All over the world, at various depths beneath land and sea, there are accumulations of hydrocarbons formed long ago by decomposition of animal and vegetable remains. Hydrocarbons are compounds of hydrogen and carbon that, at normal temperatures and pressures maybe gaseous, liquid or solid according to the complexity of their molecules. The natural deposits are correspondingly gaseous, liquid or solid depending upon the relative proportion of the various hydrocarbons present in the mixture.

Crude oils are widely in appearance and consistency from country to country and from field to field. They range from the yellowish brown, mobile liquids to black, viscous semi-solids. But they all consist essentially of hydrocarbons. Their differences are due to the different proportions of the various molecular types and sizes of hydrocarbons. One crude oil may contains mostly paraffins, another mostly napthenes. Whether paraffinic or napthenic, some may contain a large quantity of lower hydrocarbons and be mobile and contain a lot of dissolved gases. Some consist mainly of higher hydrocarbons and be highly viscous with little or no dissolved gases. The nature of crude governs to a certain extent the nature f the products manufactured from it their suitability for special applications. A napthenic crude will be more suitable for the production of asphaltic bitumen, a paraffinic crude for wax. A napthenic crude or even more so an aromatic one will yield lubricating oils that are rather viscosity sensitive to temperature. However, modern refining process permit greater flexibility in their use of crudes to produce any desired type of product. Crudes are usually classified into three groups, according to the nature of the hydrocarbons they contain.

Paraffin Base Crude Oils
These contain paraffin wax, but little or no asphaltic matter. They consist mainly of paraffinic hydrocarbons and usually give good yields of paraffin and high-grade lubricating oils.

Asphaltic Base Crude Oil
These contain little pr no paraffin wax, but asphaltic matter is usually present in large proportions. They consist mainly of napthenes and yield lubricating oils that are more viscosity sensitive to temperature than those from paraffin base crudes, but can be made equivalent to the later by special refining methods. These crudes are now often referred to as napthene base crude oils.

Mixed Base Crude Oils
These contain substantial amounts of both paraffin wax and asphaltic matter. Both paraffins and napthenes are present together with certain proportion of aromatic hydrocarbons.

There is no overall or generally accepted method of classifying crude oils. What is termed an ultimate analysis gives the composition as a percentage of the principal elements carbon, hydrogen, nitrogen, oxygen and sulphur. These types of analysis are valuable in formulating a general idea of the usefulness of crude in producing various refinery products, but the analyses give little indication of the quantity of various products that can be produced.

THE CHEMISTRY OF CRUDE OIL
Crude oils are organic in nature and are made up of carbon and hydrogen (hydrocarbons) with lesser amounts of sulphur, nitrogen, oxygen and traces of different metals such as cobalt, nickel, potassium, calcium, sodium, silicon, copper vanadium etc.

Elemental composition of crude oil can be summarized thus: Element/Component Percentage Weight Carbon 85.0 Hydrogen 12.0 Sulphur 0.8 Nitrogen 0.6 Oxygen 0.6 Trace metals (e.g. Fe, Al, Ni, Ca etc) 0.2 Sediments 0.8

The hydrocarbon contents of crude oils are complex but are principally:
1. Paraffins (Alkanes) 2. Napthenes (Cycloparaffins) 3. Aromatics
The predominance of one group gives the petroleum certain properties valuable in formulating a general idea of the usefulness of the crude in producing various refinery products.

THE PARAFFINS
These are represented by general formula CnH2n+2 and maybe gases, liquids and solids.
C1 - C4 - Gases e.g. methane C5 - C15 - lIquids e.g. Kerosene C16 and Above - Solids e.g. Waxes

Paraffins maybe normal paraffins or isoparaffins.
Paraffins constitute the bulk of domestic and motor fuels produced from crude oil. The total contents of n-alkanes in gasolines and kerosene after distillation of crude oil maybe upto 50-70 %

OLEFINS
This is represented by the general formula: CnH2n. the olefins are the unsaturated hydrocarbons occurring in very small quantities in crude oil. They maybe normal or iso-olefins. They are highly reactive and usually unwanted components of motor fuels and lubricating oils.

NAPTHENES (CYCLOPARAFFINS)
This has the general formula CnH2n and includes cyclopentance, cyclohexane and their methyl derivatives.

Methylcyclopentance can undergo isomerization to cyclohexane and can further be dehydrogenated to benzene.
The napthenes are the most important component of motor fuels and lubricating oils. Monocyclic napthenes with long parffinnic side-chains are desirable components in diesel fuels because the viscosity changes very little with the changes in temperature. Thus they posses higher density than the paraffins.

AROMATIC HYDROCARBONS
This has the general formula C2H2N-6. They are cyclic unsaturated hydrocarbons petroleum contains considerable quantities of these aromatic hydrocarbons, the simplest of which is benzene C6H6.

These hydrocarbons are physically and chemically different from the paraffins and the napthenes. They contain the benzene ring which is unsaturated but very stable. The benzene hydrocarbons are distinguished by their ability to combine in groups of 2, 3 and more rings and also to combine with napthene rings.
Aromatics are valuable components of gasoline but undesirable in diesel fuels because of deterioration and the characteristics of their combustion. Aromatics with side - chains of iso-paraffinic nature give good viscosity - temperature properties to lubricating oils.

