Q & A > Question Details
What's a wash tray/section in a CDU?
And how it is vulnerable to coking?
09/03/2018 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
In the vaccuum column a certain amount of vacuum residue is entrained up the column with the VGO vapors. That entrainment adds metals and concarbon to the VGO product. To limit entrainment some HVGO is returned to the column underneath the draw tray through a spray header over a packed bed (the wash bed). The combination of wash oil and the packing surface knocks out the bulk of the entrained resid, allowing good quality VGO to be produced.
The temperature in the flash zone can and the wash bed can be over 750 F. Much of wash oil flashes when it contacts hot vapor in the wash bed. If the wash oil rate is too low, it will not adequately wet all the packing and a dry point can be reached somewhere in the packed bed. At this point there is no flow to wash entrained resid off the packing and it can start to coke at the high temperatures. Over time the packing plugs off with coke and flash zone pressure builds, which hurts vacuum tower yields.
The same thing is going on in the atmospheric column but generally it is a more forgiving service. The wash oil will be whatever the bottom cut is on the tower. Temperatures in the flash zone are lower than the vacuum column, transfer line velocities are lower so entrainment tends to be less severe, and atmos resid entrainment is less prone to coke formation than vac resid entrainment. In the atmospheric column wash bed you are making sure that atmospheric resid is not entrained into the lighter products of the crude column.
09/03/2018 A: keith bowers, B and B Consulting, kebowers47@gmail.com
A 'wash tray/section' is utilized in a heavy ends distillation column in the bottom portion or just above the 'flash zone' as a means to remove entrained liquid droplets. These 'internals' need adequate liquid 'wash oil' to keep the surfaces wet with wash oil to keep it clean and the entrained droplets 'washed off' to prevent coking. The entrained droplets contain non-volatile residue that will form solid 'coke' deposits if they are not 'washed down' back into the bottoms product.
This IS NOT a 'fractionation' operation but is analogous to a mesh type 'mist eliminator' at the outlet of a vessel or distillation column.
09/03/2018 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
The coking process is affected by time and temperature. Coking is to be avoided except in a coking process unit. The lower packed section of the vacuum column fractionates but is sometimes referred-to as the wash section. If there is insufficient liquid downflow or poor distribution of the liquid, the temperature is high enough to promote coking. Some of the vapor experiences the high temperature for too long a period. Not good.
Some atmospheric columns have packed sections for heat removal. They do not fractionate. We use one theoretical tray in simulation calculations to make the simulator work. Packed sections are also used to replace trays when more capacity is needed. Those sections do frationate. If there is a packed section just above the flash zone, coking may result there if there is insufficient overflash or poor distribution of the liquid flow. This section is seldom called a wash section however.