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Gas-Liquid and Liquid-Liquid Separators

Maurice Stewart Ken Arnold

AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO Gulf Professional Publishing is an imprint of Elsevier

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Gulf Professional Publishing is an imprint of Elsevier 30 Corporate Drive, Suite 400, Burlington, MA 01803, USA Linacre House, Jordan Hill, Oxford OX2 8DP, UK Copyright 2008, Elsevier Inc. All rights reserved. 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. Permissions may be sought directly from Elseviers Science & Technology Rights Department in Oxford, UK: phone: (44) 1865 843830, fax: (44) 1865 853333, E-mail: permissions@elsevier.com. You may also complete your request online via the Elsevier homepage (http://elsevier.com), by selecting Support & Contact then Copyright and Permission and then Obtaining Permissions. Library of Congress Cataloging-in-Publication Data

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. ISBN: 978-0-7506-8979-3 For information on all Gulf Professional Publishing publications visit our Web site at www.elsevierdirect.com Printed in the United States of America 08 09 10 10 9 8 7 6 5 4 3 2 1

ContentsA Note from the Authors About the Book 1. Basic Principles 1.1 Introduction 1.2 Fluid Analysis 1.3 Physical Properties 1.4 Flash Calculations 1.5 Use of Computer Programs for Flash Calculations 1.6 Approximate Flash Calculations 1.7 Other Properties 1.8 Phase Equilibrium 2. Process Selection 2.1 Introduction to Field Facilities 2.2 Controlling the Process 2.3 Reservoir Fluid Characteristics 2.4 Basic System Configuration 2.5 Well Testing 2.6 Gas Lift 2.7 Offshore Platform Considerations 3. Two-Phase GasLiquid Separators 3.1 Introduction 3.2 Functional Sections of a GasLiquid Separator 3.3 Equipment Description 3.4 Selection Considerations 3.5 Vessel Internals 3.6 Potential Operating Problems 3.7 Design Theory 3.8 Separator Design Nomenclature 4. Three-Phase Oil and Water Separators 4.1 Introduction 4.2 Equipment Description vii ix 1 1 1 1 19 23 24 25 26 31 31 33 37 37 58 59 62 65 65 70 72 82 84 104 109 114 129 131 131 133

vi Contents

4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10

Vertical Separators Selection Considerations Vessel Internals Potential Operating Problems Design Theory Separator Design Separating Oil Droplets from Water Phase Vertical Separators Sizing Nomenclature

141 144 146 147 147 151 154 160 173 175 175 176 185 186 189 200 200 205 219

5. Mechanical Design of Pressure Vessels 5.1 Introduction 5.2 Design Considerations 5.3 Inspection Procedures 5.4 Estimating Vessel Weights 5.5 Specification and Design of Pressure Vessels 5.6 Pressure Relief Devices 5.7 Corrosion Protection Glossary of Terms Index

CHAPTER 1

Basic Principles

1.1 IntroductionBefore describing gasliquid (2-phase) and liquidliquid (3-phase) separation equipment used in oil and gas production facilities and design techniques for selecting and sizing that equipment, it is necessary to review some basic principles and fluid properties. We will also discuss some of the common calculation procedures, conversions, and operations used to describe the fluids encountered in the production operations.

1.2 Fluid AnalysisAn example fluid analysis of a typical gas well is shown in Table 1.1. Note that only paraffin hydrocarbons are shown. This is not correct, even though they may be the predominant series present. Also note that all molecules of heptane and larger ones are lumped together as heptanes plus fraction.

1.3 Physical PropertiesAn accurate estimate of physical properties is essential if one is to obtain reliable calculations. Physical and chemical properties depend upon:l l l

Pressure Temperature Composition

Most hydrocarbon streams are mixtures of hydrocarbons that may contain varying quantities of contaminants such asl l l

Hydrogen sulfide Carbon-dioxide Water

2 Gas-Liquid and Liquid-Liquid Separators TABLE 1.1 Example fluid analysis of gas well Component Methane (C1) Ethane (C2) Propane (C3) i-Butane (i-C4) n-Butane (n-C4) i-Pentane (i-C5) n-Pentane (n-C5) Hexanes (C6) Heptanes plus (C7) Nitrogen Carbon dioxide Total mol % 35.78 21.46 1.40 5.35 10.71 3.81 3.07 3.32 3.24 0.20 1.66 100.00

The more similar the character of the mixture molecules, the more orderly their behavior. A single component system composed entirely of a simple molecule, like methane, behaves in a very predictable, correctable manner. The accuracy of calculations decrease in the following order:l l l l

Single component system Mixture of molecules from the same homologous series Mixture of molecules from different homologous series Hydrocarbon mixtures containing sulfur compounds and/or carbon dioxide

Performance data for a single component system can be accurately correlated in graphical or tabular form. For all others, one must use either pressure/volume/temperature (PVT) equations of state or the Weighted Average. The Weighted Average assumes that the contribution of an individual molecule is in proportion to its relative quantity in the mixture. The more dissimilar the molecules, the less accurate the prediction becomes. Table 1.2 lists some of the physical properties of some of the paraffin hydrocarbon series. Water in liquid or vapor form is present to some degree in all systems. Liquid water is essentially immiscible in hydrocarbons. However, in the vapor phase it represents a small percentage (seldom more than one part per thousand, by weight). Since normal phase behavior calculations do not apply for water, special procedures must be used. Equations of state use the values of P, V, and T at the critical point. Each molecular species has a unique critical point.

