STATUS OF THESIS GRAPHICAL EXERGY ANALYSIS METHOD FOR OPTIMUM Title of thesis DESIGN AND OPERATIONS OF SEPARATION PROCESSES I, KHOA TA DANG , hereby allow my thesis to be placed at the Information Resource Center (IRC) of Universiti Teknologi PETRONAS (UTP) with the following conditions: 1. The thesis becomes the property of UTP. The IRC of UTP may make copies of the thesis for academic purposes only. This thesis is classified as Confidential Non–confidential If this thesis is confidential, please state the reason: The contents of the thesis will remain confidential for years.
Remarks on disclosure: Endorsed by Signature of Author Signature of Supervisor Permanent address: Name of Supervior: Faculty of Chemical Engineering Assoc. Shuhaimi Mahadzir Hochiminh City University of Technology HCMUT, Hochiminh City, Vietnam Date: Date: i UNIVERSITI TEKNOLOGI PETRONAS GRAPHICAL EXERGY ANALYSIS METHOD FOR OPTIMUM DESIGN AND OPERATIONS OF SEPARATION PROCESSES by KHOA TA DANG The undersigned certify that they have read, and recommend to the Postgraduate Studies Programme for acceptance this thesis for the fulfilment of the requirements for the degree stated. Signature: Main Supervisor: Assoc. Shuhaimi Mahadzir Signature: Head of Department: Assoc.
Mohd Azmi B Bustam @ Khalil Date: ii GRAPHICAL EXERGY ANALYSIS METHOD FOR OPTIMUM DESIGN AND OPERATIONS OF SEPARATION PROCESSES by KHOA TA DANG A Thesis Submitted to the Postgraduate Studies Programme as a Requirement for the Degree of DOCTOR OF PHILOSOPHY CHEMICAL ENGINEERING DEPARTMENT UNIVERSITI TEKNOLOGI PETRONAS BANDAR SERI ISKANDAR, PERAK SEPTEMBER 2012 iii DECLARATION OF THESIS GRAPHICAL EXERGY ANALYSIS METHOD FOR OPTIMUM Title of thesis DESIGN AND OPERATIONS OF SEPARATION PROCESSES I, KHOA TA DANG , hereby declare that the thesis is based on my original work except for quotations and citations which have been duly acknowledged. I also declare that it has not been previously or concurrently submitted for any other degree at UTP or other institutions. Witnessed by Signature of Author Signature of Supervisor Permanent address: Name of Supervisor: Faculty of Chemical Engineering Assoc. Shuhaimi Mahadzir Hochiminh City University of Technology HCMUT, Hochiminh City, Vietnam Date: Date: iv ACKNOWLEDGEMENTS First of all, I would like to express my deep gratitude to my supervisor, Assoc.
Shuhaimi Mahadzir, for his encouragements and advices. Without his comments and supports, this work could not run on the right path and succeed beyond expectation. He is willing to discuss and give comments on the coming ideas, as well as the drafts. I am grateful to Universiti Teknologi PETRONAS (UTP) for providing me with this opportunity to pursue Doctor of Philosophy under graduate assistantship scheme (GA).
Universiti Teknologi PETRONAS (UTP) is a trully research environment which helps me a fresh and open mind to create. I also have a high appreciation of helps from Professors and Lecturers in Chemical Engineering Department, especially Assoc. Mohd Ibrahim Abd Mutalib. Lastly, I would not forget the encouragements from Vietnamese Committee Student (VSC), Universiti Teknologi PETRONAS (UTP).
They are good friends and always accompaning me till the last station. v DEDICATION To my parents, without your knowledge and supervisions, these ideas could not be generated. To my darling, without your supports and encouragements, this thesis could not also be completed. To my beloved kids, without your smiles, this work could not be achieved completely, too.
vi ABSTRACT Enhancing the performance of energy savings is imperative to solve the issues of natural resources depletion and energy demand increase. Improvement of energy efficiency in chemical process industry leads to reduction of operations cost and environmental emission. In chemical plants, distillation and absorption are important separation processes that are widely used for product recovery and purification. These processes are energy intensive.
