Chapter 3
If we assume that the water fraction in each layer does not change during
sedimentation, the sedimentation velocity within the emulsion can be obtained by
applying a mass balance across the sedimentation front. If there is negligible
sedimentation in the concentrated emulsion with volume fraction φmax, the
sedimentation velocity of water droplets in emulsion above the front is given by:
‰er = ^max~^e^∙ (Iowerfront) [3.40]
<Pe dt
In sample 3 (no solids, no PR5, Figure 3.16) a sharp front moving upward
from the bottom is less evident. However, a front moving downward from the top
of sample 3 (though less clearly in sample 1 ) can be seen with nearly water-free
oil above and emulsion below (Figure 3.23). A similar mass balance yields:
⅛ = = (upper front) [3.41]
φe at dt
In these equations h is front position, v∣0wer and vupper are the sedimentation
velocity of water droplets in the emulsion, whose volume fraction φe is assumed
as 0.50. The average water fraction in the concentrated emulsion layer 0.75 can
be used as the φmax value, and the average water fraction in the clean oil layer
φmin is close to zero.
The predicted sedimentation velocity of the emulsion can be calculated with
the following equation, which is an empirical modification of Stokes Law:[19L [20]
82
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