Hydrocarbons explosive limit

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). 

Some vent streams, such as light hydrocarbons, can be discharged directly to the atmosphere even though they are flammable and explosive. This can be done because the high-velocity discharge entrains sufficient air to lower the hydrocarbon concentration below the lower explosive limit (API RP 521, 1997). Toxic vapors must be sent to a flare or scrubber to render them harmless. Multiphase streams, such as those discharged as a result of a runaway reaction, for example, must first be routed to separation or containment equipment before final discharge to a flare or scrubber. 

Apparently only the lower explosive limit of 100 C is avail tble for this substance (see Part Three). The empirical formula linking the LEL to both temperatures could be used this would give an LEL of 0.93% at 20°C. What result does this approach provide, if in accordance with Hass, this substance is considered as belonging to entropic group 2 (hydrocarbons) and determine its pressure at the flashpoint temperature ... 

If a significant volume of gas (caused by a leak, for example) is exposed to an ignition source and this gas is mixed with air in proportions that are close to stoichiometric, the gas cloud can cause a lot of damage when it gives rise to a detonation. The accident at Flixborough is one example. The lower explosive limit of hydrocarbons is extremely low. If the carbon chain length exceeds 8, the autoinflammation temperature of a linear hydrocarbon is close to 200°C. All these parameters decrease with pressure. The table below shows to which extent pressure influences the AIT of ethylene ... 

Explosion limits have been estimated for mixtures containing Ci -C3 hydrocarbons. Air, Rust... 

They are sensitive to all flammable gases, and they give approximately the same response to the presence of the lower explosive limit (LEL) concentrations of all the common hydrocarbon gases and vapors. However it should be remembered that gas detectors do not respond equally to different combustible gases. The milli-volt signal output of a typical catalytic detector for hexane or xylene is roughly one half the signal output for methane. 

Indeed, in developing complete mechanisms for the oxidation of CO and hydrocarbons applicable to practical systems over a wide range of temperatures and high pressures, it is important to examine the effect of the H02 reactions when the ratio is as high as 10 or as low as 0.1. Considering that for air combustion the total concentration (M) can be that of nitrogen, the boundaries of this ratio are depicted in Fig. 3.3, as derived from the data in Appendix C. These modem rate data indicate that the second explosion limit, as determined... 

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. 

At temperatures around 300-400°C and slightly higher, explosive reactions in hydrocarbon-air mixtures can take place. Thus, explosion limits exist in hydrocarbon oxidation. A general representation of the explosion limits of hydrocarbons is shown in Fig. 3.9. 

Hydrocarbons exhibit certain experimental combustion characteristics that are consistent both with the explosion limit curves and with practical considerations these characteristics are worth reviewing ... 

FIGURE 3.9 General explosion limit characteristics of stoichiometric hydrocarbon-air mixture. The dashed box denotes cool flame region. 

If the reacting system is initiated under conditions similar to point 4, pure thermal explosions develop and these explosions have thermal induction or ignition times associated with them. As will be discussed in subsequent paragraphs, thermal explosion (ignition) is possible even at low temperatures, both under the nonadiabatic conditions utilized in obtaining hydrocarbon-air explosion limits and under adiabatic conditions. 

Equations (7.23) and (7.24) define the thermal explosion limits, and a plot of In (P / E02) versus (1/7)) gives a straight line as is found for many gaseous hydrocarbons. A plot of P versus T0 takes the form given in Fig. 7.3 and shows the similarity of this result to the thermal explosion limit (point 3 to point 4 in Fig. 3.5) of hydrocarbons. The variation of the correlation with the chemical and physical terms in B should not he overlooked. Indeed, the explosion limits are a function of the surface area to volume ratio (S/V) of the containing vessel. 

Soils with high moisture or clay contents may reduce the efficiency of the LTTD system. Hot spots in the influent soils may cause fluctuations in the dryer temperature. The vaporized hydrocarbon concentration in the kiln must be kept below the lower explosive limit (LEL). 

One useful blend currently being employed as a very effective silicone solvent on an industrial scale is a mixture of 80 vol % hydrocarbon heptane and 20 vol % perfluoropentane, called 1.-12808 (see Table 6.7). 1.-12808 is useful for applying silicone lubricants to numerous medical devices, such as needles, IV spikes, blood filters, and catheters. This mixture shows no flash point and no explosion limits in air. The presence of the more volatile PFC, relative to the HC,... 

