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Trevor Hallberg Sarah Scribner Slide 2 Outline Mixed Integer Linear Programming (MILP) Pinch Technology Theory for Retrofit Improvements on Pinch Technology Crude Distillation Unit Example Discussion Slide 3 Small Conceptual Example H1 C1 C2 15 4 HU CU 170 C 200 C 10 kW/C 9 kW/C 11 kW/C 200C 30C 25C 30C 1 2 2 3 HU 55 kW 1000 kW 925 kW 420 kW1110 kW 109.1C 76.7C 128C35.5C 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 Slide 4 Based on transportation-transshipment model Heat transfer Zones Temperature Intervals Energy and flow balances Hot stream, i Cold stream, j 345678 Slide 5 Heat Transfer Zones Two Zones Pinch Technology (Above and Below the Pinch) One Zone Find minimum total cost, even if cross-pinch transfer occurs Slide 6 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 Slide 7 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% Slide 8 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 Slide 9 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 Slide 10 INPUT Parameters Stream data (F Cp) Stream temperatures (Inlet & Target) Cost functions OUTPUT Data Optimized objective function Exchanger locations Cost requirements Utility savings Slide 11 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 Slide 12 Based on thermodynamic principles Uses Temperature vs. Enthalpy diagrams Called composite curves Slide 13 Composite Curves - Combined Slide 14 The pinch ( T min ) Splits the process into 2 regions that are analyzed separately Heat Sink region above the pinch Heat Source region below the pinch Slide 15 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 Slide 16 Total Network Area - A ideal Q = UA interval T LM Vertical Heat Transfer- vertical enthalpy regions Assumes equal area of each exchanger Vary T min A ideal = A 1 +A 2 ++A i Slide 17 Retrofitting Energy vs. Area diagram Blue curve = A ideal for various T min values Want to improve ineffective use of area Want to decrease energy requirements Slide 18 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 Slide 19 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 Slide 20 Area Efficiency ( ) Based on utilities of current process A grassroots = optimal area for current process Assume new design has new current The best can be is 1 (cannot be better than ideal) Slide 21 Area Efficiency Assume = and A existing A retrofit A ideal A grassroots Can now calculate A retrofit based on constant curve Process: T min Q u,min A ideal A retrofit Slide 22 Which do we use ? Infinite amount of values At least want current The best is = 1 Larger = smaller A retrfit Assume value of 1 Slide 23 We need the optimum T min value Total Annualized Cost (TAC) vs. T min diagram for constant Optimum T min value corresponds to the minimum Now that we have A retrofit ? Slide 24 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. T min Slide 25 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 200C 30C 25C 1 2 2 3 HU 55 kW 1000 kW 925 kW 420 kW1110 kW 109.1C 76.7C 128C 35.5C 40C 30C 40C 30C 1 Slide 26 HEN Design Start design at pinch Matching streams AP: (FCp) hot (FCp) cold BP: (FCp) cold (FCp) hot Maximize exchanger loads Q = FCpT (for each stream) H1 C1 C2 15 4 HU CU 170 C 200 C 10 kW/C 11 kW/C 9 kW/C 200C 30C 25C 1 2 2 3 HU 45 kW 1870 kW 55 kW 1300 kW230 kW 109.1C 76.7C 128C 35.5C 40C 30C 40C 30C Slide 27 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 15 4 HU CU 170 C 200 C 10 kW/C 11 kW/C 9kW/C 200C 30C 25C 1 2 2 3 HU 45 kW 1870 - X kW 55 kW 1300 - X kW 230 + X kW 109.1C 76.7C 128C 35.5C 40C 30C 40C 30C 6 6 X kW Slide 28 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 Slide 29 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) Slide 30 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 Slide 31 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 Slide 32 Controllers set stream target temps Calculator assigns cost equations Optimizer minimizes cost function Slide 33 Crude Distillation Unit MILP Process Pinch Process Pinch Improvements Pro-II Simulation Retrofit Considerations Stream splitting Addition of new exchangers Allow & disallow exchanger relocation Slide 34 Original Network 10 Hot streams 3 Cold streams 18 exchangers 2 hot utilities 3 cold utilities Slide 35 No Relocation Allowed COMPARISON Original 18 exchangers MILP 8 new exchangers Process Pinch 9 new exchangers Slide 36 Slide 37 Allow Relocation Original MILP Process Pinch COMPARISON Original 18 exchangers MILP 5 new exchangers 5 relocated Process Pinch 9 new exchangers 7 relocated Slide 38 Retrofit Results Slide 39 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 Slide 40 We believe the problem is with T min The optimum value is determined prior to design Assumes equal area of every exchanger Optimization occurs after the value is chosen Slide 41 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 Slide 42 Slide 43 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 Slide 44 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 H1228.51159770.4 H220.41267880.3 H353.81343900.25 HU (hot utility)15004990.53 C193.31261270.15 C2196.111182650.5 CU (cold utility)120400.53 H1 H2 H3 C1 C2 35 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 Slide 45 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 38 9 5 HU 7 CU 6 Slide 46 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 C2 159 0 127 0 267 343 265 77 88 90 26 118 New -A +A NEW SPL 1 2 38 6 4 5 HU 7 CU 6 +A Slide 47 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 49 12 +A, NS 5 6 HU CU -A CU 7 8 Slide 48 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 12 +A, NS 10 HU CU -A 5 7 6 69 8 4 Slide 49 EXAMPLE 1 RESULTS BEYOND THIS POINT Slide 50 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 Slide 51 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 Slide 52 Results (No Relocation) Example 1 Slide 53 Results (Relocation Allowed) Example 1