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CHE 341-PROCESS CONTROL

SPRING 1999

Tutorial 3

DYNAMICS OF PROPYLENE GLYCOL REACTOR

Developed by V. Bahl under the guidance of Prof. A. Linninger
Version 2: Chandra Singavarapu (April 14, 2000)

 

PROCESS

  • We have a dynamic simulation model of the CSTR. We are producing propylene glycol through the reaction of propylene oxide and water at atmospheric pressure in a CSTR.<download>
  • The reaction is exothermic, so a coolant is circulated to the reactor to maintain the temperature.
  • The process flow diagram shows the control scheme for the reactor, which includes the following three controllers:
  1. Reactor Level Controller (LC) : Controls the level of the liquid in the reactor by manipulating the outflow.

    2.  
  2. Reactor temperature controller (TC) : Maintains the reactor temperature by manipulating the coolant flow.

    3.  
  3. Glycol concentration controller: Controls the concentration of propylene glycol in the reactor outlet stream.

 
 

CONTROLLER PARAMETERS
  1. PV Cell : Shows the current value of the process variable or the controlled variable. For example, the PV for the reactor LC is percent liquid level in the reactor.

    2.  
  2. OP Cell: Displays the current value of the controller output in percent (e.g. the position of the valve).

 

PROCEDURE

We start our experiment from empty conditions i.e. the reactor has no liquid initially.
  • We specify the maximum liquid level in the reactor, which can be attained. Specifying the set point in the LC at 70% does this.
  • We fill the vessel with water until level set point is reached; at which time the reactor LC will open the outlet valve to maintain the level at the set point. Note that the temperature of the liquid in the reactor is the same as the temperature of the water feed i.e. 23C.
  • Once the steady state conditions have been reached we start production of propylene glycol. Because the reaction is exothermic we need a temperature controller. In the temperature controller specify the set point for the temperature say 35C. For production of propylene glycol we have to feed some propylene oxide into the reactor. We open the valve so as to allow the propylene glycol to enter the reactor (Having the glycol concentration controller in the manual mode can do this. Click on the OP Cell of the controller and specify some value 10. This means that the control valve for the propylene oxide feed will open by 10%).

Observation1: We observe that the outlet flow increases to maintain the reactor level because of increased feed. Some propylene glycol appears in the product stream.
  • We now switch the manual mode to auto mode in the glycol concentration controller. We specify a set point for the glycol concentration to say 0.25.

Observation1: We observe that the propylene oxide feed increase because the composition controller wants to raise the concentration of glycol in the product stream.

Observation2: The reactor level rises as a result of higher rate and the product flow increases to maintain the level set point.

Observation3: The reactor temperature rises because the reaction is exothermic and the coolant flow increases to compensate the temperature change.
 
 

RECOMMENDED EXERCISES

  • Repeat the experiment by changing either of the set points and observing the response.
  • Observe what happens when the liquid set point has been specified and the system is integrated from empty conditions to attain the steady state value. What changes occur in the liquid level of the tank? Does the system strictly follow the set point?
  • Since the reaction is exothermic we are maintaining a constant temperature by using a temperature controller. If the set point for the temperature is 35C does the system strictly follow this set point. What is the maximum temperature in the Reaction mix?