Page 242 - Petroleum and Gas Field Processing

the horizontal) and the plate pack is placed inside a pressure vessel
(vertical or horizontal) such that the water flow is perpendicular to the
axis of corrugations in the plates. Vertical vessels are generally preferred
for handling sediments and sand problems. Regular CPI units are less
expensive and more efficient than cross-flow devices, but the later should
be used for treatment under pressure and for water containing large
amounts of sand or sediments.

9.4.5 Serpentine-Pipe Packs
The serpentine-pipe pack (SP pack) is another device that is used to
promote coalescence of the oil droplets and thus facilitates their separation
by gravity. The coalescence concept for the SP pack is, however, different
from that of the previously described equipment. Water is forced to flow
through a serpentine path that is properly sized to create turbulence that is
sufficient to cause coalescence without causing shearing of oil droplets
below a specified size. The SP packs are available in standard dimensions
ranging from 2 to 8 in. in diameter for handling water flow ranging from
900 to 73,000 bbl/day (BPD). Such packs are designed to develop a drop
size distribution curve with a maximum drop size of 1000 m. By
producing such a drop size distribution, gravity settling becomes very
efficient. In fact, SP packs can result in about 50% additional oil removal
as compared to gravity settling alone. The SP pack is normally placed
inside any gravity settling vessel with the water inlet diameter being the
same as the SP diameter. SP packs can be staged in series to allow
successive coalescence and removal of oil as the water flows from one
stage to the next.

9.4.6 Flotation Units

Flotation units utilize a completely different concept is removing oil
droplets from water. In this type of treatment equipment, a large number
of small gas bubbles are produced within the water. As the gas bubbles
rise upward, they carry the oil droplet to the surface, where they
accumulate and are then skimmed out of the unit. Flotation units are
classified into two types based on the method by which the gas bubbles are
produced. These are the dissolved gas units and the dispersed gas units.

Dissolved Gas Flotation Units
As shown in Figure 6, a portion of the treated water (between 20% and
50% of the effluent) is taken and saturated with natural gas in a contactor
at a pressure between 20 and 40 psi. The amount of gas used in standard

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