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SEPARATOR SIZE SELECTION
Must satisfy two constraints:(1)Gas capacitydetermines droplet velocity
(2) Liquid capacityliquid must be retained in separator
long enough for equilibrium (retention time) for gas
to evolve from solution
Liquid
Desired path of liquid dropletgas
Well
stream
gas
liquid
gas bubbles evolve,
travel to gas phase
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SEPARATOR SIZE SELECTION
1. GAS CAPACITY CONSTRAINT
Forces acting on a liquid droplet as it drops in
the gravity settling section of separator
gravity
drag = buoyant force
liquid droplet
gas
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SEPARATOR SIZE SELECTIONGAS CAPACITY CONSTRAINT
(1).........2
2
g
VACF tgDD
Drag force, FD, given by
meter10micron1
micronsdroplet,liquidofdiameter
ft,10281.3
ftdroplet,liquidofdiameter
ft/s32.2constant,nalgravitatio
ft/svelocity,terminallb/ftdensity,liquid
lb/ftdensity,gas
ftarea,sectional-crossdroplet
tcoefficiendrag
6-
6
2
3
3
2
m
m
t
l
g
D
d
dD
D
g
V
A
C
Buoyant force, FB, given by
(2).........6
3D
F glB
Equating forces gives
terminal velocity (Vt):
(3).........0119.02
1
D
m
g
glt
C
dV
34.0Re
3
Re
2421 DC
Vdmg0049.0Re
NOT Stokes law, CD=24/Re,
because gas is compressible
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SEPARATOR SIZE SELECTIONGAS CAPACITY CONSTRAINT
Liquid droplet size:
100 micron size removed in gravity settling
section,
10-100 micron size removed by mist
extractors
Retention time = liquid droplet must staylong enough in separator to have gas-liquid
equilibrium, typically 0.53 minutes
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SEPARATOR SIZE SELECTIONGAS CAPACITY CONSTRAINT
P= separator pressure, psig
T = separator temperature, R
z = separator z-factor at T and P
Qg= separator gas flow rate, MMSCF/D
d= separator diameter, in
liqu
idLiquid
gas
liquid
Qg
4).........()ft(
)/sft()ft/s(
2
3
g
gA
QV
Assume separator half-filled with liquid.
)........(5367
in2
2 dAg
Sizing of Horizontal Separators
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SEPARATOR SIZE SELECTIONGAS CAPACITY CONSTRAINT
From real gas law,
(6).........
327.0
P
zTQ
Q g
Subst. (5) and (6) in (4):
......(7)
120
2Pd
zTQ
V
g
g
(8)..........120
2
effeff
Pd
zTQL
V
Ltgg
g
Time for gas to traverse length of separator:
LiquidLeff
liqu
id
gas
liquidQg d
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SEPARATOR SIZE SELECTIONGAS CAPACITY CONSTRAINT
Residence time = time for liquid droplet to fall to gas-liquid
interface
(9)..........242 tt
dV
d
V
Dt
Subst. (3) in (9):
....(10)..........0119.024
2
1
m
D
gl
gd
d
Cdt
Setting traverse time, tg= residence time, td, gives
.....(11)4202
1
eff
m
D
gl
gg
d
C
P
zTQdL
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2. LIQUID CAPACITY CONSTRAINT
(12)......../s)(ftrateliquid
)(ftvolume(s)3
3rt
3)........(11073.214442
1eff
23eff2
LdLd
volume
14).........()b/d(1049.6/s)(ftrateLiquid 53 lQ
Subst. (13) and (14) in (12):
l
eff2
Q0.42 Ld
tr(15).....................
minutes)in(7.0
eff2
rlr t
QtLd
That is,
SEPARATOR SIZE SELECTIONLIQUID CAPACITY CONSTRAINT
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Seam-Seam Length, Lss
Making allowance for separator internals:
(16)..........12
:constraintcapacitygasFor effd
LLss
).......(173
4
:constraintcapacityliquidFor effLLss
Slenderness ratio, Lss/d
....(18)..........4to3ss
d
L
Smaller ratios lead to turbulence, waves, re-entrainment
at gas-liquid interface.
