Page 211 - Petroleum and Gas Field Processing

A cold feed stabilizer normally operates with a fixed top and bottom
temperatures. The former is kept as low as possible to maximize recovery,
whereas the latter is controlled to maintain the product bottom pressure. It
is of interest to mention that the overhead gas temperature is identical
to the liquid feed temperature because the ratio of masses of vapor leaving
the column to liquid feed entering is rather small.
Most stabilizers operate above 200 psia and consist of 20 bubble
trays. High-pressure stabilizers have more trays because of the higher
temperature gradient between the top and the bottom trays. More trays
allow the column to operate closer to equilibrium. Columns less than
20 inches in diameter generally use packing rather than trays. A useful rule
2
of thumb is that 1 ft of tower area could handle about 100 bbl/day of stock
tank liquid. In some designs, the cold feed is introduced several trays
below the top tray, using the upper trays as a scrubber in order to prevent
liquid carry over during burping.
Field operation of a stabilizer is described as follows. Relatively cool
liquid (oil) exiting the GOSP is fed to the top plate of the column where
it contacts the vapor rising from below. The rising vapors strip the lighter
ends from the liquid (i.e., acting as a stripping agent). At the same time,
the cold liquid—acting as an internal reflux—will condense and dissolve
heavier ends from the rising vapor, similar to a rectification process.
The net separation is very efficient as compared to stage separation
(3–7% more).
To have a stabilized product of certain specifications, in theory
a stabilizer can be operated at multiple combinations of tower pressure
and bottom temperature. In general, as the tower pressure is increased,
more light ends will condense in the bottom. In normal operation, it is best
to operate the tower at the lowest possible pressure without losing
too much of the light ends at the initial feed flash. This will minimize
‘‘burping’’ and cause the column to operate near equilibrium. In addition,
lower operating pressures require less reboiler duty with less fuel
consumption.
Operating data for a 40,000-bbl/day nonrefluxed stabilizer are given
in Figure 5.

7.4.3 Main Features and Applications of Stabilizers

Stabilizers used for oil production field operations should have the
following features:

They must be self-contained and require minimum utilities that are
available in the field, such as natural gas for fuel.

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