2 OBJECTIVES: Fractional Distillation of Crude Oil Definition and Formation of Crude OilFractional Distillation of Crude OilCracking and Reforming of Crude OilFractionsImpact of the Oil Industry on theEnvironment
3 Definition and Formation of Crude Oil Crude oil is a complex mixture of naturallyoccurring hydrocarbons found within the earth.It consists mainly of a variety of alkanes,cycloalkanes and aromatic hydrocarbons.Crude oil was formed from the remains ofmicroscopic plants and animals that were buriedand preserved in rocks millions of years ago.
4 These remains slowly decayed as they were buried deeper within layers of rocks and with time andtemperature they were converted to crude oil.The composition and appearance of crude oildepends on its source. Most crude oil lookslike thin, brown treacle (molasses), while someare almost colourless volatile(easily evaporated atnormal temp.) liquids and others are thick blackoils.
5 Crude oil must be separated into various components and selectively modified before it can be useful.This process is called refining and occurs in anoil refinery. The primary process for separating thecomponents of crude oil is fractional distillation.
7 The typical fractions of crude oil and their boiling point ranges are shown in the table below. Length of carbonchainBoiling temperaturerange/⁰CRefinery gasC₁ - C₄< 20Gasoline & NaphthaC₅ - C₁₀KeroseneC₁₁ - C₁₈Gas oil (Diesel oil)C₁₈ - C₂₅Residue> C₂₅> 350
8 Percentage of Crude Oil FractionsPercentage of Crude OilUsesRefinery gas1 - 2%Gaseous fuel or it can be liquefied underpressure to produce liquefied petroleum gas (LNG). It can also be used as feedstock for making petrochemicals.Gasoline &Naphtha15 – 30%Gasoline is used as motor fuel for internal combustion engines. Naphtha is used as chemical feedstock for conversion to other products.Kerosene10 – 15%Jet fuel and for domestic heating.Gas oil(diesel oil)15 – 20%As a fuel in diesel engines and industrial furnaces .Residue40 – 50%Fuel oil used is used as fuel for ships and power stations, lubricating oils and waxes are used for motor oil, grease and other lubricants and bitumen is used for road surfacing and roofing material.
9 Cracking Of Crude Oil Fractions Cracking is a process in which large hydrocarbon molecules are broken down into smaller molecules. For example, dodecane could break into decane and ethene: C₁₂H₂₆ C₁₀H₂₂ + C₂H₄ or into nonane and propene: C₁₂H₂₆ C₉H₂₀ + C₃H₆ Cracking is carried out either using heat (thermal cracking) or a catalyst (catalytic cracking).
10 Thermal CrackingThermal cracking involves rapidly heating the hydrocarbon to temperatures of about 800⁰C and then cooling it. This process occurs within a second. The high temperatures can cause the C - C bond to undergo homolytic fission, leaving each carbon with a single unpaired electron. For example, octane could break down into a hexyl radical and an ethyl radical:
11 C₁₂H₂₆ C₁₀H₂₁˙ + C₂H₅ ̇ The raised dot ( ̇) indicates an unpaired electron. These free radicals can then undergo further reactions. For example, they can lose a hydrogen atom to form a stable molecule. The ethyl radical becomes ethene in such a reaction: C₂H₅ ̇ C₂H₄ + H ̇
12 The hydrogen atom, which is a free radical, can combine with the decyl radical to form decane. H˙ + C₁₀H₂₁˙ C₁₀H₂₂ Or two hydrogen atoms can combine together to form hydrogen gas: H ̇ + H ̇ H₂ Thermal cracking is generally used for cracking the residue fraction of crude oil.
13 Catalytic CrackingCatalytic cracking (cat-cracking) involves the use of a catalyst at lower temperatures to break the bonds of the hydrocarbon molecules. The reaction is usually catalyzed by a powdered mixture of alumina and silica (Al₂O₃/SiO₂) at about 500⁰C. The C-C bond undergoes heterolytic fission which results in a mechanism involving carbocations.
14 Catalytic cracking is used to crack the distilled fractions such as diesel oil and kerosene into smaller molecules. The cracked compounds are important to produce more valuable fuel as well a chemical feedstock for the petrochemical industry.
15 Past Paper Question1.) a). Briefly describe the principles involved in i). fractional distillation (3 marks) ii). cracking. (2 marks) b). Give a balanced equation to illustrate cracking. (1 mark) c). Explain the importance of EITHER fractional distillation OR cracking. (1 mark) d). Suggest ONE adverse effect associated with the extraction of crude oil and comment on the environmental problems that result. (3 marks)