VACUUM DISTILLATION CONTROL by JOHN JOSEPH ANDERSON, B.S., M.S. A DISSERTATION IN CHEMICAL ENGINEERING Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY Chairperson of the Cdoin^ttee Accepted Deftniof ttie Graduate S(^hool December, 1998 Copyright 1998, John Joseph Anderson ACKNOWLEDGMENTS The past four years at Texas Tech University have been an exciting period of time in my life. This time in Lubbock has been a wonderful opportunity for personal growth and to increase my knowledge. I would like to extend my sincere thanks to Dr. James B. Riggs for inviting me to join his program in 1994. He has allowed me to greatly expand my control knowledge and gave me the confidence necessary to enter a plant environment and practice the craft. I have learned a great number of skills under his stewardship and I greatly appreciate the guidance he has provided in the last four years. I stand at the threshold of my future and thank him for providing the tools to embrace it. I would also like to thank my doctoral committee: Dr. Richard Wm. Tock, Dr. Theodore F. Wiesner, and Dr. Donald C. Wunsch. In addition, I wish to thank the Texas Tech Process Control and Optimization Consortium and the United States Department of Energy (Contract No. DR-PC04-94 AL 98747) for their financial support of my studies here at Texas Tech. Finally, I would like to thank AspenTech Inc. for the use of their DMC package as well as the help Scott Boyden and Rob Hawkins have provided. I wish to thank my parents, John Howard and Mary Ann Anderson for the support and love they have given me since they brought me into the world. You have always been there for me, encouraging me when I have needed it, and teaching me the rights and wrongs of life. I look at my parents and admire their work, not only for me, but for my brothers and sister. You both have done a fantastic job with us and I cannot begin to tell you how much I appreciate it. I wish to thank my brothers, Chris and Michael; they are testaments of courage and strength in all that they have faced. You two are incredible for what you have done and thanks for your support and love. Finally, for my sister whom has grown up while I have been in college, you are also something else and I love you. There are extended brother and sisters I would like to thank too. First, Shannon Adams, thank you for your friendship. Thanks for the reality checks when I have needed them. For Lori Dabney Neild, I am so glad to have you in my life and admire the challenges you have also overcome. Finally, Katy Linehan, thanks for the times of sharing and ii fhendship we have shared. You three occupy a special place in my heart and I love you dearly. My time here in Lubbock has also been a great period of personal development. I have made a great number of friends here; friends whom I will always cherish. I wish to thank Marshall Duvall, my office partner, fellow computer hacker, and ski and scuba buddy. Thanks for always being there when I needed someone to turn to. I really appreciate all you have done for me these last four years. For Andrew Jackson, my roommate and friend, thanks for all the Christian fellowship and outdoor adventures we have had over the past few years. It is good to have someone to share the mountains with. And for Tony Durm, another gift of God in my life. Thank you for the debates, the fellowship, and the fi"iendship we have shared since we met at church. You are an incredible person with all you have done in your life and thanks for encouraging me when I needed it. I wish to thank the members of my Sunday School class at First Church of the Nazarene; Dee, who has been a mom to me here; Norma, whom helped me with one of the hardest crises in my life, thank you; Hampton, a father I could turn to; Carolyn, I would like to thank you for all the lessons you have taught me; and Linda, you are a special woman and I am glad I opened my eyes finally. Another family I gained while in Lubbock has been the faculty and staff in Chemical Engineering. Thank you, Mary Beth, for your wonderful smile and help up in the office. I appreciate the extra work you have done. Bob Spruill, thanks for discussions in the morning and little quirks of life you shared. Drs. Robert and Nancy Bethea, I wish I could say how much I admire you both. I can look to you to see integrity, professionalism, and trust. In addition, I will cherish the time I shared with my fellow research companions: Scott Hurowitz, Rodney Thompson, Xuan Lee, and Natasha Mikel. Finally, I thank God for His Love and His support throughout my life! And one final comment (for which I am sure Andy will have me committed!). Ill If you ever plan to motor west: Travel my way, the highway that's the best. Get your kicks on Route 66! It winds from Chicago to L.A., More