Cracking and Reforming

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In today’s class, we will be talking about cracking and reforming. Enjoy the class!

CRACKING AND REFORMING

CRACKING:

Cracking is the process used in breaking down large hydrocarbon molecules into two or more smaller hydrocarbon molecules. This is the method used in increasing the quantity of petrol. The fraction from which petrol is produced (C₆ – C₁₂) is small compare with other fractions with a greater number of carbon atoms. The petroleum refineries find it difficult to cope with large demands of petrol from users, while on the other they are left with a large surplus of the less volatile fractions like kerosene and diesel oil. They are therefore been forced to think of a method of converting these less volatile fractions into petrol. This method is known as CRACKING.

There are two types of cracking in use in the petroleum industry:

1. THERMAL CRACKING: This involves vaporizing the oil fractions of the long carbon chain (C₁₂ – C₁₈) and heating them for a short time to the temperature around 600⁰C under very high pressure of about 300atoms.
2. CATALYTIC CRACKING: The long-chain hydrocarbons are heated in the presence of a silica-alumina or zeolite catalyst. The catalyst speeds up the process which requires less energy. The pressure needed is lower and high grades of petrol are produced by increasing the octane number of petrol. The temperature is still about 500⁰C. This catalytic cracking is more widely used. Catalytic cracking is better because:

(i) The process is more controllable; i.e. the conditions can be adjusted such that desirable products of certain chain lengths are obtained. This process thus yields a source of alkenes which serve as raw materials for a great variety of organic chemicals.

(ii) The process does not only yield more petrol but also gives petrol high quality. This petrol is a higher grade of petrol than the one obtained directly from the petrol fractions during the distillation of crude oil.

C₁₆H₃₄

$\underset{}{\to }$

C₈H₁₈  +   C₈H₁₆.

CH₃(CH₂)₈CH₃

$\underset{}{\to }$

CH₃C (CH₃)₂CH₂CH(CH ₃)CH₃  +  C₂H₄

The overall benefits of the cracking process are:

(i) It increases the yield of petroleum

(ii) It provides a petrol mixture rich in branched-chain hydrocarbons with an attendant increase in octane number.

(iii) It yields as a by-product, a large quantity of ethane, propane, butane etc. used for making plastics, synthetic rubber, detergent and many important chemicals like ethanol and phenol.

 

REFORMING:

This is the process used in converting long-chain hydrocarbons to shorter and branched-chain molecules to improve its anti-knock properties. The process usually takes place in the presence of catalysts such as oxides of silicon and aluminium at about 600⁰C and pressure between 8 and 15 atm to increase its octane number and to produce high-grade petrol.

Cracking is a breaking down process while reforming is an isomerisation process (i.e. changing a compound into its isomers)

OCTANE NUMBER

The octane number of octane rating of petrol is a mixture of the proportion of branched-chain hydrocarbons to the straight-chain hydrocarbons in a given blend of gasoline (petrol).

Gasoline is composed of C7 – C9 hydrocarbons i.e. heptane, octane and nonane. These hydrocarbons are present in their straight-chain or branched-chain isomers. It has been shown that straight-chain hydrocarbons (e.g. n-heptane) burn too rapidly in the car engine thus, causing irregular motion of the pistons which result in rattling noise. The rattling noise is known as ‘KNOCKING’.

Petrol containing a higher percentage of straight-chain hydrocarbons causes more knocking that petrol containing a higher percentage of branched-chain hydrocarbons

A straight-chain alkane like heptanes is assigned an octane number of O while a highly branched-chain alkane like 2,2,4 trimethylpentane is assigned an octane number of 100 which burns very smoothly in engines. Therefore the quality of any petrol is rated according to its octane number, i.e. the percentage of heptanes to 2,2,4 – trimethylpentane in a mixture.

CH₃ – CH ₂– CH ₂– CH ₂– CH ₂– CH₂ – CH₃

Heptane                   octane number = 0

Octane number is a measure of the performance of the fuel in engines and the rating is given as the percentage of iso-octane (2,2,4 – trimethyl pentane) to straight-chain hydrocarbon present. E.g. gasoline with an octane rating of 94 is understood to contain 94% iso-octane and 6% straight-chain heptane. Similarly, a fuel with an octane number of 50 has a performance equivalent to 50 – 50 mixture of heptane and 2,2,4 – trimethylpentane.

When low-grade petrol is used in some automobile engines, there is a tendency for the engine to knock. The difference in the grade of petrol is, therefore, a difference in their octane numbers. The motor car engines are known as petrol engines.

