EXPERIMENTAL EQUILIBRIUM BETWEEN ACID GASES AND ETHANOLAMINE SOLUTIONS By ABDULLATIF M. BHAIRI Bachelor of Science University of California Davis, California 1974 Bachelor of Science University of California Davis, California 1974 Master of Science Oklahoma State University Stillwater, Oklahoma 1980 Submitted to the Doctoral Advisory Corrvnittee in Partial Fulfillment of the Requirements for the Candidacy for the Degree of DOCTOR OF PHILOSOPHY July 1984 EXPERIMENTAL ; ' ! : ' ,' AC ID GASES AND ETHANOLAM I NE SOLUTIONS Thesis Approved: ii PREFACE This research is concerned with the measurement of the equilibrium solubility of acid gases in various alkanolamine solutions. The acid gases considered were carbon dioxide and hydrogen sulfide. Amines investigated were monoethanolamine, diglycolamine, diethanolamine and methyldiethanolamine. Measurements of the solubility were made at acid gas partial pressures to 1000 psia and temperatures from 77 to 240°F. I wish to express my sincere gratitude and deep appreciation to my major advisor throughout Tl\Y graduate work, Or. Robert N. Maddox. His concern, help and intelligent guidance are invaluable. I am also indebted to Dr. Gilbert J. Mains for his interest and valuable discussions. Thanks are also due to the committee members, Dr. Jan Wagner, Dr. Anthony L. Hines, and the late Dr. John H. Erbar for their advisement in the course of this work. I would like also to thank Mr. Ross Fox of Fluid Properties Research, Inc. for his constant technical assistance. The help, encouragement and friendship provided by my graduate colleagues is acknowledged. Thanks are due the Physical Properties Laboratory for financial support throughout the years of research and to the Gas Processors Association for their support to parts of this research. iii My parents, my wife, brothers and sisters and my children deserve my gratitude and appreciation for their patience, moral support and understanding. iv TABLE OF CONTENTS Chapter Page I • INTRODUCTION. • • 1 II. LITERATURE SURVEY • 4 MEA Data ••••••••••••• 4 DEA Data • • • • • • • • • • • • • 5 OGA Data • • • • • • • • • • • 9 MDEA Data. • • • • • • • • • • • • • • • • • • • • • • • 9 Experimental Techniques for Solubility Studies 10 The Chemistry of Acid Gas Reactions •• , ••• 11 Amine-H2s Reactions, • • • . ••••••. 16 Amine-co 2 Reactions, •••• , ••••• 17 III. EXPERIMENTAL APPARATUS •• 23 Constant Temperature Bath •• 23 Equilibrium Cell ••• 25 Pressure Gauges •••• 25 Gas Feed Systems ••• 26 Liquid Feed Systems •• 27 Auxi 1 i ary Equipment. 27 Material ••••• 28 IV. EXPERIMENTAL PROCEDURES •• 29 Operational Procedure. • •••••• 29 Determination of Experimental Solubility • . 31 V. EXPERIMENTAL RESULTS AND DISCUSSION. 33 MEA Data • • . 33 DGA Data • • . 36 DEA Data •• . 45 MDEA Data •• 45 VI. CONCLUSIONS AND RECOMMENDATIONS 56 Conclusions •••• 56 Recommendations. 57 BIBLIOGRAPHY ••••••• 58 APPENDIX A - EXPERIMENTAL DATA • • • • 62 Chapter Page APPENDIX B - EQUILIBRIUM CELL CALIBRATION . 86 APPENDIX C - THERMOCOUPLE CALIBRATION ••• . 88 APPENDIX D - CALIBRATION OF THE PRESSURE GAUGES • 91 APPENDIX E - PHYSICAL PROPERTIES OF AMINE SOLUTIONS •• . 95 APPENDIX F - SAMPLE CALCULATION . 104 APPENDIX G - ERROR ANALYSIS ••• . 