Halogen-containing vapor phase

The per pass ethylene conversion in the primary reactors is maintained at 20—30% in order to ensure catalyst selectivities of 70—80%. Vapor-phase oxidation inhibitors such as ethylene dichloride or vinyl chloride or other halogenated compounds are added to the inlet of the reactors in ppm concentrations to retard carbon dioxide formation (107,120,121). The process stream exiting the reactor may contain 1—3 mol % ethylene oxide. This hot effluent gas is then cooled ia a shell-and-tube heat exchanger to around 35—40°C by usiag the cold recycle reactor feed stream gas from the primary absorber. The cooled cmde product gas is then compressed ia a centrifugal blower before entering the primary absorber. 

As mentioned previously, halogen-based flame retardants are the most widely used for styrenic polymers. Since halogen-based flame retardants act primarily in the vapor phase, the halogen-containing compounds need to decompose and evolve HX in the same temperature range in which polystyrene pyrolyzes (> 300 °C). Another consideration is that the flame retardant needs to be sufficiently thermally stable to be melt compounded with polystyrene. 

Flame-retardant styrenic polymers find utility in applications such as building insulation (expanded polystyrene foam) and electronic enclosures (flame-retardant HIPS, ABS and styrenic blends). The most effective flame retardants are halogen-(particularly bromine)-containing compounds these flame retardants act by inhibiting the radical combustion reactions occurring in the vapor phase. Flame-retardant plastics are in a state of flux, due to influences of... 

Halogen lamps are timgsten lamps whose glass bulbs also contain iodine [42]. When the coil is heated incandescent volatile tungsten iodine compounds are produced in the vapor phase and these are thermally decomposed at the glowing coil. This causes a reduction in the deposition of tungsten on the surface of the... 

Class III consists of halogen-containing polymeric materials. Some of them form a small char residue, others form none at all. These polymers are inherently flame retardant because halogen radicals act as radical scavengers in the vapor phase and, therefore, inhibit combustion, as described earlier. Also the splitting-off of noncombustible gases such as HCl, Hf, and CnF, seals the polymer surface from the combustion air and is thereby partly responsible for the flame retardancy. All these factors thus influence the interaction between pyrolysis and ignition. 

As in polyester resins, reactive halogens containing fire-retardant chemicals are most often used in epoxy materials. Tetrabromobisphenol A is perhaps the most widely used component for flame-retarding epoxy resins. Nara and Matsuyama (24) and Nara et al. (25) described the thermal degradation and flame retardance of tetrabrominated bisphenol A diglycidyl ether compared to the nonbrorainated structure. Their results indicate that bromine acts by vapor-phase as well as condensed-phase mechanisms of flame inhibition. 

Corrosion of Aluminum by Halogenated Solvents. Both liquid and vapor-phase halogenated solvents used for the production of ICs and PCs corrode aluminum-containing components. Water contamination of the solvent increases the time to corrosion on the one hand and increases the corrosion rate on the other hand. Dilution of the stabilized solvents with alcohol results in the breakdown of halogenated solvents and the decomposition product, chloride ion corrodes aluminum, and aluminum-copper alloys. 

Additives functioning in the vapor phase exhibit the highest efficiency because they interfere with the combustion chemistry. Total load levels as low as 3 wt% are sufficient for certain flammability performance requirements. The halogen-containing systems that function in the vapor phase are characterized by higher heat releases than systems operating in the condensed phase. The steps involved in the combustion of polymers as described by Troitzsch (10), shown in Fig. 4.20, are summarized below. 

Improvements in acrylonitrile yield are also reported with other vapor phase promoters. A patent assigned to Monsanto Co. (125) describes the use of sulfur and sulfur-containing compounds in the feed gas mixture for production of acrylonitrile or methacrylonitrile from propane or isobutane over metal oxide catalysts. Examples of effective sulfur-containing compounds include alkyl or dialkyl sulfides, mercaptans, hydrogen sulfide, ammonium sulfide, and sulfiir dioxide. Best results are apparently achieved using a molar ratio of sulfur (or sulfur compound) to hydrocarbon of 0.0005 1 to 0.01 1. Nitric oxide has also been examined as a gas-phase promoter for propane and isobutane ammoxidation (126). However, it does not appear to be as effective as halogen or sulfur. Selectivities to acrylonitrile from propane are only about 30% over an alumina-supported chromium-nickel oxide catalyst. 

Some recent developments involving the use of novel vapor phase detectors, such as the flame thermionic ionization detector (FTID), which responds to compounds containing nitrogen and halogen atoms, and the flame emission photometric detector (33), which detects substances containing sulfur and/or phosphorus as well as chemiluminescent nitrogen detector, coupled on-line with FID (33a), should be able to widen the range of possible applications of the coated rod TLC-FID systems. 

Lagow has carried out reactions between lithium atoms and organo-halogen compounds in the gas phase (19, 20). For example, when CC14 vapor was bled into a dense stream of lithium vapor and the mixture condensed at -196°C, the condensate behaved chemically as if it contained Li4C, e.g.,... 

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