Heat transfer in boiling mixtures

In industrial processes liquid mixtures of two or more components often have to be vaporized in order to separate the components from each other. Examples of this include concentrating solutions, the recovery of solvents, distillation of sea water to gain drinking water or the separation of substances by boiling in distillation. Heat and mass transfer are closely linked with each other in evaporation, and the amount of vapour generated will, in contrast to the evaporation of pure substances, be determined by the mass transfer. It is known from earlier experiments 

A large number of binary mixtures of organic liquids and of water with organic liquids were first investigated by Bonilla and Perry [4.100]. In Fig. 4.56 the heat transfer coefficients found from their measurements with an ethanol-water mixture are presented along with the results from Preufier [4.101]. Heat flux and pressure are indicated in the diagram. As we can see, the heat transfer coefficients a of the mixture are noticeably smaller than the values aid, which would be obtained, if we had interpolated linearly between the heat transfer coefficients of the pure 

4 Convective heat and mass transfer. Flows with phase change 

Two methods have proved themselves in the reproduction of heat transfer measurements. One starts from empirical correlations for the pure substances. These correlations normally contain dimensionless numbers, that now have to be formed with the properties of the binary mixture. The reduction in heat transfer because of inhibited bubble growth caused by diffusion is taken into account by the introduction of an extra term. This type of equation has been presented by 

As Preufier showed a considerable fraction of the reduction in heat transfer compared to pure substances is caused by the change in the thermal properties, whilst the additional term in the square brackets provides a comparatively small contribution. For most hydrocarbon mixtures and those of hydrocarbons with water it lies between 0.8 and almost 1. All the other methods and models presented in the following are used to avoid the extensive calculations of the property data for the mixture. 

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