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Why is the diameter of top and/or bottom of a VDU smaller than rest of the column?
 
Answers
22/01/2020 A: Joel Kaye, Retired from UOP & Tesoro, kaye13@gmail.com
The tower diameter is set by vapor velocity. The bottom is a stripping section containing stripped vapors and steam, the top is vacuum diesel or LVGO, typically a smaller stream than the VGO. All these vapors travel through the larger main column.
17/01/2020 A: Roxana Constantin, *, Adelgazi930123@gmail.com
The top is smaller in order to facilitate the vacuum, the bottom is smaller in order not to have stationary residue which could transform into coke.
17/01/2020 A: John Asuncion, Oil and Gas, johnconradasuncion@yahoo.com
Can you provide details on Unit Capacity and Expected product yields? thanks!
17/01/2020 A: John Asuncion, Oil and Gas, johnconradasuncion@yahoo.com
Can you give the capacity and expected product yield of your VDU?
13/01/2020 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
The top section is smaller because the gasoil pumparound(s) condense most of the vapour traffic flowing up from the flash zone. If the top section were the same diameter as the rest of the tower, the superficial velocity in the top section would be too low to get good contact. Capital cost is also reduced by making the top section smaller.

The vapour traffic below the flash zone is also very small – just the stripping steam and stripped vapours. In towers with no stripping steam, the vapour traffic is solely from disentrainment and release of cracked gases. Hence the bottom section can be a smaller diameter, bringing three benefits:
1. Reduced capital cost.
2. In towers with stripping trays, the small diameter helps to keep the vapour velocity high enough to stop the trays weeping.
3. Reduced residence time of residue in the sump. This reduces thermal cracking.
08/01/2020 A: Bruce Carr, Suncor Inc, bcarr@suncor.com
The smaller section at the bottom is to reduce residence time. The liquid at the bottom is hot and prone to thermal cracking if allowed too much time. This leads to increased production of olefins that combine to form coke. It will plug up the bottom of the tower. The top is smaller because it is mostly vapour traffic that doesn't require a lot of internal reflux. It reduces the working area of the trays to avoid trays running dry...
08/01/2020 A: Rajkumar Chate, Sulzer, rajkumar.chate@sulzer.com
The column diameter is depending on the vapor/liquid load or vapor velocity inside the column. More generic term for to define the velocity in the column is C-factor or F-factor. The vapor rate to bottom section of the column (below LVGO bed) is much higher compared to the top section therefore, top section requires smaller column ID while bottom section is with larger ID.