Distillation and Hydrocarbon Processing Practices [1st ed.] 978-1-59370-343-1

Table of contents :
Distillation and Hydrocarbon Processing Practices......Page 0
Front Cover......Page 1
Contents......Page 4
Preface......Page 8
Acknowledgments......Page 10
CH 1 - Distillation Practices......Page 12
CH 2 - Distillation Tower Auxiliaries......Page 126
CH 3 - Crude, Vacuum, and Condensate Distillation......Page 188
CH 4 - Hydrotreating and Hydrocracking......Page 282
CH 5 - Gas Amine Absorbers......Page 328
CH 6 - Amine Regeneration Units......Page 344
CH 7 - Gas Concentration Units......Page 356
CH 8 - Sour Water Strippers......Page 366
CH 9 - Natural Gas Processing......Page 378
CH 10 - Delayed Coker Units......Page 386
CH 11 - Fluid Catalytic Cracking Units and Residue Fluid Catalytic Cracking Units......Page 414
CH 12 - Catalytic Reformer Units......Page 446
CH 13 - General Process Calculations......Page 466
CH 14 - Isomerization......Page 482
CH 15 - Troubleshooting......Page 492
Index......Page 514
Back Cover......Page 541

Citation preview

Ashis Nag

Disclaimer. The recommendations, advice, descriptions, and the methods in this book are presented solely for educational purposes. The author and publisher assume no liability whatsoever for any loss or damage that results from the use of any of the material in this book. Use of the material in this book is solely at the risk of the user. Copyright© 2016 by PennWell Corporation 1421 South Sheridan Road Tulsa, Oklahoma 74112-6600 USA 800.752.9764 +1.918.831.9421 [email protected] www.pennwellbooks.com www.pennwell.com Marketing Manager: Sarah De Vos National Account Executive: Barbara McGee Coons Director: Mary McGee Managing Editor: Stephen Hill Production Manager: Sheila Brock Production Editor: Tony Quinn Book Designer: Susan E. Ormston Cover Designer: Karla Womack Library of Congress Cataloging-in-Publication Data

Nag, Ashis. Distillation and hydrocarbon processing practices / Ashis Nag. pages cm Includes bibliographical references and index. ISBN 978-1-59370-343-1 1. Distillation, Fractional. 2. Extractive distillation. 3. Hydrocarbons. 4. Petroleum refineries. I. Title. QD526.N34 2015 665.5'32--dc23 2014044494 All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transcribed in any form or by any means, electronic or mechanical, including photocopying and recording, without the prior written permission of the publisher. Printed in the United States of America 1 2 3 4 5  20 19 18 17 16

Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi 1 Distillation Practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Distillation Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Feed Tray Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.3 Effect of Feed Preheat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1.4 Location of Multiple Feeds with Different Compositions . . . . . . . . . 28 1.5 Features of Two-Enthalpy Feeds with the Same Compositions . . . . 31 1.6 Industrial Applications of Two-Enthalpy Feeds . . . . . . . . . . . . . . . . . 39 1.7 Use of Intercoolers and Inter-Reboilers . . . . . . . . . . . . . . . . . . . . . . . . 42 1.8 Heat-Coupled Distillation/Multi-Effect Distillation . . . . . . . . . . . . . . 45 1.9 Thermally Coupled Distillation Schemes (TCDS) or Distributed Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 1.10 Vapor Compression in Cryogenic Distillation . . . . . . . . . . . . . . . . . . . 79 1.11 Azeotropic Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 1.12 Extractive Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 1.13 Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 2 Distillation Tower Auxiliaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 2.1 Estimation of Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 2.2 Reboilers in Distillation Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 2.3 Condensers and Column Pressure Control . . . . . . . . . . . . . . . . . . . . 131 2.4 Vapor–Liquid Contacting Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 2.5 Tray Flooding Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 2.6 Common Problems Encountered in Distillation Columns . . . . . . . 154 2.7 Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 2.8 Packed Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 v

vi  Distillation and Hydrocarbon Processing Practices



2.9 Common Installation Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 2.10 Calculation of Nozzle Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 2.11 Typical Residence Time in Vessels, Tower Bottom . . . . . . . . . . . . . . 174