than 2,000 miles ail the way, Get your kicks on Route 66! © Bobby Troup, 1946, Londontovm Music IV TABLE OF CONTENTS ACKNOWLEDGEMENTS ii ABSTRACT xi LIST OF TABLES xiii LIST OF FIGURES xvii LIST OF SYMBOLS xxvi CHAPTER 1. INTRODUCTION 1 2. LITERATURE REVIEW 5 2.1 Distillation Modeling and Simulation 5 2.2 Distillation Dynamics 7 2.3 Distillation Control 8 2.3.1 Single-Ended Composition Control 9 2.3.2 Dual-Ended Composition Control 10 2.4 Decentralized PI Controller Tuning 13 2.5 Model Predictive Control 15 2.5.1 Generic Model Control 15 2.5.2 DMC Control 16 3. MODEL DEVELOPMENT AND CONSIDERATIONS 19 3.1 Xylene/Toluene Vacuum Column Modeling Assumptions 19 3.2 Xylene/Toluene Liquid-Vapor Equilibrium and Energy Modeling 22 3.3 Vapor and Liquid Hydraulics Modeling 24 3.3.1 Vapor Flowrate Calculation 24 3.3.2 Liquid Flowrate Calculation 25 3.4 Xylene/Toluene Vacuum Column Design 26 3.4.1 Determining the Number of Trays and Feed Tray Location 26 3.4.2 Xylene/Toluene Column Design Parameters 28 3.5 Xylene/Toluene Vacuum Column Dynamic Modeling 30 3.5.1 Tray Material and Component Balances 30 3.5.2 Feed, Accumulator and Reboiler Material and Component Balances 32 3.5.3 Dynamic Simulation Development 33 3.5.4 Level Controllers 34 3.6 Xylene/Toluene Dynamic Model Responses and B enchmarking 3 6 3.6.1 High Purity Column 39 3.6.2 Low Purity Column 39 3.6.3 High/Low Purity Column 44 3.6.4 Low/High Purity Column 44 3.6.5 Inverse Purity Column 49 3.7 Ethylbenzene/Styrene Modeling Assumptions 52 3.8 Ethylbenzene/Styrene Vapor-Liquid Equilibrium and Energy Calculations 56 3.9 Vapor and Liquid Hydraulic Modeling 58 3.9.1 Vapor Flowrate Calculation 58 3.9.2 Liquid Flowrate Calculation 59 3.10 Ethylbenzene/Styrene Vacuum Column Steady-State Design 59 3.11 Determining the Number of Stages and Feed Stage 62 3.11.1 Tray Material and Component Balances 62 3.11.2 Accumulator and Reboiler Material and Component Balances 63 3.11.3 Dynamic Simulator Development 63 3.11.4 Level Controllers 63 3.12 Styrene/Ethylbenzene Dynamic Model Responses and Benchmarking 64 4. XYLENE/TOLUENE SINGLE-ENDED PI COMPOSITION CONTROL 68 4.1 Controlled Variable-Manipulated Variable Pairings 69 4.2 Single-Ended Composition Controller Tuning Approach 71 VI 4.2.1 Autotune Variation 71 4.2.2 Tyreus-Luyben Tuning Approach 75 4.3 Single-Ended PI Control Results for the Xylene/Toluene Columns 78 4.3.1 Overhead Impurity Control 79 4.3.1.1 High Purity Column 79 4.3.1.2 Low Purity Column 83 4.3.1.3 High/Low Purity Column 86 4.3.1.4 Low/High Purity Column 91 4.3.1.5 Overhead Impurity Control Summary 92 4.3.2 Bottoms Impurity Control 94 4.3.2.1 High Purity Column 94 4.3.2.2 Low Purity Column 96 4.3.2.3 High/Low Purity Column 99 4.3.2.4 Low/High Purity Column 104 4.3.2.5 Bottoms Impurity Control Summary 107 4.4 Discussion of Single-Ended Control Results 108 5. XYLENE/TOLUENE DUAL-ENDED PI COMPOSITION CONTROL 109 5.1 Configuration Selection 109 5.2 Steady-State Analysis 110 5.2.1 RGA Analysis 110 5.2.2 RGA on Xylene/Toluene Columns 111 5.2.2.1 High Purity Column 111 5.2.2.2 Inverted Purity Column 112 5.2.2.3 Low Purity Column 112 5.2.2.4 High/Low Purity Column 113 5.2.2.5 Low/High Purity Column 113 5.2.2.6 RGA Summary 114 5.2.3 Other Configuration Selection Methods 115 5.3 Dual-Ended PI Composition Controller Tuning Approach 116 Vll 5.4 High Purity PI Control Results 118 5.4.1 Setpoint Tracking 118 5.4.2 Unmeasured Disturbance Rejection 124 5.4.3 Discussion of Results 135 5.5 Low Purity PI Control Results 136 5.5.1 Setpoint Tracking 137 5.5.2 Unmeasured Disturbance Rejection 145 5.5.3 Discussion of Results 152 5.6 High/Low Purity PI Control Results 153 5.6.1 Setpoint Tracking 154 5.6.2 Unmeasured Disturbance Rejection 162 5.6.3 Discussion of Results 169 5.7 Low/High Purity PI Control Resuhs 170 5.7.1 Setpoint Tracking 170 5.7.2 Unmeasured Disturbance Rejection 179 5.7.3 Discussion of Results 186 5.8 Inverse Purity PI Control Results 187 5.8.1 Setpoint Tracking 188 5.8.2 Unmeasured Disturbance Rejection 196 5.8.3 Discussion of Results 204 5.9 Dual-Ended PI Composition Control with Decoupling 205 5.9.1 Standard Decoupler Development 206 5.9.2 Simple Decoupler Development 208 5.9.3 Setpoint Tracking Control Results 211 5.9.4 Unmeasured Disturbance Rejection Control Results 216 5.9.5 Discussion of Results 220 5.10 Dual-Ended PI Composition Control with Feedforward Compensation 222 5.10.1 Feedforward Element Development 222 5.10.1.1 [L/D,V/B] Feedforward Development 223 5.10.1.2 [L,V] Feedforward Development 224 Vlll 5.10.1.3 [L,V/B] Feedforward Development 225 5.10.2 Measured Disturbance Rejection Control Results 226 5.10.2.1 [L/D,V/B] Feedforward Development 226 5.10.2.2 [L,V] Feedforward Development 227 5.10.2.3 [L,V/B] Feedforward Development 233 5.10.3 Discussion of Results 235 6.