Optimal design and control of silane off-gas recovery process with dividing wall column under periodic disturbances from parallel batch reactors

학위논문 (박사)-- 서울대학교 대학원 : 화학생물공학부, 2016. 8. 이종민.This thesis has presented a rigorous design and a plant-wide control structure scheme for the effective separation of off-gas from a polysilicon plant with periodic disturbances in the feed. Solar power generation is eco-friendly energy resource, polysilicon is used as a raw material to manufacturing solar cells. In order to obtain the high efficiency of solar cells, it requires ultra-high purity of polysilicon which essentially involves the advanced recovery processes. The target process studied in this thesis produces 10,000 MT polysilicon per year with 28 chemical vapor deposition reactors. The reaction kinetic models were constructed, and their parameters were identified using a genetic algorithm, and that were utilized to design the separation process which separates into hydrogen, hydrogen chloride, and silanes. Though proportional-integral controllers were installed to remove the periodic disturbances, they were not enough to meet the active constraints. A centralized supervisory layer based on model predictive control to minimize the fluctuation of the recovered concentration, was proposed. Proportional-integral controllers regulates the inventory levels, thereby operating the process stable, and a model based controller showed a good performance rejecting the periodic disturbances. Using a rigorous plant-wide dynamic model, four cases were studied with different control schemes with and without buffer tanks. Separated silane off-gas is consist of dichrloro silane, trichloro silane, and silane tetrachloride. Among them, dichloro silane and trichloro silane go into the reactors again. Dividing wall column was designed to achieve the lower energy consumption for the ternary mixture, and control structures were studied. From the novel approach, the energy consumption was decreased up to 37 % compared with classical sequential columns, and the settling time also reduced by proposed research results.Abstract i 1. Introduction 1 2. Overview of process and methodology 5 3. One particular reference point based modeling of off-gas recovery process 11 3.1 Introduction 11 3.2 Identification of CVD reactor kinetics 12 3.3 OP-RP process simulation 16 3.4 Optimization with OP-RP process 19 3.5 Conclusion 23 4. Dynamic modeling and control studies of silane off-gas recovery process 24 4.1 Introduction 24 4.2 Dynamic modeling of OP-RP process 25 4.3 Conventional control schemes for silane off-gas recovery 28 4.3.1 Case1. PID with feed-forward control 28 4.3.2 Case2. PID with feed-forward control + buffer tank 32 4.4 Proposed control scheme 34 4.4.1 Supervisory control based on MPC 34 4.4.2 Case3. MPC based supervisory control 38 4.4.3 Case4. MPC based supervisory control + buffer tank 42 4.5 Comparison of four control schemes 45 4.6 Case studies of the unexpected disturbances 46 4.6.1 Case5. Change of reactor operation intervals 46 4.6.2 Case6. Reactor shut-down case 47 4.6.3 Case7. Increasing hydrogen purity 49 4.7 Conclusion 49 5. Application of dividing wall column in silane off-gas recovery process 51 5.1 Introduction 51 5.2 Design and optimization of direct sequential configuration 54 5.3 Design and optimization of DWC 58 5.4 DWC control case studies 67 5.4.1 Selection of control variables 68 5.4.2 Three-point control structure 70 5.4.3 Four-point control structure-CASE1 72 5.4.4 Four-point control structure-CASE2 74 5.4.5 Four-point control structure-CASE3 76 5.4.6 Four-point control structure-CASE4 77 5.4.7 Six-point control structure 79 5.5 Conclusion 81 6. Concluding remarks 83 Bibliography 84Docto