8.3 Hydrotreating

8.3.1 Introduction

• • In hydrotreating, sulphur, nitrogen and metals removal is targeted along with the saturation of olefins and aromatics. Thus the operating conditions of the hydrotreater unit will be the same for the simultaneous removal of S, N & O as H₂S, NH₃, H₂O respectively. These products will be separated in the off gas and the off gas will be further subjected to gas treating.

• The boiling point range of the products is not significantly different from that of the feedstocks.

• From chemistry perspective, inorganic sulphur is converted to H₂S.  Organic sulphur compounds such as mercaptans, sulphides, disulphides, thiophenes are converted to corresponding saturated hydrocarbons along with the generation of H₂S. Similarly, denitrification reactions also take place during hydrotreating in which compounds such as pyrrole, pyridine and quinoline are convered to saturated hydrocarbons. Ammonia is generated as a byproduct in the denitrification reactions.

• The olefins are converted to both cyclic and non-cyclic saturated hydrocarbons.Similarly, aromatic hydrocarbons such as toluene, naphthalene and tetrahydronaphthalene are converted to corresponding cyclic hydrocarbon compounds

• The removal of vanadium and nickel is usually difficult. This is due to the reason that they are held within asphaltene molecules and intense operating conditions (higher T, P and molar ratio of hydrogen to hydrocarbon feed) are required.

8.3.2 Process Conditions

• • Depending upon the feed conditions, the operating conditions are  

     » Temperature: 290 – 430°C

     »  Pressure: 7 – 180 bar

• •
• Higher pressure and temperature are used when heavier feed stocks are used.

• Catalysts: Cobalt, Molbdenum catalysts are used when effective sulphur removal is targeted. Nickel, molybdenum catalysts are used when effective nitrogen removal is targeted.

• The hydrotreating reactions are exothermic and therefore, the reactor product needs to be cooled.