Evaporation from Multicomponent Liquid System

In many industrial processes, the many components contained in the liquid evaporate simultaneously. Evaporation of individual components is easy to determine. For multicomponent liquid systems, the individual evaporation rates are summed to obtain the total evaporation rate. 

Applying mass transfer theory to a component / in the liquid, assumint good mixing and neglecting atmospheric concentrations, the evaporation molar rate of a single component can be expressed as 

Integrating Eq. (4.331) and knowing that is the initial number of moles of component i yields 

Equation (4.333) assumes a constant ratio, rhe total mass is 

The total evaporation mass flow rate is obtained as 

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Before closing this section we should mention several relatively recent papers that provide additional details relevant to the overview given above. The effects of evaporation, back-reaction, diffusion, and dust enrichment on isotopic fractionation in forsterite have been discussed in great detail by Tsuchiyama et al. (1999) and Nagahara and Ozawa (2000) and extended to multicomponent systems in Ozawa and Nagahara (2001). Richter et al. (2002) combined theoretical and experimental approaches to study elemental and isotopic fractionation effects due to evaporation from CMAS liquids and included consideration of the effects of temperature, gas composition, and diffusion in both the residue and in the surrounding gas. 

Marangoni effects can be encountered in both single- and multicomponent liquid systems. In a pure liquid, surface tension gradients result from differences in temperature (or evaporation rate) from one point to another in the system. It is generally found that an increase in temperature lowers ctlv so that where hot spots occur, liquid flows away to cooler regions of the liquid (Fig. 6.13 ). The result of such a phenomenon can be the formation of dimples in a surface that dries or solidifies under uneven temperature conditions. 

In multicomponent systems (e.g., surfactant solutions), surface tension gradients usually are due to adsorption-related phenomena or, where possible, to different rates of evaporation from the system (although simple temperature variations can also be important). If the system contains two liquid components of differing volatility, the more volatile liquid may evaporate more quickly from the LV interface, resulting in localized compositional—and therefore surface tension—differences. It is also commonly found that when two or more components are present, one will be preferentially adsorbed at the LV interface and lower ctlv of the system. If a surface-active component... 

The significant differences between kinetic coefficients derived from TGA and DSC result in significant differences in the basic data obtained by these two methods. The relation ship between weight loss rates registered by TGA and energy flow rates measured by DSC can be clearly understood for a simple physical or chemical process, but with the complex pyrolysis process combined with the partial evaporation of liquids produced, the relations between heat absorption and weight loss are more complex, especially in the case of multicomponent systems. 

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