Q & A > Question Details
We have recently observed a high production of off-gases from our Vacuum Distillation Column. We performed the distillation for the feed and observed that the initial and 5% boiling point had reduced significantly. Could this be the reason for the increase in the off-gases?
On the other hand, we suspect an air ingress into the system. Is there is any way to detect/prove that there is an air ingress into the system?

Added by questioner:
Thank you very much for your input. I gained a lot of insight from your answers.
After closely studying all your suggestions, we took into consideration a number of points and implemented them.
One of them was sampling the off-gases, from which we found a high % of Nitrogen which clearly indicated air ingress. As a result, we tightened the transfer line gasket and the off-gases flow reduced by 50%.
Thanks once again.
06/12/2015 A: A S Vishnu Ram, RIL, vishnu.selvaraj@ril.com
Additionally you may check the following,
1. In most of the vacuum system design, it is practiced to recycle slop wax (heater recycle oil). If the CCR value of the stream is more, it induce cracking in Vacuum furnace. If dumping option is given by design to VR stream, partial dumping may be practiced for few days and reduction in cracked off gasses may be observed.
2. In addition to the VR bottom temperature RCO bottom temperature shall be maintained not exceeding the recommended operating range to avoid cracking due to residence time. In that case, atmospheric heater COT shall be reduced a bit and the cracked off gases from vacuum tower can be monitored.
3. It is always better to have a thermography survey conducted, to find if there is any hot spots over the radiation tubes, in addition to the tube metal temperature monitoring. As the lighters reduced in the feed, velocity steam shall be increased to maintain the same mass flux across the heater tube.
18/11/2015 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
If nitrogen is not found in the sample of off gas, you are likely "laying down" coke somewhere, and the coil outlet temperature should be lowered.
17/11/2015 A: Daniel Vargas, Saudi Aramco, vargasdj@aramco.com
Regarding the effects of light material increasing in the long residue (VDU feed):
- Overloading the top pumparound section, then the VDU top temperature could increase and produce overload of the vacuum system. This effect will depend of the VDU top pumparound and the vacuum system condensers overcapacity. Please let us know if you have also problems with the vacuum pressure.
- Increase the vacuum system slop production which produces economic losses.
Regarding the sources for cracked gas:
- Vacuum Heater and flash zone according severity
- VDU column bottom: Please check the column level registers, which of course will affect the residence time, and also the level indicator, if the column has not stripping section, the flash zone pressure and the VR pump suction pressure could be used to double check the reliability of the level indicator. Bottom temperature should be below 680 °F to avoid problems.
In case of your bottom VR quench heat exchanger is preheating your crude feed or if you have some long residue (LR) feed from tank which was previously cooled with crude preheating, then if you have a leak from crude side to VR quench side or LR side, you could overload the vacuum system with non-condensable, please check the saturates/olefins ratio in your off-gas composition.
17/11/2015 A: keith bowers, B and B Consulting, kebowers47@gmail.com
The significant lowering of the IBP and 5% (evaporated or recovered?) indicates more non-condensable light material will be in the 'off-gas'

17/11/2015 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
The presence of "light" ends in the vacuum unit feed is not normally related to gases in the vacuum unit off gas. Vacuum unit off gases are almost always either products of thermal cracking in the furnace or from air ingress. GC analysis of a sample of the off gas will normally tell you what the source of the gas is. If the air leakage is in hot parts of the unit, the oxygen may get converted to CO or CO2. CO2 can also come from thermal degradation naphthenic acids, so be sure to also look for nitrogen. If there is significant air leakage there will always be a significant amount of N2 present.
A change in feedstocks, a decrease in furnace charge rate (increase in residence time), and/or an increase in furnace outlet temperature can all lead to an increase in cracked gas formation. Also an increase in the residence time of vac resid in the bottom of the tower or the quenched vac resid temperature can also result in an increase in gas make.
17/11/2015 A: NS Murthy, GE, murthy.ns@ge.com
Check the following: -
1. Cracking reactions do occur in VDU charge heater. Any change in feed quality and increase operating severity can lead to such. Higher velocity steam and lower COT (by couple of deg) can help to control excessive gas make. It may be worth tracking TMT and the off gas flow.
2. Crudes with high amount of reactive sulphur in long residue can also lead to excessive gas generation. Check H2S level in the off gas.
3. As regards, air ingress, one can check again from extent of O2 in off gas. Ideally, it will be less than 0.5% vol. This also mostly due to sampling error.