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

Fig. 2

Fig. 3

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

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[1] Li Zengrong, Xu Hui, Pang Quanshi, Shi Xichang and Wang Hengzhang. Exploitation Technology Progress and Plan for Development of Magnesium in Salt Lake. Cyclic Economy. 2010;1:3-16.

[2] Madsen J. Computational and experimental study of sprays from the breakup of water sheets: Aalborg University; 2006.

[3] Santolaya J, Aisa L, Calvo E, Garc¨ªa I, Cerecedo L. Experimental study of near-field flow structure in hollow cone pressure swirl sprays. Journal of propulsion and power. 2007;23(2):382-9.

[4] Yang J, Chen A, Yang S, Huang K. Flow analysis of spray patterns of pressure-swirl micro atomizers. Proceedings of PSFVIP-4. 2003.