Distillation Troubleshooting

1. Troubleshooting Distillation Simulations 398

1.1 VLE 398

1.2 Chemistry, Process Sequence 402

1.3 Does Your Distillation Simulation Reflect the Real World? 404

1.4 Graphical Techniques to Troubleshoot Simulations 407

1.5 How Good Is Your Efficiency Estimate? 407

1.6 Simulator Hydraulic Predictions: To Trust or Not to Trust 409

2. Where Fractionation Goes Wrong 410

2.1 Insufficient Reflux or Stages; Pinches 410

2.2 No Stripping in Stripper 412

2.3 Unique Features of Multicomponent Distillation 412

2.4 Accumulation and Hiccups 413

2.5 Two Liquid Phases 419

2.6 Azeotropic and Extractive Distillation 421

3. Energy Savings and Thermal Effects 424

3.1 Energy-Saving Designs and Operation 424

3.2 Subcooling: How It Impacts Towers 428

3.3 Superheat: How It Impacts Towers 430

4. Tower Sizing and Material Selection Affect Performance 431

4.1 Undersizing Trays and Downcomers 431

4.2 Oversizing Trays 431

4.3 Tray Details Can Bottleneck Towers 433

4.4 Low Liquid Loads Can Be Troublesome 434

4.5 Special Bubble-Cap Tray Problems 436

4.6 Misting 437

4.7 Undersizing Packings 437

4.8 Systems Where Packings Perform Different from Expectations 437

4.9 Packed Bed Too Long 438

4.10 Packing Supports Can Bottleneck Towers 439

4.11 Packing Hold-downs Are Sometimes Troublesome 440

4.12 Internals Unique to Packed Towers 440

4.13 Empty (Spray) Sections 440

5. Feed Entry Pitfalls in Tray Towers 441

5.1 Does the Feed Enter the Correct Tray? 441

5.2 Feed Pipes Obstructing Downcomer Entrance 441

5.3 Feed Flash Can Choke Downcomers 441

5.4 Subcooled Feeds, Refluxes Are Not Always Trouble Free 442

5.5 Liquid and Unsuitable Distributors Do Not Work with Flashing Feeds 442

5.6 Flashing Feeds Require More Space 443

5.7 Uneven or Restrictive Liquid Split to Multipass Trays at Feeds and Pass Transitions 443

5.8 Oversized Feed Pipes 444

5.9 Plugged Distributor Holes 444

5.10 Low ΔP Trays Require Decent Distribution 445

6. Packed-Tower Liquid Distributors: Number 6 on the Top 10 Malfunctions 446

6.1 Better Quality Distributors Improve Performance 446

6.2 Plugged Distributors Do Not Distribute Well 448

6.3 Overflow in Gravity Distributors: Death to Distribution 451

6.4 Feed Pipe Entry and Predistributor Problems 454

6.5 Poor Flashing Feed Entry Bottleneck Towers 455

6.6 Oversized Weep Holes Generate Undesirable Distribution 456

6.7 Damaged Distributors Do Not Distribute Well 457

6.8 Hole Pattern and Liquid Heads Determine Irrigation Quality 458

6.9 Gravity Distributors Are Meant to Be Level 459

6.10 Hold-Down Can Interfere with Distribution 460

6.11 Liquid Mixing Is Needed in Large-Diameter Distributors 460

