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 xix

Foreword xxi

Foreword for Greg Benz xxiii

1 Purpose of Agitator Design 1

References 2

2 Major Steps in Successful Agitator Design 3

Define Process Results 3

Define Process Conditions 5

Choose Tank Geometry 6

Calculate Equivalent Power/Airflow Combinations for Equal Mass Transfer Rate 7

Choose Minimum Combined Power 7

Choose Shaft Speed; Size Impeller System to Draw Required Gassed Power 7

Decision Point: D/T and Gassing Factors OK? 8

Mechanical Design 8

Decision Point: Is the Mechanical Design Feasible? 8

Repeat to Find Lowest Cost 8

Repeat for Different Aspect Ratios 9

Repeat for Different Process Conditions 9

Finish 9

Summary of Chapter 10

List of Symbols 10

References 10

3 Agitator Fundamentals 11

Agitated Tank Terminology 11

Prime Mover 11

Reducer 13

Shaft Seal 13

Wetted Parts 13

Tank Dimensions 14

How Agitation Parameters Are Calculated 14

Reynolds Number 15

Power Number 16

Pumping Number 17

Dimensionless Blend Time 17

Aeration Number 18

Gassing Factor 18

Nusselt Number 18

Froude Number 19

Prandtl Number 19

Geometric Ratios 20

Baffle Number 20

Dimensionless Hydraulic Force 20

Thrust Number 21

Typical Dimensionless Number Curves 21

A Primer on Rheology 25

Newtonian Model 26

Pseudoplastic

or Shear Thinning, Model (Aka Power Law Fluid) 27

Bingham Plastic 27

Herschel–Bulkley 27

Impeller Apparent Viscosity 29

A Bit of Impeller Physics 29

Summary of Chapter 31

List of Symbols 31

Greek Letters 32

References 32

4 Agitator Behavior under Gassed Conditions 35

Flooding 35

Kla Method 35

Power Draw Method 36

Visual Flow Pattern Method 37

Effect on Power Draw 38

Holdup 39

Example of Holdup Calculation 40

Holdup “War Story” 40

Variable Gas Flow Operation 40

Mechanical Effects 42

Summary of Chapter 42

List of Symbols 42

References 43

5 Impeller Types Used in Fermenters 45

Impeller Flow Patterns 45

Axial Flow 46

Radial Flow 47

Mixed Flow 47

Chaos Flow 48

Examples of Axial Flow Impellers 49

Low Solidity 49

High Solidity 52

Up-pumping vs. Down Pumping 55

Examples of Radial Flow Impellers 56

Straight Blade Impeller 56

Disc, aka Rushton, Turbines 57

Smith Turbines 62

CD-6 Turbine by Chemineer; aka Smith Turbine by Many Manufacturers 62

Deeply Concave Turbines 66

Deep Asymmetric Concave Turbine with Overhang (BT-6) 68

Examples of Mixed Flow Impellers 73

Examples of Chaos Impellers 74

Shear Effects 76

Specialty Impellers 78

Summary of Chapter 80

List of Symbols 80

References 81

6 Impeller Systems 83

Why Do We Need a System? 83

Reaction Engineering 83

Fermenter History 84

Steps to Impeller System Design 85

Choose Number of Impellers 86

Choose Placement of Impellers 86

Choose Type(s) of Impellers 87

Choose Power Split or Distribution Among Impellers 93

Choose D/T and/or Shaft Speed 93

D/T Effects with Variable Gas Flowrates 96

Conclusions on D/T Ratio 98

Design to Minimize Shear Damage 99

Sparger Design 100

Ring Sparger 100

Pre-dispersion 103

Fine Bubble Diffuser 104

Summary of Chapter 105

List of Symbols 106

References 106

7 Piloting for Mass Transfer 109

Why Pilot for Mass Transfer 109

Methods for Determining kla 112

Sulfite Method 112

Dynamic Method; aka Dynamic Gassing/Degassing Method 112

Steady-State Method; aka Mass Balance Method 113

Combined Dynamic and Steady-State Method 114

Equipment Needed for Scalable Data 114

Data Gathering Needs 120

Experimental Protocol 121

Summary of Chapter 128

List of Symbols 128

References 129

8 Power and Gas Flow Design and Optimization 131

What This Chapter Is about 131

Where We Are in Terms of Design 131

Design with no Data 131

Design with Limited Pilot Data 133

Design with Full Data 135

Choose Minimum Combined Power 136

State of Design Completion 141

Additional Considerations 142

Summary of Chapter 142

List of Symbols 142

References 142

9 Optimizing Operation for Minimum Energy Consumption per Batch 145

Purpose of This Chapter 145

Prerequisite 145

Conceptual Overview 145