GRADE OF PETROL

Petrol can be graded as:

(a) Super or extra               (b) Regular or Ordinary

Petrol which is graded as super or extra has an octane number closer to 100 than petrol that is graded as regular or ordinary.

The octane number of some hydrocarbons.

Straight chain Hydrocarbon	Relative molecular mass	Octane number
Propane	44	100
Butane	58	92
Pentane	72	61
Hexane	86	25
Heptanes	100	0
Octane	144	-27
Nonane	128	-45

The octane number of straight-chain hydrocarbons is related to their molecular mass, i.e. the lower the molecular mass of the hydrocarbon, the higher the octane number. Some fuels which are superior to 2,2,4 – trimethylpentane have an octane number greater than 100.

 SYNTHETIC PETROL

Synthetic Petrol is made from materials such as coal, coke and hydrogen which do not occur in

crude petroleum. Synthetic petroleum can be gotten from two sources:

1. From Coal: When powdered coal is heated with hydrogen in the presence of Fe or Sn as the catalyst at 500⁰C and 20 atmospheric pressure, it is converted into an oily mixture of hydrocarbons. The mixture is separated by distillation into a petrol fraction boiling at 200⁰C and heavy oil residue which can be further treated with fresh coal to obtain more petrol.
2. From Coke: When steam is passed over heated coke at 100⁰C, a mixture containing equal volumes of CO and hydrogen known as water gas is obtained.

C + H₂O  →   CO + H₂

Water-gas

The water-gas can be hydrogenated to a mixture of hydrocarbons by adding hydrogen and passing it over finely divided nickel as catalysts at 200⁰C. About half of the product is petrol, the less volatile fraction being used as fuel for diesel engines.

PETROCHEMICALS

Petrochemicals are substances that are manufactured from the by-products of petroleum. These include plastics, synthetic rubber and fibres. Petroleum and natural gas are now used in increasing amounts to produce many inorganic compounds. Examples include ethanol, ethane, propane 1, 2, 3 – triol, benzene and toluene. These small molecules organic compounds are in turn used to make large – molecule organic compounds like plastic, synthetic rubber, insecticides, detergents and synthetic fibres like nylon and Dacron.

PROBLEMS OF PETROLEUM CHEMISTRY

1. The petrol produced is too small to meet the demand of the world
2. The quality of the petrol produced is too low and may result in knocking of the petrol engine

SOLUTION TO THE PROBLEMS

These problems can be solved by increasing the amount of petrol produced through

(a)  Breaking down of larger hydrocarbon molecule into smaller hydrocarbon molecule (Pyrolysis)

C₁₀H₂₂     H₂ + C  C₅H₁₂ + CH₃ – CH = CH – CH₃

But – 2 – ene (isobutene)

(b) By the addition of lead – tetraethyl (known as ‘anti-knock’ compounds) to prevent knocking and thereby increases the octane number.

Note; Cracking, catalytic reforming and uses of additives (tetraethyl lead) are used to improve the yield and quality of petrol.

EVALUATION

1. What are petrochemicals?
2. What is the octane number?
3. List two types of cracking

WEEKEND ASSIGNMENT

1. Oil deposits in Nigeria are (a) on land and offshore (b) only offshore (c) mainly imported (d) downstream
2. Fractional distillation involves the following processes (a) Boiling (b) Boiling and condensation (c) Boiling, evaporation and condensation. (d) Condensation and collection.
3. Which is the odd one out of the following (a) petroleum ether, petroleum gases, kerosene

(b) Gas oil and diesel lubricating oil (c) Petroleum ether and bitumen (d) Haematite and asphalt.

4. Which of the petrol samples are likely to cause knocking? (a) octane (b) 2,2,3,3-tetramethyl butane (c) 2,2,3-trimethyl pentane (d) 2,2,3-trimethyl pentane.
5. Nigeria earns money from (a) petroleum gas and liquids only (b) petroleum liquids and solids only (c) petroleum liquids like petrol and kerosene oil (d) petroleum gases, liquids and solids.

THEORY

1. (a) Distinguish between cracking and reforming. Of what importance are the two processes in the petroleum industry. (b) State the problem associated with oil-producing areas.
2. Consider the following reactions schemes.
3. Petroleum Petroleum Fractions
4. C₁₆H₃₄ C₈H₁₈ + X

(i) State the type of process involved in each of the stages labelled I and II

(ii) Identify X

 

In our next class, we will be talking about Applied Chemistry.  We hope you enjoyed the class.

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