3 replies to this topic

[jack123]

Hi all,

 

I have some questions regarding distillation control. As a example I would like to take a standard distillation column. Pressure is steady by the block and bleed valve. Tray temperature is cascaded with reflux flow contoller. The reflux drum level is cascaded with distillate flow controller. The bottom flow controller is cascaded with level in the column bottom. The column is heated by the reboiler where the steam input set point is operated by operator.  There are three temperature indicators at the bottom, regulated tray and at the top of the column. I would like to discuss some situations that take place during process runing.

 

1. First I would like to take a look at the situation when the feed composition is steady, but the feed flow has increased. In my opinion the column control will decrease reflux to keep desired tray temperature. Reflux reduction will cause change in R/F, it will drop so the separation will be poor. As a result we would have more light component in the bottom and more heavy component at the overhead. So to keep desired bottom and overhead composition the operator should increase column heating (increase reboiler steam flow) then the R/F will reach desired value.

 

2. The opposit situation, feed flow is being reduced. Then the reflux flow will increase to keep desired tray tempearute but it could result higher R/F and column will start to flood because of exscesive heating. So the separation will also be poor (same situation like in point 1) In both of this situations distillate and bottom flows will change as the feed flow changes (decrease or increase). 

 

3. The third situation is when the feed flow is steady but the feed composition is being changed. In this particural case the feed will be richer in heavy component. The reflux flow will increase to keep desired tray temperature (in my opinion in this situation tray tempearute will increase). Distillate flow will decrease, bottom flow will increase. The R/F ratio will increase so the operator should decrease steam flow to the reboiler to keep desired R/F ratio. 

 

4. The opposit situation when the feed flow is richer in lighter component. Then the reflux will decrease so the operator should increase steam flow to the reboiler to keep desired R/F ratio to keep good separation. The distillate flow will increase and the bottom flow will decrease.

 

5. The situation when the feed compostion and flow are steady but whe want to change the composition of bottom and distillate flows. For example we want to have distillate flow richer in heavy component then we increase the set point on tray temperatue the reflux flow will decrease, distillate flow will increase, but to keep steady and desired R/F ratio the operator should also increase steam flow to the reboiler.

 

 

My questions are first of all am I correct in what i have written?? What will happened with the temperatures in all this situations i mean when the column will be overheated or will have insufficient heating, how will the temperatures at the bottom and at the top react??( I know that the first think to see the column flooding is column DP increase)

And point 6. when the analysis show that there is for example too much heavy component in the distillate flow (bottom composition is ok) what should the operator do? Should increase heating to the column to increase reflux what will cause better separation? or should decrease tray temperature set point? or should the operator first take a look at the reflux flow and assess the R/F ratio and if the reflux is low then there should do some heating adjustment( heat up) or when the R/F ratio is correct should decrease the tray temperature set point and decrease heating (same philosophy as in point 5) to keep R/F ratio steady?

 

When the analysis show that the bottom and distillate compositons are bad then the main problem is bad separation am I right??

 

Thanks for response

 

 

 

[Bobby Strain]

You should get Shinskey's book. It's old, but it is a good text. If you follow his guides for material balance control, you will get a better control that will allow operation nearer to the specifications. Also, a must read are Henry Kister's books.

 

Bobby

Edited by Bobby Strain, 24 March 2013 - 10:05 PM.

[jack123]

ok, thanks for your help.

 

Maybe is there someone who works professionaly with distillation columns and can admitt if those "guidelines" are correct during column control operations??

Edited by jack123, 25 March 2013 - 07:32 AM.

[Art Montemayor]

Greg Shinskey has the answers to your query in his book(s).

 

Go to:  http://www.cheresour...skey#entry18872

 

for information as to how to obtain his book.