Z

Plate 1

Ooi 0.2 03 0.4- 0.5 0.6 0.7 0.& 0.9 KG Mol. Fraction of Benzene Fig. 11-8. Operation of benzene-toluene column.

results are shown in Fig. 11-9. The amount of xylenes was small so that a large proportion of toluene was present on the bottom plate. The highest concentration of toluene occurred on the fifth plate, this component thus tending to segregate in the column. Under ordinary conditions, a column would be operated at a lower temperature level so that the benzene at the top would have contained less toluene, thus delivering the toluene and xylene together from the bottom for subsequent separation in a second column. This experimental column had insufficient plates to do this.

The column was then operated with total reflux on a cracked petroleum distillate obtained from a Winkler-West Texas crude oil. The liquid samples from the several plates in the column were then analyzed in a true-boiling-point still, being separated into components of 5°C.

I 0.2 03 0.4- 0.5 0.6 0.7 0.& 0.9 I.C Mol.Frae+ion of Benzene boiling-point range. Each of these components was indicated by its mid-temperature. Thus a component boiling on the true boiling-point apparatus between 75 and 80°C. was called the 77.5°C. component. The results of this experiment are given in Fig. 11-10, where each component is indicated by a concentration curve.

It will be noted that each component tends to segregate in the column, the segregation point depending on its boiling point. This segregation of a component in a continuous column is the basis for the type of still frequently found in petroleum refineries where streams or cuts

Condenser

TopPh-h

Plate 1

Mot Fraction of Coroponenf Fig. 11«9. Operation of column on benzene-toluene-xylenes mixture.

are taken from the central portions of the column as well as from the top and bottom. It is evident from Fig. 11-10 that such side cuts cannot be all pure or free from other components; in the commercial column, where total reflux is n6t employed, the segregation is much less pronounced than is indicated in Fig. 11-10.

The maxima ill concentrations of the fractions shown in Figs. 11-9 and 11-10 are definitely related to the volatility of the component in question and the temperature in the distillation column. These curves were obtained at total reflux, and for this condition the composition of the vapor entering a plate is equal to the liquid leaving the plate for all components, i.e., t/n and

giving

Xn+l Xn

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