RETROFIT DESIGN OF HEAT-INTEGRATED CRUDE OIL DISTILLATION SYSTEMS

Transcription

1 RETROFIT DESIGN OF HEAT-INTEGRATED CRUDE OIL DISTILLATION SYSTEMS A thesis submitted to the University of Manchester Institute of Science and Technology for the degree of Doctor of Philosophy by Mamdouh Ayad Gadalla Under the direction of Prof. Robin Smith Dr. Megan Jobson Department of Process Integration University of Manchester Institute of Science and Technology P.O. Box 88, Sackville Street Manchester M60 1QD January 2003

2 Declaration No portion of the work referred to in this thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institution of learning. Mamdouh Gadalla i

3 To my Mum Neamat and my Brother Nabil, who continuously gave me the courage, inspiration and the love in doing this work, but did not live to see it. Now, your memories will last forever through this work. To Susana Arnal, for her being with me, encouraging and supporting me. To my dad, brothers and my whole family, for being always supportive. ii

4 ACKNOWLEDGMENTS Although words and space are not enough, I would like to deeply express my gratitude to both my supervisors, Prof. Robin Smith and Dr. Megan Jobson, for their guidance, encouragement and support throughout my research and study. I found invaluable their contribution and inspiration which put me in the right direction towards accomplishing my research. I specially thank Dr. Megan Jobson for her great cooperation during the stage of writing-up. Special thanks to the Department of Process Integration for giving me the opportunity and the sponsorship to study at UMIST and the chance to meet many people. Throughout my study, I spent a pleasant time with so many wonderful people from different parts of the globe and of many different cultures; I found one common thing they all have and share that is kindness. My sincere thanks go to all staffs at the department of Process Integration, for their help and support whenever needed. I would like to thank Chris for his help in the programming part of my research; he really deserves a very big thank. Special thanks to Steve for his valuable discussion and explanations during my research work. I am also grateful to all students at the department. Thanks to them for creating a friendly atmosphere and making a great time in which I spent and worked. Many colleagues and friends left and new ones came, but they will always stay in my memory. I cannot name them all because they are too many; I can only say that they are those people whom I met since I arrived at the department. I would like lastly to thank all people unmentioned and no longer exist who were at the background of my life but their effects reach so deep up to the front. I would like to continuously send deep thanks and love to my mum, Neamat and my brother, Nabil; I wish they could be here to share and touch their dream. Special thanks go to Susana Arnal, for being with me when I needed. I also thank my father, my brothers and the whole family for their support and remembrance. Last but not least, I give unlimited thanks to the Lord who provides all I need and ask. iii

5 ABSTRACT Heat-integrated crude oil distillation systems are energy and capital intensive, and have a very complex structure with strong interactions between the individual units. Retrofit of these systems is of major interest to petroleum refiners. Retrofit objectives are various and preferably achieved with minimum capital expenditure, while equipment constraints are met. Traditional approaches to retrofit design of crude oil distillation systems identify promising modifications based on experience or pinch analysis. Later, sequential approaches to retrofit design were developed, in which distillation and heat recovery units are modified individually. Recent approaches considered simultaneously the distillation column and heat integration targets, rather than the existing heat recovery system. That shortcut models for retrofit design of distillation columns are not available is an additional limitation of established methodologies. In this thesis, a new approach is presented for retrofit design of heat-integrated crude oil distillation systems. Shortcut models are developed for distillation retrofit, including reboiled and steam-stripped columns. These models are based on the Underwood equation and are appropriate for retrofit design of simple columns and various complex column arrangements. Models are also proposed for exchanger network retrofit, addition of new columns to the existing distillation unit, modifying column internals, enhancing heat transfer in exchanger tubes and for evaluating CO 2 emissions in existing crude oil distillation units. The retrofit design methodology is optimisation-based, and considers the existing distillation process simultaneously with the details of the associated heat recovery system. Existing equipment limitations, such as the hydraulic capacity of the distillation column, exchanger network pressure drop and bottlenecked exchangers, are accounted for. The approach considers various structural modifications and design options resulting in significant benefits. Examples of these are the installation of preflash and prefractionator units to the existing column configuration, replacement of column internals with packing, enhancement of exchanger heat transfer and integration of a gas turbine with an existing furnace. The optimisation framework comprises column and exchanger network retrofit models, cost models and suitable objective functions. The approach optimises all operating conditions of the existing distillation process and any new columns to minimise or maximise a specified objective function, while satisfying existing constraints. The objective function is flexible and varies according to retrofit objectives. Several objectives are taken into account, such as reducing energy consumption and overall cost, increasing capacity, improving profit and reducing CO 2 emissions. The approach allows these objectives to be met by considering several design alternatives. The new retrofit approach is applied to different industrial cases of crude oil distillation units, for energy and total cost savings, throughput enhancement, product yield changes, profit increase and emissions reduction. Typical results conclude that retrofit goals can be achieved with substantial savings in energy and total cost, and improved profit with minimal capital investment. iv

6 Table of contents Chapter 1: Introduction Retrofit of heat-integrated crude oil distillation systems Motivation and objective of the work Overview of the thesis 5 Chapter 2: Literature review Introduction Grassroots design of heat-integrated crude oil distillation systems Shortcut models for design of distillation columns Retrofit design of heat-integrated crude oil distillation systems Modifications-based retrofit methods Pinch analysis-based retrofit approaches Retrofit design of crude oil distillation systems Retrofit design of heat recovery systems Concluding remarks 27 Chapter 3: Shortcut models for retrofit design of distillation columns Introduction Retrofit models for design of reboiled distillation columns Retrofit shortcut model for simple reboiled distillation columns Retrofit shortcut model for complex configurations of reboiled distillation columns Summary Retrofit models for design of steam-stripped distillation columns Retrofit shortcut model for simple steam-stripped distillation columns Retrofit shortcut model for complex configurations of steam-stripped distillation columns Summary Retrofit modelling for design of refinery distillation columns Illustrative example an atmospheric crude oil distillation column 69 v

