Hydrocarbon Processing - June 2021 - 26

Process Optimization
API 12J provides ranges of K factors for vertical and horizontal
vapor-liquid separators with demisters.3
The values are
presented in TABLE 1. The design of the internals (demister),
separator orientation and separator dimensions affect the K
factors. The Gas Processors Suppliers Association's GPSA Engineering
Data Book indicates that the higher values of K for
long horizontal separators are generally considered to be overly
optimistic.4
The GPSA Engineering Data Book indicates that
K = 0.5 ft/sec is normally used as an upper limit for horizontal
separators equipped with wire-mesh mist extractors.
Residence time method. The residence time method is used
to design flare KO drums with sufficient vapor retention time
for liquid droplets to settle. Therefore, the liquid droplet settling
time is important in the design of flare KO drums. The
liquid droplet settling time is shown in Eq. 5:
θ =
hv
ud
where
θ = the liquid droplet settling time, sec
hv
uv =
where
Qv
Av
Qv
Av
= the vapor volumetric flowrate at KO drum
operating conditions, ft3
/sec
= the vapor space cross-sectional area (as shown in
FIG. 2), ft2
.
The vapor residence time is calculated by using Eq. 7:
L
τ =
Uv
where
τ = the vapor residence time in the KO drum, sec
TABLE 1. API 12J guidelines for vapor-liquid separator designs
Separator type
Vertical
Horizontal
Inlet
Ud
hv, Av
Qv, ft3/s
Uv = Qv/Av
L
Height or length, ft
5
10
10
Other lengths
Typical K factor range
0.12-0.24
0.18-0.35
0.4-0.5
0.4-0.5 × (L/10)0.56
Vapor outlet
(7)
(5)
= the vertical drop height for liquid droplets (as shown in
FIG. 2), ft.
The axial vapor velocity is calculated by using Eq. 6:
(6)
Preventing re-entrainment from the liquid surface in
the KO drum. High axial vapor velocities can cause liquid reentrainment
across the liquid surface in the settled section of
the KO drums. This re-entrainment should be checked to ensure
that there is no liquid re-entrainment in the KO drums.
Ishii and Grolmes5
suggested Eq. 8 to estimate the onset liquid
re-entrainment axial vapor velocity in horizontal lines:
4
uv−reentrainment = 4.3105 ×
where
uv - reentrainment
⎡
⎣
⎢
⎢
⎢
velocity, ft/sec
The Center for Chemical Process Safety (CCPS) guidelines use
this onset liquid re-entrainment velocity from Eq. 8 as a limiting
axial vapor velocity to prevent liquid re-entrainment from the
liquid surface in the flare KO drum.6
σ = the liquid surface tension, lbm/sec2
μf
= the viscosity of liquid in centipoise (cP).
However, the onset liquid
re-entrainment vapor velocity obtained from Eq. 8 is only applicable
to dry flare vapors if there are combustible liquids in the
flare KO drum. For wet flare vapors, the maximum vapor velocity
allowed is much less. This is to ensure that the fine liquid
droplets in the wet vapors are removed prior to leaving the flare
KO drum. The proposed vapor velocity limit guidelines for wet
flare vapors are described in the following section.
New vapor velocity limit guidelines for wet (condensable)
flare vapors. This method attempts to estimate a limit
to the axial vapor velocity in horizontal flare KO drums. This
method applies to wet flare vapors. The procedure is as follows:
1. Calculate the onset liquid re-entrainment vapor velocity,
using Eq. 8.
Liquid outlet
FIG. 2. Schematic of a typical horizontal KO drum.
26 JUNE 2021 | HydrocarbonProcessing.com
2. Calculate the liquid droplet particle size, using Eq.
9 at the onset liquid re-entrainment vapor velocity,
based on the critical Weber number (We). The Weber
number represents the ratio of disruptive hydrodynamic
forces to the stabilizing surface tension force. The
liquid droplet starts to break up at the critical Weber
number. According to Ishii and Grolmes, the critical
ρf
ρg
⎛
⎝
× ⎜
σ
ρg
⎞
⎠
⎟ ×
⎛
⎝
⎜
⎜
g × ρf −ρg
µf
(
)
= the onset liquid re-entrainment vapor
⎞
⎠
⎟
⎟
2
⎤
⎦
⎥
⎥
⎥
0.1
(8)
L = the vapor flow path length (as shown in FIG. 2), ft
Uv
= the axial vapor velocity in the KO drum, ft/sec.
API 521 uses this residence time method to size horizontal KO
drums. The KO drum diameter and length are optimized-by
trial and error-to provide a larger retention time than the liquid
droplet settling time; however, no velocity considerations
are involved in the optimizing process. As a result, the flare KO
drums are often designed with axial vapor velocities exceeding
the onset liquid re-entrainment velocity, as the method provided
in API 521 does not check for possible liquid re-entrainment
from the liquid surface. API 521 indicates that flare vapor velocities
exceeding 9.8 ft/sec or 13.1 ft/sec could entrain liquid
droplets up to 1,000 µm in size. Liquid droplets 300 µm and
larger may drop out of the gas stream at less than 6.6 ft/sec.
However, the API 521 sample calculations for a flare KO drum
show an axial vapor velocity greater than 13.1 ft/sec. It was for
this reason that this article was developed.
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