Foams, solid

Yu et al. [36] used metal foams as catalyst supports for a microstructured methanol reformer and investigated the infiuence of the foam material on the catalyst s selectivity and activity. Some electrically conducting materials such as silicon carbide (SiC) [37] are used as a foam as well as internal heating element, and the reactor can be heated very rapidly to temperatures in the range 800-1000 °C. Moreover, as a result of the high thermal conductivity of metal or SiC foams, axial and radial temperature profiles are minimized and nearly isothermal reactor operation is facihtated. 

The mass transfer coefficient can be estimated based on the relations similar to those presented for packed beds. Giani et al. [19] proposed the following equation  

The Sherwood number and the Reynolds number are defined with the mean strut thickness, d, as characteristic length Shf = and RCf = 

Incera Garrido and Kraushaar-Czarnetzki [38] modified the relation and introduced an anisotropy factor in order to get a general mass transfer correlation applicable for isotropic and anisotropic foams. 

The pressure drop in ceramic and metallic foams can thus be estimated based on a modified Ergun equation [39]. Recently, Dietrich [40] evaluated the pressure drop measurements of nearly 100 different foams reported by about 25 authors. He could describe the experiments with an error of 40% in a broad range of Re numbers (10 Re 10 ) with the following correlation  

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Gas Liquid Solid — Foam Aerosol Emulsiom Aerosol Suspension Solid foam Solid emulsion Solid disersion... 

Solid Foam Solid Foam Solid Foam ... 

The stiffness ratios (i.e. stiffness of the foam sandwich beam relative to the original solid beam) are also given in Fig. 2.21. In both cases the values given are independent of the original solid material or its dimensions, so this provides a good design chart. The design of solid/foam sandwich structures is also considered in Chapter 3 in the laminate analysis. 

Colloids are classified according to the phases of their components (Table 8.9). A colloid that is a suspension of solids in a liquid is called a sol, and a suspension of one liquid in another is called an emulsion. For example, muddy water is a sol in which tiny flakes of clay are dispersed in water mayonnaise is an emulsion in which small droplets of water are suspended in vegetable oil. Foam is a suspension of a gas in a liquid or solid. Foam rubber, Styrofoam, soapsuds, and aerogels (insu-... 

Surface films much more viscous than the bulk of the material occur also at a constant temperature. Thus, proteins denature at interfaces and produce interfacial films. Evaporation of the solvent into the atmosphere leaves a layer of higher concentration on the surface of the solution. As, usually, higher concentration means higher viscosity, a viscous surface film results. Again, the loss of solvent may be so great that the films solidify and a solid foam of an indefinite persistence is obtained. As a first approximation, the foam on milk consists of evaporated milk. 

A series of 2- and 4-nitroaniline derivatives and analogues when heated with cone, sulfuric acid to above 200°C undergo, after an induction period, a vigorous reaction. This is accompanied by gas evolution which produces up to a 150-fold increase in volume of a solid foam, and is rapid enough to be potentially hazardous if confined. 

Gas Solid Solid foam Expanded polystyrene, cushion foam... 

Solid-fluid reactions, 21 343-344 Solid-fluid separation, 22 52 Solid foams, 7 272t occurrences of, 7 273t Solid fuel combustion technology, 7 463-467... 

Solid foam—expanded (polystyrene) Insulating foam... 

The methyl ester of 9 (1.72 g, 1.91 mmol) was hydrolyzed in dioxane (17mL) and H20 (17 mL) containing NaOH (2.9 mmol) at rt. After 1 h, the soln was diluted with H20 (20 mL), the acid liberated with 0.5 M H2S04 and extracted into EtOAc. Washing with brine, drying (Na2S04), and concentration afforded a solid foam yield 1.70 g (-100%) TLC (AcOH/BuOH/H20 1 3 1) Rf 0.85. 

When you pour out a glass of fizzy drink, the frothy part at the top of the drink is a gas/liquid mixture called a foam. The gas, carbon dioxide, has formed tiny bubbles in the liquid but has not dissolved in it. If left to stand, foams like this one collapse as the tiny bubbles join together to form bigger bubbles which then escape. It is possible to form solid foams where the gases are trapped in a solid structure. This happens in foam rubber and bread (Figure 2.38). 

Making a solid foam. The ingredients for making a polyurethane foam can be purchased as a demonstration kit. 

Table 1.1 Types of dispersions. Porous solids have a bicontinuous structure while in a solid foam the gas phase is clearly dispersed.

Solid gas porous solids foam styrofoam, souffl6s... 

Foams have a large variety of applications. Solid foams are widely used as insulating materials. Due to the presence of air bubbles they have a low thermal conductivity. Polyurethane foams and Styrofoam are examples. Styrofoam is also used as a packing material. The light weight of polymer foams makes them attractive as filling materials to stabilize otherwise hollow structures. A natural solid foam is pumice stone. Metal foams are used in the automotive and aerospace industry as light and stable materials [567], Ceramic foams are developed for electronic applications as piezoelectric transducers and low dielectric constant substrates [568],... 

To make a solid foam we start from a liquid foam and induce solidification. This can be achieved by a chemical polymerization (Styrofoam), by lowering the temperature (pumice stone or a souffle), or by increasing the temperature to induce a structural transition (baking of bread). Porous solids can appear as solid foams because of their low density and their high content of gas. The difference between the two is that in a porous solid we have a bicontinuous structure while the individual cavities in a foam are closed. This is an important difference because porous solids tend to adsorb liquids due to capillary effects and then completely change their properties. 

If the liquid laminae of a foam system can be converted to impermeable solid membranes, the film viscosity can be regarded as having become infinite, and the resulting solid foam will be permanent. Likewise, if the laminae are composed of a gingham plastic or a thixotrope, the foam will be permanently stable for bubbles whose buoyancy does not permit exceeding the yield stress. For other non-newtonian fluids, however, and for all newtonian ones, no matter how viscous, the viscosity can only delay but never prevent foam disappearance. The popular theory, held since the days of Plateau, that foam life is proportional to surface viscosity and inversely proportional to interfacial tension, is not correct, according to Biker-man (op. cit., p. 161), who points out that it is contradicted by experiment. 

A 5g sample, from which a few drops had been taken, was stored for four years. It was then observed to have become viscous and was set aside for disposal. Within three days it blew off the cap of its bottle, and that of a plastic secondary container. The contents had become a solid foam. Trace contamination cannot be absolutely excluded, but this looks like an autocatalytic Friedel Crafts polymerisation of a benzyl halide. It would represent serious danger on a larger scale. 

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