Agitator Design for Gas-Liquid Fermenters and Bioreactors

Gregory T. Benz, P.E., is the President of Benz Technology International, an engineering consulting firm and Chinese business development corporation. He has authored numerous articles, conducted more than 50 seminars, and holds a patent on a nonrotating, nonseal method of mechanically agitating tanks.

• Preface xixForeword xxiForeword for Greg Benz xxiii1 Purpose of Agitator Design 1References 22 Major Steps in Successful Agitator Design 3Define Process Results 3Define Process Conditions 5Choose Tank Geometry 6Calculate Equivalent Power/Airflow Combinations for Equal Mass Transfer Rate 7Choose Minimum Combined Power 7Choose Shaft Speed; Size Impeller System to Draw Required Gassed Power 7Decision Point: D/T and Gassing Factors OK? 8Mechanical Design 8Decision Point: Is the Mechanical Design Feasible? 8Repeat to Find Lowest Cost 8Repeat for Different Aspect Ratios 9Repeat for Different Process Conditions 9Finish 9Summary of Chapter 10List of Symbols 10References 103 Agitator Fundamentals 11Agitated Tank Terminology 11Prime Mover 11Reducer 13Shaft Seal 13Wetted Parts 13Tank Dimensions 14How Agitation Parameters Are Calculated 14Reynolds Number 15Power Number 16Pumping Number 17Dimensionless Blend Time 17Aeration Number 18Gassing Factor 18Nusselt Number 18Froude Number 19Prandtl Number 19Geometric Ratios 20Baffle Number 20Dimensionless Hydraulic Force 20Thrust Number 21Typical Dimensionless Number Curves 21A Primer on Rheology 25Newtonian Model 26Pseudoplasticor Shear Thinning, Model (Aka Power Law Fluid) 27Bingham Plastic 27Herschel–Bulkley 27Impeller Apparent Viscosity 29A Bit of Impeller Physics 29Summary of Chapter 31List of Symbols 31Greek Letters 32References 324 Agitator Behavior under Gassed Conditions 35Flooding 35Kla Method 35Power Draw Method 36Visual Flow Pattern Method 37Effect on Power Draw 38Holdup 39Example of Holdup Calculation 40Holdup “War Story” 40Variable Gas Flow Operation 40Mechanical Effects 42Summary of Chapter 42List of Symbols 42References 435 Impeller Types Used in Fermenters 45Impeller Flow Patterns 45Axial Flow 46Radial Flow 47Mixed Flow 47Chaos Flow 48Examples of Axial Flow Impellers 49Low Solidity 49High Solidity 52Up-pumping vs. Down Pumping 55Examples of Radial Flow Impellers 56Straight Blade Impeller 56Disc, aka Rushton, Turbines 57Smith Turbines 62CD-6 Turbine by Chemineer; aka Smith Turbine by Many Manufacturers 62Deeply Concave Turbines 66Deep Asymmetric Concave Turbine with Overhang (BT-6) 68Examples of Mixed Flow Impellers 73Examples of Chaos Impellers 74Shear Effects 76Specialty Impellers 78Summary of Chapter 80List of Symbols 80References 816 Impeller Systems 83Why Do We Need a System? 83Reaction Engineering 83Fermenter History 84Steps to Impeller System Design 85Choose Number of Impellers 86Choose Placement of Impellers 86Choose Type(s) of Impellers 87Choose Power Split or Distribution Among Impellers 93Choose D/T and/or Shaft Speed 93D/T Effects with Variable Gas Flowrates 96Conclusions on D/T Ratio 98Design to Minimize Shear Damage 99Sparger Design 100Ring Sparger 100Pre-dispersion 103Fine Bubble Diffuser 104Summary of Chapter 105List of Symbols 106References 1067 Piloting for Mass Transfer 109Why Pilot for Mass Transfer 109Methods for Determining kla 112Sulfite Method 112Dynamic Method; aka Dynamic Gassing/Degassing Method 112Steady-State Method; aka Mass Balance Method 113Combined Dynamic and Steady-State Method 114Equipment Needed for Scalable Data 114Data Gathering Needs 120Experimental Protocol 121Summary of Chapter 128List of Symbols 128References 1298 Power and Gas Flow Design and Optimization 131What This Chapter Is about 131Where We Are in Terms of Design 131Design with no Data 131Design with Limited Pilot Data 133Design with Full Data 135Choose Minimum Combined Power 136State of Design Completion 141Additional Considerations 142Summary of Chapter 142List of Symbols 142References 1429 Optimizing Operation for Minimum Energy Consumption per Batch 145Purpose of This Chapter 145Prerequisite 