2 replies to this topic

[Clay]

Dear all,

I need some help. I am working now on a depropanizer distillation column (NGL - fractionation train), trying to see how the column react on a top stage pressure change.

Here's the control philosophy of the column :
Top Pressure controller : PT0076
38th stage temperature controller: TT0256 cascade loop with hot oil reboiler
Reflux flow controller : FT0024

I don't understand why, when the top pressure increase, Top and bottom temperature (TT0096 and TT0087) increased while the temperature of the 38 Stage, start to decrease ?

You can also see that the Top pressure has not a real influence on the compositions!
An another hand, do you know (any explication) why the differential pressure between middle and bottom increased (flooding) when the top pressure increase ? (usual it's the contrary, no!)

Please find the attached files.

Many thanks in advance.

Attached Files

•  Top pressure Vs Temperature.jpg   119.84KB   43 downloads
•  Top pressure VS compositions.jpg   114.65KB   34 downloads
•  Top pressure Vs differential bottom pressure.jpg   70.32KB   24 downloads

[Robert Montoya]

Good friend, I was analyzing the case. The fractionation tower I guess having 50stages. Typically the pressure in the top of the tower varies when makes the pressure in the reflux drum. When increasing the pressure in the reflux accumulator, the pressure increases upstream and top vapor is started to condense along the tower and it is possible to cool the tower and increase the percentage of propane at the bottom of the tower. When the vapor condenses into the tower, the liquid is cooled and the steam heated in the plates, when the sensor temperature in tray 38 senses the temperature, increases the flow of hot oil or steam in the reboiler and observed an increase in temperature in the bottom of the tower.
Is interesting to note, under the same scenario if the vapor of the top remains unchanged and the percentage of volatiles in the bottom should increase.
Importantly, if possible, the temperature profile in the various stages.
Reflux is cascaded with any tray in the tower?
If you have the temperature profile along the tower you can send. for better understanding.
I hope I've helped.

[S.AHMAD]

1. You have not fully explained the control philosophy of the unit control system. For example, the distillate flow conroller is it cascaded with accumulator drom level? or only level controller?
2. Have you giving enough time for the system to reach steady state? Normally for a testrun like this we may allow enough time for the unit to stabilize and maintain for 24hrs before collecting any representative data.
3. My understanding (plkease correct me if I am wrong) is that, if pressure increases and distillate flowrate and quality is maintained, all trays temperature should increase correspondingly. Since temperature of tray 38 reduced, this an indication that the system has not yet reached a steady state.
4. When pressure increased, initially condensensation increase this will result in higher liquid traffic. That explain why pressure per each tray increases correspondingly and this condensation resullt in more C3 conbtents in each tray and hence lowering the tray temperature (lower bubble point).
5. In order to correct the situation, tray 38 set temperatuere should be increased to higher temperature in order to compensate for higher pressure.
6. If adjustment to tray 38 temp. not made, giving enough time, the bottom will contains more C3. Distillate composition may not change very much, but the flowrate reduced and bottom flowrate increased corresponding ly.
7. Since your field observation contradicted to theory, I suspect that the unit had not yet raeched a steady state.
8. If you provide enough data before and after the testrun, we probably can analyzed better. Data should be complete, partial data may not help much.