Dewvaporation Dewvaporation “Carrier Carrier- -Gas Enhanced Atmospheric Gas Enhanced Atmospheric “ Pressure Desalination” ” Pressure Desalination ���������������������� �������
Outline Outline � Existing Technologies Existing Technologies � � Reverse Osmosis Reverse Osmosis � � Thermal Processes Thermal Processes � � Dewvaporation Dewvaporation Explanation Explanation � � Mathematical Model Mathematical Model � � Cost Calculations Cost Calculations �
Desalination Desalination � Process of purifying seawater Process of purifying seawater � � A solution to water shortages around the A solution to water shortages around the � world world � Existing technologies Existing technologies � � Reverse Osmosis Reverse Osmosis � � Thermal Evaporation Thermal Evaporation � � Dewvaporation Dewvaporation �
Factors of Comparison Factors of Comparison � Purity of water Purity of water � � Economics Economics � � Energy efficiency Energy efficiency � � Production rate Production rate � � Regional factors Regional factors � � Resources vary from region to region Resources vary from region to region � � Proximity to ocean Proximity to ocean � � Availability of fuel Availability of fuel �
Reverse Osmosis Reverse Osmosis � Most common in the Most common in the � USA USA � Solvent forced through Solvent forced through � membrane membrane � Energy consumption Energy consumption � from pressure from pressure � Susceptible to fouling, Susceptible to fouling, � scaling and degradation scaling and degradation
Process of Reverse Osmosis Process of Reverse Osmosis � Pressurized feed Pressurized feed � � Applied pressure > Applied pressure > � Osmotic pressure Osmotic pressure � Semi Semi- -permeable permeable � membrane membrane � Incomplete salt removal Incomplete salt removal � (different rates) (different rates)
Typical RO Plant Typical RO Plant Membrane Module Saline Feed Pure Water Pretreatment Post Treatment Pump Brine
Problems Problems � Membrane fouling Membrane fouling � � Caused by micro organisms and particles Caused by micro organisms and particles � � Reduce water quality Reduce water quality � � Add chemical e.g. chlorine Add chemical e.g. chlorine � � Ultra Ultra- -filtration of suspended solids filtration of suspended solids � � Scaling Scaling � � Formation of salt precipitate e.g. CaCO Formation of salt precipitate e.g. CaCO 3 � 3 � Reduces efficiency Reduces efficiency � � Add anti Add anti- -scalant scalant e.g. H e.g. H 2 SO 4 2 SO � 4
RO Statistics RO Statistics � Operating costs Operating costs � 2 • 2.50 2.50 – – 4.00 $/1000gal of product 4.00 $/1000gal of product 2 • � Energy requirements Energy requirements � • 26 KWh/1000gal of product 26 KWh/1000gal of product 2 2 • � Capital cost for sea water desalination Capital cost for sea water desalination � 2 • 4.00 4.00 - - 10.00 $/gal 10.00 $/gal- -day day 2 •
Thermal Process Thermal Process (Evaporation) (Evaporation) Fresh water � Phase separation Phase separation � Heat � Heat saline Heat saline � water/condense vapor water/condense vapor Saline feed � Reduce pressure Reduce pressure � Evaporation / Condensation Pretreatment Column � Energy required for heat Energy required for heat � of vaporization of vaporization � Large energy costs, less Large energy costs, less � common in USA common in USA Brine
Multi Stage Flash Distillation Multi Stage Flash Distillation � 80% of world 80% of world’ ’s thermal s thermal � desalination product desalination product � Energy needed for heat Energy needed for heat � � Recycles heat Recycles heat � � Two heat sources for Two heat sources for � incoming saline feed incoming saline feed � External External � � Heat of vaporization Heat of vaporization �
Schematic of MSF Schematic of MSF Heat Exchanger Tubes Condensate Trays � Additional heat Additional heat � Saline � Pressure Pressure Feed � released in first released in first chamber chamber Heat Vapor Pure � Water boils Water boils � Water quickly quickly Pump � Evaporation and Evaporation and � Waste condensation condensation Brine
Problems Problems � Scale formation Scale formation � � Extra heat transfer layer Extra heat transfer layer � � Reduces heat transferred Reduces heat transferred � � Reduces efficiency Reduces efficiency � � Erosion and Corrosion Erosion and Corrosion � � Use stainless steel Use stainless steel �
Evaporation Statistics Evaporation Statistics � Energy requirements Energy requirements � 2 • 56 KWh/1000gal of product 56 KWh/1000gal of product 2 • � Costs are very high Costs are very high � • Because of expensive energy, prices are in the Because of expensive energy, prices are in the • 5 in USA range of $12 to $14 per 1000 gallons 5 in USA range of $12 to $14 per 1000 gallons • Only economically feasible in regions like the Only economically feasible in regions like the • Middle East, where fuel is cheap and water is Middle East, where fuel is cheap and water is scarce scarce
Dewvaporation Dewvaporation � Developed by James Developed by James � Beckman Beckman � Arizona State University Arizona State University � � Relies on air circulation Relies on air circulation � � Air moves in a cycle Air moves in a cycle � � Works to recycle heat Works to recycle heat � � Waste heat Waste heat � � Atmospheric pressure Atmospheric pressure �
Dewvaporation Apparatus Added heat ( Q boiler ) P S u a r l e i Air n w e Evaporating water a t f Heat e e e r Condensing water d Saline feed Outlet Air Inlet Ambient Air Air Blower
Economic Analysis Economic Analysis � The cost has two main components The cost has two main components � � Operational costs associated with the heat Operational costs associated with the heat � added added � Heat required to created a larger temperature Heat required to created a larger temperature � difference from dew formation to evaporation difference from dew formation to evaporation side side � Cost associated with equipment Cost associated with equipment � � Modeled as a heat exchanger Modeled as a heat exchanger �
Differential Analysis Differential Analysis
Heat Transfer Model Region 1 FD z G, GV z Region 1 Region 2 dW d FD z+dz G, GV z+dz Mass Balance: = + GV GV dW + z z dz d Heat Balance: ( ) ( ) ( ) ( ) ( ) + = + + + + ∆ + − Gh T GVh T Gh T dT GVh T dT h dW hL T T dz 1 1 1 1 1 2 a v a V vap d
Heat Transfer Model Heat Transfer Model
Deriving Differential Equations Deriving Differential Equations
Deriving Differential Equations Deriving Differential Equations (Continued) (Continued)
Equations Used Equations Used
Solving Differential Equation in Solving Differential Equation in Spreadsheet Spreadsheet
Heating the Air Heating the Air � Heat needs to be added to achieve a Heat needs to be added to achieve a � temperature difference from dewvaporation dewvaporation to to temperature difference from evaporation side evaporation side � Can be added as steam Can be added as steam � � Adding steam keeps air saturated Adding steam keeps air saturated � ∆ T and G of the air stream above the This made ∆ � This made T and G of the air stream above the � tower design parameters tower design parameters Q to achieve � T Humid Air G, G, Humid Air GV s GV s e d 1 2 3 4 5 Dry Air Dry Air
Results of Model Results of Model G, G, GV s GV s e d 1 Humid 2 3 4 5 Dry Air Air T 5 � Model considered credible if temperature profile was Model considered credible if temperature profile was � appropriate appropriate � Temperature of evaporation side air had to reach ambient air Temperature of evaporation side air had to reach ambient air � temperature (25º temperature (25 ºC) at bottom of column C) at bottom of column � Air flow rate (G) had the most dramatic effect on product Air flow rate (G) had the most dramatic effect on product � flow and heating requirements flow and heating requirements
Temperature Profile Temperature Profile Temperature Down the Tower 80.00 70.00 ∆ T 60.00 Temperature (deg. C) 50.00 T Air Dewformation Side T Pure Water Product 40.00 T Seawater/Brine T Air Evaporation Side 30.00 20.00 10.00 0.00 0 100 200 300 400 500 600 700 800 Distance from Tower Top (cm)
Equipment Cost and Energy Cost Equipment Cost and Energy Cost Calculations Calculations Qboiler Operating Cost Design G mol/h FD gal/day FB gal/day FAC $ J/hour $/1000gallons 1 1000 290700 187.85 954.77 $1,557.14 $5.86 2 2000 581400 272.06 870.56 $1,665.91 $4.27 3 3000 872100 562.05 580.56 $1,725.26 $1.81 4 4000 1162800 749.33 393.29 $1,773.35 $1.30 5 5000 1453500 936.60 206.01 $1,830.24 $1.00 6 6000 1744200 1123.76 18.85 $1,867.10 $0.79
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