Controller Design Using CHEMCAD

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

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.

Control Valve Design

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

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

PID-Dynamic Vessel Example

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

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)

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)

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.