| Titanium dioxide is a kind of inorganic pigment with excellent performance, which is widely used in many industrial fields and daily life. The chloride process has become the primary technology of domestic and international titanium industry. The gas phase oxidation of TiCl4 in the oxidation reactor is the core technology in the production of titanium dioxide. The fluid dynamic behavior of feeding gas directly affects the mixing condition of O2 and TiCl4 in the oxidation reactor, which have a significant influence on the morphology, quality and size distribution of the titanium dioxide product. The main chemical reaction for the production of titanium dioxide is a strong exothermic reaction. The badly mixing condition will cause the local temperature is too high, which results in the particle size of the product is not uniform, and even causes the sintering and scarring of the product. Furthermore, if the operating conditions are not properly controlled, it will produce a backflow and form tiny particles, which would cause the blockage of the jet hole. Therefore, the quality of titanium dioxide products is determined by the mixing effect. It is meaningful to study the flow and mixing in the oxidation reactor, which will reveal the flow characteristics and the mixing mechanism of the fluid in the TiCl4 gas phase oxidation process. The obtained interesting results are hoped to provide a theoretical basis and guidance for the design and optimization of oxidation reactor.In this paper, the hydrodynamic model of the industrial oxidation reactor is established, which could illustrate the flow characteristics and pressure distribution of TiCl4 in the feed ring. The penetration depth and mixing effect of TiCl4 and O2 are studied in detail, and the optimization and improvement measures are put forward for the mixing process and the oxidation reactor structure. In addition, the variation regularity of flow, temperature and concentration distribution with time during the transient mixing process is analyzed. The main contents and results are given as follows:(1) A three-dimensional fluid dynamics model of gas flow in the oxidation reactor has been developed and the model is valid since the relative errors are within 5% by comparing the simulating results with open published experiment results. Based on the verified numerical model, the flow characteristics of TiCl4, pressure distribution and the influencing factors in annular channel are simulated and characterized. The results show that TiCl4 gas is an uneven flow in annular channel and the maximum velocity and the minimum static pressure appear near 60° section. The flow in annular channel is fully developed turbulent flow when the Reynolds number Re>9.96×104. The inlet Reynolds number and the holes density of the jet ring have little effect on the pressure distribution in annular channel. However, increasing flow flux or decreasing diameter of inlet pipe can make the pressure curve smoother, which is conducive to achieve equivalent distributary in the outlet of the reactor.(2) Based on the study on the flow characteristics of TiCl4 in annular channel, the rectifying rings of different structure are installed in annular channel of the distributor with the purpose to achieve the uniform distribution of TiCl4 gas in annular channel. The annular channel is divided into the outer channel and the inner channel by the rectifying ring. Rotational flow of TiCl4 still exists in the outer channel and the velocity and pressure distribution of the fluid are not uniform. However, there is no rotating flow in the inner channel, where the distribution uniformity of the pressure and outlet velocity is significantly improved. Compared to the reactor without the rectifying ring, the non-uniformity of the pressure and outlet velocity can be reduced by up to 91% and 69%respectively. Moreover, the turbulent kinetic energy loss will decrease since the circulating flow in annular channel can be effectively reduced by installing a rectifying ring, which will reduce the resistance loss in annular channel and the total energy loss.(3) A three-dimensional model for the mixing of TiCl4 and O2 in the oxidation reactor is established. By comparing the temperature difference predicted by the model with published experiment results, the relative error is less than 2%, which indicates that the model is reliable. Based on the verified numerical model, the influence of jet penetration depth on mixing condition is analyzed, and the reasonable range of the jet penetration depth is 0.56<h/R<0.72. The relationship between the jet penetration depth (h/R) and the momentum flux ratio (J), the number of jet holes (n) and the jet hole spacing (S) is studied. The result shows that the relationship between the jet penetration depth and J/n2 is well consistent with the logarithmic function:hlR= 0.7274+0.202281n(J/n2+0.04587). The predicted jet penetration depths agree well with the experimental results. The mixture can be evenly mixed within the distance of x/D= 2 when the range of J/n2 is about 0.38-0.75. The shortest distance needed for mixing uniformity is about x/D= 0.9 for J/n2= 0.42. In addition, it is need to be at least 16 holes on the chamber with a diameter of 0.2 m, which is favorable for the rapid mixing of TiCl4 and O2.(4) The three-dimensional transient simulation of the mixing process of TiCl4 and O2 in the reactor is carried out and the flow field, temperature and concentration distribution of TiCl4 and O2 in the reactor are analyzed. The results show that the reactants are mixed at 0.08 s, and the mixture reaches a steady state at 0.42 s. The whole mixing process of TiCl4 and O2 lasted about 0.36 s. The flow rate in the reactor is not uniform since the mixing of TiCl4 and O2 is more severe during the initial stage of mixing about 0.08-0.2 s. After about 0.42 s, the flow rate tends to be uniform at about 5.5 m-s"1 and the temperature difference is less than 90 K in the cross section of the x/D= 2. During the initial stage of mixing about 0.08-0.2 s, the non-uniformity is about 1.2 near the downstream area, which indicates that the uniformity of the mixture is very poor. After about 0.42 s, the non-uniformity of mixing is about 0.3 on the cross section of the x/D= 1/2 since the mixing condition cannot be uniform near the jet hole. However, the value of non-uniformity Mean be reduced to below 0.05 on the downstream cross section of the x/D= 3/2 and x/D= 2, which can achieve the degree of mixing uniformity.(5) In order to achieve the best mixing effect of the reactants, the optimization of distributor, reactor and control for jet mixing of TiCl4 and O2 can be carried out by CFD simulation. The result shows that the mixing effect can be improved by the rectifying ring with double baffles. The number of jet holes equal to or greater than 16 must be used with ranges of J/n2 at 0.38-0.75. In order to obtain rapid and uniform mixing of TiCl4 and O2, the reasonable penetration depth should be controlled at 0.56<h/R<0.12 and the reasonable spacing between adjacent jets is 2.45<S<7.85. |
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