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Control of Distillation Column （精馏塔控制）

Control of Distillation Column （精馏塔控制）. Lei Xie Institute of Industrial Control, Zhejiang University, Hangzhou, P. R. China 2012/05/23. Contents. Distillation Principle & Control Problems Column Pressure Control Material Balance ( 物料平衡 ) Control Product Purity Control

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Control of Distillation Column （精馏塔控制）

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  1. Control of Distillation Column（精馏塔控制） Lei Xie Institute of Industrial Control, Zhejiang University, Hangzhou, P. R. China 2012/05/23

  2. Contents • Distillation Principle & Control Problems • Column Pressure Control • Material Balance (物料平衡) Control • Product Purity Control (1) Distillate Purity Control (2) Bottom Purity Control (3) Both Distillate & Bottom Purity Control • Other Control Schemes

  3. A Typical Distillation Columnwith Two Products Process Description, Balance Conditions, Control Objectives & CVs Selection ?

  4. Control Objectives • Guarantee operation safety • Decrease disturbances • Control purity and composition of products • Increase the product yield • Reduce the operational energy

  5. Selection of CVs & MVs MVs Selection ?

  6. Control Problems of Distillation Column

  7. Pressure Control Problem CV: P MVs : QC, DG , V, L DVs : F, xF MV Selection & Control Schemes ?

  8. Pressure Control Schemes Scheme 1.1: MV is the unconden-sable vapors (不凝气) vent flow Scheme 1.2: MV is the coolant flow to control column pressure Application case based on effectiveness

  9. Level Control Problem (or Material Balance Control) CVs: LD, LB MVs: L or D, QVor B DVs : F, xF CVs & MVs Pairing ?

  10. Level Control Scheme 2.1 Advantages & disadvantages ?

  11. Level Control Scheme 2.2 Advantages & disadvantages ?

  12. Level Control Scheme 2.3 Advantages & disadvantages ?

  13. Measurement of Product Purity • Direct measurement approaches (1) On-line composition/quality analyzers (在线分析仪), (2) soft sensors / inferential measurement (软测量仪). • Indirect measurement approaches (1) Sensitive tray temperature (灵敏板温度) or bubble point of the liquid, if the column pressure is constant; (2) Pressure compensated temperature; (3) Difference between tray temperature and top/bottom temperature.

  14. Material Balance (物料平衡) for a Column with Binary Mixture Overall material balance: Balance for light component: Material balance line: Relation of D/F to 1-xD, xB ?

  15. Separation factor for a Binary Mixture Separation factor S (分离度)： Separation curve : or Relation of S to 1-xD, xB ?

  16. Variables to Influence the Separation Factor For variable reflux ratio L/D（回流比）： E: average stage efficiency relative volatility (相对挥发度) Influence of L/D, xF to S ? n: number of stages

  17. Operating Points for a binary-component column Operating condition: xF = 0.52, n = 16, E = 0.816, Analyze the influence of MVs !

  18. Effects of V/F to D/F & L/D in Distillate & Bottom If the feed is in the form of liquid, and the material in the condenser keeps balance, then • If L/F is constant, V/F↑→D/F↑& D/L↑; • If D/F is constant, V/F↑→ D/L↓; • If V/F is constant, D/F↑→ L/F↓→ L/D ↓.

  19. Distillation Product Purity Control Problems (A) (B) (C)

  20. Distillate Purity Control Problem CVs: LD, TR, LB MVs: L, D, QVor B DVs : F, xF CVs & MVs Pairing ?

  21. Distillate ProductPurity Control Scheme 3.1A • Advantages & disadvantages: • Fast & strong control effect of L to TR, • Negative influence of reflux (回流量) fast change to column operation, • Difficult to control LT22 if D<< L.

  22. Distillate ProductPurity Control Scheme 3.1B • Advantages & disadvantages: • Positive influence of reflux smooth change， • Easy to control LT22 if D<< L, • Slow control effect of D to TR, • Strong coupling between TC32 & LC22.

  23. Distillate ProductPurity Control Scheme 3.1C • Advantages & disadvantages: • Week coupling between TC32 & LC22, and smooth reflux change, • Easy to control LT22 for any D/L, • Fast control effect of D to TR.

  24. Bottom Purity Control Problem CVs: LD, TS, LB MVs: L, D, QVor B DVs : F, xF CVs & MVs Pairing ?

  25. Bottom ProductPurity Control Scheme 3.2A • Advantages & disadvantages: • Fast control effect of QV to TS, • Need enough reflux resulting in large energy consumption.

  26. Bottom ProductPurity Control Scheme 3.2B • Advantages & disadvantages: • Easy to control LD if B<<V, • Slow control effect of B to TS, • Strong coupling between loops, • Need enough reflux resulting in large energy consumption.

  27. Bottom ProductPurity Control Scheme 3.2C • Advantages & disadvantages: • Fast control effect of QV to TS, • Automatic balance between L & V, to reduce energy consumption, • Strong coupling between control loops TC31 & LC22.

  28. Bottom ProductPurity Control Scheme 3.2D Overall material balance can not be satisfied (Why ?). Ex. light component in feed increases ……

  29. Both Product Purity Control Problem CVs: LD, LB ,TR, TS MVs: L, D, QV, B DVs : F, xF CVs & MVs Pairing ?

  30. Controlled Process #1 Analyze the dynamic process to the change of u1, u2 as well as the steady-state gain matrix

  31. Controlled Process #2 Analyze the steady-state gain matrix to the change of u1, u2.

  32. Both ProductPurity Control Scheme 3.3A Coupling Analysis between TC31 & TC32 ?

  33. Both ProductPurity Control Scheme 3.3B Coupling Analysis for TC31、TC32 & LC21 ?

  34. Both ProductPurity Control Scheme 3.3C Coupling Analysis for TC31、TC32 & LC22 ?

  35. Scheme Discussion to Control Both Product Purity • Not easy to control both distillate & bottom product purity simultaneously, but sometime necessary to save energy or other requirements; • Only one of product flow can be used to control product purity, the other product purity has to be controlled by V or L; • Possible to exist strong coupling among loops, especially between purity control loops; • To reduce the coupling, special control strategies can be used such as decoupling, predictive control, and other APC (Advanced Process Control) algorithms

  36. Other Control Systems for distillation column • Feed Enthalpy Control (进料热焓控制) to reduce the effect of feed enthalpy change on product composition control. • Feed Flow Feedforward Control (进料前馈控制) to reduce the effect of feed flow change on product purity control. • Minimize Energy Consumed in Column Operation • Analyzer Based Control for Product Composition Key technology: On-line composition analyzer

  37. Feed Flow-rate Feed-forward Control How to reduce the influence of other main disturbances ?

  38. Application of online analyzerin product purity control How to introduce the purity information If the output of online analyzer is not continuous ?

  39. Dynamic Simulation for a Column with Binary Mixture

  40. Influence of Reflux Flow on System

  41. Other Simulation Study 1. Level Control Scheme: D LD, B LB (1.1) Influence of V, L on CVs & D, B; (1.2) Distillate Purity Control: L xD, V const.; (1.3) Bottom Purity Control: V xB, L const.; (1.4) Both Product Control: L xD, V xB. 2. Level Control Scheme: L LD, B LB 3. Level Control Scheme: L+D LD, B LB

  42. Summary • Column Control Objectives • Regular Control Schemes Column pressure control, feed enthalpy control, product purity control including four detailed objectives , etc. • Application of APC Systems On-line analyzer control, multivariable predictive control, soft sensor/inferential measurement, etc.

  43. Integrated Exercises • See the attached DOC file

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