Q & A > Question Details
What is the relation between the amount of overflash used in a vacuum distillation tower and the stripping steam injected at the bottom of this tower?
21/04/2017 A: Rajkumar Chate, Sulzer, rajkumar.chate@sulzer.com
There is no relation between overflash and stripping steam, overflash is maintained to have enough reflux to wash bed so that HVGO remains on spec and no coke deposit on on wash bed packing. Stripping steam is maintained to certain amount to control the loss of HVGO with residue and to have enough vapor in stripping section so that stripping trays hydraulics works reasonably good. However if you have too much overflash and there is no loss of HVGO with residue then you may play little bit with stripping steam and try to reduce it but effect will not be too big.

21/04/2017 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Strictly speaking, there is no overflash at the bottom of this column. The spent wash oil is withdrawn from a total trapout tray. In fact, care is taken in the fabrication of the tray to avoid even leakage into the flash zone. Such leakage results in excess bottoms product, usually an adverse effect on revenue.
The wash oil flow rate is whatever it takes to meet concarbon (metals, actually) spec on the heavy gas oil product. In some units spent wash oil is combined with the vacuum bottoms product (unless asphalt is being produced). Maybe that is why it is misconstrued as overflash. But it does not "see" the flash zone.
20/04/2017 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
There really isn't one as they are both independently controlled variables. I suppose you could try to increase the stripping steam in the bottom to offset the yield loss from a high overflash, but the effect would not be large.