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Routing of Vacuum Unit Off-gas

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sshep

Chemical
Feb 3, 2003
761
My friends,

We have many distillation towers running under vacuum. The vacuum can be pulled by jets, liquid ring pumps, vacuum pumps, or combinations of these. The design is usually for some small air engress, typically less than 100 lbs/hr. The off-gas typically goes to an incinerator or to atmosphere, but in no cases to the plant flare system. This decision is probably driven by concern about the possibility of air going to flare.

The operations manager wants to reroute the vacuum unit off-gas to flare. I am doing a survey of practices elsewhere. How are you guys handling vacuum unit off-gas in your units?

best wishes,
Sean
 
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Your off gases are probably nitrogen or CO2 possibly rather than air and some uncondensed light components. It is fairly customary to route vac tower off gases to flare. If the material is flammable then most definitely it should go to flare (see BP TXC 2005)
 
SSHEP
Is the driver to reduce fuel gas consumption for the flare header? Seams reasonable but maybe you need to get some GC samples to determine the composition of the streams. You also have to consider how you will handle startup where a large amount of air could be introduced. I would like to say we do this, but we vent everything no flare or incinerator.

Regards
StoneCold
 
Thanks for the replies. This query came from a unit manager who has been having reliability problems with the incinerator where these gasses normally go, and wants to do something other than using the atmospheric routing which is supposed to be a short term option. I have worked in at least half a dozen plants which route vacuum off-gas to a heater or atmosphere, but none to flare; and yet I suspect that many plants probably do go to flare.

Please help me to collect a few more examples and understand the reasoning (could be back pressure just as much as fears about O2). The start-up consideration is a good thought.

best wishes,
sshep
 
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