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Published byAngel Sandow Modified about 1 year ago

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Trevor Hallberg Sarah Scribner

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

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Small Conceptual Example H1 C1 C HU CU 170 ° C 200 ° C 10 kW/°C 9 kW/°C 11 kW/°C 200˚C 30˚C 25˚C 30˚C HU 55 kW 1000 kW 925 kW 420 kW1110 kW 109.1˚C 76.7˚C 128˚C35.5˚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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Based on transportation-transshipment model Heat transfer Zones Temperature Intervals Energy and flow balances Hot stream, i Cold stream, j

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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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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 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% Area Reduction Plug Tubes By-pass fluid Enforce realistic area adjustments Area reduction ≤ 50%

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Re-piping scenarios 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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Objective Functions Maximize value of savings Value of Savings = Utility Cost Savings – Annual Capital Cost Maximize net present value NPV = ∑(DiscountFactor i * Utility Cost Savings i ) – Capital Cost Maximize return on investment ROI = Annual Utility Cost Savings / Capital Cost

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INPUT Parameters 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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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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Based on thermodynamic principles Uses Temperature vs. Enthalpy diagrams Called composite curves

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Composite Curves - Combined

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The pinch ( ΔT min ) Splits the process into 2 regions that are analyzed separately “Heat Sink” region above the pinch “Heat Source” region below the pinch

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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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Total Network Area - A ideal Q = UA interval ΔT LM “Vertical Heat Transfer”- vertical enthalpy regions Assumes equal area of each exchanger Vary ΔT min A ideal = A 1 +A 2 +…+A i

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Retrofitting Energy vs. Area diagram Blue curve = A ideal for various ΔT min values Want to improve ineffective use of area Want to decrease energy requirements

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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

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

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Area Efficiency ( α) Based on utilities of current process A grassroots = optimal area for current process Assume new design has α new ≥ α current The best α can be is 1 (cannot be better than ideal)

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Area Efficiency Assume α = β and A existing A retrofit A ideal A grassroots Can now calculate A retrofit based on constant curve Process: ΔT min Q u,min A ideal A retrofit

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Which α do we use ? Infinite amount of α values At least want α current The best is α = 1 Larger α = smaller A retrfit Assume value of 1

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We need the optimum ΔT min value Total Annualized Cost (TAC) vs. ΔT min diagram for constant α Optimum ΔT min value corresponds to the minimum Now that we have A retrofit ?

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N min = [N h +N c +N u -1] AP + [N h +N c +N u -1] BP Still assuming that the total area of the network is distributed evenly among the exchangers TAC vs. Δ T min

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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 HU 55 kW 1000 kW 925 kW 420 kW1110 kW 109.1˚C 76.7˚C 128˚C 35.5˚C 40˚C 30˚C 40˚C 30˚C 1

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HEN Design Start design at pinch Matching streams AP: (FCp) hot ≤ (FCp) cold BP: (FCp) cold ≤ (FCp) hot Maximize exchanger loads Q = FCpΔT (for each stream) H1 C1 C HU CU 170 ° C 200 ° C 10 kW/°C 11 kW/°C 9 kW/°C 200˚C 30˚C 25˚C HU 45 kW 1870 kW 55 kW 1300 kW230 kW 109.1˚C 76.7˚C 128˚C 35.5˚C 40˚C 30˚C 40˚C 30˚C

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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 C HU CU 170 ° C 200 ° C 10 kW/°C 11 kW/°C 9kW/°C 200˚C 30˚C 25˚C HU 45 kW X kW 55 kW X kW X kW 109.1˚C 76.7˚C 128˚C 35.5˚C 40˚C 30˚C 40˚C 30˚C 6 6 X kW

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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

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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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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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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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Controllers set stream target temps Calculator assigns cost equations Optimizer minimizes cost function

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

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

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

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Retrofit Results

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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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We believe the problem is with ΔT min 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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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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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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Original Network 7 exchangers 1 heater (E7) 2 coolers (E5,E6) Stream F kg/s Cp kJ/kg.C Tin o C Tout o C H kW/m 2. o C H H H HU (hot utility) C C CU (cold utility) H1 H2 H3 C1 C HU CU ˚ 343˚ 265˚ 77˚ 88˚ 90˚ 26˚ 118˚

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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 C ˚ 343 ˚ 265 ˚ 77 ˚ 88 ˚ 90 ˚ 26 ˚ 118 ˚ New -A +A NEW SPL HU 7 CU 6

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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 Example 1 – Process Pinch H1 H2 H3 C1 C ˚ 343 ˚ 265 ˚ 77 ˚ 88 ˚ 90 ˚ 26 ˚ 118 ˚ New -A +A NEW SPL HU 7 CU 6 +A

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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 A, NS 5 6 HU CU -A CU 7 8

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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 A, NS 10 HU CU -A

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EXAMPLE 1 RESULTS BEYOND THIS POINT

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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 Example 1

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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 Example 1

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Results (No Relocation) Example 1

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Results (Relocation Allowed) Example 1

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