Superheated Water

Extraction (SWE)

A. Ahmadpour

Chemical Eng. Dept.

Ferdowsi University of Mashhad

2

Contents

Introduction

Change of properties with temperature

Explanation of anomalous behavior

Effect of pressure

Solubility in superheated water

Separations Using Superheated Water

Applications of superheated water extraction

Comparisons with other extraction methods

Equipment

Conclusion

3

References

1. Green Separation Processes. Edited by C. A. M. Afonso and J. G. Crespo, 2005

2. US patent website, http://patft.uspto.gov

3. R.M. smith/ J. Chromatogr. A 975 (2002) 31-46

4. http://www.wikipedia.org/superheated-water.mht

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Introduction

We will concentrate on work using

superheated water as a replacement for

organic solvents for extractions,

chromatography and related processes

5

Cont.

? What is superheated water?

Superheated water is liquid water under

pressure at temperatures between the usual

boiling point (100°C) and the critical

temperature (374°C).

6

Cont.

The pressures required to maintain a condensed state of water are moderate, 15 bar at 200°C and 85 bar at 300°C

Note:

If the pressure drops below the boiling point at any pressure, superheated steam is formed.

That behaves quite differently as an extraction solvent to superheated water.

7

Change of properties with

temperature

The properties of all materials change with

temperature,

but water shows changes which are much

greater than would be expected from

temperature considerations alone.

8

Cont.

Viscosity

Surface tension of drops

Diffusivity

Specific heat capacity @ p=cte

Dielectric constant

When T

9

Cont.

Specific heat capacity at constant pressure

increases with temperature, from 4.187 kJ/kg

at 25°C to 8.138 kJ/kg at 350°C.

10

Cont.

The dielectric constant (relative permittivity)

decreases significantly as the temperature

rises, which has a significant effect on the

behavior of water at high temperatures.

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Explanation of anomalous behavior

Many of the anomalous properties of water are

due to very strong hydrogen bonding.

12

Cont.

Over the superheated temperature range, the extensive hydrogen bonds break down.

Water effectively becomes less polar and behaves more likean organic solvent such as methanol or ethanol.

Solubility of organic materials and gases increases by several orders of magnitude.

13

Cont.

Water is a polar molecule

In an applied electric field, the molecules align with the

field

In water, the extensive hydrogen bonded network tends

to oppose this alignment, and the degree to which this

occurs is measured by the relative permittivity

(dielectric constant)

14

Cont.

Because of its extensive hydrogen bonds, water has a high relative permittivity, about 80 at room temperature (Ɛ=80). This allows water to dissolve salts.

As the temperature increases, the thermal motion of the molecules disrupts the hydrogen bonding network, and therefore the relative permittivity

decreases with temperature, to about 7 at the critical temperature.

15

Cont.

At 205°C the relative permittivity has fallen to 33

(Ɛ=33), the same as methanol at room temperature.

Thus, from 100°C to 200°C water behaves like a

water / methanol mixture.

16

Cont.

Green Separation Processes. Edited by C.A.M. Afonso and J.G. Crespo

17

Effect of pressure

At temperatures below 300°C water is fairly

incompressible, which means that:

Pressure has little effect on the physical properties of

water, provided it is sufficient to maintain liquid state.

This pressure is given by the saturated vapor pressure, and

can be looked up in steam tables.

18

Cont.

For example:

the saturated vapor pressure at :

121°C is 100 kPa

150°C is 470 kPa

200°C is 1550 kPa

The critical point is 21.7 MPa at a temperature

of 374°C.

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Solubility in superheated water

Organic compounds

Salts

Gases

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Solubility of organic compounds

Organic molecules often show a dramatic increase in solubility in water as the temperature rises. There are 2 reasons for this behavior:

I. the polarity change

II. the solubility of sparingly soluble materials tends to increase with temperature as they have a high positive

enthalpy of solution

21

Cont.

Some organic compounds which can dissolve

in superheated water are:

Polycyclic aromatic hydrocarbon (PAHs)

Naphthalene

Polychlorinated biphenyl (PCBs)

22

Cont.

The solubility of PAH’s

increased by 5 orders of

magnitude from 25°C to

225°C and naphthalene,

for example, forms a 10

wt% solution in water at

270°C.

