Vapour space

Figure Bl.27.6. A calorimeter for enthalpies of mixing in the absence of a vapour space. (Reproduced with pennission from Larkin J A and McGlashan ML 1961 J. Chem. Soc. 3245.)...

The pressure in the vapour space of an incompletely full, sealed vessel containing liquid cannot be reduced by partially draining off liquid. 

Vapour space inside storage tank Storage tank outside buildings... 

The testing of vapour phase inhibitors, usually referred to as volatile corrosion inhibitors, is essentially a matter of placing a test specimen in the vapour space of a closed vessel containing an aggressive atmosphere — frequently water vapour, perhaps with SO2 present—and a quantity of the inhibitor. Variations on the basic technique include provision for circulation of the vapour, the use of paper impregnated with inhibitor, provision for temperature cycling, etc. 

It is independent of the volume of the vapour-space, provided that liquid is always present. 

It is almost independent of the presence of indifferent gases in the vapour-space (Law of Partial Pressures). 

Many solid substances (camphor, iodine, naphthalene, etc.), are known which are appreciably volatile at ordinary temperatures. Others, such as the metals, are apparently quite fixed, but they probably possess a definite, although very small vapour-pressure, even at ordinary temperatures. Thus, if magnesium is heated to 550° for a few hours in a magnesia boat enclosed in a vacuous tube it sublimes in beautiful crystals on the cool part of the tube. The vaporisation of a solid without previous fusion is called sublimation the vapour-pressure (like the vapour-pressure of a liquid), is definite for each temperature, is independent of i the volume of the vapour space, and increases with rise of temperature. 

Equation (7) is true for volatile as well as involatile solutes, provided n denotes the number of mols of solute in the liquid phase, andp is the -partial pressure of the vapour of the solvent, the latter being independent of the presence of other gases in the vapour space. The sole remaining problem is therefore the determination of the partial pressure of the solute, or, what will lead to this, the total pressure in the vapour space. The partial pressure of the solvent is, from Raoult s law ... 

A centrifugal pump is required to circulate a liquid of density 800 kg/m2 and viscosity 0.5 x 10 3 Ns/m" from the reboiler of a distillation column through a vaporisor at the rate of 0.004 m3/s, and to introduce the superheated vapour above the vapour space in the reboiler which contains a 0.07 m depth of liquid. If smooth-bore 25 mm diameter pipe is to be used, the pressure of vapour in the reboiler is 1 kN/m2 and the Net Positive Suction Head required by the pump is 2 m of liquid, what is the minimum height required between the liquid level in the reboiler and the pump ... 

Vapour space inside Within the vapour space of the tank Zone 0... 

Distilled water is produced from sea water by evaporation in a single-effect evaporator working on the vapour compression system. The vapour produced is compressed by a mechanical compressor of 50 per cent efficiency, and then returned to the calandria of the evaporator. Extra steam, dry and saturated at 650 kN/m2, is bled into the steam space through a throttling valve. The distilled water is withdrawn as condensate from the steam space. 50 per cent of the sea water is evaporated in the plant. The energy supplied in addition to that necessary to compress the vapour may be assumed to appear as superheat in the vapour. Calculate the quantity of extra steam required in kg/s. The production rate of distillate is 0.125 kg/s, the pressure in the vapour space is 101.3 kN/m2, the temperature difference from steam to liquor is 8 deg K, the boiling-point rise of sea water is 1.1 deg K and the specific heat capacity of sea water is 4.18 kJ/kgK. 

A single-effect evaporator with a heating surface area of 10 m2 is used to concentrate a NaOH solution flowing at 0.38 kg/s from 10 per cent to 33.3 per cent. The feed enters at 338 K and its specific heat capacity is 3.2 kJ/kg K. The pressure in the vapour space is 13.5 kN/m2 and 0.3 kg/s of steam is used from a supply at 375 K. Calculate ... 

Production of distillate = 0.125 kg/s, pressure in vapour space = 101.3 kN/m2, temperature difference from steam to liquor = 8 deg K, boiling point rise of sea water =1.1 deg K, specific heat capacity of sea water = 4.18 kJ/kg deg K. The sea water enters the evaporator at 344 K from an external heater. 

Apart from increasing the efficiency of the ejector, the economy of the system might be improved by operating with a higher live-steam pressure, increasing the pressure in the vapour space, and by using the vapour not returned to the ejector to preheat the feed solution. 

Volume of vapour space in reactor Total reactor volume... 

Suppose we have an evacuated vessel (Figure 32.3(a)) into which we introduce a liquid which is to form a volatile component A (Figure 32.3(b)) as part of a liquid mixture. It will be observed that the pure A exerts a vapour pressure, P, when confined alone in the containing vessel. This can also be referred to as the saturated vapour pressure , Psat in the sense that it equates to the saturation of the vapour space with only the pure liquid. The vapour pressure itself can be recorded by means of a manometer attached to the vapour space. This vapour pressure emerges because solvent molecules are volatile and leave the surface of the liquid, so creating a pressure in the vapour (or gas) above the liquid (which is the solvent). 

A direct experimental verification of equation (8) has been carried out by Thoma,1 by comparing the vapour pressures of various liquids over plane surfaces and over the curved surfaces in capillary tubes of a few tenths of a millimetre diameter. The very small differences of vapour pressure were measured by recording, with an interferometer, the movements of a membrane separating the vapour spaces over the two liquid surfaces. Verification of equation (8) was achieved within an accuracy of... 

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