Categories of Engineering Practice 1Sources of Information for Process Design 2 Codes, Standards, and Recommended Practices 2 Material and Energy Balances 3 Economic Balance 4 Safety Fac
Trang 3SERIES EDITOR ADVISORY EDITORS
California Institute of Technology
E BRUCE NAUMANRensselaer Polytechnic InstituteROBERT K PRUD’HOMMEPrinceton University
SERIES TITLES
Chemical Process Equipment Stanley M Walas
Constitutive Equations for Polymer Melts and Solutions
Ronald G Larson
G a s S e p a r a t i o n b y A d s o r p t i o n P r o c e s s e s Ralph T Yang
Heterogeneous Reactor Design Hong H Lee
Molecular Thermodynamics of Nonideal Fluids Lloyd L Lee
Phase Equilibria in Chemical Engineering Stanley M Walas
Transport Processes in Chemically Reacting Flow Systems
Daniel E Rosner
Viscous Flows: The Practical Use of Theory
Stuart Winston Churchill
RELATED TITLES
Catalyst Supports and Supported Catalysts Alvin B Stiles
Enlargement and Compaction of Particulate Solids
Nayland Stanley-Wood
Fundamentals of Fluidized Beds John G Yates
Liquid and Liquid Mixtures J.S Rowlimon and F L Swinton
M i x i n g i n t h e P r o c e s s I n d u s t r i e s N Harnby, M F Edwards,
and A W Nienow
S h e l l P r o c e s s C o n t r o l W o r k s h o p David M Prett and
Manfred Morari
Solid Liquid Separation Ladislav Svarovsky
Supercritical Fluid Extraction Mark A McHugh and
Val I Krukonis
Trang 4Chemical Process Equipment
Selection and Design
Stanley M WalasDepartment of Chemical and Petroleum Engineering University of Kansas
Trang 5and to my wife, Suzy Belle
Copyright 0 1990 by Butterworth-Heinemann, a division of Reed
Publishing (USA) Inc All rights reserved
The information contained in this book is based on highly regarded
sources, all of which are credited herein A wide range of references
is listed Every reasonable effort was made to give reliable and
up-to-date information; neither the author nor the publisher can
assume responsibility for the validity of all materials or for the
consequences.of their use
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, or otherwise, without the
prior written permission of the publisher
Library of Congress Cataloging-in-Publication Data
Walas, Stanley M
Chemical process equipment
(Butterworth-Heinemann series in chemical
engineering)
Includes bibliographical references and index
1 Chemical engineering-Apparatus and supplies
I Title II Series
TP157.w334 1988 660.2’83 87-26795
ISBN 0-7506-9385-l (previously ISBN o-409-90131-8)
British Library Cataloguing in Publication Data
Walas, Stanley M
Chemical process
equipment.-(Butterworth-Heinemann series in chemical engineering)
series in chemical engineering)
1 Chemical engineering-Apparatus and
Trang 6Categories of Engineering Practice 1
Sources of Information for Process Design 2
Codes, Standards, and Recommended Practices 2
Material and Energy Balances 3
Economic Balance 4
Safety Factors 6
Safety of Plant and Environment 7
Steam and Power Supply 9
Cascade (Reset) Control 42
Individual Process Variables 4.2
Steam Turbines and Gas Expanders 62
Combustion Gas Turbines and Engines 6 5
Operating Conditions 73Power Consumption and Pressure Drop 7 4Mechanical Conveyors and Elevators 76Properties of Materials Handled 76Screw Conveyors 76
Belt Conveyors 76Bucket Elevators and Carriers 78Continuous Flow Conveyor Elevators 82Solids Feeders 83
Fittings and Valves 95Orifices 95
Power Requirements 98Pipeline Networks 98Optimum Pipe Diameter 100Non-Newtonian Liquids 100Viscosity Behavior 100Pipeline Design 106Gases 109Isentropic Flow 109Isothermal Flow in Uniform Ducts 110
Adiabatic Flow 110
Nonideal Gases 111
Liquid-Gas Flow in Pipelines 111Homogeneous Model 113Separated Flow Models 114Other Aspects 114Granular and Packed Beds 117 Single Phase Fluids 117 Two-Phase Flow 118 6.10 Gas-Solid Transfer 119 Choking Velocity 119
Pressure Drop 1196.11 Fluidization of Beds of Particles with Gases 120Characteristics of Fluidization 123
Sizing Equipment 123References 127
CHAPTER 7 FLUID TRANSPORT EQUIPMENT 129
Pump Theory 131
Basic Relations 131Pumping Systems 133
Ideal Gases 153
Real Processes and Gases 156
Work on Nonideal Gases 156
Trang 7Mean Temperature Difference 172
Single Pass Exchanger 172
Individual Film Coefficients 180
Metal Wall Resistance 18.2
Dimensionless Groups 182
Data of Heat Transfer Coefficients 182
Direct Contact of Hot and Cold Streams 185
Pressure Drop in Heat Exchangers 188
Types of Heat Exchangers 188
Plate-and-Frame Exchangers 189
Spiral Heat Exchangers 194
Compact (Plate-Fin) Exchangers 194
Tube Side or Shell Side 199
Design of a Heat Exchanger 199
Tentative Design 200
Condensers 200
Condenser Configurations 204
Desien Calculation Method 205
The Silver-Bell-Ghaly Method 206
Absorption Refrigeration 229Cryogenics 229
References 229
9 DRYERS AND COOLING TOWERS 231
9.1 Interaction of Air and Water 231
9.2 Rate of Drying 2349.3
Laboratory and Pilot Plant Testing 237Classification and General Characteristics of
D e s i g n 2 7 69.11 Theorv of Air-Water Interaction in PackedTowers 277
Tower Height 2799.12 Cooling Towers 280Water Factors 285Testing and Acceptance 285References 285
CHAPTER 10 MIXING AND AGITATION 28710.1 A Basic Stirred Tank Design 287
The Vessel 287Baffles 287Draft Tubes 287Impeller Types 287Impeller Size 287Impeller Speed 288Impeller Location 28810.2 Kinds of Impellers 28810.3 Characterization of Mixing Quality 29010.4 Power Consumption and Pumping Rate 29210.5 Suspension of Solids 295
10.6 Gas Dispersion 296Spargers 296Mass Transfer 297System Design 297Minimum Power 297Power Consumption of Gassed Liquids 297Superficial Liquid Velocity 297
Design Procedures 29710.7 In-Line-Blenders and Mixers 30010.8 Mixing of Powders and Pastes 301
Trang 8Compressibility-Permeability (CP) Cell
Measurements 314
Another Form of Pressure Dependence 315
Pretreatment of Slurries 315
11.4 Thickening and Clarifying 315
11.5 Laboratory Testing and Scale-Up 317
Fluidized and Spouted Beds 362
Sintering and Crushing 363
Binary x-y Diagrams 375
Single-Stage Flash Calculations 375
Bubblepoint Temperature and Pressure 376
Dewpoint Temperature and Pressure 377
Flash at Fixed Temnerature and Pressure 377
Flash at Fixed Enthalpy and Pressure 377
Equilibria with KS Dependent on Composition 377
Evaporation or Simple Distillation 378
Multicomponent Mixtures 379
Binary Distillation 379
Material and Energy Balances 380
Constant Molal Overflow 380
Basic Distillation Problem 382
Unequal Molal Heats of Vaporization 382
Material and Energy Balance Basis 382
Number of Free Variables 395
Estimation of Reflux and Number of Travs
Actual Number of Theoretical Trays 397
Feed Tray Location 397
13.8.
13.9.
CONTENTS Vii
Tray Efficiencies 397Absorption Factor Shortcut Method of Edmister 398Seoarations in Packed Towers 398
Miss Transfer Coefficients 399Distillation 401
Absorption or Stripping 40113.10 Basis for Computer Evaluation of MulticomponentSeparations 404
Specifications 405The MESH Equations 405The Wang-Henke Bubblepoint Method 408The SR (Sum-Rates) Method 409
SC (Simultaneous Correction) Method 41013.11 Special Kinds of Distillation Processes 410Petroleum Fractionation 411
Extractive Distillation 412Azeotropic Distillation 420Molecular Distillation 42513.12 Tray Towers 426Countercurrent Trays 426Sieve Trays 428
Valve Trays 429Bubblecap Trays 43113.13 Packed Towers 433Kinds of Packings 433Flooding and Allowable Loads 433Liquid Distribution 439
Liauid Holdup 439Pressure Drop 43913.14 Efficiencies of Trays and Packings 439Trays 439
Packed Towers 442References 456CHAPTER 14 EXTRACTION AND LEACHING 45914.1 Equilibrium Relations 459
14.2 Calculation of Stage Requirements 463Single Staee Extraction 463
Crosscurrent Extraction 464Immiscible Solvents 46414.3 Countercurrent Operation 466Minimum Solvent/Feed Ratio 468Extract Reflux 468
Minimum Reflux 469Minimum Stages 46914.4 Leaching of Solids 47014.5 Numerical Calculation of MulticomponentExtraction 473
Initial Estimates 473Procedure 47314.6 Equipment for Extraction’ 476Choice of Disperse Phase 476Mixer-Settlers’ 477
Spray Towers 478Packed Towers 478Sieve Tray Towers 483Pulsed Packed and Sieve Tray Towers 483Reciprocating Tray Towers 485
Rotating Disk Contactor (RDC) 485Other Rotary Agitated Towers 485Other Kinds of Extractors 487Leaching Equipment 488References 493
CHAPTER 15 ADSORPTION AND IONEXCHANGE 495
15.1 Adsorption Equilibria 49515.2 Ion Exchange Equilibria 49715.3 Adsorption Behavior in Packed Beds 500Regeneration 504
Trang 915.4 Adsorption Design and Operating Practices 504
15.5 Ion Exchange Design and Operating Practices 506
Electrodialysis 508
15.6 Production Scale Chromatography 510
15.7 Equipment and Processes 510
16.1 Solubilities and Equilibria 523
Phase Diagrams 523
Enthalpy Balances 524
16.2 Crvstal Size Distribution 525
16.3 The Process of Crystallization 528
Conditions of Precipitation 528
Supersaturation 528
Growth Rates 530
16.4 The Ideal Stirred Tank 533
Multiple Stirred Tanks in Series 536
Applicability of the CSTC Model 536
18.7 Mechanical Design of Process Vessels 6 2 1
Design Pressure and Temperature 623Shells and Heads 624
Formulas for Strength Calculations 624References 629
CHAPTER 19 OTHER TOPICS 631
CHAPTER 17 CHEMICAL REACTORS 549
Rate Equations and Operating Modes 549
Material and Energy Balances of Reactors 555
Nonideal Flow Patterns 556
Residence Time Distribution 556
Conversion in Segregated and Maximum Mixed
Kinds of Catalvzed Organic Reactions 563
Physical Characteristics of Solid Catalysts 564
Kilns and Hearth Furnaces 575
Fluidized Bed Reactors 579
Heat Transfer in Reactors 582
Stirred Tanks 586
Packed Bed Thermal Conductivity 587
Heat Transfer Coefficient at Walls, to Particles, and
Overall 587
Fluidized Beds 589
Classes of Reaction Processes and Their Equipment 592
Homogeneous Gas Reactions 592
Homogeneous Liquid Reactions 595Liquid-Liquid Reactions 595Gas-Liquid Reactions 595Noncatalytic Reactions with Solids 595Fluidized Beds of Noncatalytic Solids 595Circulating Gas or Solids 596
Fixed Bed Solid Catalysis 596Fluidized Bed Catalysis 601
Gas-Liquid Reactions with Solid Catalysts 604References 609
CHAPTER 18 PROCESS VESSELS 611
19.1 Membrane Processes 631Membranes 632Equipment Configurations 632Applications 632
Gas Permeation 63319.2 Foam Separation and Froth Flotation 635Foam Fractionation 635
Froth Flotation 63619.3 Sublimation and Freeze Drying 638Equipment 639
Freeze Drying 63919.4 Parametric Pumping 63919.5 Seoarations bv Thermal Diffusion 64219.6 Electrochemical Syntheses 645Electrochemical Reactions 646Fuel Cells 646
Cells for Synthesis of Chemicals 64819.7 Fermentation Processing 648Processing 650
Operating Conditions 650Reactors 654
References 660CHAPTER 20 COSTS OF INDIVIDUALEQUIPMENT 663
Trang 10Material Balance of a Chlorination Process with Recycle 5
Data of a Steam Generator for Making 250,000 lb/hr at 450
psia and 650°F from Water Entering at 220°F 9
Steam Plant Cycle for Generation of Power and Low
Pressure Process Steam 11
Pickup of Waste Heat by Generating and Superheating
Steam in a Petroleum Refinery 11
Recovery of Power from a Hot Gas Stream 1 2
Constants of PID Controllers from Response Curves to a
Step Input 42
Steam Requirement of a Turbine Operation 65
Performance of a Combustion Gas Turbine 67
Conditions of a Coal Slurry Pipeline 70
Size and Power Requirement of a Pneumatic Transfer
Line 77
Sizing a Screw Conveyor 80
Sizing a Belt Conveyor 83
Comparison of Redler and Zippered Belt Conveyors 88
Density of a Nonideal Gas from Its Equation of State 9 1
Unsteady Flow of an Ideal Gas through a Vessel 93
Units of the Energy Balance 94
Pressure Drop in Nonisothermal Liquid Flow 9 7
Comparison of Pressure Drons in a Line with Several Sets of
Fittings Resistances 101
A Network of Pipelines in Series, Parallel, and Branches:
the Sketch, Material Balances, and Pressure Drop
Equations 101
Flow of Oil in a Branched Pipeline 101
Economic Optimum Pine Size for Pumping Hot Oil with a
Motor or Turbine Drive 102
Analysis of Data Obtained in a Capillary Tube
Viscometer 107
Parameters of the Bingham Model from Measurements of
Pressure Drops in a Line 107
Pressure Drop in Power-Law and Bingham Flow 110
Adiabatic and Isothermal Flow of a Gas in a Pipeline 112
Isothermal Flow of a Nonideal Gas 113
Pressure Drop and Void Fraction in Liquid-Gas Flow 116
Pressure D r p in Flow of Nitrogen and Powdered
Coal 120
Dimensions of a Fluidized Bed Vessel 125
Application of Dimensionless Performance Curves 132
Operating Points of Single and Double Pumps in Parallel
Compression Work with Variable Heat Capacity 157
Polytropic and Isentropic Efficiencies 158
Finding Work of Compression with a Thermodynamic
Chart 160
Compression Work on a Nonideal Gas 160
Selection of a Centrifugal Compressor 1 6 1
Polytropic and Isentropic Temperatures 162
Three-Stage Compression with Intercooling and Pressure
Loss between Stages 164
Equivalent Air Rate 165
Interstage Condensers 166
Conduction Throueh a Furnace Wall I70
Effect of Ignoring the Radius Correction of the Overall
Heat Transfer Coefficient 171
A Case of a Composite Wall: Optimum Insulation
Thickness for a Steam Line 1 7 1
Performance of a Heat Exchanger with the F-Method 180
8.5 8.6 8.7 8.8 8.9 8.10 8.11 8.12 8.13 8.14 8.15 8.16 8.17 9.1 9.2 9.3 9.4 9.5 9.6 9.1 9.8
3:Yo
9.11
10.110.210.310.411.111.211.311.412.112.213.113.213.313.413.513.613.713.8
13.913.1013.1113.12
ix
Application of the Effectiveness and the 8 Method 182
Sizing an Exchanger with Radial Finned Tubes 193Pressure Drop on the Tube Side of a Vertical Thermosiphon
Comparison of Three Kinds of Reboilers for the SameService 209
Peak Temperatures 214
Effect of Stock Temperature Variation 214
Design of a Fired Heater 2 1 7
Annlication of the Wilson-Lobo-Hottel eauation 219
Two-Stages Propylene Compression Refrigeration withInterstage Recycle 225
Conditions in an Adiabatic Dryer 234Drying Time over Constant and Falling Rate Periods withConstant Gas Conditions 237
Drying with Changing Humidity of Air in a TunnelDryer 238
Effects of Moist Air Recycle and Increase of Fresh Air Rate
in Belt Conveyor Drying 239Scale-Up of a Rotary Dryer 256Design Details of a Countercurrent Rotary Dryer 256Description of a Drum Drying System 260
Sizing a Pneumatic Conveying Dryer 266Sizing a Fluidized Bed Dryer 2 7 2
Sizing a Spray Dryer on the Basis of Pilot Plant Data 279Sizine of a Cooling Tower: Number of Transfer Units and
Height of Packing- 281
Impeller Size and Speed at a Specified Power Input 293Effects of the Ratios of impeller and Tank Diameters 294Design of the Agitation System for Maintenance of aSlurry 299
HP and rpm Requirements of an Aerated Agitated
Tank 301
Constants of the Filtration Equation from Test Data 310Filtration Process with a Centrifugal Charge Pump 311
Rotary Vacuum Filter Operation 312
Filtration and Washing of a Compressible Material 314 Sizing a Hydrocyclone 341
Power Requirement for Grinding 342Correlation of Relative Volatility 375Vanorization and Condensation of a Ternarv Mixture 378Bubblepoint Temperature with the Virial add WilsonEquations 379
Batch Distillation of Chlorinated Phenols 383Distillation of Substances with Widely Different MolalHeats of Vaporization 385
Separation of an Azeotropic Mixture by Operation at TwoPressure Levels 387
Separation of a Partially Miscible Mixture 388Enthalpy-Concentration Lines of Saturated Vapor andLiquid of Mixtures of Methanol and Water at a Pressure of
2 aim 390Algebraic Method for Binarv Distillation Calculation 392Shorcut Design of Multicomponent Fractionation 396Calculation of an Absorber by the Absorption FactorMethod 399
Numbers of Theoretical Trays and of Transfer Units with
Two Values of k,/k, for a Distillation Process 402
Trang 11Trays and Transfer Units for an Absorption Process 403
Representation of a Petroleum Fraction by an Equivalent
Number of Discrete Components 413
Comparison of Diameters of Sieve, Valve, and Bubblecap
Trays for the Same Service 4 3 1
Performance of a Packed Tower by Three Methods 4 4 1
