1. Heat and Power Integration CHEN 4460 – Process Synthesis, Simulation and Optimization Dr. Mario Richard Eden Department of Chemical Engineering Auburn University Lecture No. 8 – Heat and Power Integration: Targeting October 23, 2006 Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel

2. Lecture 8 – Objectives  Compute the pinch temperatures  Compute the Maximum Energy Recovery (MER) targets using graphical and/or algebraic methods Given data on the hot and cold streams of a process, you should be able to:

3. Motivating Example What is wrong with this process from an energy viewpoint? No integration of energy!!!!

4. Short Bibliography Early pioneers –Rudd @ Wisconsin (1968) –Hohmann @ USC (1971) Central figure –Linnhoff @ ICI/UMIST (1978) –Currently: President, Linnhoff-March Recommended text –Seider, Seader and Lewin (2004): Product and Process Design Principles, 2 ed. Wiley and Sons, NY –Linnhoff et al. (1982): A User Guide on Process Integration for the Efficient Use of Energy, I. Chem. E., London Most comprehensive review: –Gundersen, T. and Naess, L. (1988): The Synthesis of Cost Optimal Heat Exchanger Networks: An Industrial Review of the State of the Art, Comp. Chem. Eng., 12(6), 503-530

5. Capital vs. Energy 1:3 The design of Heat Exchanger Networks (HENs) deals with the following problem: Given:  N H hot streams, with given heat capacity flowrate, each having to be cooled from supply temperature T H S to targets T H T  N C cold streams, with given heat capacity flowrate, each having to be heated from supply temperature T C S to targets T C T Design: An optimum network of heat exchangers, connecting between the hot and cold streams and between the streams and cold/hot utilities (furnace, hot-oil, steam, cooling water or refrigerant, depending on the required duty temperature)

6. Capital vs. Energy 2:3 Optimality –Implies a trade-off between CAPITAL COSTS (cost of equipment) and ENERGY COSTS (cost of utilities). Network for minimal energy cost ? Network for minimal equipment cost ?

7. Capital vs. Energy 3:3 Numerical Example Design A:  (AREA) = 13.3 [ A = Q/U  T lm ] Design B:  (AREA) = 20.4 [ A = Q/U  T lm ]

8. Some Definitions 1:3 T S = Supply temperature ( o C) T T = Target temperature ( o C) H = Stream enthalpy (MW)CP= Heat capacity flowrate (MW/ o C) = Flowrate x specific heat capacity = m x C p (MW/ o C)

9. Some Definitions 2:3 Minimum Allowable Temperature Driving Force  T min Which of the two counter-current heat exchangers illustrated below violates  T  20°F (i.e.  Tmin = 20°F) ? 20 o 10 o 20 o 30 o Clearly, exchanger A violates the  T min constraint

10. Some Definitions 3:3 Exchanger Duty (Q): Data:Hot stream CP = 0.3 MW/ o C Cold stream CP = 0.4 MW/ o C Heat Transfer Area (A): Data: Overall heat transfer coefficient, U=1.7 kW/m 2 o C (Alternative formulation in terms of film coefficients) Check: T 1 = 40 + (100 - 60)(0.3/0.4) = 70 o C  Q = 0.4(70 - 40) = 0.3(100 - 60) = 12 MW  T lm = (30 - 20)/log e (30/20) = 24.66 So, A = Q/(U  T lm ) = 12000/(1.7  24.66) = 286.2 m 2

11. Simple Example Design a network of steam heaters, water coolers and exchangers for the process streams. Where possible, use exchangers in preference to utilities. Utilities: Steam @ 150 o C, CW @ 25 o C

12. Simple Example - Targets Units:4 Steam:60 kW Cooling water:18 kW Are these numbers optimal??

13. Temperature-Enthalpy Diagram Correlation between  T min, Q Hmin and Q Cmin More in, More out! Q Hmin + x  Q Cmin + x

14. The Composite Curve 1:2 Three (3) hot streams

15. The Composite Curve 2:2 Three (3) hot streams

16. Simple Ex. – Hot Composite Not to scale!!

17. Simple Ex. – Cold Composite Not to scale!!

18. Thermal Pinch Diagram Move cold composite horizontally until the two curves are exactly ΔT min apart

19. Simple Ex. - Pinch Diagram Q Cmin = 6 kW Q Hmin = 48 kW T Cpinch = 60 T Hpinch = 70 Maximum Energy Recovery (MER) Targets!

20. The Pinch The “pinch” separates the HEN problem into two parts: Heat sink - above the pinch, where at least Q Hmin utility must be used Heat source - below the pinch, where at least Q Cmin utility must be used. +x +x x +x +x

21. Significance of the Pinch Do not transfer heat across pinch Do not use cold utilities above the pinch Do not use hot utilities below the pinch

22. Algebraic Targeting Method Temperature scales –Hot stream temperatures (T) –Cold stream temperatures (t) Thermal equilibrium –Achieved when T = t Inclusion of temperature driving force ΔT min –T = t + ΔT min –Thus substracting ΔT min from the hot temperatures will ensure thermal feasibility at all times

23. Algebraic Targeting Method Exchangeable load of the u’th hot stream passing through the z’th temperature interval: Exchangeable capacity of the v’th cold stream passing through the z’th temperature interval:

24. Algebraic Targeting Method Collective load of the hot streams passing through the z’th temperature interval is: Collective capacity of the cold streams streams passing through the z’th temperature interval is:

25. Algebraic Targeting Method Heat balance around each temperature interval:

26. Algebraic Targeting Method The enthalpy cascade –r 0 is zero (no hot streams exist above the first interval) –Feasibility is insured when all the r z 's are nonnegative –The most negative r z corresponds to the minimum heating utility requirement (Q Hmin ) of the process –By adding an amount (Q Hmin ) to the top interval a revised enthalpy cascade is obtained

27. Algebraic Targeting Method The revised enthalpy cascade –On the revised cascade the location of r z =0 corresponds to the heat-exchange pinch point –Overall energy balance for the network must be realized, thus the residual load leaving the last temperature interval is the minimum cooling utility requirement (Q Cmin ) of the process

28. Algebraic Targeting Method Example –Two hot streams and two cold streams –ΔT min = 10°F Step 1: Temperature intervals –Substract ΔT min from hot temperatures –250°F  240°F  235°F  180°F  150°F  120°F

29. Algebraic Targeting Method Step 2: Interval heat balances –For each interval calculate the enthalpy load –  H i = (T i  T i+1 )  (  CP Hot  CP Cold )

30. Algebraic Targeting Method Step 3: Enthalpy cascade Most negative residual T Cpinch = 180°F Q Hmin Q Cmin

31. Summary – Heat Integration  Compute the pinch temperatures  Compute the Maximum Energy Recovery (MER) targets using graphical and/or algebraic methods On completion of this part, given data on the hot and cold streams of a process, you should be able to:

32. Other Business Review of Midterm Exam –Tuesday October 24 during lab –Will meet in Ross Hall Auditorium –You will get your tests back to look at during solution review –Test MUST be returned after presentation Next Lecture – October 30 –Heat and Power Integration: Network Design (SSL pp. 316-346)