Table 116

B, fractions running into Benzine distillate.

C, ,, ,, ,, Benzine-kerosene distillate for redistillation. K, ,, ,, „ Kerosene distillate.

M, ,, „ ,, Solar-kerosene for redistillation.

S, „ „ „ Solar oil.

The five distillates so obtained would be by no means sharply cut. That from the first still might be used as a straight run benzine (after chemical treatment), but would probably require redistillation for conversion into a better grade. The distillates from the second and third stills would require redistilling to split them into kerosene and benzine fractions ; that from the fourth still would be straight kerosene distillate, while that from the fifth would require redistillation to separate off its higher boiling constituents. A detailed description of the working of a simple battery of four stills is given by Wadsworth in the U.S. Bureau of Mines Bulletin, No. 162. Such a simple system of stills is, however, capable of considerable modification, the efficiency being thereby much improved.

The vapour pipe of each still may be and often is fitted with traps of the type, already described (vide p. 326), which arrest and return to the still any fine particles of crude mechanically carried over in the form of spray. It may be further fitted with one or more dephlegmators or fractional condensers, so that the vapour from any one still may be subdivided into two or three fractions, which are separately cooled and collected. By this simple device much otherwise necessary redistillation may be avoided.

Table 116 is given to exemplify the operation of a battery of 10 stills so fitted, working up a rather heavy crude from which about 25 per cent of distillate was being taken off.

It will be noticed that still 7 is yielding three distinct types of distillate, and several of the others two. This indicates how the arrangement of fractional condensers on the vapour line from a still reduces to some extent the redistillation necessary.

A further example of the distillation of a lighter crude worked in a similar bench of six continuous stills is here given (Table 117).

The efficacy of the fractional condensers is here again well exemplified. In the case of still 3, for example, distillate (1) might go into the kerosene fraction, (2) into the benzine-kerosene fraction for redistillation, and (3) into the benzine fraction. The dephlegmators or fractional condensers used in both the cases above cited were those of type C, Fig. 131.

There is apparently no reason why the efficiency of such a continuous bench of stills could not be considerably improved by the introduction of more efficient fractional condensers.

Further examples of the use of such dephlegmators will be found in Chapter XXXII (Continuous Distillation Under Atmospheric Pressure (Continued)).

As one of the chief objects to be aimed at is low fuel consumption, it will doubtless occur to the reader that much heat might be saved by utilising not only the heat of the residue, but also that of the distillate vapours, condensing those by crude oil instead of by water. This principle is indeed applied in many modern continuous benches.