Dispersion systems solid particles

Just as with emulsions and foams, suspensions can exist with additional dispersed phases present. They may contain, in addition to solid particles and a continuous liquid phase (and possibly a stabilizing agent), emulsified droplets and/or gas bubbles. Figure 2.4 (in Section 2.2.1) shows photomicrographs of a practical suspension that contains suspended oil droplets in addition to the particles. The terminology used to describe such systems can become confusing. Consider an aqueous dispersion of solid particles and emulsion droplets. If the solid particles are adsorbed on the emulsion droplets then it is an emulsion that also contains solids. If, however, the particles and droplets are not mutually associated then the system is at once a suspension and an emulsion. Which term is used becomes a matter of choosing the most appropriate context frequently one or the other is considered to be the primary dispersion while the other phase is considered to be an additive or a contaminant. 

The concept of the free volume of disperse systems can also be correlated with the change in the structure of the composite of the type solid particles — liquid — gas during its compaction. In that case the value of the maximum packing fraction of filler (p in Eq. (80b) remains valid also for systems containing air inclusions, and instead of the value of the volume fraction of filler, characteristic for a solid particles — liquid dispersion-system solid particles — liquid — gas should be substituted. This value can be calculated as follows the ratio of concentrations Cs x g/Cs, to the first approximation can be substituted by the ratio of the densities of uncompacted and compacted composites, i.e. by parameter Kp. Then Eq. (80b) in view of Eq. (88), for uncompacted composites acquires the form ... 

A dispersion Is a system made of discrete objects separated by a homogeneous medium In colloidal dispersions the objects are very small In at least one dimension. Colloidal sizes range from 1 to 100 nm however these limits are somewhat arbitrary, and It Is more useful to define colloids as dispersions where surface forces are large compared to bulk forces. Here we are concerned with systems where the dispersion medium Is a liquid examples are droplets In emulsions or mlcroemulslons (oll/water or water/oll), aggregates of amphiphilic molecules (surfactant micelles), foams, and all the dispersions of solid particles which are used as Intermediates In the manufacture of ceramics. At this stage we are not too concerned with the nature of the constituents, but rather with the structures which they form this Is a geometrical problem, where the system Is characterized by Its surface area A, by the shapes of Its Interfaces (curvatures - b ), and by the distances between opposing surfaces (d — concentration parameter). 

Pharmaceutical suspensions are dispersions of solid particles in a suspending medium or vehicle (usually aqueous in nature). When the suspended solids are less than 1 pm, the system is referred to as a colloidal suspension. When the particle sizes are greater than about 1 pm, the system is called a coarse suspension. The practical upper limit for particles in a coarse suspension is approximately 50-75 pm. Depending on the affinity or interaction between the dispersed phase and the dispersion medium, a colloidal dispersion can be classified as lyophilic (hydrophilic) or lyophobic (hydrophobic). ... 

Most paint formulations consist of disperse systems (solid in liquid dispersions) [2]. The disperse phase consists of primary pigment particles (organic or inorganic) which provide the opacity, colour and other optical effects these are usually in the submicron range. Other coarse particles (mostly inorganic) are used in primers and undercoats to seal the substrate and enhance adhesion of the top coat The continuous phase consists of a solution of polymer or resin which provides the basis for a continuous film that seals the surface and protects it from the outside environment Most modem paints contain latexes which are used as film formers. These latexes - which typically have a glass transition temperature (Tg) below... 

The population balance equation is a framework for the modeling of particulate systems. These include dispersions involving solid particles, liquid drops, and gas bubbles spanning a variety of systems of chemical engineering interest. The detailed derivation of the population balance equation and its applications can be found in Ramkrishna (1985, 2000). Publications pioneering the general application of population balance are by Hulburt and Katz (1964), Randolph and Larson (1964), and Frederickson et al. (1967). 

Colloidal systems exist in both nature and industry and can consist of either solids or liquids dispersed in either fluids or gases. Blood is a dispersion of the red blood cells (which are similar to self-assembling colloids) in serum and emulsions or microemulsions (see Chapter 8) are dispersions of oil in water or water in oil. Fog, mist, and smoke are dispersions of small particles in gases, while pollution control deals with dispersions of solid particles in air. Foams (dispersion of liquid in a gas at relatively high volume fractions of liquid) are familiar from toothpastes to beer. Many industrial processes make use of colloidal dispersions of solid particles in fluids to tailor the hydrodynamic properties of the fluid or sometimes to produce a system with large amount of internal surface area for catalytic applications. 

Neat solution-type liquid explosives are molecular mixture of all components with the best dispersion, mixing homogeneousness, and density consistency. Liquid explosives with suspended solid particles has the liquid primary explosive as the continuous media to form a sol-gel with the help of thickening agents, and their solid phase is suspended homogeneously in the system to form a mixture system. And the solid particles are surrounded by the liquid phase solution, and there are relatively ideal dispersion and uniformity of every component. Therefore, both of these two liquid explosive mixtures have sufficient explosion thermochemical reaction conditions, which makes almost aU chemical potentials of the explosive system can be released in the explosion reaction zone. And the calculation of liquid explosives with the dispersion of solid particles can be done according to the explosion property parameters of general explosive mixtures. 

