Coalescers

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 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

${\text{Re}}={\frac {\rho _{c}V_{c}d_{h}}{\mu _{c}}},$

(1)

where ρ_c = density of the continuous phase, kg/m³; μ_c = dynamic viscosity of the continuous phase, kg/(m∙s) or N∙s/m²; V_c = mean velocity of the continuous phase, m/s; and d_h = equivelent diameter of the flow channel.

For a plate pack with a perpendicular gap spacing of d_pp, the hydraulic diameter is approximately equal to 2 d_pp. 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 d_pp/cos(α), where d_pp 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.

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  Fig. 8—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

$V_{r}=V_{h}\times {\frac {d_{pp}}{L\cdot \cos \alpha }},$

(2)

where V_r = drop/rise velocity, m/s; V_h = horizontal water velocity, m/s; L = plate-pack length, m; and d_pp = 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

$V_{r}={\frac {(\rho _{w}-\rho _{o})\times g\times D_{o}^{2}}{18\mu _{w}}},$

(3)

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

and

D_o = 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.

$D_{o}={\sqrt {{\frac {18\mu _{w}}{(\rho _{w}-\rho _{o})\cdot g}}\times {\frac {V_{h}\cdot d_{pp}}{L\cdot \cos \alpha }}}}.$

(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, m

Noteworthy papers in OnePetro

Al-Qahtani, A.A. 2012. Vessel Internal Electrostatic Coalescer technology (VIEC). SPE-156087-MS presented at the SPE International Production and Operations Conference & Exhibition, Doha, Qatar, 14-16 May. http://dx.doi.org/10.2118/156087-MS.

External links

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See also

Oil and gas separators

PEH:Oil_and_Gas_Separators