Why Vacuum Distillation ?

The reasons for producing vacuum gasoil in the first place are well known, but not always the same:
• Minimise heavy residual fuel (heavy fuel oil)
• Provide components (maybe all) of the feed to a fluid catalytic cracking (FCC) unit or a hydrocracker
• Provide components for lubricating oil production
• Provide specialty cuts for the production of other specialty products (petrolatum, transformer oils, etc.)
Generally speaking, the final requirement is almost always the same – to improve the monetary returns on the heavy portion of the crude oil processed in a refinery – albeit not the method.
For fuels refineries, the products that generally bring the greatest monetary returns are transportation fuels – gasoline and diesel fuel, and perhaps LPG, depending upon market location. Conversion of atmospheric residue to transportation fuels is possible directly, but the capital cost for the units to do this is often prohibitive. Consequently, the classical solution is vacuum distillation, followed by either FCC or hydrocracking (or both) of the vacuum distillates and either further processing (destruction?) of the vacuum residue, or sales as heavy fuel oil.
The process unit chosen for separating hydrocarbon components boiling at between 340°C and up to 565°C or more is usually a vacuum distillation unit, operating at a flash zone absolute pressure of around 50 mm HG. The reason for distillation under a vacuum is to allow the distillation to take place at temperatures that are not (usually) high enough to produce thermal cracking of the vacuum gasoil components. The distillation tower internals historically were designed to be low pressure-drop systems (at least as conceived at the time of design) since the best maintainable absolute pressure at the top of the tower was generally about 10 mm HG. Because of the qualities of some of the components processed, it was necessary to utilise special metallurgy to assure the safety and continued long-term operation of the vacuum distillation unit. Carbon steel alone was not sufficient.

An example of what was originally possible (1950-1960 designs) is a vacuum tower operating at 30 mm HG at the top of the tower (two- or three-stage condensing steam ejector system). The flash zone could be less than 100 mm HG absolute pressure, with steam stripping in the tower to maintain the partial pressure of the hydrocarbons low enough that thermal cracking/coking in the bottom of the tower was minimised.
The typical products (in a fuels refinery) would be light vacuum gasoil (LVGO - boiling range about 315-400°C), heavy vacuum gasoil (HVGO - boiling range about 370-510°C), vacuum residue, and a slop cut drawn below the HVGO in order to minimize the amount of light products getting into the residue, and also to trap out the heavy material splashing up and entrained material from the bottoms (overflash). The slop cut was usually recycled back to the vacuum feed heater to extinction, although it was sometimes used as refinery fuel. There were two other small product streams, (1) the condensed liquid from the steam ejector vacuum-producing system, consisting of some lighter hydrocarbons, typically an atmospheric gasoil boiling range, partly from entrainment from the atmospheric distillation, and partly from thermal cracking in the vacuum tower system, and (2) water from the ejector system and condensed stripping steam. At 510°C endpoint, the HVGO was a relatively light feed for today’s FCC units with all riser cracking and “exotic” catalysts. It could even cause problems today with a lack of Concarbon to maintain temperature control in a modern unit designed for heavier feedstocks. It also left a relatively large amount of light material in the vacuum residue. Unless the FCC and/or hydrocracker was/were already full, and there was no other outlet, this was an extreme waste of money!
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What to Do?
In a modern fuels refinery, the objective of the vacuum distillation unit is as a feed preparation unit for conversion units. A “conversion unit” could be an FCC, a hydrocracker, or even a thermal cracker, visbreaker, or coker.