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Liquid/Liquid Separation

Technical Profile

The purpose of this process is to remove free oil from produced water prior to discharge or injection. Liquid/Liquid cyclones are the most often used equipment for this separation.

Liquid hold up in a hydro cyclone is in the range of seconds. The liquid-liquid (L/L) separation is based on centrifugal forces and the difference between specific gravity of oil and water/solids. Produced water is injected under pressure tangentially. The conical shape of the cyclone causes an increase of speed, resulting in large centrifugal forces and separation of oil from water and solids. Water/solids will move in a vortex towards the exit of the cyclone, whereas the oil/gas will move in a secondary vortex in the centre of the cyclone towards the top of the hydro cyclone.

Dissolved components, such as benzene and heavy metals will not be removed. Oil removal is good for droplets larger than 15-30 microns (depending on the specific gravity of the oil) and outlet concentrations around 60 mg/l. Inlet oil content exceeding 1000 mg/l will cause effluent performance to drop. Smaller droplets and emulsified/dissolved oil are not removed. The effluent oil concentration of a hydro cyclone will mostly be just above the limit set for open sea discharge. This makes additional treatment mandatory.

Hydro cyclones are 94 to 95% efficient at high throughput. They have a compact footprint desirable for offshore application. They are reliable with no moving parts, but they are sensitive to process fluctuations. They have high energy requirements for inlet pressure; although, often the pressure needed for operation may be supplied by the wells.

In most cases, L/L cyclones are placed before S/L cyclones. There are advantages in doing so. Gas will come off with oil, which will cause less cavitation in downstream equipment. This arrangement provides a better pressure profile since the outlet of the L/L cyclone can go to the S/L cyclone without repressurization. On the downside, oil sticking to solid particles may separate due to shear in the S/L hydro cyclone and exit with the water requiring further removal treatment downstream. Due to high shear forces in cyclones, solids have a very abrasive action on the liners of hydro cyclones. It is more expensive to protect two stages of hydro cyclones against erosion than one.

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