Heat Integration - Introduction Sub-Topics Design of Heat Exchanger Networks Selection of Utilities Ø Consumption and Production Correct Integration Ø Distillation and Evaporation Ø Heat Pumps Ø Turbines (Heat Engines) Heat and Power Considerations Project Applications New Design ( grassroot ) Reconstruction ( retrofit ) Heat Integration Introduction T. Gundersen Heat 1
Pure Countercurrent Heat Exchanger T h,in mcp h T c,out Q h = Q c = Q ΔT LM =?? x dx L T c,in A, U mcp c Q h = ΔH h = mcp h T h,in T h,out T h,out ( ) ( ) Q c = ΔH c = mcp c T c,out T c,in Q = ( A U ) ΔT LM Heat Integration Introduction T. Gundersen Heat 2
WS-1: Heat Integration Stream T s T t mcp ΔH C C kw/ C kw H1 300 100 1.5 300 H2 200 100 5.0 500 C1 50 250 4.0 800 Steam 280 280 (var) Cooling Water 15 20 (var) Specification: ΔT min = 20 C Find: Q H,min, Q C,min T pinch, U min U min,mer and Network Notice: 1) H1 and H2 provide as much heat as C1 needs (800 kw) 2) T s (C1) < T t (H1,H2) - 20 C & T s (H1) > T t (C1) + 20 C Heat Integration Introduction T. Gundersen Heat 3
Possible Solutions A C1 50 C1 300 200 H1 H2 I II C H2 H1 I II H H C1 C1 H1 I H2 I H2 II C H1 II H H B C C C D Procesintegration Heat Integration i industrien Introduction T. Gundersen Heat 4
E C1 III H2 I H1 Question: 1) Minimum Energy? 2) Which Network? C II More Solutions H H1 H2 I C1 H II C F Heat Integration Introduction T. Gundersen Heat 5
Summary for WS-1 Design Q H = Q C (kw) A 280 B 280 C 380 D 142.5 E See later.... F Around 149 (optimal split) X Any better??? We need Best Performance Targets!! Heat Integration Introduction T. Gundersen Heat 6
Phases in the Design of Heat Exchanger Networks Data Extraction (assume R/S given) Performance Targets Energy, Area, Units / Shells Total Annual Cost (gives value for ΔT min ) Process Modifications (change R/S?) Design of Network (minimum energy) Decomposition at the Pinch Qualitative and Quantitative Tools Optimization (given the basic structure) Heat Load Loops & Paths and Stream Splits Heat Integration Introduction T. Gundersen Heat 7
Illustrating Example 50 Feed 210 210 Reactor Compressor 130 270 160 160 Condenser Distillation Column 220 Reboiler 60 Product Heat Integration Introduction T. Gundersen Heat 8
Phase 1: Data Extraction Time consuming (80%) and Critical Phase New Design: Material / Energy Balances Retrofit: Various Sources of Data Measurements (that are often incorrect) Simulations (M+E balances è Consistent) Design Basis (original, but was it updated?) Should keep Degrees of Freedom open Practical Limitations must be included, but Cost Effects should be evaluated before such Constraints are accepted Heat Integration Data Extraction T. Gundersen Heat 9
Necessary Data (1) Process Streams Flowrates m kg/s, tons/h Specific Heat Capacity Cp kj/kg C Supply Temperature T s C Target Temperature T t C Heat of Vaporization ΔH vap kj/kg Film Heat Transfer Coeffs. h kw/m 2 C Utility Systems Temperature(s) and Specific Heat Content Price per Energy Unit (Amount) Heat Integration Data Extraction T. Gundersen Heat 10
