Chemical Engineering July 2011 - 40

Figure 3a
Figure 3b
Engineering Practice
thereby increasing the annular velocity.
The annular velocity is modeled
as a linear function with vertical distance,
and the annular velocity is zero
at the bottom of the cartridge and increases
to a maximum value at the top
of the cartridge.
Once the coalesced droplets are
formed, they immediately drain vertically
downward in the coalescer-medium
pack. As a direct consequence of
the treatment, the coalesced droplets
are shielded from the upward gas flow
in the annulus in most of the length of
the coalescer cartridge. The coalesced
droplets are first exposed to the annular
gas flow when they appear on
the external face of the coalescer medium
pack at the bottom third of the
coalescer cartridge (Figure 3a). Once
the coalesced droplets are released to
the annular space they are subjected
to the force of the upward flowing gas.
The trajectory of the coalesced droplets
is modeled on a force balance between
gravity settling and the drag
force created by the gas flow past the
droplets. This analysis leads to the
calculation of a critical annular velocity
for re-entrainment (vc).
The use of a surface treatment on
high-performance vertical liquid-gas
coalescer cartridge systems has been
proven to significantly enhance performance
by allowing higher flowrates or
smaller housing diameters compared
to untreated coalescers [5].
Due to the surface treatment, there
are minimal coalesced droplets present
in the annulus above the drainage
point at the bottom third of the
coalescer cartridge. For a coalescer
cartridge that is not specially surface
treated, the coalesced liquids are
present throughout the length of the
coalescer in the annulus space, and
the critical annular velocity for re-entrainment
is given for the top of the
element (Figure 3b). For the treated
coalescer, it is allowable to have annular
velocities greater than the critical
value for re-entrainment in the
portion of the annulus space where
there are no liquids present. This permits
the maximum annular velocity
at the top of the coalescer cartridge to
be about three times the critical reentrainment
value needed at the vertical
position of the lower one third
FIGURE 3. Surface
treatment of the media
reduces the coalesced
droplets present in the
annulus above the drainage
point at the bottomthird
of the coalescer cartridge.
As a result, treated
coalescers (a) can have
annular velocities greater
than the critical value for
re-entrainment than for
untreated coalescers (b)
V
C
V
C
Surface
treated coalescer
of the cartridge height where liquids
are present.
Determination of minimum housing
diameter. The housing diameter
is determined from the area of the
annulus and the area of the coalescer
end-caps. The maximum annular
velocity at the top of the coalescer
cartridges is used to determine the
annular area required. The value of
the maximum annular velocity [vann
(max)], at the top of the coalescer cartridges
is dependent on the critical annular
velocity for re-entrainment (vc)
and the vertical location at which the
coalesced droplets are present in the
free annulus space. This relationship
can be described as follows:
vann (max) = kavc
(5)
where ka is the annular velocity enhancement
factor due to drainage.
For the untreated coalescer medium,
the coalescer cartridge is completely
wetted and coalesced droplets
are present in the annulus space up
to the top of the annulus where the
annular velocity is highest. There is
no drainage enhancement, and ka = 1.
The maximum annular velocity to prevent
re-entrainment is then equal to
the critical value for re-entrainment:
Untreated coalescer:
vann (max) = vc
(6)
The effect of the surface treatment
is to greatly increase the drainage,
and the annular velocity at the top
of the coalescer cartridge can now be
40 CHEMICAL ENGINEERING WWW.CHE.COM JULY 2011
Untreated
coalescer
significantly higher than the critical
value since there are no coalesced
droplets present in the annulus except
in the bottom third of the cartridge.
The maximum annular velocity is now
determined, with ka = 3.1, as follows:
Surface treated coalescer:
vann (max) = 3.1vc
(7)
Convincing evidence for the enhanced
maximum annular velocity
given by Equation (5) has been demonstrated
by laboratory tests [6, 7,
8] and is presented in Figure 3. Visual
observations during these tests
also confirm that liquids are present
on the outside of the coalescer pack
only at the bottom third for the surface
treated coalescer and are present
throughout the length of the wetted
untreated coalescer.
LABORATORY
TEST PROCEDURES
A description of laboratory methods
that have been reported by a number
of gas-filter and coalescer manufacturers
and their relevance to actual field
operation is provided below.
DOP test
An organic liquid, dioctyl phthalate
(DOP), is first vaporized by heating
and then cooled down, allowing the
DOP to condense and create a nearly
mono-disperse drop-size distribution
at 0.3 µm. A portion of the aerosol
mist created is mixed with carrier air
and flowed through a filter disc used
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Chemical Engineering July 2011

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