Internal surface area

The components in catalysts called promoters lack significant catalytic activity tliemselves, but tliey improve a catalyst by making it more active, selective, or stable. A chemical promoter is used in minute amounts (e.g., parts per million) and affects tlie chemistry of tlie catalysis by influencing or being part of tlie catalytic sites. A textural (structural) promoter, on tlie otlier hand, is used in massive amounts and usually plays a role such as stabilization of tlie catalyst, for instance, by reducing tlie tendency of tlie porous material to collapse or sinter and lose internal surface area, which is a mechanism of deactivation. 

Thus, the porosity of an aerogel is ia excess of 90% and can be as high as 99.9%. As a consequence of such a high porosity, aerogels have large internal surface area and pore volume. 

Foams have a wide variety of appHcations that exploit their different physical properties. The low density, or high volume fraction of gas, enable foams to float on top of other fluids and to fiU large volumes with relatively Httle fluid material. These features are of particular importance in their use for fire fighting. The very high internal surface area of foams makes them useful in many separation processes. The unique rheology of foams also results in a wide variety of uses, as a foam can behave as a soHd, while stiH being able to flow once its yield stress is exceeded. 

Catalyst Effectiveness. Even at steady-state, isothermal conditions, consideration must be given to the possible loss in catalyst activity resulting from gradients. The loss is usually calculated based on the effectiveness factor, which is the diffusion-limited reaction rate within catalyst pores divided by the reaction rate at catalyst surface conditions (50). The effectiveness factor E, in turn, is related to the Thiele modulus,

first-order rate constant, a the internal surface area, and the effective diffusivity. It is desirable for E to be as close as possible to its maximum value of unity. Various formulas have been developed for E, which are particularly usehil for analyzing reactors that are potentially subject to thermal instabilities, such as hot spots and temperature mnaways (1,48,51). 

The soHds used as catalysts are typicady robust porous materials with high internal surface areas, typicady, hundreds of square meters per gram. Reaction occurs on the internal catalyst surface. The typical soHd catalyst used ia iadustry is a composite material with aumerous components and a complex stmcture. 

Six caustic soda evaporators were anodically protected against stress corrosion in the aluminum industry in Germany in 1965 [27]. Each evaporator had an internal surface area of 2400 m. The transformer-rectifier had a capacity of 300 AJ 5 V and was operated intermittently for many years. Automatic switching on of the protection current only took place in case of need when the drop in potential reached... 

Activated carbon is an amorphous solid with a large internal surface area/pore strucmre that adsorbs molecules from both the liquid and gas phase [11]. It has been manufactured from a number of raw materials mcluding wood, coconut shell, and coal [11,12]. Specific processes have been developed to produce activated carbon in powdered, granular, and specially shaped (pellet) forms. The key to development of activated carbon products has been the selection of the manufacturing process, raw material, and an understanding of the basic adsorption process to tailor the product to a specific adsorption application. 

All commercially available precoated plates are manufactured with great care. But they are active layers which, on account of the numbers and structures of their pores, possess a very large internal surface area, on which water vapor and other volatile substances can condense, particularly once the packaging has been opened. In order to prevent this as far as possible the precoated plates are packed with the glass or foil side upwards. 

Ai = initial vessel relief area, sq in. or sq meters Ag = second vessel relief area, sq in. or sq meters A3 = exposed surface area of vessel, sq ft A5 = Internal surface area of enclosure, sq ft or sq meters... 

Zeolite lattices have a network of very small pores. The pore di uneter of nearly all of today s FCC zeolite is approximately 8.0 angstroms (°A). These small openings, with an internal surface area of roughly 600 square... 

This is very important as several other properties are dependent upon it. If the porosity is too high, the article will be weak and will not retain liquid. The pore structure should also be taken into account. When a ceramic material is hred, although the internal surface area decreases as the material approaches zero porosity, the mean radius of the pores increases. Thus, when the internal surface area is 3 mVg the mean pore radius may be of the order of 10 m, while when the internal surface has dropped to 0-5 mVg the mean pore radius may be about 4-5 x 10 m. The mean pore radius may reach a value as high as 9 x 10 m as the ware approaches zero porosity during firing. It is thus obvious that at some point the pores must start to close up. This closing of the pores with the approach of vitrification is borne out by results of permeability measurements. 

Supports such as silica, alumina and carbon usually contain pores that offer a high internal surface area. The pore system of a support is usually rather irregular in shape and contains macropores, due to the spaces between individual crystallites, with diameters of the order of 100 nm, and micropores with characteristic dimensions of 5-10 nm. A good support offers... 

Optimized microfabrication and advanced assembly led to the use of thin platelets, in an original version 100 pm thick with a 80 pm micro channel depth, so that very thin walls (20 pm in the case sketched) remain for separating the fluids. Therefore, also the total inner reaction volume with respect to the total construction volume or the active internal surface area is very large. The latter surface amounts to 300 cm (for both the heat transfer and reaction sides) at a cubic volume of 1 cm. Indeed, the micro heat exchangers exhibited high heat transfer coefficients for gas [46] and liquid (Figure 3.10) [47, 48] flows. 

In addition to the general improvement of transfer in micro reactors, there is evidence that the voltage of electroosmotic flow (for EOF see [14]) in combination with the large internal surface area in glass chips can induce hydroxide ion formation [6]. Concerning catalyst loss, there is no obvious direct correlation rather, micro reactors can act as mini fixed beds fixing heterogeneous catalyst particles. 

Cross sectional area Coefficient matrix Chemical species Coefficient matrix Numerical coefficients Advanced silicon etching Internal surface area Attenuated total reflection Numerical coefficient... 

Let the net (overall) current density of the porons electrode be i. Under conditions of uniform work of the full internal surface area, the value of i would be y times larger than the current density t o of a smooth electrode, working at the same valne of polarization and in an electrolyte having the same composition. This case is rare in practice, and it is more common to And that i is smaller than its maximum value i = yi. The ratio between these parameters. 

The porosity and internal surface area of the nodules are very high. Due to high porosity, raw nodules generally contain 30-40% sea water, together with its contained salts. Several... 

In order to extrapolate laboratory animal results to humans, an interspecies dose conversion must be performed. Animals such as rodents have different physical dimensions, rates of intake (ingestion or inhalation), and lifespans from humans, and therefore are expected to respond differently to a specified dose level of any chemical. Estimation of equivalent human doses is usually performed by scaling laboratory doses according to observable species differences. Unfortunately, detailed quantitative data on the comparative pharmacokinetics of animals and humans are nonexistent, so that scaling methods remain approximate. In carcinogenic risk extrapolation, it is commonly assumed that the rate of response for mammals is proportional to internal surface area... 

Specific surface area (square meters per gram or square centimeters per gram) the sum of the external and the internal surface areas. 

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