Distillation Troubleshooting E-Book

Henry Z. Kister

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 749

Veröffentlichungsjahr: 2011

Beschreibung

THE FIRST BOOK OF ITS KIND ON DISTILLATION TECHNOLOGY The last half-century of research on distillation has tremendously improved our understanding and design of industrial distillation equipment and systems. High-speed computers have taken over the design, control, and operation of towers. Invention and innovation in tower internals have greatly enhanced tower capacity and efficiency. With all these advances, one would expect the failure rate in distillation towers to be on the decline. In fact, the opposite is the case: the tower failure rate is on the rise and accelerating. Distillation Troubleshooting collects invaluable hands-on experiences acquired in dealing with distillation and absorption malfunctions, making them readily accessible for those engaged in solving today's problems and avoiding tomorrow's. The first book of its kind on the distillation industry, the practical lessons it offers are a must for those seeking the elusive path to trouble-free distillation. Distillation Troubleshooting covers over 1,200 case histories of problems, diagnoses, solutions, and key lessons. Coverage includes: * Successful and unsuccessful struggles with plugging, fouling, and coking * Histories and prevention of tray, packing, and internals damage * Lessons taught by incidents and accidents during shutdowns, commissioning, and abnormal operation * Troubleshooting distillation simulations to match the real world * Making packing liquid distributors work * Plant bottlenecks from intermediate draws, chimney trays, and feed points * Histories of and key lessons from explosions and fires in distillation towers * Prevention of flaws that impair reboiler and condenser performance * Destabilization of tower control systems and how to correct it * Discoveries from shutdown inspections * Suppression of foam and accumulation incidents A unique resource for improving the foremost industrial separation process, Distillation Troubleshooting transforms decades of hands-on experiences into a handy reference for professionals and students involved in the operation, design, study, improvement, and management of large-scale distillation.

Leseprobe

Contents

Cover

Half Title page

Title page

Disclaimer

Copyright page

Dedication

Preface

Acknowledgements

How to Use this Book

Abbreviations

Chapter 1: Troubleshooting Distillation Simulations

Case Study 1.1 Methanol in C3 Splitter Overhead?

Case Study 1.2 Water in Debutanizer: Quo Vadis?

Case Study 1.3 Beware of High Hydrocarbon Volatilities in Wastewater Systems

Case Study 1.4 A Hydrocarbon VLLE Method Used for Aqueous Feed Equilibrium

Case Study 1.5 Ternary Mixture Using Binary Interaction Parameters

Case Study 1.6 Very Low Concentrations Require Extra Care in VLE Selection

Case Study 1.7 Diagrams Troubleshoot Acetic Acid Dehydration Simulation

Case Study 1.8 Everything Vaporized in A Crude Vacuum Tower Simulation

Case Study 1.9 Crude Vacuum Tower Simulation Underestimates Residue Yield

Case Study 1.10 Misled by Analysis

Case Study 1.11 Incorrect Feed Characterization Leads to Impossible Product Specifications

Case Study 1.12 Can You Name the Key Components?

Case Study 1.13 Local Equilibrium for Condensers in Series

Case Study 1.14 Simulator Hydraulic Predictions: To Trust or Not to Trust?

Case Study 1.15 Packing Hydraulic Predictions: To Trust or not to Trust

Case Study 1.16 Do Good Correlations Make the Simulation Hydraulic Calculations Reliable?

Chapter 2: Where Fractionation Goes Wrong

Case Study 2.1 No Reflux, No Separation

Case Study 2.2 Heavier Feedstock Impedes Stripping

Case Study 2.3 Poor H2S Removal from Naphtha Hydrotreater Stripper

Case Study 2.4 Heavies Accumulation Interrupts Boil-Up

Case Study 2.5 Interreboiler Drives Tower to A Pinch

Case Study 2.6 Temperature Multiplicity in Multicomponent Distillation

Case Study 2.7 Composition Profiles are Key To Multicomponent Distillation

Case Study 2.8 Composition Profile Plot Troubleshoots Multicomponent Separation

Case Study 2.9 Water Accumulation Causes Corrosion in Chlorinated Hyrocarbon Tower

Case Study 2.10 Hiccups in A Reboiled Deethaimizer Absorber

Case Study 2.11 Water Accumulation in Reboiled Deethanizer Absorber

Case Study 2.12 Water Accumulation and Hiccups in A Refluxed Gas Plant Deethanizer

Case Study 2.13 Hiccups in A Coker Debutanizer

Case Study 2.14 Hiccups in A Solvent Recovery Column

Case Study 2.15 Three-Phase Distillation Calculations and Trapped Components

Case Study 2.16 Hiccups in an Ammonia Stripper

Case Study 2.17 Excess Preheat Leads to Hiccups

Case Study 2.18 Recycling Causes Water Trapping

Case Study 2.19 Impurity Buildup in Ethanol Tower

Case Study 2.20 Interreboiler Induces Stubborn Hydrates in A C2 Splitter

Case Study 2.21 Siphoning in Decanter Outlet Pipes

Case Study 2.22 Hiccups in Azeotropic Distillation Tower

Case Study 2.23 Hiccups in An Extractive Distillation Tower

Chapter 3: Energy Savings and Thermal Effects

Case Study 3.1 Excess Preheat Bottleneck Capacity

Case Study 3.2 A Column Revamp that Taught Several Lessons

Case Study 3.3 Bypassing A Feed Around the Tower

Case Study 3.4 Heat Integration Spin

Case Study 3.5 Change in Cut Point Floods Tower

Case Study 3.6 Simulation Diagnoses Heat Removal Bottleneck

Case Study 3.7 Remember the Heat Balance

Chapter 4: Tower Sizing and Material Selection Affect Performance

Case Study 4.1 Extremely Small Downcomers Induce Premature Flood

Case Study 4.2 Extremely Small Downcomers Flood Prematurely

Case Study 4.3 Dumping Leads to Fluctuations in A Depropanizer

Case Study 4.4 Low Depropanizer Feed Capacity

Case Study 4.5 Minor Tray Design Changes Eliminate Capacity Bottleneck

Case Study 4.6 Establishing Downcomer Seal Can be Difficult

Case Study 4.7 A Troublesome Process Water Stripper

Case Study 4.8 Does Your Distillation Simulation Reflect the Real World?

Case Study 4.9 Flood Testing of A Packed Vacuum Tower

Case Study 4.10 In Special Applications, Spray Towers Do Better Than Packings

Chapter 5: Feed Entry Pitfalls in Tray Towers

Case Study 5.1 Flashing Feed Generates A 12-Year Bottleneck

Case Study 5.2 Flashing Feed Entry Can Make Or Break A Tower

Case Study 5.3 Flashing Feed Piping Bottlenecks Demethanizer

Case Study 5.4 Flashing Feed Entry Can Bottleneck A Tower

Case Study 5.5 A Good Turn Eliminates Hydraulic Hammer

Case Study 5.6 Distribution Key To Good Shed Deck Heat Transfer

Chapter 6: Packed-Tower Liquid Distributors: Number 6 On The Top 10 Malfunctions

Case Study 6.1 Maldistribution Can Originate From A Multitude of Sources

Case Study 6.2 Improved Distribution and Pumparounds Cut Emissions

Case Study 6.3 Keeping Solids out of Packing Distributors

Case Study 6.4 Plugged Distributors

Case Study 6.5 Distributor Overflows

Case Study 6.6 A Hatless Vapor Riser Prevents Proper Scrubbing

Case Study 6.7 Feed Pipes Need Proper Changes When Replacing Trays by Packings

Case Study 6.8 Slug Flow in A Debutanizer Feed Pipe

Case Study 6.9 Slug Flow in Feed Pipe

Case Study 6.10 Collector Drip Bypasses Distributor

Case Study 6.11 How Not To Modify A Liquid Distributor

Case Study 6.12 Tracer Analysis Leads to A Hole in A Distributor

Case Study 6.13 Tilted Distributors Give Poor Irrigation

Chapter 7: Vapor Maldistribution in Trays and Packings

Case Study 7.1 Overflowing Vapor Distributor Causes Packing Flood

Case Study 7.2 Vapor Cross-Flow Channeling

Case Study 7.3 Center Downcomer Obstructs Bottom Feed

Case Study 7.4 Channeling Initiating at A Chimney Tray

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

Case Study 8.1 Base Liquid Level Can Make or Break A Fractionator

Case Study 8.2 High-Liquid-Level Damage

Case Study 8.3 Event Timing Analysis Diagnoses High-Liquid-Level Damage

Case Study 8.4 Can Improved Level Monitoring Avoid High-Level Damage?

