Faradaic mass transfer


Film Theory. Many theories have been put forth to explain and correlate experimentally measured mass transfer coefficients. The classical model has been the film theory (13,26) that proposes to approximate the real situation at the interface by hypothetical "effective" gas and Hquid films. The fluid is assumed to be essentially stagnant within these effective films making a sharp change to totally turbulent flow where the film is in contact with the bulk of the fluid. As a result, mass is transferred through the effective films only by steady-state molecular diffusion and it is possible to compute the concentration profile through the films by integrating Fick s law  [c.21]

Mixture Effects. Care must be taken in determining the oxidation kinetics for a mixture of chemicals (29). In principle, given one set of conditions and a two-component mixture, the overall conversion of one component A may be controlled by mass transfer to the catalyst surface and the conversion of another component B by surface-reaction kinetics. Of course, the controlling regime (mass transfer or reaction) can change with temperature. Thus for two independent parallel reactions the combined effect of diffusional and reaction rate resistances can have a considerable influence on the relative rate of the two reactions. Additionally, a third, fourth, or nxh. component can conceivably affect the other components by, for instance, competing more successfully for active surface sites than B while simultaneously influencing the mass transfer of A. Thus even for a simple two-or three-component mixture, interpretation of observed results can be difficult. Extrapolation of mixture behavior from single-component data is ill-advised.  [c.505]

Methods for estimating the height of the active sec tion of counterflow differential contactors such as packed towers, spray towers, and falling-film absorbers are based on rate expressions representing mass transfer at a point on the gas-hquid interface and on material balances representing the changes in bulk composition in the two phases that flow past each other. The rate expressions are based on the interphase mass-transfer principles described in Sec. 5. Combination of such expressions leads to an integral expression for the number of transfer units or to equations related closely to the number of theoretical plates. The paragraphs which follow set forth convenient methods for using such equations, first in a general case and then for cases in which simplifying assumptions are vahd.  [c.1354]

The result is that the small-angle scattering intensity as a fimction of energy loss (the energy-loss spectrum) looks like the optical absorption spectrum. In fact, oscillator strengths are frequently more accurately and conveniently measured from electron-impact energy-loss spectra than from optical spectra, especially for higher-energy transitions, since the source intensity, transmission and detector sensitivity for an electron scattering spectrometer are much more nearly constant than in an optical spectrometer. The proportionality of the generalized oscillator strength and the optical dipole oscillator strength appears to be valid even for incident-electron energies as low as perhaps 200 eV, but it is strictly limited to small-angle, forward scattering that minimizes momentum transfer in a collision [5]. Of course 0 is inaccessible in an inelastic collision there must be at least sufficient momentum transferred to account for the kinetic energy lost by die projectile in exciting the target. For very-high-energy, small-angle scattering, the minimum momentum transfer is relatively small and can be ignored. At lower energies, an extrapolation technique has been employed in very accurate work.  [c.1318]

In the dry system, the coating layer consists of two or three layers, for which a solvent-soluble linear polyurethane and a two-component cross-linkable polyurethane are employed. The former is used for the top and/or middle layers, and the latter for the bottom layer, ie, adhesive layer. The solvent contains dimethylformamide (DMF), methyl ethyl ketone (MEIQ, 2-propanol, toluene, or other solvents, to accelerate drying. Manufacture proceeds by the following sequence (/) 100 g/m of a 10% polyurethane solution is appHed onto a transfer paper which carries a grain pattern, and dried in an oven (2) 100 g/m of a 40% two-component polyurethane solution which is composed of a polymer diol and a polyisocyanate, is appHed on the first layer and then slightly dried to a tacky state, to form an adhesive layer (J) a substrate is laminated onto the adhesive layer thus formed and passed through an oven and roUed up, and the roU is cured at 40—60°C for 2—3 days and (4) after completion of the cross-linking, the transfer paper is removed from the finished urethane-coated fabric. There are several modifications (/) the first operation is repeated for a middle layer before the second operation (2) drying is omitted in the second operation (J) a linear polyurethane solution is used for second operation and (4) the fourth operation is foUowed by a finishing process if requited, such as color shading. For the substrate, a woven or knit fabric made of cotton (qv), rayon, nylon, polyester, or thein blends is used.  [c.93]

