Dispersion units

Rubber-mill chips are dissolved similarly to resins, to provide color concentrates. Dough mixer and chip concentrates must be diluted with solvent and other vehicles to make finished inks. Media milling is becoming a method of choice in both flexo and gravure ink manufacturing. Other high speed dispersing units, such as the Morehouse, Cowles, Kady, and others, are also used. 

Bleach tower Reverse cleaners Forward cleaners Vtfesher Bleach tower Thickener Dispersion unit Flotation cells Reverse cleaners Vtfesher Bleach tower Vtfesher Bleaching Thickener Dispersion unit... 

Asymptotic Solution Rate equations for the various mass-transfer mechanisms are written in dimensionless form in Table 16-13 in terms of a number of transfer units, N = L/HTU, for particle-scale mass-transfer resistances, a number of reaction units for the reaction kinetics mechanism, and a number of dispersion units, Np, for axial dispersion. For pore and sohd diffusion, q = / // p is a dimensionless radial coordinate, where / p is the radius of the particle, if a particle is bidisperse, then / p can be replaced by the radius of a suoparticle. For prehminary calculations. Fig. 16-13 can be used to estimate N for use with the LDF approximation when more than one resistance is important. 

Sometimes a system of mixing and dispersing is composed of one or more batch units providing a feed to a continuous intensive dispersion unit. Another possibihty would be a batch mixer and surge bin which provide a continuous feed to a final dispersion unit. Various combinations of this type with adequate samphng at the proper points may be used when continuous flow would be beneficial provided that certain features could be overcome. 

Surface-active agents and hquids immiscible in water can form tiny dispersed units called reverse micelles. These can extract biochemicals from water or permit complexing or reacting in ways not possible in simple aqueous systems. 

This dispersion of the gas passes through several stages depending on the gas feed rate to the underside of the impeller and the horsepower to the impeller, varying from inadequate dispersion at low flow to total gas bubble dispersion throughout the vessel. The open, without disk, radial flow type impeller is the preferred dispersing unit because it requires lower horsepower than the axial flow impeller. The impeller determines the bubble size and interfacial area. 

LIGA-dispersion unit atiaotied to reactior and cooling charnels... 

Fig. 77 P. 0.34 in a long-oil alkyd resin system. Influence of the particle size on the dispersibility. Dispersion unit paint shaker, organic pigment Ti02 = l 5. Sample 1 fine particle pigment,...

Stokes s law and the equations developed from it apply to spherical particles only, but the dispersed units in systems of actual interest often fail to meet this shape requirement. Equation (12) is sometimes used in these cases anyway. The lack of compliance of the system to the model is acknowledged by labeling the mass, calculated by Equation (12), as the mass of an equivalent sphere. As the name implies, this is a fictitious particle with the same density as the unsolvated particle that settles with the same velocity as the experimental system. If the actual settling particle is an unsolvated polyhedron, the equivalent sphere may be a fairly good model for it, and the mass of the equivalent sphere may be a reasonable approximation to the actual mass of the particle. The approximation clearly becomes poorer if the particle is asymmetrical, solvated, or both. Characterization of dispersed particles by their mass as equivalent spheres at least has the advantage of requiring only one experimental observation, the sedimentation rate, of the system. We see in sections below that the equivalent sphere calculations still play a useful role, even in systems for which supplementary diffusion studies have also been conducted. 

Figure 2.9 is a plot of possible combinations of hydration and asymmetry for protein particles in water. Similar curves could be drawn for other materials as well. For the human hemoglobin molecule discussed in Table 2.1, the combination of sedimentation and diffusion measurements gives an /// value that lies within the domain defined by the 1.15 and 1.20 contours of Figure 2.9. The current picture of the structure of human hemoglobin, deduced from x-ray diffraction studies, suggests that the molecule may be regarded as an ellipsoid with height, width, and depth equal to 6.4, 5.5, and 5.0 nm, respectively. Applying these dimensions to the dispersed unit leads us to describe the particle as being hydrated to the extent of about 0.4-0.5 g water (g protein)...

Calculate the radius of the gold particles (pAu = 19.3 g cm 3), treating the dispersed units as equivalent spheres. 

In this equation A, B, C,. . . are constants with values to be determined. This much is evident As the concentration of a dispersion goes to zero, its viscosity must go to that of the continuous phase. Therefore, A = 7j0, the viscosity of the medium. Furthermore, the constants B, C,. . . might reasonably be expected to depend on the size, shape, orientation, and so on of the dispersed units. 

Equation (45) shows that as long as balances, volumetric flasks, and viscometers are available, [17] can be determined. All that is required is to measure viscosity at a series of concentrations, work up the data as (l/c)[(ij/i70) — 1], and extrapolate to c = 0. If the experimental value of [17] turns out to be 2.5 (V2/M2), then the particles are shown to be unsolvated spheres. If [17] differs from this value, the dispersed units deviate from the requirements of the Einstein model. In the next section we examine how such deviations can be interpreted for lyophobic colloids. 

As the system is subjected to ongoing, low-level mechanical agitation, the network structure is rearranged to a dispersion of more compact floes that display both a lower yield value and a lower apparent viscosity than the initial dispersion (curve 2). A certain amount of time is required for the dispersed units to acquire a size and structure compatible with the prevailing low level of agitation. This is why intermediate cases (not shown in Fig. 4.14a) are observed before the actual stationary-state condition is obtained. 

The viscosity of a polymer solution is one of its most distinctive properties. The spatial extension of the molecules is great enough so that the solute particles cut across velocity gradients and increase the viscosity in the manner suggested by Figure 4.8. In this regard they are no different from the rigid spheres of the Einstein model. What is different for these molecules is the internal structure of the dispersed units, which are flexible and swollen with solvent. The viscosity of a polymer solution depends, therefore, on the polymer-solvent interactions, as well as on the properties of the polymer itself. 

With the assumed uniformity of d0 values, Equation (84) shows that the coagulation of dissimilar particles becomes energetically unfavorable when the surface tension of the medium is intermediate between the surface tensions of the two kinds of dispersed units. 

EXAMPLE 13.2 Variation of Particle Concentration Due to Rapid Coagulation. An aqueous dispersion initially contains 109 particles cm -3. Assuming rapid coagulation, calculate the time required for the concentration of the dispersed units to drop to 90% of the initial value. The viscosity of water is 0.010 P at 20°C, which may be used for the temperature of the experiment. 

It may not be adequate to describe the interaction between Agl particles — especially at relatively close range —in terms of the radii of the dispersed units. In fact, the radii of surface protuberances rather than the dimensions of the particle as a whole may affect the short-range interaction. 

The authors applied this concept to both gas/liquid (see Figure 3.75) and liquid/ liquid systems (see Figure 3.76). This set-up consisted in the core of a tubular reactor with an interdigital micro mixer as dispersion unit (compare Figure 3.77). The peripheral equipment consisted of an automated pipetting robot, a fraction collector and a gas-chromatograph equipped with an automatic injector. 

Liquid inlets. Liquid enters the top tray via a hole in the column shell, often discharging against a vertical baffle or weir, or via a short, down-bending pipe (Fig. 17), or via a distributor. Restriction, excessive liquid velocities, and interference with tray action must be avoided, as these may lead to excessive entrainment, premature flooding, and even structural damage. Disperser units (e.g., perforations, values) must be absent in the liquid entrance area (Fig. 17) or excessive weeping may result. 

The total pressure drop across a tray is the sum of the pressure drop across the disperser unit, hd (dry hole for sieve trays dry valve for valve trays), and the pressure drop through the aerated mass hh i.e.,... 

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