108 vi LIST OF TABLES Table Page I. Literature Survey of Published Data on MEA-Acid Gases . 6 II. Literature Survey of Published Data on DEA-Acid Gases . 8 III. Physical Properties for Ethanolamines . 14 IV. Solubility of co 2 in 2.5 N MEA Solution at 25°C . 63 v. Solubility of co2 in 2.5 N MEA Solution at 60°C . 64 VI. Solubility of co 2 in 2.5 N MEA Solution at 80°C . 65 VII. Solubility of H2S in 2.5 N MEA Solution at 25°C . 66 VI II. Solubility of co 2 in 20% by Weight DGA Aqueous Solution at 25°C . • . • . • . • . 67 IX. Solubility of co2 in 40% by Weight DGA Aqueous Solution at 40 and 60°C. 68 x. Solubility of Hts in 40% by Weight DGA Aqueous Solution a 40°C • • • . 69 XI. Solubility of Hts in 40% by Weight DGA Aqueous Solution a 60°C • • • . 70 XII. Solubility of co 2 in 60% by Weight DGA Aqueous Solution at 50°C ••••••••••.•• . 71 XIII. Solubility of Hts in 60% by Weight DGA Aqueous Solution a 50°C • • • . • . 72 XIV. Solubility of H€S in 60% by Weight DGA Aqueous Solution a 80°C ••••••••.•••• . 73 xv. Solubility of co 2 on 0.5 N DEA Solution at 25°C • • 74 XVI. Solubility of C0 2 in 2.0 N DEA Solution at 25°C . 75 XVII. Solubility of C02 in 1.0 N MDEA Solution at 25°C. 76 XVIII. Solubility of H2S in 1.0 N MDEA Solution at 25°C. 77 vii Table Page XIX. Solubility of co 2 in 2.0 N MDEA Solution at 25°C. 78 XX. Solubility of C0 2 in 2.0 N MDEA Solution at 50°C. 79 XXI. Solubility of co 2 in 20% by \.Jeight MDEA Aqueous Solution at 100°F ••••••••••••••••••• 80 XXII. Solubility of C0 2 in 20% by Weight MDEA Aqueous Solution at 150°F ••••••••••••••••••• 81 XXIII. Solubility of C0 2 in 20% by Weight MDEA Aqueous Solution at 240°F ••••••••••••••••••• 82 XXIV. Solubility of H2S in 20% by Weight MDEA Aqueous Solution at 100°F ••••••••••••••••••• 83 XXV. Solubility of H2S in 20% by Weight MDEA Aqueous Solution at 150°F ••••••••••••••••••• 84 XXVI. Solubility of H2S in 20% by Weight MDEA Aqueous Solution at 240°F. • • • • • • • • • • • • 85 XXVI I. Calibration of Cell Thermocouple •• . • • • • • 89 XXVIII. Calibration of Bath Thermocouple. 90 XXIX. Calibration of 0-60 psi Gauge •• . • ••• 92 XXX. Calibration of 0-3000 psi Gauge 93 XXXI. Calibration of 0-300 psi Gauge •• 94 XXXII. MINI*SIM Flash Calculation Output •• • 107 viii LIST OF FIGURES Figure Page 1. Chemical Structure of Formulas for Alkanolamines of Interest in This Study ••••••••• . 13 2. Concentration of Major Species When 2.0 N MEA Reacts With Pure H2S. • • • • • • • • • • • • • • • • 19 3. Concentration of Major Species When 2.0 N MEA Reacts With Pure co2• • • • • • • • • • • . • . • . • . • 20 4. Equilibrium Loading of MEA, DEA and MDEA of Different Acid Gas Partial Pressures •• 22 5. Schematic Diagram of the Apparatus Used 24 6. Solubility of C0 2 in 2.5 N MEA Solution at 25, 60 and 80°C. 34 7. Solubility of H2s in 2.5 N MEA Solution at 25°C • • • • • • • 35 8. Solubility of co2 and H2S in 2.5 N MEA Solution at 25°C • 37 9. Solubility of co2 in 20% by Weight DGA Aqueous Solution at 25°C ••••••••••••• 38 10. Solubility of co2 in 40% by Weight DGA Aqueous Solution at 40 and 60°C •••••••••• 39 11. Solubility of H2S in 40% by Weight DGA Aqueous Solution at 40 and 60°C •••••••••• . 