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**Heat Exchanger Network Retrofit**

Trevor Hallberg Sarah Scribner

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**Heat Exchanger Network (HEN) Retrofit**

Outline Mixed Integer Linear Programming (MILP) Pinch Technology Theory for Retrofit Improvements on Pinch Technology Crude Distillation Unit Example Discussion

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**Heat Exchanger Network (HEN) Retrofit**

Small Conceptual Example H1 C1 C2 1 5 4 HU CU 170°C 200°C 10 kW/°C 9 kW/°C 11 kW/°C 200˚C 30˚C 25˚C 2 3 55 kW 1000 kW 925 kW 420 kW 1110 kW 109.1˚C 76.7˚C 128˚C 35.5˚C Retrofit Options : Adjust existing area Relocate existing exchangers Add new exchangers Introduce stream splits Optimal Results : Reduce utility usage Increase process-process exchange Maintain network integrity

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**Mixed Integer Linear Programming (MILP)**

Based on transportation-transshipment model Hot stream, i Cold stream, j 3 4 5 6 7 8 Heat transfer Zones Temperature Intervals Energy and flow balances

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**Mixed Integer Linear Programming (MILP)**

Heat Transfer Zones Two Zones Pinch Technology (Above and Below the Pinch) One Zone Find minimum total cost, even if cross-pinch transfer occurs

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**MILP Sophisticated cost analysis Model parameters**

Area adjustment Existing heat exchangers New heat exchangers Re-piping Ability to tailor-fit model for a variety of HEN scenarios

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**Area Adjustments Area Reduction Area Addition Plug Tubes By-pass fluid**

Enforce realistic area adjustments Area reduction ≤ 50% Area Addition Existing Exchangers Increase area of existing shell Install a new, larger shell New Exchangers Limit number of new exchangers Limit area (size) of new exchangers Enforce realistic area adjustments Area addition ≤ 20%

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**Re-piping Re-piping scenarios Re-piping cost assignments**

Exchanger relocation Stream splitting Exchanger by-pass Re-piping cost assignments Model compares number of split streams in retrofitted network to original network Assigns user-defined fixed cost to number of new splits

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**MILP Objective Functions Maximize value of savings**

Value of Savings = Utility Cost Savings – Annual Capital Cost Maximize net present value NPV = ∑(DiscountFactori * Utility Cost Savingsi ) – Capital Cost Maximize return on investment ROI = Annual Utility Cost Savings / Capital Cost

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**MILP INPUT Parameters OUTPUT Data Stream data (F·Cp)**

Stream temperatures (Inlet & Target) Cost functions OUTPUT Data Optimized objective function Exchanger locations Cost requirements Utility savings

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**Pinch Technology The 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

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**Pinch Technology Based on thermodynamic principles**

Uses Temperature vs. Enthalpy diagrams Called 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)

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**Pinch Technology Composite Curves - Combined**

Displays the process minimum approach temperature (ΔTmin) Displays maximum process-to-process heat recovery Displays minimum utility requirements Minimum hot utility = QH,MIN Minimum cold utility = QC,MIN

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**Pinch Technology The pinch (ΔTmin)**

Splits the process into 2 regions that are analyzed separately “Heat Sink” region above the pinch “Heat 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

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**Pinch Technology 3 Pinch Rules**

No heat transfer across the pinch No external cooling above the pinch (only HU) No external heating below the pinch (only CU) Violating these results in cross-pinch heat transfer Increases heat requirements

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**Pinch Technology Total Network Area - Aideal Q = U•Ainterval•ΔTLM**

“Vertical Heat Transfer”- vertical enthalpy regions Assumes equal area of each exchanger Vary ΔTmin Aideal = A1+A2+…+Ai Q = heat transferred U = overall heat transfer coefficient ΔTLM = 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

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**Pinch Technology Retrofitting Energy vs. Area diagram**

Blue curve = Aideal for various ΔTmin 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 design

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**Pinch Technology Retrofitting continued**

Why would we increase area and energy? Could theoretically work but the point is to use area better and decrease energy Pinch 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)

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**Pinch Technology Need a way to compare energy and area**

Want 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 Need a way to compare energy and area Which path do we choose?

