Atmosphere reducing

The Type K thermocouple (Table 11.59) is more resistant to oxidation at elevated temperatures than the Type E, J, or T thermocouple, and consequently finds wide application at temperatures above 500°C. It is recommended for continuous use at temperatures within the range — 250 to 1260°C in inert or oxidizing atmospheres. It should not be used in sulfurous or reducing atmospheres, or in vacuum at high temperatures for extended times. 

High Carbon Yield. Furfuryl alcohol and furfural are reactive solvents (monomers) and are effective in producing high carbon yield (heat induced carbonization in a reducing atmosphere). They function as binders for refractory materials or carbon bodies. Furfuryl alcohol usually requires acidic catalysis and furfural basic catalysis. Mixtures of furfuryl alcohol and furfural are generally catalyzed with acid although some systems may be catalyzed with base. 

The hydrogen can be used for organic hydrogenation, catalytic reductions, and ammonia synthesis. It can also be burned with chlorine to produce high quaHty HCl and used to provide a reducing atmosphere in some appHcations. In many cases, however, it is used as a fuel. 

Orga.nic Carbon. Organic materials interfere with plant operation because these compounds react with sulfuric acid under furnace conditions to form sulfur dioxide. There is a reducing atmosphere in the furnace which may reduce sulfur dioxide to elemental sulfur, which results in sulfur deposits in the gas handling system. 

In the reducing atmosphere of the reactor, sulfur compounds form hydrogen sulfide and small amounts of carbonyl sulfide [463-58-1J, COS, in a molar ratio of approximately 24 1. 

The GdAlgB O QiCe ", Tb " is synthesized by a soHd-state firing of the rare-earth coprecipitated oxide plus boric acid and MgCO at 900° C in a slightly reducing atmosphere. As in the case of the lanthanum phosphate phosphor, a flux is usually used. The synthesis of this phosphor is further comphcated, however, by the fact that it is a ternary system and secondary phases such as gadolinium borate form and must then react to give the final phosphor. 

In a reducing atmosphere, molybdenum is resistant at elevated temperatures to hydrogen sulfide, which forms a thin adherent sulfide coating. In an... 

Sihcon nitride can be heated ia air up to 1450—1550°C. In nitrogen, inert gas, or reducing atmosphere, Si N can be heated up to 1750°C. Above 1750°C, decomposition and sublimating evaporation become severe. When in the presence of carbon, however, Si N stabiUty depends on temperature and pressure. The equiUbrium temperature for the reaction... 

Liquid nitrogen is used in cold traps to remove and recover solvents or volatile organic compounds from gas streams to reduce atmospheric emissions. Liquid nitrogen can be used to accelerate the cooldown time for process reactors (29). 

Process condensate from reforming operations is commonly treated by steam stripping. The stripper is operated at a sufficiently high pressure to allow the overhead stripping steam to be used as part of the reformer steam requirement (71). Contaminants removed from the process condensate are reformed to extinction, so disposal to the environment is thereby avoided. This system not only reduces atmospheric emissions, but contributes to the overall efficiency of the process by recovering condensate suitable for boiler feedwater make-up because the process is a net water consumer. 

Dissociates above 2450°C in reducing atmosphere and is readily oxidized above 1650°C. 

Thermal Applications. The protection of precious-metal thermocouples in high temperature pyrometry is an important appHcation of vitreous siHca. Although satin tubing is usually employed, transparent tubes are superior for protecting couples when used in a reducing atmosphere (220). 

In contact with various refractory materials (qv), tungsten is stable in vacuum. In reducing atmospheres, however, the temperatures are lower. 

Uses. Hot-pressed hBN is useful for high temperature electric or thermal insulation, vessels, etc, especially in inert or reducing atmospheres, and for special materials such as IITV semiconductors (qv). Its low thermal expansion makes it resistant to thermal shock. The powder can be used as a mold release agent or as thermal insulation. Boron nitride is also available in fiber form (19). BN deposited pyrolyticaHy on refractory substrates at 1200—1800°C has a turbostratic stmcture and low porosity it has greater chemical resistance and is impervious to helium. 

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