7 3.5. Summary and conclusions 79 Chapter 4: Modelling for retrofit of heat-integrated crude oil distillation systems Modelling existing crude oil distillation columns Hydraulic analysis for existing crude oil distillation columns Modelling existing heat exchanger networks Retrofit curve for an existing heat exchanger network Modelling the installation of preflash units to existing crude oil distillation units Illustrative example: an existing crude unit with a preflash drum Modelling the installation of prefractionator columns to existing crude oil distillation units Illustrative example: an existing crude unit with a prefractionator Modelling carbon dioxide emissions from an existing refinery distillation for reducing environmental impact Introduction Model for CO 2 emissions Modelling heat transfer enhancement in existing heat exchanger networks Retrofit model for enhanced heat exchanger networks Pressure drop considerations in enhanced exchanger networks Modelling packed sections in existing crude oil distillation columns Summary and conclusions 120 Chapter 5: Retrofit design of heat-integrated crude oil distillation systems Features of heat-integrated crude oil distillation systems Retrofit design philosophy in refinery distillation systems Interactions between distillation operating conditions, heat recovery potential and column hydraulics New approach for retrofit of heat-integrated crude oil distillation vi

8 systems Retrofit design strategies for heat-integrated crude oil distillation systems Objective functions for optimisation of heat-integrated crude oil distillation systems Optimisation procedure for existing heat-integrated crude oil distillation systems Implementation of retrofit models and applications of retrofit approach Retrofit for energy reduction Retrofit for throughput enhancement Retrofit for profit increase Retrofit for CO 2 emissions reduction Retrofit for product/feed specification changes Summary and conclusions 143 Chapter 6: Case studies Reducing energy consumption of an existing atmospheric crude oil distillation tower Base case problem data Retrofit objective and approach results Comparison of new retrofit approach with previous work Increasing profit of an existing atmospheric crude oil distillation tower Increasing throughput of an existing atmospheric crude oil distillation tower Increasing current throughput by 20% over base case capacity Increasing current throughput to maximum capacity Reducing CO 2 emissions from an existing crude oil distillation unit Changing product yields of an existing crude oil distillation unit Installing preflash drum or prefractionator column in existing crude oil distillation unit Installing a preflash drum for reducing energy consumption vii

9 and increasing capacity Installing prefractionator column for reducing energy consumption and increasing capacity Heat transfer enhancement in preheat train for retrofit of crude oil distillation tower Case studies comparison Summary and conclusions 203 Chapter 7: Conclusions and future work Conclusions Shortcut models for retrofit design of distillation columns Retrofit modelling for heat-integrated crude oil distillation systems Retrofit design methodology and its application Future work Closing remarks 213 References 214 Appendix A: Pressure drop correlations 221 A.1. Parameters for pressure drop correlations of Polley et al. (1990) 221 A.2. Pressure drop for enhanced HEN 221 A.3. Pressure drop correlation for packed beds 224 Appendix B: Heat exchanger costs 225 Appendix C: Data for case study C.1. Problem data 227 C.2. Retrofit curve data for existing HEN 229 C.3. Data of retrofit results for optimum unit 231 C.4. Comparison of new retrofit approach with previous work 233 Appendix D: Data for case study Appendix E: Data for case study E.1. Problem data 239 E.2. Data of retrofit results for 20% capacity increase on base case 241 E.3. Retrofit curve data for existing unit with 20% capacity increase 243 E.4. Data of retrofit results for optimum unit with 20% capacity increase 246 viii

10 Appendix F: Data for case study F.1. Problem data 249 F.2. Results for retrofit with integrated gas turbine 249 F.3. Results for retrofit without integrated gas turbine 252 Appendix G: Data for case study Appendix H: Data for case study Appendix I: Data for case study Appendix J: Data for case study ix

11 List of figures Figure 3.1: Simple distillation column with reboiler 32 Figure 3.2: Sequences of two simple reboiled distillation columns 39 Figure 3.3: Decomposition of reboiled complex distillation columns (with full thermal coupling), showing distribution of existing number of stages 42 Figure 3.4: Simple uncoupled sequences versus thermally coupled sequences 43 Figure 3.5: Retrofit algorithm for complex column configurations 44 Figure 3.6: Thermal coupled complex column configurations (direct and indirect sequences) 47 Figure 3.7: Different configurations of complex columns with prefractionators 50 Figure 3.8: Modelling of column with a prefractionator and Petlyuk column 51 Figure 3.9: Simple distillation column using stripping steam 56 Figure 3.10: Retrofit algorithm for stripping sections 58 Figure 3.11: Sequences of direct and indirect simple steam-stripped distillation columns 59 Figure 3.12: Decompositions of steam-stripped complex columns, showing distribution of existing number of stages (full thermal coupling) 63 Figure 3.13: Thermal coupled complex columns with steam (direct and indirect sequences) 66 Figure 3.14: Atmospheric crude oil distillation column, showing the equivalent sequence of simple columns 73 Figure 3.15: Light naphtha composition for shortcut and rigorous models 75 Figure 3.16: Heavy naphtha composition for shortcut and rigorous models 75 Figure 3.17: Light distillate composition for shortcut and rigorous models 76 Figure 3.18: Heavy distillate composition for shortcut and rigorous models 76 Figure 3.19: Residue composition for shortcut and rigorous models 77 Figure 3.20: True boiling curves of light naphtha for shortcut and rigorous models 77 Figure 3.21: True boiling curves of heavy naphtha for shortcut and rigorous models 78 x

