| The distillation process of trichlorosilane mixed system is the key unit operation in solar-grade polysilicon production, it affects the final polysilicon product quality. After cryogenic treatment, the reaction liquid of synthesis section in polysilicon production is sent into the new type three-towers distillation system, the three towers are defined as following, the pre-separation tower, the trichlorosilane tower and the silicon tetrachloride tower. BCl3, SiH2Cl2, HC1 and H2 are separated in the pre-separation tower, SiHCl3, SiCl4 and other high-boiling residues are withdrawn as the bottom products, then feed into the trichlorosilane tower. The mass fraction of SiHCl3 is 99.93%at the top of the trichlorosilane tower, SiCl4 and other high-boiling residues are withdrawn at the bottom, then feed into the silicon tetrachloride tower. POCl3, ZnCl2 and CuCl2 are separated in the silicon tetrachloride tower, product SiCl4 that mixes of thimbleful SiHCl3 are withdrawn at the top, then return to the storage tank. The high-boiling residues are withdrawn at the bottom, finally, as the stripping section side-draw of the silicon tetrachloride tower, SiCl4 mass fraction is 99.85%.This article project mainly aims at simulating and optimizing of the new type three-towers distillation process. By using the chemical process simulation software-Aspen Plus, to analyse the best operational conditions and expenditure of energy for those three rectification columns, and propose two different energy saving reformations according to the characteristics of rectifying columns’energy consumptions.Firstly, the article analyses the vapor-liquid phase equilibrium data of mixed system. Gain a variety of SiHCl3-SiCl4 thermodynamics data through literature review, and select thermodynamic model-PENG ROB method to conduct the regression analysis of the data and the fitting of mathematics equation, then obtain the two-element interaction parameters, aij, by and cij.Secondly, using the sensitivity analysis function and design spec module to confirm the optimum operating conditions of the rectifying columns. After optimizing, the pre-separation tower’s optimum operating conditions are as follows, the operating pressure is 0.150 MPag, the number of the theoretical plate is 20, and the feed stage is the sixth one. The reflux ratio is 27.0, the distillate flow rate is 435 kg/h, the mass fraction of BC13 impurity substance is 1.92×10-8, less than 20 ppb, the results can fulfill the technological process requirements. The trichlorosilane tower’s optimum operating conditions are as follows, the operating pressure is 0.150 MPag, the number of the theoretical plate is 25, and the feed stage is the eleventh one. The reflux ratio is 3.0, the distillate flow rate is 4785 kg/h. The silicon tetrachloride tower’s optimum operating conditions are as follows, the operating pressure is 0.250 MPag, and the number of the theoretical plate is 18. The product SiCl4 is side drawn among the 12-14th stages, and the side-draw flow rate is 894 kg/h. The mass fraction of POC13 impurity substance at the top is 2.45×10-6, and the mass fraction of PCl3 impurity substance is 6.13×10-6, the proper feed temperature is 25～30℃. On the basis of simulation and optimization, the new type three-towers distillation results can satisfy the distillation technology demand.At the last part of the article, by comparing the three-towers’ series-connection distillation process with conventional two-towers distillation and three-towers’parallel-connection distillation process, to show the high efficiency and low heat consumption of the series-connection process. As the light component which is also impurity substance cannot be removed completely, and the mass fraction of BC13 is beyond the process standard, so we leave the two-towers distillation technology out of consideration. The three-towers’parallel-connection distillation can meet the rectification task requirement, but the total quantity of reflux flow exceed the required number, and the total heat duty of the column reboilers is 38.45% much more than the three-towers’series-connection distillation process. According to the disadvantages of the above two processes, and analyse the heat duty and the temperature at the top and the bottom of the three-towers’series-connection distillation process, the more effective distillation process is proposed. Make the vapor out of the top of trichlorosilane tower to be the heat source of pre-separation tower’reboiler, then complete the double-effect distillation process simulation, the results show that it can efficiently use the low grade heat energy and reduce the cost of cooling equipment.By carrying out feasibility analysis and performance evaluation of heat pump distillation for three-towers series-connection distillation process, design an opening A-type heat pump distillation to modify the trichlorosilane tower, it can save energy greatly by using its own energy. Calculate to determine the proper outlet pressure and compression ratio of the compressor, compare the original process with the improved process, the results show that, it saves 2.0526 million of utilities cost and 81.41% energy every year, this transformation has considerable benefit both in economic returns and environment. |
