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Heat Integration of Multiple Effect Evaporators by Pinch Analysis

Heat Integration of Multiple Effect Evaporators by Pinch Analysis . B. P. Thapliyal , R. M. Mathur, A.K.Goel and A. G. Kulkarni Central Pulp And Paper Research Institute P.O.Box. 174, Saharanpur. Pinch Analysis. The term introduced by Linhoff & Vredeveld.

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Heat Integration of Multiple Effect Evaporators by Pinch Analysis

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  1. Heat Integration of Multiple Effect Evaporators by Pinch Analysis B. P. Thapliyal, R. M. Mathur, A.K.Goel and A. G. Kulkarni Central Pulp And Paper Research Institute P.O.Box. 174, Saharanpur

  2. Pinch Analysis • The term introduced by Linhoff & Vredeveld. • Represents a set of thermodynamically based methods. • A systematic methodology to identify and overcome the performance limiting constraints (or “pinch”) in any process

  3. Objective • To achieve financial savings by better process heat integration (maximizing process-to-process heat recovery and reducing the external utility loads). • Systematically designing an optimal scheme of utility exchange between producers and consumers.

  4. Key Principles • Pinch Analysis looks at the whole process instead of looking at the individual components. • After understanding the overall process, it is easier to identify the opportunities for improvement.

  5. Basic Concept • Pinch is viewed as the weakest link in a chain. • All systems have at least one Pinch. Some may have multiple pinches. • When one pinch is relieved some other factor will cause a new pinch. There will always be a pinch. • The technique consists of identifying the pinch, and then taking appropriate corrective action.

  6. Types of Optimisation • Structural:- Concerns with the selection of appropriate process configuration or topology, e.g. sequence of streams selected. • Parametric:- This concerns the numerical values of the process parameters such as flow, temperature, pressure, etc.

  7. Contd…. In existing processes, the process topology is already determined, therefore all that remains is to “optimize” the parametric values. But, it is a challenge to find out that a chosen process topology is the best one? This is where we apply – Pinch Analysis

  8. Structural & Parametric Optimization by Pinch • Typical gains in Energy Efficiency by; • Structural Optimization – 15 to 35% • Parametric optimization – 3 to 7% • Structural and Parametric optimizations are complementary and for best results both should be used. • Pinch analysis offers benefits of both approaches.

  9. Classic Applications • Energy Pinch optimization for process heat recovery • Total plant energy analysis • Power conservation and recovery opportunities through integrated CHP designs. • Water Pinch for optimum water reuse / recycle.

  10. Pinch Analysis in Pulp and Paper Industry • After its introduction in 1984, Pinch Analysis has been applied in a number of mills. Documented results reported in the literature have shown; • Energy cost reduction by 15-40%. • Capacity de-bottlenecking by 5-15% for retrofits. • Capital cost reduction by 5-10% for new design. • Improved operational flexibility. • CPPRI has taken the lead in bringing Pinch Technology to Indian Paper Industry.

  11. Steps for Pinch Analysis IDENTIFICATION OF HOT, COLD & UTILITY STREAMS IN THE PROCESS THERMAL DATA EXTRACTION FOR PROCESS & UTILITY STREAMS SELECTION OF INITIAL DTmin VALUE CONSTRUCTION OF COMPOSITE CURVES & GRAND COMPOSITE CURVE ESTIMATION OF MINIMUM ENERGY COST TARGETS ESTIMATION OF HEN CAPITAL COST TARGETS ESTIMATION OF OPTIMUM DTmin VALUE ESTIMATION OF PRACTICAL TARGETS FOR HEN DESIGN DESIGN OF HEAT EXCHANGER NETWORK (HEN)

  12. In Pulp & Paper industry, pinch analysis can be very effectively used for heat integration of digester house, multiple effect evaporators, paper machines and power network optimization. • It is useful to understand the heat duty analysis of evaporators and to explore the possibilities of heat integration in multiple effect evaporators.

  13. Optimization of MEE’s • Among the Kraft pulp mill operations, black liquor evaporation is one of the highest steam consumers. • It is economical to remove maximum quantity of water from the evaporator bodies leading to improved steam economy and higher solids. This can significantly improve recovery boiler performance. • The best way to accomplish this is by applying process control system, evaporator optimization and all possible heat integration within the evaporator train.

  14. Heat Integration studies in MEE’s were taken up to address • Methodologies for improving the heat recovery in existing evaporator train by looking at it as a whole; and • For optimizing the interrelationship of its constituent parts, rather than improving each unit by itself.

  15. Methodology • Studies conducted in two mills • To carryout vaporization duty analysis of evaporator bodies in MEE’s and • To identify the potential of energy conservation by process integration and modernization using pinch analysis. • In mill-1, heat integration evaluated on hot & cold stream from pulp mill and evaporator sections. • In mill-2, data from evaporator streams only used to carry out the targeting of evaporator section.

