Physical & chemical properties of Petroleum…

What is petroleum.

Petroleum: A latin word “petra” means rock and “oleum” means oil

therefore “rock oil” Hydrocarbons Hydrocarbon generaly another term used with word

petroleum for any of its forms. The most important substances present are hydrocarbons, which are compounds containing only two elements, hydrogen and carbon also small amount of Oxygen, Sulpher and Nitrogen (NSO compounds).

Mostly in oils the hydrogen and carbon comprises 95% to 99% of the total.

Chemical properties of Petroleum

Hydrocarbons are found in nature in many different forms, mainly as:

Liquid Petroleum: known as “crude oil” to distinguish it from “refined oil” . It is most important commercially. Natural Gas: which is the lighter fraction of hydrocarbons, can be free or dissolved. Asphalt, Tar, Pitch: these are solid or semi-solid forms of hydrocarbons, the heavy fraction.

Hydrocarbon Series A saturated hydrocarbon (sometime called Alkane)

is one which the valence of all the carbon atoms is satisfied by single bonds For each carbon atom is connected to each other carbon atom by a single covelant bond e.g Parrafins.

An unsaturated hydrocarbon is one in which the valence of some of the carbon atom is not satisfied by single bond, so that these atoms are connected to one another with two or more covelant bonds e.g Benzene.

It is generally agreed that Hydrocarbons of Four different Series or Types are present in important Quantities in Petroleum.

Parafins

Paraffins are also called alkanes and have the general formula of CnH2n+2, where n is the number of carbon atoms. Paraffins from C1 to C40 usually appear in crude oil and represent up to 20% of crude by volume. Since paraffins are fully saturated (no double bond), they are stable and remain unchanged over long periods of geological time.

Napthenes Naphthenes or cycloparaffins are ring or cyclic saturated hydrocarbons with the general formula of CnH2n. Thermodynamic studies show that naphthene rings with five and six carbon atoms are the most stable naphthenic hydrocarbons. The content of cycloparaffins in petroleum may vary up to 60%.

Aromatics

Aromatics are an important series of hydrocarbons found in almost every petroleum mixture from any part of the world.

This series of aromatics is called alkylbenzenes and have a general formula of CnH2n-6 (where n ≥ 6).

Its example are Benzene C6H6

Asphaltenes They are composed of fused benzene-

ring network, but they contain impurity atoms and are not true hydrocarbons.These impurities are the high in moleculer weight compound previously referred to as NSO compounds.

Asphaltenes are heavy compounds of crude oil and the major components in many natural tars and asphalts.

Chemical composition of typical PetroleumElement Natural Gas Crude oil Asphalt

Carbon 65-80 82-87 80-85

Hydrogen 1-25 12-15 9-11

Sulphar 0.2 0.1-6 2-8

Nitrogen 1-15 0.1-2 0-2

Oxygen 0 0.1-5 0

Classification of crude oils Crude oil may be classified by their relative

enrichment in the four hydrocarbon groups or series describe above.most normal crude oils fall within only three of these fields.They can be either.

1. Paraffinic oil: rich in paraffins.2) Paraffinic-Nephthenic Oil: They can have nearly

equal amounts of Paraffins and naphthenese which togeather make up more than 50% of the crude.

3) Aromatic intermediate Oil: They can have subequal amount of Paraffins and nephthenes, which total less than 50% and the composition is dominated by the aromatics and asphaltenes.

Physical properties… The physical properties are most commonly used in

petroleum are as under: Density Specific gravity Volume Viscosity Refractive index Fluoresence Optical activity Colour Odour Boiling point

Specific Gravity Specific gravity of oil generally lies in

between 0.73 and very slightly above 1.0

Paraffin oils are commonly lies asphalt base oils almost in variably high. The gravity is conventionally signified by the Greek letter rho ρ.

The gravity was formerly express in degrees of the European Beaumé scale read directly hydrometer this means the degree goes up as the density goes down. The high gravity is not a heavy oil.

Beaumé value ,with the density standardize to 15.6 °C, is given by the equation.