12 Figure 3.22: True boiling curves of light distillate for shortcut and rigorous models 78 Figure 3.23: True boiling curves of heavy distillate for shortcut and rigorous models 79 Figure 3.24: True boiling curves of residue for shortcut and rigorous models 79 Figure 4.1: Existing heat exchanger network of a crude oil distillation unit 85 Figure 4.2: Retrofit curve of zero and one modification to the HEN topology 88 Figure 4.3: Retrofit curve of an existing HEN 89 Figure 4.4: Procedure of obtaining retrofit model of an existing HEN 90 Figure 4.5: A crude oil distillation column with a preflash unit 91 Figure 4.6: Sketch for preflash calculation variables 92 Figure 4.7: Preflash vapour mixing scheme 93 Figure 4.8: An atmospheric crude oil tower with a prefractionator column 97 Figure 4.9: Sources of CO 2 emissions from a crude oil distillation unit 102 Figure 4.10: Retrofit curve for enhanced heat exchanger networks 110 Figure 4.11: Regression of flooding data from Seader and Henley (1998) 116 Figure 4.12: Regression of density data from Seader and Henley (1998) 117 Figure 4.13: Regression of viscosity data from Seader and Henley (1998) 118 Figure 5.1: Heat-integrated crude oil distillation system including atmospheric and vacuum towers and naphtha splitter 124 Figure 5.2: Simultaneous retrofit strategy for heat-integrated distillation systems 132 Figure 5.3: CO 2 emissions reduction strategy for refinery distillation systems 136 Figure 5.4: Simultaneous retrofit strategy for heat-integrated distillation systems 137 Figure 5.5: Overall procedure for heat-integrated crude oil distillation systems 141 Figure 6.1: Atmospheric crude oil distillation column, showing the equivalent sequence of simple columns (numbers in (a) refer to section numbers, while in (b) refer to column numbers) 147 Figure 6.2: Structure of existing heat exchanger network (streams data are given in Table C.1.5, Appendix C) (light shaded exchangers indicate coolers; dark shaded exchangers indicate heaters) 147 Figure 6.3: Modifications to existing heat exchanger network 152 Figure 6.4: Relocating exchanger number 4 in the existing exchanger network 153 xi

13 Figure 6.5: Existing crude oil distillation column (numbers refer to section numbers) 168 Figure 6.6: Preheat train structure of existing crude oil distillation unit 169 Figure 6.7: Simulated stage diameter for current operation (stages from top of column to bottom) (tray sizing using HYSYS) 170 Figure 6.8: FUA plot for existing distillation column (column stages: top-bottom) 170 Figure 6.9: Simulated stage diameter for maximum capacity increase (stages from column top-down) (tray sizing using HYSYS) 171 Figure 6.10: Temperature profile for existing unit with preflash 172 Figure 6.11: Crude oil stream routing in existing HEN with preflash 183 Figure 6.12: Location of preflash vapour to enter main column 184 Figure 6.13: Temperature profile for optimum unit with preflash 185 Figure 6.14: Column diameter for base case and unit with preflash (stages from top to bottom of the column) (tray sizing using HYSYS) 187 Figure 6.15: Column diameter for 38% capacity increase of unit with preflash, compared with existing unit and base case (stages: top-bottom) 188 Figure 6.16: Reconfiguration of existing crude oil distillation tower with installed prefractionator 190 Figure 6.17: Location of installed prefractionator in existing preheat train 191 Figure 6.18: FUA curve for unit with prefractionator (stages from column top-down) 197 Figure 6.19: Simulated column diameter for 59% increased throughput to unit with prefractionator (number of stages is top-down) (tray sizing using HYSYS) 198 Figure B.1: Exchanger cost data from various sources (45 and 35% represent the ratios of the installation cost of exchangers to their purchase costs as recommended by Peters and Timmerhaus, 1980) 225 Figure B.2: Exchanger cost curve, showing model regression parameters (data used for regressions are in Table B.1) 226 Figure C.2.1: Retrofit curve and data regression for existing exchanger network (Aret: area obtained from retrofit study, Amodel: area obtained from regressed model) 230 xii

14 Figure C.2.2: Relocation of exchanger 4 in existing HEN (points 2, 2 ) 231 Figure C.2.3: Relocation of exchanger 9 in existing HEN (points 3, 3 ) 231 Figure C.2.4: Relocation of exchanger 10 in existing HEN (points 4, 4 ) 231 Figure C.2.5: Introduction of new exchanger in existing HEN (points 5, 5 ) 231 Figure C.4.1: Relocation of exchanger 4 for network pinch approach results 234 Figure D.1: Modifications to existing heat exchanger network 236 Figure E.2.1: Modifications to existing heat exchanger network for optimum unit with 20% capacity increase 241 Figure E.3.1: Retrofit curve and data regression (Aret: area obtained from retrofit study, Amodel: area obtained from regressed model) 244 Figure E.3.2: Relocation of exchanger 1 in existing HEN (points 2, 2 ) 245 Figure E.3.3: Relocation of exchanger 9 in existing HEN (points 3, 3 ) 245 Figure E.3.4: Introduction of new exchanger in existing HEN (points 4, 4 ) 245 Figure E.3.5: Relocation of exchanger 3 in existing HEN (points 5, 5 ) 245 Figure E.3.6: Comparison of retrofit model of unit with 20% increased throughput with existing throughput (vertical dotted line represents existing energy demand) 246 Figure E.4.1: Modifications to existing heat exchanger network for optimum unit with 20% capacity increase 247 Figure F.2.1: Relocation of exchanger 4 for optimum network (with integrated gas turbine) 251 Figure F.3.2: Relocation of exchanger 4 for optimum network 252 Figure H.1: Relocation of exchanger 8 for optimum unit with preflash 261 Figure H.2: FUA curve for existing unit with preflash (stages from column top to bottom) 261 Figure I.1: Redistribution of stages into sections of decomposed column sequence of main distillation column 262 Figure I.2: Heat exchanger network for unit with prefractionator (see Table I.2, for stream references) 264 Figure I.3: Relocation of exchanger 7 in existing for optimum unit with prefractionator 270 Figure I.4: Stage diameter of unit with prefractionator (stages from column top to bottom) 270 Figure J.1: Existing exchangers to be enhanced, crude oil is split into 2 xiii

