REACTION ACCOMPANIED TviASS 'l'RANSFER BETWEEN LIQUID PHASES - AN EXPERIMENTAL STUDY by P. SETO A Thesis Submitted to the Faculty of Graduate Studies in Par·cial Fulfilment o.f the Requ.irer:lents for the Degree Doctor of Philosophy Jc:muary 1969 DOCTOR OF PHILOSOPHY (1969) (Chemical Engineering) McMaster University Hamilton, Ontario. TITLE AUTHOR SUPERVISORS NUMBER OF PAGES SCOP.E AND CONTENTS: Reaction Accompanied 1vass Transfer Betv1een Liquid Phases - an Experimental Study P. Seto, B.A.Sc. (University of Toronto) M. Eng. (McMaster University) Professors A. I. Johnson and A. E. Hamielec xxiii, 44.0 This dissertation describes an experimental study of the simultaneous mass transfer and chemical reaction at a plane liquid-liquid interface involving the saponification of simple esters transferring into aqueous caustic solutio:ns. The transfer experiments \vere carried out vlith both liquid phases stirred and unsti.rred respectively. Special emphasis uas placed on the stagnant-phase systems. Turbulent reaction layer propagation rates WBre measured for ester phases ( pre·-saturated 1d th \vater) in contact \·lith aqueous caustic solutions (at various concentration levels). 'fhe distortion of the moire pattern was used to indicate the position of the propeigating layer front o The cause and nature of the tur·bulent layer \vel'·e elucidated. An apparatus, capable of \vit hdrawing sm .. -=tll samples of the liquid \vlth probes precisely located in the aqueous phase, \WS const:cucted. Experimental t echniqu.es 'Nere developed to measure (ii) the abnormal concentration profiles of the reactants and tho products vd.thin the turbulent layer. From the component concentration distribution data, the turbulent layer thicknesses, the reaction zone (\'lith in the turbulent layer) thicknesses, the mass transfer rates, and the enhancem::mt factors \'!ere deduced. The effects of turbulence in the aqueous phase were estimated in terms of the derived eddy diffusi vit ies and of the differences between the experimentally measured and the theoretically predicted (by molecular diffusion with reaction equation) enhancement factors. In the steady state transfer study using stirring in both phases, transfer rate data and enhancement factor data were obtained for three formate-sodium hydroxide systems. In addition to the mass transfer studies, a preliminary investigation on the diffusion coefficient measurement in binary and ternary liquid systems employing the moire' pattern method was carried out. A diffusion cell 1vas designed and built to enable the quick acquisition of the experimental data uith reasonable accuracy. (iii) ACKNOt•JLEDGEMENTS The author is grateful to Dr. A. I. Johnson and Dr. A. E. Hamielec for their guidance and supervision throughout the course of this l·mrk and the preparation of this thesis. He is also indebted to Dr. C. Calvo for hj.s many helpful discussions and advice. He wishes to thank Dr. W. F. Furter for his guidance and encouragement on various aspects of tM.s study during the author's t11o-year lecturershi.p at the Royal Military College of Canada. The assistance of Mr. M. Li in dravling the figures and :V.trs. P. Rogerson j_n typing the thesis is greatly appreciated. Partial financial support in the constructj_ on of the apparatus ''m.s obtained from the Royal Milit~ry College of Canada. He lJishes to thank his l'life, :Mavis, for her encouragement, understanding and valuable support throughout the course of this study and for proof­ reading the thesis. (iv) TABLE OF CONTENTS Page 1 INTRODUCTION 1 2 LITERATURE REVIEW 5 2.1 Interfacial Turbulence 6 2.1.1 Previous Experimental Observations in Physical :rt.ass Transfer Systems 6 2 .1.1.1 T\'V'o-Phase Liquid-Liquid Systems 6 2.1.1.1.1 Plane Interface, Static 6 Systems 2.1.1.1.2 Dynamic Systems 8 2.1.1.1.3 Drops 9 2.1.1.1.4 Spontaneous Emulsifications 11 2.1.1.2 THo-Phase Gas-Liquid Systems 11 2.1.2 Previous Experimental Observations in Reaction Mass Transfer Systems 12 2 .1. 2.1 TvJo-Phase Liquid-Liquid Systems 12 2.1.2.1.1 Plane Interface, Static Systems 12 2.1.2.1.2 Dynamic Systems 13 2.1.2.2 Two-Phase Gas-Liquid Systems 14 2.1.3 Interfacial Turbulence Produced by Temperature Gradient c..t the Interface 15 2.1.4 Interfacial Turbulence Produced by Density Variation 15 2.1.5 Theoretical Treatment of Interfacial Turbulence 16 2.1.6 Recent Trenffi of Interfacial Turbulence Study 19 2. 2 IntGrpha se Mass Transfer 21 2.2.1 Theoretical Considerations 21 2.2.1.1 Physical Mass Transfer 21 2.2.1.2 Jviass Transfer "tJith Chemical Reaction 22 2.2.2 Experimental Studies 24 . 2.2.2.1 Physical Mass Transfer 25 2.2.2 .. 2 Reaction Mass Transfer 25 2.3 Diffusion Coefficient Measurement 29 ( v) Page 3 THEORETICAL PRINCIPLES 34. 3.1 Mass Transfer 11ith Chemical Reaction 34 3 .1.1 Estimation of the Reaction Zone Thickness 38 3.1.2 Estimation of the Enhancement Factor 40 3.1.2.1 Penetration Theory (unsteady state) /+0 3.1.2.2 Film Theory (steady state) 42 3.2 Molecular Diffusion in Homogene:ous Binary Liquid Systems 43 3.2.1 Diffusion Coefficient 43 4 EXPERIMENTATION 47 4.1 - PART A - Qualitative and Quantitative Investigations on the Cause and Nature of the Turbulent Layer in Ester-Aqueous Caustic Systems 49 4.1.1 Qualitative Investigations of the Turbulent Layer by Means of Colour Indicators 49 4.1.1.1 Experimental Details 50 4.1.1.1.1 Apparatus 50 ly.l.l.l.2 Procedure 50 4.1.1.1.3 Number of Ester-Caustj_c Systems Investigated 52 4.1.1.2 Results of the Colour Indicator Experiments 54 4.1.1.2.1 Ethyl Formate-Sodium Hydroxide System 54 4.1.1.2.2 Ethyl Acetate-Sodium Hydroxide System 54 4.1.1.2.3 Methyl Acetate-Sodium 56 Hydroxide System 4.1.1.3 Discussion of the Colour Indicator Studies of the TurbuJ.ent Laye1'! 57 4.1.2 Measurement of the Turbulent Layer Propagation 61 4.1.2.1 Experimental Details 62 4.1.2.1.1 Apparatus 62 4.1.2.1.2 Procedure 62 4.1.2.1.3 Temparature Control 62 4.1.2.1.4 Measurement of the Tur- bulent Layer Propagation 62 4.1.2.1.5 Number of Expe1~iments Performed 64 4.1.2.2 Results of the Measurement of Tur- bulent Layer Propagation 66 (vi) Page 4.1.2.2.1 Methyl Ester-Aqueous Caustic Solution Systems 66 lt--1.2.2.2 Ethyl Ester-Aqueous Caustic Solutioa Systems 72 4.1.2.2.3 Propyl Ester-Caustic Systems 74 4.1.2.3 Discussion of the Measurement of the Turbulent Layer Propagation 76 4.1.2.3.1 Reaction Runs 76 4.1.3 Qualitative Study of the Interfacial Phenomena j.n Various 'I\1o-Component Ester­ Water Systems and in the Three-Component Ester-So1u.te·~Water Systems 83 4.1.3.1 Experimental Details 84 4.1.3.1.1 Apparatus and Procedure 84 4.1.3.1.2 Number of Experiments Perfor~ed 84 4.1.3.2 Results of the Physical Transfer Runs 85 4.1.3.2.1 Ester-Water Systems 85 4.1.3.2.2 Three-Component Systems 86 4.1.3.3.Discussion of the Physical Transfer Runs 88 4.1.4 Investigation of the Factors Affecting the Turbulent Layer Propagation 90 4.l.l~ol Factors for the Formation of the Turbulent Layer 91 4.1.4.1.1 Alcohol 91 4.1.4.1.2 Surface Tension 91 4.1.4.1.3 The Interface 91 4.1.4.2 Factors Affecting the Turbulent Layer Propagation 92 4.1.4.2.1 Density and Viscosity 92 4.1.4.2.2 Viscosity 93 4.1.4.3 Results of the Investigation of the Factors Affecting the Turbulent Layer Propagation 94 4.1.1-}.3•1 Factors for the Formation of the Turbulent Layer 94 4.1.4.3.2 Factors Affecting the Turbulent Layer Propagation '96 4.l.h.h Discussion of the Investigation of the Factors Affecting the 'h.1rbulent Layer Propagation 99 4.1.4.4.1 Factors Affecting the Zone Forna. ti on 99 4.1.4.4.2 Factors Affecting the Speed of Layer Propagation 100 ( vi.i) 4.2 - PART B - Measurement of the Concentration Distribution \·lithin the Turbulent Layer in the Ester-Aqueous Caustic Systems 1.1ith Slow Chemical Reaction · 4.2.1 Experimental Details 4.2.1.1 The Apparatus 4.2.1.2 Experimental Procedure 4.2.1.2.1 Reaction Runs 4.2.1.2.2 Physical Runs 4.2.1.3 Number of Concentration Probing Experiments Performed 4.2.1.3.1 Physical Runs 4.2.1.3.2 Reaction Runs 4.2.1.4 Method of Calculation 4.2.1.4.1 Reaction Runs 4.2.1.4.2 Physical Runs 4.2.2 Results 4.2.2.1 Physical r~ss Transfer Runs 4.2.2.1.1 n-Butanol-Water System 4.2.2.1.2 Ester-Water Systems 4.2.2.2 Reaction Mass Transfer Runs 4.2.2.2.1 Ethyl Acetate-Aqueous Sodiu~ Hydroxide System 4.2.2.2.2 Methyl Propionate-Sodium Hydroxide System 4. 2. 2. 3 Mass Transfer of the Ester Across the Liquid-Liquid Interface 4. 2. 3 Discussion 4. 2.3 .1 General Assumptions 4.2.3.2 Experimental Errors 4.2.3.2.1 Temperature 4.2.3.2.2 Sampling 4. 2. 3. 2. 3 Mechanical Vibration 4 .• 2.3.2.4 Analysis of Samples 4. 2. 3. 2. 5 Con centra ti on Profiles 4.2.3.2.6 Initial Mixing During the Phase Contact 4.2.3.3 Results of Physical Transfer Runs 4.2.3.3.1 n-Butanol-Water System 4.2.3.3.2 Ethyl Acetate-Water System 4.2.3.3.3 Methyl Propionate-Water System 4.2.3.1+ Results of Reaction Transfer Runs 4.2.3.1:'"1 Ethyl Acetate-Sodium Hydroxide System 4.2.3.4.2 Methyl Propionate-Sodium Hydroxide System 4.2.3.4.3 Induction Based on the Experimental Cone entrati on Profiles (viii) Page 102 103 103 108 108 109 110 110 111 113 113 115 117 117 117 117 123 123 124 129 132 132 134 134 135 136 136 138 l/+0 141 141 145 150 151 151 156 159 4. 2. 3. 4. 1+ Further Informati on Derived from the Exper:i.­ mental Concentration Page Profiles 165 4.3 - PART C - Steady State Mass Transfer Rate Measurements in Ester-Aqueous Caustic Systems 202 4.3.1 Experimental Details 203 4 .• 3 .. 