Heat Exchanger Network Retrofit

Heat Exchanger NetworkRetrofitTrevor HallbergSarah ScribnerHeat Exchanger Network (HEN)RetrofitOutlineMixed Integer Linear Programming (MILP)

Pinch Technology Theory for Retrofit

Improvements on Pinch Technology

Crude Distillation Unit Example

DiscussionHeat Exchanger Network (HEN)RetrofitSmall Conceptual Example

H1C1C2154HUCU170C200C10 kW/C9 kW/C11 kW/C200C30C25C30C1223HU55 kW1000 kW925 kW420 kW1110 kW109.1C76.7C128C35.5CRetrofit Options :

Adjust existing areaRelocate existing exchangersAdd new exchangersIntroduce stream splits

Optimal Results :

Reduce utility usageIncrease process-process exchange Maintain network integrity

Mixed Integer Linear Programming (MILP)Based on transportation-transshipment model

Heat transfer ZonesTemperature IntervalsEnergy and flow balancesHot stream, iCold stream, j345678Mixed Integer Linear Programming(MILP)Heat Transfer Zones

Two ZonesPinch Technology (Above and Below the Pinch)

One ZoneFind minimum total cost, even if cross-pinch transfer occursMILPSophisticated cost analysis

Model parametersArea adjustment Existing heat exchangersNew heat exchangersRe-piping

Ability to tailor-fit model for a variety of HEN scenariosArea AdjustmentsArea AdditionExisting ExchangersIncrease area of existing shellInstall a new, larger shell

New ExchangersLimit number of new exchangersLimit area (size) of new exchangers

Enforce realistic area adjustmentsArea addition 20%

Area ReductionPlug TubesBy-pass fluid

Enforce realistic area adjustmentsArea reduction 50%

Re-piping Re-piping scenariosExchanger relocationStream splittingExchanger by-pass

Re-piping cost assignmentsModel compares number of split streams in retrofitted network to original networkAssigns user-defined fixed cost to number of new splits

MILPObjective Functions

Maximize value of savingsValue of Savings = Utility Cost Savings Annual Capital Cost

Maximize net present valueNPV = (DiscountFactori * Utility Cost Savingsi ) Capital Cost

Maximize return on investmentROI = Annual Utility Cost Savings / Capital Cost

MILPINPUT ParametersStream data (FCp)Stream temperatures (Inlet & Target)Cost functions

OUTPUT DataOptimized objective functionExchanger locationsCost requirementsUtility savings

Pinch TechnologyThe root of heat integration technology

Systematic methodology for retrofitting

Identifies locations where process change will reduce the overall energy consumption

Allows us to set these energy targets

Allows us to set total cost targets

Pinch TechnologyBased on thermodynamic principlesUses Temperature vs. Enthalpy diagramsCalled composite curves

HCC profiles the heat availability in the process CCC profiles the heat demands in the process combinations of straight line representations of the hot and cold streams based on the heat capacity flow rate (CP = FCp)12Pinch TechnologyComposite Curves - Combined

Displays the process minimum approach temperature (Tmin) Displays maximum process-to-process heat recovery Displays minimum utility requirements Minimum hot utility = QH,MIN Minimum cold utility = QC,MIN13Pinch TechnologyThe pinch (Tmin)Splits the process into 2 regions that are analyzed separately Heat Sink region above the pinchHeat Source region below the pinch

Quantifies how close the composite curves can be without violating the second law of thermodynamics 2nd Law: no temperature crossover within any of the enthalpy intervals

14Pinch Technology3 Pinch RulesNo heat transfer across the pinchNo external cooling above the pinch (only HU)No external heating below the pinch (only CU)

Violating these results in cross-pinch heat transferIncreases heat requirements

Pinch TechnologyTotal Network Area - AidealQ = UAintervalTLMVertical Heat Transfer- vertical enthalpy regionsAssumes equal area of each exchangerVary TminAideal = A1+A2++Ai

