tillation carried out under rectification conditions. Usually a laboratory distillation column equivalent to a large number of plates is employed, and the separation is made at a high reflux ratio to obtain efficient fractionation. Because it is a batch operation, low liquid holdup in the column is important, and packed columns are generally employed. Ideally a true-boiling-point curve for a mixture corresponding to the simple distillation curve A of Fig. 11-1 might be represented by curve B of this figure, showing individual horizontal lines for each component with sharp increases in temperature in going from one constituent to the next. In most petroleum mixtures the curve obtained is similar to B of Fig. 11-2, and no definite steps are obtained.
This result is obtained because (1) the number of components is very large and no single step would be very significant and (2) the degree of fractionation usually, employed is not sufficient to give the sharp breaks in the curve. The sharpness of the fractionation between the different components is also lowered by the formation of azeotropes and by other solution abnormalities. However, the true-boiling-point curve probably represents a fairly high degree of separation in most cases. It is interesting to compare curves A and B of Fig. 11-2. The true-boiling-point curve begins at a lower and ends at a higher temperature than the simple distillation curve because the latter gives an averaging effect. Actually a simple distillation should give the same final temperature as the true-boiling-point distillation, because the last material to be vaporized should be the pure highest boiling component in both cases, but in general the distillations cannot be carried to 100 per cent distilled. A number of methods have been proposed for calculating
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