Pressure saturated vapour

Flere is the volume of gas required to saturate the monolayer, V the total volume of gas adsorbed, P the sample pressure, P the saturation vapour pressure and C a constant related to the enthalpy of adsorption. The resulting shape of the isothemi is shown plotted in figure Bl.26.6 for C = 500. A plot of P/V(P - Pq) against P/Pq should give a straight line having a slope (C - )/y C and an intercept The BET surface area is... 

The model implies that at any pressure below the saturation vapour pressure, the fractions of the surface covered with 1, 2,. .., i molecules will be 6 02,.., 0i respectively, so that the thickness of the adsorbed layer will not be constant throughout. On the specific surface area A, therefore, the total number Z of molecules adsorbed will be... 

As already mentioned, the choice of the supercooled liquid as reference state has been questioned by some workers who use the saturation vapour pressure of the solid, which is measured at the working temperature in the course of the isotherm determination. The effect of this alternative choice of p° on the value of a for argon adsorbed on a number of oxide samples, covering a wide range of surface areas, is clear from Table 2.11 the average value of is seen to be somewhat higher, i.e. 18 OA. ... 

Following the pioneer work of Beebe in 1945, the adsorption of krypton at 77 K has come into widespread use for the determination of relatively small surface areas because its saturation vapour pressure is rather low (p° 2Torr). Consequently the dead space correction for unadsorbed gas is small enough to permit the measurement of quite small adsorption with reasonable precision. Estimates of specific surface as low as 10 cm g" have been reported. Unfortunately, however, there are some complications in the interpretation of the adsorption isotherm. 

The working temperature, 77 K, is well below the triple point of krypton, 116 K, but if the solid is taken as the reference state the isotherm shows an unusually sharp upward turn at the high-pressure end. The usual practice, following Beebe, is therefore to take p° as the saturation vapour pressure of the supereooled liquid (p° = 2-49 Torr at 77-35 K and 27-5 Torr at 90-2 K). 

From the Kelvin equation it follows that the vapour pressure p over a concave meniscus must be less than the saturation vapour pressure p°. Consequently capillary condensation of a vapour to a liquid should occur within a pore at some pressure p determined by the value of r for the pore, and less than the saturation vapour pressure—always provided that the meniscus is concave (i.e. angle of contact <90°). 

The saturated vapour pressure above solid and liquid sulfur is given in Table 15.7. The molecular composition of the vapour has long been in contention but, mainly as a result of the work of J. Birkowitz and others " is uow known to contain all molecules S with 2 < < 10 including odd-numbered specie.s. The actual conccntralion... 

Saturated vapour pressure of HjO at the dew point Saturated vapour pressure of HjO at ambient temp. 

Moisture precipitation Apart from wetting by sea-spray, moisture may either be deposited on a surface by rainfall or dew formation. For a known ambient humidity the dew point can be calculated, using the expression given previously, from standard tables giving the saturated vapour pressure of... 

The relative lowering of the saturated vapour pressure of water is described by the Thomson equation ... 

If air and water are present together in a confined space, a balance condition will he reached where the air has become saturated with water vapour. If the temperature of the mixture is known, then the pressure of the water vapour will be the pressure of steam at this temperature (see also Section 1.3) (Table 23.1). Dalton s Taw of partial pressures (see also Section 1.5) states that the total pressure of a mixture of gases is equal to the sum of the individual pressures of the constituent gases, taken at the same temperature and occupying the same volume. Since the water saturation vapour pressure will remain constant, depending on temperature and not on volume, this pressure can be obtained from steam tables as below. The partial pressure exerted by the dry air must therefore be the remainder. 

The heat of volatilisation of the pure solvent under its saturated vapour pressure, tt, at the temperature T is similarly ... 

For a bubble to be formed in a liquid, such as steam in water, for example, it is necessary for a surface of separation to be produced. Kelvin has shown that, as a result of the surface tension between the liquid and vapour, the vapour pressure on the inside of a concave surface will be less than that at a plane surface. As a result, the vapour pressure Pr inside the bubble is less than the saturation vapour pressure P, at a plane surface. The relation between Pr and P, is ... 

Hinchley and Himus(40) measured the rate of evaporation from heated rectangular pans fitted flush with the floor of a wind tunnel (0.46m wide by 0.23m high), and showed that the rate of vaporisation was proportional to the difference between the saturation vapour pressure of the water Ps and the partial pressure of water in the air Plv. The results for the mass rate of evaporation W were represented by an empirical equation of the form ... 

The polycaprolactam waste is contacted with superheated steam in the absence of added catalyst at a temperature of about 250 to 400C and a pressure in the range of about 1.5 to 100 atm. and substantially less than the saturated vapour pressure of water at the temperature at which a caprolactam-containing vapour stream is formed. The resulting caprolactam may then be used in the production of engineered resins and fibres. 

Rau, H. Kutty, T.R.N. and Guedes de Carvalho, J.R.F. (1973a) High temperature saturated vapour pressure of sulfur and the estimation of its critical quantities. J. Chem. Thermodyn., 5, 291-302. 

Pw Saturation vapour pressure corresponding to wall temperature ML T... 

The standard term p is the chemical potential of the pure component i (i.e. when Xj = 1) at the temperature of the system and the corresponding saturated vapour pressure. According to the Raoult law, in an ideal mixture the partial pressure of each component above the liquid is proportional to its mole fraction in the liquid,... 

The pressure exerted by a vapour, either alone or in a mixture of gases. When a mixtrure of gases is involved the partial (vapour) pressure is a measure of the relative concentrations of the constituents of the mixture. The saturated vapour pressure, the pressure of a vapour in contact with its liquid form is often termed simply vapour pressure. 

Thermodynamic properties of liquid 4He at saturated vapour pressure [30]... 

Figure 6.1 shows the x-T phase diagram of the 3He-4He mixture at saturated vapour pressure. Some important characteristics of these mixture are to be noted. In the dilution of 4He with 3 He, the temperature of the superfluid phase transition of the former is lowered and the transition disappears for 3He concentration above 67.5%. At such concentration... 

Fig. 6.1. Phase diagram for a mixture of3 He-4He at the saturated vapour pressure [4].

The concentrations of the two isotopes on the phase-separation branches at the saturated vapour pressure is given by [5] and [6-8] ... 

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