column bottom temp control

[130684]

Dears,

I have an issue regarding the control strategy of the bottom temperature of a distillation column.
The system consists of a distillation column with 2 vertical parallel thermosyphon reboilers heated with hot water.

Problem: how is better to control the bottom temperature of the column?
1. manipulating the hot water flow through each thermosyphon (through a calculation block)?
2. manipulating the total flow of hot water to the reboilers

Notes: the hot water route (pipe length) from the header to each reboiler is different (approx. 7 meter more for one of them)

I would go for the second alternative since one control rule states that "a process variable can be cobtrolled by changing only one manipulated variable". What's your opinion?

Thanks,

 

[25362]

By reading F.G.Shinskey's Distillaton Control, McGraw-Hill, in particular the sections dealing with Decoupling and Multiple-Output Systems, you may find an answer to your query.

 

[hacksaw]

Shinsky is your best bet, just curious, where have you found distallation columns where the process variable was only controlled by one manipulated variable? Sounds like you need to set up your interaction matrix and perform some process testing...

 

[MortenA]

The fluid in the bottom will be boiling and the boiling point is dependent on composition and pressure. So in fact its the stripping of the light components (and sending them overhead together with a fraction of the heavy components) that determines your bottom temperature (assuming that the heating medium side is "hot enough"). I hope that you have taken this into consideration. So if your bottom temperature is too low (compared to whaqt you expect) - maybe something changed in your input fluid?

Best regards

Morten