Vaporization characteristics, bulk

This chapter complements Refs. 21 and 22 in reviewing the progresses made on the transient, convective, multicomponent droplet vaporization, with particular emphasis on the internal transport processes and their influences on the bulk vaporization characteristics. The interest and importance in stressing these particular features of droplet vaporization arise from the fact that most of the practical fuels used are blends of many chemical compounds with widely different chemical and physical properties. The approximation of such a complex mixture by a single compound, as is frequently assumed, not only may result in grossly inaccurate estimates of the quantitative vaporization characteristics but also may not account for such potentially important phenomena as soot formation when the droplet becomes more concentrated with high-boiling point compounds towards the end of its lifetime. Furthermore, multi-... 

The d -law assumes a constant Tg. However, in many practical situations the temperature of the droplet when introduced into the evaporator is far below this final, equilibrium value. Hence an initial transient heating period exists during which y, and Tf all increase whereas H decreases. Furthermore it can be estimated also that the sensible heat required to heat the droplet is of the same order as the latent heat of vaporization. Hence droplet transient heating effects on the bulk vaporization characteristics are expected to be significant. Two such models, representing extreme rates of internal heating, will be discussed. 

General Discussion. It was shown in the previous section that the bulk vaporization characteristics of a single-component droplet do not depend too sensitively on the detailed description of the internal heat transfer mechanisms. However, for multicomponent droplet vaporization qualitatively different behavior is expected for different internal transport mechanisms. This is because the vaporization characteristics (for example, the vaporization rate, the flame temperature and location, and the... 

The devolatilization of a component in an internal mixer can be described by a model based on the penetration theory [27,28]. The main characteristic of this model is the separation of the bulk of material into two parts A layer periodically wiped onto the wall of the mixing chamber, and a pool of material rotating in front of the rotor flights, as shown in Figure 29.15. This flow pattern results in a constant exposure time of the interface between the material and the vapor phase in the void space of the internal mixer. Devolatilization occurs according to two different mechanisms Molecular diffusion between the fluid elements in the surface layer of the wall film and the pool, and mass transport between the rubber phase and the vapor phase due to evaporation of the volatile component. As the diffusion rate of a liquid or a gas in a polymeric matrix is rather low, the main contribution to devolatilization is based on the mass transport between the surface layer of the polymeric material and the vapor phase. 

The rate of bubble collapse Rcl is primarily important in the first transition zone where the bulk liquid is subcooled. A number of studies have been published on subcooled boiling as well as the prediction of the point of net vapor generation, characteristics defining Transition Zone I, and the onset of nucleation. These studies all result in empirical correlations, and have not led to quantitative conclusions which can be generalized. The radial velocity... 

Using PCA, Cramer found that more than 95% of the variances in six physical properties (activity coefficient, partition coefficient, boiling point, molar refractivity, molar volume, and molar vaporization enthalpy) of 114 pure liquids can be explained in terms of only two parameters which are characteristic of the solvent molecule (Cramer 111, 1980). These two factors are correlated to the molecular bulk and cohesiveness of the individual solvent molecules, the interaction of which depends mainly upon nonspecific, weak intermolecular forces. 

Vapor (Gas) Irritant Characteristics — Since MSDSs often provide non-qualifying statements, the most appropriate of five statements listed below is given. (Source National Academy of Sciences, Committee on Hazardous Materials, "Evaluation of the Hazard of Bulk Water Transportation of Industrial Chemicals, A Tentative Guide," Washington, D.C., 1970.)... 

The rate and characteristics of surface evolution depend on the particular transport mechanisms that accomplish the necessary surface motion. These can include surface diffusion, diffusion through the bulk, or vapor transport. Kinetic models of capillarity-induced interface evolution were developed primarily by W.W. Mullins [1-4]. The models involving surface diffusion, which relate interface velocity to fourth-order spatial derivatives of the interface, and vapor transport, which relate velocity to second-order spatial derivatives, derive from Mullins s pioneering theoretical work. 

Qualitative observations made in the pioneering experiments in heavy metal halide radiochemistry evidenced some correlation between the vaporization and the adsorption energies of the compounds. This is important when judging bulk properties of new elements and compounds from their adsorption parameters. So every attempt was directed toward quantitative evaluation of the characteristics like adsorption energy or enthalpy. 

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