Copyright and Citation Considerations for This Thesis/ Dissertation
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COPYRIGHT AND CITATION CONSIDERATIONS FOR THIS THESIS/ DISSERTATION o Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use. o NonCommercial — You may not use the material for commercial purposes. o ShareAlike — If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original. How to cite this thesis Surname, Initial(s). (2012) Title of the thesis or dissertation. PhD. (Chemistry)/ M.Sc. (Physics)/ M.A. (Philosophy)/M.Com. (Finance) etc. [Unpublished]: University of Johannesburg. Retrieved from: https://ujcontent.uj.ac.za/vital/access/manager/Index?site_name=Research%20Output (Accessed: Date). ATTAINABLE REGIONS FOR OPTIMAL REACTION NETWORKS: APPLICATION OF ΔH-ΔG PLOT TO DIRECT SYNTHESIS OF DIMETHYL ETHER FROM SYNGAS By THULANE PAEPAE Submitted In Fulfilment of the Requirements of MASTERS TECHNOLOGIAE In CHEMICAL ENGINEERING In the FACULTY OF ENGINEERING AND THE BUILT ENVIRONMENT Of the UNIVERSITY OF JOHANNESBURG Supervisor: Dr Tumisang Seodigeng Co-supervisor: Prof Freeman Ntuli Johannesburg, 2015 DECLARATION I hereby declare that this dissertation which I submit in fulfilment for the qualification MAGISTER OF TECHNOLOGIAE IN CHEMICAL ENGINEERING to the University of Johannesburg, Department of Chemical Engineering is, apart from the recognized assistance from my supervisors, my own work which has not previously been submitted by me or any other person to any institution to obtain a degree. On this Day of i ABSTRACT Industrial processes pose a serious threat to the world’s natural resources for they consume them in high proportions as sources of energy for driving chemical processes that provide raw materials for many industrial chemicals. The chemical and petrochemical industries consume about 61% of global industrial energy and emit about 36% of carbon dioxide to the environment. A significant portion of the energy demand is entirely for feedstock, which cannot be reduced through energy efficiency measures. The responsibility therefore for cutting back on the amount of energy needed for chemical processes rests on improving the efficiency of the processes used. The purpose of this research is to demonstrate the use of a novel method of synthesizing process flowsheets, using a graphical tool called the GH-space and in particular to look at how it can be used to compare the reactions of a combined simultaneous process with regard to their thermodynamics. This allows us to synthesize flow-sheets that are reversible and which meet the process targets by implementing mass and energy integration. It also provides guidance on what design decisions would be best suited to developing new processes that are more effective and make lower demands on raw material and energy usage. The approach also provides useful information for evaluating processes through likely limiting extents with respect to the reaction pathways, and comparison between the research findings and their theoretical targets in order to identify any possible energy savings that can be made. The GH-space technique uses fundamental thermodynamic principles to allow the mass, energy and work balances locate the attainable region for chemical processes in a reactor. Furthermore, processes and unit operations can be defined as vectors in the GH-space. Using the targets, one can combine the vector processes in such a way as to approach the target. These vector processes, and the way they are combined, can then be interpreted in terms of flowsheets. This is opposite to what is normally done and allows the process balances to determine what the best flowsheet might look like, allowing for great innovation from the very start of a design. In addition to this, probably the greatest advantage of the GH-space technique is that processes of great complexity can all be analysed on a set of two- dimensional axes. After finding the attainable region, its boundaries can be interpreted in ii terms of the prospective limiting extents through mass balances. By these means we can deduce the process reaction pathways. The technique allows for easy and rapid interpretation of the results, such as the effects of changes in operating conditions. It also provides insight into the likely reactions achievable in the reactor under different process conditions. This approach was applied to the direct synthesis of dimethyl ether from syngas. The graphical plots show that the introduction of carbon dioxide in the feed changes the shape and size of the attainable region, resulting in multiple reaction pathways. They also demonstrate that the reaction pathways leading to the product and the change in ΔG across the reactor are interlinked. It was shown in this work that with clear understanding of the flows of mass, energy and work within a process, the reaction path analysis could become an important tool in the preliminary stages of process design since it can identify the most desirable reaction routes. iii DEDICATION This dissertation is utterly dedicated to the Lord Almighty, for His immeasurable mercy, compassion and guidance throughout this work. iv ACKNOWLEDGMENTS I owe thanks to many people, whose assistance was fundamental to the accomplishment of this project. Firstly, I express my sincere gratitude to my supervisor, Dr Tumisang Seodigeng for his technical guidance and constructive advice, continual suggestions, and encouragement to develop me into an independent researcher. Many thanks for his help which extended beyond my research. I would like to thank Prof Freeman Ntuli for his assistance in the accomplishment of this project. I am indebted to the National Research Foundation (NRF), and the University of Johannesburg for the financial support. Above all, I would like to thank my family: my mother, brothers and sisters for their support, encouragement and love throughout my life. Lastly, I offer my regards and blessings to all of those whose names are not mentioned here but supported me in one way or another towards the success of this research. v TABLE OF CONTENTS LIST OF FIGURES ............................................................................................................. viii LIST OF TABLES .................................................................................................................. xi LIST OF ACRONYMS ......................................................................................................... xii CHAPTER 1: INTRODUCTION ........................................................................................... 1 1.1 Background and Motivation.................................................................................... 1 1.2 Research Aims and Objectives ................................................................................ 5 1.2.1 Aims ........................................................................................................................ 5 1.2.2 Objectives................................................................................................................ 5 1.3 Significance and Impact of this Research ................................................................ 6 1.4 Thesis Overview ...................................................................................................... 7 CHAPTER 2: LITERATURE REVIEW ............................................................................ 12 2.1 Introduction ........................................................................................................... 12 2.2 Optimal Reactor Networks .................................................................................... 12 2.3 Attainable Region Targeting ................................................................................. 14 2.3.1 Kinetically attainable region ................................................................................. 14 2.3.2 Thermodynamically attainable region .................................................................. 16 2.4 Dimethyl Ether Synthesis ...................................................................................... 18 2.4.1 Introduction ........................................................................................................... 18 2.4.2 Chemistry .............................................................................................................. 19 2.4.3 Reaction kinetics ................................................................................................... 22 2.4.4 Essential factors to consider for dimethyl ether production ................................. 23 vi CHAPTER 3: METHODOLOGY ....................................................................................... 42 3.1 Introduction ........................................................................................................... 42 3.2 Thermodynamic Analysis of the Process .............................................................. 42 3.2.1 Effect of Temperature on the Equilibrium Conversions of Chemical Reactions .. 44 3.2.2 Effect of Pressure on the Equilibrium Conversions of Chemical Reactions ......... 50 3.3.3 Effect of Initial Composition on the Equilibrium Conversions of Chemical Reactions ............................................................................................................... 51 CHAPTER 4: RESULTS, ANALYSIS AND DISCUSSION ............................................