Tray Efficiency for the Separation of Acetone and
Benzene 451
The Equations for Tieline Data 465
Tabulated Tieline and Distribution Data for the System
A = I-Hexene, B = Tetramethylene Sulfone, C = Benzene,
Represented in Figure 14.1 466
Single Stage and Cross Current Extraction of Acetic Acid
from Methylisobutyl Ketone with Water 468
Extraction with an Immiscible Solvent 469
Countercurrent Extraction Represented on Triangular and
Rectangular Distribution Diagrams 470
Stage Requirements for the Separation of a Type I and a
Type II System 471
Countercurrent Extraction Employing Extract Reflux 472
Leaching of an Oil-Bearing Solid in a Countercurrent
Battery - 472
Trial Estimates and Converged Flow Rates and
Compositions in all Stages of an Extraction Batterv for a
Sizing of Spray, Packed, or Sieve Tray Towers 486
Design of a Rotating Disk Contactor 488
Application of Ion Exchange Selectivity Data 503
15.215.316.1
16.2 16.3 16.4 16.5 16.6 16.7 16.8 18.1 18.2 18.3 18.4 18.5 18.6 19.1 19.2 20.1 20.2
Adsorption of n-hexane from a Natural Gas with SilicaGel 505
Size of an Ion Exchanger for Hard Water 513Design of a Crystallizing Plant 524
Using the Phase Diagrams of Figure 16.2 528Heat Effect Accompanying the Cooling of a Solution ofMgSO, 529
Deductions from a Differential Distribution Obtained at aKnown Residence Time 533
Batch Crystallization with Seeded Liquor 534Analysis of Size Distribution Data Obtained in aCSTC 537
Crystallization in a Continuous Stirred Tank with SpecifiedPredominant Crystal Size 538
Crystallization from a Ternary Mixture 544Separation of Oil and Water 614
Ouantitv of Entrainment on the Basis of Sieve TravCorrelations 6 1 7
Liquid Knockout Drum (Empty) 618Knockout Drum with Wire Mesh Deentrainer 620Size and Capacity of Cyclone Separators 6 2 1
Dimensions and Weight of a Horizontal PressureDrum 628
Applications of the Equation for Osmotic Pressure 633Concentration of a Water/Ethanol Mixture by ReverseOsmosis 642
Installed Cost of a Distillation Tower 663Purchased and Installed Cost of Some Equipment 663
Trang 12This book is intended as a guide to the selection or design of the
principal kinds of chemical process equipment by engineers in
school and industry The level of treatment assumes an elementary
knowledge of unit operations and transport phenomena Access to
the many design and reference books listed in Chapter 1 is
desirable For coherence, brief reviews of pertinent theory are
provided Emphasis is placed on shortcuts, rules of thumb, and data
for design by analogy, often as primary design processes but also for
quick evaluations of detailed work
All answers to process design questions cannot be put into a
book Even at this late date in the development of the chemical
industry, it is common to hear authorities on most kinds of
equipment say that their equipment can be properly fitted to a
particular task only on the basis of some direct laboratory and pilot
plant work Nevertheless, much guidance and reassurance are
obtainable from general experience and specific examples of
successful applications, which this book attempts to provide Much
of the information is supplied in numerous tables and figures, which
often deserve careful study quite apart from the text
The general background of process design, flowsheets, and
process control is reviewed in the introductory chapters The major
kinds of operations and equipment are treated in individual
chapters Information about peripheral and less widely employed
equipment in chemical plants is concentrated in Chapter 19 with
references to key works of as much practical value as possible
Because decisions often must be based on economic grounds,
Chapter 20, on costs of equipment, rounds out the book
Appendixes provide examples of equipment rating forms and
manufacturers’ questionnaires
Chemical process equipment is of two kinds: custom designed
and built, or proprietary “off the shelf.” For example, the sizes and
performance of custom equipment such as distillation towers,
drums, and heat exchangers are derived by the process engineer on
the basis of established principles and data, although some
mechanical details remain in accordance with safe practice codes
and individual fabrication practices
Much proprietary equipment (such as filters, mixers, conveyors,
and so on) has been developed largely without benefit of much
theory and is fitted to job requirements also without benefit of much
theory From the point of view of the process engineer, such
equipment is predesigned and fabricated and made available by
manufacturers in limited numbers of types, sizes, and capacities
The process design of proprietary equipment, as considered in this
book, establishes its required performance and is a process of
selection from the manufacturers’ offerings, often with their
recommendations or on the basis of individual experience
Complete information is provided in manufacturers’ catalogs
Several classified lists of manufacturers of chemical process
equipment are readily accessible, so no listings are given here
Because more than one kind of equipment often is suitable forparticular applications and may be available from severalmanufacturers, comparisons of equipment and typical applicationsare cited liberally Some features of industrial equipment are largelyarbitrary and may be standardized for convenience in particularindustries or individual plants Such aspects of equipment design arenoted when feasible
Shortcut methods of design provide solutions to problems in ashort time and at small expense They must be used when data arelimited or when the greater expense of a thorough method is notjustifiable In particular cases they may be employed to obtaininformation such as:
1 an order of magnitude check of the reasonableness of a resultfound by another lengthier and presumably accurate computa-tion or computer run,
2 a quick check to find if existing equipment possibly can beadapted to a new situation,
3 a comparison of alternate processes,
4 a basis for a rough cost estimate of a process
Shortcut methods occupy a prominent place in such a broad surveyand limited space as this book References to sources of moreaccurate design procedures are cited when available
Another approach to engineering work is with rules of thumb,which are statements of equipment performance that may obviateall need for further calculations Typical examples, for instance, arethat optimum reflux ratio is 20% greater than minimum, that asuitable cold oil velocity in a fired heater is 6ft/sec, or that theefficiency of a mixer-settler extraction stage is 70% The trust thatcan be placed in a rule of thumb depends on the authority of thepropounder, the risk associated with its possible inaccuracy, and theeconomic balance between the cost of a more accurate evaluationand suitable safety factor placed on the approximation Allexperienced engineers have acquired such knowledge Whenapplied with discrimination, rules of thumb are a valuable asset tothe process design and operating engineer, and are scatteredthroughout this book
Design by analogy, which is based on knowledge of what hasbeen found to work in similar areas, even though not necessarilyoptimally, is another valuable technique Accordingly, specificapplications often are described in this book, and many examples ofspecific equipment sizes and performance are cited
For much of my insight into chemical process design, I amindebted to many years’ association and friendship with the lateCharles W Nofsinger who was a prime practitioner by analogy, rule
of thumb, and basic principles Like Dr Dolittle of the-Marsh, “he was a proper doctor and knew a whole lot.”
Trang 13Puddleby-on-Although experienced engineers know where to find information
and how to make accurate computations, they also keep a minimum
body of information in mind on the ready, made up largely of
shortcuts and rules of thumb The present compilation may fit into
such a minimum body of information, as a boost to the memory or
extension in some instances into less often encountered areas It is
derived from the material in this book and is, in a sense, a digest of
the book
An Engineering Rule of Thumb is an outright statement
regarding suitable sizes or performance of equipment that obviates
all need for extended calculations Because any brief statements are
subject to varying degrees of qualification, they are most safely
applied by engineers who are substantially familiar with the topics
Nevertheless, such rules should be of value for approximate design
and cost estimation, and should provide even the inexperienced
engineer with perspective and a foundation whereby the
reason-ableness of detailed and computer-aided results can be appraised
quickly, particularly on short notice such as in conference
Everyday activities also are governed to a large extent by rules
of thumb They serve us when we wish to take a course of action
but are not in a position to find the best course of action Of interest
along this line is an amusing and often useful list of some 900 such
digests of everyday experience that has been compiled by Parker
(Rules of Thumb, Houghton Mifflin, Boston, 1983)
Much more can be stated in adequate summary fashion about
some topics than about others, which accounts in part for the
spottiness of the present coverage, but the spottiness also is due to
ignorance and oversights on the part of the author Accordingly,
every engineer undoubtedly will supplement or modify this material
in his own way
COMPRESSORS AND VACUUM PUMPS
1 Fans are used to raise the pressure about 3% (12in water),
blowers raise to less than 40 psig, and compressors to higher
pressures, although the blower range commonly is included in
the compressor range
2 Vacuum pumps: reciprocating piston type decrease the pressure
to 1 Torr; rotary piston down to 0.001 Torr, two-lobe rotary
down to 0.0001 Torr; steam jet ejectors, one stage down to
lOOTorr, three stage down to 1 Torr, five stage down to
0.05 Torr
3 A three-stage ejector needs 1OOlb steam/lb air to maintain a
pressure of 1 Torr
4 In-leakage of air to evacuated equipment depends on the
absolute pressure, Torr, and the volume of the equipment, V
cuft, according to w = kVz3 lb/hr, with k = 0.2 when P is more
than 90 Torr, 0.08 between 3 and 20 Torr, and 0.025 at less than
1 Torr
5 Theoretical adiabatic horsepower (THP) = [(SCFM)T1/8130a]
[(PJPJ - 11, where Tt is inlet temperature in °F+ 460 and
a = (k - 1)/k, k = CJC,,.
6 Outlet temperature & = T,(P,/P,)“
7 To compress air from lOO”F, k = 1.4, compression ratio = 3,
theoretical power required = 62 HP/million tuft/day, outlet
temperature 306°F
8 Exit temperature should not exceed 350-400°F; for diatomic
gases (C,/C, = 1.4) this corresponds to a compression ratio of
CONVEYORS FOR PARTICULATE SOLIDS
1 Screw conveyors are suited to transport of even sticky andabrasive solids up inclines of 20” or so They are limited todistances of 150ft or so because of shaft torque strength A12in dia conveyor can handle 100@3000cuft/hr, at speedsranging from 40 to 60 ‘pm
2 Belt conveyors are for high capacity and long distances (a mile ormore, but only several hundred feet in a plant), up inclines of30” maximum A 24in wide belt can carry 3OOOcuft/hr at aspeed of lOOft/min, but speeds up to 6OOft/min are suited tosome materials Power consumption is relatively low
Bucker elevators are suited to vertical transport of sticky andabrasive materials With buckets 20 x 20 in capacity can reach
1000 cuft/hr at a speed of 100 ft/min, but speeds to 300 ft/minare used
Drug-type conveyors (Redler) are suited to short distances in anydirection and are completely enclosed Units range in size from
3 in square to 19 in square and may travel from 30 ft/min (flyash) to 250 ft/min (grains) Power requirements are high
Pneumatic conveyors are for high capacity, short distance (400 ft)transport simultaneously from several sources to severaldestinations Either vacuum or low pressure (6-12psig) isemployed with a range of air velocities from 35 to 120ft/secdepending on the material and pressure, air requirements from 1
to 7 cuft/cuft of solid transferred
COOLING TOWERS
1 Water in contact with air under adiabatic conditions eventuallycools to the wet bulb temperature
2 In commercial units, 90% of saturation of the air is feasible
3 Relative cooling tower size is sensitive to the difference betweenthe exit and wet bulb temperatures:
7 Countercurrent induced draft towers are the most common inprocess industries They are able to cool water within 2°F of thewet bulb
8 Evaporation losses are 1% of the circulation for every 10°F ofcooling range Windage or drift losses of mechanical draft towers
Trang 14Xiv R U L E S O F T H U M B : S U M M A R Y
are O.l-0.3% Blowdown of 2.5-3.0% of the circulation is
necessary to prevent excessive salt buildup
CRYSTALLIZATION FROM SOLUTION
Complete recovery of dissolved solids is obtainable by
evaporation, but only to the eutectic composition by chilling
Recovery by melt crystallization also is limited by the eutectic
composition
Growth rates and ultimate sizes of crystals are controlled by
limiting the extent of supersaturation at any time
The ratio S = C/C,,, of prevailing concentration to saturation
concentration is kept near the range of 1.02-1.05
In crystallization by chilling, the temperature of the solution is
kept at most l-2°F below the saturation temperature at the
prevailing concentration
Growth rates of crystals under satisfactory conditions are in the
range of 0.1-0.8 mm/hr The growth rates are approximately the
same in all directions
Growth rates are influenced greatly by the presence of impurities
and of certain specific additives that vary from case to case
DISINTEGRATION
1 Percentages of material greater than 50% of the maximum size
are about 50% from rolls, 15% from tumbling mills, and 5%
from closed circuit ball mills
2 Closed circuit grinding employs external size classification and
return of oversize for regrinding The rules of pneumatic
conveying are applied to design of air classifiers Closed circuit is
most common with ball and roller mills
3
4
5
6
Jaw crushers take lumps of several feet in diameter down to 4 in
Stroke rates are 10@300/min The average feed is subjected to
8-10 strokes before it becomes small enough to escape
Gyratory crushers are suited to slabby feeds and make a more
rounded product
Roll crushers are made either smooth or with teeth A 24in
toothed roll can accept lumps 14in dia Smooth rolls effect
reduction ratios up to about 4 Speeds are 50-900 rpm Capacity
is about 25% of the maximum corresponding to a continuous
ribbon of material passing through the rolls
Hammer mills beat the material until it is small enough to pass
through the screen at the bottom of the casing Reduction ratios
of 40 are feasible Large units operate at 900 rpm, smaller ones
up to 16,OOOrpm For fibrous materials the screen is provided
with cutting edges
Rod mills are capable of taking feed as large as 50 mm and
reducing it to 300 mesh, but normally the product range is 8-65
mesh Rods are 25-150mm dia Ratio of rod length to mill
diameter is about 1.5 About 45% of the mill volume is occupied
by rods Rotation is at 50-65% of critical
7 Ball mills are better suited than rod mills to fine grinding The
charge is of equal weights of 1.5, 2, and 3 in balls for the finest
grinding Volume occupied by the balls is 50% of the mill
volume Rotation speed is 70-80% of critical Ball mills have a
length to diameter ratio in the range l-1.5 Tube mills have a
ratio of 4-5 and are capable of very fine grinding Pebble mills
have ceramic grinding elements, used when contamination with
metal is to be avoided
8 Roller mills employ cylindrical or tapered surfaces that roll along
flatter surfaces and crush nipped particles Products of 20-200
mesh are made
DISTILLATION AND GAS ABSORPTION
For ideal mixtures, relative volatility is the ratio of vaporpressures rri2 = P,/P,.
