Vapour/liquid equilibria experimental determination

HAL1 Hala, E. Experimental determination of vapour-liquid equilibrium normal and low pressure region. Collect. Czech.Chem. Commun. 54 (1988) 839-861. 

In industrial practice it is often necessary to separate the products obtained from the primary reaction by use of a suitable phase change. For example, products may be fractionally distilled so as to separate individual components by means of the difference in composition that often exists between a vapour and liquid mixture in equilibrium. The design of fractionating equipment depends on the use of vapour-liquid equilibrium data and it is now widely recognized that the experimental measurement of vapour-liquid equilibrium properties is difficult. Fortunately, however, experimental measurements on vapour-liquid systems are usually made in such a manner that the properties of the system are over-determined in a thermodynamic sense. For example, the activity coefficient of each component of a mixture can usually be derived from the experimental observations, and these are not independent but are linked by the Gibbs-Duhem... 

The decomposition into groups of the hydrocarbons (linear, branched or cyclic) is very easy, that is as simple as possible. No substitution effects are considered. No exceptions are defined. For these 21 groups, we had to estimate 420 parameters (21QA/a and 210Bh values) the values of which are summarized in Table 4. These parameters have been determined in order to minimize the deviations between calculated and experimental vapour-liquid equilibrium data from an extended data base containing roughly 100,000 experimental data points (56,000 bubble points + 42,000 dew points + 2,000 mixture critical points). 

The composition of the vapour in equilibrium with a liquid of given composition is determined experimentally using an equilibrium still. The results are conveniently shown on a temperature-composition diagram as shown in Figure 11.3. In the normal case shown in Figure 11.3a, the curve ABC shows the composition of the liquid which boils at any... 

The wide varity of the properties of chemical compounds does not enable the use of a universal apparatus for the measurement of thermodynamic properties for pure components and mixtures at high pressure. In the case of two-phase equilibria like vapour-liquid equilibria, the typical set of data to be determined is the pressure, the temperature, and the composition of the two phases at equilibrium. Some experimental apparatus also allows the... 

Several authors [3-9] studied the solubility of polymers in supercritical fluids due to research on fractionation of polymers. For solubility of SCF in polymers only limited number of experimental data are available till now [e.g. 4,5,10-12], Few data (for PEG S with molar mass up to 1000 g/mol) are available on the vapour-liquid phase equilibrium PEG -CO2 [13]. No data can be found on phase equilibrium solid-liquid for the binary PEG S -CO2. Experimental equipment and procedure for determination of phase equilibrium (vapour -liquid and solid -liquid) in the binary system PEG s -C02 are presented in [14]. It was found that the solubility of C02 in PEG is practically independent from the molecular mass of PEG and is influenced only by pressure and temperature of the system. 

For ideal solutions the vapour composition that is in equilibrium with a liquid of mole fraction can be easily calculated. In the case of non-ideal solutions on the other hand, the composition of the vapour in equilibrium with a given solution is calculated from the experimentally determined vapour pressures of the two components. 

If the liquid is volatile, the gas phase will contain its vapour. In consequence, even though both the fluid phases are nominally pure components, there is in general finite adsorption at the Solid-Gas interface. The equilibrium value of ysa for this interface will be, therefore, lower than its pure-component value yso by an experimentally determinable quantity, which is called the two-dimensional or surface pressure tisg [4, 5]. For the one-component gas phase under the conditions of sufficiently low pressures in contact with an inert solid adsorbent, the value of tzsg can be evaluated using the adsorption isotherm for the vapour of the liquid on the solid surface ... 

The methods of measurement of the surface energy of liquids may be divided into two classes the static and dynamic methods. In general for pure unassociated liquids in contact with their vapour alone the values of the surface energy determined by the two methods do not differ beyond the range which may be attributed to experimental error. In other cases, however, marked divergence between the values obtained by the two methods is to be noted. This divergence is, as we shall have occasion to note, due to the comparatively slow rate of attainment by diffusion of equilibrium in the surface phase of solutions. 

Two examples illustrate this point. The enthalpy of formation of gaseous 1-heptene was determined from the enthalpy of hydrogenation to be —62.6 1.6 kJmol-1. The enthalpies of combustion and of vapourization (35.6 0.2 k.I mol 1) of the liquid were later measured, from which the enthalpy of formation of the gas was determined to be — 62.3 1.0 kJ mol-1. Yet Ped-ley did not incorporate the former measurement in the selected value although it is listed among the literature cited. This choice of experimental values is in contrast to a selected composite enthalpy of formation for gaseous 1-pentene which includes an enthalpy of formation derived from the enthalpies of combustion and vapourization of the liquid (—21.5 0.8 kJmol-1) and an enthalpy of formation derived from the equilibrium isomerization of 1-pentene and trails-2-pentene (—21.0 1.4 kJ mol-1). 

The vapour pressures of the species Se2(g), and Se5(g)-Seg(g) in equilibrium with selenium were determined for the temperature range 429 to 575 K using a mass spectrometer and a Knudsen cell. The data were divided into two sets, one for the equilibrium with solid selenium and one for the equilibrium with liquid selenium. In the evaluation of the experimental data in Figure 3 of the paper, the straight lines should intersect at the melting point of selenium. However, the intersection is at 479 K and not at the true... 

After the optimum additive has been chosen the liquid-vapour equilibria of the ternary or multicomponent systems have to be determined as exactlj- as possible in extractive as well as in azeotropic distillation. For this reason Null and Palmer [76] looked for methods allowing equilibrium values to be obtained from as few experimental data as possible. 

The experimental difficulty of determining critical behaviour from the shape of the isotherm has led to a preference for determining the critical temperature and pressure visually and the critical density by extrapolating liquid-vapour equilibrium measurements to the critical temperature. 

The determination of the properties of the 1-g interface of a dipolar fluid has been performed for a Stockmayer system and a system of diatomic particles which, in addition to the point dipole interaction, interact by site-site LJ potentials in [202]. The estimates of the surface tension are shown to be in reasonable agreement with experimental results for 1,1-difluoroethane when state variables are reduced by the critical temperature and density. The preferential orientation of the dipoles is parallel to the interface. This work also contains methodological aspects of the simulation of thin liquid films in equilibrium with their vapour. In particular, a comparison is made between the results obtained for the true (Eq. 25) and slab-adapted Ewald potentials. The agreement between the two numerical determinations of the dipolar energy is quite satisfactory asserting the validity of the use of the 3D Ewald approach for the simulation in a slab geometry. 

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