Why fractions from crude oil are cracked

That leaves the carbon atom with a positive charge. Ions like this are called carbonium ions or carbocations. Reorganization of these leads to the various products of the reaction. Thermal cracking gives mixtures of products containing high proportions of hydrocarbons with double bonds - alkenes. Thermal cracking does not go via ionic intermediates like catalytic cracking.

Instead, carbon-carbon bonds are broken so that each carbon atom ends up with a single electron. In other words, free radicals are formed. 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. Whilst the fundamentals of the process have not changed in recent decades, improvements continue to be made to the energy efficiency of the furnace, ensuring that the cost of production is continually reduced. A catalyst allows lower reaction temperatures to be used. In fluidised catalytic cracking, the feedstock is gas oil which is vaporised and passed through a zeolite, produced as a fine powder Unit 2 , heated to about K in the reactor.

It is so fine that it behaves like a fluid and continuously flows out of the furnace with the cracking products. The temperature, residence time and the catalyst determine the product proportions.

After cracking, the catalyst is separated from the products, regenerated by burning off deposited carbon in air K , and subsequently recycled. Figure 5 A catalytic cracker as used to produce alkenes from gas oil. The relative proportions of the products, as noted above, can be altered by changing the catalyst and temperature.

One of several zeolites can be used. For example, if the chosen zeolite contains ZSM-5, the propene yield is increased. A variant of the process is known as hydrocracking. The cracking is carried out with hydrogen at a pressure of 80 atm and a catalyst of finely divided platinum on silica or alumina.

Because excess hydrogen is present no alkenes are formed, and high proportions of branched alkanes, cycloalkanes and aromatics are produced which are essential in the formulation of high grade 'green' petrol. The hydrogen also decreases the tendency for the hydrocarbons to form finely divided carbon on the catalyst surface.

The reaction products are separated by fractionation. Hydrocracking is also used to crack heavy gas oils which have over 20 carbon atoms in the hydrocarbon molecule to shorter chain molecules similar to those in naphtha, which can then be steam cracked to form alkenes. Fractions containing large hydrocarbon molecules are heated to vaporise them. They are then either:. These processes break covalent bonds in the molecules, causing thermal decomposition reactions.

The process of fractional distillation is fairly simple, but is powerful in the way that it separates all the different, complex components of crude oil. First, the crude oil is heated to vapourize it and is fed into the bottom of a distillation tower. The resulting vapour then rises through the vertical column. As the gases rise through the tower, the temperature decreases. As the temperature decreases, certain hydrocarbons begin to condense and run off at different levels. Each fraction that condenses off at a certain level contains hydrocarbon molecules with a similar number of carbon atoms.

After this rough refinement, individual fuels may undergo more refinement to remove any contaminants or undesirable substances, or to improve the quality of the fuel through cracking. There are several ways of classifying the useful fractions that are distilled from crude oil. One general way is by dividing into three categories: light, middle, and heavy fractions.

Heavier components condense at higher temperatures and are removed at the bottom of the column.