Fractional Distillation and Cracking

Transcription

1 Fractional Distillation and Cracking N Goalby Chemrevise.org Fractional Distillation Petroleum is a mixture consisting mainly of alkane hydrocarbons 20 C fuel gas (bottled) 40 C petrol/gasoline Crude oil Different components (fractions) of this mixture can be drawn off at different levels in a fractionating column because of the temperature gradient. Furnace 350 C 110 C 180 C 250 C 300 C 340 C naptha (chemicals) kerosene (jet fuel) diesel oil fuel oil (ship boilers) lubricating oils bitumen 1

2 Key points to remember about fractional distillation Oil is pre-heated then passed into column. The fractions condense at different heights The temperature of column decreases upwards The separation depends on boiling point. Boiling point depends on size of molecules. Similar molecules (size, bp, mass) condense together Small molecules condense at the top at lower temperatures and big molecules condense at the bottom at higher temperatures. Vacuum Distillation fuel gas (bottled) Vacuum distillation unit Lowering the pressure over a liquid will lower its boiling point. Heavy residues from the fractionating column are distilled again under a vacuum. Crude oil This means that the heavier fractions can be further separated without high temperatures which would break them down. Furnace petrol/gasoline naptha (chemicals) kerosene (jet fuel) diesel oil fuel oil lubricating oils Click here for animation of distillation Furnace bitumen 2

3 CRACKING Involves the breaking of C-C bonds in alkanes Economics The demand for some fractions exceeds supply from crude oil distillation Alkenes (C2H4) and smaller alkanes have a high demand but relatively low abundance The larger alkanes (C10H22) have a low demand but high abundance Cracking turns the larger hydrocarbons into higher value and more useful smaller hydocarbons cracking also saves having to extract more crude oil and so preserves crude oil stocks. Ethene is used to make polymers and ethanol Smaller alkanes (octane) are used for petrol or fuels Conditions: HIGH PRESSURE kpa HIGH TEMPERATURE C to 900 C THERMAL CRACKING Bonds can be broken anywhere in the molecule by C-C bond fission and C-H bond fission. Example Equations C8H18 C6H14 + C2H4 C12H26 C10H22 + C2H4 PRODUCES MOSTLY ALKENES... e.g. ETHENE for making polymers and ethanol PRODUCES HYDROGEN... used in the Haber Process and in margarine manufacture 3

4 CATALYTIC CRACKING Conditions: SLIGHT PRESSURE HIGH TEMPERATURE C ZEOLITE CATALYST ZEOLITES are crystalline aluminosilicates; clay like substances PRODUCES BRANCHED, and CYCLIC ALKANES, and AROMATIC HYDROCARBONS USED FOR PRODUCING MOTOR FUELS Branched and cyclic hydrocarbons burn more cleanly and are used to give fuels a higher octane number Cheaper than thermal cracking because it saves energy as lower temperatures and pressures are used Click here for animation of a catalytic cracker Cracking Cracking involves breaking up large alkane molecules into smaller alkanes and alkenes. A mixture of products is formed. ceramic fibre soaked in paraffin Catalyst of aluminium oxide Al 2 O 3, and a high temperature needed. mixture of gaseous products Bunsen valve (prevents suck back aluminium oxide granules The gaseous alkenes formed will react with bromine water. 4

5 Examination Question 1. The petroleum and petrochemicals industries use a small number of processes to produce a large number of final products. Three of these processes are fractional distillation, vacuum distillation and cracking. (a) Describe briefly how fractional distillation can be used to convert crude oil into a small number of fractions. (4) (b) The residue from the fractional distillation process is usually vacuum distilled. This process allows the compounds in the residue to boil at temperatures much lower than their normal boiling points. State whether the sizes of the molecules of the compounds in the residue are smaller or larger than those in the other fractions and suggest why the process of vacuum distillation is used. (2) Examination Question (c)cracking involves free radical reactions. It is carried out by heating long-chain hydrocarbons, often in the presence of steam and a catalyst. Steam cracking is used to produce a mixture of alkanes and alkenes in reactions such as the following: C9H20 C6H14 + C3H6 (i) Give a name for each compound produced in this reaction. (2) (ii)nonane, C9H20, can be cracked to give other products. One molecule of nonane can give two molecules of ethene, one molecule of ethane and one molecule of another substance. What is the molecular formula of the other substance? (1) 5

6 Examination Question 2. The petrochemicals industry produces a vast range of organic compounds derived from crude oil. Some of the most important processes involve simple molecules containing two carbon atoms. (a) Give the name of the first stage in the refining of crude oil and state why this process is used. (2) (b)ethene can be produced by cracking alkanes. State two conditions used in cracking processes and write an equation for a reaction in which ethene is formed from butane. (3) 3. Petrol is obtained, not only by fractional distillation of crude oil, but also by cracking of hydrocarbons from heavy fractions. State why hydrocarbons from heavy fractions are cracked and explain why these hydrocarbons are less easy to ignite than those in petrol. (4) 6