Controller Design Using CHEMCAD

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Controller Design Using CHEMCAD
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Lecture Contents

• Design of control valve
• Control_valve.ccx
• Design of PID controller
• PID_Dynamic_Vessel.ccx
• Dynamic distillation with controls perspective

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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.

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Control Valve Design
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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

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PID Scheme
Set Point
C_Set
Error Function
C_Out
Sensor Function
PID
C_In
Measured Variable
(To Control Valve)
(milli amps)
(Engineering Units)
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
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PID-Dynamic Vessel Example
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Distillation Control

• Take-home messages
• Greater the vapor-liquid contact, better the
  separation
• Holdup is directly related to flow rates of vapor
  and liquid

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Distillate Control
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)
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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)
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Condenser Specs

• Constant
• Heat Duty - Fixed amount of Liquid
  Condensate (C)
• Holdup - CondensateLD
• 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.