Tower operating pressure is determined most often by thetemperature of the available condensing medium, lOO-120°F ifcooling water; or by the maximum allowable reboilertemperature, 150 psig steam, 366°F
Sequencing of columns for separating multicomponent tures: (a) perform the easiest separation first, that is, the oneleast demanding of trays and reflux, and leave the most difficult
mix-to the last; (b) when neither relative volatility nor feedconcentration vary widely, remove the components one by one
as overhead products; (c) when the adjacent orderedcomponents in the feed vary widely in relative volatility,sequence the splits in the order of decreasing volatility; (d)when the concentrations in the feed vary widely but the relativevolatilities do not, remove the components in the order ofdecreasing concentration in the feed
Economically optimum reflux ratio is about 1.2 times theminimum reflux ratio R,.
The economically optimum number of trays is near twice theminimum value N,,,
The minimum number of trays is found with the Underwood equation
Fenske-Nn = W[~l(l -~)lovtdM~ - ~)ltxrns~/~~~ a.
Minimum reflux for binary or pseudobinary mixtures is given bythe following when separation is esentially complete (xD = 1)and D/F is the ratio of overhead product and feed rates:
R,D/F = l/(cu - l), when feed is at the bubblepoint,
(R, + l)D/F = a/((~ - l), when feed is at the dewpoint
A safety factor of 10% of the number of trays calculated by thebest means is advisable
Reflux pumps are made at least 25% oversize
For reasons of accessibility, tray spacings are made 20-24 in.Peak efficiency of trays is at values of the vapor factor
F, = ~6 in the range 1.0-1.2 (ft/sec) B This range of
F, establishes the diameter of the tower Roughly, linearvelocities are 2ft/sec at moderate pressures and 6ft/sec invacuum
The optimum value of the Kremser-Brown absorption factor
A = K(V/L) is in the range 1.25-2.0
Pressure drop per tray is of the order of 3 in of water or 0.1 psi.Tray efficiencies for distillation of light hydrocarbons andaqueous solutions are 60-90%; for gas absorption andstripping, lo-20%
Sieve trays have holes 0.25-0.50 in dia, hole area being 10% ofthe active cross section
Valve trays have holes 1.5 in dia each provided with a liftablecap, 12-14 caps/sqft of active cross section Valve trays usuallyare cheaper than sieve trays
Bubblecap trays are used only when a liquid level must bemaintained at low turndown ratio; they can be designed forlower pressure drop than either sieve or valve trays
Weir heights are 2 in., weir lengths about 75% of tray diameter,liquid rate a maximum of about 8 gpm/in of weir; multipassarrangements are used at high liquid rates
Trang 1520 Packings of random and structured character are suited
especially to towers under 3 ft dia and where low pressure drop
is desirable With proper initial distribution and periodic
redistribution, volumetric efficiencies can be made greater than
those of tray towers Packed internals are used as replacements
for achieving greater throughput or separation in existing tower
shells
21 For gas rates of 500 cfm, use 1 in packing; for gas rates of
2000 cfm or more, use 2 in
22 The ratio of diameters of tower and packing should be at least
15
23 Because of deformability, plastic packing is limited to a lo-15 ft
depth unsupported, metal to 20-25 ft
24 Liquid redistributors are needed every 5-10 tower diameters
with pall rings but at least every 20ft The number of liquid
streams should be 3-5/sqft in towers larger than 3 ft dia (some
experts say 9-12/sqft), and more numerous in smaller towers
25 Height equivalent to a theoretical plate (HETP) for
vapor-liquid contacting is 1.3-1.8ft for 1 in pall rings,
2.5-3.0 ft for 2 in pall rings
26 Packed towers should operate near 70% of the flooding rate
given by the correlation of Sherwood, Lobo, et al
27 Reflux drums usually are horizontal, with a liquid holdup of 5
min half full A takeoff pot for a second liquid phase, such as
water in hydrocarbon systems, is sized for a linear velocity of
that phase of 0.5 ft/sec, minimum diameter of 16 in
28 For towers about 3 ft dia, add 4ft at the top for vapor
disengagement and 6 ft at the bottom for liquid level and
reboiler return
29 Limit the tower height to about 175 ft max because of wind load
and foundation considerations, An additional criterion is that
Efficiency is greater for larger machines Motors are 85-95%;
steam turbines are 42-78%; gas engines and turbines are
28-38%
For under IOOHP, electric motors are used almost exclusively
They are made for up to 20,000 HP
Induction motors are most popular Synchronous motors are
made for speeds as low as 150rpm and are thus suited for
example for low speed reciprocating compressors, but are not
made smaller than 50HP A variety of enclosures is available,
from weather-proof to explosion-proof
Steam turbines are competitive above 1OOHP They are speed
controllable Frequently they are employed as spares in case of
power failure
Combustion engines and turbines are restricted to mobile and
remote locations
Gas expanders for power recovery may be justified at capacities
of several hundred HP; otherwise any needed pressure reduction
in process is effected with throttling valves
DRYING OF SOLIDS
1 Drying times range from a few seconds in spray dryers to 1 hr or
less in rotary dryers and up to several hours or even several days
in tunnel shelf or belt dryers
2 Continuous tray and belt dryers for granular material of natural
size or pelleted to 3-15 mm have drying times in the range of
lo-200 min
3 Rotary cylindrical dryers operate with superficial air velocities of
5-lOft/sec, sometimes up to 35 ft/sec when the material is
coarse Residence times are S-90 min Holdup of solid is 7-8%
An 85% free cross section is taken for design purposes Incountercurrent flow, the exit gas is lo-20°C above the solid; inparallel flow, the temperature of the exit solid is 100°C Rotationspeeds of about 4rpm are used, but the product of rpm anddiameter in feet is typically between 15 and 25
4 Drum dryers for pastes and slurries operate with contact times of3-12 set, produce flakes 1-3 mm thick with evaporation rates of15-30 kg/m2 hr Diameters are 1.5-5.Oft; the rotation rate is2-10rpm The greatest evaporative capacity is of the order of
3000 lb/hr in commercial units
5 Pneumatic conveying dryers normally take particles l-3 mm diabut up to 10 mm when the moisture is mostly on the surface Airvelocities are lo-30m/sec Single pass residence times are0.5-3.0 set but with normal recycling the average residence time
is brought up to 60 sec Units in use range from 0.2 m dia by 1 mhigh to 0.3 m dia by 38 m long Air requirement is severalSCFM/lb of dry product/hr
6 Fluidized bed dryers work best on particles of a few tenths of a
mm dia, but up to 4 mm dia have been processed Gas velocities
of twice the minimum fluidization velocity are a safeprescription In continuous operation, drying times of l-2minare enough, but batch drying of some pharmaceutical productsemploys drying times of 2-3 hr
7 Spray dryers: Surface moisture is removed in about 5sec, andmost drying is completed in less than 60 sec Parallel flow of airand stock is most common Atomizing nozzles have openings0.012-0.15 in and operate at pressures of 300-4OOOpsi.Atomizing spray wheels rotate at speeds to 20,000 rpm withperipheral speeds of 250-600 ft/sec With nozzles, the length todiameter ratio of the dryer is 4-5; with spray wheels, the ratio is0.5-1.0 For the final design, the experts say, pilot tests in a unit
of 2 m dia should be made
EVAPORATORS
1 Long tube vertical evaporators with either natural or forcedcirculation are most popular Tubes are 19-63 mm dia and12-30 ft long
2 In forced circulation, linear velocities in the tubes are15-20 ft/sec
3 Elevation of boiling point by dissolved solids results indifferences of 3-10°F between solution and saturated vapor
4 When the boiling point rise is appreciable, the economic number
of effects in series with forward feed is 4-6
5 When the boiling point rise is small, minimum cost is obtainedwith 8-10 effects in series
6 In backward feed the more concentrated solution is heated withthe highest temperature steam so that heating surface islessened, but the solution must be pumped between stages
7 The steam economy of an N-stage battery is approximately0.8N lb evaporation/lb of outside steam
8 Interstage steam pressures can be boosted with steam jetcompressors of 20-30% efficiency or with mechanical compres-sors of 70-75% efficiency
EXTRACTION, LIQUID-LIQUID
1 The dispersed phase should be the one that has the highervolumetric rate except in equipment subject to backmixingwhere it should be the one with the smaller volumetric rate Itshould be the phase that wets the material of construction lesswell Since the holdup of continuous phase usually is greater,that phase should be made up of the less expensive or lesshazardous material
Trang 16Xvi RULES OF THUMB: SUMMARY
There are no known commercial applications of reflux to
extraction processes, although the theory is favorable (Treybal)
Mixer-settler arrangements are limited to at most five stages
Mixing is accomplished with rotating impellers or circulating
pumps Settlers are designed on the assumption that droplet
sizes are about 150 pm dia In open vessels, residence times of
30-60 min or superficial velocities of 0.5-1.5 ft/min are provided
in settlers Extraction stage efficiencies commonly are taken as
8 0 %
Spray towers even 20-40ft high cannot be depended on to
function as more than a single stage
Packed towers are employed when 5-10 stages suffice Pall rings
of l-l.5 in size are best Dispersed phase loadings should not
exceed 25 gal/(min) (sqft) HETS of 5-10 ft may be realizable
The dispersed phase must be redistributed every 5-7 ft Packed
towers are not satisfactory when the surface tension is more than
10 dyn/cm
Sieve tray towers have holes of only 3-8 mm dia Velocities
through the holes are kept below 0.8 ft/sec to avoid formation of
small drops Redispersion of either phase at each tray can be
designed for Tray spacings are 6-24 in Tray efficiencies are in
the range of 20-30%
Pulsed packed and sieve tray towers may operate at frequencies
of 90 cycles/min and amplitudes of 6-25 mm In large diameter
towers, HETS of about 1 m has been observed Surface tensions
as high as 30-40 dyn/cm have no adverse effect
Reciprocating tray towers can have holes 9/16in dia, 50-60%
open area, stroke length 0.75 in., 100-150 strokes/mitt, plate
spacing normally 2 in but in the range l-6 in In a 30in dia
tower, HETS is 20-25 in and throughput is 2000 gal/(hr)(sqft)
Power requirements are much less than of pulsed towers
Rotating disk contactors or other rotary agitated towers realize
HETS in the range 0.1-0.5 m The especially efficient Kuhni
with perforated disks of 40% free cross section has HETS 0.2 m
and a capacity of 50 m3/m2 hr
FILTRATION
1 Processes are classified by their rate of cake buildup in a
laboratory vacuum leaf filter: rapid, 0.1-10.0 cm/set; medium,
O.l-lO.Ocm/min; slow, O.l-lO.Ocm/hr
2 Continuous filtration should not be attempted if l/8 in cake
thickness cannot be formed in less than 5 min
3 Rapid filtering is accomplished with belts, top feed drums, or
pusher-type centrifuges
4 Medium rate filtering is accomplished with vacuum drums or
disks or peeler-type centrifuges
5 Slow filtering slurries are handled in pressure filters or
sedimenting centrifuges
6 Clarification with negligible cake buildup is accomplished with
cartridges, precoat drums, or sand filters
7 Laboratory tests are advisable when the filtering surface is
expected to be more than a few square meters, when cake
washing is critical, when cake drying may be a problem, or when
precoating may be needed
8 For finely ground ores and minerals, rotary drum filtration, rates
may be 1500 lb/(day)(sqft), at 20 rev/hr and 18-25in Hg
vacuum
9 Coarse solids and crystals may be filtered at rates of 6000
lb/(day)(sqft) at 20 rev/hr, 2-6 in Hg vacuum
FLUIDIZATION OF PARTICLES WITH GASES
1 Properties of particles that are conducive to smooth fluidization
include: rounded or smooth shape, enough toughness to resist
The other extreme of smoothly fluidizing particles is typified bycoarse sand and glass beads both of which have been the subject
of much laboratory investigation Their sizes are in the range150-500 pm, densities 1.5-4.0 g/mL, small bed expansion, aboutthe same magnitudes of minimum bubbling and minimumfluidizing velocities, and also have rapidly disengaging bubbles.Cohesive particles and large particles of 1 mm or more do notlluidize well and usually are processed in other ways
Rough correlations have been made of minimum fluidizationvelocity, minimum bubbling velocity, bed expansion, bed levelfluctuation, and disengaging height Experts recommend,however, that any real design be based on pilot plant work.Practical operations are conducted at two or more multiples ofthe minimum fluidizing velocity In reactors, the entrainedmaterial is recovered with cyclones and returned to process Indryers, the fine particles dry most quickly so the entrainedmaterial need not be recycled
4 Shell side is for viscous and condensing fluids
5 Pressure drops are 1.5 psi for boiling and 3-9psi for other
‘services
6 Minimum temperature approach is 20°F with normal coolants,10°F or less with refrigerants
7 Water inlet temperature is 90”F, maximum outlet 120°F
8 Heat transfer coefficients for estimating purposes,Btu/(hr)(sqft)(“F): water to liquid, 150; condensers, 150; liquid
to liquid, 50; liquid to gas, 5; gas to gas, 5; reboiler, 200 Maxflux in reboilers, 10,000 Btu/(hr)(sqft)
9 Double-pipe exchanger is competitive at duties requiring
Air coolers: Tubes are 0.75-1.00 in OD, total finned surface15-20 sqft/sqft bare surface, U = 80-100 Btu/(hr)(sqft baresurface)( fan power input 2-5 HP/(MBtu/hr), approach50°F or more
Fired heaters: radiant rate, 12,000 Btu/(hr)(sqft); convectionrate, 4000; cold oil tube velocity, 6 ft/sec; approx equal transfers
of heat in the two sections; thermal efficiency 70-75%; flue gastemperature 250-350°F above feed inlet; stack gas temperature650-950°F
INSULATION
1 Up to 650”F, 85% magnesia is most used
2 Up to 1600-19OO”F, a mixture of asbestos and diatomaceousearth is used
Trang 173 Ceramic refractories at higher temperatures.
