Alternative Approaches to Design Evaporator and Crystallizer Systems using OLI Software Ken Martins/CH2M HILL October 17, 2012
Outline Key Drivers for Zero Liquid Discharge (ZLD) Processes Overview of Softener, Evaporator (EVAP) and Crystallizer (CX) Processes Single Stream Analyzer based Evaluation (with Excel) Iterative Stream Analyzer Evaluation Aspen plus with OLI Chemistry Engine (Aspen OLI) Evaluation CH2M HILL Process Modeler Tool SOURCE
Why ZLD is Becoming a More Viable Option Discharge permit water quality standards changing Lowered for certain parameters (e.g., metals, chlorides) New parameters added (e.g., TDS and Whole Effluent Toxicity [WET]) Regional water scarcity concerns driving internal reuse, further increasing concentrations of all dissolved salts EPA recommending ZLD (Steam Electric Power Generating Point Source Category: Final Detailed Study Report, EPA 821-R-09-008)
Why ZLD is Becoming a More Viable Option (con t) Fewer alternatives for disposal in some regions Intensive permitting restrictions for deep well injection and surface impoundments in California New wastewater streams (e.g., flue gas desulfurization) more difficult to treat with conventional technologies
Increasing Market trends for ZLD Global Water Intelligence, Dec 2009 Total capital investment in ZLD systems around the world is estimated to be between $100-200 million per year (2009) The predominance of ZLD projects has increased from none in 1970 to about 100 in 2009 Relatively few of these systems (a total of just over 100 worldwide), are designed purely as ZLD systems (e.g., avoiding saline water discharge)
Overview of ZLD Treatment Processes Lime/Soda Softener Evaporator Crystallizer Waste Solids Distillate Distillate Waste Solids
Softener Process with Post Influent Hydrated Lime Soda Ash Sulfuric Acid Filtration Sulfuric Acid Antiscalant Evaporator Crystallizer Waste Solids to Dewatering Reactor- Clarifier Gravity Media Filter Distillate Distillate Waste Solids
Softener Precipitation Chemistry K W = [H + ][OH - ] K 2 = [10 -ph ][CO 2-3 ]/[HCO 3- ] K SP (CaCO3) =[Ca 2+ ][CO 2-3 ] K SP (Mg(OH)2) = [Mg 2+ ][OH - ] 2 K SP (CaF 2 ) = [Ca 2+ ][F - ] 2 K SP (CaSO 4 ) = [Ca 2+ ][SO 4 ]
Softener Chemistry 2
Modeling Softener Chemistry in SA First Step: Add Lime, as Needed to Attain Target Soluble Mg 2 Second Step: Add Soda Ash, as Needed to Attain Target Soluble Ca
Lime Addition 1 st Step for Softener Modeling
Soda Ash Addition 2 nd Step for Softener Modeling
Vapor Recompression-driven Evaporator Compressor Lime/Soda Softener CaSO4 Crystallizer Waste Solids Recirc Pumps Caustic Distillate Waste Solids Preheater and Deaerator not shown for clarity Evaporator Distillate
Key Design Factors for Evaporators Soften water to avoid CaCO3 scale Adjust ph, strip CO2, N2, O2 Add antiscalant For Mechanical Vapor Recompression driven Evaporators, Limit Boiling Point Rise to 5.5 to 6.5 F Seed evaporator liquor with CaSO4 solid Hold between 10-15% solids in slurry
Use Stream Analyzer to Predict Key Parameters of Interest Caustic dosage to achieve desired ph of evaporator concentrate Composition and mass of solids Composition of liquid phase Boiling point of liquid phase Volume of distillate
Boiling Point (ºF) Boiling Point Rise Boiling Point vs. Fraction Evaporated 228 226 224 222 220 218 216 Limit of 218.5 F BPR 5,000 mg/l Cl-, 10,000 mg/l TDS 10,000 mg/l Cl-, 20,000 mg/l TDS 15,000 mg/l Cl-, 25,500 mg/l TDS 20,000 mg/l Cl-, 40,000 mg/l TDS 214 212 210 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 Brine (fraction of influent flow) Developed using OLI s StreamAnalyzer Copyright 2011 TM program by CH2M HILL, INC