  16. PROCESS FLOW DIAGRAM FOREVAPORATOR SECTION PROCESS FLOW DIAGRAM FOREVAPORATOR SECTION PR PR cond cond 1 1 PR PR C Cond ond 2 2 Lower heati Lower heating duty Lower heating duty ng duty LP Steam LP Steam 4F 4F WBL WBL A/B/C A/B/C Mixing Mixing Tank Tank 2 2 3 3 4 4 5 5 6 6 Foul Foul Tank Tank Flash Flash PH PH Tank Tank Product Product PH PH Flash Flash 2 2 Flash Flash 1 1 Tank Tank Tank Tank Flash Flash Tank Tank Hot Hot Wate Wate r r Tank Tank SB SB L L Feed Water Feed Water Tank Tank Process Flow Diagram of Evaporator Section in Mill -1 Case Studies Mill – 1: Mill has 6 batch digesters. MEE consists of six effects (5-LTV and 3-FF)

  17. Steam Surface Co ndenser B y F p C a s s Hot water tank used mud 3 2 1 washer in causticizer Pure Condensate 70 CT Tank 5 4 B/H SBL WBL Storage Storage Tank Tank • Mill –2: • MEE in this mill has six effects of five LTV and one forced circulation evaporator. Process Flow Diagram of Evaporator Section in Mill -2

  18. 180 Hot Utility Target 11.2 Mkcal/hr Temperature (0C) 160 140 120 Pinch at 1000C 100 HOT UTILITY TARGET 11.2 Mkcal/hr AGAINST 12.7 Mkcal/hr ACTUAL CONSUMPTION 80 60 Cold Utility Target 6.2 MMkcal/hr 40 20 0.0 2.0 4.06.0 8.010.0 12.0 14.0 16.0 18.0 20.0 22.0 Heat Flow (Mkcal/hr) Pinch Analysis Studies in the Mills: Composite Curve for pulp mill & Evaporator streams in Mill-1.

  19. Hot Utility Target 6.2 MMkcal/hr HOT UTILITY TARGET 6.2 MMkcal / hr AGAINST 6.2 MMkcal / hr ACTUAL CONSUMPTION Cold Utility Target 6.6 MMkcal/hr Pinch at 63.5 °C Evaporator Composite Curve for Mill -1

  20. Hot utility target 11.2 MMkcal/hr Blow Vapour Pocket for additional heat recovery Vapour from Effect 6 Evaporator Fresh Water Fresh Water Cold utility target 6.2 MMkcal/hr Grand Composite Curve for pulp mill & Evaporator streams in Mill-1

  21. Effect 1 Duty Effect 2 Duty Effect 3 Duty Effect 4 Duty Effect 5 Duty Low Vaporization duty Effect 6 Duty Grand Composite Curve for Evaporator streams in Mill-1

  22. Re-compressor Lower heating duty PROCESS FLOW DIAGRAM FOREVAPORATOR SECTION PR cond 1 PR Cond 2 LP Steam 4F A/B/C WBL Mixing Tank 2 3 4 5 6 Foul Tank Flash Tank PH 2 Product Flash Tank PH 1 Flash Tank Flash Tank Hot Water Tank SBL Feed Water Tank

  23. Temperature,C 200 Hot utility target is 8.717 Mkcal/hr while actual consumption is 8.717 Mkcal/hr DTmin 7 C 180 Energy Target (Heating): 8.717 Mkcal/hr 160 140 120 100 80 60 0 30 45 35 40 50 10 15 20 25 5 Heat Flow (Mkcal/hr) Composite Curve (Temperature VS Heat Flow) for evaporator streams in Mill -2

  24. Tint(C) 190 Energy Target (Heating): 8.717 Mkcal/hr 170 150 130 110 90 70 50 1 2 3 4 5 6 7 8 9 Grand Composite Curve for Evaporator streams in Mill-2

  25. Hot utility target after Falling Film Evaporator introduction is 7.01 Mkcal/hr while actual consumption is 8.71 Mkcal/hr T0(C) 180 Dmin 7 0C 160 Energy Target (Heating): 7.01 Mkcal/hr Energy Target (Heating): 7.01 Mkcal/hr 140 120 100 80 60 30 45 35 40 0 10 15 20 25 5 Heat Flow, Mkcal/hr Composite Curve after Introduction of FF Evaporators in Finisher Stage in Mill-2.

  26. Tint(0C) 190 170 Energy Target (Heating): 7.01 Mkcal/hr 150 130 110 90 70 50 1 2 3 4 5 6 7 Heat Flow (Mkcal/hr) Grand Composite Curve after Introduction of FF Evaporator in Finisher Stage in Mill-2

  27. The energy saving schemes and their benefits

  28. Conclusions • Studies show that in spite of the operation of MEEs in a very well integrated manner, there is scope for improvement leading to reduction in energy consumption. • Process integration may be useful to suggest methodologies for improving the heat recovery in existing evaporator systems.

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