6.12 Notched Distributors Have Unique Problems 461

6.13 Others 461

7. Vapor Maldistribution in Trays and Packings 462

7.1 Vapor Feed/Reboiler Return Maldistributes Vapor to Packing Above 462

7.2 Experiences with Vapor Inlet Distribution Baffles 465

7.3 Packing Vapor Maldistribution at Intermediate Feeds and Chimney Trays 465

7.4 Vapor Maldistribution Is Detrimental in Tray Towers 466

8. Tower Base Level and Reboiler Return: Number 2 on the Top 10 Malfunctions 468

8.1 Causes of High Base Level 468

8.2 High Base Level Causes Premature Tower Flood (No Tray/Packing Damage) 470

8.3 High Base Liquid Level Causes Tray/Packing Damage 471

8.4 Impingement by the Reboiler Return Inlet 472

8.5 Undersized Bottom Feed Line 475

8.6 Low Base Liquid Level 475

8.7 Issues with Tower Base Baffles 476

8.8 Vortexing 476

9. Chimney Tray Malfunctions: Part of Number 7 on the Top 10 Malfunctions 477

9.1 Leakage 477

9.2 Problem with Liquid Removal, Downcomers, or Overflows 478

9.3 Thermal Expansion Causing Warping, Out-of-Levelness 479

9.4 Chimneys Impeding Liquid Flow to Outlet 480

9.5 Vapor from Chimneys Interfering with Incoming Liquid 480

9.6 Level Measurement Problems 481

9.7 Coking, Fouling, Freezing 482

9.8 Other Chimney Tray Issues 482

10. Drawoff Malfunctions (Non–Chimney Tray): Part of Number 7 on the Top 10 Malfunctions 484

10.1 Vapor Chokes Liquid Draw Lines 484

10.2 Leak at Draw Tray Starves Draw 486

10.3 Draw Pans and Draw Lines Plug Up 488

10.4 Draw Tray Damage Affects Draw Rates 488

10.5 Undersized Side-Stripper Overhead Lines Restrict Draw Rates 488

10.6 Degassed Draw Pan Liquid Initiates Downcomer Backup Flood 489

10.7 Other Problems with Tower Liquid Draws 489

10.8 Liquid Entrainment in Vapor Side Draws 490

10.9 Reflux Drum Malfunctions 490

11. Tower Assembly Mishaps: Number 5 on the Top 10 Malfunctions 491

11.1 Incorrect Tray Assembly 491

11.2 Downcomer Clearance and Inlet Weir Malinstallation 491

11.3 Flow Passage Obstruction and Internals Misorientation at Tray Tower Feeds and Draws 492

11.4 Leaking Trays and Accumulator Trays 493

11.5 Bolts, Nuts, Clamps 493

11.6 Manways/Hatchways Left Unbolted 493

11.7 Materials of Construction Inferior to Those Specified 494

11.8 Debris Left in Tower or Piping 494

11.9 Packing Assembly Mishaps 495

11.10 Fabrication and Installation Mishaps in Packing Distributors 496

11.11 Parts Not Fitting through Manholes 498

11.12 Auxiliary Heat Exchanger Fabrication and Assembly Mishaps 498

11.13 Auxiliary Piping Assembly Mishaps 498

12. Difficulties during Start-Up, Shutdown, Commissioning, and Abnormal Operation: Number 4 on the Top 10 Malfunctions 499

