Forward Osmosis Energy Use Comparisons to RO, MSF, MED

R. McGinnis, J. McCutcheon M. ElimelechDept. of Chemical EngineeringEnvironmental Engineering ProgramYale University

NAMS 2007, Orlando, FL

Outline• Overview of NH3 /CO2 FO

• Solute Removal / Recycle Methods

• Modeling Methods

• FO Energy Estimates

• Comparison to RO, MSF, MED

• Impact of Membrane Efficiency on Heat Use

The Ammonia-Carbon Dioxide FO Process

Reverse OsmosisSaline Water

Brine

Membrane

Product Water

HP Pump

EnergyRecovery

EnergyInput ( )πσΔ−Δ= PAJw

Forward Osmosis Process

EnergyInput

( )PAJw Δ−Δ= πσ0

Saline Water

Draw SolutionNH3 /CO2

ProductWater

Brine

Membrane

Draw Solute Recovery

Potential Benefits• Low Energy Costs

– Can use low temperature heat– Low cost of heat favors low total water costs

• Lower Energy Use than Other Thermal Methods– Phase change of solute, not solvent

• High Feedwater Recovery – No osmotic pressure limitation to recovery– Pretreatment to prevent scaling will be limiting factor

• Reduced Brine Discharge

• May be Effective for Difficult Feedwaters– FO uses so far: food concentration, landfill leachate treatment,

anaerobic sludge centrate treatment

Solute Removal / Recycle Methods

Seawater Distillation1≈=

steam

water

KgKgGOR

Image from Wikipedia

steam

water

KgKgGOR =

Multi-Stage Distillation

128−≈=steam

water

KgKgGOR

steam

water

KgKgGOR =

Image from Encarta Encyclopedia Online

MED DesalinationTemperature,

Pressure

Brine

Product Water

Feed

Energy

148−≈=steam

water

KgKgGOR

steam

water

KgKgGOR =

Forward Osmosis Process

DistillationColumn

Saline Water

Draw SolutionNH3 /CO2

ProductWater

Brine

Membrane

Draw Solute Recovery

Single Distillation Column

Reboiler

Column

Product Water

To Membrane System

From Membrane System

Heat Duty (Steam in, Condensate out)

Multi-Stage Column Distillation

• Benefits from Heat Reuse Efficiencies of MSF / MED

• Reduces Quantity of Energy Required by 60-70%

• May use Higher Temperatures than MSF or MED (to increase range of increased efficiency)

Multiple Column OperationTemperature,

PressureFrom Membrane System

Reboiler

Reboiler

ColumnProduct Water

To Membrane System

ColumnProduct Water

To Membrane System

From Membrane System

Heat DutyFrom Membrane System

Multi-Stage Column Distillation (MSCD)

Product Water

Feed

Energy

Draw Solution

Temperature, Pressure

McGinnis, Elimelech, “Energy Requirements of Ammonia–Carbon Dioxide ForwardOsmosis Desalination”, Desalination, 207 (2007) 370-382.

Modeling Methods

• AspenTech Hysys Chemical Process Modeling Software

• OLI Electrolyte Property Database (high ionic strength)

• Draw Solution Concentrations Based on Experimental Flux Data

Hysys / OLI

50 100 150 200 2500

100

200

300

400 MED MSF FO MSCD

Hea

t Dut

y (M

J/m

3 )

Heat Temp. (oC)

Heat Duty versus Temperature of Heat

MED plot based on Performance Ratios (lbs water / 1000 BTU) of 8 - 14.73 for heat temperatures of 70-131 oC; MSF for GOR of 12 for 115 oC (from Morin, O.J., Design and Operating Comparison of MSF and MED Systems. Desalination,93(1-3) (1993) 69-109)

Electrical Power:MED = 1.60 kWh/m3

MSF = 2.65 kWh/m3

FO = 0.24 kWh/m3

0 50 100 150 200 25005

101520253035

GO

R

Heat Temp. (oC)

Heat Duty vs. Temp. of Heat

( ) ( )[ ](kJ/kg)FODuty Heat

(kJ/kg)35 @ water HsteamHGOR0C−

=steam

water

KgKgGOR =

Equivalent WorkDecreasing Steam Pressure, Decreasing Ability to do Work

[ ] electurbineoutinC

waterequiv W

MJkWhEHH

GORmW +⎟

⎠⎞

⎜⎝⎛ ×××−⎟

⎟⎠

⎞⎜⎜⎝

⎛= −6

35

3

1077.2)(0

GOR =Kgwater

Kgsteam

LP Steam TurbineSteam In Steam Out

Condenser

Steam to Desalination Process

Mechanical work not done by steam

[ ] electurbineoutinC

waterequiv W

MJkWhEHH

GORmW +⎟

⎠⎞

⎜⎝⎛ ×××−⎟

⎟⎠

⎞⎜⎜⎝

⎛= −6

35

3

1077.2)(0

0 50 100 150 200 2500

1

2

3

Equi

v. W

ork

(kW

h/m

3 )

Heat Temp. (oC)

Equivalent Work vs Heat Temperature (FO MSCD)

RO

0

1

2

3

4

5

6

kW

h/m

3

MSF MED-TVC MED-LT RO FO-LT

Comparison of Desalination Technologies Based on Equivalent Work

Contribution fromElectrical Power

Draw Concentration vs. Energy Use

0.5 1.0 1.50

10

20

30

40 160 C 40-44C

GO

R

Draw Conc. (M)

Adapted from: McGinnis, Elimelech, “Energy Requirements of Ammonia–Carbon Dioxide Forward Osmosis Desalination”, Desalination, 207 (2007) 370-382.

0.5 1.0 1.50

1

2

3

4 160 C 40-44 C

Equ

iv. W

ork

(kW

h/m

3 )

Draw Conc (M)

Concluding Remarks• Availability of heat determines best

desalination method– If heat is available for cogeneration, FO is likely

preferable to RO in energy cost– If only electricity or fuel is available, RO is best

• Multiple columns useful to produce the most water yield for a given heat source

• FO membrane performance drives energy use of FO

Acknowledgements

• Funding by the Office of Naval Research

• Hydration Technologies

• Dow Filmtec

• Cascade Designs