Oxygen absorption, 6.26

High purity acetaldehyde is desirable for oxidation. The aldehyde is diluted with solvent to moderate oxidation and to permit safer operation. In the hquid take-off process, acetaldehyde is maintained at 30—40 wt % and when a vapor product is taken, no more than 6 wt % aldehyde is in the reactor solvent. A considerable recycle stream is returned to the oxidation reactor to increase selectivity. Recycle air, chiefly nitrogen, is added to the air introducted to the reactor at 4000—4500 times the reactor volume per hour. The customary catalyst is a mixture of three parts copper acetate to one part cobalt acetate by weight. Either salt alone is less effective than the mixture. Copper acetate may be as high as 2 wt % in the reaction solvent, but cobalt acetate ought not rise above 0.5 wt %. The reaction is carried out at 45—60°C under 100—300 kPa (15—44 psi). The reaction solvent is far above the boiling point of acetaldehyde, but the reaction is so fast that Httle escapes unoxidized. This temperature helps oxygen absorption, reduces acetaldehyde losses, and inhibits anhydride hydrolysis. 

Antioxidants have been shown to improve oxidative stabiHty substantially (36,37). The use of mbber-bound stabilizers to permit concentration of the additive in the mbber phase has been reported (38—40). The partitioning behavior of various conventional stabilizers between the mbber and thermoplastic phases in model ABS systems has been described and shown to correlate with solubiHty parameter values (41). Pigments can adversely affect oxidative stabiHty (32). Test methods for assessing thermal oxidative stabiHty include oxygen absorption (31,32,42), thermal analysis (43,44), oven aging (34,45,46), and chemiluminescence (47,48). 

The function of aeration in a wastewater treatment system is to maintain an aerobic condition. Water, upon exposure to air, tends to estabUsh an equihbrium concentration of dissolved oxygen (DO). Oxygen absorption is controlled by gas solubiUty and diffusion at the gas—hquid interface. Mechanical or artificial aeration may be utilised to speed up this process. Agitating the water, creating drops or a thin layer, or bubbling air through water speeds up absorption because each increases the surface area at the interface. 

Hydrogen and Nitrogen. Silver dissolves about 0.02 mL of hydrogen per mL of Ag at 600° C, and 0.05 mL at 900°C. Nitrogen, insoluble in silver, is a suitable atmosphere for casting and melting silver to avoid oxygen absorption. 

FIG. 14-78 Liqnid-film resistance in absorption of gases in wetted-wall columns. Theoretical lines are calculated for oxygen absorption in water at 55 F. To convert feet to meters, multiply by 0.3048 C = /9 ( F — 32). (Sheixuood and Vigfoid, Absorption and Extraction, 2d ed., McGraw-Hill, New York, 1952. )... 

The initial black solution gradually turns green by oxygen absorption. 

A study was done measuring the thermal oxidative stability of polyurethanes made from PPG polyols, varying the isocyanate curative. Oxygen absorption was... 

Rafts are designed to restrict the surface area exposed to air so as to reduce oxygen absorption. They are subject to damage by direct waste contact and can achieve efficiencies of up to 90 percent. 

Floating hollow spheres are made of polypropylene with projections to prevent rotation. They restrict oxygen absorption and reduce emissions with an effectiveness of 80 to 90 percent. High winds can pose serious problems by blowing the spheres away (University of Arkansas and Louisiana State University, 1985). 

Various methods have been suggested and used for the determination of stability, based on oxygen absorption. The fat is allowed to deteriorate in a closed system containing air or oxygen, and the amount of oxygen absorbed is plotted against time. Such methods are rather cumbersome and difficult to operate, although in recent years some very clever devices have been constructed which are almost automatic in operation (10,... 

Fig. 21. Kinetic curves of photosensitized oxygen absorption by thermally treated PAN samples during irradiation with white light. ( ) PAN (450 °C,...

Fig. 22. Kinetic curves of oxygen absorption by ascorbic acid in the presence of polytpro-pionitrile). (1) in the dark (2) in red light (3), (4) in white light...

