Vacuum Tower Pressure
Vacuum Tower Pressure
(OP)
Can anyone help please?
We have recently started-up a mothballed vacuum crude distillation tower, and are running at the design condition of approx 60 mmHg in the tower top. We use 3 (steam) ejector stages to achieve this. We are seeking guidance for how we situate ourselves as concerns our operation, with a view to being able to cut deeper.
What sort of pressure today is typical, as well as achieveable, in vacuum towers, and how many ejector stages are used to achieve this? Has anyone heard of achieving vacuum by using diesel as the motivation fluid, rather then steam (this is being used in Russia)? Can anyone recommend suppliers for vacuum systems.
Thank you
We have recently started-up a mothballed vacuum crude distillation tower, and are running at the design condition of approx 60 mmHg in the tower top. We use 3 (steam) ejector stages to achieve this. We are seeking guidance for how we situate ourselves as concerns our operation, with a view to being able to cut deeper.
What sort of pressure today is typical, as well as achieveable, in vacuum towers, and how many ejector stages are used to achieve this? Has anyone heard of achieving vacuum by using diesel as the motivation fluid, rather then steam (this is being used in Russia)? Can anyone recommend suppliers for vacuum systems.
Thank you





RE: Vacuum Tower Pressure
A "dry" vacuum tower doesn't need the condenser and uses little stripping steam, if at all. Since the volume of the hydrocarbon vapors increases markedly at the reduced absolute pressures there is not much change in the amount of distillates and the size of the tower. The pressures could be as low as 3mm Hg at the top and about 28 mm Hg in the flash zone. You wouldn't gain anything by replacing steam with a lighter hydrocarbon because of that reason. On the other hand, you probably can dispense with the stripping trays at the tower bottoms.
The total amount of steam consumed wouldn't drop much because the amount of steam used in the flash zone (60-70% mole) would have to be replaced by more steam to the ejectors.
In general, dry vacuum towers are generally preferred because they are a bit more cost-effective, and operationally because slugs of water, sometimes accompanying the bottom stripping steam, wouldn't appear dislodging the bottom trays therein.
Elements of Petroleum Processing by D.S.J. Jones by Wiley and Sons can be of help in reviewing the design of the exisitng vacuum tower. Good luck.
RE: Vacuum Tower Pressure
Previously steam was used for the eductors and was condensed with direct water contact to produce the tower vacuum. Unfortunately this method creates lots of pollution with hydrocarbons (HC) being vented with the non-condensible air and contaminated water. The contaminated water is recycled through a cooling tower where more pollution is created as HC's are air stripped. This is a very environmentally nasty process.
A cost effective solution (cheap) is to use HC as the eductor fluid and condensing media. The HC also acts as a lean oil to absord HC from the air. The rich HC can be recycled to the process. In this case some diesel is vaporized to power the eductor and the heat is recovered by preheating diesel for feed to a recovery process.
This method allows for reuse of all the old vacuum equipment. The alternative is to spend a lot of money on heat exchangers or vacuum pumps which are less reliable due to moving parts.
RE: Vacuum Tower Pressure
RE: Vacuum Tower Pressure
Good Luck.
rmw