Evaporation rate analysis

By assuming the Langmuir expression for the evaporation of a droplet with the Rosin-Rammler size distribution law, Sacks (74) found that the theoretical evaporation rate of a kerosine spray was about 100 times the experimentally observed values. He concluded that the Langmuir expression is based on the single drop and neglects the vapor pressure of the surrounding air, which would tend to inhibit vaporization in a spray. Consideration of the effects of dissociation of combustion products plus the effects of thermal conductivity for the vapors enabled Graves (33) to derive a theoretical curve for combustion rate which compared favorably with experimental data. However, the use of Probert s analysis to determine combustion efficiency, yielded efficiencies which were much lower than experimentally observed results. 

Evaporation rates for pheromone components under field conditions were measured by residual analysis. 

Designers of evaporation systems strive to achieve high heat transfer rates. This can be justified by a cost/benefit analysis. High rates of heat transfer in theory must often be proved in practice. Evaporators designed for high rates of heat transfer are generally more affected by traces of scale or non-condensable gases. 

The final results depend upon the water analysis, which is no better than 3 significant figures because of the evaporation rate of the water. 

Methods for determining surface cleanliness are too involved for online evaluation, so that simple tests such as water break are employed. Recently, Buser reported a rapid method based on surface tension (15). Evaporative rate analysis (16) has also been used to determine surface cleanliness as well as more sophisticated methods employing scanning auger spectroscopy (17. 18). 

These two models illustrate how the properties of the compound influence the rate of evaporation from water under static conditions. Environmental conditions such as wind speed and turbulence in the water phase will have a marked influence on rates of evaporation that would reduce gradients and also reduce the width of the interfacial diffusion layers and systematic analysis of these effects have been discussed. Other variables will affect evaporation rates by controlling the actual concentration of the compound in solution. Suspended sediments and/or DOM would act in this manner. Weak acids and bases would only evaporate as the neutral species since the complementary anions or cations would be more water soluble and essentially have no vapor pressure. Consequently, environmental pH relative to pA values will be a consideration. It should be mentioned that compounds may distribute into the vapor phase by other processes than evaporation. Formation of aerosols, for example, can be a factor. 

The present model assumes that ingredients diffuse and evaporate independently, whereas thermodynamic and mass transport considerations dictate that interactions must occur in concentrated mixtures (Cussler, 1997). Careful analysis of the evaporation rates in Vuilleumier etal. (1995) shows this to be the case The musk ingredient, compound XII in Table 10.2, depressed the initial evaporation rates of... 

Vuilleumier, C., Flament, I., and Sauvegrain, R (1995). Headspace analysis study of evaporation rate of perfume ingredients applied onto skin. Ini J. Cosmet. Sci, 17, 61-76. 

Typical spray simulations do not resolve the temperature and species gradients around each droplet to compute the rate of evaporation. Instead, evaporation rates are estimated based on quasi-steady analysis of a single isolated drop in a quiescent environment [27, 28]. Multiplicative factors are then applied to consider the convective and internal circulation effects. 

Oliveira WP, Freire IT. Analysis of evaporation rate in the spouted bed zones during drying of liquid materials using a three region model. In StrumiUo C, Pakowski Z, eds. Drying 96. Proc. lO Inti. Drying Symposium (IDS 96), Krakow, Poland, 1996, pp 504-512. 

To achieve solvent evaporation rates suitable for LC at flow rates of 200-400 pi min vapors must be discharged at a rate of 50-300 ml min This presupposes a solvent vapor exit (SVE) between the precolumn (sometimes consisting of two pieces, mostly of 0.53 mm i.d.) and the separation column. The SVE is fully open during solvent evaporation and switched to a strong restriction to achieve a small purge flow during analysis. 

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