Larger ratios (> 4 or 5) lead to re-entrainment
SEPARATOR SIZE SELECTIONLIQUID CAPACITY CONSTRAINT
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SEPARATOR SIZE SELECTION
Sizing of Vertical Separators
9)........(1183in2
2 d
Ag
......(20)60
2Pd
zTQV
gg
Derivation similar to that for horizontal separators.
1. GAS CAPACITY CONSTRAINT
impliestg VV
......(21)50402
1
m
D
gl
gg
d
C
P
zTQd
Liquid
gas
liquid
Qg Leff
d
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SEPARATOR SIZE SELECTION
2. LIQUID CAPACITY CONSTRAINT
).......(22..........12.0
2 lrQthd
Derivation similar to that for horizontal separators.
h= height of liquid in separator, inches
Seam-Seam Length, Lss
(23)..........12
76h
Lss ....(24)..........12
40
dhLssor
.....(25)..........4to3ss d
L
Slenderness ratio, Lss/d
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SEPARATOR SIZE SELECTION
CALCULATION PROCEDURE FOR SIZING
HORIZONTAL SEPARATORS
Step 1Calculate dand Leffthat satisfy gas capacity
constraint (Eq. 11)
Step 2Calculate dand Leffthat satisfy liquid capacity
(retention time) constraint (Eq. 15)
minutes)in(7.0
eff2
rlr t
QtLd
.....(11)4202
1
eff
m
D
gl
gg
d
C
P
zTQdL
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SEPARATOR SIZE SELECTION
Step 3Estimate seam-to-seam length, Lss
12:16)(Eq.constraintcapacitygasFor eff
dLLss
eff
3
4:17)(Eq.constraintcapacityliquidFor LLss
Step 4Select slenderness ratio, Lss/d (Eq. 18)
4to3ss d
L
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SEPARATOR SIZING PROCEDURE
ASSUMING 100 MICRON DROPLET
100420420
2
1
2
1
eff
D
gl
gg
m
D
gl
gg C
P
zTQ
d
C
P
zTQdL
Horizontal separators:
Eq. 11
reduces to ..(26)..........0.42eff KPzTQ
dL g
where (27)..........constant,2
1
D
gl
gCK
What value of drag coefficient CDto
use to calculate Kin Eq. 27?
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SEPARATOR SIZING PROCEDURE
ASSUMING 100 MICRON DROPLET
Vertical separators:
Eq. 21
reduces to
where2
1
D
gl
g CK
Again, what value of CDto use to calculate K?
2
1
2
1
10050405040
D
gl
gg
m
D
gl
gg C
P
zTQ
d
C
P
zTQd
......(28)0.504 KP
zTQd
g
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TWO METHODS TO ESTIMATE CDor K
1. Graphical method
- assumes 100 micron droplet
2. Iterative method on CD
- more exact
- does not assume dm= 100 micron
TWO METHODS TO ESTIMATE C K
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1. Graphical method to estimate K
R]inispsia,inis1),(airs.g.gasis:[Note
graph.fromoffreadandCalculate
TPS
KSP/T
TWO METHODS TO ESTIMATE CDor K
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TWO METHODS TO ESTIMATE CDor K
Step 3Calculate Re
Step 4Calculate CD
0119.0
2
1
D
m
g
gl
t C
d
V
tmg Vd
0049.0Re
34.0
Re
3
Re
2421 DC
Step 5Calculate Vt
Step 6If Vt in Steps 5 and 2 differ significantly, go to
Step 3 and recalculate Re usingVtfrom Step 5
Note: = gas viscosity at
separator pressure andtemperature, cp
E l Si i h i t l t
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ExampleSizing a horizontal separator
(from Arnold-Stewart)
Example Sizing a horizontal separator
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ExampleSizing a horizontal separator
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Example Sizing a horizontal separator
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ExampleSizing a horizontal separator
E ample Si ing a ertical separator
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ExampleSizing a vertical separator
(from Arnold-Stewart)
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ExampleSizing a vertical separator
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