three documents concern is raised about the validity and relevance of models used to predict exposures, such as EUSES, which rely on the substance s lipophilicity as estimated by its octanol-water partitioning coefficient (Kow) and other chemical characteristics. The reason for this is that these models are generally not applicable to substances with a very high lipophilicity. Furthermore, the determination of some main physico-chemical properties such as lipophilicity, water solubility and vapour pressure is also stated as sources of uncertainty for substances with very high lipophilicity. 

Rordorf BF (1987), Thermochimica Acta 112 117-122.. .Prediction of vapour pressures, boiling points and enthalpies of fusion for twenty-nine halogenated dibenzo-p-dioxins ... 

Eamus, D., Taylor, D.T., Maclimis-Ng, C.M.O., Shanahan, S., and De Silva, L. 2008. Comparing model predictions and experimental data for the response of stomatal conductance and guard cell turgor to manipulations of leaf-to-air vapour pressure difference and temperature feedback mechanisms are able to account for all observations. Plant Cell Environ. 31 269-277. 

Boublik, T., Fried, V., Hala, E. (1984) The Vapour Pressures of Pure Substances. 2nd Edition, Elsevier, Amsterdam, The Netherlands. Bowden, D.J., Clegg, S.L., Brimblecombe, P (1998) The Henry s law constants of the haloacetic acids. J. Atmos. Chem. 29, 85-107. Branson, D.R. (1978) In Predicting the Fate of Chemicals in the Aquatic Environment from Laboratory Data. ASTM STP 657. 

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