TWO-PHASE OIL AND GAS SEPARATION

In oil and gas separator design, we mechanically

separate from a hydrocarbon stream the liquid and

gas components that exist at a specific temperature

and pressure.

Separators are classified as "two-phase" if they

separate gas from the total liquid stream and "three-

phase" if they also separate the liquid stream into its

crude oil and water components.

Separators are designed in either horizontal, vertical,

or spherical configurations.

Horizontal Separator

Vertical Separator

spherical Separator

PRINCIPLES OF SEPAEATION

The fluid enters the separator and hits an inlet diverter causing a sudden change in momentum. The initial gross separation of liquid and vapor occurs at the inlet diverter.

The force of gravity causes the liquid droplets to fall out of the gas stream to the bottom of the vessel where it is collected.( liquid collection section)

The liquid collection section provides the retention time required to let entrained gas evolve out of the oil and rise to the vapor space. The liquid then leaves the vessel through the liquid dump valve. The liquid dump valve is regulated by a level controller.

The gas flows over the inlet diverter and then horizontally through the gravity settling section above the liquid. As the gas flows through this section, small drops of liquid that were entrained in the gas and not separated by the inlet diverter are separated out by gravity and fall to the gas liquid interface.SEPARATOR INTERNALS1-Inlet DivertersThere are two main types of inlet diverters are:i-Baffle plates ii-Centrifugal divertersA baffle plate can be a spherical dish, flat plate, angle iron, cone, or just about anything that will accomplish a rapid change in direction and velocity of the fluids and thus disengage the gas and liquid.

DIVERTER BAFFLE

TANGENTIAL BAFFLE

Centrifugal inlet diverters use centrifugal force to

disengage the oil and gas. Centrifugal diverters work

well in initial gas separation and help to prevent

foaming in crude’s.

2-Wave Breakers

In long horizontal vessels it is necessary to install

wave breakers, which are nothing more than vertical

baffles spanning the gas-liquid interface and

perpendicular to the flow.

3-Mist Extractor

Mist extractors can be made of wire mesh, vanes, centrifugal force devices, or packing.

Wire mesh pads are made of finely woven mats of stainless steel wire wrapped into a tightly packed cylinder. The liquid droplets impinge on the matted wires and coalesce.

Vane eliminators force the gas flow to be laminar between parallel plates that contain directional changes.

In vane eliminators, droplets impinge on the plate surface where they coalesce and fall to a liquid collecting spot. They are routed to the liquid collection section of the vessel.

Centrifugal mist eliminators that cause the liquid

drops to be separated by centrifugal force. These can

be more efficient than either wire mesh or vanes and

are the least susceptible to plugging. However, they

are not in common use in production operations

because their removal efficiencies are sensitive to

small changes in flow. In addition, they require

relatively large pressure drops to create the

centrifugal force.

CENTRIFUGAL MIST ELIMINATORS

Random packing is sometimes used for mist

extraction. The packing acts as a coalesces.

THE ADVANTAGES OF EACH TYPE OF SEPARATOR:

1-VERTICAL SEPARATOR

Liquid level control not as critical.

Will handle large quantities of sand.

Easier to clean.

Has greater liquid surge capacity.

Is smaller plot area.

2-HORIZONTAL SEPARATOR

Successfully used in handling foaming crude.

Cheaper than vertical separator.

More economical an efficient for processing

large volumes of gas.

Smaller diameter for a given gas capacity.

More flexible choice of nozzle arrangement.

3-SPHERICAL SEPARATOR

Cheaper than either horizontal or vertical

separator.

Better clean out and bottom drain features

than vertical type.

More compact than other types

SEPARATOR DESIGN PROCEDURES

FACTORS AFFECTING SEPARATION

The following factors must be determined before separator design:

• Gas and liquid flow rates (minimum, average, and peak)

• Operating and design pressures and temperatures

• Surging or slugging tendencies of the feed streams

• Physical properties of the fluids such as density and compressibility

• Presence of impurities (paraffin, sand, scale, etc.)