Q = heat transferred U = overall heat transfer coefficient TLM = log mean temperature difference of the enthalpy intervals A1 and A5 represent the areas of the utility exchangers These will change as our process utility requirements change Provides the minimum total 16Pinch TechnologyRetrofittingEnergy vs. Area diagram

Blue curve = Aideal for various Tmin values

Want to improve ineffective use of area

Want to decrease energy requirements

Optimum grassroots design curve assumes vertical heat transfer for the HEN Total network area is calculated using the vertical heat transfer method described earlier We have a few options for retrofitting from our existing design17

Pinch TechnologyRetrofitting continuedWhy would we increase area and energy?Could theoretically work but the point is to use area better and decrease energyPinch recommends not decreasing area in which we have already invested - ???For now we will assume this is our retrofit path

If we are going to increase area, why not try to decrease the energy at the same time? (blue) Do not want to decrease area because we have already invested in it (pink) Limitation but we will assume it is correct for now and will look into it more later (green)18Pinch TechnologyWant a curve similar to the optimum design curve

Need a way to determine the most economical solution on the new curve

We use something called area efficiency

Which path do we choose?Need a way to compare energy and area 19Pinch TechnologyArea Efficiency ()

Based on utilities of current process

Agrassroots = optimal area for current process

Assume new design has new current

The best can be is 1 (cannot be better than ideal)

Agrassroots is calculated using the vertical heat transfer method as described earlier for the current process The higher current, the closer Aexisting is to Agrassroots20Pinch TechnologyArea Efficiency

Assume = and

AexistingAretrofitAidealAgrassroots

Can now calculate Aretrofit based on constant curveProcess:TminQu,minAidealAretrofit21Pinch TechnologyWhich do we use?Infinite amount of values

At least want current

The best is = 1

Larger = smaller Aretrfit

Assume value of 1

22Pinch TechnologyWe need the optimum Tmin value

Total Annualized Cost (TAC) vs. Tmin diagram for constant

Optimum Tmin value corresponds to the minimum

Now that we have Aretrofit?

23Pinch TechnologyNmin = [Nh+Nc+Nu-1]AP + [Nh+Nc+Nu-1]BP

Still assuming that the total area of the network is distributed evenly among the exchangersTAC vs. Tmin

24Now we need to design the HENEliminate cross-pinch heat exchangers (E2)Reuse the other exchangers (usually more economic)Design sections above and below pinch separatelyPinch TechnologyH1C1C254HUCU170C200C10 kW/C11 kW/C9kW/C200C30C25C1223HU55 kW1000 kW925 kW420 kW1110 kW109.1C76.7C128C35.5C40C30C30C40C30C30C1 Design is based on knowing the optimum Tmin value maximizing the heat load for each exchanger ensure the minimum number of units Designing above the pinch separately from below the pinch ensures no cross-pinch heat transfer Match one hot and one cold stream per exchanger25HEN DesignStart design at pinchMatching streamsAP: (FCp)hot (FCp)coldBP: (FCp)cold (FCp)hotPinch TechnologyMaximize exchanger loadsQ = FCpT (for each stream)

H1C1C2154HUCU170C200C10 kW/C11 kW/C9 kW/C200C30C25C1223HU45 kW1870 kW55 kW1300 kW230 kW109.1C76.7C128C35.5C40C30C30C40C30C30C 26HEN DesignAdd E6 to reduce utilitiesUse loops and paths to make design more flexibleGive E6 a duty of XA web of exchangers is affectedPinch TechnologyH1C1C2154HUCU170C200C10 kW/C11 kW/C9kW/C200C30C25C1223HU45 kW1870 - X kW55 kW1300 - X kW230 + X kW109.1C76.7C128C35.5C40C30C30C40C30C30C66X kW Loops give the process a flexibility can be used to make old exchangers fit new duties A path gives flexibility also and is a connection through streams and exchangers between two utilities Loops and paths allow us to shift heat loads in a network27Individual Heat Exchanger Area