Case Study 8.5 High-Base-Level Damage Incidents

Case Study 8.6 Reboiler Return Impingement on Liquid Level Destabilizes Tower

Case Study 8.7 Insufficient Surge Causes Instability

Case Study 8.8 Baffling Baffles

Case Study 8.9 A 7-Ft Vortex

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

Case Study 9.1 Heat Balances Can Identify Total Draw Leaks

Case Study 9.2 Another Leaking Total-Draw Chimney Tray

Case Study 9.3 Chimney Tray Overflow Tarnishes Successful Revamp

Case Study 9.4 Leaking Chimney Tray Upsets FCC Fractionator Heat Balance

Case Study 9.5 Flat Hats Can Induce Leaks

Case Study 9.6 Hydraulic Gradient on A Chimney Tray

Case Study 9.7 “Leak-Proof” Chimney Trays in An FCC Main Fractionator

Case Study 9.8 Liquid-Level Measurement on A Chimney Tray

Case Study 9.9 A Chimney Tray Bottlenecking FCC Main Fractionator

Chapter 10: Draw-Off Malfunctions (Non-Chimney Tray) Part of Number 7 on the Top 10 Malfunctions

Case Study 10.1 Choking of Downcomer Trap-Out Line

Case Study 10.2 Fractionator Draw Instability

Case Study 10.3 A Nonleaking Draw Tray

Case Study 10.4 Leak Tests Are Key To Product Recovery

Case Study 10.5 Downcomer Unsealing At Draw Pan

Case Study 10.6 Liquid Entrainment in Vapor Draw

Case Study 10.7 Weep Into A Vapor Side Draw

Case Study 10.8 Aeration Destabilizes Reflux Flow

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

Case Study 11.1 Should Valve Floats be Removed Before Blanking?

Case Study 11.2 Directional Valve Installation

Case Study 11.3 Can Picket Fence Weirs Cause Early Flooding?

Case Study 11.4 Inspecting Seal Pans is A Must

Case Study 11.5 A Good Simulation Leads to Open Manways

Case Study 11.6 Lube Oil Vacuum Tower Problem

Case Study 11.7 Debris in Liquid Distributor Causes Entrainment

Case Study 11.8 Poor Random Packing Installation Loses Capacity, Fractionation

Case Study 11.9 Coming to Grips With Random Packing Handling

Case Study 11.10 Structured Packing Installation

Case Study 11.11 Correct Feed into Parting Boxes

Case Study 11.12 Inverted Chimney Hats

Case Study 11.13 Problems with Fabrication and Installation of Packing Liquid Distributors

Case Study 11.14 One Heat Exchanger Causing Problems in Two Towers

Case Study 11.15 Liquid Leg in Vent Line Leads to Tower Upset

Case Study 11.16 Is Your Cooling Water Flowing Backward?

Chapter 12: Difficulties During Start-Up, Shutdown, Commissioning, and Abnormal Operation: Number 4 on the Top 10 Malfunctions

Case Study 12.1 Commissioning of Lean-Oil Still Reboiler

Case Study 12.2 Reverse Flow Leads to Corrosion and Flooding

Case Study 12.3 Caustic Wash Can Dissolve Deposits

Case Study 12.4 On-Line Wash Overcomes Salt Plugging

Case Study 12.5 Simulation Identifies Draw Pan Damage

Case Study 12.6 Unique Control Problem in Total-Reflux Start-Ups

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

Case Study 13.1 Side-stripper Pressure Surge Can Damage Main Fractionator

Case Study 13.2 Damage Due to Water Entry into Hot Towers

Case Study 13.3 Interface Control Leads to Pressure Surge in Quench Tower

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

Case Study 14.1 Preventing Structured Packing Fires

Case Study 14.2 Preventing Structured Packing Fires

Case Study 14.3 Other Packing Fire Experiences

Chapter 15: Undesired Reactions in Towers

Case Study 15.1 Lowering Bottom Temperature Can Stop Reaction

Case Study 15.2 Reaction, Azeotroping, Accumulation, and Foaming

Case Study 15.3 Do Not Prejudge the Desirability of A Reaction

Chapter 16: Foaming

Case Study 16.1 Conclusive Test For Foaming

Case Study 16.2 Poor Operation of Amine Absorber

Case Study 16.3 Too Much Antifoam is Worse Than Too Little

Case Study 16.4 Static Mixer Helps Antifoam Injection

Case Study 16.5 Gamma Scans Diagnose Foaming

Case Study 16.6 Low Downcomer Velocities are Critical For Foaming Systems

Case Study 16.7 Enlarged Downcomer Clearances Mitigate Foaming

Case Study 16.8 Hardware Changes Debottleneck Foaming

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

Case Study 17.1 Packed-Bed Damage

Case Study 17.2 Fouling of Wire-Mesh Structured Packings

Chapter 18: the Tower as A Filter: Part B. Location of Plugging—Number 1 on the Top 10 Malfunctions

Case Study 18.1 Valve Trays in Sticky Chemicals Service At High Rates

Case Study 18.2 Fouling Behind Interrupter Bars and Inlet Weirs

Case Study 18.3 Effect of Tray Hole Size on Fouling

Case Study 18.4 Valve Sticking: Numerous Experiences

Case Study 18.5 Plugging Incident: Trays Versus Structured Packings

Case Study 18.6 Plugging Incident: Packing Versus Packing

Case Study 18.7 Plugging in A Packed-Tower Gas Inlet

Case Study 18.8 Overcoming Top-Tray Plugging in A Crude Fractionator

Case Study 18.9 Partially Plugged Kettle Draw Does Not Impair Tower Operation

Chapter 19: Coking: Number 1 on the Top 10 Malfunctions

Case Study 19.1 Coking in A Tall, Efficient Wash Zone

Case Study 19.2 Too Many Stages Lead to Wash Bed Coking

Case Study 19.3 Vacuum Tower Coking

Case Study 19.4 Coking of Grid in Fcc Main Fractionators

Case Study 19.5 Coking of Baffle Trays

Chapter 20: Leaks

Case Study 20.1 Tracers Diagnose Leaking Reboiler

Case Study 20.2 Preheater Leak Identified From A Simple Field Test

Case Study 20.3 Several Leaks in One Heat Exchange System

Case Study 20.4 Bottom Leak Disrupts Flow in Upper Pumparound

Chapter 21: Relief and Failure

Case Study 21.1 Atmospheric Crude Tower Relief to Atmosphere and Overpressure

Case Study 21.2 Relief Action Causes Tray Damage

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

Case Study 22.1 Short Tray Holddown Clips Unable to Resist A Pressure Surge

Case Study 22.2 Uplifting of Poorly Fastened Trays

Case Study 22.3 Packing Collapse Due to Quenching and Rapid Boiling

Case Study 22.4 Rapid Pressure Fall At Start-Up

Case Study 22.5 Tray Uplift During Compressor Start-Up

Case Study 22.6 Internal Damage During Hook-Up of Vacuum Equipment

Case Study 22.7 Valve Pop-Out: Numerous Experiences

Case Study 22.8 Vapor Gap Damage

Case Study 22.9 Loss of Vacuum Damages Trays

Case Study 22.10 Fouling and Damage in An Extractive Distillation Aldehyde Column

