Page 164 - Petroleum and Gas Field Processing

density is more pronounced than on water density. The result is that the
difference in density (or specific gravity) increases as the emulsion is

heated. For example, if oil and water are heated from 60 F to 150 F, the
following change in their relative specific gravity takes place:

at 60 F at 150 F
Oil specific gravity 0.83 0.79
Water specific gravity 1.05 1.03
Difference in specific gravity 0.22 0.24

With reference to Eq. (1) an increase in
increases the settling
velocity and, therefore, promotes the separation of water droplets from the
bulk of oil. The change in the specific gravity difference is, however, small.
Therefore, this effect is not as significant as the effect of viscosity. In fact,
we may completely ignore the effect of specific gravity on the process up

to a temperature of 200 F. For some specific crude oils, increased
temperature may cause a reverse effect on the difference in specific gravity.
For some heavy oils, the specific gravity of the oil and water will be equal
at certain temperature. This situation must be avoided, as it will stop the
separation process completely. Therefore, care should be exercised when
determining the treating temperature for a specific crude oil.
Another beneficial effect of heating is that the increased temperature
promotes movements of the small water droplets, which upon collision
with one and the other, may form larger size droplets. The increased
droplet size significantly speeds the settling process, as indicated by Eq.
(1). Heat will also help to destabilize (weakening) the emulsifying film,
hence breaking the emulsion. Further, heating will dissolve the small
paraffin and asphaltenes crystals and, thus, neutralize their potential effect
as emulsifiers.
Despite of all of the above-discussed benefits of heating, there are
some drawbacks associated with this method of treatment. Heating of the
oil can result in significant loss of the lighter hydrocarbon ends and thus

results in loss of the oil volume. For example, heating 35 API oil from

100 F to 150 F results in losing more than 1% of the oil volume. Of
course, the evaporates (light ends) could be collected and sold with the
gas. This, however, will not make up for the loss of revenue resulting from
oil losses. In addition to oil losses, evaporation of the light ends leaves the
treated oil with lower API gravity (i.e., lower quality), which will be sold
at a lower price. Finally, heating requires additional investment for heating
equipment and additional operating cost for the fuel gas and maintenance.

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