Evaluate the specific exchanger areas by accounting for temperature cross within the exchangers

Need these areas to calculate the capital cost

Pinch Technology

Q = heat load on the exchanger U = overall heat transfer coefficient (based on the two matched streams) TLM = log mean temperature difference of the exchanger28Cost comparisonsNo longer assume equal areas for each exchanger

CC (Capital Investment Cost)Based on area change for each exchanger in the networkFixed and variable costsIncludes area addition, reduction, and new exchanger cost

Operating Costs (OC)

TAC (Total Annualized Cost)ROI (Return on Investment)

Pinch Technology 29How can we improve it?Allowing relocation of all exchangersMay be able to cut down on area change expensesMay decrease the number of new exchangers needed

Incorporate Pro-II simulation

Pinch Technology

30Pinch Technology ImprovementIncorporate Pro-II OptimizationPro-II simulation based on Pinch Technology results Exchanger location

Minimize total costVary heat exchanger areaVary stream split ratioFix stream target temperaturesPro-II Optimization

Controllers set stream target tempsCalculator assigns cost equationsOptimizer minimizes cost function

HEN Retrofit ResultsCrude Distillation UnitMILPProcess PinchProcess Pinch ImprovementsPro-II Simulation

Retrofit ConsiderationsStream splittingAddition of new exchangersAllow & disallow exchanger relocation

Original Network

10 Hot streams3 Cold streams

18 exchangers2 hot utilities3 cold utilities

Crude Distillation Unit

34No Relocation Allowed

Original MILP Process Pinch

COMPARISON

Original18 exchangers

MILP8 new exchangers

Process Pinch9 new exchangers

E9 satisfies the heat requirement for hot stream 3 below the pinch temperature and so E6 is not needed as a utility exchanger a split is needed on cold stream 135Retrofit Results

MILP has 1 less exchangerMILP has more area but requires less energyPinch has less area but requires more energyPro-II simulation further optimizes the pinch technology

36Allow RelocationOriginal MILP Process Pinch

COMPARISON

Original18 exchangersMILP5 new exchangers5 relocatedProcess Pinch9 new exchangers7 relocated

E9 satisfies the heat requirement for hot stream 3 below the pinch temperature and so E6 is not needed as a utility exchanger a split is needed on cold stream 137Retrofit Results

E9 satisfies the heat requirement for hot stream 3 below the pinch temperature and so E6 is not needed as a utility exchanger a split is needed on cold stream 138Computation Time Comparison

MILP is most time-efficientMILP only requires input of dataPro-II requires large majority of manual laborPinch requires manual labor onlyDiscussion

39We believe the problem is with Tmin

The optimum value is determined prior to design

Assumes equal area of every exchanger

Optimization occurs after the value is chosenWhere Does Pinch Go Bad? 40Why?Considers the greatest number of variablesConsiders all solutionsNo limiting assumptions or methodologyOptimization based on several cost parameters

Computer does everything a person can doOnly requires input of dataDo not need experience with the methodologyMILP = The Best 41The End 42Why does pinch overlook MILP solutions?Optimum solution based on min number of unitsIt then optimizes the area distribution and utilities

For large networks, there are many (if not infinite) combinations of exchangers and heat loads to satisfy a processCannot efficiently consider all heat loops and pathsProcess Pinch Limitations 43Original Network7 exchangers1 heater (E7)2 coolers (E5,E6)

Example 1StreamFkg/sCpkJ/kg.CTinoCToutoCHkW/m2.oCH1228.51159770.4H220.41267880.3H353.81343900.25HU (hot utility)15004990.53C193.31261270.15C2196.111182650.5CU (cold utility)120400.53H1H2H3C1C23514267HUCUCU159 0 127 0 228.520.453.893.3196.126734326577889026118 44Retrofitted Network (No relocation Allowed)8 exchangers (E6 becomes non-operational)1 heater (E7), 1 cooler (E5)E8 and E9 added to increase process exchange

