The explosion limits

1 THE EXPLOSION LIMITS AND THE SLOW COMBUSTION OF CARBON MONOXIDE-OXYGEN MIXTURES 

Because of the extreme sensitivity of the position of the explosion peninsula to small amounts of hydrogenous impurity, it is not possible to separate the attempts to measure the position of the explosion region for dry mixtures on a P-T diagram from those in which impurities or additives were definitely present. 

Effect of inert gas on lower explosion limit of CO + Oj mixture at 600 °C [355] (Pressures in torr) 

Perhaps the closest approach to measuring the explosion limits of a pure CO/O2 mixture was made by Dickens et al. [358]. They went to considerable lengths to exclude water and other impurities by fractional distillation and storage of the reactants at liquid oxygen temperatures. Five different cylindrical quartz vessels were used, and reproducible results for a particular reaction vessel were only obtained after some weeks of experimentation. However, there were marked differences in the behaviour of the gases in the different reactors. Experiments carried out 

The effect of ethane and the temperature dependence of the limits are shown in Fig. 56. 

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The explosive limits of hydrazine in air are 4.7—100 vol %, the upper limit (100 vol %) indicating that hydrazine vapor is self-explosive. Decomposition can be touched off by catalytic surfaces. The presence of inert gases significantly raises the lower explosive limit (10) (Table 2). 

Properties of the principal hydrocarbons found in commercial hexane are shown in Table 9. The flash point of / -hexane is —21.7 °C and the autoignition temperature is 225°C. The explosive limits of hexane vapor in air are 1.1—7.5%. Above 2°C the equiUbrium mixture of hexane and air above the Hquid is too rich to fall within these limits (42). 

Diketene is a flammable Hquid with a flash point of 33°C and an autoignition temperature of 275°C. It decomposes rapidly above 98°C with slow decomposition occurring even at RT. The vapors are denser than air (relative density 2.9, air air = 1). The explosive limits in air are 2—11.7 vol % (135). In case of fire, water mist, light and stabilized foam, as well as powder of the potassium or ammonium sulfate-type should be used. Do not use basic extinguisher powders and do not add water to a closed container. 

As for the selectivity of DBO, the higher the reaction pressure and the lower the reaction temperature, the higher the selectivity. As for the reaction rate, the higher the reaction temperature, the larger the rate. Therefore, the industrial operation of the process is conducted at 10—11 MPa (1450—1595 psi) and 90—100°C. In addition, gas circulation is carried out in order to keep the oxygen concentration below the explosion limit during the reaction, and to improve the CO utili2ation rate and the gas—Hquid contact rate. 

Hydroxylamine sulfate is produced by direct hydrogen reduction of nitric oxide over platinum catalyst in the presence of sulfuric acid. Only 0.9 kg ammonium sulfate is produced per kilogram of caprolactam, but at the expense of hydrogen consumption (11). A concentrated nitric oxide stream is obtained by catalytic oxidation of ammonia with oxygen. Steam is used as a diluent in order to avoid operating within the explosive limits for the system. The oxidation is followed by condensation of the steam. The net reaction is... 

Many polymer films, eg, polyethylene and polyacrylonitrile, are permeable to carbon tetrachloride vapor (1). Carbon tetrachloride vapor affects the explosion limits of several gaseous mixtures, eg, air-hydrogen and air-methane. The extinctive effect that carbon tetrachloride has on a flame, mainly because of its cooling action, is derived from its high thermal capacity (2). 

For combustible dusts, the explosibility limits do not have the same meaning as with flammable gases and flammable vapors, owing to the interaction between dust layers and suspended dust. This protective measure can, for example, be used when dust deposits are avoided in operating areas or in the air stream of clean air lines after filter installations WTiere in normal operation the lower explosibility limit is not reached. However, dust deposits must be anticipated with time. When these dust deposits are whirled up in the air, an explosion hazard can arise. Such a hazard can be avoided by regular cleaning. The dust can be extracted directly at its point of origin by suitable ventilation measures. 

Oxygen, the second requirement for combustion, is generally not limiting. Oxygen in the air is sufficient to support combustion of most materials within certain limits. These limitations are compound specific and are called the explosive limits in air. The upper and lower explosive limits (UEL and LEL) of several common materials... 

The speed of a combustion reaction will be at a maximum at a certain fuel-air ratio that is generally close to the stoichiometric composition. It will be lower, however, for compositions closer to each of the explosive limits. The... 

A silver-gauze catalyst is still used in some older processes that operate at a relatively higher temperature (about 500°C). New processes use an iron-molyhdenum oxide catalyst. Chromium or cohalt oxides are sometimes used to dope the catalyst. The oxidation reaction is exothermic and occurs at approximately 400-425 °C and atmospheric pressure. Excess air is used to keep the methanol air ratio helow the explosion limits. Figure 5-6 shows the Haldor Topsoe iron-molyhdenum oxide catalyzed process. 

