Condensation of vapours

C. T. R. Wilson (Cambridge) method of making the paths of electrically charged particles visible by condensation of vapour. 

Hulden, B., Condensation of Vapours from Gas-Vapour Mixtures. An Approximate Method of Design, Chem. Eng. Science, V. 7, p. 60 (1957). 

HEAT TRANSFER IN THE CONDENSATION OF VAPOURS 9.6.1. Film coefficients for vertical and inclined surfaces... 

Cot. BURN, A.P. Proceedings of the General Discussion on Heat Transfer, September, 1951. p. [.Problems in design und research on condensers of vapours and vapour mixtures. (Inst, of Mech. Eng. and Am. Soc. Mech. Eng.). 

Dehumidification of air can be effected by bringing it into contact with a cold surface, either liquid or solid. If the temperature of the surface is lower than the dew point of the gas, condensation takes place and the temperature of the gas falls. The temperature of the surface tends to rise because of the transfer of latent and sensible heat from the air. It would be expected that the air would cool at constant humidity until the dew point was reached, and that subsequent cooling would be accompanied by condensation. It is found, in practice, that this occurs only when the air is well mixed. Normally the temperature and humidity are reduced simultaneously throughout the whole of the process. The air in contact with the surface is cooled below its dew point, and condensation of vapour therefore occurs before the more distant air has time to cool. Where the gas stream is cooled by cold water, countercurrent flow should be employed because the temperature of the water and air are changing in opposite directions. 

Condensation of vapours must be avoided since it causes a deterioration of the oil water vapour condense at 234 torr at 70°C, which is the typical working temperature of the pump. To avoid condensation, most pumps have a ballast valve in the high-pressure stage. After the gas has been closed off from the inlet, some gas is bled in through the ballast valve. This causes the discharge valve to open sooner, with a reduction of the compression ratio. Rotary piston pumps have similar characteristics. 

Weak liquor will enter the plant and pass to the calandria where it will be drawn up as a thin film by the partial vacuum caused by ultimate condensation of vapour in the condenser. Vaporisation will take place due to heat transfer from condensing ammonia in the calandria, and the vapour and concentrated liquor will then pass to a separator from which the concentrated liquor will be... 

Recovery of solvent by the condensation of vapours. This is one of the oldest methods which once was widely used in many countries (France, U.S.A.) and well checked in practice. 

Physisorption (or Physical Adsorption) is adsorption in which the forces involved are intermolecular forces (van der Waals forces) of the same kind as those responsible for the imperfection of real gases and the condensation of vapours, and which do not involve a significant change in the electronic orbital patterns of the species involved. The term van der Waals adsorption is synonymous with physical adsorption, but its use is not recommended. 

There are a number of instances in which (with the aid of sensitive measurements) well-defined transitions between gaseous and coherent states are observed as the film is compressed. The tt-A curves show a marked resemblance to Andrews p-V curves for the three-dimensional condensation of vapours to liquids. The tt-A curve for myristic acid, given as an example, has been drawn schematically to accentuate its main features (Figure 4.26). Above 8 nm2 molecule-1 the film is gaseous and a liquid-expanded film is obtained on compression to 0.5 nm2 molecule-1. Fluctuating surface film potentials verify the heterogeneous, transitional nature of the surface between 0.5 nm2 molecule-1 and 8 nm2 molecule-1. 

Colloidal dispersions can be formed either by nucleation with subsequent growth or by subdivision processes [12,13,16,25,152,426], The nucleation process requires a phase change, such as condensation of vapour to yield liquid or solid, or precipitation from solution. Tadros reviews nucleation/condensation processes and their control [236], Some mechanisms of such colloid formation are listed in Table 7.1. The subdivision process refers to the comminution of particles, droplets, or bubbles into smaller sizes. This process requires the application of shear. Some of the kinds of devices used are listed in Table 7.2 [228]. 