Example 1- Process PinchH1H2H3C1C2159 0 127 0 26734326577889026118NewNew-A-A+A+A+A+ANEW SPL14NEW SPL23895HU7CUCU6 E9 satisfies the heat requirement for hot stream 3 below the pinch temperature and so E6 is not needed as a utility exchanger a split is needed on cold stream 145Retrofitted Network (Relocation Allowed)8 exchangers (E6 is relocated a process exchanger)1 heater (E7), 1 cooler (E5)E8 added to meet increase process exchangeExample 1 Process PinchH1H2H3C1C2159 0 127 0 26734326577889026118New-A-A+A+A+A+ANEW SPL1NEW SPL238645HU7CUCU6+A E9 satisfies the heat requirement for hot stream 3 below the pinch temperature and so E6 is not needed as a utility exchanger a split is needed on cold stream 146Retrofitted Network (No relocation Allowed)9 exchangers1 heater (E7), 2 coolers (E5, E6)E8,E9 added to increase process exchange

Example 1- MILP77H1H2H3C1C2889026118267343265159127NewNew+A , NS+A , NSNEW SPLNEW SPL-A-A34912+A, NS56HUCU-A CU78 E9 satisfies the heat requirement for hot stream 3 below the pinch temperature and so E6 is not needed as a utility exchanger a split is needed on cold stream 147Retrofitted Network (Relocation Allowed)9 exchangers1 heater (E7), 2 coolers (E5, E6)E8,E9 added to increase process exchange

Example 1- MILP77I1I2I3J1J28890261182673432651591275NewNewNew+A+A, NSNEW SPL312+A, NS10HUCUCUCU-A5766984 E9 satisfies the heat requirement for hot stream 3 below the pinch temperature and so E6 is not needed as a utility exchanger a split is needed on cold stream 148EXAMPLE 1 RESULTS BEYOND THIS POINTResults (No relocation)MILP has 1 more exchanger than pinchMILP has more area but requires less energyPinch has less area but requires more energyPro-II simulation further optimizes the pinch technologyExample 1

E9 satisfies the heat requirement for hot stream 3 below the pinch temperature and so E6 is not needed as a utility exchanger a split is needed on cold stream 150Results (Relocation allowed)MILP has 2 more exchangers than pinchMILP has more area but requires less energyPinch has less area but requires more energyPro-II simulation further optimizes the pinch technologyExample 1

E9 satisfies the heat requirement for hot stream 3 below the pinch temperature and so E6 is not needed as a utility exchanger a split is needed on cold stream 151Results (No Relocation)Example 1

E9 satisfies the heat requirement for hot stream 3 below the pinch temperature and so E6 is not needed as a utility exchanger a split is needed on cold stream 152Results (Relocation Allowed)Example 1

