SEPARATOR SIZING

Theory and Calculation

SEPARATION THEORY

Gas/Liquid Separation Theory

n Liquid droplet settling¨ Liquid drops separated from gas phase when its

velocity reach terminal (settling) velocity¨ Terminal velocity when Drag Force = Buoyant Force ¨ Drag Force depends on Drag Coefficient

n Re < 10

n Re > 1000

DC

Re24

=DC

34.0Re

3Re24

2/1 ++=DC

Cont.

¨ Terminal velocity equations for different conditions

n Re < 10

n Re > 1000

¨ The value of is estimated and then used in the calculation of gas capacity constraint

( )µ

26 ..1078.1 mt

dGSV

∆×=

−

2/1

0199.0

−=

D

m

g

glt C

dV

ρρρ

DC

Cont.

n But in production facility, flow almost always has Re > 1000. So how to find ?¨ Start with Re >>1000 so that ~ 0.34¨ Use = 0.34 to calculate ¨ Use to calculate Re¨ Use Re to calculate new ¨ Repeat process until values beginning to be the same¨ Use this latest value in the gas capacity equation…

DC

DCDC

DC

DC

tV

DC

tV

Cont.

n Liquid retention time¨ Retention time is average time a liquid molecule is

retained in vessel¨ To ensure liquid and gas reach equilibrium so that

gas molecule can evolve from liquid phase¨ Retention time = Volume of liquid storage in vessel

Liquid flow rate¨ Usually 1 to 3 minutes

Oil/Water Separation Theory

n Oil drop/water drop settling¨ Flow around oil drops in water or water drops in oil is

laminar – so water droplets fall at their terminal velocity

n Oil/water retention time¨ Need certain amount of oil storage so that oil reaches

equilibrium, entrained gas liberated, and ‘free’ water coalesced to fall into water storage

¨ Need certain amount of water storage for entrained large droplets of oil have time to coalesce and rise to oil-water interface

¨ Retention time 3 – 30 minutes

SEPARATOR SIZING: TWO-PHASE SEPARATOR

General sizing procedure

CALCULATE1. Gas capacity constraint

¨ Minimum vessel diameter OR Relationship between diameter and effective length that satisfy gas capacity constraint

2. Liquid capacity¨ Relationship between diameter and effective length OR height

that satisfy liquid capacity constraint3. Seam-to-seam length, Lss

¨ For Gas capacity and Liquid capacity4. Slenderness ratio

¨ For each Lss calculated

SELECT reasonable vessel size (diameter and length)!

VERTICAL SEPARATOR

Liquid capacity

Seam-to-seam Length Lss

Diameter d

Height h

Liquid Outlet

InletGas capacity

Mist extractor

Gas outlet

Vertical separator sizing procedure

1. Determine CD using iterative procedure

2. Calculate d for gas capacity constraint using

3. Calculate d2h for liquid capacity constraint

2/1

2 5040

−

=

m

D

gl

gg

dC

PTZQ

dρρ

ρ

12.0

2 lrQthd =

Cont. vertical4. Set retention time tr to be 1, 2 and 3 minutes

(usual case)

5. For each tr , calculate and tabulate values ofa) d b) hc) Lss

§ OD < 36”à

§ OD > 36”à

d) Slenderness Ratio (SR), (12)Lss/d

1276+

=h

Lss

1240min ++= dhLss

Cont. vertical

n From table, select possible choices of separator size (d x Lss) based on the values of (12)Lss/d Ø Select (12)Lss/d values range 3 – 4Ø d values must be greater than the calculated minimum

vessel diameter for gas capacity constraint (Step 2)

q Your final selection should be based on your judgment on the costs of each possible separator

Example of separator selection

Possible size

36” x 10’

30” x 10’

30” x 10’ or30” x 8’

HORIZONTAL SEPARATOR

Liquid capacity (50%)

Gas capacity (50%)

Seam-to-seam Length Lss

Effective Length Leff

Inlet Liquid Outlet

Gas molecule flowing at average gas velocity, Vg

Liquid droplet dropping at settling velocity Vt relative to gas phase

Gas-oil interface

Diameter d

Gas outlet

Horizontal separator sizing procedure

1. Determine CD using iterative procedure

2. Calculate dLeff for gas capacity constraint

3. Calculate d2Leff for liquid capacity constraint

7.0

2 lreff

QtLd =

2/1

P420

=

− mdCTZQ

dL Dggeff

gl ρρ

ρ

Cont. horizontal

4. Set retention time tr to be 1, 2 and/or 3 minutes (usual case)

5. For each tr , calculate and tabulate values of a) d b) Leff for

Ø Gas capacity à from equation Step 2Ø Liquid capacity à from equation Step 3

Cont. horizontal

c) Lss for Ø Gas Capacityà

Ø Liquid capacity à

d) Slenderness ratio (SR), (12)Lss/d

12dLL effss +=

effss LL34=

Cont. horizontal

n From table, compare the values of Leff for each gas and liquid capacity that governs the design of the separator¨ The one with larger required length governs

n Then, select possible choices of separator size (d x Lss) based on the values of SR Ø Select SR values range 3 – 5Ø Lss values selected are the one that governs the design

Example of separator selection

1.66.23.70.848

2.17.44.90.942

39.16.61.136

5.112.79.51.3303

9.919.914.91.724

17.128.521.4220

33.544.733.52.516

SRLiquid

LssGas Lss

Liquid LeffGas Leffdtr

Horizontal Separator ExampleDiameter vs. Length

Liquid capacity constraint governs since it has the largest required length

Use the liquid Lss valuesto select separator size

Possible size

36” X 10’

That’s basically it.