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Cost effective management of a High-Vacuum Unit revamp
High-vacuum unit (HVU) revamp can have significant impact on vacuum gasoil yield improvement, according to Alie Hoksberg – Licensing Technology Manager, Shell Global Solutions International B.V.
About the technology
Cost effective management of a High-Vacuum Unit revamp
There are numerous options, so choosing the right HVU makes a big difference. Vacuum distillation revamp is one of the key opportunities, according to the Shell pentagon model of options for short-term revamps, during times of constrained capex. Shell’s deep-flash high vacuum unit is considered to be at the leading edge, enabling significant diesel yield improvement.
Vacuum units take the 350+ fraction out of crude oil and distil it into a light vacuum gas oil for diesel purposes; it might make a medium and a heavy vacuum gasoil or they might be combined. They can feed into thermal crackers, cat feed hydrotreaters, or into a hydrocracker. The vacuum residue can go off to delayed okers, residue hydroconversion units or Visbreakers: ie a whole range of options. All products have their own limitations, and we will see just how we manage some of those.
Basically, a vacuum distillation unit consists of a vacuum system, condensation and separation sections to produce diesel and VGO fractions, a vacuum residue (VR) in the bottom of the column, a VR de-entrainment section to keep the bottom of the column from coking and the furnace. Traditionally ie 15-20 years ago, the furnace outlet temperature was limited to 395-400oC because of coking issues. Every year or two the unit would have to be shut down, steam-air decoked and the furnace cleaned out.
Since that time, Shell has developed a technology called suppressed vaporisation that controls the flow regime in a furnace. The liquid coats the walls continuously throughout the furnace and there is vapour through the middle; completely avoiding the mist flow region where the droplets can hit the very hot walls and form coke. Consequently, the units can operate for up to five-year cycles problem-free and at much higher temperatures of 435oC.