4. SEPARATOR INTERNALS
Separator internals can be studied by following sections
4.1. Primary Separation Section
4.2. Secondary Separation Section
4.3. Mist Extraction Section
4.4. Liquid Degassing Section
4.1. PRIMARY SEPARATION SECTION
In the primary separation section the liquid is removed from the inlet stream. It is required to reduce the velocity of the entering stream, and to separate slugs and large droplets of liquid from the gas stream to minimize gas turbulence and re-entrainment of liquid particles. This is usually achieved by changing the direction of the fluid which causes reduction in turbulence and enlargement of particle size and to do so, some devices (internals) are constructed inside the vessels.
Many types of inlet devices are used.
1. Schoepentoeter’ for high GLR.
2. Half-open pipe for low gas liquid ratio (GLR)
3. Inclined plates
- Wave breakers
Inclined plates and Wave breakers are good for breaking the emulsion also. When the inlet stream pass through theses devices the velocity is reduced . Due to the reduction of velocity gas comes out of solution. Gas being lighter than air it will rise, whereas the heavier oil falls to the bottom, However, due to the difference in volumes, the oil is dispersed throughout the gas in small droplets.
To prevent ( For vertical separators ) these droplets from being carried upwards with the rising high velocity gas, the inlet pipe of a separator is placed so as to enter the separator tangentially. This is done in cyclone separators. A swirl tube is used to generate centrifugal force in the incoming fluid particles. The oil and gas mixture is swirled round and due to the resultant centrifugal force the droplets of liquid falls to the bottom and gas particles will be thrown to the top of the vessel.
4.2. SECONDARY SEPARATION SECTION
Secondary Separation Section is used for removing maximum of smaller liquid droplets from the gas. The major separation principle in this section is gravity settling from the gas stream after the velocity has been drastically reduced.
The efficiency of this section primarily depends upon
A. Gas and liquid properties.
B. Liquid drop size.
C. Degree of gas turbulence.
The turbulence factor is often minimized by inlet arrangement and properly designed and positioned straightening vanes.
Figure – 5 Straightening Waves
Another consideration of primary importance is the normal direction of gas stream travel. If the principal direction of travel from inlet to outlet is vertical, then the upward gas velocity must be limited to that value which permits the settlement of liquid particles counter currently in the opposite direction.
4.3. MIST EXTRACTION SECTION
The upper exit section of the separator consists of what is known as a “Mist Extractor”.
In this section it was found from field experience that it is not practical to rely on gravity settling for removal very small particles (droplets) remaining in the gas stream after primary and secondary separation. Therefore additional method (devices) were required to remove the small particles from the gas and these devices make use of the following mechanisms.
- Centrifugal force
In each case the gas passes through a series of baffles involving several changes of direction. Tiny droplets are collected on a surface where they are drained away from the gas stream and form larger droplets that can fall back into the liquid accumulation section.
There are two general types of impingement mist extractors mist mats and Vane packs
Mist mats are knitted wire mesh pads. The primary mechanism is impingement but also centrifugal and gravity forces are utilized.
Vane packs are composed of metal plates which are formed into a labyrinth in which the direction of the gas stream changes many times, generating large centrifugal forces. The collected liquid must be protected from being re-entrained in the turbulence gas flow that passes through the Pack.
A vane pack is often used instead of a mistmat, when plugging components may be present like wax, asphalt or drilling mud.
- Centrifugal force
In this mechanism a swirl tube is used in combination with either a mistmat or a vane pack. It increases the gravitational forces on the liquid droplets. Many of these tubes are fitted in a tray, which is called a swirl tray. The inlet stream is allowed to enter the vessel at a tangential direction. When the particles swirl around the swirl tube the centrifugal force generated will increase the gravitational force in the particles.
If the liquid particles entrained in the gas is too small they may not be able to separate from the gas on gravity difference. We can separate them only after allowing them to join togther and become a big particle. Coalescence is the property of very samll liquid drops joins togther and become a big particle and drops down on density difference. of small particles into those large enough to settle by gravity is provided by two mechanism
coalescence needs agitation and surface to hit. Some of the particles will be absorbed by the wet surface. These particles on the wet surface joins together and become big and then get separated on difference in gravity.
4.4. LIQUID DEGASSING SECTION
The oil now flows into the liquid section of the separator passing through a series of baffles, causing the oil to flow down and allowing more gas to escape.
A certain volume of oil is retained for a time in the separator by means of a controlled rate of outlet flow. This gives a chance for the gas that may still be trapped in the oil to escape upwards into the gas section. The larger the volume of oil kept in the separator, i.e. the higher the liquid level, the longer the oil remains in the liquid section ( residence time more ). Residence time depends up on the volume of the oil retained in the separator and the rate of production.
If, however, the level of the oil is increased too much, the oil surface may rise too close to the turbulent zone, causing agitation, with the chance of oil being picked up by the gas stream and carried over into gas outlet lines and to the compressor section,( known as liquid carry over) or into the flare lines. There is a chance to form vortex at the liquid outlet nozzle and gas may go with the liquid to the next processing section. To avoid this vortex breakers are provided.