1. Introduction
Unit operations, such as absorption, rectification, and catalytic
oxidation, occur in columns, and columns are widely used in the chemical
industry and refining and pharmaceutical production processes. The
columns can be divided into packed and tray columns based on their
internal structure 1,2. A packed column is a type of
differential contact equipment used for mass transfer. The internal
components are fillers, which have the advantages of a low-pressure
drop, high separation efficiency, simple structure, etc. The cost of the
filler is high, and the plugging resistance of the packing is poor3-5. For graded contact equipment, the tray column has
a large operating elasticity, low cost, and extensive applicability and
can be fed on the side of the column. The core component of the tray
column is the tray. The current mainstream trays are the sieve tray6, bubble cap tray 7, and valve tray8,9, which have a large processing capacity, excellent
performance, and mature technology. Thus, the development of novel
tridimensional trays has increased in recent years. This type of tray
has a high mass transfer efficiency and large flux and can use the inner
space for mass transfer. Representatives of this type of tray are the
new vertical screen plate 10, combined trapezoid spray
tray 11, and flow-guided jet packing tray12.
The contact method for the gas-liquid phase of the above-mentioned trays
is mainly spraying contact, and a higher separation efficiency is
required in flue gas desulfurization, dust removal, and absorption13. Among the numerous forms of gas-liquid contact,
gas-liquid rotational flow is widely used in separation, mixing, and
other operations 14,15. Numerous scholars have
conducted in-depth research on the rotatinal flow. Li et al.16 studied gas-liquid flow in a vertical pipe
containing a swirler with four helical vanes and classified three flow
patterns by observation: rotational gas-liquid flow, rotational
intermittent flow, and rotational annular flow. They also built a
self-organizing neural network to identify the rotational flow regimes.
In the double-alkali desulfurization process, Bao et al.17 adopted a heterogeneous condensation technology
using a rotating-steam tray to remove particles. The steam addition
method improved the removal efficiency of the particles. K. H. Javed et
al. 18 investigated rotational gas flow in a spray
tower. The experimental investigations were based on the
air–NH3/H2O system. The rotational flow
in the gas phase enhanced the mass transfer coefficient up to 20%,
compared to that in axial flows. However, although rotational flow for
the gas-liquid phase increases the separation efficiency, a large amount
of kinetic energy is lost in the process of fluid swirl, resulting in an
increase in the pressure drop and corresponding energy consumption.
In addition to rotational flow, perforated flow is a common form of
gas-liquid flow. For the sieve tray column, the gas-liquid phase
perforates the sieve holes in a countercurrent method, which causes
bubbling of the liquid layer on the sieve tray, and then produces
constantly updated foam flow. Mass transfer occurs in the foam19,20. The perforated flow makes the gas-liquid
contact more uniform, reduces pressure drop, and reduces energy
consumption. Numerous researchers have studied the flow pattern
characteristics of perforated flow. The gas-liquid concurrent downward
flow through an orifice plate was experimentally investigated by Min et
al. 21. The trickling, continuous, semi-dispersed, and
perfect-dispersed flows were defined by observation. The transition
mechanism of the flow patterns was studied by the differential pressure
pulsation method. A model for the film thickness around the orifice rim
was also proposed. Maidana et al. 22 studied the
air-water slug flow under the disturbance by an orifice plate in a
horizontal tube. The results showed that the orifice disturbances have a
significant influence on the void fraction, bubble nose velocity, and
frequency of passage. For the orifice plate, Rahimi et al.6 developed a 3-D two-phase CFD model to study sieve
tray efficiency, hydraulics, and mass transfer. Two types of sieve trays
with different diameters were investigated using a simulation. The sieve
plate with smaller sieve holes had a higher mass transfer efficiency
when the flow pattern was close to the plug flow.
In addition, the operation mode of most trays is gas-liquid
countercurrent flow, which has a higher mass transfer efficiency.
However, when the gas volume reaches the upper limit of the tray load,
flooding will occur, greatly reducing the mass transfer efficiency23. Concurrent flow can effectively avoid flooding and
has a lower pressure drop than that of countercurrent flow. Therefore,
the flux of the gas-liquid phase is higher, increasing the processing
capacity and reducing the volume of the column equipment. For mass
transfer, researchers have shown that, in the chemical absorption
process or a low phase equilibrium constant, the concurrent flow has the
same mass transferability as that of countercurrent flow24. Therefore, for chemical absorption, exhaust gas
treatment, and plant exhaust desulfurization processes, the concurrent
flow has the advantages of low energy consumption, high efficiency, and
good economy.
In conclusion, there is a high spatial utilization rate for the
tridimensional tray, a good separation effect of the rotational flow,
and a low-pressure drop and high mass transfer of the sieve tray. Tang
et al. proposed a novel tridimensional tray, referred to as the
tridimensional rotational flow sieve tray (TRST) 25.
In Fig. 1, the TRST consists of several blades with a specific twist
angle and internal and external support rings with even sieve holes on
the blades. During operation, the internal flow pattern of the TRST can
be divided into two flow modes: rotational flow and perforated flow.
Rotational flow can enhance the turbulence intensity and mass transfer
performance of the gas-liquid phase. The perforated flow enhances the
fluid mixing between the adjacent rotational flow channels, shears and
breaks the large bubbles or air masses, and further increases the
contact area of the gas-liquid phase, enhancing mass transfer again. The
TRST has no downcomer. It is a cross-flow tray, with a larger effective
flow area. If used with the gas-liquid concurrent flow, the flux for the
gas-liquid phase can be further enhanced, while the low-pressure drop is
guaranteed, providing potential application in the field of flue gas
desulfurization and dust removal 26.
Recently, a systematic study on the hydrodynamic properties of the TRST
was performed by Tang et al. The experiment investigated the pressure
drop and flooding of the tray under countercurrent and concurrent flow,
respectively. The pressure drop was mainly affected by the flux for the
gas-liquid phase, structures of the TRST, and installation quantity and
mode. Compared to countercurrent flow, no flooding occurred during
concurrent flow, which has a lower pressure drop and a larger operating
range. The gas and liquid volume range increased by at least 80% and
60%, respectively 27. In addition, Tang investigated
the internal gas flow field of the TRST using CFD technology and the
distribution of axial, radial, and circumferential velocities in the
TRST. The flow process of the gas-phase through the TRST was divided
into the stages of initial and full development by distinguishing the
direction of the gas-phase velocity. For different structures of the
TRST, the rotational flow transition point was approximately 2/5 of the
tray 28.
Although a preliminary understanding of the hydrodynamic properties of
the TRST has been obtained, the key issues, such as the flow pattern,
interaction intensity, and proportional distribution of the rotational
and perforated flow for the gas-liquid phase, are not clearly
understood. It is difficult to observe the internal flow field of the
entire tray because the twisted blades block each other. Therefore, the
research object is simplified, and the blade unit is extracted (see Fig.
1). The methods of experimental observation, photography, and
differential pressure pulsation are combined to define and discriminate
the flow patterns in the unit, and the gas-liquid phase interaction of
each flow pattern is analyzed. A new experimental method is designed to
measure the proportion distribution ratio of the gas-liquid phase
rotational and perforated flow. The mechanism of rotational and
perforated flow is analyzed, and a prediction model for the rotational
flow ratio is proposed. The results of this study provide a theoretical
foundation for the optimization of the structures of the TRST and mass
transfer investigations for further research.