(494b) The Study on Mechanism of Flow Distribution and Its Optimization in a Spray Column

The study on mechanism
of flow distribution and its optimization in a spray column

Spray
column is widely used as an absorption apparatus in wet flue gas
desulfurization (WFGD) technology due to its efficiency and reliability[1].
Because of the stirred slurry tank in the bottom of the column, flue gas enters
the column from the side typically in a radial direction. As a result, the gas
velocity is unevenly distributed across the cross section since the turning and
expanding gas flow. With increasing attention of the environment problem, WFGD
spray column has the trend of very large-scale, extremely high efficiency and
low resistance. Especially a spray column with large diameter will deteriorate
the maldistribution of the velocity[2],
which will seriously affect the removal rate. For engineering practice, it is
necessary to research flow distribution in a spray column. In spray column
without internals, the only factor to adjust gas distribution is falling
droplets, while the mechanism of the adjustment of droplets to gas distribution
is not studied in the literatures. In spray column with rod bank internal, gas
flows around the rod bank under the countercurrent condition between droplets
and gas, and less attention was paid to this flow field. In this paper, the theoretical
analysis and numerical simulation for both mechanism
of flow distribution is combined. Firstly, the self-adjustment effect with
single spraying layer is proposed. Then explorations are conducted for a spray
column with a rod bank internal and a spray column with 4 spraying layers, and
an optimization scheme for flow distribution and mass transfer is also
obtained.

Different
from drag distributed mechanism[3] for traditional
internal, the adjustment of droplets to gas is derived from the momentum
transfer between two phases. During the momentum transfer, the motion of droplets tend to optimize the flow distribution,
so we call it droplet self-adjustment effect. This effect equalizes the gas
velocity across whole column, however, it only works
under the countercurrent condition.

In
the spray column with single layer, an internal is necessary to assist the flow
equalization if droplets are too small to make the droplet self-adjustment
effect work. The adjustment effect of a rod bank internal to gas velocity is
based on drag distributed mechanism which has no precondition, but it works only
within a limited range near the rod bank. When these two effects work together,
a rod bank internal can firstly mitigate
the gas maldistribution and reduce the potential of
the concurrent condition in the lower part of the column, then
the gas velocity profile will be affected by the droplet self-adjustment effect
across the whole column. A synergistic effect exists between these two effects
which is shown in Fig.1 and Tab.1.

Fig.1 Distributions of gas velocity in a spray column
(plane y=0)

Table 1 nonuniform
coefficient in the upper part of a spray column

The
spray column usually has 4 spraying layers. The flow distribution in the column
with 4 layers becomes better from bottom to top, hence the distribution problem
across whole column turns into the problem in the lowest layer. Reactive
absorption is the key step for spray column, so a dilemma appears: droplets in
small diameter are beneficial for larger interfacial area, while droplets in
large diameter are beneficial for droplet self-adjustment effect. Since spray
column with multilayer can regulate operating conditions for each layer, a
scheme to optimize the flow distribution and mass transfer is proposed. A small
amount of spraying liquid with droplets in large diameter in lowest layer can
be used to achieve the flow distribution requirement, while the rest of
spraying liquid with small droplets in the upper layers can optimize the mass
transfer. Through numerical simulation about the influence of droplet diameter
and spraying rate for flow distribution, an optimal diameter and spraying rate
is obtained at d=3mm in the lowest layer, and d=2mm in the rest layers.
Compared with the spray column with uniform diameter d=2.5mm (under the given
L/G, it is the smallest droplet diameter for uniform distribution), the optimization
scheme increases 15% in liquid hold-up and 52% in interfacial area.

1.             Neveux,
T. and Y. Le Moullec, Wet industrial flue
gas desulfurization unit: model development and validation on industrial data.
Industrial & Engineering Chemistry Research, 2011. 50(12): p. 7579-7592.

2.             Nygaard,
H.G., et al., Full-scale measurements of
SO 2 gas phase concentrations and slurry compositions in a wet flue gas
desulphurisation spray absorber. Fuel, 2004. 83(9): p. 1151-1164.

3.             Idelchik,
I.E., N.A. Decker, and M. Steinberg, Fluid
Dynamics of Industrial Equipment: Flow Distribution Design Methods.
Hemisphere Publishing Corporation, USA, 1991.