SULPHUR COMPOUNDS PRESENT IN CRUDE OIL
The sulphur content of crude oil is an important quality because the complexity and expense of the refining operation increase tremendously as the sulphur content of the crude increases. The sulphur content of the crude oil varies from practically nothing to as high as several precent depending upon the origin and chemical composition of petroleum. The most important compounds of sulphur found in crude include:
MERCAPTANS
(RSH) where R = alkyl group e.g. CH3SH methylmercaptan, a gas which with a boiling point of - 59 degrees C, Ethylmercaptan, CH3CH2SH, a liquid insoluble in water.

CHEMICAL PROPERTIES
1. When heated they liberate H2S
2. With oxidizing agents they can form sulphonated acid
R-SH HNO3 R-SO3H H2SO4

SULPHATES R-S-R1 (THIOETHERS)
e.g. CH3 - S- CH3 Dimethylsulphide
CH3 -S-CH2CH3 Methylethylsulphide
CH3CH2-S CH2CH3 Diethylsulphide
The sulphides have low boiling points and are evolved airing the distillation process. Generally, the other sulphur compounds concentrate in the heavier distillation residue and are often barred as fuel by environmental laws.

THEOPHANS OR CYCLIC SULPHIDES
These are saturated hydrocarbons with rings containing 5-6 carbon atoms with sulphur.
Sulphur in crude oil is usually determined by oxidizing a sample and converting the sulphur compounds to barium sulphate which is determined gravimetrically. One estimation of hydrogen sulphide can be made distilling out the dissolved gas absorbing it in a suitable chemical solution, and analyzing the solution.

A more frequently used method for determining hydrogen sulphide and mercaptan sulphur is by potentiometric titration in which silver sulfide and silver mercaptides are determined in an non-aqueous solution.
Other adverse effects of sulphur in petroleum include:
1. Corrosion of processing units. 2. they promote char formation on the wick when percent in kerosene 3. they re capable of imparting bad odor to cleaned goods and paints when used for dry cleaning and paint formulations 4. Environment: SO2 in air leading to acid rains

OXYGEN COMPOUNDS IN CRUDE OIL
Oxygen compounds found in crude oil are naptheric acids, ketones, amides, esters, phenols etc. the percentage of oxygen compounds in crude oil is about 0.1% to 2 % depending on the origin. Of the various oxygen compounds present in crude oil, napthenic acids are the most abundant while the content of phenols is very low because of the acid nature of most oxygen compounds, they are readily separated from crude oil and its fractions.
Petroleum acids are highly corrosive and many corrode the processing units. Also they m,ay give rise to insoluble salts (sodium napthenates), the presence of which is also dangerous.

Small percentages of phenolic compounds found in petroleum are objectionable because they increase the gum forming characteristics and degrade the color stability of these products. Washing with concentrated caustic soda is usually effective in removing these materials and thereby improving the distillate quality.

In lube oil, these objectionable compounds (present as resins) are generally removed by sulphuric acid treatment of by solvent extraction. However in asphalt, the resins impart adhesive qualities and have desirable dispersion effects on the asphaltenes.

NITROGEN COMPOUNDS IN CRUDE OIL
The average content of organic nitrogen compounds present in crude oil is 2-3 %. Two groups of nitrogen compounds have been identified.
The nitrogen bases often cause discoloration of petroleum products especially when they occur in combination with phenolic compounds. Nitrogen compounds are therefore removed with sulphuric acid for color stability. Nitrogen compounds also tend to poison catalyst used in cracking and reformation.

ORGANOMETALLIC COMPOUNDS IN CRUDE OIL
The metal content of crude oil wary from a few parts per million to more than 1000 ppm. The major metallic contaminants in crude oil include nickel, vanadium, iron, arsenic, aluminum, silicon and calcium. Lead is often a serious contaminant in refinery distillates. The greatest source of lead being trtraethyl lead (TEL) used for octane improvement of gasoline.
Vanadium compounds are particularly destructive, concentrations above 2ppm in fuel oils can lead to severe corrosion of turbine blades and deterioration of refractory furnace linings and stacks.
These metallic components are also known to be catalyst poisons. The metallic content of crude oil maybe reduced by solvent extraction with propane or similar solvents. Numerous laboratory techniques are available to analyze a crude oil for metallic contaminants. However atomic absorption method is the preferred method in most modern laboratories.

WATER AND SEDIMENTS
In refining operations, the presence of water and sediment leads to major difficulties such as corrosion, uneven heating and plugging in heaters and exchangers and adverse effects on product quality. Sediments normally exist in crude oils as extremely fine, well dispersed solids. Solids that originate in the reservoir from which the crude came, or in drilling fluids used to drill the wells, may take the form of sand, clay, shale or rock particles. Other sediments such as scale can be picked up from tubing, pipe, tanks, and other production and transportation equipment. Water may appear in the crude as droplets or as emulsion and contain chemical salts and other harmful substances.
In the refinery, the water is measured by distillation and the sediment by extraction.