CHAPTER VIII.

CRUDE OIL.

THE properties, occurrence and genesis of petroleum have been fully discussed in Chapters III, IV, V and VI. For practical purposes the theoretical speculations regarding the origin and chemical composition of petroleum are of secondary interest. In the practice the criteria for the determination of the value of a crude oil are of a very definite character, fractional distillation, which gives a clear picture of the composition of the oil, i. e., of the proportions of essences, illuminating and heavy oils, being first in order, associated with the determination of the specific gravity.

In the practice these two examinations suffice, as a rule, for the determination of the value of a crude oil, the determinations of resin, sulphur, creosote oil and acid belonging to the scope of more detailed examinations. Photometric measurements for the determination of the burning value of the fractions requires a special expense for apparatus and contrivances, so that they can be used in rare cases only for the determination of the value of crude oil.

Fractional distillation, determination of the specific gravity and tests for resin, etc., will be discussed in this chapter, while photometric measurements will be treated of and explained in Chapter XI.

Fractional Distillation.

Since oils from various districts differ in regard to their chemical composition, the determination of the specific gravity alone does not suffice for the practice. Although it is generally accepted as a rule that specifically heavier oils are of less value, this holds good only for comparing the oils of one dis

trict with one another. Petroleum being a mixture of many and heterogeneous hydrocarbons, the relative quantities of which in the particular varieties vary very much and do not occur in conformity with any law, a specifically heavier oil may, under certain circumstances, yield in the practice better results than a specifically lighter oil.

While, for instance, Pennsylvania oil of 0.855 specific gravity is considered of low value, an oil of the same specific gravity from the Caucasus belongs to the best varieties. In the first kind the relative proportion of illuminating oil to other portions of the oil is less than in the Caucasus oil. Hence, fractional distillation is the only sure guide in regard to the yield of illuminating oil.

In handbooks, etc., numerous analyses of crude oil made in the laboratory are as a rule given, which as far as manufacturing on a large scale is concerned generally bear the stamp of inaccuracy, they presenting in very rare cases only a true picture of the yield of illuminating oil attained in practice.

This is a well known fact to the practical refiner, and is due to the circumstance that laboratory analyses only give the result of what is known in practice as first distillation. The yield in operating on a large scale is, as a rule, greater, the difference varying, according to the derivation of the oil, between quite wide limits (5 to 25 per cent.). In executing an analysis the derivation of the oil should always be taken into consideration, and the properties of the fractions distilling over before and after the illuminating oil should be determined. While in the separation of the various fractions, the essences distilling over up to 302° F. cannot be separated as belonging to illuminating oil, a small portion of them-that having the highest boiling point-may be mixed with the petroleum without injury to its quality, when the heavy oils distilling over after the illuminating oil can also be used for illuminating oil, this having been proved by experiments on a large scale. In this case the specific gravity of the heavy oils should not be too high and their color not too dark; further, they must not suffer partial

decomposition, this being of special importance, otherwise the yield of illuminating oil will be decreased by the formation of volatile hydrocarbons, and the burning value impaired by the formation of non-saturated hydrocarbons.

A few examples will suffice to show the difference between laboratory analyses and the results on a large scale:

Bradford crude oil, analyzed in the factory-laboratory of Dr.

The same oil distilled on a large scale, gave, after several weeks' distillation, the following average result:

Similar results were obtained with a Roumanian oil, which on a small scale yielded 15 per cent. benzine and 40 to 45 per cent. illuminating oil, and on a large scale, 60 per cent. illuminating oil and only 10 per cent. benzine.

The difference between the results obtained in the laboratory and in the practice is greater with crude oils containing benzine, and smaller with oils poor in benzine, or with oils with specifically heavier fractions after the illuminating oil, an ex

ample of the latter being the Russian oils, in the case of which the two results nearly agree.

A few of the many distilling apparatuses for laboratory use may here be mentioned. The principal difference is in the condensation, which may consist of a simple tube condenser or surface condensers or air condensers of various shapes. The most simple form is shown in Fig. 62. The retort, which is filled with the fluid to be distilled, is either directly connected, as shown in the illustration, with a Liebig condenser by a tube

fitted in the side of the neck, or is provided on top with a tubulure having one, two or more bulbs. In this tubulure a thermometer is placed. From the side of the tubulure a small tube leads to the cooling contrivance where the vapors condense to be caught in the receiver. The object of the tubulure is to force the heavier volatile fluids which condense in the bulbs to flow back into the retort, and allow only the volatile portions corresponding to the temperature to escape through the lateral tube, thus rendering possible a thorough separation of the different portions.

Linnemann has improved this method with the tubulure by placing in the wide tube Fig. 63, in which the vapors ascend, from five to eight small cups of platinum wire, Fig. 64, eight such cups being used for fluids with a boiling point below 320° F., six for fluids with a boiling point up to about 356° F., and five for fluids with a boiling point up to 482° F. Le Bel's arrangement, Fig. 65, is similar to the one just described. In these cups, as well as in the bulbs, more fluid is condensed than can flow through the meshes; the vapors are thus washed and come successively in contact with layers of fluid whose temperature is lower, so that only the most volatile portions can reach the condenser.

Since with this arrangement of the apparatus the tube and the bulbs become gradually filled with fluid, and since all the vapor condenses, the flame has from time to time to be removed to allow the fluid to flow back. In order not to retard the progress of distillation it is more suitable to use the Henninger tubulure, Fig. 66, in which are also placed cups or balls of platinum wire; the condensed fluid, however, runs off through the lateral tubes. To limit the radiation of heat, the tube, during the further progress of distillation, is surrounded with a sheet-iron jacket, so arranged that the escape towards the top of the hot current of air is gradually cut off, and the air surrounding the tube is constantly more strongly heated. With any one of these apparatuses (Linnemann's, Le Bel's, Henninger's) a sufficient separation of the different fractions is obtained and, what is of special importance, parallel determinations yield results which perfectly agree.

In France Regnault's apparatus, Fig. 67, is employed by custom houses. A is a small copper cylinder provided at b with a small tube bent downward. This tube passes into the brass condenser B, which ends, above and below, in two narrow metallic tubes g and i. The condenser is secured in the metallic cylinder f, which is arranged as a cooler for the reception and discharge of water. The apparatus is secured to the trevet P. The latter is provided with a horizontal board which