145Conceptual Overview 145Detailed Procedure 146Minimizing Total Energy Usage 150Practical Design 150Additional Considerations 150Summary of Chapter 152List of Symbols 152References 15310 Heat Transfer Surfaces and Calculations 155Purpose of This Chapter 155Design Philosophy 155Overview of the Problem 156Heat Sources 156Cooling Sources 157Heat Exchange Surface Overview 158Principle of Heat Transfer Calculation 164Calculations By Type of Surface 166Vessel Jacket, Agitated Side 166Simple Unbaffled Jacket, Jacket Side 167Dimple Jacket, Jacket Side 167Half-Pipe Coil, Jacket Side 169Helical Coil, Inside 171Helical Coil, Process Side 171Vertical Tube Bundle, Inside 173Vertical Tube Bundle, Process Side 174Plate Coil, Inside 175Plate Coil, Process Side 176Example Problem: Vertical Tube Bundle 176Problem Statement 176Problem Solution 177Additional Consideration: Effect on Power Draw 182Additional Consideration: Forces on Heat Exchange Surfaces Used as Baffles 183Additional Consideration: Wall Viscosity 184Additional Consideration: Effect of Gas 185External Heat Exchange Loops 186Summary of Chapter 187List of Symbols 187References 189Further Readings 18911 Gasses Other Than Air and Liquids Other Than Water 191General Principle 191Comments on Some Specific Gasses 191Ammonia 191Carbon Dioxide 192Carbon Monoxide 192Hydrogen 192Methane 192Oxygen 192Economic Factors 192Disposal Factors 193Effects of Different Gasses on kla 193Effects of Different Gasses on Driving Force 195Operating Condition Effects 195Constraints on Outlet Concentration 196Safety 196Liquids Other Than Water 198Summary of Chapter 198List of Symbols 198References 19912 Viscous Fermentation 201General Background 201Sources of Viscosity 201Viscosity Models for Broths 202Effect of Viscosity on Power Draw 203Example Problem 204Example Problem Answer 204Effect of Viscosity on kla 205Effect of Viscosity on Holdup 207Effect of Viscosity on Blend Time 207Effect of Viscosity on Flooding 209Caverns 209Estimating Cavern Size 211Xanthan and Gellan Gums 212Viscosity Models for Gums 213Installation Survey 214Effect of D/T and No. and Type of Impellers on Results in Xanthan Gum 217Production Curve 218Heat Transfer 218All-Axial Impeller Design 218Invisible Draft Tube vs. Axial/Radial Combination 222Mycelial Broths 223Typical Viscosity Model 224Morphology Effects 224Recommendations 225Summary of Chapter 227List of Symbols 227References 22813 Three Phase Fermentation 231General Problem 231Effect on Mass Transfer 231Effect on Foam 233Emulsion vs. Suspension 233Complexity: How to Optimize Operation 233Summary of Chapter 234List of Symbols 234References 23414 Use of CFD in Fermenter Design 237Purpose of This Chapter 237Basic Theory 237Methods of Presenting Data 239Velocity Distribution 240Cavern Formation 240Blending Progress 242Flow Around Coils 245Bubble Size, kla, Holdup 247DO Distribution 248Summary of Chapter 250List of Symbols 250References 25015 Agitator Seal Design Considerations 251Introduction 251Terminology 251Main Functions of Fermenter Shaft Seals 252Common Types of Shaft Seals 254Material Considerations 265Methods of Lubricating Seals 267Seal Environmental Control and Seal Support System 267Seal Life Expectations 272Special Process Considerations 272Summary of Chapter 275Reference 27516 Fermenter Agitator Mounting Methods 277Introduction 277Top Entering Methods 277Direct Nozzle Mount 278Beam Gear Drive Mount with Auxiliary Packing or Lip Seal; Beams Tied into Vessel Sidewall 281Beam Gear Drive Mount with Auxiliary Mechanical Seal; Beams Tied into Vessel Sidewall 283Beam Gear Drive Mount with Auxiliary Mechanical Seal; Beams Tied into Building Structure 284Complete Drive and Seal Mount to Beams Tied into Vessel Sidewall, with Bellows Connector 285Complete Drive and Seal Mount to Beams Tied into Building Structure, with Bellows Connector 287Bottom Entering Methods 287Direct Nozzle Mount 288Floor Gear Drive Mount with Auxiliary Packing or Lip Seal 288Floor Gear Drive Mount with Auxiliary Mechanical Seal 289Floor Integrated Drive and Seal Mount with Bellows Connector 291Summary of Chapter 292References 29217 Mechanical Design of Fermenter Agitators 293Introduction 293Impeller Design Philosophy 294Discussion on Hydraulic Force 295Shaft Design Philosophy 297Shaft Design Based on Stress 298Simple Example Problem 302Sample Problem with Steady Bearing 304Shaft Design Based On Critical Speed 304Cantilevered Designs 306Example Problem 308Units with Steady Bearings 311Solid Shaft vs. Hollow Shaft 315Role of FEA in Overall Shaft Design-Simplified Discussion 319Agitator Gear Drive Selection Concepts 319Early History 320Loads Imposed 320Handle or Isolate Loads? 