11 replies to this topic

[jack123]

Hi
I have some questions related to distillation column operations. Fist of all i would like to present my example of the column. The column bottom is heated by the reboiler with steady heat input. Bottom level is in cascade with FC of the bottom stream. The top vapours are condensed in reboiler with FC on liquid output. The reflux drum level is in cascade with reflux FC. The distillate stream is controlled with FC. Column feed is steady.
Now i would like to present the situation Im concerned about. The objective is to change
a temperature profile in column.
1) decrease SP on distillate stream
2) reflux drum level increase is compensated with reflux flow
3) higher reflux is moving temp. profile down
4) bottom stream is increased
My question is related to column pressure. What will happened with the top pressure? because first reaction when reflux is increased the top is being more cooled down so the pressure should decreased. On the other hand it will take a while to change a temperature profile in the column. When the lighter composition is achieved on the top trays the liquid phase should be easier to vapourise so pressure should increase.
What should be done in this situation to keep the same pressure when the temp. profile is being changed. Should we increase a heat input on the column or decrease the flow rate on the top reboiler?

[pavanayi]

Jack,
You have mentioned about the level and flow controllers in the column. But to help you answer your questions, we need more information like
1. What is the pressure control philosophy?
2. What is the quality control philosophy?

Instead of writing a large body of text, please sketch out the column PFD with streams and instrumentation so it is much easier to understand.

Is the condenser of the distillation column of interest thermally coupled with another reboiler for another column? Or is it a typo that you mention the top vapours are condensed in a reboiler?

Why do you need to change the temperature profile of the column? Quality change in feed? Or is this an academic question?

Edited by pavanayi, 03 April 2012 - 07:04 AM.

[jack123]

Pressure in the column is being controlled by the flow valve on liguid output on the top reboiler. Reboiler is "attached" with other process stream but let assume that the condition of this stream is steady and does not interfere with the column. Quality of the fractionation is controlled by analyzer.
Feed composition and ratio is stable. I would like to know how will column react on temp. profile changes, theoriticaly and reality.
RIght now i am not able to create drawing of my problem.
Thanks for answers

[S.AHMAD]

Dear Jack
1. The pressure of a distillation column is depending on the vapor pressure of the bottom liquid which is temperature dependence. The pressure at the top is the difference between bottom pressure and trays/piping pressure drop.
2. If you increase the bottom temperature, the vapor pressure of bottom liquid increases and thus the the tower pressure also increases. The temperature for each tray will increases correspondingly as well as the top tray temperature. The distillate product quality may be affected since more heavy ends will go into the overhead.
3. Reflux shall be increased to get the right temperature at the top tray in order to get the right product quality
4. By the way what is the reason for changing the temperature profile? Operating at higher temperature requires more energy. Efforts should be directed to minimize energy consumption not to utilize extra energy unnecessarily.
5. The pressure of distillation column is normally determined based on the temperature of cooling medium at the overhead condenser. If the pressure is too low, the overhead vapor could not be condensed. If pressure is too high, energy is wasted.

Edited by S.AHMAD, 03 April 2012 - 11:10 PM.

[pavanayi]

Jack,
In a distillation column, each tray is atleast theoretically in equilibrium. The vapour leaving the tray is in equilibrium with the liquid leaving the tray. (Vapour Liquid Equilibrium). Assuming the composition is constant, there is only one degree of freedom. This means, you either specify the pressure or temperature. They can not be varied independenty. Specifying one will automatically set the other value.

In your case, first let us assume that all controllers are in manual. As you mentioned, if you increase the reflux rate into the column top, the cold liquid will lead to a drop in the top tray tempearture. The amount of vapours exiting the column will decrease. The pressure at the top of the column will start to decrease to a point where it achieves the vapour pressure corresponding to the temperature of the tray. (As I said earlier, if you change one, the other changes automatically).

In reality, since all your controllers are in auto, the response will be more dynamic than the above description. Once you increase the reflux, the top tray temperature decreases. The vapour flowrate exiting the top tray decrease as well. Since your pressure control is in auto, it will reduce the amount of vapours condensed to keep the pressure to its set value. This will lead to your reflux drum level dropping as the liquid input rate to the drum decreases. Since reflux flow is cascased to drum level, it drops automatically.
In the meantime, the column composition profile would also have changed.

Also, since your feed to the column is constant and you have decreased the top product rate, the bottom level will eventually increase and the difference will flow out through bottom product line.

The rate of these changes will depend on your column material holdup and response time for the instruments.

Hopefully that has cleared the questions in your mind.

Edited by pavanayi, 04 April 2012 - 04:24 AM.

[jack123]

Thank You for answer.
According to my example. When I keep the flow steady on the inlet to the reflux drum then when i for example decrease distillate stream 50 kg/h the reflux will increase 50kg/h. In that case the top of the column will be more cooled so there will be temperature drop and pressure drop. In my opinion this state will be kept until the composision on trays will change. If it is obligatory to keep the design pressure then the flow valve on the outlet of the top reboiler should be also closed (lets say the set point on this valve should be decreased 50 kg/h) to keep the pressure steady or on the other hand there should be more heat input to the column bottom. Please confirm if my thinking is right.
Thanks again

[jack123]

Could anyone confirm if my thinking is correct?
Thanks for your help

[S.AHMAD]