Case Study 22.11 Tray Damage By Gas Lifting of Reflux Drum Liquid

Case Study 22.12 Tray Damage as A Result of Steamout Followed By A Water Wash

Case Study 22.13 Rapid Condensing At Feed Zone Damages Trays

Case Study 22.14 Preventing Water Stripper Damage

Case Study 22.15 Preventing Another Water Stripper Damage

Case Study 22.16 Betraying Mitigates Flow-Induced Vibrations

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

Case Study 23.1 Reboiler Surging

Case Study 23.2 Separation of Two Liquid Phases in A Reboiler

Case Study 23.3 Leaking Draw Tray Makes Once-Through Reboiler Start-Up Difficult

Case Study 23.4 Liquid-starved Once-Through Reboiler

Case Study 23.5 Surging in A Extractive Distillation Reboiler System

Case Study 23.6 Reboiler Feed Blockage

Case Study 23.7 Thermosiphon That Would Not Thermosiphon

Case Study 23.8 Establishing Thermosiphon Action in A Demethanizer Reboiler

Case Study 23.9 Film Boiling

Case Study 23.10 Loss of Condensate Seal in A Demethanizer Reboiler

Case Study 23.11 Preventing Loss of Condensate Seal

Case Study 23.12 Inability to Remove Condensate From Reboiler

Chapter 24: Condensers That Did Not Work

Case Study 24.1 Pressure and Level Surging

Case Study 24.2 Inadequate Condensate Removal

Case Study 24.3 Noncondensables Can Bottleneck Condensers and Towers

Case Study 24.4 Entrainment From C3 Splitter Knockback Condenser

Case Study 24.5 Experience with A Knockback Condenser with Cooling-Water Throttling

Chapter 25: Misleading Measurements: Number 8 on the Top 10 Malfunctions

Case Study 25.1 Poor Steam Ejector Performance Or Column Vacuum Measurement Issue?

Case Study 25.2 Incorrect Readings Can Induce Unnecessary Shutdowns

Case Study 25.3 Can Lying Pressure Transmitters Bottleneck Tower Capacity?

Case Study 25.4 Missing Baffle Affects Level Transmitter

Case Study 25.5 Bottom-Level Transmitter Fooled By Froth

Case Study 25.6 Bottom-Level Transmitter Fooled By Light Liquid

Chapter 26: Control System Assembly Difficulties

Case Study 26.1 C2 Splitter Composition Controls

Case Study 26.2 Controlling Temperature At Both Ends of A Lean-Oil Still

Case Study 26.3 Inverse Response

Case Study 26.4 Inverse Response with No Reflux Drum

Case Study 26.5 Reboiler Swell

Case Study 26.6 Base Baffle Interacts with Heat Input Control

Case Study 26.7 Good Reflux Control Minimizes Crude Tower Overflash

Case Study 26.8 Vapor Sidedraw Control

Chapter 27: Where Do Temperature and Composition Controls Go Wrong?

Case Study 27.1 Amine Regenerator Temperature Control

Case Study 27.2 Composition Control From the Next Tower

Chapter 28: Misbehaved Pressure, Condenser, Reboiler, and Preheater Controls

Case Study 28.1 Liquid Leg Interferes with Pressure Control

Case Study 28.2 Pressure/Accumulator Level Controls Interference

Case Study 28.3 Equalizing Line Makes Or Breaks Flooded Condenser Control

Case Study 28.4 Inerts in Flooded Reflux Drum

Case Study 28.5 Poor Hookup of Hot-Vapor Bypass Pipes

Case Study 28.6 Pressure Control Valve in the Vapor Line to the Condenser

Case Study 28.7 Can Condenser Fouling By Cooling-Water Throttling Be Beneficial?

Case Study 28.8 Control to Prevent Freezing in Condensers

Case Study 28.9 Valve in Reboiler Steam Induces Oscillations During Start-Up

Case Study 28.10 Condensate Drums Eliminate Reboiler Start-Up Oscillations

Chapter 29: Miscellaneous Control Problems

Case Study 29.1 Natural Flooding Or Hydrates in A C2 Splitter?

Distillation Troubleshooting Database of Published Case Histories

References

Index

About the Author

Distillation Troubleshooting

DISCLAIMER

The author and contributors to “Distillation Troubleshooting” do not represent, warrant, or otherwise guarantee, expressly or impliedly, that following the ideas, information, and recommendations outlined in this book will improve tower design, operation, downtime, troubleshooting, or the suitability, accuracy, reliability or completeness of the information or case histories contained herein. The users of the ideas, the information, and the recommendations contained in this book apply them at their own election and at their own risk. The author and the contributors to this book each expressly disclaims liability for any loss, damage or injury suffered or incurred as a result of or related to anyone using or relying on any of the ideas or recommendations in this book. The information and recommended practices included in this book are not intended to replace individual company standards or sound judgment in any circumstances. The information and recommendations in this book are offered as lessons from the past to be considered for the development of individual company standards and procedures.

Copyright ©2006 by John Wiley & Sons, Inc. All rights reserved.

Published by John Wiley & Sons, Inc., Hoboken, New Jersey.Published simultaneously in Canada.

No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400, fax [phone], or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, [phone], fax [phone], or online at www.wiley.com/go/permission.

Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.

For general information on our other products and services please contact our Customer Care Department within the U.S. at 800-762-2974, outside the U.S. at 317-572-3993 or fax [phone].

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print, may not be available in electronic format. For more information about Wiley products, visit out web site at www.wiley.com.

Library of Congress Cataloging-in-Publication Data:

Kister, Henry Z.Distillation troubleshooting / Henry Z. Kister.p. cm.Includes bibliographical references.ISBN-13 978-0-471-46744-1 (Cloth)ISBN-10 0-471-46744-8 (Cloth)1. Distillation apparatus—Maintenance and repair. I. Title.TP159.D9K57 2005660’.28425—dc222004016490

To my son, Abraham and my wife, Susana, who have been my love, inspiration, and the lighthouses illuminating my path,

and to my life-long mentor, Dr. Walter Stupin — it is easy to rise when carried on the shoulders of giants.

Preface

“To every problem, there’s always an easy solution—neat, plausible, and wrong.”

—Mencken’s Maxim

The last half-century has seen tremendous progress in distillation technology. The introduction of high-speed computers revolutionized the design, control, and operation of distillation towers. Invention and innovation in tower internals enhanced tower capacity and efficiency beyond previously conceived limits. Gamma scans and laser-guided pyrometers have provided troubleshooters with tools of which, not-so-long-ago, they would only dream. With all these advances, one would expect the failure rate in distillation towers to be on the decline, maybe heading towards extinction as we enter the 21st century. Our recent survey of distillation failures (255) brought disappointing news: Distillation failures are not on the path to extinction. Instead, the tower failure rate is on the rise and accelerating.

