Location of Thermocouple in Distillation Towers
Location of Thermocouple in Distillation Towers
(OP)
What is the best location for a tray thermocouple? I have a thermocouple located at the outlet of the tray downcommer. The temperature fluctuates by 10-15 degrees and is hard to control. Should the thermocouple be on the vapor or the liquid phase? In the downcommer where I can guarantee there is always a liquid level?





RE: Location of Thermocouple in Distillation Towers
Your problem looks like an oscillating control system. With proper tuning of the PID controller parameters you me get rid of the 10 - 15 degrees fluctuations.
A nice site is: http://www.controlstation.com
But of course with any control system it starts with a good measurement. Personally I would never use thermocouples but Pt100 elements, much more reliable in my opinion. What you could do as well is measure pressure. Since you have saturated vapour (liquid / vapour system) pressure and temperature are 1 to 1 related, just as in saturated steam.
Please give some more detail about the control strategy,
CARF
RE: Location of Thermocouple in Distillation Towers
martymartinez126@hotmail.com I will check out your link. I need a crash course on controls!
RE: Location of Thermocouple in Distillation Towers
ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee
RE: Location of Thermocouple in Distillation Towers
The TC701 (Temprerature Controller) is generating setpoints for the FC405 (Flow Controller). This is called cascade or master-slave control. The fast flow control loop is controlling the fast fluctuations in steam flow, whereas the slow temperature control loop is telling the FC405 what steam flow-setpoint is required to reach the desired temperature (= temperature setpoint). Very simple.
Now please don't listen to that friend anymore; the two most likely reasons you do not reach the required temperature setpoint are:
1) Too small steam control valve (check if control valve is 100% open all the time!).
2) Bad TC701 controller tuning (PID-parameters).
If the temperature fluctuates in a sine like wave it is called an oscillating controller and you need to adapt
the PID-parameters in the Controller or Process Control Computer (PLC).
I strongly suggest you connect a datalogger on this system and have a good look on the historical data!
Hope this helps,
CARF
RE: Location of Thermocouple in Distillation Towers