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# Coalescers

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Liquid-liquid coalecers are also widely used in oil refining industry to remove traces of contaminants.

## Use in separators

For liquid/liquid coalescence in three-phase separators, FWKO, and other separators in which it is desired to have separate liquid outlets for oil/water, plate packs provide less turbulent/more laminar flow and a smaller distance over which drops have to settle. Plate packs also have been installed to promote degassing.

Laminar flow is indicated by the flow Reynolds number, which is defined as

................(1)

where

ρc = continuous phase density, kg/m3 μc = continuous phase dynamic viscosity, kg/(m∙s) or N∙s/m2 Vc = continuous phase velocity, m/s

and

dh = hydraulic diameter.

For a plate pack with a 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. 8 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.

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

................(2)

where

Vr = drop/rise velocity, m/s Vh = horizontal water velocity, m/s L = plate-pack length, m

and

dpp = plate-pack perpendicular gas spacing, m.

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

................(3)

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, cm.

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.

................(4)

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, cm

## References

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