MODELLING OF KRAFT PULP MILL TOTAL REDUCED SULPHUR EMISSIONS by ALLAN STEWART JENSEN B.A.Sc, University of British Columbia, 1989 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Chemical and Biological Engineering) THE UNIVERSITY OF BRITISH COLUMBIA October 2007 © Allan Stewart Jensen, 2007 Abstract Atmospheric release of odorous total reduced sulphur (TRS) emissions from kraft pulp mills has been an ongoing concern worldwide. Organic TRS compounds are formed in the digester by ^undesired side reactions during the kraft pulping of wood. These, along with HS, being highly 2 volatile, are released from mill processes, such as brown stock washing. TRS gases are extremely noxious, described as rotting eggs or rotting cabbage, and have a threshold of odour detectability in the range of a few parts per billion. The general objective of this work was to develop and test a method to predict emissions of these TRS compounds from kraft pulp mills using a vapour-liquid equilibrium (VLE) model. This technique could then be utilized to optimize the process to minimize emissions, or to optimize the design and operation of TRS control systems. A mill TRS sampling and testing program was conducted around the brown stock washing area of the Howe Sound pulp mill located in Port Mellon, British Columbia. Over 90% of the sulphur emissions from the TRS compounds were in the form of dimethyl sulphide. This compound was subsequently chosen as a surrogate compound for modelling of TRS. Analytical testing was done on black liquor and various solutions containing the substances found in the highest concentrations in black liquor, including mixed sodium salts solutions to 6 wt% and lignin mixtures to 6 wt%. The VLE of dimethyl sulphide, in the form of an activity coefficient, for these solutions was determined using a headspace gas chromatographic method. The adjustable parameters that provided the best fit for the electrolyte non-random two-liquid (eNRTL) equation for the experimental activity coefficients were determined using statistical regression techniques. Simulation software, incorporating a VLE model using the eNRTL correlation, was used to construct a base-case heat and mass balance representing current operation of the Howe Sound brown stock washing process. This balance was used to evaluate potential process modifications and their effect on TRS emissions. Table of Contents Abstract i List of Tables vi List of Figures viii Nomenclature xii Glossary xiv Acknowledgments xxiii Dedication xxiv Chapter 1 Introduction 1 Chapter 2 Background 4 2.1 Kraft Pulp Mill Non-Combustion Source TRS Emissions 4 2.2 TRS Formation 5 2.2.1 Hydrogen Sulphide Formation 6 2.2.2 Organic TRS Formation 7 2.2.3 Kinetics of Organic TRS Formation 9 2.2.4 TRS Formation in Black Liquor Evaporators 13 2.3 TRS Properties 13 2.4 TRS Health Effects 15 2.4.1 TRS Odour and Health Threshold Levels 18 2.4.2 Odour Detection Threshold 20 2.4.3 Odour Identification Threshold 20 2.4.4 Nuisance Odour Threshold 21 2.4.5 Health Effects Threshold 22 2.4.6 What to Make of These Threshold Levels? 22 2.5 Kraft Pulp Mill Environmental Regulations 23 2.5.1 Environmental Guidelines in Canada 23 2.5.1.1 British Columbia 25 2.5.1.2 Alberta 25 . 