3 replies to this topic

[PIN2]

Hello everybody,

I am a newbie in a lube refinery and looking after a vacuum distillation unit . The problem faced here is poor vacuum in the tower . The flash zone of the tower experiences a pressure of 150 torr ( vs des. 129 torr) and the overhead of the tower experiences a pressure of 110 torr ( vs des. 71 torr ) . But the make of the tower overhead gas is quite high ( actual 3.5 wt% vs des. 1.5 wt% ) . The tower overhead temp. remains around 112 - 114 deg C ( vs des. 112 deg C ) .
My doubt is that if the top pressure remains high , then it should have led to less vaporisation in the tower overhead with the current amount of top pumparound owing to increase in dew point and thus leading to low make of overhead gases . But what is happening is exactly opposite to what I am thinking .

Please suggest any possible reason and remedial actions for the above and also any possible flaw in my thought process.
Please feel free to ask for any additional process data required to be provided .

Thank you,

Edited by PIN2, 16 February 2011 - 11:44 AM.

[Zauberberg]

If you are 100% positive with regards to the actual gas yield, then you have probably identified the culprit of poor equipment performance. Whatever is your vacuum system, it has certain gas/vapor handling capacity and anything above that will cause additional backpressure on the column.

Look at the heater outlet temperature, and see if decreasing the same brings any benefits. Do you use stripping and/or heater coil ("velocity") steam? Do your overhead condensers perform as designed? And finally, is this a problem that started occur recently, or it has been there since the unit was commissioned?

Good luck,

[PIN2]

If you are 100% positive with regards to the actual gas yield, then you have probably identified the culprit of poor equipment performance. Whatever is your vacuum system, it has certain gas/vapor handling capacity and anything above that will cause additional backpressure on the column.

Look at the heater outlet temperature, and see if decreasing the same brings any benefits. Do you use stripping and/or heater coil ("velocity") steam? Do your overhead condensers perform as designed? And finally, is this a problem that started occur recently, or it has been there since the unit was commissioned?

Good luck,

Thank u Zauberberg for your response..
Yes, we use coil steam in the furnace and we also use steam as a stripping medium in the vacuum tower as well as in the sidestream strippers..The vacuum ejectors use medium pressure steam as a motive fluid ( ~ 10 kg/cm2)..Decreasing the heater O/L temp. has had but little effect on the overhead gases make..Although there is a problem with the overhead condensers..As per design the condensers should see 1500 m3/hr of salt cooling water through them, but the maximum they see is 1200 m3/hr..
This problem of high overhead gases and poor vacuum has been there for many years now..
But my doubt still remained unanswered..I can understand poor equipment performance leading to poor vacuum in the tower..but what is the reason for high gas yield at the top?? Shouldn't it had come down due to already existing high pressure at the top ??
The high gas yield at the top is deteorating the vacuum further..bcoz the already water starved overhead condensers are not able to cool all the gases which are giving backpressure to the tower ...
Please give your thoughts on it..and also any further data you require ...

Thank you..

Edited by PIN2, 17 February 2011 - 11:14 AM.

[Zauberberg]

Obviously, as you have noted yourself, the overhead condensers operate with 80% of design coolant flow. That translates into approximately the same reduction of the jet ejectors processing capacity. That is one side of the problem.

The other side is that there are additional quantities of overhead gas, irrespective of the overhead system performance. This could be due to excessive cracking in the furnace, excessive steam flow (both velocity and/or stripping steam), massive in-leakage of air somewhere in the system, or even partially flooded surface condensers in the overhead system.

What I would do, is to assess gravity of situation, and test all these factors, one by one, and watch their influence on the overhead system. You can't convince me that decreasing the heater outlet temperature or steam flow(s) does not make any changes in system performance, unless there is really an outstanding ingress of air into the vacuum system, or flooded vacuum condenser due to fouling or air leak through the barometric leg of the condenser(s).