Steam/water distillation vapor pressure

This carbon dioxide-free solution is usually treated in an external, weU-agitated liming tank called a "prelimer." Then the ammonium chloride reacts with milk of lime and the resultant ammonia gas is vented back to the distiller. Hot calcium chloride solution, containing residual ammonia in the form of ammonium hydroxide, flows back to a lower section of the distiller. Low pressure steam sweeps practically all of the ammonia out of the limed solution. The final solution, known as "distiller waste," contains calcium chloride, unreacted sodium chloride, and excess lime. It is diluted by the condensed steam and the water in which the lime was conveyed to the reaction. Distiller waste also contains inert soHds brought in with the lime. In some plants, calcium chloride [10045-52-4], CaCl, is recovered from part of this solution. Close control of the distillation process is requited in order to thoroughly strip carbon dioxide, avoid waste of lime, and achieve nearly complete ammonia recovery. The hot (56°C) mixture of wet ammonia and carbon dioxide leaving the top of the distiller is cooled to remove water vapor before being sent back to the ammonia absorber. 

Steam distillation is used to lower the distillation temperatures of high boiling organic compounds that are essentially immiscible with water. If an organic compound is immiscible with water, both Hquids exert fliU vapor pressure upon vaporization from the immiscible two-component Hquid. At a system pressure of P, the partial pressures would be ... 

The Hausbrand vapor-pressure diagram [127, 128] in Figure 8-40 is a useful approach for the steam distillation calculation. This particular diagram was prepared for six organic compounds and the corresponding water vapor pressure as (it - ps) for three system pressures of 760, 300, and 70 mm Hg versus temperature,... 

This is just Dalton s Law of partial pressures. PTotal is P tm for a steam distillation. So the vapor pressure of the organic oil is now less than that of the atmosphere and the water, and codistills at a much lower temperature. 

As an example, suppose you were to try to directly distill quinoline. Quinoline has a boiling point of 237°C at 1 atm. Heating organic molecules to these temperatures may often be a way to decompose them. Fortunately, quinoline is insoluble in water and it does have some vapor pressure at about the boiling point of water (10 torr at 99.6° C). If it had a much lower vapor pressure at the boiling point of water, (say 0.1 torr), there wouldn t be enough of it vaporizing to make even steam distillation worthwhile. 

For many years water has been separated from many liquids by azeotropic distillation. Water is substantially insoluble not only in the oils but also in the diluents or solvents. Thus, the vapor pressures of the water and diluent are additive as in the familiar steam distillations. A heterogeneous azeotrope is formed this always boils below the boiling point of water because pressure must be reduced for water to boil at the vapor pressure of the hydrocarbon. 

Cholesterol is a low-volatile compound, but it is more volatile than the major triglycerides of milkfat. Superheated steam can be bubbled through the oil, heating it indirectly, which provides for the latent heat of vaporization of the distilling compounds and prevents steam condensation. Thus, the temperature and pressure can be varied independently. When the sum of the partial vapor pressures of water vapor and the distillates is equal to the total pressure, water vapor and the low-volatile components, such as cholesterol and free fatty acids, distill over. 

Some liquids with a relatively high vapor pressure can be isolated and purified by steam distillation, a process in which the organic compound codistills with water at a temperature below the boiling point of water. The apparatus for this process are shown in the chapter on steam distillation. 

The ratio of immiscible organic liquid to water in the distillate is increased if the former has a high molecular weight or a high vapor pressure. Steam distillation under vacuum may be employed when the thermal stability of the material prohibits temperatures of approximately 100°C. 

Steam distillation depends upon a substance having an appreciable vapor pressure at the. boiling point of water by lowering the vapor pressure, intermolecular hydrogen bonding inhibits steam distillation of the m- and p-isomers. 

Steam distillation. If two liquids are nearly insoluble in each other, neither one lowers the vapor pressure of the other therefore the total vapor pressure of a mixture of these two liquids will be the sum of their vapor pressures. If the mixture is heated, boiling begins when the combined vapor pressure of the two immiscible components equals the pressure of the atmosphere. The vapor and hence the distillate contain both components in the ratio of their vapor pressures. If, for example, at the temperature of the distillation 95 per cent of the vapor pressure is due to component A (of a mixture A and B), then the composition of the distillate will be 95 mole per cent A and 5 mole per cent B. This principle is applied in the separation of organic compounds from a mixture, at temperatures which are far below their boiling point, by distillation with steam. Consider, for example, a mixture of aniline, which boils at 184°, and water. At 100° the vapor pressure of aniline is 45 mm and that of water 760 mm at 98° the vapor pressure of water is 727 mm and that of aniline 40 mm. Therefore the combined vapor pressure of a mixture of water and aniline at 100° is 805 mm, and at 98°, 767 mm. It is evident that near 98° the total vapor pressure will be one atmosphere and the mixture will boil., The distillate will contain water and aniline in the mole ratio of their partial pressure. The process is called steam distillation and is further discussed in Experiment 27 (page 163). 

However in many heat and mass transfer processes in fluids, condensing or boiling at a solid surface play a decisive role. In thermal power plants water at high pressure is vaporized in the boiler and the steam produced is expanded in a turbine, and then liquified again in a condenser. In compression or absorption plants and heat pumps, boilers and condensers are important pieces of equipment in the plant. In the separation of mixtures, the different composition of vapours in equilibrium with their liquids is used. Boiling and condensing are, therefore, characteristic for many separation processes in chemical engineering. As examples of these types of processes, the evaporation, condensation, distillation, rectification and absorption of a fluid should all be mentioned. 

When the vapor pressure of a substance is appreciable only at a temperature considerably above the boiling-point of water, it may often be separated from less volatile compounds by distillation with superheated steam. In this case the flask containing the substance is heated in an oil-bath, and steam which has been passed through a hot coil of copper is conducted through it. 

Application of Equation (12.48) with the Antoine equation [Equation (12.13)] shows that steam distillation can be used to distill an organic compound at much lower temperatures than would otherwise be possible. Considering the system pressure P as constant, then the more steam introduced, the lower will be the partial and vapor pressures of the organic and thus the lower its boiling point. The other key element of the choice of water as the entrainer is that it is easily separated from the product (organic) by simple condensation followed by decanting. 

Raoult s law is the basis for steam distillation. Chapter 5, and its reverse, immiscible solvents distillation, also in Chapter 5. When the sum of the individual vapor pressures reaches 760 torr at sea level, the mixture will boil. By adding steam to a liquid, the vapor pressure of the water is quite high compared to the other components, and they will distill at a much lower temperature. This can minimize destruction caused by overheating, and is usually applied to liquids immiscible with water. The reverse, using a liquid such as toluene, can be used to remove large amounts of water from something like a watermelon section prior to additional chemical analysis. 

At about 99 "C, water ( f.w. = 18) has a vapor pressure of 544 tonr and diethylaniline (f.w. = 149) has a value of 15 torr. How many grams of diethylaniline would steam distill if 100 grams of water were collected ... 

Packing characteristics. We have shown that the optimum steam rate that leads to minimum tower volume and minimum power is that of the ideal cascade (13.14). The optimum typ>e of packing, optimum pressure, and optimum vapor velocity is that which makes the expression in braces (1327) a minimum. We shall not attempt to evaluate a number of types of packing, but shall use Spraypak no. 37 packing as an example of the selection of optimum vapor velocity and pressure. This is the type of packing recommended by McWilliams and co-workers [M4] for a water distillation plant. 

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