E9 satisfies the heat requirement for hot stream 3 below the pinch temperature and so E6 is not needed as a utility exchanger a split is needed on cold stream 153Energy AlgorithmSection ChangeUser InputHot Stream DataCold Stream DataOutputEnergy AlgorithmStreammCpTinToutHmCpH=mCpTTotal Hkg/skJ/kg.CoCoCkW/m2.oCkJ/sCkJ/skJ/sHot StreamsI353.81343900.2553.8-13611.4-36000I220.41267880.320.4-3651.6I1228.51159770.4228.5-18737HU (hot utility)15004990.53Cold StreamsJ2196.111182650.5196.19423.338250J193.31261270.1593.328826.7CU (cold utility)120400.53Tmin10Max HU-36000Max CU38250Min HU2250Min CU0Temperature IntervalsTinToutTinToutI334390I334390I226788I226788I115977I115977J2118265J2128275J126127J136137ABCDETintInterval iTintmCpintQintQcasRcas3430x34300010645.22751xx275-53.8-3658.43658.414303.62672xxx267142.31138.4252013165.21593xxxx159121.913165.2-10645.20Pinch1374xxxxx137-106.6-2345.2-83002345.21285xxxxx128-13.3-119.7-8180.32464.9906xxxx90-209.4-7957.2-223.110422.1887xxx88-155.6-311.288.110733.3778xx77-135.2-1487.21575.312220.5369x3693.33825.3-22508395.2streamI3I2I1J2J1mCp-53.8-20.4-228.5196.193.3Energy TargetsMin HU10645.2Min CU8395.2Pinch T159Hot Composite Curve (HCC)ABCDInterval iThSum mCp,hQint,hCumQh0x770001xx88228.52513.52513.52xxx90248.9497.83011.33xxx159302.720886.323897.64xx26774.28013.631911.25x34353.84088.836000streamI3I2I1mCp53.820.4228.5Cold Composite Curve (CCC)ABCDInterval iTcSum mCp,cQint,cCumQc6x26008395.27xx11893.38583.616978.88xx127289.42604.619583.49x265196.127061.846645.2streamJ2J1mCp196.193.3Grand Composite Curve (GCC)THeat Flow (kW)HCCCCCCCCHCCabs(CCC-HCC)ThCumQhTcCumQc34346645.23600010645.2770368395.227546645.232342.214303882513.512816978.826745080.331911.213169.1903011.313719583.415923897.623897.6015923897.627546645.213719583.417236.12347.326731911.212816978.814512.12466.7343360009013433.43011.310422.18813246.82513.510733.37712220.5012220.5368395.208395.2Retrofit based on constant h-valuesExchangerReal Area (m^2)1609.7610.12579.2584.1531008.51009.874117.96121.535787.5852.46104.695.067246.75246.813454.213519.92HCCABCDInterval iThSum mCp,hQint,hCumQh0x770001xx88228.52513.52513.52xxx90248.9497.83011.33xxx159302.720886.323897.64xx26774.28013.631911.25x34353.84088.836000streamI3I2I1mCp53.820.4228.5CCCABCDInterval iTcSum mCp,cQint,cCumQc6x2600155107xx11893.38583.624093.68xx127289.42604.626698.29x265196.127061.853760streamJ2J1mCp196.193.302513.5ThCumQhHot VertexTcCumQcCold Vertex3011.3770A2615510a15510882513.5B11824093.6b23897.6903011.3C12726698.2c24093.615923897.6D26553760d26698.226731911.2E31911.234336000F3600053760Pinch TableauT = 2Qh,min17597Qc,min15510VerticesABCaDbcEFdQ02513.53011.31551023897.624093.626698.231911.23600053760Th7788.090.0131.2907168814159.0267.0283.6254787341316.9006920648343.0xTcxxx26.095.096.6118.0127.0134.1xTxxxx64.0170.4165.6189.9208.9xLMTDixxxx82.9567219534108.6801296218168.0100963435177.4864904801199.2692007834xsum(mCp/h)hxxxx394.5333333333283.2215.2215.2215.2xsum(mCp/h)cxxxx1307.33333333336226221014.2392.2xsum(Q/h)xxxx101100.07285182231588.076180261416899.483937403916288.62590875498385.1584914094xAixxxx1218.7086286818290.6518081105100.586121341591.773891436442.0795509715xAideal1743.8000005417Aex2315.360.7531442197Q-shiftscold vertexaaabbcchot vertex shiftDEFEFEFQ-shift8387.616401.2204907817.611906.452139301.8T = 2Qh,min17597Qc,min15510VerticesABCDaEbcFdQ02513.53011.323897.623897.631911.232481.235085.83600062147.6Th7788.090.0159.0159.0267.0320.5390714238337.1645501579343.0xTcxxxx26.0111.9118.0127.0130.2265.0Txxxx133.0155.1202.5210.2212.8xLMTDixxxxx143.7714410386177.7709153263206.3283261328211.4999777522xsum(mCp/h)hxxxxx283.2215.2215.2215.2xsum(mCp/h)cxxxxx6226221014.2392.2xsum(Q/h)xxxxx84009.615321.608170395812705.6030235842494.728819842xAixxxxx584.327453304386.187372902261.579538116411.7954093724xAideal743.8897736953Aex2315.360.3212847133Q-shift8387.60A770A772513.5B882513.5B883011.3C903011.3C9023897.6D15923897.6D15931911.2E26723897.6a2636000F34331911.2E26723897.6a2632481.2b11832481.2b11835085.8c12735085.8c12736000F34362147.6d26562147.6d26516401.20A770A772513.5B882513.5B883011.3C903011.3C9023897.6D15923897.6D15931911.2E26731911.2E26736000F34331911.2a2631911.2a2636000F34340494.8b11840494.8b11843099.4c12743099.4c12770161.2d26570161.2d265Vertical Heat Transfer LinesHT0004000

