Distilling general distilling processes are employed: (a) the intermittent, (b) the continuous.
Intermittent the intermittent system, the oil is heated in one tank to varying ranges of temperature. The prod uct of the first range of temperature, say from 175 to 325°F., is called the first cut. The product from the second range of tem perature, 325 to 475°, is called the second cut.
The temperature is then raised from 475 to 625°; higher tem peratures will result in "cracking." The number of cuts de pends upon the quality of the oil, and varies for different oils. The final cut leaves a residue of coke or asphalt in the still. Such a system is called the intermittent system of refining.
Continuous the continuous system of refining, a battery of stills (see Fig. 91, page 206) is used. These stills are I heated independently of one another, but are so connected that the oil flows from the first still into the second still by gravity flow, and is there subjected to a higher range of temperature. It then flows to the third still and finally to the last still in the series. In each still the temperature is higher than in the preceding one, and different products are obtained. The vapors from the oil are taken from the domes at the top of each still and are run through condensers and then into the agitators (see Fig. 92, page 207) for treatment.
The oil is, of course, first pumped into the first still but no pumping is necessary until the last stills are reached. Continuous distillation is widely employed.
Types of Distillation.—Stills vary with the several different types of distillation, which are: 1. Destructive distillation, in which the stills are heated directly by coal, gas, or oil fires alone.
2. Vacuum, in which the heat is applied to the still by a direct fire while a vacuum of 15 in. of mercury is maintained on the contents of the still and within the entire condenser system.
3. Fire and steam (reduced pressure), in which fire is applied to the still and open steam is continuously bubbled through the boiling oil.
4. Steam, in which there is no direct fire, all of the heat required for the distillation being supplied by closed steam coils arranged within the still.
Vacuum distillation is not generally employed except for lubri cating oils. The main system used is the fire and steam method. Open steam in the oil minimizes the chemical decomposition occurring in the distillation of heavy hydrocarbons.
Separation and Finishing of the Groups or Cuts.—After the separation into groups is carried out the groups are then reduced or broken into other products which are refined as shown in Fig. 93, Scheme I and Chart 1, and Figs. 94, and 95, Scheme II, Charts 1 and 2.
The various products from each cut are collected in separate tanks. The first cut may carry not only the lighter benzines and gasolines, but also have a small percentage of kerosene. A second distillation will separate the gasolines from the lower hydrocarbons. Similarly the next cut may carry some lubri cating stock and a second distillation will separate this. All the gasoline is finally run into one tank, all the fuel distillates into another, and the kerosenes into another as shown in Fig. 93.
Treating Crude Gasoline Distillate—Scheme I, Chart 1.— The crude gasoline is drawn from tank (1), Fig. 93. It is run to the agitator (2). This first product has a bad odor and dark color. This odor is due principally to unsaturated hydrocarbon compounds which are treated with sulphuric acid and agitated with air. The acid reacts on impurities. It is then washed with a steam jet which dilutes the sulphuric acid and carries off the sludge. To neutralize the acid, soda ash is mixed with the product and finally is washed out. The product is now clear white gasoline. The next step is to re-refine the product in the steam still (3), heated entirely by steam coils inside. The product is the finished gasoline which is put in tank (4). It may be divided in refining into two or more grades and run to different tanks.
Variations in early refiners produced a gasoline having a specific gravity of 6213e. As the demands for gasoline have increased there has been little of this grade of gasoline available. The specific gravity has been lowered until at the I present writing, May, 1921, the specific gravity of gasoline for automobile use is 52136.