Page 192 - Environmental Control in Petroleum Engineering

178 Environmental Control in Petroleum Engineering

Chemical Coagulants

The removal of small, suspended oil droplets can be aided by adding
chemicals that coagulate and flocculate the droplets (American Petrol-
eum Institute, 1990). These chemicals typically overcome the electro-
static repulsion charges on the individual droplets, allowing them to
coagulate into larger drops. These larger drops can then be more
efficiently removed with gravity separation equipment. Common
chemicals used include lime, alum, and polyelectrolytes. The use of
dithiocarbamate has also been reported (Durham, 1993).

Electric Field Separation

Another way to separate oil from water is by applying an electric
field (voltage) to the water to electrostatically remove the oil. These
fields can be applied through either a direct or an alternating current.
Oil droplets in an oil-in-water emulsion have a negative surface charge
(zeta potential) that can be manipulated to facilitate their removal.
When a direct current is applied to the water containing such an
emulsion, the oil will migrate toward the positive electrode. The migration
velocity of the drops in many systems is on the order of 1 mrn/min, which
requires separators using very narrowly spaced parallel plates (Fang et
al., 1991). This process, however, can only be used with saline water.
When an alternating current is applied, the droplets may flocculate
if a metal hydroxide is present (Farreil, 1991). This process is known
as alternating current electrocoagulation. In this process, a metal
hydroxide is added to the water and an alternating current is used to
overcome the electrostatic repulsion charges on the particles. When
the electrostatic repulsion charges have been neutralized, the particles
can flocculate and be more easily separated from the water by other
methods. Iron and aluminum hydroxides have been successfully used.

Biological Processes

Biological processes rely on bacterial degradation of hydrocarbons.
They have limited application in the removal of free hydrocarbons
from most wastewater streams in the petroleum industry because they
are too slow and are not appropriate for high oil concentrations. Large
quantities of free oil can limit mass transfer of oxygen and nutrients