Model Investigation of Initial Fouling Rates of Protein
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MODEL INVESTIGATION OF INITIAL FOULING RATES OF PROTEIN SOLUTIONS IN HEAT TRANSFER EQUIPMENT by Ian C. Rose B.E. (1993) University of Auckland, N.Z. A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES Department of Chemical and Bio-Resource Engineering We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA April, 1999. ©Ian Rose, 1999. In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. 1 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 CK^ICAI <?vj 7j(e- R^a^r^ -C^^e.*^^ The University of British Columbia Vancouver, Canada Date / Apr-'i \ tftl DE-6 (2/88) ii Abstract As protein solutions are heated, denaturation and aggregation processes give rise to deposition on the heated surface. The present study undertakes the problem of understanding how process variables such as fluid velocity and temperature affect the initial fouling rate. Previous studies of chemical reaction fouling for other systems have demonstrated contrasting behaviour of the initial fouling rate with respect to fluid velocity: some investigations report an increase, others a decrease, and still others both an increase followed by a decrease of initial fouling rate with increasing fluid velocity. In this work a theoretical model for initial chemical reaction fouling in turbulent flow, where attachment is treated as a physico-chemical rate process in series with mass transfer (Epstein, 1994), was examined. According to the model, mass transfer is directly proportional to the friction velocity, and attachment is inversely proportional to the square of this velocity. Therefore, at a given wall temperature, it follows that if the initial fouling rate is mass transfer controlled (low fluid velocity), the deposition flux increases as the fluid velocity increases. If, however, the initial fouling rate is attachment controlled (high fluid velocity), the deposition flux will decrease as the fluid velocity increases. Therefore as the velocity is lowered the initial fouling rate goes through a maximum at a given wall temperature. In addition, this maximum initial fouling rate can be expected to increase and to move towards higher critical velocities as the wall temperature increases. Two separate experimental studies were performed, one using a 1 wt. % whey protein solution at pH 6, and the other a 1 wt. % lysozyme solution at pH 8. These experiments were performed over film Reynolds numbers of 2950 - 22730, clean inside wall temperatures of Ill 59 - 102°C and bulk temperatures of 30 - 57°C in a 9.017 mm i.d. electrically heated, stainless steel tube. The above features of the model were qualitatively demonstrated with both protein solutions, i.e. a maximum in experimental initial fouling rate at a given wall temperature over a range of fluid velocities, and an increase in the maximum rate and in the corresponding critical velocity as the wall temperature increased. Lysozyme fouling results showed that as the mass flux increased from 200 kg/m2s to 1101 kg/m2s, the fouling activation energy, AEf, increased from 29 kJ/mol to 118 kJ/mol. This observation was consistent with the model, since the optimum model prediction, with an average absolute percent deviation of 23.3 %, was obtained with a kinetic reaction order of 0.75 and chemical activation energy, AE, of 161 kJ/mol. Thus at low velocity, when mass transfer dominated, AEf was low, but as the velocity was increased and chemical attachment became increasingly more important, AEf increased, but never to the value for the pure chemical reaction, AE, since mass transfer could never be entirely neglected. Whey protein modeling results, after rejection of renegade data points, showed an optimum solution, with an average absolute percent deviation of 24.5 % from the fit of the model, with a kinetic reaction order of 0.99 and a chemical activation energy, AE, of 201 kJ/mol. These values were compatible with the kinetic parameters for whey protein denaturation in the literature. Using estimates of the deposit physical properties from the whey protein fouling experiments, the dimensionless mass transfer constant (k') for both protein fouling experiments were found to lie between the isothermal value of Metzner and Friend (1958), and the non- iv isothermal value of Vasak and Epstein (1996), where all physical properties had been evaluated at the wall temperature. In contrast, the present work evaluated the physical properties in the mass transfer term at the film temperature, and therefore