• Foaming tendencies of the crude oil

• Corrosive tendencies of the liquids or gas

1-Procedures for sizing horizontal separators

For sizing a horizontal separator it is necessary to

choose a vessel length and a diameter. This choice

must satisfy the conditions:

1- For gas capacity : that allow the liquid drops to fall

from the gas to the liquid volume.

2- For sufficient retention : to allow the liquid to reach

equilibrium.

Step 1:

Tabulate the physical properties of the fluids to be

separated

Step 2:

Calculate values of, vessel internal diameter(d) and effective length (Le ) , that satisfy the gas capacity constraint.

Step 3:

Calculate values of, d and Le , that satisfy the retention time constraint.

Step 4:

Estimate vessel length (L)

i- For gas capacity:

ii- For liquid capacity:

Step 5:Calculate slenderness ratios (SR)“ select a size of reasonable diameter and length for (SR) on order of 3 to 4 are common”Where:d= Vessel internal diameter, inLe= Effective length of the vessel where separation occurs, ftT = Operating temperature, 0RP = Operating pressure , psiaQg =Gas flow rate, MMscfdQL =Liquid flow rate , bpddm =Liquid drop to be separated , micronCD = Drag coefficienttr =Desired retention time for the liquid, minrg = Density of gas , lb/ft3rL = Density of liquid , lb/ft3

Example: Sizing a horizontal separator:Given:Gas flow rate =10 MMscfdLiquid flow rate =2000 bpd Operation pressure =1000 psiaOperation temperature =600FDensity of gas =3.71 lb/ft3Density of liquid =51.5 lb/ft3Oil viscosity = 0.013 cpGas compressibility = 0.84Liquid drop = 140 micronRetention time =3 minDrag coefficient =0.851Find: Vessel length and diameter

Solution:1-Calculate Le and d for gas capacity

2- Calculate Le and d for liquid capacity

3-Assume d, [ 24, 30, 36, 42 ] , and calculate Le for gas and liquid capacity.

4- Calculate L based on large value of Le

5- Calculate (SR)

dGas (Le)Liquid (Le)

LSR=12L/d

241.6614.8819.849.92

301.339.5212.695.08

361.116.616.812.94

420.954.866.481.85

SR5.0842.94

L12.84L6.81

d30d36

6-We choose L and d based on (SR):By interpolation:

i-For length:

ii-For diameter:

2-Procedures for sizing vertical separators

In vertical separators:

i- A minimum diameter must be maintained to allow liquid drops to separate turn the vertically moving gas.

ii- The liquid retention time requirement specifies a combination of diameter and liquid volume height.

iii- Any diameter greater than the minimum required for gas capacity can be chosen.

Step 1:

Tabulate the physical properties of the fluids to be separated.

Step 2:

Calculate minimum diameter based on gas capacity .

Step 3:

Assume diameter greater than the minimum diameter required for gas capacity .

Step 4:

Compute combinations of diameter (d) and height of the liquid volume (h) for variant’s assume diameter based on liquid capacity constrain.

Step 5:

Compute length (L) .

Where: d is the minimum diameter for gas capacity .

Step 6:

Calculate slenderness ratios (SR)

“ select a size of reasonable diameter and length for (SR) on order of 3 to 4 are common”

Example: Sizing a vertical separator:Given:Gas flow rate =10 MMscfd At 0.6 specific gravityLiquid flow rate =2000 bpd At 40 APIOperation pressure =1000 psiaOperation temperature =600F Density of gas =3.71 lb/ft3 Density of liquid =51.5 lb/ft3Oil viscosity = 0.013 cpGas compressibility = 0.84Liquid drop = 140 micronRetention time =3 minDrag coefficient =0.851Find: Vessel length and diameter

Solution:

Minimum diameter for gas capacity

2- Assume diameter [24 ,30, 36, 42 ]

3-Compute combinations of diameter (d) and height of the liquid volume (h)

Tr (min)

d (in)h (in)L=(h+76)/12 (ft)SR=12L/d

324303642

86.8055.5638.6028.35

13.5710.969.558.69

6.794.383.182.48

4- Choose:Diameter=36 inLength=9.55=10 ft

THREE-PHASE OIL AND GAS SEPARATIONThree – phase separator are designed as either horizontal or vertical pressure vessels1-Horizontal Separator

PRINCIPLES OF SEPAEATION

Fluid enters the separator and hits an inlet diverter.

