Introduction: Oil & Gas separation is a common method in upstream of oil & gas wells which is generally composed of crude and natural gas.
What is a separator: Separator refers to a pressure vessel used to separate feed mixture from crude wells. This crude well contains a gaseous-liquid mixture.
Classification of separator: Classification of the separator is based on the different factors, as listed below:
- Based on installation: Installation of the separator can be offshore or onshore.
- Offshore: Offshore refers to the separator installed inside the sea region.
- Onshore: Onshore refers to the Installation of the separator onto the land.
- Based on the installation location in the plant: depending upon the location, the separator can be one of the following:
- Production Separator: The production Separator is usually situated near the well. This is used to stabilize the crude, stabilization of the crude can be in a multi-stage type process. Production Separator operates at a higher pressure.
- Test Separator: The test Separator is also placed near to the well or against the production Separator. It is simply used to measure or quantify the small flow rate of each stream of production separator through a diverting route towards the test separator.
- MP or IP separator (Medium or Intermediate Pressure): This type of separator is situated downstream of the production Separator, these separators typically operate at lower pressure in comparison to the production separator.
- LP Separator: LP is refers to the low pressure, this separator is operates at low pressure amongst them all listed.
- Based on the orientation of the Vessel: generally vessel orientation can be vertical, horizontal, -and spherical. The selection of Orientation is a critical parameter that depends upon the several factors discussed below:
- Vertical: Vertical orientation is selected in cases where the plot plan area is small, it also has an advantage in pumping scenarios of higher NPSH in comparison to horizontal orientation.
- Horizontal: Horizontal orientation can be equipped where required space is available and significant capacity separation to be achieved.
- Spherical: Spherical orientation is to be used where the distance between the equipment is minimal, this also helps in ease of transportation.
- Based on the fluids phases availability: Fluid for separation can be two or three phases i.e. Gas, oil & water (Includes Wax, solids, asphalt, etc. within the Inlet fluid before separation).
Important terms used in the separation mechanism:
- Momentum: Momentum is simply the force that is carried by the fluid due to its velocity. Momentum plays an important role in the nozzle sizing (Feed inlet, gas & liquid Outlet). The unit of measurement for Momentum is Pascal (Pa).
Momentum = Fluid density * Velocity 2
Momentum criteria for nozzle sizing are as follows:
Feed Inlet Nozzle:
- With no inlet device ≤ 1000 Pa
- With half open pipe as inlet device ≤ 1500 Pa
- With Schoepentoeter as inlet device ≤ 6000 Pa
Gas Outlet Nozzle: Momentum should be equal to or less than (≤) 3750 Pa.
Liquid Outlet Nozzle: Liquid outlet nozzle velocity shall not exceed by 2 m/s.
Note: Indicated guidelines are based on the D.E.P Standards, and may change based on the project.
- Mist Eliminator: Mist eliminator is a device that is used for liquid removal which is entrained in the gas phase. Generally liquid droplet of size. Liquid droplets are collected at the element of the mist eliminator.
- Weir Plate: Weir plat is also a type of internally used to separate oil and water needed. A Weir plate is simply a barrier for highly dense fluid and allows the less dense fluid to pass through the outlet.
- Retention time: Retention time is the period for which liquid remains available in the liquid separation section. Retention time can be calculated by the liquid volume inside the Separator divided by the liquid volumetric flow rate.
- Terminal Velocity: Terminal velocity is referred to as the free-falling velocity in the liquid separation section. In this section, terminal velocity gets reduced and settles within the section.
Working principle of Separator in the Oil & Gas Industry:
Separator works on the principle of “Principle of momentum and gravity settling theory”. Inlet feed (Generally crude) from well enters from the feed inlet nozzle.
Principle of Momentum: According to the principle of momentum, inlet feed momentum to be absorbed to minimize the entrapment of liquid particles, and turbulence & changes the direction downwards or towards the liquid accumulator section. The activity in this section is called as Primary separation section.
Gravity settling theory: Gravity settling theory stated that “Settling of dispersed droplets takes place in condition when the gravitational force acting on the particle is greater than the buoyant and drag force acting on the same droplet”.
Liquid accumulator section: This section in which liquid settles, enables the efficient separation process. Gas separated from the settled liquid is passed through the mist eliminator placed at the gas outlet nozzle.
“Weir plate plays an important role in three phase separation where water & oil are present in the liquid accumulator section, water with high density remains in the bottom of the section and lower density fluid (oil) flowing above the water surface passed through the weir plate towards the Oil outlet”.
Thus, liquid separated from gas is taken out via the liquid outlet and gas through the gas outlet.
Gravity settling theory law: different laws are applicable based on the Reynolds number as described below:
- For Reynolds number (less than < 2): Gravity settling stokes law region is applicable, where this law consists of drag coefficient and terminal velocity. Whereas Reynolds number is a function of the shape & settling of particles, the nature of the flowing fluids.
- For Reynolds number (between 2 and 500): Gravity settling intermediate law region is applicable.
- For Reynolds number (between 500 and 200000): Gravity settling newton law region is applicable.
Gravity settling laws contains the same functional parameters but the equations may vary.