Gas-Liquid And Liquid-Liquid Separators

228 Pages·2008·8.58 MB·English

by  Maurice Stewart

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A Note from the Authors
Gulf Equipment Guides series serves as a quick reference for the
design, selection, specification, installation, operation, testing, and
trouble-shooting of surface production equipment. The Gulf Equip-
ment Guides series consists of multiple volumes, each of which cov-
ers a specific area in surface production equipment. These guides
cover essentially the same topics included in the “Surface Production
Operations” series but omit the proofs of equations, example pro-
blems and solutions which belong more properly in a handbook. This
book contains fewer pages and is therefore more focused. The reader
is referred to the corresponding volume of the “Surface Production
Operations” series for further details and additional information
such as derivations of some of the equations, example problems and
solutions and suggested test questions.
About the Book
Gas–Liquid and Liquid–Liquid Separators is the first volume in the
Surface Production Facilities Engineering Handbook series. Each vol-
umeprovidesacompleteandup-to-dateresourcemanualonaspecific
area of Facilities Engineering. The series provides the most compre-
hensive coverage you’ll find today dealing with surface production
facilities in its various stages, from initial entry into the flowline
through gas–liquid and liquid–liquid separation; emulsions, oil and
water treating; water injection; hydrate prediction and prevention;
gas dehydration; and gas conditioning and processing equipment to
the exiting pipeline. The series has volumes devoted to pumps, com-
pressorsanddrivers;plantpiping andpipelines;heattransferandheat
exchangers; plant piping and pipelines; instrumentation, process con-
trol and safety systems; project management; and risk assessment.
Featured in thisvolume aresuch important topics as basic principles,
process selection, gas–liquid separators, liquid–liquid separators, and
mechanical design of pressure vessels, and many other related topics.
AllvolumesoftheSurfaceProductionFacilitieshandbookseries
serve the practicing engineer and senior field personnel by providing
organized design procedures; details on suitable equipment for appli-
cationselection;andcharts,tables,andnomographsinreadilyuseable
form. Facility engineers, process engineers, designers, operations
engineers, and senior production operators will develop a “feel” for
theimportantparametersindesigning,selecting,specifying,andtrou-
ble-shootingsurfaceproductionfacilities.Readerswillunderstandthe
uncertainties and assumptions inherent in designing and operating
the equipment in these systems and the limitations, advantages, and
disadvantages associated with their use.
CHAPTER 1
Basic Principles
1.1 Introduction
Beforedescribinggas–liquid(2-phase)andliquid–liquid(3-phase)separa-
tionequipmentusedinoilandgasproductionfacilitiesanddesigntech-
niquesforselectingandsizingthatequipment,itisnecessarytoreview
somebasicprinciplesandfluidproperties.Wewillalsodiscusssomeof
the common calculation procedures, conversions, and operations used
todescribethefluidsencounteredintheproductionoperations.
1.2 Fluid Analysis
An 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 Properties
Anaccurateestimateofphysicalpropertiesisessentialifoneistoobtain
reliablecalculations.Physicalandchemicalpropertiesdependupon:
l Pressure
l Temperature
l Composition
Most hydrocarbon streams are mixtures of hydrocarbons that may
contain varying quantities of contaminants such as
l Hydrogen sulfide
l Carbon-dioxide
l Water
2 Gas-LiquidandLiquid-LiquidSeparators
TABLE1.1
Examplefluidanalysisofgaswell
Component mol%
Methane(C ) 35.78
1
Ethane(C ) 21.46
2
Propane (C ) 1.40
3
i-Butane (i-C ) 5.35
4
n-Butane(n-C ) 10.71
4
i-Pentane(i-C ) 3.81
5
n-Pentane(n-C ) 3.07
5
Hexanes (C ) 3.32
6
Heptanes plus(C7þ) 3.24
Nitrogen 0.20
Carbondioxide 1.66
Total 100.00
The more similar the character of the mixture molecules, the more
orderlytheir behavior.Asinglecomponent system composedentirely
of a simple molecule, like methane, behaves in a very predictable,
correctable manner.
The accuracy of calculations decrease in the following order:
l Single component system
l Mixture of molecules from the same homologous series
l Mixture of molecules from different homologous series
l 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 contribu-
tion of an individual molecule is in proportion to its relative quantity
in the mixture. The more dissimilar the molecules, the less accurate
thepredictionbecomes.Table1.2listssomeofthephysicalproperties
of some of the paraffin hydrocarbon series.