Suspensions can be formed either by nudeation or by subdivision and dispersion. The nudeation process requires a phase change, such as condensation of vapour to yield solid, or precipitation of a salt from a supersaturated solution. In the latter case a supersaturated solution must be formed. The supersaturation condition is then alleviated by condensation on nudei (which need not be composed of the same material) already present, or else by formation of nuclei with subsequent condensation. The nudei eventually grow to microscopic, or macroscopic, size. Additional details of this process are discussed elsewhere [49,320],... 

Condensation of vapours can cause severe problems with oil-sealed pumps. By the use of gas ballast (a continuous, controlled admission of air or inert gas to the pump), this can be significantly reduced. The gas ballast increases the pressure in the pump chamber to 100 mbar just before compression starts. The compression ratio k is thereby reduced to 10 to 15 1, and vapours which might otherwise have condensed may remain in the vapour phase. 

Condensers are heat exchangers that transfer the heat of condensation of vapour to a coolant. They are useful devices in the removal of large amounts of vapour from a gas stream. Depending on the application, it may be useful to condense the majority of the vapour generated in a vacuum process by inserting a condenser between the vessel and the pump, and use a small vacuum pump to remove non-condensable gases. 

A smooth tube in a condenser which is 25 mm internal diameter and 10 m long is carrying cooling water and the pressure drop over the length of the tube is 2 x 104 N/m2. If vapour at a temperature of 353 K is condensing on the outside of the tube and the temperature of the cooling water rises from 293 K at inlet to 333 K at outlet, what is the value of the overall heat transfer coefficient based on the inside area of the tube If the coefficient for the condensing vapour is 15,000 W/m2 K, what is the film coefficient for the water If the latent heat of vaporisation is 800 kJ/kg, what is the rate of condensation of vapour ... 

Vapour pressure over a curved surface of liquid. The vapour pressure over a convex surface is greater than that over a plane and over a concave surfaoe it is less. The difference depends on the fact that condensation of vapour on a small convex drop of a liquid increases its surface... 

Freundlich,1 Rideal,2 and Hiickel3 have summarized the results of adsorption measurements on porous substances. Very roughly, it may be said that the ease of adsorption is proportional to the ease with which the gases can be liquefied, a fact which has been held to indicate that there is sometimes actual condensation of vapour in the smallest pores. Numerous empirical formulae, of which Freundlich s equation ( 9) is the best known, have been developed, but. none seem to fit the data with any accuracy over a considerable range of pressures. 

It has already been shown (p. 33) that red phosphorus probably is not a unary substance, and that the difference between the vapour pressures of red and violet phosphorus below about 400° C. are probably due to the non-equilibrium conditions in the red form. Even in the case of the more uniform violet modification, however, time is required for the establishment of equilibria with vapour, and the values of the pressures even up to 500° C. are affected by an uncertainty on this account. Condensation of vapour proceeds in general more slowly than vaporisation, and especially is this the case where there is a great difference between the molecular complexities of the vapour and of the solid. 

Adsorption is brought about by the interactions between the solid and the molecules in the fluid phase. Two kinds of forces are involved, which give rise to either physical adsorption (physisorption) or chemisorption. Physisorption forces are the same as those responsible for the condensation of vapours and the deviations from ideal gas behaviour, whereas chemisorption interactions are essentially those responsible for the formation of chemical compounds. 

After reaching point D, the system can be adiabatically compressed to point A with further condensation of vapour into liquid water. 

A different type of compound (Figure 6.23) has been made by co-condensation of vapourized lanthanide atoms with 1,3,5-tri-f-butylbenzene at 77 K. These compounds are thermally stable for many lanthanides at room temperature and above, and are strongly coloured. 

It is generally accepted that the phenomenon of adsorption hysteresis has its origin in capillary condensation, i.e. condensation of vapour in capillaries under conditions differing from those for bulk phases. The steep rise in the amount adsorbed, that is observed at certain p(sat) below unity, indicates that pores... 

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