Levels Of Process Control

What are the Levels of Process Control?

  • Basic Regulatory Control
  • Advanced Regulatory Control
  • Multivariable, Model-Based Control
  • Constrained Economic Optimization
  • Multi-unit constrained Economic Optimization
  • First principle Economic Optimization (RTO)

Basic Regulatory Control (BRC) is called a Single sensing element. Input -Single final control element Output (SISO) control. All advanced control methods depends on Basic Control. The primary goal of this control is operational stability & disturbance rejection and provides operators with a handle to control process. For Simple Cascade Controllers, Ratio Controllers.

Advanced Regulatory Control (ARC) is called Multiple Input Single Output (MISO) control. The primary goals are enhanced stability & enhanced performance. It improves over performance achieved with BRC. It is normally still limited to DCS tools.

For example

  • Pressure or temperature compensation
  • Feedforward based on measurements
  • Simple process calculations based on measurements (heat and material balance, simple inferential estimations, duty control)

Multivariable, Model-Based Process Control (MPC):

It is called Multiple Input Multiple Output (MIMO) control. It simultaneously considers many controlled variables (CVs) e.g. process qualities by adjusting many manipulated variables (MVs), e.g. flowrates temperatures and optionally observing feedforward or disturbance variables (DVs). Internal model of process behavior predicts future paths of controlled variables.

Constrained Economic Optimization:

User specifies an objective function & assign costs or value to variables and translates into a direction to move the manipulated variables. Optimizer finds best position (corner) within space constrained by controller and manipulated variables limits. The Optimizer is a “steady state controller” calculates one set of MV moves to maximize or minimize objective function. It is generally too aggressive to implement directly in closed loop and typically used in conjunction with a multivariable control engine to implement solution.

Multi-Unit Constrained Economic Optimization:

Constrained Economic Optimization are applied to multi units.

First Principle Economic Optimization (RTO):

In RTO, the optimum values of the set points are recalculated on a regular basis (e.g., every hour or every day). These repetitive calculations involve solving a constrained steady-state optimization problem.

Necessary Information:

  • Steady-state process model
  • Economic information (e.g., prices, costs)
  • A performance Index to be maximized (e.g., profit) or minimized (e.g., cost). The last two points sometimes referred to as an economic mode

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