Simultaneous Water and Energy Optimization in Chemical Plants

Transcription

1 Simultaneous Water and Energy Optimization in Chemical Plants Thokozani Majozi NRF/DST Chair: Sustainable Process Engineering School of Chemical and Metallurgical Engineering

2 Group Structure NRF/DST CHAIR: SUSTAINABLE PROCESS ENGINEERING BATCH PROCESSES: Novel Scheduling Techniques CONTINUOUS PROCESSES: Utility Systems Debottlenecking Heat Integration Wastewater Minimization Design, Synthesis and Optimization Water & Energy Optimization Cooling Water System Design Clean Coal Technology: IGCC Hot Utility System Design Comprehensive Utility System Optimization

3 1 Outline Background & Motivation Problem Statement Model Development Illustrative Examples Case Study Results & Discussion Conclusion

4 2 Background Water and energy Each resource is consumed to produce the other Hydroelectric Geothermal Steam turbines Waterenergynexus Extraction Production Distribution Treatment Considers the interdependence of water and energy resources and their effect on the environment Increasing demand Stricter environmental regulations Sustainable use of water and energy Process integration techniques Environmentally benign Economically feasible

5 3 Motivation ENERGY WATER

6 4 Motivation ENERGY WATER

7 TRADEOFF 5 Motivation ENERGY Cost Minimum Total Cost Energy Water WATER Membrane Performance

8 6 Given: Problem Statement Sources with known flowrates and contaminant concentrations Sinks with fixed flowrates and known maximum allowable concentration Water regeneration units (known design parameters) Freshwater source with known concentration and unlimited supply Wastewater sink with maximum allowable concentration and unlimited capacity Determine: Minimum flowrate of freshwater into sinks Minimum wastewater flowrate Optimum design variables of regenerators for minimal energy usage Optimum water network configuration

9 7 Model Development Superstructure Splitter Mixer ED RO Sources 1 Sinks J=FW I=WW

10 Water balances for regeneration unit 8 jj ed j de ee fe ge Fed jj ro j dr er fr gr Fro

11 9 Model Development Concentration balance for regeneration unit Max. allowable conc. into ED unit jj ed j C x j,co ed C Ped co ee C Fro Red co fe C Pro co ge C Rro co C Ue co coco Max. allowable conc. into RO unit jj ro j C x j,co dr C Ped co er C Fro Red co fr C Pro co ge C Rro co C Ur co coco

12 10 Model Development Modelling the ED unit Electric current Cell Width (w) AEM Diluate CEM Concentrate AEM Cell thickness I d C Fz co N Stack length L A 2wN coco Stack Length L d C co d Feed Cell pair c C co N c

13 11 Model Development Pressure drop Rectangular pipe channel Fluid is Newtonian & behaves like a continuum Flow is steady, laminar, fully developed & incompressible P 12vL 2 Cell Width (w) Stack Length L AEM Diluate d C co d CEM Concentrate c C co c AEM Cell thickness Total annualised cost Feed TAC e K t mb max A el Ped pump spec AOT K E E Cell pair N

14 12 Model Development Modelling the RO unit Feed pressure P F P P 2 shell P P Water flux F N A P C water p Fro S coco Cco

15 13 F Nwater AP P CS Fro C F Nwater AP P CS Fro C F Nwater AP P CS Fro C Model Development Number of RO modules N m A P S m Pro P OSC Fro co coco Total annualised cost TAC r K mod Fro N m K K chemicals Pump ( P pump ) AOT P 0.65 turbine K turb turb K ( P elec turb ) AOT 0.43 P Pump K elec Pump AOT

16 14 Model Development Performance of regeneration units Removal ratio RR ed Red Fed C C Red co Fed co coco RR ro Rro Fro C C Rro co Fro co coco Liquid recovery LR ed Ped Fed LR ro Rro Fro

17 Model Development Objective function Capital cost Operational cost Min AF Membrane cost Piping cost Regeneration energy cost Freshwater cost Wastewater treatment cost Piping cost Model structure MINLP Continuous and integer variables Nonlinear constraints GAMS BARON 15

18 16 Illustrative Example I Typical pulp and paper plant Pulp Mill Water in Water in Bleached paper plant Water in Chemicals Wood chips Digester Washer Slurry Stripper 1 Screening Water out Water out Stripper 2 Bleaching Water out Bleached pulp to paper making White Liquor Clarifier Causticizer slaker Multiple Effect Evapourator (MEE) Water in Kiln offgas Lime Kiln Washer/ Filter Water out Lime mud Concentrator Solids return Offgas ESP Recovery Furnace Dissolve Tank Furnace exhaust Smelt Green Liquor Clarifier

