Automated Continuous Process Control

CARLOS A. SMITH is Professor of Chemical Engineering and Associate Dean of Academics in the College of Engineering, University of South Florida, Tampa. He not only teaches students but also teaches numerous short courses on process control to professionals. He is the coauthor of the bestselling textbook Principles of Automatic Process Control (Wiley).

Preface xi

1 Introduction 1

1-1 Process Control System 1

1-2 Important Terms and Objective of Automatic Process Control 3

1-3 Regulatory and Servo Control 4

1-4 Transmission Signals, Control Systems, and Other Terms 5

1-5 Control Strategies 6

1-5.1 Feedback Control 6

1-5.2 Feedforward Control 8

1-6 Summary 9

2 Process Characteristics 11

2-1 Process and Importance of Process Characteristics 11

2-2 Types of Processes 13

2-3 Self-Regulating Processes 14

2-3.1 Single-Capacitance Processes 14

2-3.2 Multicapacitance Processes 24

2-4 Transmitters and Other Accessories 28

2-5 Obtaining Process Characteristics from Process Data 29

2-6 Questions When Performing Process Testing 32

2-7 Summary 33

Reference 33

Problems 34

3 Feedback Controllers 38

3-1 Action of Controllers 38

3-2 Types of Feedback Controllers 40

3-2.1 Proportional Controller 40

3-2.2 Proportional–Integral Controller 44

3-2.3 Proportional–Integral–Derivative Controller 48

3-2.4 Proportional–Derivative Controller 50

3-3 Reset Windup 50

3-4 Tuning Feedback Controllers 53

3-4.1 Online Tuning: Ziegler–Nichols Technique 53

3-4.2 Offline Tuning 54

3-5 Summary 60

References 60

Problems 60

4 Cascade Control 61

4-1 Process Example 61

4-2 Implementation and Tuning of Controllers 65

4-2.1 Two-Level Cascade Systems 65

4-2.2 Three-Level Cascade Systems 68

4-3 Other Process Examples 69

4-4 Closing Comments 72

4-5 Summary 73

References 73

5 Ratio, Override, and Selective Control 74

5-1 Signals and Computing Algorithms 74

5-1.1 Signals 74

5-1.2 Programming 75

5-1.3 Scaling Computing Algorithms 76

5-1.4 Significance of Signals 79

5-2 Ratio Control 80

5-3 Override, or Constraint, Control 88

5-4 Selective Control 92

5-5 Designing Control Systems 95

5-6 Summary 110

References 111

Problems 112

6 Block Diagrams and Stability 127

6-1 Block Diagrams 127

6-2 Control Loop Stability 132

6-2.1 Effect of Gains 137

6-2.2 Effect of Time Constants 138

6-2.3 Effect of Dead Time 138

6-2.4 Effect of Integral Action in the Controller 139

6-2.5 Effect of Derivative Action in the Controller 140

6-3 Summary 141

Reference 141

7 Feedforward Control 142

7-1 Feedforward Concept 142

7-2 Block Diagram Design of Linear Feedforward Controllers 145

7-3 Lead/Lag Term 155

7-4 Extension of Linear Feedforward Controller Design 156

7-5 Design of Nonlinear Feedforward Controllers from Basic Process Principles 161

7-6 Closing Comments on Feedforward Controller Design 165

7-7 Additional Design Examples 167

7-8 Summary 172

References 173

Problem 173

8 Dead-time Compensation 174

8-1 Smith Predictor Dead-Time Compensation Technique 174

8-2 Dahlin Controller 176

8-3 Summary 179

References 179

9 Multivariable Process Control 180

9-1 Pairing Controlled and Manipulated Variables 181

9-1.1 Obtaining Process Gains and Relative Gains 186

9-1.2 Positive and Negative Interactions 189

9-2 Interaction and Stability 191

9-3 Tuning Feedback Controllers for Interacting Systems 192

9-4 Decoupling 194

9-4.1 Decoupler Design from Block Diagrams 194

9-4.2 Decoupler Design from Basic Principles 196

9-5 Summary 197

References 197

Problem 198

Appendix A Case Studies 199

Case 1: Ammonium Nitrate Prilling Plant Control System 199

Case 2: Natural Gas Dehydration Control System 200

Case 3: Sodium Hypochlorite Bleach Preparation Control System 201

Case 4: Control System in the Sugar Refining Process 202

Case 5: Sulfuric Acid Process 204

Case 6: Fatty Acid Process 205

Reference 207

Appendix B Processes For Design Practice 208

Installing the Programs 208

Process 1: NH 3 Scrubber 208

Process 2: Catalyst Regenerator 211

Process 3: Mixing Process 213

Index 215