Introduction to Process Control and Instrumentation

1. Chemical Plants and Their Objective

  • A chemical plant is constituted by assembling chemical units such as reactors, distillation columns, pumps, and compressors.
  • The primary objective of a chemical plant is to receive raw material (input) and, using available sources of energy, produce products (output) in the most economical way.

2. Requirements for Meeting Plant Objectives

To meet the objective of producing products economically from raw materials, the following must be ensured:

  • Safety: E.g., a reactor designed to operate within a specific pressure limit (e.g., 100 psig) must stay below that limit through external intervention.
  • Production Specifications: The process must meet both quality and quantity goals for the product.
  • Environmental Requirements: Regulatory compliance such as:
    • Chemical concentrations in effluent
    • SO₂ emissions
    • Wastewater discharge limits
  • Operational Constraints: Examples include:
    • No flooding in distillation columns
    • Tanks not overflowing or drying out
    • Reactor temperatures within safe limits
  • Economics: Optimize raw material, energy, and labor use for minimum cost and maximum profit.

3. Need for a Control System

  • All the above requirements need external intervention, provided by a control system.

4. Important Issues in Control Systems

A control system deals with:

  • Influence of external disturbances
  • Stability of the chemical process
  • Performance optimization

Goal: Suppress disturbances, maintain stability, and optimize performance.

5. Dealing with External Disturbances

Example: Tank Heating System

  • Components:
    • Inlet: Flow rate ( F_i ), Temperature ( T_i )
    • Outlet: Flow rate ( F ), Temperature ( T )
    • Steam: Flow rate ( F_{st} )
    • Variables to control: Height ( h ), Temperature ( T )

Objective:

Maintain: - ( T = T_d ) (desired temperature) - ( h = h_d ) (desired height)

Control System Operation:

  1. Measure ( T ) with a sensor.
  2. Compare ( T ) with ( T_d ).
  3. Calculate error: ( e = T_d - T ).
  4. Controller generates signal to adjust ( F_{st} ).

Controller Response:

  • If ( e > 0 ): Increase ( F_{st} )
  • If ( e < 0 ): Decrease ( F_{st} )

Disturbance Example: If ( F_i ) increases (but ( F_{st} ) is unchanged), then: - ( T ) drops - Controller detects ( e > 0 ), increases ( F_{st} )

6. Ensuring Process Stability

  • Stable Process: Returns to steady state automatically after disturbance.
  • Unstable Process: Does not return to steady state without intervention.

Even for stable processes, control may be needed to manage disturbances or optimize efficiency.

7. Optimizing Process Performance

Example: CSTR with Reactions

( A B C )

  • B: Desired product
  • C: Undesired product

Economic Goal:

Maximize: - Revenue from ( B ) - Minimize costs of: - Reactant ( A ) - Coolant (to remove exothermic heat)

Control System assists in optimizing this profit.

8. Classification of Variables

Input Variables:

  • Load/Disturbance Variable (LV)
  • Manipulated Variable (MV)

Output Variables:

  • Measured Output
  • Unmeasured Output

Controlled Variable (CV): Often a measured output but may also be unmeasured.

9. Variable Classification Examples

Simple Liquid Tank

  • Input: ( F_i )
  • Output: ( F_0 ), ( h )
  • CV: ( h ), MV: ( F_0 )
  • Control action: adjust ( F_0 ) to maintain ( h = h_d )

Heating Tank

  • Inputs: ( F_i ), ( F_{st} ), ( T_i )
  • Outputs: ( F ), ( T ), ( h )
  • CVs: ( T ), ( h )
  • MVs: ( F_i ), ( F_{st} )
  • LV: ( T_i )

10. Introduction to Control Configurations

  • Defines how variables interact in a control system.
  • The simplest is feedback control, discussed next.

11. Feedback Control Scheme

Example: Distillation Column

Setup:

  • Feed ( F ), Composition ( Z )
  • Overhead vapor → condenser → reflux drum
  • Output streams:
    • Distillate: ( D ), Composition ( x_d )
    • Bottoms: ( B ), Composition ( x_b )

Objective:

Maintain ( x_d ) at desired value

Feedback Control Setup:

  1. Measure ( x_d )
  2. Compare with desired ( x_d )
  3. Calculate error signal
  4. Controller adjusts reflux flow rate ( R )

In feedback, CV is measured and used to compute control action.

12. Future Topics

  • Feedforward control
  • Inferential control
  • Advanced control structures