Introduction:

Petroleum is any naturally-occurring flammable mixture of the hydrocarbons found in the geologic formations, like rock strata. Most of the petroleum is a fossil fuel, made up from the action of intense pressure and heat on buried dead zooplankton and algae. Theoretically, the word petroleum merely refers to crude oil, however at times it is applied to explain any solid, liquid or gaseous hydrocarbons.

Geologists basically use the science of petroleum chemistry to help in exploration. From the composition of oil and variations of properties in a trend or basin, they might find out clues on how the crude was made millions or hundreds of millions of years ago. Reservoir engineers are mainly concerned by how the liquid and gas properties will vary in the reservoir in response to fluid withdrawals. For the production and facility engineers, petroleum chemistry is very important in the design of wells, artificial lift and facilities. Corrosion and toxicity of the produced gas might be a critical factor in the design of facilities and wells. The environmental engineer, having a focus in safety, health and the environment requires understanding the chemistry of the produced fluids and gases. This is specifically significant nowadays whenever new platforms are positioned in environmentally sensitive regions. At last, the composition of the petroleum once it has been processed will assist in determining its market value.

The hydrocarbons can exist in all three states or phases (that is, gas, liquid and solid). Petroleum or crude oil is a naturally occurring, flammable, complex and variable mixture of hydrocarbons plus other organic compounds in liquid phase. Petroleum consists of 83 to 87% carbon, 10 to 14% hydrogen, having trace amounts of nitrogen, oxygen and sulphur. The sulphur content of oil can be as high as 6%. In common, crude oil having sulphur content in surplus of 1% is considered sour crude. Beneath 0.05% content of sulphur, the crude oil is considered sweet and less expensive to refine, therefore it commands a higher price.

Petroleum Fractions:

The PNA groups (that is, paraffinic, naphthenic and aromatics) of petroleum fractions are general categorizations of the most general components in petroleum. Paraffins comprise the alkane series, naphthenes comprise the cycloalkanes and aromatics comprise all compounds which have one or more ring structures identical to benzene.

1) Alkanes:

The series encompass single carbon bonds having the general formula, C_nH_2n+2. All the alkanes after ethane have isomers, which encompass the similar number of carbon and hydrogen atoms, having single bonded carbon atom. The n-butane is the normal single chain of carbons, whereas the i-butane consists of a Y arrangement, or a branched chain molecular structure. The isomers encompass similar molecular structure, however rather different properties.

Fig: Alkanes-Petroleum Fractions

Dry gas comprises of mostly methane (CH₄) and ethane (C₂H₆). Alkane components having higher molecular weights, C₃H₈ (butane) to C₈H₁₈ (octane), might be present in the separator gas or wet gas at reservoir conditions.

Table: Alkane and impurities properties

2) Cycloalkanes:

As the name proposes, these are similar or identical to the alkanes; though there are one or more ring structures. The elements of cycloalkanes are bound via single bonds.

Fig: Cycloalkanes-Petroleum Fractions

3) Aromatics:

Most of the times it is termed as the benzene series, as the series consist of an identical cyclic structure to benzene. Benzene is C₆H₆ the first compound of this series. The structure of benzene is illustrated in the two diagrams below. Both exhibit six carbon atoms joined in a hexagonal geometry. One hydrogen atom is associated to each and every carbon atom. The diagram on left is more correct, as there is no double bond in the benzene. The C-C bonds were illustrated from X-ray diffraction to encompass all the similar length. The electrons for C-C bonds are equally distributed. Therefore, to exhibit the delocalization nature of bonding, the bonds are conceptually illustrated as a circle. This structure is general in organic chemistry.

The aromatics like benzene, toluene and xylene are significant industrial chemicals, utilized in drugs, plastics, solvents, synthetic rubber and dyes.

Benzene is a renowned carcinogen and as such, its use in gasoline is restricted. Human exposure to benzene is a global problem.

Fig: Aromatics-Petroleum Fractions

4) Other non-hydrocarbon elements:

Sulphur is found to certain extent in all crudes and produced gas. It might be in the form of free sulphur, H₂S and organic sulphur compounds. The lower API gravity crudes are more probable to have sulphur.

Almost all crude oils include small quantities of nitrogen.

Helium can be present in the trace amounts in natural gas.

Oxygen is as well found in crude oils either as free oxygen or as part of the radical of a larger compound.

a) Kerogen:

There is huge interest in oil shale production. Kerogen or the kerabitumen is a mixture of organic chemical compounds which by heat can be transformed to one or more hydrocarbon compounds. The chemical compositional explanation of Kerogen is not possible. Usually, the organic compounds encompass very high molecular weights and are insoluble in the normal organic solvents.

Enormous Kerogen deposits have been found out in the shales. Generally, these have been termed to as 'oil shale' deposits, which strictly speaking, Kerogen is distinctly dissimilar from petroleum. As well, Kerogen deposits are termed to as 'green oil', in that the organic matter has not had enough time nor heat to transform it to petroleum.

b) Solids Precipitation:

Hydrocarbons can exist as the solids, liquids or gases. Solids can form in the reservoir, wellbore, pipelines and surface facilities. The deposited solids can comprise of asphaltenes, waxes or a mixture of such components. Solids or highly viscous liquids might have resins, crude oil, fines, sands and water.

c) Asphaltenes:

Resins and asphaltenes are the sub-classes of aromatics however some resins have only naphthenic rings. Asphaltenes precipitation for the duration of primary depletion takes place more generally in light to medium oils, initially highly undersaturated. Maximum asphaltenes precipitation takes place around the saturation pressure. Interestingly, heavier crudes having higher asphaltene content encompass less asphaltene problems as they can dissolve more asphaltenes. Asphaltene precipitation can as well take place as an outcome of hydrocarbon or CO₂ gas injection. The precipitation generally takes place at the producer at breakthrough; though it can take place anywhere in the displaced fluids region.

d) Wax Deposition:

Waxes primarily comprised of normal alkanes crystallize in large flat plates and are termed to as paraffinic waxes. The waxes comprised primarily of cycloalkanes and i-alkanes form microcrystalline structures. Waxes form mainly to decrease in temperature and a weak relationship with pressure.