Currently, exergy concept has gained renewed interest to analyse the performance of energy intensive processes. In this work, a general framework is proposed to establish the correlations between design and operations parameters to exergy lost of separation processes. A new graphical method, three dimensional exergy analysis diagram, is constructed from the general framework for optimisation of distillation and absorption columns. The diagram provides visualisation of the full energy savings potential and the path to achieve the optimum state of design and operations.
In xylene distillation column, one of the feasible solutions found from the graphical method is reduction of reflux ratio and installation of new feed pre–heaters. These modifications result in 15.5% reduction of exergy lost, which is translated to 26.9% savings of reboiler duty. In the sulphur dioxide recovery process, a lower lean solvent temperature cuts the exergy lost by 22. This work also proposes a new model of exergy costing, namely global energy costing model that considers both the quantity and quality in energy cost.
The model allows the calculation of global cost of steam at different level. For complex separation system, viz. the Selexol process, the combination of the graphical method and the global energy costing model gives a feasible process modification that results in 19.4% total exergy savings and 5.57 Million USD/yr operation cost savings. The general framework for exergy analysis together with the three dimensional exergy analysis diagram and global energy costing model have been demonstrated as effective methods to optimise design and operations of separation processes.
Hence this work contributes to the effort of environmental and energy conservation. vii ABSTRAK Peningkatan prestasi penjimatan tenaga adalah penting dalam mencari penyelesaian masalah pengurangan sumber asli dan peningkatan permintaan tenaga. Penambahbaikan kecekapan tenaga di dalam industri proses kimia membuka jalan ke arah pengurangan kos operasi dan pengurangan pencemaran alam sekitar. Di dalam loji proses kimia, penyulingan dan pencerapan adalah antara proses–proses pemisahan penting serta digunakan secara meluas untuk perolehan dan pemurnian produk.
Proses–proses pemisahan ini mempunyai keperluan tenaga yang intensif. Ketika ini, konsep exergy telah menarik kembali minat para penyelidik dalam menganalisis prestasi proses–proses intensif tenaga. Di dalam tesis ini, suatu rangkakerja umum dicadangkan untuk membina hubungan matematik di antara parameter rekabentuk dan operasi kepada kemusnahan exergy dalam proses pemisahan. Satu peranti grafik yang baru, rajah analisis exergy tiga dimensi, dibina dari rangkakerja umum yang telah dicadangkan bagi mengoptimumkan rekabentuk dan operasi alat penyulingan dan pencerapan.
Rajah analisis exergy tiga dimensi memberi gambaran potensi penjimatan tenaga yang menyeluruh serta jalan untuk mencapai keadaan rekabentuk dan operasi yang optima. Di dalam loji penyulingan xylene, salah satu penyelesaian praktik dari peranti grafik ialah pengurangan kadar refluks dan pemasangan alat pra–pemanasan baharu pada aliran masuk. Hasil pengubahsuaian yang dilakukan membawa kepada 15.5% pengurangan kemusnahan exergy yang diterjemahkan kepada 26.9% penjimatan tenaga pada dandang penyuling. Di dalam proses perolehan sulfur dioksida, pengurangan suhu pelarut bersih membawa kepada pengurangan kemusnahan exergy sebanyak 22.
Tesis ini juga mencadangkan model baharu pengiraan harga exergy, iaitu model global kos tenaga, yang mengambilkira faktor kuantiti tenaga bersama–sama dengan faktor kualiti tenaga. Model ini membolehkan pengiraan kos global stim pada tahap yang berlainan. Bagi sistem pemisahan yang kompleks seperti proses Selexol, gabungan peranti grafik dan model global kos tenaga memberi penyelesaian praktik yang membawa kepada 19.4% penjimatan exergy viii menyeluruh dan pengurangan kos operasi tahunan sebanyak 5. Rangkakerja umum analisis exergy bersama–sama dengan rajah analisis exergy tiga dimensi dan model global kos tenaga telah ditunjukkan sebagai peranti berkesan untuk mengenalpasti keadaan optima rekabentuk dan operasi proses pemisah.
