The liquid/liquid separation process is gravity driven. As a result, additional separation aids are often required to meet or increase the separation efficiency, allowing increased throughput, or offering an opportunity to reduce the size and weight of the vessel. Many three-phase (gas/liquid/liquid) or two-phase (liquid/liquid) separators therefore incorporate a Plate Pack Coalescer to achieve these objectives.

Plate Pack Coalescers contain numerous parallel plates at a fixed angle, the angle being dependent on the operational duty, the process fluids, and whether solids are present. Due to the fact that the flow between the plates lies in the laminar regime, smaller droplets can be separated than would be achieved by gravity alone. Furthermore, since the distance the dispersed phases have to travel to the plate surfaces is small, the residence times required for effective separation and, consequently, the vessel size, can be reduced, for a similar efficiency to that obtained with a gravity-settlement system.

It should also be noted that Plate Pack Coalescers, when installed with a well-designed baffling system, have very good motion dampening capabilities, enabling their use to counter surface waves in separation equipment. For this reason, they are often employed on Floating Production Units (FPUs).

Advantages of a Plate Pack Coalescer:

- Increases liquid/liquid separation efficiency
- Minimises the vessel size on a new build project (reducing CAPEX)
- Allows a greater fluid throughput for a set vessel size, on a revamp project
- Increases separation efficiency for a set vessel size, on a revamp project
- Wide operational range - typically, providing 100% turndown
- Suitable for modular or framed assembly
- Easy to install in both new and existing separators
- Includes no moving parts so no maintenance is needed (for clean duties)
- Reduces surface waves and sloshing effects.

For liquid/liquid coalescence in three-phase separators and other separators in which it is desired to separate liquid outlets for oil/water, plate packs can enhance separation performance by improving the local flow condition and reducing the distance over which drops have to travel to settle. Plate packs also have been installed to promote gas/liquid separation for degassing application.

The Reynolds number of fluid flow in a plate pack can be defined as

where ρc = density of the continuous phase, kg/m3; μc = dynamic viscosity of the continuous phase, kg/(m∙s) or N∙s/m2; Vc = mean velocity of the continuous phase, m/s; and dh = equivelent diameter of the flow channel.

For a plate pack with perpendicular gap spacing of dpp, the hydraulic diameter is approximately equal to 2 dpp. Transition to turbulent flow occurs in the Re range of 1,000 to 1,500.

To determine the drop size that can be removed, consider the schematic in Fig. 1 of an oil droplet rising in a waterflow between plates. The distance a drop has to settle is dpp/cos(α), where dpp is the perpendicular spacing of the plate, and α is the inclination angle. For liquids with “nonsticky” solids, the plate spacing and the angle of inclination can be increased to mitigate plugging.

Fig. 1—Depiction of droplet rising between parallel plates (courtesy of CDS Separation Technologies Inc.).

For the plate pack to be effective, the drop must reach the plate before exiting the pack. A ballistic model of the drop results in

where Vr = drop/rise velocity, m/s; Vh = horizontal water velocity, m/s; L = plate-pack length, m; and dpp = plate-pack perpendicular gap spacing, m.

For a low-drop Reynolds number, the drop/rise velocity is given by Stokes’ law, which is written as

where ρw = water density, kg/m3; ρo = oil density, kg/m3; μw = water dynamic viscosity, kg/(m∙s) or N∙s/m2; g = gravitational acceleration, 9.81 m/s2;

and

Do = drop diameter, m.

For a higher-drop Reynolds number, a more general form of Eq. 3 can be used. For a given plate-pack geometry and fluid conditions, the minimum drop that can be removed by the plate pack is obtained from Eqs. 2 and 3.

For water drops in oil, the water viscosity in Eq. 4 is replaced with the oil viscosity, and the horizontal velocity is that of the oil phase. Typical design drop size removal in plate packs is approximately 50 μm.

Other designs use mesh and matrix packing for liquid/liquid coalescing. However, plugging issues should be addressed when selecting the coalescer. In general, if solids are present in significant quantities, no coalescing internals are installed.

Nomenclature

ρc |
= |
continuous phase density, kg/m3 |

μc |
= |
continuous phase dynamic viscosity, kg/(m∙s) or N∙s/m2 |

Vc |
= |
continuous phase velocity, m/s |

dh |
= |
hydraulic diameter |

Vr |
= |
drop/rise velocity, m/s |

Vh |
= |
horizontal water velocity, m/s |

L |
= |
plate-pack length, m |

dpp |
= |
plate-pack perpendicular gas spacing, m |

ρw |
= |
water density, kg/m3 |

ρo |
= |
oil density, kg/m3 |

μw |
= |
water dynamic viscosity, kg/(m∙s) or N∙s/m2 |

g |
= |
gravitational acceleration, 9.81 m/s2 |

Do |
= |
drop diameter, m |