Oxidation concentration

Chemical Reaction Measurements. Experimental studies of incineration kinetics have been described (37—39), where the waste species is generally introduced as a gas in a large excess of oxidant so that the oxidant concentration is constant, and the heat of reaction is negligible compared to the heat flux required to maintain the reacting mixture at temperature. The reaction is conducted in an externally heated reactor so that the temperature can be controlled to a known value and both oxidant concentration and temperature can be easily varied. The experimental reactor is generally a long tube of small diameter so that the residence time is well defined and axial dispersion may be neglected as a source of variation. Off-gas analysis is used to track both the disappearance of the feed material and the appearance and disappearance of any products of incomplete combustion. 

The classical experiment tracks the off-gas composition as a function of temperature at fixed residence time and oxidant level. Treating feed disappearance as first order, the pre-exponential factor and activation energy, E, in the Arrhenius expression (eq. 35) can be obtained. These studies tend to confirm large activation energies typical of the bond mpture mechanism assumed earlier. However, an accelerating effect of the oxidant is also evident in some results, so that the thermal mpture mechanism probably overestimates the time requirement by as much as several orders of magnitude (39). Measurements at several levels of oxidant concentration are useful for determining how important it is to maintain spatial uniformity of oxidant concentration in the incinerator. 

Computer Models, The actual residence time for waste destmction can be quite different from the superficial value calculated by dividing the chamber volume by the volumetric flow rate. The large activation energies for chemical reaction, and the sensitivity of reaction rates to oxidant concentration, mean that the presence of cold spots or oxidant deficient zones render such subvolumes ineffective. Poor flow patterns, ie, dead zones and bypassing, can also contribute to loss of effective volume. The tools of computational fluid dynamics (qv) are useful in assessing the extent to which the actual profiles of velocity, temperature, and oxidant concentration deviate from the ideal (40). 

Oxides of nitrogen, NO, can also form. These are generally at low levels and too low an oxidation state to consider water scmbbing. A basic reagent picks up the NO2, but not the lower oxidation states the principal oxide is usually NO, not NO2. Generally, control of NO is achieved by control of the combustion process to minimize NO, ie, avoidance of high temperatures in combination with high oxidant concentrations, and if abatement is required, various approaches specific to NO have been employed. Examples are NH injection and catalytic abatement (43). 

The equiHbrium approach should not be used for species that are highly sensitive to variations in residence time, oxidant concentration, or temperature, or for species which clearly do not reach equiHbrium. There are at least three classes of compounds that cannot be estimated weU by assuming equiHbrium CO, products of incomplete combustion (PlCs), and NO. Under most incineration conditions, chemical equiHbrium results in virtually no CO or PlCs, as required by regulations. Thus success depends on achieving a nearly complete approach to equiHbrium. Calculations depend on detailed knowledge of the reaction network, its kinetics, the mixing patterns, and the temperature, oxidant, and velocity profiles. 

Analysis for Poly(Ethylene Oxide). Another special analytical method takes advantage of the fact that poly(ethylene oxide) forms a water-insoluble association compound with poly(acryhc acid). This reaction can be used in the analysis of the concentration of poly(ethylene oxide) in a dilute aqueous solution. Ereshly prepared poly(acryhc acid) is added to a solution of unknown poly(ethylene oxide) concentration. A precipitate forms, and its concentration can be measured turbidimetricaHy. Using appropriate caUbration standards, the precipitate concentration can then be converted to concentration of poly(ethylene oxide). The optimum resin concentration in the unknown sample is 0.2—0.4 ppm. Therefore, it is necessary to dilute more concentrated solutions to this range before analysis (97). Low concentrations of poly(ethylene oxide) in water may also be determined by viscometry (98) or by complexation with KI and then titration with Na2S202 (99). 