323Handle Loads Option 1: Oversized Commercial Gear Drive 323Handle Loads Option 2: Purpose-Built Agitator Drive 324Isolate Loads Option 1: Hollow Quill Integrated Drive with Flexibly Coupled Extension Shaft 325Isolate Loads Option 2: Outboard Support Bearing Module 328Bearing Life Considerations 329Noise Considerations 330Torsional Natural Frequency 332Important or Useful Mechanical Design Features 332Summary of Chapter 333List of Symbols 333Greek Letters 334References 33418 Sanitary Design 335Introduction 335Definitions 336Construction Principles 336Wetted Parts Construction Methods 336Welded Construction 336In-Tank Couplings 338Mounting Flange Area 341Axial Impellers 344Radial Impellers 345Bolts and Nuts 347Steady Bearings 348Use of Castings, 3-D Printing 349Polishing Methods and Measures1: Polishing vs. Burnishing 350Polishing Methods and Measures2: Lay 351Polishing Methods and Measures3: Roughness Average 353Electropolish 355Passivating 357Effect on Mechanical Design 357Summary of Chapter 357Additional Sources of Information 358List of Symbols 358References 35819 Aspect Ratio 359Acknowledgment 359Definition and Illustration of Aspect Ratio 359What Is the Optimum Aspect Ratio? 360Effects of Z/T on Cost and Performance at a Given Working Volume 361Vessel Cost 361Agitator Shaft Design Difficulty 361Power Required for Mass Transfer 361Agitator Cost 362Airflow Requirements 362Compressor Power 362DO Uniformity 362Heat Transfer Capability 363Real Estate/Land Usage Issues 363Building Codes; Noise 363Illustrative Problem Number 1 363Vessel Dimensions 364Airflow and Power 366Heat Transfer Data and Assumptions 367Heat Transfer Results 369Blend Time, DO Uniformity 371Capital Cost (Agitator Plus Vessel Only) 372Other Operating Costs 372So What Is the Optimum Aspect Ratio for This Problem? 373Illustrative Problem Number 2 373Illustrative Problem Number 3 376Summary of Chapter 380List of Symbols 381References 38120 Vendor Evaluation 383Product Considerations 383Gear Drive Ruggedness 384Design Technology 384Impeller Selection 384Shaft Design 385Company Considerations 385Reputation with Customers 385Company Size 386Years in Business 386Years Under New Ownership 386Employee Turnover 387Vertical Integration 387R&D Program and Publications 388Depth of Application Engineering 389Testing Laboratory 389ISO Certification (Necessary vs Sufficient) 391Quality Control Program (Not Lot Sample; 100%) 391Rep vs Direct Sales (a Good Rep Annoys the Manufacturer) 392Service Capability 393Typical Delivery Times and Performance 393Parts Availability 394Price (Least Important) 395Willingness to Work with Consultants 395Vendor Audit Checklist 396Use of an Outside Consultant 397Summary of Chapter 399List of Symbols 399References 400A. Appendix to Chapter 20 40021 International Practices 401Introduction 401North America 401Vendors 401Design Practices 402Selling/Buying Practices 402Degree of Vertical Integration 403Role of Design Firms 403R&D 404Culture 404EU 405Vendors 405Design Practices 405Selling/Buying Practices 405Degree of Vertical Integration 406Role of Design Firms 406R&D 406Culture 407Japan 407Vendors 407Design Practices 407Selling/Buying Practices 407Degree of Vertical Integration 408Role of Design Firms 408R&D 408Culture 408China 409Vendors 409Design Practices 409Selling/Buying Practices 411Degree of Vertical Integration 412Role of Design Firms 412R&D 412Culture 413Summary of Chapter 413Cultural Resources 413Afterword 415Index 417