Our survey further showed that the rise is not because distillation is moving into new, unchartered frontiers. By far, the bulk of the failures have been repetitions of previous ones. In some cases, the literature describes 10–20 repetitions of the same failure. And for every case that is reported, there are tens, maybe hundreds, that are not.

In the late 1980s, I increased tray hole areas in one distillation tower in an attempt to gain capacity. Due to vapor cross flow channeling, a mechanism unknown at the time, the debottleneck went sour and we lost 5% capacity. Half a year of extensive troubleshooting, gamma scans, and tests taught us what went wrong and how to regain the lost capacity. We published extensively on the phenomenon and how to avoid. A decade later, I returned to investigate why another debottleneck (this time by others) went sour at the same unit. The tower I previously struggled with was replaced by a larger one, but the next tower in the sequence (almost the same hydraulics as the first) was debottlenecked… by increasing tray hole areas!

It dawned on me how short a memory the process industries have. People move on, the lessons get forgotten, and the same mistakes are repeated. It took only one decade to forget. Indeed, people moved on: only one person (beside me) that experienced the 1980s debottleneck was involved in the 1990s efforts. This person actually questioned the debottleneck proposal, but was overruled by those who did not believe it will happen again.

Likewise, many experiences are repeatedly reported in the literature. Over the last two decades, there has been about one published case history per year of a tower flooding prematurely due to liquid level rising above the reboiler return nozzle, or of a kettle reboiler bottleneck due to an incorrectly compiled force balance. One would think that had we learned from the first case, all the repetitions could have been avoided. And again, for every case that is reported, there are tens, maybe hundreds that are not.

Why are we failing to learn from past lessons? Mergers and cost-cuts have retired many of the experienced troubleshooters and thinly spread the others. The literature offers little to bridge the experience gap. In the era of information explosion, databases, and computerized searches, finding the appropriate information in due time has become like finding a needle in an evergrowing haystack. To locate a useful reference, one needs to click away a huge volume of wayward leads. Further, cost-cutting measures led to library closures and to curtailed circulation and availability of some prime sources of information, such as, AIChE meeting papers.

The purpose of this book is pick the needles out of the haystack. The book collects lessons from past experiences and puts them in the hands of troubleshooters in a usable form. The book is made up of two parts: the first is a collection of “war stories,” with the detailed problems and solutions. The second part is a database mega-table which presents summaries of all the “war stories” I managed to find in the literature. The summaries include some key distillation-related morals. For each of these, the literature reference is described fully, so readers can seek more details. Many of the case histories could be described under more than one heading, so extensive cross references have been included.

If an incident that happened in your plant is described, you may notice that some details could have changed. Sometimes, this was done to make it more difficult for people to tell where the incident occurred. At other times, this was done to simplify the story without affecting the key lessons. Sometimes, the incident was written up several years after it occurred, and memories of some details faded away. Sometimes, and this is the most likely reason, the case history did not happen in your plant at all. Another plant had a similar incident.

The case histories and lessons drawn are described to the best of my and the contributors’ knowledge and in good faith, but do not always correctly reflect the problems and solutions. Many times I thought I knew the answer, possibly even solved the problem, only to be humbled by new light or another experience later. The experiences and lessons in the book are not meant to be followed blindly. They are meant to be taken as stories told in good faith, and to the best of knowledge and understanding of the author or contributor. We welcome any comments that either affirm or challenge our perception and understanding.

If you picked the book, you expressed interest in learning from past experiences. This learning is an essential major step along the path traveled by a good troubleshooter or designer. Should you select this path, be prepared for many sleepless nights in the plant, endless worries as to whether you have the right answer, tests that will shatter your favorite theories, and many humbling experiences. Yet, you will share the glory when your fix or design solves a problem where others failed. You will enjoy harnessing the forces of nature into a beneficial purpose. Last but not least, you will experience the electric excitement of the “moments of insight,” when all the facts you have been struggling with for months suddenly fall together into a simple explanation. I hope this book helps to get you there.

HENRY Z. KISTER

March 2006

Acknowledgments

Many of the case histories reported in this book have been invaluable contributions from colleagues and friends who kindly and enthusiastically supported this book. Many of the contributors elected to remain anonymous. Kind thanks are due to all contributors. Special thanks are due to those who contributed multiple case histories, and to those whose names do not appear in print. To those behind-the-scenes friends, I extends special appreciation and gratitude.

Writing this book required breaking away from some of the everyday work demands. Special thanks are due to Fluor Corporation, particularly to my supervisors, Walter Stupin and Paul Walker, for their backing, support and encouragement of this book-writing effort, going to great lengths to make it happen.

Recognition is due to my mentors who, over the years, encouraged my work, immensely contributed to my achievements, and taught me much about distillation and engineering: To my life-long mentor, Walter Stupin, who mentored and encouraged my work, throughout my career at C F Braun and later at Fluor, being a ceaseless source of inspiration behind my books and technical achievements; Paul Walker, Fluor, whose warm encouragement and support have been the perfect motivators for professional excellence and achievement; Professor Ian Doig, University of NSW, who inspired me over the years, showed me the practical side of distillation, and guided me over a crisis early in my career; Reno Zack, who enthusiastically encouraged and inspired my achievements throughout my career at C ]F Braun; Dick Harris and Trevor Whalley, who taught me about practical distillation and encouraged my work and professional pursuits at ICI Australia; and Jack Hull, Tak Yanagi, and Jim Gosnell, who were sources of teaching and inspiration at C F Braun. The list could go on, and I express special thanks to all that encouraged, inspired, and contributed to my work over the years. Much of my mentors’ teachings found their way into the following pages.

Special thanks are due to family members and close friends who have helped, supported and encouraged my work—my mother, Dr. Helen Kister, my father, Dr. John Kister, and Isabel Wu—your help and inspiration illuminated my path over the years.

Last but not least, special thanks are due to Mireille Grey and Stan Okimoto at Fluor, who flawlessly and tirelessly converted my handwritten scrawl into a typed manuscript, putting up with my endless changes and reformats.

H.Z.K.

How to Use this Book

The use of this book as a story book or bedtime reading is quite straight forward and needs no guidance. Simply select the short stories of specific interest and read them.

More challenging is the use of this book to look for experiences that could have relevance to a given troubleshooting endeavor. Here the database mega-Table in the second part of the book is the key. Find the appropriate subject matter via the table of contents or index, and then explore the various summaries, including those in the cross-references. The database mega-Table also lists any case histories that are described in full in this book. Such case histories will be prefixed “DT” (acronym for Distillation Troubleshooting). For instance, if the mega-Table lists DT2.4, it means that the full experience is reported as case history 2.4 in this book.

The database as well as many of the case histories list only some of the key lessons drawn. The lessons listed are not comprehensive, and omit nondistillation morals (such as the needs for more staffing or better training). The reader is encouraged to review the original reference for additional valuable lessons.

For quick reference, the acronyms used in Distillation Troubleshooting are listed up front, and the literature references are listed alphabetically.

Some of the case histories use English units, others use metric units. The units used often reflect the unit system used in doing the work. The conversions are straightforward and can readily be performed by using the conversion tables in Perry's Handbook (393) or other handbooks.

The author will be pleased to hear any comments, experiences or challenges any readers may wish to share for possible inclusion in a future edition. Also, the author is sure that despite his intensive literature search, he missed several invaluable references, and would be very grateful to receive copies of such references. Feedback on any errors, as well as rebuttal to any of the experiences described, is also greatly appreciated and will help improve future editions. Please write, fax or e-mail to Henry Z. Kister, Fluor, 3 Polaris Way, Aliso Viejo, CA 92698, phone [phone]; fax [phone]; e-mail [email protected].

