Control of Distillation Column （精馏塔控制） Dai Lian-Kui Shen Guo-jiang Institute of Industrial Control, Zhejiang University

Distillation Column

Problem Discussion CVs ( 被控变量 ): P, L D, L B, x D, x B MVs ( 操作变量 ): D, B, L, Q H, Q C, D G DVs : F, x F, T F To design your multi-loop control system to satisfy the above requirements and reduce interaction

Contents Distillation Principle & Control Problems Steady Characteristics Material Balance ( 物料平衡 ) Control Material Balance ( 物料平衡 ) Control Column Pressure Control Product Purity Control (1) Distillate Purity ControlDistillate Purity Control (2) Bottom Purity ControlBottom Purity Control (3) Both Distillate & Bottom Purity ControlBoth Distillate & Bottom Purity Control Other Control Schemes

A Typical Distillation Column with Two Products Process Description, Balance Conditions, Control Objectives & CVs Selection ?

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

Selection of CVs & MVs MVs Selection ?

Control Problems of Distillation Column

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-x D, x B ?

Separation factor for a Binary Mixture Separation factor S ( 分离度 ) ： Separation curve : or Relation of S to 1-x D, x B ?

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

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

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 (1)If L/F is constant, V/F↑→D/F↑& D/L↑; (2)If D/F is constant, V/F↑→ D/L↓; (3)If V/F is constant, D/F↑→ L/F↓→ L/D ↓.

Level Control Problem (or Material Balance Control) CVs: L D, L B MVs: L or D, Q V or B DVs : F, x F CVs & MVs Pairing ?

Level Control Scheme 2.1 Advantages & disadvantages ?

Level Control Scheme 2.2 Advantages & disadvantages ?

Level Control Scheme 2.3 Advantages & disadvantages ?

Pressure Control Problem CV: P MVs : Q C, D G, V, L DVs : F, x F MV Selection & Control Schemes ?

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

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.

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

Distillate Purity Control Problem CVs: L D, T R, L B MVs: L, D, Q V or B DVs : F, x F CVs & MVs Pairing ?

Distillate Product Purity Control Scheme 3.1A Advantages & disadvantages: (1)Fast & strong control effect of L to T R, (2)Negative influence of reflux ( 回流量 ) fast change to column operation, (3)Difficult to control LT22 if D<< L.

Distillate Product Purity Control Scheme 3.1B Advantages & disadvantages: (1)Positive influence of reflux smooth change ， (2)Easy to control LT22 if D<< L, (3)Slow control effect of D to T R, (4)Strong coupling between TC32 & LC22.

Distillate Product Purity Control Scheme 3.1C Advantages & disadvantages: (1)Week coupling between TC32 & LC22, and smooth reflux change, (2)Easy to control LT22 for any D/L, (3)Fast control effect of D to T R.

Bottom Purity Control Problem CVs: L D, T S, L B MVs: L, D, Q V or B DVs : F, x F CVs & MVs Pairing ?

Bottom Product Purity Control Scheme 3.2A Advantages & disadvantages: (1)Fast control effect of Q V to T S, (2)Need enough reflux resulting in large energy consumption.

Bottom Product Purity Control Scheme 3.2B Advantages & disadvantages: (1)Easy to control L D if B<<V, (2)Slow control effect of B to T S, (3)Strong coupling between loops, (4)Need enough reflux resulting in large energy consumption.

Bottom Product Purity Control Scheme 3.2C Advantages & disadvantages: (1)Fast control effect of Q V to T S, (2)Automatic balance between L & V, to reduce energy consumption, (3)Strong coupling between control loops TC31 & LC22.

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

Both Product Purity Control Problem CVs: L D, L B, T R, T S MVs: L, D, Q V, B DVs : F, x F CVs & MVs Pairing ?

Controlled Process #1 Analyze the dynamic process to the change of u 1, u 2 as well as the steady-state gain matrix

Controlled Process #2 Analyze the steady- state gain matrix to the change of u 1, u 2.

Both Product Purity Control Scheme 3.3A Coupling Analysis between TC31 & TC32 ?

Both Product Purity Control Scheme 3.3B Coupling Analysis for TC31 、 TC32 & LC21 ?

Both Product Purity Control Scheme 3.3C Coupling Analysis for TC31 、 TC32 & LC22 ?

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

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

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

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

Dynamic Simulation for a Column with Binary Mixture

Influence of Reflux Flow on System

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

Control Scheme Discussion If the flow of bottom product is determined to meet the need of the following process, please update the control scheme

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.