Criticality conditions

To illustrate calculations for a binary system containing a supercritical, condensable component. Figure 12 shows isobaric equilibria for ethane-n-heptane. Using the virial equation for vapor-phase fugacity coefficients, and the UNIQUAC equation for liquid-phase activity coefficients, calculated results give an excellent representation of the data of Kay (1938). In this case,the total pressure is not large and therefore, the mixture is at all times remote from critical conditions. For this binary system, the particular method of calculation used here would not be successful at appreciably higher pressures. 

As a liquid approaches its critical conditions, its density decreases and consequently the distance between molecules increases resulting in a rapid decrease in viscosity. 

In the startup of a reactor, it is necessary to have a source of neutrons other than those from fission. Otherwise, it might be possible for the critical condition to be reached without any visual or audible signal. Two types of sources are used to supply neutrons. The first, appHcable when fuel is fresh, is califomium-252 [13981-174-Jwhich undergoes fission spontaneously, emitting on average three neutrons, and has a half-life of 2.6 yr. The second, which is effective during operation, is a capsule of antimony and beryUium. Antimony-123 [14119-16-5] is continually made radioactive by neutron... 

Supercriticalfluid solvents are those formed by operating a system above the critical conditions of the solvent. SolubiHties of many solutes ia such fluids often is much greater than those found for the same solutes but with the fluid at sub atmospheric conditions. Recently, there has been considerable iaterest ia usiag supercritical fluids as solvents ia the production of certain crystalline materials because of the special properties of the product crystals. Rapid expansion of a supercritical system rapidly reduces the solubiHty of a solute throughout the entire mixture. The resulting high supersaturation produces fine crystals of relatively uniform size. Moreover, the solvent poses no purification problems because it simply becomes a gas as the system conditions are reduced below critical. 

An example of this is in a condenser where the corrosion probe is in a region where the temperature is lower than that at the critical condition of interest. Local scale buildup is another example of this type of situation, as is formation of a crevice at a specific location. 

Incorrect information can result if the probe is made of the wrong material and is not heat treated in the same way as the process equipment (as well as because of other problems). The probe must be as close as possible to the material from which the equipment of interest is made. Existence of a critical condition, such as weldments or galvanic couples or occluded cells in the eqmpment of concern, makes the fabrication, placement, and maintenance of the probes and monitoring system or critical importance, if accurate and useful data are to be obtained. 

Stress corrosion can arise in plain carbon and low-alloy steels if critical conditions of temperature, concentration and potential in hot alkali solutions are present (see Section 2.3.3). The critical potential range for stress corrosion is shown in Fig. 2-18. This potential range corresponds to the active/passive transition. Theoretically, anodic protection as well as cathodic protection would be possible (see Section 2.4) however, in the active condition, noticeable negligible dissolution of the steel occurs due to the formation of FeO ions. Therefore, the anodic protection method was chosen for protecting a water electrolysis plant operating with caustic potash solution against stress corrosion [30]. The protection current was provided by the electrolytic cells of the plant. 

The "critical condition" in the specification sheet is entered as the contingency which determines the valve size, i.e., fire or operating failure. 

Radiation sensor fails Fail to detect critical condition and fails to initiate bed separation High Redundant detectors are used. In addition technicians have personal audible (cherper) nioniioi s to alert to an incipient critical condition. 

Data on operating conditions can also be obtained from shift logsheets, where critical conditions are often recorded, or by conducting a survey of conditions in the control room and in the field. 

A natural ex-plosion is also an uncontrolled and unexpected event. However, unlike an accidental c.xplosion it cannot be prevented. Some e.xamples of natural explosions include lightning and olcanic eniptions. Under the right critical conditions natural e.xplosions also trigger and cause accidental explosions. 

At critical conditions, the maximum flow through the nozzle or orifice is [29]... 

The separation performance of these systems (usually low-pressure, not close to critical conditions, and with similar components) can be predicted by Raoult s Law, applying to vapor and liquid in equilibrium. 

