I am in the process of starting up a small recycled lube oil plant. The process is as so:
1. Recycled lube oil feedstock has water removed in a oily water separator and preheated to 575F.
2. Feedstock enters an atmospheric column with steam injection, yielding Naphtha, diesel, and a bottoms stream.
3. The bottoms stream is preheated to 585F and enters a vacuum column under -26"Hg of vacuum, steam is also injected into the column, yielding LVGO (top draw) , HVGO (middle draw) , and asphalt residue (bottom).
It is a relatively standard atmospheric/vacuum oil distillation unit, and the atmospheric column is working well, yielding the diesel and naphtha as expected. The problem we are facing is that the LVGO level simply will not establish itself. The only time any significant LVGO level was observed in the unit was when we accidentally overflowed the HVGO draw tray slightly to about 200% of level (nowhere near bringing liquid to the LVGO tray). After shutting the vac column down and checking out the internals it was observed that there was damage and displacement to the HVGO tray. We had been running the column during the start-up phase with the bottom of the column flooded so level could be established at the HVGO draw.
My initial thoughts are:
- Feed preheat temperature may be too low, causing the distilled fluid in the tower to condense before reaching the LVGO tray. We need to avoid cracking so are limited in how high a temperature we can raise the feed to. In your experiences what is the highest acceptable temperature you have seen lube oil preheated to in similar columns?
- LVGO tray does not have proper gaskets to hold liquid level. We poured water into the LVGO tray while the tower was shut down and saw that water slowly makes it's way through - but with rising vapor below it in operation in theory this should still hold some level. Does this need to be a water tight tray?
Any tips would be greatly appreciated.