12.1 Blinding/Unblinding Lines 499

12.2 Backflow 500

12.3 Dead-Pocket Accumulation and Release of Trapped Materials 501

12.4 Purging 501

12.5 Pressuring and Depressuring 502

12.6 Washing 502

12.7 On-Line Washes 504

12.8 Steam and Water Operations 506

12.9 Overheating 506

12.10 Cooling 507

12.11 Overchilling 507

12.12 Water Removal 508

12.12.1 Draining at Low Points 508

12.12.2 Oil Circulation 508

12.12.3 Condensation of Steam Purges 508

12.12.4 Dehydration by Other Procedures 508

12.13 Start-Up and Initial Operation 509

12.13.1 Total-Reflux Operation 509

12.13.2 Adding Components That Smooth Start-Up 509

12.13.3 Siphoning 509

12.13.4 Pressure Control at Start-Up 510

12.14 Confined Space and Manhole Hazards 510

13. Water-Induced Pressure Surges: Part of Number 3 on the Top 10 Malfunctions 512

13.1 Water in Feed and Slop 512

13.2 Accumulated Water in Transfer Line to Tower and in Heater Passes 513

13.3 Water Accumulation in Dead Pockets 513

13.4 Water Pockets in Pump or Spare Pump Lines 514

13.5 Undrained Stripping Steam Lines 515

13.6 Condensed Steam or Refluxed Water Reaching Hot Section 516

13.7 Oil Entering Water-Filled Region 517

14. Explosions, Fires, and Chemical Releases: Number 10 on the Top 10 Malfunctions 518

14.1 Explosions Due to Decomposition Reactions 518

14.2 Explosions Due to Violent Reactions 523

14.3 Explosions and Fires Due to Line Fracture 524

14.4 Explosions Due to Trapped Hydrocarbon or Chemical Release 527

14.5 Explosions Induced by Commissioning Operations 528

14.6 Packing Fires 529

14.7 Fires Due to Opening Tower before Cooling or Combustible Removal 533

14.8 Fires Caused by Backflow 534

14.9 Fires by Other Causes 535

14.10 Chemical Releases by Backflow 536

14.11 Trapped Chemicals Released 536

14.12 Relief, Venting, Draining, Blowdown to Atmosphere 537

15. Undesired Reactions in Towers 539

15.1 Excessive Bottom Temperature/Pressure 539

15.2 Hot Spots 539

15.3 Concentration or Entry of Reactive Chemical 539

15.4 Chemicals from Commissioning 540

15.5 Catalyst Fines, Rust, Tower Materials Promote Reaction 540

15.6 Long Residence Times 541

15.7 Inhibitor Problems 541

15.8 Air Leaks Promote Tower Reactions 542

15.9 Impurity in Product Causes Reaction Downstream 542

16. Foaming 543

16.1 What Causes or Promotes Foaming? 543

16.2 What Are Foams Sensitive To? 546

16.3 Laboratory Tests 547

16.4 Antifoam Injection 548

16.5 System Cleanup Mitigates Foaming 551

16.6 Hardware Changes Can Debottleneck Foaming Towers 555

17. The Tower as a Filter: Part A. Causes of Plugging—Number 1 on the Top 10 Malfunctions 558

17.1 Piping Scale/Corrosion Products 558

17.2 Salting Out/Precipitation 559

17.3 Polymer/Reaction Products 560

17.4 Solids/Entrainment in the Feed 561

17.5 Oil Leak 561

17.6 Poor Shutdown Wash/Flush 562

17.7 Entrainment or Drying at Low Liquid Rates 562

17.8 Others 562

18. The Tower as a Filter: Part B. Locations of Plugging—Number 1 on the Top 10 Malfunctions 563

18.1 Trays 563

18.2 Downcomers 564

18.3 Packings 565

18.4 How Packings and Trays Compare on Plugging Resistance 565

18.5 Limited Zone Only 567

18.6 Draw, Exchanger, and Vent Lines 569

18.7 Feed and Inlet Lines 570

18.8 Instrument Lines 570

19. Coking: Part of Number 1 on Tower Top 10 Malfunctions 571

19.1 Insufficient Wash Flow Rate, Refinery Vacuum Towers 571

19.2 Other Causes, Refinery Vacuum Towers 572

19.3 Slurry Section, FCC Fractionators 573

19.4 Other Refinery Fractionators 574

19.5 Nonrefinery Fractionators 574

20. Leaks 575

20.1 Pump, Compressor 575

20.2 Heat Exchanger 575