2.5.1.3 Ontario 26 2.5.1.4 Quebec 27 ii 2.5.1.5 Other Provinces 27 2.5.1.6 Canadian National Pollutant Release Inventory (NPRI) 28 2.5.2 Environmental Guidelines in the U.S.A 28 2.5.3 Environmental Guidelines in Rest of the World 30 2.6 NCG Collection and Treatment Systems 31 2.7 Kraft Mill Process Streams 37 2.7.1 Black Liquor Composition 37 Chapter 3 Literature Review 41 3.1 TRS Phase Equilibria Behaviour 41 3.1.1 Vapour-Liquid Equilibrium 41 3.1.2 Activity Coefficient Models 45 • 3.1.3 Factors Affecting TRS Systems 48 3.1.3.1 pH Effects on VLE 49 3.1.3.2 Inorganic Dissolved Solids Effects on VLE 51 3.1.3.3 Organic Dissolved Solids Effects on VLE 51 3.1.3.4 Other Effects on VLE 52 3.1.4 Henry's Constants and Activity Coefficients for TRS 52 3.1.5 TRS Vapour Pressure 58 3.2 Modelling of the Kraft Pulping Process 58 3.2.1 Computer Software Modelling Tools 60 3.2.2 Previous Work Modelling Non-Combustion TRS Emissions 61 3.3 TRS Emissions Factors 66 3.4 TRS Sampling and Testing Considerations 68 Chapter 4 Research Objectives 71 Chapter 5 Materials and Methods 72 5.1 Gas Chromatographic Equipment 72 5.2 Phase Equilibria Testing 74 5.2.1 Phase Equilibria Testing Method 74 5.2.2 Recovery of Alkali Lignin From Black Liquor 76 5.2.3 Phase Equilibria Sample Solution Preparation 76 5.2.4 Phase Equilibria Testing Procedure 80 in 5.2.5 Testing of Solutions for Other Properties 82 5.3 Mill Testing 83 5.3.1 The Howe Sound Pulp and Paper Mill 84 5.3.2 Mill Testing Method 86 5.3.3 Sample Collection Procedure 87 5.3.4 Sample Collection Considerations 88 5.3.5 Mill Testing Procedure 89 5.3.6 Mill Testing Considerations 91 5.4 Activity Coefficient Modelling 95 5.4.1 Regression Analysis of Phase Equilibria Testing Data 97 5.5 VLE Modelling 98 5.5.1 Phase Equilibrium Equation 99 5.5.2 Mole Balance Equation 100 5.5.3 Energy Balance Equation 100 5.5.4 Temperature (Boiling Point Rise) Equation 101 5.5.5 Solution of VLE Model 101 5.5.6 Commercial Software VLE Modules 104 Chapter 6 Results and Discussion 105 6.1 Phase Equilibria Testing using Published Data 105 6.2 Phase Equilibria Testing Results 110 6.2.1 Other Properties of Tested Solutions 113 6.2.2 Activity Coefficient as a Function of Conductivity 115 6.2.3 Activity Coefficient as a Function of Sodium Concentration 117 6.3 Phase Equilibria Testing Data Regression 121 6.3.1 Fitting of Henry's Constant Equation 121 6.3.2 Fitting of NRTL Equation for DMS-Water System 124 6.3.3 Comparison of NRTL Fit to Other Sources 126 6.3.4 Range of Validity for Infinite Dilution 128 6.3.5 Fitting of eNRTL Equation for DMS-Water-Sodium Salts System . . 128 6.3.6 Fitting of eNRTL Equation for H2S-, MM- and DMDS-Water -Sodium Salts Systems 131 iv 6.4 Mill Sampling and Testing Results 133 6.4.1 Howe Sound Equipment Dimensions and Operating Conditions .... 140 6.4.2 Time to Equilibrium for DMS in Black Liquor 144 6.5 Modelling of Howe Sound Equipment 146 6.5.1 Model Sensitivity Analysis 147 6.5.2 Equipment Modelling Results for DMS 148 6.5.3 Equipment Modelling Results for other TRS Compounds 152 6.5.4 Equipment Modelling Analysis 154 6.6 Mill Testing Data Compared to Emission Factors 160 6.7 Howe Sound Mill Modelling 165 6.7.1 Base-Case Balance of Howe Sound Process 167 6.7.2 Predicted Effect on DMS Emissions from Process Changes 175 6.7.3 Reducing Brown Stock Washing DMS Emissions 180 6.8 The Future of NCG Collection Systems 182 Chapter 7 Conclusions and Future Work 183 7.1 Conclusions 183 7.2 Future Work 187 References 189 Appendix A U.S. E PA Cluster Rule Summary 206 Appendix B N CG Collection and Treatment Systems 209 Appendix C Matlab V LE Module Program 226 Appendix D NCASI Mill Testing Data 235 Appendix E Phase Equilibria Testing Raw Data 250 Appendix F Howe Sound Mill Testing Raw Data 261 Appendix G Howe Sound M O PS Historian Data 286 v List of Tables 2.1 Total organic TRS formed during kraft pulping of softwood (spruce) at a cooking temperature of 170°C for 1, 2, 3, and 4 hour cooks (kg S/tonne dry wood) ... 