Abbreviations

ACAnalyzer controlAGOAtmospheric gas oilaMDEAActivated MDEAAMSAlpha-methyl styreneAPCadaptive process controlARon-line analyzerASTMAmerican Society for Testing and Materialsatmatmospheres, atmosphericBBottomsbargbars, gaugeBFWBoiler feed waterBMD2-bromomethyl-1, 3-dioxolaneBPDBarrels per dayBPHBarrels per hourBSDbottom side drawBTEXBenzene, toluene, ethylbenzene, xyleneBTXBenzene, toluene, xyleneC1, C2, C3…Number of carbon atoms in compoundCATcomputed axial tomographyCatCatalyticC-factorVapor capacity factor, defined by equation 2 in Case Study 1.14CFDcomputational fluid dynamicsCHPcumene hydroperoxideCO2Carbon dioxideCo.CompanyCSCarbon steelCTChimney TrayCTCCarbon tetrachlorideCWCooling waterCWRCooling water returnCWSCooling water supplyDDistillateD86ASTM atmospheric distillation test of petroleum fractionDAAdiacetone alcoholDC1DemethanizerDC2DeethanizerDC3DepropanizerDC4DebutanizerDC5DepentanizerDCMDichloromethaneDCSDistributed control systemDEADiethanol amineDFNB2, 4-difluoronitrobenzeneDIBDeisobutanizerDMACdimethylacetamideDMCDynamic matrix controlDMFDimethylformamideDMSODimethyl sulphoxideDODecant oildPSame as ΔPDQIDistribution quality indexDRDdistillation region diagramdTSame as ΔTDTDistillation troubleshooting (this book)EBEnergy balance; ethylbenzeneEDextractive distillationEDCEthylene dichlorideEGEthylene glycolEGEEEthylene glycol monoethyl etherEOethylene oxideEOREnd of runETFEEthylene tetrafluoroethylene, a type of teflonFCFlow controlFCCFluid catalytic crackerFIFlow indicatorfphfeet per hourFRFlow recorderFSFlow SwitchftFeetgalgallonsGCGas chromatographsGC-MSGas chromatography—mass spectrometrygpmgallons per minuteGSA process of concentrating deutrium by dual-temperature isotope exchange between water and hydrogen sulfide with no catalysthhoursH2HydrogenH2OWaterH2SHydrogen sulfideHAHydroxyl amineHAZOPHazard and operability studyHCHydrocarbonHCGOHeavy coker gas oilHClHydrogen chlorideHCNHydrogen cyanideHCOHeavy cycle oilHDHeavy dieselHETPHeight equivalent of a theoretical plateHFHydrogen fluorideHgMercuryHKHeavy keyHNHeavy naphthaHPHigh pressureHRHigh refluxHSSHeat-stable saltsHVhand valveHVGOHeavy vacuum gas oilIBPInitial boiling pointICOintermediate cycle oilIDInternal diameterIKIntermediate keyin.inchIPAIsopropyl alcoholIPEIsopropyl etherIRInfraredIVCInternal vapor controlkPaKilopascalskPagKilopascals gagelbpoundsLCLevel controlLCGOLight coker gas oilLCOLight cycle oilLDLight dieselLILevel indicatorLKLight keyLLLiquid—liquidLMTDLog mean temperature differenceLPLow pressureLPBLoss Prevention BullletinLPGLiquefied petroleum gas; refers to C3 and C4 hydrocarbonsLRLow refluxLTLevel transmitterL/VLiquid-to-vapor molar ratioLVGOLight vacuum gas oilmmetersMBMaterial balanceMDEAMethyl diethanol amineMEAMonoethanol amineMEKMethyl ethyl ketoneMFMain fractionatorminMinutes or minimumMISOMultiple inputs, single outputmmmillimetersMNTMononitrotolueneMOCManagement of changeMPMedium PressureMPCModel predictive controlmpymils per year, refers to a measure of conosion rates. 1 mil is 1/1000 inchMSDSMaterial safety data sheetsMTSRefers to a proprietary liquid distributor marketed by Sulzer under license from Dow ChemicalMVManual valveMVCMultivariable control, or more volitle componentN2nitrogenNCNormally closedNGLNatural gas liquidsNNFNormally no flowNONormally openNPSHNet positive suction headNRTLNonrandom two liquid; refers to a popular VLE prediction methodNRUNitrogen rejection unito2oxygenORSOxide redistillation stillOSHAOccupational Safety and Health AdministrationPAPumparoundP&IDProcess and instrumentation diagramPCPressure controlPCVPressure control valvePIPressure indicatorPRPeng—Robinson; refers to a popular VLE prediction methodpsipounds per square inchpsiapsi absolutepsigpsi gaugePSVPressure safety valvePTPressure transmitterPVCPolyvinyl chloridePVDFPolyvynilidene fluorideR22Freon 22R/DReflux-to-distillate molar ratioRef.ReferenceRefrigRefrigerationRORestriction orificeRVPReid vapor pressuressecondsSBEDi-Sec-butyl ethersec.secondarySGspecific gravitySPASlurry pumparoundSRKSoave, Redlich, and Kwong; refers to a popular VLE methodssStainless steelSTMSteamT/ATurnaroundTBPTrue boiling pointTCTemperature controlTCETrichloroethyleneTDCTemperature difference controllerTEATriethanol amineTEGTriethylene glycolTITemperature indicatorTiTitaniumTRCtemperature recorder/controllerUNIQACUnified quasi-chemical; refers to a popular VLE prediction methodVAMVinyl acetate monomerV/BStripping ratio, i.e., molar ratio of stripping section vapor flow rate to tower bottom flow rateVCFCVapor cross-flow channelingVCMVinyl chloride monomerVGOVacuum gas oilVLEVapor—liquid equilibriumVLLEVapor—liquid—liquid equilibriumVOCVolatile organic carbonvolVolumew.g.water gagewtby weightΔPPressure differenceΔTTemperature difference

Chapter 1

Troubleshooting Distillation Simulations

It may appear inappropriate to start a distillation troubleshooting book with a malfunction that did not even make it to the top 10 distillation malfunctions of the last half century. Simulations were in the 12th spot (255). Countering this argument is that simulation malfunctions were identified as the fastest growing area of distillation malfunctions, with the number reported in the last decade about triple that of the four preceding decades (252). If one compiled a distillation malfunction list over the last decade only, simulation issues would have been in the equal 6th spot. Simulations have been more troublesome in chemical than in refinery towers, probably due to the difficulty in simulating chemical nonidealities. The subject was discussed in detail in another paper (247).

The three major issues that affect simulation validity are using good vapor—liquid equilibrium (VLE) predictions, obtaining a good match between the simulation and plant data, and applying graphical techniques to troubleshoot the simulation (255). Case histories involving these issues account for about two-thirds of the cases reported in the literature. Add to this ensuring correct chemistry and correct tray efficiency, these items account for 85% of the cases reported in the literature.

A review of the VLE case studies (247) revealed major issues with VLE predictions for close-boiling components, either a pair of chemicals [e.g., hydrocarbons (HCs)] of similar vapor pressures or a nonideal pair close to an azeotrope. Correctly estimating nonidealities has been another VLE troublespot. A third troublespot is characterization of heavy components in crude oil distillation, which impacts simulation of refinery vacuum towers. Very few case histories were reported with other systems. VLE prediction for reasonably ideal, relatively high volatility systems (e.g., ethane—propane or methanol—ethanol) is not frequently troublesome.