As in dry compounding, acid acceptors must be incorporated into neoprene latices because of the wide use of these latices in coating fabrics and metals. The hydrochloric acid that forms during service life has a particularly destmetive effect on coated cotton fabrics that are not adequately protected. High zinc oxide concentration (ca 15 parts) and use of 0.4 parts AJ-phenyl-AT(p-toluenesulfonyl)-/)-phenylenediamine (Aranox, Uniroyal) as an antioxidant provides adequate protection. 

Ore Processing. Vanadium is recovered domestically as a principal mine product, as a coproduct or by-product from uranium—vanadium ores, and from ferrophosphoms as a by-product in the production of elemental phosphoms. In Canada, it is recovered from cmde-oil residues and in the Repubhc of South Africa as a by-product of titaniferous magnetite. Whatever the source, however, the first stage in ore processing is the production of an oxide concentrate. 

The Phalaborwa complex ia the northeastern Transvaal is a complex volcanic orebody. Different sections are mined to recover magnetite, apatite, a copper concentrate, vermicuhte, and baddeleyite, Hsted in order of aimual quantities mined. The baddeleyite is contained in the foskorite ore zone at a zirconium oxide concentration of 0.2%, and at a lesser concentration in the carbonatite orebody. Although baddeleyite is recovered from the process tailings to meet market demand, the maximum output could be limited by the requirements for the magnetite and apatite. The baddeleyite concentrate contains ca 96% zirconium oxide with a hafnium content of 2% Hf/Zr + Hf. A comminuted, chemically beneficiated concentrate containing ca 99% zirconium oxide is produced also. 

The te ci r l fielfi S0 y%hasin et al (1980) is a search for the temperature where ethylene oxide concentration in the discharge reaches... 

The LEL occurs at about 50% of the stoichiometric oxidation concentration at ambient temperature and pressure. 

Oxidation processes may rely on pH adjustment to enhance the chemical reaction. Figure 16 illustrates the typical configuration of a chemical oxidation process. The m or engineering considerations for chemical oxidation include reaction kinetics, mass transfer, by-products, temperature, oxidant concentration, pH and vent gas scrubbing. 

Dale and co-workers examined this reaction in considerable detail some years later and utilized a mixture of HF and BFj in dioxane as catalyst. They noted that this catalyst mixture was stable for months at room temperature and did not etch glass. It was useful for initiating the cyclooligomerization reaction which led to a product mixture. The composition of the mixture was apparently independent of the ethylene oxide concentration and the reaction was apparently not kinetically controlled. 

There are different ways, both passive and active, to provide this desired protection against deflagrations and detonations. Methods inclnde DBAs, venting, pressnre containment, oxidant concentration rednction (inerting and fnel enrichment), combnstibles concentration rednction (ventilation or air dilntion), deflagration snppression, and eqnipment and piping isolation. These are discnssed in more detail in Chapter 3. 

This book covers many aspects of DBA design, selection, specification, installadon, and maintenance. It explains how varions types of flame arresters differ, how they are constrncted, and how they work, ft also describes when a flame arrester is an effective solntion for mitigation of deflagrations and detonations, and other means of protection (e.g., oxidant concentration rednction) that may be nsed. It also briefly covers some aspects of dnst deflagration protection. 

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. 

A safety margin between the LOG and the normal oxidant concentration in the process equipment or piping system is mandated by NFPA 69 (NFPA 1997) as follows ... 

Where the oxidant concentration is continually monitored, a safety margin of at least 2 volume percent below the measured worst credible case LOG shall be maintained, unless the LOG is less than 5 volume percent, in which case, the equipment or piping shall be operated at no more than 60% of the LOG. 

Wliere the oxidant concentration is not continually monitored, the oxidant concentration shall be maintained at no more than 60% of the LOG or 40% of the LOG if the LOG is below 5 volume percent. If the oxidant concentration is not continually monitored the oxidant concentration shall be checked on a regularly scheduled basis. 

This example shows how to calculate the limiting oxidant concentration of a vapor if an experimentally determined value is not available. 

Limiting Oxidant Concentration (LOG) The concentration of oxidant below which a deflagration cannot occur in a specified mixture. 

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