Vacuum level VS Top pumparound rate 1

[MaNaTMoS]

What are the causes of loosing vacuum when increasing the top reflux rate in the vacuum bottoms stripper? Does it necessarily mean a liquid phase entrainment to the ejector system?

 

[25362]

Some reasons I could think of:

1. If the net result is a cooler tower top, it might promote a new flash equilibrium behaviour. A cooler tower top would result in a smaller amount of liquid condensed in a pre-condenser. Thus less light ends are absorbed and the load to the ejectors increases causing a pressure rise.

2. If the increased p.a. flow rate involves more heater firing to maintain the temperature profile, one may get additional cracking and consequently overloaded ejectors.

3. Do you have also a side GO stripper that may become destabilized by the larger sudden p.a. withdrawal ?

Kindly comment.

 

[MaNaTMoS]

1. That could be one of the possible reasons, but I doubt somehow; the quantity of stripping steam is constant, and we have this kind of problem only when processing lighter crude. It's interesting that first (main) column doesn't suffer loss of vacuum at all.
2. No automatic temperature control on bottoms. Heater outlet temperature (when processing light crude) is 385C.
3. No, there's no side GO stripper. Only second column, which is used for bottoms' stripping, instead of quenching vacuum residue in a single-stage VDU.


Could it be possible that too much HVGO is recovered and "burped" into the ejector system? I didn't notice any significant increase of HVGO level in the side-draw pan.

 

[MaNaTMoS]

I've forgot to mention: this lighter crude is paraffinic. Maybe we have pour point problem and difficult drainage from the booster condensers?

 

[alsayed]

I think of it as you have more liquid pump around you will have more pressure drop across that section of the column section, check you dp across the column sections with a pressure survey.

Lieberman books mentioned similar case of vacuum loss with a precondenser in the top.

 

[MaNaTMoS]

hmmm...
well, i guess it would be OK if vacuum loss with increasing pumparound rate was 1-2 mmHg; but in my plant this deltaP goes up to 40mmHg. It's interesting that we can't hold design temperature for stripper's top (80C) - current temperature is 125C and vacuum is 725mmHg. When I increase the stripped HVGO pumparound rate (or lower its return temperature through bypass controller - in order to maintain 80C), vacuum starts to fall.
Anyway, thank you, alsayed.
Maybe I'm too young to think about all these things, or maybe I need a long vacation in Rio De Janeiro

siNceReLLy,
manatmos