The major problem in simulation validation appears to be obtaining a reliable, consistent set of plant data. Getting correct numbers out of flowmeters and laboratory analyses appears to be a major headache requiring extensive checks and rechecks. Compiling mass, component, and energy balances is essential for catching a misleading flowmeter or composition. One specific area of frequent mismatches between simulation and. plant data is where there are two liquid phases. Here comparison of measured to simulated temperature profiles is invaluable for finding the second liquid phase. Another specific area of frequent mismatches is refinery vacuum towers. Here the difficult measurement is the liquid entrainment from the flash zone into the wash bed, which is often established by a component balance on metals or asphaltenes.

The key graphical techniques for troubleshooting simulations are the McCabe—Thiele and Hengstebeck diagrams, multicomponent distillation composition profiles, and in azeotropic systems residue curve maps. These techniques permit visualization and insight into what the simulation is doing. These diagrams are not drawn from scratch; they are plots of the composition profiles obtained by the simulation using the format of one of these procedures. The book by Stichlmair and Fair (472) is loaded with excellent examples of graphical techniques shedding light on tower operation.

In chemical towers, reactions such as decomposition, polymerization, and hydrolysis are often unaccounted for by a simulation. Also, the chemistry of a process is not always well understood. One of the best tools for getting a good simulation in these situations is to run the chemicals through a miniplant, as recommended by Ruffert(417).

In established processes, such as separation of benzene from toluene or ethanol from water, estimating efficiency is quite trouble free in conventional trays and packings. Problems are experienced in a first-of-a-kind process or when a new mass transfer device is introduced and is on the steep segment of its learning curve.

Incorrect representation of the feed entry is troublesome if the first product leaves just above or below or if some chemicals react in the vapor and not in the liquid. A typical example is feed to a refinery vacuum tower, where the first major product exits the tower between 0.5 and 2 stages above the feed.

The presentation of liquid and vapor rates in the simulation output is not always user friendly, especially near the entry of subcooled reflux and feeds, often concealing higher vapor and liquid loads. This sometimes precipitates underestimates of the vapor and liquid loads in the tower.

Misleading hydraulic predictions from simulators is a major troublespot. Most troublesome have been hydraulic predictions for packed towers, which tend to be optimistic, using both the simulator methods and many of the vendor methods in the simulator (247, 254). Simulation predictions of both tray and packing efficiencies as well as downcomer capacities have also been troublesome. Further discussion is in Ref. 247.

CASE STUDY 1.1 METHANOL IN C3 SPLITTER OVERHEAD?

Installation Olefins plant C3 splitter, separating propylene overhead from propane at pressures of 220–240 psig, several towers.

Background Methanol is often present in the C3 splitter feed in small concentrations, usually originating from dosing upstream equipment to remove hydrates. Hydrates are loose compounds of water and HCs that behave like ice, and methanol is used like antifreeze. The atmospheric boiling points of propylene, propane, and methanol are -54, -44, and 148°F, respectively. The C3 splitters are large towers, usually containing between 100 and 300 trays and operating at high reflux, so they have lots of separation capability.

Problem Despite the large boiling point difference (about 200°F) and the large tower separation capability, some methanol found its way to the overhead product in all these towers. Very often there was a tight specification on methanol in the tower overhead.

Cause Methanol is a polar component, which is repelled by the nonpolar HCs. This repulsion is characterized by a high activity coefficient. With the small concentration of methanol in the all-HC tray liquid, the repulsion is maximized; that is, the activity coefficient of methanol reaches its maximum (infinite dilution) value. This high activity coefficient highly increases its volatility, to the point that it almost counterbalances the much higher vapor pressure of propylene. The methanol and propylene therefore become very difficult to separate.

Simulation All C3 splitter simulations that the author worked with have used equations of state, and these were unable to correctly predict the high activity coefficient of the methanol. They therefore incorrectly predicted that all the methanol would end up in the bottom and none would reach the tower top product.

Solution In most cases, the methanol was injected upstream for a short period only, and the off-specification propylene product was tolerated, often blended in storage. In one case, the methanol content of the propylene was lowered by allowing some propylene out of the C3 splitter bottom at the expense of lower recovery.

Related Experience A very similar experience occurred in a gas plant depropanizer separating propane from butane and heavier HCs. Here the methanol ended in the propane product.

Other Related Experiences Several refinery debutanizers that separated C3 and C4 [liquefied petroleum gases (LPGs)] from C5 and heavier HCs (naphtha) contained small concentrations of high-boiling sulfur compounds. Despite their high boiling points (well within the naphtha range), these high boilers ended in the overhead LPG product. Sulfur compounds are polar and are therefore repelled by the HC tray liquid. The repulsion (characterized by their infinite dilution activity coefficient) made these compounds volatile enough to go up with the LPG. Again, tower simulations that were based on equations of state incorrectly predicted that these compounds would end up in the naphtha.

In one refinery and one petrochemical debutanizer, mercury compounds with boiling points in the gasoline range were found in the LPG, probably reaching it by a similar mechanism.

CASE STUDY 1.2 WATER IN DEBUTANIZER: QUO VADIS?

Installation A debutanizer separating C4 HCs from HCs in the C5–C8 range. Feed to the tower was partially vaporized in an upstream feed-bottom interchanger. The feed contained a small amount of water. Water has a low solubility in the HCs and distilled up. The reflux drum was equipped with a boot designed to gravity-separate water from the reflux.

Problem When the feed contained a higher concentration of water or the reflux boot was inadvertently overfilled, water was seen in the tower bottoms.

Cause The tower feed often contained caustic. Caustic deposits were found in the tower at shutdown. Sampling the water in the tower bottom showed a high pH. Analysis showed that the water in the bottom was actually concentrated caustic solution.

Prevention Good coalescing of water and closely watching the interface level in the reflux drum boot kept water out of the feed and reflux. Maximizing feed preheat kept water in the vapor.

CASE STUDY 1.3 BEWARE OF HIGH HYDROCARBON VOLATILITIES IN WASTEWATER SYSTEMS

Benzene was present in small concentration, of the order of ppm, in a refinery wastewater sewer system. Due to the high repulsion between the water and benzene molecules, benzene has a high activity coefficient, making it very volatile in the wastewater.

Poor ventilation, typical of sewer systems, did not allow the benzene to disperse, and it concentrated in the vapor space above the wastewater. The lower explosive limit of benzene in air is quite low, about a few percent, and it is believed that the benzene concentration exceeded it at least in some locations in the sewer system.

The sewer system had one vent pipe discharging at ground level without a gooseneck. A worker was doing hot work near the top of that pipe. Sparks are believed to have fallen into the pipe, igniting the explosive mixture. The pipe blew up into the worker’s face, killing him.

Morals

Beware of high volatilities of HCs and organics in a wastewater system.Avoid venting sewer systems at ground level.

CASE STUDY 1.4 A HYDROCARBON VLLE METHOD USED FOR AQUEOUS FEED EQUILIBRIUM

Contributed by W. Randall Hollowell, CITGO, Lake Charles, Louisiana

Installation Feed for a methanol—water separation tower was the water—methanol phase from a three-phase gas—oil—aqueous separator. Gas from the separator was moderately high in H2S and in CO2. Tower preliminary design used a total overhead condenser to produce 95% methanol. Methanol product was cooled and stored at atmospheric pressure. Off gas from storage was not considered a problem because the calculated impurities in the methanol product were predominantly water.

