Volatile fractions

FIGURE 2 11 Distillation of crude oil yields a series of volatile fractions having the names indicated along wih a nonvolatile residue The number of carbon atoms that characterize the hydrocarbons in each frac tion IS approximate... 

Many valuable chemicals can be recovered from the volatile fractions produced in coke ovens. Eor many years coal tar was the primary source for chemicals such as naphthalene [91-20-3] anthracene [120-12-7] and other aromatic and heterocycHc hydrocarbons. The routes to production of important coal-tar derivatives are shown in Eigure 1. Much of the production of these chemicals, especially tar bases such as the pyridines and picolines, is based on synthesis from petroleum feedstocks. Nevertheless, a number of important materials continue to be derived from coal tar. 

Distillation. Distillation separates volatile components from a waste stream by taking advantage of differences in vapor pressures or boiling points among volatile fractions and water. There are two general types of distillation, batch or differential distillation and continuous fractional or multistage distillation (see also Distillation). 

Most distillations conducted commercially operate continuously, with a more volatile fraction recovered as distillate and a less volatile fraction recovered as bottoms or residue. If a portion of the distillate is condensed and returned to the process to enrich the vapors, the Hquid is called reflux. The apparatus in which the enrichment occurs is usually a vertical, cylindrical vessel called a stiU or distillation column. This apparatus normally contains internal devices for effecting vapor—Hquid contact the devices may be categorized as plates or packings. 

Volatility This denotes how much of the total amine supplied will be present in the steam and thus available to neutralize the carbon dioxide (also in the steam). In water, a portion of the total amine hydrolyzes to form an ammonium ion and a hydroxyl ion (the dissociation reaction) the balance of the amine (the free-amine portion) is volatile. Clearly, it is important to know the size of this volatile fraction, which depends on the particular amine selected and the pH of the system. In turn, the pH depends on the concentration of total amine originally present so that, the higher the pH, the greater the volatile fraction. 

To identify the volatile components, gas chromatography-mass spectrometry (GC-MS) is still the method of choice. A comparison of the GC fingerprints of B. carter a and B. serrata reveals the different composition of the volatile fractions (Figure 16.1). Common monoterpenes, aliphatic, and aromatic compounds of olibanum are, e g., pinene, limonene, 1,8-cineole, bomyl acetate, and methyleugenol (Figure 16.2). 

Lane 1 displays the separation of the resin of B. carterii and lane 2 and lane 3 its volatile fractions received by hydrodistillation. Accordingly, lane 8 displays the... 

The brownish colored zone (Rj 0.28) of incensole (compound 3), which occurs in both the resin and the volatile fractions of B. carterii, draws the hue between the volatile diterpenes and the nonvolatile triterpenes. B. carterii reveals two further colored prominent spots, a yellowish-ochre (Rf 0.65) of incensole acetate (compound 2) and a violet-colored spot (Rj 0.98) of verticilla-4(20),7,ll-triene (compound 1). Lane 2 and lane 3 reveal a light blue area (Rj 0.60) of 1,8-cineol that is only visible in freshly distilled oils. 

Most of the studies on the thermal degradation of carotenoids analyzed the volatile fraction, as the identification of nonvolatile fractions was probably more complex to analyze. A study was published recently on the volatile compounds generated by the thermal degradation of carotenoids in... 

Kanasawud and Crouzet have studied the mechanism for formation of volatile compounds by thermal degradation of p-carotene and lycopene in aqueous medium (Kanasawud and Crouzet 1990a,b). Such a model system is considered by the authors to be representative of the conditions found during the treatment of vegetable products. In the case of lycopene, two of the compounds identified, 2-methyl-2-hepten-6-one and citral, have already been found in the volatile fraction of tomato and tomato products. New compounds have been identified 5-hexen-2-one, hexane-2,5-dione, and 6-methyl-3,5-heptadien-2-one, possibly formed from transient pseudoionone and geranyl acetate. According to the kinetics of their formation, the authors concluded that most of these products are formed mainly from all-(E) -lycopene and not (Z)-isomers of lycopene, which are also found as minor products in the reaction mixture. 

MacKinnon MD (1977) The analysis of the total organic carbon in seawater a. Development of methods for the quantification of TOC b. Measurement and examination of the volatile fraction of the TOC. PhD Dissertation. Dalhousie University... 

As for all of the fractions of organic material in seawater, the volatile organic carbon fraction is defined by the method by which it is collected. In one of the earliest estimates, Skopintsev [93] defined the volatile fraction as the difference between total organic carbon values, as measured by evaporation and dry combustion, when the evaporations were carried out at room temperature and at 60 °C. Thus Skopintsev s volatile fraction consists of those compounds that are volatile from acidified solution taken to dryness at 60 °C but not at 20 °C. This fraction was found to be between 10 and 15% of the total organic carbon. He also noted a 15% difference in measured organic carbon with his dry combustion method when samples were dried at different temperatures and concluded that this difference was due to the loss of volatiles. 

The volatile fraction as defined by the various wet oxidation methods and most of the direct injection methods would be that fraction removed by acidification and purging with inert gas at room temperature. In the freeze-drying method of Gordon and Sutcliffe [29] the volatile fraction is that fraction lost by sublimation in vacuo. There have been no actual determinations of these losses, and for the most part Skopintsev s numbers were accepted as valid for all of these methods, largely because they are the only numbers available. 

MacKinnon [91] and Wangersky [180] have made direct determinations of the volatile fractions from a variety of depths and stations in the North Atlantic. The volatile fraction as defined by MacKinnon s method is that fraction which can be removed from solution by purging with an inert gas at 80 °C and a pH of 8 for 10-12 hours, then at 65 °C for a further 10-12 hours. The inert gas stream is flushed through an ice-packed condenser to remove water, then into a trap packed with Tenax GC followed by a U-shaped stainless steel cold trap held at -78 °C. 

Gershey et al. [58] have pointed out that persulfate and photo-oxidation procedures will determine only that portion of the volatile organics not lost during the removal of inorganic carbonate [30,79,92,181]. Loss of the volatile fraction may be reduced by use of a modified decarbonation procedure such as one based on diffusion [98]. Dry combustion techniques that use freeze-drying or evaporation will result in the complete loss of the volatile fraction [72,79, 92,93],... 

In further work to that discussed above, MacKinnon [183] has discussed in detail a method for the measurement of the volatile fraction of total organic carbon in seawater. 

We take as an example the fate of benzene ((/Ur,) as it migrates with groundwater flowing through an aquifer. Benzene is a common contaminant because it makes up much of the volatile fraction of gasoline and other petroleum products. It is a suspected carcinogen with an MCL (maximum contamination level) set by the US Environmental Protection Agency of 5 qg kg-1. 

VRDS Isomax [Vacuum residua desulphurization] A hydrodesulfurization process adapted for processing the residues from the vacuum distillation of the least volatile fraction of petroleum. An extension of the RDS Isomax process, developed and piloted by Chevron Research Company in the early 1970s. In 1988, one unit was under construction and one was being engineered. 

Bicchi, C., Cordero, C., Liberto, E., Rubiolo, P. and Sgorbini, B. (2004) Automated headspace solid-phase dynamic extraction to analyse the volatile fraction of food matrices. J. Chromatogr. A 1024, 217-226. 

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