Transient foam

The numerous separations reported in the literature include surfactants, inorganic ions, enzymes, other proteins, other organics, biological cells, and various other particles and substances. The scale of the systems ranges from the simple Grits test for the presence of surfactants in water, which has been shown to operate by virtue of transient foam fractionation [Lemlich, J. Colloid Interface Sci., 37, 497 (1971)], to the natural adsubble processes that occur on a grand scale in the ocean [Wallace and Duce, Deep Sea Res., 25, 827 (1978)]. For further information see the reviews cited earlier. 

Some mildly surface-active compounds will stabilize weakly stable, transient foams. Examples include short-chain alcohols, short chain fatty acids, aniline, phenol, and pine oil. These weak frothers tend to produce foam films having stabilities on the order of seconds. More strongly surface-active compounds can stabilize quite strong, meta-stable foams. Examples include long-chain alcohols and fatty acids, and proteins. These strong frothers tend to produce foam films having stabilities on the order of minutes to hours. 

A transient foam that has no thin-film persistence and is therefore very unstable. Such foams exist only where new bubbles can be created faster than existing bubbles rupture. Examples air bubbles blown rapidly into pure water the foam created when a champagne bottle is opened. 

The latter, however, do not have the capacity, once adsorbed, to stabilise the foam, it is well established that pure liquids do not foam. Transient foams are obtained with solutes such as short-chain aliphatic alcohols or acids which lower the surface tension moderately really persistent foams arise only with solutes that lower the surface tension strongly in dilute solution - the highly surface-active materials such as detergents and proteins. The physical chemistry of the surface layers of the solutions is what determines the stability of the system. 

In kinetic terms, foams may be classified as either unstable, transient foams (with a hfetime of seconds), or metastable, permanent foams (with lifetimes of hours or days). 

Unstable (transient) foams, which have a life time of seconds, are generally produced using mild surfactants, for example short-chain alcohols, aniline, phenol, pine oil, and short-chain undissociated fatty acids. Most of these compounds are sparingly soluble and may produce a low degree of elasticity. 

On the other hand, if the solution is too dilute, then the surface tension of the solution will approach that of the pure solvent, and then the restoring force, which is the difference between the surface tension of the clean surface (than of the pure solvent) and the equilibrium surface tension of the solution, will be too small to withstand the usual thermal and mechanical shocks. Thus, according to this mechanism, there should be an optimum concentration for maximum foaming in any solution producing transient foams. (In these solutions the foam stabilization effects are much less important than the foam-producing effects, and therefore the latter can be measured more or less independently of the former.) This maximum in the foam valume-concentration curve of solution producing transient foams has been well verified experimentally. 

Kinetics of Surfactant Adsorption in a Transient Foam Body... 

In this chapter, we discuss much of the work accomplished since Fried, but without attempting a complete review. Useful synopses are available in the articles and reports of Hirasaki (2, 3), Marsden (4), Heller and Kuntamukkula (5), Baghidikian and Handy (6), and Rossen (7). Our goals are to present a unified perspective of foam flow in porous media to delineate important pore-level foam generation, coalescence, and transport mechanisms and to propose a readily applicable one-dimensional mechanistic model for transient foam displacement based upon gas-bubble size evolution [i.e., bubble or lamella population-balance (8, 9)]. Because foam microstructure or texture (i.e., the size of individual foam bubbles) has important effects on flow phenomena in porous media, it is mandatory that foam texture be accounted for in understanding foam transport. 

The core is initially completely saturated with aqueous foamer solution with rock adsorption satisfied. Nitrogen and aqueous surfactant solution are then injected at fixed flow rates until steady state is achieved. Transient pressure and aqueous saturation profiles are monitored for a wide range of gas and liquid flows. Only one transient foam displacement is reported here. Additional results are available elsewhere (78-80). 

To model the measured transient foam displacements, equations 2 through 12 are rewritten in standard implicit-pressure, explicit-saturation (IMPES) finite difference form, with upstream weighting of the phase mobilities following standard reservoir simulation practice (10). Iteration of the nonlinear algebraic equations is by Newton s method. The three primitive unknowns are pressure, gas-phase saturation, and bubble density. Four boundary conditions are necessary because the differential mass balances are second order in pressure and first order in saturation and bubble concentration. The outlet pressure and the inlet superficial velocities of gas and liquid are fixed. No foam is injected, so Qh is set to zero in equation... 

Figure 14 reports the calculated transient foam bubble density, np as a function of dimensionless distance. At all time levels, foam bubbles are coarsely textured near the inlet, but within the first fifth of the core, texture becomes much finer. Beyond the first fifth of the core, the limiting capillary-pressure regime develops foam texture in this region is nearly constant as is the liquid saturation in Figure 12. Foam texture also increases rapidly with respect to time. At 0.23 PV, foam bubble density im-...

Only the case of steady coinjection of surfactant solution and gas into a one-dimensional core initially filled with surfactant solution is addressed. Calculated transient foam displacement well represents both the measured wetting liquid saturations and pressure profiles with physically meaningful parameter values. It is predicted and experimentally verified that foam moves in a piston-like fashion through a linear porous medium presaturated with surfactant solution. Moreover, the proposed population-balance predicts the entire spectrum of unique steady foam-flow behavior in the capillary-pressure regime. 

Liu, D. Brigham, W. E. Transient Foam Flow in Porous Media with Cat Scanner Topical Report U.S. Department of Energy Washington, DC, March 1992. 

Unstable (transient) foams are frequently prepared from aqueous solutions of short-chain (low-molecular-weight) alcohols and fatty acids. The lifetimes of these foams range from several seconds to about 20 seconds. Mild surfactants, such as short-chain alcohols (ethyl, propyl, isobutyl, etc.), aniline, phenol, pine oil and short-chain undissociated fatty acids (formic, propionic, etc.) belong to this group of weak frothers. The lifetime t) of these unstable foam appears to be sensitive to the concentration of surfactant in solution and usually shows a maximum value at a critical concentration. 

The dynamic foam stability is usually measured by the volume of foam at a specific equilibrium flow rate, while the static foam stability is measured by the rate of collapse. Dynamic measurements are particular relevant for transient foams, while for foams of high stability, the static or equilibrium methods are usually more useful, particular for highly stabilized foams such as protein-stabilized foam systems. 

Mass transport in distillation and fractionation towers can sometimes be adversely affected by the generation of unwelcome, but transient, foam, which is a product of the intrinsic properties of the relevant liquids rather than any inadvertent contaminant. Ross and coworkers have drawn attention to the role played by partial miscibility of those liquids in determining that foam behavior (see, e.g., references [134-137]). Their studies concerned both binary and ternary mixtures of low molecular weight molecules, most of which were non-aqueous. Unlike the aqueous eth-oxylated and propoxylated non-ionic surfactant and polymer systems considered in Section 4.6.3.2, these binary systems often exhibit higher critical temperatures so that miscibility occurs with increasing temperature. 

For convenience, foams have been classified into two extreme types unstable or transient foams with lifetimes of seconds and metastable (or pamanrait) foams with lifetimes measured in days. Metastable foams can withstand ordinary disturbances (Brownian fluctuations), but coUq)se from abnormal disturbances (e.g. evq)oration or temperature gradients). 

If the surfactant solution is too dilute, the surface tension of the solution will not differ sufficiently from that of pure solvent for the restoring force to counteract the effects of casual thermal and mechanical agitation. This will lead to a very transient foam. Experimental data have shown that the optimal concentration to use is usually within a factor of two of the critical micelle concentration. 

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