TABLE 1.2 Physical properties of paraffin hydrocarbonsComponent Molecular weight Boiling point @ 14.696 psia, F Freezing point @ 14.696 psia, F Vapor pressure @ 100 F, psia Methane Ethane Propane iso-Butane n-Butane iso-Pentane n-Pentane n-Hexane n-Heptane n-Octane n-Nonane n-Decane 44.097 43.75 58.124 10.78 255.28 72.58 58.124 31.08 217.05 51.71 72.151 82.12 255.82 20.445 72.151 96.92 201.51 15.574 86.178 155.72 139.58 4.960 100.205 209.16 131.05 1.620 114.232 258.21 70.18 0.5369 128.259 303.47 64.28 0.1795 142.286 345.48 21.36 0.0609

16.043 30.070 258.73 127.49

296.44 297.49 305.73 (5000.) (800.) 188.4

Density of liquid @ 60 F and 14.696 psia (0.3) 0.3562 Relative density @ 60 F/60 F API (340.) 265.6 Absolute density, (2.5) 2.970 lbm/gal (in vacuum) Apparent density, (2.5) 2.960 lbm/gal (in air) Density of gas @ 60 F and 14.696 psia Relative density (air 0.5539 1.0382 1), ideal gas 42.28 79.24 lb/M ft3, ideal gas Volume @ 60 F and 14.696 psia Liquid, gal/lb-mol (6.4) (59.1) Ft3 has/gal liquid, ideal gas 10.13 37.48

0.5070 147.3 4.227 4.217

0.5629 119.8 4.693 4.683

0.5840 110.7 4.870 4.861

0.6247 95.1 5.208 5.198

0.6311 92.7 5.262 5.252

0.6638 81.60 5.534 5.524

0.6882 74.08 5.738 5.729

0.7070 68.64 5.894 5.885

0.7219 64.51 6.018 6.008

0.7342 61.23 6.121 6.112

1.5225 116.20 10.43 36.375

2.0068 153.16 12.39 30.64

2.0068 153.16 11.94 31.79

2.4911 190.13 13.85 27.39

2.4911 190.13 13.72 27.67

2.9755 227.09 15.57 24.37

3.4598 264.06 17.46 21.73

3.9441 301.02 19.38 19.58

4.4284 337.98 21.31 17.81

4.9127 374.95 23.45 16.33

(Continued)

TABLE 1.2 (Continued)Component Ratio, gas/liquid, in vacuum Critical conditions Temperature, F Pressure, psia Methane (442.) Ethane 280.4 Propane iso-Butane n-Butane iso-Pentane n-Pentane n-Hexane n-Heptane n-Octane n-Nonane n-Decane 272.1 229.2 237.8 204.9 207.0 182.3 162.6 146.5 133.2 122.2

116.67 666.4

89.92 706.5

206.06 616.0 21489 21653 2516.1 90830 23.87

274.46 527.9 21079 21231 3251.9 98917 31.03

305.62 550.6 21136 21299 3262.3 102911 31.03

369.10 490.4 20891 21043 4000.9 108805 38.19

385.8 488.6 20923 21085 4008.9 110091 38.19

453.6 436.9 20783 20942 4755.9 115021 45.35

512.7 396.8 20679 20838 5502.5 118648 52.52

564.22 360.7 20607 20759 6248.9 121422 59.68

610.68 331.8 20543 20700 6996.5 123634 66.84

652.0 305.2 20494 20651 7742.9 125448 74.00

Gross calorific value, combustion @ 60 F Btu/lb, liquid 22181 Btu/lb, gas 23891 22332 1016.0 1769.6 Btu/ft3, ideal gas Btu/gal, liquid 65869 Volume air to burn one 9.54 16.71 volume, ideal gas Flammability limits @ 100 F and 14.696 psia Lower, volume % in air 5.0 2.9 Upper, volume % in air 15.0 13.0 Heat of Vaporation @ 14.696 psia Btu/lb @ boiling point 219.45 Specific heat @ 60 F and 14.696 psia Cp gas, Btu/(lb- F), ideal 0.5267 gas Cv gas, Btu/(lb- F), ideal 0.4029 gas 1.307 K Cp/Cv, ideal gas Cp liquid, Btu/(lb- F)

2.0 9.5 183.01 0.3885 0.3435 1.131 0.6200

1.8 8.5 157.23 0.3867 0.3525 1.097 0.5707

1.5 9.0 165.93 0.3950 0.3608 1.095 0.5727

1.3 8.0 147.12 0.3844 0.3869 1.077 0.5333

1.4 8.3 153.57 0.3882 0.3607 1.076 0.5436

1.1 1.7 143.94 0.3863 0.3633 1.064 0.5333

1.0 7.0 163.00 0.3845 0.3647 1.054 0.5280

0.8 6.5 129.52 0.3833 0.3659 1.048 0.5241

0.7 5.6 124.36 0.3825 0.3670 1.042 0.5224

0.7 5.4 119.65 0.3818 0.3678 1.038 0.5210

211.14 0.4078 0.3418 1.193 0.9723

Basic Principles

5

For each of the pure components shown in the tables, the critical