19 17 Illustrative Example I Source/Sink identification Sources Process units Sinks Process units 1 Stripper 1 1 Washer 2 Screening 2 Screening 3 Stripper 2 3 Washer/Filter 4 Bleaching 4 Bleaching FW FW WW WW

20 18 Illustrative Example I Pulp and paper case study j Flowrate (t/h) Sources, j Concentration (mg/l) i Flowrate (t/h) Sinks, i Max. concentration (mg/l) FW 0 WW 600 Design parameters of ED and RO units Economic data for the case study Data for Manhattan distances

21 19 Results Removal ratio RR e RR r Total freshwater use (t/h) Scenario 1 Scenario 2 Scenario 3 Base case Fixed RR Variable RR Fixed RR Variable RR Freshwater savings 37.50% 46.30% 36.40% 43.70% Total wastewater generated Wastewater saved 43.70% 53.70% 42.30% 50.90% Total cost of water network millions($/year) CPU time (s)

22 20 Results Results Optimal results for case 3 variable RR ED RO FW WW

23 21 Results Optimum water flowsheet for scenario 3 (Variable RR) 2.64 Water in Pulp Mill Wood chips Digester Water in Washer Water in Slurry Screening 0.62 Water out Stripper 1 Water out Stripper 2 Bleached paper plant Water out Bleaching Chemicals Bleached pulp to paper making Freshwater Source White Liquor Clarifier Causticizer slaker Multiple Effect Evapourator (MEE) Water out Water in Kiln offgas Lime Kiln Washer/ Filter Lime mud Concentrator 0.02 Solids return Offgas ESP Recovery Furnace Dissolve Tank Furnace exhaust Smelt Green Liquor Clarifier ED Regeneration unit RO Regeneration unit 0.12 Wastewater Sink

24 22 Results Optimal design results of ED and RO unit Variable Value Area of ED 54m 2 Number of cell pairs in the ED unit 50 Number of RO modules 20 Length of ED unit 0.82 m Specific Energy J/s Pumping Energy J/s Electric Current 12 A Voltage across the ED unit 30 V Pressure drop on the shell side 4.5x10 5 kpa Osmotic pressure 1.6 kpa Feed pressure 5.7x10 5 kpa

25 23 Results Energy savings for ED and RO units Variable RR case Scenario2 ( Blackbox ) Scenario3 (Detailed) Energy requirement of RO unit kwh/annum ED Desalination Energy kwh/annum Total >12% savings in energy

26 24 Case Study Application to Eskom Kriel Power Station 3.6 GW [43 GW]

27 25 Case Study Current water utilisation network flowsheet at Kriel power plant Vaal Raw Water 1. Raw Water from Source 2. Floor Washing Floor Wash RAW WATER TREATMENT River Water Clarifiers Sludge to Effluent Sand Filtration 4. Backwash Water to Effluent COOLING NORTH 10. Backwash Water 5. Portable Water Portable Water Tank 13. Demin Water Feed 3 rd Parties 12. To Station Domestic Use Ion Exchange 20. Portable to Sewage 21. Portable to Drains 15 & 18 Demin Water to boilers 29. Usutu Water 22. Vaal Raw Water 24. Vaalpan North Cooling Tower Clarifier 10% Stream 16. CPP to Regen Boilers 26. Blowdown to Ash COOLING SOUTH 27. Sludge 28. Spent Regenerants Condensate to Polishing CPP 19. Demin to Drains 22. Vaal Raw Water 23. Usutu Water South Cooling Tower Clarifier POWER GENERATION 10% Stream 31. Coal Stock Yard Dam CPP Spent Regenerants Vaalpan 32. Blowdown to Ash 33. Sludge to Effluent High Level Effluent Dam Maturation Pond Ash Water Dam 40. Ash Conditioning 30. Maturation Pond Coal Stock Yard Dam 37. To Ash Dam 25 WWTW Dust Suppression 41. Dust Suppression Storm Water and Water Leaks into Drains 36. CW Recovery 38. To River 35. Kriel Mine Sewage 34. Power Station Sewage 39. Sludge to Drying Bed

28 Identified sources and sinks for the case study 26 Unit Operations Sources Sinks Variables Usutu raw water X Vaal raw water supply X Floor washing X 3 rd Parties X Sand filter backwash water X Dirty sand filter backwash water X Power station potable water use (bathrooms, kitchen etc) X Power station potable water leaking into drains X Power generation: Demin water X Power generation: Demin water to drainsmostly tanks overflows X Power generation: CPP spend regenerants X Ion exchange: Spent regenerants X Effluent dam X North cooling Tower X X South cooling tower X X WWTW X Ash dam/ash conditioning X Dust suppression X Vaalpan mostly from leaks from process units X

