Page 191 - Environmental Control in Petroleum Engineering

Waste Treatment Methods 177

Filtration

One way to remove oil droplets from water is to pass the water
through water-wet filters or membranes. These filter media use capil-
lary pressure to trap oil and prevent it from passing out of the filter.
Advanced filtration processes include crossflow membranes such as
microfiltration and ultrafiltration (Chen et al., 1991). These processes
consist of a hydrophilic microfiltration membrane that passes water
(and dissolved material), but not oil droplets. The shape of the filter
is typically a small diameter capillary tube that the emulsion flows
through. A schematic of a microfiltration capillary is shown in Fig-
ure 6-3. The emulsion leaving the tube without passing through the
filter can be recycled through the filter a number of times to further
concentrate the emulsion for other types of treatment or disposal.
Microfiltration processes are usually ineffective for hydrocarbon
removal, however, because the filters and membranes foul easily by
oil and have short useful lifetimes.

Filtration Coalescence

Another type of filtration is to pass the water through oil-wet filters.
The oil droplets attach to the filter matrix and coalesce into larger
ones. When the filter medium has become saturated, larger oil drops
will flow out of the filter, either by continued injection or by back-
washing. These larger droplets can be more easily removed from the
water by subsequent gravity separation. Sand, gravel, or glass fibers
are common media used for this process.

Filtered water out

Emulsion I _ I I I II I I Emulsion

Filtered water out

Figure 6-3. Schematic of a microfiltration capillary tube.