1.1 Apparatus 204 4.3.1.2 Procedure 206 4.3.1.3 Number of Ester-Caustic Systems Investigated 208 4.3.1.4 Method of Calculation 209 4.3.1.4.1 ¥~ss Transfer Rates 209 4.3.1.4.2 f!Jass Transfer Coefficients 209 4.3 .1.4.3 The Enhancemant Factors 211 4.3.2 Results 212 4.3.2.1 Ethyl ForTIBte-Sodium Hydroxide System 212 4.3.2.1.1 Mass Transfer Rate Measure- ments 212 4.3.2.1.2 Enhancement Factors 212 4.3.2.2 1~ethyl Formate-Sodium Hydroxide System 216 4.3.2.2.1 M:tss Tre.nsfcr Rate Measurements 216 4~3.2.2.2 Enhancement Factors 216 4.3.2.3 Propyl Formate-Sodium Hydroxide System 219 4.3.2.3.1 Mass Transfer Rate Determinations 219 4.3.3 Discussion of Results 221 4.3 .3 .1 General Assumptions 221 L:-·3·3·1.1 Measurement of Steady State Mass Transfer Rates 221 4.3.3.1.2 Salt Effects 221 4.3.3.1.3 Alcohol Effects 222 4.3 .3.1.4 Application of the "Film Theory" to the Ester- Caustic Systems 222 4.3.3.2 Ethyl Formate-Sodium Hydroxide System 224 4.3.3.2.1 Comparison of the Transfer Rates between Groups I and II 224 4.3.3.2.2 Comparison of Transfer Rate ResQlts Obtained by Other Workers 224. 4.3.3.2.3 Comparison of the Experi­ mental. and Theo:cetical Enhancement Factors 225 ( ix) 4.3.3.2.4 Possible Factors Governing the Reduced Enhancement of Transfer Page Coefficients 225 4.3.3.3 Methyl Formate-Sodium Hydroxide System 228 4.3.3.4 Propyl Formate-Sodium Hydroxide System 229 4.4 - PART D - Measurement of the Lj_quid Diffu&ion Coefficients 230 4.4.1 Experimental Details 231 4.4.1.1 Apparatus 231 4.4.1.2 Procedure 234 4.4.1.3 Principle of the Moire Pattern 235 4.4.2 Liquid Systems under Study and the Experimental Results 236 5 CONCLUSIONS 5.1 Unsteady State 1•'J:ass Transfer Study 5.2 Steady State Mass Transfer Study 6 RECmJ1MENDATIONS 6.1 Unsteady State Mass Transfer Study 246 6.1.1 Further Investigations on the Cause and Nature of the Turbulent Layer in Ester- Aqueous Caustic Systems 246 6.1.1.1 Layer Propagation Measurements on More Ester-Caustic Systems 2L~6 6.1.1.2 Investigations on the Effects of Salt and Density on the Layer · Propagation Rate 24 7 6.1.1.3 The Effects of Heat of Reaction and Alcohol on the Layer Propagation Rate 2/+ 7 6.1.2 Further Measurements on the Concentration Distribution l:lithin the Turbulent Layer in the Ester-Aqueous Caustic Systems 248 6.1.2.1 Concentration Profile Measurements on more Ester-Caustic Systems 248 6.1.2.2 ImprovemEmt on the Ex-perimffi"ltal Techniques and the Apparatus to give Better Results 2l~o9 6.1.2.3 The Effects of Alcohol and the Heat of Reaction on the Ester 'l'ransfer Rate 250 (x} Page 6.1.3 :Mathematical Modelling 251 6.2. Steady State Mass Transfer Study 253 6.2.1 Ext.ension of the Present Experimental vJork 253 6.2.2 Analysis of Data using Other Theoretical Models 254 6.3 Measurement of the Liquid Diffusion Coefficients 255 6.3.1 Improvement on the Present Existing Apparatus 255 6.3.1.1 Modification of the Apparatus to Minim)_ze Liquid Leakages 25 5 6.3.1.2 Modification of the Apparatus to Provide a Better Way of Filling the Test Solutions 256 6.3.2 Improvement on the Analysis of Data 256 6.3.3 Extension of the Present Experimental Systems 258 7 SUivlr·1ARY OF CONTRIBUTIONS TO KNOWLEDGE APPENDICES I II III IV v VI VII VIII IX The Chemicals Used in the Experiments Sanmle Calculation of the Measurement of Layer Propagation Detailed Description of the Apparatus Experimental Details for the Measurement of Concentration Distribution in the Turbulent Layer Layer Propagation Measurements in the Round Test Cell of 21.2 em. in Diameter Temperature Profile Across the Turbulent Layer for the Ethyl Acetate-Sodium Hydroxide System Sample Calculations and Error Analyses for the Concentratj_on Probing Experiments Estimation of the Extent of Reaction 'Within the Sampling Needle during Sampling j_n the Concentration Probing Experimants Analog Computer Study (xi) 259 261 266 269 278 294 301 305 325 329 X XI XII XIII XIV XVI XVII XVIII XIX REFERENCES Calculation of the Enhancement Factors and the Reaction Zone Thicknesses in the Concentration Probing Experiments Preliminary Experiments in the Ethyl Acetate-Water System Preliminary Experiments on the Measurement of Concentration Distribution of Compon­ ents in the Ethyl Acetate-Sodium Hydroxide System Measurement of the Solubilities of Esters in Aqueous Salt Solutions Comparison of the Experimental and the Theoretical Concentration Profiles for the n-Butanol-Vlater and the Ethyl Acetate­ Water systems Experimental Concentration Data of Ethyl Acetate-Aqueous Caustic Systems and Ethyl Acetate-Water System Eddy Diffusivi ties of Various Components as Derived from the Simulation of the Experimental Conc..:entration Profiles Concentration Profile Data of Ester­ Aqueous Caustic Systems Turbulent Reaction Layer Propagation Rate Data of Ester-Aqueous Caustic Systems Comparison of the Calculated and the Experimentally Measured Reactant Concentrations in the Lm'Ver Phase of the Steady FlO\v Reactor {xii) Page 336 352 358 373 378 393 402 409 427 Table 1 2 3 4 5 6 7 LIST OF TABLES Comparison of the Properties of Various Esters Densities of Various Aqueous Ester Solutions Experimentally Determined at 22°C Summary of Physical Mass Transfer in Three Component Systems Filling Rates of the Aqueous Phase for the Five Runs in the n-Butanol~Vlater System Regression Data of Total Ester Transferred across the Liquid Interface as a Function of Phase Contact Time Integrated Areas under the Concentration Profiles in Figure 13 in the n-Butano1- V!ater System Comparison of Experim~mtal and Theoretical Mass Transfer Rates (Ethyl Acetate-Water System) Comparison of Experimental and Equilibrium Interfacial Concentration of Ester Page 81 . 87 110 131 llr9. (Ester-Aqueous Caustic Systems) 167 9 Mass Balance in the Ester-Caustic Systems (Amount of the Salt in the Aqueous Phase/ Amount of the Caustic Reacted) 170 10 Time Averaged Ester Transfer Rates and Enhancement Factors in the Ethyl Acetate­ Sodium Hydroxide and the Methyl Propionate~ Sodium Hydroxide Systems 194 11 Comparison of Tux·bulent Layer Thickness as Obtained by Direct Measurements and as Derived from the Experimental Ester Concentration Profiles 197 12 Comparison of the Reaction Zone Thickness as Measured Experimentally and as Approximated from the Pseudo-Steady State Model Based on Time- Averaged Ester 'l'ransfer Rates 199 (xiii) Table II.l V.l VII.l VIII.l IX.l X.l X.2 X.3 XII.l XII.2 XII.3 XV .. l XV.2 XV.3 Experimental Data: Measurement of Turbulent Layer Propagation (Methyl Acetate-0./r ·N NaOH) Comparison of the Turbulent Layer Propagation Speed in the Round Cell (21.2 em in Diameter) and in the Optical Cell (4.8 em X 4.9 em). Estimation of the Error Range in Measuring the Sample Volume in Cornwall Syringes Amount of Reactants Disappeared in the Sampling Needle during Sampling Scaling of the Variables in Analog Simulation Instantaneous Ester 1·'Iass Transfer Rates and Enhancement Factors in the Ethyl Acetate­ Sodium Hydroxide and the Methyl Propionate­ Sodi urn Hydroxide Systems Comparison of Enhancement Factors as Measured from Experiments and as Predicted from the Penetration Theory Comparison of the Reaction Zone Thicknesses as Measured Experimentally and as Approximated from the Pseudo-Steady State Model (Based on Instantaneous Ester Transfer Rates) Eddy Diffusi.vity Data from the Sj.mulation of Concentration Profiles (Ethyl Acetate-0.5 N NaOH: Preliminary Experiments) Eddy Diffusivity Data from the Sinrnlation of Concentration Profiles (Ethyl Acetate-1.0 N NaOH: Preliminary Experiments) Eddy Diffusi vi ty Data from the Simulation of Concentration Profiles (Ethyl Acetate-1.5 N NaOH: Preliminary Experiments) Concentration Data of Ethyl Acetate-Water System Concentration Data of Ethyl Acetate-1.0 N NaOH System (Run //lt--1) Concentx·ation Data of Ethyl Acetate-1.0 N NaOH System (Run //h-2) {xiv} Page 268 299 321 328 330 343 345 370 390 391 Table Page XVI.l Eddy Diffusivity Data from the Simulation of Concentration Profiles (Ethyl Acetate- 0.2 N NaOH) 394 XVI.2 Eddy Diffusivity Data from the Simulation of Concentration Profiles (Ethyl Acetate- 0.4 N NaOH) 395 XVI.3 Eddy Diffusivity Data from the Simulation of Concentration Profiles (Ethyl Acetate- 0.6 N NaOH) 396 XVI.4 Eddy Diffusivity Data from the Simulation of Concentration Profiles (Ethyl Acetate- 397 1.0 N NaOH) XVI.5 Eddy Diffusivity Data from the Simulation of Concentration Profiles (Ethyl Acetate- 1.4 N NaOH) 398 XVI.6 Eddy Diffusivity Data from the Simulation of Concentration Profiles (Methyl Propionate- 0.4 N NaOH) 399 XVI.7 Eddy Diffusivity Data from the Simulation of Concentration Profiles (Methyl Propionate- 0.6 N NaOH) 400 XVI.8 Eddy Diffusivity Data from the Simulation of Concentration Profiles (Methyl Propionate- 1.0 N NaOH) 401 (xv) Figure 1 2 3 4 5 6 7 9 10 11 12 13 LIST OF FIGURES Schematic Diagram of the Concentration Profiles for Two Chemical Species Reacting near an Interface Experimental Set-Up for the Colour Indicator Runs Photograph of the Turbulent Layer in Ethyl Formate-1.0 N NaOH System vlith B.D.H. Universal Indicator Experimental Set-Up for the Measurement of Turbulent Layer Propagation Photograph of the Turbulent Layer in Methyl Acetate-0.4 N KOH System (Turbulent Layer Thickness) 2 vs. Phase Contact Time in Methyl Acetate-NaOH System and Methyl Propionate-NaOH System Rate of Propagation of Turbulent Layer in Ester-Caustic Systems Photograph of the Ethyl Acetate-Water System immediately after the Injection of 0.05 ml. of 8.16 N Aqueous Ethanol Solution Photograph of the \Vater Saturated \vith Ethyl Acetate in Contact with 0.5 N Aqueous NaOH Solution (one phase) Interfacial Turbulence in Ester Saponification Experimental Set-Up for