Controller Design Using CHEMCAD

Lecture Contents

• Design of control valve– Control_valve.ccx

• Design of PID controller– PID_Dynamic_Vessel.ccx

• Dynamic distillation with controls perspective

Control Valve

Facts to know:1. Control valve is always used in conjunction with a controller.2. Control valve regulates the flow of stream according to the signal

obtained from controller.3. Control valve is always placed on the stream whose flow rate is

the manipulated variable.4. Needs to be run in steady state first before running in dynamic

mode.5. Downstream pressure from control valve is always less than that

at upstream.6. Valve flow coefficient necessary for simulation is found by sizing

the control valve 7. It is necessary to understand what the emergency mode of

operation of valve is.

Control Valve Design

PID Controllers in CHEMCAD

Facts to Know:

1. PID controller is always on either sides of a control valve.

2. The manipulated variable is always a flow.

3. The measured variable can be any stream or unit operation parameter.

4. Always run the steady state simulation first: This will initialize the streams and also the steady state valve and controller outputs will be calculated.

5. Proportional band(PB)=100/Kc

PID Scheme

SensorFunction

Set Point

Measured Variable

C_Set

C_In

(milli amps)

Error Function PID

C_Out

(Engineering Units)

(To ControlValve)

Variable minimum value corresponds to 4 milli ampsVariable 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

PID-Dynamic Vessel Example

Distillation Control

Tray

Vapor, in

Vapor, outLiquid, in

Liquid, out

Vapor, out

Down Comer

Liquid, out

LiquidHoldup

Take-home messages:1. Greater the vapor-liquid contact, better the separation2. Holdup is directly related to flow rates of vapor and liquid

Distillate Control

Parameters:1. Condenser Duty

2. Condenser Holdup

3. Reflux Ratio

4. Liquid Reflux Rate

5. Distillate RateDistillate (D)

Bottoms (B)

Reflux Drum

Condenser Holdup

Feed(F)

Vapor

Liquid Reflux (L)

Condensate (C)

Bottoms Control

Feed (F)Distillate (D)

Bottoms (B)

Vapor Boil-up (V)

Reboiler

Reboiler Holdup

Parameters:1. Reboiler Duty

2. Reboiler Holdup

3. V/B Ratio

4. Vapor Boilup Rate

5. Bottoms Rate

Liquid

Condenser SpecsConstantHeat Duty - Fixed amount of Liquid Condensate (C)Holdup - Condensate=L+DReflux Ratio - Fixed ratio of L and DReflux Rate - Fixed LDistillate 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.