Where is the HVGO coming from? 3

[Alfamath]

The vacuum tower in Synthol Fractionation is giving us problems. The Vacuum Unit has low Cutpoint. I have read a lot of articles on vacuum revamp and considered so many things that can lead to such a problems. Here are the things that I have considered:

o Liquid misdistribution
o Internals Damage (those above the flash zone) due to high liquid levels
o Leakage of VGO from draw-off trays.

Is there a way of determining the above without shutting-down the Unit?

The Operation involved is dry (The bottoms is quenched by recycling cold VTB)

Operating Conditions of the Tower:

FZ Temperature = 285 0 C
FZ Pressure = 18 kPa
Pressure at Top = 11.46 kPa
Inlet Temperature = 290 - 295 0 C

% Recovered at 362 0 C is between 10 and 24 when the VTB is vacuum distilled in the lab.

Design Conditions of the Tower:

FZ Temperature = 291 0 C
FZ Pressure = 49 kPa
Pressure at Top = 48 kPa
Inlet Temperature = 293 0 C
Cutpoint = 370 – 420 0 C

There are two wash beds, one for hot slop wax reflux and another for hot HVGO reflux.
The Unit does not have a pre-condenser. It uses a Single stage twin ejector system to pull the vacuum. The tower has four packings; grid and pall rings are as wash beds and the other two for pumparound duties.
Four years ago, the vacuum tower was operating properly under similar operating conditions as the current one.

How does viscosity and density impact the fractionation in the Vacuum tower?
What can cause the lighter fractions to go in VTB?

I appreciate all the assistance I can have….

 

[0707]

Make a TruTec gamma secanning, it works!

 

[hanon]

Maybe you have a flooded wash bed due to coke accumulation during this run. It could be possible according to such a high dP compared to design. Try to do a pressure profile along the tower to see where the problems are located. If you have internal levels in the tower be aware of them: maybe they have a bad indication.

 

[0707]

go to

http://www.tracerco.com/products-processdiagnostics-towerscan.htm

luis

 

[25362]

The present [Δ]P, higher than in the design data, can be explained as the result of the lower absolute pressures (~1/3) which affect the vapor volume at equal mass loads.

All the mentioned factors, as well as excessive leakage of HVGO, or trim oil, into the bottom section are to be considered. So should be potentially fallible instrument readings.

Why do you suspect the density and viscosity of the streams may influence results? Is the composition of the feed to the vacuum tower much unlike that of four years ago ?