External area

The detailed consideration of these equations is due largely to Kozeny [50] the reader is also referred to Collins [51]. However, it is apparent that, subject to assumptions concerning the topology of the porous system, the determination of K provides an estimate of Ao- It should be remembered that Ao will be the external area of the particles and will not include internal area due to pores (note Ref. 52). Somewhat similar equations apply in the case of gas flow the reader is referred to Barrer [53] and Kraus and co-workers [54]. 

To convert the core area into the pore area ( = specific surface, if the external area is negligible) necessitates the use of a conversion factor R which is a function not only of the pore model but also of both r and t (cf. p. 148). Thus, successive increments of the area under the curve have to be corrected, each with its appropriate value of R. For the commonly used cylindrical model,... 

If a Type I isotherm exhibits a nearly constant adsorption at high relative pressure, the micropore volume is given by the amount adsorbed (converted to a liquid volume) in the plateau region, since the mesopore volume and the external surface are both relatively small. In the more usual case where the Type I isotherm has a finite slope at high relative pressures, both the external area and the micropore volume can be evaluated by the a,-method provided that a standard isotherm on a suitable non-porous reference solid is available. Alternatively, the nonane pre-adsorption method may be used in appropriate cases to separate the processes of micropore filling and surface coverage. At present, however, there is no reliable procedure for the computation of micropore size distribution from a single isotherm but if the size extends down to micropores of molecular dimensions, adsorptive molecules of selected size can be employed as molecular probes. 

Sampling ndAnalysis Guidelines. As a general safety consideration, ah gases should be vented to an external area and whenever possible, inert gases should be used as the test gas for piping systems. 

Aoe for effective area of finned surface At for total external area of finned tube Ad for surface area of dirt (scale) deposit ... 

Temporary problems of building pollution may occur during construction and engineering activities, refurbishment, painting and decorating, and cleaning in internal, or sometimes external, areas. The sources are, generally, more easily traced. 

Table 3-7 gives the external area of fin tubes per square foot of bundle surface area. From this data the area of bundle surface area can be calculated from ... 

External Area of Fin Tube Per Ft of Bundle Surface Area (APSF) for 1-in. OD Tubes... 

If the external area of the vessel is 10 m2 and the heat transfer coefficient to the surroundings at 290 K is 8.5 W/m2 K, what will be the time taken to heat the reactants over the same temperature range and what is the maximum temperature to which the reactants can be raised ... 

A factor of 2 scaleup at constant t increases both u and L by a factor of 2, but the pressure drop increases by a factor of 2 - = 6.73. A factor of 100 scaleup increases the pressure drop by a factor of 316,000 The external area of the reactor, IttRL, increases as S, apace with the heat generated by the reaction. The Reynolds number also increases as S and the inside heat transfer coefficient increases by 5 (see Chapter 5). There should be no problem with heat transfer if you can tolerate the pressure drop. 

Depart from Geometric Similarity. Adding length to a tubular reactor while keeping the diameter constant allows both volume and external area to scale as S if the liquid is incompressible. Scaling in this manner gives poor results for gas-phase reactions. The quantitative aspects of such scaleups are discussed... 

The 1 1 kaolin structures are chemically simpler the tetrahedral sites are occupied by silicon and the octahedral sites by aluminum. There is a minor amount of substitution, largely of ferric iron for aluminum, but the amounts are generally only a few tenths of a percent by weight of oxide. The kaolin minerals do not expand in the presence of water and their surface area, approximately 10 to 15 m2/g, represents the external area of the crystals. 

Clp External area of adsorbent per unit volume of adsorbent m-1 L-1... 

Wheeler [16] proposed that the mean radius, r, and length, L, of pores in a catalyst pellet (of, for that matter, a porous solid reactant) are determined in such a way that the sum of the surface areas of all the pores constituting the honeycomb of pores is equal to the BET (Brunauer, Emmett and Teller [17]) surface area and that the sum of the pore volume is equed to the experimental pore volume. If represents the external surface area of the porous particle (e.g. as determined for cracking catalysts be sedimentation [18]) and there are n pores per unit external area, the pore volume contained by nSx cylindrically shaped pores is nSx nr L. The total extent of the experimentally measured pore volume will be equal to the product of the pellet volume, Vp, the pellet density, Pp, and the specific pore volume, v. Equating the experimental pore volume to the pore volume of the model... 

If peUets are packed in a cubic configuration, then there are (2.5 x 10 ) catalyst peUets per liter. The external area of packed spheres in 1 liter is 8 x 10 m. The internal area of these porous pellets is 1.6 x 10 — 1.6 x lO km for 1 liter... 

By using these ratios and the relationships (3.18) and (3.19), we can alter the material balance expressions and the corresponding solutions of the reactor models if we use other rate expressions. It should be noted that in practice, in fixed-beds and slurries of porous particles, the external area of the particle and thus the parameters au and ac are used, respectively. 

The external area and the volume of a single spherical particle are... 

This primary current distribution becomes important in practical electrochemical devices, e.g., fuel cells. Here, one uses porous electrodes to try to increase the active electrode area for a unit apparent external area. This seems a good idea at first, but in reality the resistance of the solution down the pores prevents ions produced in the fuel cell reaction from "getting out and often only a small length of the pore in a porous electrodes is active. 

Surface areas reported in Table II show some of the difficulties in estimating a significant surface area for coal. Areas from the initial nitrogen and methane isotherms at —195°C. probably represent only the external area of... 

In a simple heterogeneous reaction the rate per gram is proportional to the sum of the external area, a , and the available internal area for reaction, at or... 

Table III presents the reaction rate and the induction time for reaction (as defined in Figure 2) for the five coals studied. These data were taken under the following conditions 109.5°C., 1 N sodium hydroxide, and 8.5 atm. of oxygen partial pressure. The mesh size of the coals was through 60 mesh on 100 mesh. The reaction rates are based on the external area of the coal particles.

In 1933, Francis (4) measured the rate of the alkaline permanganate oxidation of coal at screen cuts of through 60 on 100 and through 100 on 200 mesh. He found that the rate of reaction was directly proportional to the external area of the different coal samples, thus inferring that there is little internal area available for reaction. In 1936 Cauzelin and Crussard (8) reached the same conclusion in similar experiments. Since 1933 a vast and complicated pore structure of coal has been discovered. However, the discovery of this pore structure does not invalidate the experimental results of Francis as some authors (19) have suggested. 

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