3 Phase Separators (‘Separators’)

ASCOM designed separators guarantee a very high performance because of:

  1. Application of state-of-the-art design rules based on in-house expertise using CFD and empirically verified by testing.
  2. Use of the proprietary range of high performance ASCOM HiPer internals.

Design philosophy: 4 sections of a separator

Generally, in horizontal orientated gas/liquid separators, the incoming multiphase mixture flows into the vessel through an ‘inlet device’.

The inlet section (1) ensures preliminary gas/liquid(s) separation and a good flow distribution across the vessel. The design of separators generally includes a gas/liquid gravity separation section (2), that ensures the creation and fall out of large liquid droplets. The gravity separation section can include a coalescer stage (either a mesh type or vane type coalescer). After the fall out of the bulk liquids, the demisting section (3) ensures the removal of finer mist particles. In the liquid separation section (4) liquids are degassed and the light and heavy liquid phases are separated. Designed based on the fluid properties and liquid-liquid separation requirements, this section is large enough to provide sufficient residence time for gas bubbles to escape from the liquids and oil and water droplets to reach the oil-water interface.

For separators located on FPSO’s or moving facilities it is essential to be thoroughly evaluated by Computational Fluid Dynamics and where necessary by finite element analysis strength calculations. ASCOM has these capabilities and this will ensure the end user of a robust design under a wide range of operating conditions.

Inlet section

In horizontal three phase gas/liquid separators (‘separators’), the incoming multiphase mixture flows into the vessel through an inlet device. In the processing industry many types of inlet devices are used ranging from inlet baffles, half open pipes to more elaborate vane type and cyclonic inlet devices. Irrespective of the service, only vane type inlet devices and properly designed cyclonic inlet devices will handle the fluids at the inlet nozzle adequately and will maximise the separation performance. All other inlet devices will result in a very poor gas distribution AND in an increase of the liquid content entrained in the gas due to the high shear forces exerted by these devices.

A poorly designed or wrongly chosen inlet device can result in a poor gas and liquid distribution, increase the liquid content entrained in the gas and create smaller water in oil and oil in water droplets due to the high shear forces exerted by these devices. This will result in a poor liquid-liquid separation performance and potentially to liquid control issues. With ASCOM HiPer internals these problems will be avoided!

Gravity separation of liquids entrained in the gas phase

Downstream of the inlet device the gas section of the separator ensures the pre-separation of entrained liquids. The gravity separation section of the separator is basically an open volume in which larger liquid droplets will fall out by gravity. This effect will be stronger when the gas is well distributed over de cross section of the vessel. The gas distribution can be improved by the application of well-designed distribution baffles. 

In this case the fluids are introduced into the vessel by means of a battery of HiPer Inlet Cyclones whereby the bulk of the liquid is introduced through the bottom of the cyclone tubes and the gas will enter the vessel through the top. As the gas jets from the cyclones it will need to be evenly distributed as to maximise the liquid drop out in the gravity separation section (2) from the perforated distributor plates to the mist eliminating equipment (3).   



Coalescence aids in the gravity separation section

In clean services, i.e. no presence of wax or fouling tendencies, a HiPer Mesh Coalescer can be used to increase the liquid handling capacity of the gravity separation section. The wire mesh structure is designed to coalesce small liquid droplets into larger ones. The specific arrangement of the HiPer Mesh Coalescer ensures very good drainage of the liquid accumulated within the mesh. This device will increase the turndown properties of the scrubber as the mode of operation changes from a flooded mode to a demisting mode once the gas velocity is reduced to below the transition point. In situations where the fluids do have a fouling tendency, alternatively a HiPer Vane Coalescer can be considered.



Mist elimination

After the gas has travelled through the gravity separation section, it will only contain fine mist droplets. The third section is the mist eliminating section (3) which ensures the separation of these fine mist particles. Essentially, the overall gas-liquid separation efficiency of a separator depends on the fraction of liquid entrained as mist in the gas, the droplet size distribution, the volumetric amount of the liquid in mist form and the efficiency of the mist eliminating equipment under those conditions.



