Volatile component formation

In principle, extractive distillation is more useful than azeotropic distillation because the process does not depend on the accident of azeotrope formation, and thus a greater choice of mass-separating agent is, in principle, possible. In general, the solvent should have a chemical structure similar to that of the less volatile of the two components. It will then tend to form a near-ideal mixture with the less volatile component and a nonideal mixture with the more volatile component. This has the effect of increasing the volatility of the more volatile component. 

The pressure used in producing gas wells often ranges from 690— 10,300 kPa (100—1500 psi). The temperature of the inlet gas is reduced by heat-exchange cooling with the gas after the expansion. As a result of the cooling, a liquid phase of natural gas liquids that contains some of the LPG components is formed. The liquid is passed to a set of simple distillation columns in which the most volatile components are removed overhead and the residue is natural gasoline. The gas phase from the condensate flash tank is compressed and recycled to the gas producing formation. 

Volatiles or Aroma. The essential oil, or aroma, of tea provides much of the pleasing flavor and scent of green and black tea beverages. Despite this, volatile components comprise only - 1% of the total mass of the tea leaves and tea infusions. Black tea aroma contains over 300 characterizing compounds, the most important of which are terpenes, terpene alcohols, lactones, ketones, esters, and spiro compounds (30). The mechanisms for the formation of these important tea compounds are not fully understood. The respective chemistries of the aroma constituents of tea have been reviewed... 

In distillation operations, separation results from differences in vapor-and liquid-phase compositions arising from the partial vaporization of a hquid mixture or the partial condensation of a vapor mixture. The vapor phase becomes enriched in the more volatile components while the hquid phase is depleted of those same components. In many situations, however, the change in composition between the vapor and liquid phases in equihbrium becomes small (so-called pinched condition ), and a large number of successive partial vaporizations and partial condensations is required to achieve the desired separation. Alternatively, the vapor and liquid phases may have identical compositions, because of the formation of an azeotrope, and no separation by simple distillation is possible. 

Small concentrations of volatile components in a liquid mixture may accumulate in the vapor space of a container over time and appreciably reduce the flash point relative to the reported closed-cup value. This may be the result of degassing, chemical reaction or other mechanism. An example is bitumen [162]. Similarly, if a tank truck is not cleaned between deliveries of gasoline and a high flash point liquid such as kerosene or diesel oil, the mixture might generate a flammable atmosphere both in the tmck tank and the receiving tank. Contamination at the thousand ppm level may create hazards (5-1.4.3 and 5-2.5.4). Solids containing upward of about 0.2 wt% flammable solvent need to be evaluated for flammable vapor formation in containers (6-1.3.2). 

The formation of droplets, which range from 50 to 200 nm in diameter, gives a very large snrface area from which evaporation may take place rapidly. The desolvation chamber is maintained virtually at ambient temperature by providing snfficient heat to overcome the latent heat of vaporization of the mobile phase. While the volatile components vaporize, the less volatile components, such as... 

Calcinating limestone (composed of calcium carbonate) removes its volatile component (carbon dioxide) and results in the formation of quicklime (composed of calcium oxide) (see Textbox 33). 

If the primeval Earth s atmosphere was indeed formed only from volatile components emitted by the primitive, newly formed Earth s crust, its composition must have depended on the time at which it was formed, i.e., whether this was before or after the formation of the iron-rich Earth s core (Joyce, 1989) ... 

The early period of the Earth s history, known geologically as the Hadean, is associated with huge bombardment of the surface of the planet by meteors and comets. The sequence of events in the Earth s formation is shown in Figure 7.3, starting with the Hadean. Any volatile materials on the surface of dust grains or planetesimals deposited on the Earth will be removed and become part of the atmosphere, or more generally the volatile component inventory of the planet. 

Consider a situation in which a concentrated polymeric solution enters the extraction zone of, say, an extruder in circumstances when the pressure in the extraction zone. Pa, is less than the equilibrium partial pressure of the volatile component in the feed solution. Under these conditions the solution will be supersaturated at the extraction pressure, flashing of the volatile component will occur, gas bubbles of radius Rq will be formed, and the concentration will immediately fall from Wi to wq. If bubble formation occurs by homogeneous nucleation, the rate at which these bubbles will be formed per unit volume of solution should depend on the difference between the equilibrium partial pressure of the volatile component and the devolatilization pressure. Since this pressure difference is greatest when the solution first enters the extraction zone, the rate of formation of bubbles will at first be high but as devolatilization pro-... 