4 Cyrogenic equipment (-200°F) employs insulants with fine pores
in which air is trapped
5 Optimum thickness varies with temperature: 0.5 in at 2OO”F,
l.Oin at 400”F, 1.25 in at 600°F
6 Under windy conditions (7.5 miles/hr), lo-20% greater
thickness of insulation is justified
MIXING AND AGITATION
Mild agitation is obtained by circulating the liquid with an
impeller at superficial velocities of O.l-0.2ft/sec, and intense
agitation at 0.7-1.0 ft/sec
Intensities of agitation with impellers in baffled tanks are
measured by power input, HP/1000 gal, and impeller tip speeds:
Operation HP/1000 gal Tip speed (ft/min)
Proportions of a stirred tank relative to the diameter D: liquid
level = D; turbine impeller diameter = D/3; impeller level above
bottom = D/3; impeller blade width = D/15; four vertical baffles
with width = D/10.
Propellers are made a maximum of 18 in., turbine impellers to
9ft
Gas bubbles sparged at the bottom of the vessel will result in
mild agitation at a superficial gas velocity of 1 ft/min, severe
agitation at 4 ft/min
Suspension of solids with a settling velocity of 0.03 ft/sec is
accomplished with either turbine or propeller impellers, but
when the settling velocity is above 0.15 ft/sec intense agitation
with a propeller is needed
Power to drive a mixture of a gas and a liquid can be 25-50%
less than the power to drive the liquid alone
In-line blenders are adequate when a second or two contact time
is sufficient, with power inputs of 0.1-0.2 HP/gal
PARTICLE SIZE ENLARGEMENT
1 The chief methods of particle size enlargement are: compression
into a mold, extrusion through a die followed by cutting or
breaking to size, globulation of molten material followed by
solidification, agglomeration under tumbling or otherwise
agitated conditions with or without binding agents
2 Rotating drum granulators have length to diameter ratios of 2-3,
speeds of lo-20 rpm, pitch as much as 10” Size is controlled by
speed, residence time, and amount of binder; 2-5 mm dia is
common
3 Rotary disk granulators produce a more nearly uniform product
than drum granulators Fertilizer is made 1.5-3.5 mm; iron ore
lo-25 mm dia
4 Roll compacting and briquetting is done with rolls ranging from
130mm dia by 50mm wide to 910mm dia by 550mm wide
Extrudates are made l-10 mm thick and are broken down to size
for any needed processing such as feed to tabletting machines or
to dryers
Tablets are made in rotary compression machines that convert
powders and granules into uniform sizes Usual maximum
diameter is about 1.5 in., but special sizes up to 4in dia are
possible Machines operate at 1OOrpm or so and make up to
10,000 tablets/min
Extruders make pellets by forcing powders, pastes, and melts
through a die followed by cutting An 8 in screw has a capacity
of 2000 Ib/hr of molten plastic and is able to extrude tubing at150-3OOft/min and to cut it into sizes as small as washers at8OOO/min Ring pellet extrusion mills have hole diameters of1.6-32 mm Production rates cover a range of 30-200Ib/(hr)(HP)
Prilling towers convert molten materials into droplets and allowthem to solidify in contact with an air stream Towers as high as60m are used Economically the process becomes competitivewith other granulation processes when a capacity of 200-
409 tons/day is reached Ammonium nitrate prills, for example,are 1.6-3.5 mm dia in the 5-95% range
Fluidized bed granulation is conducted in shallow beds 12-24 in.deep at air velocities of 0.1-2.5 m/s or 3-10 times the minimumfluidizing velocity, with evaporation rates of 0.005-1.0 kg/m* sec One product has a size range 0.7-2.4 mm dia
PIPING
1 Line velocities and pressure drops, with line diameter D in inches: liquid pump discharge, (5 + D/3) ft/sec, 2.0 psi/100 ft; liquid pump suction, (1.3 + D/6) ft/sec, 0.4 psi/100 ft; steam or gas, 200 ft/sec, 0.5 psi/100 ft.
2 Control valves require at least 10 psi drop for good control
3 Globe valves are used for gases, for control and wherever tightshutoff is required Gate valves are for most other services
4 Screwed fittings are used only on sizes 1.5 in and smaller,flanges or welding otherwise
5 Flanges and fittings are rated for 150, 300, 600, 900, 1500, or
2500 psig.
6 Pipe schedule number = lOOOP/S, approximately, where P is the
internal pressure psig and S is the allowable working stress
(about 10,000 psi for A120 carbon steel at 500°F) Schedule 40 ismost common
Normal pump suction head (NPSH) of a pump must be in excess
of a certain number, depending on the kind of pumps and theconditions, if damage is to be avoided NPSH = (pressure at theeye of the impeller - vapor pressure)/(density) Common range
is 4-20 ft
Specific speed N, = (rpm)(gpm)0.5/(head in ft)‘.“ Pump may bedamaged if certain limits of N, are exceeded, and efficiency isbest in some ranges
Centrifugal pumps: Single stage for 15-5000gpm, 500ft maxhead; multistage for 20-11,000 gpm, 5500 ft max head Efficiency
45% at 100 gpm, 70% at 500 gpm, 80% at 10,000 gpm
Axial pumps for 20-100,000 gpm, 40 ft head, 65-85% efficiency.Rotary pumps for l-5000 gpm, 50,OOOft head, 50-80%efficiency
Reciprocating pumps for lo-10,000 gpm, l,OOO,OOO ft head max.Efficiency 70% at 10 HP, 85% at 50 HP, 90% at 500 HP
REACTORS
1 The rate of reaction in every instance must be established in thelaboratory, and the residence time or space velocity andproduct distribution eventually must be found in a pilot plant
2 Dimensions of catalyst particles are 0.1 mm in fluidized beds,
1 mm in slurry beds, and 2-5 mm in fixed beds
3 The optimum proportions of stirred tank reactors are withliquid level equal to the tank diameter, but at high pressuresslimmer proportions are economical
Trang 18Xviii RULES OF THUMB: SUMMARY
4 Power input to a homogeneous reaction stirred tank is 0.5-1.5
HP/lOOOgal, but three times this amount when heat is to be
transferred
5 Ideal CSTR (continuous stirred tank reactor) behavior is
approached when the mean residence time is 5-10 times the
length of time needed to achieve homogeneity, which is
accomplished with 500-2000 revolutions of a properly designed
Batch reactions are conducted in stirred tanks for small daily
production rates or when the reaction times are long or when
some condition such as feed rate or temperature must be
programmed in some way
Relatively slow reactions of liquids and slurries are conducted
in continuous stirred tanks A battery of four or five in series is
most economical
Tubular flow reactors ate suited to high production rates at
short residence times (set or min) and when substantial heat
transfer is needed Embedded tubes or shell-and-tube
construction then are used
In granular catalyst packed reactors, the residence time
distribution often is no better than that of a five-stage CSTR
battery
For conversions under about 95% of equilibrium, the
performance of a five-stage CSTR battery approaches plug
A ton of refrigeration is the removal of 12,000 Btu/hr of heat
At various temperature levels: O-50”F, chilled brine and glycol
solutions; -50-40”F, ammonia, freons, butane; -150 5O”F,
ethane or propane
Compression refrigeration with 100°F condenser requires these
HP/ton at various temperature levels: 1.24 at 20°F; 1.75 at 0°F;
3.1 at -40°F; 5.2 at -80°F
Below -80”F, cascades of two or three refrigerants are used
In single stage compression, the compression ratio is limited to
about 4
In multistage compression, economy is improved with interstage
flashing and recycling, so-called economizer operation
Absorption refrigeration (ammonia to -3O”F, lithium bromide to
+45”F) is economical when waste steam is available at 12 psig or
so
SIZE SEPARATION OF PARTICLES
1 Grizzlies that are constructed of parallel bars at appropriate
spacings are used to remove products larger than 5 cm dia
2 Revolving cylindrical screens rotate at 15-20 rpm and below the
critical velocity; they are suitable for wet or dry screening in the
range of lo-60 mm
3 Flat screens are vibrated or shaken or impacted with bouncing
balls Inclined screens vibrate at 600-70@0 strokes/min and are
used for down to 38 pm although capacity drops off sharply
below 200pm Reciprocating screens operate in the range
30-1000 strokes/min and handle sizes down to 0.25 mm at the
higher speeds
4 Rotary sifters operate at 500-600 rpm and are suited to a range
of 12 mm to 50 pm
5 Air classification is preferred for fine sizes because screens of 150
mesh and finer are fragile and slow
6 Wet classifiers mostly are used to make two product size ranges,
oversize and undersize, with a break commonly in the range
between 28 and 200 mesh A rake classifier operates at about 9
strokes/min when making separation at 200 mesh, and 32
strokes/min at 28 mesh Solids content is not critical, and that ofthe overflow may be 2-20% or more
7 Hydrocyclones handle up to 6OOcuft/min and can removeparticles in the range of 300-5 pm from dilute suspensions Inone case, a 20in dia unit had a capacity of 1000 gpm with apressure drop of 5 psi and a cutoff between 50 and 150 pm
UTILITIES: COMMON SPECIFICATIONS
Steam: 1.5-30 psig, 250-275°F; 150 psig, 366°F; 400 psig, 448°F;
600 psig, 488°F or with lOO-150°F superheat
Cooling water: Supply at 80-90°F from cooling tower, return at115-125°F; return seawater at llO”F, return tempered water orsteam condensate above 125°F
Cooling air supply at 85-95°F; temperature approach to process,40°F
Compressed air at 45, 150, 300, or 450 psig levels
Instrument air at 45 psig, 0°F dewpoint
Fuels: gas of lOOOBtu/SCF at 5-lopsig, or up to 25psig forsome types of burners; liquid at 6 million Btu/barrel
Heat transfer fluids: petroleum oils below 600”F, Dowthermsbelow 750”F, fused salts below lloo”F, direct fire or electricityabove 450°F
Liquid drums usually are horizontal
Gas/liquid separators are vertical
Optimum length/diameter = 3, but a range of 2.5-5.0 iscommon
Holdup time is 5 min half full for reflux drums, 5-10 min for aproduct feeding another tower
In drums feeding a furnace, 30 min half full is allowed.Knockout drums ahead of compressors should hold no less than
10 times the liquid volume passing through per minute.Liquid/liquid separators are designed for settling velocity of2-j in./min
Gas velocity in gas/liquid separators, V = kw ft/sec,with k = 0.35 with mesh deentrainer, k = 0.1 without meshdeentrainer
Entrainment removal of 99% is attained with mesh pads of4-12 in thicknesses; 6 in thickness is popular
For vertical pads, the value of the coefficient in Step 9 isreduced by a factor of 213
Good performance can be expected at velocities of 30-100% ofthose calculated with the given k; 75% is popular
Disengaging spaces of 6-18in ahead of the pad and 12in.above the pad are suitable
Cyclone separators can be designed for 95% collection of 5 pmparticles, but usually only droplets greater than 50 pm need beremoved
VESSELS (PRESSURE)
1 Design temperature between -20°F and 650°F is 50°F aboveoperating temperature; higher safety margins are used outsidethe given temperature range
2 The design pressure is 10% or 10-25 psi over the maximum ating pressure, whichever is greater The maximum operatingpressure, in turn, is taken as 25 psi above the normal operation
oper-3 Design pressures of vessels operating at 0-1Opsig and 1000°F are 40 psig
Trang 19600-4 For vacuum operation, design pressures are 15 psig and full
vacuum
5 Minimum wall thicknesses for rigidity: 0.25 in for 42 in dia and
‘under, 0.32 in for 42-60 in dia, and 0.38 in for over 60 in dia
6 Corrosion allowance 0.35 in for known corrosive conditions,
0.15 in for non-corrosive streams, and 0.06 in for steam drums
and air receivers
7 Allowable working stresses are one-fourth of the ultimate
strength of the material
8 Maximum allowable stress depends sharply on temperature
Temperature 1°F) - 2 0 - 6 5 0 750 850 1000
Low alloy steel SA203 (psi) 18,750 15,650 9550 2500
Type 302 stainless (psi) 18,750 18,750 15,900 6250
VESSELS (STORAGE TANKS)
1 For less than 1000 gal, use vertical tanks on legs
2 Between 1000 and 10,OOOgal, use horizontal tanks on concretesupports
3 Beyond 10,000 gal, use vertical tanks on concrete foundations
4 Liquids subject to breathing losses may be stored in tanks withfloating or expansion roofs for conservation
5 Freeboard is 15% below 500 gal and 10% above 500 gal capacity
6 Thirty days capacity often is specified for raw materials andproducts, but depends on connecting transportation equipmentschedules
7 Capacities of storage tanks are at least 1.5 times the size ofconnecting transportation equipment; for instance, 7500 gal tanktrucks, 34,500 gal tank cars, and virtually unlimited barge andtanker capacities
Trang 201 INTRODUCTION
design of individual equipment, some mention a n d e n e r g y b a l a n c e s a n d r a t e p r o c e s s e s T h i s c h a p t e r w i l l
- w i t h s e v e r a l o t h e r s i n a p l a n t , and the range of its required of flowsheets.