15 Figure J.2: Figure J.3: branches 273 Retrofit curve and data regression for existing HEN with heat transfer enhancement (HTE) (horizontal dotted line represents the existing exchanger total area) 276 Retrofit modifications to existing preheat train, showing types of exchanger additional area 278 xiv

16 List of tables Table 3.1: Typical degrees of freedom for design of reboiled distillation columns 39 Table 3.2: Feed data and column specifications 40 Table 3.3: Retrofit shortcut and rigorous model results (HYSYS) 41 Table 3.4: Typical degrees of freedom for design of fully thermally coupled sequences of reboiled distillation columns 45 Table 3.5: Column specifications for thermally coupled 46 Table 3.6: Retrofit shortcut and rigorous model results 46 Table 3.7: Retrofit shortcut and rigorous model results 49 Table 3.8: Data for column with a prefractionator 53 Table 3.9: Results for column with prefractionator 54 Table 3.10: Typical degrees of freedom for steam-stripped distillation columns 60 Table 3.11: Feed mixture data and column specifications 61 Table 3.12: Retrofit shortcut and rigorous simulation (HYSYS) results 61 Table 3.13: Column and product specifications 65 Table 3.14: Retrofit shortcut and rigorous model results 65 Table 3.15: Retrofit shortcut and rigorous model results 68 Table 3.16: Typical degrees of freedom for atmospheric crude oil tower 69 Table 3.17: Crude oil assay data 71 Table 3.18: Key components for the separation of each pair of products 72 Table 3.19: Feed composition of crude oil mixture (derived from assay data) 72 Table 3.20: Specifications of atmospheric crude oil distillation column 73 Table 3.21: Results of atmospheric crude oil distillation column 74 Table 4.1: Preflash column results 95 Table 4.2: Product flow rates of an existing crude oil unit 99 Table 6.1: Crude oil assay data 148 Table 6.2: Key components for separation of crude oil products (recoveries are presented in Table 6.4) 148 Table 6.3: Number of stages and existing diameters of distillation tower sections 148 Table 6.4: Operating conditions of base case for crude oil distillation tower 148 xv

17 Table 6.5: Optimisation variables for crude oil distillation tower 150 Table 6.6: Optimum values of operating conditions 151 Table 6.7: Energy consumption and operating costs for optimum unit 152 Table 6.8: Comparison of new approach results with previous work 156 Table 6.9: Comparison of additional area to existing exchanger 156 Table 6.10: Products income and operating costs of existing unit 158 Table 6.11: Retrofit results of optimum unit with maximum profit 159 Table 6.12: Optimum values of operating conditions for maximum profit 159 Table 6.13: Comparison of energy reduction and increasing profit optimum cases 160 Table 6.14: Energy consumption, operating costs and profit for base case 162 Table 6.15: Column diameter required for base case and actual diameter 162 Table 6.16: Column diameter for 20% increase in throughput on base case 163 Table 6.17: Process requirements for 20% increase in throughput 163 Table 6.18: Optimisation results for optimum unit with 20% increase in capacity 165 Table 6.19: Optimum values of operating conditions for 20% capacity increase 166 Table 6.20: Column diameter for optimum unit with 20% capacity increase 166 Table 6.21: Number of stages and existing diameters of distillation tower sections 168 Table 6.22: CO 2 emissions from base case of crude oil distillation tower 172 Table 6.23: CO 2 emissions from optimised unit with integrated gas turbine 173 Table 6.24: CO 2 emissions from optimum unit without integrated gas turbine 175 Table 6.25: Product yields of base case 176 Table 6.26: Key components of separation for product yield changes 176 Table 6.27: Product yields of new unit with yield changes 177 Table 6.28: Energy consumption and operating costs for new product yield units 177 Table 6.29: Retrofit results for base case with preflash 180 Table 6.30: Energy consumption and operating costs for optimum unit with preflash 181 Table 6.31: Energy consumption and operating costs for optimum unit with preflash 184 Table 6.32: Comparison of results of unit with different locations of preflash xvi

18 vapours 185 Table 6.33: Retrofit results for existing unit with prefractionator 192 Table 6.34: Optimum results of unit with prefractionator 194 Table 6.35: Optimum results of unit with prefractionator (all operating conditions) 195 Table 6.36: Energy and cost requirement and profit for maximum capacity increase 198 Table 6.37: Optimisation results of optimum unit with maximum capacity increase 199 Table 6.38: Retrofit approach results for optimum unit with heat transfer enhancement (HTE) 201 Table 6.39: Case studies comparison 202 Table B.1: Exchanger area and cost data 226 Table C.1.1: Feed composition of crude oil mixture 227 Table C.1.2: Utility, stripping steam and exchanger unit costs 227 Table C.1.3: Energy consumption and operating costs for existing unit 228 Table C.1.4: Heat exchanger data 228 Table C.1.5: Process and utility stream data for existing unit 229 Table C.2.1: Retrofit data for 229 Table C.3.1: Product flow rates of optimum unit 232 Table C.3.2: Key component recoveries of products for optimum unit 232 Table C.3.3: Process stream data for optimum unit 232 Table C.3.4: Additional area and cost for case study Table C.4.1: Additional area and cost for network pinch approach (no modifications to column operating conditions) 234 Table C.4.2: Comparison of optimisation results with minimum energy approach of Bagajewicz (1998) 234 Table C.4.3: Additional area and cost for energy-based approach 235 Table D.1: Product value and crude oil price 236 Table D.2: Additional area and capital cost 237 Table D.3: Process stream data for optimum unit of maximum profit 237 Table D.4: Key component recoveries of products for maximum profit 238 Table E.1.1: Operating conditions of base case 239 Table E.1.2: Heat exchanger data 239 xvii

