Oxidant concentration reduction

One of the most widely nsed methods of prevendng deflagrations and detonations is oxidant concentration rednction. This method can be applied to process eqnipment and vent manifold systems. The prevendon of deflagrations or detonations can be accomplished by either inerdng or fnel enrichment. 

In the case of inerdng, die oxidant (nsnally oxygen) concentration is rednced by the addition of inert gas to a valne below the limidng oxidant 

FIGURE 3-9. Effect of various gases on the flammability limits of methane-inert-gas-air mixtures at 25°C (77°F) and atmospheric pressure (Zabetakis 1965). 

Overview of Deflagration and Detonation Prevention and Proteotion Praotioes 

FIGURE 3-10. Flammability diagram for methane-oxygen-nitrogen mixtures. (Courtesy of L. G. Britton.) 

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Oxidant Concentration Reduction The technique of maintaining the concentration of the oxidant in a closed space below the concentration required for combustion to occur. 

Ghapter 7 Deflagration Prevention by Oxidant Concentration Reduction... 

Although considered an active participant in the process cycle, the tetrahydroaLkylanthraquinone (10) may not be a significant part of the catalytic hydrogenation because, dependent on the concentration in the working solution, these could all be converted to the hydroquinone by the labile shift per equation 17 and not be available to participate. None of the other first- or second-generation anthraquinone derivatives produce hydrogen peroxide, but most are susceptible to further reaction by oxidative or reductive mechanisms. 

The situation illustrated in Figure 4 allows both species to coexist. Either of the two sets of curves can be considered the oxidized species the other is the reduced species. The choice depends on whether oxidation or reduction is occurring at the surface. Assume the upper curve is the reduced species and the lower curve is its oxidized form. An appHed voltage has maintained fixed surface concentrations for some period of time including and The concentration profile of the oxidized species decreases at the electrode surface (0 distance) as it is being reduced. Electrolysis therefore results in an increase in the concentration of reduced species at the surface. The concentration profiles approach bulk values far from the surface of the electrode because electrolysis for short times at small electrodes cannot significantly affect the concentrations of species in large volumes of solution. 

If the concentrations (or, more accurately, the activities) of the oxidant and reductant are equal, E2i- = E°, i.e. the standard reduction potential. It follows from this expression that, for example, a ten-fold change in the ratio of the concentrations of the oxidant to the reductant will produce a change in the potential of the system of 0.0591/n volts. 

The oxidation and reduction should be reversible. At a potential E the ratio of the concentrations of the two forms is given by the Nernst equation ... 

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

Small amounts of molecular oxygen can influence the value of ff=0.675 With the rise of the 02 concentration in the electrolyte solution, the form of the Z", E curve changes and the value of (7=0 shifts toward less negative values. However, the effect is weak after saturation of the solution with molecular hydrogen and holding the pc-Bi electrode for 30 min at E = -1.35 V (SCE), the original shape of the Z", E curves and the original value of Eff=0 is restored. This indicates that oxidation and reduction of a pc-Bi electrode surface are reversible processes. 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

When a zinc strip is dipped into the solution, the initial rates of these two processes are different. The different rates of reaction lead to a charge imbalance across the metal-solution interface. If the concentration of zinc ions in solution is low enough, the initial rate of oxidation is more rapid than the initial rate of reduction. Under these conditions, excess electrons accumulate in the metal, and excess cationic charges accumulate in the solution. As excess charge builds, however, the rates of reaction change until the rate of reduction is balanced by the rate of oxidation. When this balance is reached, the system is at dynamic equilibrium. Oxidation and reduction continue, but the net rate of exchange is zero Zn (.S ) Zn (aq) + 2 e (me t a i)... 

The vast number of thermodynamically possible reactions obtained by permuting oxidants and reductants within the scope of this review present major problems of classification and selection. To only a limited extent is the modernity or detail of a paper indicative of its relevance, some of the definitive papers having been published before 1950. Discussion has been concentrated, therefore, at points where a kinetic investigation of a reaction has resulted in a real advance in our understanding both of its mechanism and of those of related reactions, and work which has been more of a confirmatory nature will not receive comparable consideration. Detailed reference to products, spectra, etc. will be made only when the kinetics produce real ambiguities. 

It should also be appreciated that the oxygen concentration may determine the outcome of a reaction, or even the possibility of either oxidative or reductive reactions ... 

PCPE-1 differs greatly from RICEWQ and PADDY in that the sediment layer is divided into an oxidative layer and a reductive layer because the 0-1-cm depth of sediment is oxidative, where most agrochemicals are adsorbed, and below 1 cm it is reductive. Agrochemical degradation can be different in the oxidative and reductive layers of the sediment. The prediction accuracy of agrochemical concentrations is improved sharply by this consideration. 

The key factor in voltammetry (and polarography) is that the applied potential is varied over the course of the measurement. The voltammogram, which is a current-applied potential curve, / = /( ), corresponds to a voltage scan over a range that induces oxidation or reduction of the analytes. This plot allows identification and measurement of the concentration of each species. Several metals can be determined. The limiting currents in the redox processes can be used for quantitative analysis this is the basis of voltammetric analysis [489]. The methods are based on the direct proportionality between the current and the concentration of the electroactive species, and exploit the ease and precision of measuring electric currents. Voltammetry is suitable for concentrations at or above ppm level. The sensitivity is often much higher than can be obtained with classical titrations. The sensitivity of voltammetric... 

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