1.1 PROCESS DESIGN
Process design establishes the sequence of chemical and physical
operations; operating conditions; the duties, major specifications,
and materials of construction (where critical) of all process
equipment (as distinguished from utilities and building auxiliaries);
the general arrangement of equipment needed to ensure proper
functioning of the plant; line sizes; and principal instrumentation
The process design is summarized by a process flowsheet, a material
and energy balance, and a set of individual equipment
specifi-cations Varying degrees of thoroughness of a process design may be
required for different purposes Sometimes only a preliminary
design and cost estimate are needed to evaluate the advisability of
further research on a new process or a proposed plant expansion or
detailed design work; or a preliminary design may be needed to
establish the approximate funding for a complete design and
construction A particularly valuable function of preliminary design
is that it may reveal lack of certain data needed for final design
Data of costs of individual equipment are supplied in this book, but
the complete economics of process design is beyond its scope
1.2 EQUIPMENT
Two main categories of process equipment are proprietary and
custom-designed Proprietary equipment is designed by the
manufacturer to meet performance specifications made by the user;
these specifications may be regarded as the process design of the
equipment This category includes equipment with moving parts
such as pumps, compressors, and drivers as well as cooling towers,
dryers, filters, mixers, agitators, piping equipment, and valves, and
even the structural aspects of heat exchangers, furnaces, and other
equipment Custom design is needed for many aspects of chemical
reactors, most vessels, multistage separators such as fractionators,
and other special equipment not amenable to complete
stan-dardization
Only those characteristics of equipment are specified by process
design that are significant from the process point of view On a
pump, for instance, process design will specify the operating
conditions, capacity, pressure differential, NPSH, materials of
construction in contact with process liquid, and a few other items,
but not such details as the wall thickness of the casing or the type of
stuffing box or the nozzle sizes and the foundation
dimensions although most of these omitted items eventually must be known
before a plant is ready for construction Standard specification
forms are available for most proprietary kinds of equipment and for
summarizing the details of all kinds of equipment By providing
suitable check lists, they simplify the work by ensuring that all
needed data have been provided A collection of such forms is in
Appendix B
Proprietary equipment is provided “off the shelf’ in limited
sizes and capacities Special sizes that would fit particular
appli-cations more closely often are more expensive than a larger
standard size that incidentally may provide a worthwhile safetyfactor Even largely custom-designed equipment, such as vessels, issubject to standardization such as discrete ranges of head diameters,pressure ratings of nozzles, sizes of manways, and kinds of trays andpackings Many codes and standards are established by governmentagencies, insurance companies, and organizations sponsored byengineering societies Some standardizations within individualplants are arbitrary choices from comparable methods, made tosimplify construction, maintenance, and repair: for example,restriction to instrumentation of a particular manufacturer or to alimited number of sizes of heat exchanger tubing or a particularmethod of installing liquid level gage glasses All such restrictionsmust be home in mind by the process designer
VENDORS’ QUESTIONNAIRES
A manufacturer’s or vendor’s inquiry form is a questionnaire whosecompletion will give him the information on which to base a specificrecommendation of equipment and a price General informationabout the process in which the proposed equipment is expected tofunction, amounts and appropriate properties of the streamsinvolved, and the required performance are basic The nature ofadditional information varies from case to case; for instance, beingdifferent for filters than for pneumatic conveyors Individualsuppliers have specific inquiry forms A representative selection is
in Appendix C
SPECIFICATION FORMS
When completed, a specification form is a record of the salientfeatures of the equipment, the conditions under which it is tooperate, and its guaranteed performance Usually it is the basis for
a firm price quotation Some of these forms are made up byorganizations such as TEMA or API, but all large engineeringcontractors and many large operating companies have other formsfor their own needs A selection of specification forms is inAppendix B
1.3 CATEGORIES OF ENGINEERING PRACTICE
Although the design of a chemical process plant is initiated bychemical engineers, its complete design and construction requiresthe inputs of other specialists: mechanical, structural, electrical, andinstrumentation engineers; vessel and piping designers; andpurchasing agents who know what may be available at attractiveprices On large projects all these activities are correlated by a jobengineer or project manager; on individual items of equipment orsmall projects, the process engineer naturally assumes this function
A key activity is the writing of specifications for soliciting bids andultimately purchasing equipment Specifications must be written soexplicitly that the bidders are held to a uniform standard and aclear-cut choice can be made on the basis of their offerings alone
1
Trang 21% of Total Project Time
Figure 1.1 Progress of material commitment, engineering
manhours, and construction [Matozzi, Oil Gas J p 304, (23 March
1953)].
% of Total Project Time
Figure 1.2 Rate of application of engineering manhours of various
categories The area between the curves represents accumulated
manhours for each speciality up to a given % completion of the
project [Miller, Chem Eng., p 188, (July 1956)].
For a typical project, Figure 1.1 shows the distributions of
engineering, material commitment, and construction efforts Of the
engineering effort, the process engineering is a small part Figure
1.2 shows that it starts immediately and finishes early In terms of
money, the cost of engineering ranges from 5 to 15% or so of the
total plant cost; the lower value for large plants that are largely
patterned after earlier ones, and the higher for small plants or those
based on new technology or unusual codes and specifications
1.4 SOURCES OF INFORMATION FOR PROCESS DESIGN
A selection of books relating to process design methods and data is
listed in the references at the end of this chapter Items that are
especially desirable in a personal library or readily accessible are
identified Specialized references are given throughout the book in
connection with specific topics
The extensive chemical literature is served by the bibliographic
items cited in References, Section 1.2, Part B The book by
Rasmussen and Fredenslund (1980) is addressed to chemical
~engineers and cites some literature not included in some of the
other bibliographies, as well as information about proprietary data
banks The book by Leesley (References, Section 1.1, Part B) has
much information about proprietary data banks and design
methods In its current and earlier editions, the book by Peters and
Timmerhaus has many useful bibliographies on classified topics
For information about chemical manufacturing processes, the
main encyclopedic references are Kirk-Othmer (1978-1984),
McKetta and Cunningham (1976-date) and Ullmann (1972-1983)
(References, Section 1.2, Part B) The last of these is in German,
but an English version was started in 1984 and three volumes peryear are planned; this beautifully organized reference should bemost welcome
The most comprehensive compilation of physical property data
is that of Landolt-Bornstein (1950-date) (References, Section 1.2,Part C) Although most of the material is in German, recentvolumes have detailed tables of contents in English and somevolumes are largely in English Another large compilation,somewhat venerable but still valuable, is the International CriticalTables (1926-1933) Data and methods of estimating properties ofhydrocarbons and their mixtures are in the API Data Book(1971-date) (References, Section 1.2, Part C) More generaltreatments of estimation of physical properties are listed inReferences, Section 1.1, Part C There are many compilations ofspecial data such as solubilities, vapor pressures, phase equilibria,transport and thermal properties, and so on A few of them arelisted in References, Section 1.2, Part D, and references to manyothers are in the References, Section 1.2, Part B
Information about equipment sizes and configurations, andsometimes performance, of equipment is best found in manufac-turers’ catalogs Items 1 and 2 of References, Section 1.1, Part D,contain some advertisements with illustrations, but perhaps theirprincipal value is in the listings of manufacturers by the kind ofequipment Thomas Register covers all manufacturers and so is lessconvenient at least for an initial search The other three items ofthis group of books have illustrations and descriptions of all kinds ofchemical process equipment Although these books are old, one issurprised to note how many equipment designs have survived
1.5 CODES, STANDARDS, AND RECOMMENDED PRACTICES
A large body of rules has been developed over the years to ensurethe safe and economical design, fabrication and testing ofequipment, structures, and materials Codification of these ruleshas been done by associations organized for just such purposes,
by professional societies, trade groups, insurance underwritingcompanies, and government agencies Engineering contractors andlarge manufacturing companies usually maintain individual sets ofstandards so as to maintain continuity of design and to simplifymaintenance of plant Table 1.1 is a representative table of contents
of the mechanical standards of a large oil company
Typical of the many thousands of items that are standardized inthe field of engineering are limitations on the sizes and wallth,icknesses of piping, specifications of the compositions of alloys,stipulation of the safety factors applied to strengths of constructionmaterials, testing procedures for many kinds of materials, and so
o n Although the safe design practices recommended by profes-sional and trade associations have no legal standing where they havenot actually been incorporated in a body of law, many of them havethe respect and confidence of the engineering profession as a wholeand have been accepted by insurance underwriters so they arewidely observed Even when they are only voluntary, standardsconstitute a digest of experience that represents a minimum re-quirement of good practice
Two publications by Burklin (References, Section 1.1, Part B)are devoted to standards of importance to the chemical industry.Listed are about 50 organizations and 60 topics with which they areconcerned National Bureau of Standards Publication 329 containsabout 25,000 titles of U.S standards The NBS-SIS servicemaintains a reference collection of 200,000 items accessible by letter
or phone Information about foreign standards is obtainablethrough the American National Standards Institute (ANSI)
A listing of codes and standards bearing directly on process
Trang 22TABLE 1.1 Internal Engineering Standards of a Large
1 6 Heat exchangers (IO)
1 7 Instruments and controls (45)
1 8 Insulation (IO)
1 9 Machinery (35)
2 0 Material procurement and disposition (20)
2 1 Material selection (5)
2 2 Miscellaneous process equipment (25)
2 3 Personnel protective equipment (5)
2 4 P i p i n g (150)
2 5 Piping supports (25)
2 6 Plant layout (20)
2 7 Pressure vessels (25)
2 8 Protective coatings (IO)
2 9 Roads and railroads (25)
3 0 Storage vessels (45)
3 1 Structural (35)
3 2 Symbols and drafting practice (15)
3 3 Welding (10)
‘Figures in parentheses identify the numbers of distinct standards.
TABLE 1.2 Codes and Standards of Direct Bearin on
Chemical Process Design (a Selection
A American Institute of Chemical Engineers, 345 E 47th St., New York,
NY 10017
1 Standard testing procedures; 21 have been published, for
example on centrifuges, filters, mixers, firer heaters
B American Petroleum Institute, 2001 L St NW, Washington, DC 20037
2 Recommended practices for refinery inspections
3 Guide for inspection of refinery equipment
4 Manual on disposal of refinery wastes
5 Recommended practice for design and construction of large, low
pressure storage tanks
6 Recommended practice for design and construction of pressure
r e l i e v i n g d e v i c e s
7 Recommended practices for safety and fire protection
C American Society of Mechanical Engineers, 345 W 47th St., New
York, NY 10017
8 ASME Boiler and Pressure Vessel Code Sec VIII, Unfired
Pressure Vessels
9 Code for pressure piping
10; Scheme for identification of piping systems
D American Society for Testing Materials, 1916 Race St., Philadelphia,
PA 19103
11 ASTM Standards, 66 volumes in 16 sections, annual, with about
30% revision each year
E American National Standards Institute (ANSI), 1430 Broadway, New
York, NY 10018
12 Abbreviations, letter symbols, graphical symbols, drawing and
1.6 MATERIAL AND ENERGY BALANCES 3
14 Chemical safety data sheets of individual chemicals
G Cooling Tower Institute, 19827 Highway 45 N, Spring, TX 77388
15 Acceptance test procedure for water cooling towers of mechanical draft industrial type
H Hydraulic Institute, 712 Lakewood Center N, 14600 Detroit Ave., Cleveland, OH 44107
16 Standards for centrifugal, reciprocating, and rotary pumps
17 Pipe friction manual
I Instrument Society of America (ISA), 67 Alexander Dr., Research Triangle Park, NC 27709
18 Instrumentation flow plan symbols
19 Specification forms for instruments
20 Dynamic response testing of process control instrumentation
J Tubular Exchangers Manufacturers’ Association, 25 N Broadway, Tarrytown, NY 10591
A American Concrete Institute, 22400 W 7 Mile Rd., Detroit, Ml 48219
1 Reinforced concrete design handbook
2 Manual of standard practice for detailing reinforced concrete structures
B American Institute of Steel Construction, 400 N Michigan Ave., Chicago, IL 60611
3 Manual of steel construction
4 Standard practice for steel buildings and bridges
C American Iron and Steel Institute, 1000 16th St NW, Washington, DC 20036
5 AISI standard steel compositions
D American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRE), 1791 Tullie Circle NE, Atlanta, GA 30329
6 Refrigerating data book
E Institute of Electrical and Electronics Engineers, 345 E 47th St., New York, NY 10017
7 Many standards
F National Bureau of Standards, Washington, DC
8 American standard building code
9 National electrical code
G National Electrical Manufacturers Association, 2101 L St NW, Washington, DC 20037
10 NEMA standards
design is in Table 1.2, and of supplementary codes and standards inTable 1.3
1.6 MATERIAL AND ENERGY BALANCES
Material and energy balances arc based on a conservation law which
is stated generally in the forminput + source = output + sink + accumulation
The individual terms can be plural and can be rates as well asabsolute quantities Balances of particular entities are made around
a bounded region called a system Input and output quantities of anentity cross the boundaries A source is an increase in the amount
Trang 23of the entity that occurs without a crossing of the boundary; for
example, an increase in the sensible enthalpy or in the amount of a
substance as a consequence of chemical reaction Analogously,
sinks are decreases without a boundary crossing, as the
dis-appearance of water from a fluid stream by adsorption onto a solid
phase within the boundary
Accumulations are time rates of change of the amount of the
entities within the boundary For example, in the absence of sources
and sinks, an accumulation occurs when the input and output rates
are different In the steady state, the accumulation is zero
Although the principle of balancing is simple, its application
requires knowledge of the performance of all the kinds of
equipment comprising the system and of the phase relations and
physical properties of all mixtures that participate in the process As
a consequence of trying to cover a variety of equipment and
processes, the books devoted to the subject of material and energy
balances always run to several hundred pages Throughout this
book, material and energy balances are utilized in connection with
the design of individual kinds of equipment and some processes
Cases involving individual pieces of equipment usually are relatively
easy to balance, for example, the overall balance of a distillation
column in Section 13.4.1 and of nonisothermal reactors of Tables
17.4-17.7 When a process is maintained isothermal, only a
material balance is needed to describe the process, unless it is also
required to know the net heat transfer for maintaining a constant
temperature
In most plant design situations of practical interest, however,
the several pieces of equipment interact with each other, the output
of one unit being the input to another that in turn may recycle part
of its output to the inputter Common examples are an
absorber-stripper combination in which the performance of the
absorber depends on the quality of the absorbent being returned
from the stripper, or a catalytic cracker-catalyst regenerator system
whose two parts interact closely
Because the performance of a particular piece of equipment
depends on its input, recycling of streams in a process introduces
temporarily unknown, intermediate streams whose amounts,
com-positions, and properties must be found by calculation For a
plant with dozens or hundreds of streams the resulting mathematical
problem is formidable and has led to the development of many
computer algorithms for its solution, some of them making quite
rough approximations, others more nearly exact Usually the
problem is solved more easily if the performance of the equipment
is specified in advance and its size is found after the balances are
completed If the equipment is existing or must be limited in size,
the balancing process will require simultaneous evaluation of its
performance and consequently is a much more involved operation,
but one which can be handled by computer when necessary
The literature of this subject naturally is extensive An early
book (for this subject), Nagiev’s Theory of Recycle Processes in
Chemical Engineering (Macmillan, New York, 1964, Russian
edition, 1958) treats many practical cases by reducing them to
systems of linear algebraic equations that are readily solvable The
book by Westerberg et al., Process Flows/reefing (Cambridge Univ
Press, Cambridge, 1977) describes some aspects of the subject and
has an extensive bibliography Benedek in Steady State Flowsheering
of Chemical Plants (Elsevier, New York, 1980) provides a detailed
description of one simulation system Leesley in Computer-Aided
Process Design (Gulf, Houston, 1982) describes the capabilities of
some commercially available flowsheet simulation programs Some
of these incorporate economic balance with material and energy
balances A program MASSBAL in BASIC language is in the book
of Sinnott et al., Design, Vol 6 (Pergamon, New York, 1983); it
can handle up to 20 components and 50 units when their several
outputs are specified to be in fixed proportions
distillation column (2) Ar) designates the amount of component A
in stream k proceeding from unit i to unit j Subscripts 0 designates
a source or sink beyond the boundary limits I designates a totalflow quantity
A key factor in the effective formulation of material and energybalances is a proper notation for equipment and streams Figure1.3, representing a reactor and a separator, utilizes a simple type.When the pieces of equipment are numbered i and j, the notation
A$!‘) signifies the flow rate of substance A in stream k proceedingfrom unit i to unit j The total stream is designated IF) Subscript tdesignates a total stream and subscript 0 designates sources or sinksoutside the system Example 1.1 adopts this notation for balancing areactor-separator process in which the performances are specified
considera-in operatconsidera-ing labor cost Somewhere considera-in the summation of thesefactors there is a minimum which should be the design point in theabsence of any contrary intangibles such as building for the future
or unusual local conditions
Costs of many individual pieces of equipment are summarized
in Chapter 20, but analysis of the costs of complete processes isbeyond the scope of this book References may be made, however,
to several collections of economic analyses of chemical engineeringinterest that have been published:
1 AIChE Student Contest Problems (annual) (AIChE, NewYork)
Trang 241.7 ECONOMIC BALANCE 5
Material Balance of a Chlorination Process with Recycle
A plant for the chlorination has the flowsheet shown From pilot
reactor and separator no 3 returns 90% of the benzene Bothplant work, with a chlorine/benzene charge weight ratio of 0.82, the recycle streams are pure Fresh chlorine is charged at such a ratethat the weight ratio of chlorine to benzene in the total chargecomposition of the reactor effluent is remains 0.82 The amounts of other streams are-found by material
A C,H, balances and are shown in parentheses on the sketch per 100 lbs of
2 Bodman, Industrial Practice of Chemical Process Engineering
(MIT Press, Cambridge, MA, 1968)
3 Rase, Chemical Reactor Design for Process Plants, Vol II, Case
Studies (Wiley, New York, 1977)
4 Washington University, St Louis, Case Studies in Chemical
Engineering Design (22 cases to 1984)
Somewhat broader in scope are:
5 Wei et al., The Structure of the Chemical Processing Industries
(McGraw-Hill, New York, 1979)
6 Skinner et al., Manufacturing Policy in the Oil Industry (Irwin,
Homewood, IL., 1970)
I Skinner et al., Manufacturing Policy in the Plastics Industry
(Irwin, Homewood, Il., 1968)
Many briefer studies of individual equipment appear in some
books, of which a selection is as follows:
l Happel and Jordan, Chemical Process Economics (Dekker, New
York, 1975):
