Q & A > Question Details
In Hydrocracker fractionator, ATF draw temperature is 220 Deg C which is going to side stripper. Side stripper is having reboiler which is taking heat from diesel pump around. The reboiler outlet temperature is about 250 Deg C.It is observed that when the ATF draw increases beyond 50 M3/hr. it is failing in Flash while maintaining the temperature condition same. Column is operating at 2.1 Kg/cm2g pressure.
Where is the problem?
14/03/2018 A: Emmanuel Aunde, Refining Company, lemma012@yahoo.com
For a Hydrocracker fractionator, the ATF draw off rate controls the ATF draw temperature. The higher the draw, the higher the temperature (drawing more will send more hot vapour from the side stripper into the fractionator which will heat up the ATF draw zone). This is use to effect a swing between maximum diesel production and maximum kerosene production by changing the cut point. In your case, increasing the draw off beyond 50m3/hr increased the ATF cut point in the fractionator beyond what is typically required to meet the flash point requirement.
23/09/2017 A: Celso Pajaro, Sulzer Chemtech, celso.pajaro@sulzer.com
In any side stripper you need to monitor the stripping ratio (Vapor/liquid) no temperature. For a reboiler side stripper this can be done by dividing the vapor overhead flowrate over the bottom liquid product flowrate. If there is not flowmeter for the overhead vapor you could use the heat input to the reboiler (amount of pumparound flow to the reboiler multiply by the temperature difference across the reboiler) and divided by the side stripper bottom product flowrate.
20/09/2017 A: Rajkumar Chate, Sulzer, rajkumar.chate@sulzer.com
What do you mean by failing in Flash? if you mean ATF is lighter (flash point lower) when you increase the draw rate then duty in reboiler is not enough or there can be possibility that you have premature flooding in side stripper trays. Check the temperature at bottom and top of side stripper with normal draw rate and increased draw rate, if you have same temperatue at bottom but still you do not meet the light end quality then most probably you need to look in to side stripper tray design. If your bottom temperaure is lower then you need to increase the duty in reboiler (reduce bypass and send more diesel PA to side stripper reboiler) to stripp light ends from ATF.
19/09/2017 A: Abdulkarim Aldabass, Saudi Aramco, dabassaa@yahoo.com
The flash point is determined by the distillation 5% and the initial boiling point which is may due to light material excess withdraw. The column conditions draw temp and pump around shall be control in order to have the required cuts that met the spec
19/09/2017 A: Dave Collings, Jacobs, dave.collings@jacobs.com
The information available here is limited. What's happening to the other parameters on the tower? If the ATF is at a nearly constant 95degF and you draw more, that means that you are bringing more naphtha molecules into the ATF. Eventually, this will cause you to fail on flash at constant stripping, constant ATF draw temperature, and constant pressure.
You may want to consider maximizing your ATF 95%/EP to a limit and maximize side-stripping.
It is better to rely on lab data rather than temperatures, but temperatures are a good indication directionally on what is happening. Consider having additional samples taken so that you may optimize.
Your company may want to consider investing in advanced process control so that operators and engineers do not have to be hands-on in such matters.