40 12. Solubility of co2 and H2s in 40% by Weight DGA Aqueous Solution at 40 and 60°C. • • • • • • • • • • • • • 42 13. Solubility of co2 in 60% by Weight DGA Aqueous Solution at 50°C ••••••••••••• 43 14. Solubility of H2S in 60% by Weight DGA Aqueous Solution at 50 and 80°C •••••••••• 44 15. Solubility of co2 in 0.5 and 2.0 N DEA Solution at 25°C 46 16. Solubility of co2 in 1.0 N MDEA Solution at 25°C •••• . 47 ix Figure Page 17. Solubility of H2S in 1.0 N MDEA Solution at 25°C. 48 18. Solubility of co2 in 2.0 N MDEA Solution at 25 and 50°C 50 19. Solubility of co2 in 1.0 and 2.0 N MDEA Solution at 25°C. 51 20. Solubility of co2 in 20% by \4ei ght MDEA Aqueous Solution at 100, 150 and 240°F • • • • • • . 52 21. Solubility of Hts in 20% by Weight MDEA Aqueous Solution a 100, 150 and 240°F •••••• . 53 22. Solubility of co2 and H~S in 20% by Weight MDEA Aqueous Solution at 100, 1 0 and 240°F . • . 55 23. Density of MEA Aqueous Solutions. 96 24. Vapor Pressure MEA Aqueous Solutions. 97 25. Density of DGA Aqueous Solutions. 98 26. Vapor Pressure of DGA Aqueous Solutions • . 99 - 27. Density of DEA Aqueous Solutions. 100 28. Vapor Pressure of DEA Aqueous Solutions • . 101 29. Vapor Pressure of MDEA Aqueous Solutions. 103 x CHAPTER I INTRODUCTION Acid gases such as hydrogen sulfide (H2S) and carbon dioxide (C0 2), when dissolved in an aqueous medium, dissociate to form a weak acid. The removal of such gases from the parent gas is designated as "gas sweetening". Hydrogen sulfide and carbon dioxide are frequently present in natural gas, refinery gas, and synthesis gas. Their presence is of great concern economically and environmentally. Acid gases are a source of severe corrosion in pipelines and process equipment. Environmental­ ly, hydrogen sulfide is extremely toxic; its combustion products sulfur dioxide and trioxide, when combined with particulates and moisture, produce the most damaging effects attributed to atmospheric air pollution. Acid rain is also attributed to sulfur oxides (1). There are several techniques available for the removal of acid gases from the parent gas. However, alkanolamines are the most widely used of the many available solvents. The first alkanolamine used as an absorbent for acid gases is credited to Bottoms (2, 3), who in the early 30 1 s used Triethanolamine (TEA) in sweetening sour natural gas. Monoethanolamine (MEA) and diethanolamine (DEA) have displaced TEA largely because of its low capacity, low reactivity and its relatively l 2 poor stability (4). Blohm and Reisenfeld (5) introduced in 1955 a somewhat different type of alkanolamine, s, S1 -hydroxyaminoethylether, commercially known as Diglycolamine (DGA). This amine couples the reactivity and stability of MEA, but has a vapor pressure several fold lower than that of MEA. This has the advantage of reducing the vaporization losses and avoiding degradation reactions which frequently occur with primary amines, such as MEA. Methyldiethanolamine (MDEA) was introduced as a potential acid gas absorbent in the early 1950 1 s (6, 7). MDEA has been described as a selective absorbent for H2S in the presence of co2• Although its use has been limited, an increasing role for MDEA is expected in connection with enriching the H2S content of the acid gas feed to Claus sulfur recovery plants and the feed to processes used for the synthesis of sulfur derivatives (8).