9 2.2 Physical properties of selected TRS compounds 14 2.3 TRS health effects data 16 2.4 HS threshold concentrations 19 2 2.5 Ontario POI and AAQC limits 26 2.6 Typical combined CNCG composition 32 2.7 U.S. EPA typical TRS emissions for CNCG and DNCG sources (ppmv) 33 2.8 NCASI measured and averaged TRS emissions for CNCG and DNCG sources(ppmv) 34 2.9 Components of weak black liquor solids (wt% on water-free basis) 38 2.10 Typical inorganic salt composition of black liquor 39 2.11 Elemental composition of softwood black liquor solids 40 3.1 Henry's constants for hydrogen sulphide in water 54 3.2 Henry's constants for hydrogen sulphide in various salt solutions 55 3.3 Henry's constants for methyl mercaptan in water 55 3.4 Henry's constants for methyl mercaptan in various salt solutions 55 3.5 Henry's constants for dimethyl sulphide in water 56 3.6 Henry's constants for dimethyl sulphide in various salt solutions 56 3.7 Henry's constants for dimethyl disulphide in water 57 3.8 Henry's constants for dimethyl disulphide in various salt solutions 57 3.9 Constants for vapour pressure equation 3.21 58 3.10 U.S. EPA Emissions Factors for Chemical Wood Pulping 67 5.1 Sodium salt composition used for preparing salt solutions 78 5.2 Dimethyl sulphide solutions used for phase equilibria testing 79 6.1 Methanol(I) in water(j) parameters for NRTL equation 106 6.2 Vapour-liquid equilibrium activity coefficients for DMS solutions tested Ill 6.3 pH, total dissolved solids, ash, and sodium content of solutions tested 114 6.4 Conductivities of alkali lignin mixtures, sodium salt solutions, and black liquor .... 115 6.5 Regressed Henry's constants for dimethyl sulphide in solutions tested 121 vi 6.6 Regressed NRTL equation parameters for DMS(i)-water(j) system 124 6.7 DMS(i) in waterfj) parameters for NRTL equation from other sources 126 6.8 Regressed eNRTL equation parameters for DMS(i)-water(j)-black liquor sodium salts(ca) system 129 6.9 DMS-black liquor activity coefficients 130 6.10 Regressed eNRTL equation parameters for H2S-, MM- and DMDS(i)-water(j)-sodium salts (ca) system . 132 6.11 Mill test results for September 16, 2005 134 6.12 Mill test results for September 21, 2005 134 6.13 Mill test results for September 22, 2005 135 6.14 Mill test results for September 28, 2005 135 6.15 Mill test results for September 29, 2005 135 6.16 Mill test results for September 30, 2005 136 6.17 Mill test results for November 22, 2005 136 6.18 Mill test results for November 23, 2005 137 6.19 Mill test results for November 24, 2005 137 6.20 Mill test results for November 30, 2005 138 6.21 Mill test results for December 1,2005 138 6.22 Mill test results for December 2, 2005 139 6.23 Equipment dimensions and operating conditions 141 6.24 Root mean square error (RMSE) of VLE model vent vapour concentration prediction 154 6.25 VLE model correction factor for systematic error 155 6.26 Brown stock washing DMS emission factors 163 6.27 Vapour sample testing data for November 23, 24, 30, and December 1, 2005 168 6.28 Liquid sample testing data for November 23, 24, 30, and December 1, 2005 169 6.29 MOPS historian data for November 23, 24, 30, and December 1,2005 171 6.30 Predicted effect on Howe Sound brown stock washing DMS emissions as a result of operational or equipment changes 176 vii List of Figures 2.1 Organic TRS formation as a function of time for kraft pulping of softwood (spruce) at 170°C and a sulphidity level of 30.5% 10 2.2 Organic TRS formation as a function