Common surface area/volume ratios

In use, the ELM Is dispersed In a continuous phase and separates two miscible phases. Under agitation, the ELM phase separates Into spherical globules of emulsion which have typical diameters of 10 ym to 1 mm. Each globule contains many droplets of encapsulated Inner or receiving phase with a typical size of 1 to 10 ym In diameter. The formation of many globules of emulsion produces large surface area/volume ratios of 1000 to 3000 mVm for very rapid mass transfer (20). Due to this dispersed emulsion configuration, ELMs or liquid surfactant membranes are commonly referred to as double emulsions. 

Figure 1.1 Common particle reinforcements and their respective surface area/volume ratios [4],...

Since the number of atoms on the surface of a bulk metal or metal oxide is extremely small compared to the number of atoms in the interior, bulk materials are often too costly to use in a catalytic process. One way to increase the effective surface area of a valuable catalytic material like a transition metal is to disperse it on a support. Figure 5.1.5 illustrates how Rh metal appears when it is supported as nanometer size crystallites on a silica carrier. High-resolution transmission electron microscopy reveals that metal crystallites, even as small as 10 nm, often expose the common low-index faces commonly associated with single crystals. However, the surface to volume ratio of the supported particles is many orders of magnitude higher than an equivalent amount of bulk metal. In fact, it is not uncommon to use catalysts with 1 nm sized metal particles where nearly every atom can be exposed to the reaction environment. 

Surface Area Determination The surface-to-volume ratio is an important powder property since it governs the rate at which a powder interacts with its surroundings, e.g., in chemical reactions. The surface area may be determined from size-distribution data or measured directly by flow through a powder bed or the adsorption of gas molecules on the powder surface. Other methods such as gas diffusion, dye adsorption from solution, and heats of adsorption have also been used. The most commonly used methods are as follows ... 

Adsorption is a process by which organics are retained on the surface of granulated solids. The solid adsorbent particles are highly porous and have very large surface-to-volume ratios. Gas molecules penetrate the pores of the adsorbent and contact the large surface area available for adsorption. Activated carbon is the most common adsorbent for organic removals. 

The surface mean diameter is the diameter of a sphere of the same surface area-to-volume ratio as the actual particle, which is usually not a perfect sphere. The surface mean diameter, which is sometimes referred to as the Sauter mean diameter, is the most useful particle size correlation, because hydrodynamic forces in the fluid bed act on the outside surface of the particle. The surface mean diameter is directly obtained from automated laser light diffraction devices, which are commonly used to measure particle sizes from 0.5 to 600 p.m. X-ray diffraction is commonly used to measure smaller particles (see Size TffiASURETffiNT OF PARTICLES). 

The peracid—exotherm control agent mixtures can be granulated using a variety of techniques common in the industry, including agglomeration. As with peracid precursors, the surface area to volume ratio can impact the stabiUty of the peracid. Particles are thus made as large as possible to maintain stabihty (141). 

A dispersion factor, defined as the ratio of the number of surface atoms to the total number of atoms ia the particle, is commonly used to describe highly dispersed systems that do not exhibit a particularly high surface-area-to-volume ratio (22). Representative values for 10-, 100-, and 1000-nm particles are, respectively, on the order of 0.15—0.30, 0.40, and 0.003—0.02, depending on the specific dimensions of the atoms or molecules that comprise the particles. Other quantities can be used to describe the degree of dispersion (6,7), but these tend to assume, at least, quasi-equUibrium conditions that are not always met (7,23). 

Two immiscible fluids, in contact with each other, share a common surface, called the interface. Operations involving transfer of matter or of heat across an interface are very common in chemical industry. In such operations a large interfacial area per unit volume is necessary if the desired transfer is to be obtained rapidly in equipment of finite size. Three common methods of providing a high ratio of interfacial area to volume are now discussed. 

---