Vapour composition

Owing to the nature of the impurities described from commercial samples of phosgene, the vapour phase composition of a sample of phosgene obtained, from a cylinder say, may be considerably different from that indicated by analysis of the liquid. As an example, consider 

VAPOUR-UQUID EQUILIBRIUM COMPOSITION OF A TYPICAL COMMERCIAL SAMPLE OF PHOSGENE 

In particular, it should be noted that a phosgene sample containing only 0.1% of carbon dioxide, as measured in the liquid phase, would produce a vapour containing more that 11% CO 2, owing to the higher volatility of COj relative to COClj- This COj, and other very volatile contaminants, can be removed by distillation or by simple bleeding of the phosgene cylinder. 

In addition, it should be noted that the vapour pressure of commercial samples of phosgene will differ from those recorded for the pure material (see Chapter 6). The vapour pressure corresponding to the sample instanced in Table 4.5 is illustrated in Table 4.6 as a function of temperature [ICI78]. 

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Assuming that the liquid and vapour compositions in the still are in equilibrium, i.e., that the still acts as a theoretical stage... 

The above equation also applies to the liquid and vapour compositions of the still, where equilibrium plate behaviour is again assumed. 

The situation for any plate n, with liquid composition x corresponding to an equilibrium vapour composition y , but with actual vapour composition y , is represented on a small section of the McCabe-Thiele diagram in Fig. 3.62. 

More complex situations where ideal behaviour can no longer be assumed require the incorporation of activity coefficient terms in the calculation of the equilibrium vapour compositions. Assuming ideal behaviour in the gas phase, the equilibrium relation for component i is... 

Assuming theoretical plate behaviour for both the column plates and the still, the corresponding vapour compositions are given by... 

Knowing the outlet liquid composition, the outlet vapour composition can be calculated from the v-l-e relationship. 

In the storage of flammable liquids, if the composition of the vapour-air mixture above the liquid surface falls within the flammability limits, a floating roof tank would be used or the tank blanketed with inert gas. Check if the vapour composition for liquids listed below will fall within their flammability range, at atmospheric pressure and 25°C. 

In addition to the equations arising from the material and energy balances over a stage, and the equilibrium relationships, there will be a fourth relationship, the summation equation for the liquid and vapour compositions ... 

These equations are linear, with slopes L/V and L /V. They are referred to as operating lines, and give the relationship between the liquid and vapour compositions between stages. For an equilibrium stage, the compositions of the liquid and vapour streams leaving ... 

The calculation is continued stage-by-stage up the column to the feed point (stage 7 from the top). If the vapour composition at the feed point does not mesh with the top-down calculation, the assumed concentration of the non-keys in the bottom product is adjusted and the calculations repeated. 

Murphree plate efficiency (Murphree, 1925), defined in terms of the vapour compositions by ... 

Estimate the liquid and vapour compositions leaving the reboiler ... 

During the batch distillation of a binary mixture in a packed column the product contained 0.60 mole fraction of the more volatile component when the concentration in the still was 0.40 mole fraction. If the reflux ratio used was 20 1, and the vapour composition y is related to the liquor composition x by the equation y = 1.035x over the range of... 

Referring to the data given, at total reflux, the conditions on actual plates in the column are shown as points A, B, C, and D. Considering point A, if equilibrium were achieved on that plate, point E would represent the vapour composition and point F the liquid composition on the next plate. The liquid on the next plate is determined by B however so that the line AGE may be located and the efficiency is given by AG/AE = 0.59 or 59 per cent... 

Assuming theoretical plate behaviour in the column, the corresponding equilibrium vapour compositions are calculated by... 

Figure 11.4. Vapour composition as a function of liquid composition at constant pressure...

For many systems K is constant over an appreciable temperature range and Equation 11.11 may be used to determine the vapour composition at any stage. The method is particularly suited to multicomponent systems, discussed further in Section 11.7.1. 

The Margules and van Laar equations apply only at constant temperature and pressure, as they were derived from equation 11.21, which also has this restriction. The effect of pressure upon y values and the constants and 2i is usually negligible, especially at pressures far removed from the critical. Correlation procedures for activity coefficients have been developed by Balzhiser et al.(ll Frendenslund et alSls>, Praunsitz et alS19>, Reid et al. 2 ) van Ness and Abbott(21) and Walas 22 and actual experimental data may be obtained from the PPDS system of the National Engineering Laboratory, UK1-23). When the liquid and vapour compositions are the same, that is xA = ya, point xg in... 

Latent heat and vapour density should be evaluated on the basis of the composition of the vapour composition, and so will not be affected by the presence of solids. 

An initial guess for the pressure is assumed and the fugacity coefficient of each component in the liquid phase ( ) can be calculated. An initial guess is also assumed for the fugacity coefficient of each component in the vapour phase ( v), and consequently a first estimate of the vapour composition is evaluated. With this value of y, the fugacity coefficients in the vapour phase are recalculated using the equation of state and a second estimate for y,- is evaluated. This iterative procedure is continued until the difference between two successive values of the composition are below a predetermined error. At this point, the sum of y, is checked if the sum is different from unity a new value of the pressure is assumed for a new iteration. The iterative procedure ends when the y, differs from unity by less then a given value. 

In this equation, (P) is the parachor and the p values are the liquid- and vapour- molar densities in mol/cm3. Then a is obtained in mJ/m2. For the case of mixtures in the presence of vapour phase, the surface tension of a mixture is calculated in terms of the liquid and vapour composition, as ... 

The vapour pressures of all lanthanide trifluorides have been measured, including P111F3. Various techniques have been used, which can be divided in indirect techniques (effusion and boiling point methods) which provide accurate data for the total vapour pressure, and direct mass spectrometric techniques, which are less precise, particularly in case of quadrupole mass spectrometry, but give information on the vapour composition and the presence of dimeric or polymeric molecules or dissociation products. 

In type I, the vapour pressure of pure A is the lowest and that of pure B is the highest, therefore, the boiling point of pure A will be highest and that of pure B will be lowest. Since the vapour pressures of the mixtures of A and B lie in between the vapour pressures of pure components, their boiling points will also lie in between as shown in figure 11 (iv). At a given temperature, the vapour pressure will be richer in the more volatile component B, the composition of the vapour phase will be always richer in B than that of the liquid phase. Thus, the vapour composition curve will lie above the liquid composition curve. 

Equations (6.8) to (6.10) show the relationship between the vapour and liquid compositions when the two phases are in equilibrium. The dependence of the vapour composition on total pressure can be expressed as ... 

Figure 34.4 Pressure-composition diagram for Ideal Liquid Mixture of toluene and benzene showing liquid and vapour compositions. The region of the plot enclosed and labelled liquid + vapour corresponds to the area of stability of both liquid and vapour.

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