Necessary Data (2) Cost Data for Heat Exchangers Relation between Area and Purchase Price Example: Cost = a + b * (A) C Economical Data Number of Operating Hours per year Specification on required Payback Time Installation Factors Maximum Investment (for Retrofit Projects) Minimum allowed Approach Temperature (ΔT min ), possibly based on Pre-optimization Heat Integration Data Extraction T. Gundersen Heat 11
Data Extraction for the Example Stream ID T s T t mcp ΔH h C C kw/ C kw kw/m 2 C Reactor outlet H1 270 160 18 1980 0.5 Product H2 220 60 22 3520 0.5 Feed Stream C1 50 210 20 3200 0.5 Recycle C2 160 210 50 2500 0.5 Reboiler C3 220 220 2000 1.0 Condenser H3 130 130 2000 1.0 HP Steam HP 250 250 ( 40 bar) (var.) 2.5 MP Steam MP 200 200 ( 15 bar) (var.) 2.5 LP Steam LP 150 150 ( 5 bar) (var.) 2.5 Cooling Water CW 15 20 (var.) 1.0 Heat Integration Data Extraction T. Gundersen Heat 12
Phase 2: Targeting Basis: Minimum Approach Temperature: ΔT min Results: Minimum External Heating: Q H,min Minimum External Cooling: Q C,min Minimum Number of Units: U min Minimum Heat Transfer Area (total): A min Pre Optimization: Near optimal ΔT min Q H,min, Q C,min, U min, A min = f (ΔT min ) TAC = f(q H,min, Q C,min, U min, A min ) = f (ΔT min ) T. Gundersen Heat 13
270 220 160 Composite Curves T( C) B A C Stream T s T t mcp ΔH H1 270 160 18 1980 H2 220 60 22 3520 270 220 160 T( C) B + C A 60 D H(kW) 60 D H(kW) 2000 4000 6000 2000 4000 6000 T. Gundersen Heat 14
Composite Curves for the Example 300 250 200 150 100 50 T ( C) Q C,min ΔT min Q Recovery Pinch Q H,min Note: The Column Reboiler / Condenser are not included H (kw) 0 2000 4000 6000 T. Gundersen Heat 15
300 250 200 150 100 50 Trade-off: Area vs. Energy T ( C) Q C,min Q H,min +δ +δ H (kw) 0 2000 4000 6000 T. Gundersen Heat 16
Pinch Point and Decomposition 300 250 200 150 100 50 T ( C) Q C,min + δ Surplus δ Deficit Q H,min + δ H(kW) 0 2000 4000 6000 T. Gundersen Heat 17
Summary of Targeting by Graphical Methods Provides an Overview and Understanding Identifies Key Information about the Heat Recovery Problem, such as: Minimum External Heating, Q H,min Minimum External Cooling, Q C,min The Process Pinch Point (Bottleneck) More Graphical Diagrams later Time consuming and subject to Errors T. Gundersen Heat 18
H1 200 kw 220ºC - - - - - - - 200ºC 720 kw 2000 kw - 1200 H2 ST 270ºC - - - - - - - 250ºC 720 kw + 720 230ºC - - - - - - - 210ºC 500 kw 180 kw - 520 880 kw 180ºC - - - - - - - 160ºC 800 kw 360 kw 400 kw + 400 440 kw 160ºC - - - - - - - 140ºC ΔT min = 20 C 1980 kw 1800 kw + 180 70ºC - - - - - - - - 50ºC 220 kw + 220 60ºC - - - - - - - - 40ºC CW C1 C2 Numerical Methods Intervals Heat Cascade T. Gundersen Heat 19
Pinch Point changes with ΔT min 300 250 200 150 100 50 T ( C) 0 0 2000 4000 6000 8000 10000 H (kw) T. Gundersen Heat 20
Pinch Point Candidates T ( C) 300 250 200 150 100 50 0 0 2000 4000 6000 8000 10000 Pinch Candidates ( ) are the Supply Temperatures of Streams or Stream Segments Total mcp is then increasing H (kw) T. Gundersen Heat 21
Motivating Example Remember? Area = 629 m 2 Area = 533 m 2 T. Gundersen Heat 22
