Physical and Chemical Properties of Oil

Oil is described by the physical properties of density, color, viscosity, thermal expansion and other properties related to the number of carbon atoms in the molecules.

Petroleum can be of different colors. Oil colors vary in a very wide range from oilfield to oilfield: from pale yellow, yellow and even colourless to dark grey, green and dark brown shades.

The SI unit is kg\ m³at a reference temperature, typically 15⁰C. Knowledge of density is required for quantity calculations. In the USA and some other countries the density of petroleum products is defined in terms of API gravity. This is an arbitrary scale adopted by the American Petroleum Institute for expressing the relative density of oils. The API gravity scale is read "backwards". The higher the API number, expressed as degrees API, the less dense (lighter) the oil is. Conversely, the lower the degrees API, the more dense (heavier) is the oil.

Density of oils range from 0.65 to 1.0 gr\cm³ and more at 20⁰C. According to density, oils may be light, medium and heavy. Light oil is characterized by the density of 0..5 - 0.87; medium oil: 0.871 - 0.910 and heavy oil is described being as 0.910 - 1.05 gr\cm3.

Viscosity is a property of fluids that indicates their resistance to flow, defined as ratio of shear stress to shear rate. Crude oils range in consistency from water-like to tar-like solids. Fluid with a high viscosity such as syrup deforms more slowly than fluid with a low viscosity such as water. Absolute viscosity is measured in Poise. The oil specific viscosity is usually defined as ratio of absolute viscosity of a given fluid to absolute viscosity of water at the same temperature. The viscosity of oil is dependent upon temperature, pressure and shear rate. Viscosity decreases at temperature increases because molecules vibrate and interact less. Conversely, the viscosity of oil increases at temperature decreases and it can become grease-like at very low temperature.

The volume of given oil mass increases with temperature, therefore, its density decreases. The degree of expansion is expressed as the coefficient of thermal expansion. Thermal expansion is useful to determine the size of container needed when the oil is heated. Thermal expansion is expressed as the ratio of volume change to initial volume after heating 10 ⁰C.

Crude oils are complex mixtures containing hundreds of different hydrocarbon compounds that vary in appearance and composition from oil field to oil field, therefore, in various oil fields the oil composition can vary significantly. All hydrocarbons are divided into two groups: saturated hydrocarbons and unsaturated hydrocarbons. Saturated hydrocarbons are not capable of attaching atoms and molecules while unsaturated hydrocarbons are capable of attaching atoms and molecules. The latter take part in chemical reactions easier. Hydrocarbons can be as simple as methane, but many are highly complex molecules and can occur as gases, liquids or solids.

An "average" crude oil contains about 84% carbon, 14% hydrogen, 1-5% sulfur, and less than 1% of nitrogen, oxygen, metals and salts. In the refinery, most of these non-hydrocarbon substances are removed and the oil is broken down into various compounds and blended into useful products.

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