in black and white
Main menu
Home About us Share a book
Biology Business Chemistry Computers Culture Economics Fiction Games Guide History Management Mathematical Medicine Mental Fitnes Physics Psychology Scince Sport Technics

Supercritical fluid cleaning - McHardy J.

McHardy J., Sawan P.S. Supercritical fluid cleaning - Noyes publications, 1998. - 304 p.
Download (direct link): spercrificalfluidcleaning1998.pdf
Previous << 1 .. 2 3 4 5 < 6 > 7 8 9 10 11 12 .. 101 >> Next

Figure 1. The schematic temperature dependence of the chemical potential of the solid, liquid, and gas phases. The phases with the lowest chemical potential are most stable at that temperature (shaded area).
Figure 2 is a phase diagram or temperature-pressure chart showing the phase transitions for a substance such as carbon dioxide. It is a graphical way to summarize the conditions under which the different states of that substance are stable. It maps the ranges or regions of pressure and temperature over which the various phases are thermodynamically stable (i.e., with lowest Gibbs energy, AG = AH - TAS). The phase boundaries separating different regions delineate the values of P and T over which two phases coexist in equilibrium. As one can see, the solid-liquid phase boundary is a plot of the freezing point at various pressures, the liquid-vapor boundary is a plot of vapor pressure of the liquid against temperature, and the solid-vapor boundary is a plot of the sublimation vapor pressure against temperature. Solid, liquid and gas phases are all in the equilibrium at the intersection of these three boundaries, known as the triple point.
Figure 2. A phase diagram showing the different phases and supercritical region. The critical point, critical pressure, and critical temperature are indicated.
Figure 3 shows a series of isotherms (constant temperature lines) for a substance close to its critical point. Consider what happens when the volume of a sample of gas in a closed cylinder (initially in the state marked A in Fig. 3) is compressed at constant temperature by applying pressure with a piston. Near the point A, pressure of the gas rises in approximate agreement with Boyles law (the pressure and volume of a fixed amount of gas at constant temperature are related by Boyles law. PV = constant). When the volume has been reduced to B, serious deviations from that law begin to appear. At a point (-60 atm for C02), all similarity to ideal gas behavior is lost and suddenly the piston slides in without any further rise in pressure; this state is represented by the horizontal dashed line CDE. Examination of the vessel reveals that just to the left of a liquid appears, and there are two phases separated by a sharply defined surface. As the volume is decreased from to ? through D, the amount of liquid increases and the gas condenses without any further resistance. The pressure corresponding to the line CDE, when both liquid and vapor are present in equilibrium, is called the vapor pressure of liquid at the experimental temperature. At point E, the sample is entirely liquid and any further reduction of volume requires the exertion of considerable pressure, as is indicated by the sharply rising line to the left of E. Note that even a small reduction of volume from EtoF requires a great increase in pressure.
The isotherm (plot of pressure versus volume) at the temperature Tc plays a special role in the theory of the states of matter. An isotherm behaves in accordance with the gas laws slightly below Tc. At certain pressure, a liquid condenses from gaseous state and is distinguishable from it by the presence of a visible interface. If, however, the compression takes place at 7^ a surface separating two phases does not appear and the volumes at each end of the horizontal part of the isotherm have merged to a single point, critical point of the gas. The temperature, pressure, and molar volume at the critical point are called the critical temperature critical pressure Pc, and
critical molar volume Vc of the substance respectively. Collectively, Pc, and Tc are the critical constants of a substance.
Vm/(L 1)
Figure 3. Experimental isotherms of carbon dioxide at several temperatures. The critical isotherm is the isotherm at the critical temperature (31.04C).
At and above Tc, the sample has a single phase that occupies the entire volume of the container. Thermodynamically such a phase is, by definition, a gas. However, the unusual physical properties of a substance close to its critical point warrant special terminology. The single phase that fills the entire volume 3tT>Tc may be much denser than one normally considers typical of gases, and the name supercritical fluid is preferred and the area above and to the right of the critical point in Fig. 2 is known as the supercritical region. A fluid in this region is neither a liquid nor gasbut possesses some of the properties of each. At this stage, no matter how much pressure is applied, a fluid in this region will not condense and no matter how much the temperature is increased, it will not boil. Since these materials are neither liquid or gas, we call them fluids.
The significance of the supercritical state may also be appreciated by comparing liquid-vapor transitions at constant pressure (open vessel) and constant volume (closed vessel). The behavior of a liquid heated in an open vessel differs significantly from that of a liquid in a sealed vessel (Fig. 4). In an open vessel, the liquid vaporizes from its surface as it is heated. The vaporization can occur throughout the bulk of the liquid at the temperature at which its vapor pressure would be equal to the external pressure, and the vapor can expand freely into the surroundings. The condition of free vaporization throughout the liquid is called boiling. Temperature at which the vapor pressure of a liquid is equal to the external pressure is called the boiling temperature at that pressure. Note that a liquid does not suddenly start to form a vapor at its boiling temperature; even at lower temperatures there is an equilibrium between the liquid and its vapor. At the boiling point, the vapor pressure is great enough to overcome atmospheric pressure and vaporization can occur freely. For the special case of an external pressure of 1 atm, the boiling temperature is called the normal boiling point.
Previous << 1 .. 2 3 4 5 < 6 > 7 8 9 10 11 12 .. 101 >> Next