Reaction surface, impermeable

If the surface is permeable to the flow as, for example, with a membrane, then there is also a finite flux normal to the surface. The extent to which the species permeates will depend on the rejection characteristics of the membrane, and we shall discuss this boundary condition later in connection with membrane filtration. Here, we suppose the surface to be impermeable so that u = 0, in which case at the reaction surface... 

There have been many instances of examination of the effect of additive product on the initiation of nucleation and growth processes. In early work on the dehydration of crystalline hydrates, reaction was initiated on all surfaces by rubbing with the anhydrous material [400]. An interesting application of the opposite effect was used by Franklin and Flanagan [62] to inhibit reaction at selected crystal faces of uranyl nitrate hexa-hydrate by coating with an impermeable material. In other reactions, the product does not so readily interact with reactant surfaces, e.g. nickel metal (having oxidized boundaries) does not detectably catalyze the decomposition of nickel formate [222],... 

TerraTherm Environmental Services, Inc., a subsidiary of Shell Technology Ventures, Inc., has developed the in situ thermal desorption (ISTD) thermal blanket technology to treat or remove volatile and semivolatile contaminants from near-surface soils and pavements. The contaminant removal is accomplished by heating the soil in sim (without excavation) to desorb and treat contaminants. In addition to evaporation and volatilization, contaminants are removed by several mechanisms, including steam distillation, pyrolysis, oxidation, and other chemical reactions. Vaporized contaminants are drawn to the surface by vacuum, collected beneath an impermeable sheet, and routed to a vapor treatment system where contaminants are thermally oxidized or adsorbed. 

The inner PbO layer is formed because of the impermeability of PbS04 layer for S04 ions only Pb +, OH , and H+ ions can transfer across this film. Thus, in the course of anodic scan, H+ ions can flow from the reaction site into solution, resulting in alkaline medium formation near the electrode surface. With increasing H2SO4 concentration, the lead sulfate layer is more compact and electrolyte ions... 

If the reacting core is impermeable, reaction will take place at the surface of the core, whereas if the core has some degree of porosity the combination of chemical reaction and limited core diffusivity will give rise to a more extended reaction zone. In either case, the overall rate of reaction will be proportional to the area of the reaction front. 

In an SOFC, the electrochemical reactions take place in the electrodes in the functional layer, that is, a zone within a distance of less than 10-20 pm from the electrolyte surface [5,136-138], The portion of the electrode beyond this width is principally a current collector structure, which has to be porous to permit the admission of gas to the functional layer where the oxidation and reduction reactions occur. Besides, the electrolyte has to be gas impermeable to avoid direct combination and combustion of the gases [137], The essential parts of the SOFC, that is, the electrolyte, the anode, and the cathode, are made of ceramic materials produced with appropriate electrical conducting properties, chemical and structural stabilities, similar expansion coefficients, and negligible reactivity properties [135],... 

The contaminant s aqueous solubility and density greatly influence its final disposition in water-solid systems. Dense nonaqueous phase liquids (DNAPLs) are chemicals with densities greater than water and typically low aqueous solubilities. DNAPLs naturally partition away from the aqueous phase and towards the solid phase, often pooling on top of an impermeable solid layer. On the other hand, light nonaqueous phase liquids (LNAPLs) possess densities less than water and have a tendency to pool on the water s surface where they may be prone to volatilization and photolysis reactions. Various liquid-solid partition coefficients have been... 

Inner membrane. The inner membrane is folded into numerous cristae, greatly increasing its total surface area. It contains proteins with three types of functions (1) those that carry out the oxidation reactions of the electron transport chain, (2) the ATP synthase that makes ATP in the matrix, and (3) transport proteins that allow the passage of metabolites into and out of the matrix. An electrochemical gradient of H+, which drives the ATP synthase, is established across this membrane, so the membrane must be impermeable to ions and most small charged molecules. 

Impermeable silicon carbides of both types, sintered and reaction bonded, perform generally better than the permeable refractories as shown in Table 19-3 Both reaction bonded and sintered products can be exposed to higher temperature for longer periods of time with lower weight loss than the oxide, Si3N4 or Si20N2 bonded refractories. This is due to the lower surface area available for reaction and to the greater relative inertness of their bond phases. 

For impermeable materials such as the reaction bonded and sintered products, true corrosion rates can usually be calculated, as the reactions generally occur only on the surface of the parts. 

These materials are added to plastics to slow the deterioration of the finished product that occurs from exposure to ozone. They typically work by migrating to the surface of the product and then create an ozone-impermeable barrier or skin on the surface. More expensive antiozonants work by initiating a reaction with the ozone that prevents their damaging effects. 

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