Periodic Redistillation

Certain of the products obtained from the crude oil distillation require redistillation, e.g. a portion of the distillate which requires sharply separating into benzine and kerosene, and a portion which requires separating into kerosene and gas or solar oil. The redistillation of the latter calls for no special comment, being usually carried out in a crude oil still fitted with simple dephlegmators as above described. The redistillation of a mixture of benzine and kerosene, and the working up of benzine into products of limited boiling point ranges, requires, however, more accurate fractionation. This is usually conducted in a so-called steam still, fitted with much more efficient fractionating apparatus according to requirements.

The earlier types of redistillation plant, many examples of which are still in use, consisted merely of large stills, often of the cheese-box type, near the bottoms of which were laid series of perforated steam pipes. The distillation was conducted merely by the blowing in of live steam. Large quantities of water were thereby condensed and drawn off from time to time. Such stills were, in fact, simply crude oil stills minus the fire heating arrangements. They have now been to a large extent replaced by more efficient forms, as more efficient fractionation and a sharper cut between kerosene and benzine is required.

Fig. 132. - Vertical condenser.

Redistillation or steam stills differ from crude oil stills in that they are fitted with nests of steam coils, connected to steam traps. Each of these coils should preferably be in one piece in order to avoid joints inside the still. They should be of drawn steel piping about 1 1/2 inches to 2 inches diameter, and should be placed near the bottom of the still. Steam at pressure up to ten atmospheres is generally used, which enables a temperature of about 160° C. to be obtained in the still. Such stills are often set in brickwork so that direct firing may be used to supplement the steam heating.

Fig. 133. - Petroleum redistillation and fractionating still.

Fractionating columns of all types, such as simple columns fitted with baffle plates, highly efficient bubbling columns of the Heckmann type and columns of the absorption tower type, are fitted to steam redistillation stills.

The following description of an actual petroleum redistillation and fractionating still may be taken as typical. The plant (Fig. 133) consists of a still of about 30 tons capacity (1) fitted with three or four heating coils of 2 inches diameter (2) having a total heating surface of about 600 square feet. Each coil is connected to a separate steam trap (3). On top of the still is placed a Heckmann column (4) (shown in detail in Fig. 134), 18 feet long and •

Fig. 134. - Heckmann columi.

4 1/2 feet in diameter, fitted with 23 plates, each plate being fitted with 46 cups. The vapours escaping from the top of the column pass into the dephlegmator (5) which is practically a reflux condenser. This is 9 feet high and 3 feet in diameter, and has a cooling surface of 320 square feet. The water supply to this dephlegmator is controlled, so that the quantity of heavy distillate returned to the column by the trapped pipe (6) may be varied according to requirements. The vapours which pass the dephlegmator go on to the tubular condenser (7), 20 feet long, 3 feet in diameter, of cooling surface 900 square feet, whence the condensate passes away by pipe (8) fitted with gas vent (9).

Such a still can distil at the rate of from 500 gallons an hour or less according to the amount of fractionation required. The degree of fractionation is controlled by varying the water supply to the dephlegmator and by the rate of distillation. For relatively high boiling fractions the supply of water to the dephlegmator may be heated. Table 114 gives some actual results obtained by the above described plant.

Table 114. Engler Distillation

The spirit distilled in this case was a petroleum spirit rich in aromatic hydrocarbons, the object being to obtain a fraction into which the toluene was largely concentrated. The fractionation was, therefore, closely controlled only over the range in which the toluene would be found, i.e. fractions 13 to 22. The original spirit boiled between 47° and 170° C. in an Engler flask under normal conditions, and had a specific gravity of .785 at 15° C. The distillation was carried out at first at 300 gallons per hour, the rate being reduced to 200 gallons during the periods of careful fractionation. The samples were taken hourly and the distillation figures given are those obtained by distilling in an Engler flask under normal conditions. The table clearly shows the effect of the intensive fractionation. Fraction number 20 contained as much as 70 per cent of toluene, the original spirit containing only 14.

Table 115 gives some idea as to what takes place in the fractionating column. The same benzine as in the previous case was used. Five samples of vapour were drawn simultaneously from five points at different levels in the column, condensed and analysed, with the following results :Temperature of benzine in still, 117° C.

Temperature of water entering dephlegmator, 29° C , leaving, 81° C.