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

Figure 20. Refinery process flow diagram (
Source: Wikipedia
) 
 
Most of these reactions take place at elevated temperature and pressure 
over a catalyst such as platinum or rhenium or sometimes iron, and need 
precise control for optimal throughput. A few process flow diagrams have 
been included to give an indication of the complexity of these processes 
compared to the relative simplicity of many upstream processes.  

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82 
 
 
Atmospheric distillation is the fractional distillation unit already described. In 
actual designs, it is combined with vacuum distillation. Raw crude cannot be 
heated to more than 370-380 °C. It is often called the Crude Oil Distillation 
Unit (CDU) 
 
Vacuum distillation unit (VDU) further distills the black oils into fuel oils and 
residual bitumen and coke to avoid overheating the crude and to extract 
additional valuable product that could be upgraded. 
 
Naphtha hydrotreater: Various sulfur compounds are present in the 
hydrocarbon mixture and, if burnt with the other carbons, will cause sulfuric 
emissions. The hydrotreater uses hydrogen to remove some of these 
compounds.
 
As an example, the hydrodesulfurization (HDS) reaction for 
ethanethiol can be expressed as: 
 
ethanethiol + hydrogen  
→ ethane + hydrogen sulfide
C
2
H
5
SH + H
2
 
 
→ C
2
H
6
 + H
2
S 
 
A catalytic reformer unit is used to convert the naphtha molecules (C5-C12) 
into higher octane reformate (reformer product). These are mixed with raw 
gasoline to achieve a higher octane product. The process creates more 
aromatics (ring formed hydrocarbons) by dehydrocyclization or more 
complex hydrocarbons with double bonds or side groups by 
dehydrogenation. These processes release hydrogen which is recovered 
and can be reused in hydrotreaters or hydrocrackers.