Process Unit Control System Design

1. Introduction

2. Influence of process design

3. Control degrees of freedom

4. Selection of control system variables

5. Process safety

Introduction

Control system requirements

» Safe operation

» Satisfy environmental regulations

» Stable plant operation

» Satisfy specifications on product quality and production rate

» Economic plant operation

Control system design

» Select controlled, manipulated and measured variables

» Specify control structure and controller type

» Select controller tuning parameters

Multivariable Control Strategies

Multivariable processes » Multiple input and output variables

» Requires multivariable control strategy

Multiloop (decentralized) control » Manipulated inputs and controlled outputs are paired

together

» A controller is designed for each input/output pair

» Simple to design and often provides satisfactory performance

Multivariable control » Manipulated inputs and controlled outputs are not

explicitly paired

» All the inputs are used simultaneously to control all the outputs

» Often necessary for processes with slow dynamics, strong variable interactions, and/or variable constraints

Influence of Process Design

Process design

» Invariably performed prior to control system

design

» Based on steady-state analysis

» May produce processes that are very difficult to

control dynamically

Integrated process design and control

» Dynamic operability considered during the

process design stage

» Currently a popular research topic

» Starting to be practiced in industry

Heat Integration

• Increase interactions between columns

• Lose second column reboiler duty as a

manipulated variable

• Increase interactions between units

• Introduces positive feedback

• Lose hot oil flow as a manipulated variable

Control Degrees of Freedom

Process degrees of freedom (NF) » NF = NV - NE

» NV = number of process variables

» NE = number of independent model equations

Control degrees of freedom (NFC) » NFC = number of process variables that can be

independently controlled

» NFC = NF - ND

» ND = number of disturbance variables

» NFC usually can be calculated as the number of independent material and energy flows that can be manipulated

Feedback control usually does not change NFC

Distillation Column Example

Manipulated variables: B, D, R, qc, qh

NFC = 5

Controlled variables: xB, xD, hD, hB, P

Selection of Control System Variables

Output variables » Measured variables – variables that can be

measured

» Controlled variables – usually a subset of the measured variables

Input variables » Manipulated variables – inputs that can be

manipulated

» Disturbance variables – inputs that cannot be manipulated and are determined outside the control system

Basic requirement » Number of manipulated variables must be greater

than or equal to the number of controlled variables

Selection of Controlled Outputs

All outputs that are not self-regulating must be controlled » Liquid level in a tank with outlet pump

Select outputs that must be maintained within equipment and operating constraints » Polymer reactor temperature

Select outputs that directly represent product quality or strongly affect product quality » Distillation column overhead product composition

Select outputs that strongly interact with other output variables » Steam header pressure in steam generation unit

Select outputs that have favorably steady-state and dynamic characteristics » Large steady-state gains, small time constants, and

small time delays

Selection of Manipulated Inputs

Select inputs that have large effects on controlled variables

» Large steady-state gains and large input ranges

Select inputs that rapidly affect the controlled outputs

» Small time constants and small time delays

Select inputs that directly (rather than indirectly) affect the controlled outputs

» Condenser duty rather than reboiler duty for column overhead composition control

Avoid selecting inputs that recycle disturbances

» Flow rate of column product stream recycled to chemical reactor

Selection of Measured Variables

Select measurements that are reliable and accurate

» Stream temperature rather than composition

Select measurements that exhibit sensitivity to the manipulated inputs

» High purity distillation column tray temperature

Select measurements that minimize time constants and time delays

» Distillation column product composition measurement with gas chromatograph

Evaporator Example

NFC = 3

Controlled outputs: h, xB, P

Manipulated inputs: B (for h), D (for P), Ps (for xB)

Measured variables: h, xB, P, B, D, Ps

Evaporator Control System

Process Safety

Alarms and Interlocks