Partial Evaporation or Distillation

If an organic solution or mixture of solvents is known to contain components of different volatility, partial evaporation or selective distillation can provide preferential enrichment of one or more components. The collection of spectra at different stages of the evaporation process can provide an insight into the nature of the key components. Careful use of spectral subtraction can help in the numerical extraction of chemical components from the different fractions. Successive subtractions may be used, with moderate success, in multicomponent situations. The main obstacle in this case is spectral variations due to mutual interaction of components in solution. Obviously, if nonvolatile components are present, this approach provides a practical separation of some of the main components—by complete evaporation. 

Normally, the analyses mentioned previously are performed on the residual materials. However, there is nothing to prevent measurements being made on the separated fractions or on condensate materials. In the case of samples containing water, be aware that the condensate will be enriched with water. An interesting approach to the controlled evaporation of materials is to use the combined techniques of thermogravimetric analysis and infrared. Although this technique is typically used for the determination of thermal decomposition products, it may also be used for controlled and reproducible selective removal of volatile components, with on-line monitoring of the volatile components. 

Note that the use of a hydrocarbon solvent is required to swell the membrane and to generate the required molecular porosity. Other membranes may be available for other chemical systems and for the use of other solvents. 

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The term coal tar pitches refers to the residues obtained by the partial evaporation or distillation of coal tar obtained by the removal of gaseous components from bituminous coal. Such pitches include gas-works coal tar pitch, coke-oven coal tar pitch, blast-furnace coal tar pitch, producer gas coal tar pitch, and the like. 

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