06 Introduction To Heat Exchanger Network Synthesis.ppt

6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,1,054402 Design and Analysis II,LECTURE 6:INTRODUCTION TO HEAT EXCHANGER NETWORK SYNTHESISDaniel R.LewinDepartment of Chemical EngineeringTechnion,Haifa,Israel,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,2,Schedule Introduction to HEN Synthesis,Unit 1.Introduction:Capital vs.EnergyWhat is an optimal HEN designA Simple Example(Class Exercise 1)Setting Energy TargetsUnit 2.The Pinch and MER DesignThe Heat Recovery PinchHEN RepresentationClass Exercise 2Unit 3.The Problem TableClass Exercises 3 and 4,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,3,Schedule Advanced HEN Synthesis,Unit 4.Loops and SplitsMinimum Number of Units by Loop BreakingClass Exercise 5Stream Split Designs Class Exercise 6Unit 5.Threshold ProblemsClass Exercise 7,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,4,Schedule Heat and Power Integration,Unit 6.Data ExtractionClass Exercise 8Unit 7.Heat Integration in DesignGrand Composite CurveHeat-integrated DistillationHeat EnginesHeat Pumps,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,5,Part One:Objectives,The first part of this three-part Unit on HEN synthesis serves as an introduction to the subject,and covers:The“pinch”The design of HEN to meet Maximum Energy Recovery(MER)targetsThe use of the Problem Table to systematically compute MER targets Instructional Objectives:Given data on hot and cold streams,you should be able to:Compute the pinch temperaturesCompute MER targetsDesign a simple HEN to meet the MER targets,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,6,A Short Bibliography.,Early pioneers:RuddWisconsin(1968)HohmannUSC(1971)Central figure:LinnhoffICI/UMIST(1978)Currently:President,Linnhoff-MarchRecommended texts:Seider,Seader and Lewin(1999):Process Design Principles,Wiley and Sons,NYLinnhoff et al.(1982):A User Guide on Process Integration for the Efficient Use of Energy,I.Chem.E.,LondonMost up-to-date review:Gundersen,T.and Naess,L.(1988):“The Synthesis of Cost Optimal Heat Exchanger Networks:An Industrial Review of the State of the Art”,Comp.Chem.Eng.,12(6),503-530,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,7,UNIT 1:Introduction-Capital vs.Energy,The design of Heat Exchanger Networks deals with the following problem:Given:NH hot streams,with given heat capacity flowrate,each having to be cooled from supply temperature THS to targets THT.NC cold streams,with given heat capacity flowrate,each having to be heated from supply temperature TCS to targets TCT.Design:An optimum network of heat exchangers,connecting between the hot and cold streams and between the streams and cold/hot utilities(furnace,hot-oil,steam,cooling water or refrigerant,depending on the required duty temperature).What is optimal?Implies a trade-off between CAPITAL COSTS(Cost of equipment)and ENERGY COSTS(Cost of utilities).,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,8,Example,Network for minimal energy cost?,Network for minimal equipment cost?,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,9,Numerical Example,Design B:(AREA)=13.3,Design A:(AREA)=20.4 A=Q/UTlm,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,10,Some Definitions,TS=Stream supply temperature(oC)TT=Stream target temperature(oC)H=Stream enthalpy(MW)CP=(MW/oC)=Heat capacity flowrate(MW/oC)=Stream flowrate specific heat capacity,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,11,Which of the two counter-current heat exchangers illustrated below violates T 20 oF(i.e.Tmin=20 oF)?,Clearly,exchanger A violates the Tmin constraint.,DTmin-Example,Tmin=Lowest permissible temperature difference,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,12,Definitions(Contd),6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,13,Utilities.Steam150 oC,CW25oC Design a network of steam heaters,water coolers and exchangers for the process streams.Where possible,use exchangers in preference to utilities.,Tmin=10 oC,Class Exercise 1,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,14,Setting Energy Targets,Summary of proposed design:Are 60 kW of Steam Necessary?,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,15,The Temperature-Enthalpy Diagram,One hot stream,Two hot streams,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,16,The Temperature-Enthalpy Diagram,Correlation between Tmin,QHmin and QCminMore in,More out!QHmin+x QCmin+x,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,17,The Composite Curve,Hot Composite Curve,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,18,The Composite Curve(Contd),Cold Composite Curve,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,19,The Composite Curve(Contd),Method:manipulate hot and cold composite curves until required Tmin is satisfied.This defines hot and cold pinch temperatures.,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,20,UNIT 2:The Pinch,The“pinch”separates the HEN problem into two parts:Heat sink-above the pinch,where at least QHmin utility must be usedHeat source-below the pinch,where at least QCmin utility must be