Bulk foams, drying

When the foam is stable and the time for the process is short, the accumulation caused by internal collapse is not substantial. For foams of low stability and prolonged process, the accumulation resulting from internal foam collapse and foam column decay is comparable with that occurring in the adsorption layers. In addition, if accumulation is carried out with increase in the expansion ratio in the course of gravitational drainage or by the method of capillary foam drying, then the excess of the substance that accumulates in the bulk sections... 

Foam A dispersion of gas bubbles in a liquid or solid in which at least one dimension falls within the colloidal size range. Thus a foam typically contains either very small bubble sizes or, more commonly, quite large gas bubbles separated by thin liquid films. The thin liquid films are called lamellae (or laminae). Sometimes distinctions are drawn between concentrated foams, in which liquid films are thinner than the bubble sizes and the gas bubbles are polyhedral (Dry Foam and Polyederschaum), and low concentration foams in which the liquid films have thicknesses on the same scale or larger than the bubble sizes and the bubbles are approximately spherical (Gas Emulsions, Gas Dispersions, Wet Foams, and Kugelschaum). Bulk foams may also be distinguished from lamellar foams. See also Aerated Emulsion, Foam Texture, Froth. 

Aside from aforementioned hybrid technologies, there are two basic techniques for convective drying of foamed materials, namely, drying of bulk foams (foam-mat drying) and drying of dispersed foams (spray drying). 

An example of the bulk volume structure of foam-dried particles (e.g., maltodextrin/sodium caseinate powder) is shown in Fig. 6.3 (Schoonman et al, 2001). Here, the solid matrix, voids, open and closed pores and bubbles, micropores and cracks create a complex structure that affects both heat and mass transfer during drying. 

Fig. 63 Bulk volume of foam-dried particles (maltodextrin/sodium caseinate powder), according to Schoonman etal. (2001) 1 Solid matrix 2 Voids 3 Open pores 4 Closed pores 5 Cracks 6 Connected pores.

Foam spray dryiag coasists of forcing gas, usuaHy air or nitrogea, iato the product stream at 1.38 MPa (200 psi) ahead of the pump ia the normal spray dryer circuit. This method improves some of the characteristics of dried milk, such as dispersibHity, bulk deasity, and uniformity. The foam—spray dryer can accept a condensed product with 60% total soHds, as compared to 50% without the foam process. The usual neutralization of acid whey is avoided with the foam—spray dryer (see Drying Foams Sprays). 

Polystyrene. Polystyrene [9003-53-6] is a thermoplastic prepared by the polymerization of styrene, primarily the suspension or bulk processes. Polystyrene is a linear polymer that is atactic, amorphous, inert to acids and alkahes, but attacked by aromatic solvents and chlorinated hydrocarbons such as dry cleaning fluids. It is clear but yellows and crazes on outdoor exposure when attacked by uv light. It is britde and does not accept plasticizers, though mbber can be compounded with it to raise the impact strength, ie, high impact polystyrene (HIPS). Its principal use in building products is as a foamed plastic (see Eoamed plastics). The foams are used for interior trim, door and window frames, cabinetry, and, in the low density expanded form, for insulation (see Styrene plastics). 

Several features of this experiment (5 — 7) as a radiotracer method are as follows (1) since the relative adsorbability has been determined from the ratio of radiocounts of dried samples, it is not affected by errors in the absolute values for radioactivity and (2) specific activity, which has to be large usually for the study of surface phenomena, may be very small and yet the radioactive measurements yield accurate results. The concentrations of solutions examined were all close to but less than the c.m.c. values of respective surfactant solutions. The foams were stable enough to carry the experiment in this concentration range and the mole ratio of counterions in the bulk was equal to the stoichiometric mole ratio —i.e., there was no shift in mole ratio of dispersed counterions owing to the micelle formation at the present study. 

Thus, the real modulus of bulk elasticity would be higher than Ev and the ratio between the modulus of elasticity of wet and dry foam would be... 

Figure 5 shows the 3D reconstruction of a PE foam (Figure 5a), and the bulk (Figure 5b) and porous structure (Figure 5c) of a wastewater sludge after drying completion. 

Figure 5. (a) 3D reconstruction of a PE foam, (b) 3D reconstruction of the bulk and (c) the pore structures of a pasty soft material after drying completion. 

MERCURIC THIOCYANATE (592-85-8) Hg(SCN)i Moderately unstable solid. Possible violent reaction with strong oxidizers strong acids organic peroxides, peroxides and hydroperoxides potassium chlorate potassium iodate, silver nitrate, sodium chlorate, nitric acid. Incompatible with ammonia, chlorates, hydrozoic acid, methyl isocyanoacetate, nitrates, nitrites, perchlorates, sodium peroxyborate, trinitrobenzoic acid, urea nitrate. When heated, this material swells to many times its original bulk. Attacks aluminum in the presence of moisture. Decomposes above 329°F/165°C, releasing toxic mercury and cyanide fumes, and sulfur and nitrogen oxides. On small fires, use dry chemical powder (such as Purple-K-Powder), alcohol-resistant foam, or COj extinguishers. MERCURIC (Spanish) (7439-97-6) see mercury. 

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