Rectification of Complex Mixtures

The design methods considered for multicomponent mixtures in Chap. 9 were based on a limited number of definitely known components. In some cases, the mixtures are so complex that the composition with reference to the pure component is not known. This is particularly true of the petroleum naphthas and oils which are mixtures of many series of hydrocarbons, many of the substances present having boiling points so close together that it is practically impossible to separate them into the pure components by fractional distillation or any other means. Even if it were possible to determine the composition of the mixture exactly, there are so many components present that the methods of Chap. 9 would be too laborious. It has become customary to characterize such mixtures by methods other than the amount of the individual components they contain, such as simple distillation or true-boiling-point curves, density, aromaticity (or some other factor related to types of compounds), refractive index, etc. 

The simple distillation curve is the temperature as a function of the per cent distilled in a simple or Rayleigh type of distillation. This type of distillation is approximated by the laboratory A.S.T.M. distillation which is widely used to characterize petroleum fractions. The A.S.T.M. procedure gives some reflux and rectification, and the results are not exactly equal to the simple batch distillation, although the difference is not large. The temperature normally measured is the condensation temperature of the vapor flowing from the still to the condenser. Curve A of Fig. 11-1 is typical for the simple distillation of a complex mixture. The temperature at any point is the averaged result of a large number of components and includes all the effects of nonideality in the solutions. Thus in most cases it is impossible to relate such a curve to the volatility of the individual components involved. As a result, such simple distillation curves are not of much direct value for the solution of rectification problems. 

The true-boiling-point curve is an attempt to separate the complex mixture into its individual components. Actually it is a batch dis- 

It has been found possible to use true-boiling-point curves to dehne the compositions for distillation calculation. A fraction distilling over a narrow range is taken as an individual component. Thus the fraction coming over as distillate between 39 and 40 per cent in curve B of Fig. 11-2 might be considered as a component, the boiling point of which, at the pressure at which the distillation was carried out, being 

If the distillate during a true-boiling-point distillation were to be divided into two fractions at some convenient point. A, corresponding to the temperature h, and rimple distillation and true-boiling-point curves obtained for the two fractions, the results would resemble the 

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