If more than one component is present in the droplet, the evaporation rate depends on the miscibility of the components. For totally miscible components, distillation of the more volatile component(s) occurs, and the droplet composition changes as evaporation proceeds. If the components are partially miscible or insoluble, evaporation may be greatly retarded by the formation of a surface layer. In the case of evaporation of a surfactant... 

This experiment showed that some volatile component was formed in the thermal decomposition of tetramethyllead and that this compound consumed a cold lead mirror with formation of a volatile product. If, instead, a zinc mirror was first deposited and allowed to be consumed by the volatile product from decomposition of tetramethyllead, dimethylzinc could be identified as the product. Paneth concluded that free methyl radical was formed in the thermal reaction and could determine its half-life to be 0.006 seconds under the reaction conditions employed. Also, free ethyl radicals could be formed in... 

The selective effect that a salt can have on the volatilities of the two liquid components, and hence on the composition of the equilibrium vapor, comes about primarily through effects exerted by the salt ions and/or molecules on the structure of the liquid phase. The most likely effect to be expected is that the salt would induce formation of association complexes, or clusters, of molecules of the volatile components about its ions. This effect would lower both of their volatilities but by differing amounts depending on the degree of selectivity of the particular salt in the preference of its ions for clustering with the molecules of one volatile component over those of the other. A preference for associating with the less volatile component would result in an increase in relative volatility and hence in ease of separation, and a preference for the more volatile component would have the opposite effect. 

The combination of arsenic with dry nascent hydrogen was observed by Vournazos,14 who obtained a mixture of hydrogen and arsine by heating rapidly to 400° C. in a round-bottomed flask a mixture of three parts of powdered arsenic with eight parts of dry sodium formate. The addition of sodium hydroxide or lime to the mixture prevents the formation of sodium oxalate and hence of carbon monoxide. Arsenious oxide, sodium arsenite or arsenic acid may be used in place of arsenic, but the yields are small. The gas is also formed if arsenic vapour is passed over heated sodium formate. Also, if the sulphide or phosphide of arsenic is heated with the formate, hydrides of both components of the arsenic compound are formed but with metallic arsenides the hydride of the non-volatile component is not formed. 

As stated earlier, distillation is a widely used separation technique for liquid mixtures or solutions. The formation of these mixtures is straightforward, and is usually spontaneous, but the separation of a mixture into its separate constituents requires energy. One of the simplest distillation operations is flash distillation. In this process, part of the feed stream vaporizes in a flash chamber, and the vapor-liquid mixture, which is at equilibrium, is separated. The vapor is rich in the more volatile component, but complete separation is usually not achieved. A simple schematic showing the necessary equipment for flash distillation is given in Figure 10.3. We will illustrate the concepts by using a simple case of the flash distillation of a binary mixture. 

When the components A are mixed, there is rapid formation of adduct C in equilibrium with the other compounds. On fractionation in vacuo at low temperatures, the most volatile component, dimethylamine, is readily removed, giving good yields of FaSiColCO) ... 

The volatile components identified from the reaction of cystine and DMHF in aqueous medium are shown in Table I. 2,4-Hexanedione, 3,5-dimethyl-l,2,4-trithiolanes and thiophenes are the major compounds. The mechanistic relationship of the three thiophenones produced has been postulated (23). The major groups of volatile components identified from the reaction in the glycerol medium are 1,3-dioxolanes and thiazoles (Table II). 1,3-Dioxolanes are formed by the reaction of glycerol and the degraded carbonyls by ketal or acetal formations. Comparison of the reaction of cystine and DMHF in water and in glycerol is outlined in Table III. 

Various kinds of heterocycles and two unsaturated methylketones were identified as characteristic components in the volatiles from cooked small shrimps. Without exception, they were all thermally generated compounds. Some volatile components from cooked small shrimps were in common with those of other animal protein foodstuffs like meat however, various types of compounds found in another foodstuffs were composed of the volatiles from specific shrimp species. Both the precursors and the formation pathways for the typical aroma compounds have already been elucidated, even though it is difficult to explain the different constituents of the volatile components among shrimp species. In future, it will be necessary to investigate the key factors which define the possible pathway to form characteristic volatiles in each foodstuff. 

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