1. Controller Design Using CHEMCAD

2. Lecture Contents Design of control valve Design of PID controller
Control_valve.ccx
Design of PID controller
PID_Dynamic_Vessel.ccx
Dynamic distillation with controls perspective

3. Control Valve Facts to know:
Control valve is always used in conjunction with a controller.
Control valve regulates the flow of stream according to the signal obtained from controller.
Control valve is always placed on the stream whose flow rate is the manipulated variable.
Needs to be run in steady state first before running in dynamic mode.
Downstream pressure from control valve is always less than that at upstream.
Valve flow coefficient necessary for simulation is found by sizing the control valve
It is necessary to understand what the emergency mode of operation of valve is.

4. Control Valve Design

5. PID Controllers in CHEMCAD
Facts to Know:
PID controller is always on either sides of a control valve.
The manipulated variable is always a flow.
The measured variable can be any stream or unit operation parameter.
Always run the steady state simulation first: This will initialize the streams and also the steady state valve and controller outputs will be calculated.
Proportional band(PB)=100/Kc

6. PID Scheme Set Point C_Set Error Function C_Out Sensor Function PID
C_In
Measured Variable
(To Control
Valve)
(Engineering Units)
(milli amps)
Variable minimum value corresponds to 4 milli amps
Variable maximum value corresponds to 20 milli amps
C_Out is the controller output in milli amps
Set Point value is scaled between 4 and 20 milli amps

7. PID-Dynamic Vessel Example

8. Distillation Control Vapor, out Vapor, out Liquid, in Liquid Holdup
Down Comer
Liquid, out
Liquid
Holdup
Tray
Vapor, in
Vapor, out
Liquid, in
Liquid, out
Take-home messages:
Greater the vapor-liquid contact, better the separation
Holdup is directly related to flow rates of vapor and liquid

9. Distillate Control Parameters: Condenser Duty Condenser Holdup
Condensate (C)
Parameters:
Condenser Duty
Condenser Holdup
Reflux Ratio
Liquid Reflux Rate
Distillate Rate
Vapor
Condenser
Holdup
Reflux Drum
Feed
(F)
Liquid
Reflux (L)
Distillate (D)
Bottoms (B)

10. Bottoms Control Parameters: Reboiler Duty Reboiler Holdup V/B Ratio
Vapor Boilup Rate
Bottoms Rate
Distillate (D)
Feed (F)
Reboiler Holdup
Bottoms (B)
Liquid
Reboiler
Vapor Boil-up (V)

11. Condenser Specs Constant
Heat Duty Fixed amount of Liquid Condensate (C)
Holdup Condensate=L+D
Reflux Ratio Fixed ratio of L and D
Reflux Rate Fixed L
Distillate Rate - Fixed D
Take-Home Messages:
1. Identify Free and Fixed parameters before attempting controller design.
2. Only Free paramters can be manipulated by controllers.