Heat Integration - Introduction

Transcription

1 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

2 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

3 WS-1: Heat Integration Stream T s T t mcp ΔH C C kw/ C kw H H C Steam (var) Cooling Water (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

4 Possible Solutions A C1 50 C 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

5 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

6 Summary for WS-1 Design Q H = Q C (kw) A 280 B 280 C 380 D E See later.... F Around 149 (optimal split) X Any better??? We need Best Performance Targets!! Heat Integration Introduction T. Gundersen Heat 6

7 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

8 Illustrating Example 50 Feed Reactor Compressor Condenser Distillation Column 220 Reboiler 60 Product Heat Integration Introduction T. Gundersen Heat 8

9 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

10 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

11 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

12 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 H Product H Feed Stream C Recycle C Reboiler C Condenser H HP Steam HP ( 40 bar) (var.) 2.5 MP Steam MP ( 15 bar) (var.) 2.5 LP Steam LP ( 5 bar) (var.) 2.5 Cooling Water CW (var.) 1.0 Heat Integration Data Extraction T. Gundersen Heat 12

13 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

14 Composite Curves T( C) B A C Stream T s T t mcp ΔH H H T( C) B + C A 60 D H(kW) 60 D H(kW) T. Gundersen Heat 14

15 Composite Curves for the Example T ( C) Q C,min ΔT min Q Recovery Pinch Q H,min Note: The Column Reboiler / Condenser are not included H (kw) T. Gundersen Heat 15

16 Trade-off: Area vs. Energy T ( C) Q C,min Q H,min +δ +δ H (kw) T. Gundersen Heat 16

17 Pinch Point and Decomposition T ( C) Q C,min + δ Surplus δ Deficit Q H,min + δ H(kW) T. Gundersen Heat 17

18 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

19 H1 200 kw 220ºC ºC 720 kw 2000 kw H2 ST 270ºC ºC 720 kw ºC ºC 500 kw 180 kw kw 180ºC ºC 800 kw 360 kw 400 kw kw 160ºC ºC ΔT min = 20 C 1980 kw 1800 kw ºC ºC 220 kw ºC ºC CW C1 C2 Numerical Methods Intervals Heat Cascade T. Gundersen Heat 19

20 Pinch Point changes with ΔT min T ( C) H (kw) T. Gundersen Heat 20

21 Pinch Point Candidates T ( C) Pinch Candidates ( ) are the Supply Temperatures of Streams or Stream Segments Total mcp is then increasing H (kw) T. Gundersen Heat 21

22 Motivating Example Remember? Area = 629 m 2 Area = 533 m 2 T. Gundersen Heat 22

23 The Actual Flowsheet & Data Extraction Focus: Thermal Energy and the Energy Balance First: Checking the Mass Balance: Symbol: kg/s (1) = (OK) (2) = (OK) (3) = (OK) T. Gundersen Heat 23

24 Simple Data Extraction C 2 3 T in ºC, ΔH in kw and mcp in kw/ºc Str. Ts Tt ΔH mcp 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

25 Composite Curves and Energy Targets T ( C) Q (kw) PRO_PI1: ΔT min = 10 C Q H,min = 1068 kw and Q C,min = 0 kw T. Gundersen Heat 25

26 Energy Requirements vs. ΔT min Q (kw) Q H,exist = 1722 kw 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

27 200ºC Reactor Q: What about Phase changes & C p =C p (T)? 200ºC ST H2 70 kw 190ºC II 153ºC 40ºC 141ºC ST 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 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 = = kw/ C = = kw/ C Q: Can we solve this Puzzle? T. Gundersen Heat 27

28 T ( C) T ( C) Q (kw) S i m p l e Q (kw) D e t a i l e d T. Gundersen Heat 28

29 Energy Requirements vs. ΔT min 2500 Q (kw) Q H,exist = 1722 kw HRAT exist = 42.4 C 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

30 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

31 Fewest Number of Units ST C H C H1 CW 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 = = 6 U min = (N-1) T. Gundersen Heat 31

32 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 H C C1 15 CW 160 T. Gundersen Heat 32