3 Crude, Vacuum, and Condensate Distillation . . . . . . . . . . . . . . . . . . . . . . 177 3.1 Crude Distillation Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 3.2 Vacuum Distillation Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 3.3 Condensate Distillation Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 3.4 Crude Vacuum Units in Crude Oil Upgraders . . . . . . . . . . . . . . . . . 253 4 Hydrotreating and Hydrocracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 4.2 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 4.3 Different Hydrocracker Configurations . . . . . . . . . . . . . . . . . . . . . . . 281 4.4 Methodologies for Debottlenecking and Revamping . . . . . . . . . . . 291 4.5 Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305 4.6 Metallurgy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314 5 Gas Amine Absorbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 5.2 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 5.3 Design Concerns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322 5.4 Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325 5.5 LPG Amine Contactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329 6 Amine Regeneration Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 6.1 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 6.2 Degradation Products of Amine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 6.3 Design Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336 6.4 Calculation of Amine Circulation Rate . . . . . . . . . . . . . . . . . . . . . . . . 336 6.5 Fundamentals in Amine Regeneration . . . . . . . . . . . . . . . . . . . . . . . . 337 6.6 Calculation of Water Loss in an Amine Absorption Unit (AAU) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338 6.7 Calculation of Reboiler Duty in the ARU . . . . . . . . . . . . . . . . . . . . . . 338 6.8 Common Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 6.9 Methods Used for Revamp of Amine Absorber and Regenerator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342 7 Gas Concentration Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 7.1 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 7.2 Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348

  Contents vii

8 Sour Water Strippers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 8.1 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 8.2 Feed Impurities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357 8.3 Process Chemistry and Operating Conditions . . . . . . . . . . . . . . . . . 358 8.4 Unique Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 8.5 Simulation and Design Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362 8.6 Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363 9 Natural Gas Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 9.2 Recovery Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370 10 Coker Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 10.2 Description of the Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377 10.3 Description of the Decoking Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . 387 10.4 Design Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390 10.5 Capacity Augmentation through Revamp . . . . . . . . . . . . . . . . . . . . . 395 10.6 Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397 11 Fluid Catalytic Cracking Units and Residue Fluid Catalytic Cracking Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403 11.2 Heat Balance across Regenerator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412 11.3 Concepts Used for Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 11.4 Pressure Balance in the Catalyst Circulation System . . . . . . . . . . . . 419 11.5 Typical Design Data for the Catalyst Standpipes and Slide Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423 11.6 FCCU Fractionators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424 11.7 Concepts of FCCU Revamps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428 11.8 Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430 12 Catalytic Reformer Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435 12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435 12.2 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435 12.3 Fixed-bed CRU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439 12.4 Continuous Catalytic Reforming Unit (CCRU) . . . . . . . . . . . . . . . . 441 12.5 Rating the Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445 12.6 Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452

viii  Distillation and Hydrocarbon Processing Practices

13 General Process Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455 13.1 Refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455 13.2 Refrigeration in an SDU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455 13.3 Refrigeration with Propylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457 13.4 Furnace Pressure Drop Calculation (Vacuum Furnace) . . . . . . . . . 459 13.5 Furnace Flue Gas Side Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463 13.6 Solvent Recovery System in Solvent Extraction Units . . . . . . . . . . . 466 14 Isomerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471 14.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471 14.2 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472 14.3 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476 15 Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481 15.1 Troubleshooting Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481 15.2 Good Design Practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482 15.3 Case Studies of Pump Failure in Different Process Units . . . . . . . . 484 15.4 Case Study of Repeated Rich Gas Compressor Failure in a CCRU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488 15.5 Case Study of the Failure of the Top Dome of a Crude Fractionator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 15.6 Case Study where Vacuum Was not Achievable in a Vacuum Column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492 15.7 Case Study of Heavy Corrosion in a Crude Column Overhead Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492 15.8 Case Study of FCCU Fractionator Flooding . . . . . . . . . . . . . . . . . . . 493 15.9 Case Study of the Perils of Decreasing the Stripping Steam in a Crude Column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493 15.10 Case Study of a VGO Hydrotreater Unit Stripper that Experienced Frequent Choking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494 15.11 Case Study of Troubleshooting a High Pressure Drop in a Residue Fluid Catalytic Cracking Unit’s Main Fractionator . . . . . . 495 15.12 Case Study of Troubleshooting a Partial Conversion Hydrocracker Effluent Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 495 15.13 Case Study of an HCGO Quality Problem in a Coker Unit . . . . . . 495 15.14 Case Study of a Corrosion Problem in a Diesel Hydrotreater . . . . 497 15.15 Case Study of Deactivating Catalyst in the Second Stage of a Two-Stage VGO Hydrocracker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 15.16 Case Study of Low Propylene Recovery in a Propylene Recovery Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503