Therefore, for flooding in vertical tubes for a range of these conditions, the tube f.D. must be greater than 0.26 in. generally, the recommendation is to use 0.5-1.0-in. f.D. tubes, approximately, to move far enough away from the critical condition. 

Gases and Vapors Hydrocarbons Reference Symbols i Chemical Formula Mol. Wt. 1r> 1/7 I a Mol. Wt. Critical Conditions Boiling Point (F) 14,7 Psia Specific Volume Cu ft/lb 14.7 Psia 81 60F (Z Facior Accounted For) Latent Heat of Vaporization (Btu/lb 14.7 Psia) Specific heat Constant Pressure (Cp 60F) Specific heat Constant Volume Specific heat ratio K = Cp/Cv... 

Margules, and Scatchard-Hildebrand) are particular mathematical solutions to Eq. (48) these models do not satisfy Eqs. (45) and (46), except in the limiting case where the right-hand sides of these equations vanish. This limiting case provides a good approximation for mixtures at low pressures but introduces serious error for mixtures at high pressures, especially near critical conditions. 

Chao and Seader assume that the partial molar volumes are independent of composition this assumption is equivalent to saying that at constant temperature and pressure there is no volume change upon mixing the pure liquid components, be they real or hypothetical. The term on the right-hand side of Eq. (46) is assumed to be zero for all temperatures, pressures, and compositions. This assumption is very poor near critical conditions, and is undoubtedly the main reason for the poor performance of the Chao-Seader correlation in the critical region. 

Besides the spontaneous, complete wetting for some areas of application, e.g., washing and dishwashing, the rewetting of a hydrophobic component on a solid surface by an aqueous surfactant solution is of great importance. The oil film is thereby compressed to droplets which are released from the surface. Hydrophobic components on low-energy surfaces (e.g., most plastics) are only re wetted under critical conditions. For a complete re wetting of a hydrophobic oil on polytetrafluoroethylene (PTFE) by an aqueous solution, the aqueous solution-oil interface tension must be less than the PTFE-oil interface tension... 

Laminar flow ceases to be stable when a small perturbation or disturbance in the flow tends to increase in magnitude rather than decay. For flow in a pipe of circular cross-section, the critical condition occurs at a Reynolds number of about 2100. Thus although laminar flow can take place at much higher values of Reynolds number, that flow is no longer stable and a small disturbance to the flow will lead to the growth of the disturbance and the onset of turbulence. Similarly, if turbulence is artificially promoted at a Reynolds number of less than 2100 the flow will ultimately revert to a laminar condition in the absence of any further disturbance. 

Similarly it may be shown that, at the critical conditions, the flowrate is a maximum for a given value of the specific energy J. At the critical velocity, (ir/gD) is equal to unity. This dimensionless group is known as the Froude number Fr. For velocities greater than the critical velocity Fr is greater than unity, and vice versa. It may be shown that the velocity with which a small disturbance is transmitted through a liquid in an open channel is equal to the critical velocity, and hence the Froude number is the criterion by which the type of flow, tranquil or rapid, is determined. Tranquil flow occurs when Fr is less than unity and rapid flow when Fr is greater than unity. 

It is thus seen that the velocity of an elementary wave is equal to the critical velocity, at which the specific energy of the fluid is a minimum for a given flowrate. The criterion for critical conditions is therefore that the Froude number, (u2/gD), must be equal to unity. 

Figure 20. Pit-dissolution current density pit radius and ion concentration buildup AC in the pit electrolyte corresponding to the critical condition for growing pits on 18Cr-8Ni stainless steel to passivate at different repassivation potentials, EK, in 0.5 kmol m 3 H2S04 + 0.5 kmol m-3 NaCl during cathodic potential sweep at different sweep rates.7 (From N. Sato, J. Electrochem. Soc. 129,261,1982, Fig. 1. Reproduced by permission of The Electrochemical Society, Inc.)...

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