1. introduction 2. influence of process design 3. control degrees … · 2014-09-17 · process...

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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

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Page 1: 1. Introduction 2. Influence of process design 3. Control degrees … · 2014-09-17 · Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control

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

Page 2: 1. Introduction 2. Influence of process design 3. Control degrees … · 2014-09-17 · Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control

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

Page 3: 1. Introduction 2. Influence of process design 3. Control degrees … · 2014-09-17 · Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control

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

Page 4: 1. Introduction 2. Influence of process design 3. Control degrees … · 2014-09-17 · Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control

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

Page 5: 1. Introduction 2. Influence of process design 3. Control degrees … · 2014-09-17 · Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control

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

Page 6: 1. Introduction 2. Influence of process design 3. Control degrees … · 2014-09-17 · Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control

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

Page 7: 1. Introduction 2. Influence of process design 3. Control degrees … · 2014-09-17 · Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control

Distillation Column Example

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

NFC = 5

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

Page 8: 1. Introduction 2. Influence of process design 3. Control degrees … · 2014-09-17 · Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control

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

Page 9: 1. Introduction 2. Influence of process design 3. Control degrees … · 2014-09-17 · Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control

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

Page 10: 1. Introduction 2. Influence of process design 3. Control degrees … · 2014-09-17 · Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control

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

Page 11: 1. Introduction 2. Influence of process design 3. Control degrees … · 2014-09-17 · Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control

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

Page 12: 1. Introduction 2. Influence of process design 3. Control degrees … · 2014-09-17 · Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control

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

Page 13: 1. Introduction 2. Influence of process design 3. Control degrees … · 2014-09-17 · Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control

Evaporator Control System

Page 14: 1. Introduction 2. Influence of process design 3. Control degrees … · 2014-09-17 · Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control

Process Safety

Page 15: 1. Introduction 2. Influence of process design 3. Control degrees … · 2014-09-17 · Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control

Alarms and Interlocks