Q & A > Question Details
I am shift superintendent working in a CDU/VDU unit. We face a problem of maintaining the flow rate of Heater recycle oil (Overflash in Vacuum column which normally contains 50-50 of HVGo-VR). We process the API of 24 and 29. We take LVGO, HVGO and VR products and the HRCO is being recycled back to the vacuum column through heater.
The HRCO generation suddenly drops down and increases due to some problem. We raise the HVGO IR and alter the coil outlet temp (COT) of the vacuum column to maintain the HRCO generation fearing that we could end up in coking up the packing bed. Furthermore we experience the packing bed Delta Pressure high across it. Hence we could not raise the temp of COT and reach the deep cut HVGO.
Please explain any of your experience into this particular subject.
 
Answers
16/05/2011 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
Heater Recycle oil is otherwise known as Slop distillate recycle / dirty slop distillate etc.,. This recycle acts as wash oil for washing down the metals / heavy components from the FCC feed stream. Steady generation of the HRCO is a function of health of the Slop distillate packed bed in the vacuum column. The slop distillate bed is kept wet using hot Heavy Vacuum Gas Oil (HVGO) reflux. Piped distributors fitted with spray nozzles are used generally for this hot HVGO reflux. However going by the nature of the Slop distillate material, probability of spray nozzle chokages are observed to be high. Choked spray nozzles would lead to improper wetting of the bed, leading to dry spots in the packed bed and results in the Coking of the bed. Once the bed is coked up, we are in a vicious cycle. Off late the piped distributors are being replaced with trough / channel / gutter distributors (similar to the liquid distributors used in Delayed Coker Fractionator bottom sections) in this service. These distributors are difficult to choke with Coke etc., and will help minimising the bed fouling in the HRCO section of Vacuum column.
24/11/2009 A: Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
if you are having fouling problems in wash oil section, it is quite possible spray nozzles on wash oil pump around are plugged. This can result in ineffective washing and build up of coke in the packings eventually requiring a shutdown due to high dP and maldistribution of wash oil. It's quite a safety issue as well. Inspection of the spray nozzles on the wash oil system should be done to check if this is indeed the case or not. It may be that the existing filters or strainers on the wash oil system are inadequate, bypassing, too course a micron rating or possibly don't exist.
We have seen similar situations before at a European oil major with VDU spray nozzles fouling and this has been rectified by installing duplexed largebore sintered woven metallic Rigimesh Highflow pleated baskets strainers with either 225 or 450um um absolute micron rating. The Rigimesh filters require cleaning out about once per 2 to 4 weeks. Since installation about 4-5 years ago wash oil spray nozzles fouling is reduced if not eliminated and dP build up improved so that unscheduled VDU shutdown has been prevented. We'd happy to assist further if needed.
24/11/2009 A: keith bowers, B and B Consulting, kebowers47@gmail.com
You already have excellent suggestions from Ralph Ragsdale. A question: Is the wash bed pressure drop suddenly changing when HRCO production changes? If so, this indicates flooding of that bed, probably caused by coke build-up from poor irrigation of the packing.
If the HRCO (the relative rate % of feed we do not know) really contains 50% VR, the entry horn is not doing its job well and an excessive amount of VR is entrained in the vapor entering the wash bed.
Many plant operators do not understand that when operating at 'deep cut' conditions it is imperative that enough HVGO be condensed in the wash bed and 'left in the VR' product such that adequate wash oil rates and QUALITY are maintained. Too little HVGO 'left in the VR product' means wash oil composition is high in VR and the wash bed WILL coke quickly.
24/11/2009 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Correction. If you reroute the spent wash oil as I described, since it is already being pumped, the change may able to be made with hot taps and a flying switch. You would still need to replace the packing in the wash oil section during a shutdown to correct the channeling problem.
24/11/2009 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
As you suspect, the high pressure drop through the wash oil section indicates flow channeling resulting in fouling of the packing. This could have been caused by low wash oil flow, a malfunction of the liquid spray system over the top of the bed, or a poor design of that system. I don't know if the unit is old or relatively new. If old, consult Scott Golden for revamp recommendations.
Recycling spent wash oil through the heater was not designed for several decades for reasons like the one you describe, i.e., operating problems. In my opinion, the better design is where the spent wash oil is pumped to combine with the bottoms product if asphalt is not being produced, and pumped out as a separate product if asphalt is being produced. That change would involve a shutdown tie-in project.
Furthermore, maintaining consistently good separation of HVGO and bottoms as well as gas oil free bottoms (except for the spent wash oil in my preferred design) can be achieved only if the chimney tray below the wash oil section is well designed such that it will not leak liquid down to the flash zone anytime during the run between turnarounds.