Experimental Study on Separation Characteristics of a Gas-Liquid Cyclone Separator in WGS

Wet Gas Scrubbing (WGS) flue gas
desulfurization scrubber is an important equipment for Fluid Catalytic Cracking
(FCC) to wet desulfurization. It is mainly used to separate and purify flue gas
into the atmosphere and lye droplets which can be collected and recycled after
separation. Sinopec Daqing Company found that there was serious rain and ice
formation around the scrubbing tower, which affected the stable operation of
the device due to the low gas-liquid separation efficiency of the scrubbing
tower in actual operation. In order to improve the separation efficiency of the
scrubber, it is necessary to understand the internal separation rule. In this
experiment (Fig.1), a large-scale cold gas-liquid cyclone separator was
established by the industrial device based on the similar amplification
principle. The pressure and velocity field information inside the device was
measured by pressure sensors and five-hole probes.

Fig.1 Schematic
diagram of device structure and measuring point

Then we analyzed separation rule of
the inside of the device. The results are as follows.

1. The pressure drop of the gas
liquid separator has good predictable performance and the total pressure drop
is very small less than 30 Pa and the Eu
number (
) keeps at
 at
different inlet gas velocities in the range of 3.7 to 9.8 m/s.

Fig.2 The
variation of pressure drop with inlet gas velocity

2. Through the analysis of mean,
standard deviation of pressure and velocity field at different measuring points
of the gas-liquid separator, we found that the tangential velocity peaks at the
region of r/R=0.12, the boundary of inner and outer swirling flow in the
separation space is in the region of r/R=0.15-0.2 (Defined r/R=1 as the wall,
r/R=0 as the center), and the boundary of upward flow and downward flow at
r/R=0.57. The two boundary radial positions vary little in the whole separation
space. In the free space, all flow is upward, and the variation of tangential
velocity along the radial direction is the same as that in the stable
separation space. In the whole space, the static pressure value is the lowest
at the center and the maximum at the wall. The standard deviation at the center
is the maximum and the value at the side wall is the minimum. A small amount of
outer swirling gas along the separation space will enter the inner swirling
flow, and the amount of gas increased gradually with the increase of the axial
altitude. Most of the outer swirling flow reverses to the inner swirling occurs
in the region of Z/D = 3.98 ~ 4.70 (Z- distance from the center of the feed
pipes; D- inner diameter of the cylinder).

Fig.3 Time-average and standard deviations pressure at different tappings

Fig.4 Tangential
and axial velocity at different tappings

3. Through the power spectrum
analysis of the pressure in different positions of the experimental device, the
pressure fluctuation caused by the unstable intake air will be transmitted in
the whole device. The high-frequency signal can be easily eliminated in the air
flow movement but the low-frequency signal can not be easily eliminated which
caused the high-amplitude clutter signal peaks in the center. In the separation
space, the inner and outer swirling frequencies of 155 Hz and 45 Hz are found
at the center, and the inner and outer swirling frequencies of 145 Hz and 35 Hz
are found at the wall. It shows that the frequency and amplitude has certain
transferability along the radial direction, and the frequency attenuates less
from the center to the side wall, and the amplitude attenuates more. Because of
the intersection of outer and inner swirling, the unique frequency peaks of 88
Hz was found at Z/D= -2.30, 3.98 and 4.70 sections in the free space and
separation space.

Fig.5 Power spectral density at different tappings

4. When there is a certain height
of initial liquid level in the separation space, the liquid level will rotate
under the influence of rotating airflow. The maximum distance and the minimum
distance of the liquid level will increase with the increase of cross-sectional
gas velocity (V). When the
cross-sectional Reynolds number Re_c (
) is greater than 4.69¡Á10⁴, the rotating
distance will remain unchanged. Compared to the condition of no level, the
dimensionless static pressure defined as pressure value of the measuring point
divide value of the inlet and the standard deviation of static pressure
decrease greatly at the center in the whole space, but the change is not
obvious at the wall. The change of pressure in free space has nothing to do
with the initial liquid height. In the separation space, it is found that the
dimensionless static pressure and the static pressure standard deviation is
similar in the whole section when the liquid level DH/D (DH- distance from the
initial liquid level to the center of the feed pipes) is constant measured at
Z/D=0.62. With the increase of the initial liquid level, the dimensionless
static pressure does not change, but the static pressure standard deviation increases
first then decreases and approach the peak when the initial liquid level is
DH/D=3.01. In the separation space, when measured at the same distance above
the initial liquid level, it is found that there is little difference in
dimensionless static pressure and standard deviation of the whole section, that
is, dimensionless static pressure and standard deviation in the center and the
wall keeps the same change because of the existence of liquid level.

Fig.6
The distance of the liquid level in different cross section gas velocity.

Fig.7 The dimensionless mean pressure and standard deviation in free space

Fig.8 The dimensionless mean pressure and standard deviation in separation space

Fig.9 The dimensionless mean pressure and standard deviation at the same distance above the initial liquid level

5. The separation efficiency
increased from 82% to 93% with the increase of liquid concentration at 30 g/m³-60
g/m³ while the cross-section gas velocity remained unchanged. And the
separation efficiency increased with the increase of cross-section velocity. If
the water evaporation of import and export is not considered, the separation
efficiency deviation is 5%-7%. It is also found that the mass flow rate of the
separated water in the free space is approximately the same, that is, the
separated water in the free space may be independent of the concentration of
the liquid.

Fig.10
Relationship between separation efficiency and liquid concentration

Fig.11
Relationship between separation efficiency and cross-section gas velocity

According
to the existing analysis, it has great potential structure to improve
separation efficiency of the gas liquid cyclone separator.