Stationary foam

To explain the role of the medium capillary pressure upon foam coalescence, consider a flat, cylindrical, stationary foam lamella of thickness, 2h, circa 1000 A, and radius, R (i.e., 50 to 100 /xm), subject to a capillary pressure, P, at the film meniscus or Plateau border, as shown in Figure 3. The liquid pressure at the film meniscus is (P - P ), where P is the gas pressure. g c g... 

Mobility of the gas phase is reduced in two ways when a foam is used. First, liquid lamallae become trapped and block off a portion of the porous media. This results in a stationary foam fraction that can be nearly unity at low flow rates. Also lamellae present in the flowing foam fraction create an additional resistance to flow. The apparent viscosity of foam can be several orders of magnitude larger than the gas viscosity. 

Interaction parameters, a(x,t), calculated from the bubble population balance itself, e.g., the total bubble density in flowing and stationary foam, the higher moments of the bubble number density distribution, etc. 

The continuum form of the bubble population balance, applicable to flow of foams in porous media, can be obtained by volume averaging. Bubble generation, coalescence, mobilization, trapping, condensation, and evaporation are accounted for in the volume averaged transport equations of the flowing and stationary foam texture. 

The zeroth order moments of the volume averaged bubble population equations, i.e., the balances on the total bubble density in flowing and stationary foam, have the form of the usual transport equations and can be readily incorporated into a suitable reservoir simulator. 

The functional forms and relative importance of mechanisms which change the density of bubbles in flowing and stationary foam still are not well known. In particular, the functional form of d, h, i=f,t, and that of y and 6 in Equations (5) and (6) needs to be investigated more thoroughly. Also, a model linking the flowing fraction of foam, X, to the gas flux and predicting the conditions of total bubble mobilization should be developed. 

This idea has been utilized in elaboration of similar mixture of alkaline hydroperoxide with foam-generating detergents recently [10] but with a lower efficiency (tested only on GD - DI50 12.9). It is obvious that such emulsion cannot be used for primary decontamination in the first line due to a very limited stability (in the order of hours only) and also due to unsufficient effectiveness against HD. Such emulsion has thus limited utility. It could be used as a means of choice in stationary facilities to utilise its excellent effectiveness against V agents. 

Several approaches towards monolithic GC columns based on open pore foams prepared in large diameter glass tubes were reported in the early 1970s [26,27, 110]. However, these columns had poor efficiencies, and the foams possessed only limited sample capacities in the gas-solid GC mode. Subsequent experiments with polymerized polymer layer open tubular (PLOT) columns where the capillary had completely been filled with the polymer were assumed to be failures since the resulting stationary phase did not allow the gaseous mobile phase to flow [111]. 

T.W. Patzek Self-Similar Collapse of Stationary Bulk Foams. AIChE J. 39, 1697 (1993). 

In the 1970s, several research groups came up with foam-filled columns for GC and HPLC [14-17]. These open pore polyurethane foam stationary phases, which were prepared via in situ polymerization, were shown to possess comparatively good column performance and separation efficiency. They could, however, not achieve general acceptance and broader application due to insufficient mechanical stability and strong swelling behavior. 

During synthesis of a polymer, particularly of polyurethane, gaseous products can appear. Therefore, a complete model of the process must take into account (at least in some cases) the possibility of local evaporation and condensation of a solvent or other low-molecular-weight products. Such a complex model is discussed for chemical processing of polyurethane that results in formation of integral foams in a stationary mold.50 In essence, the model is an analysis of the effects of temperature in a closed cell containing a solvent and a monomer. An increase in temperature leads to an increase in pressure which influences the boiling temperature of the solvent and results in an increase in cell volume. The kinetics of polymerization is described by a simple second-order equation. The... 

You have undoubtedly seen and felt the effects of some of these titles already. The air pollution and motor vehicle titles have been addressed by new laws requiring oxygenated motor fuels and low-sulfur coal for electric power plants. The Clean Air Act has affected refrigerants in automotive and stationary air conditioning and refrigeration equipment and the manufacture of some types of foamed plastics. 

The device, presented in Fig. 2.25 (without electrodes 1 and 6) has been used in [136] for the determination of K by the so-called stationary bubble method . A foam film forms on the porous plate acquiring the shape of a hemisphere. The radius of curvature R is practically equal to the radius of the perimeter at the base of the hemispherical bubble. Because of the gas passing from the bubble through the foam film into the atmosphere, R decreases with time t. The values of r are measured and K is calculated from... 

Foam (5) is a collection of gas bubbles with sizes ranging from microscopic to infinite for a continuous gas path. These bubbles are dispersed in a connected liquid phase and separated either by lamellae, thin liquid films, or by liquid slugs. The average bubble density, related to foam texture, most strongly influences gas mobility. Bubbles can be created or divided in pore necks by capillary snap-off, and they can also divide upon entering pore branchings (5). Moreover, the bubbles can coalesce due to instability of lamellae or change size because of diffusion, evaporation, or condensation (5,8). Often, only a fraction of foam flows as some gas flow is blocked by stationary lamellae (4). 

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