Preface This book is intended primarily for practicing process engineers and students of chemical engineering. I have noticed that some process principles, their details, and design considerations are often not well-documented for use by the engineers engaged in process design and troubleshooting. My role in both process engineering as well as plant operation made me realize that such pieces of knowledge lie scattered over different textbooks, research papers, design codes, safety codes, and individual notes of experienced engineers and operators. This work is my humble attempt to compile and elucidate the practices in the hydrocarbon industry and their fundamental principles in a concise form. The advanced practices in distillation are explained and illustrated with the help of examples, using process simulations along with the basic principles. The practical examples and design guidelines are based on all the knowledge and experience I have gathered in my professional career and from various other resources, including personal interaction with masters in the field. I sincerely hope that this work shall add value to your academic and professional pursuits.

ix

Acknowledgments I’d like to express my sincere thanks to Indian Oil Corporation Limited for providing the opportunity to work in different process plants of its several refineries. I am indebted to my friend Mr. Subhabrata Ray, a faculty member in the Department of Chemical Engineering, Indian Institute of Technology, Kharagpur, India, who inspired me to write this book based on practical experiences supplemented by the underlying process principles and case study examples. I also wish to thank my colleagues Rakesh Gagat, Prabhat Ranjan Choudhary, Mukesh Kumar Sharma, and S. G. Venkatesh for their support through numerous useful discussions and suggestions. The contribution of Mr. M. M. Khan, my colleague, in Chapters 7 and 10 is also gratefully acknowledged. I wish to thank PennWell’s Professional Education Products for agreeing to publish the work, and Mr. Stephen Hill, acquisitions editor, and Mr. Tony Quinn, production editor, for their continued association and support. I cannot afford to forget my wife Indrani and my daughter Shreya for their unconditional support and continuous encouragement. I shall be pleased to hear any comments or responses of any reader at my personal email [email protected].

xi

Distillation Practices

1

1.1  Distillation Fundamentals Distillation, a well-known and much practiced process, separates products from a homogeneous mixture based on their boiling point difference or relative volatility. In a typical distillation tower, which contains a reboiler and a condenser, heat is applied in the reboiler to vaporize the bottom liquid and condense it to produce overhead product and generate reflux and high-boiling product from the bottom. At a specified reflux ratio, a minimum number of trays is required for desired separation. A minimum reflux is always required to attain the specified separation. The requirements for minimum stages and minimum reflux are discussed in the section Short-Cut (FUG or FUEM) Procedure. Distillation is the main unit operation in a refinery. Distillation alone consumes around 40% of a refinery’s energy requirement. Thus, the high energy needed to perform distillation processes has become a major concern. As a result, process designers have begun putting more emphasis on energy-efficient design and retrofitting in addition to the recovery of products of a specified purity and yield.

