Also known as: hydrodesulfurization, HTU, HDS unit
The purpose of a hydrotreater unit is primarily to remove sulfur and other contaminants from intermediate streams before blending into a finished refined product or before being fed into another refinery process unit.
Hydrotreaters have become increasingly important as sulfur limits have been lowered in finished products. Also, for some key conversion units such as the reformer, feed must be hydrotreated to keep contaminants from poisoning the conversion catalyst.
Hydrotreaters also saturate aromatics and olefins if operated at high pressure, which is great for diesel quality (raises cetane) but bad for gasoline (reduces octane).
Types of hydrotreaters
It is quite common for a refinery to have multiple hydrotreaters. Some of the more common are:
- Naphtha hydrotreater - Hydrotreats heavy naphtha before it is fed to the reformer
- Kerosene hydrotreater - Hydrotreats kerosene to remove sulfur and to improve smoke point properties for jet fuel
- Distillate hydrotreater - Hydrotreats diesel blendstocks to remove sulfur and nitrogen and raise cetane
- VGO hydrotreater - Hydrotreats VGO before feeding into the FCC. This reduces sulfur, resulting in lower sulfur products (FCC gasoline, cycle oil)
- FCC gasoline hydrotreater - Hydrotreats the FCC gasoline coming out of the FCC, which is often high in sulfur
- Resid hydrotreater - Hydrotreats atmospheric resid or vacuum resid to reduce sulfur, sometimes before sending it to an RCC unit for upgrading
How it works
The hydrocarbon is mixed with hydrogen and heated to 500-750F. The mixture is injected into a reactor vessel filled with a solid metal catalyst (cobalt-molybdenum or nickel-molybdenum).
In the presence of the catalyst and heat, the hydrogen reacts with the hydrocarbon, removing sulfur (to form H2S), removing nitrogen (to form ammonia), and saturating olefins and aromatics with hydrogen.
Typically, there is also a small amount of hydrocarbon cracking to form methane, ethane, propane, and butane.
The operating pressure of any hydrotreating unit is elevated to reduce the amount of coke laydown on the catalysts, which are normally in fixed beds. In general, the heavier the type of feedstock, the higher the operating pressure of the unit.