The Actual Flowsheet & Data Extraction 2 1 3 Focus: Thermal Energy and the Energy Balance First: Checking the Mass Balance: Symbol: kg/s (1) 1.089 + 1.614 = 2.703 (OK) (2) 6.931 + 2.703 = 9.634 (OK) (3) 7.841 + 0.179 + 1.614 = 9.634 (OK) T. Gundersen Heat 23
Simple Data Extraction 4 1 40 C 2 3 T in ºC, ΔH in kw and mcp in kw/ºc Str. Ts Tt ΔH mcp 1 5 200 2233 11.451 2 40 200 413 2.581 3 35 126 992 10.901 4 200 35 2570 15.576 Average Ref.: Ian C. Kemp, Pinch Analysis and Process Integration: A User Guide on Process Integration for the Efficient Use of Energy, 2nd. Ed., IChemE, Elsevier, 2007 T. Gundersen Heat 24
Composite Curves and Energy Targets T ( C) 250 200 150 100 50 0 0 500 1000 1500 2000 2500 3000 3500 4000 Q (kw) PRO_PI1: ΔT min = 10 C Q H,min = 1068 kw and Q C,min = 0 kw T. Gundersen Heat 25
Energy Requirements vs. ΔT min Q (kw) 2000 1800 1600 1400 1200 1000 800 600 400 200 Q H,exist = 1722 kw 0 0 10 20 30 40 50 60 70 80 Global temperature difference (K) HRAT exist = 64 C PRO_PI1: ΔT min = 22 C Q H,min = 1068 kw and Q C,min = 0 kw ΔT min = 23 C Q H,min = 1083 kw and Q C,min = 15 kw ΔT thrsh = 22.1 C Threshold Process becomes Pinched T. Gundersen Heat 26
200ºC Reactor Q: What about Phase changes & C p =C p (T)? 200ºC ST H2 70 kw 190ºC II 153ºC 40ºC 141ºC ST 2 1652 kw H1 CW 654 kw C 35ºC 114ºC 115.5ºC 128ºC I 1 5ºC Feed Recycles 3 III 4 Flash To Column I I I 1 4 II II II II 1 2 4 III III III 3 4 H1 H1 1 H2 H2 H2 1 2 III 4 "tot" 4 Q =Δ H =Δ H =?? Q =Δ H +Δ H =ΔH Q =Δ H =Δ H = 992 kw Q =Δ H = 1652 kw Q =Δ H +Δ H = 70 kw mcp mcp 126ºC Q =Δ H = 654 kw C C 4 992 = = 39.68 kw/ C 153 128 2570 = = 15.576 kw/ C 200 35 Q: Can we solve this Puzzle? T. Gundersen Heat 27
T ( C) T ( C) 250 200 150 100 50 0 0 500 1000 1500 2000 2500 3000 3500 4000 250 200 150 100 50 0 Q (kw) 0 500 1000 1500 2000 2500 3000 3500 4000 S i m p l e Q (kw) D e t a i l e d T. Gundersen Heat 28
Energy Requirements vs. ΔT min 2500 Q (kw) 2000 1500 1000 500 Q H,exist = 1722 kw HRAT exist = 42.4 C 0 0 10 20 30 40 50 60 70 80 Global temperature difference (K) PRO_PI1: ΔT min = 10 C Q H,min = 1068 kw and Q C,min = 0 kw ΔT min = 11 C Q H,min = 1072 kw and Q C,min = 4 kw ΔT thrsh = 10.5 C Threshold Process becomes Pinched T. Gundersen Heat 29
Lessons learned from the Example Data Extraction is a Critical Activity We have to solve a Puzzle Plant Data are often missing or incorrect Collecting and Processing Data is Time consuming (80% of Conceptual Studies) Remember: Garbage in means Garbage out Energy Targeting is much Simpler Next: Heat Exchanger Network Synthesis (Design & Optimization) is manageable T. Gundersen Heat 30
Fewest Number of Units 1000 1000 ST C1 3200 3520 H2 2200 1320 C2 2500 1180 1980 H1 CW 800 800 L = Loops S = Subnetwork U = Units Euler s Rule: U = N + L - S Assume: L = 0, S = 1 N = Number of Streams and Utilities N = n H + n C + n util = 2 + 2 + 2 = 6 U min = (N-1) T. Gundersen Heat 31
Maximum Energy Recovery requires Decomposition at Pinch Point(s) 250 HP 270 H1 210 U min,mer = (N-1) above + (N-1) below Process Pinch 180 220 60 H2 210 160 C2 20 160 50 C1 15 CW 160 T. Gundersen Heat 32