More Concentrated Systems

Most industrial liquid-liquid applications fall into the category of being more concentrated systems. We identify more concentrated systems as 4) 0.20 by volume fraction of dispersed phase. Industrial examples include suspension and emulsion polymerization, extraction, and separations, including decantation, centrifugation, and electrostatic precipitation. Because practice is as much an art as a science, much of the industrial experience on concentrated systems is proprietary and not published, contrasting the vast amount of academic work published for dilute and clean systems. 

As discussed in previous sections, turbulent eddies are affected by high dispersed phase concentrations. ElasticUke behavior of deformable drops cushion eddies, reducing momentum trauspoit. This means drop dispersion is limited to a smaller region closer to the impeller than for dilute systems. 

Surface-active materials are used to stabilize dispersions in industrial applications when coalescence must be prevented, as for suspension and emulsion polymerization processes. Concentrated dispersions are more likely to undergo phase inversion. This complex coalescence-dominated phenomenon is discussed later in this section. 

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Their results show that quadrupole coupling persists in domains at high dilution, for which a reasonably accurate magnitude and the sign may be determined, even in more concentrated systems and even in the presence of substantial... 

The absolute values of the reductions in moduli, or increases in Mj, can be Interpreted in terms of small, inelastic loops. The small-strain, static moduli measured are consistent with affine chain behaviour, showing, on the basis of one-membered loops, that between about 10 and 20% of groups react to form inelastic loops by the end of a polymerisation. For the more concentrated systems a significant proportion of this comes from post-gel intramolecular reaction (Pr,e Pr,c) ... 

Photon correlation spectroscopy, carried out under very dilute conditions, has unambiguously demonstrated the expansion of carboxylic emulsion polymers at high pH, but it may not always be useful in predicting properties of practical interest. Of special concern is the apparent decrease in the intrinsic ionization constant of surface carboxyls at very low concentration. Since most uses of emulsion polymer occur at high concentrations, the measurement of particle-particle interactions is of great practical importance (21J. It has been found that the sedimentation and viscometric techniques closely reflect viscosity changes in latexes at much higher solids. Extension of the PCS approach to more concentrated systems is underway but not without problems (22). 

The results are generally consistent with a broader treatment of the techniques for measuring particle swelling of carboxylic emulsion polymer latexes reported elsewhere in this Monograph (9). The broader study, which was carried out independently but concurrently, has shown that the magnitude and pH of maximum expansion depends on dilution and ionic strength. Studies of the concentration dependence in the dilute regime and more concentrated systems are underway. 

A kind of foam in which the gas bubbles have an unusually thick stabilizing film and exist clustered together as opposed to either separated, nearly spherical bubbles or the more concentrated, system-filling polyhedral bubbles. A microgas emulsion will cream to form a separate phase from water. Also termed aphrons or colloidal gas aphrons . 

The associated and paralleled developments in GISANS will provide access to not only the surface, but also to the near surface structure, and this will enhance considerably our ability to study ordered and relatively more concentrated systems. 

The influence of the water content of the gel on the crystallization kinetics of ZSM-23 is shown in Figure 6. Obviously, crystallization is faster for the more concentrated systems. This observation can be easily rationalized if it is considered that with decreasing amount of water in the synthesis mixtures the concentration of the reactants and hence pH increases. This has the same effect as an increase in 0H"/Si02 ratio, viz. higher supersaturation of the mother liquor. The consequences have been discussed before. 

The aim of this first section is to describe the rupturing mechanisms and the mechanical conditions that have to be fulfilled to obtain monodisperse emulsions. A simple strategy consists of submitting monodisperse and dilute emulsions to a controlled shear step and of following the kinetic evolution of the droplet diameter. It will be demonstrated that the observed behavior can be generalized to more concentrated systems. The most relevant parameters that govern the final size will be listed. The final drop size is mainly determined by the amplitude of the applied stress and is only slightly affected by the viscosity ratio p. This last parameter influences the distribution width and appears to be relevant to control the final monodispersity. 

First, the observed critical concentration l 0.65 may be compared with the maximum kinematic concentration (0max = n/4 a 0.785) possible for a two-dimensional suspension of circular disks undergoing simple shear. That the actual theoretically predicted one may be rationalized in terms of spatially periodic packings allowing the existence of more concentrated systems than disordered packings. According to Berryman... 

These results can be effectively explained by supposing that colloidal stability plays a major role in determining miniemulsion stability. In fact, it is clear that addition of surfactant stops the droplet growth, which is explained by the enhanced colloidal stability. Moreover, in more concentrated systems, where the rate of droplet coalescence is larger, one obtains larger droplets, as... 

The basic technique is only applicable to dilute suspensions where multiple scattering does not occur and this technique is sometimes referred to as through dynamic light scattering. The introduction of the controlled reference method has extended it to more concentrated systems [275]. 

More Concentrated Systems 4> > 0.2. This range is common in industry. Fast coalescence is probable for clean systems. Sprow " found that with coalescing systems, drop sizes were position dependent within the vessel. This behavior is very complex and extremely difficult to scale-up, as coalescence and dispersion dominate in different regions of the vessel. 

During the last 15 years, interest in the study of aquatic humic substances has increased. The problems associated with isolating and concentrating this material from aqueous solution largely have been overcome, and humic substances can be easily extracted from any aquatic sample. Humic substances have been successfully isolated from waters with very low DOC values, such as seawater and groundwater, as well as more concentrated systems. 

