Vapour pressure of water

Phosphorus(V) oxide is an extremely effective desiccating agent, reducing the vapour pressure of water over it to a negligibly small... 

As an example of steam distillation, let us consider bromobenzene which has a normal boiling point of 155°. The vapour pressures of water and bromobenzene at different temperature.s are given in the following table. 

The pressure is reduced by means of a high capacity water pump, preferably constructed of metal the lowest pressure that can be attained is that of the vapour pressure of water at the particular temperature (for a detailed discussion, see Section 11,21). In winter when the temperature of the water may be 6-8° the vapour pressure of water is 7-8 mm., but in summer when the temperature of the water may reach 22°, the corresponding vapour pressure is 20 mm. Thus the vacuum ... 

The high-pressure water supply service is employed for the operation of the ordinary filter pump, which finds so many applications in the laboratory. A typical all metal filter pump is illustrated in Fig. 11, 21, 1. It is an advantage to have a non-return valve fitted in the side arm to prevent sucking back if the water is turned off or if the water pressure is suddenly reduced. Theoretically, an efficient filter pump should reduce the pressure in a system to a value equal to the vapour pressure of the water at the temperature of the water of the supply mains. In practice this pressure is rarely attained (it is usually 4 10 mm. higher) because of the leakage of air into the apparatus and the higher temperature of the laboratory. The vapour pressures of water at 5°, 10°, 15°, 20° and 25° are respectively 6-5, 9-2,12-8, 17 5 and 23 8 mm. respectively. It is evident that the vacuum obtained with a water pump will vary considerably with the temperature of the water and therefore with the season of the year in any case a really good vacuum cannot be produced by a filter pump. 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

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

Regnault investigated the vapour-pressures of water and benzene, both in the liquid and solid states, and represented his results graphically. He con-. eluded that the curves for the liquid and solid joined at the melting-point, and gave a continuous curve. This was shown theoretically to be incorrect. by Ivirchhoff (1858), who proved, by a method to be described later, that the... 

At a pressure of 760 mm. a mixture of nitrobenzene and water boils at 99°. The vapour pressure of water at this temperature being 783 mm., that of nitrobenzene will be... 

Wet materia], containing 70% moisture, is to be dried at the rate of 0.15 kg/s in a countercurrent dryer to give a product containing 5% moisture (both on a wet basis). The drying medium consists of air heated to 373 K and containing water vapour equivalent to a partial pressure of 1.0 kN/m2. The air leaves the dryer at 313 K and 70% saturated. Calculate how much air will be required to remove the moisture. The vapour pressure of water at 313 K may be taken as 7.4 kN/m-. 

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. 

At 40°C the vapour pressure of water over 45 per cent HN03 is 29 mmHg (Perry s Chemical Engineers Handbook, 5th edn, pp. 3-65). Take the total pressure as 8 atm. The mol fraction of water in the outlet gas stream will be given by the ratio of the vapour pressure to the total pressure ... 

The relevance of the method to this problem is illustrated in Figure Ilf where the vapour pressure of chlorobenzene is plotted as a function of temperature. On the same graph (P - PB) is plotted where P = 18 kN/m2 (130 mm Hg) and Pb is the vapour pressure of water at the particular temperature. These curves are seen to intersect at the distillation temperature of 323 K. 

Sulphur dioxide is recovered from a smelter gas containing 3.5 per cent by volume of SO2, by scrubbing it with water in a countercurrent absorption tower. The gas is fed into the bottom of the tower, and in the exit gas from the top the SO2 exerts a partial pressure of 1.14 kN/m2. The water fed to the top of the tower is free from SO2, and the exit liquor from the base contains 0.001145 kmol SC /kmol water. The process takes place at 293 K, at which the vapour pressure of water is 2.3 kN/m2. The water flow rate is 0.43 kmol/s. 

The minimum pressure achievable with a water pump equals the vapour pressure of water, and has a value of about 28 mmHg. 

Worked Example 4.9 The pressure inside a water pump is the same as the vapour pressure of water (28 mmHg). The pressure of gas inside a flask is the same as atmospheric pressure (760 mmHg). What is the change in Gibbs function per mole of gas that moves Take T = 298 K. 

SAQ 4.7 A flask of methyl-ethyl ether (V) is being evaporated. Its boiling temperature is 298 K (the same as room temperature) so the vapour pressure of ether above the liquid is the same as atmospheric pressure, i.e. at 100 kPa. The source of the vacuum is a water pump, so the pressure is the vapour pressure of water, 28 mmHg. 

On the Theory of Steam Distillation.—The ideal case occurs when the substance to be distilled is insoluble, or, more accurately, sparingly soluble in water (examples toluene, bromobenzene, nitrobenzene) so that the vapour pressures of water and the substance do not affect each other, or hardly so. The case of substances which are miscible with water (alcohol, acetic acid) is quite different and involves the more complicated theory of fractional distillation. Let us consider the first case only and take as our example bromobenzene, which boils at 155°. If we warm this liquid with water, its vapour pressure will rise in the manner shown by its own vapour pressure curve and independently of that of water. Ebullition will begin when the sum of the vapour pressures of the two substances has become equal to the prevailing atmospheric pressure. This is the case, as we can find from the vapour pressure curves, at 95-25° under a pressure of 760 mm. 

A is the area of interface for heat and mass transfer, and Pw) is the difference between the vapour pressure of water at the surface and the partial pressure in the air. 

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