A fourth risk is from combined damages, ie, thermal, physical, and chemical, sustained by small organisms that are pumped through the cooling system, ie, entrainment. These damages were among the eadiest to be recognized, but the early emphasis on thermal stresses alone retarded examination of the physical components of the damage, which are appreciated as the principal risks at many installations (7). During entrainment, any organisms, including phytoplankton, zooplankton, larval fish, invertebrates, and many small fish that caimot swim against the induced current at the cooling-water intake, are drawn into the cooling circuit unless they are large enough to be screened out initially. In the cooling circuit, they receive in rapid sequence (usually <1 min) a series of stresses. These include a pressure drop in front of the pump impeller, risk of physical impact with the impeller or shear stress of a near miss, rapid pressurization downstream of the pump, shear stress as the cooling water is divided among hundreds of condenser tubes ca 2.5 cm in diameter, rapid temperature elevation as heat is transferred to the water through condenser tubes, maintenance of high temperature (usually 8—10°C above ambient) through the discharge system, decreasing pressure in discharge piping (sometimes below atmospheric), followed by turbulent mixing and cooling as the condenser water rejoins the source water body (8). Many entrained organisms do not survive. During periods of biocide treatment to remove heat-transfer-retarding biological slimes from condenser tubes, entrained organisms are also exposed to lethal concentrations of a toxicant, usually chlorine.  [c.473]

Vitrification. Vitrification is an innovative treatment that turns contaminated soils into a glasslike monolithic mass. Heat is appHed through electrodes placed in the ground, the soil reaching temperatures of 1600 to 2000°C. A layer of graphite [7782-42-5] and glass frit is first placed on the surface of the ground between the electrodes to act as the initial conductive starter path. The conductive layer and adjacent soils become a molten mass that becomes the primary electrical conductor and heat transfer medium. As heat continues to be suppHed by the electrodes, the molten mass moves both outward and downward. As organic contaminants in the soil are heated, they begin to vaporize and eventually pyrolize with the rising temperature. Inorganic contaminants are immobilized in the molten material. Off-gases, which include vaporized organics and the by-products of organic and inorganic pyrolysis, are captured above the site and treated to meet air emissions standards. Once cooled, the vitrified mass is very stable with low leaching potential.  [c.172]

Temperature affects rates of reaction, degradation of catalysts, and equilibrium conversion. Some of the modes of heat transfer applied in reactors are indicated in Figs. 23-1 and 23-2. Profiles of some temperatures and compostions in reactors are in Figs. 23-3 to 23-6, 23-22, and 23-40. Many reactors with fixed beds of catalyst pellets have divided beds, with heat transfer between the individual sections. Such units can take advantage of initial high rates at high temperatures and higher equilibrium conversions at lower temperatures. Data for two such cases are shown in Table 23-2. For SO9 the conversion attained in the fourth bed is 97.5 percent, compared with an adiabatic single bed value of 74.8 percent. With the three-bed ammonia reac tor, final ammonia concentration is 18.0 percent, compared with the one-stage adiabatic value of 15.4 percent. Some catalysts deteriorate at much above 500°C, another reason for hmiting temperatures.  [c.2075]

Weeping Liquid flow through sieve-plate perforations occurs when the gas pressure drop through the perforations is not sufficient to create bubble surface and support the static head of froth above the perforations. Weeping can be deleterious in that liquid tends to short-circmt the primary contacting zones. On the other hand, some mass transfer to and from the weeping liqmd occurs. Usual practice is to design so that deleterious weeping does not occur, based on a correlation such as that shown in Fig. 14-27.  [c.1375]