1 Absorption of ethanol from a gas containing CO, (p 403).
2 A reactor-separator for simultaneous chemical reactions (p
419)
3 Distillation of a binary mixture (p 38.5)
4 A heat exchanger and cooler system (p 370)
5 Piping of water (p 353)
6 Rotary dryer (p 414)
l Jelen et al., Cost and Optimization Engineering (McGraw-Hill,New York, 1983):
7 Drill bit life and replacement policy (p 223)
8 Homogeneous flow reactor (p 229)
9 Batch reaction with negligible downtime (p 236)
l Peters and Timmerhaus, Plant Design and Economics for Chemical Engineers (McGraw-Hill, New York, 1980):
10 Shell and tube cooling of air with water (p 688).
l Rudd and Watson, Strategy of Process Engineering (Wiley, NewYork, 1968):
11 Optimization of a three stage refrigeration system (p 172).
l Sherwood, A Course in Process Design (MIT Press, Cambridge,
MA, 1963):
12 Gas transmission line (p 84)
13 Fresh water from sea water by evaporation (p 138)
l Ulrich, A Guide to Chemical Engineering Process Design and Economics (Wiley, New York, 1984):
14 Multiple effect evaporator for Kraft liquor (p 347)
l Walas, Reaction Kinetics for Chemical Engineers (McGraw-Hill,New York, 1959):
15 Optimum number of vessels in a CSTR battery (p 98)
Since capital, labor, and energy costs have not escalatedequally over the years since these studies were made, theirconclusions are subject to reinterpretation, but the patterns of studythat were used should be informative
Because of the rapid escalation of energy costs in recent years,
Trang 25closer appraisals of energy utilizations by complete processes are
being made, from the standpoints of both the conservation laws and
the second law of thermodynamics In the latter cases attention is
focused on changes in entropy and in the related availability
function, AB = AH - &AS, with emphasis on work as the best
possible transformation of energy In this way a second law analysis
of a process will reveal where the greatest generation of entropy
occurs and where possibly the most improvement can be made by
appropriate changes of process or equipment Such an analysis of a
cryogenic process for air separation was made by Benedict and
Gyftopolous [in Gaggioli (Ed.), Thermodynamic Second Law
Analysis, ACS Symposium Series No 122, American Chemical
Society, Washington, DC, 19801; they found a pressure drop at
which the combination of exchanger and compressor was most
economical
A low second law efficiency is not always realistically
improv-able Thus Weber and Meissner (Thermodynamics for Chemical
Engineers, John Wiley, New York, 1957) found a 6% efficiency for
the separation of ethanol and water by distillation which is not
substantially improvable by redesign of the distillation process
Perhaps this suggests that more efficient methods than distillation
should be sought for the separation of volatile mixtures, but none
has been found at competitive cost
Details of the thermodynamic basis of availability analysis are
dealt with by Moran (Availability Annfysb, Prentice-Hall,
Englewood Cliffs, NJ, 1982) He applies the method to a cooling
tower, heat pump, a cryogenic process, coal gasification, and
par-ticularly to the efficient use of fuels
An interesting conclusion reached by Linnhoff [in Seider and
Mah (Eds.), Foundations of Computer-Aided Process Design,
AIChE, New York, 19811 is that “chemical processes which are
properly designed for energy versus capital cost tend to operate at
approximately 60% efficiency.” A major aspect of his analysis is
recognition of practical constraints and inevitable losses These may
include material of construction limits, plant layout, operability, the
need for simplicity such as limits on the number of compressor
stages or refrigeration levels, and above all the recognition that, for
low grade heat, heat recovery is preferable to work recovery, the
latter being justifiable only in huge installations Unfortunately, the
edge is taken off the dramatic 60% conclusion by Linnhoff’s
admission that efficiency cannot be easily defined for some
complexes of interrelated equipment For example, is it economical
to recover 60% of the propane or 60% of the ethane from a natural
gas?
1.8 SAFETY FACTORS
In all of the factors that influence the performance of equipment
and plant there are elements of uncertainty and the possibility of
error, including inaccuracy of physical data, basic correlations of
behavior such as pipe friction or tray efficiency or gas-liquid
distribution, necessary approximations of design methods and
calculations, not entirely known behavior of materials of
con-struction, uncertainty of future market demands, and changes in
operating performance with time The solvency of the project, the
safety of the operators and the public, and the reputation and
career of the design engineer are at stake Accordingly, the
experienced engineer will apply safety factors throughout the design
of a plant Just how much of a factor should be applied in a
particular case cannot be stated in general terms because
cir-cumstances vary widely The inadequate performance of a
particular piece of equipment may be compensated for by the
superior performance of associated equipment, as insufficient trays
in a fractionator may be compensated for by increases in reflux and
reboiling, if that equipment can take the extra load
With regard to specific types of equipment, the safety factorpractices of some 250 engineers were ascertained by a questionnaireand summarized in Table 1.4; additional figures are given by Petersand Timmerhaus (References, Section 1.1, Part B, pp 35-37).Relatively inexpensive equipment that can conceivably serve as abottleneck, such as pumps, always is liberally sized; perhaps asmuch as 50% extra for a reflux pump In an expanding industry it is
a matter of policy to deliberately oversize certain major equipmentthat cannot be supplemented readily or modified suitably forincreased capacity; these are safety factors to account for futuretrends
Safety factors should not be used to mask inadequate orcareless design work The design should be the best that can bemade in the time economically justifiable, and the safety factorsshould be estimated from a careful consideration of all factorsentering into the design and the possible future deviations from thedesign conditions
Sometimes it is possible to evaluate the range of validity ofmeasurements and correlations of physical properties, phaseequilibrium behavior, mass and heat transfer efficiencies and similarfactors, as well as the fluctuations in temperature, pressure, flow,etc., associated with practical control systems Then the effects ofsuch data on the uncertainty of sizing equipment can be estimated.For example, the mass of a distillation column that is relateddirectly to its cost depends on at least these factors:
1 The vapor-liquid equilibrium data
2 The method of calculating the reflux and number of trays
3 The tray efficiency
4 Allowable vapor rate and consequently the tower diameter at agiven tray spacing and estimated operating surface tension andfluid densities
5 Corrosion allowances
Also such factors as allowable tensile strengths, weld efficiencies,and possible inaccuracies of formulas used to calculate shell andhead thicknesses may be pertinent
When a quantity is a function of several variables,
that is, the relative uncertainty or error in the function is relatedlinearly to the fractional uncertainties of the independent variables.For example, take the case of a steam-heated thermosyphonreboiler on a distillation column for which the heat transferequation is
q = UAAT.
The problem is to find how the heat transfer rate can vary when theother quantities change U is an experimental value that is known
Trang 261.9 SAFETY OF PLANT AND ENVIRONMENT 7
TABLE 1.4 Safety Factors in Equipment Design: Results of a Questionnaire
Equipment Design Variable Range of Safety Factor 1 % ) Compressors, reciprocating piston displacement
H a m m e r m i l l s power input Filters, plate-and-frame a r e a
Heat exchangers, shell and tube for a r e a
l i q u i d s Pumps, centrifugal impeller diameter Separators, cyclone diameter Towers, packed d i a m e t e r Towers, tray d i a m e t e r Water cooling towers volume
B Based on pilot plant tests.
[Michelle, Beattie, and Goodgame, Chem Eng frog 50,332 (1954)).
11-21 8-21 15-21”
ll-218 14-20’
11-18
7 - 1 4 7-11 11-18 lo-16 12-20
only to a certain accuracy AT may be uncertain because of possible
fluctuations in regulated steam and tower pressures A, the effective
area, may be uncertain because the submergence is affected by the
liquid level controller at the bottom of the column Accordingly,
dq dU dA d(AT)
that is, the fractional uncertainty of q is the sum of the fractional
uncertainties of the quantities on which it is dependent In practical
cases, of course, some uncertainties may be positive and others
negative, so that they may cancel out in part; but the only safe
viewpoint is to take the sum of the absolute values Some further
discussion of such cases is by Sherwood and Reed, in Applied
Mathematics in Chemical Engineering (McGraw-Hill, New York,
1939)
It is not often that proper estimates can be made of
uncertainties of all the parameters that influence the performance or
required size of particular equipment, but sometimes one particular
parameter is dominant All experimental data scatter to some
extent, for example, heat transfer coefficients; and various
cor-relations of particular phenomena disagree, for example, equations
of state of liquids and gases The sensitivity of equipment sizing to
uncertainties in such data has been the subject of some published
information, of which a review article is by Zudkevich [Encycl
Chem Proc Des 14, 431-483 (1982)]; some of his cases are:
1 Sizing of isopentane/pentane and propylene/propane splitters
2 Effect of volumetric properties on sizing of an ethylene
compressor
3 Effect of liquid density on metering of LNG
4 Effect of vaporization equilibrium ratios, K, and enthalpies on
cryogenic separations
5 Effects of VLE and enthalpy data on design of plants for
coal-derived liquids
Examination of such studies may lead to the conclusion that some
of the safety factors of Table 1.4 may be optimistic But long
experience in certain areas does suggest to what extent various
uncertainties do cancel out, and overall uncertainties often do fall in
the range of lo-20% as stated there Still, in major cases the
uncertainty analysis should be made whenever possible
1.9 SAFETY OF PLANT AND ENVIRONMENT
The safe practices described in the previous section are primarily for
assurance that the equipment have adequate performance over
anticipated ranges of operating conditions In addition, the design
of equipment and plant must minimize potential harm to personnel
and the public in case of accidents, of which the main causes are
a human failure,
b failure of equipment or control instruments,
c failure of supply of utilities or key process streams,
d environmental events (wind, water, and so on)
A more nearly complete list of potential hazards is in Table 1.5, and
a checklist referring particularly to chemical reactions is in Table1.6
Examples of common safe practices are pressure relief valves,vent systems, flare stacks, snuffing steam and fire water, escapehatches in explosive areas, dikes around tanks storing hazardousmaterials, turbine drives as spares for electrical motors in case ofpower failure, and others Safety considerations are paramount inthe layout of the plant, particularly isolation of especially hazardousoperations and accessibility for corrective action when necessary.Continual monitoring of equipment and plant is standardpractice in chemical process plants Equipment deteriorates andoperating conditions may change Repairs sometimes are made with
“improvements” whose ultimate effects on the operation may not
be taken into account During start-up and shut-down, streamcompositions and operating conditions are much different fromthose under normal operation, and their possible effect on safetymust be taken into account Sample checklists of safety questionsfor these periods are in Table 1.7
Because of the importance of safety and its complexity, safetyengineering is a speciality in itself In chemical processing plants ofany significant size, loss prevention reviews are held periodically bygroups that always include a representative of the safety depart-ment Other personnel, as needed by the particular situation, arefrom manufacturing, maintenance, technical service, and possiblyresearch, engineering, and medical groups The review considersany changes made since the last review in equipment, repairs,feedstocks and products, and operating conditions
Detailed safety checklists appear in books by Fawcett andWood (Chap 32, Bibliography 1.1, Part E) and Wells (pp.239-257, Bibliography 1.1, Part E) These books and the large one
by Lees (Bibliography 1.1, Part E) also provide entry into the vastliterature of chemical process plant safety Lees has particularlycomplete bibliographies A standard reference on the properties ofdangerous materials is the book by Sax (1984) (References, Section1.1, Part E) The handbook by Lund (1971) (References, Section1.1, Part E) on industrial pollution control also may be consulted
Trang 27TABLE 1.5 Some Potential Hazards
Energy Source
Process chemicals, fuels, nuclear reactors, generators, batteries
Source of ignition, radio frequency energy sources, activators,
Spillage, leakage, vented material
Exposure effects, toxicity, burns, bruises, biological effects
Flammability, reactivity, explosiveness, corrosivity and fire-promoting
properties of chemicals
Wetted surfaces, reduced visibility, falls, noise, damage
Dust formation, mist formation, spray
Fire hazard
Fire, fire spread, fireballs, radiation
Explosion, secondary explosion, domino effects
Noise, smoke, toxic fumes, exposure effects
Collapse, falling objects, fragmentation
Process state
High/low/changing temperature and pressure
Stress concentrations, stress reversals, vibration, noise
Structural damage or failure, falling objects, collapse
Electrical shock and thermal effects, inadvertent activation, power
source failure
Radiation, internal fire, overheated vessel
Failure of equipment/utility supply/flame/instrument/component
Start-up and shutdown condition
Maintenance, construction and inspection condition
Environmental effects
Effect of plant on surroundings, drainage, pollution, transport, wind
and light change, source of ignition/vibration/noise/radio
interference/fire spread/explosion
Effect of surroundings on plant (as above)
Climate, sun, wind, rain, snow, ice, grit, contaminants, humidity,
ambient conditions
Acts of God, earthquake, arson, flood, typhoon, force majeure
Site layout factors, groups of people, transport features, space
limitations, geology, geography
Processes
Processes subject to explosive reaction or detonation
Processes which react energetically with water or common
contaminants
Processes subject to spontaneous polymerisation or heating
Processes which are exothermic
Processes containing flammables and operated at high pressure or
high temperature or both
Processes containing flammables and operated under refrigeration
Processes in which intrinsically unstable compounds are present
Processes operating in or near the explosive range of materials
Processes involving highly toxic materials
Processes subject to a dust or mist explosion hazard
Processes with a large inventory of stored pressure energy
Operations
The vaporisation and diffusion of flammable or toxic liquids or gases
The dusting and dispersion of combustible or toxic solids
The spraying, misting or fogging of flammable combustible materials
or strong oxidising agents and their mixing
The separation of hazardous chemicals from inerts or diluents
The temperature and pressure increase of unstable liquids
(Wells, Safety in Process P/ant Design, George Godwin, L o n d o n ,
3 What unwanted hazardous reactions can be developed through unlikely flow or process conditions or through contamination?
4 What combustible mixtures can occur within equipment?
5 What precautions are taken for processes operating near or within the flammable limits? (Reference: S&PP Design Guide No 8.) (See Chap 19)
6 What are process margins of safety for all reactants and intermediates in the process?
7 List known reaction rate data on the normal and possible abnormal reactions
8 How much heat must be removed for normal, or abnormally possible, exothermic reactions? (see Chaps 7, 17, and 18)
9 How thoroughly is the chemistry of the process including desired
and undesired reactions known? (See NFPA 491 M, Manual of
Hazardous Chemical Reactions)
10 What provision is made for rapid disposal of reactants if required by
14 Review provisions for blockage removal or prevention
15 What raw materials or process materials or process conditions can
be adversely affected by extreme weather conditions? Protect against such conditions
16 Describe the process changes including plant operation that have been made since the previous process safety review
(Fawcett and Wood, Safety and Accident Prevention in Chemical
Operations, Wiley, New York, 1982, pp 725-726 Chapter references
refer to this book.)