an intermediate estimate of k' was to be expected. In general, both protein fouling studies show results that conform in the mass transfer control region with the Epstein (1994) model, but in the attachment control region, the inverse dependence of the initial fouling rate on the friction velocity is even greater than the second power dependence predicted by that model. V Table of Contents Abstract ii Table of Contents v List of Tables ix List of Figures xii Acknowledgments xix 1. Introduction 1 1.1 Fouling of Heat Exchangers 1 1.2 Fouling Categories 4 1.2.1 Factors Affecting Fouling 7 1.3 Objectives 8 2. Literature Review 11 2.1 Chemical Reaction Fouling 11 2.1.1 Effect of Wall and Bulk Temperatures 13 2.1.2 Effect of Fluid Velocity 14 2.2 Theoretical Fouling Model 18 2.2.1 Mathematical Model Development (Epstein, 1994) 19 2.2.2 Application of Model 24 2.3 Milk Based Fluid Fouling 26 2.3.1 Fouling Components of Milk 26 2.3.2 Factors Affecting Fouling 29 2.3.3 Biochemistry of Whey Protein Solutions 32 2.3.4 P-lactoglobulin and a-lactalbumin Denaturation Kinetics 37 2.3.5 Previous Fouling Studies into the Controlling Mechanisms of Deposition 39 2.3.6 Whey Protein Deposit Properties 47 2.4 Lysozyme Fouling 50 2.4.1 Properties of Lysozyme 50 2.4.2 Effect of Electrostatic Forces on Fouling 51 2.4.3 Thermal Inactivation Kinetics of Lysozyme 52 3. Experimental Apparatus and Methods 55 3.1 Tube Fouling Unit (TFU) 5 5 3.1.1 TFU Apparatus (Wilson, 1994) 55 3.1.2 Wall Temperature Measurement 61 vi 3.1.3 Data Collection 63 3.1.4 TFU Operating Procedures 65 3.2 Whey Protein Solutions 69 3.2.1 Microbial Contamination Testing Procedure 70 3.2.2 Deposit Property Analysis 74 3.3 Lysozyme 77 3.3.1 Enzymatic Activity Assay 77 4. Experimental Results and Discussion 81 4.1 Data Handling Methods 81 4.2 Whey Protein Fouling 84 4.2.1 Physical Properties 85 4.2.2 Microbial Contamination 86 4.2.3 Velocity Effect 89 4.2.4 Effect of Particulate Material 104 4.2.5 Deposit Morphology 112 4.2.6 Physical Properties of Whey Protein Fouling Deposits 125 4.3 Lysozyme Fouling 137 4.3.1 Physical Properties 137 4.3.2 Particulate Material 138 4.3.3 Steady State Conditions and the Measurement of Fouling 139 4.3.4 Effect of Fluid Velocity 146 4.3.5 Effect of pH on the Fouling Behaviour 149 4.3.6 Effect of Enzymatic Activity 152 4.4 Kinetic Compensation Effect (KCE) 155 5. Mathematical Modeling and Discussion 157 5.1 Development of FORTRAN 77 Program 157 5.2 Modeling Whey Protein Solution Fouling 159 5.2.1 Input Data 159 5.2.2 Mathematical Model Predictions 162 5.3 Modeling Lysozyme Fouling 172 5.3.1 Input Data 172 5.3.2 Mathematical Model Predictions 175 5.3.3 The Optimum Model Solution 182 5.3.4 Elimination of Low Reynolds Number Data 184 5.3.5 Discussion of the Model Response 188 5.4 Interpretation of Optimum Model Predictions 192 6. Conclusions 199 6.1 Whey Protein Solution Fouling 199 6.2 Lysozyme Solution Fouling 202 vii 7. Recommendations for Further Study 206 Nomenclature 208 References 214 Appendix 1 Calibrations 221 Appendix 2 Acid Catalyzed 2-Furaldehyde Fouling 223 A2.1 Literature Review 223 A2.1.1 2-Furaldehyde as a Decomposition Product 223 A2.1.2 2-Furaldehyde Decomposition Kinetics 227 A2.2 Experimental Apparatus and Methods 228 A2.2.1 Stirred Cell Reactor (SCR) 228 A2.2.2 UV Spectrophotometer 230 A2.2.3 Fluid Physical Properties 232 A2.2.4 Tube Fouling Unit (TFU) 234 A2.3 Kinetic Experiments 236 A2.4 Fouling Experiments 239 A2.5 Application of Experimental Results to Model 243 Appendix 3 Whey Protein Solutions 246 A3.1 Composition of WPC-80 Powder 246 A3.2 Fouling Resistance for TFU 200 Experiments used for Deposit Property Analysis 246 A3.3 Thermocouple Location with respect to Cut Tube Sections 248 A3.4 Velocity and Temperature Distribution Calculations for Whey Protein Fouling Experiments 248 Appendix 4 Lysozyme Solutions 259 A4.1 Analysis of Food Grade Lysozyme Chloride (Powder) 259 A4.2 Enzymatic Assay of Lysozyme 259 A4.3 Spectrophotometer Data for Substrate and Blank Runs 266 Appendix 5 Mathematical Modeling 267 A5.1 Algorithm for Levenberg-Marquardt Non-Linear Curve Fitting Method 267 A5.2 FORTRAN Program used for Non-Linear Multi-Parameter Regression of Experimental Data According to Epstein's Mathematical Model (1994) 269 A5.3 Whey Protein Solution Input Data 277 A5.4 Lysozyme Solution Input Data 278 A5.5 Statistical Analyses of Optimum Model Fit of Whey Protein Data to Experimental Results 280 viii A5.6 Statistical Analyses of Optimum Model Fit of Lysozyme Data to Experimental Results 281 Appendix 6 Kinetic Compensation