Near critical state

By adding propane to the reaction mixture, we have attained extremely high reaction rates, up to 1000 times higher than with the traditional technique. These results occur when the whole mixture of propane, oil and hydrogen is in a supercritical or near-critical state. 

In supercritical extraction, a compressed gas at a temperature above and near its critical temperature is used as a solvent to dissolve a solid component. At near-critical states, molecular clusters are formed in abundance the formation of clusters of small solvent molecules about a large solute molecule effectively contributes to the solvent power of the compressed gas. The fugacity coefficient ([) is reduced, and the solubility y is raised. 

U.S. 4,147,624 Wastewater treatment with desorbing of an adsorbate from an adsorbent with a solvent in a near critical state, Apr. 3, 1979. 

Thompson, P.A. Kim, Y.-G. and Chan, Y. Nonequilibrium, near-critical states in shock-tube experiments. Shock Tubes and Waves (H. Gronig, ed.), 343-349, VCH Publ. 1987. 

In the third case, beginning from a supercritical state on the critical isentrope, a reversible expansion wave of suitable amplitude will produce a stationary near-critical state by reflection from the endwall. This process has the advantage that the nominally uniform, near-critical state may be long in duration. An interesting feature is that the (ideal) termination reflected wave travels at the critical soundspeed. As the critical point approached, the isochoric heat capacity increases asymptotically according to the power law... 

Figure 1. Idealized space-time wave diagrams for the creation of a near - critical state in a shock tube (a) Direct shock compression in a retrograde fluid (b) Shock compression with reflection in a retrograde fluid (c) Isentropic expansion with reflection in a regular or retrograde fluid.

Having included the extra parameter in the model for the influence parameter, the calculated interfacial tensions appear to be much more in accordance with the experimental tensions, which is shown in Figures 2a and b. Comparison of the computed interfacial tensions of the APACT model with those obtained from the PR equation of state shows that the APACT yields similar results. However, the results obtained with the APACT at near critical states appear to be not as accurate as was found for the PR equation of state. This is due to the fact that the APACT theoiy predicts a critical point while the PR equation of state is fixed in that point. Other differences between the two equations of state are almost completely absorbed by the fitting of the calculated interfacial tensions to experimental data. 

For multicomponent systems, experiments with synthetic methods yield less information than with analytical methods, because the tie lines cannot be determined without additional experiments. A common synthetic method for polymer solutions is the (P-T-m ) experiment. An equilibrium cell is charged with a known amount of polymer, evacuated and thermostated to the measuring temperature. Then flie low-molecular mass components (gas, fluid, solvent) are added and the pressure inereases. These eomponents dissolve into the (amorphous or molten) polymer and the pressure in the equilibrium eell deereases. Therefore, this method is sometimes called pressure-decay method. Pressure and temperature are registered after equilibration. No samples are taken. The composition of the liquid phase is often obtained by weighing and using the material balance. The synthetic method is particularly suitable for measurements near critical states. Simultaneous determination of PVT data is possible. Details of experimental equipment can be found in the original papers compiled for this book and will not be presented here. 

A postulated fast reactor accident that has been extant for many years is the collapse of the core following partial melting of the fuel, hidependently of the cause or probability of such a situation, certain cases can be defined and some facets of the dynamic history of these cases can be described with more than enough accuracy for safety considerations.. The situation considered is that during which a fair fraction of the fuel has become molten and has reassembled in a nearly critical state somewhere in or below the customary location of the core. 

Systems in a near-critical state (z.e., in the vicinity of a critical or... 

The main ideas and results of scaling theory can be demonstrated through a simplified mesoscopic model. Let us assume that the near-critical state (in the one-phase region) is a t/-dimensional ideal lattice gas of fluctuation clusters with a mesoscopic lattice spacing 2 and size L. Then the excess pressure, reads... 

The main advantages of the analytical methods are that systems with more than two components can be studied, several isotherms or isobars can be studied with one filling, and the coexistence data are determined directly. The main disadvantage is that the method is not suitable near critical states or for systems where the phases do not separate well. Frrrthermore, dynamic methods can be difficult in their application to highly viscous media like concentrated polymer solutions where foaming may cause further problems. 

With respect to high-pressure application, some speciflcs on heat and mass transfer are of importance. Especially for gases in near-critical state, the described thermophysical anomalies (see Figures 2.15-2.18) result in maxima of heat capacity... 

Figure 2.20 Effect of heat transfer enhancement in a gas cooler operated in near-critical state.

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