Justeru itu, tesis ini membawa sumbangan kepada usaha pemuliharaan alam sekitar dan tenaga. ix In compliance with the terms of the Copyright Act 1987 and the IP Policy of the university, the copyright of this thesis has been reassigned by the author to the legal entity of the university, Institute of Technology PETRONAS Sdn Bhd. Due acknowledgement shall always be made of the use of any material contained in, or derived from, this thesis. © KHOA TA DANG, 2012 Institute of Technology PETRONAS Sdn Bhd All rights reserved.
x TABLE OF CONTENTS ACKNOWLEDGEMENTS -------------------------------------------------------------------- v DEDICATION----------------------------------------------------------------------------------- vi ABSTRACT ------------------------------------------------------------------------------------ vii ABSTRAK --------------------------------------------------------------------------------------viii COPYRIGHT ------------------------------------------------------------------------------------ ix TABLE OF CONTENTS ---------------------------------------------------------------------- xi LIST OF FIGURES ---------------------------------------------------------------------------- xiv LIST OF TABLES --------------------------------------------------------------------------- xviii ABBREVIATIONS ---------------------------------------------------------------------------- xix NOMENCLATURES --------------------------------------------------------------------------xx CHAPTER 1: INTRODUCTION -------------------------------------------------------------- 1 1.1 The motivation of research ----------------------------------------------------------- 1 1.1 Concept of exergy -------------------------------------------------------------- 2 1.2 Classification of exergy ------------------------------------------------------- 6 1.5 Objectives of research ----------------------------------------------------------------17 1.6 Scope of work -------------------------------------------------------------------------18 CHAPTER 2: LITERATURE REVIEW ----------------------------------------------------20 2.1 Energy savings techniques for distillation and absorption processes ----------20 2.2 Pinch technology and graphical methods ------------------------------------------23 2.3 Exergy analysis methods -------------------------------------------------------------26 2.4 Exergy and economics ---------------------------------------------------------------34 2.5 Summary of literature review -------------------------------------------------------35 CHAPTER 3: METHODOLOGY ------------------------------------------------------------37 xi 3.1 Exergy analysis for distillation column ----------------------------------- 40 3.2 Exergy analysis for absorption column ----------------------------------- 51 3.1 Investment cost estimation -------------------------------------------------- 58 3.2 Operation cost estimation --------------------------------------------------- 60 3.3 Application of global energy costing model ----------------------------- 63 3.4 Comparison between conventional and global energy costing models -- ---------------------------------------------------------------------------------- 64 3.5 Economics estimation for a process improvement project ------------- 66 3.3 Process simulation ------------------------------------------------------------------- 67 CHAPTER 4: RESULTS AND DISCUSSIONS ------------------------------------------ 71 4.1 Comparison of simulation results -------------------------------------------------- 71 4.2 Three dimensional exergy analysis diagram ------------------------------------- 73 4.1 Three dimensional exergy analysis diagram for distillation column ---- ---------------------------------------------------------------------------------- 73 4.2 Three dimensional exergy analysis diagram for absorption column ----- ---------------------------------------------------------------------------------- 79 4.3 Case study 1: Exergy analysis and improvements of xylene distillation column --------------------------------------------------------------------------------- 81 4.1 Base case of xylene distillation column----------------------------------- 81 4.2 Analysis of base case -------------------------------------------------------- 82 4.3 The effects of reflux ratio --------------------------------------------------- 84 4.4 The effects of feed thermal condition ------------------------------------- 87 4.5 Process