Why cracking is used in the oil industry

By kind permission of BASF. Petrol gasoline contains a mixture of hydrocarbons, with 5 to 10 carbon atoms. The mixture of C 5 -C 10 hydrocarbons obtained directly from the distillation of crude oil contains a high proprtion of straight-chain alkanes. However, if this mixture is used as petrol, it does serious damage to a car's engine. Petrol containing a high proportion of straight chain alkanes tends to ignite in the cylinder of the car engine as the piston increases the pressure and before the cylinder reaches the optimum position.

Ideally, the mixture of petrol vapour and air is ignited with a spark at a predetermined position of the piston in the cylinder. This problem of premature ignition is referred to as pre-ignition and also as engine knock. The term knock is used as pre-ignition can be heard. Severe knock can cause serious engine damage. However, branched-chain alkanes, cycloalkanes and aromatic hydrocarbons are much more resistant to knock and straight-chain alkanes are converted into them in a series of processes in the refinery which are described in this unit.

The resistance of petrol to knock is measured in terms of an octane rating octane number. The higher the number, the less likely is a fuel to pre-ignite. The octane rating is on a scale where heptane is given an arbitary score of 0 and 2,2,4-trimethylpentane iso-octane one of A rating of 95 does not mean that the petrol contains just iso-octane and heptane in these proportions, but that it has the same tendency to knock as this mixture.

The octane rating of petrols usually available for cars range from 95 upwards and contain a mixture of straight-chain, branched, cyclic and aromatic hydrocarbons, produced by the processes described below.

These processes are also used to convert staight-chain hydrocarbons to hydrocarbons which are much more useful to make chemicals which are then used to make a huge range of compounds from polymers to pharmaceuticals.

Cracking, as the name suggests, is a process in which large hydrocarbon molecules are broken down into smaller and more useful ones, for example:. The cracking products, such as ethene, propene, buta-1,3-diene and C 4 alkenes, are used to make many important chemicals. Others such as branched and cyclic alkanes are added to the gasoline fraction obtained from the distillation of crude oil to enhance the octane rating.

Steam cracking plants Figures 1 and 2 use a variety of feedstocks, for example. Very recently a cracking plant has come on stream in Singapore in which crude oil itself is the feedstock, the first time that this has been done. The advantages of this are that it cuts out the expensive distillation processes needed, for example to produce naphtha, and that it produces a wider range of products.

However the disadvantage is that it may not produce the product that is needed in high enough yield. F or example, if you want a high yield of ethene it is better to make it from ethane or naphtha. This disadvantage can be overcome by having more than one plant on the same site. Figure 2 Naphtha is used as the feedstock to the row of furnaces on this steam cracking plant located at Wilton, UK.

Modern steam cracking plants are very large, usually producing million tonnes of products annually and several have been built recently that can have an output of nearly 3 million tonnes a year and cost about 1 billion dollars to build,.

The reactant gases ethane, propane or butane or the liquids naphtha or gas-oil are preheated and vaporised, are mixed with steam and heated to K in a tubular reactor Figure 3. They are converted to low relative molecular mass alkenes plus by-products. The proportion of different products from steam cracking depends essentially on two factors. The composition of the the products depends crucially on the feedstock used.

For example, a much higher proportion of ethene to other products is formed from ethane and propane than from other feedstocks. However, if more RPG raw pyrolysis gasoline , a mixture of C 5 -C 8 hydrocarbons, is needed, one would choose naphtha or gas oil. More details are given in Table 1. The conditions chosen for the furnace temperature and the flow rate of the heated reactants depend on the products that are needed, as shown in Table 2.

It is important to ensure that the feedstock does not crack to form carbon, which is normally formed at this temperature. This is avoided by passing the gaseous feedstock very quickly and at very low pressure through the pipes which run through the furnace. There is however, a problem; if the plant is run at sub-atmospheric pressure, there may be a leak that allows air to enter into the gases and form an explosive mixture.

This is prevented by mixing the feedstock with steam. The steam also acts as a diluent and inhibits carbonisation. This endothermic reaction occurs in less than a second as the hydrocarbon mixture passes through tubes within the radiant section of the cracking furnace.

The products are cooled rapidly quenched to prevent loss via side reactions and separated in a series of processes including compression, absorption, drying, refrigeration, fractionation and selective hydrogenation. Figure 4 A view of the steam cracking unit at Wilton in the north-east of England. The products from steam cracking include a mixture of C1 - C4 hydrocarbons and are separated by fractional distillation.

Some of the columns are: 1 A debutaniser which separates the C4 hydrocarbons from the C1 - C3 hydrocarbons 2 A depropaniser which separates out the C3 hydrocarbons 3 A deethaniser which separates out the C2 hydrocarbons 4 A demethaniser which separates out the methane 5 A C3 splitter which separates propene from propane 6 A C2 splitter which separates ethene from ethane By kind permission of SABIC Europe. A steam cracker is one of the most technically complex and energy intensive plants in the chemical industry.

It has equipment operating from K to K and near vacuum to atm. The slurry oil can be blended with residual fuel oil or further processed in the coker. Carbon is deposited on the catalyst during the cracking process. This carbon, known as catalyst coke, adheres to the catalyst, reducing its ability to crack the oil. The coke on the spent catalyst is burned off, which reheats the catalyst to add heat to the FCC process. Regeneration produces a flue gas that passes through environmental control equipment and then is discharged into the atmosphere.

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Cracking is a critical stage in the process of refining crude oil. Other petroleum products, such as heating oil, diesel fuel, and gasoline, rely on cracking. Following its extraction from a well, crude oil in its raw form contains a blend of large and complex hydrocarbon molecules.

Although the crude oil is valuable even in its raw form, its economic usefulness is relatively limited until it has been subject to additional refining processes. In order to help render the crude oil into a form that can be more widely utilized, the first and foremost stage in the refining process is to break up, or " crack ," the unprocessed hydrocarbon molecules into smaller components. This stage—commonly referred to as "cracking"—makes it possible to turn crude oil into a variety of marketable fuels, lubricants, and other products.

Although the basic concept is the same in all cases, the process of cracking can be implemented in a variety of ways. A common application is what is known as fluid catalytic cracking FCC , which is used in the production of gasoline as well as various distillate fuels.

A single product crack reflects the difference between the prices of one barrel of crude oil and one barrel of a specified product. For example, from crude oil into gasoline. Refiners and investors also implement crack strategies against multiple products.