used.,+x,+x,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,21,Significance of the Pinch,Do not transfer heat across pinchDo not use cold utilities above the pinchDo not use hot utilities below the pinch,Summary of modified design:,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,22,HEN Representation,Where is the pinch?,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,23,HEN Representation with the Pinch,The pinch divides the HEN into two parts:the left hand side(above the pinch)the right hand side(below the pinch)At the pinch,ALL hot streams are hotter than ALL cold streams by Tmin.,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,24,Class Exercise 2,For this network,draw the grid representation Given pinch temperatures at 480 oC/460 oC,and MER targets:QHmin=40,QCmin=106,redraw the network separating the sections above and below the pinch.Why is QH QHmin?,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,25,Class Exercise 2-Solution,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,26,Class Exercise 2-Solution(Contd),This can be fixed by reducing the cooling duty by 10 units,and eliminate the excess 10 units of heating below the pinch.,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,27,Design for Maximum Energy Recovery(MER),Step 1:MER Targeting.Pinch at 90o(Hot)and 80o(Cold)Energy Targets:Total Hot Utilities:20 kWTotal Cold Utilities:60 kW,Example,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,28,Design for MER(Contd),Step 2:Divide the problem at the pinch,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,29,Design for MER(Contd),Step 3:Design hot-end,starting at the pinch:Pair up exchangers according to CP-constraints.Immediately above the pinch,pair up streamssuch that:CPHOT CPCOLD(This ensures that TH TC Tmin),6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,30,Design for MER(Contd),Step 3(Contd):Complete hot-end design,by ticking-off streams.,90,240,Add heating utilities as needed(MER target),QHmin=20 kW,20,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,31,Design for MER(Contd),Step 4:Design cold-end,starting at the pinch:Pair up exchangers according to CP-constraints.Immediately above the pinch,pair up streamssuch that:CPHOT CPCOLD(This ensures that TH TC Tmin),6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,32,Design for MER(Contd),Step 4(Contd):Complete cold-end design,by ticking-off streams.,Add cooling utilities as needed(MER target),QCmin=60 kW,30,90,60,35o,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,33,Design for MER(Contd),Completed Design:,Note that this design meets the MER targets:QHmin=20 kW and QCmin=60 kW,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,34,Design for MER(Contd),Design for MER-Summary:,MER Targeting.Define pinch temperatures,Qhmin and QCminDivide problem at the pinchDesign hot-end,starting at the pinch:Pair up exchangers according to CP-constraints.Immediately above the pinch,pair up streams such that:CPHOT CPCOLD.“Tick off”streams in order to minimize costs.Add heating utilities as needed(up to QHmin).Do not use cold utilities above the pinch.Design cold-end,starting at the pinch:Pair up exchangers according to CP-constraints.Immediately below the pinch,pair up streams such that:CPHOT CPCOLD.“Tick off”streams in order to minimize costs.Add heating utilities as needed(up to QCmin).Do not use hot utilities below the pinch.Done!,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,35,Class Exercise 3,Design a network of steam heaters,water coolers and exchangers for the process streams.Where possible,use exchangers in preference to utilities.,Tmin=10 oC.Utilities:Steam150 oC,CW25oC,QHmin=48,QCmin=6,54,120,43oC,6,100,8,40,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,36,UNIT 3:The Problem Table,Tmin=10 oF.,Example:,Step 1:Temperature Intervals(subtract Tmin from hot temperatures)Temperature intervals:250F 240F 235F 180F 150F 120F,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,37,UNIT 3:The Problem Table(Contd),Step 2:Interval heat balances For each interval,compute:Hi=(Ti Ti+1)(CPHot CPCold),6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,38,UNIT 3:The Problem Table(Contd),Step 3:Form enthalpy cascade.,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,39,Class Exercise 4-Now try again!,Calculate the Problem Table.Predict QHmin and QCmin.Draw the Enthalpy Cascade.,Tmin=10 oC.,Step 1:Temperature Intervals(subtract Tmin from hot temperatures)Temperature intervals:,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,40,Class Exercise 4(Contd),Step 2:Interval heat balances For each interval,compute:Hi=(Ti Ti+1)(CPHot CPCold),6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,41,Class Exercise 4(Contd),Step 3:Form enthalpy cascade.,6-Intro HEN Synthesis,DESIGN AND ANALYSIS II-(c)Daniel R.Lewin,42,Introduction to HEN Synthesis-Summary,Unit 1.Introduction:Capital vs.EnergyWhat is an optimal HEN designSetting Energy TargetsUnit 2.The Pinch and MER DesignThe Heat Recovery PinchHEN RepresentationMER Design:(a)MER Target;(b)Hot-and cold-side designs Unit 3.The Problem Tablefor MER Targeting,Next week:Advanced HEN Synthesis,