Study On HCL Recovery And Chlorosilane Double Effect Distillation Processes In Polysilicon Production

Polysilicon is basic raw material in information technology and new energyresource industry. At present, it is mainly produced by means of modified Siemensprocess. HCl absorption-desorption and chlorosilane distillation process are twoimportant processes of this technique. However, both of the two processes require alot of energy and often produce low purity products.Steady-state simulation process for HCl absorption-desorption system isestablished using the software Aspen Plus. First, by investigating various factorsaffecting absorption effect the optimum operational parameters for absorption columnare obtained, in which column pressure is1.45MPa, theoretical tray number is13,liquid-gas ratio is1.0and absorbent temperature is-40oC. Second, the optimaltheoretical stage number (13) is determined on the basis of minimum annual cost andboth reboiler and condenser duty reach the minimum when stage4is chosen to befeed stage. Moreover, by analyzing the influence of feed temperature of desorptioncolumn on reboiler duty, it can be concluded that hot feed process has remarkableenergy saving effect. Then, an improvement process is proposed. Reboiler duty andoperating cost for desorption tower reduced by40%and25%respectively than themfor conventional desorption column. Finally, economic evaluation result demonstratesan important role of rich liquid outlet temperature of rich-lean liquid heat exchangerin determining capital cost and operating cost for HCl absorption-desorption systemand gave the optimum temperature (25oC).Control performance of absorption-desorption improvement process is studiedsystematically by employing Aspen Dynamics flowsheet simulator. We found thesensitive stage is stage2by means of slope and sensitivity criterion. Two alternativecontrol configurations are studied using relative gain array. The result obtained showscontrol of reflux drum liquid level by manipulating distillation rate and temperatureon tray2by handling reboiler heat input is a reasonable control method. Results ofclosed-loop dynamic response show that this control configuration has good controlfeature on disturbance from feed flow rate and composition.Based on distillation techniques a novel double effect distillation processconsisting of three columns is proposed in this paper. Optimal operating parameters ofeach column is obtained using Aspen Plus simulator. Impurities like HCl, BCl3as wellas PCl3can be removed efficiently under optimal process condition. Product purity of SiHCl3and SiCl4is up to99.9999%and99.9%respectively. Recovery ratio of SiHCl3and SiCl4reaches90.6%and99.9%respectively. In addition, low-pressure andhigh-pressure distillation columns can be thermally integrated completely throughenergy matching. Total condenser and reboiler duty of this new process cut down by28.2%and25.7%respectively compared with conventional distillation columnswithout heat integration. The results show a remarkable effect in energy saving.