Surface Facilities 2 1

8/11/2019 Surface Facilities 2 1

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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


2/27


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


4/27


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


5/27


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


6/27


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


7/27


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


8/27

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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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


11/27


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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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


13/27


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


14/27

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?


15/27

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


16/27

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


17/27

1. Graphical method to estimate K

R]inispsia,inis1),(airs.g.gasis:[Note

graph.fromoffreadandCalculate

TPS

KSP/T



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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


20/27

ExampleSizing a horizontal separator

(from Arnold-Stewart)

Example Sizing a horizontal separator


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Example Sizing a horizontal separator


23/27


E ample Si ing a ertical separator


24/27

ExampleSizing a vertical separator

(from Arnold-Stewart)


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ExampleSizing a vertical separator


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Surface Facilities 2 1