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Liquid/Solid Separation

Technical Profile

The purpose of this process is to remove suspended solid particles from produced water prior to discharge or injection. Solid/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 solids-liquid (S/L) separation is based on centrifugal forces and the difference between specific gravity of water and 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 solids from liquid. The heavier solids will move in a vortex towards the exit of the cyclone, whereas the lighter liquid will move in a secondary vortex in the centre of the cyclone towards the top of the hydro cyclone.

Hydro cyclones are 94 to 95% efficient at high throughput. They have a compact footprint desirable for offshore application. Hydro cyclones are generally placed between primary separation and filtration. 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.

The solids to be separated by means of a S/L hydro cyclone are mostly sand particles having a specific gravity of 2.5 or greater thereby differing quite a lot from that of water. Separation performance is thus better than that of a liquid-liquid (L/L) hydro cyclone with less effort. For that reason lower G-forces can be applied, which means lower pressure and wider units. This in turn leads to higher liner capacity and lower overall investment cost. Particle removal is typically:

Particle Size

Removal Rate

>10 micron

98%

5-10 micron

95%

In most cases, S/L cyclones are placed after L/L cyclones. When possible, there are advantages for placing S/L cyclones before L/L cyclones. 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. Also, oil sticking to solids may separate due to shear in the S/L hydro cyclone. If S/L cyclones are place ahead of L/L cyclones oil will separate into the oil phase rather than leaving with water where it must be removed in downstream equipment. However, placement of S/L before L/L cyclones requires repressurization between the two separation processes.

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