Waterinliquidorvaporformispresenttosomedegreeinallsys-
tems.Liquidwaterisessentiallyimmiscibleinhydrocarbons.However,
in the vapor phase it represents a small percentage (seldom more than
onepartperthousand,byweight).Sincenormalphasebehaviorcalcula-
tionsdonotapplyforwater,specialproceduresmustbeused.Equations
ofstateusethevaluesofP,V,andTatthecriticalpoint.Eachmolecular
specieshasauniquecriticalpoint.
TABLE1.2
Physicalpropertiesof paraffinhydrocarbons
Component Methane Ethane Propane iso-Butane n-Butane iso-Pentane n-Pentane n-Hexane n-Heptane n-Octane n-Nonane n-Decane
Molecularweight 16.043 30.070 44.097 58.124 58.124 72.151 72.151 86.178 100.205 114.232 128.259 142.286
[email protected] (cid:3)258.73 (cid:3)127.49 (cid:3)43.75 10.78 31.08 82.12 96.92 155.72 209.16 258.21 303.47 345.48
psia,(cid:2)F
[email protected] (cid:3)296.44 (cid:3)297.49 (cid:3)305.73 (cid:3)255.28 (cid:3)217.05 (cid:3)255.82 (cid:3)201.51 (cid:3)139.58 (cid:3)131.05 (cid:3)70.18 (cid:3)64.28 (cid:3)21.36
psia,(cid:2)F
Vaporpressure@100(cid:2)F, (5000.) (800.) 188.4 72.58 51.71 20.445 15.574 4.960 1.620 0.5369 0.1795 0.0609
psia
Densityofliquid@60(cid:2)Fand14.696psia
Relativedensity@ (0.3) 0.3562 0.5070 0.5629 0.5840 0.6247 0.6311 0.6638 0.6882 0.7070 0.7219 0.7342
60(cid:2)F/60(cid:2)F
(cid:2)API (340.) 265.6 147.3 119.8 110.7 95.1 92.7 81.60 74.08 68.64 64.51 61.23
Absolutedensity, (2.5) 2.970 4.227 4.693 4.870 5.208 5.262 5.534 5.738 5.894 6.018 6.121
lbm/gal(invacuum)
Apparentdensity, (2.5) 2.960 4.217 4.683 4.861 5.198 5.252 5.524 5.729 5.885 6.008 6.112
lbm/gal(inair)
Densityofgas@60(cid:2)Fand14.696psia
Relativedensity(air¼ 0.5539 1.0382 1.5225 2.0068 2.0068 2.4911 2.4911 2.9755 3.4598 3.9441 4.4284 4.9127
1),idealgas
lb/Mft3,idealgas 42.28 79.24 116.20 153.16 153.16 190.13 190.13 227.09 264.06 301.02 337.98 374.95
Volume@60(cid:2)Fand14.696psia
Liquid,gal/lb-mol (6.4) 10.13 10.43 12.39 11.94 13.85 13.72 15.57 17.46 19.38 21.31 23.45
Ft3has/galliquid,ideal (59.1) 37.48 36.375 30.64 31.79 27.39 27.67 24.37 21.73 19.58 17.81 16.33
gas
(Continued)
TABLE1.2 (Continued)
Component Methane Ethane Propane iso-Butane n-Butane iso-Pentane n-Pentane n-Hexane n-Heptane n-Octane n-Nonane n-Decane
Ratio,gas/liquid, (442.) 280.4 272.1 229.2 237.8 204.9 207.0 182.3 162.6 146.5 133.2 122.2
invacuum
Criticalconditions
Temperature,(cid:2)F (cid:3)116.67 89.92 206.06 274.46 305.62 369.10 385.8 453.6 512.7 564.22 610.68 652.0
Pressure,psia 666.4 706.5 616.0 527.9 550.6 490.4 488.6 436.9 396.8 360.7 331.8 305.2
Grosscalorificvalue,combustion@60(cid:2)F
Btu/lb,liquid – 22181 21489 21079 21136 20891 20923 20783 20679 20607 20543 20494
Btu/lb,gas 23891 22332 21653 21231 21299 21043 21085 20942 20838 20759 20700 20651
Btu/ft3,idealgas 1016.0 1769.6 2516.1 3251.9 3262.3 4000.9 4008.9 4755.9 5502.5 6248.9 6996.5 7742.9
Btu/gal,liquid – 65869 90830 98917 102911 108805 110091 115021 118648 121422 123634 125448
Volumeairtoburnone 9.54 16.71 23.87 31.03 31.03 38.19 38.19 45.35 52.52 59.68 66.84 74.00
volume,idealgas
Flammabilitylimits@100(cid:2)Fand14.696psia
Lower,volume%inair 5.0 2.9 2.0 1.8 1.5 1.3 1.4 1.1 1.0 0.8 0.7 0.7
Upper,volume%inair 15.0 13.0 9.5 8.5 9.0 8.0 8.3 1.7 7.0 6.5 5.6 5.4
[email protected]
