B. P. Thapliyal Central Pulp And Paper Research Institute P.O.Box. 174, Saharanpur.

1. PROCESS INTEGRATION THROUGH PINCH ANALYSIS FOR ENERGY CONSERVATION IN PULP & PAPER INDUSTRY B. P. Thapliyal Central Pulp And Paper Research Institute P.O.Box. 174, Saharanpur.

2. Pinch Analysis • The term “PINCH TECHNOLOGY” was introduced by Linhoff & Vredeveld. • Pinch analysis is a process integration technique based on straightforward application of thermodynamics, which uses it in a practical way. • The approach is simple and largely non- mathematical. • It represents set of Thermodynamically based methods that guarantee minimum energy levels in design of heat exchanger networks.

3. Objective • The prime objective of Pinch Technology is to achieve financial savings by better process heat integration (maximizing process-to-process heat recovery and reducing the external utility loads.) • Pinch Technology does this by making an inventory of all producers and consumers of these utilities and then systematically designing an optimal scheme of utility exchange between these producers and consumers. • Energy and water reuse are important function of the Pinch Technology.

4. Benefits of Pinch Analysis • Major advantages are; • Determine the minimum practical amount of energy required to operate a process. • Determine beneficial changes to the process itself and the site energy supply systems • Identify the measures that must be implemented in order to achieve the minimum energy consumption targets. • Identify the minimum cost schemes that maximise the savings consistent with planning and operating schemes. • Contd….

5. Determine the minimum heating (steam) and cooling (water) requirements. • Determine Cogeneration opportunities. • Pinch can replace the old energy studies with a live study that can be easily updated using simulation.

6. 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.

7. Steps for Pinch Analysis

8. Mass flowrate W(kg/s) Sp.heat capacity Cp (kj/kg0C) Heat capacity flowrate CP (kW/0C) Initial (supply) temp.Ts (0C) Final (target) Temp.TT (0C) Heat load H(Kw) CPdT, Cold stream Hot stream 0.25 0.4 4 4.5 1.0 1.8 20 150 200 50 -180 +180 Consider this simple process . . . STEAM FEED PRODUCT REACTOR 80 200 200 90 CW

9. Consider this simple process . . . STEAM FEED PRODUCT REACTOR 80 200 200 90 CW T STEAM 200 90 CW 80 H

10. Improved Design . . . 200 200 REACTOR 200 Heat recovery will result in saving of steam for heating. STEAM FEED 80 The question is - How much heat can be actually recovered and how big the heat exchanger should be ? This can be addressed with pinch analysis using simple temperature enthalpy diagram which is a helpful method of visualization cw 90 PRODUCT

11. Temperature-Enthalpy diagram after heat recovery T STEAM 200 90 CW 80 H

13. Process Steam type Heat capacity flowrate (kW/ 0C) Initial (Supply) temperature (0C) Final (target) temperature (0C) Stream heat load (kW) (positive for heat release) Cold 2.0 20 135 2.0 x (20-135)=-230 Hot 3.0 170 60 3.0 x (170-60)=330 Cold 4.0 80 140 4.0 x (80 – 140)= -240 Hot 1.5 150 30 1.5 x(150-30) =180

14. Composite Curves HOT COLD COMPOSITE COMPOSITE

16. Interval temperature Interval temperature Temperature Pinch Head load Head load Head flow c b a Composite Curves a. Converting stream temperature to interval temperature b. Identifying heat flow data c. Grand composite curve

17. Temperature 0C Heating 400 units Heating 300 units 140 Curve B Curve A 100 60 Pinch Curve B Curve A 20 Cooling 210 units Cooling 310 units Heat flow 100units Fig Grand composite curve

18. Principals of Pinch Analysis • No external cooling above the pinch point because it is already heat deficit. • No external heating below the pinch point because it is already heat surplus. • No heat transfer across the pinch point, it will lead to increase in both hot and cold utility consumption.

20. The determination of pinch is very useful as it provides us simple insight into following simple and effective concepts. Targets:- Once the composite curve and problem tables are known, we know how much external heating is unavoidably required. Near optimal and non- optimal processes are thus identified with confidence. The Pinch:- This tells as where to place furnace, stream heaters, coolers etc. More in – More out- An off target process requires more then minimum external heating and cooling for every unit of external heat in a process, we have to provide additional heat transfer equipment. Thus optimization allows to improve both heat energy and capital cost.