of sulphidity for kraft pulping of softwood (spruce) at 170°C for 4 hours 11 2.3 Organic TRS formation as a function of H-factor for kraft pulping of softwood (loblolly pine) at 170°C 12 3.1 Relative volatility of common kraft mill contaminants 45 3.2 Dissociation of hydrogen sulphide and methyl mercaptan 50 3.3 Effect on activity coefficient of methanol due to the presence of dissolved inorganic and organic matter in black liquor 63 3.4 Comparison of methanol vent stack model predictions with mill measurements for various process equipment 64 3.5 Comparison of GEMS prediction versus mill data for Mill E for methanol concentration in the vapour phase 65 3.6 Comparison of GEMS prediction versus mill data for Mill E for TRS concentration in the vapour phase 66 5.1 Typical gas chromatograph output 73 5.2 Time for a 1 ppm (mol) solution of DMS in 10 ml of water in a 24.1 ml sample vial at 90°C to reach equilibrium 81 5.3 Howe Sound mill brown stock washing area overview showing liquid sample (LS) and vapour sample (VS) point locations 85 5.4 Gas sampling equipment, including phosphoric acid impinger, Teflon tubing, 3-way valves, vacuum pump and Tedlar bag 87 5.5 Typical sample point (blow tank vent) being tested using the velometer 88 5.6 Typical liquid standard DMS calibration curve at the testing temperature of 80°C ... 91 5.7 Degradation of TRS liquid standard held at 20°C and at an initial concentration of30ppm(mol) 92 5.8 Degradation of TRS gas standard held at 80°C and at an initial concentration of 5 ppm (mol) 93 viii 5.9 VLE module used to predict emissions of volatile compounds 98 5.10 Summary of VLE module calculation block non-linear equations that require simultaneous solution using a Newton-Raphson or equivalent technique 102 5.11 VLE Module vapour and liquid by-passes 103 6.1 Vacuum drum washer filtrate tank vent stack measured methanol concentration compared to predicted concentration based on outlet filtrate concentration 107 6.2 Decker washer hood vent stack measured methanol concentration compared to predicted concentration based on inlet wash liquor concentration 108 6.3 Diffusion washer vent stack measured methanol concentration compared to predicted concentration based on inlet wash liquor concentration 109 6.4 Weak black liquor tanks vent stack measured methanol concentration compared to predicted concentration based on liquor outlet concentration 110 6.5 Temperature effect on DMS-water activity coefficient at various alkali lignin concentrations (solutions 4 and 5) 112 6.6 Temperature effect on DMS-water activity coefficient at various sodium salts concentrations (solutions 6, 7, and 8) 112 6.7 Temperature effect on DMS-water activity coefficient at various total dissolved solids concentrations in black liquor (solutions 11 and 12) 113 6.8 Conductivity of alkali lignin mixtures, sodium salts solutions, and black liquor as a function of sodium concentration 116 6.9 Activity coefficient at 90°C of alkali lignin mixtures, sodium salt solutions and black liquor, as a function of sodium concentration 117 6.10 Activity coefficient at 90°C (up to 0.5 wt% sodium) of alkali lignin mixtures, sodium salt solutions and black liquor, as a function of sodium concentration 118 6.11 Sodium concentration effect on DMS-water activity coefficient for sodium salts and black liquor at 20°C 119 6.12 Sodium concentration effect on DMS-water activity coefficient for sodium salts and black liquor at 50°C 119 6.13 Sodium concentration effect on DMS-water activity coefficient for sodium salts and black liquor at 70°C 120 ix