19 Table E.1.3: Process and utility stream data for base case 240 Table E.2.1: Operating conditions of 20% capacity increase on base case 241 Table E.2.2: Additional area and cost for 20% increase on base case 242 Table E.2.3: Process and utility stream data for 20% increase of base case capacity 243 Table E.3.1: Retrofit data 243 Table E.4.1: Operating conditions of optimum unit with 20% capacity increase 246 Table E.4.2: Additional area and cost for optimum unit with 20% capacity increase 247 Table E.4.3: Process stream data for optimum unit with 20% capacity increase 248 Table F.1.1: Data of hot utilities for case study 249 Table F.1.2: Data for heating fuels in CO 2 emissions 249 Table F.2.1: CO 2 emissions from optimum unit with integrated gas turbine 249 Table F.2.2: Cost and economic parameters for CO 2 emissions calculation 250 Table F.2.3: Optimum values of operating conditions 250 Table F.2.4: Additional area and cost for optimum unit 251 Table F.2.5: Process stream data for optimum unit 251 Table F.3.1: Optimum values of operating conditions 252 Table F.3.2: Additional area and cost for optimum unit 252 Table F.3.3: Process stream data for optimum unit 253 Table G.1: Operating conditions for new product yields 254 Table G.2: Additional area and cost for unit with new product yields 254 Table G.3: Process stream data for unit with new product yields 255 Table G.4: Optimum operating conditions for unit with new product yields 255 Table G.5: Additional area and cost for optimum unit with new product yields 256 Table G.6: Process stream data for optimum unit with new product yields 256 Table H.1: Process stream data for base case with preflash 257 Table H.2: Additional area and cost for base case with preflash 257 Table H.3: Optimum operating conditions for unit with preflash drum 258 Table H.4: Process stream data for optimum unit with preflash 258 Table H.5: Additional area and cost for optimum unit with preflash 259 Table H.6: Optimum operating conditions for unit with preflash drum 259 Table H.7: Product flow rates of optimum unit with preflash 259 Table H.8: Additional area and cost for optimum unit with preflash 260 xviii

20 Table H.9: Process stream data for optimum unit with preflash 260 Table I.1: Number of stages and of main distillation tower sections 262 Table I.2: Design specifications of prefractionator column 262 Table I.3: Operating conditions of main column of unit with prefractionator 263 Table I.4: Product flow rates of unit with prefractionator 263 Table I.5: Key component recoveries of products for unit with prefractionator 263 Table I.6: Process streams for base case and unit with prefractionator 264 Table I.7: Process stream data for unit with prefractionator 265 Table I.8: Additional area and cost for unit with prefractionator 265 Table I.9: Operating conditions of main column of optimum unit with prefractionator 266 Table I.10: Product flow rates of optimum with prefractionator 266 Table I.11: Key component recoveries for optimum unit with prefractionator 266 Table I.12: Process stream data for optimum unit with prefractionator 267 Table I.13: Additional area and cost for optimum unit with prefractionator 267 Table I.14: Optimum operating conditions for unit with prefractionator 268 Table I.15: Product flow rates of optimum with prefractionator 268 Table I.16: Key component recoveries for optimum unit with prefractionator 268 Table I.17: Process stream data for optimum unit with prefractionator 269 Table I.18: Additional area and cost for optimum unit with prefractionator 269 Table I.19: Optimum operating conditions for unit with prefractionator (59% increase) 270 Table I.20: Optimum design specifications of unit with prefractionator (59% increase) 271 Table I.21: Product flow rates of optimum with prefractionator (59% increase) 271 Table I.22: Recoveries of products for unit with prefractionator (59% increase) 271 Table I.23: Additional area for optimum unit with prefractionator (59% increase) 272 Table I.24: Process stream data for optimum unit with prefractionator (59% increase) 272 Table J.1: Data for exchanger units 273 Table J.2: Maximum area for heat transfer enhancement 273 Table J.3: Calculations of model parameters for retrofit with enhancement 274 xix

21 Table J.4: Calculations of model parameters for retrofit with enhancement 274 Table J.5: Optimum operating conditions for unit with heat transfer enhancement 276 Table J.6: Product flow rates of optimum unit with enhancement 277 Table J.7: Key component recoveries of products for optimum unit with enhancement 277 Table J.8: Additional and enhancement area for optimum unit with enhancement 277 Table J.9: Process stream data for optimum unit with heat transfer enhancement 278 xx

22 Nomenclature and abbreviation a 1, a 2 Constant parameters for equation 4.51 A Exchanger area (m 2 ) A en Enhanced exchanger area (m 2 ) A ex, B ex, C ex Cost parameters in equation 5.1 ($, $/m 2, dimensionless) A exist Existing area of heat exchanger network (m 2 ) A o, B o, C o Constant parameters for flooding limits in equation 4.1 A req Additional exchanger area for retrofit (m 2 ) A ret Retrofit exchanger area given by equation 4.9 (m 2 ) b 0, b 1, b 2, b 3 Retrofit model parameters of polynomial form in equation 4.8 B Molar flow rate of bottom product (kmol/h) c 1, c 2 Cost parameters in equation 5.2 ($, $/m 2 ) C% Carbon mass percent in fuel CC Composite curve CGCC Column grand composite curve CO 2 Emiss Carbon dioxide emissions (kg/h) Cost FInst Installed cost of preflash ($) Cost GT Capital cost of gas turbines (k$) Cost pack Installed cost of packed section ($) GT Cost Power Cost of power generated by gas turbines ($/h) Cost PrShell Shell installed cost of prefractionator column ($) Cost PrStages Stages installed cost of prefractionator column ($) C P Specific heat capacity of flue gases (kj/kg o C) C SB Souder and Brown flooding constant (capacity factor) (ft/s) d Tube diameter in Appendix A (m) D Molar flow rate of top product (kmol/h) DC Ratio of down-comer area to cross sectional area of stage ( 12%) d e Tube bundle equivalent diameter in Appendix A (m) D Flash Diameter of preflash (m) d i Molar flow rate of component i in top product (kmol/h) D pack Diameter of packing (ft) D PrColumn Diameter of prefractionator column (m) (DR) HK Distribution ratio of heavy key component (DR) i Distribution ratio of component i D T Stage diameter (ft) E Energy consumption of existing HEN (MW) EnCost ex Cost of enhancement material ($) f Fraction of flooding velocity (0.5 to 0.7) F Molar flow rate of feed (kmol/h) F LV F m Flow parameter Correction factor for column material of construction in equation 4.20 F p Correction factor for column pressure in equation 4.20 F pack Packing factor (ft 2 /ft 3 ) F pd Dry-bed packing factor (ft -1 ) FUA Fractional of utilised area Fuel Fact Fuel factor defined in equation 4.26 f{f u o } Actual velocity function defined (calculated from equation 4.64) xxi

