Vacuum Tower Pressure

If you have much steam as needed for the heater (velocity steam) and for stripping the products, including the bottom residue, you'd probably have a pre-condenser to your set of ejectors. This condenser would dictate the minimum attainable top pressure concomitant with the vapor pressure of condensed water and light hydrocarbons at the prevailing temperature, plus the pressure drop through the condenser. This is what is called a "wet" vacuum tower.

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.

Using diesel as the eductor motive fluid was probably done for environmental purposes instead of performance, though there may also be a performance improvement.

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.

You should see if you can get a new set of nozzles designed for your motive fluid and not for steam. That should help some. But the venturi diameters are also dependent on what's flowing through them, so having a different motive fluid will change it's performance. If you had jets designed for steam, and you're running them with vaporized hydrocarbons, and you're getting 90 mm absolute, congratulations. I don't have anything in front of me to do a design check, but the venturis might need to be opened up a little. Any access to a good machine shop? See if you can get the original manufacturer to help you.

For a supplier with a good track record on crude vacuum towers, see
Good Luck.

rmw