the Concentration Probing Experiments Apparatus for Sampling Concentrntion Profiles (n-Butanol-\Vater System) (xvi) · Page 37 51 55 68 70 71 95 97 99 104 105 118 Figure 14 15 16 17 18 19 20 21 22 23 24 25 Variation of Ester Concentrations \'lith Phase Contact Time (Ethyl Acetate-Water System) Concentration Profiles (Ethyl Acetate­ Water System) Concentration Profiles of Methyl Propionate­ Water System Variation of the Reactant and Product Concentrations 1rlith Phase Contact Time (Ethyl Acetate-1.0 N NaOH System) Concentration Profiles of Reactants and Products in the Aqueous Phase (Ethyl Acetate-1.0 N NaOH) Concentration Profiles of Reactants and Products in the Aqueous Phase (Methyl Propionate-0.4 N NaOH) Total Ethyl Ester Transferred Across the Liquid Interface as a Function of Phase Contact Time. Ethyl Acetate-tT~OH System and Methyl Propionate-NaOH System Comparison of Theoretical and Experimental Dimensionless Concentration Profiles in the n-~Butanol-VJater System Comparison of Theoretical and Experimental Dimensionless Concentration Profiles (Ethyl Acetate-'\'later System) · Replot of the Concentration Profiles (Ethyl Acetate-0.6 N NaOH:50 Minutes after Initial Phase Contact) Simulated Concentration Profiles of the Reactants and Products in the Aqueous Phase (Ethyl Acetate-1.0 N NaOH and Methyl Propionate-0.4 N NaOH) Variation of .Mean Eddy Diffusi vi ties as a Function of Phase Contact Time and as a Function of Initial Aqueous Phase NaOH Concentrations (Ethyl Acetate-NaOH System and TV::othyl Propionate-NaOH System) (:xvii) Page 120 121 122 125 126 128 130 144 147 162 182 Figure 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 Time Averaged Mean Eddy Diffusivities as a function of Initial Aqueous Phase NaOH Concentration Time Averaged Mean Eddy Diffusivities versus Turbulent Layer Propagation Rates Total Amount of Reactants in the Aqueous Phase as a Function of Contact Time: Ethyl Acetate-NaOH System and Methyl Propionate-NaOH System Steady State Transfer Apparatus Transfer Rates vs. Input NaOH Concentration (Ethyl Formate-NaOH System) Reciprocal of Reaction Mass Transfer Rate vs. Time {Ethyl Formate-0.5 N NaOH) Comparisons of Theoretical and Experimental Enhancement Factors (Ethyl Fonnate-NaOH System) 'l'ransfer Rates vs. Input NaOH Concentration {Methyl Formate-NaOH System) Comparison of Theoretical and Experimental Enhancement Factors (Methyl F'orma te-NaOH System) Transfer Rates vs. Input NaOH Concentration (Propyl Formate-NaOH System) Schematic Diagram of the Diffusion Cell The Experimental Set-Up Moire' Photograph of the Sucrose~Water System Effect of Concentration on Diffusion Coefficient (Ethyl Acetate-Water System) Proposed Modifications of the Diffusion Cell Proposed Modifications of the Diffusion Cell (xviii) Page 184 184 190 205 213 214 215 217 218 220 232 233 239 240 257 257 Figure Page I.l Apparatus for the Distillation of the Esters 263 III.l The Caustic Delivery Tube 270 III.2 Cross-Section of the Test Cell and the Supporting Wooden Plate 272 III.3 Sampling Needles Arrangement 274 IV.l Tripod Stand for Measuri n_g the Sampling Needle Inlet Positions 279 V.l Turbulent Layer Thickness vs. Phase Contact Time (Ethyl Acetate-NaOH System) 295 V .2 Turbulent Layer Thickness vs. Phase Contact Time (Methyl Propionate-NaOH System) 296 V.3 (Turbulent Layer Thickness) 2 vs. Phase Contact Time {Ethyl Acetate-NaOH System) 297 V.h (Turbulent Layer Thickness) 2 vs. Phase Contact Time (l-1ethyl Propionate-NaOH System) 298 VI.l Qualitative Study of Temperature Distribution in the Turbulent Layer of Ethyl Acetate- 1.0 N NaOH System 303 VII.l Magnitude of the Probable Error Range Involved in The Measurement of NaOH Concentrations 315 VII.2 Magnitude of the Probable Error Range Involved in the Measurement of Ester Concentration 316 VII.3 Magnitude of the Probable Error Range Involved in the Measurement of Ethanol Concentration 317 VII.h Magnitude of the Probable Error Range Involved in the Measurement of Salt Concentrations 318 VII. 5 Magnitude of the Probable Error Range Involved in the Measurement of Methanol Concentration 319 VIII.l Reaction Rate Constant vs. Temperature (Ethyl Acetate-NaOH System) 327 IX.l Analog Circuit Diagram 331 (xix) Figure Page IX.2 Simulated Concentration Profiles of the Reactants and the Products in the Aaueous Phase (Ethyl Acctate-0.6 N NaOH) • 333 IX.3 Simulated Concentration Profiles of the Reactants and Products in the Aqueous Phase (Ethyl Acetate-1.4 N NaOH) 334 IX.4 Simulated Concentration Profiles of the Reactants and Products j_n the Aqueous Phase (Ethyl Acetate-0.4 N NaOH) 335 X.l Enhancement Factor Calibration Curves XI.l XI.2 , XI.3 XII.l XII.2 XII.3 XII.h XII.5 XII.6 XII.? Apparatus for the Preliminary Concentration Probing Experiments in the Ethyl Acetate­ Water System Concentra.tion Profiles of Ethyl Acetate­ Water System Preliminary Experiments Total ~thyl Ester Transferred Across 353 cm2 of the Liquid Interface as a Function of Phase Contact Time Apparatus for the Preliminary Concentration Probing Experi~ents in the Ethyl Acetate­ ~raO!-I System Distillation Apparatus Concentration Profiles of Components in the Aqueous Phase (Ethyl Acetate-1.0 N NaOH) Total Ethyl Ester Transferred Across the Liquid Interface as a Function of Phase Contact Time (Ethyl Acetate-NaOH System) Preliminary Experiments Total Ethyl Ester Transferred Across 1 cm2 of the Liquid Interface as a Function of Phase Contact Time S5.mulati on of the Concentration Profiles of Components in the Aqueous Phase (Ethyl Acetate·-1.0 N NaOH) MIMIC Program Listings (:xx) 342 353 355 356 359 359 362 366 367 372 Figure Page XIII.l Solubility of Ethyl Acetate in Aqueous Sodium Acetate Solutions 374 XIII.2 Solubility of Methyl Propionate in Aqueous Sodium Propionate Solutions 375 XIII.3 Solubility of Methyl Formate in Aqueous Sodium Formate Solutions 376 XIII.4 Solubility of Ethyl Formate in Aqueous Sodium Formate Solutions 377 XIV.l FORTRAN Listings for the Solution of Diffusion Equation \·lith Variable Diffusion Coefficient 380 XIV.2 Effect of Concentration on Diffusion Coefficient(n-Butanol-Water System) 382 XIV .3 Comparison of Theoretical and Experimental Dimensionless Concentration Profiles ( n-Butanol·~Water Sys tern Run #3) 384 XIV.!} Comparison of Theoretical and Expe.riment,al Dimensionless Concentration Profiles ( n-Butanol--Water System Run #4) 385 XIV.5 Comparison of Theoretical and Experimental Dimensionless Concentration Profiles ( n-Butanol-Water System Run //5) 386 XIV .6 Comparison of Theoretical and Experimental Dimensionless Concentration Profiles (Ethyl Acetate-Water System) 387 XV .1 Variation of the Ester Con centra ti ons l'li th Phase Contact Time (Ethyl Aceta te-NaOH Systom) 392 XVII.l Concentration Profiles of Reactants and Products in the Aqueous Phase (Ethyl Acetate-0.2 N NaOH) 403 XVII.2 Concentration Profiles of Reactants and Products in the Aqueous Phase (Ethyl Acetate-0.4 N NaOH) 404 XVII.3 Concentration Profiles of Reactants and Products in the Aqueous Phase (Ethyl Acetate-Oo6 N NaOH) 405 (xxi) Figure XVII.4 XVII.5 XVII.6 XVIII.l XVIII.2 XVIII.3 XVIII .I+ XVIII.5 XVIII.6 XVIII.7 XVIII.8 XVIII.9 XVIII.lO XVIII.ll Concentration Profiles of Reactants and Products in the Aqueous Phase (Ethyl Acetate-1.4 N NaOH) Concentration Profiles of Reactants and Products in the Aqueous Phase (Methyl I'':r·opiona te-O. 6 N NaOH) Concentration Profiles of Reactants and Products in the Aqueous Phase (Methyl Propionate-1.0 N NaOH) {Turbulent Layer Thickness) 2 vs. Phase Contact Time (Methyl Acetate-KOH System) (Turbulent Layer Thickness) 2 vs. Phase Contact Time (Methyl Acetate-NaOH System, Adjusted to Simulate the Viscosity and the Density of 1.0 N NaOH) {Turbulent Layer Thickness) 2 vs. Phase Contact Time (Methyl Propionate-KOH System) (Tu:t:·bulent Layer Thickness) 2 vs. Ph::tse Contact Time (Ethyl Acetate-NaOH System) (Turbulent Layer Thickness) 2 vs. Phase Contact Time (Ethyl Acetate-KOH System) (Turbulent Layer Thj.ckness) 2 vs. Phase Contact Time (Ethyl Propionate-NaOH System) (Turbulent Layer Thickness) 2 vs. Phase Contact Time (Ethyl PropionG.te-KOH System) (Turbulent Layer Thickness) 2 vs. Phase Contact Time (Propyl Formate~NaOH System) (Turbulent Layer Thickne·ss) 2 vs. Phase Contact Time (Propyl Formate-KOH System) Photograph of the Turbulent Layer in Propyl Formate-0.4 N NaOH System (Turbulent Layer Thickness)2 vs. Phase Contact Time (Ethyl Form:tte-NaOH System, Adjusted to Simulate the Viscosity and the Density of 1.0 N NaOH) (xxii) Page 406 408 410 411 412 413 414 415 416 417 418 419 420 Figure XVIII.l2 XVIII.l3 XVIII.l4 XIX.l (Turbulent Layer Thickness) 2 vs. Phase Contact Time (Ethyl Formate-0.1 N NaOH System, Adjusted to Simulate the Viscosity and the Density of 1.0 N NaOH) (Turbulent Layer Thickness) 2 vs. Phase Contact Time (Ethyl Acetate-NaOH System, Adjusted to Simulate the Viscosity of the 1.0 N NaOH) Turbulent Layer Thickness vs. Phase Contact Time (Ethyl Forrnate-NaOH System with Carboxy Methyl Cellulose Added) Resistance vs. Concentration of NaOH: Calibration Curve for Run #7 Ethyl ForrrB.te­ NaOH System (Constant Total Concentration of NaOH + HCOONa = 0.5 N) (xxiii:) Page 421 422 423 425 CHAPTER l ~-- - INTRODUC'l'ION - -·- ----- Ever since the development of chemical engineering, j_nterphase mass transfer has been one of the vital operations in the field. Important practical examples are numerous; such as the gas absorption of sulphur trioxide and nitrograter. They found that when protein or detergent v.ras adsorbed at the interface, the interfacial turbulence \'las inhibited, but the emulsification vm.s not appl~ecia bly affected. Kaminski and McBain ( Kl) examined the emulsion \vi th turbulence formed l'lhen benzene, toluene or xylene was placed gently on strong solutions of dodecylamine hydrochloride. They found that either the dilution of the hydrochloride (to less than 0.1 N) or the presence of a detergent vvould inhibit the emulsion formation. Indeed, interfacial turbulence has been advanced as one of the three probable mechanisms of spontaneous emulsification. An excellent revi e1:1 on the subject of spontaneous emulslfi cation 1:1as given by Davies and Haydon ( DlO) • 2.1 o 1. 