The Simulation And Optimization Of Polysilicon Production

At present, the production of polysilicon is mainly used improved Siemens method in the world. But the polysilicon in our country was still produced by traditional siemens method several years ago. Recently, the combination of our own improved technology and that introduced from Russian has been adopted. At present there are two major problems in polysilicon manufacture at home and abroad: first, the actual yield is relatively low; second, there is too much by-product. Therefore, a technical breakthrough about how to increase polysilicon actual yield and lessen byproduct is urgent for our country.The major aims of this paper are SiHCl₃ and SiCl₄ hydrogenation reactions and tail gas separation during production of polycrystalline silicon. For the hydrogenation reactions of SiHCl₃ and SiCl₄, first step is to understand the thermodynamics characteristics of the reacting process, and then, the optimal design of reacting technology is carried out by minimization of Gibbs free energy under different temperature, pressure and ratio of inputs. For the process of tail gas separation, the optimal design is carried out using segmented design method, and then, the exhaust gas separation sequence, scheme and operating conditions are decided. The most suitable operating conditions and process of tail gas separation were obtained to minimumize the energy consumption.1. The thermodynamics analysis of SiHCl₃ hydrogenation reaction systemsThe conventional process of polysilicon with SiHCl₃ is carried out at 1373K,1atm and the ratio of H2/SiHCl₃ 10/1. In this process, however, the actual yield is low and the amout of by-product is relatively high. A new product technics has been proposed to increase the actual yield and reduce the amount of by-product. The major innovations of the new process are as follows:(1) Conventional SiHCl₃ hydrogenation reactionThe thermodynamics analysis of conventional SiHCl₃ hydrogenation reaction was carried out using minimization of Gibbs free energy. According to the results, high-temperature and high ratio of H₂/SiHCl₃ are in favor of the generation of polycrystalline silicon. If the temperature of reaction and ratio of H₂/SiHCl₃ are increased appropriately, the equilibrium yield of polycrystalline silicon may be up to 98% or even higher. Meanwhile, the equilibrium yield of SiCl₄ and SiH₂Cl2 which are undesired by-product will drop to less than 0.5%.Furthermore, low pressure is in favor of the production of polycrystalline silicon. When pressure is 10.13KPa , the equilibrium yield of polycrystalline silicon could be up to almost 100% and the equilibrium yield of by-products are minimum. Therefore, the SiHCl₃ hydrogenation reaction could be operated at low pressure.(2) SiHCl₃ hydrogenation reaction without producing SiCl₄There is a mass of SiCl₄ generated in traditional SiHCl₃ hydrogenation reaction, which could cause serious environmental contamination. Through simulation, it's found that generation of SiCl₄ could be depressed by adding certain SiCl₄ in feed. Therefore, the amount of SiCl₄ remains unchanging before and after the reaction, and the thermal effect of reaction system has been improved. The SiHCl₃ hydrogenation reaction system without producing SiCl₄ could also be operated at high temperature and low pressure.2. The thermodynamics analysis of SiCl₄ hydrogenation reaction systems(1) SiCl₄ conversion systemBased on the principles of hydrogenation reaction, two SiCl₄ hydrogenation methods have been induced: direct hydrogenation and hydrogenation using silicon as additive. Thermodynamics analysis have been carried out for the two methods separately. The conversion rate of SiCl₄ and the equilibrium yield of SiHCl₃ is increasing as the temperature is increasing in the direct hydrogenation system. In the hydrogenation using silicon as additive, however, the conversion rate of SiCl₄ and the equilibrium yield of SiHCl₃ is decreasing with increasing temperature, decreasing pressure and H₂/SiCl₄ ratio. Therefore, the operation conditions should be choosed cautiously.(2) SiCl₄ hydrogenation reaction to produce polysilicon at ultrahigh temperature Traditional SiCl4 hydrogenation reaction to produce polysilicon is also operated at 1473K and normal pressure.Through the simulation, it was found that the conversion of SiCl4 was increased, the thermal effect of reaction system has been improved and the equilibrium yield of polysilicon may achieve almost 100% under the superior high temperature, low pressure and high H₂/SiCl4 ratio. Based on these results, a new technology to produce liquid polysilicon with SiCl4 method is proposed at superior high temperature and low pressure.3. The optimal design of the tail gas separation system in polysilicon productionIn this paper, the tail gas separation technology was improved according to the shortcomings of harsh operation conditions. Ascertain the tail gas separation sequence and feasible scheme by using segmented design method to fit distillation and absorption together. The results indicate that the whole process could be reached a good separation effects under comparative mild operation conditions.