Produced water ordinarily enters the water-treatment framework from either a two or three-phase separator, a free water knockout, a gun barrel, a heater treater, or other primary separation unit processes. It presumably incorporates small amounts of free or dissolved hydrocarbons and solids that must be removed before the water can be re-used, injected or discharged.

The level of removal (especially for hydrocarbons) and disposal options are typically specified by province, state, or national regulations. This blog will discuss procedures for the removal of free and dissolved hydrocarbons.

Gravity Separation devices

Water-treating equipment that makes use of gravity separation incorporates:

Skim tanks

API separators

Plate coalescers

Skim piles

These devices are very simple and inexpensive; however, because of the large residence times necessary for separation, they are substantial and require large footprints. These devices are commonly utilized on both land-based and offshore fixed structure facilities; however, they are motion-sensitive and find restricted use on floating facilities.

It is important to know both the oil concentration in the influent water and the molecule-size distribution to appropriately design a gravity separator to meet a specific effluent quality. This information can be determined precisely only by sampling the treated water stream. Laboratory testing can provide indicative data for scale-up and correlation, while curves can provide an initial estimate from which to work. These data will vary with the oil and water properties and process conditions.

Skim tanks and vessels

The easiest form of treatment equipment is a skim tank or pressure vessel. These are typically intended to provide large residence times during which coalescence and gravity separation can occur. They can be either pressure vessels or atmospheric tanks.

Skim tanks can be either vertical or horizontal in setup. They may be set up for vertical descending streams of water with or without inlet spreaders or outlet collectors. They may likewise be designed as horizontal vessels in which the water enters on one side and flows over a weir on the far end.

In vertical vessels, the oil droplets must flow upward against the descending velocity of the water. Consequently, vessels are more efficient in gravity separation of the two liquid phases.

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