This second-order decomposition of hydrogen peroxide has been studied independently, and its rate parameters are known. The activation energy is ca. 48 keal., and its lifetime is about 1 second at about 900°K. At temperatures of ca. 450° to 550°C., it proceeds at a sufficiently rapid rate to be responsible for initiating the normal explosion limit, which one finds for stoichiometric hydrocarbon-oxygen mixtures. 

The H + O2 competition is responsible for several important aspects of combustion phenomena. For example, the second explosion limit for hydrogen-oxygen mixtures is explained by the competition between H + O2 branching and termination (Section 13.2.6). The observed reduction in hydrocarbon-air flame speeds with increased pressure between 1 and 10 atm is caused by the branching-termination competition. For a given temperature, as the pressure increases, the concentration of [M] increases, which favors the termination reaction. Thus the chain branching competes less favorably for a greater portion of the flame, which diminishes the flame speed [427]. 

It would seem worth while, therefore to restudy the explosion limits of methane-oxygen and ethane-oxygen and also to study the effects of these hydrocarbons on the carbon monoxide-oxygen limits, with a view toward establishing whether these systems are connected in any way. In any case, valuable clues to the mechanisms of combustion of hydrocarbons can probably be obtained. 

H2/02 reaction - an example of a reaction with explosion limits, 249-252 Cool flames in oxidation of hydrocarbons - an analysis of the chemical behaviour and reactions involved, 254—259... 

The oxidation of butane (or butylene or mixtures thereof) to maleic anhydride is a successful example of the replacement of a feedstock (in this case benzene) by a more economical one (Table 1, entry 5). Process conditions are similar to the conventional process starting from aromatics or butylene. Catalysts are based on vanadium and phosphorus oxides [11]. The reaction can be performed in multitubular fixed bed or in fluidized bed reactors. To achieve high selectivity the conversion is limited to <20 % in the fixed bed reactor and the concentration of C4 is limited to values below the explosion limit of approx. 2 mol% in the feed of fixed bed reactors. The fluidized-bed reactor can be operated above the explosion limits but the selectivity is lower than for a fixed bed process. The synthesis of maleic anhydride is also an example of the intensive process development that has occurred in recent decades. In the 1990s DuPont developed and introduced a so called cataloreactant concept on a technical scale. In this process hydrocarbons are oxidized by a catalyst in a high oxidation state and the catalyst is reduced in this first reaction step. In a second reaction step the catalyst is reoxidized separately. DuPont s circulating reactor-regenerator principle thus limits total oxidation of feed and products by the absence of gas phase oxygen in the reaction step of hydrocarbon oxidation [12]. 

Marin et al. (250) attempted to model a reactor similar to that used by Alonso and co workers. Their simulations were compared with simulations representing a fixed-bed reactor operated under similar conditions. They concluded that the membrane reactor (with the external fluidized bed) was a viable technology for n-butane oxidation, but that it offered only a modest increase in MA yields relative to those realized in a fixed-bed reactor. Nonetheless, the safer operating conditions which keep the O2 and hydrocarbon flows separate, particularly with the oxidation of butane to MA, are desirable. Presently, MA yields are chiefly governed by the explosive limits of butane in air (i.e., 1.8%). Increasing the butane concentration with an optimized membrane reactor may increase overall MA yields. 

The cool flame phenomenon seems to be closely tied to the formation of aldehydes and peroxides in oxidation systems. In Fig. XIV. 10 is shown a typical example of the explosion limits for a hydrocarbon-oxygen mixture. The explosion region, except for a region of positive slope, resembles the limit curve for a thermal explosion. The transition between slow combustion and explosion is characterized by an intense luminous blue flame which appears after a short induction period and is followed by explosion. The induction periods are not more than a few seconds. 

The explosion limit dropped to lower laser fluence upon increasing the pressure of the (fluoromethyl)silanes. Below the explosion limit, chemical changes of the (fluoromethyl)silanes could only be detected after irradiation with as many as 10 laser pulses. The reaction products revealed an almost quantitative reduction of the CF bond and afforded the gaseous fluorosilanes Sip4 and Sip3H, the hydrocarbons CH4 and C2H2, and different solid Si/C/F/H materials. 

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