Enzymes can be immobihzed in sheets. One design had discs of enzymes fastened to a rotating shaft to improve mass transfer, and an alternate design had the feed stream flowing back and forth through sandwiches of sheets with enzyme. However, volumetric efficiency of such reactors is low because sheets with finite spacing offer less area than that of packed particles.  [c.2150]

Orthanilic acid. Fit a 1-litre three-necked flask with a liquid-sealed mechanical stirrer and a reflux condenser. Place 60 g. of o-nitrobenzene-sulphonyl chloride, 30 g. of anhydrous sodium carbonate and 180 ml. of water in the flask. Heat the mixture to boiling, with stirring the hydrolysis of the sulphonyl chloride to the sulphonic acid is complete within 40 minutes after the compound has melted. Filter the orange red solution and acidify (to litmus) with acetic acid (about 7-5 ml. are required). Transfer the solution to the original flask (which has been thoroughly rinsed with water) and equipped as before. Heat the solution to boiling, and add 105 g. of finely-divided iron filings (about 20 mesh) with vigorous stirring at the rate of about 7 -5 g. every 15 minutes. The mixture soon becomes deep brown and exhibits a tendency to froth. Complete the reaction by stirring for a further 4 hours, i.e., until a test portion when filtered yields an almost colourless filtrate if the filtrate is orange or red, the heating and stirring must be continued. When the reduction is complete, add 2 g. of decolourising carbon, filter the hot reaction mixture at the pump, and wash the residue with three 15 ml. portions of hot water combine the washings with the main solution. Cool the filtrate to about 15°, and add 28 -5 ml. of concentrated hydrochloric acid slowly, and cool to 12-15°. Filter the acid with suction on a Buchner funnel, wash with a little cold water, followed by a little ethyl alcohol, and dry upon filter paper in the air. The yield is 97 g. the orthanilic acid has a purity of 97-99 per cent. If required perfectly pure, it may be recrystaUised from hot water it decomposes at about 325°.  [c.588]

Application of an AC voltage of high frequency to a piezoelectric crystal causes faces of the crystal to move back and forth at the same frequency. This is a piezotransducer. If one of the piezocrystal faces is immersed in a liquid, the oscillatory motion transmits longitudinal ultrasound waves into the liquid. At the surface, the longitudinal waves disrupt it as compression and rarefaction waves arrive there. The breakup of the surface forms drops which spray away from the surface. The rate of formation of drops is similar to the frequency of surface disruption. Thus, a 100-kHz ultrasound wave produces about 10- droplets per second. This rate of formation of droplets is some thousands of times greater than their rate of formation in pneumatic nebulizers. Therefore, with normal argon flows, much more material is carried toward the plasma flame per unit time than is the case with pneumatic devices. Even so, there may be a need for a desolvation chamber to remove as much solvent as possible before the droplets or particulates reach the flame, if the performance of the latter is not to be affected. The ultrasonic devices have greater need still for desolvation because of their higher rate of droplet formation and higher rate of transfer of sample solution. However, at the highest frequencies (1 MHz), droplet size is small, and natural desolvation by evaporation is so rapid that a special desolvation chamber becomes unnecessary.  [c.148]

Figure 2(a) shows the EPI profile as a function of distance for PdsoRhso alloy. The dominant pair interaction Vi is negative and the plot illustrates the rapidly convergent properties of the pair interactions as a function of distance. In table 2 we present the comparison between the numerical values of pair interactions up to fourth nearest neighbors for PdsoRhso as obtained by the KKR-CPA-GPM and in the present methodology. The agreement of the numerical values between two different methodologies is reasonable and better compared to PdV alloy system. Pd-Rh being a simpler system, in the context of much smaller charge transfer and the amount of disorder present between the constituents, the difference between the two methodologies may not have significant contributions.  [c.28]


See pages that mention the term Faradaic mass transfer : [c.68]    [c.170]    [c.1195]    [c.1382]    [c.1382]    [c.1382]    [c.2026]    [c.215]    [c.1935]    [c.713]    [c.1815]    [c.128]    [c.365]   
Corrosion, Volume 2 (2000) -- [ c.2 , c.136 ]