Hydrogenation is a generic name for treating fuels and lubricants at elevated temperatures, while in the presence of hydrogen and a catalyst.

Hydrotreating (often called hydrofinishing) and hydrocracking both are different names for hydrogenation. Hydrotreating is a non-destructive process, whereas hydrocracking is a destructive process.
This paper is restricted to the subject of conversion of PLO (Processed Lube Oil) from API Gr I to Gr II by means of Hydrotreatment process.

API classification of Gr I and Gr II base oils are as below:
Group Sulphur (%m) Saturates (%m) Viscosity Index
I >0.03 <90 >80
II ≤0.03 >90 80 to 120
(III) (≤0.03) (>90) (>120)

Hydrotreatment removes objectionable materials by selectively reacting these materials with hydrogen in a reactor at high temperatures and pressures in the presence of a catalyst. Hydrotreating removes hetero atoms and saturates carbon-carbon bonds with the result that materials such as Sulphur, nitrogen, oxygen and metals are removed; and olefinic and aromatic bonds are saturated. The process improves the colour, odor, VI and stability.

The hydrotreating process is shown schematically in Figure 1 and is described below.
The three sections of the process are:

  • Reaction Section
  • Steam Stripping Section
  • Vacuum Drying Section

Reaction Section:
The reaction section consists of the following equipment:

  • Feed/effluent exchanger
  • Reactor charge heater
  • Reactor
  • Reactor effluent condenser
  • Product separators
  • Recycle hydrogen compressor
  • Makeup hydrogen compressor
  • Amine scrubber

Steam Stripping Section:
The stripping section consists of:

  • Liquid feed vaporiser
  • Stripping column
  • Top condenser
  • Reflux drum
  • Reflux pump

Vacuum Drying Section:
The drying section consists of:

  • Vacuum column
  • Ejector or vacuum pump
  • Base oil transfer pump

The reactants, namely a mixture of base oil feed and high-pressure hydrogen heated to the desired temperature, enter the top of the reactor containing layers of the catalyst. As the reactants flow downward through the catalyst bed, various exothermic reactions occur and the temperature increases along the catalytic bed. The average process temperature may be reckoned at (1/3 rd of Tinlet + 2/3 rd of Toutlet).

Typical process parameters of the feed entering the reactor are; pressure 25 to 90 bar, temperature 350 to 400°C, depending upon the severity of the process and the properties of the feed.

A feed/reactor effluent exchanger preheats feed before entering the reactor charge heater. This recovers as much heat as possible from the heat of reaction.

The processes involved in hydrotreating are:
HDS: Hydro-desulfurization:

  • Sulphur is objectionable because it leads to catalyst poisoning; more importantly it is the
    primary contributor to air pollution and the cause of corrosion in the engine.
  • Organic sulphur is converted to hydrogen sulphide (H₂S).

HDA: Hydro-dearomatization:

  • The aromatics are the most reactive components in the lube oil; and they consume large
    quantities of hydrogen.
  • Aromatic saturation which takes place within the reactor converts some of the aromatic
    compounds to naphthenes.
  • Removal of aromatics also improves the lubricating quality of the base oil.

HDO: Hydro-deolefining:

  • Olefins decreases the saturation level. Olefines also cause product fouling by the
    formation of gums or insoluble materials. Hydrotreatment converts olefins to more
    stable compounds and increases the saturation level.

HDN: Hydro-denitrogenation:

  • Nitrogen inhibits catalytic activity. Nitrogen compounds reduce the effective surface
    area of the catalyst.
  • Organic nitrogen compounds are converted to ammonia (NH₃)

Hydrotreating employs catalysts which increase the VI.

H₂S and NH₃ formed during the desulphurization and denitrogenation reactions respectively are removed by circulating wash water before the product stream from the reactor enters the condenser in order to prevent salt formation.

A high-pressure separator separates gas from sour water (water containing H₂S + NH₃) and the recycle gas is sent to an amine scrubber to remove most of the H₂S. The hydrogen exiting the amine scrubber is compressed and recycled. A part of the nitrogen exiting the scrubber may have tobe purgedto prevent excessive hydrogen gas pressure build-up.

Efficient removal of H₂S is very important as H₂S reduces the hydrogen partial pressure and suppresses the catalyst activity.

The pressure of the vapour/liquid hydrocarbon mixture exiting the high-pressure separator is reduced in a pressure reducing station and enters a low pressure separator where the liquid hydrocarbon is separated from the gas.

Deactivation and Regeneration of Catalyst:

Depending upon the properties of the feed and operating conditions, the catalyst deactivates due to polymerization reactions and coke formation over the catalyst. The process of deactivation can partially be controlled by increasing the outlet temperature of the process steam. However, after several cycles of operation it needs to be regenerated before it is put to use again and again.

Stripping Section:

The hydrocarbon liquid exiting the reactor section is vaporized before it enters the stripping column. Steam is used as a stripping medium. The steam reduces the partial pressure of light hydrocarbons and lowers the vaporization temperature of the light hydrocarbons, which are condensed and removed from the reflux drum.

Vacuum System:

The bottom product of the stripping column is subjected to vacuum of approx. 100mbar. Water evaporates and the base oil separates out and is pumped out.