Page 198 - Petroleum and Gas Field Processing

Figure 6 Dual polarity ac/dc field.

3. Variable gradient field for maximum salt reduction. If the field
gradient is increased beyond a certain limit (E c ), this will shatter
the drops; it is expressed as

1=2

E c k
d
where k is the dielectric constant,
is the interfacial tension, and d
is the drop diameter. Thus, the drop size can be controlled by the
field gradient. The electric field can be used both to mix and
separate the drops.
By cycling the field strength, the process can be repeated many times
during the retention time of the drops within the electric field. Voltage
modulation will create mixing, coalescence, and settling, as shown in
Figure 7. The electrostatic field causes the dipolar water molecules to align
with the field.
In the electrical desalting process, a high potential field (16,500–
33,000 V) is applied across the settling vessels to help coalescence, as
shown in Figure 8. The chemielectric concept utilizing both chemical
agents and electrical field is schematically illustrated in Figure 9.
In the desalting process, it is a common practice to apply enough
pressure to suppress any loss of hydrocarbon due to vaporization of
the oil. The pressure normally used in a desalting process is in the range
50–250 psi.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.