Hydrocarbon Processing - June 2021 - 25

Special Focus Process Optimization
J. S. KIM, H. J. DUNSHEATH and J. LOPEZ,
Covestro, Baytown, Texas
Vapor velocity guidelines for horizontal
flare knockout drums
Flare knockout (KO) drums are important components
of effluent handling systems. A horizontal drum is often considered
more economical than a vertical one if the vapor flow
is high and a large liquid holdup is required. The function of
the flare KO drum is to remove liquid droplets in the range of
300 μm-600 μm from the vapor feed stream to the flare burner.
Flare stacks should not discharge larger liquid droplets, as these
can cause smoke or burning rain. Burning rain creates a significant
safety hazard to people and can cause damage to structures
and equipment. The most effective way to prevent burning rain
is to install a properly designed KO drum.
American Petroleum Institute (API) Standard 521 presents
a method for sizing horizontal flare KO drums based on a residence
time method that does not verify vapor axial velocities.
High axial vapor velocities are generally not desirable because
the high axial vapor velocities may entrain the liquid droplets in
the vapor phase. Therefore, the liquid droplets are carried along
with the continuous vapor flow. The residence time method
should be supplemented by vapor velocity limit guidelines for
horizontal flare KO drums; however, there are presently no appropriate
guidelines for the vapor velocity limits. This article
proposes a new guideline for the axial vapor velocity in the horizontal
flare KO drums.
Liquid droplet settling velocity. In horizontal flare KO
drums, liquid droplets are separated from a continuous vapor
phase. To separate the liquid droplets, the vapor phase velocity
should be sufficiently low to allow the liquid droplets to settle
(i.e., to fall out of the vapor). API 521 uses Eq. 1 to calculate the
liquid droplet settling velocity in flare KO drums:1
ud = 1.1547 ×
where
ud
g × Dp × ρf −ρg
ρg × C
(
)
g = the acceleration due to gravity, 32.174 ft/sec2
Dp
= the liquid droplet settling velocity, ft/sec
= the liquid droplet diameter, ft
C = the drag coefficient (i.e., API 521, Figure 5).
API 521 Figure 5 relates the drag coefficient (C) to a function
of the drag coefficient and Reynolds number.1
ρf = the density of liquid at operating conditions, lb/ft3
ρg = the density of vapor at operating conditions, lb/ft3
The drag
coefficient can also be obtained by a curve fit equation (Eq. 2):
(1)
))
x = C × Re2 =
where
μv
= viscosity of vapor in cP.
The values obtained from Eq. 2 agree closely with those from
API 521, Figure 5. The relationship between Eqs. 2 and 3 is
shown in FIG. 1.
In vertical flare KO drums, API 521 recommends using the
liquid droplet settling velocity from Eq. 1 as the limiting vapor
velocity. For horizontal flare KO drums, API 521 uses the liquid
droplet settling velocity as a starting point for optimizing
the KO drum designs.
General vapor velocity guidelines for two-phase separators.
The maximum vapor velocity for two-phase separators
has been calculated using the empirical Sauders-Brown equation
(Eq. 4). Walas recommended a K value of 0.14 ft/sec for
vertical separators without a demister.2
For sizing horizontal
separators without a demister, it is recommended to use higher
values by a factor of 1.25, since the upward drag of the vapor is
largely absent in horizontal separators:
uv = K
f
g
where
K = the vapor load empirical factor
uv
= the vapor velocity, ft/sec.
64
32
16
8
4
2
1
0.5
0.25
10
100
1,000
x, C.Re2
10,000
FIG. 1. Drag coefficient vs. Reynolds number.
Hydrocarbon Processing | JUNE 2021 25
100,000
1,000,000
g
(4)
C = EXP(6.494 - 1.158 × ln(x) + 0.056 × ln(x2
0.00074 × ln(x3
9.5 × 107 × ρg × ρf −ρg
(
µv
2
) -
) × Dp
3
(2)
(3)
Drag Coefficient, C
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