Pressure crossover

The crossover value and the chamber volume V result in the crossover pressure p to which the vacuum chamber must be evacuated first before opening the valve leading to the cryopump. The following may serve as a guide ... 

The time which a system takes to achieve a pressure (the crossover pressure pj at which the high vacuum pump can be switched into the system is given by Equation (1.5) ... 

It is necessary to observe, in particular, the effect of temperature on the solubility. At pressures below a given value, which is typical for each binary solute-solvent system, the solubility increases with decreasing temperature. At higher pressures the opposite effect is observed. This characteristic pressure is normally referred to as crossover pressure and it is very important when a process involving solids must be optimized. 

Crossover pressure. If a refrigerator cryopump is attached to a vacuum system via a valve and the pump is already cold, the crossover pressure... 

A common feature of solubility data for single solutes dissolved in a supercritical fluid is the existence of what has been termed crossover pressure (Chimowitz and Pennisi 1986). This is a pressure around which isotherms at various near-critical temperatures tend to converge. Figure 3 shows this behavior for phenanthrene solubility in carbon dioxide. Below the crossover pressure, an isobaric increase in temperature causes a solubility decrease, so that the solute is retrograde. 

FIGURE 3 Mole fraction solubility (y) of phenanthrene in supercritical carbon dioxide versus pressure. (From Anitescu, G. and Tavlarides, L. L., J. Supercrit. Fluids, 10(3), 175-189, 1997.). Three isothermal lines cross at the crossover pressure of 165 bar. 

Above the crossover pressure, the opposite eifect occurs. This behavior can be understood by considering two opposing effects of temperature on solubility (Chimowitz 2005). The vapor pressure of the solid solute always increases with temperature, while the density (or solvent power) of supercritical carbon dioxide decreases. Below the crossover pressure where the compressibility is larger, the density effect dominates, and the solubility decreases with increasing temperature. At pressures above the crossover pressure, the vapor-pressure effect dominates hence solubility increases with temperature. 

The effect of temperature on solubility is more complex and involves both a consideration of the solute vapor pressure as well as the density of the SCF. The solubility isotherms shown in Figure 1.2-9 are typical of most solid-SCF systems in that they intersect within a narrow range of pressure. For any two isotherms, the point of intersection, or crossover pressure, represents a change in the temperature dependence of solubility. 

The region of pressure below the crossover pressure is known as the retrograde region. In this range of pressure, solubility decreases with an increase in temperature because the density of the SCF falls sharply. The decrease in density is sufficient to overcome any increases in solute vapor pressure that would normally lead to an increase in solubility. Above the crossover pressure, the decrease in solvent density is less sensitive to temperature and so solubility increases with temperature because the vapor pressure effect becomes dominant. 

The solubility isotherms cross one another. Namely, at pressures roughly ranging from 40 bar to 170-180 bar (zone B of the plot) a retrograde behavior is seen, where the solubility is decreased by a temperature increase, whereas above the crossover pressure the usual dependence is encountered (zone C of the plot). 

Solubility properties of the solute in SC-CO2 depend on the extraction temperature and pressure, which are the main variables that affect the efficiency of the extraction process. It is well known that extraction yield increases with an increase in pressure if other factors are fixed, due to the inCTease in density. Temperature, however, has the opposite effects on extraction yields. An increase in temperature results in a reduction in flnid density that negatively affects the extraction yield. On the other hand, increasing the tanperature also increases the solute vapor pressure, which enhances the solubility. At a crossover pressure, where the temperature does not show any balanced effect, the two competing effects are equal. At lower pressures than the crossover pressure, the change in density is predominant at higher pressures the vapor pressure is predominant. 

When there are two solid solutes (extracted into the SCF) each having a different crossover pressure, we have a crossover regime (Figure 4.1.17). At an intermediate pressure between the two crossover pressures PJ and Pj for solutes 1 and 2, considerable selectivity can be developed between the two solutes by changing the temperature. Suppose the SCF is initially at Th and is then cooled to a temperature Ti, [equilibrium solubility of species 2 is increased. On the... 

Figure 20.1.9. The crossover pressure and retrograde behavior supercritical fluid and two solutes.

Time to crossover pressure (roughing to high vacuum pumping)... 

Time to reach the crossover pressure i.e., the pressure at which point the system is changed from roughing to high vacuum pumping. 

Crossover pressure (vacuum technology) The chamber pressure at which the vacuum pumping system is switched from the rough pumping mode (roughing) to the high vacuum pumping mode. 

Rough vacuum (vacuum technology) Pressure from atmospheric to about SOmTorr. Alternatively, pressure from atmospheric pressure to the crossover pressure. See also Crossover pressure. 

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