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Thermodynamics of Clean Production

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  1. Thermodynamics of Clean Production D.S.H. Wong Department of Chemical Engineering National Tsing Hua University Thermodynamics of Clean Production

  2. Contents • Resource Thermodynamics • Efficiency • Dispersion • Alternate Energy Source • Biofuel • Fuel Cell • Clean Production Technology • Pinch • Integration • Intensification Thermodynamics of Clean Production

  3. Resource Thermodynamics Thermodynamics of Clean Production

  4. Problems of the 21st Century --Lack of Resource Thermodynamics of Clean Production

  5. Problems of the 21st Century --Pollution of Environment Thermodynamics of Clean Production

  6. Twin Brothers of Trouble --Energy and Pollution Thermodynamics of Clean Production

  7. Laws of Thermodynamics • Energy is conserved • Processes are irreversible • This is our source of problem Thermodynamics of Clean Production

  8. Efficiency of a Process Thermodynamics of Clean Production

  9. Irreversibility of Mixing Thermodynamics of Clean Production

  10. Solution to the Energy Problem • Alternate energy source • Biofuel • Fuel cell • ... • Energy saving technology • Pinch • Integration • Intensification Thermodynamics of Clean Production

  11. Energy Efficiency V. Energy Issues

  12. Energy Consumption by Sector Thermodynamics of Clean Production

  13. Energy Saving Technology --Pinch, Integration and Intensification Thermodynamics of Clean Production

  14. Basic Theory of Pinch Thermodynamics of Clean Production

  15. A heat recovery problem • Consider a system with two hot streams to be cooled and two cold streams to be heated. To obey the Second Law, thermal energy transfer is limited by the temperature difference. Assume that there must be a 5 K difference in temperature for heat to be efficiently transferred. What are the minimum heating and cooling that must be supplied from external utilities (i.e.: high pressure steam and cooling water? Thermodynamics of Clean Production

  16. Simple heat balance • Heat supplied • due to H1 = 1000*(250-120) = 130,000 • due to H2 = 4000*(200-100) = 400,000 • total = 530,000 • Heat demand • due to C1 = 3000*(150-90) = 180,000 • due to C2 = 6000*(190-130)= 360,000 • total = 540,000 • Constraint due to second law has not been considered: heat must be transferred from high to low temperature Thermodynamics of Clean Production

  17. Heat exchanger Tho Thi Tco Tci • The smaller DT is, the less is the heat recovered, and the less is heat exchange area required. This leads to decrease in capital cost but increase in utility used • A rule of thumb is to let DTmin = 5 K Thermodynamics of Clean Production

  18. Heating composite curve Thermodynamics of Clean Production

  19. Cooling composite curve Thermodynamics of Clean Production

  20. Pinch and minimum utility Minimum heating Minimum cooling Pinch point Thermodynamics of Clean Production

  21. Heat cascade Heating Utility Pinch Cooling Utility Thermodynamics of Clean Production

  22. Design Heuristics Based on Pinch Theory Thermodynamics of Clean Production

  23. Do not exchange heat across pinch +X +X -X -X -X -X -X -X +X +X +X +X Thermodynamics of Clean Production

  24. Power generation by waste heat recovery below pinch is most desirable -X -X -X -X -X -X +X-W +X-W -W -W -W Thermodynamics of Clean Production

  25. Cogeneration above pinch is also acceptable +X -X -X -X +W-X -X +W-X -X +W-X +X-W +X-W -W Thermodynamics of Clean Production

  26. Cogeneration across pinch is not worthwhile +X -X -X -X -X -X -X -X +X-W +X-W -W +X-W Thermodynamics of Clean Production

  27. Pinch in Separation Thermodynamics of Clean Production

  28. Gas treating • The theory of pinch can be extended to design a separation network, one such application is in the removal of hydrogen sulfide from coke oven gas • El-Halwagi and Manousiouthakis, AICHE J. 1989m v.35, p.1233-1244 • The basic process is shown in the following diagram Thermodynamics of Clean Production

  29. Utility and process resource • Specifications • Ammonia is available at 2.3 kg/s as a byproduct of the process • Chilled methanol is available as a external mass separating agent • Henry’s law of hydrogen sulfide in ammonia and methanol are 1.45 and 0.26 respectively • Targets for cleaning and inlet conditions of solvent are as follows: • What is the minimum amount of methanol required Thermodynamics of Clean Production

  30. Saturation Targets • Due to mass transfer limit • since ammonia is responsible for cleaning the bulk gas, therefore its saturation target cannot be greater than the equilibrium composition of the richest inlet composition 0.070/1.45=0.0483 • since inlet ammonia contains 0.0006 H2S, the cleanest target it can achieve is greater than 0.0006*1.45=0.00087 • What is the minimum extra capacity of ammonia that is left • What is the maximum amount of methanol required Thermodynamics of Clean Production