This sudden change in momentum does the initial

gross separation of liquid and vapor .

The inlet diverter contains a down comer that

directs the liquid flow below the oil/water interface.

This forces the inlet mixture of oil and water to mix

with the water continuous phase in the bottom of the

vessel and rise through the oil/water interface. This

process is called "water-washing," and it promotes

the coalescence of water droplets which are entrained

in the oil continuous phase.

The liquid collecting section of the vessel provides sufficient time so that the oil and emulsion form a layer or "oil pad" at the top. The free water settles to the bottom.

The weir maintains the oil level and the level controller maintains the water level. The level of the oil downstream of the weir is controlled by a level controller that operates the oil valve.

An interface level controller senses the height of the oil/water interface. The controller sends a signal to the water pump valve thus allowing the correct amount of water to leave the vessel so that the oil/water interface is maintained at the design height.

The gas flows horizontally and out through a mist extractor to a pressure control valve that maintains constant vessel pressure. The level of the gas/oil interface can vary from half the diameter to 75% of the diameter depending on the relative importance of liquid/gas separation.

Alternate configuration known as a "bucket and weir" design. This design eliminates the need for a liquid interface controller.

Both the oil and water flow over weirs where level control is accomplished by a simple displacer float. The oil overflows the oil weir into an oil bucket where its level is controlled by a level controller that operates the oil valve.

The water flows under the oil bucket and then over a water weir. The level downstream of this weir is controlled by a level controller that operates the water dump valve.

HORIZONTAL SEPARATOR [bucket and weir design]

It is critical to the operation of the vessel that the

water weir height be sufficiently below the oil weir

height so that the oil pad thickness provides

sufficient oil retention time.

If the water weir is too low and the difference in

specific gravity is not as great as anticipated, then the

oil pad could grow in thickness to a point where oil

will be swept under the oil box and out the water

outlet. Normally, either the oil or the water weir is

made adjustable so that changes in oil/water specific

gravities or flow rates can be accommodated.

To obtain a desired oil pad height, the water weir should be set a distance below the oil weir, which is calculated by:

Setting the pressure at point A

Where:

Dh = distance below the oil weir, in.

h0 = desired oil pad height, in.

r0 = oil density, lb/ft3

rw = water density, lb/ft3

However, in heavy oil applications or where large amounts of emulsion or paraffin are anticipated it may

be difficult to sense interface level. In such a case bucket and weir control is recommended.

Vertical Separator

Flow enters the vessel through the side as in the horizontal separator, the inlet diverter separates the bulk of the gas.

A down comer is required to transmit the liquid through the oil-gas interface . A chimney is needed to equalize gas pressure between the lower section and the gas section.

The spreader or down comer outlet is located at the oil-water interface. From this point as the oil rises any free water trapped within the oil phase separates out.

The water droplets flow countercurrent to the oil. Similarly, the water flows downward and oil droplets trapped in the water phase tend to rise countercurrent to the water flow.

SEPARATOR DESIGN PROCEDURES

1-Horizontal Separators

For sizing a horizontal three-phase separator it is necessary to specify a vessel diameter and a seam-to-seam vessel length.

The gas capacity and retention time considerations establish certain acceptable combinations of diameter and length.

The need to settle 500-micron water droplets from the oil establishes a maximum diameter.

Step1:

Select oil retention time (tr)0 and water retention time (tr)w

Step2:

Calculate maximum oil pad thickness. (ho)max , allow the water droplets to settle out time (tr)0 .