Be=(140/ ρ )-130 The Beaumé scale was long ago

superseded by the scale of the American Petroleum Institute, called the API Scale. The relation between two scales is given by: API value=(1.010 71*Be)-0.107 14in relation to the density, this is equivalent to API value=(141.5/ ρ)-131.5

The value of API Gravity is high correspond to low Specific Gravity. And the low API Gravity value correspond to High Specific Gravity. Between oils in the same reservoir rocks but in separate traps. And between oils within the same reservoir rocks but different structural position.

The most favorite grade of crude oil is about 37° API, equivalent to a relative density of 0.84.

Very light crude above 40° API, occur in large quantity in Algeria, south eastern Australia and in some Indonesian and Andean fields.

Very heavy crudes dominant production from California, Mexico,Venezuela and Sicily.

Gravity of Crude oil at different temperature..

Gravity at 60 °F change in gravity for each 1°F change in T.

Specific gravity

API Specific gravity

0.90 25.7 0.00036

0.80 45.4 0.00039

0.70 70.6 0.00049

Colors Paraffinic oils are light color: Yellow

to Brown by transmitted light. Asphalt-base oils are commonly brown to black; many of them are known as “Black oils”. Color is commonly determined with the Saybolt Colorimeter.

Refractive Index Absolute refractive index (RI) of a

substance is the inverse ratio of the speed of light. The range of refractive indices for petroleum is from1.42 to 1.48. The lower indices are the lighter oils. The refractive index is dependent on the density of the oils, the heavy (lower API Gravity) oils have the higher indecies.

Refractive Indices of Representative OilsAPI Degrees Density Refractive

Index6 1.029 1.56632 0.918 1.50944 0.802 1.44858 0.742 1.41772 0.691 1.390

Fluorescence The all oils show more or less

fluorescence. The aromatic oils being the most fluorescent. The fluorescent colors of crude oils range from Yellow through Green to Blue.

Fluorescence is observed under ultraviolet radiation that most generally used for Petroleum having wavelengths of 2,537 and 3,650 angstrom unit.

Odor Due to the light hydrocarbons some

oils is agreeable like gasoline odor. Aromatics impart pleasant odors. Oils containing sulphur and certain

nitrogen compounds usually a disagreeable odor.

Viscosity. Viscosity is the internal friction of fluid causing it is

resistant put change of form.(viscosity is conventionally defied by the Greek letter eta,  η).

It is the ratio of stress to shear per unit time. Shear with liquid is not a constant but is proportional to time

viscosity is defined by the ratio, force*distance / area*velocity The CGS unit of viscosity is the poise which is too

larger unit of practical purpose in the oil industry. Viscosity of oil are therefore conventionally measured in centipoises.

Such a unit is a saybolt universal second(SUS):

SUS = viscosity in centipoises*4.635 / relative density

Hydrocarbons having viscosity higher then 10,000 mPa are now to be called natural tar.

A useful indicator of the viscosity of a crude oil is it pour point. This is the lowest temperature at which the crude will flow under described controlling conditions. pour point is above 40 degree(more than 100 F) a relatively common among crudes having highest contents of paraffin's wax.

Volume Oil in the reservoir contains dissolved

gas, and the volume of the solution depends upon the formation gas-oil ratio and the reservoir pressure.

Gas may be dissolved in oils under increasing pressure and increase the volume in solution.

The volume of liquid petroleum, at constant pressure.

The volume of surface equivalent gas will dissolve in a unit volume of reservoir oil. Increases as the reservoir pressure increases until the oil is finally saturated with gas and no more gas will dissolve in the oil.

Boiling Point Atmospheric true boiling point (TBP) data are obtained through distillation of a petroleum mixture using a distillation column with 15-100 theoretical plates at relatively high reflux ratios (1-5 or greater).

The high degree of fractionation in these distillations gives accurate component distributions for mixtures. The lack of standardized apparatus and operational procedure is a disadvantage, but variations between TBP data reported by different laboratories for the same sample are small.

ASTMD 86 and TBP curves for a kerosene sample.