For more concentrated systems, I Q) also depends on the interference effects arising from particle-particle interactions,... 

Characterization of immiscible liquid-liquid systems into broad groups based on coalescence rates can sometimes help reduce the complexity that must be considered. Noncoalescing systems can be treated as dispersions only. Examples include dilute dispersions, typically <5% dispersed phase, and stabilized, more-concentrated systems. Many industrial systems are either noncoalescing or very slowly coalescing. 

Both static and dynamic light-scattering techniques will continue to find wide applications in the study of the structure of more concentrated systems. 

Table 12.5. Although the basis of the comparison presented therein is slightly different (per particle vs per doublet), the results predicted by both extreme models are not too widely disparate. The configurational free energy change associated with flocculation can obviously be as large as 10 A T in systems of usual interest. Dilute dispersions are clearly more stable on this basis than more concentrated systems. What this means in terms of the effect on the critical flocculation point depends critically upon the nature of the particular system concerned.

It might be noted here that accordingly the intrinsic viscosity [rj] as defined in Eq. (4) will be given by 2.5/p for compact spheres, where p is the density of these spheres. However, Eq. (5) is valid only for relatively dilute systems (volume fraction < 0.02). For more concentrated systems this expression has to be augmented by higher order terms of the volume fraction, i.e., one has to expand Eq. (5) by higher orders of O, as... 

A limited number of studies have been carried out on more concentrated systems using variations of the traditional electrophoretic method, e.g., the tracer and mass-transport methods. Reed and Morrison (35) have shown that, for d.c. fields, even in highly concentrated systems, the hydrodynamic and electrostatic interactions cancel one another when the double layers are thin, and the only effect which must be taken into account is the reverse flow of fluid displaced by the moving particles. Zukoski and Sav-ille (3 6), using red blood cells mixed with ghosts, have verified that this is so and that the d.c. mobility, of a concentrated system of volume fiaction d> is given by the simple relation ... 

Suspensions of insoluble material are commonly found in many food and feed systems. Some of their properties are summarized in Table 2.3. The particles are almost always obtained from the grinding operation of a biological tissue or of a pure solid food material (typically sucrose or protein). These processes generate particles with a typical size range of 5-500 pm. The sedimentation of particles of these sizes is rapid under dilute conditions, which thus makes such dilute suspensions unstable. More concentrated systems produce stable suspensions by the formation of a concentrated network, which then only slowly change due to a consolidation process. 

Monte Carlo simulations provide a rewarding and invaluable approach to solving these systems, and computer simulations and theory can isolate the molecular factors that control polyelectrolyte conformations in solution. Therefore, they are exU cmely useful to address the optimization of colloid-polymer mixtures and guide the design of new experiments. A simple model involving one chain interacting with one particle has been described, but the same model can be extended to more concentrated systems, e.g. involving several chains (and/or colloidal particles) with explicit counter ions, co-ions and solvent molecules. 

McCabe-Thiele Analysis for More Concentrated Systems... 

If absorption or stripping can be assumed 1) to be isothermal and 2) to have negligible heat of absorption, then the energy balances will be satisfied. In this case the McCabe-Thiele analysis procedure can be adapted to more concentrated systems where the total flow rates L and V are not constant. We will have the desired straight operating line if we define... 

To have a straight operating line for the more volatile conponent in distillation we assumed that constant molal overflow (CMO) was valid, which meant that in each section total flows were constant. For absorption, stripping, and extraction we could make the assunption that total flows were constant if the systems were very dilute. For more concentrated systems we assumed that there was one chemical species in each phase that did not transfer into the other phase then the flow of this species (carrier gas, solvent, or diluent) was constant. In general, we have to assume either that total flows are constant or that flows of nontransferred species are constant. 

Since Pick cast his equation in a familiar form and since Eqs. ri5-2bi and fl5-4ai fit data for isothermal dilute binary systems very well, this equation rapidly became enshrined as Pick s law (sometimes known as Pick s first law). However, problems arose when other researchers extended Pick s work to more concentrated systems. In Section 15.2.3 we will see that when there is significant convection in the diffusion direction, the diffusion flux J needs to be related to the flux N with respect to a fixed coordinate system (N is the flux needed to design equipment). This conplicates the picture but does not invalidate Pick s law. As we shall see later, when extended to concentrated, nonideal systems or to multiconponent systems. Pick s law often requires very large adjustments of the molecular diffusivity—sometimes with negative values—as a function of concentration to predict behavior. Said in clearer terms. Pick s law no longer applies. We should not blame Pick for this lack of agreement. His law works fine for the conditions that he developed it for. 

Interested readers should consult Basmadjian (1997), LeVan et al. (1997), Ruthven f19841 or Yang (198Z). These more concentrated systems can also be simulated with commercial simulators. 

Because of the underlying assumption that the particles are noninteracting, which implies here that the flow pattern around any particle is not affected by the presence of the others, Equation 17.7 applies to very dilute dispersions only. Roughly speaking, Equation 17.7 is valid for rij/rio < 1.03. For more concentrated systems, t s can be approximated in an expansion of powers of O ... 

For more concentrated systems other approaches have been given. Several authors have made experimental studies of the effects of concentration on the... 

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