Detailed Procedure 146

Minimizing Total Energy Usage 150

Practical Design 150

Additional Considerations 150

Summary of Chapter 152

List of Symbols 152

References 153

10 Heat Transfer Surfaces and Calculations 155

Purpose of This Chapter 155

Design Philosophy 155

Overview of the Problem 156

Heat Sources 156

Cooling Sources 157

Heat Exchange Surface Overview 158

Principle of Heat Transfer Calculation 164

Calculations By Type of Surface 166

Vessel Jacket, Agitated Side 166

Simple Unbaffled Jacket, Jacket Side 167

Dimple Jacket, Jacket Side 167

Half-Pipe Coil, Jacket Side 169

Helical Coil, Inside 171

Helical Coil, Process Side 171

Vertical Tube Bundle, Inside 173

Vertical Tube Bundle, Process Side 174

Plate Coil, Inside 175

Plate Coil, Process Side 176

Example Problem: Vertical Tube Bundle 176

Problem Statement 176

Problem Solution 177

Additional Consideration: Effect on Power Draw 182

Additional Consideration: Forces on Heat Exchange Surfaces Used as Baffles 183

Additional Consideration: Wall Viscosity 184

Additional Consideration: Effect of Gas 185

External Heat Exchange Loops 186

Summary of Chapter 187

List of Symbols 187

References 189

Further Readings 189

11 Gasses Other Than Air and Liquids Other Than Water 191

General Principle 191

Comments on Some Specific Gasses 191

Ammonia 191

Carbon Dioxide 192

Carbon Monoxide 192

Hydrogen 192

Methane 192

Oxygen 192

Economic Factors 192

Disposal Factors 193

Effects of Different Gasses on kla 193

Effects of Different Gasses on Driving Force 195

Operating Condition Effects 195

Constraints on Outlet Concentration 196

Safety 196

Liquids Other Than Water 198

Summary of Chapter 198

List of Symbols 198

References 199

12 Viscous Fermentation 201

General Background 201

Sources of Viscosity 201

Viscosity Models for Broths 202

Effect of Viscosity on Power Draw 203

Example Problem 204

Example Problem Answer 204

Effect of Viscosity on kla 205

Effect of Viscosity on Holdup 207

Effect of Viscosity on Blend Time 207

Effect of Viscosity on Flooding 209

Caverns 209

Estimating Cavern Size 211

Xanthan and Gellan Gums 212

Viscosity Models for Gums 213

Installation Survey 214

Effect of D/T and No. and Type of Impellers on Results in Xanthan Gum 217

Production Curve 218

Heat Transfer 218

All-Axial Impeller Design 218

Invisible Draft Tube vs. Axial/Radial Combination 222

Mycelial Broths 223

Typical Viscosity Model 224

Morphology Effects 224

Recommendations 225

Summary of Chapter 227

List of Symbols 227

References 228

13 Three Phase Fermentation 231

General Problem 231

Effect on Mass Transfer 231

Effect on Foam 233

Emulsion vs. Suspension 233

Complexity: How to Optimize Operation 233

Summary of Chapter 234

List of Symbols 234

References 234

14 Use of CFD in Fermenter Design 237

Purpose of This Chapter 237

Basic Theory 237

Methods of Presenting Data 239

Velocity Distribution 240

Cavern Formation 240

Blending Progress 242

Flow Around Coils 245

Bubble Size, kla, Holdup 247

DO Distribution 248

Summary of Chapter 250

List of Symbols 250

References 250

15 Agitator Seal Design Considerations 251

Introduction 251

Terminology 251

Main Functions of Fermenter Shaft Seals 252

Common Types of Shaft Seals 254

Material Considerations 265

Methods of Lubricating Seals 267

Seal Environmental Control and Seal Support System 267

Seal Life Expectations 272

Special Process Considerations 272

Summary of Chapter 275

Reference 275

16 Fermenter Agitator Mounting Methods 277

Introduction 277

Top Entering Methods 277

Direct Nozzle Mount 278

Beam Gear Drive Mount with Auxiliary Packing or Lip Seal; Beams Tied into Vessel Sidewall 281

Beam Gear Drive Mount with Auxiliary Mechanical Seal; Beams Tied into Vessel Sidewall 283

Beam Gear Drive Mount with Auxiliary Mechanical Seal; Beams Tied into Building Structure 284

Complete Drive and Seal Mount to Beams Tied into Vessel Sidewall, with Bellows Connector 285

Complete Drive and Seal Mount to Beams Tied into Building Structure, with Bellows Connector 287