23 f{u o } Flooding velocity function defined in equation 4.64 f{µ L } Liquid viscosity correction factor given by equation 4.67 f{ρ L } Liquid density correction factor given by equation 4.66 g 0, g 1, g 2, g 3, g 4 Regression parameters in equation 4.66 g Acceleration due to gravity (32.2 ft/s 2 ) GT Gas turbine h 0, h 1, h 2, h 3, h 4 Regression parameters in equation 4.64 H Column Enthalpy of crude oil before entering distillation tower (kw) HD Heavy distillate product HETP Height equivalent to a theoretical plate (ft) H F Enthalpy of crude oil before preflash exchanger (kw) H FI Enthalpy of crude oil before entering preflash column (kw) H Flash Height of preflash (m) HK Light key component H L Enthalpy of bottom liquid from preflash (kw) H V Enthalpy of top vapour from preflash (kw) H mix Enthalpy of vapour mixture (kw) HN Heavy naphtha product H n Enthalpy of vapour from column n (kw) HP High pressure H PrColumn Height of prefractionator column (m) Enthalpy of steam delivered to process (kj/kg) h Proc h shell Shell side heat transfer coefficient (W/ m 2 o C) Shell side heat transfer coefficient in parallel arrangement (W/ m 2 o C) HTE Heat transfer enhancement exist h tube Tube side heat transfer coefficient in parallel arrangement (W/ m 2 o C) en h tube Enhanced tube side heat transfer coefficient (W/ m 2 o C) HXCost Nex Capital cost of heat exchanger ($) K Scaling parameter for penalty function in equation 5.3 K 1, K 2 Parameters for pressure drops (equations 4.49, 4.50) k 1, k 2, k 3 Parameters for pressure drop correlation in Appendix A k 1, k 2, k 3 Parameters for pressure drop correlation in Appendix A L Flow rate of liquid (kmol/h) (only in equation 4.2, units are kg/h) LD Light distillate product L F Gas loading factor in Robbins correlation (lb/h ft 2 ) L Flash Molar flow rate of bottom liquid from preflash (kmol/h) L flux Mass flow rate of liquid flux (lb/h ft 2 ) LK Light key component LN Light naphtha product m 0, m 1 Retrofit model parameters in equation 5.4 (m 2 /MW, m 2.h/MW.kmol) m, c Retrofit model parameters of power form in equation 4.7 (m 2 /MW, -) m', c' Retrofit model parameters of power form in equation 4.48 (m 2 /MW, -) M FG Flow rate of flue gas (kg/s) MINLP Mixed integer non-linear programming h shell exist xxii

24 MS Index Marshall and Swift index for equipment costs scaling up ( for the year 2001) n 0, n 1 Retrofit model parameters in equation 5.4 (-, h/kmol) N Total number of theoretical stages N Actual Actual total number of stages in distillation column NHV Fuel net heating value (kj/kg) NLP Non-linear programming N min Minimum number of stages at total reflux N OVLDTray Number of overloaded (bottlenecked) trays N R Number of theoretical stages in rectifying section N RActual Number of actual stages in rectifying section N S Number of theoretical stages in stripping section N SActual Number of actual stages in stripping section N Spacing Stage spacing (m) ( 0.60) NTP Number of theoretical plates P Pressure (Pa) PA Pump-around PCKV cost Cost of packing per unit volume ($/ft 3 ) Penalty Value of penalty function P mix Pressure of vapour mixture (Pa) P n Pressure of vapour from column n (Pa) Pr Prandtl Number in Appendix A P t Tube pinch in Appendix A (m) PUnit Cost Unit cost of power produced in gas turbines ($/kw h) p, z Pressure drop model constant parameters (Pa/MW, -) q 1, q 2 Constants in Robbins correlation given by equation 4.70, 4.71 q Liquid fraction of feed at feed stage conditions Q Heat load of heat exchanger (kw) Q Fuel Amount of fuel burnt in utility devices (kw) Q Preflash Heat duty of required for preflash column (kw) Q Preheat Heat duty of furnace (kw) Q Proc Heat duty required by process (kj/kg) Furn Q Fuel Amount of fuel consumed in a furnace (kw) GT Q Fuel Amount of fuel consumed in a gas turbine (MW) PS Q Fuel Reduction in fuel consumption at a power station (kw) Q Furn Process heat duty provided by a furnace (kw) Q GT Process heat duty provided by a gas turbine (kw) r Split ratio for the new to existing shell branches R Reflux ratio RES Residue product R Flash Ratio of height to diameter of preflash R HK Recovery of heavy key component to bottom product R LK Recovery of light key component to top product R min Minimum reflux ratio SS Side-stripper TBP True boiling point ( o C) T Exhaust Exhaust gas temperature from a gas turbine ( o C) T FTB Flue gases flame temperature in boilers ( o C) T FTF Flue gases flame temperature in furnaces ( o C) xxiii