1. It is not easy to answer you question since the purpose of you changing the variables are not known. For example, by reducing the distillate flowrate by 50 kg/h, so what is the purpose? What are you trying to achieved? We simply do not change distillation parameters unless we have a good reason such as distillate does not meet product specification.
2. Distillation pressure is FIXED by design which is selected based on the temperature of condensing medium available for the overhead condenser. Once the pressure is selected, it is no longer used as a variable. It must be controlled at the selected design pressure. NO LONGER AN OPTION.
3. For educational sake, let we analyzed what would happen if the distillate flowrate is reduced, with the same pressure controller in your example.
4. If we do not change anything else, once the steady state achieved, the distillate flowrate will returns to its INITIAL VALUE, before the changing made unless the pressure controller is not working as it is designed for.
5. Let assume now that the pressure controller is bypassed. The liquid level in the overhead drum will eventually increased to maximum and this will triggered the reflux flowrate to increase. (cascaded). Increasing the reflux will in turn reduced the top tray temperature. This temperature will change the liquid composition correspond to the pressure at the top of the column (overhead vapor contains more light ends) and thus will eventually change the quality as well as yield of distillate.
6. What happen next? Since the reflux increased, the liquid loading on each tray will also increased. This lead to higher liquid height on each tray and causing higher pressure drop. High pressure drop, result in lower pressure at the top of the column.
7. As the liquid goes down he column, it absorbs more and more light ends. So finally the bottom product contains more light ends and c hanging the quality. The bottom product flowrate increased corresponding to the distillate reduction (our initial adjustment).
8. Composition of bottom contains more light ends, this lead to lower vapor pressure and thus the top pressure will also change such that:
Top pressure = Bottom Pressure - tray pressure drop.
9. Once the system has reached thermodynamic equilibrium, the end results would be summarized as below:
a. Distillate yield will be reduced and contains more light ends.
b. Top temperature will be reduced
c. Reflux flowrate will be reduced level control will be controlling all the time)
d. Bottom flowrate increases and contains more light ends.
e. The bottom pressure (vapor pressure of bottom product) is expected to increase if temperature is maintained and top pressure depends on the tray pressure drop.

Edited by S.AHMAD, 08 April 2012 - 09:54 PM.

[S.AHMAD]

1. Post no#8 is only half of the story.
2. As the bottom temperature is maintained, hot vapor starts to climb up the column. This results in higher trays temperature and thus more vapor to the condenser. More vapor means drum level will increase and the process is repeating until a steady state is achieved.

[jack123]

S.AHMAD
Thank You for the description. I have been thinking about this problem and still some of the problems aren't 100% clear for me.
According to Your description:
Post nr.8
You Have mentioned that the bottom contains more light ends so the bottom pressure will decrease. In this moment the deltaP will return to its initial value (first the top pressure had been reduced so deltaP increased but when more liquid went down to the column bottom and reduced the temperature and pressure the deltaP should return to its initial value?).
You have mentioned that the bottom temeperature is maintaned. I understand that the heat input should be increased to increase vapour flow through the column to the condenser. This will lead to achieve the initial pressure value of the column. My question is: can we instead of increasing the heat input to the bottom of the column reduce the flow on the output of the top reboiler? In my opinion it would cause the same effect.
Post no#9
c) In case when there is just a set point on the flow on the top reboiler output the reflux flow should increase not decrease.
Please correct me if I am wrong.

I have been thinking about this column operation when the top reboiler output flow is in cascade with the column top pressure. When distillate flow is reduced to change the temp. profile first the reflux flow will increase. This will cause the top temp. and top pressure drop. While the top pressure is cascaded with the top reboiler output the flow on the output should reduce itself to keep the pressure fixed. This reduction will cause the decrease in reflux drum level which is cascaded with the reflux flow so the reflux flow will be reduced.
The pressure is fixed but while the composition of the column is changed the temp. profile should be changed ( top and bottom temperature will decrease). So now the column works on fixed pressure but decreased temperatures so there will be more liquid flow through the column which will increase the bottom stream (cascaded with bottom level). In this position can we assume that the steady state has been achieved? Keeping the pressure fixed and changing the temp. profile can we assume that V/L ratio will decrease because the V flow is constant and the L flow has been increased. When pressure is fixed should the deltaP change while changing the temp. profile?
Please correct me if I miss something.

[jack123]

"Keeping the pressure fixed and changing the temp. profile can we assume that V/L ratio will decrease because the V flow is constant and the L flow has been increased."

I am confused in this part because V = R + D, and in this situation D has been reduced and R also has been reduced or lets assume it hasnt increased much (first increased because od the refluf drum level cascade and then decreased to keep the pressure steady). By the way how will the reflux flow behave in this situation? will it be increased according to its initial value?

Edited by jack123, 18 April 2012 - 08:06 AM.

[S.AHMAD]

Dear Jack
1. My explanation in post#8 and 9 is what happen at the initial stage of the adjustment which is not yet a steady state.
2. The end results at steady state will depend on the control philosophy of the system (quality, temperature and pressure) You should respond to Pavanayi request in post#2.
3. Let we see what happen to the overhead material balance where V=R+D. When D is reduced, initially V is still maintained and R is also maintained. By material alance, since what goes in is more than what goes out, there is material accumulation in the drum. Since the R is cascaded to drum level, than the R will increase to drop the level back to its set point. At steady state, R will be maintained (flow control). Therefore, V will be reduced such that the material balance is maintained V = R + D. Since V is reduced but R is maintained, this will increase the L/V ratio in the column and hence the top tray temperature will be reduced such that the liquid composition corresponds to the tray temperature. Since pressure is maintained, temperature is reduced, this will result in lighter distillate, the composition will be corresponding to the dew point at the top tray temperature.
4. The situation will be different if you have reflux flowrate is cascading to the top/overhead temperature and distillate flow control is cascading to the drum level control.

Edited by S.AHMAD, 18 April 2012 - 08:12 PM.