Problem Tower feed had been calculated with a standard gas-processing vapor—liquid—liquid equilibrium (VLLE) method (Peng—Robinson equation of state). A consultant noted that the VLLE method applied only to aqueous phases that behaved like pure water and only to gas-phase components that had low solubility in the aqueous phase.

The large methanol content of the aqueous phase invalidated these feed composition calculations. Every gas component was far more soluble in the tower feed than estimated. The preliminary tower design would have produced a methanol product with such a high H2S vapor pressure that it could not be safely stored in the atmospheric tank.

Better Approach Gas solubility in a mixed, non-HC solvent (methanol and water) is a Henry’s constant type of relationship for which process simulation packages often do not have the methods and/or parameters required.

Addition of a pasteurization section to the top of a tower is a common fix for removing light impurities from the distillate product. After condensing most of the overhead vapor, a small overhead vent gas stream is purged out of the tower to remove light ends. Most or all of the overhead liquid is refluxed to minimize loss of desired product in the purges. The pasteurization section typically contains 3–10 trays or a short packed bed, used to separate light ends from the distillate product. The distillate product is taken as a liquid side draw below the pasteurization trays. The side draw may be stripped to further reduce light ends. The vent gas may be refrigerated and solvent washed or otherwise treated to reduce loss of desired product.

Solution An accurate, specific correlation (outside of the process simulation package) was used to calculate H2S and CO2 concentration in the methanol—water tower feed. Solubility of HC components was roughly estimated because they were at relatively low concentrations in the tower feed. A high-performance coalescer was used to minimize liquid HC droplets in the tower feed.

A pasteurization section was added to the top of the tower. The overhead vent gas purge stream was designed to remove most of the H2S, CO2, and light HCs. Downstream recovery of methanol from the vent gas and stripping of the methanol product side draw were considered but found to be uneconomical.

Moral Poor simulation and design result from poor selection of VLE and VLLE methods. Computer output does not include a warning when the selected VLE method produces garbage.

CASE STUDY 1.5 MODELING TERNARY MIXTURE USING BINARY INTERACTION PARAMETERS

Contributed by Stanislaw K. Wasylkiewicz, Aspen Technology, Inc., Calgary, Alberta, Canada

This case study describes a frequently encountered modeling problem during simulation of heterogeneous azeotropic distillation. Phase diagrams are invaluable for troubleshooting this type of simulation problems.

Distillation Simulation A sequence of distillation columns for separation of a mixture containing water and several organic alcohols was set up in a simulator. Since some of the alcohols are not fully miscible with water, a nonrandom two-liquid (NRTL) model was selected to model VLLE in the system. At atmospheric pressure, the vapor phase was treated as an ideal gas.

Problem Simulation of the sequence of distillation columns never converged, giving many warnings about flash failures.

Investigation For the three key components (methanol, water, and n-butanol) a phase diagram was created (508) (Fig. 1.1a). As expected, the water—methanol and methanol—n-butanol edges are homogeneous and the water—n-butanol edge contained an immiscibility gap. Surprisingly, the three-liquid region and three two-liquid regions covered almost the entire composition space. Since water and methanol, as well as butanol and methanol, are fully miscible, the diagram should have been dominated by a single-liquid region. Just looking at the phase diagram one can conclude that the model is not correct.

Analysis Binary interaction parameters for activity models used for VLLE calculations are published for thousands of components [see, e.g., DECHEMA (158) series]. They are regressed based on various experimental data and usually fit the experimental points quite well. NRTL, UNIQUAC, and Wilson models extend these binary data to multicomponent systems without requiring additional ternary, quaternary, and so on, interaction parameters. That is why these models are so popular for modeling VLE for strongly nonideal azeotropic mixtures. This extension, however, is not always performed correctly by the model.

For the ternary mixture methanol—water—n-butanol, the binary interaction parameters have been taken from DECHEMA (158). Some of them are recommended values. All of them describe all the binary pairs very well. But what they predict when combined together can be seen in Figure 1.1a. Notice that to create this VLLE diagram an extremely robust flash calculation with stability test is essential. Without a reliable global stability test, flash calculation can easily fail at some points in this component space or give unstable solutions (526).

Figure 1.1 Phase diagram for nonideal system methanol—water—n-butanol, based on extension of good binary data using NRTL model: (a) incorrect extension; (b) correct extension.

Solution Another set of binary interaction parameters was carefully selected and a new phase diagram was recreated (34). The VLLE changed dramatically (Fig. 1.1b). There is no more three-liquid phase region and only one two-liquid phase region covers only a small part of the composition space. After proper selection of interaction parameters of the thermodynamic model, the sequence of distillation columns converged quickly without any problems.

Morals

To simulate multicomponent, nonideal distillation, the behavior of the mixture must be carefully verified, starting from binary mixtures, then ternary subsystems, and so on.Since there may be many pairs of binary interaction parameters of an activity thermodynamic model that describe behavior of a binary mixture equally well, it is recommended to select one with the lowest absolute values. It is our experience that such values extrapolate better to multicomponent mixtures.To correctly create a multicomponent, nonideal VLLE model, an extremely robust VLLE calculation routine with a reliable global stability test is a must [even if liquid—liquid (LL) split is not expected].Because of their visualization capabilities, VLLE phase diagrams are invaluable (for ternary and quaternary mixtures) for verification of thermodynamic models used in distillation simulations.

CASE STUDY 1.6 VERY LOW CONCENTRATIONS REQUIRE EXTRA CARE IN VLE SELECTION

Contributed by W. Randall Hollowell, CITGO, Lake Charles, Louisiana

Problem Bottoms from a tower recovering methanol from a methanol—water mixture contained 6 ppm methanol, exceeding the maximum specification of 4 ppm required for discharging to the ocean.

Investigation A consultant pointed out that unusual hydrogen-bonding behavior had been reported at very low concentration of methanol in water. He recommended use of the UNIQUAC equation.

Wilson’s equation is generally the method of choice for alcohol—water mixtures when there is no unusual behavior. The more complex NRTL equation is the usual choice for systems that cannot be handled by Wilson’s equation. The UNIQUAC equation often applies to systems with chemicallike interactions (i.e., hydrogen bonding, which behaves like weak chemical bonding) that neither Wilson’s nor the NRTL equations can represent.

Solution Schedule constraints precluded independently developing UNIQUAC parameters. Various process simulation packages were checked for methanol—water VLE with Wilson’s, NRTL, and UNIQUAC equations. All of the equations in all of the packages gave essentially the same VLE, except that UNIQUAC in one major simulator gave lower methanol relative volatilities (by as much as 15%) at very low methanol concentrations. This package executed much slower than the other alternatives. The only methanol concentration predictions that were in line with the field data came from this UNIQAC equation.

Postmortem Exceptions to the typical choices of chemical VLE methods are often not reflected in process simulation packages. For this case, the same data base was probably used by all of the process simulation packages for the regression of UNIQUAC parameters. Predicting VLE for high-purity mixture often requires extrapolation of activity coefficients. Only one method and one simulation package did a good extrapolation to the low-methanol end. Cross checking of VLE equations and packages is a useful way to identify potential problems.

CASE STUDY 1.7 DIAGRAMS TROUBLESHOOT ACETIC ACID DEHYDRATION SIMULATION

Contributed by Stanislaw K. Wasylkiewicz, Aspen Technology, Inc., Calgary, Alberta, Canada

This case study describes a typical thermodynamic modeling problem in distillation simulation and an application of residue curve maps for troubleshooting and proper model selection. The problem described here happened far too many times for many of our clients.