Basic definitions For a two-component system with components A and B: Relative volatility α = Y(1 – X)/X(1 – Y) or Y = αX/(1 + (α – 1)X), where X and Y are the liquid and vapor phase mole fractions respectively for the more volatile component. α = KA/KB = PA/PB when ideal in vapor and liquid phase, where KA, KB are equilibrium constants and PA, PB are the vapor pressures of components A and B. 1

2  Distillation and Hydrocarbon Processing Practices

The stages and reflux ratio can be estimated by the Fenske–Underwood– Gilliland (FUG) or Fenske–Underwood–Erbar-Maddox (FUEM) correlations. The correlations can estimate the number of stages and reflux ratio required for a specified separation. A rigorous calculation using a steady state simulator can also be used to determine the number of stages and reflux ratio.

Short-cut (FUG or FUEM) Procedure The Fenske equation calculates the minimum number of stages required at total reflux. Thus, the Fenske equation is used for estimating Nm (minimum number of stages) as Nm = log[XLK/XHK]D[XHK/XLK]B/log αavg. LK – HK ,where αavg. LK – HK = (αtop × αbottom)1/2 or αavg. LK – HK = (αtop × αfeed × αbottom)1/3, where [XLK/XHK]D and [XHK/XLK]B are the ratio of concentrations (mole fraction) of light key to heavy key in the overhead and heavy key to light key in the bottom product, respectively. αavg. LK – HK is the relative volatility of the light key with respect to the heavy key. αavg. LK – HK can also be taken as the arithmetic mean of αtop and αbottom or can simply be evaluated at feed tray temperature. The Underwood equation calculates the minimum reflux ratio required for a given separation as i=n α X i iF = 11–– qq (Equation 1.1.1) α i=1 i –θ

Σ

where q depends on the condition of the feed (defined in section 1.1.2) and θ root of the Underwood equation and derived from the above equation. i=n α X i iD = Rm + 11 (Equation 1.1.2) α i=1 i –θ

Σ

XiF is the mole fraction of the ith component in the feed, XiD is the mole fraction of the ith component in the overhead distillate, Rm is the minimum reflux, and n is the total number of components in the feed.

Feed tray location using the Kirkbride equation: log(N ÷ M) = 0.206log 0.206log log(N/M)

B.XFHK D.XFLK

XBLK XDHK

2



(Equation 1.1.3)

In this equation, N and M are the number of stages above feed and below feed, respectively. D and B are molar flows of the top and bottom products, respectively. XFHK, XFLK are concentrations (mole fractions) of heavy and light key in the feed. XBLK and XDHK are concentrations (mole fractions) of light and heavy key

Chapter 1 

Distillation Practices 3

in the bottom product and top product. A Gilliland chart, an Erbar-Maddox correlation (chart), or an Eduljee correlation calculates the number of stages required at actual reflux based on the data of minimum reflux and minimum number of stages calculated using the above equations of the FUEM correlation. The method outlined above is popularly known as the Short-cut Method or FUG/FUEM method.

1.1.1 Reflux ratio, heat and material balance Reflux ratio Reflux ratio is the ratio of reflux flow to distillate product, or R:D. Actual reflux rate is derived from the minimum reflux rate required for a given separation. Distillation columns are typically designed to operate at 1.2–1.5 times the minimum reflux ratio (Rm) based on economic considerations. A higher reflux ratio will call for a lower number of stages to achieve the desired separation but would need higher reboiler/condenser duties. As a result, the tower’s height will be lower, resulting in lower capital cost. However, the operating cost will increase due to higher reboiler/condenser duties. Additionally, with an increase in reflux ratio, the diameter of a column may increase, adding to capital cost.

Heat and material balance Fig. 1.1.1 demonstrates the heat and material balance of a distillation column. QC

V R

D

Feed (F)

