XII. Speak on:

1. Petroleum composition.

2. Physical and chemical properties of crude oil.

SUPPLEMENTARY READING

Physical properties of petroleum

The colour of petroleum varies from light yellow to red, while some very dark grades are opaque. The higher the specific gravity, the darker the oil.

The nature and composition of crude oil usually determines its odour. Unsaturated hydrocarbons, sulphur and certain nitrogen compounds are responsible for disagreeable odours. Crude petroleum from Russia, Rumania and California has an aromatic odour. Pennsylvania petroleum has an agreeable gasoline-like odour. Other oils vary in odour depending on the quantity of light hydrocarbons present in the oil, and of the impurities present. The petroleum from South Texas and Mexico has a strong disagreeable odour, due to hydrogen sulphide and other sulphur compounds. Petroleum is lighter than water. The specific gravity is influenced by physical factors and by the chemical composition of the crude oil. It varies from 0.7684 to 0.992.

The viscosity, or the measure of the resistance to flow, is an important factor with lubricating oils. It is usually stated in terms of the time¹ necessary for a definite volume of oil at a definite temperature to flow through a small opening. The actual work is carried out in an instrument known as a viscosimeter, of which there are several standard makes in use at the present time. Crude oils are quite different in viscosity. Some are very mobile, while others, like Mexican crude, are quite viscid. Oils composed of hydrocarbons belonging to the series CnHⁿ₂-2 and CnHⁿ₂-4 are viscous. Heavy petroleum in general is composed to a large extent of these hydrocarbons. The viscosity increases with the density.² The viscosities of oils of the same specific gravity but from different sources, are not always the same. This is due to the difference in chemical composition.

Notes

¹ It is usually stated in terms of the time – она обычно определяется промежутком времени

² increases with the density – увеличивается с увеличением плотности

Petroleum

Colour, odour, specific gravity, and viscosity are the main physical properties of petroleum. Another important determination made in connection with petroleum products is that of the flash and burning points. The flash point may be defined as the temperature at which the vapours arising from the heated oil will ignite with a flash of very short duration, when a small flame is passed over the surface. This is of the utmost importance, as it indicates the temperature at which oil will ignite. The specific heat of the hydrocarbons in each series decreases with increase in molecular weight. The heat of vaporization decreases with increase in molecular weight.

Crude oils differ markedly in their boiling points and in the amount of distillate which is obtainable at definite temperatures. The boiling point of the crude oil will depend largely on the hydrocarbons. Petroleum of low specific gravity will usually contain a larger proportion of the more volatile hydrocarbons, whereas in a crude oil of high specific gravity, high boiling hydrocarbons generally predominate.