improvement options for xylene distillation column---------- 88 4.4 Case study 2: Exergy analysis and improvements of sulphur dioxide absorption column ------------------------------------------------------------------- 93 4.1 Base case of sulphur dioxide recovery process -------------------------- 93 4.2 Analysis of base case -------------------------------------------------------- 94 4.3 The effects of (Lm/Gm) ratio------------------------------------------------- 95 4.4 The effects of solvent temperature ----------------------------------------- 97 4.5 Process improvement options for sulphur dioxide absorption column -- ---------------------------------------------------------------------------------- 98 xii 4.5 Case study 3: Exergy analysis and improvements of Selexol process ------ 101 4.1 Introduction of Selexol process ------------------------------------------- 101 4.2 Base case of acid gas removal in the Selexol process ----------------- 101 4.3 Analysis of base case ------------------------------------------------------- 104 4.4 The effects of (Lm/Gm) ratio------------------------------------------------ 105 4.5 The effects of reflux ratio -------------------------------------------------- 106 4.6 The effects of lean solvent temperature ---------------------------------- 107 4.7 The effects of feed thermal condition in distillation column --------- 109 4.8 Three dimensional exergy analysis diagrams for columns ------------ 111 4.9 The effects of (Lm/Gm) and reflux ratios on exergy lost in Selexol process ------------------------------------------------------------------------ 112 4.10 Process improvement options for Selexol process --------------------- 115 4.11 Economic estimation ------------------------------------------------------- 117 CHAPTER 5: CONCLUSIONS AND FUTURE WORKS ----------------------------- 121 5.2 Future works ------------------------------------------------------------------------- 123 REFERENCES -------------------------------------------------------------------------------- 126 PUBLICATION LISTS ---------------------------------------------------------------------- 140 APPENDIX A: Compositions of feed stream in xylene column ----------------------- 141 APPENDIX B: Plant data of process streams in xylene column ----------------------- 142 APPENDIX C: HYSYS process model and streams data ------------------------------- 143 xiii LIST OF FIGURES Figure 1.1: Power generation by steam turbine ---------------------------------------------- 2 Figure 1.2: Sankey diagram for energy and exergy analysis ------------------------------- 5 Figure 1.3: Model of exergy balance for a system or unit operation---------------------- 8 Figure 1.4: Adiabatic distillation column --------------------------------------------------- 10 Figure 1.5: McCabe–Thiele diagram for distillation column ---------------------------- 11 Figure 1.6: Diabatic distillation column ---------------------------------------------------- 13 Figure 1.7: Typical absorption column------------------------------------------------------ 14 Figure 1.8: Mole ratio diagram for absorption column ----------------------------------- 15 Figure 3.1: The general framework for constructing exergy lost model of a process --- --------------------------------------------------------------------------------------- 38 Figure 3.2: McCabe–Thiele diagram for reversible operation state of diabatic column --------------------------------------------------------------------------------------- 41 Figure 3.3: McCabe–Thiele diagram for ideal adiabatic column with infinite number of stages, perfect efficiency and minimum reflux ratio --------------------- 42 Figure 3.4: McCabe–Thiele diagram for adiabatic column with infinite number of stages, finite efficiency and minimum reflux ratio--------------------------- 43 Figure 3.5: McCabe–Thiele diagram for adiabatic column with finite number of stages, perfect efficiency and minimum reflux ratio------------------------- 44 Figure 3.6: McCabe–Thiele diagram for adiabatic column with finite number of stages, perfect efficiency and actual reflux ratio ----------------------------- 44 Figure 3.7: McCabe–Thiele diagram for adiabatic column with actual number of stages, perfect efficiency and theoretical reflux ratio------------------------ 45 Figure 3.8: McCabe–Thiele diagram for actual column with actual number of stages, finite efficiency and actual reflux ratio ---------------------------------------- 46 Figure 3.