Prepare integrated mass balance, integrating mass flows and chemistry calculations Use Stream Analyzer in tandem with Excel Start with design basis values of flows and concentration in Excel Use Stream Analyzer to calculate chemistry and phase changes for first unit operation Transfer Stream Analyzer output to Excel, Calculate feed to next unit process Use Stream Analyzer to calculate chemistry and phase changes for second unit operation and so on
Make Adjustments to Account for the Perfections of Stream Analyzer Within integrated mass balance, we make adjustments to account for : Lack of complete equilibrium Add excess to drive to near completion Equipment imperfections Floc carryover Undissolved lime Need to create methods and a tool to release to others at CH2M HILL
Sequential Process Steps Modeled in SA
Example Screen Shot of Spreadsheet Portion of Integrated Mass Balance
Vapor Recompression-driven Crystallizer Lime/Soda Softener Evaporator Multistage Compressors Caustic Waste Solids Distillate Condensor Waste Solids Distillate Crystallizer
Key Design Factors for Crystallizers Soften water to avoid CaCO3 scale Adjust ph, strip CO2, N2, O2 Add antiscalant For Mechanical Vapor Recompression driven Crystallizer, Limit Boiling Point Rise to 6.5 F, otherwise steam-driven or vacuum crystallizer designs Maintain crystallizer recirculated between 10-15% solids in slurry
Use Stream Analyzer to Predict Key Parameters of Interest Composition and mass of solids Composition of liquid phase CaCl2 and NO3 often removed strictly as salt cake moisture (Check by mass balance) Boiling point of liquid phase Mass of distillate
If Boiling Point Rise Too High for Mechanical Recompression.. Can use vacuum crystallizer BP is reduced when operated under vacuum Ca/Mg salt solubility decreases as temperature is lowered Can use steam fed crystallizer Compressor Salt (CaCl 2, MgCl 2 ) Expansion Valve Vacuum Pump Hot Distillate
Alternative use of Stream Analyzer as Iterative method Labor intensive method Replicates process as if starting up Water surveyed to create solids Take solids cut at 15% solids (normal design point for CX Mix concentrate back with fresh influent 1:1 Check solids with moisture for element balance Repeat
First Concentration (Water Survey) of Example Project 85% Aq phase
20th Concentration (Water Survey) of Example Project 85% Aq phase
Issues with iterative method Tedious - Each iteration requires Determine closest cut to 85% aqueous Save Aq phase, Check mass balance of solubles Blend with raw influent, Water survey again Prone to error Easy to select wrong percent liquids cut Time consuming
Speeding up the Iteration Approach Method 1 Mix 4 parts concentrate to a part make-up for first couple iterations, then just 1 part concentrate to 1 part make-up Method 2 Water survey to dry salt, Identify soluble species, artificially increase those about 40 times, mix 1 part concentrate to 1 part make-up
Aspen plus with OLI Engine (Aspen OLI) Iterative method using Stream Analyzer identified need to iterate and cycle-up concentration to reach equilibrium Aspen plus does exactly that
Aspen OLI Model for Crystallizer Only
Aspen Output is Clunky
Aspen OLI Input and Output can be Linked with Excel
Excel Allows use of Convenient Graphics and Calculations 34
Future CH2M HILL Software SOURCE with OLI Engine SOURCE is a CH2M HILL developed process modeler similar to Aspen Unit processes are selected and linked together Chemistry is entered and the model calculates the chemistry changes at each unit process iteratively SOURCE has developed the ability to use OLI s chemistry engine (call basis) Strictly for internal CH2M HILL use
Future CH2M HILL Source with OLI Engine Current SOURCE unit processes Lime/Soda softeners, ph adjustment, Clarifiers, Filters, Microfilters, Reverse osmosis, Electrodeionization, Ion Exchange Future planned SOURCE unit processes Evaporators, Crystalyzers, Dewatering Systems
Screen Shot of SOURCE
Screen Shot of SOURCE
Screen Shot of SOURCE
Screen Shot of SOURCE
Questions?