9.1 Basic control concepts and terminology

Process control is crucial in chemical engineering, ensuring systems run smoothly and efficiently. It involves managing key variables like temperature and pressure to maintain desired setpoints, optimizing processes, and enhancing safety.

Understanding basic control concepts and terminology is essential for grasping how control systems work. From setpoints and process variables to feedback loops and control elements, these fundamentals form the backbone of effective process management in chemical engineering.

Process Control Terminology

Key Terms and Definitions

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• Setpoint: The desired value or target for a process variable that the control system aims to maintain
  • Example: Setting the temperature setpoint of a reactor to 350°C
• Process variable: A measurable quantity or parameter that is being controlled or monitored in a process, such as temperature, pressure, flow rate, or composition
  • Example: Monitoring the pH level of a solution in a neutralization process
• Manipulated variable: An adjustable parameter that can be changed by the controller to influence the process variable and bring it closer to the setpoint
  • Example: Adjusting the flow rate of steam to control the temperature of a heat exchanger
• Controlled variable: Another term for the process variable, referring to the quantity being controlled to maintain it at the desired setpoint
  • Example: Controlling the liquid level in a storage tank
• Error: The difference between the setpoint and the actual value of the process variable at any given time
  • Example: If the setpoint is 100°C and the measured temperature is 95°C, the error is 5°C
• Feedback: The information about the current state of the process variable that is sent back to the controller for comparison with the setpoint
  • Example: A thermocouple sends the current temperature reading back to the controller for comparison with the desired setpoint

Process Control System Components

• Sensor or measuring device: Measures the current value of the process variable and sends the information to the controller for comparison with the setpoint
  • Example: A pressure transducer measures the pressure inside a reactor vessel
• Controller: Receives the measured process variable value, compares it to the setpoint, and calculates the necessary adjustment to the manipulated variable based on the control algorithm
  • Example: A PID controller determines the appropriate change in valve position to maintain the desired flow rate
• Final control element: Receives the control signal from the controller and adjusts the manipulated variable accordingly to influence the process variable
  • Examples include valves, pumps, or heating elements
  • Example: A control valve adjusts its opening to regulate the flow of coolant in a heat exchanger
• Process: The system or equipment being controlled, where changes in the manipulated variable affect the process variable
  • Example: A distillation column where the reflux ratio is manipulated to control the product composition

Process Control Benefits

Optimization and Efficiency

• Process control aims to maintain process variables at their desired setpoints, ensuring stable and efficient operation of chemical engineering systems
  • Example: Controlling the feed rate and temperature of a reactor to maximize product yield
• Implementing process control helps to optimize product quality by keeping key variables within acceptable ranges, reducing variability and off-spec products
  • Example: Maintaining the pH of a fermentation process within the optimal range for cell growth and product formation
• Effective process control improves process efficiency by minimizing energy consumption, raw material usage, and waste generation
  • Example: Implementing advanced control strategies to minimize utility consumption in a distillation process

Safety and Consistency

• Process control enhances safety by preventing variables from reaching dangerous levels and enabling rapid response to disturbances or abnormal conditions
  • Example: Implementing safety interlocks to shut down a process if the pressure exceeds a critical threshold
• Automated process control reduces the need for manual intervention, allowing for more consistent operation and freeing up personnel for other tasks
  • Example: Using a distributed control system (DCS) to automate the operation of a large-scale chemical plant

Feedback Control Loop Components

Measurement and Comparison

• Sensor or measuring device: Measures the current value of the process variable and sends the information to the controller for comparison with the setpoint
  • Example: A flow meter measures the flow rate of a liquid in a pipeline
• Controller: Receives the measured process variable value, compares it to the setpoint, and calculates the necessary adjustment to the manipulated variable based on the control algorithm
  • Example: A proportional-integral (PI) controller determines the appropriate change in valve position to maintain the desired level in a tank

Actuation and Process Interaction

• Final control element: Receives the control signal from the controller and adjusts the manipulated variable accordingly to influence the process variable
  • Examples include valves, pumps, or heating elements
  • Example: A variable speed drive adjusts the speed of a pump to control the flow rate of a process stream
• Process: The system or equipment being controlled, where changes in the manipulated variable affect the process variable
  • Example: A heat exchanger where the cooling water flow rate is manipulated to control the outlet temperature of the process fluid
• Feedback loop: The continuous cycle of measuring the process variable, comparing it to the setpoint, and adjusting the manipulated variable to minimize the error and maintain the desired setpoint
  • Example: A temperature control loop continuously measures the temperature, compares it to the setpoint, and adjusts the heating power to maintain the desired temperature

Open-Loop vs Closed-Loop Control

Open-Loop Control Characteristics

• Open-loop control operates without feedback, where the controller adjusts the manipulated variable based on a predefined sequence or schedule, regardless of the actual process variable value
  • Example: A timer-based control system that turns on a valve for a fixed duration to add a certain amount of reactant to a batch reactor
• In open-loop control, there is no direct measurement of the process variable, and the system assumes that the desired outcome will be achieved by following the predetermined control actions
  • Example: A conveyor belt that moves at a constant speed, assuming that the desired product flow rate will be achieved without measuring the actual flow

Closed-Loop Control Characteristics

• Closed-loop control, also known as feedback control, continuously measures the process variable, compares it to the setpoint, and adjusts the manipulated variable based on the error signal
  • Example: A level control system that measures the liquid level in a tank, compares it to the desired level, and adjusts the inlet or outlet flow rates to maintain the level at the setpoint
• Closed-loop control actively responds to disturbances and deviations from the setpoint, making necessary corrections to maintain the process variable at the desired value
  • Example: A pH control system that measures the pH of a solution, compares it to the desired pH, and adjusts the flow rate of an acid or base to maintain the pH at the setpoint
• Closed-loop control is more effective in handling process disturbances and maintaining stable operation compared to open-loop control, as it adapts to changing conditions based on real-time feedback
  • Example: A closed-loop temperature control system can maintain a constant temperature in a reactor despite changes in the feed temperature or composition, while an open-loop system would not be able to compensate for these disturbances