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Model Experiments of Autoxidation Reaction Fouling

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Model Experiments of Autoxidation Reaction Fouling MODEL EXPERIMENTS OF AUTOXIDATION REACTION FOULING by David Ian Wilson M.A. (1988) Jesus College, Cambridge M.Eng. (1989) Jesus College, Cambridge A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES Department of Chemical Engineering We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA March, 1994 © D.I.Wilson, 1994 ____________________ In presenting this thesis in partial fulfilment of the requirements for an advanced degree the at University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of The University of British Columbia Vancouver, Canada Date fl t9- DE-6 (2188) Abstract Chemical reaction fouling of heat exchangers is a severe problem in the petrochemicals industry, where deposits can be formed by a wide range of undesirable reactions. Autoxidation has been identified as a prime source of deposit formation in oxygenated process streams and fuel storage systems but the fouling mechanism has not been fuily investigated. The fouling of heat exchangers subject to autoxidative fouling was studied using model solutions of an active alkene, indene, in inert solvents saturated with air. The heat exchangers were operated at moderate surface temperatures (180-250°C) and at turbulent flow velocities. The experiments featured air pressures of 342-397 kPa and heat fluxes of 2. The effects of chemical reaction surface 90-280 kW/m rate, temperature and flow velocity were investigated and compared with existing chemical reaction fouling models. Chemical initiators were used to eliminate the chemical induction periods observed under ‘natural’ thermal initiation and permitted the study of the chemical reaction rate and surface temperature as separate variables. The chemistry of the physical system and the complex reaction mechanism prevented extensive model development. Two fouling probes were used: an annular probe which allowed visual inspection of deposit formation and a novel tubular heat exchanger constructed during this work which allowed inspection of the deposit in situ after an experiment. The same fouling mechanism was found to generate deposit in both probes. Chemical analyses were developed to monitor the autoxidation reaction during the batch fouling experiments. The results confirmed that fouling was caused by the deposition of insoluble polyperoxide gums generated by the reaction of indene and oxygen. The gums il aged on the heat exchanger surface to form complex oxygenated solids which were not easily removed. These results confirmed the hypotheses of Asomaning and Watkinson (1992). The kinetics of indene autoxidation was studied in a separate series of semi-batch stirred tank experiments. The rate of formation of polyperoxides was influenced by the solvent nature, temperature, oxygen concentration and mode of initiation. The aromatic polyperoxides exhibited limited solubility in aliphatic solvents. The kinetics of indene autoxidation could not be described by the schemes reported in the literature and were found to be subject to oxygen mass transfer effects. The fouling resistance behaviour was controlled by conditions in the bulk fluid. The initial, linear fouling rate decreased with increasing flow velocity and increased with bulk reaction rate and surface temperature. A simple fouling model, involving generation of deposit in the reaction zone next to the heat transfer surface and an attachment factor related to the mean fluid residence time, was fitted to the experimental data. Once the solubility limit was reached, the fouling resistance showed increasing rate behaviour, caused by the deposition of globules of insoluble gum. The effect of an antioxidant on the fouling process was studied. The efficiency of the antioxidant, di-t-butyl-4-methylphenol, was found to be severely reduced under the enhanced thermal conditions in the heat exchanger. Simulated ageing experiments were performed to investigate the fate of polyperoxides exposed to the enhanced temperatures on the heat exchanger surface. The studies confirmed that the insoluble polyperoxide gums undergo ageing processes after deposition. U’ Table of Contents Abstract ii Table of Contents iv List of Tables vii List of Figures x Acknowledgement xvi INTRODUCTION 1 2. LITERATURE REVIEW 7 2.1 The Role of Autoxidation in Chemical Reaction Fouling 7 2.2 Experimental Studies in Autoxidation Reaction Fouling 11 2.2.1 Fuel Stability Studies 13 2.2.2 Thermal Fouling Studies 18 2.3 Autoxidation Chemistry 21 2.3.1 Autoxidation Mechanisms