Steam distillation vapor pressure-temperature

Batch distillation (see Fig. 3) typically is used for small amounts of solvent wastes that are concentrated and consist of very volatile components that are easily separated from the nonvolatile fraction. Batch distillation is amenable to small quantities of spent solvents which allows these wastes to be recovered onsite. With batch distillation, the waste is placed in the unit and volatile components are vaporized by applying heat through a steam jacket or boiler. The vapor stream is collected overhead, cooled, and condensed. As the waste s more volatile, high vapor pressure components are driven off, the boiling point temperature of the remaining material increases. Less volatile components begin to vaporize and once their concentration in the overhead vapors becomes excessive, the batch process is terrninated. Alternatively, the process can be terrninated when the boiling point temperature reaches a certain level. The residual materials that are not vaporized are called still bottoms. 

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 ... 

For example, for steam (saturated vapor, no liquid) distillation with one organic compound (liquid), there are two phases, two components, and two degrees of freedom. These degrees of freedom that can be set for the system could be (1) temperature and (2) pressure or (1) temperature and/or (2) concentration of the s) stem components, or either (1) pressure and (2) concentration. In steam distillation steam may be developed from water present, so there would be both a liquid water and a vapor phase water (steam) present. For such a case, the degrees of freedom are F = 2 + 2- 3 = l. 

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,... 

The results of the differential distillation end the same as the flash distillation, although the mechanism is somewhat different. This is a batch type operation distilling differentially. All sensible and latent heat are supplied separately from the steam or by superheat in the steam. Steam acts as an inert in the vapor phase, and quantity will vary as the distillation proceeds, while temperature and pressure are maintained. 

Example 15.4 A reboiler is required to supply 0.1 krnol-s 1 of vapor to a distillation column. The column bottom product is almost pure butane. The column operates with a pressure at the bottom of the column of 19.25 bar. At this pressure, the butane vaporizes at a temperature of 112°C. The vaporization can be assumed to be essentially isothermal and is to be carried out using steam with a condensing temperature of 140°C. The heat of vaporization for butane is 233,000 Jkg, its critical pressure 38 bar, critical temperature 425.2 K and molar mass 58 kg krnol Steel tubes with 30 mm outside diameter, 2 mm wall thickness and length 3.95 m are to be used. The thermal conductivity of the tube wall can be taken to be 45 W-m 1-K 1. The film coefficient (including fouling) for the condensing steam can be assumed to be 5700 W m 2-K 1. Estimate the heat transfer area for... 

If that s all there were to it, we d be talking about Class 4 Steam Distillations and the like, where the components aren t soluble, and we could quit. Here, the two are soluble in each other. The individual vapor pressures of each component (PA, PB) depend not only on the temperature, but also on their mole fraction. 

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. 

With the introduction of dist.illa.tion and fractionation under high vacuum in the presence of steam, it was possible to effect a relatively efficient separation of oil from the asphalts. However, in the case of the more highly paraffinic crudes, a complete separation between oil and asphaltic materials could not be made, because the high viscosity oil fractions which have very low vapor pressures are thermally decomposed at the relatively high temperatures required in the distillation. 

The word distillation in the chemical laboratory is usually confined to a process in which the liquid or mixture in the boiler actually reaches the pressure allowed within the apparatus so that rapid bulk flow of vapor takes place from boiler to condenser. The speed of transfer rises enormously when this pressure is reached. If water were not present in a steam distillation, the other components would not reach the necessary pressure at the same temperature, and only slow diffusive transfer in the gas phase would occur. The presence of water does not assist diffusive transfer but only lowers the temperature at which diffusion gives place to bulk flow. The use of steam as a carrier gas allows easy collection. 

The bottoms of the CD, also known as atmospheric residue, are charged to a second fired heater where the typical outlet temperature is about 750-775°F. From the second heater, the atmospheric residue is sent to a vacuum tower. Steam ejectors are used to create the vacuum so that the absolute pressure can be as low as 30-40 mm Hg (about 7.0 psia). The vacuum permits hydrocarbons to be vaporized at temperatures below their normal boiling point. Thus, the fractions with normal boiling points above 650°F can be separated by vacuum distillation without causing thermal cracking. In this example (Fig. 18.14), the distillate is condensed into two sections and withdrawn as two sidestreams. The two side-streams are combined to form cracking feedstocks vacuum gas oil (VGO) and asphalt base stock. 

In a 500-cc. flask fitted with a 24-cm. fractionating column (Notes 1 and 4) attached to a water-j acketed condenser, are placed 148 g. (1 mole) of dry powdered cinnamic acid, 2 g. of hydroquinone and several small pieces of day plate. One-half gram of hydroquinone is also placed in the flask in which the distillate is to be collected (Note 2). The acid is rapidly heated over a wire gauze with a free flame until phenylethylene begins to distil, the heating being regulated so that the temperature of the vapors at the head of the column never exceeds 130°, and mainly remains below 120° (Notes 3 and 4). The cinnamic acid refluxes, but very little should escape and collect in the condenser. The decomposition is complete in three and one-half to five hours, when no more phenylethylene distils and the temperature at the head of the column rises rapidly. The distillate consists of a straw-colored oil and a little water a dark tarry residue (50-60 g.) remains in the reaction flask. About 100 cc. of water is added to the distillate and the aqueous mixture is distilled. The phenylethylene is easily volatile with steam and separates in the distillate as a colorless oil. The oil (45-48 g.) is separated, dried with a small amount of calcium chloride, and carefully distilled under diminished pressure, cool-... 

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. 

Stripping Steam—Injection of steam into the oil modifies the vapor pressure of the materials to be distilled to effect deodorization. Adequate stripping steam, consistent with the temperature and pressure, is required however, too much live steam may cause hydrolysis, recreating FFA. 

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. 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). 

Essential oils or ti-limonene are recovered from oil-water emulsions by steam distillation at a reduced temperature. This is not a typical fractional distillation, as practiced when distilling cold-pressed oil, but rather, it is a bulk separation of the volatile compounds in the condensed vapor. When this distillation is applied to pure ti-limonene, the mass ratio of water d-limonene at atmospheric pressure is in the range of 8-10 1. In the actual oil emulsions, considerably more water must be distilled to recover a given quantity of rf-limonene, because this component is adsorbed by the pulp particles in the emulsion. If the distillation is performed in a continuous manner under pressure, the ratio of water removed per amount of (i-limoncne recovered can be reduced to 3 4 1 (Braddock, 1999). However, when the steam temperature rises above 120°C, recovery of d-limonene decreases due to formation of some water-soluble alcohols and epoxides, which are soluble in the aqueous phase and are not recovered in the d-limonene phase above the condensate. 

The vapor issuing from the flask consists of a mixture of steam and the volatile substance to be distilled. When this is condensed, two layers are formed. The theory of the process is briefly as follows When a mixture of two immiscible liquids is heated, each substance vaporizes independently of the other. When the sum of the vapor pressures of the two liquids is equal to the pressure of the atmosphere, the mixture distils. The relation between the weight of the two substances obtained is determined by their molecular weights and their vapor pressures at the temperature of distillation. The case of nitrobenzene... 

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. 

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