Vapour pressure-temperature relationship

In order to assess a separation problem we need to know the vapour pressure-temperature relationship. By plotting the pressure of the saturated vapours against the temperature we may gain some idea of the pressure at which the separation appears most promising the abscissa is best drawn on a logarithmic or related scale (Fig. 39). The choice of the pressure depends on whether we wish to perform 

In general we should choose a pressure for the distillation such that, at the one extreme, the temperature in the column head is adequate for complete condensation with the cooling medium available and, at the other extreme, that the temperatures are not high enough to cause decomposition. 

In analytical distillation it should be the aim to achieve a maximum separating effect the relative volatility should therefore be as large as possible, and in an ideal mixture this is so at the pressure at which the ratio of the vapour pressures is a maxi- 

Although numerous data are hence already available, it may be necessary for the laboratory worker to determine vapour pressures himself. The more reliable course is to measure data experimentally, though several methods of calcidation, described in section 4.4.2, can be followed. 

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It is transported in high pressure steel cylinders equipped with brass valves. Physical properties are summarized in Table 9.21 and the vapour pressure/temperature relationship ... 

More accurate estimates, suitable for most engineering purposes, can be made from a knowledge of the vapour pressure-temperature relationship for the substance. Several correlations have been proposed see Reid et al. (1987). 

Properties.— The salt separating from its vapour is orange-red, as are thin films, but in mass the sublimed material appears black. It sublimes readily at pressures below 10" mm. at above 90°. Attempts to determine the vapour pressure-temperature relationship failed because pressures developed below 130° are small in comparison with the probable error, and the compound reacts with the glass container above that temperature. When heated in a sealed tube it melts to a viscous liquid at 219° and evolves a little gas. But decomposition is incomplete at this temperature since A -ray photographs still show the characteristic cubic phase. 

The emerald green solid, OsOFe. m. p. 59-2, b. p. 100-6. is the major product of the fluorinatlon of osmium dioxide. Vapour pressure-temperature relationships for the compound are 32—59 (solid), log Pmu + 9-064 ... 

Figure 5 Water vapour pressure-temperature relationship...

Depending upon the vapour pressure-temperature relationship of the condensing material, its freezing point and effective concentrations in the gas phase, condensation may occur directly to the solid rather than the liquid phase. In this case the particle size of the powder produced will depend upon the crystal growth rate from the vapour since coalescence will not be effective, and the particles will tend to have regular crystallographic facets. MgO condensed from an arc, for example, consists of small cubic crystals... 

It is possible to estimate the flash point of a multicomponent mixture provided only one of the components is flammable and its flash point and vapour pressure/temperature relationship is known [5]. The flash point temperature of the mixture is estimated by determining the temperature of the mixture at which the vapour pressure of the flammable component in the mixture, obtained using Raoult s Law, is equal to its vapour pressure at its flash point. Thus to determine the flash point of a mixture of 75% methanol and 25% water by weight, it is necessary to know the flash point of methanol (11°C) and that its vapour at this temperature is 53 mmHg. The mole fraction, a, of the flammable component in the mixture is needed (in this case the mole fraction of methanol is 0.63) in order to apply Raoult s Law. This is used to calculate the vapour pressure (Psat) of pure methanol based on the partial pressure (p) required at the flash point ... 

The vapour pressure/temperature relationship for methanol is then used to determine the temperature that would result in this pressure. This temperature, in this case 17°C, is the flash point of the mixture. 

In the SGHWR design, this turns out to be the most severe condition because the pressure applied to the pressure tube decreases with decrease of temperature in accordance with the saturation vapour pressure/temperature relationship. 

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