2 T'\1o-Phase Gas-Liauid S:ys terns Surface instability also occurred in the gas-liquid systems. A classi.c example \ms furnished by Langmuir's 12 experiments(Ll) on the evaporation of ether from a saturated solution in water. Talc sprinkled on the surface shovred abrupt local movements, caused by the differences in surface tension in the different regions of the surface. A monolayer of oleic acid on the surface \"las observed to reduce the turbulence considerably. A similar effect vras found by Groothuis and Kramers(Gl2 ) for the absorption of sulphur dioxide in n-heptane. The surface of the liquid became violently agitated. Using Schlieren and interferometric techniques, Kroepelin and Prott(KlJ) showed that the transfer of methanol vapor to \'Tater \'las an example of eruptive transfer. Ramshaw· a~d Thornton (R3) shov1ed that \'Then eithe·r acetone or methanol vapor '"vas absorbed into \"rater, considerable interfacial turbulence occurred with ripple formation and surface oscillations. Using a capacitance probe and photographic techniques, Ellis and Biddulph{El) measured the amplitude of the uaves induced during the absorption of acetone or methanol into \'Tater. One of the common examples of surface-tension- driven flows is the ripples and tears at the surface in a glass of strong wine. 2 .1. 2.1 !_vr_o-:~.h~l!i£uid-L_ig_uid Syst~ 2 .1. 2 .1 .. 1 ,P_..lanLfut erfa ce..J-_Sta "U:c S:ts !:,ems Interfacial turbulence phenomenon is more pronounced in systems \"lith chemical reaction. Gad{GZ) and Brtlcke(Bl9) noticed that ·when solutions of a fatty acid (e.g. lauric) in oil vrere placed very gently upon the sodium hydroxide solution, an emulsion with interfacial turbulence was formed in the aqueous phase. Sherwood and Wei ( Sl5) studied a 13 number of ternnry and quaternary reaction systems and concluded that the interfacial turbulence lvas pronounced in the case of exothermic neut~ralizations. Seto et al. ( S9), l'lhile studying the reaction r~ss transfer in ester-caustic systems, observed the formation of a turbulent reaction layer \'lith turbulent liquid motion at and near the interface. They also found that the addition of a trace of surface active agent '\rTOuld reduce the layer turbulence as well as the layer propagation rates. Cho and Ranz(Cl) used the Schlieren method to follo1r1 the diffusion~controlled reaction zones resulting from the contact of two reactive liquid phases. They reported that the abnormally high rates of reaction zone propagation and transfer in some systems were due to the interfacial turbulence. 2.1.2.1.2 Qygamic ~.Y§tems Searle and Gordon(S6 ), Shervrood and \'lei(Sl5), and Osborne(06} performed transfer experiments on systems with instantaneous reaction in stirred vessels. They reported mass transfer rates several times as great as those predicted from Hatta' s theory(H7). Bakker et al. (B2} used a \·retted-wall apparatus to study the reaction transfer of the acetic acid in the iso- butanol-aqueous sodium hydroxide system. The hydrodynamic 14 flov; conditions of the tl·To phases Here Hell-defined. They confirmed the abnormally high transfer results of Sherv10od and Wei~ Gaven(Gld and Sherwood et al. (Sl5) reported the "miniature explosion" of a drop of benzene oontaining acetic acid as it rose through an aqueous solution of ammonia. 2.1.2.2 T\'vo-Phase Gas-Lig}l~d _?ys_!;~ms Interfacial turbulence also occurred during the gas absorption with reaction. Using a highly sensitive inter- ferometer, the optical study of interfacial turbulence occurring during the absorption of carbon dioxide into mono­ ethanolamine solution \·ras carried out by Thomas and Mcl\Nicholl (T4) in order to determine the conditions under \vhich interfacial turbulence occurred. They also obtained the relation ship of the time lapse (after the phase contact and before the onset of interfacial turbulence) versus the MEA concentration. Danck~rrerts and Da Silva (D5) observed the surface instability during the absorption of carbon dioxide by a drop of 1 N mono- ethanolamine solution. Other literature data on the absorption of carbon dioxide into monoothanolamine under \·;ell-defined hydrodynamic conditions lvere obtained by many investigators using different kinds of experimental apparatus including Clarke(C2 ) and Astari ta ( A3) using laminar jets; Emmert et al. ( E2 ) and Brian et al. (Bl8) using Netted-\'l'all columns. Most of the absorption data by these investigators fell above the theoretically 15 predicted values due to the occurrence of interfacial turbulence. 2.1.3 Interfacial Turbulence Produced by Te~Gradient at the Interface Interfacial turbulence can also be induced by the local variation of temperature alone at the interface. Early in 1900f Bdnard(B6) described his observations of a cellular deformation produced on the free surface of a liquid film (0.5 to 1.0 mm. thick), the bottom of which \vas uniformly heated and hotter than the top surface. He also observed a cellular flol-v associated with the deformation. Experiments have shown(P4) that drying paint films often display a steady cellular circulatory flow of the same type as that observed in the case of fluid layers heated from belov1. Block{B9) demonstrated experimentally that the Benard cells observed were caused by surface-tenBion-driven flo\'lS which, in turn, \vere induced by the temperature gradients at the surface. He also observed that the addition of a monolayer at the free surface of the film would inhibit the formation of the flow cells. Mitchell and Quinn(M9) investigated the convective flow j..n at.1.in liquid layer heated from belm'J" and· confirmed the findings of Block. Furthermore, they observed oscillating temperature and velocity fields in thin horizontal films. Interfacial Turbulence Produced .2.L!f.ensi~.r.t.9 tTon -- Interfacial turbulence can also be induced by the action of buoyancy forces a.s a result of the density variation. Experimental investigations by Zierep(Zl) and Schmidt et al.(S)) revealed a cellular motion in a liquid layer bounded by horizontal plates {to eliminate the surface-tension-driven flows) , when the bottom plate was hotter than the top plate. 16 A small disturbance analysis was carried out by Rayleigh(R5) and others for the case of free-surface instability due to density gradients. They obtained a critical film thickness ( 2 rnm.) belol'J which there 'tvas stability. Block(B9) performed experiments \'lith a thin film heated from the bottom whereby the Benard cells (due to surface tension variation) "Vlere first produced and then removed by covering "Vlith a monolayer. Then the thickness of the liquid film (covered by a monolayer) was increased until cellular surface deformation and circulation cou~d again be seen. The thickness of the film at the reappearance of the cellular motion \".:as 2 mm., which 1.1as about the predicted critical depth for the onset of convective instab.ility due to density variation. 2.1.5 !l!E?O:re-g.cal Treatment of Interfacial Tvrbulence It is w·ell knm~rn that local variations of interfacial tension not only cause movementi in a liquid surface but also bring forces to bear on the underlying liquid, setting it in motion. This has been call0d the Marangoni effect(B?). The surface tension variation can be brought about by the local variations of component concentration at the int.erface as the result of mass transfer. Alternatj_vely, a temperature variation at the interface, produced by uneYen distribution of heat of solv:t.i.on or heat of reaction, will also cause the 17 local variation of interfacial tension. The development of detailed theory relating the 1~rangoni effect to the inter­ facial turbulence proceeded at a slow rate mving to the inherent complexity of a reasonable mathematical descriptiono Haydon(H9) proposed a kicking mechanism to explain the drop oscillation duriP..g mass transfer. Later Davies and Ha.ydon(Dll) derived the equations of motion for the pendent drop oscillation, thus cnabliP~ the prediction of oscillation frequencies and energy dissipated against viscous forces during the damping of the oscillations. Recent available analyses usually dealt ~dth the stability of the transfer systems to~Jards small disturbances. The onset of a steady, cellular convection driven by surface tension gradients (which, in turn, \'lere caused by surface temperature gradients) on a thin layer of liquid was examined by Pearson(P4). His results indicated the existence of a critical "V~rangoni number" for the onset of instability. Sternling and Scriven(S22) formulated a theoretical model describing surface-tension-driven interfacial activity ('Hhich in turn vras caused by surface concentre.tion gradients) at plane liquid-liquid interfaces. They mathematically analyzed the simplified, ti.