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In a naphtha stabilizer, top tray (tray 46) temperature is controlled by the reflux and the bottom tray (tray 3)temperature is cascaded with the hot light gas oil flow (reboiler's hot medium). Top pressure (overhead vapor pressure not the OVHD drum pressure) is controlled by the removal of non-condensed vapors from OVHD accumulator. Here, when I increase the bottom tray temperature, top pressure is getting increased slightly (very minimum) and then OVHD vapor pressure is getting decreased faster. Can anyone explain this phenomenon? Note: Distillation tower - 48 trays column; OVHD product - LPG; Bottom product - Stabilized naphtha.
 
Answers
08/04/2019 A: Krishna Chaitanya Voodi, Schneider Electric India Limited, krishnachaitanyavoodi@yahoo.in
So, If you increase the bottom temperature set point. Re boiling Hot Oil Medium will increase to reach the set point of the bottom tray temperature.(This might be slightly aggressive response. That's OK.)
Now Since the bottom fluid started increasing its temperature(PV). The flow of vapours from the Column starts to increase with an increase in Top Temperature as well as Top Pressure. Now the Master Controller at the top takes action to control the set point of Top Temperature by increasing the reflux since its cascaded. Now if the Vapour pressure is getting decreased faster that means either its condensing more OR Check the trend of DP of the Overhead Condenser VS Column Pressure as DP is directly proportional to reflux flow. Check this, if the response is linear then slightly reduce the P Value in the Overhead Master PID Controller.
14/03/2019 A: Bruce Carr, Suncor Inc, bcarr@suncor.com
What is it you are trying to accomplish by increasing the bottom tray temperature?
It's all about heat balance if you put more heat in the bottom eventually the top temperature will increase and the reflux rate will increase to maintain the temperature set point. If you are trying to maximize the separation you want to have the highest delta T between the bottom of the tower and the top. This is accomplished by reducing the top temperature, increasing the reflux while maintaining the bottom tray temp.
Note: if you reduce the top temperature there should be an increase in the duty of the reboiler as eventually the increase in reflux will cool the whole tower down. The reboiler will have to increase to compensate. If the reboiler duty doesn't increase then you have too much reflux and passed the incipient flood point and you are starting to top flood.
14/03/2019 A: Jayaraj Jayam, Chennai Petroleum Corporation Limited, njayaraj@gmail.com
When reboiling temp is raised, more vaporization takes place causing as you said mild raise in pressure. However, the additional temperature vaporizes the heavy ends compare with the vapor generated at normal temperature. This slightly heavy vapor readily condensed when meets low temperature at OVHD condenser. Due to the activation of condensation, it tends to condense more lighterends along with comparatively heavy ends. Point to note is RVP of your bottom product ie stabilized naptha will get reduced further which is generally not an issue. LPG weathering test to be checked, adjust reflux flow to maintain your tray#46 temperature.