Destruction thermal

TABLE 24-4 Various Activation Energies for Thermal Destruction... 

Thermal destructive techniques have been widely used for many years to control some of these emissions. Thermal oxidizer sizes range from 100 SCFM up to 100,000 SCFM. Each industry has operations that dictate the exhaust flow that must be processed. 

References 124 through 130 provide additional information on thermal destructive technologies, design and scale-up principles, as well as operational guidance. 

Upon thermal destruction of polyethylene the chain transfer reactions are predominant, but depolymerization proceeds to a much lesser extent. As a result, the products of destruction represent the polymeric chain fragments of different length, and monomeric ethylene is formed to the extent of 1-3% by mass of polyethylene. C—C bonds in polypropylene are less strong than in polyethylene because of the fact that each second carbon atom in the main chain is the tertiary one. 

The other pattern of breaking the carbon-carbon bonds which results in the formation of free radicals is observed to much lesser degree and is responsible for an insignificant propylene content upon thermal destruction. 

Thus, suppression of the radical-chain thermal destruction reaction of olefins necessitates an addition of substances having the ability to react with active macroradicals and to yield inactive or low-reactivity products. 

It has been only found that some antioxidants and light stabilizers show the ability for partial inhibition of thermal destruction of the polyolefins. 

Organic phosphites POR(OR )OR where R=Ci—C30 represents aliphatic, cycloaliphatic, or aromatic radical, are also able of inhibiting the thermal destruction of polyolefin [19]. Of light stabilizers, benzo-phenone derivatives have the ability for inhibiting thermal destruction of polyolefins, too. 

Phthalic anhydride also shows the ability to inhibit thermal destruction of polyolefins [21]. Among the organometallic compounds may be quoted organotin compounds R2Sr(OR )2, where R2 means alkyl, aryl, or cycloalkyl OR means alkoxyl, acyl, or R2Sn(CH2COORi)2, where Rj—Ci—Cm means alkyl, allyl, or benzyl Ro represents chloro-, mono-, or triorga-notin mercaptans [22,23]. 

It should be noted that the aforementioned few compounds behave as stabilizers of thermal destruction of polyolefins only at temperatures from 200-250°C. 

Thermal stabilization of polyolefins has been first demonstrated for low-molecular models-normal structure alkanes [29]. It has been shown that metallic sodium and potassium hydroxide with absorbent birch carbon (ABC) as a carrier are efficient retardants of thermal destruction of n-heptane during a contact time of 12-15 s up to the temperature of 800°C [130]. Olefins and nitrous protoxide, previously reported as inhibitors of the hydrocarbon thermal destruction, are ineffective in this conditions. 

High inhibitive efficiency relative to thermal destruction of n-alkanes is displayed by hydrides and amides of alkali metals [33-35]. 

Hydroxides of alkali metals are effective as inhibitors of thermal destruction of polyolefins even without the carrier, yet at lower temperatures (Table 4). 

Thermal destruction of low-pressure polyethylene with molecular weight of 34,800 and of high-pressure polyethylene is completely retarded by potassium hydroxide. The molecular weight of high-molecular polyethylene decreases by a factor of 1.8, and without an... 

At 300°C and in the presence of KOH an increase in the molecular weight is observed, i.e., the reaction of macropolymerization is realized [38,39]. Potassium hydroxide is effectively inhibiting thermal destruction of polyethylene at temperatures from 350-375°C. The per cent change in molecular weight is half or one-third as high as that without the use of an inhibitor. At 400°C the efficiency of inhibition is insignificant. Potassium hydroxide with an ABC carrier is effective up to the temperature of 440°C due to the increased contact surface of the inhibitor with macroradicals. 

Fiber glass provides effective inhibition of polyethylene thermal destruction up to 400°C. The inhibitive efficiency increases with increased content of sodium oxide from 0.7-16% (Table 5). 

A similar situation is observed when studying the effect of temperature on inhibition of thermal destruction of polyethylene by fiber glass of varying composition (Table 6). The molecular weight of polyethylene is practically unchanged when exposed over a period of 6 hours at 350°C with 30% of fiber glass containing 16%... 

Table 3 Inhibition of Polyolefin Thermal Destruction by KOH with ABC Carrier in Nitrogen Atmosphere...

Hydroxides of alkali metals and alkali metals provide for inhibition of polyolefin thermal destruction following the radical-chain pattern. These substances fail... 

Table 5 Inhibition of Polyethylene Thermal Destruction by Filler—Fiberglass of Varying Alkalinity...

According to the ionization potential and electron-transfer work, alkali metals form the following series Li > Na > K, and their hydroxides are arranged in the sequence KOH > NaOH > LiOH as to their inhibitive efficiency relative to thermal destruction of polyolefins. And the efficiency of alkali metals can be represented by the sequence Na > K > Li. This seems to be due... 

The inhibitive efficiency of alkali metal hydroxides increases with increased branching of polyethylene. This is confirmed by more pronounced effect of these hydroxides diminishing the yield of propane and propylene than in case of ethane and ethylene. The decreased yield of propane and propylene is also conditioned by more efficient inhibition of the macroradical isomerization stage by alkali metal hydroxides. Upon thermal destruction of polyethylene with the use of inhibitors the... 

At the first stage of polyethylene thermal destruction the metallizing of polyethylene macroradical by the metal radical takes place. 

One would think that thermal destruction of polyethylene should be inhibited by hydroxides of alkali metals according to the following scheme, as with phenols ... 

Metallizing is supported by the fact that thermal destruction of polyethylene is inhibited by alkali metals. 

Boric acid esters provide for thermal stabilization of low-pressure polyethylene to a variable degree (Table 7). The difference in efficiency derives from the nature of polyester. Boric acid esters of aliphatic diols and triols are less efficient than the aromatic ones. Among polyesters of aromatic diols and triols, polyesters of boric acid and pyrocatechol exhibit the highest efficiency. Boric acid polyesters provide inhibition of polyethylene thermal destruction following the radical-chain mechanism, are unsuitable for inhibition of polystyrene depolymerization following the molecular pattern and have little effect as inhibitors of polypropylene thermal destruction following the hydrogen-transfer mechanism. 

Inhibition of polyethylene thermal destruction by polypyrocatechin borate could be represented as follows. The initial molecular-chain scission of branched... 

Polyethylene cured by the chemical and radiation-chemistry methods undergoes thermal destruction upon heating as in normal polyethylene. Thermoslabiliz-... 

A series of polyamine disulphides (polyaniline disulphide, polyamine disulphide, and polyparaphenylenedi-amine disulphide) represent effective thermostabilizers of cured polyethylene, and provide a decrease in gel fraction 2.5-3 times as large as that in case of inhibited thermal destruction. Stabilizers of normal polyethylene (Neozone D , Santonox R ) are inefficient as stabilizers of cured polyethylene, these substances decompose and even initiate thermal destruction of cured polyethylene. 

An investigation into the effect of the concentration of polyaniline disulphide on inhibition of thermal destruction in case of cured polyethylene has demonstrated that polyaniline disulphide is efficient even at the concentration of 0.25%. An increase in the concentration over the range 0.25-1.0% results in the increased efficiency, while further increase in the concentration leads to a slight drop in inhibition. 

Polyamine disulphides as inhibitors of thermal destruction of cured polyethylene are effective over a long period of time. 

Table 8 Inhibition of Thermal Destruction of Low-Density Cured Polyethylene in Vacuum (10- torr)...

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