25 T inlet Inlet temperature to the gas turbine (combustion temperature) ( o C) T o Ambient temperature ( o C) T outlet Outlet temperature from the gas turbine (flue gas temperature) ( o C) T Stack Stack temperature ( o C) U Overall heat transfer coefficient for existing exchanger (W/ m 2 o C) U des Design velocity actual velocity (ft/s) U en Overall heat transfer coefficient for enhanced exchanger (W/ m 2 o C) U max Flooding velocity (ft/s) u o Flooding velocity in a packed column (ft/s) U ratio Ratio of design velocity to flooding velocity (70-80%) V Flow rate of vapour (kmol/h) (only in equation 4.2, units are kg/h) V F Gas loading factor in Robbins correlation (lb/h ft 2 ) V Flash Molar flow rate of top vapour from preflash (kmol/h) V flux Mass flow rate of liquid flux (lb/h ft 2 ) V L Volumetric flow rate of flashed liquid (m 3 /s) V min Minimum molar vapour flow rate in top section (kmol/h) V min Minimum molar vapour flow rate at bottom pinch (kmol/h) V min,top : Minimum molar vapour flow rate in top section (kmol/h) V mix Flow rate of vapour mixture (kmol/h) V n Flow rate of vapour from column n (kmol/h) V S Volumetric flow rate in shell side in Appendix A (m 3 /s) V T Volumetric flow rate in tube side in Appendix A (m 3 /s) V V Volumetric flow rate of vapour (ft 3 /s)) W GT Power produced in a gas turbine (kw) X Molar composition of bottom liquid from preflash x blk Mole fraction of light key component in bottom product x dhk Mole fraction of heavy key component in top product x dlk Mole fraction of light key component in top product x f,i Mole fraction of component i in feed x fhk Mole fraction of heavy key component in feed x flk Mole fraction of light key component in feed x HKFS Liquid mole fraction of heavy key component on feed stage x i Dependent variable in optimisation, equation 5.3 x LKFS Liquid mole fraction of light key component on feed stage x i,lim Constrained value for variable in optimisation, equation 5.3 x, y Number of carbon and hydrogen atoms respectively in equation 4.24 XX Location of entering vapours from preflash to distillation column Y Molar composition of top vapour from preflash Y mix Molar composition of vapour mixture Y n Molar composition of vapour from column n Z Molar composition of feed to preflash xxiv

26 Greek letters α Ratio of molar masses of carbon dioxide and carbon (=3.67) α HK Volatility of heavy key component relative to a reference component α i Volatility of component i relative to a reference component α LK Volatility of light key component relative to a reference component A Additional area to an existing exchanger unit (m 2 ) A max Maximum area that can be achieved by enhancing an exchanger (m 2 ) max A total Total maximum area that can be achieved by enhancing a HEN (m 2 ) H PrColumn Extra height added to prefractionator column (m) P Pressure drop for a plain tube (Pa) P C.O. Pressure drop for the crude oil stream in preheat train (Pa) P en Pressure drop for an enhanced exchanger (Pa) P shell Pressure drop on shell side (Pa) exist P shell Pressure drop of existing shell (Pa) new P shell Pressure drop of new shell (Pa) total P shell Total pressure drop of existing and new shells (Pa) P flood Pressure drop in a packed bed at flooding (in. H 2 O/ft height) P pack Pressure drop in a packed bed (in. H 2 O/ft height) P tube Pressure drop in tube sides (Pa) T Temperature drop through pump-around ( o C) T LM Logarithmic mean temperature difference ( o C) T min Minimum difference temperature approach ( o C) φ Roots of Underwood equation φ Fensk Fenske term given by equation 3.6 to simplify equation 3.5 φ Kirk Kirkbride term given by equation 3.17 to simplify equation 3.16 Φ Friction factor in Appendix A η C Carnot Factor of gas turbine η Furn Furnace efficiency η GT Gas turbine heat recovery efficiency η OP Overall plate efficiency η OTray Overall tray efficiency of distillation column η RTray Tray efficiency of rectifying section η STray Tray efficiency of stripping section κ, υ Constants in Coker correlation for pressure drop in Appendix A λ Thermal conductivity in Appendix A (W/m 2 K) λ Proc Latent heat of steam delivered to process in Appendix A (kj/kg) µ Viscosity (kg/m s) µ F,avg Average viscosity of feed at section temperature and pressure (cp) µ L Liquid viscosity (cp) θ R Residence time in preflash (s) ρ Density in Appendix A (kg/m 3 ) ρ 2 Density of water (62.4 lb/ft 3 ) H O xxv

27 ρ L Mass density of liquid (kg/m 3 ) (in equations 4.74, 4.75 only, units are lb/ft 3 ) ρ V Mass density of vapour (kg/m 3 ) (in equations 4.72, 4.73 only, units are lb/ft 3 ) ϕ Steam Stripping steam term given by equation 3.31 to simplify equation 3.30 σ Surface tension (dyne/cm) ξ Term defined by equations 2.8, 3.9 and 3.34 ψ Gill Gilliland term in equations 3.7, 3.32 Subscripts and superscripts C C.O. des en exist F,avg FG Fensk flood Furn Gill GT HK Kirk lim LK LM max min mix OP OTray OVLDTray Pack Proc PS R RActual req ret RTray S SActual SB shell Steam STray tube Carnot factor Crude oil stream Design Heat transfer enhanced Existing unit Feed average property Flue gases Fenske equation Flooding condition Furnace Gilliland equation Gas turbine Heavy key component Kirkbride correlation Limiting value for a variable Light key component Logarithmic mean Maximum condition Minimum condition Mixture property Overall plate property Overall tray property Overloaded or bottlenecked trays Packed section Distillation process Central power station Residence time, Rectifying section Rectifying actual trays Required Retrofit design Rectifying tray section Stripping section Stripping actual trays Souder and Brown constant Shell-side Stripping steam Stripping tray section Tube-side xxvi

