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Secondary Oil/Water Separation

Technical Profile

Secondary oil/water separation generally utilizes gas flotation, to attach gas bubbles to small particles thereby increasing their buoyancy, to remove contaminants. The objective of secondary oil/water separation is to remove oil from water.

Often oil and solid particles are so small they distribute themselves evenly in the water phase and will not settle by gravity or cyclone separation. Secondary separation involves some type of mechanical or hydraulic mechanism to remove oil and solids. Equipment used in secondary separation includes induced gas flotation (IGF) and dissolved gas flotation (DGF). Since space and weight are so critical on offshore platforms, DGF is not used since it needs larger retention times to make good separations

Induced gas flotation involves the introduction of fine gas bubbles that attach to oil and fine solids and float them to the surface where they are removed to slop oil systems. Flotation units can remove up to 95% of free oil. They are mostly used as a separation step downstream of gravity and cyclone separation to meet discharge limits for oil and grease. Inlet oil content should not exceed 300 mg/L as outlet oil removal suffers above this limit. Produced gas recirculation is generally used to generate micro bubbles. The introduction of air is not recommended for potentially hydrocarbon rich environments due to risk of explosion and dissolved oxygen content can have detrimental effects on equipment and subsurface production or disposal formations.

The most common flotation separators in offshore produced water use are vertical hydraulic IGF units. They have low retention times and therefore are smaller units. IGF is efficient at elevated water temperatures, which are often produced from the wellhead. Without chemical addition flotation units should remove particles larger than 25 microns. With chemical addition to coagulate oil and solids, particles less than 10 microns may be removed.

In induced gas flotation, gas is introduced into water hydraulically. The result is a turbulent mixture of gas and liquid. Rising gas bubbles attach to oil and solids and float these particles to the surface where they are removed into a skimming launder as float recycle. Retention time is generally between 4 and 8 minutes depending on the method of inducing gas.

IGF scum (float material) is watery and needs further thickening or recycle. Subnatant from oil thickening can be recycled upstream of the IGF. Recovered float material may also be recycled to upstream separation equipment. Skimming devices exist to reduce water content of the float and lower recycle rates. Still float recycle can account for 3-15% of feed flow.

It should be noted that demulsifying chemicals may be added in primary separation to aid removal of water from oil. These chemicals may have a negative effect on flotation cell performance. For this reason, some type of foaming agent may be needed to improve performance.

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