QR B

Fig. 1.1.1  Heat and material balance of a column

4  Distillation and Hydrocarbon Processing Practices

From Fig. 1.1.1, overall material balance can be written as F = D + B, and the component balance equation can be written as FXF = DXD + BXB, where XF is the liquid phase mole fraction of one component in the feed, XD is the liquid phase mole fraction of the same component in the overhead product (D), XB is the liquid phase mole fraction of the same component in the bottom product (B), and F is molar flow of feed. Overall heat balance is written as FhF = DhD + BhB + QC – QR , where hF is the molar enthalpy of feed, hD is the molar enthalpy of overhead liquid product, hB is the molar enthalpy of bottom liquid product, QC is the condenser duty, and QR the reboiler duty. QC is a function of reflux rate and overhead product rate, column top temperature, and reflux drum temperature. The vapor leaving the column top is V = R + D: QC, which can be estimated as QC = (R + D)(Htop – hdrum), where Htop is the enthalpy of top vapor and hdrum is the enthalpy of drum liquid. Once QC is calculated, QR can be calculated from the overall energy balance equation: QF = QD + QB+ QC – QR, where QF is equal to FhF, QD is equal to DhD, and QB is equal to BhB. Thus, all the parameters, including condenser duty and reboiler duty, can be estimated once the reflux rate (reflux ratio) is estimated for a given number of stages. Reboiler duty can be also lowered by introducing heat at any location in the column below the feed tray. If reboiler duty is inadequate then additional heat can be injected above the reboiler (but below the feed tray) of the column. The heat supplemented below the feed tray is often called a staged reboiler (inter-reboiler). No heat should be injected above the feed tray in a distillation column (except for side strippers in some cases) so as not to raise the condenser duty. Fig. 1.1.2 demonstrates heat material balance with an inter-reboiler. QC

V R

D

Feed (F) Q inter-reboiler

QR

Fig. 1.1.2  Heat material balance with an inter-reboiler

B

Chapter 1 

Distillation Practices 5

Heat balance with an inter-reboiler (Fig. 1.1.2) can be written as QR + QF + Q inter-reboiler = QD + QB + QC. Thus, reboiler duty with an inter-reboiler will be less than reboiler duty without an inter-reboiler. Similarly, when an intercooler is added above the feed tray, the heat balance can be written as QR + QF = QD + QB + QC + Q intercooler. Thus the condenser duty with an intercooler will be less than the condenser duty without an intercooler.

1.1.2 McCabe–Thiele method The McCabe–Thiele method provides insight for the analysis of many distillation problems. The basic assumption of the McCabe–Thiele is equal molar heat of vaporization and condensation of components, and it reduces to equal molar liquid–vapor rates in all stages in the sections above the feed stage and equal molar liquid and vapor flow rates in all the stages below the feed stage. Fig. 1.1.3 shows the enrichment section and stripping section envelopes used to derive the enrichment section equation and stripping section equation. Rectifying Section

Stripping Section Lm – 1

Vm Ym

Xm – 1

Stage m Stage 1

Stage N-1

Stage 2

Stage N

Stage n D

Vn + 1 Yn + 1

Ln Xn

XD

X

B

B

Fig. 1.1.3  Envelopes of the rectification and stripping section

Enrichment line equation The material balance equation on the enrichment section is V = L + D. Yn + 1 = (L/(L + D))Xn + (D/(L + D))XD is the equation of the enrichment line for nth stage. The enrichment line equation can be rewritten as Yn + 1 = (R/(R + 1))Xn + (1/(R + 1))XD with substitution of R = L/D in the equation. Thus, (R/(R + 1)) is the slope and (1/(R + 1))XD is the intercept of the enrichment line and can be plotted on an X-Y chart.

6  Distillation and Hydrocarbon Processing Practices

Stripping line equation Ym = (L'/V')Xm – 1 – (B/V')XB

q line equation Y = (q/(q – 1))X + (1/(q – 1))XF q/(q – 1) is the slope of the line, where q = (heat required to vaporize one mole of feed/molar latent heat of vaporization of the feed). Thus, L' = L + qF, V = V' + (1 – q)F, where F is the molar flow rate of the feed and XF is the mole fraction of the more volatile component in the feed. V, L are the molar flow rates of vapor and liquid in the rectification section, and V', L' are the molar flow rates of vapor and liquid respectively in the stripping section. Table 1.1.1 gives the values of q with the thermal conditions of the feed. Table 1.1.1  Values of q with thermal condition of feed

Condition BP liquid DP vapor Subcooled Superheated vapor Part vapor

Value of q 1.0 0 >1.0