Fractional Distillation of Crude Oil Industrial Cracking Thermal Cracking Catalytic Cracking By Andrew Ingham

Fractional Distillation, what is it? Fractional distillation is the breaking down of a mixture into it’s component parts, these are known as FRACTIONS. FRACTIONS are identified by their boiling points and chain length, and are separated into their fractions by heating them past the boiling points. The main use of fractional distillation is to break down crude oil into it’s fractions.

What are these fractions? There are 8 fractions that are produced when crude oil is distilled. These separate when the temperature reaches their respective boiling points. Name Approx boiling temperature (C) Uses Petrol 40 Vehicle Fuel Naptha 180 Industrial cleaners and solvents Kerosene (paraffin) 200 Used as a gas for some heaters Diesel Oil 250 Vehicles e.g. lorries Lubricating Oil 280 Reduce friction between multiple parts Fuel Oil 310 The fuel used in ships and power stations Greases and Wax 340 Used to make things like candles Bitumen 367 Used for road tar and roofing

How is it done? The shorter the chain, the lower Fractional distillation is done in a series of 4 main stages. Crude oil is passed over a high temperature heater/furnace and then added to the bottom of the distillation tower. The crude oil is heated to a point where it’s fractions begin to evaporate and these gases rise up through the tower, with the temperature gradually cooling as you get higher up the tower. When a fractions vapour rises to a temperature lower than it’s boiling point, condensation takes place. The vapour condenses and collects in the tray at the point of condensation. These tray’s are drained off and the products are attained. The shorter the chain, the lower the boiling point, so condensation happens higher up the tower

Why do we need to distil crude oil? In it’s raw form crude oil is not very useful. Crude oil is mainly made up of alkanes, both branched and unbranched. These are useful, however crude oil also contains small amounts of other compounds which are dissolved into it coming from the animals and plants that the oil was formed from. An example of one of these compounds is sulphur. When burned, sulphur dioxide is formed, which is one of the problems leading to acid rain. By distilling the oil, you can separate it into it’s useful fractions.

Industrial Cracking Fractional distillation is useful, however it doesn’t always meet the industries demand and by using industrial cracking you can make shorter chains with a higher economic value from longer chains that are in much higher supply. Fractions like Naptha are in much higher demand than fractions of bitumen. The general idea is that the shorter chain hydrocarbons are the ones in high demand. Industrial cracking is the process of taking a large chain fraction and breaking (cracking) it into smaller, more useful chains. There are two main methods of doing this, thermal and catalytic cracking.

Thermal Cracking Thermal cracking is the process of breaking down large hydrocarbons into smaller ones by using high temperatures and pressure. Temperatures between 700-1200 K are needed and a pressure of around 7000kPa. These conditions cause the carbon-carbon bonds to break so that 1 electron from the covalent bond goes to each carbon atom. So these new chains end with a carbon atom and 1 free electron. However as there are not enough hydrogen's to make 2 alkanes, one of the chains has a double bond and becomes an alkene. . C H R - C H R - . C H R - Thermal Cracking C H R - C H R = +

Catalytic Cracking Catalytic cracking is the other method of industrial cracking. It is conducted in a similar way to thermal cracking just with alternative conditions. This method requires less heat than thermal cracking, around 720K, less pressure and uses a zeolite catalyst. This catalyst often consists of silicon dioxide and aluminium oxide. These catalysts have a honeycomb structure and have a huge surface area. In both catalytic and thermal cracking the products attained are separated by using fractional distillation.