Hydrocarbons, inflammation limits

Jones [Jones, Inflammation Limits and Their Practical Application in Hazardous Industrial Operations, Chem. Rev., 22(1) 1-26 (1938)] found that for many hydrocarbon vapors the LFL and UFL can be estimated from the stoichiometric concentration of fuel ... 

The theoretical air required for the oxidation of toluene to benzoic add, 1.5 mols of oxygen per mol of tolnene, amounts to about 30 cn. ft. of air measured at 20° C. (6S° F.) and one atmosphere, per pound of toluene oxidized. Similarly, the theoretical air for oxidation to bcnzalcle-hyde amounts to 20 cu. ft. per pound of tolnene oxidized. The explosive limits of toluene at ordinary pressures and temperatures as measured in narrow tubes or small vessels are lower limit, 0.003375 lbs. toluene per cu. ft. air (296.5 cu. ft. air per lb. toluene) and upper limit, 0.01927 lbs. toluene per cu. ft. air (52 cu. ft. air per lb. toluene).1151 In the case of the lower aliphatic hydrocarbons as methane and ethane it is well known that the explosive or inflammability limits widen with rise in temperature so that it could well be expected that mixtures of toluene and air leaner than the one given for the lower limit and richer than the one for the upper limit would become inflammable as the temperature becomes higher. The same is true for increase in size of the explosion chamber. Hence, although theoretical mixtures of toluene and air for formation of benzaldehyde and benzoic acid lie on the rich side of the inflammable range,... 

It is only in a few cases that the limits of inflammability of a vapour mixture can be calculated, thanks to the limits of inflammability of the components in the pure state of a mixture. In fact, Le Chatelier s law only applies to mixtures of saturated aliphatic hydrocarbons and to two or three other mixtures of inorganic substances (CO, H2) alone or with methane. 

The agreement between the observed and calculated values of L is very striking, and seems to point to a definite and dominating relationship between the calorific values of the combustibles named in the table —the paraffin hydrocarbons—and their lower limits of inflammation when mixed with air. 

The effect of pressure upon the limits of inflammability of saturated gaseous hydrocarbons is similar to that shown by Figure 12 for methane. 

In a chain reaction, once a source of energy activating the chlorine has been applied, the rate of reaction of chlorine gas with vapors of paraffin hydrocarbons is a function of the molar composition of the mixture. Within certain limits this reaction will become so rapid that inflammation of the mixture will occur because of lack of heat dissipation, and within still narrower limits the flame velocity will increase into detonation rates. An approximate range for these limits of propane and chlorine is presented in Fig. 6-4. 

Aliphatic hydrocarbons possess a high foaming efficiency and they are nontoxic. However, they are highly inflammable and safety precautions must be taken by the producers this is a severe limitation to their use. Chlorinated hydrocarbons do not present the flammability problem and they are largely used as physical blowing agents. For instance, chlorinated ethylenes are recommended to foam PVC and epoxy resins. 

When considering the combustion of hydrogen or some hydrocarbons, however, if we plot a curve of the limit of inflammation of the mixture in a pressure-temperature space, we obtain curve DCBA as shown in Figure 15.2. This curve divides the plane into two regions one on the right in which any mixture ignites spontaneously and a second region to the left of the curve in which the oxidation reaction occurs without inflammation. 

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