-vo-dirnensional model (roll cells) by means of the theory of linearized stability coupled \·r.i.. th the traditional hydrodynamic and diffusion principles. They developed criteria for the onset of instability and determined the nature of the dominant dj_sturbance, predicting its vrave 18 length, amplification factor, speed of propagation, and temporal period. The Sternling and Scriven theory has been partially confirmed by many vlorkers(05} (112) (113). The Pearson's analysis v1as later modified by Scriven and Sternling( S5) to include the effects of surface viscosity and by Smith(S2l) to include the effects of surface viscosity and gravity v;aves. Recently, Vidal and Acrivos (V3) extended Pearson's analysis to include the effect of nonlinear conductive temperature profiles on the stability of liquid layers subject to surface tension gradients. Ruckenstein and Berbente(R12) extended the Sternling and Scriven's analysis(S22) to the case \'lhere diffusion Has accompanied by a first-order chemical reaction in one of the t1rvo phases. They established conditions und cr which the Marangoni effect produced interfacial turbulence in reacting systems and concluded that even a slm·1 first-order reaction might cause instabilities in an othe1'"'\·lise stable system. Ruckenstein (RlO) also examined the influence of the Marangoni effect on the mass transfer in the continuous phase from a spherical drop or bubble when Re < 1. He concluded that the Marangoni effect should be a factor only in transfer from small drops or bubbles. Surface instability due to buoyancy effects is probably, in most cases, considered of secondary importance and has, accordingly, received much less attention. Rayleigh(R5) analyzed the stability \vith respect to buoyancy-driven 19 convection of a fluid layer heated from underneath. His analysis \'JaS subsequently extended by Jeffreys ( Jl}, Lot;(Ll5 ) and Pellm'l et al. ( P5). Conditions for instability were established. Recently, Sani(Sl) analyzed the convective instability of a system undergoing heat and mass transfer, to buoyancy-driven finite amplitude roll cell disturbances. The distortion of the mean temperature and concentration fields by the disturbance and the transport of heat and mass at a finite amplitude roll cell equilibrium state were investigated. 2.1.6 Recent Tren9-sof Interfacial Turbulence Study Apart from the continuing interest in theoretical stability analysis, recent developments· are also concerned with the enhancement of physical m9.ss transfer under controlled mechanical interfacia 1 turbule nee. Boyd and Marchello ( Bl2} studied the influence of small surface "t-:aves on the absorption of carbon dioxide into water. Small amplitude,progressive, ti'To-dimensional waves were mechanically generated in a sealed ripple tank. Goren et al.(Gll} investigated the effect of standing Haves of controlled amplitude and frequency on the steady state rate of mass transfer through thin horizontal liquid layers. Both groups of investigators reported a transfer increase due to the agitations. At the same time, attention is directed to the study of physical mass transfer under falling vmvy film conditions. Thus Banerjee et al.(B4) developed a physical model describing a small eddy structure near tho in'c erfa ce. They shovved that the mass transfer process was likely to be controlled by small eddies for which viscous di ssipa ti on w·as important. An expression for the mass transfer coefficient in terms of the viscous dissipation near the surface has been obtained. Another model \'las proposed by Banerjee et al. (B3), using a partial surface renevral concept, to predict mass transfer rates under falling wavy liquid films. The model was based 20 on the action of large single eddies associated with the \"lave motion. Comparison of the computed values of mass transfer coefficient from the large-eddies-model with data from the literature gave good agreement at lar.r Reynolds numbers (100 to BOO). Holmrd and Lightfoot (Hl7) applied the surface­ stretch model to predict rates of gas absorption into laminar rippling films in terms of surface velocities. Oliver and Atherinos (04) measured the enhancement of physical mass transfer to liquid films on an inclined plane. due to rippling. The enhancement \'vas All the above-mentioned studies may be a step in the direction tmvards the understanding of the effects of turbulence (at or near the interface) on the mass transfer rates. 2. 2 Inte.rpha~e Mass Transfer 2.2.1 Theoretical Considerations 2.2.1.1 Physis;§l.l Mass Transfer 21 Ever since the advent of Nernst's diffusion layer theory (NJ)(N2}, a considerable number of models have been proposed to describe the mechanism (and thus to predict the rates) of physical mass transfer across an interface. The over­ simplified two-film theory of Le\•ds and Whitman(L6} has been superseded by the more elegant concepts such as the penetration theory of Higbie { Hll) and the surface renelval theory of Danckv.rerts {DJ). Both the 'Whitman and Higbie-Dancb·.rerts models have been elaborated on (Hanratty(H2), Perlmutter(P6)) or combined and elaborated on (Toor and N'ia.rchello(T8) (M2), Harriott(H6), Dobbino(Dl6 ), Ruc:kenstein(R9}), uuc p:L'ogressively improved agreement \'lith observation has only been achieved at the expense of increasing the complexity. Alternatively, a steady state phenomenological approach has been advanced by Kishinevskii (K6 ) (K7}, in v1hich the turbulent convective effects give rise to an eddy diffusivity Nhich is obtained from exreriments. Derivations of mass transfer rates have been developed(LB)(Sl3) based on the boundary layer theories first proposed by Prandtl et al.(PlO)(V6). More recently developed models tend to be more realistic in describing the actual experimental interphase transfer conditions. \'lith particular attention to the flov.r very near the stu""face (e.g. the quasi­ steady large scale model(Fl} and the small eddy structure 22 model(B4)). A generalized penetration theory for unsteady convective mass transfer taking into account the convective motions along the interface and along the normal at the interface \'Vas proposed by Ruckenstein (Rll). Szekely et al. { 29) obtained an analogue computer solution for transient diffusion in two-phase systems with the bulk flow and concentration dependent diffusion co efficient. :Multicomponent diffusion problems are important in many chemical and physical processes. Exact treatment of such problems is seldom feasible, since the governing differential equations are complicated and nonlinear. For small concen- tration differences, however, a linearized treatment of the multicomponent mass transfer equations leads to a set of equivalent binary equations. Solutions of the linearized equation~ of multicomponent mass transfer \vere obtained by Toor{T6) and Ste\vart and Prober(S24-). Theoretical studies on multicomponent mass transfer have been developed by Toor et al.(T5), and by Roper et al.(RS) on gas absorption. The mass transfer equations obtained differed in 'form from the usual binary component equations and they pre­ dicted qualitative as v.rell as large quantitative differences between binary and ternary transfer. 2.2.1.2 Mass Transfer \'lith Chemical Reaction All of the above models concerning the mechanism of interphase mass transfer combined vd th chemical kinetics of a reaction can lead tmvard the predictions of the effect of a 23 simultaneous chemical reaction upon the rate of rre.ss transfere Several cases have been treated (Bl6)(Vl)(P7)(D2)(D6)(Dl)(H7) (Hl9)(D4}(P2){F3)(Sl3) • The effect of ions diffusing in mixed electrolytes has been discussed(Sl4)(V4) and, in several cases, it has been combined ,,lith existing theories(Sl4) (Vl~o) (Ml} (V2 ) (Bl4). In general (except under certain conditions as specified in Reference (Blh), e.g. equal ionic self-diffusion coefficients of the ionic species) it '\vas found that mass transfer rates calculated taking into account ion diffusion effects departed significantly from those which '\'lould be obtained 1dth molecular diffusion(Bl4). The effect of heat of reaction on mass transfer vlith chemical reaction has been investigated by Kobayashi and Saito ( KB) • The effect of bulk flo'.v on mass transfer accompanied by homogeneous chemical reaction \'Jas discussed by Szekely( 828 ) in terms of the "film model". Some problems of analyzing processes of mass transfer '."lith chemical reaction \t·rere discussed by Mutriskov and Maminov ( MlO} • Brian et al.(Bl5} presented theoretical solutions for gas absorption accompanied by a two-step chem:ical reaction involving a transient intermediate species. The type of chemical system which has received the most attention is that in which a gaseous species dissolves and then reacts irreversibly \'lith a non-volatile solute already present in the liquid phase. In the case of a second-order chemical reaction betv1eon the dissolv:i.ng species and the 24 non-volatile solute, an approximate solution to the film theory \'ras presented by Van Krevelen and Hoftijz·ar(Vl}, and the accuracy of this approximated solution was demonstrated by Peaceman ( P3}. The penetration-theory solution for this case was obtained numerically by Perry and Pigford(P?} for a limited range of variables and later by Brian et al. (Bl6} for a much \'rider range of variables. Generalized penetration theory solutions vTith respect to the order of the chemical kinetic equation v.rere presented by Brian(Bl3} and Hikita et al.