21. Pinch 170 90 60 2 C 90 kW 90 150 30 4 135 110 20 1 H 180 kW 50 kW 140 80 3 240 kW Common sense network design Heating duty – 50 kW Cooling duty – 90 kW With Energy Targets Heating duty – 20 kW Cooling duty – 60 kW

22. APPLICATION OF THE PINCH ANALYSIS IN PULP AND PAPER INDUSTRY • Pulp mill • Evaporator Section • Steam & power network utilization • Whole mill analysis

23. Pinch Analysis in Digester Section • Number of Digester, – 6 (Indirect Heating) • Number of Cooks per day, – 21 • Material and Energy balance was carried out and CC and GCC were constructed. • Based on stream data, range targeting was carried out for process to process heat recovery. • DTmin of 10oC was chosen for further analysis. Contd…

24. Composite curve shows hot utility target of 11.2 Mkcal/hr can be archived against actual consumption of 12.7 Mkcal/hr. • 1.5 Mkcal/hr is envisaged by process modification. • Suggestions; • Online heating of white liquor with preheater condensate (saving potential 0.8 t/hr of steam). • White liquor tank heating with preheater condensate (saving potential 2.0 t/hr of steam).

25. condenser After cond. New Blow Tank PROCESS FLOW DIAGRAM FOR PULPMILL SECTION Sat. Steam 13.33 t/hr 167 oC Pulp Dorr Washing Street Pulp To Soda Recovery To BL Storage Tank Wood Chips Liquor trap Black Liquor White Liquor DIGESTER EMICO Washing Street Accumulator Warm water to washing Plant PRE HEATER Old Blow Tank Fresh Water Warm water Black Liquor Tank Black Liquor Tank White Liquor Tank Cond. tank Cond to feed water To Soda Recovery New Blow Tank To BL Storage Tank Air Condensate 13.33 t/hr 160 oC Dilution from washing Plant

26. condenser After cond. New Blow Tank SCHEME 1 FOR ONLINE HEATING OF WHITE LIQUOR Sat. Steam 13.33 t/hr 167 oC Pulp Dorr Washing Street Pulp To Soda Recovery To BL Storage Tank Wood Chips Liquor trap Black Liquor White Liquor DIGESTER EMICO Washing Street Accumulator Warm water to washing Plant PRE HEATER Old Blow Tank Fresh Water Warm water Black Liquor Tank Black Liquor Tank White Liquor Tank Cond. tank Cond to feed water To Soda Recovery New Blow Tank To BL Storage Tank 160 oC 90 oC Air Dilution from washing Plant 70 oC 33.3 t/hr New Exchanger 110 oC

27. condenser After cond. New Blow Tank SCHEME 2 FOR FOR WHITE LIQUOR TANK HEATING Sat. Steam 13.33 t/hr 167 oC Pulp Dorr Washing Street Pulp To Soda Recovery To BL Storage Tank Wood Chips Liquor trap Black Liquor White Liquor EMICO Washing Street DIGESTER Accumulator Warm water to washing Plant PRE HEATER Old Blow Tank Fresh Water Warm water Black Liquor Tank Black Liquor Tank White Liquor Tank Cond. tank Cond to feed water To Soda Recovery New Blow Tank To BL Storage Tank 95 oC 160 oC Air 70 oC 33.3 t/hr Dilution from washing Plant New Exchanger 105 oC New Pump

28. Composite Curves for Pulp Mill Section Hot Utility Target 11.2 Mkcal/hr Pinch at 100 °C HOT UTILITY TARGET 11.2 Mkcal / hr AGAINST 12.7 Mkcal / hr ACTUAL CONSUMPTION Cold Utility Target 6.2 MMkcal/hr

29. Hot utility target 11.2 Mkcal/hr Blow Vapour Pocket for additional heat recovery Vapour from Effect 6 Evaporator Fresh Water Fresh Water Cold utility target 6.2 Mkcal/hr Grand Composite Curve for Pulp Mill Section

30. Evaporator Optimization • Mill has 5 LTV evaporators with one falling film evaporator in the final stage. • Based on the stream data range targeting was carried out to establish the optimum DTmin (7oC). • The composite curve shows that evaporator section is already well integrated. • Analysis of Grand Composite curve reveal that vaporization duty in effect 4 is less than 5th effect.

