Volatilization, theoretical treatment

Fractional distillation. The aim of distillation is the separation of a volatile liquid from a non-volatile substance or, more usually, the separation of two or more liquids of different boiling point. The latter is usually termed fractional distillation. The theoretical treatment of fractional distillation requires a knowledge of the relation between the boiling points, or vapour pressures, of mixtures of the substances and their composition if these curves are known, it is possible to predict whether the separation is difficult or easy or, indeed, whether it will be possible. 

The theoretical treatment which has been developed in Sections 10.2-10.4 relates to mass transfer within a single phase in which no discontinuities exist. In many important applications of mass transfer, however, material is transferred across a phase boundary. Thus, in distillation a vapour and liquid are brought into contact in the fractionating column and the more volatile material is transferred from the liquid to the vapour while the less volatile constituent is transferred in the opposite direction this is an example of equimolecular counterdiffusion. In gas absorption, the soluble gas diffuses to the surface, dissolves in the liquid, and then passes into the bulk of the liquid, and the carrier gas is not transferred. In both of these examples, one phase is a liquid and the other a gas. In liquid -liquid extraction however, a solute is transferred from one liquid solvent to another across a phase boundary, and in the dissolution of a crystal the solute is transferred from a solid to a liquid. 

Multiple publications (Pankow et al. 1997 Ingebrethsen et al. 2001 Pankow et al. 2003 Watson et al. 2004) have discussed measuring free-base nicotine directly, addressed the importance of free-base nicotine delivery, and examined the chemical properties of nicotine in cigarette smoke as an important determinant of the effective delivery and bioavailability of nicotine from cigarettes. Pankow et al. (1997) examined how ammonia influences nicotine delivery in tobacco smoke and concluded that conversion of nicotine to the free-base form could be facilitated by ammonia. Based on a theoretical treatment, Pankow et al. (1997) concluded that, under certain circumstances, up to 40% of the nicotine could be available as the volatile free-base form. These authors also concluded that the rate of volatilization was more rapid than that previously measured by Lewis et al. (1995) using denuder technology to examine the properties of mainstream cigarette smoke. 

Theoretical treatments have also been given to other situations of interest. In zone leveling, the impurity level is homogenized either by moving the zone back and forth along a linear rod, or by movement around a ring (1). Evaporation and condensation of a volatile impurity (17—18), as weU as decomposition of the material itself (13), have also been treated. 

In 1977 Kolb and Pospisil proposed a method for the quantitative analysis of volatiles in solid samples [48] by using headspace extraction and gas chromatographic detection. The method, termed discontinuous gas extraction, is based on stepwise gas extraction, followed by a subsequent analysis of the extracted volatiles. The method theoretically calculates the total amount of analyte in a soUd sample after a few successive extractions and makes the quantitation of volatile analytes in soUd matrices possible. The proposed method was validated by measuring the styrene content in polystyrene by discontinuous gas extraction and by a procedure proposed by Rohrschneider in which the polystyrene is dissolved in dimethyl formamide (DMF) [49]. The two methods were in good agreement, which supported the validity of the discontinuous gas extraction. Kolb and Pospisil later elaborated the theoretical treatment of discontinuous gas extraction and in 1981 the method was re-named as multiple headspace extraction (MHE) [50]. 

It calculates solid heat conduction, surface convection and radiation in a manner similar to the models related in Table 14.3. It also presents a theoretical treatment of the mass transfer of volatiles out of the composite, and the additional gas-phase heat supplied to the composite during gas-phase combustion. 

The following scheme of classification has been found to work well in practice it is not a rigid one since some of the anions belong to more than one of the subdivisions, and, furthermore, it has no theoretical basis. Essentially the processes employed may be divided into (A) those involving the identification by volatile products obtained on treatment with acids, and (B) those dependent upon reactions in solution. Class (A) is subdivided into (i) gases evolved with dilute hydrochloric acid or dilute sulphuric acid, and (ii) gases or vapours evolved with concentrated sulphuric acid. Class (B) is subdivided into (i) precipitation reactions, and (ii) oxidation and reduction in solution. 

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