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**Pinch Technology Area Efficiency (α)**

Based on utilities of current process Agrassroots = optimal area for current process Assume new design has αnew ≥ αcurrent The best α 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 αcurrent, the closer Aexisting is to Agrassroots

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**Pinch Technology Area Efficiency Assume α = β and**

Aexisting Aretrofit Aideal Agrassroots Can now calculate Aretrofit based on constant curve Process: ΔTmin Qu,min Aideal Aretrofit

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**Pinch Technology Which α do we use? Infinite amount of α values**

At least want αcurrent The best is α = 1 Larger α = smaller Aretrfit Assume value of 1

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**Pinch Technology Now that we have Aretrofit?**

We need the optimum ΔTmin value Total Annualized Cost (TAC) vs. ΔTmin diagram for constant α Optimum ΔTmin value corresponds to the minimum

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**Pinch Technology TAC vs. ΔTmin Nmin = [Nh+Nc+Nu-1]AP + [Nh+Nc+Nu-1]BP**

Still assuming that the total area of the network is distributed evenly among the exchangers

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**Pinch Technology Now we need to design the HEN**

Eliminate cross-pinch heat exchangers (E2) Reuse the other exchangers (usually more economic) Design sections above and below pinch separately H1 C1 C2 5 4 HU CU 170°C 200°C 10 kW/°C 11 kW/°C 9kW/°C 200˚C 30˚C 25˚C 1 2 3 55 kW 1000 kW 925 kW 420 kW 1110 kW 109.1˚C 76.7˚C 128˚C 35.5˚C 40˚C Design is based on knowing the optimum ΔTmin 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 exchanger

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**Pinch Technology HEN Design Start design at pinch**

Matching streams AP: (F•Cp)hot ≤ (F•Cp)cold BP: (F•Cp)cold ≤ (F•Cp)hot Maximize exchanger loads Q = F•Cp•ΔT (for each stream) H1 C1 C2 1 5 4 HU CU 170°C 200°C 10 kW/°C 11 kW/°C 9 kW/°C 200˚C 30˚C 25˚C 2 3 45 kW 1870 kW 55 kW 1300 kW 230 kW 109.1˚C 76.7˚C 128˚C 35.5˚C 40˚C

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**Pinch Technology HEN Design Add E6 to reduce utilities**

Use loops and paths to make design more flexible Give E6 a duty of X A web of exchangers is affected H1 C1 C2 1 5 4 HU CU 170°C 200°C 10 kW/°C 11 kW/°C 9kW/°C 200˚C 30˚C 25˚C 2 3 45 kW X kW 55 kW X kW 230 + X kW 109.1˚C 76.7˚C 128˚C 35.5˚C 40˚C 6 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 network X kW

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**Pinch Technology Individual Heat Exchanger Area**

Evaluate the specific exchanger areas by accounting for temperature cross within the exchangers Need these areas to calculate the capital cost Q = heat load on the exchanger U = overall heat transfer coefficient (based on the two matched streams) ΔTLM = log mean temperature difference of the exchanger

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**Pinch Technology Cost comparisons**

No longer assume equal areas for each exchanger CC (Capital Investment Cost) Based on area change for each exchanger in the network Fixed and variable costs Includes area addition, reduction, and new exchanger cost Operating Costs (OC) ∆TAC (Total Annualized Cost) ROI (Return on Investment)

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**Pinch Technology How can we improve it?**

Allowing relocation of all exchangers May be able to cut down on area change expenses May decrease the number of new exchangers needed Incorporate Pro-II simulation

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**Pinch Technology Improvement**

Incorporate Pro-II Optimization Pro-II simulation based on Pinch Technology results Exchanger location Minimize total cost Vary heat exchanger area Vary stream split ratio Fix stream target temperatures

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**Pro-II Optimization Controllers set stream target temps**

Calculator assigns cost equations Optimizer minimizes cost function

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**HEN Retrofit Results Crude Distillation Unit Retrofit Considerations**

MILP Process Pinch Process Pinch Improvements Pro-II Simulation Retrofit Considerations Stream splitting Addition of new exchangers Allow & disallow exchanger relocation

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**Crude Distillation Unit**

Original Network 10 Hot streams 3 Cold streams 18 exchangers 2 hot utilities 3 cold utilities

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**Original MILP Process Pinch**

No Relocation Allowed Original MILP Process Pinch COMPARISON Original 18 exchangers MILP 8 new exchangers Process Pinch 9 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 1

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**Retrofit Results MILP has 1 less exchanger**

