Page 178 - Petroleum and Gas Field Processing

time, and the viscosity of the oil affect the water droplet size that must be
removed.
Heating the emulsion results in a state of excitation where water
droplets collide and coalesce into larger droplets. Therefore, by increasing
the temperature of the emulsion, the water droplet size that must be
removed increases.
Increasing the retention time allows water droplets to grow larger.
However, after a certain initial retention time, increasing the time does not
significantly affect the water droplet size. For practically sized treaters,
the retention time is normally kept between 10 and 30 min. The higher
retention time is usually associated with heavier oils.
The viscosity of the oil is the most influencing factor on the water
droplet size that can be removed. The more viscous the oil, the larger the
droplet size that can be removed (settled) within a reasonable time. In a
recent article by Thro and Arnold [3], the following equations relating
the water droplet diameter, d m (that must be removed to achieve 1%
water content in the treated oil), to oil viscosity were developed using
field data:
For o < 80 cP

d m ¼ 200 o mm ð10Þ
For electrostatic treaters, 3 cP < o < 80 cP

d m ¼ 170 o mm ð11Þ

The above equations are useful in the absence of actual laboratory
measurements, which are very difficult to obtain.
With the water droplet size that must be removed determined from
the above equations, the dimensions of the coalescing/settling section of
the treater must be sufficient to allow settling of such water droplets and
also allow for the required retention time. These two conditions could be
used to develop two equations that govern the dimensions of the treater as
detailed in the following subsections for horizontal and vertical treaters.

5.8.1 Sizing Horizontal Treaters
Water Droplets Settling Constraint
We start with the settling equation [Eq. (3)]

ð
Þd m ft
6
u ¼ 1:787 10 ð3Þ
o s

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