Page 190 - Environmental Control in Petroleum Engineering

176 Environmental Control in Petroleum Engineering

Hydrocyclones for separating oil and water are limited to cases where
the inlet pressure is sufficient to drive the flow (Flanigan et al., 1989).
For low-pressure operations, the fluid may need to be pumped into the
hydrocyclone. A progressive cavity pump with low shear has been found
to be an effective way to increase the fluid pressure without shearing the
oil into smaller drop sizes. The drop size is a critical parameter in the
effectiveness of hydrocyclones in separating oil from water.
A related way to enhance gravity separation is through a decanting
centrifuge. In this device, the produced water enters the spinning
centrifuge, where the oil is separated from the water because of its
lower density. Centrifuges differ from hydrocyclones in that the
spinning is mechanically driven in a centrifuge, while it is induced
by the inlet velocity of the water in a hydrocyclone. A centrifuge can
also have internal plates to enhance separation, making it a spinning
plate separator. Centrifuges can remove oil droplets down to about 2
micrometers in diameter (Van Den Broek and Plat, 1991).

Heater Treaters

Oil and water can also be separated by heating the mixture. The
higher temperature lowers the fluid viscosity of the mixture and alters
the interfacial tension between the phases, allowing the oil and water
to separate faster.

Gas Flotation

Suspended oil droplets can also be removed from water by gas
flotation. If gas bubbles are passed through an emulsion of oil-in-water,
the oil droplets will attach to the bubbles and be carried to the top of
the mixture where they can be easily removed. Air bubbles are norm-
ally pumped through the water, although the expansion of dissolved
air is also used. Gas flotation is often aided by the addition of chemical
coagulants. Carbon dioxide has also been used as the flotation gas
(Burke et al., 1991). Gas flotation, however, can create a foam that is
difficult to break.
Gas flotation systems can reduce oil concentrations to 15-100 mg/1,
with a typical average of 40 mg/1 (Simms et al., 1990). Like other
separation methods, gas flotation systems can have operational difficul-
ties, as summarized in Table 6-1.