Btu/lb@boilingpoint 219.45 211.14 183.01 157.23 165.93 147.12 153.57 143.94 163.00 129.52 124.36 119.65
Specificheat@60(cid:2)Fand14.696psia
C gas,Btu/(lb-(cid:2)F),ideal 0.5267 0.4078 0.3885 0.3867 0.3950 0.3844 0.3882 0.3863 0.3845 0.3833 0.3825 0.3818
p
gas
C gas,Btu/(lb-(cid:2)F),ideal 0.4029 0.3418 0.3435 0.3525 0.3608 0.3869 0.3607 0.3633 0.3647 0.3659 0.3670 0.3678
v
gas
K¼C /C ,idealgas 1.307 1.193 1.131 1.097 1.095 1.077 1.076 1.064 1.054 1.048 1.042 1.038
p v
C liquid,Btu/(lb-(cid:2)F) – 0.9723 0.6200 0.5707 0.5727 0.5333 0.5436 0.5333 0.5280 0.5241 0.5224 0.5210
p
BasicPrinciples 5
For each of the pure components shown in the tables, the critical
values represent the maximum pressure and temperature at which a
two-phase,vapor–liquidsystemcanexist.AboveP andT ,onlyasingle
c c
phase is possible. For mixtures, pseudo-critical values are calculated,
which are correlation constants only and are not a point on the phase
diagram.
1.3.1 Equations of State
The correlations that follow are commonly used for hydrocarbon sys-
temsandaresuitableforuseformostcalculations.Anyequation cor-
relatingP,V,andTiscalledanequationofstate.Theidealequationof
state is sometimes called ideal gas law, perfect gas law, or general gas
law and is expressed by Equation (1.1).
PV ¼nRT (1.1)
where
P ¼ absolute pressure
V ¼ volume
n ¼ number of moles of gas of volume V at P and T
R ¼ Universal gas constant (refer to Table 1.3)
T ¼ absolute temperature
Equation (1.1) is valid up to pressures of about 60 psia (500kPa,
4 bara). As pressure increases above this level, its accuracy becomes
lessandthesystemshouldbeconsideredanon-idealgas.Table1.3lists
the values of the universal gas constant for different unit systems.
1.3.2 Molecular Weight and Apparent Molecular Weight
The number of moles is defined as follows:
Mass
Mole¼ (1.2)
Molecular weight
TABLE1.3
Universalgasconstant
P V T R
kPa m3 K 8.314(kPa)(m3)/(kmol)(K)
MPa m3 K 0.00831(MPa)(m3)/(kmol)(K)
bar m3 K 0.08314(bar)(m3)/(kmol)(K)
psi ft3 (cid:2)R 10.73(psia)(ft3)/(lb(cid:3)mol)((cid:2)R)
lb/ft2 ft3 (cid:2)R 1545(psia)(ft3l/(lb(cid:3)mol)((cid:2)R)
6 Gas-LiquidandLiquid-LiquidSeparators
expressed as
m
n¼ (1.3)
M
or in units as
lb
lb(cid:3)mole¼ (1.4)
lb
lb(cid:3)mole
Molecular weight is defined as the sum of the atomic weights of the
various elements present.
Example 1.1: Molecular Weight Calculation
Given:
Determine the molecular weight of ethane, C H
2 6
Solution:
Element No. of Atoms Atomic Weight Product
C 2 (cid:4) 12 ¼ 24
H 6 (cid:4) 1 ¼ 6
Molecular weight ¼ 30lb/(lb(cid:3)mol)
Up to now, we have addressed only pure substances. We now have
to consider hydrocarbon mixtures. However, first we must discuss
apparent molecular weight and specific gravity. It is not correct to
say that a hydrocarbon mixture has a molecular weight; rather, it is
an apparent molecular weight. Apparent molecular weight is defined
as the sum of the products of the mole fractions of each component
times the molecular weight of that component. This is shown in
Equation (1.5):
X
MW¼ yiðMWÞi (1.5)
where
yi ¼ molecular fraction of ith component
PMWi ¼ molecular weight of ith component
yi ¼ 1
Now, let us look at an example of the application of apparent
molecular weight that will also result with a number that we will
use often throughout this book.