As is the case in most discussions of interfacial systems and their applications, definitions and nomenclature can play a significant role in the way the material is presented. The definition of an emulsion to be followed here is that they are heterogeneous mixtures of at least one immiscible liquid dispersed in another in the form of droplets, the diameters of which are, in general, greater than 0.1 (.m. Such systems possess a minimal stability, generally defined rather arbitrarily by the application of some relevant reference system such as time to phase separation or some related phenomenon. Stability may be, and usually is, enhanced by the inclusion of additives such as surfactants, finely divided solids, and polymers. Such a definition excludes foams and sols from classification as emulsions, although it is possible that systems prepared as emulsions may, at some subsequent time, become dispersions of solid particles or foams. 

A particular focus of this chapter is colloidal dispersions of solid particles in a liquid. These are both industrially important but also scientifically interesting since model systems can be prepared with which we can probe the intermolecular interactions responsible for colloidal aggregation. As indicated in Table 3.1, such systems are termed sols. Sometimes they are also known as lyophobic solids. This reflects a now-outmoded classification of colloids into those that are solvent hating (lyophobic) and those that are solvent loving (lyophilic). Some examples of sols are described in Section 3.9, whilst the aggregation of model sols is discussed in Section 3.15. Other examples of commonly encountered colloids are described in Sections 3.10 to 3.14. 

The oxidation rate has also to be known precisely as the formation of an oxide scale has several consequences. First it consumes the material and it leads then to a decrease of the sound material. Moreover if the oxide scale is too thick, it can be spalled, leading to the dispersal of solid particles in the coolant. These particles can cause erosion of other components of the system or plug the narrowest parts of the circuits. Finally, if the oxide scale is too thick, it can lead to an important decrease in the heat conductivity of the material. This phenomenon will then have consequences on the heat transfer capability of the system, on the thermomechanical resistance of the steel, and on the oxidation kinetics itself as it is a thermally activated process. 

Dilute This is a fully expanded condition in which the solids particles are so widely separated that they exert essentially no influence upon each other. Specifically, the solids phase is so fuUy dispersed in the gas that the den sity of the suspension is essentially that of the gas phase alone (Fig. 12-29). Commonly, this situation exists when the gas velocity at all points in the system exceeds the terminal setthng velocity of the solids and the particles can be lifted and continuously conveyed by the gas however, this is not always true. Gravity settling chambers such as prilling towers and countercurrent-flow spray diy-ers are two exceptions in which gas velocity is insufficient to entrain the sohds completely. 

Information on gas holdup and axial dispersion in bubble-columns containing suspended solid particles is scarce reference will therefore also be made to significant studies of bubble-columns with no particles present, results obtained for these systems being probably of some relevance to the understanding of bubble-column slurry operations. 

Mass transfer across the liquid-solid interface in mechanically agitated liquids containing suspended solid particles has been the subject of much research, and the data obtained for these systems are probably to some extent applicable to systems containing, in addition, a dispersed gas phase. Liquid-solid mass transfer in such systems has apparently not been studied separately. Recently published studies include papers by Calderbank and Jones (C3), Barker and Treybal (B5), Harriott (H4), and Marangozis and Johnson (M3, M4). Satterfield and Sherwood (S2) have reviewed this subject with specific reference to applications in slurry-reactor analysis and design. 

The expression gas-liquid fluidization, as defined in Section III,B,3, is used for operations in which momentum is transferred to suspended solid particles by cocurrent gas and liquid flow. It may be noted that the expression gas-liquid-solid fluidization has been used for bubble-column slurry reactors (K3) with zero net liquid flow (of the type described in Sections III,B,1 and 1II,V,C). The expression gas-liquid fluidization has also been used for dispersed gas-liquid systems with no solid particles present. 

In terms of the two-phase system which comprises dispersions of solids in liquids, the minimum energy requirement is met if the total interfacial energy of the system has been minimized. If this requirement has been met, chemically, the fine state of subdivision is the most stable state, and the dispersion will thus avoid changing physically with time, except for the tendency to settle manifest by all dispersions whose phases have different densities. A suspension can be stable and yet undergo sedimentation, if a true equilibrium exists at the solid-liquid interface. If sedimentation were to be cited as evidence of instability, no dispersion would fit the requirements except by accident—e.g., if densities of the phases were identical, or if the dispersed particles were sufficiently small to be buoyed up by Brownian movement. 

The electrokinetic processes can actually be observed only when one of the phases is highly disperse (i.e., with electrolyte in the fine capillaries of a porous solid in the cases of electroosmosis and streaming potentials), with finely divided particles in the cases of electrophoresis and sedimentation potentials (we are concerned here with degrees of dispersion where the particles retain the properties of an individual phase, not of particles molecularly dispersed, such as individual molecules or ions). These processes are of great importance in particular for colloidal systems. 

Often, dispersed solids are active in defoaming in suitable formulations. Some liquid defoamers are believed to be active only in the presence of a solid. It is believed that a surface-active agent present in the system will carry the solid particles in the region of the interface and the solid will cause a destabilization of the foam. 

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