MILP has more area but requires less energy Pinch has less area but requires more energy Pro-II simulation further optimizes the pinch technology 36

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**Original MILP Process Pinch**

Allow Relocation Original MILP Process Pinch COMPARISON Original 18 exchangers MILP 5 new exchangers 5 relocated Process Pinch 9 new exchangers 7 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 1 37

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Retrofit 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 1 38

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**Discussion Computation Time Comparison MILP is most time-efficient**

MILP only requires input of data Pro-II requires large majority of manual labor Pinch requires manual labor only

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**Where Does Pinch Go Bad? We believe the problem is with ΔTmin**

The optimum value is determined prior to design Assumes equal area of every exchanger Optimization occurs after the value is chosen

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**MILP = The Best Why? Considers the greatest number of variables**

Considers all solutions No limiting assumptions or methodology Optimization based on several cost parameters Computer does everything a person can do Only requires input of data Do not need experience with the methodology

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The End

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**Process Pinch Limitations**

Why does pinch overlook MILP solutions? Optimum solution based on min number of units It 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 process Cannot efficiently consider all heat loops and paths

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**Example 1 Original Network 7 exchangers 1 heater (E7)**

2 coolers (E5,E6) Stream F kg/s Cp kJ/kg.C Tin oC Tout H kW/m2.oC H1 228.5 1 159 77 0.4 H2 20.4 267 88 0.3 H3 53.8 343 90 0.25 HU (hot utility) 500 499 0.53 C1 93.3 26 127 0.15 C2 196.1 118 265 0.5 CU (cold utility) 20 40 H1 H2 H3 C1 C2 3 5 1 4 2 6 7 HU CU 159 0 127 0 228.5 20.4 53.8 93.3 196.1 267˚ 343˚ 265˚ 77˚ 88˚ 90˚ 26˚ 118˚

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**Example 1- Process Pinch**

Retrofitted Network (No relocation Allowed) 8 exchangers (E6 becomes non-operational) 1 heater (E7), 1 cooler (E5) E8 and E9 added to increase process exchange H1 H2 H3 C1 C2 159 0 127 0 267˚ 343˚ 265˚ 77˚ 88˚ 90˚ 26˚ 118˚ New -A +A NEW SPL 1 4 2 3 8 9 5 HU 7 CU 6 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 1

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**Example 1 – Process Pinch**

Retrofitted Network (Relocation Allowed) 8 exchangers (E6 is relocated a process exchanger) 1 heater (E7), 1 cooler (E5) E8 added to meet increase process exchange H1 H2 H3 C1 C2 159 0 127 0 267˚ 343˚ 265˚ 77˚ 88˚ 90˚ 26˚ 118˚ New -A +A NEW SPL 1 2 3 8 6 4 5 HU 7 CU +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 1 46

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**Example 1- MILP Retrofitted Network (No relocation Allowed)**

9 exchangers 1 heater (E7), 2 coolers (E5, E6) E8,E9 added to increase process exchange 77˚ H1 H2 H3 C1 C2 88˚ 90˚ 26˚ 118˚ 267˚ 343˚ 265˚ 159˚ 127˚ New +A , NS NEW SPL -A 3 4 9 1 2 +A, NS 5 6 HU CU 7 8 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 1

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**Example 1- MILP Retrofitted Network (Relocation Allowed) 9 exchangers**

1 heater (E7), 2 coolers (E5, E6) E8,E9 added to increase process exchange 77˚ I1 I2 I3 J1 J2 88˚ 90˚ 26˚ 118˚ 267˚ 343˚ 265˚ 159˚ 127˚ 5 New +A +A, NS NEW SPL 3 1 2 10 HU CU -A 7 6 9 8 4 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 1

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**EXAMPLE 1 RESULTS BEYOND THIS POINT**

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**Example 1 Results (No relocation) MILP has 1 more exchanger than pinch**

MILP has more area but requires less energy Pinch has less area but requires more energy Pro-II simulation further optimizes the pinch technology 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 1 50

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**Example 1 Results (Relocation allowed)**

MILP has 2 more exchangers than pinch MILP has more area but requires less energy Pinch has less area but requires more energy Pro-II simulation further optimizes the pinch technology 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 1 51

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**Example 1 Results (No Relocation)**

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 1 52

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**Example 1 Results (Relocation Allowed)**

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 1 53

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