4 replies to this topic

[omarax]

Dear all,
I am working on my graduation Project and i am asked to make a complete simulation for a simple binary distillation coloumn control using LabView software.
So far I have studied the control strategies and the different control schemes so please correct me if wrong.

The control variables are:

Top and bottom Compositions
Condenser drum level
Bottom level

Compositions are controled by controlling bottom and top trays temperatures.
Bottom temperature is varied by varying the steam flow rate in the reboiler

Levels are controlled by the bottoms and distillates flow

I assumed that Pressure is perfectly controlled.

so

The coloumn control is a MIMO and i want to break it first into SISOs

how can i get a model or transfer function for these variables or should i develop a mathematical model from the mass, material and energy balances???

also i have a question..
Is the reflux ratio kept constant for the coloumn??? i.e when controlling top temperaure using reflux flow, does the distillate flow change as well to keep L/D constant.

Thanks in advance

[abhi_agrawa]

Omarax,

a) It is very important that you should control the pressure at the top of the tower. In real world you will never get the exact same feed all the time and separation is influenced strongly by the pressure of the tower.

You say that you control the top and bottom product composition by varying the heat input to the tower. You control the distillate and bottom flow based on level, to me it seems that this could create problem in maintaining the material balance ie. feed in = product out.

c) The reflux ratio varies to get the required distillate purity. It depends on the quality of feed.

Hope this helps,
Abhishek

[omarax]

Thanks for your concern
How is Column pressure controlled? Which Manipulated variable is used?
i read that the cooling water flow in the condenser is used to control the coloumn pressure but i dont understand how pressure is affected by cooling water flow??

Is there a mathematical model or transfer function for pressure control??

Thanks

[djack77494]

Thanks for your concern
How is Column pressure controlled? Which Manipulated variable is used?
i read that the cooling water flow in the condenser is used to control the coloumn pressure but i dont understand how pressure is affected by cooling water flow??

Is there a mathematical model or transfer function for pressure control??

Thanks

Column pressure is most often controlled by controlling the overhead flowrate out of the column. Under some circumstances, you might control the top pressure using the heat removed (i.e. condenser duty). To some extent this is done by controlling the reflux flow. You imply that that is done in a different way when you talk about manipulating cooling water flow to the condenser. Though I don't see anything inherently wrong with the concept of manipulating condenser duty, it would not be wise to throttle the cooling water flowrate. this is best kept constant to minimize fouling.

[abhi_agrawa]

Oramax,

The most effective scheme to control the column pressure depends on the type of condenser. Go though the article : A Soley, Effectively Control Column Pressure, CEP January 2001.

If you can upload a sketch of your configuration we will be in a better position to advise you.

abhishek