(Hl2 ). Location and thickness of the reaction zone in liquid­ liquid systems l'rith reaction \vere derived by Scriven(S4) for unsteady state transfer. On the other hand, reaction zone thickness \vas derived by Friedlander et al. (F2 ) for steady state transfer. Although most of the theories are proposed specifically for gas absorption, their extension to certain types of liquid­ liquid system does not involve assmnptions differing signifi- cantly from t.hose postulated for gas-liquid sys~ems. 2. 2. 2 EJq2cr1Jl!.~?ntal Studies Various experimental investigations have been conducted, aimed mostly at testing the various transfer models. Probably the type of apparatus used (i.e. the flow pattern} is clearly related to the theory chosen. For example, the film theory would be expected to uork b0tter for a gent:.ly stirred vessol, '\"lhere steady state mass transfer across an unbroken interface 25 occurs, than for a packed tower, where the liquid flows over a piece of packing for a very short period of time before being mixed and then flows to the next piece. 2.2.2.1 Physical Mass_ Transfer A comprehensive summary of the reported experimental vmrk up to 1963 was given previously in Seta's Masters Thesis(sg}. Recent developments include the study of turbulent and convective transfer in apparatus such as turbulent jets ( Dl3) and agitated vessels(K3). Physical interphase mass transfer rates were measured for mechanically-controlled wavy films(B12)(Gll), vibrating spheres(N5) and cylinders(S27). Thus the eP~ance- ment of transfer due to imposed mechanical motion \·ras obtained. Other advances include mass transfer from a rotating disk into pm•Ter~lavr-liquids(H4) and the detection of interphase mass transfer by measuring electrochemiluminescence in a electrolytic cell(C3). 2.2.2.2 Reaction M~ Transfer Many experimental investigations are concerned ,,n_ th gas absorption involving a chemical reaction. Thus Nijsing et al.(N4) carried out studies on the absorption of carbon dioxide into lanunar jets and laminar falling films of aqueous solutions of sodium, potassium and lithium hydroxides. Dan ch1ert s and co-workers have carried out a series of studies on the absorption of carbon dioxide into alkaline solutions with a variety of interfacial geometries, such as rotating drum(D7), -vvetted-\'.:-all co.luwn(R7) and laminar jet(Sll). The carbon dioxide-monoethanolamine system has been the subject of many investigators ( C2) ( A3) { E2) ( Bl8) • Many studies of gas absorption '\vith chemical reaction in stirred vessels have been reported(Al) (Pl) (V2 ) (K9). A bubble of carbon dioxide absorbed by the monoethanolamine solution v1as studied by Houghton (Hl6 ). 26 Reaction mass transfer studies in liquid-liquid systems have been performed mostly by using stirred vessels. Thus Osborne(06) Searle and Gordon(S6 ) She~wood and Wei(Sl5) ' ' extracted acetic acid from isobutanol using aqueous sodium hydroxide solution. The extraction of acetic acid from water by a solution of cyclohexylamine in isobutanol and the extraction of n-butyd.c acid from benzene by aqueous sodium hydroxide -.;ere also pe rfor·n1ed by Sherwood and Wei ( Sl5) • Abnormally high transfer rates were obtained due to interfacial turbulence. On the other hand, Fujinav1a et al. (F4)(F6 ) carried out a large number of experiments in liquid- liquid extraction \·lith chemical reaction. In each of the systems under study, chemical extraction rates were found to be, constant in the range above a certain critical value of the initial sodi urn hydroxide concentration. Their conclusion was in support of the film theory. Raal and Johnson ( R2) derived a non-linear differential equation on liquid extraction where the solute, on transferring across the interface, reacted to form a dimer thrm.1gh a rapid second-order reversible reaction uith the solvent. Hmvever, experimental results 27 deviated from the predicted ones(Rl). Seto et al.(SS) studied the reaction transfer of esters into caustic solutions. Ab- normally lmv transfer rates (as compared to theory) \wre obtained. Reaction transfer a cross a liquid-liquid interface in stagnant two-phase systems \'las studied by Cho and Ranz(Cl) and by Seto et al.(S9). They reported that the abnonnal enhancement of transfer rates and reaction layer propagation rates \'Jere due to interfacial turbulence. Very little information concerning mass transfer '\'lith reaction in drops is available. Nevertheless, extraction of acetic acid and butyric acid from various solvent drops by aqueous potassium hydroxide v1as carried out by Fujinm,m and Hakaik~ ( F5) • They found that in the case of acetic acid transfel"' from solvent drops, the transfer rates, in general, decreased vlith increasing potassi urn hydroxide concentration; '\'Thile in the case of butyric acid transfer from solvent drops, the transfer rates, in general, slightly increased with increasing caustic concentration. Drop study 't'Tith reaction '\•ras also carried out by Watada 0'!2 ) of this Department. Comparisons betlveen experimental results and theoretical predictions have not only been used to check the applicability of the theory but they have also been used to infer the kinetics and the mechanisms of very fast gas-liquid reactions( Bl7) (D6) (G?) (R7) (J2) (J3) (G5) and of the liquid-liquid reaction(Nl). 28 Recent advances included mass transfer with reaction in gas-fused salt systems(Rl4) (02) (Ol} (Gl) (Rl3) (G8) and the measurement of concentration distribution of gases in a reacting trickling film by a fluorescence method(HlOa). 29 2.3 Dif-fu~iq_:g_ Coefficient Measurement The experimental and theoretical investigation on binary gaseous diffusion has been fairly adequate for moderately l01'l temperatures and pressures. A number of semi-empirical equations \vere proposed to predict the diffusion coefficient of low density gases, such as the Gilliland's(G6), the Hirschfelder, Bird and Spotzs' (Hl4), the Slattery and Bird's( 820). Equations predicting diffusion coefficients for a gas diffusing into a multi component· gas solution based on the binary diffusion coefficients have also been developed. For a three-component system, Curtiss and Hirschfelder derived(C?} an expression relating the multicomponent diffusion coefficient to the binary diffusion coefficients. Methods of evaluating multicomponent diffusion coefficients for special cases have been developed by Wilke ( \\1"5} , St ev1art ( 823) , Hsu and Bird ( HlS) • These methods simplify the calculations somel'lhat. The most well-knov-m of all is the Stefan-I··1ax\•rell equations ( B8) which are usually the starting point for the calculation of ordinary diffusion in multicomponent gas mixtures. On the contrary, the a.dvent of liquid diffusion j_s not as extensive, notHithstanding the fact that experimental liquid diffusion data are actually more essential. One of the possible explanations is that the diffusivities for most liquids vary quite markedly \'lith concentration as \'lell as viscosity even if the temperature and pressure are constant. Furthermore, there is considerable difference in the diffusion 30 process between ionic liquids and non-electrolytes. Never- theless, a few correlations determining the diffusivities at dilute solutions have been proposed. For the non-electrolytes, these are the Arnold equation(A2), the Wilke correlation(W4), the \'lilke and Chang equation(W6). Recently, a note by Shrier( Sl6) reinforced the continued use of the Wilke- Chang .correlation. An expression \'.ras also developed to account for the effect of concentration on diffusion of non-electrolytes (P9) • Among the recent developments are the analysis of Pynn and Fixrnan(L7) which allov1s the extension of the hydrodynamic model to concentrated systems, the analyses of Olander and of Gainer and Metzner(L7) which extend the Eyring theory to dilute solutions of very high viscosity. Cullinan(C6) proposed a modification of the absolute rate theory of Eyring \vhich resulted in an expression to predict the concentration dependence of binary diffusion coefficients for non-ideal solutions, in terms of tw·o coefficients at infinite dilution and a thermodynamic factor. The proposed relation '\'mrks well except for associated mixtures (those exhibit maxima). Hansen(H3) presented simple correlation factors for the rapid calculation of the true diffusion coefficients obtained by absorption and desorption methods based on an exponential dependence on concentration. He pointed out that very serious errors \vere made \>Then concentration-dependence v1as neglected. Very little v:ork has been done on the prediction of multicomponcnt diffusion in liquids. More recently, the technique developed by Pynn and Fixman has been applied by Lightfoot and Cussler(L?) to obtain a first approximation of multicomponent diffusion. This development is in semi- quantitative agreement with data of the polystyrene -toluene system. Both Toor(T?) and Kass and 0'Keefee(K2 ) discussed 31 methods fer predicting the concentration distribution and hence evaluating diffusivities that depended on concentration. Hijnlieff and Vreedenberg {M7) solved the rna ss balance for isothermal, isopiestic diffusion in a three-component electrolytic system with concentration dependent diffusivities. Included was a method of evaluating the diffusion coefficients as a function of concentration. Hays and Curd(HlO) described a variational technique for concentration dependent diffusion. The diffusion coefficient in very dilute solution of completely ionized simple univalent electrolytes \•Tas given by the Nernst equation. The case of diffusion in mixed electrolytes vras treated by Vinograd and McBain (V4) and Sherwood and Wei (Sll+). Since no adequate theory for the prediction of diffusion