Step3:

Calculate the fraction of the vessel cross sectional area occupied by the water phase.

Step4:

Calculate maximum diameter for oil pad thickness constraint

Step5:

Calculate combination of d and Le for d less than dmax that satisfy the oil and water retention time constraint

Step6:

Estimate vessel length

Step7:

Calculate slenderness ratios (SR)=(12L/d)

“ select a size of reasonable diameter and length for (SR) on order of 3 to 5 are common”

Example: Sizing a Horizontal Three-Phase SeparatorGiven:Gas flow rate =5 MMscfdOil flow rate =5000 bpdWater flow rate =3000 bpdOperation pressure =100 psiaOperation temperature =900FOil =300APISpecific gravity =1.07(water phase) Oil viscosity = 0.013 cpOil Retention time =10 minWater Retention time =10 minFind: Vessel length and diameter

Solution:

1-Calculate difference in specific gravities

2- Calculate maximum oil pad thickness

3-Calculate maximum diameter for oil pad thickness constraint

4- Calculate combination of d and Le

d(in)Le(ft)L=(4/3Le) (ft)(12L/d)

6031.5642.078.42

7023.1830.915.30

8017.7523.673.55

9014.0218.702.49

10011.3615.151.82

5- Possible choices[ 80 in x 24 ft]

2-Vertical Separators

A minimum diameter must be maintained to assure

adequate gas capacity.

The height of the three phase separator is

determined from retention time consideration.

Step 1:

Calculate minimum diameter from requirement for

water droplets to fall through oil layer. Use 500-

micron droplets if no other information is available.

Step 2:

Calculate minimum diameter from requirement for oil droplets to fall through gas. Use 100-micron droplets if no other information is available.

Step 3:

Choose the larger of the two as minimum diameter

Step 4:

Select oil retention time (tr)0 and water retention time (tr)w

Estimate the height of oil pad (h0) and height from water outlet to interface (hw) for various diameter (d)

Step 5:

Estimate the vessel length (L)

Step 6:

Select a size of reasonable diameter and length. Slenderness ratios (12 L/d) on the order of 1.5 to 3 are common.

Where:

Qw= Water flow rate ,bpd

Q0= Oil flow rate ,bpd

m = Viscosity , cp

DS.G.= Difference in specific gravities

dm= Liquid drop to be separated , microns.

T= Operating temperature, 0RZ= Gas compressibilityQg= Gas flow rate, MMscfdCD= Drag coefficientP=Operating pressure, psiarg=Density of gas ,lb/ft3rl=Density of liquid,lb/ft3h0= Height of oil pad, inhw= Height from water outlet to interface, in(tr)0= Oil retention time, min(tr)w= Water retention time, mind= Vessel diameter , inL= vessel length, ft

Example: Sizing a Vertical Three-Phase SeparatorGiven:Gas flow rate =5 MMscfdOil flow rate =5000 bpdWater flow rate =3000 bpdOperation pressure =100 psiaOperation temperature =900FOil =300APIGas specific gravity =0.6Specific gravity of water =1.07 Oil viscosity = 10 cpOil Retention time =10 minWater Retention time =10 minGas compressibility = 0.84Drag coefficient =0.89Find: Vessel length and diameter

Solution:

1. Calculate liquid and gas densities.

2- Calculate difference in specific gravities.

3- Calculate minimum diameter to satisfy gas capacity constraint.

4- Calculate minimum diameter for water droplet settling.

5-Estimate the height of oil pad (h0) and height from water outlet to interface (hw)

6 .Compute combinations of d, and h0 + hw for diameters greater than minimum diameter

d (in)h0 + hw (in)L (ft)SR=12L/d

849096102

94.582.372.364.1

18.217.717.417.2

2.62.42.22.0

7 -Choose a reasonable size :A 90-in. X 18-ft or a 96-in. X 17-ft size would be a

reasonable choice.