and Kinetics 21 2.3.2 Solvent and Additive Effects in9H8Autoxidation 25 2.3.3 The Autoxidation of Indene, C 28 2.3.4 Antioxidation 30 2.4 Mechanistic Modelling of Fouling 31 2.4.1 Transport and Adhesion Models 33 2.4.2 Transport and Reaction Models 36 2.4.3 Reaction Engineering 43 2.5 Objective 44 3. EXPERIMENTAL MATERIALS AND METHODS 46 3.1 Materials and Physical Properties 46 3.1.1 Model Solutions 46 3.1.2 Physical Properties 50 3.1.2.1 Kerosene Properties 50 3.1.2.2 Paraflex Properties 51 3.1.2.3 Concentration of Dissolved Gases 55 3.2 Portable Fouling Research Unit (PFRU) Apparatus 56 3.2.1 PFRU Heat Transfer 62 3.2.2 PFRU Experimental Procedure 64 3.3 Stirred Cell Reactor (SCR) 66 3.4 Tube Fouling Unit (TFU) 68 3.4.1 Tube Fouling Unit Apparatus 69 3.4.2 Surface Temperature Measurement 74 3.4.3 Data Collection and Processing 78 iv 3.4.4 TFUHeatTransfer 82 3.4.5 TFU Operating Procedures 86 3.5 GumAgeingOven 88 3.6 Chemical Analysis 90 3.6.1 Hydroperoxide Analysis- Peroxide Number (POx) 91 3.6.2 Polyperoxide Analysis (Gum in Solution Assay) 92 3.6.3 Indene Concentration- Gas Liquid Chromatography 95 3.6.4 Further Analysis- FTIR, SEM 99 4. AUTOXIDATION OF MODEL SOLUTIONS 100 4.1 Solvent Effects in Indene Autoxidation 100 4.2 Autoxidation Kinetics 106 4.2.1 Mass Transfer Effects in Autoxidation Kinetics 108 4.2.2 Kinetics of Autoxidation in Mass Transfer 112 4.2.3 Gas Phase Resistance Effects in Mass Transfer 117 4.2.4 Oxygen Effects in Autoxidation 121 4.3 Chemically Initiated Autoxidation of Indene 124 4.4 Temperature Effects in Autoxidation 127 4.5 Autoxidation in the Presence of Antioxidants 132 4.6 A Kinetic Model of Indene Autoxidation 135 4.7 Ageing of Polyperoxide Gums 146 4.8 Summary of Autoxidation Studies 151 5. INITIAL FOULING EXPERIMENTS 153 5.1 Model Solution Selection 153 5.1.1 Tetralin as Solvent 154 5.1.2 Hexadec-1-ene as Dopant 156 5.1.3 IndeneasDopant 160 5.1.4 Deposit Characterisation 162 5.1.5 Initial Mechanistic Insights 164 5.2 Effects of Dopant Concentration 167 5.3 Fouling in Model Solutions with Two Dopants 173 5.4 Temperature Effects in Thermally Initiated Fouling 177 5.5 Velocity and Surface Temperature Effects in Chemically Initiated Fouling 186 5.6 Stages in Autoxidation Fouling 191 5.7 Fouling in the Presence of Antioxidants 197 5.8 Summary of Initial Fouling Experiments 203 6. FOULING EXPERIMENTS IN THE TUBE FOULING UNIT 206 6.1 Autoxidation of Indene in the TFU 206 6.2 Initial Fouling Studies 209 6.2.1 TubePressureDrop 211 6.2.2 Local Fouling Behaviour 214 6.2.3 Deposit Distribution and Morphology 218 v 6.3 Surface Temperature Effects in TFU Fouling 226 6.4 Velocity Effects in TFU Fouling 229 6.5 Comparison of TFU and PFRU Fouling Probes 234 6.6 Summary of TFU Fouling Studies 237 7. MODELS OF ASPECTS OF AUTOXIDATION FOULING 240 7.1 Autoxidation Kinetics in the Fouling Apparatus 242 7.1.1 Model Development 242 7.1.2 Model Performance 246 7.2 Fouling Mechanisms in the Initial Fouling Regime 249 7.2.1 Particulate Fouling Models 249 7.2.2 Chemical Reaction Fouling Models 254 7.3 Analysis of Fouling Rate and Behaviour 257 7.3.1 Formulation of a Lumped Parameter Fouling Model 258 7.3.2 Analysis of Experimental Fouling Data 260 8. CONCLUSIONS 270 9. RECOMIVIENDATIONS FOR FURTHER STUDY 275 Abbreviations 277 Nomenclature 278 References 283 APPENDICES A. 1 Experimental Apparatus and Configuration 294 A.2 Experimental Summaries 301 A.3 Sample Calculations 307 A.4 Fouling Model Calculations 312 B.1 DataSummary 316 vi LIST OF TABLES 2.1 Examples of Chemical Reaction Fouling in Refineries 8 2.2 Summary of Autoxidation Related Fouling Studies 12 2.3 Fouling Results of Taylor (1969b) and Asomaning and Watkinson (1992) 14 2.4 Activation Energies Reported in Chemical Reaction Fouling 16 2.5 Velocity Effects Reported in Chemical Reaction Fouling 17 2.6 Summary of Chemical Reaction Fouling Models 34 3.1 Alkenes Used in Model Solutions 48 3.2 Properties of Solvents Used in Model Solutions 49 3.3 Physical Properties of Initiators and Antioxidants 48 3.4 Estimates of Dissolved Oxygen Concentration in Paraflex and Kerosene 55 3.5 Comparison of Nusselt Numbers in PFRU Heat Transfer 63 3.6 TFUAlarmMatrix 72 4.1 Solvent Effects in the Autoxidation of Model Solutions of Indene 104 4.2 Mass Transfer Effects in Batch Autoxidation of Indene 110 4.3 Regression of Thermally Initiated Autoxidation Data to Mass Transfer Models 118 4.4 Gas Phase Resistance Effects in Autoxidation Kinetics 120 4.5 Comparison of Autoxidation Kinetics in the SCR and PFRU 120 4.6 Oxygen Effects in the Initiated Autoxidation of Indene in Paraflex 123 4.7 Chemically Initiated
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