(14c) Design of a Flow Feature with Three-Dimensional Circulating Air in a Spray Pyrolysis Furnace Applied to High-Purity MgO Production

Qaidam Basin which is located in Qinghai-Tibet
plateau of China, has proven reserves of 6.5 billion tons of magnesium
accounting for 96.78%of the national reserve[1].
Along with the extraction of potassium resources in salt lakes, quantities of
surplus magnesium caused resource waste and adversely affected the extraction
of potassium resources in salt lakes. Under the guidance of low carbon, energy
saving, comprehensive and recycling utilization, producing high purity magnesia
by means of direct spray pyrolysis from large quantities of accessible hydrated
magnesium chloride is one of the competitive processes to exploit magnesium
resource in salt lakes. On the industrialization process of this spray
pyrolysis method, intensive study should be carried out to prevent equipment
from corrosion caused by HCl and reduce energy consumption. This work focus on designing
an efficient flow features in the furnace where spray and hot air are
introduced in. CFD simulations, particle image velocimetry (PIV), and High-speed
photography technologies have been used to assist the design of a complex air
motion with three-dimensional stable vortexes in a spray pyrolysis furnace.
This innovation of flow features in the furnace is expected to enhance heat and
mass transfer process in both near-field and far-field spray patterns, thus to
reduce reaction time, improve the quality of the end products, decrease exhausting
temperature, reduce corrosion of equipment and cut down energy consumption.

Firstly, the near-field flow structure in
hollow cone pressure swirl sprays is investigated. Two types of pressure
nozzles produced by Spraying Systems Co. with different inlet diameters are
tested in steady air under a wide range of upstream pressure of feeding liquid.
The spray angles and liquid flux of water and saturated brine are compared. The
spray features are evaluated by high-speed schlieren photography technique and
the velocity distribution of droplets is investigated through particle image
velocimetry (PIV). The resulted droplets velocity distribution of the axial
cross-section of the spray cone reveals two kinds of feature velocities (Fig.
1), namely high velocity at the spray periphery and the high-speed stream at
the axis of the spray cone. The flow field with high centreline velocity is
also found by other researchers[2][3][4]
(all the works without any explanation for this effect). With the assistance of
advanced CFD simulation, a detailed analysis of interactions between particles and
air motion is carried out to establish the formation of the center high-speed
stream.

Upon the acquired flow features in the near-field
spray region, a far-field air flow is added to interact with the flow features
generated by spray nozzles. A cylinder cavity with ¦µ1.2
x 1.5m³ is built to represent the spray pyrolysis furnace. CFD
simulation is carried out to guide the design of air flow in the cylinder.
The standard k-e turbulence model
and the discrete phase model with particle coalescence and breakup are selected
for a comprehensive description of the flow characteristics of multiphase flow
in the cylinder furnace. Quantities of models with different air injection
combinations are simulated and optimized to develop an axisymmetric, center
intruded, rolling air flow feature. One stage of the air motions in the cavity
is showed in Fig. 2 and Fig. 3. A cylinder furnace with six air entrance
combined with center top spray is designed upon the simulation-based
optimization. Then this new type of flow feature in the cylinder is tested in
full-scale cold model experiments,
the flow features are acquired by means of schlieren photography and PIV/V3V
techniques and are used to compare with the simulated results for further
optimization. Based on the research of the flow with near-field and far-field
sprays in the cavity, a new type of spray pyrolysis furnace with advantages of strong
transfer process, enough long resistance time, minimized size and reduced wall deposition
of droplets has been designed and optimized. This work is a preliminary
research for an efficient spray pyrolysis furnace design under high
temperatures. The influent of
cycling motion of small droplets carried by the rolling air on the size-control
of particles, and other process including heat and mass transfer will be
further researched in the future.

Fig. 1 Velocity magnitudes from PIV measurement

Fig. 2 Path lines of air motion in the cavity

Fig. 3 Top view of the air motion

References