Simulation, Modeling and Control for Crude Distillation Unit

ABSTRACT 

The control of the distillation unit in Khartoum refinery, by keeping the operating conditions in certain limits is essential to adjust disturbances which lead to deviation from the desired values. The aim of this work is to select the suitable controller and the appropriate tuning method. The control loops are designed depending upon controllability and performance. These loops were Control of furnace temperature, Control of interface level at reflux drum, Control of level at stripper one, Control of level at bottoms of the column and Control of pressure at the top of the column. The mathematical model for each loop was determined using MATLAB toolbox system identification by designing Graphical User Interface (GUI). The same was used to identify the transfer function for each loop. Having identified the transfer functions, each loop was closed and the characteristic equation was obtained from the overall transfer function. Using Z-N tuning method to get the ultimate 6 gain and ultimate period, from which the adjustable parameters were determined using Z-N criterion. These were used to investigate the offset upon a unit step change in the set point using proportional only (P), proportional integral (PI) and proportional integral derivative controller (PID).The controller that gave the minimum offset is found to be proportional integral derivative controller (PID) and has been selected. The procedure is repeated using Root-Locus and decay ratio criterions. In each case the controller that gave the lowest offset was selected. The three methods of Z-N, Roots Locus and decay ratio were compared according to their offset and found to be within good accuracy. Stability performance was examined using Bode, Nyquist and Root locus Criteria. Having closed the loops in conventional system, the same was transformed to digital computer control. The analysis of the offset investigation and system stability were performed on (Z-domains). The results are in agreement with conventional analysis. In conclusion a complete control system of a distillation unit was designed, drawn in block diagram and interfaced to a digital computer control.

Chapter one

1-Introduction

Introduction 2

Topping plant unit 2

 Importance of process control 3

Statement of problem 5

Control systems 6

Objectives 6

 Chapter two

2- Literature review

Literature Review 8

 Important of Refining 8

Crude oil Distillation Unit (CDU) 9

Basic distillation equipment 10-11

Control Historical Background 12-19

Control Problems 19-22

Controllers 18-26

Controller tuning 22-23

 Stability 23-28

Types of control systems 28-30

 Computer – aided process control 30-32

Case Study 32-35

Chapter three

3- Methodology

9

System Identification 36-39

Analyzing the model 40-41

 MATLAB Soft ware 41-46

KRC-CDU Case Study 46-47

Feedback control system 47-49

 KRC-CDU Records 49-52

Procedure 52-53

 Chapter four

4- Results and Discussion

Transfer functions identification 55-60

 Controller tuning 60

 Ziegler-Nicholas (Z-N) tuning method 61

with using MATLAB soft ware

Control of level at stripper one using Z-N 61-82

tuning method of P, PI and PID controller.

Root locus tuning method 82-86

 Decay ratio tuning method 86-87

Comparison between tuning methods 87-88

Control of Pressure at top of the column 88-89

Control of level at bottom of the column 90-91

Control of furnace temp 91-93

The stability criteria 93

Bode Criterion 94-98

Nyguist stability criterion 98-102

Root locus criterion 103-106

Digital control 106-112

Chapter five

5- Conclusion and Recommendations

Conclusion 114

Recommendations 114

Refrences 115-116

Appendix 117-129