Regenerators, inhibitor

Inhibitor regenerators— These react with intermediates or products formed in the chainstopping reaction so as to regenerate the original inhibitor or form another product capable of functioning as an antioxidant. 

Thrombin concentration 20 pg/rrtl for 80 pl/well (1,6 pg/well thrombin, 16 ng/well inhibitor) regeneration HCI... 

A simplified flow diagram for the refrigeration process is shown in figure 7.7. The inhibitor regeneration still is not shown. 

Figure 7.7 Simplified process flow diagram for a refrigeration unit (inhibitor regenerator system not shown.

Denisov, E.T. Inhibitor regeneration in oxidation. In Developments in Polymer Stabilisation Scott, G., Ed. Applied Science London, 1980 Vol. 3, 1-20. 

With hydrocarbons, two chains are normally terminated by one molecule of the inhibitor (of phenol or aromatic amine), i.e. the stoichiometric coefficient of the inhibitor is f = 2 [1]. With cyclohexanol, multiple termination of chains takes place with the same molecule of the inhibitor, as found for a-naphthylamine [76], This was observed for many aromatic amines in primary as well as in secondary alcohols [77,78], To the first approximation, the mechanism of inhibitor regeneration consists of reaction of the alcohol hydroxyperoxy radical with the radical In, reducing to InH with simultaneous termination of the chain [79], viz. 

The whole problem of inhibitor regeneration and of negative catalysis has also been recently examined by Denisov [31], using quinones and a-naphthylamine. The quinone can inhibit the oxidation of cyclohexanol by abstraction of H, viz. 

Detailed review of examples as related to inhibitor regeneration in the liquid-phase oxidation of organic substances are found in [16]. 

The experiment performed at 120 °C (see Figure 7.5) also points to the existence of such an equilibrium. The reaction comes out from the induction period at the expense of the hydroperoxide accumulation, responsible for the autoinitiation of the oxidation at an incomplete conversion of the inhibitor. This is evidence of the inhibitor regeneration by reaction (17). 

The oxidation of some classes of substances (alcohols, aliphatic amines) gives peroxyl radicals, which possess both oxidative and reductive actions. In these systems, a several inhibitors terminate chains and are regenerated again in acts of chain termination catalytic chain termination takes place. The number of chain terminations depends on the ratio of the rates of inhibitor regeneration to its irreversible consumption. In several cases, multiple chain termination is observed in polymers. Inhibitors of multiple chain termination are aromatic amines, nitroxyl radicals, and compounds of variable-valence metals. 

Eor antioxidant activity, the reaction of aminyl radicals with peroxy radicals is very beneficial. The nitroxyl radicals formed in this reaction are extremely effective oxidation inhibitors. Nitroxides function by trapping chain-propagating alkyl radicals to give hydroxylamine ethers. These ethers, in turn, quench chain propagating peroxy radicals and in the process regenerate the original nitroxides. The cycHc nature of this process accounts for the superlative antioxidant activity of nitroxides (see Antioxidants). Thus, antioxidant activity improves with an increase in stabiUty of the aminyl and nitroxyl radicals. Consequendy, commercial DPA antioxidants are alkylated in the ortho and para positions to prevent undesirable coupling reactions. 

Primary alkanolamine solutions require a relatively high heat of regeneration. Also excessive temperatures or localized overheating in reboilers cause the MEA to decompose and form corrosive compounds. An inhibitor system, such as the Amine Guard system developed by Union Carbide, is an effective method of corrosion control (52). Inhibitors permit the use of higher (25—35%) concentration MEA solutions, thus allowing lower circulation rates and subsequendy lower regeneration duty. 

MEA with inhibitors potassium carbonate potassium carbonate and regenerator(s) activated MDEA and regenerator(s)... 

Silica gels will shatter in the presence of free water and are chemically attacked by many corrosion inhibitors. The chemical attack permanently destroys the silica gels. The other desiccants are not as sensitive to free water and are not chemically attacked by most corrosion inhibitors. However, unless the regeneration temperature is high enough to desorb the inhibitor, the inhibitor may adhere to the desiccants and possibly cause coking. 

As this takes place, the inhibitor molecule is regenerated, and side methyl group is substituted by end vinyl [41]. 

Disappearance of pairs of inhibitor radicals by disporportionation leads to the regeneration of a molecule of benzoquinone or a substituted quinone. This molecule may terminate a subsequent chain. Hence, if the inhibitor radicals disappear exclusively by disproportionation,... 

The rate of consumption of initiator should therefore be constant (i.e., of zero order) under given conditions. This deduction applies regardless of whether or not inhibitor radicals may undergo regeneration (reaction 63). It emphasizes again that the observation of a well-defined induction period of duration proportional to the amount of inhibitor initially present offers no assurance of a simple stoichiometric ratio between radicals stopped and inhibitor consumed. It will be observed that the rate of consumption of inhibitor, and therefore the length of the induction period for a given amount of inhibitor, depends exclusively on Ri if 2 = 0 if it depends also... 

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