20.3 Chemicals to/from Other Equipment 577

20.4 Atmospheric 578

21. Relief and Failure 580

21.1 Relief Requirements 580

21.2 Controls That Affect Relief Requirements and Frequency 580

21.3 Relief Causes Tower Damage, Shifts Deposits 581

21.4 Overpressure Due to Component Entry 581

21.5 Relief Protection Absent or Inadequate 582

21.6 Line Ruptures 583

21.7 All Indication Lost When Instrument Tap Plugged 584

21.8 Trips Not Activating or Incorrectly Set 584

21.9 Pump Failure 585

21.10 Loss of Vacuum 585

21.11 Power Loss 585

22. Tray, Packing, and Tower Damage: Part of Number 3 on the Top 10 Malfunctions 586

22.1 Vacuum 586

22.2 Insufficient Uplift Resistance 587

22.3 Uplift Due to Poor Tightening during Assembly 587

22.4 Uplift Due to Rapid Upward Gas Surge 589

22.5 Valves Popping Out 590

22.6 Downward Force on Trays 590

22.7 Trays below Feed Bent Up, above Bent Down and Vice Versa 591

22.8 Downcomers Compressed, Bowed, Fallen 592

22.9 Uplift of Cartridge Trays 593

22.10 Flow-Induced Vibrations 593

22.11 Compressor Surge 594

22.12 Packing Carryover 595

22.13 Melting, Breakage of Plastic Packing 595

22.14 Damage to Ceramic Packing 595

22.15 Damage to Other Packings 595

23. Reboilers That Did Not Work: Number 9 on the Top 10 Malfunctions 596

23.1 Circulating Thermosiphon Reboilers 596

23.2 Once-Through Thermosiphon Reboilers 597

23.3 Forced-Circulation Reboilers 599

23.4 Kettle Reboilers 599

23.5 Internal Reboilers 602

23.6 Kettle and Thermosiphon Reboilers in Series 603

23.7 Side Reboilers 603

23.8 All Reboilers, Boiling Side 604

23.9 All Reboilers, Condensing Side 605

24. Condensers That Did Not Work 607

24.1 Inerts Blanketing 607

24.2 Inadequate Condensate Removal 608

24.3 Unexpected Condensation Heat Curve 609

24.4 Problems with Condenser Hardware 610

24.5 Maldistribution between Parallel Condensers 611

24.6 Flooding/Entrainment in Partial Condensers 611

24.7 Interaction with Vacuum and Recompression Equipment 612

24.8 Others 612

25. Misleading Measurements: Number 8 on the Top 10 Malfunctions 613

25.1 Incorrect Readings 613

25.2 Meter or Taps Fouled or Plugged 614

25.3 Missing Meter 615

25.4 Incorrect Meter Location 615

25.5 Problems with Meter and Meter Tubing Installation 616

25.6 Incorrect Meter Calibration, Meter Factor 617

25.7 Level Instrument Fooled 617

25.8 Meter Readings Ignored 619

25.9 Electric Storm Causes Signal Failure 619

26. Control System Assembly Difficulties 620

26.1 No Material Balance Control 620

26.2 Controlling Two Temperatures/Compositions Simultaneously Produces Interaction 621

26.3 Problems with the Common Control Schemes, No Side Draws 622

26.4 Problems with Side-Draw Controls 626

27. Where Do Temperature and Composition Controls Go Wrong? 629

27.1 Temperature Control 629

27.2 Pressure-Compensated Temperature Controls 632

27.3 Analyzer Control 633

28. Misbehaved Pressure, Condenser, Reboiler, and Preheater Controls 636

28.1 Pressure Controls by Vapor Flow Variations 636

28.2 Flooded Condenser Pressure Controls 637

28.3 Coolant Throttling Pressure Controls 640

28.4 Pressure Control Signal 641

28.5 Throttling Steam/Vapor to Reboiler or Preheater 641

28.6 Throttling Condensate from Reboiler 642

28.7 Preheater Controls 643

29. Miscellaneous Control Problems 644

29.1 Interaction with the Process 644

29.2 ΔP Control 644

29.3 Flood Controls and Indicators 644

29.4 Batch Distillation Control 645

29.5 Problems in the Control Engineer’s Domain 645

29.6 Advanced Controls Problems 646

References 649

Index 669

About the Author 713