1、Control of Distillation Column（精馏塔控制）Lei XieInstitute of Industrial Control,Zhejiang University,Hangzhou,P.R.China2012/05/23Problem DiscussionCVs(被控变量):P,LD,LB,xD,xBMVs(操作变量):D,B,L,QH,QC,DGDVs:F,xF,TFTo design your multi-loop control system to satisfy the above requirements and reduce interactionCon
2、tentsnDistillation Principle&Control ProblemsnColumn Pressure ControlnMaterial Balance(物料平衡)ControlnProduct Purity Control(1)Distillate Purity Control(2)Bottom Purity Control(3)Both Distillate&Bottom Purity ControlnOther Control SchemesA Typical Distillation Columnwith Two ProductsProcess Descriptio
3、n,Balance Conditions,Control Objectives&CVs Selection?Control ObjectivesnGuarantee operation safetynDecrease disturbancesnControl purity and composition of productsnIncrease the product yieldnReduce the operational energySelection of CVs&MVsMVs Selection?Control Problems of Distillation ColumnPressu
4、re Control ProblemCV:PMVs:QC,DG,V,L DVs:F,xFMV Selection&Control Schemes?Pressure Control SchemesFLDPCFLDPCScheme 1.1:MV is the unconden-sable vapors(不凝气)vent flowScheme 1.2:MV is the coolant flow to control column pressureApplication case based on effectiveness Level Control Problem(or Material Bal
5、ance Control)CVs:LD,LBMVs:L or D,QV or BDVs:F,xFCVs&MVs Pairing?Level Control Scheme 2.1Advantages&disadvantages?Level Control Scheme 2.2Advantages&disadvantages?Level Control Scheme 2.3Advantages&disadvantages?Measurement of Product PuritynDirect measurement approaches(1)On-line composition/quality
6、 analyzers(在线分析仪),(2)soft sensors/inferential measurement(软测量仪).nIndirect 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.M
7、aterial Balance(物料平衡)for a Column with Binary MixtureBDFOverall material balance:BDFBxDxFxBDFDBDBFxxxxFBxxxxFD,Balance for light component:Material balance line:BFDxFDxFDx11111Relation of D/F to 1-xD,xB?Separation factor for a Binary Mixture)1()1(11DBBDBBDDxxxxxxxxSSeparation factor S(分离度)：Separatio
8、n curve:)1(1SxSxxBBD)1(111SxxxBBDorRelation of S to 1-xD,xB?Variables to Influence the Separation Factor relative volatility(相对挥发度)For variable reflux ratio L/D（回流比）：nEFxLDS11 n:number of stages E:average stage efficiency Influence of L/D,xF to S?Operating Points for a binary-component columnOperati
9、ng condition:xF=0.52,n=16,E=0.816,0.2Analyze 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,thenDLVVR(1)If L/F is constant,V/FD/F&D/L;(2)If D/F is constant,V/F D/L;(3)If V/F is constant,D/F L/F
10、L/D.Distillation Product Purity Control Problems(C)(A)(B)Distillate Purity Control ProblemCVs:LD,TR,LBMVs:L,D,QV or BDVs:F,xFCVs&MVs Pairing?Distillate ProductPurity Control Scheme 3.1AAdvantages&disadvantages:(1)Fast&strong control effect of L to TR,(2)Negative influence of reflux(回流量)fast change t
11、o column operation,(3)Difficult to control LT22 if D L.Distillate ProductPurity Control Scheme 3.1BAdvantages&disadvantages:(1)Positive influence of reflux smooth change，(2)Easy to control LT22 if D L,(3)Slow control effect of D to TR,(4)Strong coupling between TC32&LC22.Distillate ProductPurity Con
12、trol Scheme 3.1CAdvantages&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 TR.Bottom Purity Control ProblemCVs:LD,TS,LBMVs:L,D,QV or BDVs:F,xFCVs&MVs Pairing?Bottom ProductPurity Control Scheme 3.2AAdvantage
13、s&disadvantages:(1)Fast control effect of QV to TS,(2)Need enough reflux resulting in large energy consumption.Bottom ProductPurity Control Scheme 3.2BAdvantages&disadvantages:(1)Easy to control LD if BV,(2)Slow control effect of B to TS,(3)Strong coupling between loops,(4)Need enough reflux resulti
14、ng in large energy consumption.Bottom ProductPurity Control Scheme 3.2CAdvantages&disadvantages:(1)Fast control effect of QV to TS,(2)Automatic balance between L&V,to reduce energy consumption,(3)Strong coupling between control loops TC31&LC22.Bottom ProductPurity Control Scheme 3.2DOverall material
15、 balance can not be satisfied(Why?).Ex.light component in feed increases Both Product Purity Control ProblemCVs:LD,LB,TR,TSMVs:L,D,QV,BDVs:F,xFCVs&MVs Pairing?Controlled Process#1 Analyze the dynamic process to the change of u1,u2 as well as the steady-state gain matrixControlled Process#2Analyze th
16、e steady-state gain matrix to the change of u1,u2.Both ProductPurity Control Scheme 3.3ACoupling Analysis between TC31&TC32?Both ProductPurity Control Scheme 3.3BCoupling Analysis for TC31、TC32&LC21?Both ProductPurity Control Scheme 3.3CCoupling Analysis for TC31、TC32&LC22?Scheme Discussion to Contr
17、ol Both Product PuritynNot easy to control both distillate&bottom product purity simultaneously,but sometime necessary to save energy or other requirements;nOnly one of product flow can be used to control product purity,the other product purity has to be controlled by V or L;nPossible to exist stron
18、g coupling among loops,especially between purity control loops;nTo 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 columnnFeed Enthalpy Control(进料热焓控制进料热焓控制)to re
19、duce the effect of feed enthalpy change on product composition control.nFeed Flow Feedforward Control(进料前馈控制进料前馈控制)to reduce the effect of feed flow change on product purity control.nMinimize Energy Consumed in Column Operation nAnalyzer Based Control for Product CompositionKey technology:On-line co
20、mposition analyzer Feed Flow-rate Feed-forward ControlHow to reduce the influence of other main disturbances?Application of online analyzerin product purity controlHow to introduce the purity information If the output of online analyzer is not continuous?Dynamic Simulation for a Column with Binary M
21、ixtureInfluence of Reflux Flow on System0501001502000.10.120.140.16D,kmol/min 0501001502000.9511.05LD,kmol 0501001502000.260.280.3L,kmol/min 0501001502000.920.93xD 0501001502000.160.180.20.22B,kmol/min 0501001502001LB,kmol 050100150200-1012V,kmol/min min05010015020000.10.2xB minOther Simulation Stud
22、y1.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 SummarynColumn Control ObjectivesnRegular Control SchemesColumn pressure control,feed enthalpy control,product purity control including four detailed objectives,etc.nApplication of APC SystemsOn-line analyzer control,multivariable predictive control,soft sensor/inferential measurement,etc.Integrated ExercisesnSee the attached DOC file