Bottom Entering Methods 287

Direct Nozzle Mount 288

Floor Gear Drive Mount with Auxiliary Packing or Lip Seal 288

Floor Gear Drive Mount with Auxiliary Mechanical Seal 289

Floor Integrated Drive and Seal Mount with Bellows Connector 291

Summary of Chapter 292

References 292

17 Mechanical Design of Fermenter Agitators 293

Introduction 293

Impeller Design Philosophy 294

Discussion on Hydraulic Force 295

Shaft Design Philosophy 297

Shaft Design Based on Stress 298

Simple Example Problem 302

Sample Problem with Steady Bearing 304

Shaft Design Based On Critical Speed 304

Cantilevered Designs 306

Example Problem 308

Units with Steady Bearings 311

Solid Shaft vs. Hollow Shaft 315

Role of FEA in Overall Shaft Design-Simplified Discussion 319

Agitator Gear Drive Selection Concepts 319

Early History 320

Loads Imposed 320

Handle or Isolate Loads? 323

Handle Loads Option 1: Oversized Commercial Gear Drive 323

Handle Loads Option 2: Purpose-Built Agitator Drive 324

Isolate Loads Option 1: Hollow Quill Integrated Drive with Flexibly Coupled Extension Shaft 325

Isolate Loads Option 2: Outboard Support Bearing Module 328

Bearing Life Considerations 329

Noise Considerations 330

Torsional Natural Frequency 332

Important or Useful Mechanical Design Features 332

Summary of Chapter 333

List of Symbols 333

Greek Letters 334

References 334

18 Sanitary Design 335

Introduction 335

Definitions 336

Construction Principles 336

Wetted Parts Construction Methods 336

Welded Construction 336

In-Tank Couplings 338

Mounting Flange Area 341

Axial Impellers 344

Radial Impellers 345

Bolts and Nuts 347

Steady Bearings 348

Use of Castings, 3-D Printing 349

Polishing Methods and Measures1: Polishing vs. Burnishing 350

Polishing Methods and Measures2: Lay 351

Polishing Methods and Measures3: Roughness Average 353

Electropolish 355

Passivating 357

Effect on Mechanical Design 357

Summary of Chapter 357

Additional Sources of Information 358

List of Symbols 358

References 358

19 Aspect Ratio 359

Acknowledgment 359

Definition and Illustration of Aspect Ratio 359

What Is the Optimum Aspect Ratio? 360

Effects of Z/T on Cost and Performance at a Given Working Volume 361

Vessel Cost 361

Agitator Shaft Design Difficulty 361

Power Required for Mass Transfer 361

Agitator Cost 362

Airflow Requirements 362

Compressor Power 362

DO Uniformity 362

Heat Transfer Capability 363

Real Estate/Land Usage Issues 363

Building Codes; Noise 363

Illustrative Problem Number 1 363

Vessel Dimensions 364

Airflow and Power 366

Heat Transfer Data and Assumptions 367

Heat Transfer Results 369

Blend Time, DO Uniformity 371

Capital Cost (Agitator Plus Vessel Only) 372

Other Operating Costs 372

So What Is the Optimum Aspect Ratio for This Problem? 373

Illustrative Problem Number 2 373

Illustrative Problem Number 3 376

Summary of Chapter 380

List of Symbols 381

References 381

20 Vendor Evaluation 383

Product Considerations 383

Gear Drive Ruggedness 384

Design Technology 384

Impeller Selection 384

Shaft Design 385

Company Considerations 385

Reputation with Customers 385

Company Size 386

Years in Business 386

Years Under New Ownership 386

Employee Turnover 387

Vertical Integration 387

R&D Program and Publications 388

Depth of Application Engineering 389

Testing Laboratory 389

ISO Certification (Necessary vs Sufficient) 391

Quality Control Program (Not Lot Sample; 100%) 391

Rep vs Direct Sales (a Good Rep Annoys the Manufacturer) 392

Service Capability 393

Typical Delivery Times and Performance 393

Parts Availability 394

Price (Least Important) 395

Willingness to Work with Consultants 395

Vendor Audit Checklist 396

Use of an Outside Consultant 397

Summary of Chapter 399

List of Symbols 399

References 400

A. Appendix to Chapter 20 400

21 International Practices 401

Introduction 401

North America 401

Vendors 401

Design Practices 402

Selling/Buying Practices 402

Degree of Vertical Integration 403

Role of Design Firms 403

R&D 404

Culture 404

EU 405

Vendors 405

Design Practices 405

Selling/Buying Practices 405

Degree of Vertical Integration 406

Role of Design Firms 406

R&D 406

Culture 407

Japan 407

Vendors 407

Design Practices 407

Selling/Buying Practices 407

Degree of Vertical Integration 408

Role of Design Firms 408

R&D 408

Culture 408

China 409

Vendors 409

Design Practices 409

Selling/Buying Practices 411

Degree of Vertical Integration 412

Role of Design Firms 412

R&D 412

Culture 413

Summary of Chapter 413

Cultural Resources 413

Afterword 415

Index 417