Dehydration of Acetic Acid At atmospheric pressure, there is no azeotrope in the binary mixture of water and acetic acid. However, there is a tangent pinch close to pure water. This makes this binary separation very expensive if only a small concentration of acetic acid in water is allowed (high reflux, many rectifying stages). The difficult separation caused by the tangent pinch can be avoided by adding an entrainer that forms a new heterogeneous azeotrope, moving the distillation profile away from the binary pinch toward the minimum-boiling heterogeneous azeotrope. A decanter can then be used to obtain required distillate purity in far fewer stages than in the original binary distillation (525).

Distillation Simulation A column with top decanter was set up in a simulator to remove water from a mixture containing mostly water and acetic acid. N-Butyl acetate was selected as an entrainer. The vapor phase was treated as an ideal gas [Idel (227) option]. For the liquid phase, the NRTL model was selected.

Problem Even with an extreme reflux and a large number of stages, the simulation never achieved the required high-purity water in the bottom product of the column.

Troubleshooting For the three key components (water, acetic acid, and the entrainer) a distillation region diagram (DRD) was created (227) to examine the three-component space for multiple liquid regions, azeotropes, and distillation boundaries, as shown in Figure 1.2a.

Figure 1.2 Phase diagram for dehydration of acetic acid using n-butyl acetate (n-B-C2-oate) entrainer at 1 atm: (a) with ideal vapor phase, incorrect; (b) accounting for dimerization, correct.

Analysis By examining the DRD, one can easily conclude that there is something wrong with the model. We know that there is no binary acetic acid—water azeotrope at 1 atm. The model (ideal vapor phase) is not capable of describing the system properly. It is well known that carboxylic acids associate in the vapor phase and this has to be taken into account, for example, by vapor dimerization model (158) [Dimer option (227)].

Solution Instead of Idel, the Dimer option was selected (227). The DRD for the system changed tremendously (see Fig. 1.2b). There are no more binary azeotropes between acetic acid and water or n-butyl acetate. After proper selection of the thermodynamic model, the distillation column converged quickly to the required high-purity water specifications in the bottoms.

Morals

It is important to select the proper thermodynamic model and carefully verify the behavior of the mixture.Because of their visualization capabilities, DRDs are extremely useful for evaluating thermodynamic models for ternary and quaternary mixtures.

CASE STUDY 1.8 EVERYTHING VAPORIZED IN A CRUDE VACUUM TOWER SIMULATION

Contributed by W. Randall Hollowell, CITGO, Lake Charles, Louisiana

Problem Atmospheric crude tower bottom was heated, then entered a typical, fuel-type vacuum tower. A hand-drawn curve estimated the atmospheric crude tower bottom composition from assay distillation data for a light crude oil. The simulation estimated that all of the vacuum tower feed vaporized in the flash zone. This was a preposterous result inconsistent with plant data.

Investigation The heaviest assay cuts fell progressively lower than those from another assay of the same crude oil. The heaviest cut was at 850°F atmospheric cut point, compared to the other assay at 1000°F. The assay data were extrapolated on a linear scale to 100% at 1150°F atmospheric boiling point.

The high-boiling part of crude assay data must be carefully assessed. The last several assay points are often poor, particularly when coming from laboratories that cut back on quality control for increased productivity. Crude oils have very high boiling point material. Even light crude oils have material boiling above 1500°F. Extrapolation should be done with percent distilled on a probability-type scale, particularly for light crudes where the slope increases very rapidly on a linear scale.

Solution A new boiling point curve was developed. Another assay was used up to 1000°F cut point, thus reducing the needed extrapolation range. Extrapolation and smoothing of assay data were based upon a probability scale for percent distilled.

A 95% point (whole crude oil basis) of 1400°F was estimated by this extrapolation. Simulation based upon the new boiling point curve was in reasonable agreement with plant data.

Moral Crude oil high-boiling-point data are often poor and must be extrapolated. Experience, following good procedures, and cross checks with plant data are essential for reliable results.

CASE STUDY 1.9 CRUDE VACUUM TOWER SIMULATION UNDERESTIMATES RESIDUE YIELD

Contributed by W. Randall Hollowell, CITGO, Lake Charles, Louisiana

Problem Process simulation estimated much lower vacuum residue yields than obtained from plant towers and from pilot unit runs. Vacuum tower feed boiling point curves were based upon high-temperature gas chromatography (GC) analyses.

Investigation Vacuum tower feed boiling point curves from the GC fell well below curves estimated from assays. The GC analyses assumed that all of the feed oil vaporized in the test and was analyzed.

The highest boiling part of crude oil is too heavy to vaporize in a GC test. Thus the reported GC results did not include the highest boiling part (that above about 1250°F boiling point) of the feed. Simulations based upon this GC data estimated much higher vaporization than actual because they were missing the heaviest part of the feed.

Solution The GC method was modified to include a standard that allowed estimation of how much oil remained in the GC column and was not measured. New GC data and extrapolations of assay data indicated that 10–15% of the feed oil was not vaporized and thus had not been measured by the earlier GC method.

With this improved GC data, simulations agreed well with most of the pilot data. The agreement between simulation and plant data was much better than before but was still not good. This may have been due to poor plant data. Specifically, measured flash zone pressures were often bad.

Moral The analyses used for process simulations must be thoroughly understood.

CASE STUDY 1.10 MISLED BY ANALYSIS

Contributed by Geert Hangx and Marleen Horsels, DSM Research, Geleen, The Netherlands

Problem After a product change in a multipurpose plant, a light-boiling by-product could not be removed to the proper level in the (batch) distillation. The concentration of the light-boiling component in the final product was 0.5%. It should have been (and was in previous runs) 200 ppm.

Investigation The feed was analyzed by GC per normal procedure. The concentration levels of different components looked good. No significant deviation was found. Then some changes in the distillation were performed, such as

increasing the “lights fraction” in the batch distillation,increasing the reflux ratio during the lights fraction, anddecreasing the vapor load during the lights fraction.

These changes yielded no significant improvement.

The off-specification product was redistilled. The purity was improved, but still the specification could not be met. The GC analysis was checked (recalibrated) again. Everything was OK.

As all of the above-mentioned actions did not improve the product quality, it seemed that something was wrong with the column. After long discussions it was decided to open the handhole at the top of the column and to have a closer look at the feed distributor: Nothing suspicious was found.

Then it was decided to have a closer look at the analysis again. A gas chromatography—mass spectrometry (GC-MS) analysis was performed. This method showed that the impurity was not the light-boiling component as presumed. This component was a remainder from the previous run in the multipurpose plant. Having a boiling point much closer to the end product, this component could not be separated in the column.

Moral It is a good idea to check the analysis with GC-MS before shutting down a column.

CASE STUDY 1.11 INCORRECT FEED CHARACTERIZATION LEADS TO IMPOSSIBLE PRODUCT SPECIFICATIONS

Contributed by Chris Wallsgrove

Installation A new, entirely conventional depentanizer, recovering a C5 distillate stream from a C5/C6/C7 raffinate mixture from a catalytic reformer/aromatics extraction unit, with some light pyrolysis gasoline feed from an adjacent naphtha-cracking ethylene plant. The column had 30 valve trays, a steam-heated reboiler, and a condenser on cooling water.

Problem The C5 distillate was guaranteed by the process licensor to contain a maximum of 0.5% wt. C6’s. Laboratory testing by the on-site laboratory as well as an impartial third-party laboratory consistently showed about 1.0% of C6