(440f) Distillation Process for the Purification of Trichlorosilane

Silicon is the most abundant semiconductor elements in the world. Silicon of high purity has been widely used in electronic industry for a long time. In recent years, silicon, particular high-purity polysilicon and organosilicon, has had an increasing importance as the growth of photovoltaic energy industry and progress in new materials field. The production of solar cells, which can convert solar energy into direct current energy, is the most critical step in the photovoltaic industry chain. Currently polycrystalline silicone solar cell is the most widely used one. Polysilicon is the key material of the solar energy industry and the semiconductor industry, thus making it as strategic materials for electronic information industry and photovoltaic industry. Modified Siemens Process(MSP) is now the main process to produce polysilicon, accounting for more than 75% of total output. The process includes chloration, purification, deoxidization, tail gas separation, hydrogenation, post-processing and so on. However, now MSP is still faced with some problems, such as high energy consumption, low single pass conversion, especially low distillation efficiency. In normal case, purification process during MSP purify raw material based on the distillation technology, therefore the distillation technology plays an important role in MSP, including synthesis condensation material distillation in synthesis procedure, first distillation and second distillation in refined procedure, hydrogenation condensation material distillation in hydrogenation procedure, tail gas distillation in tail gas recovery procedure. Trichlorosilane is an important raw material production of polysilicon. Trichlorosilane which has highly strict purity requirement must reach more than 99.999%, and the content of the boron, phosphorus elements must be less than 10 ppb, polymers less than 10 ppm, metal elements such as iron, arsenic, magnesium, aluminum, nickel, chromium, copper, zinc and lead less than 1 ppb, the total content of metal elements less than 10 ppb. This paper presents some novel distillation process for the purification of trichlorosilane. This distillation process is sort of high heat consumption chemical engineering units operation which has high recoverable heat, so the energy saving and recovery in distillation process is important for chemical engineering process that includes production of polysilicon. Traditional distillation technology on the MSP has less theoretical tray numbers and high reflux ratio, leading to higher energy consumption and lower yield. Therefore, it is necessary to optimize its distillation technology. Through the way of the chemical process intensification, it can reduce the theoretical tray numbers of the distillation column, minimize the equipments, reduce costs and energy consumption, and increase the purity of polysilicon. The appropriate operation parameters such as reflux ratio, overhead pressure, suitable components recovery and heat state of feed have been set up in this study by applying a new structure packing column technology, which considering optimized hydrodynamic performance, such as carrier point, flood point, liquid holdup, pressure drop, entrainment and so on. Finally, the suitable distillation tower can be chosen, the structure optimization of distillation column can reduce energy consumption, increase the separation efficiency. Boron, phosphorus, iron and other impurities especially organosilicon can be reduced significantly, improving the quality of polysilicon. In addition, for the problem of high energy consumption, this study changes the distillation flowsheet, so the different pressure thermally coupled distillation technology can be used. High purity of the trichlorosilane can be provided by two columns different pressure thermally coupled columns distillation technology, which uses two columns serial process, the distillation columns are used pressurized operation. In this way the energy consumption can be minimized while the high purity of the trichlorosilane is produced. Take the purification of trichlorosilane during the producing process of 2000 t/year polysilicon as example, feeds are as follows: the content of 1% of dichiorosilane, content of 98% of trichlorosilane, content of 1% Silicon tetrachloride, trace impurities such as Boron, phosphorus, iron, polymers and other impurities. The overhead pressure of low-pressure column operating pressure is 0.2 MPa, the top temperature of 50.7 °C, reflux feed ratio of 14, and theoretical tray number for the 180. The overhead pressure of high-pressure column operating pressure is 0.55 MPa, the top temperature of 88.5 °C, reflux feed ratio of 14, and theoretical tray number for the 180. Following the above process, the quality of high-purity trichlorosilane can achieves the solar-grade and electronic-grade. Because of the different pressure thermally coupled distillation technology, the total heat duty of low-pressure and high-pressure columns is 2528800 KCal/HR, while heat duty of the conventional distillation technology is 5059500 KCal/HR, energy saving up to 50.01%. Since the distillation process coupled with differential pressure method, the heat duty of low-pressure column and cooling duty of high-pressure are matched, so that the reboiler of low-pressure column and the condenser of high-pressure column are not necessary, saving investment on instruments and cutting energy consumption by 50%. The chemical process intensification reduces the production costs while improving production quality. This method can speed up the development of semiconductor industry, promote electronic information industry and new solar photovoltaic industry development, and might eventually make a breakthrough in the information industry and new energy industry.