Influence of the Processes Parameters on the Properties of The ...
Influence of the Processes Parameters on the Properties of The ...
Influence of the Processes Parameters on the Properties of The ...
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Chapter 2.<br />
<str<strong>on</strong>g>Processes</str<strong>on</strong>g> to Manufacture Foams and to Functi<strong>on</strong>alize <str<strong>on</strong>g>the</str<strong>on</strong>g> Surface<br />
A supercritical fluid is defined as a substance for which <str<strong>on</strong>g>the</str<strong>on</strong>g> temperature and pressure are above<br />
<str<strong>on</strong>g>the</str<strong>on</strong>g>ir critical values and which has a density close to or higher than its critical density [Darr and Poliak<str<strong>on</strong>g>of</str<strong>on</strong>g>f,<br />
1999; Span and Wagner, 1996; Angus et al., 1976]. Above <str<strong>on</strong>g>the</str<strong>on</strong>g> critical temperature, <str<strong>on</strong>g>the</str<strong>on</strong>g> vapour-liquid<br />
coexistence line no l<strong>on</strong>ger exists. <strong>The</strong>refore, supercritical fluids can be regarded as “hybrid solvents”<br />
because <str<strong>on</strong>g>the</str<strong>on</strong>g> properties can be tuned from liquid-like to gas-like without crossing a phase boundary by simply<br />
changing <str<strong>on</strong>g>the</str<strong>on</strong>g> pressure or <str<strong>on</strong>g>the</str<strong>on</strong>g> temperature. Although this definiti<strong>on</strong> gives <str<strong>on</strong>g>the</str<strong>on</strong>g> boundary values <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
supercritical state, it does not describe all <str<strong>on</strong>g>the</str<strong>on</strong>g> physical or <str<strong>on</strong>g>the</str<strong>on</strong>g>rmodynamic properties. Baldyga et al. [2004]<br />
explain <str<strong>on</strong>g>the</str<strong>on</strong>g> supercritical state differently by stating that <strong>on</strong> a characteristic microscale <str<strong>on</strong>g>of</str<strong>on</strong>g> approximately 10–<br />
100Å, statistical clusters <str<strong>on</strong>g>of</str<strong>on</strong>g> augmented density define <str<strong>on</strong>g>the</str<strong>on</strong>g> supercritical state, with a structure resembling that<br />
<str<strong>on</strong>g>of</str<strong>on</strong>g> liquids, surrounded by less dense and more chaotic regi<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> compressed gas. <strong>The</strong> number and<br />
dimensi<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g>se clusters vary significantly with pressure and temperature, resulting in high<br />
compressibility near <str<strong>on</strong>g>the</str<strong>on</strong>g> critical point. To illustrate <str<strong>on</strong>g>the</str<strong>on</strong>g> “hybrid” properties <str<strong>on</strong>g>of</str<strong>on</strong>g> supercritical fluids, Table 2.1<br />
gives some characteristic values for density, viscosity, and diffusivity. <strong>The</strong> unique properties <str<strong>on</strong>g>of</str<strong>on</strong>g> supercritical<br />
fluids as compared to liquids and gases provide opportunities for a variety <str<strong>on</strong>g>of</str<strong>on</strong>g> industrial processes.<br />
Table 2.1: Typical values <str<strong>on</strong>g>of</str<strong>on</strong>g> physical properties <str<strong>on</strong>g>of</str<strong>on</strong>g> gas, supercritical fluid and liquid.<br />
[Poling et al., 2001]<br />
<strong>Properties</strong> Gas Supercritical Fluid Liquid<br />
Density 1 100 − 800 1000<br />
Viscosity (Pa.s) 0.001 0.005 − 0.01 0.05−0.1<br />
Diffusivity D (m 2 s -1 ) 1.10 -5 1.10 -7 1.10 -9<br />
In Figure 2.16, two projecti<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> phase behaviour <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> dioxide are presented: <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
pressure-temperature (Figure A) and <str<strong>on</strong>g>the</str<strong>on</strong>g> density-pressure (Figure B) diagrams. <strong>The</strong> critical point at <str<strong>on</strong>g>the</str<strong>on</strong>g> T c<br />
critical temperature and <str<strong>on</strong>g>the</str<strong>on</strong>g> P c critical pressure marks <str<strong>on</strong>g>the</str<strong>on</strong>g> end <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> vapour-liquid equilibrium line and <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
beginning <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> supercritical fluid regi<strong>on</strong>. Density <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> pressure at different temperatures<br />
(solid lines) and at <str<strong>on</strong>g>the</str<strong>on</strong>g> vapor-liquid equilibrium line (dashed line). At <str<strong>on</strong>g>the</str<strong>on</strong>g> critical point, <str<strong>on</strong>g>the</str<strong>on</strong>g> densities <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
equilibrium liquid phase and <str<strong>on</strong>g>the</str<strong>on</strong>g> saturated vapour phases become benefits. Supercritical carb<strong>on</strong> dioxide has<br />
also desirable physical and chemical properties.<br />
Figure 2.16: Phase diagrams P-T and -P for a pure CO 2 .<br />
[Span and Wagner, 1996; Angus et al., 1976]<br />
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