TABLE 1.7 Safety Checklist of Questions About Start-up and
Shut-down Start-up Mode (54.1)
Dl Can the start-up of plant be expedited safely? Check the following: (a)
lb) (4 (d (e) (f)
(cl) (h)
(i) 0) (k) (I)
Abnormal condentrations, phases, temperatures, pressures, levels, flows, densities
Abnormal quantities of raw materials, intermediates and utilities (supply, handling and availability)
Abnormal quantities and types of effluents and emissions (91.6.10)
Different states of catalyst, regeneration, activation Instruments out of range, not in service or de-activated, incorrect readings, spurious trips
Manual control, wrong routeing, sequencing errors, poor identification of valves and lines in occasional use, lock-outs, human error, improper start-up of equipment (particularly prime movers)
I s o l a t i o n , p u r g i n g Removal of air, undesired process material, chemicals used for cleaning, inerts, water, oils, construction debris and ingress of same
Recycle or disposal of off-specification process materials Means for ensuring construction/maintenance completed Any plant item failure on initial demand and during operation in this mode
Lighting of flames, introduction of material, limitation of
Trang 28TABLE 1.7~(continued)
(m) Different modes of the start-up of plant:
Initial start-up of plant
Start-up of plant section when rest of plant down
Start-up of plant section when other plant on-stream
Start-up of plant after maintenance
Preparation of plant for its start-up on demand
Shut-down Mode [884.1,4.2)
D2 Are the limits of operating parameters, outside which remedial
action must be taken, known and measured? (Cl above)
D3 To what extent should plant be shut down for any deviation beyond
the operating limits? Does this require the installation of alarm
and/or trip? Should the plant be partitioned differently? How is
plant restarted? (59.6)
D4 In an emergency, can the plant pressure and/or the inventory of
process materials be reduced effectively, correctly, safely? What is
the fire resistance of plant (@9.5,9.6)
D5 Can the plant be shut down safely? Check the following:
(a) See the relevant features mentioned under start-up mode
(b) Fail-danger faults of protective equipment
(c) Ingress of air, other process materials, nitrogen, steam, water, lube
oil (54.3.5)
(d) Disposal or inactivation of residues, regeneration of catalyst,
decoking, concentration of reactants, drainage, venting
(e) Chemical, catalyst, or packing replacement, blockage removal,
delivery of materials prior to start-up of plant
(f) Different modes of shutdown of plant:
Normal shutdown of plant
Partial shutdown of plant
Placing of plant on hot standby
Emergency shutdown of plant
(Wells, Safety in Process Plant Design, George Godwin, L o n d o n ,
1980, pp 243-244 Paragraph references refer to this book.)
1.10 STEAM AND POWER SUPPLY
For smaller plants or for supplementary purposes, steam and powercan be supplied by package plants which are shippable and ready
to hook up to the process Units with capacities in a range ofsizes up to about 350,OOOlb/hr of steam are on the market,and are obtainable on a rental/purchase basis for emergencyneeds
Modem steam plants are quite elaborate structures that canrecover 80% or more of the heat of combustion of the fuel Thesimplified sketch of Example 1.2 identifies several zones of heattransfer in the equipment Residual heat in the flue gas is recovered
as preheat of the water in an economizer and in an air preheater.The combustion chamber is lined with tubes along the floor andwalls to keep the refractory cool and usually to recover more thanhalf the heat of combustion The tabulations of this example are ofthe distribution of heat transfer surfaces and the amount of heattransfer in each zone
More realistic sketches of the cross section of a steam generatorare in Figure 1.4 Part (a) of this figure illustrates the process ofnatural circulation of water between an upper steam drum and alower drum provided for the accumulation and eventual blowdown
of sediment In some installations, pumped circulation of the water
In plants such as oil refineries that have many streams at hightemperatures or high pressures, their energy can be utilized togenerate steam or to recover power The two cases of Example 1.4
E X A M P L E 1.2
Data of a Steam Generator for Making 25O,OOOIb/br at
450 psia and 650°F from Water Entering at 224lT
Fuel oil of 18,500 Btu/lb is fired with 13% excess air at 80°F Flue
gas leaves at 410°F A simplified cross section of the boiler is shown
Heat and material balances are summarized Tube selections and
arrangements for the five heat transfer zones also are summarized
The term A, is the total internal cross section of the tubes in
parallel, (Steam: Its Generation and Use, 14.2, Babcock and
Wilcox, Barberton, OH, 1972) (a) Cross section of the generator:
Total to water and steam 285.4 Mbtu/hr
In air heater 18.0 MBtu/hr
(c) Tube quantity, size, and grouping:
Screen
2 rows of 2&-m OD tubes, approx 18 ft long
Rows in line and spaced on 6-in centers
23 tubes per row spaced on 6-in centers
S = 542 sqft
A, = 129 sqft
Trang 29EXAMPLE 1.2-(continued)
Superheater
12 rows of 2$-in OD tubes (0.165-in thick),
17.44 ft long
Rows in line and spaced on 3$in centers
23 tubes per row spaced on 6-in centers
S = 3150 sqft
A, = 133 sqft
Boiler
25 rows of 2&in OD tubes, approx 18 ft long
Rows in line and spaced on 3$-in centers
35 tubes per row spaced on 4-m centers
Rows in line and spaced on 3-m centers
47 tubes per row spaced on 3-m centers
S = 2460 sqft
A, = 42 sqftAir heater
53 rows of 2-in OD tubes (0.083-in thick),approx 13 ft long
Rows in line and spaced on 2$-in centers
41 tubes per row spaced on 3&n centers
(a)
lb)
Steam out
Downcomer not Heated
Trang 301 1 0 S T E A M A N D P O W E R S U P P L Y 11
E XAMPLE 1.3
Steam Plant Cycle for Generation of Power and Low Pressure
Process Steam
The flow diagram is for the production of 5000 kW gross and
20,000 lb/hr of saturated process steam at 20 psia The feed and hot
well pumps make the net power production 4700 kW Conditions at
L
’ + 60,8OOw-2Op-228%-I 156h n-x
~.~00n~-400p-655%1337h O w
Reducing valve (and desuperhecled
’ 26 Ib.hq in obo
’ Iin Hg& dry and so?.
1 3 3 7
E XAMPLE 1.4
Pickup of Waste Heat by Generating and Superheating Steam
in a Petroleum Refinery
The two examples are generation of steam with heat from a
sidestream of a fractionator in a 9000 Bbl/day fluid cracking plant,
and superheating steam with heat from flue gases of a furnace
by generating steam, Q = 15,950,OOO Btu/hr (b) Heat recovery bysuperheating steam with flue gases of a 20,OOOBbl/day crudetopping and vacuum furnace
of propane is the indirect means whereby the power is recovered the rich liquor through a turbine
Trang 31E XAMPLE 1.5
Recovery of Power from a Hot Gas Stream
A closed circuit of propane is employed for indirect recovery of
power from the thermal energy of the hot pyrolyzate of an ethylene
plant The propane is evaporated at 500 psig, and then expanded to
PROPANE
34700 pph y-fFoy=TE 500 psig
195F
5600 pph
190 psig 1OOF
I
CONDENSER
100°F and 190 psig in a turbine where the power is recovered Then
the propane is condensed and pumped back to the evaporator to
complete the cycle Since expansion turbines are expensive
machines,even in small sizes, the process is not economical on the
scale of this example, but may be on a much larger scale
T U R B I N E
_ 75%eff
204.6 HP
Before a chemical process design can be properly embarked on, a
certain body of information must be agreed upon by all concerned
persons, in addition to the obvious what is to be made and what it is
to be made from Distinctions may be drawn between plant
expansions and wholly independent ones, so-called grassroots types
The needed data can be classified into specific design data and basic
design data, for which separate check lists will be described Specific
design data include:
Required products: their compositions, amounts, purities,
toxicities, temperatures, pressures, and monetary values
Available raw materials: their compositions, amounts,
toxi-cities, temperatures, pressures, monetary values, and all
pertinent physical properties unless they are standard and can
be established from correlations This information about
properties applies also to products of item 1
Daily and seasonal variations of any data of items 1 and 2 and
subsequent items of these lists
All available laboratory and pilot plant data on reaction and
phase equilibrium behaviors, catalyst degradation, and life and
corrosion of equipment
5 Any available existing plant data of similar processes.
6 Local restrictions on means of disposal of wastes.
Characteristics of raw makeup and cooling tower waters,
temperatures, maximum allowable temperature, flow rates
available, and unit costs
Steam and condensate: mean pressures and temperatures and
their fluctuations at each level, amount available, extent of
recovery of condensate, and unit costs
Electrical power: Voltages allowed for instruments, lighting and
various driver sizes, transformer capacities, need for emergency
generator, unit costs
Compressed air: capacities and pressures of plant and
in-strument air, inin-strument air dryer
Plant site elevation
Soil bearing value, frost depth, ground water depth, piling
requirements, available soil test data
Drainage and sewers: rainwater, oil, sanitary
Buildings: process, pump, control instruments, specialequipment
Paving types required in different areas
Pipe racks: elevations, grouping, coding
Battery limit pressures and temperatures of individual feedstocks and products
Codes: those governing pressure vessels, other equipment,buildings, electrical, safety, sanitation, and others
Miscellaneous: includes heater stacks, winterizing, insulation,steam or electrical tracing of lines, heat exchanger tubing sizestandardization, instrument locations
A convenient tabular questionnaire is in Table 1.8 Foranything not specified, for instance, sparing of equipment,engineering standards of the designer or constructor will be used Aproper design basis at the very beginning of a project is essential togetting a project completed and on stream expeditiously
These provide motive power and heating and cooling of processstreams, and include electricity, steam, fuels, and various fluidswhose changes in sensible and latent heats provide the necessaryenergy transfers In every plant, the conditions of the utilities aremaintained at only a few specific levels, for instance, steam atcertain pressures, cooling water over certain temperature ranges,and electricity at certain voltages At some stages of some designwork, the specifications of the utilities may not have beenestablished Then, suitable data may be selected from thecommonly used values itemized in Table 1.9
1.12 LABORATORY AND PILOT PLANT WORK
The need for knowledge of basic physical properties as a factor inequipment selection or design requires no stressing Beyond this,the state-of-the-art of design of many kinds of equipment and
Trang 32I.101 Pldnt Location
l.ltJ? PlaruCapactty Ibortons/yr
I 103 Operating Factor or Yearly Operating Hours
(For mos: modern chemical plants this figure is generally 8.000 hours per year).
I IO4 Provisions for Expansion
1.10s Raw Material Feed (Typical of the analyses required for a liquid)
Array WI per cent mitt
Impurities WI per cent maa
Characteristic specifications
Specific gravity
Distillation range ‘F
Initial boiling point ‘F
Dry end point ‘F
Viscosity centipoises
Color APHA
Heat stability color
Reaction rat6 with established reagent
Acid number
Freezing point or set point ‘F
Corrosion test
End-use test
For a solid material chemical assay, level of impurities and its physical characteristics,
such as spcciftc density, bulk density, particle size distribution and the liko are included.
This physical shape inrormation is required to assure that adequate processing and material
handling operations will be provided.
I.1051 Source
Supply conditiona at proccu
plant battery limits
Max M i n N o r m a l Storage capacity (volume or day’s inventory)
Rquircd delivery conditions at battery limits
Pressure
-Temperature
Method of transfer
I .I06 Product !ipodfiEaliON
Here again spcci6cationr w o u l d be similar IO that of the raw material in quivalent or
some-times greater detail as often traa impurities at&t the marketability of the final product.
Storage rquircmatts (volume or slays of inventory)
Type of product storage
For solid pro&eta type of wntainu or method of ship- _
1.107 Miscellaneous Chemicals and Catalyst Supply
In this section the operating group should outline how various misccllancuus chemicals and catalysts arc to be stored and handled for consumplion within the plant.
1.108 Atmospheric Conditions Barometric pressure ran*e Temperature
Design dry bulb temperature (‘F)
‘/ of summer season, this temperature is exceeded.
Design wet bulb temperature
% of summer season this temperature is uaedcd.
Minimum design dry bulb temperature winter condition (‘F) Level of applicable pollutants that could afTcct the process.
Examples of these are sulfur compounds dust and solids, chlorides and salt water mist when the plant is at a coastal location.
2.100 Utilities 2.101 Electricity
Characteristics of primary supply Voltage phases cyclu
Preferred voltage (or motors Over 200 hp
Under 200 h p Value, t/kWtt (If available and if desired detailed clatricity pricing schedule an be included for base load and incrmnmtal additional consumption.)
2.102 Supply Water Clcmlinas coNNlv- Solids content analysis Other detaila
SWPIY Return 2.103 Cooling Water Well river sea cooling tower, other.
Q u a l i t y Value
Trang 33USC for heat cxchallpf dasign
Fouling properties
D&in hdin# hcKor
Pr&md tuba material
HWP=-=.tiil
Tanparatur~, ‘F
Hoislurc %
Value par thousand lb
Medium pressure pria
Rsquircd prusura at battery limiu
Valua par thousand lb or gal
([~~qtulityolchcprocmrrtnirditkrcntlromchcIrulc-oprrtcr~boi~ reed -ICI.
separate ioformatiuo should be plovidcd.)
h ant CO*
Pet cent oxygm Rrwulco Other tmca impuritiu Quantity rvaikbk
VJW per th~0d w n
2.109 Plant Air Supply Source OUsita battuy limits (OSBL) lbrlabkcomprusor
Pl-OCUSlirSyrtcm
spaial comprcnor WPlY Pm prit 2.110 IllswumallAir supply loure (OSBL) Spoial compressor SUPPlY prar”rr pris DcrpOilll.‘F Oii din md moisture rcmovrl requiremenu
IO gcwrol , valua of planl mod indrumall air is usu8Ily no: given as lbe ycarIy over-all cost is bui&cant in ttktion to lbc utlur utiliriu required.
