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Cracking

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Cracking petroleum fractions involves heat, pressure and time. Thermal cracking exposes a fraction to high temperature and pressure that causes some of the atoms to split off and form different molecules. Catalytic cracking has the same outcome only it depends on the presence of substance called catalysts, which speed up the reactions. Both of these processes require precise control of the temperature and pressure.

Cracking is done by passing heavy oil fractions which contain big alkane molecules through a bed of powdered catalyst heated to a high temperature. Modern cracking uses zeolites as the catalyst. These are complex aluminosilicates. The large alkane molecules break up over the catalyst to form a mixture of small molecules.

Definition

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To meet the demand for petrol, ethene and hydrogen, big alkane molecules are broken up into smaller molecules in a process called "cracking".

Cracking is one of the most important reactions in the petroleum industry. It starts with alkanes that have large molecules and are too big to use in petrol - for example alkanes from the fuel oil fraction. These large molecules are broken down to give alkanes with shorter chains that can be used in petrol. Most of the cracking carried out to produce petrol is done by heating heavy oils in the presence of a catalyst called catalytic cracking.

Cracking essentially involves a chemical cleavage reaction, in which a saturated alphatic hydrocarbon molecule splits into one paraffinic and one olefinic molecule, this is the primary cracking reaction.

Catalytic Cracking

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In 1936 a new cracking process opened the way to higher octane gasoline; this process was catalytic cracking. Catalytic cracking is a conversion process that can be applied to a variety of feedstocks ranging from a gas oil to heavy oil.

Heating in the presence of catalyst not only accelerates the reaction rate of carbon to hydrogen bond breaking, but also causes carbon-carbon bond fracture. This process is not a good source of petrochemicals but is important for gasoline. The catalysts are called zeolites, molecular sieves on an aluminoslicate matrix and are acidic catalysts. Reaction products different from those produced by thermal cracking are formed, such as aromatic and branched chain molecules. The temperature is 450-550oC. An example of this is the cracking of gas oil (C15-C25) to "amylenes" a mixture of 2-methyl-1-butene and 2-methyl-2-butene. 

The schematic flowsheets of catalytic cracking processes is shown below. 

Catalytic Cracking

The cracking of hydrocarbons in the petroleum industry is catalyzed by a mixture of silica and alumina.

C8H18(g) $\overset{Al_{2}O_{3}/ SiO_{2}}{\rightarrow}$ C4H10(g) + C4H8(g)

The importance of cracking is that it converts high boiling point fractions obtained from the fractional distillation of crude oil into more valuable low boiling point fractions which are used as fuel for motor vehicles and aircraft.

Methodologies

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Three methods of cracking are
  1. Thermal cracking
  2. Catalytic cracking
  3. Hydrocracking

1. Thermal cracking


Thermal cracking was the primary process available to convert low value feed stocks into lighter products. Thermal cracking is a function of temperature and time. The reaction occurs when hydrocarbons in the absence of a catalyst are exposed to high temperatures in the range of 800oF to 1,200oF.
  • The initial step in the chemistry of thermal cracking is the formation of free radicals. They are formed upon splitting the C-C bond.
Thermal Cracking
  • Beta scission produces an olefin (ethylene) and a primary free radical which has two fewer carbon atoms. 

  • The hydrogen extraction produces methane and a secondary or tertiary free radical. 

  • The products will be an alpha-olefin and a primary free radical. 

  • This radical also extract a hydrogen atom from another paraffin to form a secondary from radical and a smaller paraffin. 
Thermal Cracking
One of the drawbacks of thermal cracking in an FCC is that a high percentage of the olefins formed during intermediate reactions polymerize and condense directly to coke.

2. Hydrocracking


Hydrocracking processes are characterized by the fact that in a catalytic operation under relatively high hydrogen pressure a heavy oil fraction is treated to give products of lower molecular weight. Hydrocracking is an extremely versatile process that can be used in many different ways, and one of the advantages of hydrocracking is its ability to break down high-boiling aromatic stocks produced by catalytic cracking or coking.

The ability of refiners to cope with the renewed trend toward distillate production from heavier feed stock with low atomic hydrogen/carbon ratios has created a renewed interest in hydrocracking. Without the required conversion units heavier crude oils produce lower yields of naptha and middle distillate. To maintain current gasoline and middle distillate production levels, additional conversion capacity is required because of the differences between the amount of distillates produced from light crude oil and the amount of distillate products produced from heavier crude oil. 

Hydrocracking product yields from various feed stocks is shown below. 


Cracking Hydrocarbons

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The hydrocarbon molecules are broken up in a fairly random way to produce mixtures of smaller hydrocarbons, some of which have carbon-carbon double bonds.

The reasons for cracking hydrocarbons are as follows.
  • Cracking converts heavy petroleum fractions like fuel oil into more useful fractions like petrol and diesel. One large petroleum molecule is broken into two smaller molecules. The small molecules are for use in petrol and diesel.
C29H42 $\overset{Cracking}{\rightarrow}$ C12H26 + C8H16
  • Cracking is used to produce large amounts of ethane which is used for making plastics, ethanol and other useful chemicals. One possible reaction involving the hydrocarbon C15H32 might be
C15H32 $\overset{Cracking}{\rightarrow}$ 2C2H4 + C3H6 + C8H18
  • Cracking is used to produce hydrogen. Hydrogen is used in the manufacture of ammonia, as a rocket fuel and for use in hydrogen fuel cells. An example is the cracking of hexane C6H14, to give a mixture of alkenes and hydrogen.
C6H14 $\overset{Cracking}{\rightarrow}$ C4H8 + C2H4 + H2