Q & A > Question Details
We have a design of a Vacuum unit where a slop wax line is going to slop wax pump. This line has a vertical leg of 35 ft the line is 6 inch but this vertical leg is 8 inch; the vertical leg has a level control which controls the recyle of slop wax
is someone familiar with this scheme; what is the logic/theory behind this?
20/02/2020 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
If slop wax is allowed to overflow the draw tray in the tower, there would be a loss of HVGO yield because a large part of the slop wax is HVGO boiling range material which would be lost in the residue stream. If the slop wax overflow pipe is undersized (or not included in the design), there is a risk that the chimneys could overflow which would cause entrainment and flooding. Hence it is common to design the tray with a total draw. The slop wax is then either recycled to get another opportunity to recover the HVGO, or sent to an upgrading unit (e.g. the FCC).

The 8” vertical pipe is performing the function of an accumulator vessel. The level controller acts to stop the level going too high (which could lead to overflow and yield loss) or too low (which could cause slop wax pump cavitation and damage).
29/01/2020 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
The slop wax stream is prone to coking (I am assuming you are referring to the liquid being drawn from the bottom of the wash bed above the flash zone.). To minimize risk of coking the tray, sometimes the tray is sloped towards the outlet to minimize residence time at high temperature and level is controlled in the outlet line. The units I am familiar with usually controlled the suction pressure on the pump and not the level directly. Controlling the suction pressure effectively controls the level because any pressure above tower pressure must come from liquid head. The key in this kind of system, whether you are controlling level or pressure, is to keep the level/pressure high enough that you don't cavitate the pump but low enough that you are not forcing liquid to build up on the collection tray below the wash bed.
28/01/2020 A: keith bowers, B and B Consulting, kebowers47@gmail.com
A high liquid level in the vertical 'standpipe' is needed to ensure adequate NPSH to the slop wax recycle pump. The larger diameter standpipe is an inexpensive way to provide adequate 'surge volume' to facilitate a steady (slow to change) rate of recycle and stabilize the column operation. This is a smart way to avoid the expense and plot space of a conventional 'reflux drum.'
28/01/2020 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Please send me a P&ID and I will comment. ralph.ragsdale@att.net. Also, is the slop wax pump a centrifugal pump?