CC77268811890127159265267343

HCCCCCEnthalpy Rate (kJ/s)Temperature (C)Composite Curves

GCC34327526715913712890887736

Pinch TemperatureProcess-to-Process Heat ExchangeMinimum Hot UtilityMinimum Cold UtilityUtilitiesEnthalpy Rate (kW)Temperature (C)Grand Composite Curve

No RelocationStreamPinchMCpI I151.3132.6I1I I159.077228.5I I77I I12709.4I II I88.0I2267159.0I I159.08820.4159.0I I88I I0.0I II II II II3343159.0I I159.09053.8159.0I I90I II I81.326.2J1I I127.02693.3I I26I I4262.71448.4I I67.1I II I160.4I I3712.2188.7I IJ2265127.0I I127.0118196.1127.0I I11814959.49899.2I I1764.9I I35.7I I2203.2I IdTminExchanger area32QMCpUTh,inTh,outTc,inTc,outLMTDAAoA-AoQcQhCU savingsHU savingsCU CostHU CostU CostA reduceA additionA CostSplittingCapitalTC19899.20.1667343159.0127188.777.7366011218763.9042931955609.7154.204293195512709.414959.42,800.62,800.6254,043.61,421,806.01,675,849.60.0149,453.1149,453.120,000.0959,328.92,635,178.523712.20.09375159.0902681.370.6231051562560.6776976519579.2-18.52230234818,742.60.08,742.634262.70.10909151.2761487965132.681.3127.036.12226199161081.7449813821008.573.2449813820.080,070.980,070.942203.20.1875267159.0127188.751.7368728782227.1184813907117.96109.15848139070.0110,848.8110,848.8512709.4490.98333975470.22796132.6772045.885603381970.8304740007787.1296137615787.5-0.37038623858,651.90.08,651.960.00.191688.088206839.22288253190104.6-104.69,173.00.09,173.0714959.454.33853977480.25728500224.7188.7265270.9010230096214.6334976282246.75-32.11650237188,810.60.08,810.681764.90.22222159.0151.3118127.032.6339158559243.3704081471243.37040814710.0243168.439782092243,168.491448.40.1159.088.026130.043.4302784194333.5000494385333.50004943850.0320409.542368756320,409.5Total50959.44212.0790225954939,328.9E7224.7>188.7Aexisting2315.36Aideal12046.8808588082E545.96188.7Aexisting2315.36Aideal12046.8808588082E545.96188.7Aexisting2315.36Aideal12046.8808588082E545.96188.7Aexisting2315.36Aideal12046.8808588082E545.96188.7Aexisting2315.36Aideal12046.8808588082E545.96188.7Aexisting2315.36Aideal12046.8808588082E545.96188.7Aexisting2315.36Aideal12046.8808588082E545.96188.7Aexisting2315.36Aideal12046.8808588082E545.96188.7Aexisting2315.36Aideal12046.8808588082E545.96188.7Aexisting2315.36Aideal12046.8808588082E545.96188.7Aexisting2315.36Aideal12046.8808588082E545.96188.7Aexisting2315.36Aideal12046.8808588082E545.96188.7Aexisting2315.36Aideal12046.8808588082E545.96188.7Aexisting2315.36Aideal12046.8808588082E545.96