vacuum column revamps
vacuum column revamps
(OP)
The vacuum column in the slo fractionation unit is giving us problems. Some of the HVGO goes with the vacuum bottoms.
How can i fix this problem....
Thank you
How can i fix this problem....
Thank you





RE: vacuum column revamps
RE: vacuum column revamps
To refilwe,
I wouldn't mind to know what slo stands for.
Anyway, can you tell us what type of operation is involved, dry or wet ? Temperatures and pressures, what are they, and are they steady ? Do you have precondensers or is the tower directly connected to a main "booster" ejector ?
Generally, some of the options to recover more HVGO are:
• Unload the tower and the heater by improving
previous atmospheric GO removal.
• Reduce thermal cracking which affects vacuum.
• Reduce ΔP and flash zone pressure by
switching from pumparound trays to packing.
• Reduce number of wash trays; results as above.
• Improve vacuum generation.
• Increase the superheat of steam to the resid
stripping trays, and check the cooling effect of this
stripping; below 15 Celsius means the contact with steam
isn't OK. It may be due to tray flooding, corrosion, or
upset. Even a bottom level controller malfunction could
be the culprit.
• Don't over-wash the lower fractionation section.
• Seal weld HVGO and dark or slop VGO draw-off trays
to minimize leakage into the residue.
• Quench the bottoms to avoid thermal cracking.
• Circulate slop VGO to extinction.
The general ROT to verify whether there is too much HVGO (up to 540 Celsius) in the residue is to vacuum distill it in the lab. 5 to 10% is reasonably good; more than 30%, points to a poor performance. It even can markedly reduce the NPSHA of the bottom pump.
You may find some of the above steps aren't applicable or contradict each other.
Please comment.
RE: vacuum column revamps
I couldn't, and not only because I don't have your expertise...
RE: vacuum column revamps
The operating conditions are as follows
Pressure: Overhead -52KPa Ab
Flash Zone -51kPag
Temperature: LVGO REFLUX 40 DEGRESS CELCIUS
HVGO REFLUX 86
BOTTOMS 260
HVGO HOT RETURN 155
The problem is that the vacuum unit experiences a low TBP cut point.
So far i was looking into increasing the furnace temperature and or reducing the flash zone pressure.
Thank you
RE: vacuum column revamps
Refilwe,
Could you refer to the following queries ?
1. The vacuum level is not impressive. Is the hydrocarbons'
partial pressure being reduced with steam ?
2. What are the temperatures at the flash zone and the top?
3. Again, what is the meaning of the acronym slo ?
4. Have you checked the unit for inward air leakages ?
5. How do you intend to improve the vacuum ?
6. Are you using velocity steam in the heater ?
7. How many wash trays do you have, and of what kind ?
8. What do you do with the black gas oil from washings ?
9. Does the unit have a top precondenser ?
10.Why do you express the absolute pressures at the top in
kPa abs., and in kPag at the flash zone ?
Epoisses, thanx for the star, the truth is my "experience" is reinforced by plenty of notes I collected over the years.
RE: vacuum column revamps
What is the design cut-point for HVGO and vacuum resid?
According to your tower pressure profile data, decent HVGO recovery should not be expected. Or, possibly, you are suffering from very intensive condensate back-up in your vacuum condensers. Do you have 1-, 2-, or 3-stage ejector system?
Also, which is very important, are these P,T data from the DCS or directly measured outside on the process equipment? What is the design pressure of tower top and FZ?
Before answering, you may want to look at some of these resources, they could help you:
http://www.graham-mfg.com/downloads/216.pdf
http://www.graham-mfg.com/downloads/220.pdf
http://www.graham-mfg.com/downloads/221.pdf
http://www.graham-mfg.com/downloads/232.pdf
RE: vacuum column revamps
RE: vacuum column revamps
I know nothing about the set up of this particular application, so I cannot comment on which scenario makes more sense. But when the data is inconsistent like this it makes me suspicious of all the data. It is very hard to trouble shoot when you don't have a clear picture.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com