Pasveer (PI) studied oxygen absorption from air bubbles by water and found that the experimental results could be adequately correlated in terms of a penetration or surface-renewal theory. 

Adlington and Thompson (Al) measured the gas-liquid interfacial area in beds of particles of from 0.3- to 3-mm diameter by oxygen absorption in a sodium sulfite solution. They found that the interfacial area decreased with decreasing bed porosity, and was less sensitive to changes in particle size. 

Probably an important pro-oxidant for all mbbers is ultraviolet (UV) hght. Blake and Bmce [ 17] performed a study of the oxygen absorption rates of unvulcanized NR mbbers under exposure to UV light. It was observed that exposure to light caused dramatic increases in the oxygen absorption... 

Second, the energy of emission activation is close to the energy of activation of adsorption of oxygen by silver - 146 kj/mol [49]. This implies that emission of O-atoms occurs due to energy of defect annihilation in the surface-adjacent layers of catalyst due to adsorption of oxygen. From the stand-point of such assumption it is obvious that depletion of emission is linked with sloroing down of the oxygen absorption rate (as shown, for instance, in Fig. 6.12). 

Fig. 6.19. Temperature dependence of triplet oxygen absorption (f) and singlet oxygen emission (2) during the heating of VgOu sample in 0.1 Torr O2.

The major fatty acids present in plant-derived fatty substances are oleic acid (9-octadecenoic, C18 l), linoleic acid (9,12-octadecadienoic, C18 2) and the conjugated isomers thereof and linolenic acid (9,12,15-octadecatrienoic, C18 3) (Scheme 31.1). Their rates of oxygen absorption are 100 40 1, respectively, hence partial hydrogenation with consequent lowering of the iodine number would lead to a significant increase in oxidative stabihty, particularly when C18 3 is reduced. 

Fig. 2.3.4 Film formation of a photoinitiated the lower surface (left) after a 90 min induction cross-linking latex coating as measured by period due to oxygen absorption. The profiles CARField. (a) The coating is exposed to air shown were recorded 10, 90, 100 and 110 min (evaporation) and light from above, (b) A sam- and 2, 3, 4, 5, 6 and 17 h after casting the layer, pie comprising a combination of only polymer (d)The full formulation film forms in the central and water dries from the upper surface (right) layers first. In this final time series, the profiles as shown by a time series of profiles, recorded shown were recorded after 10 min (dotted at 10, 20, 30, 40, 50, 60, 70, 100 and 120 min trace, T) attenuated) and then, from the top after casting the layer, (c) A combination of down, 30, 60 and 90 min and 2, 3, 6 and 17 h polymer and photoinitiator only cures from after casting the layer.

The progress of polymer degradation may be followed by a wide variety of techniques, some of them being mentioned at the right column in the Bolland-Gee scheme (Scheme 2). There are techniques that directly monitor some of the elementary reaction steps such as, for example, oxygen absorption (reaction 2), differential scanning calorimetry (DSC) (reaction 3), chemiluminescence (reaction 11) analytical and/or spectral methods of determination of hydroperoxides, etc. 

The comparison of measurements for oxidation of polyisoprene indicates that the increase of chemiluminescence precedes that of oxygen uptake (Figure 12) and that a maximum of chemiluminescence is situated around an inflexion point of the oxygen absorption curve. 

Oxygen absorption methods have limited sensitivity and require high levels of oxidation as the endpoint for induction periods (Frankel 1993). In foods, antioxidant... 

Oxidative stress Lipid oxidation Oxygen absorption Manometric, polarographic Diene conjugation HPLC, spectrophotometry (234 nm) Lipid hydroperoxides HPLC, GC-MS, chemiluminescence, spectrophotometry Iodine liberation Titration Thiocyanate Spectrophotometry (500 nm) Hydrocarbons GC Cytotoxic aldehydes LPO-586, HPLC, GC, GC-MS Hexanal and related end products Sensory, physicochemical, Cu(II) induction method, GC TBARS Spectrophotometry (532-535 nm), HPLC Rancimat Conductivity F2-iP GC/MS, HPLC/MS, immunoassays... 

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