3.101 Wosta Disposal Rcqubwna~u
In plrarL tbcrc UC tirea typo of wuu 10 be cunsidcrcdz liquid, solid and gaseous The destination aud dkposal of ucb of lbcp clsuents b usually diUucn~ Typical items arc as followl:
Daliwtiocl of liquid dnualu C4tolilyrrterblowdown
stormwmw HclbOdorchanialveo~fwliquid~a Pdemd matuiok dcofumdoo for cooli- rater bluwdowo ChEiCd- stcmllaawu Fxilitiol for dwmil lfatiq for liquid CtRomtr Fsilitia for trcatnxn~ uf - clnwats
clwnial-Solidsdispa8l
Trang 34REFERENCES 15
TABLE 1.9 Typical Utility Characteristics
Pressure (psig) Saturation (“F) Superheat (“F)
direct firing and electrical heating Refrigerants
h e l i u m Cooling Water Supply at EO-90°F
Return at 115°F with 125°F maximum
Return at 110°F (salt water)
Return above 125°F (tempered water or steam condensate)
C o o l i n g A i r Supply at 85-95°F
Temperature approach to process, 40°F
Power input, 20 HP/1000 sqft of bare surface
Fuel Gas: 5-10 psig, up to 25 psig for some types of burners, pipeline gas at
1000 Btu/SCF
Liquid: at 6 million Btu/barrel
Compressed Air Pressure levels of 45, 150, 300, 450 psig
Instrument Air
45 psig, 0°F dewpoint
REFERENCES
1.1 Process Design
A Books Essential to a Private Library
1 Ludwig, Applied Process Design for Chemical and Petroleum Plants,
Gulf, Houston 1977-1983, 3 ~01s
2 Marks Standard Handbook for Mechanical Engineers, 9th ed.,
McGraw-Hill, New York, 1987
3 Perry, Green, and Maloney, Perry’s Chemical Engineers Handbook,
Electricity Driver HP Voltage
l - 1 0 0 220,440, 550
200-2500 2300,400O Above 2500 4000, 13,200
processes often demands more or less extensive pilot plant effort.This point is stressed by specialists and manufacturers of equipmentwho are asked to provide performance guaranties For instance,answers to equipment suppliers’ questionnaires like those ofAppendix C may require the potential purchaser to have performedcertain tests Some of the more obvious areas definitely requiringtest work are filtration, sedimentation, spray, or fluidized bed orany other kind of solids drying, extrusion pelleting, pneumatic and
slurry conveying, adsorption, and others Even in such thoroughlyresearched areas as vapor-liquid and liquid-liquid separations,rates, equilibria, and efficiencies may need to be tested, particularly
of complex mixtures A great deal can be found out, for instance,
by a batch distillation of a complex mixture
In some areas, suppliers make available small scale equipmentthat can be used to explore suitable ranges of operating conditions,
or they may do the work themselves with benefit of their extensiveexperience One engineer in the extrusion pelleting field claims thatmerely feeling the stuff between his fingers enables him to properlyspecify equipment because of his experience of 25 years withextrusion
Suitable test procedures often are supplied with “canned” pilotplants In general, pilot plant experimentation is a profession initself, and the more sophistication brought to bear on it the moreefficiently can the work be done In some areas the basic relationsare known so well that experimentation suffices to evaluate a fewparameters in a mathematical model This is not the book to treatthe subject of experimentation, but the literature is extensive.These books may be helpful to start:
1 R.E Johnstone and M.W Thring, Pilot Plants, Models and
Scale-up Method in Chemical Engineering, McGraw-Hill, NewYork, 1957
2 D.G Jordan, Chemical Pilot Plant Practice, Wiley-Interscience,New York, 1955
3 V Kafarov, Cybernetic Metho& in Chemtitry and Chemical Engineering, M i r P u b l i s h e r s , M o s c o w , 1 9 7 6
4 E.B Wilson, An Introduction to Scientific Research, Hill, New York, 1952
McGraw-McGraw-Hill, New York, 1984; earlier editions have not been obsolescedentirely
4 Sinnott, Coulson, and Richardsons, Chemical Engineering, Vof 6, Design, Pergamon, New York, 1983.
Trang 353 Backhurst and Harker, Process Plant Design, Elsevier, New York, 1973.
N e w Y o r k , 1 9 8 0
Press, Cambridge, MA, 1968.
vols.
Standards, Gulf, Houston, 1979; also, Design codes standards and
Dekker, New York, 1982.
London, 1956-1965, 12 ~01s.
9 Crowe et al., Chemical Plant Simulation, Prentice-Hall, Englewood
Cliffs, NJ, 1971.
Plants, Gulf, Houston, 1979, 2 ~01s.
Y o r k , 1 9 7 2
Processes, McGraw-Hill, New York, 1981.
M i r P u b l i s h e r s , M o s c o w , 1 9 7 6
15 Leesley (Ed.), Computer-Aided Process Plant Design, Gulf, Houston,
1982.
1 7 N o e l , Petroleum Refinery Manual, R e i n h o l d , N e w Y o r k , 1 9 5 9
Engineers, McGraw-Hill, New York, 1980.
Y o r k , 1 9 5 7
McGraw-Hill, New York, 1981.
1968.
Engineers, McGraw-Hill, New York, 1979.
1963.
W i l e y , N e w Y o r k , 1 9 8 4
McGraw-Hill, New York, 1983.
McGraw-H i l l , N e w Y o r k , 1 9 5 9
London, 1973.
C Estimation of Properties
1 AIChE Manual for Predicting Chemical Process Design Data, AIChE,
New York, 1984-date.
Fluids, Pergamon, New York, 1971; larger Polish edition, Warsaw, 1962.
Estimation Methods: Environmental Behavior of Organic Compounds,
McGraw-Hill, New York, 1982.
McGraw-Hill, New York, 1987.
Corresponding States Methods, Elsevier, New York, 1979.
S t o n e h a m , M A , 1 9 8 4
D Equipment
1 Chemical Engineering Catalog, Penton/Reinhold, New York, annual.
2 Chemical Engineering Equipment Buyers’ Guide, McGraw-Hill, New
York, annual.
Y o r k , 1 9 6 4
6 Thomas Register of American Manufacturers, Thomas, Springfield IL, annual.
E Safety Aspects
Operations, Wiley, New York, 1982.
2 Lees, Loss Prevention in the Process Industries, Buttenvorths, London,
1980, 2 ~01s.
1971.
McGraw-Hill, New York, 1983.
6 Sax, Dangerous Properties of Industrial Materials, Van Nostrand/ Reinhold, New York, 1982.
7 Wells, Safety in Process Plant Design, George Godwin, Wiley, New
and Design, Dekker, New York, 1976-date.
Weinheim, FRG, German edition 1972-1983; English edition 1994(?).
1984-B Bibliographies
1 Fratzcher, Picht, and Bittrich, The acquisition, collection and tabulation
of substance data on fluid systems for calculations in chemical
3 Mellon, Chemical Publications: Their Nature and Use, McGraw-Hill,
N e w Y o r k , 1 9 8 2
Kemiigeniorgruppen, Lyngby, Denmark, 1980.
C General Data Collections
Rejining, API, Washington, DC, 1971-date.
annual.
vols.
International Critical Tables, McGraw-Hill, New York, 1926-1933.
Science and Technology, Springer, New York, 1950-date.
Lange’s Handbook of Chemistry, 13th ed., McGraw-Hill, New York, 1984.
Program for Scientific Translations, Jerusalem, 1970.
1987.
Perry’s Chemical Engineers Handbook, McGraw-HiIl, New York, 1984.
Physico-Chemical Properties for Chemical Engineering, Maruzen Co., Tokyo, 1977-date.
Trang 36R E F E R E N C E S 1 7
Hemisphere, New York, 1976.
Hemisphere, New York, 1983.
15 Yaws et al., Physical and Thermodynamic Properties, McGraw-Hill, New
Y o r k 1 9 7 6
D Special Data Collections
DECHEMA, Frankfurt/Main, FRG, 1977-date.
Vapor-Liquid Equilibria, Elsevier, New York, 1976.
Units, 1978.
Thermodynamic Properties of Non-reacting Binary Systems of Organic
Substances, Texas A & M Thermodynamics Research Center, College
Station, TX, 1977-date.
R u s s i a n ) , M o s c o w , 1 9 6 6
Thermodynamic Properties of Organic Aqueous Systems, Engineering Science Data Unit Ltd, London, 197%date.
Russian), Moscow, 1971; data of 21,069 systems.
Compounds, Pergamon, New York, 1979, 7 ~01s.
Compounds, Wiley, New York, 1969.
Selected Values for Inorganic and C, and C, Organic Substances in SI Units, American Chemical Society, Washington, DC, 1982.
Trang 382 Flowsheets
pictures An engineer thinks naturally in terms of the
sketches and drawings which are his “pictures ”
Thus, to solve a material balance problem, he will
start with a block to represent the equipment and then will
show entering and leaving streams with their amounts and
properties Or ask him to describe a p r o c e s s a n d h e w i l l b e g i n
to sketch the equipment, show how iris interconnected, and
w h a t t h e f l o w s a n d o p e r a t i n g c o n d i t i o n s a r e
Such sketches develop into flow sheets, which are more
elaborate diagrammatic representations of the equipment, the sequence of operations, and the expected performance of a proposed p/ant or the actual performance of an already operating one For clarity and to meet the needs of the
v a r i o u s p e r s o n s e n g a g e d i n d e s i g n , c o s t e s t i m a t i n g , purchasing, fabrication, operation, maintenance, and management, several different kinds of flowsheets are necessary Four of the main kinds will be described and
i l l u s t r a t e d
2.1 BLOCK FLOWSHEETS
At an early stage or to provide an overview of a complex process or
plant, a drawing is made with rectangular blocks to represent
individual processes or groups of operations, together with
quantities and other pertinent properties of key streams between
the blocks and into and from the process as a whole Such block
flowsheets are made at the beginning of a process design for
orientation purposes or later as a summary of the material balance
of the process For example, the coal carbonization process of
Figure 2.1 starts with 1OO,OOOIb/hr of coal and some process air,
involves six main process units, and makes the indicated quantities
of ten different products When it is of particular interest, amounts
of utilities also may be shown; in this example the use of steam is
indicated at one point The block diagram of Figure 2.2 was
prepared in connection with a study of the modification of an
existing petroleum refinery The three feed stocks are separated
into more than 20 products Another representative petroleum
refinery block diagram, in Figure 13.20, identifies the various
streams but not their amounts or conditions
2.2 PROCESS FLOWSHEETS
Process flowsheets embody the material and energy balances
between and the sizing of the major equipment of the plant They
include all vessels such as reactors, separators, and drums; special
processing equipment, heat exchangers, pumps, and so on
Numerical data include flow quantities, compositions, pressures,
temperatures, and so on Inclusion of major instrumentation that is
essential to process control and to complete understanding of the
flowsheet without reference to other information is required
particularly during the early stages of a job, since the process
flowsheet is drawn first and is for some time the only diagram
representing the process As the design develops and a mechanical
flowsheet gets underway, instrumentation may be taken off the
process diagram to reduce the clutter A checklist of the
information that usually is included on a process flowsheet is given
in Table 2.1
Working flowsheets are necessarily elaborate and difficult to
represent on the page of a book Figure 2.3 originally was 30in
wide In this process, ammonia is made from available hydrogen
supplemented by hydrogen from the air oxidation of natural gas in a
two-stage reactor F-3 and V-S A large part of the plant is devoted
to purification of the feed gases of carbon dioxide and unconverted
methane before they enter the converter CV-1 Both commercial
and refrigeration grade ammonia are made in this plant
Com-positions of 13 key streams are summarized in the tabulation
Characteristics of the streams such as temperature, pressure,enthalpy, volumetric flow rates, etc., sometimes are convenientlyincluded in the tabulation In the interest of clarity, however, insome instances it may be preferable to have a separate sheet for avoluminous material balance and related stream information
A process flowsheet of the dealkylation of toluene to benzene
is in Figure 2.4; the material and enthalpy flows and temperatureand pressures are tabulated conveniently, and basic instrumentation
is represented
2.3 MECHANICAL (P&l) FLOWSHEETS
Mechanical flowsheets also are called piping and instrument (P&I)diagrams to emphasize two of their major characteristics They donot show operating conditions or compositions or flow quantities,but they do show all major as well as minor equipment morerealistically than on the process flowsheet Included are sizes andspecification classes of all pipe lines, all valves, and all instruments
In fact, every mechanical aspect of the plant regarding the processequipment and their interconnections is represented except forsupporting structures and foundations The equipment is shown ingreater detail than on the PFS, notably with regard to externalpiping connections, internal details, and resemblance to the actualappearance
The mechanical flowsheet of the reaction section of a toluenedealkylation unit in Figure 2.5 shows all instrumentation, includingindicators and transmitters The clutter on the diagram is minimized
by tabulating the design and operating conditions of the majorequipment below the diagram
The P&I diagram of Figure 2.6 represents a gas treating plantthat consists of an amine absorber and a regenerator and theirimmediate auxiliaries Internals of the towers are shown with exactlocations of inlet and outlet connections The amount ofinstrumentation for such a comparatively simple process may besurprising On a completely finished diagram, every line will carry acode designation identifying the size, the kind of fluid handled, thepressure rating, and material specification Complete informationabout each line-its length, size, elevation, pressure drop, fittings,etc.-is recorded in a separate line summary On Figure 2.5, which
is of an early stage of construction, only the sizes of the lines areshown Although instrumentation symbols are fairly well standard-ized, they are often tabulated on the P&I diagram as in thisexample
2.4 UTILITY FLOWSHEETS
These are P&I diagrams for individual utilities such as steam,steam condensate, cooling water, heat transfer media in general,
1 9
Trang 39Figure 2.1 Coal carbonization block flowsheet Quantities are in Ib/hr.
compressed air, fuel, refrigerants, and inert blanketing gases, and
how they are piped up to the process equipment Connections for
utility streams are shown on the mechanical flowsheet, and their
conditions and flow quantities usually appear on the process
flowsheet
Since every detail of a plant design must be recorded on paper,
many other kinds of drawings also are required: for example,
electrical flow, piping isometrics, instrument lines, plans and
elevations, and individual equipment drawings in all detail Models
and three-dimensional representations by computers also are now
standard practice in many design offices
2.5 DRAWING OF FLOWSHEETS
Flowsheets are intended to represent and explain processes To
make them easy to understand, they are constructed with a
consistent set of symbols for equipment, piping, and operating
conditions At present there is no generally accepted industrywide
body of drafting standards, although every large engineering office
does have its internal standards Some information appears in ANSI
and British Standards publications, particularly of piping symbols
Much of this information is provided in the book by Austin (1979)
along with symbols gleaned from the literature and some
engineering firms Useful compilations appear in some books on
process design, for instance, those of Sinnott (1983) and Ulrich
(1984) The many flowsheets that appear in periodicals such as
Chemical Engineering or Hydrocarbon Processing employ fairly
consistent sets of symbols that may be worth imitating
Equipment symbols are a compromise between a schematic
representation of the equipment and simplicity and ease of drawing
A selection for the more common kinds of equipment appears in
Table 2.2 Less common equipment or any with especially intricate
configuration often is represented simply by a circle or rectangle
Since a symbol does not usually speak entirely for itself but alsocarries a name and a letter-number identification, the flowsheet can
be made clear even with the roughest of equipment symbols The
TABLE 2.1 Checklist of Data Normally Included on a
4 Valves essential to an understanding of the flowsheet
5 Design basis, including stream factor
6 Temperatures, pressures, flow quantities
7 Weight and/or mol balance, showing compositions, amounts, and other properties of the principal streams
6 Utilities requirements summary
9 Data included for particular equipment
a Compressors: SCFM (60°F 14.7 psia); APpsi; HHP; number of stages; details of stages if important
b Drives: type; connected HP; utilities such as kW, lb steam/hr, or Btu/hr
c Drums and tanks: ID or OD, seam to seam length, important internals
d Exchangers: Sqft, kBtu/hr, temperatures, and flow quantities in and out; shell side and tube side indicated
e Furnaces: kBtu/hr, temperatures in and out, fuel
f Pumps: GPM (6o”F), APpsi, HHP, type, drive
g Towers: Number and type of plates or height and type of packing; identification of all plates at which streams enter or leave; ID or OD; seam to seam length; skirt height
h Other equipment: Sufficient data for identification of duty and size
Trang 402.5 DRAWING OF FLOWSHEETS 21
TABLE 2.2 Flowsheet Equipment Symbols
HEAT TRANSFER FLUID HANDLING
Centrifugal pump or blower,
Rotary dryer
or kiln Evaporator