Oil and gas production handbook An introduction to oil and gas production

Figure 21. Catalytic reformer (
Source: Wikipedia
) 
 

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83 
 
Distillate hydrotreater units desulfurize distillates (such as diesel) after 
fractional distillation, in the same way as the naphtha hydrotreater. 
 
Fluid catalytic crackers (FCC) units upgrade heavier fractions into lighter, 
more valuable products. Long chain molecules (high carbon numbers) are 
split into shorter molecules to achieve more of the high value fuel 
components. A typical design uses a reactor and a regenerator. A fine 
powdered porous catalyst with zeolite (silicate and alumina) is fluidized in the  
hydrocarbon vapor, where a reaction takes place at 535 °C and 0.172 MPa. 
The catalytic reaction takes place within a few seconds, after which the 
reformate and catalyst is separated in a cyclone. The spent catalyst then 
goes back to a regenerator that heats it to 715 °C at 0.241 MPa and 
releases flue gas. The catalyst powder can then be reused. The reformates 
go to a distillation column for separation into fractions. 
 
A hydrocracker unit performs essentially the same function as the FCC when 
more  saturated hydrocarbons are desirable in the product. This means 
alkane carbon chains with single bonds, not double bonds or cyclic rings like 
aromatics, or more complex molecules. For this, additional hydrogen is 
needed. The reaction takes place with hydrogen under pressure over a 
catalyst. The relative market need for diesel, kerosene and gasoline will 
influence the choice of FCC versus hydrocracker. In the US, with a higher 
relative volume of gasoline, more FCC capacity is needed, while in Europe 
and Asia, with higher diesel consumption, more hydrocracking is used. 
 
Visbreaking units upgrade heavy residual oils by thermally cracking them 
into lower viscosity product that can be blended into lighter, more valuable 
products. Visbreaking is characterized by its thermal severity, ranging from 
mild cracking at 425 °C to severe cracking at 500 °C. Depending on the 
residual oil, as much as 15-25% lighter fractions like diesel, kerosene and 
gasoline could be obtained. The residue is tar and coke. 
 
The  Merox unit treats LPG, kerosene or jet fuel by oxidizing thiols 
(mercaptans) to organic disulfides. The purpose is to reduce strong odors 
caused by thiol presence.  
 
Coking units (delayed coking, fluid coker, and flexicoker), like the visbreaker, 
uses thermal cracking of very heavy residual oils into gasoline and diesel 
fuel. The residue is green coke, and is further processed to fuel coke or, if 
too low in sulfur and contaminants, to anode coke for the metallurgic 
industries. 
 
An  alkylation unit produces high-octane components for gasoline blending. 
The main use is to convert isobutane (C
4
H
10,
 but arranged differently than n-

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84 
 
butane) to alkylates, mainly isooctane or isoheptane by adding an alkyl 
group such as propene or butene over a strong acid catalyst, such as 
sulfuric or hydrofluoric acid.  
 
Dimerization is similar to alkylation, but uses a dimer group instead of an 
alkyl group. For example, butenes can be dimerized into isooctene, which 
may be hydrogenated to form isooctane. 
 
Isomerization units convert linear molecules to higher-octane branched 
molecules by rearranging the same atoms arranged in a different way. For 
example, C
4
H
10 
n-butane has the carbon atoms in a chain, while isobutane 
has a central carbon with one hydrogen and three CH
3
 groups attached. The 
isobutane can then be fed to the alkylation unit.  
 
Steam reforming produces hydrogen for the hydrotreaters or hydrocracker. 
Typical is the steam methane reformer (SMR), where steam reacts with 
Methane at 425 °C with a nickel catalyst to produce syngas, which is a 
source for many different reactions: 
CH
4
 + H
2
O 
↔ CO + 3 H
2 
If more hydrogen is needed, followed by a gas shift reaction with CO: 
CO + H
2
O 
↔ CO
2
 + H
2
 
Amine gas treater,  Claus unit, and tail gas treatment converts hydrogen 
sulfide from hydrodesulfurization into elemental sulfur, which is a valuable 
traded product.