Green Separation Processes. Edited

by C.A.M. Afonso and J.G. Crespo

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Solubility of salts

Despite the reduction in relative permittivity, many saltsremain very soluble in superheated water until the critical point is approached.

For example:

Sodium chloride, dissolves 37 wt% at 300°C

Note:As the critical point is approached, the solubility drops markedly to a few ppm, and salts are hardly soluble in supercritical water.

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Solubility of Gases

The solubility of gases in water is usually thought to decrease with temperature, but this only occurs to a certain temperature, then solubility increases again.

For example:

For nitrogen, this minimum is 74°C and for oxygen it is 94°C

Therefore, gases are quite soluble in superheated water at elevated pressures.

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Separations Using Superheated

Water

History

Superheated water extraction

Applications of superheated water

extraction

Comparisons with other extraction methods

Equipment

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History

Liquid water at elevated temperatures above its boiling pointhas been used for many years as an industrial solvent and cleaning agent in applications ranging from:

enhancing the extraction of oil shale

the extraction of sulphur from ore bodies to degreasing

As a vapor, steam is commonly used in :

hydro distillation for the isolation of volatile constituents of plant materials to provide essential oils of value in perfumery.

27

Cont.

The recent analytical interest in superheated

water as an extraction solvent began with the

work of Hawthorne who was interested in

environmentally friendly extraction methods

for soils and environmental solids.

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Superheated water extraction

a) Extraction using superheated water tends to be fast because diffusion rates increase with temperature.

b) Organic materials tend to increase in solubility with temperature.

Therefore,

extraction with superheated water can be both selective and rapid.

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Applications of superheated water

extraction

Examples of separations include:

1) the removal of pesticides from contaminated soil

including removal in situ,

2) the removal of organic pollutants from

wastewater,

3) the extraction of organic compounds from solids,

4) the extraction of compounds from solids coupled

with degradation,

30

Cont.

5) the extraction and degradation of chemical warfare

agents,

6) the extraction of synthesis contaminants and organic

compounds from polymers or plastics,

7) the extraction of biologically active organic

compounds from plant tissue,

8) the extraction and reaction of compounds from plant

tissue to produce flavors and fragrances,

9) as a mobile phase for liquid chromatography.

31R.M. smith, J. Chromatogr. A 975 (2002) 31-46

32

R.M. smith, J. Chromatogr. A 975 (2002) 31-46

33

R.M. smith, J. Chromatogr. A 975 (2002) 31-46

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R.M. smith, J. Chromatogr. A 975 (2002) 31-46

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Comparisons with other extraction

methods

Many of the reports have compared SWE

with previously reported methods, such as:

Soxhlet extraction, SFE and steam distillation

for plant materials.

For most environmental samples the results

were generally similar to previous methods

although there were some interesting

variations.

36

Cont.

All of these methods had attributes but water

was judged the best overall because of:

Its low price

Good availability

Environmental safety

Lower energy requirements

37

Cont.

The energy required to heat water is significantly

lower than that needed to vaporize it (for example for

steam distillation), and the energy is easier to recycle

using heat exchangers.

To heat water at 25°C to steam at 250°C and 1 atm

requires 2869 kJ/kg.

To heat water at 25°C to liquid water at 250°C and

50 atm requires only 976 kJ/kg.

38

Cont.

Therefore:

the energy use for superheated water

extraction is less than one sixth needed for

steam distillation.

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Equipment

Most SWE have employed relatively simple home made equipment because pressure is not a critical factor in SWE due to the low compressibility of water over the typical temperature ranges.

Therefore, the pressure control can be very basic and accurate measurement and control is not required.

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Cont.Laboratory-scale Extraction

Green Separation Processes. Edited by C.A.M. Afonso and J.G.

Crespo

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Cont.Pilot-plant Equipment

Green Separation Processes. Edited by C.A.M. Afonso and J.G.

Crespo

42

Conclusion

Superheated water extractions have been shown to be

feasible with particular interest in avoiding the need

for organic solvents in environmental extractions or

in pharmaceutical or food samples.

The method is thus environmentally friendly, cheap

and nontoxic.

The equipment required is relatively simple and

avoids the need for the high pressures employed in

SFE.

43

Cont.

unlike carbon dioxide, there is no problems

with cooling and condensation.

Most samples have been solid matrices, such

as soils and plant materials.