Page 240 - Petroleum and Gas Field Processing

cleaning the vessels as compared to the sand drains in horizontal vessels.
Further, vertical vessels are better than horizontal vessels with regard to
handling liquid surges. Surging in horizontal vessels tends to create
internal waves, which results in a false indication of a high liquid level
within the vessel and leads to false high-level shutdown.
Another type of skimmer is the API separator, which is basically a
horizontal, rectangular cross-section tank. This type of skimmer is mostly
used for treatment under atmospheric conditions.

9.4.4 Plate Coalescers
There are mainly two types of plate coalescer: the parallel plate interceptors
(PPI) and the corrugated plate interceptors (CPI). Both types consist of a
set of parallel plates that are spaced a short distance apart and are inclined

by an angle of 45 .
The PPI was the first form of plate coalescers where a series of
inclined parallel plates is installed inside an API separator, as shown in
Figure 3. The water flow is split between the plates; therefore, the oil
droplets need first to rise along the short distance between two consecutive
plates where coalescence occurs (Fig. 4). Due to gravity, the large oil
droplets move upward along the bottom surface of the inclined plate and
then vertically upward to the oil collection section, where oil is skimmed
out of the tank. Sediments in the water move downward to the bottom of
the tank, where they can be removed.
The CPI is the most commonly used plate interceptor in the industry.
The CPI was an improvement over the PPI, where the surface of the
parallel plates was made corrugated with the axis of the corrugations being
parallel to the direction of water flow. As shown in Figure 5, the water to
be treated flows downward through the CPI pack. The oil rises upward,
counter to the water flow and accumulates at the corrugations. The
accumulated oil flows along the axis of corrugations and upward to the
oil–water interface.

Figure 3 The parallel plate interceptor.

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