An impressive example of the impact of miniaturization on the explosion limit has been given for the oxyhydrogen reaction [18]. For a conventional reactor of 1 m diameter, explosive behavior sets in at 420 °C at ambient pressure (10 Pa). An explosion occurs at about 750 °C, when the reactor diameter is decreased to about 1 mm. A further reduction to 100 pm shifts the explosive regime further to higher pressures and temperatures. 

GP 3] ]R 3h] [R 4a] Safe operation in the explosive regime was demonstrated [103]. Catalytic runs with 1-butene concentrations up to 10 times higher than the explosion limit were performed (5-15% 1-butene in air 0.1 MPa 400 °C). A slight catalyst deactivation, possibly due to catalyst active center blockage by adsorption, was observed under these conditions and not found for lower 1-butene concentrations. Regeneration of the catalyst is possible by oxidation. 

Excess air is supplied to the oxidiser to keep the ammonia concentration below the explosive limit (see Chapter 9), reported to be 12 to 13 per cent (Chilton), and to provide oxygen for the oxidation of NO to NO2. 

For the case where the inlet naphthalene concentration is below the explosion limit and where one is successful in producing phthalic anhydride in commercial yields, the mole fraction oxygen in the reactant gases remains relatively constant. This point is readily seen by the construction of a mole table for a feed containing 0.75% naphthalene, of which 80% reacts to give phthalic anhydride and the remainder goes to C02. 

During oxy chlorination of ethylene to 1,2-dichloroethane, excess hydrogen chloride is used to maintain the reaction mixture outside the explosive limits. 

In liquid phase aerobic oxidation of p-xylene in acetic acid to terephthalic acid, it is important to eliminate the inherent hazards of this fuel-air mixture. Effects of temperature, pressure and presence of steam on the explosive limits of the mixture have been studied. 

The explosion limits have been determined for liquid systems containing hydrogen peroxide, water and acetaldehyde, acetic acid, acetone, ethanol, formaldehyde, formic acid, methanol, 2-propanol or propionaldehyde, under various types of initiation [1], In general, explosive behaviour is noted where the ratio of hydrogen peroxide to water is >1, and if the overall fuel-peroxide composition is stoicheiometric, the explosive power and sensitivity may be equivalent to those of glyceryl nitrate [2],... 

For leak detection, the sensor must be sensitive and fast enough to provide early leak detection so that action can be taken before the explosive limit in air is reached. Utilizing a fiber-optic sensor configuration could provide the best chance of meeting fast response and inexpensive and reliable goals. 

A mixture of hydrogen and chlorine gas, eventually in combination with air, can be very explosive if one of the components exceeds certain limits. In chlorine production plants, based on the electrolysis of sodium chloride solutions, there is always a production of hydrogen. It is, therefore, essential to be aware of the actual hydrogen content of chlorine gas process streams at any time. There are several places in the chlorine production process where the hydrogen content in the chlorine gas can accumulate above the explosion limits. Within the chloralkali industry, mainly two types of processes are used for the production of chlorine—the mercury- and the membrane-based electrolysis of sodium chloride solutions (brine). 

Butler, M.A., Fiber optic sensor for hydrogen concentrations near the explosive limit, Journal of Electrochemical Society, 46(138), L46,1991. 

Currently, pellistors are often used as guarding sensors in rooms where there is a risk of flammable gases leaking and causing explosion. Pellistors react to concentrations far below the explosion limits. As these pellistors have been specifically developed for this purpose, nearly all that are currently available work at an ambient temperature of below 50 °C. 

Ethylene oxide is produced by adding ethylene, oxygen, a methane diluent, and recycled carbon dioxide to a continuous reactor. Gaseous compositions are controlled carefully to keep the concentrations outside the explosion limits. 

Figure 3.2 depicts the explosion limits of a stoichiometric mixture of hydrogen and oxygen. Explosion limits can be found for many different mixture ratios. The point X on Fig. 3.2 marks the conditions (773 K latm) described at the very beginning of this chapter in Fig. 3.1. It now becomes obvious that either increasing or decreasing the pressure at constant temperature can cause an explosion.

It is informative, however, to consider the possible mechanisms for dry CO oxidation. Again the approach is to consider the explosion limits of a stoichiometric, dry CO—02 mixture. However, neither the explosion limits nor the reproducibility of these limits is well defined, principally because the extent of dryness in the various experiments determining the limits may not be the same. Thus, typical results for explosion limits for dry CO would be as depicted in Fig. 3.5. 

The higher-order hydrocarbons, particularly propane and above, oxidize much more slowly than hydrogen and are known to form metastable molecules that are important in explaining the explosion limits of hydrogen and carbon monoxide. The existence of these metastable molecules makes it possible to explain qualitatively the unique explosion limits of the complex hydrocarbons and to gain some insights into what the oxidation mechanisms are likely to be. 

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