coefficients of liquids exists, experimental measurements are generally required. Much effort has been spent to improve the design of apparatus for di. ffusion measurements. A number of diffusion cells have been developed. The most common one is the use of pseudo-steady state diffusion through a diaphragm cell(S25) (M8) (T3). Although the apparatus is simple to 32 construct, the diffusion process through the diaphragm is slo-t'IT and often requires a considerable amount of experimental time. Generally, the component allowed to diffuse is limited to a small amount; thus, rendering the analysis of concentration inaccurate. In the extreme case, e.g. polymer diffusion, such cells may not be suitable because of the difficulties encountered by the molecules passing through the pores. The diffusion coefficient measured with the diaphragm cell is the integral value over the concentration increment and at the particular experimental concentration level. Several theories l'thich include the effect of concentration dependence of diffusion coefficient have been developed for the diaphragm cell(S26), but they are difficult to use. For higher accuracy measurements, either free or restricted diffusion m0thod is used. In order to get accurate diffusi vi ties using such methods, tvJo difficulties must be overcome t'll'hile perfonning the experiment. They are (a) to form a sharp init.ial boundary of contact, (b) to probe the concentration profiles v'li thout disturbing the system. There are a nurnber of diffusion cells designed to give good contact betw·een ttlO solutions. The FUrth cell (F?) is an example of the earlier development. A thin drmv slide is used to separate the tt-;o solutions initially ins ide the FUrth cell. It is more popular to provide the initj_al contact bet~:reen the tt~o solutions by a shearing mechanism such as the Claesson cell ( Clb). An excellent design for very viscous solution is the flo~t;-junctton 33 diffusion cell (P7a). On the other hand, a simple cell for free diffusion measurements has been recommended by Lamm(Llal Damper type cells as vvell as invertible type cells were recommended by Inoue et al.(Il). Some other methods have been developed to sharpen the boundary between solutions such as that of Polson and Kahn (P?b). A fei'l methods have .been proposed to trace the concentration profiles -vlithout actual sampling. Radioactive tracers tvere used(E3) although the diffusion coefficients so obtained may not be the binary ones(M7a). Less accurate but \vell executed measurements have been made with electrical conductivity(M7a). However, the most accurate methods so far yet devised are the optical methods. These are the light absorption method(K3a), Guoy's diffusiometer(Glla) and Rayleigh interferometer(C4b), and Schlieren knife-edge method by Takamatsu et al. (Tl). More recently, Sato( 82 ) proposed a method to calculate the concen- tration profiles making use of the moire pattern. This method has also been extended to cover two-phase, three-component systems(Hl5). Similar method has been applied by Secor(S?) for polymer diffusion measurements. Of a 11 the methods discussed so far, Sato's method is the easiest to use experimentally. Even 1·li th so many experimental techniques available, diffusion coeff:L cient data are quite scarce·. Only a few systems have been studied at various concentrations(T2) (S2) (Jl:-). Further detailed literature revieVC A - a2cA· at - DA-·~· - kCA CB (4a) -c>x2 'C>CB - ?>2CB - DB~ - kCACB (4b) ot ox~ ?>Cc o2cc {4c) ot- = Dc----"2· ":* kCACB -ox~ oCD :: o2cn kCACB (4d) at" Dn~ + where the subscripts A and B represent the reacting species, the subscripts C and D represent the reaction products, k is the second order reaction rate coefficient, cm3/mole-sec. and x is the distance, em. If the reaction rate is infinitely fast, the zone of reaction is of negligible thickness as illustrated in Figure la. The transfer process becomes diffusion controlled and independent of the chemical reaction (AI+) (Sl2 ). On the other hand, if the reaction :t":ate is slow, the reaction zone tends to spn::ad throughout the solution as shovm in Figure lc. 36 The reaction becomes pseudo-first order ,.n_ th respect to A ( Sl2). Finally, if the reaction rate is finite, then the reaction zone thickness (the region in which non-negligible concentrations of both reacting species exist simultaneously) will also be finite as shovm in Figure lb. Such is the case for the reaction systems in the present study. z 0 >-I 1- FIGURE 1 CONCENTRATION PROFILES FOR TWO CHEMICAL SPECIES REACTING NEAR AN INTERFACE 1/ INTERFACE ~ INTERFACE .II" INTERFACE REACTION ZONE ~ I REACTION ZONE REACTION ZONE - I 1- z I..LJ u z 0 u CB 0 X 0 X 0 I I I X DISTANCE FROM THE INTERFACE la. lb. Diffusion Controlled Finite reaction infinite reaction rate rate 1 c. Pseudo-first order; slow reaction rate - high diffusivity of B \....) ---.J 38 J.l.l Estimation of the Reaction Zone Thickness --~-----~- (Refer to 4.2 - PART B) If the transfer process is a steady state one, equations (4a) and (4b} vdll be reduced to d2cA DA dx2 - kCACB = 0 (5a) (5b) With the assumptions of constant k and negligible thermal effects, equation (5a) is subtracted from (5b) and the resulting equation is integrated twice with the follo'.Aring boundary conditions:(F2) at X =+co. CA = o, dCA = o, dCB ~ dx·~ = ' dx DB X =- 00 e CB - o, dc11 .. o, dCA :: JA - - (IX'"" .. DA ' dx JA = JB = J; (steady state molal fluxes;moles/cm2-sec.) The relationship between the con cent rations of the reacting species is: gives: CB = 1 ( D A CB + JX) DB where, ( 6) X = x - x0 and x 0 is an arbitrary constant of integration. Substitution of (6) into (5a) yields; d2c k J ---,:A ::. -( C 2 + "~''ACAX) dXt:. DB A u ( 7) With the aid of equation (7), the dimensional analysis 39 ( 8) where, yt = X/1 l = (DADB/Jk) 11J Substitution of equation (8) back to (?) gives ( 9) By the transforrra ti on 'fA = ¢A - !>'\., equation ( 9) becomes d2nrA Y\2 YJ - nr2 -1. d~- - y; A - T; ( 10) With the following boundary conditions: at ll. =-co, d¢A ::: -~ dyt \'\ =+co d¢A -I. ' = + ~ dyt Equation (10) was solved by Friedla.nder et al. (F2 ) With the reaction zone defined as the region in which the flux of species A was reduced from 95% of its initial value to 5% of its initial value, they found that the thickness of the reaction zone was 4.4 1. The above theory should also apply to transient diffusion where the stationary-state assumption is valid. 3.1.2 Estimation of the Enhancement Factor 3.1.2.1 Penetration Theory (unsteaqy~~at~} (Refer to 4.2 - PART B) Normalizing equations (4a)and (4b)by changing the variables into dimensionless ones(Bl6}. a b l (kCBo/DA)~x- z results in: -c?a 'C>a 'C>z2" - ie = ab o2b ob -rq-- - q- - ab az2 ae with the following boundary conditions: at e = o , any 'Z > 0 z = 0, any e > 0 z- CX>' any e By defining ¢ ~ k1/k~ where a b a ob az a b - 0 - :: l = l = 0 :: 0 = 1 40 (lla} (llb} k1 = mass transfer coefficient vlith chemical reaction based on the average transfer rate over the contact time interval,cm./sec. average physical nB ss transfer coefficient for transfer lvi thout reaction, cm./sec. enhancement factor of na ss transfer involving an infinitely rapid chemical reaction ¢a :. 1/erf {o-) qff = {1 - erf {o-/Jr) )/ [erf(O'") exp(o-2(1-1k))J Also, by defining M = {11'/4)9 = kDACB0/(k~)2 Brian et al. speculated that: UM) {1 - ( {¢·-l)L{_~) )) ~ ¢ : tar~)(l _ ( {¢-l)/(¢a-l)) }) ~ (12a) { l2b)t (13) 41 They then proceeded to solve equations (lla) and {llb) numerically using the linearized time-centredJ implicit finite difference methods of Crank and Nicholson( C4a). The ¢ values so obtained from the numerical solutions were compared vdth those approximated by equation (13) by Brian et al. Calibration curves were made to adjust the ¢ values obtained by equation (13) to vnthin 3% of the ¢values by the true penetration solution. (For details of the calibration curves, Reference (B16) should be consulted.) t () is defined implicitly by equation { l2b). 3.1.2.2 Film Theory (steady_~~ate) (Refer to 4.3 - PART C) 42 By assuming the concentration of the reactant B from the lower phase (phase II) is constant throughout the pro per reaction zone in the liquid film, Van Krevelen and Hoftijzer(Vl) presented an approxim::t te solution to the film theory. 1 - L(M) (1 - ( -l)L:rgj_l_2 ¢ - tanh ((M) (1 - (¢-1)/rq)) fJ (lh) A more detailed discussion of the film theory solution js available (S8). Molecular Diffusion in Hom.2n;eneous Binary Liquid. Systems (Refer to 4.4 - PART D) 3.2.1 Diffusion Coefficient 43 Consider the diffusion process between tv;o solutions, A and B. For this pro cess, Fick' s law can be written as follmqs: (for one dimensional transfer) NA oc +XA(NA+ :: - DAB ~ NB} ( 15) ox where NA is the molar flux of the substance A NB is the molar flux of the substance B XA is the molar fraction of the substance A x distance in the diffusion direction from the boundary DAB diffusion coefficient betv1een A and Bat the concentration CA. For a constant volume diffusion cell, the net flow of volume through any plane perpendicular to the x direction must be equa 1 to zero. NAVA + NBVB = 0 where and VB = av anB Hence, we have: VA, VB; partial molar volumes of A and B. From (16) we have: (16) 1) If there is no volume change on mixing, we have: VA= VAo v 0. B , NA - - NB and (15) can be written or 2) If V 0 A -f NA - - NA -- - oCA DAB ox VB 0 and there 'OCA DABox + xA(NA DAB oCA \iA ox 1-xA ( 1-=--) VB is volume change on mixing, VA =-N ) VB A Duda et al.