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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
27/06/2019 A: keith bowers, B and B Consulting, kebowers47@gmail.com
You have posed a question that is a little ambiguous. 'Vapor Pressure' can be either REID Vapor Pressure' or True Vapor Pressure. For a mixture, the answers are different. True Vapor Pressure (at a specified temperature) is that measured when the first bubble of vapor is formed. Reid , Vapor Pressure is that indicated by a sample at 100 F, in a closed container with a 5 unit volume submerged in a 100F water bath starting with 4 volumes of water saturated air @ 100F and 1 volume of sample chilled to a temperature low enough to practically avoid any sample evaporation.
While BOTH measures give an indication of the 'light ends' content of a hydrocarbon mixture, they can be significantly different. An extreme example would be comparing the two measured values of say 99ml of diesel containing 1 ml of propane.
Distillation uses thermal energy (work) to separate low boiling components from high boiling ones. Increasing the WORK input increases the Entropy of the sample indicated by the separation of lower boiling(smaller, lighter) molecules from higher boiling (larger, heavier) molecules.
If there is only a very small amount of 'very light ends' in the sample, and separating it by more WORK so most of it is in the 'vapor space' above a small pool of equilibrium liquid (amount of liquid in the Reflux Drum), purging that 'vapor' from the small amount originally in the whole sample will more rapidly decrease the measured 'vapor pressure' of the remaining 'heavy ends' more than one might otherwise anticipate.
Measured 'Vapor Pressure' is not a good property to use for distillation control as it can be highly non-linear as in the diesel:propane example. While there may be a 'specification value', it is not appropriate for 'composition control' of distillation in most instances. ' Distillation' in most refinery situations is composition driven--separating a mix of similar molecules with differing molecular weights. '
Stripping' is the term most applicable to removing very small amounts of much lighter molecules from the bulk liquid, for example 'stripping H2S' from Jet Kerosene. The amount of the removed H2S maybe less than 0.01% of the kerosene, but the measured corrosion tendency (copper strip Corrosion test) of the jet fuel will fail the specification.
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.