AN ABSTRACT OF THE THESIS OF Abdullah A. Abu-Al-Saud for the degree of Doctor of Philosophy in Chemical Engineering, presented on August 15, 1988. Title: The Effect of Copolymer and Iron on the Fouling Characteristics of Cooling Tower Water Containing Corrosion Inhibitors. Redacted for Privacy Abstract approved: Dr. James G. Knudsen Various antifoulant treatment programs and the considerations necessary for the effective use of such programs were examined. Two different groups of tests, with and without iron contamination, have been carried out on the effectiveness of several of the state-of-the-art copolymers (PA, HEDP, AA/HPA, AA/MA, SS/MA and AA/SA) in the inhibition of the fouling of high hardness cooling tower water containing phosphate corrosion inhibitors (polyphosphates and orthophosphates). The tests were conducted on metal surfaces (SS, CS, Adm, and Cu/Ni), using simulated cooling water in a specially designed cooling tower system. For each group of tests at various pH values (6.5, 7.5 and 8.5), the effects of flow velocity (3.0, 5.5, 8.0 ft/sec) and heat transfer surface temperature (130, 145, 160°F) on thefouling characteristics of cooling tower water havebeen investigated. During the course of each test, the water quality was kept constant. For the iron tests, the effects of iron presence (2, 3 and 4 ppm Fe) on the fouling characteristics of the cooling tower water havebeen investigated and discussed for three different situations: 1. High hardness cooling tower water and iron. 2. High hardness cooling tower water, iron and phosphate corrosion inhibitors. 3. High hardness cooling tower water, iron, phosphate corrosion inhibitors and copolymers. When fouling occurred, measurements were made of the fouling thermal resistance as a function of time. Four different fouling curves were obtained, linear, concave upward, asymptotic and sawtooth curves. The fouling data obtained were correlated with the Heat Transfer Research, Inc. (HTRI) model and fouling predictive equations and charts were developed. For the two groups of tests, the effectiveness of each copolymer has been evaluated and the threshold values of surface temperature, flow velocity and pH have been identified. The Effect of Copolymer and Iron on the Fouling Characteristics of Cooling Tower Water Containing Corrosion Inhibitors by Abdullah A. Abu-Al-Saud A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Completed August 15, 1988 Commencement June 1989 APPROVED: Redacted for Privacy ProfesIgor* Chemical Enginering Department in charge of major( Redacted for Privacy Chairman of'CiAeolcal Engineering Department Redacted for Privacy Dean of Gr ate Schol Date thesis is presented August 15, 1988 Typed by K. Abu-Al-Saud for Abdullah A. Abu-Al-Saud ACKNOWLEDGEMENT My special thanks goes to my professor, Dr. James G. Knudsen, for his advice and encouragement. I would also like to thank Professor Charles E. Wicks for his concern and assistance. I am particularly grateful to my friends Khalid Al- Qusami and Hani Redha for their help. My deepest thanks and love are for my wife for without her, this work would not have come to life. This thesis is dedicated to my mother, my wife, my daughter and my sisters and their families. TABLE OF CONTENTS PAGE: I. INTRODUCTION 1 II. GENERAL REVIEW AND LITERATURE SURVEY 4 Heat Transfer Equations 4 Fouling Types 5 Fouling Mechanism 8 Important Parameters 10 Predictive Methods 12 Chemical Treatments 18 III.EXPERIMENTAL EQUIPMENT 27 Heat Exchanger System 29 Test Sections 29 Cooling Tower System 33 Data Acquisition System 35 IV. EXPERIMENTAL PROCEDURES 36 Experimental Program 36 Procedure 39 V. CALCULATION PROCEDURES 45 Calculation of FoulingResistance 45 Error Estimation 49 Calculation of Inner Wall Shear Stress 53 Fitting the Fouling Curves 55 VI. RESULTS AND DISCUSSION 57 General Data 58 Results Presentation and Data Treatment 59 Tests Without Iron 60 Iron Tests 73 Deposit Composition 99 Types of Fouling Resistance Time Curves Obtained 100 Analysis of the Fouling Data 103 Data Reduction and Discussion 111 Discussion of Model Constants E, a and b 125 Scale Strength 132 Fouling as a Function of Time and Asymptotic Fouling Resistance 133 Conditions Showing Insignificant Fouling 140 VII. APPLICATION OF RESULTS 143 Model Correlational Equations 143 Threshold Values 162 Application to other Geometries 162 VIII.CONCLUSIONS 164 BIBLIOGRAPHY 171 APPENDICES PAGE AppendixA Nomenclature 178 AppendixB Calibration Equations 183 AppendixC Chemical Analysis Procedure 185 AppendixD Sample Calculations 187 AppendixE Summary of Test Results 190 AppendixF Fouling Resistance Time Curves 213 AppendixG Composite Plots of Selected Runs 244 AppendixH Deposit Compositions 282 Appendix I Nonlinear Regression 285 AppendixJ Average Cooling Tower Water Quality 310 AppendixK Average System Flowrates 316 AppendixL Plots of Correlational Curves 320 AppendixM Solubility Information 353 LIST OF FIGURES FIGURE PAGE III-1. Schematic Flow Diagram of Experimental Equipment 28 111-2. Annular Geometry