(5b) Numerical Study of Hygroscopic Particle Removal Using a Cyclone Separator with Saturated Water Vapor

Introduction: Cyclone separator is
one of the most widely applied equipments for separating particles from the
air. However, it is difficult to separate particles with diameters smaller than
2.5 micron using the cyclone separator. In light of the particle-vapor
interaction, it is possible to increase the particle diameter via forced
condensation, especially when the particle contains soluble component. This
study has examined the possibility of using a cyclone separator to collect the
soluble particle, i.e., sodium chloride (NaCl) as test example, numerically.
Specifically, a steam of saturated water vapor with elevated pressure (0.7 MPa)
is introduced vertical to the air-particle inlet to increase the particle size
by hygroscopic growth, i.e., forming a NaCl-water droplet due to absorbing
water vapor. Comparisons of trajectories between the particles with and without
hygroscopic growth are presented.

Methods: A cyclone separator
model and its mesh, i.e., tetrahedral mesh with prism layers, were created (see
Fig 1.). The velocities of the air and the water vapor were 10 m/s and 1 m/s,
respectively. The NaCl particle density was 2165 kg/m³. The
Euler-Lagrange method was applied to predict the airflow and particle
transport. If The NaCl-water droplet collided with the cyclone surface,
deposition was assumed to occur when considering the particle-vapor interaction.
For comparison, the particle separation without hygroscopic growth was also
simulated to examine the separation efficiency. Simulations were carried out by
ANSYS Fluent with user-defined functions based on validated particle-vapor
interaction model (J. Aerosol Sci. 2017, 105: 108-127).

Results
and Discussion:
The trajectories of the 2.5 micron NaCl particles are shown in Fig. 2 when
including the effect of particle-vapor interaction. The particle / droplet size
increases rapidly when traveling through the air with high relative humidity. Thus,
these large droplets would deposit on the cyclone surface due to inertial
impaction, which leads to a high separation efficiency. The solution could be
collected at the bottom of the cyclone separator with extra water trickled from
the top of the cyclone, if the deposited droplets and condensed water cannot
form a liquid film on the cyclone surface. Fig. 3 illustrates the trajectories
of the 2.5 micron NaCl particles without hygroscopic growth.

Conclusions:
The
numerical study validated the feasibility of separating small soluble particle
(<2.5 micron) using cyclone separator by introducing the saturated water
vapor with elevated pressure. This method could be useful for the design of the
fine particle removal device with simple structure and easy-maintenance.

Acknowledgements: The authors
gratefully acknowledge the financial support of the National Natural Science
Foundation of China (grant No. 51606041), National Natural Science Foundation
of Jiangsu Prov. (grant No. BK20160688). The Fundamental Research Funds for the
Central Universities are also acknowledged.