Hydrogen back-diffusion

Johnson Matthqr developed the HotSpot fuel processor, a modular autother-mal reformer. The basic idea of the HotSpot was that hydrogen back-diffusion to the reaction front, where it would be consumed by the oxygen feed, was prevented by the spot wise feed injection into the centre of the reactor. The heat of the exothermic reaction was also distributed from the centre of the reactor to its periphery, where it was required to supply the endothermic reactions with energy... 

The diaphragm cell consists of multiple electrolytic cells having the anode plates and cathodes mounted vertically and parallel to each other. The cathodes, often flat hollow perforated steel structures that are covered with asbestos fibers, serve as the diaphragm that prevents the mixing of hydrogen and chlorine and back diffusion of hydroxide (OH) ions from the cathode to the anode. Brine fed into the cell is decomposed to approximately half of its original concentration to produce chlorine gas at the anode and hydrogen and sodium hydroxide at the cathode. 

The mucosal cells, which possess intrinsic mechanisms that resist injury, for example, by extruding back-diffused hydrogen ions using a basolateral carrier (e.g., sodium-hydrogen or sodium-bicarbonate exchange) (Figure 64.1)... 

Back-Leak dRTA. Although the kidney tubule retains the ability to secrete hydrogen ions, the gradient is not maintained because of back diffusion. Typically, this occurs in association with specific drug treatments (e.g., amphotericin B). 

On the other hand, the use of external humidification is essential at high temperatures, because the concentration gradient of H2O in the membrane of individual cells would be more uniform if both air and hydrogen streams were humidified externally. The external water supply helps to balance the combined effects of electro-osmotic drag and back diffusion permitting to maintain the performance of the membrane. External humidification is practically useful also below 60°C, at least for medium large-size FCS. 

Yamaoka, I., and H. Tsuji. 1991. The effect of back diffusion of intermediate hydrogen on methane-air and propane-air flames diluted with nitrogen in a stagnating flow. Combustion Flame 86 135-46. 

There are three main fluxes through the membrane. A proton flux that goes from anode to cathode, a water electro-osmotic flux that develops along with the proton flux, and a water-gradient flux. This last flux is sometimes known as the water-back flux or back-diffusion flux it is due to a difference in the chemical potential of water at the two sides of the membrane and may be in either direction although the direction is typically from cathode to anode due to water production at the cathode. In addition to the above three fluxes, there are also fluxes due to crossover of oxygen and hydrogen, which are described in Section 5.9. 

Pozzio et al [66] studied the anodic oxidation of pure hydrogen on gas diffusion electrodes catalysed by Pt/C, Pt-Ru/C and Pt-Mo/C in aqueous H2SO4. The substrate was a carbon paper and the GDL was composed of a mixture of carbon and PTFE. Electrochemical measurements were carried out using a conventional three-electrode cell, where the GDE was moimted on a holder furnished with a metal ring current collector and hydrogen back feeding. 

The mineral oil bubbler trap has two functions. First, it allows you to keep a pressure of hydrogen within the system that is slightly above atmospheric. Second, it prevents back-diffusion of air into the system. The functions of the other two units are self-explanatory. 

Because transit of an individual hydrogen ion cannot be traced, the conclusion that disappears by back-diffusion must be an inference based on observations that massive amounts of disappear from the lumen at a time when mucosal permeability is greatly increased and that neutralization by HCO7 is inadequate to account for the disappearance. 

At the high gas velocities required for measuring intrinsic rates at low eonversions away from equilibrium, the lack of back-diffusion of product gases may lead to oxidation of the nickel catalyst, as methane behaves as an inert in the Ni/NiO equilibrium (refer to Section 4.1). The problem is solved by addition of hydrogen to the feed. Axial dispersion also plays a signifieant role in determining the stability of carbide catalysts (refer to Section 4.1). 

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