6 replies to this topic

[USR]

HELLO EVERYONE,

It is well known that tray temperatures in distillation towers are related to the composition of their liquid present at its bubble point in equilibrium with the corresponding vapour at dew point - with the pressure remaining constant. Does that mean that increasing reflux will always bring down the tray temperatures as we are adding more lighter components into the tower? This question relates mostly for the upper trays which are more sensitive to reflux addition.

[Zauberberg]

To answer that question, you will need to come up with a simple PFD showing configuration of the column. There are multiple arrangements and wide variety of schemes involving tray temperature control - it can be control of the bottom product quality, overhead product quality, or control of any of the column side draws. In general, yes it is a sort of indirect composition control.

Based on your question, I would say that you are referring to the case when tray temperature is used for controlling heat input (reboiler duty). If reflux rate is increased, more colder liquid will cascade down the tower and cause additional condensation of upflowing vapors, making the composition of liquid phase lighter on all trays, and their temperatures will start to decrease. The tray temperature controller will act by increasing the flow of heating medium to the reboiler in order to combat against the quenching effect of additional reflux, and bring the tray temperature back to the set point.

[Padmakar Katre]

HELLO EVERYONE,

It is well known that tray temperatures in distillation towers are related to the composition of their liquid present at its bubble point in equilibrium with the corresponding vapour at dew point - with the pressure remaining constant.

Dear USR,
This particular statement is true with the theoretical tray or an actual tray with 100% efficiency. Theoretical tray will have identical temperatures for the liquid and vapor leaving the tray i.e. in mass transfer words vapor is at its dew point and liquid is at its bubble point temperatures. In industrial columns due to lower efficiencies the temperature profile is not like the theoretical trays due to lower or less than 100% efficiency.

Does that mean that increasing reflux will always bring down the tray temperatures as we are adding more lighter components into the tower? This question relates mostly for the upper trays which are more sensitive to reflux addition.

Though your both statements above are contradictory to each other but both statements are true with some provided assumptions. The increase in reflux will bring down the temperature profile since lighters will be refluxed back to column which will condense the vapors flowing upward, this is true when you have constant reboiler duty or the heat input. Whereas in case of the TIC cascaded with the heat input will try to achieve the desired set value for temperature. Increased reflux will bring higher purities but yields will be lower or the excess heat input will be required. Increased reflux is OK to a certain point called Incipient Flood Point beyond this point column will be observed for further decrease in efficiency resulting into poor separation. I will recommend you the readings from Lieberman's book called "A working Guide to process equipments", the first chapter itself will make to understand the phenomenon better.

[USR]

Dear Mr Zauberberg and Mr Padmakar,
The columns i have been operating do not control the tray temperatures by varying the heat input.We provide a fixed heat input sufficient to provide the necessary amount of reflux and without pressurising our column. The control is basically in an upper tray temperature(12th tray in a 30 tray column).I cascade the reflux drum level controller with the reflux flow controller.I have a pressure controller on my column overhead vapour line.If the tray temperature increases i.e. i have more vapours being produced the pressure controller will open up the control valve more to control the column top pressure more vapours will subsequently be condensed in the condenser(i have sufficient condenser capacity).This will increase the reflux drum level and the LC will subsequently increase the reflux to bring down the tray temperature.I think this works quiet well for a 30 tray column.I have been operating another column which has got 80 trays,this has the upper tray temp controller similar to the one I have already described and it has a provision for controlling bottom tray temperature with the heat input as you have described.This loop we are anyhow not using.What could be the impact if i use both the control loops that is control both top and bottom tray temperatures?

[Zauberberg]

USR,

It is difficult to have both control loops active at the same time, as they would tend to "catch up" with each other which will eventually lead to poor and unstable control. Each change of reflux rate or reboiler duty will automatically cause action of the other controller, and the system will never reach equilibrium. Normally, one of the loops is in MAN mode, the other one is in AUTO/CAS.

As said earlier, it's easy to upload the PFD or PID of the column for further analysis. Uploading the file will definitely take less time than typing 12 rows of text.

[AZIZ_MN]

[I cascade the reflux drum level controller with the reflux flow controller.]

hi USR,

Instead of cascading the reflux drum LIC with the reflux FIC, you try to put column tray temp TIC (which you want to maintain temp.) with reflux FIC, then only column bottom TIC cascade with heat input FIC will work, this type of control phylosophy you will find where column top section temp profile is -ve and bottom temp profile is +ve.

[sheiko]

If reflux rate is increased, more colder liquid will cascade down the tower and cause additional condensation of upflowing vapors, making the composition of liquid phase lighter on all trays, and their temperatures will start to decrease.

Zauber, i have seen another logic stating that since the top product is richer in the more volatile components (when the reflux rate is increased), the bottoms becomes richer in the less volatile components. Therefore, the bottom temperature tend to increase (if the column pressure is constant).
Now i am confused. Could you please clarify?

Edited by sheiko, 08 February 2011 - 06:03 PM.