Q & A > Question Details
We are operating CDU/VDU @ 110% throughput.
We have a Demister Pad installed at the top of the vacuum column.
The DP of the demister pad is increasing @ ~1mmHg/10 days. This is resulting into higher Flash zone pressure causing dropping of Vacuum gas oil into Vacuum residue. We operate the column at 23 mmHga top pressure.
Also we have noticed chlorides in our Vacuum diesel stream (First side cut of VDU) in the range of 10-20 ppm w.
What can be the cause of this fast increasing DP and what measures can be taken to arrest that?
05/01/2017 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
Demisters are installed for the prevention of mist into the down stream equipment. Removal of demisters will lead to bigger issues like erosion of ejector nozzles (two phase flow) and subsequent failure of ejector itself.
04/01/2017 A: Eric Hennings, Technip Stone & Webster, EHennings@technip.com
10-20 ppm organic chlorides is an extremely high concentration! Are you adding machine shop solvents to your slop? Many refiners no longer use organic chlorides for such service as they can become extremely corrosive.
I have been successful in reducing fouling of NH4HS type salts (in other process) using well distributed water wash and/or neutralization chemicals. I have also successfully used non-chlorinated chemicals (such as sulfonates) for paraffin dispersion.

03/01/2017 A: NS Murthy, Suez, murthy.ns.ext@suez.com
Demisters are installed in VDU OH to prevent hydrocarbon carry over into ejector condenser system. In one of the refineries as part of revamp we removed the demister only find excessive (long tail in D86) of hot well oil. Further, there was deterioration in condenser cooling performance owing to wax deposition. The refinery put back the demister in the next opportunity. The solution to mitigate demister plugging will be: -
a) Better desalting and effective caustic dosing to prevent Cl ingress into VDU
b) Checking C factor at VDU OH section and try to optimize the operation which include OF control in u/s CDU tower.
30/12/2016 A: NS Murthy, Suez, murthy.ns.ext@suez.com
In a refinery where demister was removed in a turnaround witnessed steady ingress of hy. oil into ejector and condenser system. The hot well oil end point shot up significantly leading to wax deposition on condenser outer surface thereby dropping heat transfer / cooling and in turn vac too. Best option will be right design of demister with good control of desalters and caustic dosing to minimize / control Chloride ingress into vac unit.
28/12/2016 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
You are dropping out ammonium chloride salts from running the top of the tower too cool. I have seen this same thing happen in multiple vacuum units, and did a presentation on the subject at NACE a couple of years ago.
Once the salts are deposited it is very difficult to reduce the pressure drop without a shutdown to wash the salts out. By increasing the temperature in the top of the tower you can slow down or stop the increase though. The trick is in finding the right balance on overhead temperature. If you let the temperature warm up too much you load up the ejectors with hydrocarbons and increase your slop oil production off of the seal drum. If you run too cool you will form salts as you are doing.
I have used some simple salt point modelling tools to help several refineries set reasonable operating targets to get out of the salt formation regime with minimum impact on ejector system operation.
25/12/2016 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Your experience with the demister is why it is not recommended to install them in vacuum columns. Same is true of eductor pre-condenser. They will foul and cause a reduction in revenue. Remove the demister at the next opportunity.
23/12/2016 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
Demisters get plugged with entrained particulate matter. This could happen when one or more of the reflux distributor spray nozzles get partially blocked and sprays profile change or finer mist. Finer mist will fail to reach the packed bed and instead take the reverse umbrella profile to take the reflux towards the demister. This will lead to increased DP across demisters. Once this process starts it will lead to other associated problems of bed coking etc., too.
22/12/2016 A: Celso Pajaro, Sulzer Chemtech, celso.pajaro@sulzer.com
There have been reported problems of demister pads being plugged with amine chloride. It is not recommended to install demister pad on top of vacuum column. Chevron type mist eliminator have been successfully used in this service. If you need more information on the type of Chevron please contact your column internal supplier.
22/12/2016 A: Ravi shankar, HPCL, prskreddy@hpcl.co.in
1) If you use water coolers in any of your CR/IR circuit check for cooler leaks.Also check your Vac OH condensers for leaks.
2) If you use caustic/Ammonia for neutralisation of OH vap circuit,consider other chemicals for neutralisation,
3) Checkyour desalter efficiency & Caustic dosing system