CRACKING
This is the process of breaking down complex long chained molecules into simpler short chained ones. In the oil industry this typically results in the breakdown of organic compounds such as kerogens or heavy hydrocarbons into smaller lighter hydrocarbon molecules. The process usually involves the breaking of the carbon bonds of the longer molecule.
The cracking process breaks down complex alkanes into short chained alkanes and alkenes.
Cracking as a chemical process involves the splitting of molecules under the influence of heat, catalysts and solvents.
Brief history
1891- Invented by Vladimir Shukov “The Shukov Process”
1908- Modified by William Merriam Burton
1912- “The Burton Process” invented by William Burton
1921- More advanced thermal cracking process developed by C.P. Dubbs; “The dubs Process”
1940- Rise of the popularity of catalytic cracking
Applications
Primarily cracking is used to generate lighter products/fractions from the crude. These fractions include LPG and gasoline, which in today’s economy are highly prized. These also are very financially lucrative. The FCC specifically produces gasoline and LPG. The hydrocracking process produces jet fuel, diesel, naptha and LPG.
Thermal cracking is used to up the quality of very heavy fractions or to produce light fractions or distillates.
Hydrocracking
A catalytic cracking process that is assisted by the presence of an elevated partial pressure of hydrogen gas. The function of the hydrogen is to purify the hydrocarbon stream from sulphur and nitrogen hetero-atoms.
The process produces saturated hydrocarbons depending on the reaction conditions they may range from ethane, LPG to heavier hydrocarbons, mostly isoparaffins.
The hydrocracking catalyst is bi-functional capable of re-arranging and breaking hydrocarbon chains as well as adding to produce alkanes and napthenes.
Major products include, diesel and jet fuel, high octane fuels and LPG are produced. They are supposed to have low sulphur content.
Steam Cracking
Saturated hydrocarbons are broken down into smaller unsaturated hydrocarbons. Principle means of producing lighter alkenes, eg. ethene and propene.
In this process gaseous or liquid hydrocarbons feed like naptha, LPG or ethane is diluted with steam and heated without oxygen present. Usually this is done briefly and at high temperatures.
To produce the products –
- The feed composition
- The hydrocarbon to steam ratio
- The temperature
- Residence time
All have to be considered.
Light hydrocarbon feeds produce rich lighter alkenes.
Heavier hydrocarbon feed produce some rich lighter alkenes, and also gives rich aromatics and those that can be included in gasoline and fuel oil.
The process often results in the generation of coke (carbon compound) on the reactor vessel walls. As such with time the reactor efficiency decreases. After a run time of a few months it must be de-coked. Steam is passed through and this converts the hard solid carbon into carbon monoxide and dioxide.
The Chemistry
Thermal cracking is homolytic, that is bond breaking is symmetrical and the thus pairs of free radicals are formed.
Catalytic cracking uses the presence of acid catalysts to promote heterolytic asymmetrical bond breaking. Ions of opposite charges are produced a carbo-cation and unstable hydride anion.
Thermal Cracking
Modern high pressure cracking operates at 7,000kPa. In this case ‘light’ hydrogen rich products are formed at the expense of heavier molecules which condense and are depleted of hydrogen. This is hemolytic fission and produces alkenes.
There are 5 main types of reactions that take place during the cracking process.
- Initiation
- Hydrogen Abstraction
- Radical Decomposition
- Radical Addition
- Termination
Initiation
C2H6 => 2CH3*
A single molecule is broken to form 2 free radicals. Only a small amount of the feed undergoes this reaction. However this is necessary to produce the free radicals that drive the other reactions. In using steam cracking the carbon to carbon (C-C) bond is broken and not the carbon to hydrogen (C-H) bond.
Hydrogen Abstraction
CH3* + C2H6 => CH4 + C2H5*
A free radical removes the hydrogen from another larger molecule thus the 2nd molecule becomes a free radical.
Radical Decomposition
C2H5* à C2H4 +H*
The radical decomposes /breaks down to form 2 molecules 1 – an alkene and 2 – the other free radical.
Radical Addition
C2H5* + C2H4 => C4H9*
The opposite of decomposition a radical reacts with as alkene to form a single larger free radical.
Termination
2 free radicals react to produce something that is not a free radical.
– Recombination
CH3* + C2H5* => C3H8
2 free radicals form 1 larger molecule.
– Disproportionation
C2H5* +C2H5* => C2H4+C2H6
1 radical transfers a hydrogen atom to the other giving an alkene and an alkane.
Thermal cracking is dominated by entropy. The high temperatures and the fragmentation of the large molecule into several smaller pieces, contributes to the higher entropy than enthalpy.
FLUID CATALYTIC CRACKING UNIT (FCCU)
Fluid catalytic cracking is used to convert high boiling, high molecular weight hydrocarbons to more gasoline, olefinic gases and other products. This process produces gasoline with a higher octane rating.
The FCC feedstock is the fraction of crude that has a boiling point of 340°C or higher at atmospheric pressure. The average molecular weight should be about 200-600 OR higher. This portion is known as heavy gas oil or vacuum gas oil.
The FCC process contacts the heavy long chained molecules with the fluidized powdered catalyst in the presence of high temperatures and moderate pressure. This breaks the long chained molecules into shorter ones. Refineries use this process to generate gasoline to make up for demand.
There are two types;
Stacked – where the reactor and the catalyst regenerator are located in a single vessel with the reactor above the catalyst regenerator.
Side by Side – where the reactor and the regenerator are two separate vessels.
Reactor and Regenerator
They are at the heart of the FCCU operation. The pre-heated feed stock enters at 315°C to 400°C and is combined with recycled slurry oil and is then injected into the catalyst riser. Within the riser it is vapourized and cracked into smaller molecules of vapour by contact and mixing with the powdered catalyst from the regenerator.
The entire reaction takes place within the riser in a 2-4 second period. Within the reactor the vapours are separated from the spent catalyst, by flowing through cyclones. The spent catalyst flows downward through the steam stripping section to remove excess hydrocarbons before being returned to the catalyst regenerator.
The catalytic process produces cake which lowers the catalysts ability to work. Injecting hot air helps to regenerate the catalyst.
Distillation Column
Reactor product flow from the top of the reactor to the bottom section of the distillation column where it is distilled into the FCC end products of cracked naptha, fuel oil and off gas. Further processing for removal of sulphur, the cracked naptha becomes a high octane component of the refinery’s gasoline blend.
Slurry oil, usually the heavy high boiling point fluids settle back down to the bottom of the column. It usually contains catalyst particles. This is usually recombined with incoming feedstock oil, which helps to recycle the catalyst particles.
Regenerator Flue Gas
The gas leaving the flue is allowed to pass through an expander; this may provide power to operate another piece of machinery for instance, the combustion air compressor. The flue gas can pass through a motor-generator combination. If the flue gas does not provide enough energy then the motor steps in. if the flue gas provides too much the generator steps in and exports to the refinery grid.
The expanded gas is routed to a steam generating boiler where the carbon monoxide is burnt as a fuel to provide steam. An electrostatic precipitator is used to remove particulate matter.
