Chapter 12
Controlling the Process
12.1 The Need for Process Control
• Process variables (ex. flow rate, temperature) vary with time
Stable, but
widely varying
Unstable
Wellcontrolled
Process
Variable
of
Interest
desired value
time
time
time
• A method of controlling the process to keep variations within a narrow band around the desired value
is needed (essential element of modern chemical processes)
12.1 The Need for Process Control
• Flow rate of HCl leaving the holding tank would need to be adjusted frequently (valve!)
12.1 The Need for Process Control
• Various types of valves
12.1 The Need for Process Control
• Many years ago, processes were controlled manually by people (operators)
: from the readings on the dials and gauges, and based on their experience and training
• A fully automated control system is much better alternative
: input signal from a sensor to controller, then output signal to valve (operate w/o human involvement)
12.1 The Need for Process Control
• Process control (definition)
: maintaining a process output (ex. conc., temp., flow rate) within desired specifications by continually
adjusting other variables in that process
• Controlled variable (liquid level), setpoint (SP, value of controlled variable, ex. 2 ft), input variables
(flow rate of HCl into and out of the tank)
• Disturbance (uncontrolled changes), error (difference between liquid level and setpoint)
12.2 Feedback Control
• Controlled variable is monitored over time via sensor (level sensor, flow meter, and so on)
→ using above values, the feedback controller adjusts an input variable via affectors (valves)
• Feedback control loop: the structure consisting of sensor, controller, affector, conmmunication lines
12.2 Feedback Control
• pH control: pH sensor → concentration controller (CC) → NaOH flow rate
• Temperature control: outlet temperature sensor → temp. controller (TC) → cooling water flow rate
12.2 Feedback Control
• The value of controlled variable must be different from the setpoint (i.e. there should be “error”)
• As the value of controlled variable approaches setpoint, the controller will make adjustments
• Goal: to drive the value of controlled variable to the setpoint as rapidly as possible (below (a) case)
12.2 Feedback Control
12.3 Feedforward Control
• Feedforward controller PREDICTS the effect of the disturbances on the output and adjusts the
manipulated variable to offset the predicted effect
• Unlike feedback control, the output (controlled) variable is not measured, but the adjustment of the
manipulated variable is based solely on expectation (from a mathematical model)
12.3 Feedforward Control
• Measuring disturbance (a sensor) → predict the effect of changes on the final pH
→ determine output signal to adjust NaOH flow rate
• If HCl flow rate increased, the feedforward controller immediately increase NaOH flow rate,
WITHOUT waiting for a change in the final pH to occur before responding
• An error or deviation from setpoint is not needed here (unlike feedback controller)
12.4 Comparison of Strategies
Feedback Control
Pros
Cons
Feedforward Control
- Actually measures the controlled
- Before controlled variable is affected
variable, and capable of responding to
- Potential of preventing deviation
all disturbances
- There must be an error
- Relies on a model to predict (no
- It may require a long time to come
direct measurement)
back to the setpoint
- If the model fails, it will lead
deviations, and cannot be corrected
• Feedback control is most commonly used, and usually combined with feedforward control (when it
has particular adventages) to achieve the best of both strategies
12.4 Comparison of Strategies
• Three feedback control loops: HCl holding tank level, pH of neutralization reaction, outlet temp.
• One feedforward loop: pH of neutralization reaction in combination with feedback loop