of Test Section 30 IV-1. Water Conditioning System 40 V-1. Cross Section of Clean and Fouled Test Section 46 VI-1A. Fouling Resistance Time Curves 101 VI-1. Normalized -In Od vs Velocity 104 VI-2. Error Plot of Deposition Rate 119 VI-3. Error Plot of Time Constant 124 VI-4. Error Plot of Fouling Resistance 136 VI-5. Curves of Constant R'*', on Grid of Velocity vs Surface Temperature 146 VI-6. Curves of Constant Surface Temperature on Grid of Rte. vs Velocity 154 LIST OF TABLES TABLE PAGE III-1. Heat Transfer Research, Inc. Rods Specification 32 IV-1. Constituents of the City Water and System Water 37 IV-2. Additives Used in Each Group of Tests 38 IV-3. Typical Volumes and Flowrates 43 VI-1. Regression Analysis 106 VI-2. Constants to be Used in Equations (6-5), (6-12) and (6-13) 116 VI-3. Comparison of Experimental Values vs Model Equations for Od, Ac and R4*-1, 137 VI-4. Threshold Value for Various Additives 141 THE EFFECT OF COPOLYMER AND IRON ON THE FOULING CHARACTERISTICS OF COOLING TOWER WATER CONTAINING CORROSION INHIBITORS I INTRODUCTION This study is part of an on-going experimental program under way at Oregon State University on the fouling characteristics of cooling-tower water. Fouling is defined as the formationof deposits on heat exchanger surfaces which impede the transfer of heat and increase the resistance to fluid flow. Cooling-tower water is utilized as a medium for heat rejection to the surroundings via heat-exchange equipment. Untreated open recirculating cooling-tower water may contain significant concentrations of scale-forming ions, such as Ca-1-, Mg4-°, PO..- and SiOa- which can form inverse solubility salts at high temperature. These salts may deposit on the hot heat-transfer surface. Control of pH and the use of scale inhibitors are the common methods by which fouling of cooling tower water can be reduced or, in the best case, prevented. As pH decreases, thesolubility of many scale forming salts increases and the deposition rate is significantly reduced. The pH is usually reduced to the range of 6.5 and 7.0 by the addition of sulfuric acid to the cooling water system. Under these conditions, however, the cooling water is slightly corrosive to the material in the 2 system, therefore corrosion treatment is necessary. Non- toxic phosphate corrosion inhibitors are now being used to meet environmental requirements. However, all of the phosphate-containing treatment programs suffer one major draw back; the phosphate level used must be very stringently controlled. Too much phosphate causes uncontrolled precipitation of calciumphosphate salts, while too little does not provide the corrosion protection required. To alleviate this problem, corrosion inhibitors are commonly used in conjunction with antifoulants. The objective of this investigation is to study the effects of state-of-the-art copolymers as antifoulants on the fouling characteristics of cooling-tower water containing phosphate corrosion inhibitors with and without iron contamination. Corrosion inhibitors used in this study are orthophosphate and polyphosphate. Antifoulants used are polyacrylates, phosphonates (HEDP), acrylic acid/hydroxypropyl acrylate (AA/HPA), acrylic acid/sulfuric acid (AA/SA), sulfonated styrene/maleic anhydride (SS/MA) and acrylic acid/maleic anhydride (AA/MA). The amount of deposit on a heat-transfer surface is measured in terms of the thermal resistance of the deposit. Generally, if the thermal resistance is less than 0.0001 ft° hr °F/Btu, the amount of fouling is tolerable, and the heat exchanger operates essentially as 3 a clean heat exchanger. The most important parameters that effect the fouling process are surface temperature and the surface condition of the heated surface, fluid bulk temperature, flow velocity/shear stress and water quality. A correlation, among the important parameters that affect and control fouling, which will reliably predict fouling resistance or determine conditions under which fouling would not occur would be most useful to the designer and operator of heat exchange equipment. 4 II GENERAL REVIEW AND LITERATURE SURVEY Heat Transfer Eauations The effect of fouling in terms of fouling resistance on thedesign of heat transfer equipment is expressed in the fundamental equation for the overall heat transfer coefficient Ur, on the outside of the surface as 1 = 1 AQ 1 + Ara + Rw (2-1) U0 hr.) Ai hi Ai Where: U = overall heat transfer coefficient h = convective heat transfer coefficient A = surface area Re = thermal resistance of deposit and subscript: 0 = outside i = inside w = wall The convective heat transfer coefficients, hi and h0, and the thermal resistance of the wall can be determined with reasonable accuracy using well established techniques and correlations. Accurate general methods for predicting the fouling resistance, which is often a major or even the dominating term in the equation, have not been developed for two reasons. First, fouling varies with time which makes it difficult to justify the assumption of a steady state.