28 Chapter 1 Introduction Chapter 1: Introduction Crude oil distillation is a process of major importance in the refining industry. This process is energy and capital intensive. An existing crude oil distillation unit is a complex structure with highly interlinked columns. The crude oil distillation column has strong connections with the associated heat recovery system, i.e. the distillation column is integrated with the heat recovery system. The operating variables of the distillation process (e.g. feed preheating temperature, steam flow rates, pump-around duties, reflux ratio, etc.) and the existing hardware of the heat recovery system affect opportunities for heat recovery and throughput enhancement. Retrofit of crude oil distillation systems for energy savings, capacity increase, emissions reductions and other objectives is of major concern to the refining industry. Retrofit objectives can be achieved by changing the operating conditions of the distillation column and increasing heat recovery or by modifying the structure of the existing distillation and heat recovery units. Few new crude oil distillation units are built; on the other hand, projects revamping existing equipment are common. Rather than install new equipment, refinery managers prefer to exploit existing units for more profit. Retrofit objectives are conventionally energy saving and increasing production capacity; however, economic and environmental incentives drive other objectives, such as improving process profit, reducing greenhouse emissions, processing new feedstock or changing product yields. Many retrofit objectives are closely related; for example, energy retrofit projects are accompanied by reducing emissions; the associated reduction in vapour loads may allow both the column and the preheat train to process more crude oil feed. Retrofit goals are best achieved by reusing existing equipment more efficiently, with minor modifications to the existing equipment, rather than installing new units and incurring greater capital investment. While achieving these objectives, the existing equipment and process constraints, such as distillation hydraulic capacity and crude oil feed stream pressure drop, must be met. The interactions between the existing distillation process and heat recovery system have a critical effect on the retrofit of the overall process. These interactions are the operating conditions of the distillation column, including feed preheating temperature, steam flow rate, pump-around duties and flow rates and reflux ratio, in addition to the existing 1

29 Chapter 1 Introduction exchanger matches and areas of the heat recovery system. Changing the operating conditions may benefit the heat recovery in the exchanger network and enhance the processing capacity of the distillation column. On the other hand, existing exchanger matches and hardware of the heat recovery system affect opportunities for heat recovery. Furthermore, modifying the column structure by adding preflash or prefractionator units, changing the column internals, or integrating a gas turbine with the furnace can increase the heat recovery, allow more capacity to be processed and help the overall system reduce the combustion emissions. This thesis presents a retrofit design methodology and modelling and optimisation framework that aims to identify quantitatively the operating conditions and structural changes, such as adding a preflash drum or a prefractionator unit, integrating a gas turbine, etc., that can best achieve a specified retrofit objective. The approach addresses both the distillation column and the heat exchanger network to maximise use of existing equipment Retrofit of heat-integrated crude oil distillation systems For retrofitting a crude oil distillation system (i.e. the distillation column, side-strippers and associated heat exchanger network), the existing system has to be modified. Which modifications will be beneficial is a key design issue that changes according to the existing system together with the retrofit goal. Modifications might be made to the distillation process or to the heat recovery system. Modifications that can be made have different cost implications, ranging from zero cost to high capital investment. Examples of these modifications and their investment requirements can be summarised as follows (Fraser and Sloley, 2000): 1. Changing the operating conditions of the distillation process. These modifications do not necessarily require any fixed cost. 2. Repiping the existing exchangers to improve the heat recovery of the process without the purchase of new exchanger units. The investment for repiping is moderate. 3. Existing equipment can be modified in various ways. For example, internals can be changed in the heat exchangers (e.g. tube bundles) or distillation columns (e.g. 2

30 Chapter 1 Introduction packing) and the external size may be modified (e.g. add to the height of a column). Clearly, these modifications require some capital investment. 4. The purchase of new equipment is normally the most costly retrofit decision, although adding a new shell to an existing heat exchanger is much cheaper than purchasing a new distillation column or a new compressor. Installing new equipment may be combined with modifying piping to change the configuration of the process. The above modifications are more beneficial for retrofit when they are applied in parallel with the exploitation of the interactions between the distillation process and the associated heat recovery system. Many efforts have been devoted to solve the retrofit design problem of heat-integrated crude oil distillation systems. Early research concentrated on applying engineering experience and heuristics to propose modifications separately to the distillation column and the heat exchanger network in order to reduce energy consumption. These modifications include the installation of new column internals with higher efficiency and the use of intermediate reboilers (e.g. Sittig, 1978), the installation of pump-arounds (e.g. Bannon and Marple, 1978), the addition of preflash drums and prefractionator columns (e.g. Harbert, 1978), and changing operating conditions (e.g. Fraser and Sloley, 2000; Liu, 2000). Other researchers used pinch analysis principles to identify modifications to distillation columns for reducing energy consumption and improving the performance of the system. (e.g. Linnhoff et al., 1983; Dhole and Linnhoff, 1993a; Dhole and Buckingham, 1994). Liebmann (1996) proposed a two-step (sequential) approach for improving the performance of refinery distillation columns, based on insights derived from pinch analysis. Bagajewicz (1998) extended the approach to optimise existing refinery distillation columns by incorporating pinch analysis principles within a rigorous-based simulation framework. Recent work was carried out by one research group (Bagajewicz and Ji, 2001; Bagajewicz and Soto, 2001; Ji and Bagajewicz, 2002a; b; c) for grassroots design of heat-integrated crude oil distillation units. The objective was to design energyefficient distillation units and heat exchanger networks that can handle different types of crude oils. The design was applied to conventional atmospheric towers, stripping-type columns (crude oil is heated to a relative low temperature, 150 o C, and fed at the top of the column, crude oil goes down the column and heated consecutively in three heaters) 3