29 26 Limiting data for case study Sources Sinks No Name Flowrate m 3 /d Conc. mg/l No Name Flowrate m 3 /d Conc. mg/l 1 SF backwash F Washing PS to Drains rd Parties PG to Drains SF Backwash CPP Regents PS Potable Spent Regents PG Demin NC Tower NC Tower SC Tower SC Tower Effluent Dam Ash Dam WWTW D Suppression Vaalpan Waste Dam Freshwater 45

30 27 Results Current Practice Direct Reuse and Recycle Blackbox Model Detailed Model Freshwater m 3 /d (3.1 l/uso) (2.1 l/uso) (1.9 l/uso) FW Savings % Wastewater m 3 /d WW Savings %

31 Optimal water network flowsheet configuration 28 98m 3 /day 3 rd Parties m 3 /day Vaal Raw Water 1. Usutu Water 17212m 3 /day 2105m 3 /day Floor Wash RAW WATER TREATMENT 444m 3 /day River Water Clarifiers Sand Filtration 4. Backwash Water to Effluent COOLING NORTH Backwash water 5. Portable Portable Water Tank Water 13. Demin Water Feed 3000m 3 /day 12. To 3000m 3 /day Station 1890m 3 /day Ion Exchange 15 & 18 Demin Water to boilers Domestic Use 21. Portable to Drains 3412m 3 /day 3412m 3 /day 34689m 3 /day 2745m 3 /day 22. Vaal Raw Water 800m 3 /day 444m 3 /day 26. Blowdown to Ash North Cooling Tower 156m 3 /day Clarifier 10% Stream 28. Spent Regenerants 1039m 3 /day Condensate to Polishing Boilers CPP 19. Demin to Drains COOLING SOUTH POWER GENERATION 45388m 3 /day 22. Vaal Raw Water 6077m 3 /day South Cooling Tower Clarifier 10% Stream 10m 3 /day CPP Spent Regenerants Vaalpan 32. Blowdown to Ash 1400m 3 /day High Level Effluent Dam Maturatio n Pond Ash Water Dam 2740m 3 /day 834m 3 /day 30. Maturation Pond Coal Stock Yard Dam 390m 3 /day WWTW 50m 3 /day Dust Suppression 10m 3 /day 1390m 3 /day RO 276m 3 /day 8299m 3 /day 364m 3 /day Waste Dam 574m 3 /day 0.556m 3 /day ED 546m 3 /day

32 29 Results Regeneration cost analysis Blackbox Model Estimated cost True cost Detailed Model Total reg. feed (m 3 /d) 3,768 3,768 1,390 Regeneration cost (ZAR) 59, , ,156 Total cost (ZAR) 588,129 1,256, ,419 Energy Savings within the Regeneration units Combined desalination and pumping Energy in kwh/annum BlackBox Detailed 5, , % savings in both desalination and pumping energy

33 30 Results Summary of model characteristics Direct Reuse and Recycle Blackbox Model Detailed Model No. of constraints No. of continuous variables No. of discrete variables Tolerance CPU time (s) Size of model Increasing number of constraints Integer Nonlinear terms Computational intensity

34 31 Conclusion Mathematical model was developed Based on a superstructure regeneration reuse/recycle Detailed regeneration units (ED & RO) The proposed model was applied to Illustrative example involving single contaminant Case study involving single contaminant Results showed 43.7% freshwater savings and 50.9% reduction in wastewater 18.7% freshwater savings and 82.4% reduction in wastewater Accurate cost representation Optimal operating parameters and design configurations Minimum network cost and Energy Savings

35 32 Conclusion Recent publications 1. BuabengBaidoo, E., Majozi, T., 2015, Effective synthesis and optimization framework for integrated water and membrane networks: A focus on Reverse Osmosis membranes, Industrial and Engineering Chemistry Research, 54: (IF: 2.567). 2. Mafukidze, N.Y., Majozi, T., 2016, Synthesis and optimisation of an integrated water and membrane network framework with multiple electrodialysis regenerators, Computers & Chemical Engineering, 85: (IF: 2.581). 3. Nezungai, C.D., Majozi, T., 2016, Optimum Synthesis of an Electrodialysis Framework with a Background Process. I: A Novel Electrodialysis Model, Chemical Engineering Science, 147: (IF: 2.570). 4. Nezungai, C.D., Majozi, T., 2016, Optimum Synthesis of an Electrodialysis Framework with a Background Process. II: Optimization and Synthesis of a Water Network, Chemical Engineering Science, 147: (IF: 2.570). 5. Abass, M., Majozi, T., 2016, Optimization of integrated water and multiregenerator membrane systems, Industrial and Engineering Chemistry Research, 55: (IF: 2.567).

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37 Acknowledgements

38 Thank You 38