Superheated Water Extraction (SWE)
A. Ahmadpour Chemical Eng. Dept. Ferdowsi University of Mashhad 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
2 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
3 Introduction
We will concentrate on work using superheated water as a replacement for organic solvents for extractions, chromatography and related processes
4 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).
5 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.
6 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.
7 Cont.
Viscosity Surface tension of drops Diffusivity When T Specific heat capacity @ p=cte
Dielectric constant
8 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.
9 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.
10 Explanation of anomalous behavior
Many of the anomalous properties of water are due to very strong hydrogen bonding.
11 Cont.
Over the superheated temperature range, the extensive hydrogen bonds break down.
Water effectively becomes less polar and behaves more like an organic solvent such as methanol or ethanol.
Solubility of organic materials and gases increases by several orders of magnitude.
12 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)
13 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.
14 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.
15 Cont.
Green Separation Processes. Edited by C.A.M. Afonso and J.G. Crespo 16 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.
17 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.
18 Solubility in superheated water
Organic compounds
Salts
Gases
19 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
20 Cont.
Some organic compounds which can dissolve in superheated water are: Polycyclic aromatic hydrocarbon (PAHs)
Polychlorinated biphenyl (PCBs)
21 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 22 Solubility of salts
Despite the reduction in relative permittivity, many salts remain 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.
23 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.
24 Separations Using Superheated Water
History Superheated water extraction Applications of superheated water extraction Comparisons with other extraction methods Equipment
25 History
Liquid water at elevated temperatures above its boiling point has 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.
26 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.
27 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.
28 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,
29 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.
30 R.M. smith, J. Chromatogr. A 975 (2002) 31-46 31 R.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
34 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.
35 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
36 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.
37 Cont.
Therefore: the energy use for superheated water extraction is less than one sixth needed for steam distillation.
38 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.
39 Cont.
Laboratory-scale Extraction
Green Separation Processes. Edited by C.A.M. Afonso and J.G. Crespo 40 Cont.
Pilot-plant Equipment
Green Separation Processes. Edited by C.A.M. Afonso and J.G. 41 Crespo 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.
42 Cont.
unlike carbon dioxide, there is no problems with cooling and condensation. Most samples have been solid matrices, such as soils and plant materials.
43