Diffusion fluid, process

A new supercritical fluid process has been developed for the continuous extraction of liquids. The most useful solvent employed in the recently patented process is supercritical or near-critical carbon dioxide(l). At the heart of the process are porous membranes. Their porosity combined with a near-critical fluid s high diffusivity create a dynamic non-dispersive contact between solvent and feed liquid. The technique is dubbed porocritical fluid extraction and will be commercialized as the Porocrit Process. 

Supercritical fluids possess characteristics that make them interesting for use as polymerization media. A supercritical fluid exists at temperatures and pressures above its critical values. In the supercritical state, the fluid exhibits physical and transport properties intermediate between the gaseous and liquid state. This is illustrated in Table 2. SCFs have liquid-like densities, but gas-like diffusivities. These intermediate properties can provide advantages over liquid-based processes. In particular, the higher diffusivities of SCFs reduce mass transfer limitations in diffusion-controlled processes. Additionally, lower energy is required for processing the supercritical fluid because its viscosity is lower than that of most liquids, and because the need to vaporize large quantities of liquid is avoided. 

Now if diffusion/dispersion processes that mix fluid elements are superimposed on the convective flow in the axial direction (z direction), then the total flow rate can be written as ... 

Cycles of pollution and cleaning of silicone rubber samples show that hydrophobicity transfer is a dynamic process that depends on the amount of freely diffusible fluid components. It seems probable that the transfer effect is present during the whole lifetime of a silicone rubber for high-voltage insulators. 

In the foregoing discussions it has been implicit that advection and the Fickian mixing processes of diffusion and dispersion are responsible for the transport of dissolved chemicals. It is not necessary, however, for a chemical to be dissolved to be transported by these fluid processes chemicals that are adsorbed onto the surfaces of particles or absorbed into particles can also be readily transported by these processes. 

The Schmidt number is defined as the ratio of molecular momentum to mass difiusivity. It is used to characterise fluid flows in which there are simultaneous momentum and mass diffusion convection processes. It is named after Ernst Schmidt and expressed as... 

The hydrophobic core of nanoparticles is mostly made of solid glassy polymers such as polycaprolactone, polylactide, and their random copolymers. Drags are physically trapped and dispersed in the core. Except for the initial burst release period, the drug release from the solid nanoparticle cores tends to be a slow diffusion-controlled process [126]. Thus, nanoparticles responding to the acidic environments of tumor intercellular fluid or intracellular acidic compartments have been developed for fast drug release. 

The coefficients a(p, c) and tj(p, c) describe chemical and physical effects on the kinetics of deposition. The transport of particles from the bulk of the flowing fluid to the surface of a collector or media grain by physical processes such as Brownian diffusion, fluid flow (direct interception), and gravity are incorporated into theoretical formulations for fj(p, c), together with corrections to account for hydrodynamic retardation or the lubrication effect as the two solids come into close proximity. Chemical effects are usually considered in evaluating a(p, c). These include interparticle forces arising from electrostatic interactions and steric effects originating from interactions between adsorbed layers of polymers and polyelectrolytes on the solid surfaces. 

The interaction in a two-body collision in a dilute suspension has been expanded to provide a useful and quantitative understanding of the aggregation and sedimentation of particulate matter in a lake. In this view, Brownian diffusion, fluid shear, and differential sedimentation provide contact opportunities that can change sedimentation processes in a lake, particularly when solution conditions are such that the particles attach readily as they do in Lake Zurich [high cc(i,j)exp]. Coagulation provides a conceptual framework that connects model predictions with field observations of particle concentrations and size distributions in lake waters and sediment traps, laboratory determinations of attachment probabilities, and measurements of the composition and fluxes of sedimenting materials (Weilenmann et al., 1989). 

There are two types of diffusion limitations in fluid/solid interactions boundary layer diffusion and pore diffusion. Both processes are weakly temperature dependent but the operating conditions that reveal their presence are different. 

These and other aspects of potential exploitation of the existence of these liquid-saturated porous media displacement waves are discussed in greater detail in another paper in these Proceedings (Dusseault 2003), along with process diagrams and additional references. However, it is important to emphasize that a great deal more development is needed to fully exploit the more complete description of inertial and diffusive fluid flow in liquid-saturated porous media. What has been done in this article is partly a re-statement of developments made in the recent past, with a more... 

Transport properties, including diffusion coefficients and viscosities, undergo changes in the critical region. As mentioned in Figure 1, these properties are useful in optimizing supercritical processes and the use of these properties should become more important as supercritical fluid process calculations develop. This discussion is presented to stress the density dependence of these properties. 

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