Wax precipitation doesn't essentially mean deposition will take place. Wax crystals might disperse. Waxes might crystallize around fine particles like asphaltenes or fines in producing crude.

Methods to prevent or remedy wax deposits comprise thermal, chemical and mechanical ways. Generally, scrapers are employed to eliminate wax deposits in the pipeline and wellbores.

Petroleum products and their uses:

Gases:

Gaseous refinery products comprise hydrogen, fuel gas, ethane, propane and butane. Mostly the hydrogen is consumed in the refinery desulfurization facilities that remove hydrogen sulphide from the gas stream and then separate that compound to elemental hydrogen and sulphur; small quantities of the hydrogen might be delivered to the refinery fuel system. Refinery fuel gas differs in composition however generally includes a significant quantity of methane; it consists of a heating value identical to natural gas and is consumed in the plant operations. Periodic variability in heating value makes it inappropriate for delivery to consumer gas systems. Ethane might be recovered from the refinery fuel system for use as the petrochemical feedstock. Propane and butane are sold as liquefied petroleum gas (LPG) that is a suitable portable fuel for domestic heating and cooking or for the light industrial use.

Gasoline:

Gasoline, a lightweight material which flows simply, spreads quickly and might evaporate totally in a few hours under temperate conditions. It poses a risk of fire and explosion due to its high volatility and flammability, and is more toxic as compare to crude oil. Gasoline is amenable to biodegradation; however the use of dispersants is not suitable unless the vapors pose an important human health or safety hazard.

Kerosene:

However it is use as an illuminant has greatly diminished, kerosene is still employed extensively all through the world in cooking and space heating and is the primary fuel for modern jet engines. Whenever burned as a domestic fuel, kerosene must generate a flame free of smoke and odor. Standard laboratory processes test these properties via burning the oil in special lamps. All kerosene fuels should satisfy minimum flash-point specifications (49 °C, or 120 °F) to limit fire hazards in the storage and handling.

Diesel oils:

The principal end utilization of gas oil is as diesel fuel for powering automobile, bus, truck and railway engines. In a diesel engine, combustion is induced via the heat of compression of the air in the cylinder under compression. Detonation that leads to injurious knocking in a gasoline engine is a need for the diesel engine. A good diesel fuel begins to burn at some locations in the cylinder after the fuel is injected. Once the flame has initiated, any more fuel entering the cylinder ignites at once.

Straight-chain hydrocarbons formed the best diesel fuels. In order to encompass a standard reference scale, the oil is matched against blends of cetane (normal hexadecane) and alpha methylnaphthalene, the latter of which provides very poor engine performance. The high-quality diesel fuels have cetane ratings of around 50, giving the similar combustion characteristics as a 50-50 mixture of the standard fuels. The large, slower engines in ships and stationary power plants can tolerate even heavier diesel oils. The more viscous marine diesel oils are heated to permit simple pumping and to provide the right viscosity at the fuel injectors for good combustion.

Fuel oils:

Furnace oil comprises largely of residues from crude oil refining. These are blended by other appropriate gas oil fractions in order to accomplish the viscosity needed for convenient handling. As a residue product, fuel oil is the only refined product of important quantity that commands a market price lower than the cost of crude oil.

As the sulphur contained in the crude oil is concentrated in the residue material, fuel oil sulphur levels are naturally high. The sulphur level is not critical to the combustion process as long as the flue gases don't impinge on cool surfaces (that could lead to corrosion via the condensation of acidic sulphur trioxide). Though, in order to decrease air pollution, most of the industrialized countries now limit the sulphur content of fuel oils. Such regulation has led to the construction of residual desulphurization units or cokers in refineries that produce such fuels.

Residual fuels might have large quantities of heavy metals like nickel and vanadium; these produce ash on burning and can foul burner systems. These contaminants are not simply eliminated and generally lead to lower market prices for fuel oils having high metal contents.

Lubricating oils:

At one time, the suitability of petroleum fractions for utilization as lubricants depended completely on the crude oils from which they were derived. Those from the Pennsylvania crude, that were largely paraffinic in nature, were recognized as having superior properties. However, with the advent of solvent extraction and hydro-cracking, the choice of raw materials has been significantly extended.

Gear oils and greases:

In gear lubrication, the oil separates metal surfaces, decreasing friction and wear. Extreme pressures build up in some gears, and special additives should be used to prevent the seizing of the metal surfaces. Such oils contain sulphur compounds that form the resistant film on the surfaces, preventing real metal-to-metal contact.

Greases are the lubricating oils to which thickening agents are added. Soaps of aluminum, lithium, calcium and sodium are generally utilized, whereas non-soap thickeners like carbon, silica and polyethylene as well are used for special purposes.

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