Dewvaporation Carrier-Gas Enhanced Atmospheric Pressure Desalination. Noah Abbas and Kehinde Adesoye

Transcription

1 Dewvaporation Carrier-Gas Enhanced Atmospheric Pressure Desalination Noah Abbas and Kehinde Adesoye

2 Outline Existing Technologies Reverse Osmosis Thermal Processes Dewvaporation Explanation Mathematical Model Cost Calculations

3 Desalination Process of purifying seawater A solution to water shortages around the world Existing technologies Reverse Osmosis Thermal Evaporation Dewvaporation

4 Factors of Comparison Purity of water Economics Energy efficiency Production rate Regional factors Resources vary from region to region Proximity to ocean Availability of fuel

5 Reverse Osmosis Most common in the USA Solvent forced through membrane Energy consumption from pressure Susceptible to fouling, scaling and degradation

6 Process of Reverse Osmosis Pressurized feed Applied pressure > Osmotic pressure Semi-permeable membrane Incomplete salt removal (different rates)

7 Typical RO Plant Membrane Module Saline Feed Pretreatment Post Treatment Pure Water Pump Brine

8 Membrane fouling Problems Caused by micro organisms and particles Reduce water quality Add chemical e.g. chlorine Ultra-filtration of suspended solids Scaling Formation of salt precipitate e.g. CaCO 3 Reduces efficiency Add anti-scalant e.g. H 2 SO 4

9 Operating costs RO Statistics $/1000gal of product 2 Energy requirements 26 KWh/1000gal of product 2 Capital cost for sea water desalination $/gal-day 2

10 Phase separation Heat saline water/condense vapor Thermal Process (Evaporation) Saline feed Heat Fresh water Reduce pressure Energy required for heat of vaporization Large energy costs, less common in USA Pretreatment Evaporation / Condensation Column Brine

11 Multi Stage Flash Distillation 80% of world s s thermal desalination product Energy needed for heat Recycles heat Two heat sources for incoming saline feed External Heat of vaporization

12 Schematic of MSF Additional heat Condensate Trays Heat Exchanger Tubes Pressure released in first chamber Heat Vapor Saline Feed Water boils quickly Pure Water Evaporation and condensation Pump Waste Brine

13 Scale formation Extra heat transfer layer Reduces heat transferred Reduces efficiency Erosion and Corrosion Use stainless steel Problems

14 Evaporation Statistics Energy requirements 56 KWh/1000gal of product 2 Costs are very high Because of expensive energy, prices are in the range of $12 to $14 per 1000 gallons 5 in USA Only economically feasible in regions like the Middle East, where fuel is cheap and water is scarce

15 Dewvaporation Developed by James Beckman Arizona State University Relies on air circulation Air moves in a cycle Works to recycle heat Waste heat Atmospheric pressure

16 Dewvaporation Apparatus Added heat (Q boiler ) Air Evaporating water Heat Condensing water Saline feed Outlet Air P u r e w a t e r S a l i n e f e e d Ambient Air Inlet Air Blower

17 Economic Analysis The cost has two main components Operational costs associated with the heat added Heat required to created a larger temperature difference from dew formation to evaporation side Cost associated with equipment Modeled as a heat exchanger

18 Differential Analysis

19 Heat Transfer Model Region 1 G, GV z FD z Region 1 Region 2 dw d G, GV z+dz FD z+dz Mass Balance: z = GVz+ dz GV + dw d Gh a Heat Balance: ( T1 ) + GVhv ( T1 ) = Gha ( T + dt1 ) + GVhV ( T + dt1 ) + hvapdwd + hl( T1 T2 ) dz

20 Heat Transfer Model

21 Deriving Differential Equations

22 Deriving Differential Equations (Continued)

23 Equations Used

24 Solving Differential Equation in Spreadsheet

25 Heating the Air Heat needs to be added to achieve a temperature difference from dewvaporation to evaporation side Can be added as steam Adding steam keeps air saturated This made T T and G of the air stream above the tower design parameters G, GV s d Humid Air Q to achieve T G, GV s e Humid Air Dry Air Dry Air

26 G, GV s d Results of Model G, GV s e 1 Humid Air Dry Air T 5 Model considered credible if temperature profile was appropriate Temperature of evaporation side air had to reach ambient air temperature (25ºC) at bottom of column Air flow rate (G) had the most dramatic effect on product flow and heating requirements

27 Temperature Profile Temperature Down the Tower T Temperature (deg. C) T Air Dewformation Side T Pure Water Product T Seawater/Brine T Air Evaporation Side Distance from Tow er Top (cm)

28 Equipment Cost and Energy Cost Calculations Design G mol/h Qboiler J/hour FD gal/day FB gal/day FAC $ Operating Cost $/1000gallons $1, $ $1, $ $1, $ $1, $ $1, $ $1, $0.79

29 Cost $/1000gallons Costs $/1000gallons vs. Flow Rate $/1000gallons Flow Rate (gallons/day)

30 Cost of Energy $/day Equipment Cost and Energy Cost Energy Cost Vs. Air Flow Flow Rate of Air Mol/hr vs. Air Flow Energy cost goes up sharply More air to heat Equipment Cost $ Equipment Cost vs. Air Flow Air Flow Mol/hr Equipment cost increases More expensive blower Slightly higher tower

31 Fixed Annualized Cost FAC vs. Air Flow FAC $ Air Flow mol/hr 10 years of operation Production of 200 to 1200 gal/day

32 Conclusions Dewvaporation is on the low end of costs for current desalination technologies Flow rates similar to Beckman s s had similar costs This is in the $1.70 to $3.70/1000gallon range Most effective in places like Arizona where the air is dry