Sublimation-condensation model

The particles can change their mass if sublimation, condensation or evaporation occurs, or if mass is lost or gained due to interfacial mass transfer (mass diffusion). The interfacial mass transfer rate can be represented by use of the simple film model and expressed in terms of a mass transfer coefficient ... 

Calculation of the density of deposited layers of sublimate, and of associated variables, as an aid towards the optimization of sublimate condenser design, is discussed by Wintermantel, Holzknecht and Thoma (1987). The starting point of the analysis is the assumption that the growth of sublimate layers is governed mainly by heat and mass transfer the model is based on conditions in the diffusion boundary layer. The main process-determining factors (growth rate, mass transfer, and gas concentration) are accounted for. The derived theoretical relationship is shown to fit experimental data. 

The L vov sublimation model. The applications (outlined in Sections 2.4.5. and 2.4.6.) of the Hertz-Knudsen-Langmuir vaporization model to decompositions by L vov et al. [94] are based on the assumption that decomposition involves an initial sublimation step, followed by condensation of the less volatile products. Because sublimation is an endothermic process, the condensation process would need to make a significant energetic contribution for the decomposition to be exothermic overall. 

The basic idea behind this model is the following snow crystals grow (respectively decrease) by the condensation (sublimation) of vapour on their bottom (top) portions because they are colder (warmer) than the average snow temperature at the considered height (Figure 1). 

Figure 1 Simplified model ofTG metamorphism the upper (respectively lower) grain is colder (warmer) than the surrounding vapour. It leads to condensation (sublimation) on the grain surface.

Figure 2 Simple faceting model based on Kossel crystal explanation. Condensation leads to faceting (a) while sublimation generates a rounding of the grains (b).

During their work on the total synthesis of diazasteroids, Burckh lter and Abramson8 investigated, as a model, the synthesis of 2-(2-ethoxy-carbonylethyl)-3,4,5,6-tetrahydropyrimidine hydrochloride (16) via the condensation of 1,3-propanediamines (14) with ethyl 3-ethoxycarbonyl-propionimidate hydrochloride (13). When 16 was heated above its melting point, and the melt was allowed to sublime, 2,3,4,6,7,8-hexa-hydro-6-oxopyrrololl,2-a]pyrimidine hydrochloride (17) was isolated in excellent yield [Eq. (3). ... 

To summarize, the calculations performed to determine the thermo-d)mamic functions of RCI3 in the condensed state and a systematic analysis of the use of these functions in calculations of the standard enthalpies of sublimation allowed us to compile a complete set of the thermodynamic parameters for modeling equilibria involving these compounds. 

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