Again the gas distribution is of significant importance to maximise the gas-liquid separation efficiency. In particular mist eliminating equipment with a low pressure drop (e.g. mesh or vane mist eliminator) is prone to mal-performance as the result of gas mal-distribution on the face of the equipment. Therefore the orientation in the gas flow is a critical design parameter. Mist eliminating cyclones typically having a slightly higher pressure drop positively contribute to an overall improved gas distribution in the entire vessel.



Oil / water separation

The liquid introduced into the vessel by either a HiPer Vane Diffuser or HiPer Inlet Cyclones is very turbulent in the inlet section. Maximising the liquid/liquid separation efficiency, the flow of liquids needs to be evenly distributed over the entire cross-sectional area of the vessel’s liquid section. For this reason, perforated baffles need to be installed in three-phase separators.

In order to further enhance the liquid/liquid separation, a HiPer Plate Coalescer can be considered. This can either reduce the vessel size required in a new built situation, or increase the throughput or improve the oil in water or water in oil outlet performance in retrofit situations. Between each of the flat parallel plates oil-water interfaces are being established which significantly reduce the distance oil or water droplets need to travel to their respective continuous phase. This directly impacts the time and distance required for liquid/liquid separation to occur, which therewith improves the minimum Tan/Tan length requirement of the separator.

When applied during the initial design stages of a new built facility, the space, weight and overall cost savings can be quite substantial due to the knock-on effects on the overall facility.

Sand removal

Fields where a large amount of sand is produced, sand will accumulate in process equipment. Horizontal and vertical separators can be equipped with a HiPer Sand Fluidising System. This will allow solid particles that have settled in the separator to temporarily being fluidised by the injection water and the slurry simultaneously being drained off to the sand handling / produced water system for physical removal from the process. This can be done both whilst the separator is offline as well as online.



Separators on floating installations

Separators that are installed on moving facilities such as FPSO’s deserve special attention for separation as well as mechanical reasons. The movement of the facility as a result of the local weather conditions / waves induces the liquid in the separator to move backward and forward, eventually leading to sloshing under the more severe weather/motion conditions. In case liquid sloshing is not adequately suppressed by wave suppression baffles, the separation process in the separator will be affected.

Effect as result of motion vs Operational consequence(s)

  1. Inlet device to become submerged → 
Obliteration of liquid, overloading of mist elimination equipment
  2. Bottom of inlet cyclones to become exposed →
 , Obliteration of liquid, overloading of mist elimination equipment, liquid level control difficulties, oil water interface disruption
  3. Mist elimination overloaded with liquid → Leading to excessive liquid carryover
  4. Mist elimination equipment to become submerged →Leading to excessive liquid carryover
  5. Oil exiting water outlet nozzle → Leading to excessive OIW carryover
  6. Water sloshing over weir → Leading to excessive WIO carryover
  7. Liquid level & interface control difficulties → Leading to level trips and consequential shut-down
  8. Reducing mechanical loads on distribution baffles → Leading to excessive forces and potential mechanical failure

During the entire design process, ASCOM takes effects and their operational consequences into consideration. This applies to both the process and the mechanical aspects. To verify the robustness of the design, the separator is subjected to a Computational Fluid Dynamics (CFD) study. The movement of the liquid will be visualised and the mechanical forces on the wave mitigation baffles will be calculated. This information is used to properly design baffles and their welded supports in the vessel and to ensure that the design and material thicknesses are adequate to handle the loads. Finite element analysis will be performed if necessary. 



Summary

ASCOM posseses expertise and CFD capabilities as well as the possibilitiy to test equipment under model and real life conditions which results in ASCOM having a very powerful design expertise. In combination with the range of proprietary HiPer separator internals, this guarantees that ASCOM separators and scrubbers will provide:



  • Maximum liquid droplet coalescence and separation from gas
  • Maximum oil from water and water from oil separation efficiency
  • Excellent turndown characteristics
  • Insensitivity to sand presence
  • Insensitivity to wave induced motion
  • Low pressure drop
  • Insensitivity to fouling service
  • No maintenance requirements due to robust design
  • Removable through Man Way
  • No welding requirements for installation
  • Easily replaces existing older internals