Page 157 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SOLID-LIQUID SEPARATION AND INTERCEPTORS

SOLID-LIQUID SEPARATION AND INTERCEPTORS 3.25

3. Chemically stabilized emulsions are made up of fine droplets that are stable because of
surface active agents.
4. Dissolved and dispersed oil is either actually dissolved or suspended in such a small size
(typically 5 μm or less) that ordinary filtration is not possible.
5. Oil-wet solids are particulates in which oil adheres to their surface.

Methods of Separation and Treatment. Oil spills and leaks are best treated in their most
concentrated state, which is at their source or as close as is reasonable. The primary methods
used to separate and remove free oil and oil-wet solids are floatation and centrifugation.
Secondary treatment, such as chemical treatment/coalescence or filtration, is then used to
break up oil-water emulsions and remove dispersed oil. Finally, tertiary treatment, such as
ultrafiltration, biological treatment, or carbon adsorption, will remove the oil to required
levels prior to discharge. This section will discuss the general principles of the primary and
secondary separation methods and devices only.
The American Petroleum Institute (API) has established criteria for the large-scale
removal of globules larger than 150 μm. In abbreviated form, they are as follows:
1. The horizontal velocity through the separator may be up to 15 times the rise velocity of
the slowest rising globule, up to a maximum of 3 ft/s.
2. The depth of flow in the separator shall be between 3 and 8 ft.
3. The width of the separator shall be between 6 and 20 ft.
4. The depth-to-width ratio shall be between 0.3 and 0.5.
5. An oil-retention baffle should be located no less than 12 in downstream from a skim-
ming device.

Gravity Separators. Gravity separation is the primary separation method. It is based on
the specific gravity difference between immiscible oil globules and water. Since all of these
liquids are lighter than an equal volume of water, gravity separators operate on the principle
of floatation. As water and oil flow through the unit, the oil floats to the top and is trapped
inside a series of internal baffles. Since the oil remains liquid, it is easily drawn off. Refer
to Table 3.3 for specific gravity of many common oils and other substances.

Floatation Devices. For service on a larger scale, the floatation of oil and oil-wet solids
to the top of the floatation chamber can be increased by the attachment of small bubbles
of air to the surface of the slow-rising oil globules. This is done by adding compressed air
to the bottom of the floatation chamber in a special manner that will create small bubbles
which will mix with, and attach themselves to, the oil globules.
Centrifugal Separators. For service on an even larger scale, the centrifugal separator is
used. This device operates on the principle of inducing the combined oil and water mixture
to flow around a circular separation chamber. The lighter oil globules will collect around a
central vortex, which contains the oil removal mechanism, and the clear water will collect at
the outer radial portion of the separation chamber. Methods have evolved that can produce
effluent water with only 50 to 70 parts per million (ppm) of oil; proprietary devices exist
that can lower oil content to 10 ppm.
Filtration. Using chemical methods first to break oil-water emulsions and then using
depth-type filters to remove the destabilized mixture have proven effective in removal of
oil globules in sizes between 1 and 50 μm. The velocity and flow rate of the mixture must
be carefully controlled to allow optimum effectiveness of the system.

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