Q & A > Question Details
Currently the slop wax from the our vacuum distillation unit is directed outside the plant. Do you know some solutions regarding recycling this low-margin product back to the process? I have read about directing it to the feed of CDU or with the long residue to the vacuum furnace or directly from the vacuum column to the evaporation section of the vacuum column (is it safe and not coke the bed?) . Our plant is combined CDU and VDU , internals of the vacuum columns are structured packings Mellapack. I wonder what your recommendations are on this subject, maybe some advantages and disadvantages of specific solutions.
31/01/2012 A: Rajkumar Chate, Sulzer, rajkumar.chate@sulzer.com
If you have a margin in column and furnace then directing slop wax to furnace will be the better option. yes it is correct that due to recycle of this slop wax there is a high chance of coking of wash be (particularly bottom 2-3 layers of packing bed), but this can be avoided by putting a stronger coke resistant packing (grid) with higher thickness. This grid packing will have a higher thickness (0.2 -0.5 mm depending on the COT and asphalt content this shall be decided) and more open area (less surface area) which will protect the packing from coking. To avoid the coking of this bottom bed dry out ration (reflux at top of wash bed/slop wax) shall be at least 0.3, this ensures a proper washing of coke settled on packing. Distributor shall be designed properly with good drip points. If you have a chimney tray at bottom of wash bed to collect the liquid from wash bed then this chimney tray shall be installed with some inclination this will help to flow the slop wax liquid on chimney tray deck to sump/ draw off box.
important thing is that first analyze the quantity of gas oil going with slop wax, if it is higher then only routing this slop wax to furnace will give benefit (some gas oil can be recovered).
21/07/2011 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
Slop wax beds are kept wet with a internal reflux stream of Hot HVGO (wash oil) failing which the packed bed will run dry and gets coked up. This stream is called as wash oil as it also washes down the metals and entrained asphaltenes from the HVGO product. So, essentially some of the slop wax is a recoverable HVGO (FCC feed). Analysis indicate that at least 40% of the slop wax product will fit the HVGO quality. Remaining 6o% is understandably the entrained asphaltenes and metals from the RCO (Vac feed). Probably this why we need to route this material out side to Fuel oil tanks when the Bitumen (asphalt) is produced. When the Bitumen production is not desired the slop wax is recycled to RCO (to the inlet of the Vac furnace) so that part of the wash oil is recovered as HVGO.
Going by the general logic all the distillation columns need some overflash returning to flash zone to help the washing down of contaminants. In case of packed bed columns considering each packed bed is necessarily accompanied by a total draw off tray, each section operates as mini distillation column. So a compulsory stream of hot reflux is returned to the section below the total draw off tray. In case of old designs of Slop wax beds we do not see this overflash arrangement. Recent upgrades of slop wax draw trays are modifying the slop wax total draw tray as semi draw tray with the remaining material has an internal overflow arrangement to return the entrained material to the flash zone of the columns. That way we can improve the product recovery, but this arrangement may not be suitable for Bitumen production directly where the short residue is fed to a conventional Bitumen Blowing unit.
18/07/2011 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Your scheme is preferred for producing road asphalt, to be able to meet asphalt specifications. As for other designs:
(1) Recycling slop wax (spent wash oil) to the vacuum heater inlet: Some refiners experienced premature fouling of equipment, particularly the heater tubes. Most modern units do not use this recycle design.
(2) Internal flow to the flash zone: A proper design will work O.K., however, the flow rate can be calculated only by computer generated overall heat balance, and that is not as accurate as being metered.
(3) Pull out, meter, and return to flash zone: A good design. Some use a pump and meter. Some simply use a low ∆P meter with no pump.
(4) Combine the slop wax with column bottoms flow: If not producing asphalt specifications, this is my preferred design. The slop wax should join the bottoms after the quench stream comes from the bottoms line.
If lube stock is being produced, there is usually a recycle stream from the middle of the vacuum column to the atmospheric section, but that is another subject.
18/07/2011 A: RENE AGA, SECOT, renelaga@telefonica.net
In my opinion, the best would be to dilute it as much as possible to reprocess it, i.e. dilute it in the feed to the CDU.