(Dl~ have treated this case in detaile However, if xA « 1, vve can V!rite: 'OCA NA - DAB-ox- or Summarily, if there is no volume change on mixing or if the concentrations involved are low, the diffusion process in a solution of A and B can be represented by the following differential equation. oCA 0 'OC A u- = ox 'n AB'"""ix) 44 Boltzmann(BrQ has treated this equation by using a similarity transformation: y = .ft for the following conditions: CA = CA 0 for x < 0 and t = 0 CA = 0 for x > 0 and t = 0 cA= CAo for X= -co and t ~ 0 CA = 0 for x = + co and t ~ 0 45 Free diffusion situation is assumed with this tr·ansformation. Upon substitution one obtains: 1 dCA - d dCp, _.,..y -- - ~- ( D A&d,:r .: ) ~ dy dy J Integrating c -~- CS A y dCA = ,_ Ao (D dCA AB-dy) c A dC - (DAn-A) D dy c Ao Noting that dCA - 0 at CA d.y . • • • If y is replaced back by _JL.. n -- - or ( 17) (18) where x is the distance, in the diffusion direction, from the boundary of the two solutions. defined by the condition (J5): c S Ao x dCA = 0 0 This boundary must be according to the law of conservation of mass. All the material of A found in the region x > 0 must come from the region x < 0. It can be seen by the expression of D, that if the concentration profile (i.e. the curve of CA vs. x) is available, D can easily be found. CHAPTER !± EXPERIMENTATION In this Chapter, the experimental work is presented. Due to the nature of this project, the experiments can best be grouped into four parts; each part is self­ contained. In 4.1 - PART A, descriptions are made of experiments concerning the qualitative and quantitative investigation of the cause and nature of the turbulent layer in various ester­ aqueous caustic systems. Experiments in /+. 2 - PART B deal with the measurement of the concentration distribution inside the turlmlent layer in t1.-.ro ester-aqueous caustic systems with slow chemical reaction. Associa.ted. l.vith the stagnant phase systems, mass transfer experiments were carried out with both phases stirred, as described in 4.3 - PART c. In each experiment in PARTS A, B and C, vli th the except ions othervvise stated, the upper phase vvas presaturated with l.V"ater at 2h ± l°C. In 4.4 - PART D, an improved design of a diffusion cell is described. The cell enables the quick measurement of the diffusion coefficient as a function of concentration in binary liquid systems. 47 It should be realized that the following experimental apparatus and the procedure sections contain only the information necessary to give the reader a basic understanding of the design and function of the experimental equipment and how the experiments were done. Further details may be found in the appropriate appendices. bL:- P A_H'f.~A QUALITATIVE .1\ND QUANTITATIVE INVESTIGATIONS ~-_,.,..,...,~_ .. ,., _____ ~ .. ~ •• -------~, ~- "A>'>=-~~ Experiments Here conducted at 24 t .1 °C vlith the following ester-caustic systems: I Ethyl formate-sodium hydroxide Nith B.D.H. Universal indicator. II Ethyl acetate-sodium hydroxide with alizarin yellow as indicator. III Ethyl acetate - sodium hydroxide with thymolphthalein as indicator. IV Methyl acetate - sodi urn hydroxide w'li th alizarin yellov1 as indicator. 53 4.1.1.2 Results of the Colour Indicator Experiments 4.1.1.2.1 Ethyl formate - soditl_!!l_hydroxide system When the B.D.H. Universal indicator was added to the ethyl formate-sodium hydroxide system, the turbulent layer revealed it self as a green colour band against the yellow upper phase and the dark blue bulk sodium hydroxide. Also, sodium hydroxide movement in the layer could be observed as circulating blue streaks. At the later stage of each run, an additional thin zone was observed vii thin the turbulent layer immediately below the interface. The colour of this zone was also yellow inside which no blue caustic streaks were noted. A typical photograph of the turbulent layer of this system is shmvn in Figure 3. 4.1.1.2.2. Ethyl acetate_- sodium hydroxide syste~ The pH of the turbulent layer in the:~ ethyl acetate- sodium hydroxide system Has previously measured to be approximately 10.2(S8! Under this pH value, the B.D.H. Universal indicator was already dark blue in colour. 54 Consequently, the B.D.H. Universal indicator was not suitable for this particular reaction system. When alizarin yellm'l Nas added to the ethyl acetate­ caustic system, the turbulent layer \vas yellow against the almost colourless ester phase and the red bulk sodium hydroxide phase. The sodium hydroxide insid8 the layer revealed itself as red patches. FIGURE 3 . T _L ETHYL FORMATE- 1.0 N NaOH SYSTEM WITH B.D.H. UNIVERSAL INDICATOR : 106 MINUTES AFTER THE INITIAL PHASE CONTACT 55 ZONE A ZONE B On the other hand, when thymolphthalein was added to the system, the turbulent layer was colourless against the 56 light blue bulk sodium hydroxide phase. The sodium hydroxide inside the layer revealed itself as light blue patches. In both cases, the caustic patches were quite uniformly distributed cross-sectionally and concentrated at the front of the turbulent layer. 4.1.1.2.3. Methyl acetate- sodium hv:.q;roxide system When alizarin yellow was added to methyl acetate­ sodium hydroxide system, the turbulent layer was mainly yellow, and red streaks were seen vlherever sodium hydroxide was present within the reaction layer. The layer was wider than the one in the ethyl acetate-sodium hydroxide system but narrower as compared to that. in the ethyl foruat.e-soclium hydroxide system. At the later stage of the run, an additional zone was again observed within the turbulent layer and located just below the liquid-liquid interface. This zone was yellow in colour and was similar in nature to that observed in the ethyl formate-sodi urn hydroxide system. Discussion of the Colour Indicator Studios O:ft~1'urbulent Layer The mainly green colour of the turbulent zone in the ethyl formate-sodium hydroxide system indicates that the pH of the liquid inside the layer is approximately 7. The saponification rate of the ester-caustic system is fairly rapid (Table 1). The concentration of sodium hydroxide inside the layer is low and the transfer of ethyl formate across the interface is fast. Consequently, the speed of turbulent layer grmvth is high. Results of the thymolphthalein and alizarin yellow 57 indicator in the ethyl acetate-sodium hydroxide experiments and alizarin yellow in the methyl acetate-sodium hydroxide systems (together vv-ith experiments using B.D.H. Universal indicator in both liquid systems) shovr that the average pH value of the liquid in these turbulent layers is higher than that in the ethyl formate-sodium hydroxide system. The turbulent layer growth rate is sloiver and the turbulence inside the layer is less. These qualitative results are as expected since the reaction velocities of the two systems are much 10\r.Jer as compared to that of the ethyl formate-sodium hydroxide system (Table 1). Examination of the colour pictures of the turbulent layer in ethyl formate-sodium hydroxide and methyl acetate- sodium hydroxide systems taken at higher caustic concentration runs (e.g. 1 N) and at longer contact times indicate that the TABLE 1 COMPARISON OF THE PROPERTIES OF VARIOUS ESTERS Methyl Methyl Methyl Formate Acetate Propionate Chemical Formula HCOOCH3 CH3COOCH3 CH3cH2COOCH3 Molecular Weight 60.05 74.08 88.10 Boiling point, <::t: 31.50 57.1 79.9 Melting point, CC -99.0 -98.1 -87.5 20 0.9274(¥} 20 Density, gm/ cc 0.975('"7+} 0.9148(-rr} Solubility, gm/100 ml water 30.4( 20} 31.9(20} 6.5(20} Diffusivity in water at infin- it~ dilution, em /see - - - Reaction constant at 25°C (with NaOH), ... litrel(MOle-min} 2400 -10 5.9 Refractive index at 20°C 1.344 1.35935 1.3779 - ----- Note: Data are obtained from the "Handbook of Chemistry and Physics" and from the "International Critical Tables" unless otherwise stated. Ethyl Formate HCOOC2H5 74.08 54-3 -80.5 0.9236(~} 11.8<25} +t l.lil0-5 1240 1.35975 •Experimentally measured diffusion coefficient (4.4- PlRT D) +Experimentally determined; conductivity method (Reference R6) • •Refer to Figure VIII.l. ++Estimated by Othmer and Thaker method. Ethyl Acetate CH3COOC2H5 88.10 77.15 -83.6 0.901 2.79< 20 } - - 1.3771 I I I \1'1 ()). 59 turbulent layer actually co ns.i sts of two separate zones "A 11 and "B" (Figure 3). Zone "A 11 is a zone of turbulent j.nter- diffusion of the ester from the upper phase. and the alcohol from the la..ver phase. The colour indicator study reveals that the caustic concentration in this zone is extremely small and is essentially equal to zero. Although no circulating colour strea.ks 1ve:ce observed in this zone, interfacial turbulence as a result of the uneven alcohol distribution at the interface does exist. Zone ttB" is a zone of reaction where the concentration of caustic is substantial. A similar structure of the turbulent layer is expected in the ethyl acetate-sodium hydroxide system. The ·two-zone phenomenon may be observed at lcr:.Jer caustic concentration runs and at shorter contact times as compared to the ethyl formate-sodium hydroxide system. Ho•"rever, the turbulent layer developed in ethyl acetate-sodium hydroxide system is quite small, and the presence of the two zones inside the layer is not conspicuous. The observation of the two zones in the tt~rbulent layer is in agreement with most of the proposed models(F2) (S4) for transient systems vli th fairly rapid to moderately slovv reactions. The models state that the reaction zone proper moves a1:1ay from the interface ivith increasin.e; contact time. According to available literature discussion (Refer to