  31. Composition levels Thermodynamics of Clean Production

  32. Composition cascade • Extra ammonia capacity = 2.3-0.04242/(0.0483-0.0006)=1.41 • Maximum methanol needed = 0.00059/(0.0001/0.26-0.0000) =1.53 Thermodynamics of Clean Production

  33. Water Pinch Thermodynamics of Clean Production

  34. Water Demand and Source Thermodynamics of Clean Production

  35. Water Cascade Table Thermodynamics of Clean Production

  36. Summary • Pinch targeting is the estimation of minimum resource required for accomplishing a task, thus identifying wasteful processes • Pinch analysis also provide guidelines of good design practice • Although pinch targeting and analysis do not provide actual solutions of the design problem, they are still very helpful in reducing energy and other resource usages. Thermodynamics of Clean Production

  37. Process Integration Thermodynamics of Clean Production

  38. Example of Process Integration -- Gas Composite Cycle Thermodynamics of Clean Production

  39. Basic Rankine Cycle Boiler Turbine Pump Condenser • Most traditional power plants employed a standard Rankine cycle, the process flow diagram of which is shown below. • Typical operating conditions are as follows • The working fluid is water • The compressor and pump operate adiabatically and reversibly • There is no heat loss • Boiler produces saturated steam at 10 MPa • Condenser produces recycled water at 310 K and 0.1 MPa. Thermodynamics of Clean Production

  40. Rankine cycle efficiency • Water at 310 K and 0.1 MPa ~ saturated water at 310 K • H=155 kJ/kG, S=0.5319 kJ/(kG K), VL=0.001007 M3/kG • Pumped to water at 10 MPa • W~VDP~0 • Boiled to saturated steam at 10 MPa • T~585.15 K; • H=2725 kJ/kG, S=5.611 kJ/(kG K), • Q=(2725-155)=2570 kJ/kG • Expanding to wet steam at 0.1 Mpa • T~373.15 K; S=5.611 kj/(kG K • SL=1.306 kJ/(kG K);SV=7.355 kJ/(kG K) : x=0.712 • HL=419 kJ/kG;HV=2676 kJ/kG: H=2025 kJ/kG • W=2725-2025 kJ/kG = 700 kJ/kG • Cooling to 310 K • Q=2025-155=1870 kJ/kG • Efficiency • 700/2570x100%=27% Thermodynamics of Clean Production

  41. Rankine cycle TS plot 3 4 1 2 • Rankine cycle • 1 to 2: pump • 2 to 3: boiler • 3 to 4: turbine • 4 to 1: condenser • Total work is the back box Thermodynamics of Clean Production

  42. Rankine Cycle vs Carnot Cycle • To provide heat for the boiler, a fuel is burned and produces a flue gas at 1000 K. To cool the low pressure steam in the condenser, the cooling water is available at 300 K. • Carnot efficiency between 1000 and 300 K is 70% • Area outside the black-lined region but inside the red box is the lost work, thermo-efficiency is 39% Thermodynamics of Clean Production

  43. Improving the Rankine Cycle • It is well known that the efficiency of Rnakine cycle can be improved by superheating and waste heat preheating Thermodynamics of Clean Production

  44. Basic Gas Cycle Fuel Combustion Chamber at 1000 K Compressor Turbine Exhaust gas Mixing with ambient air Air in 300 K • In automobile, air planes, and sometimes small power generator, power is obtained using a gas cycle Thermodynamics of Clean Production

  45. Combine cycle -- TS • A combined cycle proposed to integrate the two process to achieve higher efficiency Thermodynamics of Clean Production

  46. Combine cycle -- Process Fuel Combustion Chamber at 1000 K Boiler Turbine Compressor Turbine Heat exchanger Air in Pump Exhaust gas Condenser Thermodynamics of Clean Production

  47. Other Examples of Process Integration -- Divided Wall Column, Reactive Distillation Thermodynamics of Clean Production

  48. Concept of Divided Wall Column Thermodynamics of Clean Production

  49. Reactive Distillation • Improve Selectivity • Reduce Raw Materials Usage • Reduce Byproducts Prevent Pollution • Reduce Energy Use • Utilize Heat of Reaction for Separation • Handle Difficult Separations • Avoid Separating Reactants • Eliminate/Reduce Solvents • Enhance Overall Rates • “Beat” Low Equilibrium Constants Thermodynamics of Clean Production

  50. Traditional and Reactive Distillation Process for Methyl Acetate Production Thermodynamics of Clean Production