31. Suggested Modifications • Reducing the Pressure in effect 4. • Introducing more vapors in effect 4 through; • Recompressing low pressure vapor • Flashing high pressure condensate

32. 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

33. Composite Curves for Evaporator Section Hot Utility Target 6.2 Mkcal/hr HOT UTILITY TARGET 6.2 Mkcal / hr AGAINST 6.2 Mkcal / hr ACTUAL CONSUMPTION Cold Utility Target 6.6 Mkcal/hr Pinch at 63.5 °C

34. Effect 1 Duty Effect 2 Duty Effect 3 Duty Effect 4 Duty Effect 5 Duty Effect 6 Duty Grand Composite Curve for Evaporator Section Low Vaporization Duty

35. Suggested Modification and Saving Potential

36. Heat recovery in paper machine hood Min. heating requirement 760 kW 100 80 60 40 20 0 Cold composite curve Hot composite curve Temperature 0C Pinch at 410 C – caused by dew point of the exhaust Heat recovery opportunities Min. cooling requirement 572 kW Hot and cold composite curves for paper machine hood

37. By reducing the air infiltration to the dryers, we can ; • - increase the temperature of the dryer exhaust and • increase the relative humidity and hence the dew point temperature. • Increase in dew point temperature would alter the shape of the hot composite curve. Thus; • the scope of heat recovery would increase. • steam consumption would increase.

38. Effect of dryer exhaust conditions on the target steam consumption 80 70 60 50 Present design conditions Hood Exhaust Temperature 0C 80% 60% 50% 40% Relative humidity 700 750 800 850 900 1000 Target steam consumption

39. Heating requirement 400 kW 120 100 80 60 40 20 Pinch temperature at 70 0C Temperature 0C Heat recovery Min heat exchange temperature (20 0C) Steam saving – 47% Investment – Rs. 37 lakhs Annual savings – Rs. 30 lakhs Pay back period – 15 months Cooling requirement 310 kW Modified hot and cold composite curves for paper machine hood

40. Steam Power Network Optimization • Mill is generating 63.5 t/hr steam from the recovery boiler and two power boilers at a pressure of 20.0 kg/cm2. • Steam is distributed at 3.0, 5.2, 8.5, 10.0, 20.0 kg/cm2. • 2.84 MW power is generated from extraction turbine that accepts 20.0 kg/cm2 steam and exhausts to 8.5 kg/cm2, 3 kg/cm2 to condensing level. • 10.0 kg/cm2 and 5.2 kg/cm2 steam is generated by letting down 20.0 kg/cm2 and 8.5 kg/cm2 steam. Contd….

41. CC and GCC of the process were constructed and then analysis reveals a potential for generating of 3.2 MW of power against 2.8 MW actual power generation. • There is a potential for 0.4 MW power generation additionally.

42. DG SET Power = 6.5 MW 21.5 tph 40 tph PROCESS Rec. Blr Pwr. Blr 63.5 tph 20 kg/cm2 313 C 2 tph 13 tph 50.5 tph 13 tph 10 kg/cm2 169 C Total Power Req. 10.3 MW DG Set Gen. 6.5 MW Turb. Gen. 2.8 MW Purchase 1.0 MW Power = 2.8 MW 10.9 tph 10.9 tph 8.5 kg/cm2 252 C 34.3 tph 10.9 tph 5.2 kg/cm2 172 C 10.9 tph 34.3 tph 3 kg/cm2 185 C 5.3 tph C STEAM POWER SYSTEM

43. DG SET Power = 6.5 MW 21.5 tph PROCESS Rec. Blr 40 tph Pwr. Blr 63.5 tph 20 kg/cm2 313 C 2 tph 13 tph 50.5 tph Power = 0.3 MW 13 tph 10 kg/cm2 169 C Total Power Req. 10.3 MW DG Set Gen. 6.5 MW Turb. Gen. 3.2 MW Purchase 0.6 MW Power = 2.9 MW 10.9 tph 10.9 tph 6.5 kg/cm2 220 C 34.3 tph 10.9 tph 5.2 kg/cm2 172 C 10.9 tph 34.3 tph 3 kg/cm2 185 C 5.3 tph C MODIFIED STEAM - POWER SYSTEM

44. CO-GENERATION POTENTIAL 3.2 MW AGAINST 2.8 MW ACTUAL GENERATION Co-generation Potential

45. Recommendations • Installation of a new back pressure turbine providing 0.3 MW power by lowering extraction pressure of existing turbine from 20.0 to 10.0 kg/cm2. • Reduction of extraction pressure from 8.5 kg/cm2 to 6.5 kg/cm2 has shown additional cogeneration potential upto 3.5 %.

46. Whole Mill Analysis

47. Conclusion • Pinch study is a powerful tool to identify energy saving opportunities in pulp and paper mills. • The pinch analysis has obvious advantage of integrating entire hot and cold utilities in the mill resulting in substantial energy saving. • Energy managers in Pulp & Paper Industries should explore the potential of pinch analysis to find out • 1. minimum energy requirement by their processes • 2. ways to achieve maximum energy recovery in plant

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